Mechanistic insight of in situ electrochemical reduction of solid PbO to lead in ChCl-EG deep eutectic solvent

The in situ electroreduction mechanism of solid PbO in choline chloride-ethylene glycol deep eutectic solvent (ChCl-EG DES) is preliminarily clarified through systematical investigations at cell voltage 2.5 V and 353 K. Cyclic voltammetry of the cavity microelectrode loaded with PbO powders exhibits that solid PbO can be direct electrochemically reduced to metallic lead and increasing scan numbers is beneficial to the deoxidation process. Potentiostatic electrolysis with an assumed cathode of PbO pellet and a graphite anode shows that generated numerous gases are mainly composed of O-2 with small amounts of Cl-2 at anode whereas H-2 is released and preferentially adsorbed on the surface of reduced metallic lead at cathode. Parts of Cl-2 can remain in the electrolyte and react with it to form oxidizing Cl-3 ions. Besides, the dissolution-electrodeposition of PbO(s) -> [PbO center dot Cl center dot EG] -> Pb(s) exists simultaneously with the direct deoxidation of PbO(s) -> Pb(s) during the electrolysis process. ESI-MS analysis implies that complex anions [PbO center dot Cl center dot EG] and [ChCl center dot O center dot EG](2) are formed in ChCl-EG-PbO solution before and after the deoxidation process, respectively. Furthermore, a reduction mechanism model of PbO bulk to metallic lead in ChCl-EG DES is proposed. The in situ electrolysis process takes place initially at the current collector/PbO/electrolyte 3PIs and then new formed Pb/PbO/electrolyte 3PIs expands until the whole bulk are metallized. These findings provide a basis for scientifically in-depth discussion on the implication and optimization of the electrochemical production of metals and alloys from metal oxides. (C) 2015 Elsevier Ltd. All rights reserved.