(Invited) Critical Assessment of Aqueous Electrochemical Synthesis of NH3: N2 Reduction Versus NOx Reduction

Abstract

As an ideal alternative to the Haber-Bosch process, the renewable energy-powered electrochemical N2 reduction to NH3 is a promising approach since the electrochemical reduction can occur under mild conditions; only water and N2 are consumed during overall nitrogen reduction reaction.[1] Recently, there have been many efforts devoted to developing electrocatalysts for energy efficient NH3 synthesis from N2; however, it remains very challenging due to the thermodynamic inertness of the dinitrogen triple bond. The electrochemical reduction of N2 to NH3 (the nitrogen reduction reaction “NRR”) requires the consecutive six-electron/proton transfer reactions to proceed and this leads to the sluggish kinetics. In addition, the competitive hydrogen evolution reaction (HER, Eº = 0 V vs. RHE) can concomitantly occur at similar potentials to the NRR (Eº = 0.092 V vs. RHE), resulting in both low faradaic efficiency (< 10 %) and low yield rates (< 10-10 mole cm-2 s-1) for NH3 synthesis. Such poor conversion efficiency and yield rates also make it more difficult to confirm the origins of the NH3 production, ie whether it genuinely comes from electrocatalytic NRR, as opposed to some other readily reducible N-containing contaminants (NO, NO2, N2O and doped N atoms in the materials) under reducing potentials.Herein, we investigate the catalytic nature of nitrogen reduction reaction on three different types of preeminent electrocatalysts from the literature (bismuth, gold and N-doped carbon)[2, 3] using a rigorous experimental protocol developed by our group.[4] It is demonstrated that all of the catalysts are essentially inactive (below LOD) towards dinitrogen reduction to NH3. We also systematically unravel the origins of the reported activity, showing that other N-containing species, particularly ionic/gaseous NOx or doped N atoms in the materials, are strongly active reactants towards NH3 production. Our presentation will conclude with a summary of the critical contaminants leading to false-positive NRR and also provide further protocol recommendations to avoid this outcome.[1] S.L. Foster, S.I.P. Bakovic, R.D. Duda, S. Maheshwari, R.D. Milton, S.D. Minteer, M.J. Janik, J.N. Renner, L.F. Greenlee, Nature Catalysis, 1 (2018) 490-500.[2] Y.-C. Hao, Y. Guo, L.-W. Chen, M. Shu, X.-Y. Wang, T.-A. Bu, W.-Y. Gao, N. Zhang, X. Su, X. Feng, Nature Catalysis, 2 (2019) 448.[3] Y. Liu, Y. Su, X. Quan, X. Fan, S. Chen, H. Yu, H. Zhao, Y. Zhang, J. Zhao, ACS Catalysis, 8 (2018) 1186-1191.[4] B.H.R. Suryanto, H.-L. Du, D. Wang, J. Chen, A.N. Simonov, D.R. MacFarlane, Nature Catalysis, 2 (2019) 290-296.