Unravelling the pH-dependent mechanism of ferroelectric polarization on different dynamic pathways of photoelectrochemical water oxidation

Abstract

Ferroelectric polarization is considered to be an effective strategy to improve the oxygen evolution reaction (OER) of photoelectrocatalysis. The primary challenge is to clarify how the polarization field controls the OER dynamic pathway at a molecular level. Here, electrochemical fingerprint tests were used, together with theoretical calculations, to systematically investigate the free energy change in oxo and hydroxyl intermediates on TiO₂–BaTiO₃ core–shell nanowires (BTO@TiO₂) upon polarization in different pH environments. We demonstrate that the adsorbate evolution mechanism (AEM) dominated in acidic environments, and both positive and negative polarization resulted in a reduction in the oxo-free energy, which inhibited the reaction kinetics. In the oxide path mechanism (OPM) that occurs in alkaline conditions, the ferroelectric polarization exhibits repulsive adsorbate–adsorbate interaction for OH⁻ coverage and free energy shift of the OH⁻ groups. We elucidate that a weakly alkaline electrolyte is the optimal environment for ferroelectric polarization because the positive polarization promotes OH⁻ coverage and facilitates reaction pathway transfer from AEM to OPM; therefore, BTO@TiO₂ exhibited a record polarization enhancement to 0.52 mA cm⁻² at 1.23 V_RHE in pH = 11. This work provides a more accurate insight into the pH-dependent effect of ferroelectric polarization on the OER dynamic pathway than conventional models that are based solely on the regulation of band bending.