Molecular electrocatalysts for the oxygen reduction reaction

The past decade has seen considerable growth in the development of materials for fuel cell electrodes, and there is a desire for active electrocatalysts derived from base metals instead of noble metals. Fuels cells that consume H2 and O2 require catalysts to cleave these reactants, with the oxygen reduction reaction (ORR) — either 4H+/4e− reduction to 2H2O or 2H+/2e− reduction to H2O2 — being particularly challenging. The ORR is efficiently performed by certain metalloenzymes, and understanding the links between their structure and function aids the design of molecular ORR electrocatalysts. These bio-inspired catalysts exhibit good activity relative to previous synthetic systems and, furthermore, have provided mechanistic insights relevant to synthetic and enzymatic catalysts. This Review covers recent developments in homogeneous and heterogeneous molecular ORR catalysis, placing emphasis on reaction mechanisms and the factors governing rates and selectivities. Electrocatalysts for the oxygen reduction reaction are important components of energy technologies such as fuel cells. The study of molecular catalysts affords mechanistic insights that further the development of robust, active and energy-efficient systems. This Review describes state-of-the-art metal complexes that operate either in solution or immobilized on an electrode.