Electrolyte pH Effects
Changing electrolyte pH can change electrocatalytic rates and selectivities beyond the simple Nernstian shift that the reversible hydrogen electrode (RHE) scale removes. Proposed origins include changes in the proton donor (hydronium versus water), in mechanism or rate-determining step, in interfacial water structure and adsorbate coverage, and in the interfacial electric field: at a fixed RHE potential, a higher pH means a more negative absolute (SHE) potential relative to the electrode’s potential of zero charge, so a stronger negative field acts on adsorbates with large dipoles or polarizabilities.
Field-based explanation of ORR pH effects (2025)
Bender and co-workers proposed that pH effects on the oxygen reduction reaction arise from the field sensitivity of the intermediates in the kinetically relevant step, not from a change in that step (Bender et al. 2025). Kinetic measurements at fixed SHE potential (for H+ and OH- orders) and fixed overpotential (for O2 orders and Arrhenius barriers) showed that the rate-determining step on Pt and on Au is the same in acid and base. DFT with charged slabs and a capacitor model referenced to the potential of zero total charge showed that a more negative field strongly stabilizes *O2 on Au and Ag (rates rise sharply in base), barely changes the *O2 to *OOH energetics on Pt (rates unchanged), and moderately helps Ir, Ru and Pd. The paper also stresses that kinetic analysis of pH effects must specify the potential scale, since reaction orders in H+ or OH- are trivial on the RHE scale. Summary: bender2025.