Industrial Requirements of Materials for Electrical Double Layer Capacitors: Impact on Current and Future Applications

Abstract

active material. Since the energy storage in EDLCs is only based on the adsorption of ions without any faradaic reactions, they not only have higher power densities compared to other storage devices like batteries or fuel cells but also can easily reach a cycle life of millions of cycles. [1] However, since only the surface of the active material is used for the energy storage mechanism, the energy density of EDLCs is lower than that of batteries (2-8 Wh kg −1 ) limiting the areas in which EDLCs can be used as electrochemical storage device. [1a] Therefore, both scientific and industrial research efforts are dedicated to find ways to increase the energy density of EDLCs. The maximum energy density is calculated by the equation, where C is the capacitance of the EDLC and V is the applied voltage. In general, the capacitance is mainly related to the used active material, whereas the usable voltage is limited by the electrochemical stability of the electrolyte-electrode interface. Thus, the choice of materials is of pivotal importance for maximizing the energy storage properties of EDLCs. [1c] Since energy storage in EDLCs takes place at the electrodeelectrolyte interface, both of these components need to be improved. It is also important to notice that tailoring the electrolyte to the electrode material (or vice versa) can result in beneficial effects: By allowing the ions of the electrolyte to appropriately fit into the pores of the electrolyte, negative effects like ion sieving can be avoided and capacitance can be maximized. [1c,2] The assembly of an EDLC device requires the following components:(1) An electrode, which is comprised of active material, binder, current collector and, in most cases, a conductive agent (2) An electrolyte, which is comprised of solvent and salt, but solvent-free electrolytes are also possible (3) A separator to prevent physical contact and short circuit of the electrodes (4) Casing and mechanical components ensuring a reliable connection for high currents within the capacitor as well as a connection to the respective application.A cross section of a typical EDLC device with all its components is shown in Figure 1. The figure is also giving indications about the current cost of the individual EDLC components.Electrical double layer capacitors (EDLCs) are nowadays considered one of the most important energy storage technologies. In recent years, great efforts have been made toward the development of novel materials, active and inactive, suitable for the realization of advanced EDLCs displaying higher performance, especially in terms of energy, compared to the state-of-the-art devices. Nevertheless, the applicability of these materials in real devices and the industrial requirements related to the development of innovative EDLCs are not always properly addressed by the scientific community. This short review addresses these two fundamental aspects, with the aim to supply an updated set of information about the industrial requirements for the materials usable in commercial EDLCs. ...

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