Oxygen Reduction Reaction
The oxygen reduction reaction (ORR) is the cathode half reaction of low-temperature fuel cells, in which O2 combines with protons and electrons to form water. Its equilibrium potential is 1.23 V relative to the standard hydrogen electrode, but even on Pt, the usual cathode material, the reaction is slow and requires a sizable overpotential, which limits fuel cell efficiency. Proposed pathways include a dissociative route (O2 splits, then adsorbed O is hydrogenated to OH and water) and an associative or peroxo route that passes through adsorbed OOH. It is the reverse of the oxygen evolution reaction, and DFT studies of both commonly use the computational hydrogen electrode and a volcano plot against oxygen binding.
Thermodynamic origin of the Pt overpotential (2004)
Nørskov, Rossmeisl, Kitchin and coworkers computed the free energies of adsorbed O and OH on Pt(111) as a function of electrode potential. At 1.23 V both are so stable that the proton-electron transfers to them are uphill by about 0.45 eV, close to the observed overpotential, and all steps become downhill only below about 0.78 V, where water dissociation also sets in. Across 13 metals, Pt and Pd sit at the top of an activity volcano versus oxygen binding energy, and surfaces binding oxygen somewhat more weakly than Pt (such as Pt skins on 3d metal alloys) are predicted to be more active. The associative peroxo path dominates below about 0.8 V and on the most noble metals but hardly moves the volcano maximum (Nørskov et al. 2004). Summary: norskov2004.
Electrolyte cation effects in alkaline ORR (2024)
Bender, Sanspeur, Kitchin, Resasco and coworkers measured the alkaline ORR in 0.1 M LiOH, NaOH, KOH and CsOH. Rates rose with cation size on Pt, Ir, Ru and Au but were unchanged on Ag and Pd, and on Au larger cations also raised H2O2 selectivity. On strongly binding Pt, larger cations relieved *OH poisoning (a less negative hydroxide reaction order); on weakly binding Au they lowered the apparent activation energy for the O2 adsorption step from 40.0 to 21.7 kJ/mol. DFT estimates of the potential of zero total charge that include *OH and *O coverage placed the ORR window negative of the PZTC for Pt, Ir, Ru and Au (negatively charged surface, cations attracted) and positive of it for Ag, explaining the trend for every metal except polycrystalline Pd (Bender et al. 2024). See electrolyte cation effects. Summary: bender2024.