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

氢作为燃料将在未来的脱碳经济中发挥关键作用。然而，为了实现这一目标，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%来自甲烷，释放二氧化碳）。最有希望的途径是通过水电解（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）。这一过程的瓶颈是析氧反应（OER），因为这是一个复杂的、多步骤的电化学反应。通过选择合适的出氧电极材料，可以加快这一反应。有些材料在促进反应方面比其他材料更好，因此在相同的能量输入下，反应发生得更快——我们把这些材料称为电催化剂。我们迫切需要高效的电催化剂来提高电解槽的效率，从而将绿色氢的生产成本降低到低于化石燃料衍生氢的生产成本。然而，可供选择的材料范围非常有限，因为在水分解过程中电化学电池的阳极是一个腐蚀性极强的环境，而且大多数材料根本无法存活足够长的时间来发挥作用。这项提议的目的是探索一类新的材料，这种材料最近被称为高熵硫化物（HES），它被认为是氧演化的电催化剂。这些材料是由5种或5种以上的金属以大致相等的比例与等量的硫混合而成的。HES中的元素共享相同的晶格，金属随机分布在晶格中，这使得它们具有非常高的无序性或熵。与直觉相反，这种熵赋予HES极高的耐腐蚀性，这意味着它可以在严酷的电解条件下保持所需的稳定性。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同一个晶格，我们可以使材料承受很大的压力，压力的大小可以通过我们选择的元素来调节。这种应变反过来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——这两者对于材料的电催化性能都是至关重要的。我们相信，HES的耐腐蚀性能，加上几乎无限的调节材料特性的能力，意味着HES可能会改变氧电催化的游戏规则。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。HES上析氧反应的反应机理尚不清楚，晶格应变与材料性能之间的确切关系也不清楚。我们建议使用一种新的薄膜合成技术来快速合成各种高熵硫化物以供测试。然后，我们可以制定方案来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。到本项目结束时，我们计划对HES作为电催化剂的理解产生一个阶梯式的变化，并有一套全面的设计原则来设计出最活跃和最稳定的析氧反应电催化剂。

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.