(Invited) Challenges and Opportunities of Alkaline Water Electrolysis

Abstract

Alkaline water electrolysis has been operated on 100+ MW scale in the 20th century powered by hydropower. Because of this history, the technology is often regarded as a mature technology with limited improvement potential that is less suitable for flexible operation. As a result most research efforts in the field of water electrolysis focus on other technologies such as PEM, solid oxide and AEM electrolysis. This is a pity, since alkaline electrolysis is very well positioned to be deployed on large-scale for the production of green hydrogen in the coming decades. Main reason is that in contrast to the other technologies, alkaline technology does not depend on scarce and/or expensive noble or rare earth metals. Also, the use of a diaphragm instead of a membrane makes the technology more robust.Yet, to enable the large-scale deployment of alkaline electrolysis for production of green hydrogen from renewable electricity certain technical challenges need to be overcome. These include safety challenges associated with rare scenarios where hydrogen and oxygen can mix, flexibility limitations associated with gas crossover limiting the minimum load of the plant, and durability challenges resulting from frequent shutdowns of electrolyzers. In principle all these challenges can be overcome by rigorous plant design and the selection of suitable diaphragm and electrode materials, but they do require research efforts.Next to that, there is room to significantly drive down the costs of green hydrogen produced by alkaline electrolyzers. Traditionally, alkaline electrolyzers have been operated at low current densities of 0.2-0.4 A cm-2. This was related to the high ohmic drop associated with the use of thick diaphragms and non-zero-gap cell designs. Presently, advanced alkaline electrolyzers are under development that can operate at significantly higher current densities (~1 A cm-2) through the use of thinner diaphragms, zero-gap cell designs and improved electrode materials. Operation at these increased current densities increases the output of alkaline electrolyzers and in this way can effectively reduce the costs and footprint of water electrolysis plants. Yet, the use of thinner diaphragms and advanced electrodes also creates new challenges, since they can lead to increased gas crossover and vulnerability to impurities and reverse currents that occur upon the shutdown of electrolyzers.In this presentation the fundamentals of alkaline water electrolysis will be discussed. These include overpotentials and durability of electrode coatings, the role of electrolyte impurities, ohmic resistance, gas crossover through the diaphragm, the influence of operating temperature and pressure, the effect of bubbles, and the influence stray currents resulting from the manifold design. These fundamentals will be translated into a potential design for the perfect alkaline electrolyzer.Figure:alkaline electrolyzers operational in Norway in 1931. Source: Norsk Industriarbeidermuseum Figure 1