Electrocatalytic Oxygen Reduction Reaction

Oxygen (O2) is the most abundant element in the Earth’s crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes such as biological respiration, and in energy converting systems such as fuel cells. ORR in aqueous solutions occurs mainly by two pathways: the direct 4-electron reduction pathway from O2 to H2O, and the 2-electron reduction pathway from O2 to hydrogen peroxide (H2O2). In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O2 to superoxide (O2) can also occur. In proton exchange membrane (PEM) fuel cells, including direct methanol fuel cells (DMFCs), ORR is the reaction occurring at the cathode. Normally, the ORR kinetics is very slow. In order to speed up the ORR kinetics to reach a practical usable level in a fuel cell, a cathode ORR catalyst is needed. At the current stage in technology, platinum (Pt)-based materials are the most practical catalysts. Because these Pt-based catalysts are too expensive for making commercially viable fuel cells, extensive research over the past several decades has focused on developing alternative catalysts, including non-noble metal catalysts [1]. These electrocatalysts include noble metals and alloys, carbon materials, quinone and derivatives, transition metal macrocyclic compounds, transition metal chalcogenides, and transition metal carbides. In this chapter, we focus on the O2 reduction reaction, including the reaction kinetics and mechanisms catalyzed by these various catalysts. To assist readers, we first provide an overview of the following background information: the major electrochemical O2 reduction reaction processes, simple ORR kinetics, and conventional techniques for electrochemical measurements.