Porosity as a Morphology Marker to Probe the Degradation of IrO₂ Anode Catalyst Layers in Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane (PEM) electrolyzers are key devices for the production of green hydrogen through the electrolysis of water. Their performance and durability heavily rely on the stability of the anodic layers, typically composed of IrO 2 nanoparticles, employed to facilitate the oxygen evolution reaction. Understanding the degradation of the IrO 2 anodic layers under operation in electrolyzers is thus of utmost importance to design more robust anodic materials and enhance their long-term performance. The degradation of anodic layers can be attributed to various phenomena. While Ir dissolution in electrolyzers has been previously studied, the effect of structural changes in IrO 2 anodic layers remains elusive and difficult to prove. This is because conventional IrO 2 anodic layers exhibit a disordered and poorly defined structure, making it difficult to clearly identify any structural changes in anodic layers. In this work, we introduce the idea that ordered porosity can be used as a characteristic structural signature to unveil the morphological evolution in anodic layers during operation in electrolyzers. More specifically, we build anodic layers made of IrO 2 -based spheres with a well-defined and ordered macroporosity that can be easily identified by electron microscopy before and after electrolysis. To validate the principle, several materials exhibiting different crystallinity and stability were tested in electrolyzers. We systematically probed the evolution of performance, the Ir dissolution, and the change in morphology during or after operation. By mapping the evolution of the porosity in the layer, we reveal that the largest morphological degradation occurs in the zone in contact with the Ti fibers of the porous transport layer (PTL). This result suggests that the anodic layer/PTL interface represents the weakest part of the anodic layer, indicating that additional effort can be devoted to the optimization of the PTL design. More broadly, the concept of using porosity as the structural signature could be generalized to follow the degradation of other materials and other electrochemical devices beyond electrolyzers.