Next, we explored the possibility of hydrogenating the C=C double bond in maleic acid (Fig. ). This process is an important strategy to upgrade the biomass-derived maleic acid into succinic acid (SA), a valuable chemical feedstock which can be used as polymer precursor, food addictive and dietary supplement. The experimental conditions adopted for the ECH of maleic acid were the same as for AP, except for the electrolyte pH which was optimized to 7.7 instead of 11.8 (see Supplementary Figure ). The recorded 1 H-NMR spectra of the reaction mixtures, and the measured FESA and JSA after 1 h of electrolysis at different potentials between -0.28 and -0.68 VRHE are shown in are shown in Supplementary Figures and, respectively. Similarly to AP, the maximum ECH activity was observed at -0.48 VRHE (FESA = 75±4%, JSA = 12±1 mA cm - 2 ). Furthermore, constant potential electrolysis at -0.48 VRHE resulted in 89±3% conversion of maleic acid and 89±3% yield of succinic acid after 8 h of electrolysis (Supplementary Figure ).