Exploring the source of ammonia generation in electrochemical nitrogen reduction using niobium nitride

Abstract

In this study, niobium nitride (NbN) is prepared via the urea-glass route by annealing a mixture of NbCl₅ and urea at 650 °C under a flow of N₂, and is used as a catalyst for the electrochemical nitrogen reduction reaction (NRR). The as-prepared NbN exhibits a maximum production rate of 5.46 × 10⁻¹⁰ mol s⁻¹ cm⁻² at −0.6 V vs. RHE, along with an apparent FE of 16.33% at −0.3 V vs. RHE. In addition, the leaching of NbN is confirmed by ICP-OES, where the leached amount of Nb is almost identical to the amount of N measured by UV-vis. Moreover, ¹H NMR experiments are performed using ¹⁵N₂ as the feeder gas; the dominant detection of ¹⁴NH₄⁺ peaks strongly suggests that the produced NH₃ originates from the leaching of NbN rather than via an electrocatalytic process. Hence, for a comprehensive understanding of NH₃ generation, especially when utilizing transition metal nitride (TMN)-based NRR catalysts, a thorough investigation employing multiple analytical methods is imperative.