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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 8: 1134 - 1144

DOI: https://doi.org/10.7536/PC240124

Review

Application of Non-Precious Transition Metal Catalyst in Electrocatalytic Nitrogen Synthesis of Ammonia

  • Siyu Liu 1 ,
  • Yike Wei 2 ,
  • Yu Tan , 1, * ,
  • Weiming Yuan 1 ,
  • Kexin Liang 1 ,
  • Shenghan Zhang 1
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  • 1 College of Environmental Science and Engineering, North China Electric Power University, Baoding 071000, China
  • 2 Huaneng Dongguan Gas Turbine Cogeneration Co., LTD., Dongguan 523000, China
*e-mail:

Received date: 2024-01-29

  Revised date: 2024-03-06

  Online published: 2024-07-01

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Abstract

As an important chemical product and chemical raw material,ammonia is widely used in industry,agriculture,medicine and other industries,and plays an irreplaceable role in global economic development.At present,industrial ammonia synthesis mainly uses the traditional Haber-Bosch process,which consumes a lot of fossil energy and has a relatively low equilibrium conversion rate.Electrocatalytic nitrogen reduction of ammonia synthesis can convert N2and H2O into NH3at normal temperature and pressure,and it is easy to operate and easy to obtain raw materials,which has become an important research direction in the scientific research field.Among them,non-precious metal transition metal-based oxides,nitrides,sulfides,bimetal catalysts and heteroatom-based catalysts represented by transition metals in zone d showed good catalytic performance.This paper focuses on the recent progress of electrocatalytic ammonia production by transition metal-based electrocatalytic nitrogen reduction reaction(E-NRR),including its challenges,reaction mechanism,and different materials of E-NRR catalysts,and focuses on the structure-performance relationship.The strategies and prospects for improving the performance of E-NRR were introduced from the aspects of synthesis scheme,structure modification,activity,selective enhancement and reaction mechanism 。

Key words： electrocatalytic synthesis of ammonia; transition metals; nitrogen reduction reaction

Cite this article

Siyu Liu , Yike Wei , Yu Tan , Weiming Yuan , Kexin Liang , Shenghan Zhang . Application of Non-Precious Transition Metal Catalyst in Electrocatalytic Nitrogen Synthesis of Ammonia[J]. Progress in Chemistry, 2024 , 36(8) : 1134 -1144 . DOI: 10.7536/PC240124

1 Research background and significance

As a very important chemical raw material and inorganic chemical product,ammonia is widely used in the production of plastics,explosives,pharmaceuticals,dyes,synthetic fibers and resins.At the same time,because of its high hydrogen density(17.7 wt%),high energy density and（3 kW·h·kg−1）,it is a more competitive hydrogen energy carrier compared with other carriers,and the storage mode of ammonia is milder than that of hydrogen.Liquid ammonia has a boiling point of−33.3℃at standard atmospheric pressure and can be liquefied at 9 atmospheres at room temperature.It has a narrow range of flammability and is easy to store and transport.Therefore,ammonia is a green energy and hydrogen storage medium with great potential[1]。 At present,industrial ammonia synthesis mainly follows the traditional Haber-Bosch process,which converts high-purity N2and hydrogen H2into NH3at high temperature(350~550℃)and high pressure(150~350 atm )[2,3][4⇓~6]。 From the feedstock point of view,hydrogen production is mainly the conversion of natural gas to H2and CO2through desulfurization,methane steam reforming and water-gas shift reaction,the removal of residual CO through methanation reaction,and the removal of CO2through pressure swing adsorption process[5,7,8]。 This process requires a high temperature of 500–600°C,which will consume a large amount of fossil energy,and the energy consumption is as high as 1%–2%of the global energy.Moreover,the equilibrium conversion rate is relatively low,only 10%–15%.At the same time,the synthesis of each ton of ammonia is accompanied by about 1.87 t CO2emissions,which aggravates energy consumption and environmental pollution[5,9,10]。
According to the World Energy Outlook 2023 released by the International Energy Agency,the oil and gas industry accounts for nearly 15%of energy-related greenhouse gas emissions.The production,transportation,and processing of oil and natural gas emitted a total of 5.1G t CO2in 2022.In the IEA 2050 net-zero scenario,the relevant emission intensity is projected to decline by 50%by 2030[11]。 In this scenario,China pays more and more attention to the development and utilization of clean energy and renewable energy.Therefore,the study of low-cost and high-conversion catalysts to achieve large-scale ammonia production under mild conditions has become a hot research topic。
At present,researchers are committed to artificial ammonia production in a more environmentally friendly way under environmental conditions,including electrocatalytic ammonia production,photocatalytic ammonia production,plasma ammonia production,artificial biomimetic enzyme nitrogen fixation and so on[12]。 However,the strict demand for light,the uncertainty of solar energy and the low utilization rate limit the use of photocatalytic ammonia production,and the complex preparation conditions of nitrogenase and the strict environmental requirements for its use also make it unsuitable for industrialization.Compared with other processes,the electrochemical reduction of N2to NH3（Nitrogen reduction reaction(NRR)has the advantages of mild reaction conditions,safety,controllability and simple process,which stands out as one of the most promising alternatives to Haber process in many ammonia production processes[13,14]。 Electrochemical ammonia synthesis has the following significant advantages:firstly,its reaction device is simple,and it can use renewable energy(solar energy,wind energy,nuclear energy,geothermal energy,etc.)to convert into electricity and introduce it into the ammonia synthesis industry at normal temperature and pressure,so as To reduce the limitation of thermodynamic equilibrium on the reaction[6,15]; Secondly,water is used as the raw material to provide the hydrogen source needed for NRR,getting rid of the heavy dependence of Haber process on fossil fuels[6,16]。 These advantages can be adapted to the wide and decentralized distribution of clean energy,and can also alleviate the shortage of product supply caused by the energy crisis.However,due to the high stability of N≡N at room temperature and pressure and the slow adsorption of N≡N N2,the resistance in the electrocatalytic reduction of nitrogen to ammonia is large,and because of the competitive relationship with the hydrogen evolution reaction,the selectivity of the catalyst to nitrogen and the yield of ammonia are greatly reduced.According to the ARPA-EREFUEL program of the US Department of Energy,the performance of the NRR process should be as follows:when the current density is 300 mA·cm-2,the absorption rate of NH3reaches 10-6mol·s-1·cm-2,the FE is 90%,and the catalytic efficiency can only decrease by 0.3%after electrolysis for 1000 H,that is,it should have high stability[17]。 Therefore,there is a big gap between NRR and industrial production。
N2+e−→N2* −3.37 V vs. RHE at pH=0
N2+H++e−→N2H* −3.20 V vs. RHE at pH=0
N2+2H++2e-→N2H2* −1.10 V vs. RHE at pH=0
N2+4H++4e−→N2H4* −0.36 V vs. RHE at pH=0
N2+6H++6e−→2NH3 0.148 V vs. RHE at pH=0
2H++2e−→H2 0 V vs. RHE at pH=0
in recent years,a lot of research has been carried out on electrocatalytic ammonia production materials,mainly including precious metal materials,main group metal based materials,metal-free materials and d-block non-noble transition metal based materials.Among them,non-noble transition metal-based catalysts,represented by d-block transition metals and their oxides,nitrides,carbides and sulfides,have shown good catalytic performance,and a large number of related studies have been reported In the field of electrocatalytic NRR[18]。 Transition metal atoms in the d region have unique advantages in the field of electrocatalytic NRR due to their unfilled valence shell electron orbitals(d orbitals),both empty orbitals and occupied d orbitals.It can accept the lone pair electrons of N2molecules to form metal-nitrogen bonds,and can also donate its own electrons to the antibonding orbital of N2to achieve the purpose of weakening N≡N,so that d-region transition metals have considerable catalytic ability.A large number of transition metals,such as Fe,Co,Ni,Mo,Cu and Ti,have been reported to be studied by combining surface/interface engineering,crystal plane control and amorphization,construction of defect engineering,and construction of biomimetic sites,and a series of progress has been made in overcoming nitrogen kinetic barriers,inhibiting competitive reactions,high Faradaic efficiency,and low overpotential[19]。
non-noble transition metal based catalysts have broad application prospects in ammonia synthesis.Therefore,in This paper,the research results of non-noble transition metal elements used in E-NRR under environmental conditions in recent years are summarized and discussed from the aspects of catalyst types,preparation methods,catalytic properties,working conditions,etc.the catalytic mechanism of various non-noble transition metal element-based catalysts in electrochemical ammonia production was systematically discussed.this review can provide a reference for the preparation of high-performance E-NRR catalysts and the construction of efficient electrochemical ammonia synthesis systems。

2 Mechanism of Electrocatalytic Ammonia Synthesis

2.1 Reaction process of NRR

One of the main difficulties in the study of electrocatalytic NRR is the low selectivity of the catalyst for the product.Compared with its competing reaction,Hydrogen evolution reaction(HER),NRR is a multistep process with electron transfer,which involves six electron transfers,while HER requires only two electron transfer steps to complete the reaction without too many intermediates to produce Hydrogen evolution reaction.The various intermediates involved in the process from vs.RHE to NH3and the related reaction equations are as follows(where vs.RHE represents the potential relative to the reversible hydrogen electrode;*indicates active site):
N2+6H2O+6e−→2NH3+6OH −0.763 V vs. RHE at pH=14
2H2O+2e−→H2+2OH− −0.828 V vs. RHE at pH=14
Whether in acidic or alkaline environment,the potential of ammonia formation reaction(reaction(5),(7))is close to that of hydrogen evolution reaction(reaction(8),(6)),so the side reaction HER is often mixed in the product of NRR catalytic reaction,which brings challenges to the selectivity of NRR catalyst.in addition,the reaction potentials of the intermediates involved in NRR are all negative,which makes the formation of ammonia thermodynamically difficult.These barriers together become an inevitable problem for the improvement of selectivity and yield in the research of electrocatalytic NRR catalysts。
the reaction mechanism of NRR As described in the previous section,the electrocatalytic synthesis of ammonia from NRR involves multiple proton and electron transfers,and the reaction process may be accompanied by complex reaction mechanisms,which have not yet been accurately studied in detail.At present,the typical NRR mechanisms proposed are dissociation mechanism and binding mechanism[20]。 In the dissociation mechanism(Figure 1A),the N2molecule adsorbed on the catalyst surface undergoes triple bond cleavage at a specific temperature and pressure to generate two mutually independent activated N atoms,each of which is converted into ammonia after successive hydrogenation reactions,and the NH3molecule is finally detached from the catalyst surface.The Haber-Bosch process usually follows this path,and the need to break N≡N is an important reason for the high energy consumption of the Haber-Bosch process.In the binding mechanism,N2molecules first bind to the active sites on the surface of the catalyst,and after the completion of molecular activation,N2continuously receives protons from the environment for hydrogenation,and finally N≡N is broken,and NH3molecules are desorbed from the surface of the catalyst to complete a N2reduction.According to the sequence of hydrogenation,the binding mechanism can be divided into alternating mechanism(Fig.1b)and distal mechanism(Fig.1C )。
图1 典型的N2催化还原为NH3机理：（a）解离机制；（b）交替结合机制；（c）远端结合机制；（d）酶机制

Fig. 1 Typical mechanism of N2 catalytic reduction to NH3. (a) Dissociation mechanism; (b) Alternate binding mechanisms; (c) Distal binding mechanism; (d) Enzyme mechanism

Different from the terminal adsorption of N2in the binding mechanism,in the enzyme mechanism(Figure 1D),the N2is adsorbed on the active site of the catalyst through the side terminal configuration,and then H is alternately bound to two N atoms to produce NH3 。
For transition metal nitride catalysts,Abghoui proposed a new Mars-van Krevelen(MvK)mechanism,in which H atoms are attached to N atoms on the surface of TMNs,and the first reaction to form NH3does not require the adsorption and cleavage of N2on the catalyst surface,which can effectively reduce the energy barrier for NH3formation[21]。 The N-vacancy is replenished by the supplied N2molecule,and the hydrogenation process ensues,and the surface energy of the catalyst with N-vacancy is higher,favoring favorable adsorption of N2and cleavage of N≡N bond.Finally,the catalyst surface is regenerated and this cycle is repeated in the NRR process.Taking the electrocatalytic nitrogen reduction reaction on the surface of VN nanoparticles as an example,Kong et al.Proposed VN nanosheets doped with heterogeneous metal atoms at in-plane and edge sites as electrocatalysts for nitrogen reduction reaction through density functional theory(DFT)calculations[22]。 Bader charge analysis and charge density difference show that the additional charge injection contributes to the adsorption performance,and therefore,the introduction of heterogeneous TM atoms can effectively improve the adsorption capacity of VN for N2.Further study,the formation energy of doped TM shows that the element exhibits a higher doping rate in the first half due to the existence of negative energy.VMo shows excellent NRR performance in the edge-doped structure with a limiting potential of only 0.22 V.However,VTi not only has a small overpotential(<0.6 V)at the in-plane and edge sites,but also its negative formation energy indicates a high doping rate in the experiment.In addition,VTi and VMo have good selectivity for HER 。
In addition,Ling et al.Put forward a new point of view on the NRR mechanism of the surface of precious metal materials such as Au and Pd[23]。 Different from the traditional mechanism in which the adsorption of N2molecules on the surface of the catalyst is the first step,the reduction of H+becomes the first step and is also the determining step of the overpotential,which can explain why Au can synthesize ammonia at a lower potential,and then N2is activated by surface*H and reduced to*N2H2,which requires a large amount of energy input and determines the rate of the whole reaction 。

3 Non-noble transition metal catalyst for NRR

In the NRR process,the transition metal in the d block can bind nitrogen molecules by accepting and feeding back electrons.Specifically,the empty d orbital of the transition metal accepts the lone pair electrons of nitrogen molecules,which can enhance the adsorption of N2;At the same time,its separated d-orbital electron can also donate the antibonding orbital of the N2molecule,which weakens N≡N and decomposes N2 。

3.1 Application of Mo in NRR catalyst

Biological nitrogen fixation is one of the earliest ways to convert nitrogen into ammonia.By studying the structure and function of nitrogenase,early scholars found that the principle of nitrogen fixation is to use molybdenum-iron protein as the active center of molybdenum nitrogenase to activate nitrogen and fix nitrogen into ammonia under mild conditions.Molybdenum nitrogenase is the most common enzyme.At the same time,based on structural chemistry,chemical bond theory,complexation catalysis,chemical probe and biochemical methods,researchers have proposed dozens of nitrogen fixation models of cluster polynuclear coordination activated N2molecules,all of which use Mo as the coordination site of the substrate[24]。 Inspired by this,many researchers have speculated that Mo metal can also be a catalyst for electrochemical ammonia synthesis.Using DFT calculations,Sk Skúlason et al.Confirmed that the electrochemical synthesis of ammonia from Mo at ambient temperature and pressure is feasible[25]。 As the catalytic performance of Mo is not ideal,the microstructure of Mo can be effectively adjusted by introducing vacancies,doping heteroatoms and constructing heterostructures to further improve the performance of electrocatalytic N2to NH3 。
The Mo3Si supported on carbon paper prepared by Wang et al.Has been proved to have NRR catalytic ability by experiments[26]。 The supported Mo atom was shown to be the active site by DFT calculations.However,by studying H adsorption to evaluate the selectivity of NRR for HER,it was found that the highly active Mo could also effectively bind H atoms,resulting in an unexpected FE of the catalyst.Inspired by the fact that Mo can exist in compounds with different valence States,Tan et al.Creatively developed a feasible N2bridging strategy for electrocatalytic N2immobilization by constructing a heterodicationic Mo4+-Mo6+center at the MoO2@MoO3interface[27]。 The 4degand t2gorbitals of Mo can be synergistically optimized to accept the unoccupiedπ*orbital from the occupied electron orbital and to feed back to N≡N,thus catalyzing the progress of NRR and finally obtaining an ammonia yield of 60.9μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$.It has been found that oxygen vacancies can be used as active sites for the adsorption of N2on the surface of the catalyst,and the surface properties and electronic structure of the catalyst can be changed by introducing oxygen vacancies,so that the electrons captured by oxygen vacancies can enter the antibonding orbital of N2,which weakens N≡N,reduces the reaction energy barrier,and accelerates the reduction of nitrogen[27,28]。 In the study of Zhang et al.,oxygen-coordinated molybdenum(Mo)single atoms(Mo-O-C)anchored on carbon were synthesized via an impregnation-pyrolysis-etching synthesis route and pretreated bacterial cellulose with abundant oxygen groups and nanofiber network structure as an adsorption regulator[29]。 The catalyst has a high surface area of V vs.RHE and a porous structure,with an ammonia yield of（248.6±12.6）μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$and a faradaic efficiency of(43.8%±2.3%)at−0.2 V vs.RHE potential.The results of synchrotron X-ray absorption spectroscopy and DFT calculation show that the O-coordinated Mo structure of the Mo-(O-C2)4anchored on carbon is the most stable monatomic structure 。
it has been shown that the modification of the S atom is generally beneficial to the selective hydrogenation of nitrogen to ammonia,because It can provide more sufficient exposed active sites for NRR[30⇓⇓~33]。 Li et al.Used a modified hydrothermal method to prepare FeMoO4nanorods,and obtained sulfided nanorods composed of FeMoO4,2H-MoS2,and FeS2by sulfidation of FeMoO4[34]。 The ammonia yield is as high as V vs.,and the faradic efficiency reaches 30.9%at−0.39 V vs.RHE.In the continuous research of scholars,Zhang et al.Designed a new strategy of vacancy and phase engineering to achieve reduction and phase control by potential difference in molten sodium,and activated the synthesis of heterostructured 1T/2H-MoSxmonolayer catalyst rich in sulfur vacancies.At the same time,DFT calculation shows that the effective transition from 2 H to 1 T of MoS2forms sulfur-rich vacancy and heterojunction,which changes the local electron density in the potential-determining step related to MoSxto reduce the energy barrier of nitrogen reduction[35]。 The 1T/2H-MoSxof the catalyst showed excellent ammonia conversion 93.2μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$,and the faradic efficiency was 20.5%at−0.4 V.The catalyst operated stably within 30 H and had good stability.To comprehensively present the catalytic performance of Mo-based catalysts,Table 1 summarizes the recently published Mo-based electrocatalytic NRR catalysts 。
表1 Mo-based electrocatalytic NRR catalyst[26⇓⇓⇓⇓⇓⇓⇓⇓⇓~36]

Table 1 Mo-based electrocatalytic NRR catalysts[26⇓⇓⇓⇓⇓⇓⇓⇓⇓~36]

Catalyst Electrolyte NH3 yield FE(%) Potential Ref
MoS3 0.5 M LiClO4 51.7 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 12.8 -0.3 V vs. RHE 30
Vo-MoO2@C 0.5 M Na2SO4 9.75 μg·h-1·mg-1 3.24 -0.5 V vs. RHE 28
Mo−(O−C2)4 0.1 M Na2SO4 ~248.6 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ ~43.8 -0.2 V vs. RHE 29
VS-MoS2 0.1 M Na2SO4 29.55 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 4.58 -0.3 V vs. RHE
(ammonia yield)
-0.2 V vs. RHE (FE)		 31
FeS2-MoS2@IFx 0.1 M KOH 7.1×10-10 mol·s-1·cm-2 4.6 −0.5 V vs. RHE
(ammonia yield)
−0.3 V vs. RHE (FE)		 36
MoO2@MoO3 0.05 M H2SO4 60.9 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 23.8 −0.35 V vs. RHE 27
vulcanized FeMoO4 0.1 M Na2SO4 31.93 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 30.9 −0.39 V vs. RHE 34
SM-MoS2−x 0.05 M H2SO4 17.2 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 24.7 −0.2 V vs. RHE 32
MoS 0.1 M K2SO4 43.4±3 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 16.8 −0.3 V vs. RHE 33
1T/2H-MoSx 0.1M Na2SO4 93.2 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 20.5 −0.4 V vs. RHE 35
Mo3Si 0.1 M Na2SO4 2×10-10mol·s-1·cm-2 6.69 −0.4 V vs. RHE
(ammonia yield)
−0.3 V vs. RHE (FE)		 26

3.2 Application of Co and Ni in NRR catalyst

Because Co atom not only has the empty d orbital which can accept the long pair electron of N2molecule,but also has the occupied d orbital which can donate electron to the antibondingπ*orbital of N2molecule,Co atom can cooperate to accept the electron density from N2and donate backward.In this process,the electron density of the N2triple bond can be significantly and effectively reduced,and then the inert N2molecule is activated due to the appropriate energy level and matching symmetry.For this reason,Co atom is also the research object of many scholars in the field of NRR catalysis.Considering the adjacent positions of Co and Ni in the periodic table of elements and their similar atomic radii,they have similar effects in the catalyst 。
Du and Yang's team worked together on a new nanocomposite composed of nickel nanoparticles[37]。 The team ground a certain proportion of V,Al and C powders by ball mill and sintered them at 1300℃,then etched them by HF to obtain V4C3TxMXene,and at the same time,prepared Ni precursor by hydrothermal method.The Ni precursor and the V4C3TxMXene were mixed into ionized water,and the two types of nanosheets would stick together driven by the hydrogen bonding interaction,and finally the mixture was annealed in a 5%H2/Ar flow at 400℃to form the composite.The distal NRR pathway on the O vacancy of the V4C3TxMXene surface and the alternative NRR pathway on the Ni NPs were indicated by DFT simulation.They can synergistically promote the NRR reaction,thereby improving the overall catalytic ability of the catalyst 。
Fan et al.Prepared nanoporous NiSb by chemical dealloying of a Ni96Sb4precursor alloy[38]。 Compared with Ni or Sb,the nanoporous NiSb alloy can enhance the adsorption of N2molecules and reduce the activation energy barrier of N≡N.The results of DFT show that the coordination of Sb atoms can be separated from the binding sites of N2after alloying,thus reducing the influence of H coverage on the blocking active sites.At a potential of−0.2 V vs.RHE,the catalytic material exhibited a V vs.RHE ammonia yield with an excellent faradaic efficiency of 48% 。
Murmu et al.Identified Ni as the active site of NiPc NRs by DFT calculation.the presence of Ni makes the overpotential of NRR lower than that of HER,indicating that the Ni active site can amplify the optimal active site of NRR by inhibiting the activity of HER[39]。 The experiment showed a 85μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$ammonia yield and a faradaic efficiency of 25%.In addition,because Ni has relatively good conductivity,it is also used to modify elements with poor conductivity.Li et al.Used nickel foam as a substrate to load bismuth nanoparticles through a replacement reaction to prepare BiNPs@NF electrodes[40]。 It is worth noting that the material prepared by the replacement reaction eliminates the use of polymer binder,which can reduce the impedance between the diffusion layer and the catalytic substrate,and reduce the energy input required for ammonia production.However,the efficient passage of electrons inevitably accelerates the HER reaction,resulting in a low Faradaic efficiency of the catalyst。
Competitive hydrogen evolution reaction is an important factor affecting the efficiency of electrocatalytic reduction of ammonia,and improving the selectivity of the catalyst for NRR in aqueous solution is an important method to improve the yield of ammonia.Lu et al.Believed that the existence of strong hydrogen evolution sites could effectively attract hydrogen ions in the electrolyte,thus providing more opportunities for the active sites of NRR catalysts to contact N2molecules and directly reducing the competitive effect of HER reaction.Therefore,HER attractant(MoS2)was added by hydrothermal method.The V vs.RHE composite was prepared,and it was experimentally verified that the composite had a faradaic efficiency of 34.49%and a NH3yield of 54.66μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$at−0.2 V vs.RHE,greatly improving the NRR activity of ZIF-67-derived Co-NC[41]。
Reasonable construction of nanostructures can also effectively improve the electrocatalytic activity,and Ghorai's team prepared cobalt phthalocyanine hollow nanotubes(β-CoPc NTs)showing good NRR performance[42]。 Gram-scale catalysts can be synthesized by hydrothermal method,and the prepared CoPc NTs have one-dimensional hollow structure,which can greatly increase the specific surface area of the material and expose more active sites.Density functional theory shows that nitrogen can follow a distal binding mechanism as well as an alternating binding mechanism in this catalytic system.Compared with the iron phthalocyanine catalyst in the phthalocyanine system,the d-band center of Co is lower than that of Fe(−1.51 and−1.48 eV,respectively),which makes the cobalt phthalocyanine have a lower overpotential in the NRR reaction than the iron phthalocyanine[43]。 The effective catalytic center and reasonable surface structure makeβ-CoPc NTs have 107.9μg·h−1·$\mathrm{mg}_{\text{cat}}^{-1}$catalytic efficiency.In order to facilitate the comparison of the performance of Co and Ni-based catalysts,the yield and Faradaic efficiency of various catalysts are summarized in Table 2 。
表2 Co-based and Ni-based electrocatalytic NRR catalyst[37⇓⇓⇓⇓~42,44⇓⇓⇓⇓⇓⇓⇓⇓ ~53]

Table 2 Co-and Ni-based electrocatalytic NRR catalysts[37⇓⇓⇓⇓~42,44⇓⇓⇓⇓⇓⇓⇓⇓ ~53]

Catalyst Electrolyte NH3 yield FE (%) Potential Ref
CoS 0.05 M H2SO4 (12.1±1.4) μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 16.5±1.5 -0.15 V vs. RHE 44
CoS2/MoS2 1.0 M K2SO4 38.61 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 34.66 -0.25 V vs. RHE 45
Co3O4 nanoparticle 0.1 M Na2SO4 235.0 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 16.3 -0.3 V vs. RHE 46
CoMoO4 nanorod 0.1 M Na2SO4 79.87 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 22.76 -0.1 V vs. RHE 47
CoPi/HSNPC 1.0 M KOH 16.48 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 4.46 -0.2 V vs. RHE 48
Zn-Co3O4-10 0.1 M HCl 22.71 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 11.9 -0.3 V vs. RHE 49
Co-NC/MoS2 0.1 MHCl 54.66 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 34.49 -0.2 V vs. RHE 41
β-CoPc NTs 0.1M HCl 107.9 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 27.7 -0.3 V vs. RHE 42
BiNPs@NF 0.5 M K2SO4 9.3×10-11 mol·s-1·cm-2 6.3 -0.5 V vs. RHE 40
Ni@MX 0.1 M KOH 21.29 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 14.86 — 37
NiS@MoS2 0.1 M Na2SO4 9.66 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 14.8 -0.3 V vs. RHE
(ammonia yield)
-0.1 V vs. RHE (FE)		 50
Ni/NiFeOH 0.5 M KOH 19.74 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 23.34 -0.15 V vs. RHE 51
NiSb 0.1 M HCl 56.9 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 48.0 -0.2 V vs. RHE 38
NiPc NRs 0.1 M HCl 85 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 25 -0.3 V vs. RHE 39
NiFe-LDH 0.1 M KOH 19.44 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 19.41 -0.2 V vs. RHE 52
V-NiS2 0.1 M 47.63 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 9.37 -0.45 V vs. RHE
(ammonia yield)
-0.35 V vs. RHE (FE)		 53

3.3 Application of Fe in NRR catalyst

iron is the only element coexisting In the three biological nitrogenases,which indicates that iron plays an important role in NRR.in addition,iron is an active metal,which is easily complexed with other elements to modify into nitrides,hydroxides,oxides,sulfides and phosphides[54]。
Based on the structure of Mo-Fe nitrogenase under natural conditions,iron-molybdenum compounds have attracted much attention.the synergistic effect between Fe and Mo and the electronic coupling effect promote the charge transfer from the metal orbital to the nitrogen class,thus weakening the inert nitrogen triple bond[55]。 Han et al.Designed a bimetallic Fe-Mo atom-modified C2N to synthesize a FeMo@C2N catalyst,and calculated that the binding energy of FeMo@C2N with FeMo was 7.61 eV,which was basically greater than the corresponding Ecof Fe(4.86 eV)and Mo(6.34 eV).The bond length of Fe-Mo(on C2N)is 1.7474Å,which is significantly smaller than the sum of the atomic radii of Mo and Fe(rFe=1.40Å,rMo=1.45Å),indicating the successful diatomic coordination of Fe-Mo with good stability[56]。 The more localized electronic States of 3D/4D and 2p orbitals near the Fermi level in FeMo@C2N make it superior for NRR activity as seen by the FeMo@C2N local DOS analysis.Density functional theory calculations show that FeMo@C2N has a low limiting potential(−0.17 V through the enzyme pathway)and a high ability to inhibit the hydrogen evolution reaction(EFNRR=99.93%).Wang et al.Induced Bi2MoO6to form an irregular polyhedral structure by doping Fe3+through a one-step method,which provided more active sites for the catalyst.The synergistic effect of unsaturated Bi ions and strong Lewis acid Fe3+effectively strengthened the adsorption of N2and the cleavage of N−N,and established abundant active nitrogen molecules on Fe-BMO[57]。 The V vs.RHE yield and faradaic efficiency of Fe-BMO reached 71.01μg·h−1·$\mathrm{mg}_{\text{cat}}^{-1}$and 80.12%,respectively,in 0.1 M HCl solution at−0.1 V vs.RHE potential,and the reliability and stability of the experimental data were guaranteed by rigorous control experiments 。
Guo et al.Prepared Reduced graphene oxide by hydrothermal method and supported it on Reduced graphene oxide,in which Fe atoms have abundant surface terminal groups and low-coordinated Fe3csites after combining with Te,which can activate NRR while inhibiting HER[58]。 The study also proposed that Li ions could hinder the adsorption of H,and an ammonia yield of 39.2μg·h−1·$\mathrm{mg}_{\text{cat}}^{-1}$as well as a faradaic efficiency of 18.1%was achieved in the 0.5 M LiClO4solution 。
The transition metal diboride（TMB2）has a layered structure similar to MXene,which has been proved to play a role in NRR by researchers through theory and experiment.The amorphous FeB2porous nanosheet（a-FeB2PNS）prepared by Chu et al.Exhibited satisfactory NRR catalytic activity,reaching an ammonia yield of 39.8μg·h−1·$\mathrm{mg}_{\text{cat}}^{-1}$and a faradaic efficiency of 16.7%in 0.5 M LiClO4solution[59]。 DFT calculations demonstrate that surface amorphization can induce an upward shift of the d-band center in a-FeB2to enhance the stability of the bond*N2H intermediate and facilitate NRR 。
Lv et al.Fabricated a porous Fe-doped TiO2mesoporous nanospheres(FeHTNs)loaded with oxygen vacancies by designing morphology engineering[60]。 The ingenious porous and mesoporous structure is like the"alveolus"of animal respiration,which effectively realizes the diffusion of N2gas.Meanwhile,the abundant oxygen vacancies promote the adsorption and activation of N2.Taking advantage of the effective activation effect of Fe on NRR,it was confirmed that the doping of Fe improved the intrinsic catalytic activity of the catalyst,and the ammonia yield of the material was 2.8 times that of the undoped TiO2hollow nanospheres(HTNs),reaching the 43.14μg·h−1·$\mathrm{mg}_{\text{cat}}^{-1}$.By doping iron atoms on the surface of InVO4,Li team confirmed that the Fe-InVO4nanosheets prepared by doping had excellent NRR performance due to the formation of abundant oxygen vacancies[61]。 The synergistic adsorption and activation of N2by Fe active sites and oxygen vacancies is the key 。
Liu et al.Used a hydrothermal method to synthesize 0.50Fe-Bi2WO6with a smooth octahedral structure,and the ammonia yield reached an astonishing 289μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$[62]。 Liu's team conducted a series of strictly controlled experiments to confirm that the product ammonia comes from the catalyst's electrocatalytic effect on N2.In addition,through the electrocatalytic NRR comparative experiments of pure BiWO and FeWO,the team concluded that the doping of Fe promoted the NRR activity.In the electrochemical impedance spectroscopy(EIS)test,the charge transfer resistance of FE-BiWO is much smaller than that of BiWO,that is,the addition of Fe can enhance the electron transfer ability of Fe-BiWO catalyst,but at the same time,the efficient electron transfer and lower reaction potential also lead to the enhancement of competitive reaction HER,resulting in Fe can not achieve the ideal expectation.Hao et al.,inspired by the unique solubility of eutectic solvents(DESs),dissolved commercial Fe3O4in choline chloride/oxalic acid deep eutectic solvent,and obtained powders and nanosheet precursors after microwave heating,and then prepared porous Fe3O4nanosheets rich in oxygen vacancies by annealing in nitrogen atmosphere[63]。 It has an ammonia yield of V vs.RHE at a low potential of−0.1 V vs.RHE with a faradaic efficiency of 34.38%.A large number of autoclaves and other equipment are used in the preparation process of the catalysts reported above,which is cumbersome and the output is low.In view of this situation,He et al.Used electrochemistry to realize the rapid preparation of gram-scale catalyst Fe3O4under normal temperature and pressure.The ammonia yield of Fe3O4nanoparticles under neutral conditions can reach 12.09μg·h−1·$\mathrm{mg}_{\text{cat}}^{-1}$,and the faradic efficiency can reach 16.9%,which is of certain significance for the wide use of NNR[64]。
While reducing the catalyst cost,Kafle et al.Simplified the synthesis procedure and prepared nickel boride-cellulose paper(NiB-CP)electrode deposited by Fe3O4on non-conductive CP by a continuous two-step manufacturing method[65]。 In the scanning electron microscopy(SEM)image,Fe3O4-70/NiB-CP exhibits a uniform nanosphere array composed of nanosheets,and Fe3O4nanospheres are uniformly distributed on the surface.The high-resolution transmission electron microscopy(HR-TEM)image shows a porous framework similar to the field emission scanning electron microscopy(FE-SEM)image,which makes the catalyst have a large surface area and easy to transfer electrons and mass.The faradaic efficiency of NRR was 4.32%with a yield of V vs.RHE at−0.1 V vs.RHE.Zhang et al.Electrodeposited Fe on carbon cloth by cyclic voltammetry to prepare the working electrode by electrochemical method[66]。 While demonstrating the NRR catalytic performance of the materials,the study proposed that phosphorus and K+could effectively prevent the adsorption of H+onto the active sites on the catalyst surface,thereby preventing the generation of hydrogen.The solubility of N2in phosphate solution is better than that in other electrolyte aqueous solutions,thus effectively improving the efficiency of ammonia production.Table 3 summarizes the ammonia yield and faradaic efficiency of recently published Fe single-atom catalysts,oxides,sulfides,etc.Among them,Fe-BMO has better overall performance,and the ammonia yield can also reach 71.01μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$while the faradic efficiency is 80.12% 。
表3 Fe-based electrocatalytic NRR catalyst[57⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~71]

Table 3 Fe based electrocatalytic NRR catalysts[57⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~71]

Catalyst Electrolyte NH3 yield FE(%) Potential Ref.
C@CoFe2O4−x 0.1 M Na2SO4 30.97 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 11.65 −0.4 V vs. RHE 67
FeTe2/RGO 0.5 M LiClO4 39.2 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 18.1 −0.5 V vs. RHE
(ammonia yield)
−0.3 V vs. RHE (FE)		 58
Fe3O4 0.1 M Na2SO4 12.09 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 16.9 −0.15 V vs. RHE 64
Fe3O4 0.1 M Na2SO4 12.09 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 34.38 −0.1 V vs. RHE 63
a-FeB2PNS 0.5 M LiClO4 39.8 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 16.7 −0.3 V vs. RHE
(ammonia yield)
−0.2 V vs. RHE (FE)		 59
Fe(III)-MoO3 0.1M Na2SO4 9.66 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 13.1 −0.6 V vs. RHE 68
FeHTNs 0.1M Na2SO4 43.14 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 16.35 −0.7 V vs. RHE 60
Fe-InVO4 0.1M HCl 17.23 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 14.27 −0.4 V vs. RHE 61
Fe-BMO 0.1M HCl 71.01 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 80.12 −0.1 V vs. RHE 57
Nano-Fe 1.0 M K3PO4 79.0±5×10−11 mol·s−1·cm−2 16.68 − 66
0.50Fe-Bi2WO6 0.05 M H2SO4 289 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 1.96 −0.75 V vs. RHE 62
F-Fe:TiO2 0.05 M H2SO4 27.86 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 27.67 −0.5 V vs. RHE 69
Fe3O4-70/NiB-CP 0.1 M KOH 245 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 4.32 −0.1 V vs. RHE 65
Zn-Fe2O3 0.1 M Na2SO4 15.1 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 10.4 −0.5 V vs. RHE 70
V-Fe2O3 0.1 M HCl 68.7 μg·h−1·$\mathrm{mg}_{\text {cat }}^{-1}$ 5.7 −0.2 V vs. RHE 71

3.4 Application of other transition metals in NRR catalyst

in addition to the above transition metal elements that have been proved to be used in NRR,transition metals such as Zr,Ti,V,and Cu have also been used in the preparation of NRR catalytic materials(see Table 4)[72⇓~74][75⇓~77][78,79]。 Li's team used DFT and experiments to confirm that Zr sites with oxygen vacancies can be the adsorption and active centers of NRR terminal pathway[70]。 After that,the team adjusted the surface of the material by doping Cu on this basis,so that its catalytic performance was improved[74]。
表4 Other transition metal-based electrocatalytic NRR catalysts[66⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~82].

Table 4 Other transition metal-based electrocatalytic NRR catalysts[66⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~82]

Catalyst Electrolyte NH3 yield FE(%) Potential Ref.
ZrO2@C 0.1 M Na2SO4 6.72 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 2.68 −0.6 V vs. RHE 73
ZrO2 0.1 M Na2SO4 9.63 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 12.1 −0.7 V vs. RHE 72
Cu-ZrO2 0.1 M Na2SO4 12.13 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 13.4 −0.6 V vs. RHE 74
UiO-Zr-Ti 0.1 M Na2SO4 1.16×10-10 mol·s-1·cm-2 80.36 −0.3 V vs. RHE 80
Ti2O3 0.1 M HCl 26.01 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 9.16 −0.25 V vs. RHE 75
Ag/TiO2 0.1 M HCl 3.158×10-10 mol·s-1·cm-2 0.13 −0.6 V vs. RHE 76
Sn-TiO2 0.1 M KOH 10.5 μg·h-1·cm-2 8.36 −0.45 V vs. RHE 77
ZIF-67@Ti3C2 0.1 M KOH 6.52 μmol·h-1·cm-2 20.2 −0.4 V vs. RHE 83
VN@NSC 0.1 M HCl 20.5 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 8.6 −0.3 V vs. RHE 79
V3O7·H2O 0.1 M Na2SO4 36.42 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 14.2 −0.55 V vs. RHE 78
Cu0·1CeO2@NC 0.1 M Na2SO4 44.5 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 34.6 −0.5 V vs. RHE 84
Cu2-xS/MoS2 0.1 M Na2SO4 22.1 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 6.06 −0.5 V vs. RHE 81
np-CuMn 0.1 M Na2SO4 28.9 μg·h-1·cm-2 9.83 −0.3 V vs. RHE 85
Cu@Ce-MOF 0.1 M KOH 14.83 μg·h-1·cm-2 10.81 −0.2 V vs. RHE 82
CuO NA/CF 0.1 M Na2SO4 1.84×10−9 mol·s-1·cm-2 18.2 −0.1 V vs. RHE 86
Cu9S5/NC 0.5 M Na2SO4 10.8±0.4 μg·h-1·cm-2 5±3 −0.5 V vs. RHE 87
Cu-Nb2O5@C 0.1 M HCl 28.07 μg·h-1·$\mathrm{mg}_{\text {cat }}^{-1}$ 13.25 −0.2 V vs. RHE 88
Tan et al.used N,N-dimethylformamide(DMF)and Zr to synthesize UiO-Zr,and then Used Ti to exchange Zr in UiO-Zr to prepare a series of UiO-Zr-Ti with different exchange ratios[80]。 It is believed that the d-orbital of Ti and theπg2p orbital of N can be highly overlapped,and the formed d-π*orbital is beneficial to the activation of N2.Although Ti as the active site of the catalyst can well activate the N2,the replacement of Zr by Ti will destroy the ordered structure of 10UiO-Zr,resulting in a decrease in the specific surface area,which has a negative impact on the exposure of active sites.Through experiments,the research team prepared UiO-Zr-Ti-5d with appropriate Ti-Zr ratio,showing excellent Faradaic efficiency,reaching an astonishing 80.36%,which is higher than the reported types of NRR catalysts 。
Lv et al.Embedded VN nanodots in situ into N,S-codoped carbon matrix by pyrolysis[79]。 The stable mosaic structure effectively prevents the overflow of hydrogen,reduces the coverage of H2on the surface of the catalyst due to hydrogen overflow while ensuring that VN nanodots do not fall off and deactivate,and ensures the contact between active sites and N2.The unique structure of the catalyst allows its effective components to play a full role.The adsorption and activation of nitrogen are completed by NSC,while the conversion of nitrogen to ammonia is completed by nano-VN.The synergistic catalysis of the two shows unique nitrogen reduction activity 。
Jiang's team confirmed the catalytic effect of NRR by imitating the active site of natural nitrogenase and incorporating MoS2into the Cu2−xS/MoS2of ultra-thin core-shell spherical catalyst[81]。 as the main structure of the catalyst,the prepared Cu spherical shell structure not only improves the adsorption strength of nitrogen,but also exposes more active sites at the curved core-shell interface.Metal-organic frameworks(MOFs)have been gradually applied to NRR electrocatalysts due to their high dispersion and high specific surface area,in which Cu can be used as an excellent conductive substrate(such as copper foam)to enhance charge transfer.Inspired by the above route,Liu's team prepared Ce-MOF grown on copper mesh(Cu@CeMOF)by a simple self-sacrificial template method,which can avoid the use of polymer binders by in-situ growth[82]。

4 Conclusion and prospect

In this paper,the research progress of E-NRR catalysts based on non-noble transition metal Fe,Mo,Co,Ni and Cu based composite metal catalysts is reviewed.the dissociation mechanism,alternative binding mechanism,distal binding mechanism and enzymatic mechanism of E-NRR were explored.The preparation methods and catalytic properties of transition non-noble metal-based catalysts were summarized from the aspects of theoretical calculation and practical application,and the feasibility of electrocatalytic ammonia synthesis under ambient conditions was discussed。
By comparing the catalytic performance(such as ammonia yield,Fe and reaction potential)of various catalysts in E-NRR,it can be concluded that the non-noble transition metal-based catalysts with better comprehensive catalytic performance are FE-BMO and Mo-(O-C2)4.Among them,the V vs.RHE catalyst at a potential of−0.2 V vs.RHE can achieve an ammonia yield as high as V vs.RHE with a FE value of 43.8%,while the FE-BMO catalyst at a potential of−0.1 V vs.RHE has a FE as high as 80.12%with an ammonia yield of 71.01μg·h-1·$\mathrm{mg}_{\text{cat}}^{-1}$.It is not difficult to find that there is no E-NRR catalyst that can meet both high yield and high Faradaic efficiency in the existing research.In addition,in the same kind of catalyst,by increasing the specific surface area,constructing heterojunction and creating surface defects,the catalyst can expose more active sites and improve the mass transfer effect of N2and NH3,thus enhancing the catalytic performance of the catalyst.Therefore,in view of how to synchronously improve ammonia yield and FE in the future development of E-NRR,the following prospects can be made:
(1)The design and preparation of catalysts by introducing vacancies,heteroatom doping,heterojunction engineering and single atom doping are helpful to regulate the charge density distribution and electronic structure,improve the catalytic kinetic performance,and thus improve the surface activity of catalysts,thus providing a good electrocatalytic interface environment for E-NRR.In addition,materials with porous structure and high specific surface area(such as two-dimensional carbon nanosheets,hollow carbon nanotubes,MOF,three-dimensional metal foams,etc.)Are selected as catalyst carriers,which is conducive to improving the charge transfer rate and the mass transfer efficiency of reactants;At the same time,the catalyst which can effectively prevent the H+from being adsorbed on the active sites of the catalyst surface is found through the combination of DFT theoretical calculation and experiment,so as to reduce the production of hydrogen.The combination of the two is expected to achieve the simultaneous improvement of E-NRR yield and FE 。
(2)the structure of the electrolyzer directly affects the contact mode of the electrolyte and the reactant with the catalyst.the special flow field(such as serpentine flow field,return flow field and ripple flow field)is helpful to control the reactant concentration on the surface of the electrode catalyst and improve the current utilization efficiency,thereby improving the FE and ammonia yield.Therefore,when designing the E-NRR catalytic system,the appropriate electrolyzer can be designed according to the characteristics of the catalyst to ensure that the feed direction,feed concentration and flow field can be accurately controlled to ensure that the reaction can be carried out under optimized conditions。

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