Frontispiece: Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Oxygen catalysts are notorious for their high overpotential and sluggish kinetics. Both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) consist of four proton-coupled/decoupled electron-transfer steps with a bond-breaking (ORR) or of a bond-forming (OER) step. The rate-determining steps and intermediates for OER and ORR are different, and the binding energy between the active sites and intermediates are not identical; thus, it is tough to achieve identical oxygen binding energies of the intermediates close to the theoretical equilibrium potential. A good bifunctional oxygen catalyst should have multiple redox centers that could be responsible for OER and ORR. Composites with multiactive centers and conductive supports could offer new opportunities for bifunctional air electrode. For more details see the Review by S. Wang, Y. Cheng et al. on page 3906 ff.