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

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.

This study investigates photo-anodes based on titanium dioxide (TiO 2 ) that can be used to produce hydrogen by the photo-electrochemical decomposition of water. TiO 2 is a wide band gap semiconductor that absorbs only the UV region of the solar spectrum. Sensitization of TiO 2 to visible light by the addition of gold nanoparticles (AuNPs) was studied. AuNPs sustain localized surface plasmon resonance (LSPR) that results in the absorption of light at the resonant energy. The evidence for water splitting by Au-TiO 2 systems is discussed critically. Fabrication of arrays of AuNPs was done by; annealing sputtered gold thin films, micellar nanolithography, and nano-sphere lithography. The optical characteristics and photo-electrochemical 'water splitting' performance of AuNP coated rutile (110) electrodes were determined. Nbdoped crystals coated in AuNPs of ca. 20 nm exhibited a small photocurrent that was not present with the bare rutile electrode. Reduced un-doped rutile (110) with AuNPs did not exhibit the 'plasmonic photocurrent'. Some Nb-doped electrodes did not exhibit an effect. Batches of Nb-doped and reduced rutile were examined using voltammetry and impedance spectroscopy and it was found that the 'inactive' Nb-doped TiO 2 was partially reduced.Thin films of TiO 2 were fabricated by pulsed laser deposition (PLD) onto amorphous and single crystal substrates.The effect of growth conditions on the phase and orientation of the film were studied, and procedures to grow anatase films oriented with (100), (001), and (101) were developed. The temperature and heating regime of TiO 2 films fused silica affected the orientation of film growing. Nb doping of the films also affected the temperature of the anatase-rutile phase transition and the orientation of the films, acting to stabilize anatase at higher temperatures. Surprisingly, highly doped films were found to be non-conductive. The importance of the oxygen partial pressure in producing conductive films for use as electrodes is discussed.

Hydrogen production using renewable energy sources could enable the technological objectives to be met of decarbonising electrical power generation, energy storage to mitigate renewable energy intermittency and balancing electrical power supply and demand. The required reactors and processes need to be efficient, economic, durable, and of scalable design and fabrication. Additive manufacturing, e.g. by inkjet printing of ceramic particles offers novel means of fabricating solid oxide fuel cells and electrolysers with reproducible geometries, predictable densities of triple phase boundaries, and upscaling of electrode | electrolyte interfacial areas using imaginative 3D structures, thereby decreasing specific costs. We shall present results of using 3D inkjet printing, currently with ca. 10 mm lateral resolution, followed by sintering, for fabrication of SOFCs and SOEs. Having characterised their particle size distributions, zeta potentials and rheologies, colloidal dispersions of < ca. 200 nm yttria-stabilised zirconia (8-YSZ), NiO and lanthanum strontium manganite (LSM) particles were formulated. Having optimised inkjet printing parameters, these dispersions were used for printing 3D structures using a Ceradrop X-Serie piezoelectric DOD printer with a DIMATIX Sapphire QS-256/30 AAA printhead with 52 µm nozzle diameters. Gas-tight planar YSZ electrolyte layers were printed and sintered with ca. 10 mm thicknesses, onto porous Ni(O)-YSZ substrates. Porous LSM electrode layers also were printed on pressed YSZ substrates; cracking of printed layers due to capillary forces were mitigated by avoiding attractive depletion potentials and judicious control of sintering conditions. Detailed optimization of printing parameters, droplet size and substrate temperatures will be presented for electrolyte and electrode layers.