| Abstract | A photoelectrosynthetic solar cell is a device that converts light energy into chemical potential energy by driving an overall endothermic reaction, such as benzyl alcohol dehydrogenation. To catalyze these reactions, visible light-absorbing metal oxide semiconductor materials such as BiVO4, a-Fe2O3, and WO3 are often utilized. This work describes the fabrication and study of nanostructured BiVO4-based photoanodes to optimize the capture and conversion of light energy to chemical potential energy. Two approaches are explored for improving the photoelectrochemical (PEC) performance of BiVO4-based photoanodes: 1) increasing the surface area of the BiVO4 interface through the use of a structure directing agent (polyethylene glycol) and 2) the use of a heterojunction WO3|BiVO4 interface.
To investigate the hypothesis that increased BiVO4 surface area would improve the photocurrent generated, the concentration of polyethylene glycol (PEG), used as a templating agent, was changed in the BiVO4 deposition solution. Specifically, this study describes the fabrication of FTO|BiVO4 surfaces from solutions containing varying PEG concentrations by weight (2.5% w/w and 5% w/w) to determine the relationship between porosity and the resultant photocurrent generation. These nanostructured photoanodes were used to measure the oxidation of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), via PEC analysis. While the concentration of polymer did increase the porosity and surface area of the BiVO4 films, decreased photocurrents were observed for high surface area films compared to those with smaller surface areas.
The second approach to improving PEC performance involves establishing a type II heterojunction interface using sequential deposition of WO3 and BiVO4 layers on the electrode surface. Based on the conduction band alignment of these materials, photoexcited electrons from BiVO4 should transfer to WO3 while holes accumulate at the BiVO4-electrolyte interface. Information about the efficiency of light energy conversion to chemical energy was obtained by incident photon-to-current (IPCE) measurements. IPCE values are determined by relating the proportion of incident light power to the current produced by illuminating the WO3-BiVO4 photoanode over a small wavelength range. IPCE experiments were run for WO3-only, BiVO4-only, and WO3-BiVO4 samples. It was hypothesized that the WO3-BiVO4 heterojunction will increase the conversion of light energy into chemical energy. Comparing IPCE values for WO3-BiVO4 samples shows a clear increase compared to BiVO4-only photoanodes. These results demonstrate how coupled materials (WO3-BiVO4) can generate higher current densities upon illumination for driving photoelectrosynthetic reactions. Overall, these two approaches provide valuable insight into the optimization of BiVO4-based photoanodes for use in photoelectrosynthetic solar cells. |
