(Invited) MEA Technologies for PEM Electrolyzers: Overview and Perspectives

Abstract

Hydrogen has gained increasing attention recently as an alternative energy carrier leading towards a sustainable hydrogen economy [1]. Green hydrogen has less pollution (water as the only byproduct) and higher energy density than its counterpart fossil fuels. Additionally, it has much higher refueling speed than lithium-ion batteries, which makes it’s more suitable for mobility applications.Proton exchange membrane (PEM) water electrolysis is one of the most popular approaches for green hydrogen generation due to its high efficiency and high differential pressure operation capability [2]. The membrane electrode assembly (MEA) is the core of a PEM electrolyzer in which water molecules are spilt into hydrogen and oxygen molecules. In this overview, the challenges and paths regarding the MEA materials, operation/diagnosis/maintenance, and final recycling will be discussed.An electrolyzer MEA contains an anode and cathode made of platinum group metal (PGM) catalysts, a perfluorosulfonic acid (PFSA) membrane, and other gasketing materials. PGM catalysts are a major source of the cost of the PEM electrolyzer stack. Ir black based catalysts have been used for decades in the PEM electrolyzer industry. Due to the high cost and scarcity of Ir, efforts have been dedicated to reducing the catalyst loading and developing highly active catalysts [3]. The membrane is another major component of a PEM electrolyzer. 5 mil and 7 mil thick membranes have been used in electrolyzer industries. However, the ionic resistive losses of these relatively thick PEMs significantly reduce the efficiency of the MEA. Hence, reducing membrane thickness has been a central topic for PEM electrolyzer membrane development. With the progressive thinning of the PEM, the hydrogen gas crossover will become significant, which will compromise the safe operation of a PEM electrolyzer system. An effective gas recombination layer in the PEM is therefore urgently needed to mitigate the resulting higher hydrogen crossover. Additionally, the mechanical stresses imposed due to operation at high differential pressure operation (up to 40 bar) increases the attractiveness of a reinforcement layer.The unique properties of the membrane and catalysts in an MEA also require ultra-pure feed water. However, this cannot always be guaranteed. Metal ions will leach out from system plumbing and the DI system can’t always effectively remove all ions from tap water. The contamination will also poison the catalysts in an MEA. It is therefore critical to establish a universal method to efficiently identify the issue and recover the MEA performance.It is expected that the MEA can last 60k to 90k hours [4]. Though functionality may be compromised at the this point, the value in the PGM catalysts, and to a lesser extent membrane, and other carbon-based materials [5] remains. MEA recycling thus needs to be considered at the very early stage of the large-scale PEM electrolyzer application. Tradition calcination approaches for catalyst refinery is not optimized for PEM MEAs due to the presence of the PFSA membrane. The burning of MEAs will not only waste costly PFSA material but also poses a threat to the environment due to carbon and fluoride emissions. Current research efforts have focused on catalyst recycling and reuse/repurpose of PFSA based materials.[1] Oliveira, A. M., Beswick, R. R. & Yan, Y. A green hydrogen economy for a renewable energy society. Current Opinion in Chemical Engineering 33, 100701 (2021).[2] Ayers, K. et al. Perspectives on low-temperature electrolysis and potential for renewable hydrogen at scale. Annual review of chemical and biomolecular engineering 10 (2019).[3] Mittelsteadt, C. (Invited) Ir Strangelove: Or How I Learned to Stop Worrying and Embrace the PEM. ECS Meeting s MA2022-01, 1335-1335 (2022).[4] Schmidt, O., Future cost and performance of water electrolysis: an expert elicitation study. International journal of hydrogen energy 42 (2017), 30470-30492.[5] Sverdrup, H. U. & Ragnarsdottir, K. V. A system dynamics model for platinum group metal supply, market price, depletion of extractable amounts, ore grade, recycling and stocks-in-use. Resources, Conservation and Recycling 114, 130-152 (2016)