Thermochemical conversion of an MCl2/M (M = Co, Ni, Cu) couple as an index of electrochemical conversion voltage

The chemical-state changes in the reduction process of metal chlorides (CoCl 2 , NiCl 2 , and CuCl 2 ) supported on SiO 2 were clarified using in situ XAFS measurements. Hydrates were supported in the dried samples prepared by the incipient wetness impregnation, and all metal chlorides changed to anhydrides by heating to 200 °C under an H 2 gas flow diluted by He. Subsequently, CoCl 2 and NiCl 2 were reduced to metallic Co and metallic Ni, respectively, while CuCl 2 was reduced to metallic Cu via CuCl. The temperature at which chloride ions dissociate from anhydrous metal chloride particles was directly determined. In addition, the conversion temperature was determined by in situ XAFS analysis of the process in which metal particles on SiO 2 are oxidized (chlorinated) by the Cl 2 gas diluted by N 2 at elevated temperature. Finally, the thermochemical redox temperature between metal oxides and metals was found to be correlated to their electrochemical conversion voltages. Based on this correlation, the electrochemical conversion voltage was predicted from the thermochemical conversion temperature of the MCl 2 /M couple determined in this study. • The conversion between MCl 2 and M on SiO 2 was analyzed by in situ XAFS. • The reduction and chlorination temperatures between MCl 2 and M were determined. • The thermochemical temperature was correlated to the electrochemical voltage. • The electrochemical conversion voltage of MCl 2 /M couples was predicted.