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Enhanced Water-splitting for Hydrogen Production

Nowadays, hydrogen fuel is often produced from substances where hydrogen is only one component among other elements. Some of the substances hydrogen is obtained from, like methane, have the drawback of being harmful to the environment. To separate hydrogen from water, the oxygen evolution reaction (OER) can be used. OER generates molecular oxygen through a limiting chemical reaction, such as oxidation of water during oxygenic photosynthesisor electrolysis of water into oxygen and hydrogen. Efficiently steering this reaction is important not only for hydrogen production, but also for chemical processes in batteries.

Now (2021), scientists at the Argonne National Laboratory have discovered that perovskite oxides exhibit a shape-shifting quality under certain circumstances. Perovskite oxides are a promising material for enhancing the OER. They are usually comprised of an alkaline earth metal or lanthanides such as lanthanum and strontium in the A-site, and a transition metal such as cobalt in the B-site. In previous research the scientists had concentrated more on the bulk properties of perovskite materials and their influence on OER activity. However, they had soon realised that their attention should focus more on the surface properties of the material as they are not only quite different from the rest of the material, but also experience certain changes over time. Once the perovskite oxides are part of an electrochemical system, where they can interact with an electrolyte made of water and special salts, the perovskite surface undergoes certain changes and turns into a thin, amorphous film. To examine the exact mechanisms of these changes during the OER, the scientists applied theoretical calculations and experiments, which comprised tuning a lanthanum cobalt oxide perovskite by enhancing it with more reactive strontium. They found that the amount of strontium added significantly influenced the reactivity of the surface. They also discovered that strontium dissolution and oxygen loss from the perovskite were the driving forces behind the formation of the amorphous surface layer. Finally, they explored how small amounts of iron present in the electrolyte would influence the reaction because only recently analyses had revealed that small traces of iron were able to improve the OER on other amorphous oxide surfaces. They found that iron did the same for perovskite oxides.

Producing hydrogen through water splitting has been at the centre of scientific attention for many years. In 2016, for example, scientists generated hydrogen through a photoelectrochemical reaction with the help of films of exfoliated 2-dimensional (2D) MoS2. The film of chemically-exfoliated MoS2 layers started the water splitting reaction and led to hydrogen generation. The study demonstrated that 2D MoS2 was a promising material as a photocatalyst for light-induced hydrogen generation. The efficient photoelectrocatalytic property of the 2D MoS2 was caused by catalytically-active edge sites, as well as minimal stacking that could increase the electron transfer.

In 2020, scientists managed to produce hydrogen by means of water electrolysis through microwave-induced redox activation of solid-state ionic materials at low temperatures (<250°C). Water was brought to react with gadolinium-doped CeO2 that had been electrochemically deoxygenated by application of microwaves. The microwave-driven water reduction led to an instant rise in electrical conductivity and O2 release. Deoxygenation of low-energy molecules (H2O or CO2) caused the formation of energy carriers and enabled CH4 production when combined with a Sabatier reactor.

Shedding a light on the shape-shifting qualities of perovskite oxides has several advantages: perovskite oxides can easily start the OER while being less expensive than precious metals such as iridium or ruthenium also used for this purpose. However, perovskite oxides are not as active and efficient at keeping the OER going as the aforementioned metals, and they show a tendency to slowly degrade. However, the study has proved that these effects could be mitigated if the perovskite structure was fortified with strontium and iron. Moreover, when iron was present in the electrolyte, the iron was able to accelerate the OER, while being stabilised by cobalt-rich film which also supported its activity.

The findings of this study pave the way for creating new perovskite martials which are able to promote the OER even more efficiently. This, in turn, would enhance the efficiency of energy conversion and storage as well as make it more cost-effective.