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. As for this power generation unit, in the inner circuit, a potential is created by the triboelectric effect due to the charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity; in the outer circuit, electrons are driven to flow between two electrodes attached on the back sides of the films in order to balance the potential. Ever since the first report of the TENG in January 2012, the output power density of TENG has been improved for five orders of magnitude within 12 months. The area power density reaches 500 W/m², volume density reaches 490 kW/m³, and a conversion efficiency of ~50% has been demonstrated. The TENG can be applied to harvest all kind mechanical energy that is available but wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more. Alternatively, TENG can also be used as a self-powered sensor for actively detecting the static and dynamic processes arising from mechanical agitation using the voltage and current output signals of the TENG, respectively, with potential applications for touch pad and smart skin technologies. The TENG is possible not only for self-powered portable electronics, but also as a new energy technology with a potential of contributing to the world energy in the near future.
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.
G. Zhu, J. Chen, T. Zhang, Q. Jing, Z. L. Wang* “Radial-arrayed rotary electrification for high-performance triboelectric generator”, Nature Communication, 5 (2014) 3456.

Piezoelectric unimorphs are made of laminating a piezoelectric layer with an inert flexible substrate. They have found wide applications in sensing, energy harvesting, and actuation. We have developed a comprehensive understanding of the effects of unimorph thickness ratios on mechanical load induced voltages, charges, and energies, as well as voltage induced displacements of eight different boundary conditions, with both analytical and numerical means. By adopting an average s₃₃ stress method, the effect of d₃₃ for both point and distributed loads is taken care of and our analytical equations are able to fully capture the FEM results which are obtained by COMSOL Piezoelectric Devices Module. Non-monotonic voltage and energy generation versus thickness ratio curves have been found for load-controlled energy generation scenarios. When the unimorph is actuated by applied voltage, non-uniform maximum deflection versus thickness ratio curves are also found. Our results reveal that the optimum thickness ratios for actuation is an order higher than the optimum thickness ratios for energy generation. In conclusion, closed-form analytical solutions are now available for the thickness optimization of piezoelectric unimorphs generators and actuators, under different boundary conditions.

We report a novel class of energy harvesters for generating AC power through repeated mechanical bending and unbending. The device consists of two identical Li-alloyed Si as the electrodes, separated by electrolyte-soaked polymer separator. The device is in soft thin-film form and is highly flexible and durable. Bending induced asymmetric stresses generate chemical potential difference, driving the lithium ion flux from the compressed to the tensed electrode to generate electrical current. Removing the bending reverses ion flux and electrical current. Our thermodynamic analysis reveals that the ideal energy-harvesting efficiency of this device is dictated by the Poisson&’s ratio of the electrodes. For the thin-film based energy harvester used in this study, we show that the device has an ideal efficiency of 27.8% and achieves a generating capacity of 10%. The device is well-suited for harvesting low-frequency motion energy because of its finite current width. We provide kinetic models to explain the current width and the possibility of timescale tunability. The device demonstrates a practical use of stress-composition coupling in electrochemically active alloys to harvest low-grade mechanical energies from various low-frequency motions, such as everyday human activities that generate inhomogeneous stress.

Energy harvesting systems based on piezoelectric and triboelectric nanomaterials are in great demand, as they can provide routes for the development of self-powered devices which are highly flexible, stretchable, mechanically durable, and can be used in a wide range of applications. Our recent research interest mainly focuses on the fabrication of piezoelectric and triboelectric nanogenerators based on various kinds of nanomaterials. Flexible nanogenerators exhibit good performances and are easy to integrate which make it the perfect candidate for many applications, and therefore crucial to develop. In this presentation, I firstly introduce the fundamentals and possible device applications of nanogenerators, including their four basic operation modes. Then the different improvement parameters will be discussed. As main topics, I will also address a couple of recent achievements regarding highly stretchable piezoelectric-pyroelectric hybrid nanogenerators, transparent flexible graphene triboelectric nanogenerators, textile-based wearable triboelectric nanogenerators, etc.

This seminar introduces three recent progresses that can extend the application of self-powered flexible inorganic electronics. The first part will introduce self-powered flexible piezoelectric energy harvesting technology. Energy harvesting technologies converting external sources (such as vibration and bio-mechanical energy) into electrical energy is recently a highly demanding issue. The high performance flexible thin film nanogenerator was fabricated by transferring the perovskite thin film from bulk substrates for self-powered biomedical devices such as pacemaker and brain stimulation. The second part will introduce flexible electronics including large scale integration (LSI) and high density memory. Flexible memory is an essential part of electronics for data processing, storage, and radio frequency (RF) communication. To fabricate flexible large scale integration and fully functional memory, we integrated flexible single crystal silicon transistors with 0.18 CMOS process and memristor devices. The third part will discuss the flexible GaN/GaAs LED for implantable biomedical applications. Inorganic III-V light emitting diodes (LEDs) have superior characteristics, such as long-term stability, high efficiency, and strong brightness. Our flexible GaN/GaAs thin film LED enable the dramatic extension of not only consumer electronic applications but also the biomedical devices such as biosensor or optogenetics. Finally, we will discuss laser material interaction for flexible and nanomaterial applications. Laser technology is extremely important for future flexible electronics since it can adopt high temperature process on plastics, which is essential for high performance electronics, due to ultra-short pulse duration. (e.g. LTPS process over 1000 °C) We will explore our new exciting results of this field from both material and device perspective.
Related References (from Keon&’s group as corresponding authors)
[1] Nano Letters 11, 5438, 2011. [2] Nano Letters 10, 4939, 2010.
[3] Nano Letters 12, 4810, 2012. [4] Nano Letters 14, 7031, 2014
[5] Adv. Mater, 26, 2514, 2014. [6] Adv. Mater. 26, 4880, 2014
[7] Adv. Mater, 26, 7480, 2014 [8] Adv. Mater. 24, 2999, 2012.
[9] Adv. Mater. 10.1002/adma.201501592 [10] Adv. Mater. 27, 2866, 2015
[11] Energy Environ. Sci. 10.1039/C5EE01593F [12] Energy Environ. Sci., 7, 4035, 2014
[13] ACS Nano 7, 11016, 2013 [14] ACS Nano 9, 4120, 2015
[15] ACS Nano 7, 4545, 2013 [16] ACS Nano 7, 2651, 2013.
[17] ACS Nano 8, 9492, 2014 [18] ACS Nano 8, 7671, 2014
[19] ACS Nano, 10.1021/acsnano.5b02579 [20] Adv. Energy Mater. 3, 1539, 2013
[21] Adv. Energy Mater. 5, 1500051, 2015 [22] Adv. Func. Mater. 24, 2620, 2014
[23] Adv. Func. Mater. 24, 6914, 2014 [24] Nano Energy, 14, 111, 2015
[25] Nano Energy, 1, 145, 2012

Triboelectric nanogenerators (TENG) have been developed to harvest mechanical energy through contact/friction motions. However, the efficiency of the TENG might be limited by the energy loss during power generation process, such as heat dissipation, wearing and plastic deformation. In this work, a hybrid cell consisting of a TENG and a pyroelectric-piezoelectric nanogenerator (PPENG) was developed for high-efficient mechanical energy harvesting through multiple mechanisms. Outstanding output performances were demonstrated for both the TENG and PPENG, and the hybridized output power could provide a sustainable power source for driving a light-emitting diode with extended illumination time and charging a supercapacitor. Furthermore, the hybrid cell was also applied as a multi-functional self-powered sensor for measuring both the temperature and the normal force. These excellent performances of the triboelectric-pyroelectric-piezoelectric hybrid cell enhance the energy-harvesting efficiency significantly (by 26.2% at 1 k#8486; load resistance), improve the charging rate greatly (by 81.3%), and enable the self-powered sensing, which will lead to a variety of advanced applications.

Recently, triboelectric nanogenerators (TENG) have received increasing interest due to their potential for mechanical energy harvesting. Important advancements have been achieved to increase the output power and efficiency, even as new structures have emerged. Their robustness and endurance have increased, but there still remain some concerns about the degradation and lifetime of TENG. How will TENGs age under intensive use in daily life? To address this issue, we propose in this paper to use shape memory polymer (SMP) to extend TENG&’s lifetime and better guarantee their performance. For this purpose we have introduced a new smart SMP-TENG structure which has the capacity to be healed and recover good performance after degradation of its triboelectric layer. We studied the degradation and the self-healing process of SMP-TENG, improving their endurance, life time and thus demonstrating the huge potential of self-healing SMP-TENGs.

Computer keyboard is one of the most common, reliable, accessible and effective approaches used for human-machine interfacing. Accessing the information provided by computer from internet dictates the quality, efficiency and happiness of our work and life. A keyboard, an indispensable component of the system, is the only means for information input and control for many purposes such as information recording/outputting, financial management, bill payment, personal communications and many more. With this regard, the heavy reliance on computer incurs a major concern for its security issue. Although keyboard has been used for hundreds of years for advancing human civilization, studying human behavior by keystroke dynamics using smart keyboard remains a great challenge.
Here we report the first intelligent, self-powered, non-mechanical-punching keyboard enabled by contact electrification between human fingers and a flexible transparent thin-film layer. The intelligent keyboard (IKB) converts mechanical stimuli applied onto the keyboard into local electronic signals without applying an external power. It can not only sensitively trigger a wireless alarm system once gentle finger tapping occurs but also be capable of tracing and recording typing contents by detecting both the dynamic time intervals between and during inputting letters and the force used for each typing action. Such features promise its use as a smart security system that can realize detection, alert, recording, and identification. Moreover, the IKB is able to identify personal characteristics from different individuals if assisted by behavioral biometric of keystroke dynamics. Furthermore, the IKB can effectively harness typing motions for electricity to charge commercial electronics at arbitrary typing speed larger than 100 characters per min, with an area power density of 69.6 mWcm^-2. Given the above features, the IKB can be potentially applied not only to self-powered electronics but also to artificial intelligence, cyber security, and computer or network access control. The justified concepts and demonstrations in this work can be immediately and extensively adopted in a variety of applications, and come into effect of improving the way of our living.
References: (* indicate co-first author).
1.J. Chen*, G. Zhu*, J. Yang, Q. Jing, P. Bai, W. Yang, X. Qi, Y. Su and Z. L. Wang. ACS Nano9 (2015), 105-116.
2.J.Chen*, G. Zhu*, T. Zhang, Q. Jing and Z. L. Wang. Nat. Commun.5 (2014), 3426.
3.J. Chen*, G. Zhu*, W. Yang, Q. Jing, P. Bai, Y. Yang, T. C. Hou and Z. L. Wang. Adv. Mater.25(2013), 6094-6099.
4.J. Chen*, J. Yang*, Z. Li, X. Fan, Y. Zi, Q. Jing, H. Guo, Z. Wen, K. C. Pradel, S. Niu and Z. L. Wang. ACS Nano 9 (2015), 3324-3331.
5.J. Yang*, J. Chen*, Y. Su, Q. Jing, Z. Li, F. Yi, X. Wen, Z. Wang and Z. L. Wang. Adv. Mater. 27(2015), 1316-1326.

Transparent electronic devices built on flexible substrates are expected to meet emerging technological demands for the next generation of flexible electronic and optoelectronic devices. A suitable energy source is a vital part for realizing fully self-powered systems. Energy harvesting and conversion technology from environment in which the system will be deployed is a promising route. Recently, we have introduced a new concept on the basis of triboelectrification and electrostatic induction for successfully converting mechanical energy into electric energy at an energy conversion efficiency as high as 50%. Subsequently, transparent triboelectric nanogenerators (TENGs) using indium tin oxide (ITO) or graphene as electrodes have been demonstrated, which are the only possible power source that light can penetrate through. However, there is a need for a low-cost and large-area compatible technology for producing transparent TENGs with high-output power supply, aiming for applications such as touch screens.
In our previous work, we fabricated a transparent TENG and pressure sensor by using polyester (PET) and polydimethylsiloxane (PDMS) films as flexible substrates to integrate ITO electrodes. It has also been demonstrated that the patterned surface is an effective method to improve the output performance of the TENG. However, realizing such a structure is not trivial in mass production and the patterned structure will reduce the transparency of the device due to the light scattering effect. Here, we present a simple, cost-effective and large-scalable method for fabrication of highly transparent TENGs.
Although triboelectric effect has been known for thousands years, the underlying mechanism is actually very complex and still under debating. The competing possible mechanisms appear to include electron transfer, ion transfer, bond dissociation, chemical changes, and material transfer, and it is likely that different mechanisms may be involved depending on the specific materials and environmental conditions. [18-21] Usually, it needs specialized equipment to characterize the charged surface and study the mechanism. It is worth noting that the output performance of TENG can directly reflect the capacity of the triboelectrification. Herein, we use this feature to investigate the mechanism of the triboelectrification by examining various factors on the performance of TENG. Furthermore, another meaningful outcome of our study is that we explore the mechanism from the microscopic and molecular perspective using the methods for surface characterization, which would be beneficial to the improvement and application of TENG at a large scale.
References
F. R. Fan, Z. Q. Tian, Z. L. Wang, Nano Energy 2012, 1, 328-334.
F. R. Fan, L. Lin, G. Zhu, W. Z. Wu, R. Zhang, Z. L. Wang, Nano Lett. 2012, 12, 3109-3114.
F. R. Fan, J. J. Luo, W. Tang, C. Y. Li, C. P. Zhang, Z. Q. Tian, Z. L. Wang, J. Mater. Chem. A 2014, 2, 13219-13225.

Triboelectric nanogenerators (TENGs) have been invented as a high efficient, cost-effective and easy scalable energy harvesting technology for converting ambient mechanical energy into electricity at a total energy conversion efficiency as high as 85%. The four basic working modes of the TENG have been demonstrated, each of which has different structural design and choice of materials to accommodate the corresponding mechanical triggering conditions. A common standard is thus required to quantify the performance of the TENGs so that their outputs can be compared and evaluated. Here, starting from the plot of built-up voltage V - total transferred charges Q, we propose a general figure-of-merit (FOM) for defining the performance of a TENG, which is composed of a structural FOM related to the design of the TENG and a material FOM that is the square of the surface charge density. The structural FOM is derived and simulated to compare the TENGs with different configurations. A standard method is introduced to quantify the material FOM for a general surface. This study is likely to establish the standards for developing TENGs toward practical applications and industrialization.

Effectively harvesting energy from ambient environment as a renewable energy source has attracted much interest. But as one rich source of water, the rain drop energy still need to be exploited deeply. Previous studies employed triboelectric generator have successfully harvested its energy. However, the output performance and energy conversion efficiency were in very low level. Besides, the hydrophobic surface with micro textured structure made the device not transparent or flexible, which limits its practical applications.
To improve the energy harvesting efficiency, we developed interdigital-electrodes-based triboelectric generators (TEGs) which is totally transparent. It is composed of PET substrate, interdigital-shaped ITO electrode, PDMS dielectric layer, and ZXL-SS-02 hydrophobic layer. The contact angle of its surface with a 2 mu;L was about 120°. Compared with PET-ITO film, the fabricated device exhibits a better transmittance (more than 80%) in the visible and near infrared region.
The working principal of the device is based on the triboelectricity and electrostatic induction. The surface of the hydrophobic layer will possess negative charges once contacts with a water drop, while the water drop will be charged positive. Along with the droplet flowing, the surface will be charged successively, which can drive the current to flow between the interdigital electrodes according to the electrostatic induction effect.
The device was characterized by a 30 mu;L droplet flowing on its surface while the inclined angle theta; was fixed at 60°. The obtained peak-to-peak voltage and current were about 50 V and 800 nA separately. The maximum RMS power was 1.1 mu;W under a 70 M optimal resistance. The output electricity by a water drop could charge a 30 nF capacitor to 1.2 V, corresponding to 36 nC transferred charge. By adjusting the theta;, a maximum output energy of about 2.67 mu;J was obtained at 80°. The average energy conversion efficiency was about 4%, and a highest efficiency of about 8.7% was achieved when the angle was 10°. Compared with previous works, our device shows much better output performance and higher efficiency.
In summary, this work presents a transparent and flexible generator based on contact-electrification effect for harvesting water drop energy. The generator shown a transmittance of more than 80% in the visible and near infrared region. Facilitated by the utilization of interdigital-shaped electrode, this generator could harvest a water drop&’s energy for multiple time. Employed the simple structure and method, considerate output performance was observed. With a 30 mu;L water drop flowing on the device, 22 V effective voltage and 2.67 mu;J maximum output energy were achieved, while a maximum energy conversion efficiency of about 8.7% was observed. Thanks to its highly transparency, simple manufacturing process, and high output performance, the generator has much potential to harvest the energy from rain drop or liquid flow in microfluidics.

Recently, several interesting reports suggest a moving droplet of sea water or ionic solution over monolayer graphene using pseudocapacitive effects between graphene and a single droplet, producing electrical signals of about 19 nW as a maximum power output. This power output is very low, thus limited device application, indicating increasing power output is much challenging. Here, we demonstrate anomalous large power generation from a single moving water droplet on the basis of triboelectrification-induced pseudocapacitance between graphene and a droplet. The output power of around 2 mu;W achieved from a single moving droplet onto a monolayer graphene, which is one hundred-folds larger than that in the previous research. Very strong negative triboelectric potential on the surface of a PTFE template is generated during a graphene transfer process. The triboelectric potential induces positive and negative charge accumulation on the bottom and top surfaces of graphene, respectively. The negative charges accumulated onto the graphene top surface are driven forward by the moving droplet, charging and discharging at the front and rear of the droplet.

Micrometer-size [110] (Na_1-xK_x)NbO₃ (NKN) platelets were synthesized through the annealing of (K_8-8xNa_8x)Nb₆O₁₉middot;nH₂O (KNNH) precursors at 500^oC. The plate-like KNNH precursors were produced from (1-y)NaOH-yKOH + Nb₂O₅ specimens using the hydrothermal process at 160^oC. The size of the NKN platelets was similar to that of the KNNH precursor, but the surfaces of the NKN platelets were rough while the KNNH precursor had a smooth surface. Formation of a rough surface is related to the vigorous evaporation of the H₂O from the KNNH platelets during the annealing process at high temperature. NKN platelets with the smooth surfaces can be synthesized using KNNH platelets, which were heated to 150^oC to evaporate H₂O before annealing at 500^oC. These NKN platelets can be used for the fabrication of textured NKN ceramics. The nanogenerator was synthesized using NKN platelets and their output power will be presented in this work.

Graphene has a unique combination of electrical, mechanical, and optical properties, is being actively explored for future electronic applications. Specially, graphene has an excellent optical transparency, mechanical flexibility, high mechanical elasticity. These fascinating properties make graphene an ideal material for photodetectors, transparent, flexible electrodes in solar cells and nanogenerators.
For graphene triboelectric nanogenerator, device need high-k dielectric material for insulating layer. However high-k dielectric such as Al₂O₃ or HfO₂ doesn&’t deposit on graphene surface because graphene has sp² atomic configuration. Hence, in this present work we demonstrated the deposition of high-k dielectric material on graphene using hexagonal boron nitride (h-BN) as a buffer layer. Transmission electron and atomic farce microscopy studies show that presence h-BN layer on the top of graphene facilitates the growth of high-quality aluminum oxide (Al₂O₃) layer by atomic layer deposition (ALD). Simulation results also support the experimental observations and provide explanation for suitability of h-BN as buffer layer. The analysis of Raman and X-ray photoelectron spectroscopy (XPS) data also confirms the importance of h-BN for growth of good-quality oxide dielectric materials. Also, h-BN works as protective shield to prevent graphene from oxidation during ALD of Al₂O₃ for the fabrication of triboelectric nanogenerator.

Recently, silver nanowires (AgNWs) have had a great interest as a conducting material for flexible and transparent electrode. In this paper, we studied the network structure formation of AgNWs by stamping transfer process using polydimethylsiloxane (PDMS) with various surface treatments. Surface treatment could control the transfer yield (thickness) of AgNWs. We have designed various conditions of AgNWs surface using silane-coupling modifiers. Flexible PDMS substrate, with embedded AgNWs, was fabricated by the following processes; spraying AgNWs layer and network film formation on silicon wafer, transferring and curing of PDMS, and releasing AgNWs/ PDMS from the original substrate (glass). Transfer yield of AgNWs was adjusted by the variation of surface treatment condition (different concentration and exposure time) with silane-based modifier coating such as Methacryloxypropyltrimethoxysilane (MPS) and Octadecyltrichlorosilane (OTS) on top of AgNWs. Transparency of embedded AgNWs film has increased up to 20% at the optimum surface treatment condition. Especially, MPS treatment process was appropriate for thin film organic device substrate due to its generation of transferred AgNWs with low surface roughness profile. As a result, polymer bulk-hetero junction (BHJ) solar cells with anodes composed of AgNWs have been successfully fabricated with the following conventional device structure: embedded AgNWs film (Anode)/ poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) (45nm)/ poly[(4,8-bis-(2-ethylhexyloxy)-venzo[1,2-b;4.5-b&’]dithiophen)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophen))-2,6-diyl] (PBDTTT-c): phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) (110nm)/ LiF (0.7nm)/ Al (100nm). The organic solar cell showed 3.2% of power conversion efficiency (Jsc=10.333 mA/cm2, Fill factor=43.8%, Voc=0.71V) at the condition of the optimized MPS treatment, and this AgNWs/ PDMS embedded substrate provided a working range of substrate stretching within 15~25 %.

There is an urgent need to look for alternate and renewable energy source as fossil fuel based energy sources are depleting at an alarming rate. Recently piezoelectric materials based renewable energy source by utilizing stray vibrational energy have emerged as an alternative for this demand. Here we report facile, low temperature synthesis of large area self assembled Lead Zirconate Titanate (PZT) based micro/nanostructures using hydrothermal method for omni-directional vibrational energy harvesting. Growth morphologies are found to be dependent on concentration and pressure inside the reaction chamber. Extensive characterizations using SEM, TEM, XRD, raman spectroscopy, absorption spectroscopy etc have been performed to identify quality of synthesized material. These synthesized micro/nanostructures film were used for making functional miniaturized device for omni-directional energy harvesting.
Keywords: Lead Zirconate Titanate (PZT), Vibrational Energy, piezoelectric materials, Hydrothermal, Energy harvesting.

With the rapid growth of portable electronics and sensor networks, mobile and sustainable energy sources for these devices becomes indispensable in modern society. Recently, triboelectric nanogenerators (TENGs) based on contact electrification and electrostatic induction emerge as a promising mechanical energy harvesting technique because of their unique figure of merits, including high output power density, ultra-high energy conversion efficiency, low weight, cost-effective materials, and high adaptability design to different applications. However, because of the inherent uncontrollable and unstable characteristics of the environmental mechanical energy source, the converted electrical energy from TENG is thus unstable and hardly utilized to directly power electronic devices. Thus, understanding the integration performance of TENGs with an energy storage unit is critical for designing a practical energy harvesting system. However, until now there are still many mysteries about the charging behavior of TENGs.
In this work, the characteristics of utilizing a TENG to charge a capacitor are thoroughly studied. Through rigorous mathematical derivation, TENGs have the saturation charging behavior, which is completely analogous to utilizing a DC voltage source with an internal resistance to charge a load capacitor. The TENG with larger short circuit transferred charges and smaller inherent TENG capacitance can charge a load capacitor to a higher saturation voltage under unlimited charging cycles. Besides, the TENG with larger inherent capacitance can provide a smaller charging time constant. An optimum load capacitance that matches the TENG&’s impedance is observed for the maximum stored energy as well. However, different from unidirectional charging, this optimum load capacitance is linearly proportional to the charging cycle numbers and the inherent capacitance of the TENG. Finally, corresponding experiments were performed to further validate the above theoretical prediction to show its application in guiding experiments. [1]
With the above thorough understanding of TENG charging characteristics, we developed the first practical exclusively TENG self-powered systems through systematical design and optimization. A power management board was designed for TENG to realize maximum charging efficiency. With solely human motion as the energy source, we can continuously provide DC electrical energy for the use of practical portable electronics, including pedometers, temperature sensors, and calculators. This work clearly shows the unique TENG potential as a power source for powering consumer electronics. [2]
Reference:
1. S. Niu, Y. Liu, Y. S. Zhou, S. Wang, L. Lin, Z. L. Wang. IEEE Trans. Electron Devices, 2015, 2, 641
2. S. Niu, X. F. Wang, F. Yi, Z. L. Wang, manuscript submitted

In this study, the triboelectric generator based on conductive textile was proposed to generate energy from stretching motion. Previous triboelectric generators have limited stretchability because of non-stretchable materials or structures. Also, most of the triboelectric generators were operated by pushing motion in the vertical direction.[1][2] Proposed conductive textile based triboelectric generator was composed of corrugated stretchable conductive textile sewing on Ag textile which is coated by PDMS. Triboelectric energy is generated from electric potential difference between two materials by pushing and stretching motion, when the two material contacts; however, the corrugated structure generates a power regardless of the direction of the force distributed on the Ag textile and PDMS. If the conductive textile based triboelectric generator applies to fabric, power can be generated from the various body movements including pushing and stretching. When the conductive textile based triboelectric generator contact and releasing repeatedly, output voltage was 15 V. In addition, the voltage of 150 V was observed under the mechanical pushing force of 0.1 kgf. The experiment result represents the triboelectric generator made by corrugated stretchable textile can be integrated into clothing for generating the energy. Reference [1] Wanchul Seung et al., ACS Nano, 9, 4 (2015). [2] Yeong Hwan Ko et al., RSC Advances, 5 (2015). (This research was supported by the ITRC support program, IITP-2015-R0992-15-1021)

We fabricated new prototype triboelectric generator that can convert the mechanical energy to electrical energy efficiently. A two-ply yarn based triboelectric generator enables energy generation by a conjunction of triboelectrification and electrostatic induction. The silver conductive fiber with diameter of 160 mu;m and silver conductive fiber coated silicon rubber are used. Silver conductive fiber and Si rubber is selected as the active triboelectric materials due to flexibility, lightweight and proper stiffness. According to triboelectric series, silver is more positively charged, while the Si-rubber is negatively charged. The fibers that have opposite triboelectric polarities are twisted each other to make a single thread. As a result of triboelectric properties, the voltage output of two-ply yarn is 7 V where compressive force is 1 Newton. Also voltage output of two-ply yarn is 3 V at stretched. Through an external load, the two-ply structure based generator enables periodic contact and noncontact between materials that differed in polarity of triboelectricity yield a potential drop which will drive electrons to flow. Therefore, energy conversion is achieved with the triboelectric effect and electrostatic induction. Because of the easy fabrication, cost effectiveness, flexible properties, the two-ply based triboelectric generator expands the potential applications to harvesting energy from moving object or human motion. (This research was supported by the ITRC support program, IITP-2015-R0992-15-1021)

Colloidal perovskite oxide nanocrystals have attracted a great deal of interest owing to the ability to tune physical properties by virtue of the nanoscale, and generate thin film structures under mild chemical conditions, relying on self-assembly or heterogeneous mixing. This is particularly true for ferroelectric/dielectric perovskite oxide materials, for which device applications cover piezoelectrics, MEMs, memory, gate dielectrics and energy storage. The synthesis of complex oxide nanocrystals, however, continues to present issues pertaining to quality, yield, % crystallinity, purity and may also suffer from tedious separation and purification processes, which are disadvantageous to scaling production. We report a simple, green and scalable “self-collection” growth method that produces uniform and aggregate-free colloidal perovskite oxide nanocrystals including BaTiO₃ (BT), Ba_xSr_1-xTiO₃ (BST) and quaternary oxide BaSrTiHfO₃ (BSTH) in high crystallinity and high purity. The synthesis approach is solution processed, based on the sol-gel transformation of metal alkoxides in alcohol solvents with controlled or stoichiometric amounts of water and in the stark absence of surfactants and stabilizers, providing pure colloidal nanocrystals in a remarkably low temperature range (15 #730;C -55 #730;C). Under a static condition, the nanoscale hydrolysis of the metal alkoxides accomplishes a complete transformation to fully crystallized single domain perovskite nanocrystals with a passivated surface layer of hydroxyl/alkyl groups, such that the as-synthesized nanocrystals can exist in the form of super-stable and transparent sol, or self-accumulate to form a highly crystalline solid gel monolith of nearly 100% yield for easy separation/purification. The process produces high purity ligand-free nanocrystals excellent dispersibility in polar solvents, with no impurity remaining in the mother solution other than trace alcohol byproducts (such as isopropanol). The afforded stable and transparent suspension/solution can be treated as inks, suitable for printing or spin/spray coating, demonstrating great capabilities of this process for fabrication of high performance dielectric thin films. The simple “self-collection” strategy can be described as green and scalable due to the simplified procedure from synthesis to separation/purification, minimum waste generation, and near room temperature crystallization of nanocrystal products with tunable sizes in extremely high yield and high purity.

Regarding with triboelectric nanogenerators (TENGs), following factors including materials, architectural design for given occasion predominantly are believed to be quite important for performance of TENGs. There have been already many reports investigated for highly effective TENGs in terms of design, but choosing materials still remains great challenge.
Here, a multi-ferroelectric based triboelectric nanogenerator (MF-TENG) is developed for high power generation through taking advantage of optimum BaTiO₃ embedded P (VDF-TrFE) matrix film. Since fluorine atom originated from P (VDF-TrFE) is inclined to attract electrons once a contact electrification taken place, it induces large potential difference upon opposite triboelectric materials. While BaTiO₃ has higher dielectric constant than P (VDF-TrFE), which is associated with high output charge density compared to that of just using the single-layer of P (VDF-TrFE). MF-TENG having dimensions of 3 x 3 cm under 5 kgf exhibits high electrical performances: an instantaneous output voltage of 210 V and a current of 180mu;A, which is recordable output current as considered traditional TENGs&’ output performance, giving an average power density of approximately 4.2 mW. This MF-TENG is providing a new approach to enhance output power of TENGs using embedding ferroelectric material in order to drive various electric devices such as portable electronics, implantable and wearable devices.

Triboelectric effect has been knowing since more than a century and it can be observed only on the surface of dielectric materials such as polymers. There are speculations about the mechanism of this effect, but none of them are proven yet. Three important phenomenon are considerably strong to explain triboelectric effect, which are direct charge transfer, ion transfer and material transfer from surface to surface. Although mechanism of triboelectric effect is a conundrum, a closer look into recent studies, which partially explain the mechanism of triboelectric effect reveals that one can facilitate from this effect as a sensor or energy harvesting device. It has high capacity to drive electricity from contact and sliding of dielectric materials on their surfaces. This contact and separation phenomenon also causes separation in charges and thus induction of free charges on the electrodes coated on the surface of the dielectrics. Researchers succeeded to harvest over 1 kV and 1 mA from surrounding environmental motions. Surface structure and triboelectric polarity of materials play a major role to build high performance energy harvesting and sensor devices. Interestingly, not only solid materials but also pure water and liquid compounds were also prior to use in triboelectric devices. Many studies has been published that proving water waves are a good source of energy when freely move on a dielectric substrate. The mechanism is related to chemical polarity of the liquid as well as the surface and triboelectric properties of the dielectric substrate. The current study represents a smart energy harvesting and chemical sensing microfluidic systems that converts mechanical motion of chemicals droplets in a tubular poly(vinylidene floride) (PVDF) microfluidic channel to the electrical signals. The system consist of a single channel microfluidic channel with a PVDF micro-tubular attachment, which is produced using thermal fiber drawing technique. Channel lengths and the diameters can vary, but what effects the output voltage and current signal is the change in the speed or air - deionized water speed and size. We observed that increasing droplet speed increases the induced charge. When there is a continuous flow through the channel system generates enough power to power 6 LEDs. Finally, the system can detect the change in the ratio of water - ethanol mixture down to 5 %. Although output voltage signal is at the maximum 1 V when plane deionized water flushed, increasing ethanol percentage decreases the output voltage. Our system can be used for understanding chemical concentrations of liquid chemicals, when accurate and detailed calibration was applied for versatile solution.

Mechanical energy harvesting from the ambient environment is likely to play an important role for sustainable development of modern society, due to consumption of traditional fossil fuels and related environmental issues. In this regard, a novel approach of triboelectric nanogenerator (TENG) has been recently invented that employs the coupling effect of triboelectrification and electrostatic induction. Both high output power and numerous practical applications have been demonstrated, indicating its promising potential as a new energy technology for large scale power conversion. However, the energy conversion efficiency and device durability are still essential issues, which may be limited by the relatively large frictional resistive force between triboelectric surfaces during the operation of the TENG, especially for the working modes based on sliding electrification.
Here in this work, we designed a novel working mode of rolling triboelectric nanogenerators (RTENG) that can deliver ultrahigh energy conversion efficiency without scarifying its robustness and stability. For the first time, rolling electrification between cylinder-shaped steel rods and planar thin films of fluorinated ethylene propylene (FEP) was employed for converting the kinetic energy of rolling rods into electric power. The rolling triboelectric nanogenerator (RTENG) has a sandwiched structure composed of two FEP thin films and steel rods rolling between them. The rolling motion of the steel rods between the FEP thin films introduces triboelectric charges on both surfaces and led to the change of potential difference between each pair of electrodes on the back of the FEP layer, which drives the electrons flow in the external load. As power generators, each pair of output terminals can work independently that delivers an open-circuit voltage of 425 V, an instantaneous charge transfer of 0.145 uC, and a short-circuit current density of 5 mA/m². The two sets of output terminals can also be integrated to present an overall power density of up to 1.6 W/m². The impact of various structural factors was investigated to achieve optimization of the output performance. Owing to the low friction coefficient of the rolling movement, a high energy conversion efficiency of up to 55% has been demonstrated, with much smaller wearing of triboelectric surfaces as compared with sliding friction. Based on the basic concept of rolling electrification, many other prototypes of RTENGs have been successfully derived, like the rotating disk structure, the grating structure, and other structures based on rolling balls. This work demonstrates a new working mode of high-efficient and robust TENGs towards large scale energy harvesting.
Reference: L. Lin, Y. N. Xie, S. M. Niu, S. H. Wang, P. K. Yang, Z. L. Wang, ACS Nano, 2015, 9(1) 922-930.

Scavenging the widely existed mechanical energy from the environment and converting it into electricity through nanogenerators may provide new solutions to the current energy and environment problems. Based on piezoelectric effect and triboelectric effect, nanogenerators have been utilized to power electronic devices, cleaning air pollution, harvesting mechanical energy in large scale, etc. Combining the piezoelectric and triboelectric effects together to form hybrid nanogenerators can further promoting the practical applications, which not only enhances the power output, but also broadens the application fields. However, the investigation of the coupling between piezoelectric and triboelectric effects is currently insufficient. Therefore, in this paper, we demonstrated the theoretical and experimental studies of the coupled piezoelectric-triboelectric hybrid nanogenerators (PE-TENGs).
Firstly, theoretical comparison between piezoelectric and triboelectric nanogenerators has been demonstrated, indicating that both types of nanogenerators have similar dependence on size, material properties, and applied external force. Secondly, the coupling of piezoelectric and triboelectric effects in PE-TENGs at different structures was investigated through finite element method simulation. The results show that the structure, piezoelectric polarization direction, electrode connection, as well as the operation mode (i.e., piezoelectric and triboelectric effects work successively or simultaneously) together influence the final output performance of piezoelectric-triboelectric hybrid nanogenerators. Moreover, since the combination of piezoelectric and triboelectric effects can either enhance or reduce the total output, the obtained results may provide guidance on the design of high performance PE-TENGs. Finally, based on the theoretical and simulation results, experimental measurements of different PE-TENGs were conducted. For instance, a successive mode PE-TENG was tested at different electrode connections and the results showed highly consistency with the simulation. In addition, an r-shaped hybrid nanogenerator was designed to realize the simultaneous mode PE-TENG. Micro/nanostructured polydimethylsiloxane (PDMS) and nanostructured Al were utilized as the electrification materials in order to obtain high output performance. One cycle electric generation of this device can directly power 10 light-emitting diodes, and enables a 4-bit liquid crystal display to display continuously for more than 15 s.
References:
[1] M. Han, X. S. Zhang, B. Meng, W. Liu, W. Tang, X. Sun, W. Wang, H. Zhang, “r-Shaped Hybrid Nanogenerator with Enhanced Piezoelectricity”, ACS Nano, 2013, 7, 8554-8560.
[2] M. Han, X. Chen, B. Yu, H. Zhang, “Coupling of Piezoelectric and Triboelectric Effects: from Theoretical Analysis to Experimental Verification”, submitted.

Converting wasted energy into electrical energy has received great attention and become an important research field, especially since the commercialization of wearable device systems, wireless sensors and small electronics with low power consumption. Harvesting energy from wasted thermal energy in our environment would be a great opportunity and very important for low power consumption electronics. There is still a lack of research on energy harvesting based on the pyroelectric effect. In addition, the reported power-generating performance of pyroelectric nanogenerators (PNGs) is relatively low compared with other energy harvesters. Therefore, enhancement of the pyroelectric performance is one of the significant technical issues for meaningful usage of pyroelectric NGs as a new energy harvesting device. We innovatively designed a stretchable PNG (SPNG) based on a coupling of piezoelectric and pyroelectric effects using micro-patterned P(VDF-TrFE) and micro-patterned polydimethyl-siloxane (PDMS). We utilized the different thermal expansion characteristics of two materials to build strain on ferroelectric P(VDF-TrFE) films to generate a piezoelectric effect using only thermal energy. We compared the SPNG with a normal PNG (NPNG) which is based on the flat P(VDF-TrFE) coated onto a flat and rigid Ni/SiO₂/Si substrate, and we found that the highly sensitive SPNG operates from temperature changes that range from extremely low (#8710;T asymp; 0.64 K) to high (#8710;T asymp; 18.5 K), and generates maximum around 2.5 V of output voltage and 570 nA/cm² of output current density. Thermally induced stress, strain, and piezoelectric potential in the P(VDF-TrFE) of the SPNG and the NPNG were simulated in order to support the experimental results using COMSOL multi-physics simulation software. We successfully drove small electronic devices, such as a liquid-crystal display, light-emitting diodes, using output power from a SPNG. The results obtained in this work strongly suggest that the SPNG could be applied in various kinds of promising device applications such as self-powered wireless sensors, highly sensitive temperature imaging, and self-powered biomedical applications.

New strategies for heterogeneous integration of hard and soft materials into mechanically flexible, composite structures create opportunities in unusual classes of high performance devices for energy conversion and storage. This talk describes progress in inorganic photovoltaic modules and lithium ion batteries, both as separate devices and as co-integrated systems that can be stretched, twisted, folded or bent, by virtue of optimized mechanical designs.

According to current previsions our mobile society will have to face up an ever-increasing demand for portable energy sources. Although Li-ion batteries (LIB) are considered as the more mature technology to address this demand, LIB cathodes rely mostly on transition metal oxide materials suffering from limited resources. Consequently organic cathode materials have recently attracted vivid interest as more sustainable potential alternatives.^1-3 A promising candidate is the poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) (PTMA), which bears TEMPO side-groups able to undergo reversible redox reactions. PTMA displays excellent electrochemical features (high potential of 3.6 V vs. Li⁺/Li and high stability upon cycling) among those the most noticeable is its ultrafast charge/discharge capabilities.^1-3 Yet, this polymer suffers from its solubility in standard commercial electrolytes and from its electric insulating behavior.¹ The common approach to improve electrical transport and charge collection in PTMA electrodes involves incorporation of conductive carbon additives (typically 60 to 80 wt.%) detrimental to the specific capacity of the cathode.
To tackle these issues, we propose an original strategy towards dense grafting of PTMA on multi-walled carbon nanotubes (MWCNTs) in order to provide a chemical anchor and a more intimate contact between the polymer and the conductive carbon. Our strategy involves the surface initiated polymerization of a precursor monomer from the high-density initiator functionalized surface of MWCNTs and the oxidation of the accordingly obtained polymer precursor into PTMA, leading to MWCNT-g-PTMA composites.
Extended chemical and morphological analysis confirmed the compact core-shell morphology with a high active material mass loading of 60 wt.%. The electrodes made out of these MWCNT-g-PTMA composites display good cycling stability, good rate capabilities and an excellent specific capacity. Furthermore, with respect to common PTMA/conductive carbon blends, our polymer-grafted material displays superior power performances thanks to its enhanced charge transfer and current collection.⁴ The success of this strategy highlights the benefits of a precise distribution of the active material around the conducting network thanks to our designed core-shell morphology.
[1]H. Nishide and T. Suga, Electrochem Soc Interface, 2005, 14, 32-36. [2]T. Janoschka, M. D. Hager and U. S. Schubert, Adv Mater, 2012, 24, 6397-6409. [3]A. Vlad, N. Singh, J. Rolland, S. Melinte, P. Ajayan and J.-F. Gohy, Sci Rep, 2014, 4. [4]B. Ernould, M. Devos, J.-P. Bourgeois, J. Rolland, A. Vlad and J.-F. Gohy, J Mater Chem A, 2015, 3, 8832-8839.

In the last decade, radical polymers have emerged as promising electrode materials for secondary batteries because of their stable high voltage, sustainability and potential low-cost. Furthermore their electronic properties can be rationally tuned.
The most studied radical compounds for electrode applications are nitroxide-based polymers. Indeed, nitroxide can be reversibly n-doped to aminoxyl anion and p-doped to oxoammmonium cation in anodic and cathodic reaction, respectively. Poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) is the most common active material which has already been used as a cathode in Li-ion cells. It displays a reversible redox process around 3.6 V vs Li/Li+ with a theoretical capacity of 111 mAh/g.
High-throughput ab initio calculations could be used for the molecular engineering of new radical materials with tunable electrochemical properties. As a first step in this direction, we test the agreement between theoretical and experimental data for various organic radical polymers used as active materials in batteries. A ΔSCF method based on density functional theory is compared to the higher-level GW many-body perturbation theory and to experimental values. The results indicate that the method provides a fast and accurate estimation of the open cell voltage of these organic radical Li-ion batteries, suggesting it would be sufficiently robust for a preliminary screening of theoretically designed radicals.
In a second step we perform high-throughput simulations on modified nitroxyl radicals where the methyl groups are para-substituted with various functional groups. These new radicals show theoretical voltages ranging from 3.4 to 6.2 V and specific energies up to 564 mWh/g. Depending on the specific battery applications, the best candidates can be selected according to the desired open cell voltage and being tested experimentally.

Since the inception of flexible and wearable electronics, e.g., roll-up displayer, on-skin circuit, Google glass, and Apple iWatch, flexible energy storage devices have attracted much attention and interest. Because of their high energy density coupled with long life and passable rate capability, lithium-ion batteries (LIBs) have become the mainstream choice for consumer electronics and electric vehicles as well as grid energy storage. Therefore, it is desirable to bestow flexibility upon the LIBs. To this end, much effort has been devoted to not only planar but also fiber-shaped flexible LIBs. Some revolutions have been made compared with the traditional rigid LIBs consisting of current collector, active material, binder and conductive additives. The most pivotal change among flexible LIBs is the introduced flexible platforms, e.g., a metal mesh/fiber, a paper, a piece of cloth, a carbonaceous textile/film/fiber. The ideal platforms are required to possess good flexibility and strength, good conductivity and high specific surface area. Amongst them, the platform made of carbon nanotube (CNT) is deemed as one of the most promising types because CNT exhibits excellent mechanical, physical and chemical properties.
Herein, we have developed a series of both planar and fiber-shaped flexible LIBs based on the aligned CNTs. Firstly, aligned and spinnable CNT arrays are synthesized by chemical vapor deposition. Then, aligned CNT sheets can be directly dry-drawn from the arrays. Afterwards, the aligned CNT fibers can be prepared by twisting the aligned CNT sheets. The aligned CNT sheets and fibers directly serve as the flexible platforms on which the other battery components can be deposited, resulting in the flexible LIBs. In particularly, we have proposed a flexible planar LIB using CNT/silicon and CNT/lithium manganite respectively as the anode and cathode, a flexible and stretchable planar LIB using CNT/lithium titanate and CNT/lithium manganate respectively as the anode and cathode, and a fiber-shaped flexible LIB using CNT/silicon and CNT/lithium manganite respectively as the anode and cathode.
References
Wei Weng, Sun Q., Zhang Y., He S., Wu Q., Deng J., Fang X., Guan G., Ren J., Peng H.* Advanced Materials, 2015, 27, 1363.
Wei Weng, Sun Q., Zhang Y., Lin H., Ren J., Lu X., Wang M., Peng H.* Nano Letters, 2014, 14, 3432.
Wei Weng, Lin H., Chen X., Ren J., Zhang Z., Qiu L., Guan G., Peng H.* Journal of Materials Chemistry A, 2014, 2, 9306.
Wei Weng, Wu, Q., Sun Q., Fang X., Guan G., Ren J., Zhang Y., Peng H.* Journal of Materials Chemistry A, 2015, 3, 10942.
Lin H., Wei Weng, Ren J., Qiu L., Zhang Z., Chen P., Chen X., Deng J., Wang Y., Peng H.*. Advanced Materials, 2014, 26, 1217.
Edition, 2014, 53, 7864.

Lithium-sulfur (Li-S) batteries have attracted great attention as next-generation high specific energy density storage devices. However, the low sulfur loading in the cathode for Li-S battery greatly offsets its advantage in high energy density and limits the practical applications of such battery concepts. Flexible energy storage devices are also becoming increasingly important for future applications but are limited by the lack of suitable lightweight electrode materials with robust electrochemical performance under cyclic mechanical strain. Here, we proposed an effective strategy to obtainflexible Li-S battery electrodes with high energy density, high power density, and long cyclic life by adopting graphene foam-based electrodes. Graphene foam can provide a highly electrically conductive network, robust mechanical support and sufficient space for a high sulfur loading. The sulfur loading in graphene foam-based electrodes can be tuned from 3.3 to 10.1 mg cm^-2. The electrode with 10.1 mg cm^-2 sulfur loading could deliver an extremely high areal capacity of 13.4 mAh cm^-2, much higher than the commonly reported Li-S electrodes and commercially used lithium cobalt oxide cathode with a value of about 3-4 mAh cm^-2. Meanwhile, the high sulfur-loaded electrodes retain a high rate performance with reversible capacities higher than 450 mAh g^-1 under a large current density of 6 A g^-1 and preserve stable cycling performance with around 0.07% capacity decay per cycle over 1000 cycles.These impressive results indicate that such electrodes could enable high performance, fast charging, and flexible Li-S batteries that show stable performance over extended charge/discharge cycling.

Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hishy;eshy;rshy;ashy;shy;rshy;chical design of 3-dimensional nano-architectures. Such structural meta-materials exhibit superior thermoshy;mechanical proshy;shy;shy;pershy;shy;ties at exshy;treshy;meshy;ly low mass densities (lighter than aerogels), making these solid foams ideal for many scientific and techshy;noshy;loshy;gishy;cal applications.
We present the fabrication of 3-dimensional architected meta-materials whose constituents vary in size from several nanometers to tens of microns to millimeters. We specifically discuss the mechanics and characterization of such architected meta-materials in the scope of energy storage applications. Attention is focused on Si-anode Li-ion batteries and on nanostructured cathodes for Li-O₂ batteries. For the Li-ion system, battery performance is tested in situ, in an electrochemical half-cell inside of a scanning electron microscope (SEM) using a lithium electrode and the nano-architected Si-Cu electrode. Electrochemical-mechanical experiments and modeling suggest that these electrodes are robust against fracture while maintaining reasonable charge-discharge-recharge performance. We also demonstrate the feasibility of using 3-D architected microlattices as positive electrodes in Li-O₂ batteries. Using Au as the electrode material and DME as the electrolyte solvent, we observed the formation of toroidal-shaped Li₂O₂ as the predominant product after 1^st discharge, their complete removal upon charge, and the accumulation of Li₂CO₃, HCO₂Li and CH₃CO₂Li after multiple cycles. These findings demonstrate that 3-dimensional architected meta-materials provide a useful testbed when serving as electrodes for studying fundamental electrochemistry and discharge product morphology.

Much like other physiochemical processes such as oxidation and corrosion in metals, electrochemical cycling of electrodes features intimate coupling between insertion kinetics and mechanical stress: the insertion of the secondary species generates localized stress, which in turn mediates electrochemical insertion rates. Here we report such strong coupling in nanostructure Si (nanoparticles and nanowires). We show that on the one hand lithiation self-generated stress causes lithiation retardation; on the other crystallographic orientation-dependent chemical reaction at the lithiation front modulates stress generation and fracture of the electrode materials. We further illustrate that externally force, such as bending, applied to Si nanowires breaks the lithiation symmetry, resulting in much higher lithiation rate in the tensile side than the compressive side of the nanowires. Finally we highlight that such coupling effect can be exploited to construct mechanically rechargeable batteries.

Development of high energy density flexible wearable batteries that maintain safe and stable operation under deformation is crucial to enable autonomous operation of electronic body-wearable systems. Among other compliant architectures, wire-shaped batteries offer unique versatility in terms of integration with wearable technologies due to their omnidirectional flexibility. Over the last years considerable progress was made in developing wire-shaped battery systems. However, simultaneously achieving high specific capacity and long cycle life of these devices remains a challenge.
Here we present compliant wire battery with improved stability (over 170 stable cycles with capacity retention above 98%) and linear capacity higher than previously reported (between 1.2 to 1.8 mAh cm^-1 at 0.5C discharge rate). The battery is assembled by utilizing low cost fabrication processes and is based on the silver-zinc chemistry. This technology, in addition to being energy dense and rechargeable, utilizes aqueous electrolyte and thus, is intrinsically safer than battery chemistries that rely on organic solvents.
Traditionally silver-zinc batteries have failed prematurely due to poisoning of zinc electrode by silver ions. Here we show that the lifetime of the battery can be improved by optimizing potassium hydroxide (KOH) concentration in the electrolyte to reduce the dissolution of silver ions and by embedding cellophane membrane between the anode and cathode to inhibit migration of silver ions towards the zinc electrode.
The obtained wire battery can successfully withstand mechanical perturbations that are expected to occur in wearable applications. Its capacity remains stable even after repeated flexing up to a bending radius of 1 cm. Furthermore, to demonstrate suitability of the wire battery for wearable applications, several batteries in series were assembled into flexible and stretchable form factor to power commercially available LEDs.

Powering flexible wearable electronics remains a challenge due to the size and packaging constraints of traditional mobile devices and battery chemistries. One possible solution to the space constraint is utilizing clothing for energy storage, converting previously passive materials into electrochemical energy storage devices. In this work, we target textile-based energy storage systems — where size constraints are less stringent, battery real estate is scalable to requirements, and flexibility is possible — fabricated with automated, modular processes.
We demonstrate zinc and Prussian blue analog battery electrodes on carbon fiber substrates that are fabricated in-house using an automated system composed of low-cost, modular electronics components. Conductive carbon fibers are drawn into zinc or copper hexacyanoferrate (CuHcf) slurries for the anode and cathode, respectively. After drying, the active material-coated fibers then are embedded in a sodium-rich agarose hydrogel electrolyte at neutral pH. Preliminary charge/discharge tests of the 1.7 V cells in a flooded configuration show an initial specific discharge capacity of 51 mAhr/g CuHcf, and 43 mAhr/g after 25 cycles. Further optimization of electrode processing is likely to improve cycle life significantly. Despite this low capacity, we explain how this can be used with minimalout any encapsulation, thereby making its system level energy density on par with, if not better than, aprotic techniques requiring hermetic sealing.
Additionally, we demonstrate batteries consisting of woven fibers as a proof of concept towards realizing wearable and washable electronics. We characterize the physical properties of the fiber battery with SEM, stress loading, and washing tests. Charge cycling, cyclic voltammetry and electrochemical impedance spectroscopy is performed on the woven structures.

We report the first successful demonstration of a “π-conjugated redox polymer” simultaneously featuring a π-conjugated backbone and integrated redox sites, which can be stably and reversibly n-doped to a high doping level of 2.0 with significantly enhanced electronic conductivity. The properties of such a heavily n-dopable polymer, poly{[N,Nprime;-bis(2-octyldodecyl)-1,4,5,8-naphtha- lenedicarboximide-2,6-diyl]-alt-5,5prime;-(2,2prime;-bithiophene)} (P(NDI2OD-T2)),werecomparedvis-a-#768;vistothoseofthe corresponding backbone-insulated poly{[N,Nprime;-bis(2-octyl- dodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt- 5,5prime;-[2,2prime;-(1,2-ethanediyl)bithiophene]} (P(NDI2OD- TET)). When evaluated as a charge storage material for rechargeable Li batteries, P(NDI2OD-T2) delivers 95% of its theoretical capacity at a high rate of 100C (72 s per chargeminus;discharge cycle) under practical measurement conditions as well as 96% capacity retention after 3000 cycles of deep dischargeminus;charge. Electrochemical, impe- dance, and charge-transport measurements unambiguously demonstrate that the ultrafast electrode kinetics of P(NDI2OD-T2) are attributed to the high electronic conductivity of the polymer in the heavily n-doped state.

Molybdenum dioxide (MoO₂) is commonly utilized in electrochromic, catalytic, sensing, field emission and recording media devices; however, its demonstration in supercapacitors is very limited. It has unique physical and chemical properties such as good conductivity, good chemical stability and cyclability. In this work, we report on the fabrication and characterization of supercapacitors with nanocomposite MoO₂ and silver nanowire network electrodes to investigate their capacitive properties. MoO₂ was deposited onto Ag nanowire thin films by electrodeposition. Ag nanowire thin films on polyethylene terephthalate substrates provided high conductivity and a template for the attachment of MoO₂. Electrochemical properties such as specific capacity and cycling ability of supercapacitors with coaxial composite electrodes were then investigated through cyclic voltammetry, chronopotentiometry, electrochemical impedance spectroscopy. A specific capacitance of 435 F/g was obtained for the nanocomposite electrodes, which was higher than that of bulk MoO₂ electrodes. We will present a detailed electrochemical analysis of the fabricated flexible supercapacitors to underline their capacitive behavior. Our results reveal the potential use of the Ag nanowire/MoO₂ nanocomposites in flexible supercapacitor electrodes that can be fabricated through simple solution based methods and the method investigated herein can be simply adapted to industrial scale fabrication.

Portable and foldable electronic devices had been shown to be highly desired in the current life and further dominate the future life. To this end, it becomes critically important to develop matchable energy storage systems such as electrochemical supercapacitors to power them. The supercapacitor should be lightweight, flexible and stretchable, and could be easily integrated with high performances. However, the available supercapacitors typically appear in a heavy and rigid plate which cannot meet the above requirements. Herein, novel supercapacitor fibers in a coaxial structure have been developed from aligned carbon nanotube fiber and sheet which functioned as two electrodes. The unique coaxial structure enables a rapid transportation of ions between the two electrodes with a high maximum discharge capacitance of 59 F/g, and the high electrochemical performance has been well maintained at high currents.

Structural energy storage materials combining load-bearing mechanical properties and high energy storage performance are desired for applications in wearable devices or flexible displays. Vanadium pentoxide (V₂O₅) is a promising cathode material for possible use in flexible battery cathodes, but it remains limited by poor lithium-ion diffusion coefficient and electronic conductivity, severe volumetric changes during cycling, and limited mechanical flexibility. Here, we demonstrate a route to address these challenges by blending a diblock copolymer bearing electron- and ion-conducting blocks, poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO), with V₂O₅ to form a mechanically flexible, electro-mechanically stable hybrid battery cathode. V₂O₅ layers were arranged parallel in brick-and-mortar-like fashion held together by the block copolymer binder. This unique structure significantly enhances mechanical flexibility, toughness and cyclability without sacrificing capacity. Properties are probed as a function of composition using dynamic mechanical analysis, impedance spectroscopy, cyclic voltammetry, and galvanostatic cycling. It is found that optimum composition exists, where too little polymer results in brittle inflexible electrodes and too much polymer reduces the electrochemical activity. Only 5 wt % polymer is required to triple the flexibility (normal strain) of V₂O₅, and electrodes comprised of 10 wt % polymer have unusually high toughness (293 kJ/m³) and specific energy (530 Wh/kg), both higher than reduced graphene oxide paper electrodes. These results demonstrate a new approach for the development of structural energy storage materials.

Recently, binary and ternary composites have been widely reported by integrating multiple components, which could overcome the limitations of each individual and improve their electrochemical performance. Thus, developing efficient assembled electrodes with rational design are important for supercapacitors with good properties. Carbon cloth (CC), an inexpensive and highly conductive textile with excellent mechanical flexibility and strength, holds great promise for fabrication of flexible and freestanding electrode for energy device from the design and packaging perspectives [1]. In addition, the three-dimensional porous structure of CC is expected to facilitate the diffusion of electrolyte into the electrode material and provide more channels for rapid ion transport.
Here, two types of ternary composites were fabricated by reversing the deposition sequence of polyaniline (PANI) and MnO₂ layers on plasma treated carbon cloth (m-CC), i.e., PANI@MnO₂@m-CC and MnO₂@PANI@m-CC. By comparison, PANI@MnO₂@m-CC displayed a unique porous structure, significantly different from either the petal-like MnO₂ nanostructures or PANI nanorods. The morphological transformation of MnO₂ petals into nanoparticles while anchoring PANI nanorods increases the interactions between the two pseudoactive materials. With a rational deposition process, PANI@MnO₂@m-CC possessed a higher areal capacitance of 421.6 mF/cm² at 0.2 mA/cm², a better rate performance (58.9% retention with current densities from 0.2 to 2 mA/cm²) than and a comparable cycling stability (a capacitance of 86.5% after 5000 cycles) to that of MnO₂@PANI@m-CC. Moreover, an asymmetric supercapacitor was fabricated by using PANI@MnO₂@m-CC as the positive electrode and activated microwave exfoliated graphite oxide (‘a-MEGO&’) @m-CC as the negative electrode. The asymmetric supercapacitor showed a maximum energy density of 33.9 Wh/kg (at a power density of 319.0 W/kg) and power density of 17.2 kW/kg (at an energy density of 14.3 Wh/kg) at an operating voltage of 2.0 V. Our results indicate that a promising electrode candidate for supercapacitor of PANI@MnO₂@m-CC could be obtained with a rational design of each component, which is instructive to fabricate binary or ternary composites for energy storage devices. [1] P. Yu, Y. Li, X. Yu, X. Zhao, L. Wu and Q. Zhang, Langmuir, 2013, 29, 12051-12058.

A simple but effective method has been described to produce 3D composite thin films of 2D layered MoS₂ and reduced graphene oxide (rGO). Liquid crystal dispersions of large monolayer GO sheets have been produced via a modified Hammer process. Single to few layer MoS₂ dispersion were prepared by sonic assisted aqueous-phase exfoliation. Both 2D layered materials have been blended together to form composite thin films with excellent homogeneity and flexibility. The resulting films were reduced at 200 °C under vacuum to produce flexible conductive rGO-MoS₂ films. RAMAN analysis at different spots of the film confirms the presence of MoS₂ throughout the entire film, which is in good agreement with XRD data and SEM micrographs. The embedment MoS₂ layers between individual GO sheets was confirmed by cross-sectional SEM images of the films. The rGO-MoS₂ composite exhibits superior electrochemical performance with increased capacitance and cycling stability during the electrochemical performance test in aqueous electrolytes. This improvement of performance can be attributed to the embedding of MoS₂ sheets between rGO sheets that prevents restacking after reduction by acting as nano-spacer and produces an efficient electron transfer network. These results indicate the applicability of the novel composite film as a potential candidate for the high performance flexible supercapacitors.

We present a post synthesis self-assemble protocol that transforms the trillions of CNTs in powder form into densely packed flexible, robust and binder-free macroscopic membranes with hierarchical pore structure. The processing protocol has limited or no impact on the intrinsic properties of the CNTs. The binder-free CNT membranes could be as thin as < 10mm with mass density greater than that of water (1.0g/cc). As the thickness of the CNT membrane is increased, we observed a gradual transition from high flexibility to buckling and brittleness in the flexural properties of the CNT membranes. We correlate the mass of the CNTs to thickness of the CNT membrane. We have demonstrated the use of the CNT membranes as electrode in two-electrode 1M H₂SO₄ aqueous double layer supercapacitor that shows very high power density ~1040 kW/kg based on the mass of both electrodes and time constant of ~ 15 ms with no degradation in performance even after 10,000 cycles. Furthermore, we will show the designing of flexible 3-stack bipolar solid-state ultracapacitor and present results on energy/power densities, voltage, cyclability, temperature stability in relation to flexibility and weight. Preliminary results indicate high temperature stability >85^oC and CV voltage ~3V with very low leakage current ~ 10nA.

The development of portable electronics calls for new energy storage systems preferably in the form of biodegradable and environmentally friendly devices [1-3] as most electrochemical energy storage systems utilize toxic or scarce materials and thus need advanced waste disposal procedures. In this quest conducting polymers have been proposed as electrode material in batteries and capacitors due to their fast redox processes, high capacities, abundance and environmental friendliness [4, 5]. Conducting polymer based electrodes generally exhibit supercapacitive-like behavior including the possibility of rapid charge and discharge. The rapid self-discharge of these electrodes is, however, a problem [6] which can limit their range of application.
Composites of nanocellulose, derived from Cladophora sp. algae, and polypyrrole (PPy) can be used as electrode materials in environmentally friendly energy storage devices [7]. The PPy/cellulose composites, consisting of cellulose fibers with a diameter of ~30 nm coated by a ~50 nm thick layer of PPy, constitute porous materials with surface areas of ~80 m²/g. The thin PPy layer and the aqueous electrolyte present in the pores yield short counter ion mass transport distances enabling use of high charge and discharge rates. Symmetric energy storage devices with PPy/cellulose electrodes also exhibit capacitances of ~40 F/g (after normalization with respect to the weight of both electrodes) when operated at cell voltages up to 0.8 V.
The self-discharge rates of the PPy/nanocellulose based devices are known to be determined by an intrinsic activation controlled reaction for the positive electrode and oxidation of the negative electrode by residual oxygen [8]. The present study is aimed at suppressing the self-discharge rates and maximizing the useful cell voltage of these symmetric energy storage devices. The influence of oxygen, irreversible oxidation and the absolute potential of the oxidized PPy on the self-discharge rates are discussed. A symmetric PPy-based device, with appropriately balanced electrode weights, exhibiting 25% decrease in the cell voltage during 45 h after charging to 0.8 V is also demonstrated. As this self-discharge rate is significantly lower than the previously reported values the results are indeed very promising.
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[2] Y.H. Jung, T.-H. Chang, H. Zhang, C. Yao, Q. Zheng, V.W. Yang, H. Mi, M. Kim, S.J. Cho, D.-W. Park, Nature Commun., 6 (2015).
[3] M. Armand, J.M. Tarascon, Nature, 451 (2008) 652.
[4] P. Novák, K. Müller, K. Santhanam, O. Haas, Chem. Rev., 97 (1997) 207.
[5] J.R. Heinze, B.A. Frontana-Uribe, S. Ludwigs, Chem. Rev., 110 (2010) 4724.
[6] B.E. Conway, Electrochemical Supercapacitors, Plenum, N.Y., 1999.
[7] G. Nystro#776;m, A. Razaq, M. Stroslash;mme, L. Nyholm, A. Mihranyan, Nano Letters, 9 (2009) 3635.
[8] H. Olsson, M. Sjödin, E.J. Berg, M. Stroslash;mme, L. Nyholm, Green, 4 (2014) 27.

The importance of large-scale flexible Li-ion battery (LIB) is realized by the emergence of new markets for innovative electronic structures that demand thinness and flexibility. However, flexible LIB requires high areal and volumetric capacities in order to make it practical for energy storage application. Here w