How Electrolyte pH Affects the Oxygen Reduction Reaction
Jay T. Bender, Rohan Yuri Sanspeur, Nicolas Bueno Ponce, Angel E. Valles, Alyssa K. Uvodich, Delia J. Milliron, John R. Kitchin and Joaquin Resasco, “How Electrolyte pH Affects the Oxygen Reduction Reaction”, Journal of the American Chemical Society 147(41), 37819-37832 (2025). https://doi.org/10.1021/jacs.5c14208
This summary was written by a language model from the paper and has not been checked line by line. Rely on the published paper for details.
Question
On the reversible hydrogen electrode (RHE) scale, oxygen reduction reaction (ORR) rates on Pt are essentially the same in acid and base, while rates on Au are much faster in alkaline electrolyte. Earlier explanations (a change of rate-determining step, of mechanism, of interfacial water structure or of adsorbate coverage) were usually specific to one catalyst. The paper asks whether a single picture can explain why some metals respond strongly to pH and others do not, and proposes that pH acts through the interfacial electric field on the intermediates of the kinetically relevant elementary step (Bender et al. 2025).
Approach
ORR activity and H2O2 selectivity were measured by rotating ring-disk electrode on carbon-supported Pt, Au, Ir, Ru and Pd nanoparticles and on polycrystalline Pt, Au, Ag and Pd in 0.1 M HClO4 and 0.1 M KOH, with Koutecký-Levich analysis as a cross-check. On Pt(poly) and Au(poly), reaction orders in O2, H+ and OH- were measured at fixed ionic strength (0.2 M, balanced with NaClO4) in kinetically controlled potential windows. O2 orders and Arrhenius barriers (25 to 50 °C) were taken at fixed overpotential, correcting the equilibrium potential for temperature and O2 pressure, while H+ and OH- orders were taken at fixed potential on the SHE scale, since the RHE scale makes them trivial. Microkinetic models gave reference reaction orders and Tafel slopes for different assumed rate-limiting steps. DFT (VASP, PBE, implicit VASPsol solvent) on the (111), (100) and (110) facets of Pt, Au, Ag, Ir, Ru and Pd used the WhereWulff workflow (Sanspeur et al. 2023) to build surface Pourbaix diagrams with *OH and *O, then charged the slabs to obtain each intermediate’s binding energy as a function of field. The field was related to potential with a Helmholtz capacitor model referenced to the potential of zero total charge (PZTC), so that going from pH 1 to pH 13 at fixed RHE potential corresponds to moving from 0.7 to 0.0 V vs SHE and a more negative field (Bender et al. 2025).
Key findings
On Pt/C and Pt(poly), activity barely changed over 12 pH units and selectivity was essentially fully 4e- to water. On Au/C and Au(poly), rates rose sharply in base and selectivity shifted from mostly H2O2 (above 50% current efficiency in acid) toward water. On Pt(poly) the orders were about first in O2 (1.08 in acid, 1.04 in base), 0.96 in H and -0.90 in OH-, and Tafel slopes rose from about 60 to 120 mV/dec. Together with microkinetic modeling, these point to the first proton-coupled electron transfer (PCET), *O2 to *OOH, as the kinetically relevant step rather than *OH removal. On Au(poly) the orders were about first in O2 (1.03 and 0.92) and zero in H+ and OH- (-0.01 each), identifying non-PCET O2 adsorption as kinetically relevant in both electrolytes. The apparent barrier on Pt(poly) at 350 mV overpotential was 26.4 kJ/mol in acid and 23.5 kJ/mol in base, while on Au(poly) it fell from 42.3 kJ/mol (acid, 1000 mV) to 26.5 kJ/mol (base, 400 mV). The authors conclude that pH does not change the rate-determining step on either metal but lowers the O2 adsorption barrier on Au. They also show that Tafel slopes alone cannot discriminate between mechanisms (Bender et al. 2025).
The DFT field sensitivities rationalize these trends. On Pt(111), *O2 and *OOH respond almost identically to field, and *OH is nearly field-insensitive, so pH has little effect whichever PCET step limits. On Au, and likewise on Ag, *O2 is strongly stabilized by a more negative field. Ag(poly) behaved like Au experimentally. Ir/C, Ru/C and Pd/C showed modest gains in base. On Ir and Ru (*OH removal limiting, high *O coverage) *OH is destabilized by the negative field while other intermediates are stabilized. On Pd the *O2 and *OOH sensitivities differ more than on Pt. Stabilizing *OOH under a stronger field also explains the 2e- to 4e- selectivity shift on Au, Ag and Pd (Bender et al. 2025).
Significance
The study offers a general, cross-metal account of ORR pH effects: changing pH at fixed RHE potential shifts the absolute potential relative to the PZTC, and a rate responds only if the intermediates in its kinetically relevant step are field-sensitive. This parallels the authors’ earlier cation-effect work (Bender et al. 2024), in which Au was likewise most and Pt least sensitive, though cations act through the Helmholtz capacitance rather than the potential. The authors note that the DFT field positions depend on assumed capacitance and PZTC values and give qualitative rather than quantitative comparisons (Bender et al. 2025).