Oxygen-Involved Electrosynthesis of Nitrate from Air

The electrochemical nitrogen oxidation reaction (NOR) provides a method for converting N2 and H2O into nitrate with zero CO2 emissions, offering an alternative to the energy-intensive Haber−Bosch and Ostwald processes. This study investigates the electrosynthesis of nitrate using air-saturated electrolyte, inspired by natural nitrogen fixation during thunderstorms. By employing Pd2+ and S2−-doped SnO2 nanoparticles on n-butyl triethyl ammonium bromide-functionalized polypyrrole/graphene oxide (Pd/S-SnO2@BTAB/PPy/GO), improvements in nitrate yield and Faradaic efficiency are achieved at a potential of 1.77 V vs reversible hydrogen electrode in air-saturated electrolyte, compared to the NOR performance in a N2-saturated electrolyte. Characterization confirms that Pd sites serve as catalytic centers for NOR, with isotope labeling experiments revealing that the N element in the produced nitrate originated entirely from N2 gas, while the excess O2 involved in the reaction contributes to the partial oxygen content of the generated nitrate. Theoretical calculations propose a reaction pathway wherein O2 is first adsorbed onto the electrocatalyst before reacting with N2. Although intermediates like NO and N2O can form in air-saturated electrolyte, they still require an electrocatalyst for conversion to nitrate. The NOR process in the air-saturated electrolyte includes the traditional 10-electron NOR process as well as additional 8-electron, 6-electron, and 2-electron NOR processes. This research clarifies the role of O2 in nitrate electrosynthesis and presents a pathway for nitrate production, aligning with environmental objectives and addressing current energy challenges.