A Porous Iron Electrode for Electrochemical Ammonia Synthesis

Abstract

Ammonia is a precursor in fertilizer production and a potential carbon-free energy carrier, which is essential for the energy transition to more renewable energy sources. To that end, the current fossil fuel based method of industrial ammonia production through the Haber-Bosch process should be replaced by electrochemical ammonia synthesis. However, research into this topic faces several challenges, including a strong dinitrogen bond, competition from the hydrogen evolution reaction and low solubility of nitrogen in aqueous electrolytes, limiting the availability of nitrogen at the reaction sites on the electrode surface. In this study, in order to better understand these limitations to electrochemical nitrogen reduction to ammonia, a porous iron electrode is used in a cell design allowing both aqueous electrolyte and nitrogen gas access to the surface, which is successfully reduced electrochemically to serve as a nitrogen dissociation catalyst. Careful control of the applied potential limits the competition from the hydrogen evolution reaction. Nuclear magnetic resonance spectroscopy and ion chromatography measurements of the electrolyte were performed to check for possible contamination by nitrogen containing species. Small amounts of ammonia were measured using gas chromatography-mass spectroscopy on the gas output, but only with a high residence time of the nitrogen gas inside the cell. Since reduced iron surfaces are known to be good nitrogen dissociation catalysts, even at ambient temperature and pressure, the cause of the low ammonia production rate reported here is the abundant presence of water, which blocks nitrogen from the surface adsorption sites even when the catalytic surface is not submerged in the electrolyte. These results show that creating a surface with sufficient catalytic activity to break the dinitrogen bond is very well possible, but the main challenge is keeping the surface clean to prevent blocking of the reaction sites. This gives additional insight for future efforts in developing electrochemical ammonia synthesis methods.