Parallel water photo-oxidation reaction pathways in hematite photoanodes: implications for solar fuel production

Water photo-oxidation on stable metal-oxide photoanodes presents a critical challenge for solar fuel production. This reaction is widely considered to proceed in a sequential pathway with four stepwise hydroxide-coupled hole transfer steps, resulting in oxidized surface intermediates that trap holes while their adsorbates change forms (e.g., from –OH to [double bond, length as m-dash]O to –OOH and back to –OH) so as to maintain charge neutrality. Here we study the potentiodynamic discharge characteristics of hematite photoanodes following polarization under water photo-oxidation conditions. Upon turning the light off, some of the oxidized intermediates discharge spontaneously whereas others remain oxidized for a while. The metastable intermediates discharge in a double-peak wave during cathodic potential sweep (in the dark). The relative peak heights were found to reverse after long time delays since turning the light off. This unexpected observation indicates that the discharge proceeds in parallel pathways, suggesting the same for the reverse reaction that leads to water photo-oxidation. Complementary photoelectrochemical impedance spectroscopy measurements display distinct charge transfer features, supporting the prevalence of parallel pathways in the water photo-oxidation reaction. Through a micro-kinetic model, we derive a criterion that explains why peak reversal, as observed in our measurements, can emerge only from parallel pathways. The prevalence of parallel pathways fundamentally broadens the current paradigm of the water photo-oxidation reaction mechanism, and it may inspire new strategies to reduce the high overpotential of this reaction so as to enhance the efficiency of solar fuel production.