collector. SEM images of ErCl3 electrode before electrochemical
measurements show that ErCl3 salts were uniformly mixed with
carbon and PVDF matrix (Fig. 5c and d). The loosely aggregated carbon
spherical structures disappeared. Instead, ErCl3 salts were
filled in interspace between carbon particles and adsorbed at the
surface of carbon spheres. After electrochemical measurements,
the size of spherical structures increases due to the adsorbed
ErOOH colloids (Fig. 5a). During the phase transformation from
ErCl3 to ErOOH, the volume change was occurred. Therefore, some
porous can be found at the surface of electrode (Fig. 5b). Thus, carbon
spheres with ErOOH colloids at their surfaces were overlapped
and formed link-like structures. The specific electrode configuration
can shorten the electron transfer length and ion diffusion
pathway, which can significant enhance the electrochemical performance
of ErCl3 electrodes.
Scheme 2 shows schematic drawing of the reaction process and
electrode configuration of ErCl3 pseudocapacitor. When the as-prepared
ErCl3 electrode was dipped into KOH electrolyte, ErCl3 can be
firstly reacted with KOH to form Er(OH)3 within the electrode due
to the large Ksp value (1.3 1023).
ErðOHÞ3 $ Er3þ þ 3OH Ksp ¼ ½Er3þ½OH3 ¼ 1:3 1023 ð7Þ
ErðOHÞ3 $ ErOOH þ H2O ð8Þ
Then, Er(OH)3 were transformed to ErOOH by dehydrated reactions
occurred in the strong alkaline condition (Scheme 2a). At the same
time, the chemical coprecipitation underwent electrochemical reactions.
Two-layer ErCl3/ErOOH structures were further formed by
electric-field assisted chemical coprecipitation (Scheme 2b). The
in situ formed ErOOH colloids showed highly electrochemical activity
in alkaline electrolyte. Finally, ErOOH colloids were grown
within the carbon black/PVDF matrix (Scheme 2c and d). It is most
intriguing that the formation of electroactive ErOOH colloids and
the production of pseudocapacitance (Faradaic reaction) occurred
at the same time and same electrode. The small size of ErOOH