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

As an ideal alternative to the Haber-Bosch process, the renewable energy-powered electrochemical N 2 reduction to NH 3 is a promising approach since the electrochemical reduction can occur under mild conditions; only water and N 2 are consumed during overall nitrogen reduction reaction.[1] Recently, there have been many efforts devoted to developing electrocatalysts for energy efficient NH 3 synthesis from N 2 ; however, it remains very challenging due to the thermodynamic inertness of the dinitrogen triple bond. The electrochemical reduction of N 2 to NH 3 (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 NH 3 synthesis. Such poor conversion efficiency and yield rates also make it more difficult to confirm the origins of the NH 3 production, ie whether it genuinely comes from electrocatalytic NRR, as opposed to some other readily reducible N-containing contaminants (NO, NO 2 , N 2 O 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 NH 3 . We also systematically unravel the origins of the reported activity, showing that other N-containing species, particularly ionic/gaseous NO x or doped N atoms in the materials, are strongly active reactants towards NH 3 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.