High Entropy Sulfides as Corrosion Resistant Electrocatalysts for the Oxygen Evolution Reaction

氢气将在未来的脱碳经济中发挥关键作用。然而，要实现这一目标，必须找到大规模生产氢气而不排放二氧化碳的方法（目前生产的所有氢气中有95%以上来自甲烷，释放二氧化碳）。最有希望的方法是通过水电解（在浸入水中的两个电极之间施加电压，将水分子在一个电极上分解为氢，在另一个电极上分解为氧）。该过程中的瓶颈是析氧反应（OER），因为这是一个复杂的多步电化学反应。通过适当选择析氧电极的材料可以加速该反应。有些材料比其他材料更能促进这一反应，因此在相同的能量输入下，反应发生得更快--我们将这些材料称为电催化剂。高效的电催化剂是提高电解槽效率的迫切需要，从而将绿色氢气的生产成本降低到化石燃料氢气的生产成本以下。然而，可供选择的材料范围非常有限，因为在水裂解过程中，电化学电池的阳极是一种非常腐蚀性的环境，大多数材料根本无法存活足够长的时间。该提案旨在探索一类新的材料，这些材料最近显示出作为析氧电催化剂的前景，称为高熵硫化物（HES）。这些是由5种或更多种金属以大致相等的比例混合沿着等量的硫制成的材料。HES中的元素共享相同的晶格，金属随机分布在整个晶格中-使它们具有非常高的无序或熵。这种熵，违反直觉，赋予羟乙基淀粉异常高的耐腐蚀性，这意味着它可以拥有所需的稳定性，以生存的恶劣条件下的电解。此外，材料的无序状态为我们提供了定制材料特性以优化催化活性的机会。通过迫使许多不同大小的不同原子共享相同的晶格，我们可以将材料置于很大的应变下，应变的大小可以通过我们选择的元素来调整。这种应变反过来又会对材料的电子行为以及溶液中的分子如何与表面相互作用产生深远的影响，而这两者对材料的电催化性能至关重要。我们相信，HES的耐腐蚀性，加上几乎无限的材料性能调节能力，意味着HES可能成为氧电催化的游戏规则改变者。然而，在真正探索和优化这些材料之前，必须提高对这些材料电化学行为的基本理解。在HES上的析氧反应的反应机理是完全未知的，因为是晶格应变和材料properties之间的确切关系，我们建议使用一种新的薄膜合成技术，快速合成各种高熵硫化物测试。然后，我们可以开发方案来稳健地测试和比较它们的电催化活性和稳定性。最后，我们将使用一系列光谱表征技术来了解晶格应变和电子结构之间的相互作用，以及HES中哪些元素参与了电催化反应。我们计划制作一个步骤-改变我们对HES作为电催化剂的理解，并拥有一套全面的设计原则来设计最活跃和最稳定的电催化剂用于氧析出反应。

Hydrogen will play a pivotal role as a fuel in a future decarbonised economy. However, for this to be realised, methods must be found to produce hydrogen on a vast scale with no CO2 emissions (>95% of all hydrogen currently produced is from methane, releasing CO2). The most promising route to do this is via water electrolysis (applying a voltage between two electrodes immersed in water to split the water molecules into hydrogen at one electrode and oxygen at the other). The bottleneck in this process is the oxygen evolution reaction (OER) as this is a complicated, multi-step electrochemical reaction. This reaction can be sped up by the appropriate choice of material for the oxygen-evolving electrode. Some materials are better than others at facilitating this reaction, and hence allow the reaction to happen faster with the same energy input - we refer to these materials as electrocatalysts.Efficient electrocatalysts are desperately needed to increase the efficiency of electrolysers and therefore reduce the cost producing of green hydrogen below that of fossil-fuel-derived hydrogen. However, there is an extremely limited range of materials to choose from, as the anode of an electrochemical cell during water splitting is an exceptionally corrosive environment and most materials simply will not survive long enough to be useful.This proposal aims to explore a new class of materials which have very recently shown promise as electrocatalysts for oxygen evolution, known as High Entropy Sulfides (HES). These are materials made of 5 or more metals mixed in roughly equal proportions along with an equivalent amount of sulfur. The elements in a HES share the same crystal lattice and the metals are randomly distributed throughout this lattice - giving them a very high level of disorder, or entropy. This entropy, counterintuitively, confers the HES exceptionally high corrosion resistance, meaning it can possess the required stability to survive the harsh conditions of electrolysis. Furthermore, the disordered state of the material offers us opportunities to tailor the material properties to optimise catalytic activity. By forcing many different atoms of different sizes to share the same crystal lattice, we can place the material under a lot of strain, the amount of which is tuneable by our choice of elements. This strain can in turn have a profound impact on the electronic behaviour of the material and how molecules from the solution interact with the surface - both of which are critical for the electrocatalytic properties of the material.We believe that the corrosion resistance of HES, coupled with the almost limitless ability to tune the material properties mean that HES could be a game-changer for oxygen electrocatalysis. However, before these materials can really be explored and optimised, the fundamental understanding of the electrochemical behaviour of these materials must be improved. The reaction mechanism for the oxygen evolution reaction on HES is completely unknown, as is the exact relationship between lattice strain and material properties.We propose to use a novel thin-film synthesis technique to rapidly synthesise a wide range of high entropy sulfides for testing. We can then develop protocols to robustly test and compare their electrocatalytic activity and stability. Finally, we will use a range of spectroscopic characterisation techniques to learn about the interplay between lattice strain and electronic structure and which of the elements within the HES are participating in the electrocatalytic reaction.By the end of this project, we plan to have produced a step-change in our understanding of HES as electrocatalysts and have a comprehensive set of design principles to design the most active and stable electrocatalyst for the oxygen evolution reaction.