Influence of Gaseous Oxygen Species on Liquid-Phase, Fixed Nitrogen Products in Aqueous Plasma-Based Electrochemical Processes

Abstract

Electrified, small-scale, remote approaches are needed as an alternative to conventional centralized methods for nitrogen fixation in order to reduce our reliance on fossil fuels and ensure global food security. Plasma-based electrolytic processes offer a promising solution by directly reacting molecular nitrogen and water under mild conditions. However, the complex non-equilibrium chemistry results in a diverse range of gas-phase and liquid-phase reactions, which impacts selectivity toward desired products. In this study, we investigate the influence of feed gas composition, specifically the presence of molecular oxygen at minute quantities, on the liquid-phase nitrogen products. Specifically, oxygen gas concentrations as low as 0.1% in the gas feed are found to substantially affect the selectivity towards ammonium ions. We additionally show that the total gas flow rate has an indiscriminate effect on both ammonium and nitrate/nitrite ion yields because of the presence of water vapor. By carefully controlling these process parameters, a production rate for ammonium ions exceeding 1 mg/h with a molar selectivity of ~ 14 is achieved. Our results highlight the importance of gas-phase chemistry in plasma-based electrolytic nitrogen fixation.