Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells

THE non-biological reduction of carbon dioxide to organic compounds is of interest, as an alternative to natural photosynthesis, for the production of organic raw materials or fuel. In one approach, the required energy was supplied by irradiation with UV light, in the presence of ferrous salts, and resulted in the production of formic acid and of formaldehyde1. In another approach, the energy was supplied from an external power source by electrochemical reduction of aqueous carbon dioxide. The reduction of carbon dioxide and production of formic acid during the electrolysis of sodium bicarbonate in aqueous solutions has also been reported2, and a study of the reduction of carbon dioxide on a mercury cathode reviews earlier work3. Polarographic measurements on mercury electrodes showed that carbon dioxide, rather than the bicarbonate ion, is the electroactive species, with a half-wave reduction potential of −2.1 V (relative to SCE), and that formic acid is the only product4. We report here the photoassisted electrolytic reduction of aqueous carbon dioxide, achieved using p-type gallium phosphide as a photocathode, with part or all of the energy being supplied by light. The products were formic acid, formaldehyde and methanol.