In recent years, a significant emphasis has been placed on the production of green hydrogen from water using renewable energy sources as researchers pursue efforts to achieve net-zero carbon emissions.
A versatile energy carrier and storage medium with no greenhouse gas emissions, hydrogen has gained attention as an alternative energy source for many sectors that have proven difficult to decarbonize. Despite the increased emphasis, green hydrogen production is still a developing area of research.
“Presently, electrolytic technologies that harness renewable electricity to produce hydrogen and oxygen by splitting water are experiencing rapid development and commercialization,” said Rohini Bala Chandran, assistant professor of mechanical engineering. “However, formidable cost and stability barriers persist.”
Bala Chandran recently collaborated with colleagues from universities around the world on a study that aims to address these challenges. The study, which was published in a high-impact journal, Energy and Environmental Science, examines a new modeling framework to predict the performance of sunlight-driven photocatalytic hydrogen production with water-stable metal oxide semiconductors as light absorbers.
In this process, sunlight is absorbed by the semiconductor materials, and used to split water molecules into hydrogen and oxygen. Bala Chandran noted that this type of hydrogen production has the potential to be cost-effective, but currently faces hurdles that prevent large-scale deployment.
“Most photocatalytic systems are operative at less than 1% solar-to-hydrogen efficiency,” she said.

However, to meet the cost target of green hydrogen of $1/kg by 2031 set by the Department of Energy Hydrogen Shot Initiative, solar-to-hydrogen efficiencies of photocatalytic systems need to be significantly improved.
“To overcome this efficiency hurdle, there is a crucial need to prevent undesired and competing reactions that are prone to occur in photocatalytic systems for water splitting,” Bala Chandran added.
She explained that there is a need for better predictions of these undesired reactions, which her team’s study aims to address. They have developed rigorous, yet simple models that link fundamental material properties, kinetics, and mass transfer behavior with the extent of undesired reactions and overall performance characterized by solar-to-hydrogen efficiencies.
Notably, her modeling framework incorporates new capabilities, going beyond state-of-the-art circuit models to incorporate effects of competing reactions, mass-transfer limitations, and the presence of multiple light absorbers. This is especially relevant in practical reactor concepts consisting of a suspension of many light absorbing semiconductor particles rather than a single electrode.
“Because we include capabilities to model competing reactions, we can now start to provide insights on how to avoid or mitigate these effects,” Bala Chandran said. “Particularly, what we show is that instead of only relying on manipulating catalysts and their surfaces to influence kinetic parameters, we can achieve reaction selectivities, and therefore high efficiencies, by also manipulating the mass transfer rates of select electrolyte species.”
For example, Bala Chandran explained that this model demonstrates that using selective coatings on photocatalysts could be one approach to enhance solar-to-hydrogen efficiency by selectively lowering mass-transport rates of the reactants for the competing reactions. To this end, Daniel Esposito, a co-author of the study from Columbia University, is working toward experimentally synthesizing and demonstrating the effectiveness of selective coatings in photocatalytic materials for water splitting.

“Another interesting finding from this work is that we show that when operating under conditions where mass-transfer rates are slow and rate-limiting, for instance in batch reactors, photocatalytic systems with multiple light absorbers can outperform single light absorber systems, provided the undesired reactions are kept in check,” Bala Chandran said.
Beyond application of the model to explore effects of various thermodynamic, kinetic, and transport parameters, model predictions have been compared with experimental data obtained from particle-suspension photocatalytic reactors to provide insights and guidance to help improve future photocatalysts, selective coatings, and reactor designs.
“This field of photocatalysis for solar hydrogen production is currently struggling with the lack of materials and reactor design to achieve large enough solar-to-hydrogen conversion efficiencies,” Bala Chandran said. “This makes it challenging to bring down production costs of H2 to be cost-competitive with the state-of-the-art production by reforming natural gas. Our research provides foundational and simple modeling tools, and early discoveries of strategies to boost efficiencies to address this challenge.”
This work was funded in part by the U.S. Department of Energy, Fuel Cell Technologies Office (Award No. DE-EE0008838). Simulation data, figures, and manuscript revisions were supported as part of Ensembles of Photosynthetic Nanoreactors (EPN), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science (Award No. DE-SC0023431).
Additional co-authors: Luisa Barrera of the University of Michigan, Bradley L. Wayne of the University of California Irvine, Zejie Chen of the University of California Irvine, Kenta Watanabe of the Tokyo University of Science and the Tokyo Institute of Technology, Akihiko Kudo of the Tokyo University of Science, Daniel V. Esposito of Columbia University, and Shane Ardo of the University of California Irvine.
Full citation: “Revealing the role of redox reaction selectivity and mass transfer in current–voltage predictions for ensembles of photocatalysts,” Luisa Barrera, Bradley L. Wayne, Zejie Chen, Kenta Watanabe, Akihiko Kudo, Daniel V. Esposito, Shane Ardo, and Rohini Bala Chandran, Energy and Environmental Science 21 (2024). DOI: https://doi.org/10.1039/D4EE02005G