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FEB Lora, A Hankin, GH Kelsall – Journal of Materials Chemistry A, 2017

A semi-empirical model was developed for the prediction of photocurrent densities and implemented to predict the performance of a photo-electrochemical reactor for water splitting in alkaline solutions, using SnIV-doped α-Fe2O3 photo-anodes produced by spray pyrolysis. Photo-anodes annealed at different temperatures were characterised using photo-electrochemical impedance spectroscopy, cyclic voltammetry in the presence and absence of a hole scavenger and also the open circuit potential under high intensity illumination. Mott–Schottky analysis was used cautiously to estimate the charge carrier concentration and the flat band potential. In addition to overpotential/current distribution and ohmic potential losses, the model also accounts for absorbed photon flux, surface and bulk electron–hole recombination rates, gas desorption, bubble formation and (H2–O2) cross-over losses. This allows the model to estimate the total yield of hydrogen, charge and gas collection efficiencies. A methodology is presented here in order to evaluate the parameters required to assess the performance of a photo-electrochemical reactor in 1D and 2D geometries. The importance of taking into account bubble generation and gas desorption is discussed, together with the difficulties of measuring charge carrier concentration and electron–hole recombination in the bulk of the semiconductor, which are of major importance in the prediction of photocurrent densities.

Photo-anode characterisation

A photo-electrochemical cell of 0.06 dm3 (PVC body with a quartz window) was used for the electrochemical characterisation of photo-anodes. A potentiostat/galvanostat (Autolab PGSTAT 30) was used to control the three-electrode cell. Hematite samples acted as working electrodes, a platinised titanium mesh (Expanded Metal Company, UK) as a counter electrode and HgO|Hg as a reference electrode (0.913 V vs. the RHE and 0.109 V vs. the SHE). 1 M NaOH was used as an electrolyte (pH 13.6) at room temperature, ca. 25 °C. Hydrogen peroxide, H2O2 (30%, AnalaR Normapur, VWR) was added in some cases as a sacrificial electron donor (hole scavenger) at a concentration of 0.5 M. A Xe arc lamp (LOT-Oriel) of 300 W with Fresnel lenses was used to obtain a beam of 3 mm × 6 mm and a maximum power intensity of 3646 W m−2. The intensity of light was corrected by electrolyte and quartz attenuation to give an effective power intensity of 3000 W m−2 on the photo-anode. The light source was characterised and calibrated with a UV-vis spectrophotometer coupled to a CR2 cosine receptor (Black-Comet CXR-25, StellarNet, USA). Absorption spectra of FTO samples were obtained using a Shimadzu UV-2600 UV-vis spectrophotometer with an integrating sphere and corrected by baseline subtraction assuming reflection and scattering are wavelength independent.54 Thickness of the films deposited on FTO was measured using a stylus profilometer (TencorAlphastep200 Automatic Step Profiler). For the photo-electrochemical cell described here, i.e. small beam size to volume ratio, there was no significant increase in temperature after long term exposure to illumination.

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