Temperature effects in (photo)electrochemical devices for solar fuels and chemicals production

In contrast to electricity or heat production from solar energy, direct solar fuel generation processes are less mature and cost-competitive for large-scale commercialization. (Photo)electrochemical devices have the advantage to operate closer to ambient conditions than other technologies, but there are a variety of device configurations which fall in this broad definition. Temperature is one of the most relevant operating parameters which influences the energy conversion efficiency of (photo)electrochemical devices due to its effects over several different physical phenomena in the semiconductor, on the active sites for the electrochemical reactions or in the electrolyte. In this thesis, the effects of operating temperature in various (photo)electrochemical device configurations for solar fuel production were therefore analysed. A high-throughput system with ten photoelectrochemical cells operating simultaneously whilst controlling temperature from 26 °C to 65 °C was developed to study the effects of temperature in a hematite photoanode-electrolyte junction for oxygen evolution. The increase of surface-state-mediated charge recombination with temperature mainly penalized the energy conversion efficiency due to the reduction of the photovoltage produced. The performance and photostability of bismuth vanadate photoanodes with cobalt phosphate co-catalyst on metallic felts integrated in membrane-photoelectrode assemblies for water splitting were then assessed at temperatures from 30 °C to 56 °C. Operation with liquid water at 56 °C mainly reduced the photovoltage produced by the semiconductor due to the more severe recombination of charges. The (photoelectro)chemical corrosion reactions resulted in the dissolution of Bi, V, Mo and Co, which was accelerated by temperature. CuxO in N-doped graphdiyne (CuxO/N-GDY) catalyst for nitrate reduction to ammonia was tested at different temperatures in an anion exchange membrane electrolyzer after a preliminary optimization in a commercial H-cell. Operation at 40°C promoted the reduction reaction kinetics of NO3- to NH3, but it penalized the ammonia selectivity with respect to hydrogen evolution reaction. A physics-based model to simulate a combined hydrothermal liquefaction (HTL) of organic waste substrates, the complete oxidation of the biooil produced in a membraneless electrolyzer with a fluid dynamic separation of the streams while reducing water to hydrogen and the full-chain capture and sequestration of CO2 as solid carbonate salt was developed. Agricultural waste is the most suitable substrate due to its large fraction of lipids and it reaches a maximal integrated efficiency of 44% at 308 °C with a biomass to solvent ratio of 7.7%. The captured carbonate anions (CO32-) are sequestrated as solid carbonate salt by cooling down the solution below 0 °C and separating the liquid phase. A non-dimensional model was developed to evaluate the performance of a photocatalytic device for organic waste reforming