5. (a–d) SEM images of ErCl3 electrodes after and before electrochemical measurements. (a and b) high-magnification and low-magnification SEM images after
electrochemical measurements. (c and d) SEM images of ErCl3 electrode before electrochemical measurements. (e and f) SEM images of carbon black used as conductive agent
in electrode preparation. After electrochemical reaction, carbon matrix-constrained ErOOH colloids electrode configuration can be confirmed.