Reduction of  CO 2 on n ‐ GaAs Electrodes and Selective Methanol Synthesis

Faradaic efficiencies and current voltage characteristics for the reduction of COs to CH3OH on n-GaAs (111 As) single-crystal e ectrodes have been determined. The yield of CH3OH is- 100 % for current densities of- 140 t~A]cm 2 at-1.2 to-1.4V vs. SCE. The excess current due to COs was found to have a threshold at pH- 3.6 and a maximum at pH 4.3. The current also exhibited a rotation-independent j/v characteristic that is interpreted as a rate-determining surface chemical combination step involving adsorbed H atoms and a carbon containing intermediate. The pH dependence of the saturation current is modeled with a kinetic scheme involving the parallel reduction of H § and CO2. Over the past 40 years, there have been a number of studies dealing with COs reduction at metal electrodes (1, 2). Recently, we have been investigating the reduction of CO2 at semiconductor electrodes. One goal of the semi-conductor approach is to utilize the electrochemical prop-erties of p-type photocathodes to convert sunlight and COs into useful fuels. It is known that CO2 can be reduced to methanol and small amounts of methane on certain semiconductor electrodes in aqueous olutions (3-6). Our