Abstract
H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.
Highlights
H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties
Recent advances in haematite photoanodes have led to significant progress in improving their performance, with most of the progress focused on enhancing the photocurrent, reaching over 4 mA cm−2 for champion haematite photoanodes[5,6,7,8]
Unlike the linear models of water photo-oxidation on haematite that consider a multi-step reaction on a single surface site[24,25,26,34,35], as in Eley–Rideal (ER) reactions, our work suggests that splitting the reaction into two sites, as in LH reactions, enabled here by the presence of H2O2 in the electrolyte, gives rise to nonlinear behaviour and facilitates the collection of photogenerated holes by oxidized surface species that help to level the potential of the elementary steps involved in the reaction[20]
Summary
H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Enhancing the photocurrent and reducing the applied bias (potential) are required to compete with the efficiency of PV-powered electrolysis systems[10] Achieving these goals would benefit from detailed understanding of the underlying processes that govern the photogeneration, recombination, and charge transfer at the semiconductor electrode|electrolyte interface[9,11,12,13,14,15]. The mechanism by which H2O2 extracts the photo-generated holes remains elusive The understanding of this mechanism may lead to rational design of photoanode|electrolyte interfaces and co-catalysts that enhance hole collection and reduce deleterious surface recombination in closely related processes, such as water photo-oxidation
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