The Potential of Zero Total Charge Predicts Cation Effects for the Oxygen Reduction Reaction
Jay T. Bender, Rohan Yuri Sanspeur, Angel E. Valles, Alyssa K. Uvodich, Delia J. Milliron, John R. Kitchin and Joaquin Resasco, “The Potential of Zero Total Charge Predicts Cation Effects for the Oxygen Reduction Reaction”, ACS Energy Letters 9(9), 4724-4733 (2024). https://doi.org/10.1021/acsenergylett.4c01897
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
The identity of the alkali metal cation in the electrolyte reliably changes rates of reactions run at strongly reducing potentials, such as CO2 reduction and hydrogen evolution. For the alkaline oxygen reduction reaction (ORR), which runs at milder potentials near the potential of zero charge of many metals, cation effects appear on some catalysts and not on others. This letter asks what decides whether an ORR catalyst will respond to cation size, and proposes that the answer is the position of the ORR potential window relative to the metal’s potential of zero total charge (PZTC), the potential at which the surface, including its adsorbates, carries no excess charge (Bender et al. 2024).
Approach
Rotating ring-disk electrode measurements were made on polycrystalline Pt, Au, Ag and Pd and on carbon-supported Pt, Ir, Ru, Au and Pd nanoparticles in 0.1 M LiOH, NaOH, KOH and CsOH (pH 13). Activity was reported as the half-wave potential after subtracting the Ar-saturated background, and H2O2 selectivity came from the Pt ring held at 1.1 V vs RHE, cross-checked by Koutecký-Levich analysis. Two kinetic probes targeted the two classes of catalyst: hydroxide reaction orders on Pt(poly) in 0.1 to 1.0 M XOH, and Arrhenius measurements between 25 and 50 °C on Au(poly). On the computational side, the WhereWulff workflow (Sanspeur et al. 2023) built surface Pourbaix diagrams with *OH and *O on low-index facets of Pt, Ir, Au, Ag, Pd and Ru, then charged each slab (by changing its electron count) at every ORR intermediate at 0.80 V vs RHE, and fit the resulting free energy versus potential to a parallel plate capacitor model whose maximum gives the PZTC. Implicit solvation was used throughout (Bender et al. 2024).
Key findings
On Pt, Ir, Ru and Au, ORR activity rose in the order Li+ < Na+ < K+ < Cs+, while Ag and Pd were insensitive to the cation. All catalysts except Au were fully selective to the 4e- path to water; on Au the H2O2 selectivity grew with cation size. On Pt(poly), the negative hydroxide reaction order (a sign of *OH poisoning) became less negative with larger cations, consistent with cations weakening *OH poisoning on strongly binding metals. On Au(poly), at 400 mV overpotential the apparent activation energy fell from 40.0 to 21.7 kJ/mol from Li+ to Cs+. Because a change in site blocking would shift only the Arrhenius intercept and not its slope, the authors argue the cation acts mainly through the interfacial electric field on O2 adsorption, not by forming site-blocking complexes with *OH (Bender et al. 2024).
The calculated close-packed PZTCs (vs SHE) were 0.92 V for Pt, 0.68 V for Ir, 0.30 V for Ru, 0.33 V for Au, 0.58 V for Pd and -0.39 V for Ag, compared with measured half-wave potentials in 0.1 M KOH between about -0.11 and 0.11 V vs SHE. For Pt, Ir, Ru and Au the ORR therefore runs negative of the PZTC, the surface is negatively charged, and cations gather in the double layer; for Ag it runs positive of the PZTC and cations are repelled. Literature PZC values mispredict Ru (about -0.48 V); including its high *O coverage in the PZTC fixes this. Oxygen-covered reactive metals (Pt, Ir, Ru, Pd) have PZTCs well positive of their bare-surface PZCs, while weakly binding Au and Ag stay close. Au was correctly predicted to show cation effects even though WhereWulff found bare Au(111) and Au(110), so a high *OH coverage is not required. Pd is the one miss: all facets are predicted to show cation effects. The authors note that implicit solvation overestimates the PZC of high work function metals by about 0.40 V; applying that shift would predict a cation effect only on Pd(111), matching a single-crystal report, and the polycrystalline data may be dominated by other sites (Bender et al. 2024).
Significance
The work gives two conditions for a reaction to show electrolyte cation effects: it must run negative of the metal’s PZTC so that cations reach the interface, and its rate-controlling steps must involve intermediates or transition states with large dipoles or polarizabilities. The PZTC, which accounts for adsorbate coverage, is a better descriptor than the bare PZC for the ORR, and the combination of kinetics with automated coverage-aware DFT points to electrolyte engineering as a lever for tuning electrocatalytic rates (Bender et al. 2024).