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Ambient Electrochemical Ammonia Synthesis with High Selectivity on Fe/Fe-Oxide Catalyst Lin Hu, Asim Khaniya, Jun Wang, Gang Chen, William E Kaden, and Xiaofeng Feng ACS Catal., Just Accepted Manuscript • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018
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ACS Catalysis
Ambient Electrochemical Ammonia Synthesis with High Selectivity on Fe/Fe-Oxide Catalyst Lin Hu,† Asim Khaniya,‡ Jun Wang,‡,§ Gang Chen,∥ William E. Kaden,‡,§ Xiaofeng Feng*,†,‡,§ †
Department of Materials Science and Engineering, University of Central Florida, Orlando,
Florida 32816, United States ‡
Department of Physics, University of Central Florida, Orlando, Florida 32816, United States
§
Energy Conversion and Propulsion Cluster, University of Central Florida, Orlando, Florida
32816, United States ∥Department
of Chemistry, University of Central Florida, Orlando, Florida 32816, United States
ABSTRACT: Electrochemical reduction of N2 to NH3 under ambient conditions can provide an alternative to the Haber–Bosch process for distributed NH3 production that can be powered by renewable electricity. The major challenge for realizing such a process is to develop efficient electrocatalysts for the N2 reduction reaction (N2RR), as typical catalysts show a low activity and selectivity due to the barrier for N2 activation and the competing hydrogen evolution reaction (HER). Here we report an Fe/Fe3O4 catalyst for ambient electrochemical NH3 synthesis, which was prepared by oxidizing an Fe foil at 300 oC followed by in situ electrochemical reduction. The Fe/Fe3O4 catalyst exhibits a Faradaic efficiency of 8.29% for NH3 production at −0.3 V vs the reversible hydrogen electrode in phosphate buffer solution, which is around 120 times higher than that of the original Fe foil. The high selectivity is enabled by an enhancement of the intrinsic (surface-area-normalized) N2RR activity by up to 9-fold as well as an effective suppression of the HER activity. The N2RR selectivity of the Fe/Fe3O4 catalyst is also higher than that of Fe, Fe3O4, and Fe2O3 nanoparticles, suggesting Fe/Fe-oxide composite as efficient catalyst for ambient electrochemical NH3 synthesis.
KEYWORDS: electrocatalysis, ammonia synthesis, nitrogen reduction, selectivity, iron, iron oxide
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1. Introduction: Ammonia (NH3) is one of the most highly produced inorganic chemicals in the world, as it is widely used for the production of fertilizers, plastics, explosives, nitric acid, and intermediates for pharmaceuticals. It has been primarily produced via the Haber−Bosch process, which produces NH3 from N2 and H2 with Fe-based catalyst under harsh conditions: high temperature (400−500 oC) and high pressure (150−250 bar).1,2 However, this NH3 production process is energy-intensive and has severely relied on the consumption of fossil fuels. For example, around 146 million tons of NH3 was produced globally via the Haber−Bosch process in 2015,3 which consumes 3−5% of the world’s natural gas production, amounting to 1−2% of the global annual energy supply.4,5 It also accounts for >1% of the global CO2 emissions.4 Considering the global energy and climate challenge, there is an urgent need to develop a sustainable ammonia synthesis technology that can decrease our reliance on fossil fuels and mitigate the CO2 emissions. Numerous efforts have been made to facilitate NH3 synthesis under milder conditions or using renewable energy.6−13 Among them, the electrochemical synthesis of NH3 from N2 and H2O under ambient conditions is a promising route, which may enable sustainable, distributed production of NH3 when powered by solar- or wind-generated electricity.14−17 This electricallydriven process is compatible with intermittent renewable energy supply and can eliminate CO2 emissions in the NH3 production process. In addition, electrochemical reduction of N2 to NH3 also enables a process for renewable energy storage in NH3, which is a potential carbon-neutral fuel due to its high energy density (4.32 kWh/L), high hydrogen content (17.8 wt%), easy liquidation (boiling point −33.3 oC at 1 atm).18−20 Actually, NH3 has been proven to work efficiently in internal combustion engines21 and direct ammonia fuel cells,22,23 confirming it to be a promising liquid transportation fuel. Despite these potential advantages, the development of the electrochemical process has been hindered by the lack of efficient electrocatalysts for N2 reduction reaction (N2RR) under ambient conditions.14−17 One major challenge for N2 reduction in aqueous electrolyte is the low selectivity due to the competing hydrogen evolution reaction (HER):24,25 2H+ + 2e– ⇌ H2
E° = 0 V
(1)
Although the standard potential for N2RR is 0.058 V vs the reversible hydrogen electrode (RHE):
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ACS Catalysis
N2(g) + 2H2O + 6H+ + 6e− ⇌ 2NH4OH(aq)
E° = 0.058 V
(2)
the breaking of the strong N≡N bond requires a reduction potential where the HER readily occurs, leading to a low selectivity for NH3 production. Specifically, most studies of the N2RR in aqueous electrolyte under ambient conditions showed a Faradaic efficiency
