It is becoming more and more evident that carbon emissions are causing the deterioration of the global climate and ecosystems across the globe. Hydrogen is expected to play a key role in meeting the need for clean fuel for both energy storage and transport purposes.
Electrolysis of water is a clean method of producing hydrogen fuel. Currently, existing alkaline water electrolysis plants operate at low current densities (around 0.25 A cm-2) and can only achieve energy efficiencies of around 60%. Proton exchange membrane (PEM) water electrolyser systems demonstrate a substantial increase in efficiency and are commercially available but expensive.
Our research at the University of Exeter aims to develop anion exchange membrane (AEM) electrolysers utilising non-precious metal catalysts to enable reduced over-potentials for electrode reactions compared to acidic systems, whilst maintaining improved efficiency.
The focus of the work is therefore on developing cost- effective, efficient, and stable catalysts for both oxygen evolution reaction at the anode and hydrogen evolution reaction at the cathode, the design and fabrication of membrane electrode assemblies (MEA) and the development of efficient AEM electrolysers. Ultimately, AEM electrolysers can lead to cheaper hydrogen production. Here in this exclusive article for H2 View, we share the scale of research projects currently underway at the University of Exeter as hydrogen continues to intensify in academia.
Figure 1 | An innovative anion-exchange membrane (AEM) zero-gap water electrolyser has been scaled up from 10cm2 to 100cm2 test systems. New non-precious metal electrocatalysts combined with inexpensive membranes are greatly increasing the potential of this technology.
Xiaohong Li: Professor and Chair of Energy Storage
Professor Xiaohong Li is currently researching how to use renewable energy to produce hydrogen. Hydrogen is used as a fuel in fuel cell electric vehicles and other energy storage devices and can be used in processes such as making ammonia for use in agriculture. At present, global hydrogen is produced using fossil fuels, known as ‘black hydrogen’; as opposed to the ‘green hydrogen’ which Professor Li makes using renewables.
A key aspect of Xiaohong’s work is to make the production of green hydrogen more efficient and affordable. This involves finding cheaper and more abundant metals to use as a catalyst in the production process. Traditionally, precious metals such as platinum or palladium have been used, but Professor Li’s work has investigated using more available metals such as nickel, iron, or cobalt to reduce the costs, and make the technology more available. The end aim of the research is to be able to demonstrate the technology, so that it can be adopted and scaled up by industry.
As Professor Li says, her role in the process is to ‘make the recipe’ for a technology that could soon help us all dramatically decrease our carbon emissions. It’s a future that Professor Li is optimistic about, and she remains enthusiastic that we can meet the energy challenge of the future. Her students are very passionate and engaged in the work she’s been doing, technology has come a long way in the last decade, and Government also appears to be taking the challenge seriously. In Professor Li’s words, energy storage is the greatest technology available to us today – if we can continue to grow and develop this technology, we may be able to look forward to a much greener future.
Jack Corbin: Development of hydrogen generation from seawater
Jack is a PhD student working as part of the Energy Storage group at the University of Exeter with a focus on developing hydrogen generation from seawater for green hydrogen production within an AEM electrolyser.
Direct seawater electrolysis (DSWE) offers the opportunity for green hydrogen production on a large scale while benefitting from reduced capital costs and a simplified one-step approach by removing the need for desalination or pre-treatment systems and reducing freshwater scarcity. Significant research is being directed to this DSWE as seawater demonstrates high electrical conductivity (due to its composition being rich in a variety of ions), with a relatively neutral pH that helps to reduce chloride oxidation and worldwide abundance (seawater accounts for 75% of the earth and 97% of surface water). Due to the novelty of the research area, there is a need to overcome some challenges, most of which occur at the anode during the reaction, relating to high volumes of negatively charged chloride anions present, which corrode catalysts and substrates due to metal chloride hydroxide formation.
Currently, Corbin is working on the oxygen evolution reaction (OER) and designing electrocatalysts that can withstand the harsh environment that DSWE presents, minimising chloride corrosion and unwanted side reactions. Modified Nickel materials have proven to show good stability and activity in seawater as the OER electrocatalyst as well as demonstrating a high level of chlorine corrosion resistance. The current analysis is focusing on the corrosion of bare metals to better understand corrosion characteristics, thus mitigating them when applying modifications to the electrode surface.
Cheng Lyu: Development of electrocatalysts for alkaline exchange membrane water electrolysers
Cheng Lyu is a PhD student working in the energy storage group in University of Exeter with the focus of the development of low-cost hydrogen evolution reaction (HER) catalyst for green hydrogen production in AEM water electrolyser.
Equipped with the zero-gap design, AEM water electrolysers shall reach a higher efficiency of hydrogen production. Different from PEM water electrolysers, AEM allows the employment of low-cost transition metal-based catalysts for its less corrosive working condition. The development and employment of low-cost HER catalysts in place of platinum group metal helps to reduce the cost for hydrogen production as well as improving the efficiency of water electrolysis.
Currently Lyu is focusing on the preparation of Ni-S based HER catalyst via electrodeposition method. Ni-S based catalysts have shown a promising HER catalytic performance in alkaline environment. Optimisation of the composition and operating conditions are carried out to further improve the performance. During this period, rotating disk electrode and microelectrodes are employed for a better characterisation of the electrochemical properties. Instead of just focusing on the half-cell performance, full cell tests are also to be carried out to simulate the actual working conditions to obtain stability data.
Mikey Jones: Development of perovskite electrocatalysts for the oxygen evolution reaction in alkaline zero-gap electrolysers
This research focuses on the development of perovskite electrocatalysts for the anodic side of the water electrolyser, where oxygen evolution occurs.
The oxygen evolution reaction (OER) is an inherently sluggish process due to the need for four electrons to be transferred for a molecule of oxygen to be evolved; in comparison, the HER requires the transfer of just two electrons per molecule of hydrogen evolved. The energy input, and thus the overall efficiency of the process, is dependent on the voltage that must be applied for the OER to occur; it is essential that the over-potential is kept as low as possible.
Perovskites are a group of compounds with the general formula ABX3, where A and B are positively charged metal ions and X is a negatively charged ion such as oxygen. Over 90% of elements in the periodic table can be incorporated into at least one of these A, B or X sites, resulting in a large family of compounds with highly tuneable physical, chemical and electronic properties and making them very promising candidates for OER electrocatalysts.
Acknowledgements
We acknowledge the European Interreg 2 Seas programme 2014-2020 co-funded by the European Regional Development Fund under subsidy contract No [2S03-019].
This was originally published in H2 Review in January 2023.
