performed in 2 M KOH electrolyte. The galvanostatic charge–discharge
curves at different current densities of 3–30 A/g are shown
in Fig. 1a. According to the discharge curves under galvanostatic
conditions, the high specific capacitance of 1811 F/g at the current
density of 3 A/g and the potential window of 0.55 V was obtained
based on the weight of Er3+ ions. The non-linear charge–discharge
curves indicate the pseudocapacitive mechanism (Fig. 1a) [23]. The
present results proved that our designed inorganic salt-alkaline
electrolyte system can display highly electrochemical activity
toward pseudocapacitive reaction.
Traditionally, the specific capacitance of an electrode material is
calculated based on the weight of active materials. According to
the physical origin of electrochemical energy storage, 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]. Therefore, specific capacitance
calculated from Er3+ cations can more deeply reflect their