期刊
ACS APPLIED MATERIALS & INTERFACES
卷 9, 期 10, 页码 8649-8658出版社
AMER CHEMICAL SOC
DOI: 10.1021/acsami.6b14100
关键词
electrochemical capacitors; pseudocapacitance; double-layer charging faradaic reactions; cyclic voltammetry
资金
- Agence Nationale de la Recherche [14-CE05-0014-01]
Batteries and electrochemical double layer charging capacitors are two classical means of storing electrical energy. These two types of charge storage can be unambiguously distinguished from one another by the shape and scan-rate dependence of their cyclic voltammetric (CV) current potential responses. The former shows peak-shaped current potential responses, proportional to the scan rate v or to v(1/2), whereas the latter displays a quasi-rectangular response proportional to the scan rate. On the contrary, the notion of pseudocapacitance, popularized in the 1980s and 1990s for metal oxide systems, has been used to describe a charge storage process that is faradaic in nature yet displays capacitive CV signatures. It has been speculated that a quasi-rectangular CV response resembling that of a truly capacitive response arises from a series of faradaic redox couples with a distribution of potentials, yet this idea has never been justified theoretically. We address this problem by first showing theoretically that this distribution-of-potentials approach is closely equivalent to the more physically meaningful consideration of concentration dependent activity coefficients resulting from interactions between reactants. The result of the ensuing analysis is that, in either case, the CV responses never yield a quasi-rectangular response alpha V, identical to that of double layer charging. Instead, broadened peak-shaped responses are obtained. It follows that whenever a quasi-rectangular CV response proportional to scan rate is observed, such reputed pseudocapacitive behaviors should in fact be ascribed to truly capacitive double layer charging. We compare these results qualitatively with pseudocapacitor reports taken from the literature, including the classic RuO2 and MnO2, examples, and we present a quantitative analysis with phosphate cobalt oxide films. Our conclusions do not invalidate the numerous experimental studies carried out under the pseudocapacitance banner but rather provide a correct framework for their interpretation, allowing the dissection and optimization of charging rates on sound bases.
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