(Invited) Efficient Electrocatalysts for Alcohol Oxidation and Oxygen Reduction in Alkaline Solutions

Abstract

Alkaline fuel cells have some advantages as compared with conventional acid fuel cells. Our research group has investigated the possibility of alkaline fuel cells using anion-exchange membrane as an electrolyte. In the presentation, I would like to introduce and discuss about some topics relating to alkaline fuel cells as follows: 1) Direct oxidation fuels: alcohols We have been focusing on ethylene glycol as a direct fuel in anion-exchange membrane fuel cells (AEMFCs). Ethylene glycol has superior energy density (7.56 kWh dm−3) and higher boiling point (471 K) than some typical alcohol fuels such as methanol and ethanol. The oxidation of ethylene glycol in alkaline medium is faster than that in acid medium. And, surprisingly, ethylene glycol provides larger oxidation currents than methanol and polyols such as glycerol, erythritol, and xylitol in alkaline solutions. Thus, ethylene glycol is a promising fuel for AEMFCs, which overcomes a conventional direct methanol fuel cell (DMFC). Ethylene glycol has the above-mentioned advantageous features as DAFCs’ fuel, but it has a crucial problem derived from its inherent molecular structure. Ethylene glycol is a C2 molecule having a C−C bond in its structure. C−C bond is a stumbling block for electroorganic chemists since thus C−C bond is comparatively strong and cannot be easily cleaved. As well as ethanol oxidation, ordinal Pt catalyst cannot achieve the complete oxidation of ethylene glycol, and leaves some intermediate products. Therefore, in order to increase the efficiency of fuel utilization, active catalysts, having sufficient ability to break C−C bond in ethylene glycol, are strongly required. To our best knowledge, there is no available literature concerning the oxidation of ethylene glycol to CO2from an electrocatalytical view point. Here we introduce an effective C−C bond cleavage electrocatalyst for ethylene glycol. 2) Oxygen reduction catalysts: non-precious metal catalysts Electrochemically reduction of oxygen is an essential reaction involving in fuel cells. Among the various kinds of electrocatalysts, Pt, Pt-alloy, Ag are known to be fairly active for oxygen reduction in alkaline solutions. Precious metals, however, have some serious problems to be overcome before worldwide spread of fuel cells, such as economical cost, reproducibility and limited resources. These problems motivated many researchers to study for alternative catalysts besides a standard catalyst of Pt/C. Among proposed alternative catalysts, perovskite-type oxide is one of the promising candidates as a non-precious metal catalyst. Since Meadowcroft pointed out the catalytic activity of lanthanum-cobalt oxide for oxygen reduction, perovskite-type oxides become attractive materials as electrocatalysts for many electrochemists. However, the detailed mechanism for oxygen reduction on perovskite-type oxide electrodes has not been fully clear yet. We investigated the oxygen reduction on perovskite-type oxides using thin film electrodes and single crystal electrodes, and shed lights on the catalytic roles of oxides and carbon additives.