Fundamentals of Electrochemical Pseudocapacitors

As discussed in the previous chapter, the double-layer capacitance developed in electrochemical supercapacitors (ESs) is mainly due to net electrostatic charge accumulation and separation at the electrode-electrolyte interface. The net negative (or negative) charges such as electrons are accumulated near the electrode surface. At the same time, an equal number of positive charges such as cations are accumulated near the electrode surface at the electrolyte side, forming electric double-layers such as the Helmholtz and diffuse layers [1]. 

To increase the capacitance of ESs, some electrochemically active materials are explored for electrode use to provide much higher pseudocapacitance than double-layer capacitance. Pseudocapacitive charge storage fundamentally differs from the electrostatic mechanism that governs double-layer capacitance. For pseudocapacitance, a faradic charge transfer in the electrode porous layer occurs through a thermodynamically and kinetically favored electrochemical reduction-oxidation (redox) reaction [1]. 

Because this redox reaction is strongly dependent on the electrode potential, the change in charge quantity arising from this reaction (dq) has a relationship with the change in electrode potential (dV). The dependency of dq on dV (dq/dV) is called the pseudocapacitance created by the redox reaction. 

For electrochemical redox, each reactant molecule in the bulk phase contributes one or more charges toward the stored energy, unlike the case in a double-layer charging or discharging process where only charges can physically accumulate on the material particle s surface. Therefore, pseudocapacitance is much higher for a double-layer mechanism than the possible capacitance. 

The electrochemical reversibility of the employed redox material in a pseudocapacitor normally means that the redox process follows Nerstian behavior [2]. These redox materials include (1) electrochemically active materials that can be adsorbed strongly on an electrically conductive substrate surface such as a carbon particle and (2) solid-state redox materials that can combine with or intercalate into an electrode substrate to form a hybrid electrode layer. For example, adsorption on an electrode substrate surface is commonly observed as underpotential deposition of protons on the surface of a crystalline metal electrode (Ft, Rh, Pd, Ir, or Ru). In the case of Ru, the protons can pass through the surface into the metal lattice by an absorption process, similar to the transitional behavior seen in lithium battery intercalation electrodes. 

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Pseudocapacitors store charge based on reversible (faradaic) charge transfer reactions with ions in the electrolyte. For example, in a metal oxide (such as RUO2 or I1O2) electrode, charge storage results from a sequence of redox reactions. Electrochemical capacitors (ECs) based on such pseudocapacitive materials will have both faradaic and nonfaradaic contributions. The optimization of both EDLCs and pseudocapacitors depends on understanding how features at the nanoscale (e.g. pore size distribution, crystaUite or particle size) affect ion and electron transport and the fundamental properties of electrochemical interfaces. 

This chapter describes the fundamentals of different kinds of electrochemical capacitors (ESs), including electrical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid ESs. This serves as a foundation for understanding ES science... 

At the same time, a fundamental understanding of supercapacitor design, operation, performance, and component optimization led to improvements of supercapacitor performance, particularly increasing their energy density. To further increase energy density, more advanced supercapacitors called pseudocapacitors, in which the electroactive materials are composited with carbon particles to form composite electrode materials, were developed. The electrochemical reaction of the electroactive material in a pseudocapacitor takes place at the interface between the electrode and electrolyte via adsorption, intercalation, or reduction-oxidation (redox) mechanisms. In this way, the capacitance of the electrode and the energy density can be increased significantly. 

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