Elucidation of the Active Phase and Deactivation Mechanisms of Chromium Nitride in the Electrochemical Nitrogen Reduction Reaction

Metal nitrides have been suggested to be effective catalysts for the electrochemical nitrogen reduction reaction (ENRR) based on computational investigations; however, experimental verification has been scarce. In this work, we demonstrate that chromium nitride is an active and selective ENRR catalyst in a Nafion-based membrane electrode assembly. Both the specific ENRR rate (1.4 × 10^–11 mol cm^–2 s^–1) and faradic efficiency (0.58%) on the chromium nitride catalyst are approximately 20 times higher than those on Pt at −0.2 V vs the reversible hydrogen electrode. Although the only bulk phase identified by X-ray diffraction of the chromium nitride catalyst is pure phase Cr₂N, X-ray photoelectron spectroscopy (XPS) investigations reveal that CrN, CrN_xO_y, and CrO_x species, in addition to Cr₂N, are present on the surface of the catalyst. In contrast, a synthesized chromium nitride sample with a bulk CrN phase shows a negligible ENRR rate. XPS shows that the synthesized sample does not possess any Cr₂N species on the surface, which leads to the identification of Cr₂N as the active phase in ENRR. Batch cell testing with ¹⁵N₂ as the feed forms both ¹⁴NH₃ and ¹⁵NH₃, indicating the involvement of surface N in the activation of dinitrogen, i.e., a Mars–van Krevelen mechanism. Two mechanisms of catalyst deactivation are identified: (1) leaching of surface N at lower potentials (<−0.4 V) and (2) slow conversion of the active Cr₂N phase to the inactive CrN phase at −0.2 V.