Electrochemical CO₂ Reduction on Metal Electrodes

Carbon dioxide is a potential carbon resource abundant on earth. It is also a green house gas with a rapidly increasing atmospheric concentration during the last two centuries. Chemical fixation of CO₂ is an attractive technique for utilization of carbon resources, as well as for the reduction of the atmospheric concentration of CO₂. Nevertheless, CO₂ is the stablest among carbon based substances under the environmental conditions. It has not been incorporated as a major industrial material.

Carbon dioxide can be electrochemically reduced to useful products under mild conditions. However, the energy conversion efficiency, defined as the ratio of the free energy of the products obtained in electrochemical CO₂ reduction and that consumed in the reduction, would be roughly 30 to 40%. Such a low efficiency may discourage practical application of CO₂ reduction in the very near future. However, the significance of the CO₂ reduc- tion will be reevaluated in various aspects of science in future, since many interesting facts in basic electrochemistry have been revealed by recent studies. At the moment, electrochemical reduction of CO₂ is an interesting as well as an important topic in chemistry.

Fundamental studies of the electrochemical reduction of CO₂ started as early as the 19th century, and have been further promoted by many workers in recent years. Numerous reports have been published in the last two decades. At the same time, confusions and contradictions are found in this field.

This article attempts to provide a review of CO₂ reduction at metal electrodes. Several review papers have already appeared, which contain the history and classical works of CO₂ reduction. Thus this paper will not repeat the historical aspects of CO₂ reduction. This chapter will review recent progress, putting emphasis on basic problems and particularly on electrocatalytic aspects.