Catalyst lifetime is a primary technical bottleneck obstructing Cu-based CO₂ reduction (CO₂R), with restructuring via dissolution-redeposition being a commonly reported reason for selectivity loss. Here we examine how atomistic restructuring manifests at the microlevel of gas diffusion electrode (GDE)-based systems, ultimately compromising long-term CO₂R performance. Using a flow-cell CO₂R electrolyzer configuration and a copper-coated PTFE GDE, we first show how voltage gradients result in directional in-plane copper migration and porosity changes, causing a decrease in CO and ethylene production due to blocked catalyst pores. By the incorporation of different ionomer and inert carbon overlayers onto copper, we then demonstrate how in-plane degradation is mitigated by modulating the local pH and voltage homogeneity of the electrode, extending ethylene lifetimes by 10-fold. Ultimately, through-plane compaction of copper then becomes the limiting degradation pathway. Combined, these results provide rationale for the paradox of why copper degradation in membrane-electrode assemblies illustrates 100-fold greater stabilities than H-cell and flow-cell architecture. ... Read more