Mechanistic Insights into Electrochemical Nitrogen Reduction Reaction cm−2 after 2 h (Figure 1, B and C). No detectable level of ammonia is detected after 1 h on VN on Vanadium Nitride Nanoparticles at potentials below −0.2 V. Importantly, the ability of VN to activate dinitrogen is confirmed by conducting ENRR with 15N at −0.1 V in the batch mode in 0.05 M H SO , with clear signal 2 2 4 of 15NH +, along with 14NH +, in the 1H nuclear magnetic resonance (NMR) spectrum after 48 h Xuan Yang1, Jingguang G. Chen,2* Yushan Yan1* and Bingjun Xu1* 4 4 (Figures 1D). Control experiments show that a minor fraction of the 14NH + detected after 1Center for Catalytic Science and Technology, Department of Chemical and Biomolecular 4 ENRR could be attributed to leaching of N from VN, which is detected when the VN is soaked Engineering, University of Delaware, 150 in the same electrolyte at the open circuit potential (OCP) for 48 h. Academy Street, Newark, Delaware 19716, United States

2Department of Chemical Engineering, Columbia University, New York, New York 10027, United States *[email protected], [email protected], [email protected] Introduction The ability to fixate nitrogen on an industrial scale marked a turning point in the history of the human society, which supports the livelihood of close to half of world’s population.1 Distributed and modular ammonia synthesis via the electrochemical nitrogen reduction reaction (ENRR) at or close to ambient conditions, powered by renewable electricity, emerges as an attractive alternative to Haber-Bosch process because it allows for the on- demand, on-site production of ammonia, and in turn N-fertilizers, from ubiquitously available 2 resources, i.e., N2 and water.

In this work, we demonstrate that vanadium nitride (VN) nanoparticles are active and selective catalysts for ENRR. Both the ammonia production rate and Faradaic efficiency (FE) are approximately two orders of magnitude higher than those of noble metal catalysts. Through a combination of ex-situ and operando characterizations, vanadium oxynitride (VN0.7O0.45) is identified as the active phase, and the catalyst deactivation is caused by its conversion to VN. The ENRR is proposed to proceed via a Mars-van Krevelen mechanism based on results 15 obtained using N2 as the feed in the ENRR. We show that an ammonia production rate of 1.1 × 10−10 mol s−1 cm−2, with an FE of 1.6%, can be maintained for 116 h on VN at −0.1 V.

Materials and Methods Vanadium nitride nanoparticles were synthesized following a “urea-glass” method.3 Typically, 1 g of vanadium precursor was dissolved in 2 g of ethanol to make the dark-reddish precursor solution; then 1.04 g of urea was slowly added into the alcoholic solution; the Significance dispersion was stirred until urea was completely solubilized and the solution changed from VN0.7O0.45 is identified to be the active phase for ENRR, via a Mars-van Krevelen mechanism. dark-reddish to green; the solution was then dried into a glass or glassy film by evaporating the Figure 1. (A) Production rate and FE of VN catalysts at different potentials for 1 h tests. (B) solvent; the gel was then put into an furnace under a N2 flow at 800 °C for 3 h with a heating ramp of 3 °C/min. Time-dependent production rate and FE at −0.1 V and −0.2 V for 8 h tests, respectively. (C) Time-dependent production rate and FE at −0.1 V for 120 h tests. (D) 1H NMR spectra for the 15 14 Results and Discussion post-electrolysis 0.05 M H2SO4 at −0.1 V in N2 and Ar, and at OCP in N2 for 48 h. The VN nanoparticles are approximately two orders of magnitude more active and selective for ENRR than noble metal catalysts.2 An ammonia production rate of 3.3 × 10−10 mol References s−1 cm−2 (based on geometric area) and an FE of 6.0% are measured on the VN catalyst at −0.1 1. Erisman, J. W., Sutton, M. A., Galloway, J., Klimont, Z., Winiwarter, W. Nat. Geosci. 1, V within the first hour (Figure 1A). The ammonia production on VN at −0.1 V remains 636, (2008). constant in the first two hours with a decrease of 28% and 62% in the next two (3−4 h) and 2. McEnaney, J. M., Singh, A. R.; Schwalbe, J. A.; Kibsgaard, J.; Lin, J. C.; Cargnello, M.; four (5−8 h) hours, respectively, which is accompanied by a similar reduction in the FE (Figure Jaramillo, T. F.; Nørskov, J. K. Energy Environ. Sci. 10, 1621, (2017). 1B). The ammonia production rate at −0.1 V becomes stable after 4 h, at 1.1 × 10−10 mol s−1 3. Giordano, C., Erpen, C., Yao, W., Milke. B., Antonietti, M. Chem. Mater. 21, 5136, cm−2 for as long as 116 h, while that at −0.2 V plummets more than 95% to 1.1 × 10−11 mol s−1 (2009).