Membrane electrode assembly design for lithium-mediated electrochemical nitrogen reduction

Abstract

Ammonia is closely associated with the food supply and production of chemicals in modern society. Motived by the global shift toward green production, the synthesis of ammonia via the electrochemical nitrogen reduction reaction (NRR) has been proposed as an alternative to the traditional Haber–Bosch process. In this case, the lithium-mediated process (LiNR) is considered to be the most promising route in the field of NRR in terms of outstanding ammonia yield and faradaic efficiency; however, it is limited by its poor gas transfer, dependence on organic solvent and significant voltage loss. In this study, a feasible membrane electrode assembly (MEA) configuration is proposed as a promising solution to overcome the above-mentioned problems. The MEA was comprised of lithium-deposited stainless-steel cloth as the cathode, lithium-doped polyethylene oxide (PEO) as the polymer electrolyte and carbon paper loaded with Pt/C catalyst as the anode. A mean ammonia production rate of 2.41 ± 0.14 μmol h⁻¹ cm⁻²_geo and faradaic efficiency of 8.9 ± 1.7% were obtained at a cell voltage of 3.6 V. Lower voltage loss (ca. 0.25 V@5 mA cm⁻²_geo) was observed in the absence of ethanol. In situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were performed to reveal the transformation of lithium deposits. This study offers a new route for LiNR with the advantages of efficient gas transfer, reduced solvent consumption and compact configuration.