In the anodic catalyst layer of a proton-exchange membrane (PEM) water electrolyzer, the triple-phase boundary (TPB) is mainly distributed on the surface of ultrafine iridium-based catalysts encapsulated by the ionomer within the catalyst-ionomer agglomerate. It is found that the ionomer at the TPB acts as a barrier to mass transport and a buffer for the bubble coverage during the oxygen evolution reaction (OER). The barrier effect can decrease the OER performance of the catalysts inside the agglomerate by ≤23%, while the buffer effect can separate the bubble evolution sites from the OER sites, turning the instant deactivation caused by the bubble coverage into a gradual performance loss caused by local water starvation. However, this local water starvation still deteriorates the catalyst performance because of the affinity of the ionomer surface for bubbles. Introducing additional transport paths into the agglomerate can reduce the barrier effect and regulate the bubble behavior, reducing the overpotential by 0.308 V at 5 A cm-2.