Acid Electrolytes for Pseudocapacitors

It is worth noticing that the electrolyte nature plays a significant role in the pseu-docapacitive properties of carbon-based materials becanse the snrface functionalities can exhibit different behavior depending on the electrolytes. For example, quinone-type functionalities generate pseudocapacitive effects in the presence of acidic aqueous electrolytes such as H2SO4 since they reqnire protons to proceed as shown in Reaction 2.1, while this effect is hardly observed when alkaline electrolytes are used [76,77]. 

FIGURE 2.4 Cyclic voltammograms of unmodified AC (—) and AC modified with 11 wt.% 

Power Sources, 196, Pognon, G. et al., Performance and stability of electrochemical capacitor based on anthraqninone modified activated carbon, 4117-4122, Copyright 2011, with permission from Elsevier.) 

Apart from the pseudocapacitive contribution from specific surface functionalities, the possible intercalation of electrolyte ions into certain electrode materials may also result in the increased capacitance [86-88]. For example, Soneda et al. [87] reported a large capacitance of about 555 F g for exfoliated carbon fibers in 18 M H2SO4 electrolyte, which was due to the H2SO4 molecules intercalation into the carbon layers of exfoliated carbon fibers. 

The pseudocapacitance can also be provided by other pseudocapacitive materials such as some metal oxides and electrically conductive polymers (ECPs) that have much higher theoretical capacitance than carbon-based materials. These materials have been reviewed in detail elsewhere [89,90]. Although many materials have been reported to exhibit pseudocapacitive behavior, they are very sensitive to the type and pH of the electrolytes and few of them are suitable for application in strong acid electrolytes. As previously mentioned in Section 1.3.2, RUO2 is one of the most extensively studied pseudocapacitive materials in H2SO4 electrolytes. 

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