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

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.