capacitances [8]. It has been demonstrated that the pseudocapacitance
of transition metal oxides or hydroxides is much higher than
that of carbon-based materials behaving electric double-layer
capacitance [8]. To extract the maximum pseudocapacitance functionality
of metal oxides/hydroxides, one must carefully consider
how they are synthesized and subsequently integrated into practical
electrode structures [9]. Expressing the metal oxide/hydroxides
in a nanoscale form often enhances electrochemical utilization
(maximizing specific capacitance) and facilitates high-rate operation
for both charge and discharge [9,10]. For example, the hybrid
electrode with MnO2 nanocrystal deposited on nanoporous gold
structures displayed a specific capacitance of 1145 F/g that is
close to its theoretical value (1370 F/g) due to the enhanced conductivity
[11]. However, most synthetic pseudocapacitive materials
often show low specific capacitances compared with their
theoretical value and display much low material utilization ratio.
In order to increase the specific capacitance of pseudocapacitor,
redox-based metal cations of pseudocapacitive electrode materials
must be fully utilized in Faradaic reaction. It is because that the
pseudocapacitive behavior is associated primarily with the redox
reactions of the cations or changes in oxidation states of the cations
in electrode materials during operation [12]. Recently, we
have explored the pseudocapacitance of commercial water-soluble
inorganic salts with multiple valence state metal ions in aqueous
alkaline electrolyte [13–16]. It is interesting that the commercial