Carbon Nanospikes as A Physical Catalyst for the Electrolysis of Nitrogen to Ammonia

Y. Song, D.K. Hensley, A.J. Rondinone
Oak Ridge National Laboratory,
United States

Keywords: carbon nanospikes, nitrogen reduction, ammonia synthesis, physical catalyst

Summary:

The industrial demand for ammonia for use in nitrogen fertilizers is very large and will increase as population and the demand for food increases. Though nitrogen gas is abundant in the atmosphere, synthesizing ammonia and other useful products from nitrogen gas is difficult due to the thermodynamic energy barrier for N≡N dissociation and the sluggish kinetics. The industrial method for ammonia synthesis from nitrogen gas is the Haber-Bosch process that requires high temperatures and pressures to maintain a useful reaction rate and relies on methane from natural gas as a source of hydrogen. Motivated by a necessity to a sustainable nitrogen fixation route, we herein describe a highly textured physical catalyst comprised of N-doped carbon nanospikes (CNS)1 electrochemically reduces dissolved N2 gas to ammonia in an aqueous electrolyte under ambient temperature and pressure. The Faradaic efficiency achieves 11.56 +/- 0.85% at –1.19 V vs. reversible hydrogen electrode (RHE) and the maximum production rate is 97.18+/- 7.13 µgh–1cm–2. The energy efficiency of the reaction is estimated to be 5.25% at the current FE of 11.56%. We reported the CNS exhibited low hydrogen selectivity and high surface area, making it suitable for complex electrocatalytic reactions such as oxygen1 and CO2 reduction2 previously. In this presentation, we describe the CNS as a physical catalyst for the electrochemical reduction of dissolved nitrogen gas to ammonia. Theoretical calculations demonstrated that the electric field is concentrated at the tips, thereby promoting the electroreduction of dissolved N2 molecules near the electrode, which was proved by comparison to glassy carbon and plasma etched CNS controls. The choice of electrolyte is also critically important, as the reaction rate is dependent on the counterion type, suggesting interaction between counterion and N2 to concentrated N2 within the Stern layer. 1. L.B. Sheridan, D.K. Hensley, N.V. Lavrik, S.C. Smith, V. Schwartz, C. Liang, Z. Wu, H.M. Meyer, and A.J. Rondinone. Growth and Electrochemical Characterization of Carbon Nanospike Thin Film Electrodes. Journal of the Electrochemical Society 161, H558 (2014). 2. Y. Song, R. Peng, D.K. Hensley, P.V. Bonnesen, L. Liang, Z. Wu, H.M. Meyer, M. Chi, C. Ma, B.G. Sumpter, and A.J. Rondinone. High-Selectivity Electrochemical Conversion of CO2 to Ethanol using a Copper Nanoparticle/N-Doped Graphene Electrode. ChemistrySelect, 1, 6055 (2016).