(Invited) A Coating Strategy for Heterogeneous Photocatalysis Producing Renewable Fuels
Authors: Hu Shu 
Published: July 2022

Photocatalysts coevolve reductive and oxidative reactions in close proximity. Due to simplified reactor implementation, photocatalysis promises solar fuels production at scale. Despite decades of study, their rates and selectivity were often improved by trial and error, and their solar-to-fuel conversion efficiencies remain much lower than the theoretical limit. I will discuss an emerging coating strategy to stabilize particulate photocatalysts in a photo-reactor that promises solar energy utilization at scale. Those photocatalysts coevolve reductive and oxidative reactions in close proximity, and they potentially overcome the scale-up challenge by photoelectrochemical panels.

I will first introduce the Hu-lab invented oxide coatings to protect semiconductors, such as silicon and gallium indium phosphide, and achieve efficient and durable photocatalysis. We elucidate the coupled multi-phase processes, including charge separation, charge transfer, and chemical transport across multiple scales. We will show that the local electrochemical potentials of conduction-band electrons and the branching ratios of local charge transfer kinetics under multiple pathways are mutually dependent, and how charge transfer kinetics and surface energetics sensitively determine the charge separation behavior.[1]

Based on the holistic understanding of the photophysical, electrocatalytic, and transport processes coupled at the nanoscale, we employ stabilization coatings to coevolve H2 at a record rate of 48.5 mmol∙h-1∙g-1 or 2.5 mL H2∙h-1∙cm-2 under 1-sun solar illumination in ambient air.[2] Additionally, the discovery of new coatings offers the opportunity to tune the local energetics, kinetics, and reaction environments of supported co-catalysts. Manipulation of the electronic defect energetics enables the semiconductor photoabsorbers of 1.1 – 2.3 eV with sufficient band energetics.

Coated photocatalysts can perform H2 evolution, water oxidation, and can further achieve CO2 reduction reactions combining with CO2 capture.[3] Recently, Berlinguette and others showed a CO2 electrolyzer for directly converting dissolved bicarbonates into CO2-reduction products.[4] The analogy in photocatalysis is to locally drive pH swing to release CO2 at the oxidative sites, whereas the nearby reductive sites reduce in-situ generated CO2 into CO2R products. We show that in the presence of quinone redox couples in a bicarbonate solution, CO is produced with a 1-atm CO2-free headspace where the only source of CO2 is the (bi)carbonate anions.[6] We envision the direct solar fuels production from natural resources such as sunlight, bicarbonates from the ocean, or moisture in the air in a durable particle reactor.[5]

References:

[1] Zhenhua Pan, Yanagi Rito, Q. Wang, X. Shen, Q. Zhu, Y. Xue, J. A. Rohr, Takashi Hisatomi, Kazunari Domen, and Shu Hu, “Mutually-dependent kinetics and energetics of photocatalyst/ co-catalyst/two-redox liquid junctions”,
Energy & Environmental Science
, 13, 162–173 (2020). doi: 10.1039/C9EE02910A

[2] T. Zhao, R. Yanagi, Y. Xu, Y. He, Y. Song, M. Yang, and S. Hu, “A Coating Strategy to Achieve Effective Local Charge Separation for Photocatalytic Coevolution”,
Proceedings of National Academy of Sciences
, 16, 119(7) e2023552118 (2021). doi: 10.1073/pnas.2023552118

[3] J. Tang, D. Solanki, T. Zhao, and S. Hu, “Selective Two-Electron Hydrogen Peroxide Conversion Tailored by Surface, Interface, and Device Engineering,”,
Joule
, 6, 1432 – 1461 (2021). doi: 10.1016/j.joule.2021.04.012.

[4] Li, T.; Lees, E. W.; Zhang, Z.; Berlinguette, C. P. Conversion of bicarbonate to formate in an electrochemical flow reactor. ACS Energy Lett
2020,
5 (8), 2624-2630. doi: 10.1021/acsenergylett.0c01291

[5] X. Shen, S. Hu, et al., “Comprehensive Evaluation For Protective Coatings: Optical, Electrical, Photoelectrochemical, and Spectroscopic Characterization”,
Frontier in Energy Research
.