BiVO4-based photoanodes for photoelectrochemical water splitting

Abstract

Global warming refers to the rise in average surface temperatures on Earth and is considered as the major environmental problem humanity is facing today. An overwhelming scientific consensus maintains that climate change is primarily caused by the combustion of fossil fuels, which releases carbon dioxide and other greenhouse gases into the atmosphere. Consequently, this gives rise to a range of severe changes in the planet’s ecosystem. For this reason, humanity urgently needs a clean energy supply. This can be provided by the use of hydrogen as energy carrier which solely leaves energy and water when it is combusted. An indispensable prerequisite for the environmental friendly use of hydrogen is a sustainable hydrogen generation. Direct photoelectrochemical (PEC) water splitting using sunlight as energy source is considered as the Holy Grail of sustainable hydrogen production. In this context, one of the most promising photoanode materials for the PEC water oxidation reaction is the n-type semiconductor bismuth vanadate (BiVO4) which exhibits a range of beneficial material properties for this approach. However, the main drawback of this material is the poor bulk conductivity which limits its overall PEC performance. In this thesis, principles to overcome the sluggish electron transport properties of BiVO4 are presented. Anion doping, i.e. partial O/F substitution by fluorination of BiVO4 powder samples is demonstrated to be a viable method to improve the PEC behavior. Significant enhancement of the PEC performance is revealed for the fluorinated BiVO4 compared to its pristine counterpart. Furthermore, the development of a new bottom-up synthesis approach for the direct deposition of BiVO4 thin films is demonstrated. The new synthesis allows the adjustment of the photoanode design to the materials properties. Combining the new synthesis method with cation doping and optimizing the photoanode morphology as well as PEC-relevant properties, unleashes tremendous PEC performance regarding water oxidation accounting for photocurrent densities up to 4.6 mA/cm² at 1.23 V vs RHE (illumination of white light, 400-700 nm at 100 mW/cm² at neutral pH). Additionally, the concepts of anion and cation doping were combined to form F/Mo:BiVO4 thin film photoanodes. The co-doped BiVO4 photoanodes exhibit enhanced PEC performance in terms of photocurrent densities accounting for 5.4 mA/cm² at 1.23 V vs RHE (illumination of white light, 400-700 nm at 100 mW/cm² at neutral pH); a higher photocurrent density has not been reported so far for a single-material photoanode regarding water oxidation. With the new synthesis method in hands, the preparation of a well-performing WO3/BiVO4 heterojunction photoanode was enabled. In a first step, a WO3 sol was used for dip coating an FTO substrate resulting after subsequent calcination in a WO3 thin film homogeneously covering the rough FTO substrate morphology. Thereafter, the WO3 thin film was coated with a BiVO4 thin film in a second synthesis step. Photocurrent densities of ~ 6.8 mA/cm² at 1.23 V vs RHE (illumination of white light, 400-700 nm at 100 mW/cm² at neutral pH) were obtained for the heterojunction photoanode which easily outperforms heterojunction photoanodes of comparably facile design and also competes well with the best performing WO3/BiVO4 photoanodes reported of advanced nanostructure.