Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

https://hal.archives-ouvertes.fr/hal-02417326/document

Materials for electrochemical capacitors

Carbon properties and their role in supercapacitors

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

PEM fuel cell electrodes

Mesopore-modified zeolites: preparation, characterization, and applications.

Carbon aerogels are prepared here via pyrolysis of resorcinol-formaldehyde aerogels. Their open porous and electrically conductive structure renders carbon aerogels suitable for the application in supercapacitors. Different types of electrodes can be derived from the sol-gel-precursors of carbon aerogels: Monolithic fibre-reinforced electrodes and polymer-carbon compounds. Both carbon fibre reinforced and polymeric bound aerogel electrodes based on polytetrafluoroethylene (PTFE) have been investigated in this work with respect to their electrical conductivity, surface area and capacitive performance. The capacitance of both electrode types is above 65 F/cm3 in aqueous electrolytes and this meets the demands of supercapacitor electrodes.

Carbon Aerogels for Electrochemical Double Layer Capacitors

We prepared resorcinol–formaldehyde (RF) aerogel films with a high molar ratio of resorcinol/catalyst (R/C) of about 1500 and a low mass ratio of 30%. At the onset of the gelation process, organic fibers or carbon fiber fleece were added to the sol in order to increase the mechanical stability. Upon pyrolysis the RF-aerogel as well as the organic fibers were transformed into porous nanostructured carbon. The thickness of the films was about 500 μm and below. As previously shown, the addition of fibers modifies the gelation and thus the structure of the carbon network as well as the particle and pore sizes in characteristic ways. To investigate this influence, samples with different fiber types and contents were prepared. Scanning electron microscopy was used to determine the particle and pore sizes. The largest pore sizes obtained were in the range of several microns. The highest electrical conductivity reached was about 28 S/cm for a sample with almost 80% porosity. Tests of single cells prepared with carbon aerogel sheets show that films with a rather coarse structure in the interior and a micron thin fine structured top and bottom layer are most suitable as gas diffusion electrodes in polymer electrolyte membrane (PEM) fuel cells. The top and bottom layer promote the formation of low resistance electrical contacts between the gas diffusion electrodes and the polymeric membrane as well as the current collector.

M. Wiener

Papers

Carbon Aerogels for Electrochemical Double Layer Capacitors

Integration of carbon aerogels in PEM fuel cells