Investigating the origin of improved photoelectrochemical performance in BiVO4-based composite systems for solar water splitting using pump-probe spectroscopy Louise Oldham 1 , Tianhao He 1 , Daniele Benetti 1 , Andreas Kafizas 1,2 and James R. Durrant
1 Department of Chemistry, Imperial College London, UK, 2 The Grantham Institute, Imperial College London, UK
Solar fuels provide an important solution in the global transition to net zero, by allowing solar energy to be stored and transported in chemical form. In particular, hydrogen production from photoelectrochemical (PEC) water splitting is a promising approach for clean production of solar fuels. However, the kinetics of water oxidation at the photoanode limit the performance of PEC devices significantly. Understanding photoanode materials and how to control charge dynamics is thus essential for the development of more effective PEC systems. Due to their relatively high abundance, stability in aqueous environments and resistance to photooxidation, metal oxides have been widely studied as photoanode materials. Among these, BiVO 4 has emerged as a leading material, in large part because its relatively narrow bandgap (2.4eV) gives it a high maximum theoretical photocurrent. However, its high recombination rate limits its practical performance. 1 In order to address this challenge, various approaches have been suggested, with one of the most important being the formation of composite materials. This can involve surface modification with electrocatalysts such as NiFeOx 2 , or the formation of type II (staggered) heterojunctions 3 . A key example of a successful composite photoanode is WO 3 /BiVO 4 /CoPi, which demonstrates the record performance for a BiVO 4 -based system. 4 As the improved PEC performance of composite materials can be due to different processes, understanding the underlying mechanisms of performance enhancement is crucial for informing photoanode design. In the Durrant group, we use pump-probe spectroscopic techniques to investigate the charge carrier dynamics of PEC systems. In transient absorption spectroscopy (TAS), a laser pulse photoexcites the system and in photoinduced absorption spectroscopy (PIA) a longer LED pump is used to mimic steady state conditions. By examining the competing pathways for photogenerated charges, we can gain insight into the underlying mechanisms that govern the performance of PEC systems. In this study, TAS and PIA are utilised to investigate the charge carrier dynamics of BiVO 4 -based systems, with an emphasis on how the formation of composite materials enhances performance. Observations of improved charge separation or reduced recombination rates can be correlated with key PEC characteristics such as photocurrent, fill factor and photovoltage to understand the origin of improved performance in composite materials. This can provide insights to inform design of higher performance and higher stability PEC devices. References 1. Y. Ma, S. R. Pendlebury, A. Reynal, F. Le Formal & J. R. Durrant, Chem. Sci ., 2014, 5 , 2964–2973. 2. L. Francas, S. Selim, S. Corby, D. Lee, C. A. Mesa, E. Pastoe, K-S. Choi & J. R. Durrant, Chem. Sci .,2021, 12 , 7442-7452. 3. S. J. Hong, S. Lee, J. S. Jang & J. S. Lee, Energy Environ. Sci ., 2011, 4 , 1781-1787. 4. Y. Pihosh, I. Turkevych, K. Mawatari, J. Uemura, Y. Kazoe, S. Kosar, K. Makita, T. Sugaya, T. Matsui, D. Fujita, M. Tosa, M. Kondo and T. Kitamori, Sci. Rep. , 2015, 5 , 11141.
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