Molecular hydrogen, a valuable clean energy fuel and industrial feedstock, is commonly produced by steam reforming of fossil fuels, a process that results in the co-generation of CO2. Strategies that produce H2 without CO2 emissions focus on water-splitting powered by solar or wind energy. Here, we introduce a new approach in which energy derived from the oxidation of waste organic matter is used to subsidize the electrical energy consumed by electrochemical H2 generation. The primary gas product is H2, with minimal O2 contamination, avoiding the purification step required for conventional water electrolysis. The energy efficiency (i.e., energy from H2 combustion ÷ input electrical energy × 100%) ranges from 214% to 304%. We also demonstrate a sustained cycling operation with concomitant treatment of domestic and industrial wastewater. These findings open the door to recovery of H2 from other dilute reservoirs of organic matter, such as marine sediment.