Electrocatalysts for Hydrogen Peroxide Reduction Used in Fuel Cells

Fuel cells technology has attracted much attention owing to their high conversion efficiency in chemical energy to electrical energy with simple structures, clean emissions, insignificant scale effect, etc. Various types of fuel cells were constructed by employing metal, metal oxide and metal complexes as cathodes, where electrochemical H₂O₂ reduction proceeds. Molecular oxygen (O₂) often used as an oxidant for contracting fuel cells is abundant and free of charge, however, oxygen reduction involves four electrons and four protons to form two water molecules that is hard reaction from the kinetic point of view. H₂O₂ produced by two-electron reduction of O₂ is more easily reduced to water by further two-electron reduction (H₂O₂ + 2H⁺ + 2e^– = 2H₂O; E ^o = 1.78 V vs NHE) than O₂ by four-electron reduction (O₂ + 4H⁺ + 4e^– = 2H₂O; E ^o = 1.23 V vs NHE) from both kinetic and thermodynamic points of views. Not only active but also selective cathodes for two-electron reduction of H₂O₂ should be developed to achieve high power fuel cells using H₂O₂ as an oxidant. Herein, suitable catalysts, which are made of metal, metal oxide and metal complexes, for two-electron reduction of H₂O₂ are reviewed.