Water microdroplets containing dissolved ammonia (30−300 μM) are sprayed through a copper oxide mesh with a 200 μm average pore size, resulting in the formation of nitrate (NO 3 − ) and the release of molecular hydrogen (H 2 ). The products result from a redox process that takes place at the liquid−solid interface through contact electrification, where no external potential is applied. Oxidation is initiated by superoxide radical anions (O 2 −