<p>The semi-hydrogenation of alkynes to alkenes, especially acetylene to ethylene, is an essential transformation that delivers raw materials and scaffolds for synthetic industries. Electrocatalytic hydrogenation, which is green and mild, provides an alternative strategy to the conventional hydrogenation process, which relies on high temperature, high pressure and flammable H₂. This protocol describes an electrocatalytic semi-hydrogenation method to synthesize olefins with water as the hydrogen source under ambient temperature and pressure. Electrocatalytic semi-hydrogenation involves the adsorption and activation of alkynes and the cathodic generation of the active hydrogen (H*) intermediate from water dissociation, followed by the addition of H* to an adsorbed alkyne to yield an alkene. This process is generally assisted by Cu-based electrocatalysts (sulfur-modified Cu and Cu nanoparticles) and commercially available reaction vessels and is performed under a direct-current or constant potential power supply. Here we provide detailed procedures for catalyst design synthesis, alkene electrosynthesis and electrochemical in situ/ex situ spectroscopies for investigating reaction mechanisms. The semi-hydrogenation procedure can be performed within hours; it can also be flexibly adapted to synthetic procedures performed in batch or flow reactors and for various reaction times to meet the adjustable capacity requirements for fine or bulk chemicals. Compared with conventional approaches, the electrocatalytic semi-hydrogenation method eliminates the need for expensive and toxic hydrogenation reagents and conditions with elevated temperature and pressure. Our electrocatalytic semi-hydrogenation strategy has various advantages as a sustainable and alternative method to existing methods, including high alkene selectivity, operational simplicity, substrate universality and easily reproducible functional group compatibility.</p>