Electrolyte Cation Effects
The identity of the alkali metal cation in an aqueous electrolyte (Li+, Na+, K+, Cs+) can change electrocatalytic rates and selectivities even though the cation is not a reactant. Such effects are common for reactions at strongly reducing potentials, including CO2 reduction, hydrogen evolution and hydrogen oxidation. Two families of explanation are discussed. In the field picture, hydrated cations accumulate in the double layer of a negatively charged electrode and strengthen the interfacial electric field, which stabilizes or destabilizes adsorbates and transition states with large dipole moments or polarizabilities. Larger, more weakly solvated cations approach the surface more readily, so the effect grows with cation size. In the noncovalent-interaction picture, hydrated cation clusters bind to adsorbed hydroxide and block sites, an effect that depends on the oxophilicity of the metal. See oxygen reduction reaction and potential of zero charge.
When cation effects appear for the ORR (2024)
Bender, Sanspeur, Kitchin, Resasco and coworkers measured the alkaline ORR in 0.1 M LiOH to CsOH on Pt, Ir, Ru, Au, Ag and Pd. Activity increased with cation size on Pt, Ir, Ru and Au but not on Ag and Pd. Larger cations made the hydroxide reaction order on Pt less negative (less *OH poisoning) and lowered the apparent activation energy on Au from 40.0 to 21.7 kJ/mol, the latter being hard to explain by site blocking. They argue that cation effects require two things: the reaction must occur negative of the metal’s potential of zero total charge, so the surface is negatively charged and attracts cations, and the kinetically relevant intermediates must be field-sensitive (Bender et al. 2024). Summary: bender2024.
Cation effects on an oxide OER catalyst (2015)
Cation effects also appear at oxidizing potentials on oxyhydroxide catalysts, where alkali cations are known to intercalate into the film during redox cycling. Michael, Kitchin and coworkers found that NiOOH thin films gave about twice the OER current in purified 0.1 M CsOH as in LiOH, reversibly on electrolyte switching, with K+ and Na+ intermediate. Trace Fe changed the ordering to K+ ≈ Na+ > Cs+ > Li+ and had a far larger effect than the cation itself, so the two variables interact. In situ Raman showed slightly lower Ni-O band frequencies (longer Ni-O bonds) in CsOH than LiOH, which the authors read as a cation-induced change in the active-phase structure, though the mechanism was left open (Michael et al. 2015). Summary: michael2015; see also Oxygen evolution reaction.