The electrocatalytic semihydrogenation of alkynes to (Z)-alkenes using a proton-exchange membrane (PEM) reactor has been studied as an alternative to the Lindlar catalytic system. We recently reported that Pt-Pd electrocatalysts achieved both a high (Z)-alkene selectivity and a high (Z)-alkene production rate. However, the mechanism of hydrogenation on Pt-Pd electrocatalysts has been not elucidated. In the present study, we systematically investigated the electrocatalytic hydrogenation of alkynes in a PEM reactor using Pt-Pd electrocatalysts with various compositions and identified the active sites on the Pt-Pd electrocatalysts via detailed characterizations. In addition, the roles of both the Pt and Pd metals of the Pt-Pd electrocatalysts in the electrocatalytic hydrogenation of alkynes were clarified via voltammetric studies, and a synergistic mechanism of electrocatalytic hydrogenation was proposed.