The Characteristically Slow Proton Transfer Coupled to Platinum Oxidation in Alkaline Polyelectrolyte as Elucidated at the Molecular Level

Abstract

The proton transfer in alkaline polyelectrolyte membrane (APEM)/electrode interfaces is significantly coupled to the electrochemical reactions in energy conversion and green synthesis. The OH^– in APEM/electrode interfaces is characteristically without cations in the surroundings but ambiguous in proton-transfer-coupled electrochemical reactions at the molecular level. Here we employed in situ electrochemical surface-enhanced Raman spectroscopy and high-level quantum-chemical calculations to elucidate the proton transfer in the APEM/Pt interface by using electrochemical Pt oxidation as an indicator. To manifest the characters in APEM, a comparison to that in conventional NaOH solution was made. With the similar electron transfer of Pt oxidation in both APEM and NaOH, the driving force and rate of proton transfer were distinguished respectively according to the onset oxidation potential and morphology of Pt nanoparticles, which suggested the slow proton transfer in an APEM/Pt interface. The similar vibrational fingerprints of subsurface oxygenated intermediates in both APEM and NaOH solution evidenced the characteristically slow proton transfer in an APEM/Pt interface. The high-level quantum-chemical calculations combined with molecular dynamics simulation showed that the driving force of proton transfer in APEM was reduced since OH^– was coordinated by more water molecules in its hydration shell. The characteristically slow interfacial proton transfer may be universally coupled to electrochemical reactions in devices with APEMs.

The proton-transfer-coupled Pt oxidation in the alkaline polyelectrolyte membrane/Pt interface was elucidated to be characteristically slow due to the H₂O coordination number of interfacial OH⁻.