Photocatalysts based on Earth-abundant metals: towards a sustainable production of solar hydrogen

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In an attempt to fight against climate change and global warming, our global carbon footprint must be reduced to a minimum. This incredible challenging objective implies the use of sustainable sources of energy and raw materials, having become an intense focus of research in recent years. In this context, fuel cells are booming again, especially in long-range auto-mobility, since zero CO2 emissions are produced. However, the sustainable and efficient production of hydrogen and oxygen is still an open question and remains a major long-term endeavour. Indeed, most hydrogen is produced from fossil resources such as natural gas or coal, but also from water electrolysis that uses non-renewable electricity. Therefore, there is a clear and urgent need for generating sustainable hydrogen. To tackle this problem, photochemical water splitting offers an incredible possibility of producing hydrogen as well as oxygen from the inexhaustible solar energy. Thus, intermittent sunlight would be easily converted into chemical energy carriers for its storage, transportation and eventual utilisation. More importantly, the association of the water splitting process with that of a fuel cell, producing only water that would be fed back into the water splitting scheme, is undoubtedly a truly sustainable process. Concerning hydrogen evolution, a great deal of photocatalytic materials has been developed, but none of them would allow for a large-scale hydrogen production. In the project SunHy, we aim at developing efficient low-cost photoactive systems for photocatalytic proton reduction by mimicking Nature’s approach in leaves. Indeed, we propose exploring solutions based on metalorganic photo-systems bearing only Earth-abundant elements such as iron and cobalt for light-harvesting and redox catalysis, respectively. While the remarkable catalytic properties of cobalt for hydrogen generation have long been demonstrated, iron photosensitizers have been elusive until very recently. Moreover, assemblies bearing base metals have been barely described, and their photophysical and photochemical behaviour is still completely unknown. To this end, the consortium of the SunHy project gathers recognised expertise in chemical design and synthesis, and novel Fe/Co heterosystems will be prepared based on the recently achieved breakthroughs of Fe(II) and Fe(III) complexes with extended excited state lifetimes up to the ns range. Furthermore, expertise regarding characterisation techniques together with advanced ultrafast spectroscopy, covering the mid-IR to X-ray domains, will allow not only to understand the working principle of the assemblies but also to improve it upon chemical redesign. Thus, the expected outcome of the SunHy project is a new class of rationally designed photo-catalytic molecules for energy-efficient production of hydrogen to pave the way for long-term large-scale practical applications.