To further shed light on the reactivity trend observed in experiments, we next investigated the reaction mechanism for the ECH of AP to 1-PEA on the different metal cathodes by DFT calculations. Given that the pKa of the mono-hydrogenated form of AP (APHᐧ) is reported to be of 9.9 in water, we deemed the non-hydrogenated form of AP to dominate in solution at the working pH of 11.8. Hence, we assumed that the H atoms required for ECH must be primarily sourced from the H surface coverage, if present. As discussed above, DFT calculations predict the In surface to be bare under reaction conditions. Hence, we rationalize the poor ECH activity on this metal with the lack of surface H atoms, and no further mechanistic investigations were conducted for this cathode. For the rest of the metals, the ECH mechanism was explored on the surface coverages predicted in Fig. . This corresponds to 25% of the fcc sites covered by H (0.25 H) on Ag and Au, 75% for Cu (0.75 H), and 100% for Ni and Pt (1.00 H), as shown in Fig. .