Sunlight-driven water splitting offers the potential to store and transport transient solar energy in the form of stable chemical bonds as hydrogen. Existing technologies for solar water splitting—multijunction photovoltaic systems coupled to water electrolyzers and photoelectrochemical cells—have demonstrated efficiencies up to 18%, but are not cost-competitive with hydrogen from fossil fuels on an energy density basis. Particle-suspension reactor designs consisting of microscale photocatalysts suspended freely in the electrolyte are projected to be cost-competitive with hydrogen from gasoline and hence motivate this study. Of specific focus here is a vertically stacked, particle-suspension reactor (see figure below) evolving hydrogen (HER) and oxygen (OER) in separate compartments in the presence of a redox mediator/shuttle via a Z-scheme mechanism. Porous separators between the two reactor vessels facilitate ion transport while attenuating crossover of product gases. The tandem design facilitates the use of wide bandgap semiconductors in the top compartment and relatively narrow bandgap materials in the bottom compartment. A transient, two-dimensional, computational model has been developed to simulate the coupled light absorption, species transport and electrochemical reactions in a unit cell of the proposed reactor. The model is used to identify reactor dimensions that reduce mass transport limitations of the redox shuttle and maximize incident light absorption. Sensitivity of model results to optical properties of the photocatalyst and species diffusivities are studied. Results indicate sustainable solar-to-hydrogen conversion efficiencies of at least 1% for a chosen reactor design in the absence of forced convection, which is not only feasible with state-of-the-art photocatalytic materials but is close to a value that could be reasonably cost competitive with hydrogen from fossil sources.

Acknowledgments: This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and University of California, Irvine under award no DE-EE0006963.