Electrochemical nitrogen reduction reaction (NRR) is used to convert atmospheric nitrogen (N2) to ammonia (NH3) at ambient temperature and pressure. In general, acidic electrolytes are used for NRR to provide protons (H+) for the electrocatalytic reduction process. However, this leads to a low NH3 selectivity due to the formation of the H2 by-product via the hydrogen evolution reaction (HER). Recently, we showed that a Ti2N MXene is active and selective for NRR in acidic electrolyte. We demonstrated that this new catalyst works through a Mars-van Krevelen (MvK) mechanism instead of the conventional associative/dissociative mechanisms. However, the effect of pH and electrolyte on its catalytic activity and selectivity was not well understood. Here, we investigate these effects by performing experiments in varying electrolytic conditions and pH values and develop relations between pH, electrolyte choice, and performance. The Ti2N nitride MXene was synthesized via an oxygen-assisted molten salt fluoride etching technique and characterized through XRD and Raman and FTIR spectroscopies. Our findings showed that changing pH does not affect the onset potentials for NRR. However, it was found that as pH increases, selectivity towards NH3 production increases due to a lack of free H+ available to participate in the HER. Also, when pH is below one, the electrochemical activity drastically decreases due to material instability. Favorable NRR selectivity values were obtained in non-aqueous protic electrolytes, such as 2-picoline/trifluoro-acetic acid because of the suppression of HER. We plan to expand these findings to other materials and systems, and use the knowledge obtained from these studies to design optimal electrolytic conditions for the production of NH3 through NRR.

Effect of pH and electrolyte choice on the nitrogen reduction reaction activity of Ti2N Nitride MXene

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