Potential of Zero Charge
The potential of zero charge (PZC) is the electrode potential at which a metal surface carries no excess charge. Negative of the PZC the surface is negatively charged and attracts cations into the double layer; positive of it the surface is positively charged and repels them. To a first approximation the surface charge density scales as the gap capacitance times the difference between the applied potential and the PZC, and it is roughly proportional to the interfacial electric field. Several related quantities are distinguished: the potential of zero free charge (PZFC) refers to the bare metal without specific adsorption, while the potential of zero total charge (PZTC) includes charge transferred by adsorbates such as *OH and *O. The PZC of metals correlates linearly with their work function. Computing it is difficult: implicit solvation misses specific water adsorption on reactive metals, and explicit-solvent molecular dynamics values for Pt(111) span a wide range. See electrolyte cation effects.
PZTC as a descriptor for ORR cation effects (2024)
Bender, Sanspeur, Kitchin, Resasco and coworkers computed PZTCs with implicit solvation by charging slabs whose adsorbate coverage came from surface Pourbaix diagrams generated by the WhereWulff workflow, and fitting free energy versus potential to a parallel plate capacitor model. Close-packed values were 0.92 V (Pt), 0.68 V (Ir), 0.30 V (Ru), 0.33 V (Au), 0.58 V (Pd) and -0.39 V (Ag) vs SHE. Oxygen coverage pushes the PZTC of reactive metals well positive of their bare PZC, which corrected the cation-effect prediction for Ru, while weakly binding Au and Ag stay near their PZC. Comparing the PZTC with the measured ORR half-wave potential correctly predicted which metals show cation effects for all but polycrystalline Pd, where the authors point to an estimated 0.40 V implicit-solvent overestimate and facet dependence (Bender et al. 2024). Summary: bender2024.