Out-of-Lab Solar Photocatalytic Hydrogen Production in the Presence of Methanol Employing the Solar Concentrator SoCRatus

Abstract

Hydrogen production from water via efficient solar based photocatalytic or photoelectrochemical processes could play a major role in the energy regimes of the future. Here, intermittent solar energy is converted into the promising energy vector hydrogen for later carbon free use on demand. Although much effort has been made in the last years photocatalytic/photoelectrochemical systems with acceptable solar-to-hydrogen-efficiency for economic operation could not be introduced, yet. Within the project DuaSol simultaneous hydrogen generation and water treatment in a photoelectrochemical tandem cell is investigated as a potentially economic process. Organic contaminants are oxidised by interaction with photo-generated electron holes at the photoanode. Produced protons approach the photocathode to react with photo-generated electrons to form hydrogen. Experiments with photocatalytic systems employing DLR’s 2-axis tracking modified linear Fresnel solar concentrator SoCRatus (Solar Concentrator with a Rectangular Flat Focus) were carried out in order to set a reference for the further experimental assessment. Diverse photocatalysts based on titanium dioxide (TiO2) and tin niobate (SnNb2O6) were tested in a planar suspension reactor with two parallel reaction chambers irradiated in the focal plane of the SoCRatus. The evolution of hydrogen was measured and correlated to the overall solar input and to spectral quantities. Three temperature levels, mostly 25°C, 37.5°C, and 50°C, were considered and maintained during the experiments in order to study temperature related effects. Methanol as a sacrificial reagent or rather a model substance for organic contaminants formed part of the suspension with a volume fraction of 10% at 20°C. As expected regarding the band gaps of the considered TiO2 based photocatalysts the hydrogen output is predominately affected by the applied UV portion. The UV fraction of solar light varies significantly in the course of a day and coherently also the production of hydrogen. Hydrogen was generated at rates as high as 7386 µmol/h. Regarding the SnNb2O6 based photocatalysts the generation of hydrogen rather corresponds with the irradiance in the visible range. The solar-to-hydrogen efficiency as well as the photon efficiency in different spectral ranges could be calculated. In addition an extensive analysis of the uncertainty of experimental results was conducted. It could be confirmed that the SoCRatus is an excellent platform for the experimental assessment of photocatalytic / photoelectro-chemical systems under practical conditions.