Insights into electrolyte flooding in flexible gas diffusion electrodes for CO2 electrolysis: from mechanisms to effective mitigation strategies

The advancement of CO2 electrolysis has reached a stage where practical CO2 electrolysers show promise for high conversion rate, low manufacturing cost, and extended system durability. While gas diffusion electrodes (GDEs) as flexible cathodes play a pivotal role in flow cell electrolysers, a prevalent issue arises with the implemented GDEs. Electrolyte flooding refers to the infiltration of bulk liquid electrolyte into the GDEs' gas diffusion channels. This typically occurs when the hydrophobicity of the GDEs towards the electrolyte diminishes, and then lowers the conversion efficiency and hence forecloses the durability of CO2 electrolysis. Compared to a proven track record of reporting substantial advancements in various novel catalysts, there is a scarcity of publications addressing the fundamental challenge of electrolyte flooding. In this review, the recent advancements in flexible GDEs for CO2 electrolysis are summarized, covering the evolution of different GDE types used in CO2 electrolysis and the current design trends in various flow cell electrolysers. In addressing the critical challenge, valuable insights into the fundamental mechanisms of triggering electrolyte flooding and in situ or ex situ approaches to observe flooding are discussed. A key segment of this review covers a comprehensive summary and evaluation of state-of-the-art methods of mitigating electrolyte flooding in flexible GDEs for CO2 electrolysis, considering three distinct perspectives within flow cell electrolysers: each layer of GDEs, properties of the membrane, and operating conditions. Finally, we discuss the remaining challenges and propose prospective research directions for alleviating the persistent issue of electrolyte flooding.