ABSTRACT: Despite some impressive amount of work dedicated to the discovery of new pseudocapacitive electrodes, there is still no commercially available Electrochemical Capacitors (ECs) using this kind of materials. There are several reasons for this matter of fact. Firstly, most of the proposed pseudocapacitive materials are operated in aqueous electrolytes which require the use of tailored current collectors, separators, casing, etc. Such manufacturing aspects are rarely presented in the literature. Another reason could be the use of expensive elements in the formulation of such pseudocapacitive electrodes which is a pertinent argument when dealing with RuO2 but which is not a good one when MnO2 electrodes are involved. Of course the design of multicationic oxides make things more complicated since specific synthesis routes are required with the use of two or more cations. However, this is also the case for lithium-ion cathodes such as Li(Ni,Mn,Co)O2 which are commonly used in commercially available devices. There has also been some controversy upon the charge storage mechanism of so-called pseudocapacitive electrodes. Last, many electrode materials presented as pseudocapacitive are indeed battery type electrodes which induces some confusion for EC manufacturers, especially with respect to cycle life, cell voltage management, etc.
Moreover, most of the researchers seem to be convinced that only performance matter. Subsequently, metrics are often mistreated especially when micrograms of electrodes are used to extrapolate gravimetric energy and power densities. Following the same trend as for carbon based electrodes, all possible shapes and designs of pseudocapacitive materials have been proposed with an outbidding of fancy pictures: nanospheres, cauliflowers, starfish, etc. However, the true role of scientists relies upon the understanding of phenomena, especially with regards to charge storage mechanism. Only few studies are dedicated to this topic, and unfortunately most of them are very confusing, using battery type electrodes instead of pseudocapacitive ones (again), providing in-situ studies where operando design should be used, and finally not targeting in-depth investigations of such pseudocapacitive electrodes, whatever their use could be. As a result, the main question about the current research studies on pseudocapacitive materials is: are we getting nowhere fast?
Thierry Brousse is a Distinguished Professor of Materials Science at the University of Nantes. He received his PhD degree in 1991 and joined the University of Nantes in 1994 where he got a Full Professor position in 2005. He is a researcher at Institut des Matériaux Jean Rouxel (IMN). His research focuses on materials for electrochemical energy storage with particular emphasis on innovative and/or modified materials for electrochemical capacitors and related devices/microdevices. He is Associate Editor for the Journal of The Electrochemical Society. He has mentored 25 PhD students. With his team as well as international collaborators, he co-authored 9 books' chapters, 170 peer-reviewed journal publications, 9 patents, and 60 invited and keynote lectures at international meetings. He is vice-dean of the University of Nantes in charge of Innovation since 2013.