Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials.

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Bismuth vanadate (BiVO4) is a widely studied oxide in solar water splitting, known for its ease of synthesis, high charge extraction yields, and advantageous band alignment with water. We present a combined first-principles and experimental study of the electronic structure of the (010) surface of BiVO4 aimed at disentangling the impact of the surface and bulk oxygen vacancies on the electronic structure and transport properties. We found that oxygen vacancies are deep donors at the surface as they are in the bulk; our calculations on defect and polaron formation energies suggest that, while polarons formed from oxygen vacancies in the bulk can contribute to conductivity, those at the surface likely do not. Our results also show that out-of-plane structural relaxations at the surface contribute to the relatively immobile nature of electron polarons derived from surface oxygen vacancies. The structural model derived from first-principles calculation was validated by comparing computed results with experimental measurements of single-crystal and epitaxially grown single-crystalline BiVO4 samples. We also found a reasonably good agreement between our calculated and measured work functions for BiVO4 samples with and without oxygen vacancies.

Electrodeposition is a widely used technique for electrode synthesis in various applications. Because of its low synthesis cost and easy scalability, electrodeposition is particularly attractive for the production of semiconductor and catalyst electrodes for use in solar fuel production. For researchers who are interested in learning about or utilizing electrodeposition, the current paper describes detailed methods for electrodeposition, which include procedures for preparing electrodes and plating solutions, determining deposition conditions, and performing electrodeposition. Postdeposition treatments that can be used to prepare electrodes of more diverse compositions and photodeposition procedures that can be used to place catalyst layers on semiconductor electrodes are also provided. The methods are described using the synthesis and modification of photoelectrodes as an example, but most principles and procedures explained in this paper are general and can be applied to electrodeposition of various electrodes. In addition to methods for electrochemically preparing photoelectrodes, methods for photoelectrochemical characterization, which include light setup and calibration, photoelectrochemical characterization, and efficiency calculations, are described along with rationale for each setup and procedure. This will improve understanding and performance of various experimental procedures used for photoelectrode evaluation.

Titanium dioxide (TiO2) deposited by atomic layer deposition (ALD) has been the most commonly used protection layer to enhance chemical and photoelectrochemical stabilities of photoelectrodes. In this study, we report a new electrochemical deposition method that can place a thin, conformal TiO2 coating layer on a photoelectrode. This method takes <1 min and may serve as a practical alternative to ALD for the deposition of a TiO2 layer. The uniform quality of the TiO2 protection layer was confirmed by demonstrating the chemical stability of the BiVO4/TiO2 electrode in strongly basic media (pH 12 and 13) where BiVO4 readily dissolves. More importantly, the high-quality TiO2 protection layer made it possible to comparatively investigate photoelectrochemical properties and stabilities of the BiVO4 and BiVO4/TiO2 electrodes, which was critical to elucidate the effect that the chemical instability of BiVO4 in basic media has on the rate of photocorrosion. Systematic photoelectrochemical studies for sulfite oxidation and water oxidation provided a coherent understanding of how the interplay among the relative rates of interfacial charge transfer, surface recombination, and photocorrosion affects the photocurrent generation and photostability of BiVO4. On the basis of this understanding, stable photocurrent generation for water oxidation could be achieved at pH 12 over 20 h using a BiVO4/TiO2/FeOOH/NiOOH electrode where FeOOH/NiOOH served as oxygen evolution catalyst. The results and discussion contained in this study provide new insights into the understanding of photocurrent decay caused by photocorrosion involving dissolution, enabling the development of effective strategies to achieve stable photocurrent generation.