True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

Abstract

The development of pseudocapacitive materials for energy‐oriented applications has stimulated considerable interest in recent years due to their high energy‐storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery‐like to the pseudocapacitive‐like behavior. As a result, it becomes challenging to distinguish “pseudocapacitive” and “battery” materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery‐type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid‐state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state‐of‐the‐art progress in the engineering of active materials is summarized, which will guide for the development of real‐pseudocapacitive energy storage systems.

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“…The charge storage electrochemical kinetics of the CoWO 4 and S−CoWO 4 electrodes were further explored by analyzing the current peak response at different scan rates using the power‐law relationship shown in Equation : [24,25] …”

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“…[52][53][54] Figure 4 represents the principally important parameters related to electrode materials such as best possible composition, morphology, material dimension, doping, and chemical functionalization, that considerably augment the overall energy performance of different EES systems, particularly in terms of improved bio-compatibility, charge transport features, enhanced surface area, and mechanical flexibility to survive extreme environmental conditions collectively. [55,56] Further, as far as devising of supercapacitor cells are concerned, employing pseudocapacitive materials along with battery materials gives rise to hybrid configurations, popularly known as "hybrid capacitors" or "supercapatteries" or "supercapatteries". [35,[57][58][59] They are estimated to possess superior properties of both supercapacitors and batteries in a single gadget.…”

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“…Reproduced with permission from Refs. [12] and [13]. Copyright (2020) Wiley-VCH and (2010) Royal Society of Chemistry, respectively.…”

“…By applying the respected values in the above equation specific capacity can be calculated from GCD. [12] Hybrid supercapacitors store charge both by electrostatically and electrochemically combining the benefits of both EDLCs and Pseudocapacitors. In a three-electrode workstation, the instrument consists of a working electrode, a counter electrode, and a reference electrode.…”

“…In the above equation, i and m denote the current density, dt denotes the discharge time. By applying the respected values in the above equation specific capacity can be calculated from GCD [12] …”