Harnessing the power of water to produce clean, renewable hydrogen energy is a critical mission for humanity. Given the abundance of oceans on earth, and with freshwater supplies already depleted, the photocatalytic splitting of seawater offers vast potential for a sustainable hydrogen economy as the world transitions away from fossil fuels.
The photocatalytic process of water-splitting uses sunlight to break down water molecules into hydrogen and oxygen gases with a light-absorbing catalyst. But in the past, using seawater has been limited by poor energy conversion efficiencies, instability, and the negative effects of electrolytes in seawater at room temperature. In addition, high electricity consumption and desalination capital costs have impaired technological advancements.
However, in January 2024, breakthrough results published in Nature Catalysis revealed for the first time that the natural ionic composition of seawater can improve photocatalytic efficiency at elevated temperatures (around 270°C) without additional sacrificial reagents, addressing negative electrolyte effects and instability in seawater [1].
The research involved scientists from the University of Oxford (UK), East China University of Science and Technology (PR China), Hong Kong Polytechnic University (PR China), and Diamond Light Source (UK).
Experimental techniques included near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) deployed on Diamond’s Versatile Soft X-ray (VerSoX) Beamline [2] to monitor the surface charge polarisation among different crystal facets of the photocatalyst. A novel method of employing a probe molecule was utilised to explore photocatalyst surface charge accumulation.
The focus of the research was to understand how electrolyte-assisted charge polarisation could influence the photocatalytic performance of nitrogen-doped titanium dioxide (N-TiO2) in seawater (N-TiO2 is a semiconductor and a versatile photocatalyst). This type of polarisation could increase the lifetime of charged species on the surface of the photocatalyst for splitting water into hydrogen and oxygen.
Co-author Professor Edman Tsang, who heads up the Tsang group at the University of Oxford’s Wolfson Catalysis Centre, explained: "It is known that sunlight can instantly generate charge species (positively charged ‘holes’ and negatively charged ‘electrons’) in semiconducting materials but there is also rapid recombination to release heat. In electrolyte-assisted polarisation, electrons will accumulate temporarily on one facet of semiconductor and holes on another facet before the majority of them rapidly recombine and shed thermal energy."
Experiments conducted at high temperature revealed that a Na+ ion from seawater (the main electrolyte in seawater is Na+Cl-) attracts electrons on a facet of a semiconductor and a Cl- ion attracts holes during illumination.
The results demonstrate that N-TiO2, under electrolyte-assisted polarisation, can achieve a higher solar-to-hydrogen conversion efficiency of 15.9% at 270°C, surpassing previous conversion efficiencies of < 5% using other photocatalysts. Ionic species in seawater were also found to prolong the charge-carrier lifetime, significantly enhancing photocatalytic activity.
The Tsang group led testing and development of the facet-controlled N-TiO2 photocatalysts, in collaboration with other groups. Extensive characterisations explored the interactions between ionic species and photocatalyst surfaces.
Diamond’s B07C beamline was used to study charge separation and distribution on the photocatalyst’s surface at an atomic level. B07C is ideal because it specialises in providing gas delivery systems that deliver the well-defined compositions of gases a catalyst might be exposed to, using a computer-controlled environment that can be managed remotely.
The surface-sensitive NAP-XPS technique and trimethylphosphine (TMP) as the surface probe were deployed to investigate the surface charge transfer/accumulation on different crystal facets of the photocatalyst at different temperatures, ranging from 150 to 270 °C.
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Co-author and B07 Principal Beamline Scientist Professor Georg Held explained: "Basically, we were investigating the water-splitting reaction, which is the Holy Grail of the hydrogen economy. Charge transfer is a common way of explaining reactivity at surfaces but very difficult to visualise. What’s unique here is using TMP as a probe molecule. TMP is a phosphor atom with three carbon atoms with hydrogen atoms attached to them. The phosphor atom tends to only occupy the active side of the catalyst surface so when used for experiments like these, we are looking at a molecule that is stuck to an active side and by monitoring how it reacts to light it tells us the story about the charge transfer."
Professor Held uses the analogy of dyes in medical diagnostics to illustrate the novel method of using TMP as a probe. Such dyes mark only cancerous or non-cancerous cells, but in this case the molecule only attaches itself to active catalyst sites - the parts of the surface where the chemistry happens. When a light is shone onto these areas, the electrons coming from the phosphor can be monitored and the charge transfer visualised directly.
Professor Tsang said: "Our results show the best energy conversion, and the highest photo hydrogen production with seawater over our optimised photocatalytic semiconductor materials. We achieved a steady hydrogen evolution rate comparable to that of laboratory-scale electrolysers, using only sunlight and seawater – without sacrificial reagents; we have developed a system that operates well at high temperature; and we have proved these aspects theoretically and experimentally for the first time. An excellent research team and great collaborations, together with access to cutting edge facilities, made these exciting results possible. Georg Held and Alex Large at Diamond provided us with valuable expertise and support using NAP-XPS, which is a very powerful technique that enabled us to see and verify charge separation and understand and confirm our proposed mechanism. They also helped us identify the best materials for our research, which is ongoing. This work would not have been successful without their efforts."
He continued, “Our collaborators at East China University of Science and Technology assisted with modelling to verify our proposal, which proved very successful, and the group from Hong Kong Polytechnic University provided high resolution electron microscope images that helped us to visualise the charge separation among different crystal facets. Our colleagues in Physics at the University of Oxford’s Clarendon Lab helped us characterise our materials and optimise our methods to produce powerful light to model the system.”
Currently, only steady-state information can be observed, therefore the plan is to further develop the TMP-probed NAP-XPS technique at B07C to enable the monitoring of the dynamics/kinetics (time-dependence) of photo-generated charge carriers at different temperatures.
At Diamond, Professor Held and his team have designed a miniature reaction cell for the part of the end station that is exposed to phosphor and are working on measuring the surface dynamics. The Diamond II upgrade – planned for 2028/29 – will make it possible to do time-resolved experiments at millisecond time scales or even lower.
“For this kind of experiment, what we need is flux,” Professor Held explained. “With Diamond-II measurements will be faster, enabling pump probe experiments where surface reactions are measured rapidly as the light (the ‘pump’) is switched on and off in very quick succession. In the meantime, we look at different types of reaction and different catalysts and explore a variety of probe molecules.”
In terms of commercialisation, patent applications on behalf of Oxford University Innovation (OUI) have been made and Global Impact Ventures Inc. (GIVE) has received exclusive licensing rights from OUI to exploit and deploy the technology. Concept plans have been drawn up for a commercial scale solar farm to produce green hydrogen from thermal assisted photocatalytic splitting of seawater.
“Ultimately, the goal is to promote energy security through decentralised, local production facilities, paving the way for exploration into scaling up the technology for industrial applications,” said Yiyang (Bruce) Li, Senior Postdoctoral Research Associate in the Tsang group.
“This includes scaling-up the technology with focused solar mirrors that track the sun to provide intense light and heat which can be used and stored for hydrogen production or other applications; or generating renewable electricity to create artificial sunlight when natural sunlight is not available.”
Electrolyte-assisted polarisation could also be expanded and integrated into other material and reaction systems, such as desalination of seawater by evaporating water into superheated steam using photothermal energy. On cooling, superheated steam produces pure water, addressing freshwater scarcity and environmental pollution issues at the same time. Electrolyte-assisted charge polarisation is also applicable in artificial salty solutions or wastewater.
In other research, the Tsang group has explored prolonging the lifetime for charge species of semiconductors generated by sunlight by electric/magnetic fields. The green hydrogen produced by photo splitting could be used as clean fuel via combustion or producing electricity by fuel cells.
Alternatively, hydrogen combining with nitrogen from our atmosphere can be converted to ammonia, which can be another energy vehicle for transportation [3,4,5].
“Our findings are not only scientifically significant but also socially and environmentally impactful, aligning with global efforts toward sustainability,” said Professor Tsang. “The world cannot rely on fossil fuels; we must move to renewables. It is also very good to contribute to these efforts and involve and train students and graduates in energy research. We are now a major step forward in using natural resources to generate clean energy efficiently and to offer hope for a future with reduced carbon emissions and enhanced energy security.”
To find out more about the B07C beamline please contact Professor Georg Held, the Principal Beamline Scientist at: georg.held@diamond.ac.uk
[1] Li, Y., Zhou, H., Cai, S. et al. Electrolyte-assisted polarization leading to enhanced charge separation and solar-to-hydrogen conversion efficiency of seawater splitting. Nat Catal 7, 77–88 (2024). https://doi.org/10.1038/s41929-023-01069-1
[2] https://www.diamond.ac.uk/Instruments/Structures-and-Surfaces/B07.html accessed on 27.09.2024. See also: Held et al. (2020). J. Synchrotron Rad. 27, 1153-1166, https://doi.org/10.1107/S1600577520009157
[3] Electric-/magnetic-field-assisted photocatalysis: Mechanisms and design strategies Zihan Wang, Yiyang Li, Chen Wu, Shik Chi Edman Tsang, Joule (Cell Press), 2022/7/19. https://doi.org/10.1016/j.joule.2022.06.018
[4] Local magnetic spin mismatch promoting photocatalytic overall water splitting with exceptional solar-to-hydrogen efficiency, Yiyang Li, Zihan Wang, Yiqi Wang, András Kovács, Christopher Foo, Rafal E Dunin-Borkowski, Yunhao Lu, Robert A Taylor, Chen Wu, Shik Chi Edman Tsang, Energy & Environmental Science https://doi.org/10.1039/D1EE02222A, 2022.
[5] Energy Decarbonization via Green H2 or NH3? Simson Wu, Nicholas Salmon, Molly Meng-Jung Li, René Bañares-Alcántara, and Shik Chi Edman Tsang. ACS Energy Lett. 2022, 7, 3, 1021–1033 https://doi.org/10.1021/acsenergylett.1c02816
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