Scientific hints of developing supercapacitors
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FEATURE ARTICLE
Scientific hints of developing supercapacitors Koichi Jeremiah Aoki 1 & Jingyuan Chen 2 Received: 29 May 2020 / Revised: 29 May 2020 / Accepted: 1 June 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Since the research direction of supercapacitors (SC) has been deviated from the principle of causing capacitance, the revisit may be a future task in exploiting electric power storage. SC have been developed by applying ionic properties in order to enhance heterogeneity of ionic distributions. However, SC belong to double-layer capacitances (DLC), which are caused by orientation of solvent dipoles without any direct relation to ionic properties. DLC are always complicated with ion-independent characteristics such as frequency dispersion, voltage dependence, and participation in redox reactions, whereas these complications have not yet been considered in development of SC. Especially, pseudo capacitances should be examined through isolating the contribution of DLC from the redox properties.
Introduction
Participation in ionic distribution
Future tasks are generally directed either to development of materials which have been exploited in other fields or to challenge along social requirements. If a present task is at a deadlock or runs short of scientific backgrounds, it should be regarded as a future task by incorporating basic concepts. SC belong to this category, in that they have been thought to be caused by heterogeneity of ionic distributions. Since they are based on DLC in principle, they should obey DLC, reflecting electric properties and thermodynamically averaged properties of solvents. Some complicated features of DLC should be also included in SC. A critical task is a search for the contribution of Faradaic reactions to DLC, which is known as pseudo capacitances [1]. It is important to examine the dependence of DLC on Faradaic reaction, which has been classically regarded as independent. However, a question should be cast not only to the relation between the two but also to conditions of enhancement of pseudo capacitances from the thermodynamic viewpoints.
An electrochemical monograph describes SC as a member of DLC of which charge is stored by heterogeneous distribution of ions, discriminated from conventional capacitors composed of dielectrics. However, meticulously experimental results in the nineteenth century concluded that DLC electrochemically observed come from the inner layer rather than a diffuse layer with ionic distribution [2]. The former structure is still unknown in detail, whereas the latter can be estimated from Gouy-Chapman’s theory. As a result, a source of DLC should be still ambiguous. There may be some technical reasons for paying attention to a contribution of ions. Enhancement of performance of SC needs to increase not only conductance of solution but also the affinity between carbon powder electrodes and solution. Consequently, utilized conditions of salts become inevitably targets of research, so that they may seem to control the perfor
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