Catalytic electrode comprising a gas diffusion layer and bubble-involved mass transfer in anion exchange membrane water electrolysis: A critical review and perspectives

Production of green hydrogen through water electrolysis powered by renewable energy sources has garnered increasing attention as an attractive strategy for the storage of clean and sustainable energy. Among various electrolysis technologies, the emerging anion exchange membrane water electrolyser (AEMWE) exhibits the most potential for green hydrogen production, offering a potentially cost-effective and sustainable approach that combines the advantages of high current density and fast start from proton exchange membrane water electrolyser (PEMWE) and low-cost catalyst from traditional alkaline water electrolyser (AWE) systems. Due to its relatively recent emergence over the past decade, a series of efforts are dedicated to improving the electrochemical reaction performance to accelerate the development and commercialization of AEMWE technology. A catalytic electrode comprising a gas diffusion layer (GDL) and a catalyst layer (CL) is usually called a gas diffusion electrode (GDE) that serves as a fundamental component within AEMWE, and also plays a core role in enhancing mass transfer during the electrolysis process. Inside the GDEs, bubbles nucleate and grow within the CL and then are transported through the GDL before eventually detaching to enter the electrolyte in the flow field. The transfer processes of water, gas bubbles, charges, and ions are intricately influenced by bubbles. This phenomenon is referred to as bubble-associated mass transfer. Like water management in fuel cells, effective bubble management is crucial in electrolysers, as its failure can result in various overpotential losses, such as activation losses, ohmic losses, and mass transfer losses, ultimately degrading the AEMWE performance. Despite significant advancements in the development of new materials and techniques in AEMWE, there is an urgent need for a comprehensive discussion focused on GDEs, with a particular emphasis on bubble-associated mass transfer phenomena. This review aims to highlight recent findings regarding mass transfer in GDEs, particularly the impacts of bubble accumulation; and presents the latest advancements in designing CLs and GDLs to mitigate bubble-related issues. It is worth noting that a series of innovative bubble-free-GDE designs for water electrolysis are also emphasized in this review. This review is expected to be a valuable reference for gaining a deeper understanding of bubble-related mass transfer, especially the complex bubble behavior associated with GDEs, and for developing innovative practical strategies to advance AEMWE for green hydrogen production.