Capacitive storage of electrical energy is receiving increasing interest because of potential opportunities in large scale energy regeneration and energy storage technologies. Traditionally, electrochemical capacitors are known for their ability to provide high power, however, research on new materials and designs has led to improvements in energy density without compromising power capabilities or cycle life. The field has evolved from electrochemical capacitors based on the energy stored by electrical double layers to pseudocapcitance in which faradaic reactions occur between the electrode and electrolyte. The materials of interest span from high surface area carbons to transition metal oxides to conducting polymers. Electrochemical capacitor designs have also evolved. Hybrid capacitors, based on combining electrical double layer capacitance and faradaic reactions, represent an intriguing new direction for improved energy density. This presentation will review the current state of capacitive energy storage materials and discuss several of the scientific challenges facing the field.

This paper is concerned with electrode materials and electrolytes for high performance electrochemical capacitors. It is based primarily on information/data presented at the DOE Basic Energy Sciences Workshop on Electrical Energy Storage held in April 2007. By high performance is meant electrochemical capacitors having an energy density greater than 20 Wh/kg and a high efficiency, pulsed power density of greater than 2 kW/kg. Most of the devices considered are hybrid devices in which one of the electrodes is carbon (microporous activated carbon or a graphitic carbon) and the other electrode is a metal oxide (ex. manganese oxide or lithium titanate oxide). The electrolytes are an organic solvent plus a salt or an ionic liquid. The cell voltages of the devices are between 3-4 V. Data are presented concerning the characteristics of the electrode materials, particularly their surface area, pore size distribution and specific capacitance. Test data are also presented for small prototype cells using the various materials over a range of charge/discharge current and power. Projections of electrochemical capacitor performance based on calculations for various cell designs are presented as an indication of future material requirements and device development possibilities.

Supercapacitors have been touted as a solution to batteries inherent low power capacity for the past quarter century, but little progress has been made in increasing the material performance. The huge number of supercapacitor active material properties and electrolyte combinations makes a fully exhaustive study nearly impossible with traditional electrode processing techniques. Recent research shows the utility of using the microelectrode technique for electrochemical characterizations of supercapacitor materials[1,2]. Essentially, this technique uses small (microgram) amounts of active material in a reproducible electrically conductive cavity to perform electrochemical analysis. The small amount of material used in this technique leads to a low RC constant which allows fast analysis using high scan rate (>10 V/s) and minimal (<10 s) electrode processing.Microelectrode studies on nanodiamonds and carbon nanotubes that have been thermally treated at high temperature to modify the microstructure and the surface chemistry have shown the effect of altering the carbon surface. The high power performance of nanodimaond annealed at high temperature is in good agreement with previous results obtained on traditional electrodes either in organic electrolyte or in aqueous electrolyte, albeit with the ability to study >10 materials per day by a single person. The screening of more traditional porous carbon materials is also possible with this technique. A large systematic study of Carbide-Derived Carbons using different precursors (TiC, B₄C, ZrC, SiC, Ti₂AlC, Ti₃SiC₂,...) and synthesis temperatures from 500°C to 1000°C have been analyzed using this technique in few days by one person proving its high efficiency. Similar trends to those already demonstrated[3] were obtained showing the reliability of the method. The microelectrode technique is then a powerful tool to analyze carbon materials and quickly select the best candidates for power or energy applications in electrical double layer capacitors. It perhaps more importantly it allows various parameters to be studied simultaneously, such as electrolyte and carbon properties.
References:[1] C. Cachet-Vivier, V. Vivier, C. S. Cha, J. Y. Nedelec, and L. T. Yu, Electrochemistry of powder material studied by means of the cavity microelectrode (CME), Electrochim. Acta, 47, 181-189 (2001).[2] M. Zuleta, M. Bursell, P. Bjornbom, and A. Lundblad, Determination of the effective diffusion coefficient of nanoporous carbon by means of a single particle microelectrode technique, Journal of Electroanalytical Chemistry, 549, 101-108 (2003).[3] J. Chmiola, G. Yushin, R. Dash, and Y. Gogotsi, Effect of pore size and surface area of carbide derived carbons on specific capacitance, J. Power Sources, 158, 765-772 (2006).

An electroactive polymer-based Type IV supercapacitor uses a p-doping polymer at the anode and an n-doping polymer at the cathode, resulting in higher output voltages, higher accessible capacities, and higher power and energy densities than in other types of supercapacitors. Supercapacitor performance is affected by all of the components of the device: polymer(s), electrolyte, conductive substrate, and packaging. This paper will address material considerations for Type IV supercapacitors. We have evaluated many n-doping polymers, including poly(benzimidazobenzophenanthroline) and poly(xylyl viologen), but our best devices are derived from bis(3,4-ethylenedioxythiophene)-functionalized thiadiazoloquinoxalines and benzobisthiadiazoles. Performance of devices utilizing these polymers will be discussed. Electrolyte, substrate, and packaging considerations will also be addressed.

High permittivity polymers are attractive materials for electrical energy storage for a wide range of applications such as capacitor dielectrics. Ferroelectric polymers based on vinylidene fluoride (VDF), trifluoroethylene (TrFE), and chlorotrifluoroethylene (CTFE) exhibit some of the highest permittivities among polymers. Our work has demonstrated, for the first time, the ability to precisely control the polymers’ permittivity from 10 to 50 through a unique synthetic approach. The maximum energy density of a dielectric polymer is controlled by the polymers’ permittivity and electrical breakdown strength with higher values of both producing a higher energy density. With the goals of improving energy density, the influence of molecular weight on the electrical breakdown strength of poly(vinylidene fluoride - chlorotrifluoroethylene) P(VDF-CTFE) is explored. Three different ferroelectric polymers with number average molecular weights (Mn) ranging from 136 to 294 kg/mol of composition 78.4 mol% VDF and 21.6 mol% CTFE were suspension polymerized with polydispersities ranging from 1.10 to 1.72. The use of suspension polymerization showed a five-fold increase in Mn over a previously used solution based method allowing testing of a greater range of molecular weights. The thermal and low electric field dielectric properties of the polymers were nearly identical with a melt temperature of 110C and permittivity of 9.3 at 1 kHz. The polymers also showed similar crystallite sizes and degrees of crystallinity observed from x-ray diffraction. The mechanical properties at both low and high strains revealed significant differences between the polymers with the mechanical properties improving as Mn increases. Additionally, conduction measurements at high electric fields also revealed differences between the polymers.The DC electrical breakdown strength of the three polymers was measured and analyzed using a two-parameter Weibull distribution at -35C, -15C, and 25C. The breakdown strength increased with increasing Mn and decreasing temperature. Two electrical breakdown mechanisms are identified and compared to modeled predictions. The breakdown mechanism transitions from electromechanical at 25C to a combination of electromechanical and thermal at -15C to thermal breakdown at -35C. Polarization measured versus electric field revealed that the polymers behave like linear dielectric materials allowing the energy density to be calculated using a simple relationship. Energy density of the polymers increases an average of 77% from 25C to -35C illustrating the dominant effect that increasing breakdown strength has over the decreasing permittivity in the same temperature range.

Nanostructured materials offer enhanced electrochemical performance because of their ability to access both bulk and surface properties. As electrochemically active materials approach nanoscale dimensions, the charge storage of lithium ions from Faradaic processes occurring at the surface of the material, referred to as the pseudocapacitive effect, becomes increasingly important. In this paper we describe pseudocapacitive effects for two different forms of nanocrystalline TiO2 films; nanostructured films with particle size ≤ 10 nm and mesostructured films with pore diameters in the range of 15 - 25 nm. Monodisperse nanoparticles of 7 nm and 10 nm were prepared without surfactants and with all organic constituents removed. We have used a detailed cyclic voltammetric analysis to quantitatively establish the dependence of pseudocapacitance on nanocrystalline TiO2 particle size. At particle sizes below 10 nm, capacitive contributions became increasingly important, leading to greater amounts of total stored charge (gravimetrically normalized) with decreasing TiO2 particle size. The area normalized capacitance was determined to be well above 100 μF/cm2, confirming that the capacitive contribution was pseudocapacitive in nature. We have also produced mesoporous nanocrystalline TiO2 films by using “KLE” block copolymers as a structure-directing agent. Our voltammetry studies showed that such mesoporous crystalline films offer higher lithium-ion storage capacity and faster kinetics than non-templated films. Once again, we used detailed cyclic voltammetric analysis to separate the capacitive contributions from the total stored charge for mesoporous crystalline films. We find that the mesoporous TiO2 films offered significantly higher insertion capacity and capacitive effects than the non-templated films. The study of nanostructured and mesostructured TiO2 underscores the importance of pseudocapacitive behavior that develops in nanostructured solids. The nanodimensional metal oxides represent a direct connection between Li-ion batteries and supercapacitors, suggesting the ability to design materials that offer a balance between power density and energy density.