Case studies Chemical Engineering and Chemistry 2015-2021

Appendix B: Case studies 9 B.3. ENGINEERING REACTORS AND DEVICES FOR HYDROGEN PRODUCTION Context Nowadays, European energy and transport systems are mainly based on fossil energy carriers, leading to major concerns regarding energy supply security, climate change, air pollution and the increasing prices of energy services. In the EU Sustainable Development Scenario, wind and solar will supply more than half of the total electricity in advanced economies by 2050, with renewables reaching 80% of the total generation in the EU. An increasing share of renewables in the energy mix will strongly influence the flexibility requirements of the grid and will require adequate energy storage solutions to cope with the availability of renewable sources. Amongst energy storage solutions, hydrogen produced from electrolysis offers great promise as a flexible energy carrier that offers a perspective for both short and long-term energy storage. H 2 has the potential to ensure inter-seasonal energy storage, solving energy supply issues related to dark doldrums, for instance. Additionally, hydrogen can be transported with minimum losses, especially when stored in liquid energy carriers like ammonia. A major challenge with hydrogen produced by water electrolysis is that it is still relatively expensive. This high price is not only driven by the price of electricity but also by the capital costs of electrolyzer plants. The latter is especially relevant when the electrolyzer is operated on the basis of an excess renewable electricity supply, implying that it will not be operational all year round. The leading electrolysis techniques are alkaline and proton exchange membrane (PEM) electrolysis. PEM is a relatively new technology with high flexibility and compactness. Yet it depends on several critical raw materials (especially iridium), which is expected to be a major hurdle in its scale-up. In contrast, alkaline electrolysis is a technology that is already over a hundred years old. It is cheap but bulky and less flexible than PEM. Newer technologies such as anion exchange membranes (AEM) hold the promise of combining the advantages of alkaline and PEM but still need significant development. Our research The Sustainable Process Engineering group is developing improved alkaline and AEM electrolyzers that can achieve the same performance as PEM electrolyzers without the use of critical raw materials. This can be achieved by making improvements to the design of alkaline electrolysis, including the use of thinner separators and more active electrocatalysts, the optimization of the operating parameters (temperature and pressure) and better cell designs. A challenge is that the design of electrolyzers has traditionally been carried out in a highly empirical way with a very limited fundamental understanding of key aspects such as mass transport and the influence of bubbles on the performance of electrolysis. It is therefore difficult to predict to what extent improvements will enhance electrolyzer performance. One key ‘mystery’ is that the ohmic resistance in alkaline electrolyzers is much higher than what one would expect based on the properties of the separator. The implications of this high ohmic resistance are significant as it limits the operational current density of the electrolyzer and is hence responsible for the fact that alkaline electrolyzers are so ‘bulky’.