Effect mechanism of H2O on the electrodes process: aluminum electrochemical dissolution to deposit aluminum foil collector in EmimCl-AlCl3 ionic liquid

Conquering the anodic passivation effects is essential to achieve the validity of the electrorefining process in room temperature ionic liquids. Herein, we investigated the effect mechanism of H 2 O on the removal of AlCl 3 (s) passivation layer on the surface of aluminum anode and the deposition of aluminum foil on the cathode in EmimCl-AlCl 3 ionic liquid. The results indicate that the introduced H 2 O derives an intermediate species AlCl 3 OH − , which is the key to de-passivation. The de-passivation effect of AlCl 3 OH − is divided into three stages with the anodic potential increase: under the low anodic potential of 0.8 V, it derives the black passivation film AlCl 2 OH(s), and consumes the AlCl 3 (s) passivation layer derived by Al 2 Cl 7 −  → AlCl 4 −  + AlCl 3 (s) at the anodic potential of 1.6 V ~ 2.2 V. Moreover, the derived AlCl 2 OH(s) is simultaneously also consumed up by the reaction AlCl 2 OH(s) + AlCl 4 −  → Al 2 Cl 6 OH − . At the higher anodic potential of 2.2 V, it consumes up the derived AlCl 3 passivation layer, which ensures the continuous dissolution of the aluminum anode, resulting in the high anode dissolution current efficiency of about 100 %. On the other hand, the aluminum foil is electrodeposited by the electrochemical reduction of 4Al 2 Cl 7 − and Al 2 Cl 6 OH − species. However, the introduction of H 2 O reduces the current efficiency of the aluminum foil electrodeposition from 97 % to 90 % as the anodic potential increases from 1.6 V to 3.0 V. In addition, the lithium cobaltate half-cell with the electrodeposited aluminum foil collector shows a superior capacity retention rate of 98.33 % for 150 cycles at 0.5C current density.