Selective Electrocatalysis of a Water-Soluble Rhenium(I) Complex for CO2 Reduction Using Water As an Electron Donor

Reduction of CO2 using water as an electron donor could be deemed one of the most important reactions in artificial photosynthesis. In the current study, electrochemical properties of a water-soluble rhenium(I) tricarbonyl complex with hydroxymethyl groups [(Re(CH2OH)-OH2](+)) and its electrocatalysis for CO2 reduction were investigated in aqueous solutions. In comparison with general rhenium(I) carbonyl complexes, [Re(CH2OH)-OH2](+) shows high solubility in water even in the presence of both CO2 and OH-. In an aqueous solution, [Re(CH2OH)-OH2](+) exhibited a catalytic current due to CO2 reduction in a cyclic voltammogram at a positive potential of about 400 mV more than that in a DMF solution. It is suggested that two molecules of a one electron-reduced Re complex participated in reducing one molecule of CO2 in the aqueous solution. Electrolysis using [Re(CH2OH)-OH2](+) at -1.1 V vs NHE in a CO2-saturated aqueous solution afforded CO and HCOOH with 95 and 4% selectivity, respectively, at pH 6.9; thermodynamically favorable H-2 evolution via water reduction was almost completely suppressed. Even at pH 4.2, the selectivity of CO2 reduction was still high (CO 84%; HCOOH 3%). A full cell consisting of a cathode unit with [Re(CH2OH)-OH2]* for CO2 reduction and an IrO2/FTO anode for water oxidation electrocatalytically produced both CO and O-2 as the main products with high efficiency and selectivity.