Re-examination of methodologies used for flat band potential determination to help avoid misleading results from the Mott–Schottky approximation.

0.1×0.1 m2 tin-doped hematite photo-anodes were fabricated on titanium substrates by spray pyrolysis and deployed in a photo-electrochemical reactor for photo-assisted splitting of water into hydrogen and oxygen. Hitherto, photo-electrochemical research focussed largely on the fabrication, properties and behaviour of photo-electrodes, whereas both experimental and modelling results reported here address reactor scale-up issues of minimising inhomogeneities in spatial distributions of potentials, current densities and the resultant hydrogen evolution rates. Such information is essential for optimising the design and photon energy-to-hydrogen conversion efficiencies of photo-electrochemical reactors to progress their industrial deployment. The 2D and 3D reactor models presented here are coupled with a modified micro-kinetic model of oxygen evolution on hematite thin films both in the dark and when illuminated. For the first time, such a model is applied to a scaled-up photo-electrochemical reactor and validated against experimental data

Green hydrogen, produced using solar energy, is a promising means of reducing greenhouse gas emissions. Photoelectrochemical (PEC) water splitting devices can produce hydrogen using sunlight and integrate the distinct functions of photovoltaics and electrolyzers in a single device. There is flexibility in the degree of integration between these electrical and chemical energy generating components, and so a plethora of archetypal PEC device designs has emerged. Although some materials have effectively been ruled out for use in commercial PEC devices, many principles of material design and synthesis have been learned. Here, the fundamental requirements of PEC materials, the top performances of the most widely studied inorganic photoelectrode materials, and reactor structures reported for unassisted solar water splitting are revisited. The main phenomena limiting the performance of up‐scaled PEC devices are discussed, showing that engineering must be considered in parallel with material development for the future piloting of PEC water splitting systems. To establish the future commercial viability of this technology, more accurate techno‐economic analyses should be carried out using data from larger scale demonstrations, and hence more durable and efficient PEC systems need to be developed that meet the challenges imposed from both material and engineering perspectives.

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We revisit the fundamental constraints that apply to flat band potential values at semiconductor photo-electrodes. On the physical scale, the Fermi level energy of a non-degenerate semiconductor at the flat band condition, EF(FB), is constrained to a position between the conduction band, EC, and the valence band, EV,: |EC| < |EF(FB)| < |EV| throughout the depth of the semiconductor. The same constraint applies on the electrode potential scale, where the values are referenced against a common reference electrode: UC(FB) < UF(FB) < UV(FB). Some experimentally determined flat band potentials appear to lie outside these fundamental boundaries. In order to assess the validity of any determined flat band potential, the boundaries set by the conduction band and the valence band must be computed on both scales a priori, where possible. This is accomplished with the aid of an analytical reconstruction of the semiconductor|electrolyte interface in question. To illustrate this approach, we provide a case study based on synthetic hematite, α-Fe2O3. The analysis of this particular semiconductor is motivated by the large variance in the flat band potential values reported in the literature.