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Z-scheme photocatalytic particles in an aqueous solution with soluble redox shuttles present a promising pathway for solar water splitting to produce hydrogen. Tandem light absorbers more efficiently utilize the incident solar spectrum, and physically separating hydrogen and oxygen production sites offers a safe reactor design. Theoretically, these reactor designs are projected to achieve competitive solar-to-hydrogen efficiencies as large as 34% with idealized light absorbers, state-of-the-art hydrogen, and oxygen evolution catalysts, selective reactions, and rapid mass transfer rates1. However, practical demonstrations have been limited to significantly lower efficiencies that are less than 1%. In part, this effect is attributed to competing and undesired redox reactions occurring at the particle–cocatalyst–electrolyte interface. In this work, we develop and apply an expanded equivalent circuit model to test the effectiveness of coatings to achieve reaction selectivity by manipulating redox species transport to functional interfaces2. This model formulates light absorbers as photodiodes and accounts for the effects of kinetic and mass-transfer resistances for both the desired and competing redox reactions. Transport properties in the coatings are parametrized as a function of its thickness, species permeabilities, and solubility coefficients. Mass transport rates are computed as a function of species concentration and concentration boundary layer thickness. Additionally, spatial profiles of light intensity in a reactor with multiple light absorbers are evaluated for an ensemble of (a) optically thin semitransparent slabs, and (b) particles that can absorb and scatter incident light. Results reveal the interplay of light absorption behavior, coating transport and kinetic properties, and mass transfer rates on the predicted solar-to-hydrogen efficiencies. An optimum concentration of light absorbers is shown to maximize light absorption while minimizing the extent of competing reactions. Results are further interpreted to provide insight on design principles for selective coatings to maximize efficiency. Overall, the modeling framework provides new capabilities with a tractable approach to traverse the multidimensional dependencies of various parameters on the solar-to-hydrogen efficiencies for Z-scheme photocatalytic systems. References Keene, S., Bala Chandran, R. & Ardo, S. Calculations of theoretical efficiencies for electrochemically-mediated tandem solar water splitting as a function of bandgap energies and redox shuttle potential. Energy Environ. Sci. 12, 261–272 (2019). Barrera, L. & Bala Chandran, R. Harnessing Photoelectrochemistry for Wastewater Nitrate Treatment Coupled with Resource Recovery. ACS Sustain. Chem. Eng. 9, 3688–3701 (2021). |
