Influence of Electrolyte on the Stability and Activity of WO3 for Photoelectrochemical Oxidation Reactions

Abstract

Metal oxide semiconductors provide a platform for absorption of solar energy to generate a chemical potential gradient, driving an uphill chemical transformation in a photoelectrochemical (PEC) reaction. However, the stability of the metal oxide material is also an important consideration. In this thesis, tungsten oxide (WO3 ) films were made by spin casting aqueous ammonium metatungstate solutions on fluorine tin oxide substrate. The tungsten oxide films were used to investigate water oxidation and chloride oxidation, two important reactions in solar fuel production. WO3 has generally been regarded as a poor choice of metal oxide light absorber for performing water oxidation due to degradation in the photocurrent density during normal operation. However in this work, electrochromic tungsten bronze impurities (HxWO3) were shown to oxidize during PEC water oxidation operation, resulting in a loss of donor density, causing the loss of photocurrent density observed. The regeneration of tungsten bronze donor species can induced by light in a photochromic mechanism, resulting in a recovery in the photocurrent density and demonstrates that photocurrent density loss during water oxidation is reversible. The rate of water oxidation was determined by detecting oxygen gas, the product of water oxidation, with flow cell gas chromatography. After photochromic regeneration of tungsten bronze donors, the rate of water oxidation recovered alongside the overall photocurrent. Because of the importance of tungsten bronze species in the PEC behavior of tungsten oxide, it is referred to as HxWO3 in the thesis chapters to better describe the material used. Tungsten bronze donor species are electrochemically active and suppressing the rate of donor oxidation will increase the stability in PEC photocurrent. To this end, chloride oxidation was explored as a possible alternative to water oxidation for solar fuel formation. Improvement in PEC photocurrent stability was observed in 0.5 M sodium chloride electrolytes compared to 0.1 M potassium phosphate electrolytes at the same pH. Chloride more rapidly injects electrons into the HxWO3 material, eliminating photogenerated holes before oxidation of tungsten bronze impurities can occur. The oxidation of donors was also prevented by applying an FeOOH electrocatalyst to the surface of the Hx WO3 because the electronic states of FeOOH are well positioned to inject electron density into HxWO3 , similar to chloride oxidation. Using chloride electrolytes with HxWO3 materials leads to higher stability in the photocurrent density during solar fuel producing reactions. The hypochlorous acid produced by Hx WO3 is a strong oxidant capable of performing many useful chemical transformations. The oxidation of aqueous primary and secondary alcohols was investigated using ethanol and 2-propanol and 1 M chloride electrolytes. The yield was monitored using 1 H-NMR. The yield of products was high for both ethanol and 2-propanol. Proton activity influenced the yield of acetic acid and acetone, but no influence was observed in the acetaldehyde yield. Because hypochlorous acid undergoes light induced homolytic cleavage, generating radical species, alcohol oxidation was also performed using a platinum electrode in the same electrolyte. Comparing the product yield of alcohol oxidation using platinum in the dark and under illumination, only the acetaldehyde yield exhibited a dependence on the illumination with no difference observed in the acetic acid and acetone yields. These observations were used to rationalize the mechanism of hypochlorous oxidation for the observed products.