On the Promise of Photocatalysis for Solar Hydrogen Production: A Materials-to-Reactor Design Perspective

In the global pursuit of net-zero carbon emissions, ultra-low-emissions hydrogen (green hydrogen) production from only water and renewable resources is crucial. Presently, electrolytic technologies that produce hydrogen and oxygen by splitting water are experiencing rapid development and commercialization. However, formidable cost and stability barriers persist. Alternatively, sunlight-driven, photocatalytic hydrogen production with low-cost and water stable metal oxide semiconductors has the potential to be cost-effective, but faces low efficiency issues. Photocatalysts are conceptually simple reaction systems where nano- and micro-scale semiconductors integrated with catalysts act as light absorbers to drive a pair of redox reactions on illumination. However, undesired chemical reactions that occur due to the close proximity of anodic and cathodic reaction sites, and ineffective charge-carrier separation severely limits light-to-fuel conversion efficiency. My presentation will highlight our research efforts to better understand, predict and control reaction selectivity to reduce undesired reactions in photocatalysis. We focus on nature-inspired, Z-scheme photocatalytic systems that employ a two-step water splitting process with a soluble redox salt to relay electrons between oxygen and hydrogen evolving light absorbers. Our recent work demonstrates with simple, yet powerful circuit models, that manipulating mass transfer of select redox species can curtail the undesired reactions and improve selectivity towards hydrogen (and oxygen) production. This is a notable discovery in light of conventional expectations that rely on kinetic asymmetry in electrocatalysis to achieve reaction selectivity. Additionally, I will report on multiscale model predictions and experimentally measured performance of particle-suspension reactors for photocatalytic water splitting. Our results showcase that even with well-known materials such as TiO₂, there is an interesting interplay between particle surface charge, pH of the solution, and temperature on the reaction selectivity and the overall light-to-fuel energy conversion efficiencies. Finally, I will conclude by providing our perspectives and insights on strategies to improve the performance and stability of photocatalytic materials and reactors, and critically assess pathways towards meeting the US Department of Energy Hydrogen Shot cost targets of $1 per kg of H₂ by 2031.