Bonds over Electrons: Proton Coupled Electron Transfer at Solid-Solution Interfaces

This Perspective argues that most redox reactions of materials at an interface with a protic solution involve net proton-coupled electron transfer (PCET) (or other cation-coupled ET). This view contrasts with the traditional electron-transfer focused view of redox reactions at semiconductors, but redox processes at metal surfaces are often described as PCET. Taking a thermodynamic perspective, transfer of an electron is typically accompanied by a stoichiometric proton, much as the chemistry of lithium-ion batteries involves coupled transfers of e- and Li+. The PCET viewpoint implicates the surface-H bond dissociation free energy (BDFE) as the preeminent energetic parameter and its conceptual equivalents, the electrochemical ne-/nH+ potential versus the reversible hydrogen electrode (RHE) and the free energy of hydrogenation, Delta G degrees H. These parameters capture the thermochemistry of PCET at interfaces better than electronic parameters such as Fermi energies, electron chemical potentials, flat band potentials, or band-edge energies. A unified picture of PCET at metal and semiconductor surfaces is presented. Exceptions, limitations, implications, and future directions motivated by this approach are described.

Automated summary

This Perspective argues that most redox reactions at an interface with a protic solution involve net proton-coupled electron transfer. The view contrasts with the traditional electron-transfer focused view of redox reactions at semiconductors, but redox processes at metal surfaces are often described as PCET. The importance of surface-H bond dissociation free energy as the preeminent energetic parameter and its conceptual equivalents in understanding thermodynamics of PCET at interfaces is emphasized.