(Invited) Multiscale Evaluation of Light Absorption, Transport and Kinetics in Z-Scheme Photocatalytic Reactors for Solar Hydrogen Production

Abstract

Z-scheme photocatalytic particles in an aqueous solution with soluble redox shuttles and present in vertically stacked reaction compartments present a promising pathway for solar water splitting to produce hydrogen1–4. Key advantages of this approach include efficient utilization of the incident solar spectrum with light-absorbers that are optically connected in series while facilitating hydrogen and oxygen separation. In recent work, we theoretically demonstrated the feasibility of attaining up to 4% solar-to-hydrogen efficiencies with state-of-the-art photocatalysts and redox shuttles with passive diffusion-driven species transport5. To attain larger solar-to-hydrogen efficiencies, it is crucial to achieve selective and functional interfaces that can inhibit undesired reactions of the redox shuttle, and enhanced rates of redox shuttle transport between the two reaction compartments. Herein, I report on the development of a multiscale model-based approach, supplemented with experiments, to evaluate the coupled effects of light absorption, species transport, reaction kinetics and heat transfer on the overall performance. At the zero-dimensional scale (or the lumped system), we have developed an expanded equivalent-circuit modeling framework with notable innovations to model competing redox reactions, in addition to the desired reactions, while also accounting for mass-transfer limitations. Predictions from this model provides important insights for the design of selective coatings to mitigate competing redox reactions and identifies key physical parameters that are crucial to optimize for the efficiency of hydrogen production. Next, at the length scale of an individual particle, we assess the effects of particle size and the transport and kinetic parameters on the internal and the external quantum yields. Finally, at the reactor-scale with ensembles of particles, we assess whether natural convective currents can aid in redox shuttle transport. Two-dimensional, coupled flow and heat-transfer models of the reactor have been developed in conjunction with experimental measurements in a benchtop reactor to obtain the spatio-temporal temperature and concentration profiles. On the whole, these results provide new insights and strategies to realize highly effective materials-to-device scale designs for Z-scheme photocatalytic solar fuel reactors with soluble redox shuttles. References Pinaud, B. A. et al. Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry. Energy Environ. Sci. 6, 1983 (2013).Fabian, D. M. et al. Particle suspension reactors and materials for solar-driven water splitting. Energy Environ. Sci. (2015) doi:10.1039/C5EE01434D.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).Wang, Q. et al. Scalable water splitting on particulate photocatalyst sheets with a solar-to-hydrogen energy conversion efficiency exceeding 1%. Nat. Mater. 15, 611–615 (2016).Bala Chandran, R., Breen, S., Shao, Y., Ardo, S. & Weber, A. Z. Evaluating particle-suspension reactor designs for Z-scheme solar water splitting via transport and kinetic modeling. Energy Environ. Sci. 11, 115–135 (2018).