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

为了应对气候变化和全球变暖，我们必须将全球碳足迹减少到最低限度。这一令人难以置信的具有挑战性的目标意味着使用可持续的能源和原材料，近年来已成为研究的热点。在这种背景下，燃料电池再次蓬勃发展，特别是在远程汽车方面，因为零二氧化碳排放。然而，氢气和氧气的可持续和高效生产仍然是一个悬而未决的问题，仍然是一项长期的重大努力。事实上，大多数氢气是从天然气或煤炭等化石资源中生产出来的，但也有来自使用不可再生电力的水电解。因此，显然迫切需要生产可持续的氢气。为了解决这个问题，光化学分解水提供了一种难以置信的可能性，可以从取之不尽的太阳能中生产氢气和氧气。因此，间歇性的阳光很容易转化为化学能载体，用于储存、运输和最终利用。更重要的是，将水分离过程与燃料电池的过程相结合，只产生可反馈到水分离方案中的水，无疑是一个真正可持续的过程。在析氢方面，已经开发了大量的光催化材料，但没有一种材料能够大规模生产氢气。在SunHy项目中，我们的目标是通过模仿大自然在树叶中的方法来开发高效、低成本的光催化质子还原系统。事实上，我们建议探索基于金属有机光系统的解决方案，这些光系统仅含有地球上丰富的元素，如铁和钴，分别用于捕光和氧化还原催化。虽然钴对氢气产生的显著催化性能早已被证明，但直到最近，铁的光敏剂一直难以捉摸。此外，含有贱金属的组件几乎没有被描述，它们的光物理和光化学行为仍然完全未知。为此，SunHy项目的联合体汇集了公认的化学设计和合成方面的专业知识，并将基于最近取得的突破来制备新的Fe/Co杂化体系，其激发态寿命延长到ns范围。此外，有关表征技术的专业知识和先进的超快光谱学，涵盖中红外到X射线领域，不仅可以了解组件的工作原理，还可以在化学重新设计时对其进行改进。因此，SunHy项目的预期成果是一种新型的合理设计的光催化分子，用于节能生产氢气，为长期大规模实际应用铺平道路。

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.