The H2 crossover was measured sequentially at current densities of 50, 100, 150, 200, 300, 400, and 500 mA cm -2 . At each current density, the setpoint was maintained for 3400 s, and the hydrogen content on the anode side was measured using an electrochemical hydrogen sensor (Geopal Systems, GJ-EX with a H2/M-40000 detector) after passing through a silica gel column. To ensure consistent signal quality, a continuous nitrogen flow of 57 mL min -1 was applied to the system. The hydrogen permeability was calculated using methods described elsewhere. Figure S2