Understanding the reaction kinetics and interplay of two widely probed surface states on hematite photoanodes is crucial for unravelling the underlying reaction mechanisms for water oxidation. In this work, we first thoroughly characterized two different types of surface states on model hematite photoanode surfaces at pH 8 by photo-electrochemical impedance analysis. Rate law analysis shows that the first surface states with higher oxidative energies (S1) undergo a distinct transition from 1st to 3rd order kinetics of water oxidation when the illumination intensity surpasses a certain threshold, while the second surface states located at lower potentials (S2) exhibit unexpected transitions from 0th to 3rd order kinetics under similar light modulation. We newly derived a dynamic evolution pathway for both surface states to explain their interplay. This unique evolution model is supported by systematic fast-cathodic cyclic voltammetry analysis and related lifetime estimation of both surface states. Moreover, we demonstrate that the distributions and dynamic interactions of both surface states depend significantly on the nature of the electrolyte. Specifically, the pH-dependent surface protonation/deprotonation state modulates the kinetic behavior of S1. Overall, we reveal the decisive role of operational parameters, such as illumination intensity, nature of electrolyte, and applied potential on the dynamic interplay of both surface states. In addition, the interesting dynamic behavior of both surface states was reproduced with an alternative probing approach. These new coherent insights are crucial to facilitate the design of efficient photoanodes.

Reaction kinetics and interplay of two different surface states on hematite photoanodes for water oxidation

ACS Spring Meeting 2021

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