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Title: Photo-electrochemical reactors with hematite photo-anodes for water and hydrogen sulfide splitting
Author: Bedoya Lora, Franky Esteban
ISNI:       0000 0004 7657 6751
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The impact of CO₂ emissions on climate change, human health and local environments are driving the search for more benign sources of energy. The sun is the earth's ultimate energy source, though subject to diurnal and seasonal intermittency. This calls for a technology to effectively store the harvested solar energy, and make it readily available when needed. 'Solar fuels', such as hydrogen, have been proposed as suitable energy carriers. However, due to high costs involved in the production of hydrogen using renewable sources, coupling its production to other processes, such as waste treatment, would be advantageous. Hydrogen sulfide is a toxic and highly corrosive gas which is produced by hydrodesulfurisation processes in petroleum refineries for removal of sulfur from gas and petroleum products. Hence, hydrogen sulfide, which is thermodynamically predicted to be easier to oxidise than water, can be treated while simultaneously producing hydrogen via photo-assisted electrolysis. The performances of tin(IV)-doped hematite photo-anodes were assessed for hydrogen production using photo-assisted electrolysis of water and hydrogen sulfide in photo-electrochemical reactors. Initially, photo-electrochemical, optical and morphological properties were measured as a function of post-deposition annealing. Using these parameters, a semi-empirical model was developed for the prediction of photocurrent densities and the performance of bench-scale (< 0.1 dm³) and up-scaled (0.8 dm³) reactors for water splitting. The model was also implemented to predict optimised geometric properties of photo-anodes with the aim to decrease (H₂-O₂) cross-over losses and minimise inhomogeneities in current density distributions. Lastly, a comparison between conventional water splitting and hydrogen sulfide splitting indicated that interfacial electron-hole recombination rates are diminished effectively in presence of hydrogen sulfide ions, and the photo-anode potentials could be decreased by ca. 0.5 V. Charge yields for hydrogen and polysulfide ions were also estimated, typically as 0.9 and 1.0, respectively. A customised UV/vis absorption technique was required for in situ quantification of polysulfide ions at low concentrations in hydrogen sulfide alkaline solution. A mechanism was proposed for hydrogen sulfide oxidation to polysulfide ions on tin(IV)-doped hematite photo-anodes, on which electrodeposition of elemental sulfur occurred and polysulfide ions accumulated; this needs to be considered when selecting operating conditions for hydrogen production via photo-assisted hydrogen sulfide splitting.
Supervisor: Kelsall, Geoff Sponsor: Fondo Colombiano de Investigaciones Científicas y Proyectos Especiales Francisco José de Caldas
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral