Discovery of the Potential of Minimum Mass for Platinum Electrodes

Electrochemical quartz-crystal nanobalance (EQCN) analysis of the behavior of Pt in aqueous H2SO4 reveals that the interfacial mass reaches a minimum, the potential of minimum mass (E-pmm), at 0.045 V. A similar behavior is observed for Pt in aqueous HClO4 and NaOH. E-pmm is a new parameter describing the electrochemical interface. The value of E-pmm, coincides with the completion of the saturation layer of electroadsorbed H (H-UPD) and the commencement of H-2(g) generation or H-2(g) electro-oxidation. The value of E-pmm and the structure of the Pt/electrolyte interface are discussed in terms of the interactions of the anions H3O+, H-UPD, H-OPD, and H2O with Pt. The layer of H-UPD embedded in the Pt surface lattice minimizes the surface dipole-water dipole and surface charge water dipole interactions, thus reduces the wetting ability of Pt. Consequently, the discharge of H3O+ in the electrolytic formation of H-2(g) or the dissociative adsorption of H-2(g) that precedes its electro-oxidation to H3O+ proceed easily on Pt, because the species do not have to displace H2O molecules. Effective and inexpensive non-platinum electrocatalysts for the electrolytic H-2(g) generation in water electrolyzers or H-2(g) electro-oxidation in polymer electrolyte membrane fuel cells should mimic the interfacial behavior of Pt by minimizing the interaction of H2O molecules with the electrode.