metal salt electrode displayed highly electrochemical activity in
KOH electrolyte and showed ultrahigh specific capacitance.
Water-soluble inorganic salts including many free active cations
are promising candidates to study the pseudocapacitance of
cations. Another significant advantage of water-soluble salts
pseudocapacitors is that the use of commercial inorganic salt as
pseudocapacitor does not need complex synthesis procedures
[17–19]. During the study of the inorganic salt pseudocapacitors,
the in situ crystallized colloids during the electrochemical reaction
can deliver high specific capacitance. However, the detailed
mechanism of high capacitance was not fully understood.
Erbium compound materials, such as ErOOH and Er2O3, are recognized
as one of the most important rare earth oxides for sensing
applications in the lasers and optical amplifiers, due to intra-Er3+ 4f
shell transition from its first excited state (4I3/2) to the ground state
(4I5/2) [20]. Recently, flaky rare earth oxide including Er2O3 was
used as additives on nickel electrodes and their effects on high
temperature performance of nickel electrodes were investigated
[21]. The result proved that flaky rare earth oxides increased the
oxygen evolution potential and improved the reversibility of nickel
electrodes. To the best of our knowledge, Er-based super capacitors
have not been reported. In one case, Er(OH)3 was used to increase
structural and electrochemical stability of Co–Al layered double
hydroxide pseudocapacitor [22]. Less soluble of Er(OH)3 can adhere
to the Co–Al layered double hydroxide more strongly, stop the
directly contact of OH, and thus inhibit leaking process of Al3+.
Herein, we designed an in situ crystallization route for ErCl3 salt
pseudocapacitors in KOH aqueous electrolyte. In our designed system,
ErOOH colloids were in situ crystallized through electric-field
assisted chemical coprecipitation. These electro active ErOOH colloids
absorbed by carbon black and PVDF matrix were highly
redox-reactive and delivered high specific capacitance values of
1811 F/g exceeding the one-electron redox theoretical capacitance
(Er3+MEr2+). This work demonstrated that ErCl3 alkaline aqueous
pseudocapacitors can produce one-electron (Er3+MEr2+), two electron
(Er2+MEr) or three-electron (Er3+MEr) reactions.