By Jacob Przywolski, MSc in the Seferos group at the University of Toronto and GCI member at large
The future is going to be run on hydrogen fuels, or at least that is what many scientists are hoping to accomplish. Imagine a world where fossil fuels and their by-products won’t be filling the sky due to commuting cars in the mornings and evenings, and instead make odor-less water. Or imagine if your whole home could be powered by a hydrogen-filled crystal like in the movie Glass Onion, but much less explosive and more realistic. Now of course, these ideas are just highly idealized versions of using hydrogen as fuel, but many scientists have in fact expressed their thoughts that hydrogen is the ‘Holy Grail’ of green energy.1
Before any of this can happen, there are many factors that must be solved such as hydrogen production. The current method to make hydrogen through steam-methane reformation, which is cheap but is not environmentally friendly. It involves heating water and methane at high pressures to make hydrogen, but also releases carbon dioxide and carbon monoxide as by-products.2 Therefore, current research focuses on new methods of producing hydrogen so that one day we can replace the steam-methane reformation process.
The focus in chemistry for investigating this problem is largely through photocatalysis. This involves making molecular architectures which mimic photosynthesis – the conversion of light into chemical energy. These are made up of what are known as donor-acceptor architectures, where light can cause an electron to transfer from a photosensitizer to a catalyst, thus powering the hydrogen production reaction.
There are lots of research examples about these types of systems, such as exploring different catalysts (which are often Co-based) and different photosensitizers,3 in small molecule systems. Earlier examples include metal-based photosensitizers like Ru(bpy)3 (Figure 1a) because of their amazing light absorbing properties,4 but research has shifted towards cheaper and more environmentally friendly alternatives like organic dyes such as xanthenes instead (Figure 1b).5
Figure 1. (a) Linked metal photosensitizer-cobaloxime photocatalyst systems.4 (b) Free xanthene-based organic dye photosensitizer system showing a simplified photocatalytic process for hydrogen production with cobaloximes.5
A problem with these small molecules is that they may have issues with long-term stability and are harder to recycle. Therefore, there is a focus in making heterogeneous systems with high surface areas for this task instead, and there have been a lot of cool examples for this in the literature. A simple example (which isn’t really heterogeneous) is a water-soluble polymer with Co porphyrin catalysts as side chains and nearby CdSe nanoparticles as photosensitizers (Figure 2a).6 Another example are nanorods surrounded by BODIPY photosensitizers and cobaloxime catalysts held in close-contact with eachother (Figure 2b).7 One last example are conjugated covalent organic frameworks with high porosity and anchor sites for platinum nanoparticle catalysts to link to (Figure 2c).8
Figure 2. (a) A polymer system with appending Co porphyrins as catalysts with CdSe nanoparticle photosensitizers.6 (b) A crystallizable nanorod photocatalyst system with a corona featuring BODIPY photosensitizers in close-contact with cobaloxime catalysts.7 (c) A 2D covalent organic framework with anchored Pt-based nanoparticles.8
As research in this area continues, we are sure that more researchers will continue to come up with new and more innovative ways of producing green hydrogen, so that we can one day come up with a solution for removing steam-methane reformation once and for all!
References
(1) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42 (12), 1995–2004. https://doi.org/10.1021/ar900253e.
(2) Hydrogen Production: Natural Gas Reforming. Energy.gov. https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming (accessed 2023-11-05).
(3) Yang, L.; Fan, D.; Li, Z.; Cheng, Y.; Yang, X.; Zhang, T. A Review on the Bioinspired Photocatalysts and Photocatalytic Systems. Advanced Sustainable Systems 2022, 6 (5), 2100477. https://doi.org/10.1002/adsu.202100477.
(4) Dolui, D.; Khandelwal, S.; Majumder, P.; Dutta, A. The Odyssey of Cobaloximes for Catalytic H 2 Production and Their Recent Revival with Enzyme-Inspired Design. Chem. Commun. 2020, 56 (59), 8166–8181. https://doi.org/10.1039/D0CC03103H.
(5) Lazarides, T.; McCormick, T.; Du, P.; Luo, G.; Lindley, B.; Eisenberg, R. Making Hydrogen from Water Using a Homogeneous System Without Noble Metals. J. Am. Chem. Soc. 2009, 131 (26), 9192–9194. https://doi.org/10.1021/ja903044n.
(6) Xie, L.; Tian, J.; Ouyang, Y.; Guo, X.; Zhang, W.; Apfel, U.-P.; Zhang, W.; Cao, R. Water-Soluble Polymers with Appending Porphyrins as Bioinspired Catalysts for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2020, 59 (37), 15844–15848. https://doi.org/10.1002/anie.202003836.
(7) Tian, J.; Zhang, Y.; Du, L.; He, Y.; Jin, X.-H.; Pearce, S.; Eloi, J.-C.; Harniman, R. L.; Alibhai, D.; Ye, R.; Phillips, D. L.; Manners, I. Tailored Self-Assembled Photocatalytic Nanofibres for Visible-Light-Driven Hydrogen Production. Nat. Chem. 2020, 12 (12), 1150–1156. https://doi.org/10.1038/s41557-020-00580-3.
(8) Zhao, Z.; Zheng, Y.; Wang, C.; Zhang, S.; Song, J.; Li, Y.; Ma, S.; Cheng, P.; Zhang, Z.; Chen, Y. Fabrication of Robust Covalent Organic Frameworks for Enhanced Visible-Light-Driven H2 Evolution. ACS Catal. 2021, 11 (4), 2098–2107. https://doi.org/10.1021/acscatal.0c04820.
The Green Chemistry Initiative Blog 