Key factors dictating the energetics of fundamental electrocatalytic reactions, such as the oxygen reduction reaction that takes place in most fuel cells, have gone unnoticed, according to a theoretical investigation by Feng Tian and Alfred B. Anderson of Case Western Reserve University (J. Phys. Chem. C, DOI: 10.1021/jp1100126). The study suggests new criteria for selecting better-performing catalysts for fuel cells and other electrochemical systems. A central problem limiting fuel-cell performance and hence widespread commercialization is that O2 reduction, the reaction that converts O2 to water, is plagued by a high “overpotential.” That term refers to the additional energy relative to the thermodynamically predicted value that’s needed in practice to drive an electrode reaction. Conventional wisdom attributes this energy loss, generally as wasted heat, to molecules blocking O2’s access to catalytic sites and to slow electron-transfer kinetics. Tian and Anderson propose a previously unrecognized cause for the overpotentials—free-energy loss in non-electron-transfer steps such as O2 dissociation. The team also notes that hydrogen bonding plays a critical role in intermediate reaction steps but has not been accounted for in previous studies.