How to measure and report the capacity of electrochemical double layers, supercapacitors, and their electrode materials
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ORIGINAL PAPER
How to measure and report the capacity of electrochemical double layers, supercapacitors, and their electrode materials Yuru Ge 1 & Xuan Xie 2 & Jessica Roscher 1 & Rudolf Holze 1,3,4
&
Qunting Qu 5
Received: 25 May 2020 / Revised: 11 August 2020 / Accepted: 12 August 2020 # The Author(s) 2020
Abstract Relevant fundamentals of the electrochemical double layer and supercapacitors utilizing the interfacial capacitance as well as superficial redox processes at the electrode/solution interface are briefly reviewed. Experimental methods for the determination of the capacity of electrochemical double layers, of charge storage electrode materials for supercapacitors, and of supercapacitors are discussed and compared. Intrinsic limitations and pitfalls are indicated; popular errors, misconceptions, and mistakes are evaluated. The suitability of available methods is discussed, and practical recommendations are provided. Keywords Supercapacitor . Differential double layer capacity . Integral double layer capacity
Introduction Capacitors as a means of storing electric energy without any transformation are standard for decades; supercapacitors as devices showing capacitances and thus storage capabilities bigger by orders of magnitude are a recent addition in electrical engineering. In the search for better materials and their combinations, standards of reporting have been less than perfect; demands for more uniform reporting are almost as old as the research in this field [1]. Whenever an ionically conducting phase (e.g., an electrolyte solution) and an electronically conducting phase (e.g., a metal or graphite) are brought into contact, an electrochemical
double layer is established. Its structure, its properties, and its behavior have fascinated electrochemists, surface scientists, biologists, tribologists, and scientists from other fields for many decades. Its capacitor-like behavior known since the work of Lippmann [2, 3], Perrin [4], Stern [5], Gouy [6–8], Chapman [9, 10], Helmholtz [11], and others has been frequently treated by experimentalists more like a nuisance causing a non-Faradaic charging current. Only those interested in fundamentals of electrochemistry have studied the structure and dynamics of the electrochemical interface thoroughly and without trying to get around its effects as in, e.g., polarography. For an early overview highlighting the work by Grahame [12], see [13], for more recent ideas [14].
Dedicated to Fritz Scholz on the occasion of his 65th birthday in recognition of his numerous contributions to fundamental and applied electrochemistry and to scientific communication. And foremost to a friend. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-020-04804-x) contains supplementary material, which is available to authorized users. * Rudolf Holze [email protected] 1
Institut für Chemie, AG Elektrochemie, Technische Universität Chemnitz, D-09107 Chemnitz, Germany
2
Key Laboratory for Anisotropy and Texture o
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