Symposium EN13 : Capacitive Energy Storage---Fundamentals, Materials and Devices

Since their discovery in 2011, the family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides known as MXenes have received considerable attention for energy storage applications due to their tunable electronic and physical properties. MXenes are ideal candidates for applications requiring materials that can store and deliver large amounts of energy at high rates. MXenes have high specific capacitances due to their redox-active transition metal surfaces and when MXenes are fabricated into binder free, free-standing films they have exceptional electronic conductivity (>8000 S/cm). Different MXenes are available with a large variety of surface chemistries, all of which have demonstrated high performance in energy storage applications. Titanium, vanadium, molybdenum, and niobium based MXenes are some of the more commonly researched members of this family of materials. More recently, a new family of double-ordered MXenes, such as Mo₂TiC₂, was discovered which have unique properties due to combinations of different inner and outer transition metal atoms.
Ion intercalation and redox reactions at the transition metal surfaces enable MXenes to store more charge than traditional double layer capacitors and due to their unique chemical composition and layered structure, MXenes can store charge in a wide variety of aqueous electrolytes with various charges (H+, K⁺, Na⁺, Mg²⁺, Al³⁺). Utilizing proton induced pseudocapacitance in acidic electrolytes leads to even higher energy densities, and the water confined within the MXene nanosheets enables ion transport rates that are far superior to any other known pseudocapacitive material. By using saturated aqueous electrolytes, redox reactions between alkali ions and MXenes over an extended voltage range is also possible, leading to enhanced energy densities and safer MXene based electrochemical systems. Furthermore, MXenes are not limited to operating in aqueous environments, and reversible intercalation and deintercalation of lithium and sodium ions from organic electrolytes has been demonstrated with competitive energy power densities and high charge-discharge rates. The same mechanism was also demonstrated in organic and ionic liquid electrolytes with large imidazolium and alkylammonium cations.
The ability MXenes have for storing and delivering large amounts of energy at high power densities in numerous electrolytes paired with the versatility with which they can be processed for use as active materials in energy storage devices puts MXenes in a promising position to lead the research and development of the next generation of devices for energy storage and delivery.

We will firstly show how the careful design of nanostructured carbons can help in preparing high energy density carbons for supercapacitor applications, using conventional electrolytes (solvent plus salt), as well as neat ionic liquid and ionogel electrolytes. Moving from double layer to pseudocapacitive materials, we will show how the control of the electrodes structure can help in preparing high capacitance electrodes using 2-Dimensional MXene materials in aqueous electrolytes. In neat ionic liquid electrolyte, the combination of in-situ XRD with electrochemical techniques evidenced a difference in the charge storage mechanism depending on the electrode polarization.
This set of results helped in developing our basic understanding of the ion fluxes at the electrolyte / material interface as well as ion interactions in confined pores. From a practical point of view, they offer new opportunities for designing high energy density supercapacitors and micro-supercapacitors.

Synthesis and crystal engineering of metal oxides typically focuses on creating single-phase materials. Our team emphasizes the importance of disorder as a means to increase the electrochemical charge-storage performance of transition-metal oxides, including when the materials are inherently disordered thanks to high surface-to-volume nanoscale expression within three-dimensional electrode architectures. Establishing structure when the energy-relevant material is nanoscale, disordered, and already incorporated as the functional component in a device-ready macroscale form-factor imparts challenges for structure determination. But when disorder serves to amplify performance, structural characterization becomes a necessary challenge. Amorphous and/or nanoparticulate phases are difficult to characterize via neutron or X-ray scattering because of broad Bragg peaks, so we turn to total scattering analyses that allow atomistic modeling of these difficult to characterize—but functionally important—composites via differential pair distribution function (DPDF) analyses. Two pseudocapacitive systems will be discussed as fabricated by electroless deposition of conformal, ultrathin films: (1) ~10 nm–thick manganese oxide on carbon nanofoam paper in which the oxide “paint” can be crystal engineered in-situ via ion exchange and thermal processing and (2) ~3 nm–thick ruthenium dioxide on silica fiber paper, in which electrical conductivity of the electrode can be tuned over three orders of magnitude by calcining without ripening the ruthenia nanoparticles that comprise the conductive shell. The effects of rearranging in-situ the structural order/disorder of the charge-storing oxide will be tracked by the pseudocapacitance or faradiac (battery-like) performance of the device-ready electrodes.

Triboelectrification is an effect that is known to each and every one probably ever since the ancient Greek time, but it is usually taken as a negative effect and is avoided in many technologies. We have recently invented a triboelectric nanogenerator (TENG) that is used to convert mechanical energy into electricity by a conjunction of triboelectrification and electrostatic induction. This presentation will focus on the integration of a TENG with an energy storage device for building a self-powered power pack. The density of the energy storage unit may not be the highest, but it is being charged continuously so that the energy will not be drained out. This is a new approach toward sustainable operation of small electronic devices.

[1] Z.L. Wang, L. Lin, J. Chen. S.M. Niu, Y.L. Zi “Triboelectric Nanogenerators”, Springer, 2016. http://www.springer.com/us/book/9783319400389
[2] Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano 7 (2013) 9533-9557.
[3] Z.L. Wang, J. Chen, L. Lin “Progress in triboelectric nanogenertors as new energy technology and self-powered sensors”, Energy & Environmental Sci, 8 (2015) 2250-2282.

Supercapacitors can deliver high power density and long cycle stability, but the limited energy density due to weak electronic and ionic conductivity has been a bottleneck in many applications. In this study, NiMn layered double hydroxides (LDH) with sulfidation is used to reduce the charge transfer resistance of energy storage electrodes. Fast reversible redox reactions with notably enhanced specific capacitance can be achieved. The incorporation of graphite oxide (GO) in NiMn LDH during sulfidation leads to simultaneous reduction of GO with enhanced conductivity, lower defects and doping of S into the graphitic structure. Cycling stability of the sulfidized composite electrode is enhanced due to the alleviation of phase transformation during electrochemical cycling test.
On the other hand, Mn-doped Ni sulfide (NMS) nanoparticles can be obtained by similar sulfidation process. The NMS nanoparticles deliver higher electronic conductivity and larger sodium diffusion coefficient compare to Ni sulfide (NS) nanoparticles. NMS/reduced graphene oxide composites (NMGS) can be obtained with the reduction of GO during synthesis. Conversion-based electrochemical mechanism of NMGS in carbonate-based electrolyte was confirmed using ex-situ transmission electron microscopy technology. With the use of ether-based electrolyte, the NMGS electrode maintains a capacity of 229.2 mAh/g at high current density of 5 A/g, and retain capacity of 206.1 mAh/g at 0.5 A/g after 2000 cycles. The high specific capacity, high rate performance with excellent cycling stability emphasizes the promising potential of NMGS as sodium ion battery anodes.

Jayan Thomas^1,2,3*, Nitin Choudhary¹, Chao Li¹, Yeonwoong Jung^1,2,4
1NanoScience Technology Center, University of Central Florida, Orlando, FL 32826.
²Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816.
³College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816.
⁴Electrical & Computer Engineering, University of Central Florida, Orlando, Florida 32816.
Supercapacitors have gained considerable attention as an energy storage device for many applications that require high power density including electric cars, heavy machinery, and aircrafts. Recently, many new materials have been proposed and developed for capacitive energy storage. Among these materials, 2-dimensional (2D) transition metal dichalcogenides (TMD) are an important category because of its remarkable active surface area and mechanical robustness for capacitive energy storage. In this presentation, we will deliberate the usefulness of TMDs for supercapacitor electrode fabrication. Especially, we will discuss the recent development of a 1D/2D (1D WO₃ core/2D WS₂ shell) nanowire electrode grown on a tungsten metal foil current collector electrode. This core-shell geometry facilitates charge storage on the outside shell and charge transport through the inside core. The encouraging cycling stability coupled with high mechanical stability under mechanical deformation of the assembled supercapacitor make these electrode materials very attractive for the construction of supercapacitors.

A new family of two-dimensional transition metal carbides, nitrides, and carbonitrides – collectively referred to as MXenes – have shown great promise in a wide range of applications starting from electrochemical energy storage to electromagnetic shielding. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions are the key for high-rate pseudocapacitive energy storage in MXene electrodes. For instance, the most studied MXene, Titanium carbide (Ti₃C₂) has shown high specific volumetric capacitances in the range of 1000-1500 F/cm³ at ultra-high rates. These striking features including high conductivity and capacitance make MXenes great candidate materials for microscale on-chip and flexible energy storage devices. The ease with which MXenes can be dispersed in aqueous and organic solvents to make colloidal, functional inks allows for printing, spray coating, drop casting, and spin coating of MXenes in any pattern that can be imagined for microscale devices. Thin film MXene coatings for flexible on-chip energy storage applications can easily be produced by spray coating and direct laser patterning technique. Simple doctor blade technique has produced thick film (>100 µm) MXene microsupercapacitors for flexible, paper supported energy storage systems. Asymmetric devices were fabricated by combining negative MXene electrodes with positive carbon and metal oxide electrodes in extending the voltage window and further improving the output energy and power densities of the devices. MXene based energy storage devices produced thus far have higher energy and power densities than comparable state-of-the-art microsupercapacitors currently being researched, which opens up new avenues for further exploring MXene architectures for development of the next generation of microscale energy storage devices.

Electrochemical capacitors (ECs) provide appealing energy storage solutions, because of their higher power density, better safety and greater life span relative to lithium batteries. Rational design and synthesis of two-dimensional (2D) materials (such as graphene, MXene, transition metal dichalcogenides, etc.) has shown great impact for transformative technological advances for ECs. We will discuss the discovery of a 2D conductive supramolecular hybrid framework material, as well as its structure and relevant electrochemical properties for ECs. The 2D framework comprises periodically stacked 2D nanosheets with 1.2-nm basal spacing. In contrast to the pre-existing framework materials (for example MOFs), this conductive 2D framework has large density (~1.8 g cm⁻³) and low porosity (16.5 m² g⁻¹). The electrochemical performance (up to 732 F cm⁻³ in neutral aqueous electrolytes) and pseudocapacitive intercalation mechanism will be highlighted.

The development of novel electrode materials that exhibit both high specific power and energy is at the focal point of current research activities.¹ Synergistically combining supercapacitor and battery materials yields hybrid electrodes with improved performance metrics.² To enable high charging and discharging rates, limitations posed by both electron and ion mobility need to be overcome. Consequently, advanced electrode materials have to offer high electrical conductivity and nanostructured surfaces yielding large electrode/electrolyte interfaces. For that purpose, a nanoscopic decoration of high surface area carbons with metal oxides is explored.
In this study, we synthesized hybrid electrodes of carbon and vanadium oxide by atomic layer deposition (ALD). The carbon substrate was obtained by hard templating using SiO₂ nanospheres that were removed by hydrofluoric acid. The obtained spherical pores were especially tailored to offer high internal surface area (1000 m²/g) and mesopore volume (1.18 cm³/g) and ensured a homogenous adsorption of ALD precursors during synthesis.³ Up to 65 mass% vanadium pentoxide was deposited inside the carbon mesopores without an obstruction of internal surface area or pore blocking. This ensured a high accessibility of electrolyte during electrochemical cycling. The hybrid materials were benchmarked as lithium and sodium intercalation hosts, resulting in specific capacities of 310 and 250 mAh/g per V₂O₅ at a rate of 0.5C, respectively, and retained about 50 % of the capacity at a high rate of 100C. Charge storage behavior was predominantly pseudocapacitive, that is, showing constant voltage profiles like a capacitor. The materials also showed a remarkable cycling stability, with an increase in capacity to 116 % of the initial value (lithium) and a retention of 75 % (sodium) after 2,000 cycles. The homogenous distribution and local confinement of nanoscopic V₂O₅ domains inside the tailored carbon matrix were identified as the reason for enhanced rate handling and cyclability of the material.
(1) Augustyn, V.; Simon, P.; Dunn, B., Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energy Environ. Sci. 2014, 7, 1597-1614.
(2) Dubal, D.; Ayyad, O.; Ruiz, V.; Gómez-Romero, P., Hybrid energy storage: the merging of battery and supercapacitor chemistries. Chem. Soc. Rev. 2015, 44, 1777-1790.
(3) Fleischmann, S.; Leistenschneider, D.; Lemkova, V.; Krüner, B.; Zeiger, M.; Borchardt, L.; Presser, V., Tailored Mesoporous Carbon/Vanadium Pentoxide Hybrid Electrodes for High Power Pseudocapacitive Lithium and Sodium Intercalation. Chem. Mater. 2017, 29, 8653-8662.

Monolayer molybdenum disulfide (MoS2) has been rigorously studied following the discovery of the first monolayer (or 2D) material, graphene, in 2004. The optical and electronic properties of monolayer MoS₂ are of great interest, due to possible application in nano-scale devices such as transistors and photodetectors. In addition to these properties, monolayer MoS₂ may possess the ability to contribute to pseudocapacitive redox reactions, when used as an electrode material in an electrochemical capacitor. Here, in order investigate its participation in faradaic charge transfer processes, we perform physical and electrochemical analysis of liquid-exfoliated few-layer MoS₂ flakes in combination with carbon nanoparticles (a well-studied electric double-layer capacitor electrode material). Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves of various concentrations of MoS₂/C have already shown significant pseudocapacitive properties in the exfoliated MoS₂. Reduction peaks are visible at +0.07 V and +0.20 V vs. Ag/AgCl in 0.05 M H₂SO₄, with corresponding oxidation peaks at +0.05 V and +0.18 V. With higher concentration of MoS₂, these peaks become more prominent, supporting the hypothesis that MoS₂ participates in faradaic charge transfer processes. Analysis of physical interactions between MoS₂ nanoflakes and carbon nanoparticles have been investigated through scanning electron microscopy (SEM), transition electron microscopy (TEM), and energy-dispersive x-ray spectroscopy (EDXS).

The presence of structural water in tungsten oxides leads to a transition in the energy storage mechanism from battery-type intercalation (limited by solid state diffusion) to intercalation pseudocapacitance (limited by surface kinetics). Here, we explore how these mechanisms affect mechanical electrode properties and present a pathway to utilize the mechanical coupling for local studies of electrochemistry. In operando atomic force microscopy (AFM) dilatometry at fast voltammetry sweep rates (up to 4 second charge/discharge timescales) is utilized to measure the electrode height changes via the AFM cantilever displacement during proton intercalation in anhydrous and hydrated tungsten oxides. It is found that the local mechanical deformation of the hydrated tungsten oxide is smaller and more gradual than for the anhydrous oxide, and occurs without hysteresis for the intercalation and de-intercalation processes, highlighting the different mechanical response of battery vs. intercalation pseudocapacitance mechanisms.

Electrochemical double-layer capacitors (EDLCs) represent the most important family of today’s commercially available Electrochemical Capacitors (ECs). Such systems can store charges electrostatically in the electrochemical double-layer that arises from the separation of charges at the electrode / organic electrolyte interfaces when polarizing the electrodes, and exhibit an excellent cycling stability combined with fair gravimetric and volumetric energy densities. Other types of ECs use “pseudocapacitive” materials, i.e. materials that use fast and reversible surface redox (faradaic) reactions to store energy. To combine the advantages of both organic EDLCs and aqueous symmetric systems, the design of aqueous asymmetric supercapacitors was proposed. These devices comprise either two pseudocapacitive materials, or a pseudocapacitive and a capacitive carbon-based material which exhibit complementary electroactive windows. As a result, operating cell voltages approaching 2 V can be obtained, leading to competitive energy densities without the disadvantages of using an organic electrolyte.
Both specific energy and power densities have to be increased and improving the volumetric capacitance of supercapacitors, which is one of the limiting factors of today’s stationary applications, is essential. In order to meet those requirements, solid state chemistry is an extremely powerful tool to design new materials. Our strategy was based on the study of materials that are electrochemically active in aqueous media, combining transition metals such as Fe, that is known to exhibit a pseudocapacitve behavior when used as low voltages in negative electrode materials such as Fe3O4 or FeWO4. The use of low-temperature synthesis methods in order to get nanosized particles with high specific surface areas is also required. Synthesis conditions and materials characterizations of the electrodes and also of full devices will be detailed in the presentation, highlighting the crucial role of the electroactive elements, the crystallographic structure, and the morphology of the synthesized materials on their electrochemical performance.

Activated carbon is still the most attractive material used as a component of electrochemical capacitor irrespective of electrolyte. Actually organic medium (volatile, toxic) is generally used. However, aqueous electrolytic solutions start to be more and more interesting despite a low thermodynamic stability range (1.23V). Performance of electrochemical capacitors operating in aqueous electrolyte can be significantly improved by modification of electrode materials as well as by application of redox reactions from electrolytes. Different types of aqueous electrolytes with a redox activity based on halides (iodides, bromides) and pseudohalides have been used for capacitance enhancement. For the voltage extension, combinations of electrode/electrolyte interfaces with various pH and different composition for both negative and positive electrodes were proposed, e.g. magnesium nitrate and potassium iodide. Electrochemical capacitor with a high operating voltage (1.8V) was successfully built, especially if separation of electrolytes was realized. The detailed electrical examination of such capacitor (galvanostatic charge/discharge, cyclic voltammetry, electrochemical impedance spectroscopy, floating, self-discharge etc.) confirmed a good cycling, perfect charge dynamics as well as beneficial energy and power values. In-situ techniques (Raman spectroscopy, Mass spectrometry) allow the safe operating voltage of capacitor to be determined. Another strategy to enhance capacitor performance is based on the electrodes modification by ammonia treatment. In this case pH gradient between positive and negative electrode interfaces allows a beneficial voltage extension. High energy and power values as well as stable cycling of capacitor have been confirmed. Finally, the capacitor characteristics (Ragone plot) obtained in aqueous electrolytes are comparable to the parameters achieved in organic medium.

Batteries and electrochemical capacitors (ECs) represent the most widely used types of electrochemical energy storage devices. The fact that carbon-based ECs can deliver greater power, have much faster response times and longer cycle life than batteries makes this approach very attractive for a number of applications. An important limitation to this technology is its low energy density. For this reason, there is widespread interest in pseudocapacitance, a faradaic process involving surface or near-surface redox reactions, that can lead to high energy density at high charge-discharge rates. While there has been considerable progress in identifying pseudocapacitive materials as well as nanoscale materials that exhibit some of these responses, the fact remains that these redox materials are usually wide band gap semiconductors whose low electronic conductivity limits their kinetics when prepared in thick electrodes. The present paper provides guidance towards the development of pseudocapacitive materials for high rate energy storage. An analysis based on time-dependent multi-dimensional simulations was applied to an architecture in which a pseudocapacitive material was deposited on a conductive scaffold. The computed faradaic capacitance, which arises from reversible faradaic reactions, gave values consistent with experimental results as it decreased continuously with increasing sweep rate and pseudocapacitive layer thickness. Our research on Li⁺ insertion into Nb₂O₅ and reduced MoO₃ is in good agreement with these simulations. This analysis provides guidance on the design of electrodes for high rate energy storage and identifies the key factors for attaining increased loading of the redox material.

Manganese oxides (MnOx) have a long history as charge-storing materials in electrochemical devices, ranging from electrolytic MnO₂ in primary alkaline batteries to spinel-type Li-MnOx compositions that serve as high-performance cathodes in Li-ion batteries. More recently the application of this class of oxides has extended to aqueous-electrolyte electrochemical capacitors (ECs) in which nanostructured MnOx–based materials enable pulse-power capabilities via fast pseudocapacitance charge-storage mechanisms. The ability of MnOx to alternately express battery-like and capacitor-like functionality offers intriguing prospects to design electrode materials and corresponding devices that deliver both high energy content and rapid charge/discharge response. We are exploring such opportunities with electrode architectures comprising nanoscale MnOx coatings affixed to porous carbon frameworks [1,2,3]. By varying such factors as the oxide crystal structure (layered birnessite MnOx vs. cubic spinel LiMn₂O₄) and the composition of the contacting electrolyte (mixtures of Na⁺, Li⁺, and/or Zn²⁺), the resulting electrodes exhibit either battery- or capacitor-like behavior as well as combinations thereof. We are also developing electroanalytical protocols to deconvolve the complex electrochemical response of such systems in terms of both time scale and current–potential profile.

1. A.E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, and J.W. Long, Nano Letters 2007, 7, 281–286.
2. J.W. Long, M.B. Sassin, A.E. Fischer, and D.R. Rolison, J. Phys. Chem. C 2009, 113, 17595–17598.
3. M.B. Sassin, S.G. Greenbaum, P.E. Stallworth, A.N. Mansour, B.P. Hahn, K.A. Pettigrew, D.R. Rolison, and J.W. Long, J. Mater. Chem. A 2013, 1, 2431–2440.

The ultrafast charge-discharge rate and robust cycling performance associated with capacitive electrochemical energy storage devices, makes them more appealing than batteries in some specific applications, such as electric vehicles. However, despite their high power performance, capacitive electrochemical energy storage devices suffer from relatively low energy density. Enhancing the capacitance density of these devices is one effective way to improve their overall energy density. Usually there are three types of capacitive energy storage mechanisms in electrochemical supercapacitors: double-layer capacitance, surface pseudocapacitance, and intercalation capacitance. Much has been done to improve the double-layer and pseudocapacitance of electrode materials. However, intercalation capacitance has not been sufficiently exploited. The Li⁺ intercalation properties of layered valence-sensitive VO₂ (B), which the layers are linked by strong covalent bond, has been heavily investigated due to its stable open-framework and high theatrical capacity (323 mAh g^-1 or 1163 C g^-1). However, the expected high-capacity and cycling stability have not been experimentally achieved despite many attempts with various VO₂ (B) nanostructures, probably due to the slow reaction kinetics.
To this end, we propose a monomer-assisted strategy to fabricate atomic thin two-dimensional (2D) VO₂ (B) nanoribbons to improve its capacitive energy storage performance. We demonstrate that these 2D-VO₂ (B) nanoribbons deliver unexpectedly high energy density, rate capability (> 140 mAh g^-1 at 100C) and ultralong lifespan (> 9000 cycles at 20C). Furthermore, we show that the 2D-VO₂ (B) offers much faster charge storage kinetics and enables fully reversible Li ions uptake and removal in its lattice using cyclic voltammetry, in-situ Raman, ex-situ XRD and ex-situ XPS studies. The higher specific capacity (> 600 mAh g^-1) of VO₂ (B) is attributed to its atomic-level thickness (theoretical capacity: 884 mAh g^-1 for monolayer VO₂ (B)). What’s more, we prove that the atomic-thin 2D feature can strongly decrease the intercalation energy of Li⁺ into VO₂ (B) crystal and further lead to lower diffusion barriers using detailed theoretical and experimental methods. We believe that transformation of the atypical layered (or non-layered) materials into ultrathin 2D geometry could lead to significantly enhanced pseudocapacitive performance.

We have synthsized new two dimensional nitrogen containing covalent organic framework (COF) materials with conductive backbones that may be ideal candidates for supercapacitor applications. The materials are "bottom up" synthesized rather than top down as in modifification of graphene or graphene oxide. We can produce materials with various sized ordered nanopores that can be functionalized with virtually any functional group. For supercapacitor applications we focus on lining the pores with charged carboxylate, quaternary nitrogens and sulfonate groups. Prelimary studies indicate that, depending on the dimension of the COF backbone, we can put as many as 6 to 12 carboxylate groups in each nanopore that have radii on the order of double layer thicknesses in electrolytes. This high density of charged pores allows us to approach the theoretical capacitive energy storage density. Composites with graphene and or graphene oxide increase the conductivity to reduce the time of charge/discharge cycles in these new materials.

The performance of electrochemical energy storage devices relies largely on a scrupulous design of nanoarchitectures and smart hybridization of active materials. Nanoarray electrodes are particularly investigated for power source in microelectronics, which requires high rates, high areal capacity/capacitance and long cycle stability. When the metal-ion battery electrode materials are designed into nanostructures (e.g., 1D array, 2D nanosheets), the pseudocapacitive effect may become dominating and can boost the high-rate performance. Our group has been actively working on nanoarray materials directly on conductive substrates as electrodes for Li-ion and Na-ion storage. In this talk, I will articulate our material design strategies and how the extrinsic pseudocapacitance effect contributes to the high-rate performance. This effect applies to intercalation, conversion, as well as alloy type electrodes. These materials include VO₂, SnS, and MoSeS. Full hybrid batteries (or called Li-ion or Na-ion capacitors) will also be presented based on such subtle designed electrodes.

Formation of thick, high energy density, and flexible solid supercapacitors is challenging because of difficulties in filling gel electrolytes into porous supercapacitor electrodes. If infilling is incomplete, the result are low areal and volumetric energy densities and poor mechanical properties. Here we demonstrate several different gel infilling approaches that overcome these challenges. Wire supercapacitors, supercapacitor electrodes up to 500 μm thick, and interdigitated electrode structures were all formed from nanostructured carbons and various gel electrolytes. In one example, the mechanical flexibility and stability of a thick electrode is significantly improved to the point that it can be rolled into a radius of curvature as small as 0.5 mm without cracking, and retains 95% of its initial capacitance after 5000 bending cycles. The areal capacitance and energy density of a bottom-up filled 500 μm thick supercapacitor is 2547 mF/cm² at 2 mV/s and 226.4 μWh/cm² at 2.02 mW/cm², respectively, at least 5 times greater than previously reported.

Porous carbon materials are highly demanded for a broad range of applications, such as catalysis/electrocatalysis, adsorption processes or energy storage. Specifically, the field of energy storage and supercapacitors is dramatically increasing and urging to develop porous carbon materials with controlled textural, structural and chemical properties following sustainable, efficient and economic synthesis protocols.
Within this context, my group has developed several synthesis approximations. On one side, the use of biomass or hydrochar, i.e. hydrothermally carbonized biomass, as sustainable platform for the production of advanced porous carbons by means of chemical activation approaches. Hydrochar has several advantages as carbon precursor over biomass, such as a more a more homogeneous structure and an increased degree of aromaticity, which ensures a higher yield of product in the transformation process. By combining hydrochar with a benign substance such as potassium bicarbonate, highly porous carbons with a supercapacitor performance that can compete with that of benchmark KOH-activated carbons can be obtained [1]. However, in certain cases, the soluble and/or melting ability of certain kind of biomasses or biomass derivatives is advantageous. That is the case of using solution synthesis approaches or templating strategies. In this regard, we have recently shown the synthesis of hierarchical porous carbons with a large porosity development from a variety of biomasses or biomass derivatives (e.g. microalgae, soya flour, glucose or glucosamine). By the selection of an appropriate carbon precursor, the porosity can be tailored in the micro-mesopore range [2].
On the other side, the use of organic salts comprising metals with an activating (K, Na) or templating (Ca, Zn or Fe) function, which represents a straightforward, sustainable approximation [3-6]. By their direct thermal treatment at high temperature and the subsequent removal of the inorganic species generated, porous carbons with a variety of pore structures and particle morphology can be obtained. Worth noting is the production of highly porous carbon nanosheets by using potassium citrate [4] or sodium gluconate [5] as precursor. Besides, N-doping of the materials can be accomplished quite easily in-situ or post-synthesis with an N-rich substance. These materials have shown enhanced performance when used in supercapacitors as a result of their special morphology and/or pore structure.

References:
[1] M. Sevilla, A. B. Fuertes, ChemSusChem, 2016, 9, 1880.
[2] M. Sevilla, G. A. Ferrero, A. B. Fuertes, Chem. Mater., 2017, 29, 6900.
[3] M. Sevilla, A. B. Fuertes, J. Mater. Chem. A, 2013, 1, 13738.
[4] M. Sevilla, A. B. Fuertes, ACS Nano, 2014, 8, 5069.
[5] A. B. Fuertes, M. Sevilla, ACS App. Mater. Interfaces, 2015, 7, 4344.
[6] G. A. Ferrero, M. Sevilla, A. B. Fuertes, Carbon, 2015, 88, 239.

Supercapacitors are rapidly emerging as a new class of charge storage devices that can be used in a broad range of applications. Supercapacitors distinguish from lithium ion batteries by their ability to be charged and discharged at ultrafast rates. Unfortunately, previous studies have primarily focused on improving the charge/discharge performance of supercapacitor materials at relatively low current densities. This is one of the main obstacles to more widespread adoption of supercapacitors for ultrafast charging devices. Carbons are the most widely researched candidates for supercapacitors. The rational creation of unique multi-scale pore network in carbon structure not only significantly increases the electrode surface area, but also facilitates ion diffusion and charge transfer. In this talk, I will present some recent development in a novel carbon architecture with multi-scale pores that achieves record high specific capacitance at ultra-high current density. The carbon electrode yields a remarkable gravimetric capacitance of 374.7±7.7 F g^-1 at a current density of 1 A g^-1, and more importantly, it retains 235.9±7.5 F g^-1 at an ultrahigh current density of 500 A g^-1. The electrode retains 60% of its capacitance when current density is increased by 500 times from 1 A g^-1 to 500 A g^-1. This performance greatly exceeds the performance of other conventional carbon electrodes. Key breakthroughs that made this performance possible include: 1) the carbon electrode with multi-scale pores exhibits an extremely large surface area of 2905 m² g^-1; 2) the unique combination of multi-scale pores allows efficient ion diffusion and charge transfer, supporting much faster charge/discharge rates without significantly degrading capacitance. The findings pave a way for improving rate capability of supercapacitors and their capacitances at ultrahigh current densities, which is a long standing challenge for ultrafast charging devices.

On the basis of our recent research on multilayered graphene gel membranes, this talk will present our personal perspectives on the following questions: (1) What are the intrinsic limitations of the traditional porous carbon materials for application in supercapacitors? (2) What makes graphene unique for use as electrodes in supercapacitors beyond high specfici surface area? (3) Applications of supercaapcitors beyond energy storage.

References:
[1] Cheng, C., Jiang, G., Garvey, C. J., Wang, Y., Simon, G. P., Liu, J. L. & Li, D. Science Advances, 2, e1501272 (2016).
[2] Wang, Y., Yang, X., Pandolfo, A. G., Ding, J. & Li, D. Adv.Energ. Mater., 6, DOI: 10.1002/aenm.201600185 (2016).
[3] Yang, X., Cheng, C., Wang, Y. Qiu, L. & Li, D., Science, 341, 534-537 (2013).
[4] Cheng. C. & Li, D. Adv. Mater. 25, 13-30 (2013).
[5] Yang, X. W., Zhu, J. W., Qiu, L. & Li, D. Adv. Mater. 23, 2833 (2011).
[6] Yang, X., Qiu, L., Cheng, C., Wu, Y., Ma, Z.-F. & Li, D. Angew. Chem. Int. Ed. 50, 7325-7328 (2011).
[7] Qiu, L., Zhang, X. H., Yang, W. R., Wang, Y. F., Simon, G. P. & Li, D. Chem. Commun. 47, 5810-5812 (2011).
[8] Li, D. Muller, M. B., Gilje, S., Kaner, R. B. & Wallace, G. G. Nature Nanotechnol. 3, 101-105 (2008).

Pollen is a kind of biomass waste, which is widely available and relatively low-cost. Similar to the other biomass materials, each kind of pollens has its own unique porous micro-structure, which makes it easy to contact and react with other chemical agents, hence, can be easily tuned and modified by the designed process. The content of protein in camellia pollen is very high (27.3%), suggesting the possibility to introduce sufficient element nitrogen, which can promote the capacitive performance of the electrode in final products after carbonization. In this work, carbon materials derived from camellia pollen for supercapacitor electrode were prepared through a facile hydrothermal process with different amount of NH₄BF₄ followed by carbonization. The effects of NH₄BF₄ on the morphology, microstructure, the development of elemental state, texture structure as well as the electrochemical performance of the products were studied. Among the various products, the sample treated by 0.015g/mL NH₄BF₄ during the hydrothermal process showed the highest specific surface area as well as the largest specific capacity. Besides, doped-N also contribute to the final capacity. The cycle measurements demonstrated that it has excellent cycling stability after 10000 cycles.

Electrical double-layer capacitors (EDLCs), commonly known as supercapacitors, store energy through reversible adsorption of electrolytic ions on the charged electrode surfaces. Extensive work on carbide-derived carbons (CDCs), ACs and templated carbons revealed that the sub-micro pores (< 1 nm) contribute significantly to the overall charge storage. Carbon materials with uni-modal pore size distribution curves (PSD) are essential for such a focused analysis. However, the difficulties in synthesizing such carbon materials and obtaining their PSDs reliably through gas sorption have hampered the progress. Meanwhile, research on graphene derivatives for charge storage in SCs has shown promising advances owing to good surface areas and electrical conductivities. However, much of the current research is devoted to improving the material porosity with a focus on PSDs, similar to the research on ACs. Surprisingly, little attention has been devoted towards exploring the possible confinement of the electrolytic ions into the unique layered structures of graphitic materials. An analysis of materials with varied inter-graphene sheets distance (d-spacing) in SCs would assess the possibility of such confinement.

In this context, this study is aimed at understanding the consequences of interlayer spacing on electrochemical charge storage. A class of graphene-like materials with varying d-spacing have been synthesized using alkyl diamines cross-linking and are characterized for their application in SCs. Electrochemical analysis of the materials in SCs using electrolytes containing tetraalkylammonium cations of different sizes (alkyl: ethyl, propyl, butyl and hexyl) showed limitations of ion adsorption in different materials. The PSD curves of the materials show no presence of sub-micro pores (< 1 nm) and thus relate the observed ion limitation to d-spacing. Hence, this work demonstrates that the inter-layer galleries in graphene like materials indeed confine electrolyte ions based on the size constrictions. A correlation between the d-spacing and the electrolyte ion sizes is established and a condition for maximum capacitances versus d-spacing for a specific electrolyte is being investigated.

The net capacitance obtainable from activated carbon (AC) based electrodes, for supercapacitors, is often hampered by inadequate electrolyte accessibility within the pores of the AC, in addition to space charge capacitance (Csc), in series with the expected double-layer capacitance (Cdl). In this talk, we aim to investigate whether a capacitance increase would be facilitated through the plasma processing of AC.

It was aimed to determine whether plasma processing could contribute to enhanced capacitance and energy density of activated carbon electrode based electrochemical capacitors, through the formation of additional surface charges. While an increase of up to 35% of the gravimetric capacitance, along with ~ 20% decrease in resistance, was obtained through optimal plasma processing, increased plasma exposure yielded a drastic reduction (/increase) in the capacitance (/resistance). It was also found that the capacitance and resistance modulation was a sensitive function of sample processing as well as electrochemical testing procedure.

Considering the complexity of modeling realistic porous matrices, a metric to parameterize the reach of an electrolyte into the matrix will be posited. It will be discussed as to how an optimal plasma processing is necessary for enhanced capacitance ifor supercapacitor applications.

Wearable electronics attract remarkable attention both in academia and industry because of their wide application such as smart textiles with built-in electronic functions. All wearable electronic devices require lightweight, wearable and high-performance energy-storage components to provide power. Regarding these requisitions, developing fabric-based energy-storage devices is an ideal solution because textile fabrics exhibit intrinsically mechanical flexibility, lightweight, high surface area and most importantly, the feasibility of device integration into wearable forms. Among many energy-storage devices, a supercapacitor shows distinctive advantages due to its high power density, fast charge/discharge capability, excellent reversibility, long cycle life and safety, which are very suitable for wearable applications.

Therefore, we report herein a cost-effective and continuous one-step electrospinning method for directly fabricating the flexible and wearable supercapacitor fabrics. The key novelty of our method is to use the wearable nickel-coated cotton fabric as a conductive collecting substrate, on which a high concentration of multi-walled carbon nanotubes (MWCNTs) can be directly electrospun to form a thin carbon nanofiber web on metallic fabric at the room temperature, without the need for any post-treatment. This enables the formation of an interconnected MWCNT framework through the electrospun nanofiber web, ensuring the high electrical conductivity and ion accessibility of the as-made fabric electrode. Supercapacitor fabrics assembled with these as-made electrodes can show good electrochemical performances and more importantly, can be easily integrated into wearable forms by using simple and conventional sewing technology. Considering the wide accessibility, high scalability and good variability of the electrospinning technique, we believe that there still exists a large room for improving the performance of the supercapacitor fabrics, which are very likely to become ideal integrated energy-storage devices for next-generation wearable electronics.

N-doped graphitic carbon nanorods were synthesized by thermal transformation of zeolite imidazolate framework-8 (ZIF-8) nanorods. The one dimensional morphology and pore structure of the carbon nanorods were readily tuned by using amphiphilic surfactant of tri-block co-polymer Pluronic F127 as a soft template. The as-synthesized carbon nanorods exhibit an ultra-high surface area of up to 2088 m² g^-1 and a bimodal distribution of micro- and meso-porous structures. In addition, the Nitrogen-doping carbon nanorods provides pseudo-capacitance that promotes electrochemical performance, rending a high specific capacitance of up to 297 F g^-1 at a current density of 0.5 A g^-1 in the three-electrode system . An ultra-long cycle life was also demonstrated by recording a 97.26% preservation of capacitance after 10⁴ cycles of charge-discharge at a current density of 4.0 A g^-1. Furthermore, their electrochemical performance were evaluated with the assembled symmetrical supercapacitors that specific capacitance, energy density and power density could be as high as 130 F g^-1, 18.0Wh kg^-1 and 4000 kW kg^-1.

Boron carbon nitride (BCN) is a novel ternary hybrid material between carbon and BN with tuneable compositions of B-C-N.¹ So far, the nanostructures of BCN include zero-dimensional (0D) BCN nanoparticles, one-dimensional (1D) BCN nanoscrolls, two-dimensional BCN nanosheets and three dimensional (3D) BCN monolith.^2-4 With the controllable band gaps (0-5.5 eV), BCN nanomaterials have shown many striking properties such as cathodoluminescence (CL) emission and photoredox catalytic activities.² Besides, the hetetro-polarity of B,N bonding greatly stimulates the electrochemical activity, benefiting the versatility of BCN nanomaterials in energy conversion and storage applications such as oxygen reduction reaction (ORR) and lithium-ion batteries.^4,5 As supercapacitor electrodes, carbon-based nanomaterials such as carbon nanotube and graphene are most widely reported. Since they exhibit the merits of light weight, high surface areas and good conductivity compared with transition metal oxides and conducting polymers. Nevertheless, most carbon materials only show electrical double-layer capacitance (EDLC), which restrict the capacity. To enhance the capacitance, recently heteroatoms such as B, N doping are preferred. By introducing B or N elements, it generates a pseudo-capacitance, endowing faster reversible redox reactions and thus resulting in improved capacity. Therefore, it is believed that BCN nanomaterials are promising for supercapacitor electrodes. For one thing, B, N co-doped in the carbon network could induce larger faradic pseudo-capacitance than single element doping. For another, when contacting the aqueous electrolytes, the hetero-polar B-N bonding is capable of providing an extra dipole, thus likely facilitating the relative wettability between electrode materials and electrolytes. As a result, more ions could access the pores as well as being attached on the surface of BCN than that of pure carbon nanomaterials.¹ However, there still remain some challenges. As supercapacitor electrode, BCN nanomaterials generally maintain high capacitance at relatively low charge and discharge current. For the reason that, the faradic pseudo-capacitance induced by heteroatoms tends to be irreversible and deteriorative under fast charging/discharging. In addition, excessive B-N domains would reduce the conductivity. Hence, in the future study, it is necessary to elaborately design BCN nanomaterials with decent B, N doping and structure for better improving the electrode performance.

1.J. Wang, C. Chen, C. Yang, Y. Fan, D. Liu, W. Lei, Curr.Graph.Sci. 2017, 1, 1
2.W. Lei, D. Portehault, R. Dimova, M. Antonietti, J. Am. Chem. Soc.2011, 133, 7121.
3.J. Wang, J. Hao, D. Liu, S. Qin, C. Chen, C. Yang, Y. Liu, T. Yang, Y. Fan, Y. Chen, W. Lei
Nanoscale 2017, 9, 9787.
4. J. Wang, J. Hao, D. Liu, S. Qin, D. Portehault, Y. Lin, Y. Chen, W. Lei, ACS Energy Lett. 2017, 2, 306.
5. W. Lei, S. Qin, D. Liu, D. Portehault, Z. Liu, Y. Chen, Chem commun. 2013, 49, 352.

Development of energy storage systems with high energy, power density and long cycle life is very essential, as the global energy demand continues to grow with the growing dependence on portable electronic devices, electrically driven vehicles and smart grids. Hybrid capacitors that combine the advantages of both intercalation of cations (faradaic) and adsorption of anions (double layer) in the anode and cathode part from electrolytes, provides high energy and power density, respectively.^[1,2] Recently sodium based devices are encouraged globally due to the dwindling of lithium resources.^[3] Na-ion hybrid capacitor emerged as a vital energy storage device and there is huge interest in designing efficient novel Na-insertion electrode material as the anode. Here we present the hydrothermally synthesised flower shaped layered sodium titanium oxide hydroxide [Na₂Ti₂O₄(OH)₂] as anode material, exhibits pseudocapacitive nature with a specific capacity of 150 mAh g^-1 at 100 mA g^-1 with good electrochemical kinetics showing a capacitive behaviour of 57.2% to the total capacity (323.3 C g^-1), at 1.0 mV s^-1, while the rest is due to Na-intercalation. Also conducted the diffusion coefficient studies of Na-ions inside the anode material by using GITT and EIS methods.^[4] Further, a full cell Na-ion capacitor is assembled with Na₂Ti₂O₄(OH)₂ as anode and chemically activated Rice Husk Derived Porous Carbon (RHDPC-KOH) as cathode, by using non-aqueous electrolyte (1 M NaPF₆ in Propylene Carbonate). The device exhibits excellent electrochemical properties and delivers a remarkable energy density of ~65 Wh kg^-1 with a maximum cell voltage of 4 V with more than ~ 93% capacitive retention after 3000 cycles.

Reference
1. K.Naoi et al., Energy Environ.Sci. 5, (2012), 9363-9373.
2. B. Babu et al., Electrochim. Acta 211, (2016), 289-296.
3. M. D. Slater et al., Adv. Funct. Mater. 23, (2013), 947–958.
4. M.V. Reddy et al., Electrochimica Acta, 128 (2014) 198-202.

Simple and easily integrated design of flexible and transparent electrode materials affixed to polymer based substrates hold great promise to have a revolutionary impact on the functionality and performance of energy storage devices for many future consumer electronics. Here, we demonstrate an environmentally friendly, simple yet scalable approach to produce optically transparent and mechanically flexible all-solid-state supercapacitors. These supercapacitors were constructed on tin-doped indium oxide (ITO) coated polyethylene terephthalate (PET) substrates by intercalation of a polymer-based gel electrolyte between two reduced graphene oxide (rGO) thin film electrodes. The rGO electrodes were fabricated simply by drop-casting of graphene oxide (GO) films, followed by a novel low-temperature (~250 ^oC) vacuum-assisted annealing approach for the in-situ reduction of GO to rGO. A trade-off between the optical transparency and electrochemical performance is determined by the concentration of the GO in the initial dispersion; whereby the highest area specific capacitance (~ 650 μF cm^-2) occurs at a relatively lower optical transmittance (24%). Additional experiments demonstrated that the devices are mechanically flexible and exhibit stable cycling with a capacity retention rate above 90% under various bending angles and cycles.

Ultracapacitors are widely used in modern power electronics, automotive industry and renewable energy systems. The main component of the ultracapacitor design is the electrode system on the basis of highly-porous carbon materials that have a number of advantages: a large surface area (up to 2000 m²/g), low resistance, reasonable cost.
The aim of the work is to investigate nanoporous material – carbon fabrics, which is used as electrodes in rechargeable energy storage capacitors (ultracapacitors). The performed studies resulted in determination of the investigated carbon material structure, determination of impurities composition of carbon material and change of impurities content depending on thermal treatment in vacuum.
For the investigation of impurities composition of carbon material and change of impurities content the thermal treatment in vacuum was developed. Dimensions and structure of the fibers are tested by JEOL JSM-6610LV scanning electron microscope (SEM). SEM has Oxford Instruments Inca X-Max20 with X-ray spectroscopy energy-dispersive analyzer (EDX) which is used for the elements concentration control.
The samples annealing was carried out at temperatures of 400°C, 600°C, 800°C and 1000°C during 1, 2 and 4 hours in 10^-6 Torr vacuum. The main observed impurities are O, Al, Fe, Cr, Na, Si, Cl, Ni. On the fiber surface any modifications were not detected. The oxygen diffusion from the volume to the surface was detected. The microanalysis of the elements concentration along the fiber cross-section was made before and after annealing. The average fiber diameter was 7.5 mm. It was clearly detected than annealing leads to oxygen redistribution in the fiber. After annealing the oxygen concentration decrease in the fiber volume took place.
The carried out research on study of the structure of carbon fabrics and impurities on its surface makes it possible to establish the impurities structure and composition. The main impurity type is oxygen, which is in a bound state throughout the total volume of the material. As a result of carbon fabrics annealing in vacuum, oxygen diffuses to the surface. The regularities of oxygen diffusion in carbon are established. The annealing temperature and time increase result in increase of the ultracapacitor capacity and its parameters stabilization. Obtained results are important for the ultracapacitor use in energy storage devices.

In this work, graphene film is readily produced by an in-situ chemical reduction method, which involves simultaneous film formation and chemical reduction. In terms of versatility, graphene films produced with different types and doses of reductants, different thicknesses and areas are also successfully produced by this in-situ chemical reduction method. The graphene film produced with reductant of HI/CH₃COOH (G_(HI/CH3COOH)) has the highest electrical conductivity of 6900 S m^-1 and unique loose multilayered structure, and also exhibits excellent capacitive performance. Typically, the supercapacitor based on G_(HI/CH3COOH) exhibits the highest areal capacitance (C_A) of 152.4 mF cm^-2 at 2 mA cm^-2 and good rate performance (89% retention at 8 mA cm^-2). Notably, the C_A has no decay with the areas of G_(HI/CH3COOH) increased from 1×1 cm² to 5×5 cm², indicating the highly electrical conductivity and uniformity of produced G_(HI/CH3COOH). In addition, the flexible solid-state supercapacitor with fascinating cycling stability (97% retention after 10000 cycles) and electrochemical stability (negligible capacity loss after 4000 bending cycles) is also obtained by employing the G_(HI/CH3COOH) electrode material and polymer electrolyte. These results demonstrate that the in-situ chemical reduction method produced graphene film holds great potential for applications in wearable energy storage devices.

In the past decades, supercapacitors have attracted great attentions due to their high power density, long life cycle, and fast recharge capability. In general, the electrode materials for supercapacitors are divided into two categories on the basis of the energy storage mechanism: electrical double layer capacitors (EDLCs) and pseudocapacitors (PCs). However, PCs exhibit much larger capacitance values and energy density than EDLCs due to their fast and reversible redox reactions of the electrochemically active electrode materials.
Mixed transition-metal oxides (MTMOs), such as single-phase ternary metal oxides with two different metal cations, typically in a spinel structure (donated as A_xB_3-xO₄, A, B = Co, Ni, Zn, Mn, Fe, ie.), have captured much attention as promising electrode materials for supercapacitors. Among the MTMOs, compared with NiO and Co₃O₄, the spinel nickel cobaltite (NiCo₂O₄) exhibits better electrical conductivity and higher electrochemical activity. However, the relatively weak conductivity and small specific surface area make the capacity greatly lower than the theoretical value. Therefore, numerous efforts have been made to optimize the supercapacitors performance of NiCo₂O₄ via various methods, including control of microstructures, crystallinity, and electrical conductivity.
Herein we have designed two different NiCo₂O₄/carbon composites as electrode materials for high performance supercapacitor: (1) The CNS/NiCo₂O₄ core-shell structural sub-microspheres, in which the CNS act as a core and NiCo₂O₄ coated on the CNS surface, exhibited a high specific capacitance and excellent cycling stabilities at high current density. (2) The sandwich-like NiCo₂O₄/rGO/NiO heterostructure composite on nickel foam, wherein the components were assembled into a uniform structure and each component could partially retain its individual traits to improve electrochemical properties. The above rational combination of NiCo₂O₄ and carbon material can provide a synergistic effect for supercapacitors to enhance the electrochemical performance. The conductive carbon material not only facilitate the electron transport, but also effectively prevent the NiCo₂O₄ agglomeration and ensure the full utilization of the electroactive materials, which lead to the high supercapacitor performance of the composites.
This work was supported by the Shenzhen Science and Technology Innovation Committee 2017 basic research (free exploration) project of Shenzhen City of China (no. JCYJ20170303170542173).

Among various morphologies of nanomaterials, hollow spheres are of great interest because of their high ratio of surface to volume, large pore volume and low density, which could be exploited for applications in controlled encapsulation-release of drugs and medical diagnostic, energy storage and conversion, photocatalysis, chemical sensors, and photonic crystals. In the context of magnetism, magnetic hollow spheres can show unique physical properties compared to those of flat thin films and their solid counterparts of the same sizes, due to their confined hollow geometry and curved surfaces. It is known, that coercivity is dependent on domain-wall motion and the barrier to domain-wall propagation along a curved surface is larger than that of a flat surface. Due to growing application of nanoscale magnetic hollow spheres in biomedical end energy fields it remains important to understand the influence of growth parameters to prepare Fe₃O₄ with highly homogeneous features in terms of size and shape.
In this work, effect of hydrolyzing agents such as urea, ammonium bicarbonate (ABC), dodecylamine (DDA) on morphology, size and electrochemical activity of Fe₃O₄ nanospheres was investigated. For comparison, Fe₃O₄ nanospheres were also synthesized without hydrolyzing agent. The structural and morphological assessment of the synthesized Fe₃O₄ nanopowder was performed using x-ray diffraction, scanning electron microscopy and surface area analysis. The room temperature magnetic properties were studied via vibrating sample magnetometer. The scanning electron microscopy images showed nanospheres of Fe₃O₄ with a range of sizes (150-330 nm) which depend on hydrolyzing agents used. All the synthesized samples were crystalline in structure with distinct signature of magnetite phase. The surface area analysis indicated that these particles were mesoporous in nature. VSM measurement show that Fe3O4 prepared via hydrolyzing agent display high magnetization ~85 emu/g with average coercivity in the range of 150 Oe, Different hydrolyzing agents were observed to have minimum influence on the magnetic property of Fe3O4 hollow spheres. Electrochemical characteristics were investigated using cyclic voltammetry and galvanostatic measurements. Cyclic voltammetry measurements were performed in three different electrolytes viz. KOH, NaOH, and LiOH and observed that specific capacitance of the synthesized Fe₃O₄ depend on electrolyte used. Relatively high specific capacitance of 173.8 F/g was observed for Fe₃O₄ prepared using DDA in 3M KOH electrolyte. Fe₃O₄-DDA also showed excellent cyclic stability as well, retaining 107% of specific capacitance value at up to 5,000 cycles measured. The study clearly elucidates the effect of hydrolyzing agent on physical and morphological properties of Fe₃O₄. In addition, through electrochemical testing the study illustrates the choice of aqueous electrolyte in optimizing the electocapactive performance of Fe₃O₄ nanospheres.

The flexible supercapacitor, with high power density, rapid charge-discharge rate, and long cycle life, is one of the most promising energy storage devices, and has potential applications in portable electronic devices, such as roll-up displays, electronic papers, wearable devices, mobile phones, and computers. On account of their outstanding mechanical properties, high yield, low cost, renewability, and environmental friendliness, natural fibers such as flax and cotton have been regarded as promising substrates or precursors for flexible supercapacitor electrodes.
Free-standing and flexible activated flax fabrics (AFFs) with hierarchical meso/microporous structure have been prepared through a novel one-step synthetic strategy of rapid carbonization/activation of flax fabrics in CO2. The fast heating and the high partial pressure of CO2 inhibit the decomposition of flax during heating up and thus keep more char materials for gasification at high temperature. It is found that such a process could retain a considerable amount of oxygen groups and creates relatively large pores, bringing a one-step process to carbonize and activate flax fabrics at the same time, and offering the freedom of tuning the mesopore volume/total pore volume (Vmeso/Vtotal) ratio. Notably, the Vmeso/Vtotal ratio is significantly increased from 27.6 % to 67.0 %. As a result, the flexible electrodes show excellent electrochemical performance in aqueous electrolyte, including a large specific capacitance of 205 F g-1 or 140 F cm-3 (at current density of 0.1 A g-1), a good rate capability (139 F g-1 or 95 F cm-3 at 35 A g-1) and an excellent cycling stability (~96.6 % retention after 3000 cycles). The excellent rate performance can be attributed to the improved ion transport (due to large pore size) and the good wettability (due to the oxygen group) of electrolyte.
In another work aiming at increasing the specific capacitance of natural fiber-based flexible supercapacitor electrode, MnO2/carbonized cotton textile with high mass-loading and tunable morphology has been developed. After a simple carbonization process, the cotton cloth with high surface area (585 m2 g-1) served as 3D binder-free and flexible scaffolds to anchor MnO2 nanostructures. The morphology of MnO2 nanostructures was tuned and optimized into curled sheet-like, which provided large surface area and could also release large local stress. Electrochemical measurements showed that the curled sheet-like MnO2 had a specific capacitance of 465 F g-1 at 0.1 A g-1, and exhibited an excellent cyclic stability with a specific capacitance retention ratio of 95% after 5000 cycles (at 10 A g-1). Due to the flexible nature of cotton textile, the hybrid electrodes could be bent freely, and the capacitance and cyclability almost remained unchanged even at a bending angle of 150 degrees.

Pseudocapacitive nickel hydroxide was synthesised at particle sizes from around 5–500 nm using hydrothermal methods, with surfactant removed post synthesis using ozone. Syntheses using elctrochemical deposition to produce porous thin films have also been studied. The products have been used to study the depth of charging into the material. Specific capacitance peaked at around 10 nm particle diameter. Further reduction in particle size offered no improvements to specific capacitance, suggesting a depth of charging of 5 nm. Capacitance per unit area was not constant at all particle sizes. This suggests either the anisotropic growth of a more active crystal face or an overestimation of the electroactive surface area for small particles.

To meet the increasing demand for high power energy storage systems, supercapacitors have been intensively developed in recent years. Supercapacitors have attracted great interest among researchers because they have higher power density than batteries and higher energy density than conventional dielectric capacitors. Supercapacitors can be divided into two types: electrochemical double layer capacitance and pseudocapacitance (i.e. transition metal oxides). Transition metal hydroxides exhibit high pseudocapacitance, and nanostructured carbon materials have high electrochemical double layer capacitance as well as high electronic conductivity. Therefore, transition metal hydroxide/nanostructured carbon hybrid material can enhance the specific capacitance and electronic conductivity thus increase both energy density and power density.
In this research, we used a facile hydrothermal method to synthesize a novel ternary metal hydroxide/nanostructured carbon hybrid material as electrode for supercapacitors. This hybrid material exhibited high specific capacitance (> 1000 F g^-1) which was mainly contributed from ternary metal hydroxide, while showed good capacitance retention which was benefited from nanostructured carbon. The crystallinity and microstructure of this ternary metal hydroxide/nanostructured carbon hybrid material were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The electrochemical properties were characterized by cyclic voltammetry and galvanostatic charge/discharge cycle measurements.

Yuqing Kuai, Meitang Liu,^* Hongwen Ma, Tianlei Wang
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P. R. China
^*E-mail: mtliu@cugb.edu.cn
The imminent of fossil forces and the environment problem triggered by it have intensified studies for renewable, cleaner and cost-effective energy storage/conversion devices. [1-2] Supercapacitors, as a bridge between traditional capacitors and batteries, possess higher power density and longer cycle life than batteries. [3-4] Till date, researches based on transition metal oxides and sulphides as faradaic electrode materials have been fruitfully inspected in quantity. [5-6] However, in order to obtain enhanced capacitive performance electrode materials, the same main group selenides are explored recently due to its lower electronegativity. [7] In this work, we synthesized a 3D flower-like nanosheet nickel selenide electrode by one step selenization of flower-like nickel precursors. Comparing with flower-like nickel precursors, the nickel selenide exhibits better specific capacity and rate capability. The specific capacity almost enhanced 5 times of nickel precursors. Besides, the 3D flower-like nanosheet nickel selenide electrode also demonstrates good cycling stability. Above all results, it is believed that the proposed 3D flower-like nanosheet nickel selenide electrode could be a new potential faradaic electrode in the development of hybrid supercapacitor.

Reference
[1] X.F. Wang, B. Liu, Q.F. Wang, W.F. Song, X.J. Hou, D. Chen, Y.B. Cheng, G.Z. Shen. Advanced Materials. 2013, 25, 1479-1486.
[2] A. Banerjee, S. Bhatnagar, K.K. Upadhyay, P. Yadav, S. Ogale. ACS Applied Materials & Interfaces. 2014, 6, 18844-18852.
[3] C.L. Zhang, H.H. Yin, M. Han, Z.H. Dai, H. Pang, Y.L. Zheng, Y.Q. Lan, J.C. Bao, J.M. Zhu. ACS NANO. 2014, 8, 3761-3770.
[4] H. Peng, C.D. Wei, K. Wang, T.Y. Meng, G.F Ma, Z.Q. Lei, X. Gong. ACS Applied Materials & Interfaces. 2017, 9, 17067-17075.
[5] T.L. Wang, M.T. Liu, H.W. Ma, Nanomaterials. 2017, 7, 140-150.
[6] Y. Fu, M.T. Liu, H.W. Ma, T.L. Wang, K.R. Hu, C. Guan, Electrochimica Acta. 2016, 191,916-922.
[7] P. Xua, W. Zeng, S.H. Luo, C.X Ling, J.W. Xiao, A.J. Zhou, Y.M. Sun, K. Liao. Electrochimica Acta. 2017, 241, 41-49.

Since the first exfoliation and identification of graphene in 2004, research on layered ultrathin two-dimensional (2D) nanomaterials has achieved remarkable progress. Realizing the special importance of 2D geometry, one may naturally anticipate that the controlled synthesis of non-layered nanomaterials in 2D geometry could yield some unique properties that otherwise cannot be achieved in these non-layered systems. Herein, we report a systematic study involving theoretical and experimental approaches to evaluate the intercalation pseudocapacitive energy storage capability in 2D atomic sheets of non-layered molybdenum dioxide (MoO₂). The uniform ultrathin 2D MoO₂ are synthesized using a monomer-assisted, growth-confined solution approach. When used as electrodes for symmetrical microsupercapacitors, with PVA/LiCl gel electrolyte, these quasi-all-solid-state devices exhibit high areal capacitance (63.1 mF cm^-2 at 0.1 mA cm^-2), good rate performance (81% retention from 0.1 to 2 mA cm^-2), and superior cycle stability (86% retention after 10,000 cycles), superior to its bulk counterparts. Furthermore, we show that capacitor-like charge storage in ultrathin 2D-MoO₂ occurs to a much greater extent compared to its un-exfoliated analogue, implying faster kinetics for Li storage. In addition, in comparison with other state-of-the-art systems, our devices offer the best performance in terms of volumetric energy and power density in the parallel-plate configuration (17.2 mWh cm^-3 at 0.35 W cm^-3). More importantly, the present method is quite general and can be used to topotactically synthesize other low-valence-state metal oxides/sulfides (or even metals) with unique architecture. We believe that our work identifies a new pathway to make 2D nanostructures from non-layered compounds, which results in an extremely enhanced energy storage capability.

We introduce high-performance ultrathin supercapacitor electrode prepared by amphiphilic ligand exchange-induced layer-by-layer (LbL) assembly of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in water, amine-functionalized multi-walled carbon nanotubes (NH₂-MWCNTs) in ethanol, and oleic acid-stabilized Fe₃O₄ nanoparticles (OA-Fe₃O₄ NPs) in toluene. Consecutive ligand exchange reactions between electrochemically non-active hydrophobic OA of Fe₃O₄ NPs and SO₃⁻ groups of PEDOT:PSS as well as between this hydrophobic ligands of Fe₃O₄ NPs and NH₂ groups of the NH₂-MWCNTs can be easily converted from densely packed hydrophobic Fe₃O₄ NPs into hydrophilic Fe₃O₄ NPs within the ternary component electrodes (i.e., (PEDOT:PSS/OA-Fe₃O₄ NP/NH₂-MWCNT/OA-Fe₃O₄ NP)_n, TC_n electrodes). This unique assembly process successfully increased the loading amount of high-energy Fe₃O₄ NPs within the electrodes, facilitated charge (ion/electron) transfer throughout the electrode supported by the porous conductive MWCNT network and semiconducting polymer (PEDOT:PSS). As a result, fabricated TC_n electrode exhibited high volumetric and areal capacitance of 408 ± 4 F/cm³ and 8.79 ± 0.06 mF/cm² at 5 mV/s, respectively. In addition, the areal capacitance of can be further enhanced by increasing the periodic number (n) of TC_n electrodes. Furthermore, the TC_n electrode also displayed excellent cycling stability (98.8 % of the initial capacitance after 5000 cycles) due to the multidentate bonding between PEDOT:PSS and Fe₃O₄ NPs and likewise between MWCNTs and Fe₃O₄ NPs after amphiphilic ligand exchange.

In recent times, demand for portable electronic devices like mobile phones, cameras and laptops etc. is increasing day by day. Energy storage devices such as batteries and supercapacitors have significant importance because of their high energy density and high power density, respectively.¹ Supercapacitor is gaining great amount of attention because it uses less toxic material, offers high power density, excellent electrochemical stability, wide range of operating temperatures and durability. A facile fabrication of low cost, efficient, stable, eco-friendly and earth-abundant electrode materials for supercapacitors is critical.² Layered double hydroxide (LDH) is new class of material having positively charged hydrotalcite-like layers, weakly bound, intercalating charge compensating anions and water molecules, has showed enormous supercapacitive performance.Layered double hydroxide (LDH) is new class of material having general formula (where, M²⁺ and M³⁺ are the bivalent and trivalent metal cations and A^n- is the charge balancing anion of valence n; ), has showed enormous supercapacitive performance.³ In this work, an ionic lamellar, two-dimensional (2D) nickel-iron layered double hydroxide (NiFe LDH) nanosheets over Ni foam have been synthesized via facile, cost effective and potentially scalable hydrothermal method. The as-prepared 2D NiFe-LDH nanosheets on nickel foam (NF/NiFe-LDH) was used as supercapacitor electrode. The electrochemical characterization techniques such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were used to characterize the material for its electrochemical properties, while SEM, TEM, XRD, BET, and XPS etc. techniques have been used for their morphological, structural and physical characterization. High specific capacitance of 700 F g^-1 at 1 A g^-1 was observed in three-electrode system using 2 M KOH as electrolyte. This work signifies great potential for low cost and eco-friendly NiFe LDH nanosheets as supercapacitor material.
Keywords: Supercapacitor; layered double hydroxide; energy storage
References:
[1] A. Tyagi, K. M. Tripathi, and R. K. Gupta, J. Mater. Chem. A 3, (2015) 22507.
[2] A. Tyagi, and R. K. Gupta, Nanomaterials: A guide to fabrication and applications, (CRC Press), 261 (2015).

[3] A. D. Jagadale, G. Guan, X. Li, X. Du, X. Ma, X. Hao, and A. Abudula, J. Power sources 306 (2016) 526.

Graphene’s remarkably large specific surface area (~ 2500 m²/g) has led to speculation that graphene supercapacitors could have specific capacitance as high as ~ 550 F/g. In practice, however, a number of factors reduce the specific capacitance of graphene-based electrodes. It is an ongoing challenge to untangle and quantify the various factors affecting the specific capacitance of graphene-based electrodes. In our work, we show that the capacitance per unit area of the graphene-liquid interface is much smaller than previously assumed. We use single-layer graphene on a flat, insulating surface to achieve well-defined surface area. Charge density in the graphene is precisely determined via Hall-effect transport measurements. For a variety of electrolytes, including 1 M Na₂SO₄ and the ionic liquid BMIM-PF₆, the capacitance is less than 0.05 F/m², i.e. much less than the common benchmark value 0.2 F/m² associated with bulk metals in contact with electrolytes. We interpret our results in terms of quantum capacitance and double-layer capacitance. Combining our experimental results and capacitance model we make realistic projections for the ultimate performance of graphene-based supercapacitors.

Vanadium oxide (V₂O₅) thin films due to layered VO₅ pyramids in orthorhombic crystal structure, visible transparency (Eg =2.5 eV) and varied oxidation states have multifunctional applications. Most emergent is the V₂O₅ based pseudo-capacitive energy storage with high pulsed power capability. The V₂O₅ exhibits energy storage functionality in high pH aqueous medium. This is detrimental to stability under charge-discharge due to electrode loss via mass transport. We demonstrate energy storage function in V₂O₅ supercapacitors using the ionic liquid gel electrolyte, address the electrode stability issue and enable flat solid-state cell assembly. With visible V₂O₅ and gel electrolyte transparency, supercapcitor device over conducting glass has potential for integration with transparent electronics. We used liquid synthesis to realize V₂O₅ films in layered structure. By inducing Li+ ion intercalation in V₂O₅ via LiClO₄ dopant in ionic liquid gel electrolyte, we investigated its effect on the active V₂O₅ layer thickness and over energy storage, power density performance of supercapacitors.

The V₂O₅ film electrodes were deposited by spin coating of HVO₄⁺ sol and air annealing at 300°C for 3h. Using Raman spectra peaks at 144, 283, 403, 526, 700 and 993 cm^-1 layered crystalline V₂O₅ structural phase was established. Optical spectra show 75% transmittance in 700-480 nm range. The ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄) with 0.2M LiClO₄ dopant formed as gel in PVdF/acetone solution was applied between two V₂O₅ electrodes enabling large interfacial contact by infiltration to fabricate supercapacitor device. The energy storage mechanism is derived from oxidation-reduction peaks observed in cyclic voltammetry (CV) plots. Basically, V₂O₅ surface reduction from V(5+) to V(4+) state at cathode upon charging is shown to involve intercalation of Li+ ions accompanied by oxidation at anode. Studies on V₂O₅ electrode thickness show additional energy storage from electrical double layer charging. The supercapcitor cells in solid-state platform showed energy density of ~ 4.8-7.3 Wh/kg scaling with cell voltage. Specific capacitance values ~ 35-40 F/g show voltage scan rate dependence implying diffusive ion transport lags continuous electron exchange. This reflects on the performance during charging and discharge phase. The discharge rates in V₂O₅ cells are higher in aqueous medium compared to other oxides, and with ionic liquid, we obtained high power density in the range 20-70 KW/kg. These energy-power values with added transparency factor are attractive for powering compatible electronics. The energy-power parameters of single V₂O₅ cell can be further enhanced by multiple stacking enabled by the solid-state design. This paper will report on detailed V₂O₅ electrode, synthesis, structural and electrochemical properties and the energy storage performance of supercapacitor with emphasis on storage mechanism with ionic liquid electrolyte.

Layered structure materials (α-MoO₃, LiNi_xCo_yMn_(1-x-y)O₂, and LiCoO₂) have been widely studied as promising electrode materials of energy storage devices. In spite of the high theoretical capacity of α-MoO₃, it is not widely used as the electrode material because of its low electrical conductivity. A viable route to overcome the low electrical conductivity is the introduction of oxygen vacancies into α-MoO₃, because oxygen vacancies can work as shallow donors in α-MoO₃ and consequently enhance the electrical conductivity of α-MoO₃. In addition, the introduction of oxygen vacancies can increase interlayer spacing of α-MoO₃. Therefore, the creation of oxygen vacancies effectively promotes faster charge and discharge storage and prevents volume change during Li-ion intercalation/deintercalation. In this study, we propose a facile and fast method to incorporate oxygen vacancies into α-MoO₃ using an intense pulsed white light (IPWL) reduction method. The IPWL irradiation transfers heat energy in the form of light from a xenon lamp that emits a light spectrum in the visible region. A comprehensive study has been carried out to examine the effect of IPWL irradiation on the creation of oxygen vacancies in α-MoO₃ and the effects of oxygen vacancies on the structure and pseudocapacitive charge storage properties of α-MoO₃. Since this simple and novel strategy is applicable to other transition metal oxides, it is a promising method for not only energy storage systems, but also for water splitting and catalysis systems.

Recently, light and stable energy conversion and storage resources for portable devices have attracted much attention. Among them, flexible supercapacitors are emerging as energy storage sources for wearable devices due to their high energy and power density, light weight, and flexible mechanical properties. Of the various materials that can be used as supercapacitor electrodes, conductive polymers have received much attention due to their low cost, easy fabrication and excellent electrochemical properties. In particular, various attempts for using conductive polymers such as polypyrrole as supercapacitor electrode materials have been made due to their high electrochemical capacitance, low density and mechanical flexibility. However, its wide use has been hindered because of its low cycle stability and poor rate capability. In this study, we demonstrate a flexible electrode for supercapacitor applications based on a hybrid structure of reduced graphene oxide (r-GO), single-walled carbon nanotubes (SWCNTs), and polypyrrole nanotubes (PPNTs). The r-GO/SWCNT combination resulted in enhanced cycle stability while the use of electrochemically deposited PPNTs maximized the electrochemical capacitance. The hybrid structure of r-GO/SWCNT/PPNT films showed high conductivity and large surface area, resulting in improved electrochemical performance, good cycle and rate stability compared to previous works. We expect that our hybrid structure of r-GO/SWCNT/PPNT film can open new possibilities for high performance flexible energy storage devices and wearable electronics.

At present, maximum utilization of sustainable and renewable energy resources to accomplish the world’s growing energy demand has become the prime focus of major world power and scientific research community. In this concern, development of resourceful energy storage devices (Fuel cells, supercapacitors and Li-ion batteries) have received gigantic attention of scientists and researchers worldwide. Recently, supercapacitors due to their high power density (>10 kWkg^-1), long cycle life (>10⁵ cycles) and fast charge discharge response have captivated much interest. However, the poor energy density of supercapacitors (SCs) is the main issue, which hinders their potential industrial utilization. Basically, carbon based materials (CNT, graphene, activated carbon and mesoporous carbon) are utilized as the active electrode materials for the development of practical electric double layer supercapacitors (EDLCs). Besides this, the chemical doping of foreign atoms (such as N, B, P, I and S) in the carbon based materials is an effective tool to achieve the electron-donor characteristics, which effectively enhance the electrochemical performance of the material. Such doping of heteroatoms in graphene would provide acid/base characteristics for upgraded electrochemical properties. Among the pure graphene, N and B co-doped graphene has attracted a lot of interest because of its superior electrochemical properties, cost-effective and eco-friendly behavior. In this work we developed a simple one step hydrothermal method to prepare N, B co-doped graphene at low temperature 180°C. Melamine diborate precursor has been utilized as source of N and B. Electrodes fabrication process done by the electrophoretic deposition (EPD) technique at 50 V. Best fabricated sample, labeled as NBG-0.7, exhibits highest specific capacitance of 217 Fg^-1 at the scan rate of 5 mVs^-1 which is about 1.2 times higher than the specific capacitance (180 Fg^-1 at 5 mVs^-1) of pristine reduced graphene oxide. The structural analysis, morphological studies, thermal stability and specific surface area of pure RGO and N, B co-doped graphene have been investigated via XRD, FE-SEM, TGA, TEM and BET surface area analyzer. The electrochemical measurements have been carried out using three electrodes measurement system in 6M KOH electrolyte using cyclic voltammetry (CV), galvanostatic charge-discharge cycling (GCD) and electrochemical impedance spectroscopy (EIS).

Keywords: N, B co-doping; reduced graphene oxide; energy storage; supercapacitors

Ti₃C₂ (MXene) nanosheets obtained by selectively etching of the aluminium layer from Ti₃AlC₂ (MAX phase), has shown a unique properties such as metallic conductivity, hydrophilicity, and high energy storage performance. The extremely high volumetric capacitance (~1500 F cm^-3) suggested that electrodes made of MXene can obtain higher performance than other electrode material with the same volume. This advantage not only beneficial in increasing the energy storage performance but also significantly decreases the volume of the device. MXene supercapacitor devices developed so far, are based on films and papers. Rear reports mentioned MXene in yarn supercapacitor due to several challenges in fabricating MXene-based fibre supercapacitors. Firstly, monolayer MXene flakes are not stable in high concentration due to aggragation. Secondly, small MXene flakes suffers high contact resistance among flakes which hinder efficient charge transfer. Thirdly, there is lack of an effective fabrication technique to achieve high loading of MXene fibre-shaped devices.

MXene-coated carbon fibre (CF) yarn supercapacitors were fabricated with the assistance of PEDOT-PSS as the conducting glue. CF was selected as the coating substrate because of its high electrical conductivity and excellent mechanical strength. By optimising the ratio between MXene and PEDOT-PSS, 9:1 was selected as the best ratio to study the influence of MXene loading. The result showed that the length specific capacitance increased linearly from 25.8 to 155.6 mF cm^-1 when MXene loading increased from 0.4 to 3 mg cm^-1. The energy density and power density of the YSC device reached ~4.1 mW h cm^-1 and ~372.6 mW cm^-1 at the current density 1 mA cm^-1. Additionally, face-to-face configuration was designed and showed stable energy storage performance under various mechanical deformations such as bending and twisting. The cyclic stability of the MXene YSC showed that approximately 90 % of the device capacitance was maintained after cycling for 10,000 times. This work demonstrates that MXene is a suitable candidate for YSC. This study opens up an opportunity to fabricating high-performance YSC that can be used in portable electronic devices including wearables.

Ultracapacitors, due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells, have attracted considerable attention for commercial applications such as plug-in hybrid electrical vehicles. Rational design of nanostructured composite electrode materials have demonstrated superior electrochemical properties in producing high-performance ultracapacitors. Incorporation of metal/metal oxide nanoparticles with appropriate size distributions into the interior regions of nitrogen-doped, activated carbon powders provides a feasible route to improve the energy density of ultracapacitor devices via the contribution of pseudocapacitance and improved electrolyte transport. In this project, we structurally tuned the porosity of commercially available, high-surface-area activated carbon powders to generate large amount of mesopores with pore widths in the range of 10-30 nm, which enables incorporation of niobium/niobium oxide nanoparticles. Nitrogen-doping of the activated carbon powders further enhances their capacitive performance. Such rationally designed high-surface-area nanocomposite carbon powders are fabricated into a monolithic hybrid capacitor electrode structure for significantly enhancing the energy density of ultracapacitors based on commercially available activated carbon powders.

Supercapacitors have garnered substantial attention in recent years due to their ultra-fast charge and discharge rate, excellent stability, long cycle life, and very high power density; for future applications including electric vehicles and portable electronics. I will first talk about supecapacitors based on pillared graphene nanostructures (PGN) grown on Ni foil/foam substrates conformally decorated with transition metal oxide nanoribbons and nanoparticles. The three dimensional electrode architectures of graphene floors with carbon nanotube pillars demonstrate long cycle electrochemical stability; and its integration with the pseudocapacitive transition metal oxides including MnO₂ and RuO₂ provides superior gravimetric capacitance; exceptionally high energy density and power density. Next, I will talk about PGN supercapacitors with large operational voltage window and ultrafast cycling achieved using organic tetraethyl ammonium tetrafluoroborate electrolyte. Then, I will discuss supercapacitor architectures based on Ni nanodendrites synthesized on Ni foil/foam substrates followed by conformal decoration with RuO₂ nanoparticles as a low temperature process, indicating long cycle life, high specific capacitance and high energy denisty. Finally, I will describe equivalent circuit design/analysis for supercapacitors connected in series and in parallel via electrochemical impedance spectroscopy, which are important for practical applications.

In recent years, interest in the development of alternative energy storage/conversion devices with high power and high energy densities has increased due to current high energy demands. Electrochemical capacitors or supercapacitors are attracting wide attention, because of their high-power density, excellent reversibility and long cycle life. Many researchers found Manganese dioxide (MnO₂) as promising materials for supercapacitors applications because of its natural abundance, high theoretical capacity, low environmental toxicity, and cost effectiveness [1,2]. We have synthesized MnO2/graphene oxide (GO) materials at room temperature using deionized water as a solvent as reported earlier [3]. The XRD pattern show all distinguishable peaks at 2θ =12.82^o, 18.22^o, 25.78^o, 28.74^o, 37.64^o, 42.10^o, 49.80^o, 56.34^o, 60.34^o, 65.52^o and 69.68^o, corresponding to (100), (200), (220), (310), (211), (301), (411), (600), (521), (002) and (541) crystal planes of a pure tetragonal α-MnO₂ phase. Relatively rectangular CV curves without redox peaks indicate the ideal electrochemical reversibility and capacitive properties of the composite materials. The galvanostatic charge-discharge studies were performed at various currents e.g. 500 mA/g and between 1A/g - 5 A/g with an increment of 1A/g. The specific capacitance of MnO2/GO composite materials were found between 143 F/g at 500 mA/g and 105 F/g at 5A/g discharge current, respectively. The detailed results on electrochemical characterizations will be presented at MRS Spring 2018 meeting.

Porous carbon materials such as activated carbon have applications in electrodes and sensors. Though they provide high surface areas and porosity, they are not always able to make strong connections to increase conductivity with other active materials. Graphene and graphite are highly conductive materials but lack surface area and porosity. Herein we propose a novel material synthesized from simple esterified carbohydrate-polymer precursors which exhibit both graphite/graphene sheets and porous, amorphous carbon structures. The carbohydrate-polymer precursor is used to grow graphene and graphite sheets on a Ni sputter coated Si/SiO2 substrate. Such a precursor allows an oxidation product to undergo esterification with poly(vinyl) alcohol and grow porous carbon structures. Spin coating distributes the product solution over the substrate and annealing in reducing atmosphere allows the Ni thin film to dissolve carbon feedstock and thus grow 2D graphene and graphite sheets directly bonded to a 3D network of porous carbon structures. Porous morphology is identified by SEM and analyzed by BET. XRD showing graphite and graphene growth on Ni substrate is collaborated with RAMAN spectroscopy. I-V curves indicate Ohmic contact between the substrate and the Porous Graphene Nanoarchitecture. Such a novel material shows promising applicability in capacitors and battery electrode materials.

Transition metal chalcogenides are considered as a potential alternative of carbon materials for high power supercapacitors with fast response. However, the studies so far have typically reported an operation frequency limited to 1 Hz or even lower. Here, 2D nanosheets of MoS₂ grown on carbonized cellulose fibers were investigated as electrodes for high power fast supercapacitors. MoS₂ nanosheets were grown along vertically oriented direction encircling carbon fibers in a hydrothermal process. Plasma pyrolysis technique was adopted to produce highly conductive carbon fibers from cellulose tissue papers. Performances of the capacitors were evaluated in aqueous and organic electrolytes. Electrode capacitance densities of 151 F/g in aqueous and 60 F/g in organic electrolyte has been observed at a high CV scan rate of 10 Vs^-1. The cutoff frequency at -45^o phase angle in 2M KCl aqueous electrolyte is 103 Hz and in 1M TEABF4 organic electrolyte is 16 Hz. Satisfactory cycling stability was observed up to 100,000 cycles in both aqueous and organic electrolytes.

Supercapacitors have been extensively researched in recent years as candidates for energy storage devices, particularly for use in short-term high-power demands. Supercapacitors are now receiving attention for their use in low-temperature environments such as polar regions, high altitude locations (upper atmosphere), and in space exploration . Characterized by their fast charge and discharge times as well as excellent cycle stability, supercapacitors store energy via electric double layer capacitance (EDLC), which is largely dependent on electrode surface area. We have successfully developed a porous carbon electrode with a multi-scale pore network that exhibits excellent ion diffusion and an outstanding gravimetric capacitance of 374±7.7 F g-1 (at current density of 1 A g-1).[1] The pore structure of this electrode material are being optimized to spiro-type quaternary ammonium electrolytes to improve capacitance at low temperatures. In order to extend both the low- and high-temperature limits of non-aqueous electrolyte supercapacitors, ethers with low melting points, high boiling points, and low viscosity are being tested as co-solvents in non-aqueous electrolyte systems.

References
[1] F. Zhang, T. Liu (co-first author), M. Li, M. Yu, Y. Luo, Y. Tong, Y. Li. Nano Lett., 2017, 17, 3097-3104

Polymer structural electrolytes are required to have both superior mechanical properties along with high ionic conductivity in order to develop structural supercapacitors. In the present investigation, a solid polymer electrolyte (SPE) is prepared by varying the relative content of polyethylene glycol (PEG), lithium trfluoromethanesulfonate (LiTf), and graphene nanoplatelets (GnP) contained in an epoxy resin matrix. The mechanical, viscoelastic, and ionic properties are studied as functions of PEG, LiTf, and GnP content. Dynamic mechanical analysis (DMA) is performed to obtain mechanical properties and glass transition temperature, while impedance spectroscopy (IS) is performed to obtain ionic conductivity. It was found that with greater PEG content the ionic conductivity increases, while there is a significant decrease in the mechanical properties. On the other hand greater GnP content results in simultaneously enhancement of both mechanical and ionic properties. Network morphology of these structural electrolytes is studied under scanning electron microscopy (SEM).

Supercapacitors have advantages such as excellent power density, fast charge and discharge times, long life cycle, and relatively low cost which make them likely to replace other energy storage options. Semi-metallic (1T` phase) molybdenum ditelluride (MoTe<sub>2</sub>) has recently been considered as a promising candidate for an electrode in supercapacitors due to high mobility (3900cm<sup>2</sup>/Vs) with low carrier concentration (n<sub>e</sub> ≈ 2.3 x 10<sup>20</sup> cm<sup>-3</sup>) from MoTe<sub>2</sub> powders and flakes. Thus, an efficient fabrication technique for MoTe<sub>2</sub> electrodes with high electrical properties for supercapacitor application is needed. Here we employed two-step method of magnetron sputtering followed by chemical vapor deposition (CVD) to synthesize a few-layer 1T`MoTe₂ electrode to be used in a supercapacitor.

The pressing need for ultrahigh footprint-normalized capacitance emerges in the dimensional migration of hybrid supercapacitors.¹ Herein, we demonstrate an advanced integration protocol mimicking natural Cuscutae, in which ribbon-like vanadium oxides creep along porous tunnels in a commercially available carbon host of low density.² The biomimicry of the Cuscutae design converts the original pore network of submicrometre size into a mesoporous, aperiodic and three-dimensionally interconnected bi-continuous composite framework. Moreover, the massive infiltration of pseudocapacitive functionality onto a highly conductive carbon-cloth backbone leads to an unprecedented footprint-normalized capacitance exceeding 8 F cm^-2. The biomimetic electrode design formulates a symmetric supercapacitor rendering a maximal footprint-normalized cell capacitance more than 4 F cm^-2, a geometric energy density of 0.48 mW h cm^-2 and a geometric power density of 36.8 mW cm^-2 that are superior to commercial double-layer supercapacitors and most of the state-of-the-art hybrid supercapacitors and lithium-ion microbatteries, respectively.³

[1] Goesmann, H.; Feldmann. C. Angew. Chem. Int. Ed. 2010, 49, 1362.
[2] Chen, Y. C.; Hsu, Y. K.; Feldmann, C. 2017, in preparation.
[3] Pikul, J. H.; Zhang, H. G.; Cho, J.; Braun, P. V.; King, W. P. Nature Commun., 2013, 4, 1732.

In this work we report lithium ion capacitors (LIC) with extraordinary energy-to-power ratios based on olive-pit recycled carbons, supported with graphene as a conducting agent. LICs typically present limited energy at high power densities due to the sluggish kinetics of the battery-type electrode. In order to circumvent this limitation, the olive-pit derived hard carbon (HC) was embedded in reduced graphene oxide (rGO). The addition of rGO into the negative electrode not only wraps HC particles but forms a 3D interpenetrating carbon network, facilitating Li-ion diffusion and enhancing the electronic conductivity -which becomes critical- at high power densities. Impedance analysis reveals that charge transfer and contact resistance within the electrode is considerably inferior in the presence of rGO. Furthermore, it remains constant upon cycling, independent to the applied current density, while in the absence of rGO resistance is highly depending on the applied current density. Resistance reduction within the battery-type electrode is reflected in capacity gain at 10C, which is over 100%, rising from 75 mAh g^-1 to more than 150 mAh g^-1. This fact triggers the overall LIC performance, allowing assembling an ultrafast LIC delivering 80 Wh kg^-1 at a power ratio of 10000 W kg^-1, to our knowledge one of the best reported energy densities at such a high delivery of power.

Internal hybridization of a battery-type electrode with an electrical double-layer (EDL) electrode appears as a key for opening a path to more versatile devices which can at the same time deliver high power and high energy, while being able to display a long term cycle life. The best example is the lithium-ion capacitor (LIC) which implements an EDL positive electrode made from porous carbon and a LIB faradaic negative electrode made from graphite or hard carbon, while using a lithium salt (LiPF₆) generally dissolved in ethylene carbonate:dimethyl carbonate (EC:DMC) mixture [2]. Since lithium must be intercalated in the graphite/carbon negative electrode, the first concept of LIC included an auxiliary metallic lithium electrode which was used for pre-lithiation, hence complicating the cell design and being potentially the cause of safety issues as thermal runaway. Therefore, pre-lithiation has been proposed directly from the electrolyte [3], but it leads to a decrease of ionic concentration and conductivity, which might have a negative impact on the LIC power.

A novel strategy to lithiate the negative electrode consists in an irreversible lithium de-intercalation from a sacrificial lithiated material incorporated together with activated carbon in the positive electrode [4]. Using lithiated oxides with low band gaps, such as Li₅ReO₆, enables extracting lithium ions at a potential around 4.2 V vs. ref. Li/Li⁺ and to avoid detrimental electrochemical oxidation of the electrolyte [5]. Very promising performance was obtained with renewable organic lithium salts from which lithium is irreversibly extracted, with a capacity of ca. 350 mAh/g, at potential as low as. 3.3 V vs. Li/Li⁺; the resulting LIC cells demonstrate an excellent cycle life in the potential range from 2.2 ~ 4.0 V [6].

This presentation will briefly introduce the various strategies which have been successfully used by our group with a special focus on the structural/microstructural changes occurring after the first pre-lithiation step and on the performance of LICs in terms of energy and power densities as well as cycle life. New perspectives, using materials leaving a reduced dead-mass after pre-lithiation, will be also detailed.


References
[1] F. Béguin, E. Frackowiak, Supercapacitors: Materials, Systems and Applications, Wiley-VCH, Weinheim, 2013.
[2] T. Aida, K. Yamada, M. Morita, Electrochem. Solid-State Lett., 9 (2006) A534.
[3] V. Khomenko, E. Raymundo-Piñero, F. Béguin, J. Power Sources, 177 (2008) 643.
[4] M.-S. Park, Y.-G. Lim, J.-H. Kim, Y.-J. Kim, J. Cho, J.-S. Kim, Adv. Energy Mater., 1 (2011) 1002.
[5] P. Jezowski, K. Fic, O. Crosnier, T. Brousse, F. Béguin, J. Mat. Chem. A, 4 (2016) 12609.
[6] P. Jezowski, O. Crosnier, E. Deunf, P. Poizot, F. Béguin, T. Brousse, Nat. Mat. (in press).

Energy storage devices are some of the most important environmental technologies that are highly influential in advancing our life in future Society5.0. Specifically, electrochemical capacitor is an energy facilitator that exhibits an efficient/economical charging and discharging characteristics with long lifespan. Thus, the capacitor technology is regarded as promising due to an increasing effectiveness when combined with renewable (solar/wind/micro) energy sources. In this connection, Li-ion based hybrid supercapacitors and their functional materials are being vigorously researched in hopes to improve their capacity/voltage and therefore their energy density. Transition metal oxides are among the most popular materials utilized in this purpose. Thanks to high voltage and associated high energy density, they are tuned as both high energy and high power materials. In recent years, the structural/textural properties of oxides, including particle size, crystallinity, defects, and porosity, are successfully fine-tuned to achieve both high rate performance (>300 C) and long cycleability (>10,000 cycles). The present review will describe pseudo-capacitive nanosized oxides prepared with in-situ synthetic means of “ultracentrifugation” showing ultrafast electrochemical response even more than EDLC.

Lithium ion capacitors have been recognized as high energy density energy harvesting devices, owing to the synergetic combination of double-layer charge storage at the positive electrode and lithiated carbon negative electrode. The performance of such hybrid supercapacitors are mainly governed by the capacitive cathode, thus devices which employ higher capacitance capacitive electrodes are desired.
We have developed a new hybrid supercapacitor design based on water stable, protected Li anode technology that uses a water-stable solid electrolyte as a separator . Our preliminary cell configuration allowed the use of pseudocapacitive cathodes in aqueous electrolyte with a 4 V rated voltage [1,2]. Unfortunately, the high resistance of the protected Li anode allowed charge/discharge only at 60^oC. Here we report are recent progress of room temperature performance of 4-V aqueous hybrid supercapacitors with energy density higher than lithium ion capacitors [3]. A multi-layered lithium-doped carbon (Li_xC₆) negative electrode was developed, which consisted of Li_xC₆ anode, poly(ethylene oxide) (PEO) polymer or alginate gel electrolyte doped with lithium bis(trifluoromethansulfonyl)imide (LiTFSI) and N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI) ionic liquid as the buffer layer and LISICON-type glass ceramic (LTAP). The protected Li_xC₆ anode employing PEO-LiTFSI-PP13TFSI (Li | PEO-LiTFSI | LTAP) at 25^oC exhibited comparable performance to a protected Li negative electrode without PP13TFSI addition at 60^oC (Li | PEO-LiTFSI | LTAP), owing to a drastic increase in conductivity. Charge/discharge tests of an aqueous hybrid supercapacitor using the newly developed protected Li_xC₆ anode with PEO-LiTFSI-PP13TFSI at 25^oC showed good capacitive behavior and long-term capability. In addition, an aqueous hybrid supercapacitor employing a RuO₂ nanosheet positive electrode with specific capacitance of 1000 F/g in acetic acid-lithium acetate catholyte [4,5] (Li_xC₆ | PEO-LiTFSI-PP13TFSI | LTAP | AcOH-AcOLi | RuO₂ nanosheet) showed excellent specific capacity of 196 mAh/(g-RuO₂) and specific energy of 625 Wh/(kg-RuO₂) at 25^oC. Further decrease in cell resistance was realized by using alginate gel electrolyte in place of PEO polymer electrolyte.

[1] S. Makino, Y. Shinohara, T. Ban, W. Shimizu, K. Takahashi, N. Imanishi, W. Sugimoto, RSC Adv. 2 (2012) 12144.
[2] W. Shimizu, S. Makino, K. Takahashi, N. Imanishi, W. Sugimoto, J. Power Sources. 241 (2013) 572.
[3] S. Makino, R. Yamamoto, S. Sugimoto, W. Sugimoto, J. Power Sources. 326 (2016) 711.
[4] S. Makino, T. Ban, W. Sugimoto, Electrochemistry. 81 (2013) 795.
[5] S. Makino, T. Ban, W. Sugimoto, Towards Implantable Bio-Supercapacitors: J. Electrochem. Soc. 162 (2015) A5001.

We employed a polypyrrole hydrogel precursor to create a carbon framework that possesses both huge heteroatom content (13 wt% nitrogen and 11 wt% oxygen) and high surface area (945 m²g^-1) that is equally divided between micropores and mesopores. A sodium ion capacitor (NIC, HIC) electrode fabricated from this N and O Functionalized Carbon (NOFC) has tremendous reversible capacity and rate capability, e.g. 437 mAh g^-1 at 100 mA g^-1, and 185 mAh g^-1 at 1600 mA g^-1. This is among the most favorable reported, and is due to copious nanoporosity that enables fast ion sorption at the many N and O moieties and graphene defects. The NOFC imbues a NIC device with energy-power characteristics that are not only state-of-the-art for Na hybrids, but also rival Li systems: Ragone chart placement is 111 Wh kg^-1 and 38 Wh kg^-1 at 67 W kg^-1 and 14,550 W kg^-1, respectively, with 90% capacity retention at over 5,000 charge/discharge cycles.

Lithium ion capacitor (also called dual carbon lithium-ion capacitor, LIC) is one of the most promising hybrid capacitors as they provide relatively higher energy and power density. This will satisfy the energy and power requirements for applications that have energy storage demands between dielectric capacitors and batteries. Energy density of the LIC is limited by cathode (usually made of activated carbon) while power density is determined by anode. In this investigation, we demonstrate a high energy density lithium ion capacitor (> 100 Wh/Kg) made using a hierarchical porous carbon with bimodal pore size distribution through pyrolysis and controlled activation of polymer gel derived from furfuryl alcohol and phloroglucinol. In addition, use of nanocrystalline prelithated graphite as the anode helped us to optimize rate capability of the device.

Hybrid supercapacitors, composed of a capacitive electrode and a battery-type faradaic electrode, have demonstrated significantly higher energy density than conventional carbon-based electrical double-layer capacitors (EDLCs), due largely to the higher capacity of the battery-type electrode and the broader voltage window of the electrode pair. As a promising battery-type electrode for hybrid supercapacitor, nickel compounds-based electrodes could theoretically deliver high specific capacity and rate capability in alkaline aqueous electrolyte. However, nickel compounds-based electrodes usually suffer from an irreversible phase transition, large volume variation, and low electronic conductivity, resulting in poor durability and limited rate capability. In this presentation, we will highlight our recent progress in investigations into the charge storage mechanism of nickel compounds-based electrodes in order to develop knowledge-based materials design strategies, including in-operando resonance Raman spectroscopic measurements, density functional theory (DFT)-based calculations, and controlled synthesis of advanced nanostructures. Perspectives for a new generation of hybrid supercapacitors for various applications will be discussed as well.

Lithium ion (Li–ion) supercapacitors are a potential pathway to bridging the gap between high power and high energy electrochemical devices. Of the various materials studied as anodes for Li–ion batteries and supercapacitors, titanium dioxide nanotube arrays (TNTAs) are promising as they inherently have high surface area and fast, reversible Li–ion storage. Additionally, when grown via anodisation, the TNTA is amorphous – which tends to increase capacitance [1, 2] – and the morphology is readily tuneable through the anodisation conditions. However, there exists a trade-off when aiming to increase supercapacitor energy density through Li–ion storage, insomuch as the Li–ion storage processes have a tendency to reduce the power density and cyclability of the supercapacitor device. Interestingly, these storage mechanisms have also been shown to cause a self-improvement of the storage capabilities [3]. Hence, there is great value in investigating the charge storage mechanisms in amorphous TNTA with the aim of balancing power density and cyclability with energy density.

This study investigates the charge storage mechanisms in amorphous TNTAs hierarchically grown on titanium mesh as a means to achieve high power supercapacitor anodes. TNTAs are grown via anodisation in a bath of glycerol, ammonium fluoride and water. It is proposed that the mesh macrostructure provides greater electrolyte access for increased power density and favourable adhesion between the TNTAs and the substrate, while the nanostructured TNTAs provide sites for Li–ion storage and increased surface area for double layer storage. The effects of the amorphous nanotube dimensions (wall thickness, diameter and length) on these charge storage mechanisms are also evaluated. These charge storage mechanisms are investigated through electrochemical techniques, as well as through SEM/TEM studies of the lithiated and delithiated materials, and FIB cross-sectional studies on the adhesion between TNTA film and the bulk titanium mesh before and after cycling.
1. D. Guan, C. Cai and Y. Wang, Journal of Nanoscience and Nanotechnology 11 (4), 3641-3650 (2011).
2. H. T. Fang, M. Liu, D. W. Wang, T. Sun, D. S. Guan, F. Li, J. Zhou, T. K. Sham and H. M. Cheng, Nanotechnology 20 (22), 225701 (2009).
3. H. Xiong, H. Yildirim, E. V. Shevchenko, V. B. Prakapenka, B. Koo, M. D. Slater, M. Balasubramanian, S. K. R. S. Sankaranarayanan, J. P. Greeley, S. Tepavcevic, N. M. Dimitrijevic, P. Podsiadlo, C. S. Johnson and T. Rajh, The Journal of Physical Chemistry C 116 (4). 3181-3187 (2012)

Conductive polymers are attractive materials for use in supercapacitors due to their high specific capacitances, high electronic conductivities, and ease of processing. However, the poor cycle-life of these materials due to volume changes associated with the doping/de-doping process has prohibited their adoption in commercial devices. Research efforts have focused on improving the cycle life of conductive polymers through nanostructuring and compositing but have not yet investigated the importance of polymer chain orientation on the electrochemical properties of these materials.

Here we present a study of the electrochemical properties of PEDOT thin-films formed via oxidative chemical vapor deposition (oCVD). The oCVD process allows for simultaneous synthesis, doping, and thin-film formation of PEDOT via a surface, oxidative polymerization reaction. Initial comparisons to PEDOT:PSS thin-films reveal that oCVD PEDOT films are highly crystalline with electronic conductivities 20 times greater than amorphous PEDOT:PSS films. oCVD PEDOT films are also electrochemically active from 2.5 to 4.3 V vs. Li whereas PEDOT:PSS shows no redox behavior in this region. Many of these differences can be attributed to the doping counter-ion: oCVD PEDOT is doped with a highly-mobile chlorine anion as opposed to a bulky polyanion like polystyrene sulfonate (PSS). This enables ion exchange or “secondary doping” of oCVD PEDOT and results in the high specific capacitances observed in the material.

The deposition of PEDOT in vacuum and at moderate substrate temperatures facilitates crystal growth of the polymer. The orientation of the polymer film is dependent on the substrate temperature as oCVD PEDOT changes from predominantly [h00] to predominantly [0k0] at higher temperatures. The electrochemical performance of the [0k0]-oriented films are significantly better than their [h00] counterparts with higher capacities and the evolution of a smaller charge-transfer resistance after long-term cycling. These differences are attributed to an increased ease of doping/de-doping of the [0k0]-oriented films that lead to minimal volume changes and structural changes. These results suggest that the orientation of conductive polymers plays an important role in their electrochemical properties and that manipulation of deposition may finally enable the use of conductive polymers in commercial devices.

The integration of carbon nano-onions has opened new horizons in the use of nano-scale energy storage for applications for which conventional electrolytic capacitors are not sufficient. The use of carbon nano-onion (CNO) in the development of super capacitors now seems to be a promising venture. Ultra-capacitors or supercapacitors are electrochemical systems that store energy within their double-layered structure consisting of opposite charged materials. Supercapacitors by offering fast charging and discharging rates, and the ability to sustain millions of cycles are bridging the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities. Electrodes made from activated or porous carbon are used in the production of the highest rated super capacitors. Here we describe that CNOs can be produced by annealing nanodiamond (ND) in a vacuum furnace or an inert atmosphere.
Here, we functionalised CNOs produced as a result of graphitising pre-purified NDs at high temperature; with Nitrogen, NH4 and O3 to improve their wettability for use in supercapacitor electrodes and did characterise the resulting structures with different techniques. We also studied the effect of chemical and physical activation over the CNOs.

The effect of functionalised samples presented open pores distribution size with higher level of mesoporosity and slightly lower microporosity, alongside reduction in the initial decomposition rate compared to that of pristine CNO.

High concentration of sp2 carbon confirmed with the p-p* at high binding energy and with increasing values of sp2 area while the value of p-p* area was also increasing conversely after functionalisation.

The results have demonstrated strong effect of used activation methods in creation of porosity by creating more open mesopores and development of higher surface area. The TEM results suggested the increase of interlayer spacing of up to 0.72 nm for ACNO KOH-7M N₂ sample compared to that of Pristine CNO (0.35 nm) which is also confirmed by XRD measurements. XRD measurements confirmed reduction of crystallite size upon activation procedure. Although it has been deduced that the ultimate performance of activation with plasma is strongly dependent on the selection of chemical reagent in contrast to activation by Ultra-Violet, concluding that proper selection of chemical reagent in conjunction with a posterior physical activation method, gives desirable results for CNOs surface activation.

Finally, it has been shown that that K2CO3 chemically activated CNO presents the highest surface area of 834 m2/g with presence of narrow mesopores (4.724 nm) and larger micropore (1.822 nm), the smallest crystallite size of 1.06 nm and stability temperature of 710 °C in air, makes it perfect candidate for electrolyte accessible pores and pseudocapacitive behaviour with shorten ion diffusion pathways and reduced charge-transfer resistance.

Unlike carbon electrodes which enable charge storage through capacitive process, pseudocapacitive materials store charges through fast and reversible surface redox reactions that confers them a "capacitive like" behavior. In such peculiar materials multiple valency cations are involved in the charge storage mechanism as it was shown for MnO₂ or RuO₂ electrode. In order to understand the electrochemical behavior of pseudocapacitive materials, a deeper understanding of the role of electroactive cations is required. However, this is seldom reported in the literature. One probable reason for that is the high kinetic of charge storage mechanism, i.e. few seconds to charge or discharge an electrode, which does not ease the operando analysis of pseudocapacitive materials. Up to now, this issue has mainly been addressed by the use of in situ experiments which require long polarization steps at constant potentials that are not compatible with the time frame of electrochemical capacitor operation.
Metal tungstates (M²⁺WO₄) represent an important group of inorganic high-density oxides that were recently proposed as potential electrode for electrochemical capacitors and Li-ion batteries. Nanosized iron tungstate (FeWO₄) for example has been synthesized using various methods (polyol-mediated synthesis, hydrothermal synthesis, microwaves synthesis, ultra centrifugation synthesis). The different powders of FeWO₄ have been electrochemically tested in aqueous electrolytes, exhibiting a pseudocapacitive behavior over a limited potential window. Subsequently, operando X-ray absorption spectroscopy was used for the first time on such pseudocapacitive electrode at SOLEIL Synchrotron on the ROCK beamline (Rocking Optics for Chemical Kinetics) in order to probe the charge storage mechanism while operating the electrode using potentiodynamic conditions. XAS measurements were performed on FeWO₄ electrode at both Fe K-edge and W L3-edge, allowing very accurate determination of the oxidation state evolution of both Fe and W during cycling. Operando XAS enables to evidence the role of Fe²⁺/Fe³⁺ in charge storage while W⁶⁺ seem to act as a spectator cation. In this communication, the main results will be presented and the relationship between capacitance and the characteristics of the different FeWO₄ powders will be detailed.

Investigation of charge storage mechanisms is of great importance in developing new electrochemical energy storage materials. Here we study the intercalation mechanisms, electrochemical strains and cyclic stability of defective 2-D nanosheet δ-MnO₂ electrodes using a combination of high energy X-ray scattering and X-ray spectroscopy. MnO₂ nanosheets have a peculiar Mn3+ defect, one that is displaced from the plane of the nanosheet to form a “surface Frenkel” defect. The quantity of sodium ions intercalated correlates with the surface Mn³⁺ Frenkel defect density as determined by gravimetric and X-ray pair distribution function (PDF) analysis. Electrochemical strain manifests as contractions of the Mn-O bond lengths with increasing charge state, and the data shows that the in-plane Mn-Mn coordination also increases while the surface displaced Mn concentration decreases. This result indicates that the surface coordinated Mn³⁺ may be extracted from and re-inserted into the plane of the nanosheet during charge/discharge cycling. EXAFS studies of K and Rb intercalation show that there is significant disorder in the local environments around these cations, but do not definitively identify the intercalation sites.

MXenes have attracted great attention as the next-generation capacitive energy-storage material, but the mechanism underlying its pseudocapacitive behavior remains elusive. Here we provide an overall understanding on the surface redox process of Ti₃C₂T_x (T=O, OH), a prototypical MXene, in H₂SO₄ electrolyte based on joint density functional theory and the analysis of Gibbs free energy under the constant electrode potential. From the dependence of the O/OH ratio and configuration and the surface charge on the applied potential, we obtain a complete capacitive energy-storage picture of Ti₃C₂T_x that shows very good agreement with previous experimental findings in terms of the electron transfer number, capacitance, and redox peak position. We find a voltage-dependent redox/double-layer co-charging behavior that provides new insight into the capacitive energy storage mechanism of MXenes.

MXenes are a family of two-dimensional (2D) metal carbides or nitrides with attractive physical and chemical properties. The research on the electrochemical properties of MXenes in the past few years has revealed their outstanding electrochemical performance and unprecedented volumetric capacitances. So far, about 20 different MXene compositions with a general formula of M_n+1X_nT_x, where M is a transition metal, X denotes carbon or nitrogen, and Tx stands for surface terminations, are experimentally realized, and many of them have been investigated for their capacitive properties. This talk will present the results of our group’s recent investigations of the fabrication of MXene films and three dimensional (3D) structures and their performance as electrode materials for electrochemical capacitors. The effect of microstructure of freestanding MXene films and 3D MXene aerogels on their electrochemical performance will be discussed. A procedure to synthesize MXenes with controlled lateral dimensions will be presented, and the effects of dimension on the electrode performance will be explained. In addition, our recent efforts on controlling the orientation of the MXenes sheets in the structure of electrodes and the effect of ordering on the performance of the electrodes will be discussed. Our studies show the dependence of the electrical and electrochemical properties of MXene electrodes on the dimension of 2D sheets and their orientation in the structure. The results of this work can help to design MXene electrode with superior properties.

Dielectric properties are a challenge to measure for advanced capacitor development, since they necessitate macroscopic electrodes, pinhole-free films, and homogeneous specimens. Nanoscale dimensions are increasingly relevant, however, for multilayer, multicomponent, patterned, and generally heterogeneous capacitor designs. This motivated the development of an AFM-based approach for mapping capacitive charges and the associated local dielectric properties. The method is based on depositing charges with a known bias, followed by sequential mapping of the surface potential as it decays over time due to various dissipation processes and charge trapping energies and distributions. The results are demonstrated for oxide as well as polymer dielectric systems, revealing mcrostructure dependent charging and dissipation mechanisms. Ultimately such a nanoscale approach for mapping the local capacitance of dielectrics can provide novel insight into optimizing local dielectric performance, fundamental mechanisms near breakdown conditions, and cycling realiability.

Several forms of activated carbons have already been implemented and widely investigated as electrodes for electrochemical capacitors. Aqueous solutions based on inorganic salts demonstrated effective operating voltages around 1.6 V, and several interesting strategies have been furthermore implemented to improve these systems, including redox activity of the electrolyte.

This work is focused on the electrochemical performance of the activated carbon electrodes operating in various electrolytes containing sulphur-based species. Since sulphate (SO₄^2-)-based aqueous solutions have already been widely investigated, this study will discuss the electrolytes with containing S₂O₃ ^2-, SO₃^2- and S^2- anions and their interaction with a carbon electrode. The results obtained clearly indicated that sulphur plays an important role in the charge accumulation process and has an excellent redox chemistry. Dependently on the electrolyte pH, the ionic specimen might be easily reduced or oxidized and perfectly preserves the activated carbon surface against oxidation. Moreover, the affinity of sulphur to the carbon surface promotes a sulphur-carbon bonding and creates additional redox active site on the electrode surface.

A variety of electrochemical methods used in the study confirmed that a specific formulation of the electrolyte with sulphur-based redox shuttle might remarkably enhance the electrode capacitance (up to 300 F g^-1) and the operating voltage (up to 2.0 V). Moreover, operando Raman spectroscopy proved that sulphur-carbon bond is reversibly created while the charge transfer occurs. Furthermore, high energy density (ca. 26 Wh kg^-1) maintained at excellent power rates (1 kW kg^-1) has been preserved during 10 000 charge/discharge cycles with reversibility of 97%.

Energy storage devices often present lower performance at lower temperatures, therefore developing renewable and low-cost approaches to tackle this problem is of great importance. Here, for the first time, photothermal effect is applied to enhance the capacitance, energy density and power density of supercapacitors. The supercapacitors choose three-dimensional hierarchical graphene as the electrodes and solid-state PVA/H₃PO₄ as the electrolyte, with an absorption of 92.88% over the entire solar spectrum, a solar thermal conversion of ~18.56% and a response time of less than 200 s. The surface temperature change is ~39 ^oC under 1 solar illumination (1 kW m^-2). Under 1 solar illumination, the capacitance of the pseudocapacitor increases by ~1.5 times, and the capacitance of the electric double-layer capacitor (EDLC) increases by ~3.7 times. The mechanisms were systematically and quantitatively studied via modeling and simulation. Under solar illumination, the increases in the reaction rate constant, electric double-layer capacitance and conductivity mainly contribute to the enlarging capacitance of the pseudocapacitor while the increase in the dielectric constant mainly contributes to the increasing capacitance of the EDLC. This work provides new insights for the development of energy storage devices and offers new design options for energy storage devices with multifunctional properties.

An electrospinning technique with polyacrylonitrile fiber carbonization in NH₃ is used to prepare three-dimensional graphene nanofibers. The graphene nanosheets on the nanofibers contain between 1 and a few atomic layers with their edges exposed on the surface, the membrane show excellent electrical conductivity (184 S m^-1) and high ion-accessible surface area (850 g m^-2). As a consequence, the graphene nanofibers displays high specific capacitance of 320 F g^-1, excellent rate capability and ultrahigh energy density of 70 Wh kg^-1 in organic electrolyte.

Save and Solar - process improvement, mobility, sustainability, and electricity generation are pivotal fields of interest in nowadays energy research. Enhanced development of better membranes, more efficient fuel cell catalysts, biomass treatment and tailor-made conversion, hydrogen storage, lighter batteries with increased energy density, and organic photovoltaics got spurred over the past few years by Chemspeed's high-throughput and high-output solutions. The presentation covers a selection of case studies in these fields in order to highlight the beneficial use in synthesis, process research, formulation, application, and testing.

Nanostructured titanium oxide (TiO_2-x) has attracted attention for use in both battery and supercapacitor electrodes in recent years due to its ease of fabrication, high cycling capability with fast charging/discharging, safety and low toxicity. Although initial studies used crystalline TiO_2-x where lithium ions (Li⁺) intercalate in planes between octahedral crystal elements, more recent studies have focussed on amorphous nanostructured materials due to their increased charge storage capacity. This increased charge storage has been attributed to increased Van de Waals attraction at the roughened anodic oxide surface, with oxygen vacancies providing electrically positive sites that can trap electrons at the convex surfaces [1].
In this paper, we report the use of NMR measurements of diffusion and relaxation to probe the surface chemical environmental of self-supported amorphous TiO_2-x nanotubes in 1 M LiPF₆ in 1:1 EC:EMC. Of particular interest is the ability to use these measurements to obtain estimates of the electrolyte-accessible metal oxide surface area, surface to volume ratio and surface wettability as a function of different anodisation conditions. TiO_2-x nanotubes were first formed by anodising titanium mesh or foam in an electrolyte of fluoride-containing glycerol. Then relaxation measurements (T₁ and T₂) were performed in 1 M LiPF₆ in 1:1 EC:EMC for each species: Li⁺ (measured using ⁷Li NMR) and the electrolyte anion, PF₆^- (measured using both the ¹⁹F and the ³¹P NMR) in both the neat electrolyte and in anodised mesh soaked in electrolyte. T₁ and T₂ variations thus are being explored for different electrode charge states as well as different anodisation conditions.
Diffusion of the Li⁺ and PF₆^- species in the electrolyte by pulsed field gradient stimulated echo (PFGSTE), is also being investigated both in the absence and presence of a mesh TiO_2-x nanotubes electrode. The Li⁺ diffusion coefficient in the bulk electrolyte is consistent with previous measurements in this electrolyte [2] thus confirming the accuracy of our PFGSTE measurement system. Diffusion coefficients, measured in the presence of self-supported TiO_2-x nanotube electrodes at different states of charge, will be used to provide dynamic information about the exchange of ions between in-and ex-pore environments, and in combination with the relaxation measurements, to provide information on how both the electrolyte anion and Li⁺ interact with the surface.
[1] M. Fukahara et al. Sci. Rep., 6, 35870,2016.
[2] C. Capiglia et al., J. Power Sources, 81-82, 859,1999.

With rising energy concerns, efficient energy conversion and storage devices are required to provide a sustainable, green energy supply. It has also been predicted that next-generation electronics will be wearable. Many efforts have been devoted to developing safe, lightweight and flexible power sources with the goal to meet the urgent need for wearable electronics. Here, activated cotton textile (ACT) was directly converted from a cotton textile, which was further used as a flexible conductive substrate to construct flexible power sources. Specifically, nanostructures of active materials, including core/shell metal oxide nanowires (NiCo₂O₄@NiCo₂O₄, CoO@NiO), two-dimensional Co-Al layered double oxides (Co-Al LDH) nanoarrays and graphene oxide (GO) nanosheets were homogeneously deposited on the ACT surface to fabricate the ACT-based electrode materials for asymmetric supercapacitors. Solid-state PVA/KOH gel polymer electrolyte was used to improve the mechanical robustness of the assembled all-solid-state flexible cells. Furthermore, a cotton-textile-enabled asymmetric supercapacitor was integrated with a flexible solar cell to fabricate self-sustaining power pack via a scalable roll-to-roll manufacturing approach, which demonstrates its potential to continuously power future electronic devices. Cotton-textile-derived flexible ACT conductive substrates suggest a huge potential for fabricating next-generation flexible power sources.

Supercapacitors are electrochemical energy storage devices that combine the high energy-storage-capability of conventional batteries with the high power-delivery-capability of conventional capacitors. In this contribution we will show the results of our group recently obtained on supercapacitors with electrodes obtained using mixtures of carbonaceous nanomaterials (carbon nanotubes (CNTs), graphite, graphene, oxidised graphene). The electrode fabrication has been performed using a new dynamic spray-gun based deposition process set-up at Thales Research and Technology (patented). First, we systematically studied the effect of the relative concentrations of Multi-Walled Carbon Nanotubes (MWCNTs) and graphite on the energy and power density (using commercial aqueous electrolytes). We obtained a power increase of a factor 2.5 compared to barely MWCNTs based electrodes for a mixture composed by 75% of graphite/graphene. This effect is related to the improvement of the mesoporous distribution of the composites and to the increase of the conductance as pointed out by Coleman et al. After these preliminary results, we decided to test water as a solvent to reduce the heating temperature and to obtain a green type process without toxic solvents. To achieve stable suspensions we oxidised the graphene and the CNTs before putting them in water. After the deposition by spray the electrodes were left in an oven (to deoxidise) and recover a good conductivity. We observed that changing the Graphene Oxide concentrations we obtained different values of capacitance and energy. The best results were obtained with 90% of GO and 10% of CNTs. We obtained 120F/g and a power of 30kW/Kg (using aqueous electrolytes). Finally we performed the same measurements using high quality exfoliated graphene supplied by IIT. We fabricated graphene/single wall carbon nanotubes (SWCNTs)-based hybrid supercapacitors and we achieved a remarkable performance in terms of maximum specific energy (20.8Wh/kg) and power (92.3kW/kg), outperforming devices made both using commercial activated carbon (PICACTIF), and graphene-SWCNT bucky papers. The importance of these results is that it is the first time that these performances have been obtained for graphene related materials using an industrial fabrication suitable technique that can be implemented in roll-to-roll production. Finally, new results using mixtures of Carbon nanofibers and graphene will be also shown. These new composite allow to use ionic liquids, having larger dimensions, as electrolytes and so to increase dramatically the energy stored in the device without reducing the power.

Wire-shaped hybrid supercapacitors have potential for flexible wearable applications due to their durability under the various deformation conditions, ease of manufacturing processes, fast charge/discharge rates, and long cycle life. A wire-shaped hybrid supercapacitor based on three-dimensional (3D) nickel cobalt layered double hydroxide (NiCo LDH) with large electrochemical active sites and improved chemical and structural stability was fabricated for a high-performance supercapacitor with an outstanding energy density. The calculated gravimetric, volumetric, areal, and length capacitance of the 3D-NiCo LDH/Ni nanostructured electrode are 2446 F g^-1, 73 F cm^-3, 1.04 F cm^-2, and 0.18 F cm^-1, respectively. When tested in an asymmetric two-electrode system, the 3D- NiCo-LDH@Ni//3D-Mn₃O₄@Ni wire hybrid supercapacitor exhibited energy density of 153.3 Wh/kg and power density of 8810 kW/kg. The results verified that such wire band-like hybrid supercapacitors are promising for smart wearable and implantable devices.

Flexible all-solid-state micro-supercapacitors are an emerging field of wearable electronics, since traditional supercapacitors are too large for microelectronics, and fabrication processes are not compatible with fine electrical feature applications. In this study, we integrated an all-solid-state flexible micro-supercapacitor with high performance based on 3D-graphene/SWNT/Ag NWs on the commercial flexible film by a plotter-assisted method. The 3D-graphene/SWNT/Ag NW micro-supercapacitor exhibited a superior areal capacitance of 19 mF cm^-2 and 84% capacitance retention after 10,000 cycles. The energy density value ranged from 0.1 to 2.75 µWh cm^-2 with a maximum power density of 361 to 5.71 µW cm^-2. The capacitance retention and electrical performance did not show noticeable degradation under various mechanical deformations. The device fabrication of an all-solid state micro-supercapacitor that can be transferred directly onto fabric provides a viable method for producing versatile wearable electronic devices.

We developed a well-organized three-dimensional nitrogen-doped carbon network (NCW), composed of reduced graphene oxide(rGO)/carbon natnotube(CNT)/graphitic carbon nitride(g-C₃N₄) by a single-step hydrothermal process. CNT and g-C₃N₄ provide benefits as a conductive enhancer and a wettability enhancer, respectively, by solving the stacking problem of rGO sheets, which commonly occurs during the hydrothermal process of GO. The well-organized NCW device exhibited a very high capacitance of 338 F/g at 1 A/g in a three-electrode system, which is superior to the traditional rGO-based systems (~200 F/g). Surprisingly, almost 100% of capacitance retention was achieved for 30,000 cycles in a two electrode-system. Our symmetric NCW supercapacitor with an easy and simple fabrication method exhibited a much greater energy density, 21.5 Wh/kg at a power density of 696.5 W/kg, than that of previously reported rGO-based devices (~10 Wh/kg), which demonstrated a great potential for high-tech energy storage devices with high energy density and stability.

While orthorhombic (T-) Nb₂O₅ is one of the most promising energy storage material with rapid Li⁺ intercalation pseudocapacitive response, a key challenge remains in the realization of high-rate charge-transfer reaction when fabricated into thick electrodes. Herein, a facile gas reduction method is demonstrated for Nb₂O₅ nanoparticles treated with H₂ to create intrinsic defects. The specific capacitance for hydrogenated (H-) Nb₂O₅ at 0.5 A g^-1 reaches 649 C g^-1, exceeding that of T-Nb₂O₅ with only 569 C g^-1, which suggests numbers of active sites resulting from the formation of oxygen vacancies. In addition, the successful introduction of oxygen vacancies leads to enhanced electrical conductivity and increased donor density that accelerates charge storage kinetics and enables excellent long-term cycling stability. This work are of great importance to provide a new insight into improving the electrochemical properties of various transition metal oxides by incorporating oxygen vacancies.

Dielectric, ferroelectric ordering and phonons in a mixed doped La³⁺ and Sc³⁺ (1-y) [PbZr_0.53Ti_0.47] y[La_xSc_1-x]O_3-δ for samples with 0.0≤ x≤ 0.08 and fixed y = 0.1 are investigated using dielectric, polarization, and Raman spectroscopic measurements as a function of temperature. The system is stabilized at tetragonal phase for pure (1-y) [PbZr_0.53Ti_0.47] y[Sc]O_3-δ and the tetragonality is reduced when Sc³⁺ is partially substituted by La³⁺ (0.0≤ x≤ 0.08 ). The stochiometric of the chemical compositions are examined using energy dispersive x-ray analysis results. A uniform distribution of grains on the surface of the sample is observed from scanning electron micrographs recorded on pellets. For mixed crystal, the temperature dependent dielectric constant exhibits a broad feature in the T-range 100-600 K, measured at different frequencies (10²-10⁶ Hz). Upon increasing x, the values of dielectric constant are found to be monotonically increased and lie in the range 280-1800. Well-defined hysteresis loops are found to saturate above 1000 V. However, the loops are found to be progressively slimmer (remnant polarization remain low) with increasing La-composition x. To begin with the Raman spectra are found to be broad at ambient temperature and the presence of several Raman peaks at elevated temperature indicates the existence of disorder activated Raman bands. Based on the broad dielectric features, slim polarization loops, and disorder activated forbidden Raman scattering; the dopant samples are understood on the basis of formation polar nanoregions, which essentially manifested in relaxor behavior.

For the steady supply of renewable energy such as solar energy and wind energy, energy storage and distribution system is necessary. To establish an energy storage system, large capacity supercapacitors together with secondary batteries are essential. Supercapacitors has been developed for the last 20 years, however, intensive research have been focused last 5 years. Numerous researchers are trying to develop high performance supercapacitor electrode materials. Graphene was employed to use its excellent electrical conductivity and large surface area. As the EDLC type supercapacitors showed limitations in energy density, nanocomposite materials with psedudocapacitance, especially metal sulfides, have been investigated for supercapacitor electrodes. Composites of graphene-based metal sulfides (or oxides) have been synthesized for improving the supercapacitor performance. Recently, three dimensional electrode structure has been getting tremendous attention as it provide a large surface area and large pores that enables electrolytes and charges penetrates freely. In this study, we will introduce the new electrode materials and their fabrications to supercapacitor devices.

Supercapacitors have attracted great attention due to their high power density, rapid charge/discharge cycles and long life-time. Currently, these devices are limited by their cost and scalability for commercial manufacturing. Herein we introduce a cost-effective method for synthesizing a porous carbon structure with embedded graphite-encapsulated Ni nanoparticles and tunable structural properties. By varying the carbon content of precursors, utilizing monosaccharides and disaccharides, the overall surface area and pore size was optimized for maximum capacitance. The bulk structure was investigated with microscopy. Presence of Ni nanoparticles and graphite encapsulation was confirmed with XRD and is corroborated with Raman spectroscopy. This comparison was conducted using BET analysis and is correlated with cyclic voltammograms comparing the changes in Helmholtz double-layer capacitance, derived energy density and power density. These devices represent a cheap, environmentally benign approach to modern supercapacitors while simultaneously exerting control over bulk structural properties.