Improvement Strategies for Photoelectrochemical Water Splitting at n-Type Oxide Semiconductors: A Case Study of WO3–Based Systems
Authors: Oskam Gerko , Rodriguez Gutierrez Ingrid , Rodríguez Pérez Manuel , Vega Alberto , Rodriguez Gattorno Geonel , Vayssieres Lionel , Su Jinzhan , Souza Flavio L. 
Published: December 2020

The attraction of n-type metal oxide (MO) semiconductors for photoelectrochemical water splitting lies in their inherent stability, specifically against oxidation. Hence, n-type MO photoelectrodes are suitable for solar water splitting systems, especially in tandem configurations. The semiconducting MO must absorb sunlight efficiently, the valence and conduction band edges must be situated at appropriate energies, and the electrochemical reaction must have favorable kinetics in order to be able to compete with charge separation, transport and recombination kinetics in the MO photoelectrode.
Many n-type MO photoelectrodes have been evaluated for photo-oxidation of water in the past 10 years, including Fe2O3, WO3, BiVO4, CuWO4, TiO2, etc. In order to improve the efficiency of each material, the specific causes for the lower than desired efficiency need to be elucidated. Based on these insights, strategies need to be designed to improve performance. However, it is not straightforward to determine the precise loss mechanisms and generally non-steady state methods need to be applied. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell: the photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge separation, charge transfer and surface recombination may be determined, depending on the kinetic model can may be applied to the specific system [1].
We report on the results from IMPS studies of the charge transfer and recombination dynamics in two systems, where strategies have been applied in order to increase the conversion efficiency of WO3 nanomaterials [2-4]: (i) a WO3 nanowire array was either partially or completely converted to CuWO4 using a surface treatment; then, the array was covered with a thin BiVO4 film [3]; (ii) a planar, compact WO3 / BiVO4 heterojunction systems was compared with the nanorod array WO3 / BiVO4 heterojunction, with and without a hole scavenger. The results indicate the importance of the balance of the variety of kinetics parameters that determine performance. The best results were obtained for the WO3 nanoarray / BiVO4 heterojunction system, reaching a water oxidation photocurrent of about 2 mA cm-2 at sufficiently positive potentials [4].
This work was performed with the help of Dr. Antonio Zapien within the NANOMXCN initiative to foment scientific collaboration between Mexico and China. The authors gratefully acknowledge funding from CONACYT, SENER, and CICY through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4).

References

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“Charge Transfer and Recombination Kinetics at WO3 for Photoelectrochemical Water Oxidation”. Manuel Rodríguez-Pérez, Ingrid Rodríguez-Gutiérrez, Alberto Vega-Poot, Rodrigo García-Rodríguez, Geonel Rodríguez-Gattorno, Gerko Oskam. Electrochim. Acta, 258, 900-908 (2017).

“Photoelectrochemical Water Oxidation at FTO|WO3@CuWO4 and FTO|WO3@CuWO4|BiVO4 Heterojunction Systems: An IMPS Analysis”. Ingrid Rodríguez-Gutiérrez, Essossimna Djatoubai, Manuel Rodríguez-Pérez, Jinzhan Su, Geonel Rodríguez-Gattorno, Lionel Vayssieres and Gerko Oskam. Electrochim. Acta, 308, 317-327 (2019).

“An intensity-modulated photocurrent spectroscopy study of the charge carrier dynamics of WO3/BiVO4 heterojunction systems”. Ingrid Rodríguez-Gutiérrez, Essossimna Djatoubai, Jinzhan Su, Alberto Vega-Poot, Geonel Rodríguez-Gattorno, Flavio L. Souza, Gerko Oskam. Sol. Energ. Mater. Sol. Cells, 208, 110378 (11 pag.) (2020).