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.
Optimal descriptors and uncertainty for 4e⁻ and 2e⁻ ORR (2016)
Using BEEF-vdW with explicit water layers on fcc(111) and fcc(100) facets of Pt, Pd, Ag, Au and Pt3Ni, this paper identified ΔGOH as the descriptor that minimizes prediction error for the 4e⁻ ORR and ΔGOOH as optimal for the 2e⁻ reduction to H2O2, while showing ΔGOH is accurate enough for the 2e⁻ case to compare both pathways on one axis (Deshpande et al. 2016). BEEF-vdW placed the potential-determining step on Pt(111) at O* to OH* rather than OH* to water, a difference of only about 0.05 V attributed to overstabilized O*. Summary: deshpande2016.
Fast alchemical estimates of ORR intermediate binding (2017)
Saravanan, Kitchin, von Lilienfeld and Keith benchmarked computational alchemy for the ORR descriptors O*, OH* and OOH* on alloys derived from Pt, Pd and Ni (111) surfaces. Alchemy often reproduced PBE binding energies within about 0.1 eV at negligible cost, but OOH* errors in M3X skin alloys were generally above 0.2 eV, and O* errors were larger than OH* errors because O* has larger alchemical derivatives (Saravanan et al. 2017). Summary: saravanan2017.
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.
Electrolyte pH effects across metals (2025)
Bender and co-workers combined rotating ring-disk kinetics in 0.1 M HClO4 and 0.1 M KOH with field-dependent DFT on six metals to explain why ORR rates on Pt are pH-independent on the RHE scale while those on Au and Ag rise sharply in base (Bender et al. 2025). Reaction orders (about first order in O2 and H+ and negative first order in OH- on Pt(poly); zero order in H+ and OH- on Au(poly)) identify the *O2 to *OOH proton-coupled step as kinetically relevant on Pt and non-PCET O2 adsorption on Au, with neither changing with pH. The Au apparent barrier fell from 42.3 to 26.5 kJ/mol from acid to base, while that on Pt was about 26.4 versus 23.5 kJ/mol.
Strongly binding Ir, Ru and Pd showed modest activity gains in base, and Au, Ag and Pd shifted from 2e- (H2O2) toward 4e- selectivity. The unifying explanation is that higher pH at fixed RHE potential means a more negative interfacial field, which stabilizes *O2 strongly on weakly binding metals but hardly changes the energetics of the limiting step on Pt. Summary: bender2025.