Ammonia (NH3) is a globally important commodity for fertilizer production, but its synthesis by the Haber-Bosch process causes substantial emissions of carbon dioxide. Alternative, zero-carbon emission NH3 synthesis methods being explored include the promising electrochemical lithium-mediated nitrogen reduction reaction, which has nonetheless required sacrificial sources of protons. In this study, a phosphonium salt is introduced as a proton shuttle to help resolve this limitation. The salt also provides additional ionic conductivity, enabling high NH3 production rates of 53 ± 1 nanomoles per second per square centimeter at 69 ± 1% faradaic efficiency in 20-hour experiments under 0.5-bar hydrogen and 19.5-bar nitrogen. Continuous operation for more than 3 days is demonstrated.

Renewable energy-driven ammonia electrosynthesis by N2 reduction reaction (NRR) at ambient conditions is vital for sustainability of both the global population and energy demand. However, NRR under ambient conditions to date has been plagued with a low yield rate and selectivity (<10%) due to the more favorable hydrogen evolution reaction (HER) in aqueous media. Herein, surface area enhanced α-Fe nanorods grown on carbon fiber paper were used as NRR cathodes in an aprotic fluorinated solvent–ionic liquid mixture. Through this design, significantly enhanced NRR activity with an NH3 yield rate of ∼2.35 × 10–11 mol s–1 cmGSA –2, (3.71 × 10–13 mol s–1 cmECSA –2) and selectivity of ∼32% has been achieved under ambient conditions. This study reveals that the use of hydrophobic fluorinated aprotic electrolyte effectively limits the availability of protons and thus suppresses the competing HER. Therefore, electrode–electrolyte engineering is essential in advancing the NH3 electrosynthesis technology.

Electrosynthesis of ammonia, especially at ambient temperature and pressure, could provide a facile and efficient renewable energy transportation and storage process. However, the nitrogen reduction reaction (NRR) often exhibits low faradaic efficiencies due to (i) the low solubility of nitrogen gas (N2) in water and (ii) the hydrogen evolution reaction that is prominent in the same range of potentials in aqueous systems. Ionic liquids (ILs) have been shown to overcome these issues to some extent. In this work, we describe the synthesis and characterization of a family of phosphonium-based ILs with highly fluorinated anions, which are shown to display high N2 solubilities. Their thermal properties were examined, with perfluorosulfonate-based ILs showing high decomposition temperatures in comparison to a low, two-step decomposition process found with perfluorocarboxylate-based ILs. Their transport properties, including viscosity and ionic conductivity, were fitted to the Vogel–Tammann–Fulcher (VTF) equation over a wide temperature range, and the VTF parameters are described. The electrochemical window for all of the synthesized ILs extend past the reduction potentials required for N2 reduction. Thus, these high N2 solubility ILs show scope as nonaqueous electrolytes for the electrochemical NRR.