Electric power can be generated from renewable energy sources such as solar and wind, making electrocatalytic hydrogenation an important technology to reduce carbon dioxide emissions in the organic synthesis industry. In the present work, the electrocatalytic semihydrogenation of diphenylacetylene was carried out in a proton exchange membrane (PEM) reactor with carbon-supported Pt, Pd, and Pt-Pd alloy cathode catalysts. Diphenylacetylene introduced into the PEM reactor at less negative potentials underwent electrocatalytic hydrogenation to providecis-stilbene as a main stereoisomer, with excellent current efficiencies. Among the investigated catalysts, the Pt-Pd alloy with a composition of 1(Pt):99(Pd) was found to be the most suitable for achieving both highcis-stilbene selectivity and a high production rate (partial current density) forcis-stilbene.