Hematite Photoanodes for Solar Water Splitting: A Detailed Spectroelectrochemical Analysis on the pH-Dependent Performance

The effect of the electrolyte pH on the performance of metal oxide photoanodes for solar water oxidation has not been fully resolved, and contrasting views have been presented in recent reports. Herein, a comprehensive set of spectroelectrochemical techniques (impedance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), and operando UV-vis spectroscopy) are deployed to clearly uncover the role of pH on the performance of hematite photoanodes. Our results reveal that, despite the presence of high-valent iron-oxo active sites over a wide pH range (7-13.6), the observed performance improvement with increasing pH is mainly driven by the reduction of surface accumulated charges, in the form of reactive intermediate species, that alleviates Fermi level pinning (FLP). Interestingly, IMPS data provides compelling evidence that the mitigation of FLP originates from changes in the reaction mechanism which boost the rate of charge transfer reducing, in turn, the surface charging. Additionally, we present a phenomenological analysis of the IMVS response which brings to light the additional impact of the electrolyte pH on the surface-related recombination dynamics. Our work identifies the pH-dependent kinetics of water oxidation as the key step governing the performance, defining not only the efficiency of charge transfer across the interface but also the degree of FLP that determines both the photocurrent magnitude and onset potential.