The reduction of carbon dioxide at illuminated p-type semiconductor electrodes in nonaqueous media

Abstract

The photo-electrochemical reduction of CO 2 has been examined on a number of p-type semiconductors. The (100) plane of CdTe gave the best results: CO was the main product. The most used solution was DMF with 5% water. The onset potential was ca. −1.00 V vs sce. Reaction order with respect to CO 2 was one. The faradaic efficiency for CO formation was > 80% in DMF solutions containing water up to 25% and currents were stble for 24 h. Tetraalkylammonium salts were suitable electrolytes. DMSO and PC were also useful solvents. Increase of temperature shifted the onset potential to more negative potentials. In and Pb coatings reduced and Pt, Ru and Ni increased co-reduction of H 2O to H 2. Limiting photocurrents were proportional to light intensity. Taking into account the CO dissolved in the solution increases the current efficiency to 90%. CO 2 + e → CO − 2, followed by CO − 2 + CO 2 + e → CO + CO 2− 3 and CO − 2 + 2H + + e → CO + H 2O, is a likely mechanism. The Tafel slope is consistent with a rate-determining discharge. The effect of the tetraalkyl-ammonium salts as electrolyte may be connected with specific adsorption of the cation and its surmised positive influence on CO 2 reduction. The photo-aided electrolysis saves electrical energy corresponding to a potential shift of ca. 0.7 V (than at In) and ca. 1.0 V (than at Pb or Hg) to less negative potentials.