Journal of the Korean Electrochemical Society (전기화학회지)

The Korean Electrochemical Society (한국전기화학회)
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Electrochemical Nitrogen Reduction Reaction to Ammonia Production at Ambient Condition

상온 상압 조건에서 전기화학적 질소환원반응을 통한 암모니아 생산 연구 동향

  • Lee, Dong-Kyu (Department of Materials Science & Engineering, Chonnam National University) ;
  • Sim, Uk (Department of Materials Science & Engineering, Chonnam National University)
  • 이동규 (전남대학교신소재공학부) ;
  • 심욱 (전남대학교신소재공학부)
  • Received : 2018.11.05
  • Accepted : 2018.12.31
  • Published : 2019.02.28
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Abstract

The reduction of nitrogen to produce ammonia has been attracting much attention as a renewable energy technology. Ammonia is the basis for many fertilizers and is also considered an energy carrier that can power internal combustion engines, diesel engines, gas turbines, and fuel cells. Traditionally, ammonia has been produced through the Haber-Bosch process, in which atmospheric nitrogen combines with hydrogen at high temperature ($350-550^{\circ}C$) and high pressure (150-300 bar). This process consumes 1-2% of current global energy production and relies on fossil fuels as an energy source. Reducing the energy input required for this process will reduce $CO_2$ emissions and the corresponding environmental impact. For this reason, developing electrochemical ammonia-production methods under ambient temperature and pressure conditions should significantly reduce the energy input required to produce ammonia. In this review, we introduce the electrochemical nitrogen reduction reaction at ambient condition. Numerical studies on the electrochemical nitrogen reduction mechanism have been carried out through the computation of density function theory. Electrodes such as nanowires and porous electrodes have been also actively studied for further participation in electrochemical reactions.

암모니아 생산은 이전부터 비료, 식량과 관련되어 많은 연구가 이루어져 왔는데, 최근 신재생 에너지에 대한 관심이 커짐에 따라 암모니아 또한 에너지로서 내연기관이나 연료전지로 활용이 기대되어 더 많은 연구가 활발히 이루어 지고 있다. 하지만 암모니아를 생성하기 위해서 하버-보쉬법을 사용하는데, 150-300기압과 $350-550^{\circ}C$ 이상의 높은 압력과 온도가 필요하므로 지구 에너지의 1-2%를 사용할 만큼 암모니아 생산에 많은 에너지가 소모되며, 주로 화석연료가 사용된다. 위와 같은 반응에 사용되는 에너지를 줄임으로 이산화탄소 배출량을 줄여 환경문제에도 대응할 수 있기 때문에 반응온도 및 압력을 줄이는 노력이 필요하다. 본 총설에서는 암모니아를 생산하기 위한 방법 중 특히 상온, 상압에서의 전기화학적 질소환원반응 결과들을 소개한다. 실험 결과뿐만 아니라 밀도범함수 계산을 통하여 전기화학적 질소환원반응 메커니즘 연구가 많이 되었으며, 더 많이 전기화학반응에 참여할 수 있도록 하는, 나노 와이어, 다공성 전극과 같은 나노구조화 전극설계에 대한 다양한 연구 결과들 또한 제시한다.

Keywords

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Fig 1. Competition of ammonia production with other reduction and hydrogen evolution reaction(Ref.17,18)

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Fig 3. The free energy diagrams of NRR on (a) Mo2N (100) and (b) MoO2 (100) surfaces (c) The structures of key intermediates on Mo2N (d) The geometries of adsorbed N2 (*N2) and the N2 reduced by one (H++e-) pair (*NNH) on MoO2. An asterisk (*) denotes a surface site. Color code: Mo, cyan; H, white; O, red; N, blue or purple for highlight(Ref.22)

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Fig 2. (a) Possible pathways for the NRR to form ammonia (NH3)(Ref.20) (b-c) Free-energy diagrams for the NRR on the single Mo atom embedded into the MoS2 nanosheet at zero and applied potential (limiting or onset potential) through (b) distal and (c) alternating mechanisms (d) The volcanoes of the HER and NRR limiting (or onset) potentials as a function of the N2H* species adsorption energy(Ref.21)

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Fig 4. (a) Schematic illustration for the electrochemical NRR by catalysts of a-Au/CeOx–RGO and c-Au/RGO under ambient conditions (b) Yield of NH3 (red) and Faradaic efficiency (blue) at each given potential(Ref.25) (c) Schematic of electrochemical cell for NRR using hollow gold nanocages. (d) Ammonia yield rate and faradaic efficiency at various potentials in 0.5M LiClO4 at 20°C (e) Geometric models of an Au THH NR and exposed 24{730} facet. The {730} facet is composed of (210) and (310) sub-facets on Au NRs (f) Free energy diagram and alternating hydriding pathway for NRR on Au (210) and Au (310) at equilibrium potential (Ref.26)

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Fig 5. (a) Schematic of electrochemical synthesis of NH3 in an anion-exchange-membrane-based electrolyzer (b) NH3 formation reaction rate (left y-axis) and faradaic efficiency(right y-axis)(Ref.28) (c) Schematic view of the electrocatalytic flow reactor for ammonia synthesis using CNTs (d) Faradaic efficiency values of ammonia formation and H2 evolution under different applied voltages(Ref.29) (e) A schematic representation of ethylenediamine (EDA)-based ammonia synthesis (f) Cumulative NH3 production during the potentiostatic electrolysis with the supply of Ar or N2 at a cell voltage of 1.8V(Ref.30)

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Fig 6. (a) Schematic reaction cell for the NRR. (b) Average NH3 yields and faradaic efficiencies of VN/CC at different potentials.(Ref.31) (c) Schematic diagram for electrocatalytic NRR. (d) NH3 yields and Fes at each given potential.(Ref.22) (e) Proposed reaction pathway for nitrogen reduction on the surface of VN0.7O0.45 via a Mars–van Krevelen mechanism and the catalyst deactivation mechanism(Ref.32)

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Fig 7. (a) Schematic illustration of NRR using N-doped porous carbon(NPC) (b) Ammonia production rates and current efficiency of NPC-750 during 10 consecutive cycles at -0.9V(Ref.33) (c) Illustration of the fabrication of BVC‐A and BVC‐C NRR electrocatalysts. (d) Yield of NH3 (blue–green) and Faradaic efficiency (red) at each given potential (e) yield of NH3 with different catalysts at -0.2 V versus RHE.(Ref.34)

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Fig 8. (a) Schematic diagram of the cell. (b) Yield of ammonia over 24h obtained on different substrates: (i) Ptype silicon, (ii) bSi, (iii) GNP/bSi, (iv) GNP/bSi/Cr and (v) Au/Si/Cr after illumination with two suns and (vi) GNP/bSi/Cr in dark(Ref.58) (c) Proposed Photocatalytic Cycle for N2 Fixation on the Rutile TiO2 (110) Surface (d) Change in the amount of NH3 formed and the SCC efficiency under simulated AM1.5G sunlight irradiation (1-sun)(Ref.59)

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Fig 9. (a) Scheme of BiOBr nanosheet for nitrogen reduction photocatalyst (b) Schematic illustration of the photocatalytic N2 fixation model in which water serves as both the solvent and proton source. (c) Quantitative determination of the generated NH3 under visible light (λ > 420 nm) (Ref. 61)

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Fig 10. (a) schematic of g-C3N4 and V-g-C3N4 (b) The concentration of generated NH4+ in different systems (c) photocatalytic N2 fixation rate of g-C3N4 and V-g-C3N4 (Ref.62)

Table 1. The list of catalyst of nitrogen reduction reaction at ambient condition

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Table 1. Continued.

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