Homogeneous, Heterogeneous and Biological Catalysts for

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Review

Homogeneous, Heterogeneous and Biological Catalysts for Electrochemical N2 Reduction towards NH3 under Ambient Conditions Liu Huimin, Li Wei, Fei Liu, Zengxia Pei, Jeffrey Shi, Zhou-jun Wang, Dehua He, and Yuan Chen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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Homogeneous, Heterogeneous and Biological Catalysts for Electrochemical N2 Reduction towards NH3 under Ambient Conditions Huimin Liu a,c, Li Wei a, Fei Liu a,e, Zengxia Pei a, Jeffrey Shi a, Zhou-jun Wang b*, Dehua He d*, Yuan Chen a* a,

School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006,

Australia b,

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Energy

Environmental Catalysis, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing 100029, P.R. China c, TJU-NIMS International Collaboration Laboratory, School of Material Science and Engineering,

Tianjin University, Tianjin 300072, P. R. China d,

Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics and Molecular

Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China e,

State Key Laboratory of Applied Microbiology Southern China, Guangdong Provincial Key

Laboratory of Microbial Culture Collection and Application, Guangdong Institute of Microbiology * Corresponding Authors. [email protected] (Z.-j. Wang), [email protected] (D. He), [email protected] (Y. Chen)

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Abstract: Ammonia (NH3) synthesis is an important industrial chemical process. Recently, electrochemically converting the earth-abundant dinitrogen (N2) in the aqueous phase to NH3 at ambient conditions has been proposed as an alternative to the well-established Haber-Bosch process. Catalysts for the electrochemical N2 reduction to NH3 play crucial roles in realizing this NH3 synthesis route. Electrochemical N2 reduction has been studied for decades and, many studies have emerged in the last few years. Herein, we provide a comprehensive review to summarize various catalysts used for achieving electrochemical N2 reduction to NH3, including homogeneous, heterogeneous and biological catalysts, as well as relevant computational studies to understand their reaction mechanisms. We compare the advantages and shortcomings of these catalytic systems. Future research directions for realizing catalysts with low overpotentials, high energy efficiency, good scalability, and stability modularity are also proposed. We hope that this review provides readers an overview of this fast-growing research field and encourages more studies towards the rational design of catalysts for electrochemical N2 reduction to NH3 under ambient conditions.

Keywords: electrochemical nitrogen reduction, ammonia synthesis, homogeneous catalyst, heterogeneous catalyst, biological catalysts


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1. Introduction Ammonia (NH3) is an important fertilizer and widely used as synthetic building blocks of various pharmaceutical products. Additionally, it also works as a liquid transportation fuel in internal combustion engines. Therefore, it has been recognized as one of the mainstays in the modern world with over 160 million tons synthesized every year.1-5 However, the commonly used Haber-Bosch process for NH3 synthesis has several disadvantages: the high-cost of clean hydrogen (H2) produced from fossil resources, huge energy inputs derived from fossil fuels, relatively low NH3 yield (conversion rates of 10% - 15% from dinitrogen (N2)), and harsh reaction conditions (temperatures: 400 - 500 oC and pressures: > 60 bars).6-8 It is of great importance to develop alternative approaches for NH3 synthesis, which can be efficiently carried out at mild reaction conditions. Several potential methods are being explored. First, as a cheap and abundant hydrogenated resource, water (H2O) has been explored as a substitute for H2 in CO2 reduction.9,10 Similarly, reducing N2 using H2O has been studied to be a potential approach for the artificial synthesis of NH3.11,12 Second, photocatalytic reduction of N2 to NH3 by harnessing the inexhaustible solar light has been studied as a sustainable NH3 synthesis route.13,14 Many semiconductors14-16 and metal decorated/doped semiconductors17,18 have been tested as photocatalysts. Nevertheless, the efficiencies of these photocatalysts are still low due to their low photon utilization rate and the easy combination of excited electron-hole pairs.19-22 Third, electricity can be generated from sustainable solar or wind sources,23-30 which has been used to electrochemically reduce N2 to NH3 at moderate temperatures and atmospheric pressure. NH3 may also be used to store energy generated from intermittent renewable energy supplies, because it has a high energy density (4.32 kWh L-1) and high hydrogen content (17.8 wt. %).2,31-36 Further, it is a CO2-emission free route, 3 ACS Paragon Plus Environment

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resulting in NH3 as a carbon-neutral fuel.37-44 Based on these merits, electrochemically converting N2 with H2O to NH3 at ambient conditions has been proven as a promising alternative to the wellestablished Haber-Bosch process. Despite the above advantages, the electrochemical N2 reduction to NH3 at ambient conditions is still inefficient due to the lack of suitable electrocatalysts. The challenges are in the following two aspects. First, because N2 is an inert molecule with no dipole moment and has a very low polarity, its N≡N triple bond is one of the most stable bonds.45-48 Therefore, large overpotentials are required for the electrochemical activation of N2. Second, the required overpotential for nitrogen reduction reaction (NRR) is larger than that needed for hydrogen evolution reaction (HER). Owing to the competing HER, the selectivity to NH3 is often low when N2 reduction occurs in aqueous electrolytes.33,49-52 Many research efforts have been devoted to developing electrocatalysts for realizing efficient electrochemical N2 reduction to NH3 at ambient conditions. However, significant challenges remain in achieving low overpotentials and high NH3 selectivity.53-57 This review starts with an introduction of some basics related to the electrochemical NRR. Next, we summarize the experimental studies of different catalytic systems, including homogeneous, heterogeneous and biological catalysts (Scheme 1), as well as computational studies of their potential reaction pathways. Last, we discuss future research directions in this field.


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Scheme 1 Schematic overview of various types of catalytic systems used for electrochemical N2 reduction to NH3 at ambient conditions along with their major advantages and shortcomings.

2. Basics of electrochemical NRRs Chemical reactions are the heart of any chemical process where reactant molecules are transformed into useful products. Catalysts play crucial roles in facilitating such transformations, leading to efficient chemical processes. It is essential to understand the basics of chemical reactions for developing successful chemical processes. Here, we discuss some basics of the electrochemical N2 reduction to NH3, including the reactions occurring on cathodes and anodes, potential


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competing reactions, N2 activation pathways, active sites, evaluation criteria for catalytic performance and experimental methods.

2.1 Reactions at cathodes and anodes The electrochemical N2 reduction to NH3 in aqueous electrolytes is typically carried out in the three-electrode configuration, where oxidation of H2O takes place on anodes, while NRR occurs on cathodes. The specific anodic and cathodic reactions depend on the nature of aqueous electrolytes.58 In acidic electrolytes, protons produced on anodes are directly transferred to cathodes through a proton exchange membrane, resulting in the production of NH3 molecules. The two half-reactions occur in acidic electrolytes are represented as equations (1) and (2). Anodic side (oxidation reaction) 3H2O → 6H+ + 1.5O2 + 6e-

(1)

Cathodic side (reduction reaction) 6H+ + N2 + 6e- → 2NH3

(2)

In alkaline electrolytes, N2 combines with H2O and electrons to produce NH3 and OH- on cathodes, then OH- ions transfer through an anion exchange membrane to the anodes and are oxidized to O2. The reactions are described as equations (3) and (4). Anodic side (oxidation reaction) 6OH- → 1.5O2 + 6e- + 3H2O

(3)

Cathodic side (reduction reaction) 6 ACS Paragon Plus Environment

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N2 + 6H2O + 6e- → 2NH3 + 6OH-

(4)

In both types of electrolytes, the overall reaction can be expressed as equation (5). N2 + 3H2O → 2NH3 + 1.5O2

(5)

2.2 Competing reactions HER is the primary reaction competing with NRR when H2O is used as the hydrogenated source. The preference towards desired NH3 formation or competing reactions could be manipulated either thermodynamically or kinetically. From the thermodynamic point of view, at the same reaction condition, thermodynamic potentials of these reactions mainly govern their preferences. Unfortunately, HER from H2O splitting and NRR exhibit similar thermodynamic potentials at room temperature (Fig. 1a).2 The overpotentials can be varied by changing reaction temperatures or employing appropriate catalysts. When H2O is used as a reactant, the potential for NH3 synthesis reaction first decreases with the increase of the temperature till the boiling point of H2O, and then increases with the temperature when H2O exists as a steam (Fig. 1a).2 In contrast, the potential of HER decreases continuously with the increase of temperature from 200 to 500 oC (Fig. 1b).2 Overall, higher reaction temperatures favor NRR. When the reaction temperature is fixed as a constant, electrocatalysts can be used to kinetically manipulate the overpotentials of the two reactions to enhance the selectivity toward NH3 synthesis. Previous density functional theory (DFT) calculation results show that nearly all pure metals have more negative limiting potentials for NRR than HER.59 Using well-dispersed metal catalysts can improve the selectivity toward NRR due to the suppression of HER.60 Because transition metal nitrides can activate N2 via a Mars-van Krevelen (MvK) mechanism at reduced 7 ACS Paragon Plus Environment

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overpotentials, transition metal nitrides, such as VN, ZrN, NbN, CrN, RuN, and WN, have been proposed as selective catalysts for NRR, which are discussed in details later in Section 2.3.61

Figure 1 (a) Theoretical thermodynamic potentials for electrocatalytic NRR and HER. The potentials are calculated based on thermochemical data of individual species at different temperatures. (b) The measured HER potentials at different temperatures. Reproduced with permission from the reference.2 During the formation of NH3 via NRR (equation 6), N2H2 and N2H4 can be formed as byproducts via equations 7 and 8. Due to the inevitable competition from HER in aqueous electrolytes, to avoid the inference from HER, organic electrolytes have been explored for thermodynamic study, for example, acetonitrile.62 As shown in Figure 2a, NH3 is thermodynamically favored in acetonitrile with the potential difference of 0.43 V between NH3 and N2H4 and the difference of 1.26 V between NH3 and N2H2.62 Reaction kinetics also play critical roles in determining the selectivity towards NH3, which can be tuned by catalysts. For example, N2H4 and/or N2H2 had been detected as major products in some NRR studies.63 N2 + 6H+ + 6e- ↔ 2NH3

(6)

N2 + 2H+ + 2e- ↔ N2H2

(7) 8

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N2 + 4H+ + 4e- ↔ N2H4

(a)

(b)

(c)

(d)

(8)

Figure 2 (a) Thermodynamic data for electrochemical N2 reduction to NH3, N2N4, and N2H2. Reproduced with permission from the reference.62 (b) Schematic illustration of the distal pathway and the alternating pathway for electrochemical reduction of N2 to NH3, where “cat” represents catalyst. The red arrow is used to mark a hybrid path that shifts from the distal pathway to the alternating pathway. Reproduced with permission from the reference.64 (c) The putative cycle for the proposed Li-mediated electrochemical synthesis of NH3 from N2. Reproduced with permission from the reference.65 (d) The dissociative pathway (solid lines) and associative pathway (dashed lines) for N2 activation on the flat (black) and stepped (red) metal surfaces. Reproduced with permission from the reference.59 9 ACS Paragon Plus Environment

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2.3 N2 activation pathways The dissociation of the N≡N triple bond is the rate determining step in NRR on most of the catalysts.66 Designing active catalysts to accelerate the dissociation of the N≡N triple bond is vital for realizing efficient NH3 synthesis. On transition metal-based catalysts, N2 activation can be categorized into the following two pathways: the direct N2 dissociation67,68 and the associative N2 dissociation.69,70 In the direct dissociation, the N≡N triple bond is directly dissociated into two N atoms before the addition of H to the adsorbed N atoms.59 This pathway is suggested to apply only to early transition metals under ambient conditions. In the associative dissociation, the dissociation of the N≡N triple bond and the hydrogenation of N atoms coincide. As illustrated in Figure 2b, according to the hydrogenation sequences, the associative dissociation pathway can be further classified into the distal pathway and the alternating pathway.64 In the distal pathway, only one N atom of N2 is adsorbed on the catalyst surface. The remote N atom is hydrogenated first and released as NH3. In the alternating pathway, both N atoms are adsorbed on the catalyst surface. They are hydrogenated simultaneously and are first hydrogenated to *N2H, then dissociated into *N and *NH, and finally released as NH3. Occasionally, N2 can also be activated by a hybrid associative dissociation pathway that shifts from the distal pathway to the alternating pathway (see Figure 2b). As mentioned early, transition metal nitrides could activate N2 via the MvK mechanism,61 in which a lattice N atom on the outmost surface of metal nitrides was first hydrogenated and reduced to NH3, afterward, the N vacancy was refilled by gaseous N2 to return the catalyst to its original state. This mechanism was proposed based on

15N

2

isotope labeling experiment results. 10


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Quantitative analysis showed that N atoms in NH3 came from both N containing catalysts and gaseous N2, and more gaseous N2 contributed to NH3 synthesis with prolonged reaction time.40,42 Compared with the dissociative and associative N2 activation pathways on transition metal catalysts, N2 activation and NH3 formation on transition metal nitrides via the MvK mechanism were found to require relatively smaller overpotentials.40,42 Sakata et al. discovered that Li could electrochemically reduce N2 to NH3.71 There are contradictory opinions on whether the role of Li is a mediator or a catalyst. N2 is proposed to be activated on Li via the cycle shown in Figure 2c.65 Li salts in electrolytes first deposited on cathodes at negative cathodic potentials (e.g. -4 V vs. Ag/AgCl), and then reacted with N2 to generate Li nitride (Li3N), which then reacted with proton donors to synthesize NH3.

2.4 Brønsted-Evans-Polanyi relations and Scaling relations Activation energy is defined as the energy barrier to initiate a chemical reaction, which determines the difficulty in carrying out the reaction. An active catalyst is expected to reduce the activation energy and follow the Sabatier principle, that is, there should be medium interactions between the chemical species and catalyst, neither too strong nor too weak. In the case that the interactions are too weak, it is difficult for the chemical species to bind to the catalyst and then no reaction could take place. Conversely, if the chemical species bind strongly to the catalyst, they would cover the active sites and ultimately poison the catalyst. The interactions between catalysts and chemical species can be roughly quantified according to their bonding strength. However, due to the complexity of intermediates adsorbed on the catalyst surface, Brønsted-Evans-Polanyi


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relations and scaling relations are often used to deduce activation energies of elementary reactions and adsorption energies of adsorbed species, respectively.72,73 According to the scaling relations, the adsorption energy of H to element A (where A can be N, O, C, S or other elements) on the catalyst surface can be expressed by the equations (9) and (10). AHx denotes the species formed by adsorbing H to A, α(x) is the slope relying on the valencies of A and AHx, β is a constant affected by the geometric configuration of the species on the catalyst surface, and xmax and x represent the maximum number of H atoms could be bound to A and the actual number of H atoms bonded to A, respectively. ΔEAHx = α(x)ΔEA + β α(x) = (xmax - x) / xmax

(9) (10)

For NH3 synthesis, A is N atom, xmax = 3, x = 0, 1, or 2. With the assistance of the Scaling relations, the free energy diagrams for electrochemical NRR can be plotted as a function of adsorption energies. The activation energies can be calculated roughly from adsorption energies, which serve as a predictor for the performance of catalysts. Thus, the search for effective NRR catalysts can be facilitated by an initial screening using the Brønsted-Evans-Polanyi relations and Scaling relations without performing more complicated DFT calculations.

2.5 Active sites DFT calculation is an important tool for examining active catalytic sites for NRR and providing theoretical guidance in the search of suitable catalysts. For example, DFT calculations have been adopted to screen a wide range of transition metal surfaces under ambient conditions in acidic electrolytes,59 for both the direct dissociation67,68 and associative dissociation pathways of 12 ACS Paragon Plus Environment

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NRR.69,70 A volcano plot (Figure 2d) has been obtained with Fe, Mo, Rh, and Ru appearing on the top. However, the faradaic efficiency for NH3 synthesis on these active metals are low because they are also active for the competing HER.59 In comparison, early transition metals, such as Sc, Y, Ti, and Zr, show higher faradaic efficiency because they possess higher affinity to N adsorption than H adsorption.59 Compared with flat metal surfaces, stepped metal surfaces are predicted to be more active for N2 activation. For metal nitride catalysts, N atoms on the topmost surface of the nitride lattice and vacancies created by removing N atoms via the MvK mechanism have been found to be the active sites for N2 activation.38

2.6 Criteria for efficient NRR catalysts Activity, selectivity, and stability are the three important criteria for assessing the performance of a catalyst. An efficient catalyst is expected to exhibit high activity, high selectivity to the desired product, as well as good stability during its lifetime. For the electrochemical NRR, an efficient catalyst should be capable of activating N2 to produce NH3 at a low overpotential (high activity), producing NH3 with minimum byproducts, such as H2, N2H2 and N2H4 (high selectivity), and retaining its catalytic activity and selectivity for a long period of time (good stability). Besides the three criteria, the production yield to NH3, which is defined as the amount of NH3 produced when the reaction is carried out, frequently serves as an indicator of catalyst efficiency in electrochemical NRR. Faradaic efficiency, which depicts the efficiency with which charge/electrons is transferred in the catalytic system facilitating NRR, is also widely used as parameters to describe the efficiency of a catalyst for electrochemical NRR. Higher NH3


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production yield and higher faradaic efficiency to NH3 production suggest the better efficiency of the catalyst in electrochemical NRR towards NH3.

2.7 Performance evaluation methods for NRR catalysts Currently, the performance of NRR catalysts is typically evaluated in research labs using an electrochemical workstation in a three-electrode configuration. Up to now, four types of electrochemical reactors have been developed, including the back-to-back cell,74,75 H-type cell,76 polymer electrolyte membrane-type cell37,77 and single chamber cell.78 These four types of reactors along with their advantages and shortcomings have been reviewed in details by Amar et al.,79 Kyriakou et al.,80 and Cui et al..11 Due to the complexity of the electrochemical NRR experimental systems, several factors might affect N2 activation and NH3 production rates, including cell configurations, electrode materials, the conductivity of working electrodes, the nature of electrolytes, reaction temperatures, applied potentials/currents, partial pressures of N2, and proton fluxes. The accurate performance evaluation of catalysts for electrochemical NRR involves the selection of an appropriate reactor and optimization of the factors mentioned above. A major challenge associated with the NRR performance evaluation is the analysis of NH3 and reaction byproducts in aqueous media. Several methods have been used to detect NH3, including

ion

chromatography,81,82

ion-selective

electrode

methods,83

spectrophotometric/colorimetric methods,84,85 or fluorescence methods.86 Every method has its pros and cons, and it is common that several methods have to be used together to obtain a reliable result. Developing more selective and more accurate NH3 detecting methods is currently an urgent need. Further, NH3 is usually measured after NRR in nearly all of the current studies. It is desirable 14 ACS Paragon Plus Environment

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to have in situ detection methods to continuously monitor NH3 production under reaction conditions.

3. Progress on catalysts for electrochemical NRR E. E. van Tamelen and co-workers initiated the research of electrochemical NRR to synthesize NH3 in the 1960s. They used nichrome as the cathode, Al as the anode, and 1,2dimethoxyethane dissolved titanium tetraisopropoxide and aluminum chloride as the electrolyte.87,88 This process suffered from large N2 reduction overpotentials, low NH3 production yield, and poor stability due to the conversion of the Al anode to Al nitrides. Because of these disadvantages, electrochemical NRR received limited interests for a long time until the advent of the 21st century, when some research breakthroughs appeared by utilizing transition metals and metal nitrides as catalysts. Inspired by these breakthroughs, many recent studies have been carried out on electrochemical NRR under ambient conditions targeting the obstacles mentioned above. In the following sections, we summarize various catalytic systems that have been explored for electrochemical NRR including both historical and recent studies. We classify them into homogeneous, heterogeneous and biological catalysts according to the nature of catalysts used.

3.1 Homogeneous catalysts Homogeneous electrocatalysts used for NRR are typically molecular complexes containing transition metals as metal centers. These catalysts are first dissolved in acidic electrolytes, and then reduced on electrode surfaces during reactions, while electrodes themselves are inert for NRR. The homogeneous electrocatalytic reactions typically follow a cyclic reaction pathway, in which metal 15 ACS Paragon Plus Environment

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centers in molecular complexes first take up reactants via coordination, store charges obtained from electrodes, and then convert reactants to products. Afterward, products are released and the catalysts are converted back to their original configuration. In this section, several categories of molecular complexes used for NRR are summarized. We categorize these homogeneous electrocatalysts according to their different N2 activation pathways.

3.1.1 Molecular complexes with a {MP4} core The studies of molecular complexes with a {MP4} core, where M=Mo or W, have been inspired by the early discovery of Chatt et al. in 1975,89 reporting the formation of NH3 via the protolysis of a dinitrogen complex cis-[W(N2)2(PMe2Ph)4]. NRR was achieved through the cyclic conversion of N2 to NH3 using {MP4} as a mediator.63,90,91 NH3 production rates in the range of 1.3 to 35.8% (per mol of the complexes) were reported using Mo/W complexes containing either dinitrogen ([M(N2)2(dppe)2], M=Mo or W) or hydrazido(2-) ligands ([M(X)(NNH2)(dppe)2]X, M=Mo or W, X=Br, BF4 or HSO4).91 However, this type of complexes seldom produce NH3 exclusively, and N2H4 and N2H2 have been occasionally detected as byproducts over cis- and trans[M(N2)2(PR3)4],

trans-[M(N2)2(dppe)(PR3)2],

trans-[M(N2)2(dppe)2],

and

trans-

[M(N2)2(triphos)(PPh3)] in acidic electrolytes,91 even though NH3 would be favored thermodynamically.62 For example, both NH3 and hydrazine were formed at the ratio of 1/10 on Mo(III)-Mg2+-R3P-phospholipid.63 The ligands coordinated to center metals and the configuration of molecular complexes are speculated to be responsible for the selectivity toward different products.


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3.1.2 Titanocene dichloride Titanocene dichloride ((η5-C5H5)2TiCl2, abbreviated as Cp2TiCl2 here) can reduce N2 to NH3 under ambient conditions. An NH3 production rate up to 9.5 × 10-10 mol·cm-2·sec-1·M Cp2TiCl2-1 was reported at -1 V in H2O and a faradaic efficiency up to 0.95 % was achieved at -2 V in tetrahydrofuran.92 DFT calculations suggested that electrochemically reduced [Cp2TiCl] and/or [Cp2Ti]-based species could significantly reduce the activation barrier for the protonation of N2, and N2 was activated by Cp2TiCl2 via Cp2TiClN2 as an intermediate through the following steps: first, H2O was electrochemically split to H+ at electrode surfaces. Next, gaseous N2 bound weakly to Cp2TiCl2 to form the Cp2TiClN2 intermediate. Afterward, H+ diffused to Cp2TiClN2 and split on the bonded N2. Last, N2 dissociated and was hydrogenated to form NH3.92

3.1.3 Metal-phthalocyanine Furuya et al. reported that metal-phthalocyanine was active for electrochemical NRR under ambient conditions in 1989.93 A Faradaic efficiency of NH3 production rate of ca. 1.6% was achieved at -0.6 V vs. RHE at the beginning of electrolysis when using Fe-phthalocyanine as the catalyst, a gas-diffusion electrode loaded with Pt as the anode and 1 M Na2SO4 as the electrolyte.93 However, Fe-phthalocyanine was not stable under the reaction conditions and the Faradaic efficiency to NH3 dropped sharply to less than 0.1% after ca. 10 min of the electrolysis. The Faradaic efficiency and stability of phthalocyanine-based catalysts depended on electrolytes and center metals of phthalocyanines. Shifting the electrolyte from Na2SO4 to a mixture of KOH and KHCO3 was found to increase the Faradaic efficiency and stability of Fe-phthalocyanine.93 Among a series of metal phthalocyanine catalysts, Sn-phthalocyanine showed the best activity and stability, 17 ACS Paragon Plus Environment

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and the observed Faradaic efficiency was ranked as the following: H (a free base phthalocyanine) > Ti, Fe > Pd > Co > Pt > Ni, In > Pb > Cu > Zn > Ga, Sn > Pb.94

3.1.4 Sacconi-type tetradentate ligand complexes A family of Sacconi-type tetradentate ligands, short for P3E here, in which three phosphine donors (P) bonded to a center atom through an ortho-phenylene linker (E=B, C, Si), has been reported as active catalysts for electrochemical NRR.95-101. For instance, Fe based Sacconi-type tetradentate ligands, denoted as P3EFe (E=B, C, Si), could produce NH3 under the atmosphere of N2. 64 equivalent of NH3 per Fe site in P3BFe and 47 equivalent of NH3 per Fe site in P3CFe were reported.95 P3EFe also exhibited good stability after multiple reaction cycles.95 In situ and conventional characterization, results suggested an electrochemical N2 activation cycle reaction mechanism, in which P3BFe-N2- served as the catalyst and Fe-borohydrido-hydride complex worked as a resting state (see the illustration in Figure 3a).95 P3ECo (E=B, C, Si) complexes performed similarly in promoting the binding and activation of N2 in electrochemical NRR. (a)

(b)

(c)

Figure 3 (a) Mechanism of a P3BFe molecular complex in electrochemical NRR to NH3. Reproduced with permission from the reference.95 (b) The structure of µ2-N2Ru2(*L)2DPB (Ru2(N2)) and (c) dinitrogen complexes: µ2-diazene (Ru2(N2H2)), µ2-hydrazine (Ru2(N2H4)), and bis-ammine (Ru2(NH3)2) complexes. Reproduced with permission from the reference.102 18 ACS Paragon Plus Environment

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3.1.5 Mo-Ti-S cluster Natural nitrogenase has the structure of Fe-Mo-S. Some compounds mimicking the structure of Fe-Mo-S might behave good N2 fixation capacity. Ohki et al. reported a Mo-Ti-S [Mo3S4Ti] cluster (the cluster 2 in Figure 4) for NRR with an N2 binding cluster (the cluster 3 in Figure 4) as an intermediate.103 The bridged two [Mo3S4Ti] cubes with an N2 moiety (the cluster 3 in Figure 4) can be converted into NH3 and N2H4 by a reducing reagent.103

Figure 4. Synthesis of the cubic [Mo3S4Ti] cluster 2, the N2-binding cluster 3, and its 1e reduced form [3]–; Cp* =η5-pentamethylcyclopentadienyl; THF=tetrahydrofuran. Reproduced with permission from reference.103 19 ACS Paragon Plus Environment

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3.1.6 Other promising homogeneous catalysts Ru complexes with their structures similar to µ2-N2[Ru(NH3)5]2[BF4]4 are promising in electrochemical NRR.104 µ2-N2Ru2(*L)2DPB (short for Ru2(N2) here), produced by adding 2 equivalents of l-tert-butyl-5-phenylimidazole (*L) to a benzene solution of a metal-metal-bonded bis-ruthenium cofacial diporphyrin (Ru2DPB) under N2 atmosphere, could be further converted to µ2-diazene (Ru2(N2H2)), µ2-hydrazine (Ru2(N2H4)), and bis-ammine (Ru2(NH3)2) (the structures of these chemicals are shown in Figure 3b and 3c) via protonation.102 Due to the structural similarity of Ru2(N2) to µ2-N2[Ru(NH3)5]2[BF4]4 and the protonation capacity of Ru2(N2), Ru2(N2) is expected to be competent of activating N2 by coordinating at the center metal and thereby producing NH3.105 Pincer-ligated (PCP)Rh and (PCP)Ir (PCP=2,6-(CH2PtBu2)2C6H3) complexes are prospected to be active in electrochemical NRR, owing to (1) their structural similarity to titanocene dichloride (described in section 3.1.2), (2) their electron-rich nature, which endows them a strong affinity to N2,92 as well as (3) their capacity in forming metal-hydride species, such as (PCP)RhH2 and (PCP)IrH2, via the metal-centered regioselective protonation.106 It is speculated that (PCP)Rh and (PCP)Ir may be able to reduce N2 to NH3 under acidic conditions electrochemically.

3.1.7 Summary of homogeneous catalysts One or more representative catalysts in each of the above several categories of homogeneous catalysts for electrochemical NRR are chosen and summarized in Table 1. These homogeneous electrocatalysts have two main advantages: (1) they can be dissolved uniformly in electrolytes, 20 ACS Paragon Plus Environment

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allowing the reaction systems free from reactant mass transfer limitations, and (2) the chemical structures of homogeneous catalysts are well defined. Thus, it is straightforward to carry out indepth mechanistic studies. However, their major drawback is the difficulty in separating catalysts from reaction products if the catalysts are to be recycled for reuse. This would significantly compromise their commercialization potential.

Table 1. A summary of representative homogeneous catalysts for electrochemical NRR. Catalysts

[W(Br)(NNH2)(dppe)2]Br

Reaction conditions

H-type

cell,

30

mL

THF

Ammonia formation

Faradaic

References

yield

efficiency (%)

9.2%

8.8

91

1.5%

2.7

91

1.3%

2.1

91

35.8%

6.6

91

N/A

92

(distilled over Na) and 0.3 M nBu4NClO4 in HBr acid, Hg cathode [W(Br)(NNH2)(dppe)2]Br

H-type

cell,

30

mL

THF

(distilled over Na) and 0.3 M nBu4NClO4 in HBr acid, Pt cathode [Mo(Br)(NNH2)(dppe)2]Br

H-type

cell,

30

mL

THF

(distilled over Na) and 0.3 M nBu4NClO4 in HBr acid, Hg cathode [Mo(HSO4)(NNH2)(dppe)2]HSO4

H-type

cell,

30

mL

THF

(distilled over Na) and 0.3 M nBu4NClO4 in H2SO4, Hg cathode (η5-C5H5)2TiCl2

A self-made cell, 1.0 M LiCl, Pt

9.5×10-10

cathode

2·sec-1·(M

mol·cm-


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CP2TiCl2)-1 vs. -1 V in water Fe-phthalocyanine

Sn-phthalocyanine

H-type cell, Pt anode, 1 M

1.6%, at -0.6 V vs.

Na2SO4 as electrolyte

RHE

H-type cell, Pt anode, 1 M KOH

1.85%, at

at -0.40 V vs. RHE

P3BFe

H-type cell, Zn foil, -45 °C, 0.1

N/A

93

N/A

94

18

95

-0.40 V vs. RHE. N/A

M NaBArF4 as electrolyte

3.2 Heterogeneous catalysts Heterogeneous catalysts are often used to address the drawback of homogenous catalysts. They are generally metals or metal-based solids, such as metals, carbides, sulfides, oxides, with diverse geometric and chemical structures. Because heterogeneous electrocatalytic reactions occur at electrode/electrolyte interfaces, the surface chemistry of active catalytic sites and the structures of heterogeneous catalysts play critical roles in determining their catalytic performance. Here, we classify heterogeneous catalysts according to their active catalytic sites. Recent progress and some historical studies are reviewed in several sub-sections.

3.2.1 Metal-based catalysts

3.2.1.1 Fe-based catalysts Fe-based catalysts are currently the most widely used non-noble-metal catalysts for NH3 synthesis due to their high activity for dissociating the N≡N triple bond. Various Fe compounds, 22 ACS Paragon Plus Environment

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such as Fe2O3, Fe containing metal organic frameworks (MOFs), and other Fe containing species, have been explored for electrochemical NRR.

3.2.1.1.1 Fe2O3 Fe2O3 is active for electrochemical NRR at moderate reaction temperatures and pressures. Several factors have been found to affect its catalytic activity, including properties of electrolytes (e.g., pH, concentration and chemical properties),107 catalyst supports, reaction temperatures, and its crystalline surfaces. First, Fe2O3 usually exhibits higher activities in alkaline electrolytes. For example, γ-Fe2O3 nanoparticle-based catalysts have demonstrated a Faradaic efficiency up to 1.9% for electrochemical NRR.108 However, due to potential corrosion in alkaline media,109 it is desirable to use electrolytes at neutral pH, which often results in lower catalytic performance. For example, Fe2O3 nanorods in 0.1 M Na2SO4 electrolyte revealed a Faradaic efficiency of 0.94% at the potential of -0.8 V vs. RHE and an NH3 yield of 15.9 mg h-1 mg-1 (catalyst).110 Fe2O3-CNT catalysts in KHCO3 electrolyte showed a low Faradaic efficiency of 0.15% at -1.39 V vs. RHE.37 Second, some synergistic effects between Fe2O3 and its catalyst supports may enhance its catalytic activities. For example, Centi et al. reported that a Fe2O3/CNT catalyst demonstrated a stable NH3 production rate of 2.2 × 10-3 gNH3 m-2 h-1 for more than 60 h under an applied potential of -2.0 V vs. RHE, which was even better than that of noble metal Ru/C catalysts.37 It was proposed that the interfacial sites between Fe2O3 and CNTs contributed to the high activity.37 Total Faradaic efficiency of 95.1% was achieved on the Fe2O3/CNT catalyst. However, the Faradaic efficiency of NH3 (the selectivity toward NH3) was not high.37


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Third, the overpotential of electrochemical NRR can be tuned by regulating reaction temperatures, and the selectivity toward NH3 may be improved at higher reaction temperatures. For example, Licht et al. reported an electrochemical NRR method based on the electrolysis of air and steam over nano-Fe2O3 catalysts in high temperature molten hydroxide electrolytes.2 NH3 was produced at 1.2 V under the applied current of 2 mA cm-2 in a 200 °C molten NaOH/KOH electrolyte, and the Faradaic efficiency was 35%.2 Dispersing Fe2O3 catalysts in a molten chloride electrolyte (LiCl-KCl-CsCl) could produce NH3 at the rate of 3.00 × 10-10 mol s-1 cm-2. It was proposed that the molten chlorides triggered the tandem NRR and the reaction between reduced N2 and H2 from H2O via HER.111 Confining Fe2O3 on activated charcoal (AC), the Fe2O3/AC catalyst could inhibit the competing HER, and improve the selectivity toward NH3. In a 250 °C molten hydroxide electrolyte (NaOH/KOH), the catalyst produced NH3 at the rate of 8.27 × 10-9 mol s-1 cm-2 under 1.55 V, and the current density of 49 mA cm-2 and the Faradaic efficiency for NH3 synthesis was 13.7% under 1.15 V and 11 mA cm-2.112 Fourth, Fe2O3 with specified active surfaces may possess better catalytic performance for electrochemical NRR. For example, Nguyen et al. carried out a DFT study on the electrochemical NRR on two hematite (0001) surfaces, Fe-O3-Fe- and Fe-Fe-O3-.113 It was found that N2 could be activated on the hematite (0001) surface via an associative pathway under an applied bias of 1.1 V. The addition of the first H species to the adsorbed N2 was the rate limiting step. The hematite (0001) surface could inhibit HER and thus improve the Faradaic efficiency (selectivity) to NH3.113

3.2.1.1.2 Fe containing MOFs


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MOFs are material networks with metal nodes coordinated with organic ligands. Several MOFs having different metal nodes, such as Fe, Co, and Cu, have been explored as heterogeneous catalysts for electrochemical NRR. For example, Yin et al. discovered that a Fe based MOF had an NH3 formation rate of 2.12 × 10-9 mol s-1 cm-2 and a Faradaic efficiency of 1.43% under 1.2 V at 90 oC.114 The activity of the Fe based MOF was partially attributed to its rich micropores and large specific surface area, which provided many N2 adsorption sites and facilitated the adsorption and dissociation of N2 on the Fe metal node sites. Co and Cu based MOFs were also found to be active for electrochemical NRR with slightly inferior performance compared to that of the Fe based MOF.114

3.2.1.1.3 Other Fe-based catalysts 3Fe2O3 (s) + 2H+ + 2e- ↔ 2Fe3O4 (s) + H2O

(11)

Fe3O4 (s) + 8H+ + 8e- ↔ 3Fe (s) + 4H2O

(12)

Most of the current studies on Fe-based catalysts for electrochemical NRR reaction are based on Fe2O3, however, Fe3O4 and Fe might be formed due to the reduction of Fe2O3 (via the equations 11 and 12) during NRR. To investigate whether Fe3O4 and Fe are active for NRR, Feng et al. prepared a Fe/Fe3O4 catalyst by in situ electrochemically reducing a preoxidized Fe foil. The Fe/Fe3O4 catalyst exhibited a Faradaic efficiency of 8.29% toward NH3 production at -0.3 V vs. RHE in a phosphate buffer electrolyte, which was 120 times higher than that of the original Fe foil (see Figure 5).31 The improved selectivity was attributed to the interfaces between Fe and Fe3O4 nanocrystals, which allowed the reductive adsorption of N2 to *N2H as well as the reductive desorption of *NH2 to NH3.31 25 ACS Paragon Plus Environment

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Figure 5 The performance of the Fe/Fe3O4 catalyst and the original Fe foil for electrochemical NRR in a 0.1 M phosphate buffer electrolyte. (a) Chronoamperometric curves of the two catalysts measured at -0.3 V vs. RHE and their Faradaic efficiency toward NH3 production was marked. (b) Total current densities of the two catalysts at selected potentials, (c) the corresponding geometric NH3 yield rates (RNH3), and (d) the Faradaic efficiency toward NH3 production. (e) Surface-areanormalized NH3 yields, and (f) HER activities of the two catalysts. Reproduced with permission from reference.31

Stoukides et al. evaluated some industrial Fe catalysts (provided by Phosphoric Fertilizer Industry S.A.) for electrochemical NRR reaction in double-chamber and single-chamber proton conducting cells.115 The catalysts were pasted on proton-conducting solid electrolyte disks and served as working electrodes. The NH3 formation rate was enhanced by 3 times.115 Since the catalyst prepared by pasting method restricted the contact area, Otomo et al. enlarged the interfacial contact by loading the Fe catalysts on porous BaCe0.9Y0.1O3 (BCY) using an infiltration 26 ACS Paragon Plus Environment

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method, and more than 20 times enhancement in NH3 formation rate at -1.5 V relative to that at the open circuit voltage was achieved.66

3.2.1.1.4 Summary of Fe-based catalysts Recent studies have reported that Fe compounds in various valences and morphologies, including metallic Fe, Fe2O3, Fe3O4, Fe based MOFs, and their hybrids, are active for electrochemical NRR. In general, the properties of electrolytes (i.e., pH, concentration, and their chemical properties) strongly affect the performance of Fe-based catalysts in NRR. Alkaline electrolytes are found to favor N2 activation and NH3 production. Their catalytic performance can be further enhanced by creating synergistic effects between Fe species and catalyst substrates. Tuning reaction temperatures and designing Fe compounds with specific crystalline surfaces are also beneficial for obtaining enhanced performance.

3.2.1.2 Mo-based catalysts Inspired by the natural N2 fixation over Fe-Mo enzyme nitrogenases, continuous effects have been devoted to studying Mo-based catalysts for electrochemical NRR. Some DFT calculation studies suggested that Mo was one of the most active metals for N2 activation.59 Experimental studies have shown that the (110) surface of Mo was favorable for electrochemical NRR.116 Mo (110) could produce NH3 at an overpotential of 0.14 V with a Faradaic efficiency of 0.72%, and the maximum NH3 formation rate was 3.09 × 10-11 mol s-1 cm-2 at -0.49 V vs. RHE, which was 100 times higher than that of a Mo foil.116


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In comparison with bulk Mo-based catalysts, the catalysts containing highly dispersed metal clusters or atomically isolated metal sites may significantly improve the catalyst efficacy.117,118 Single-atom catalysts can provide the maximum atom utilization efficiency, and their electronic structures and atomic configuration can be changed radically by anchoring the active metal atoms on different substrates. In a DFT study, Zhao et al. demonstrated that individually dispersed Mo atoms on a defective boron nitride (BN) monolayer were active for NRR, and the optimized catalyst structure was displayed in Figure 6a.119 It was found that the inert N≡N bond could be sufficiently activated because the Gibbs free energy (ΔG) of N2 adsorption was -0.19 V (Figure 6b) and the Mo atoms could selectively stabilize N2H* and destabilize NH2* species (Figure 6c).119 Similarly, individual Mo atoms embedded in a MoS2 nanosheet was also theoretically proved to be active for electrochemical NRR.64 Overall, current results suggest that Mo-based catalysts, especially small Mo clusters and individually dispersed Mo atoms are active for electrochemical NRR. (a)

(b)

(c)

Figure 6 (a) The structure of a single Mo atom embedded in a BN monolayer. The unit of bond length is Å. (b) Calculated Gibbs free energies of N2 adsorption, and (c) the adsorption energies of N2H and NH2 species on different single transition metal atoms on the defective BN nanosheet. Reproduced with permission from reference.119

3.2.1.3 Ru-based catalysts 28 ACS Paragon Plus Environment

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Ru has been used for NH3 synthesis at relatively low reaction temperatures due to its capacity in dissociating the N≡N triple bond.120,121 The dissociation of the N≡N triple bond is generally regarded as the rate-determining step for NH3 synthesis from N2 and H2O. The limiting kinetic barriers required for N2 activation was -0.71 V for the dissociative pathway and -0.68 V for the associative pathway.122 The NRR on Ru-based catalysts is affected by several parameters, including reaction temperature, pressure, and catalyst morphology. For example, the NH3 formation rate would increase with the increase of the reaction temperature up to 100 °C,77 and the higher reaction pressure was beneficial for NH3 production. The size and defect sites of Ru nanoparticles play important roles in determining their catalytic activity. Studies have shown that B5 sites on Ru catalysts were the active center for N2 activation, and Ru catalysts with the sizes of 2 - 5 nm could expose more B5 sites.123 Otomo et al. synthesized Ru doped (40 at.% Ru) La0.3Sr0.6TiO3, resulting Ru nanoparticles in the range of 2 - 5 nm. This catalyst delivered an NH3 formation rate of 3.8 × 10-12 mol s-1 cm-2 at -0.1 V vs. RHE in a proton conducting fuel cell.124 It has been found that N2 dissociation preferentially occurred on stepped surfaces or defect sites of Ru particles over flat Ru surface.125-127 Therefore, engineering defects on Ru-based catalysts may be an effective strategy to enhance N2 adsorption and reduce energy barriers for N2 dissociation. A Ru/Ti catalyst with a high density of defects was prepared by galvanically coating Ru onto randomly structured Ti felts. This catalyst presented a low overpotential and a high catalytic activity with an optimal current density of 115 mA dm-2 for electrochemical NRR.128 The N≡N triple bond is one of the most stable chemical bonds.45-47 The dissociation of N≡ N triple bond is often considered as the rate determining step and the most energy demanding step in NRR. However, a Ru-based catalyst behaves differently. Hosono et al. loaded Ru on a strong 29 ACS Paragon Plus Environment

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electron donator 12CaO∙7Al2O3 (see Figure 7a) to produce a Ru/12CaO∙7Al2O3.120 As electron donator, electrons in 12CaO∙7Al2O3 were trapped at oxygen vacancy sites and octahedrally coordinated with six adjacent Ca2+ ions in CaO crystal (see Figure 7b). The electrons would transfer from substrate 12CaO∙7Al2O3 to the deposited Ru particles, which endowed the Ru/12CaO∙7Al2O3 catalyst a significantly reduced N2 activation energy (see Table 2) and tuned the rate-determining step from the dissociation of the N ≡ N triple bond to the subsequent formation of N-Hn species.129 The highly dispersed Ru atoms on the high surface area 12CaO∙7Al2O3 exhibited superior performance to conventional Ru-based catalysts, such as Ru/CaO and Ru-Cs/MgO, and an NH3 synthesis rate of 2.4 mmol g-1 h-1 was achieved.130

(a)

(b)

Figure 7 Ru/12CaO∙7Al2O3 catalyst for NH3 synthesis. (a) Schematic illustration of Ru/12CaO∙7Al2O3 catalyst. (b) Electrons in 12CaO∙7Al2O3 are trapped at oxygen vacancy sites and octahedrally coordinated with six adjacent Ca2+ ions in CaO crystal. Reproduced with permission from reference.1


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Table 2 The activation energy for NH3 synthesis and N2 desorption on different Ru-based catalysts. Reproduced with permission from reference.129 Catalyst

Activation energy (kJ mol-1) NH3 synthesis

Ru/C12A7:e-

49

∆E*

N2 exchange

N2 TPD

(N2 desorption)

(N2 desorption)

58

64

Ru/C12A7:O2- 104

133

Ru-Cs/MgO

99

139

Ru/CA

118

154

Ru/MgO

80

9 29

137

40 36

158

78

TPD: temperature-programmed desorption. ∆E* is the difference in the activation energy between N2 exchange reaction and ammonia synthesis.

Despite promising catalytic activities demonstrated on Ru-based catalysts, they suffer from competing HER in the low overpotential region, which reduces the number of active sites on catalysts for N2 adsorption/activation, resulting in significant increase of kinetic barriers for N2 activation.122 Replacing water with non-aqueous electrolyte is a promising strategy for inhibiting the occurrence of HER. Nørskov et al. predicted theoretically that when non-aqueous 2,6lutidinium (LutH+) was used as electrolyte, LutH+ could also serve as a proton donor to selectively drive the NRR rather than HER on Ru-based catalysts. Further, it could also selectively stabilize the *NNH intermediate and facilitate NH3 synthesis.131

3.2.1.4 Au-based catalysts


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In 2017, crystallized tetrahexahedral Au nanorods were reported for electrochemical NRR. The crystallized tetrahexahedral Au nanorods delivered a production yield of 1.648 µg h-1 cm-2 for NH3 and 0.102 µg h-1 cm-2 for N2H4∙H2O, with an activation energy of ca.13.704 kJ mol-1 at -0.2 V vs. RHE.132 Amorphous Au with more “dangling bonds” was found to be more active than crystallized Au.133 For example, amorphous Au anchored on CeOx-rGO achieved an NH3 production yield of 8.3 μg h-1 mg-1 catalyst and a Faradaic efficiency of 10.10% at -0.2 V vs. RHE.133 To improve both activity and selectivity towards NH3, Yan et al. prepared small-sized Au sub-nanoclusters (≈ 0.5 nm) and embedded them onto TiO2. This Au/TiO2 catalyst possessed a large amount of low-coordinated sites and delivered an NH3 production yield of 21.4 µg h-1 mg-1 catalyst and a Faradaic efficiency of 8.11% at -0.2 V vs. RHE (see Figure 8a and 8b).134 The high catalytic stability and good selectivity to NH3 (no formation of N2H4) were attributed to the formation of strong Au-O-Ti bonds between highly dispersed Au and the lattice O of TiO2, which improved the stability of Au and accelerated the electron transfer from Au to TiO2.134 The proposed reaction mechanism was illustrated in Figure 8c, in which the positively charged Au centers preferably chemisorb N2 by forming Au-N2 bond and break the N≡N triple bond by hybriding to a more stable N-H bond.134 (c)


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Figure 8 Electrochemical NRR on Au/TiO2. (a) Linear sweep voltammetric curves in N2 saturated (red line) and Ar saturated (blue line) in 0.1 M HCl aqueous solution at ambient conditions. (b) NH3 yield (blue) and Faradaic efficiency (red) at different potentials. (c) A proposed reaction pathway for the NH3 synthesis on the Au/TiO2 catalyst. Reproduced with permission from reference.134 In general, atoms buried deep inside lattice structures of bulk materials are not accessible to surface catalytic reactions.135 Thus, it is useful to adopt catalysts with a hollow structure to improve the utilization of active catalytic sites, especially for catalysts based on expensive metals. El-Sayed et al. prepared hollow Au nanocages (AuHNCs), which showed a Faradaic efficiency of 30.2% towards NH3 at -0.4 V vs. RHE and an NH3 production yield of 3.9 μg cm-2 h-1 at -0.5 V vs. RHE.136 The catalytic performance of AuHNCs could be further enhanced by engineering their size and the density of pores in their walls.137 Oschatz et al. decreased the size of Au to an atomic level and synthesized a single Au atom catalyst by stabilizing Au atoms on a hierarchical N-doped porous carbon substrate. The carbon substrate provided N2 with favorable accessibility to the active sites and improved the mass transport during the electrochemical NRR. The catalyst delivered an NH3 production yield of 2.32 µg h-1 cm-2, a Faradaic efficiency of 12.3%, and good stability in a sixcycle test under a potential of -0.2 V vs. RHE.138 Even though several types of mechanisms have been raised up for electrochemical NRR on Au-based catalysts,134 the mechanisms are based on the performance evaluation and kinetic studies while the information on reaction intermediates is still lacking. Thus, Shao et al. used surfaceenhanced infrared absorption spectroscopy (SEIRAS) to study the possible intermediates on an Au thin film.53 The H-N-H bending at 1453 cm-1, the -NH2 wagging at 1298 cm-1, and the N-N stretching at 1109 cm-1 were detected during NRR at potentials below 0 V vs. RHE. The result 33 ACS Paragon Plus Environment

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suggested that the activation of N2 on the Au film followed an associative mechanism, in which the dissociation of the N≡N triple bond coincided with H addition (see Figure 9).53

Figure 9 A reaction pathway proposed for electrochemical NRR on an Au surface. Reproduced with permission from reference.53

Overall, Au-based catalysts have shown potentials for electrochemical NRR. However, the high cost of Au may limit their industrial applications. Further, the reaction mechanism of NRR on Au-based catalysts also require further explorations.

3.2.1.5 Rh- and Pt-based catalysts A DFT calculation suggested that Rh is one of the most active metals for N2 activation, as it appears on the top of the volcano diagram (see Figure 2d).59 However, few experimental studies have been carried out on using Rh-based catalysts for electrochemical NRR. In one study, Wessling et al. used a Rh-based catalyst as a reference to demonstrate that a Ru-based catalyst had 34 ACS Paragon Plus Environment

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better performance.128 The Rh/Ti catalyst prepared by galvanically coating Rh onto a Ti felt yielded only 1/8 of the NH3 production rate of that from the Ru/Ti catalyst.128 Considering that Rh is about 17 times more expensive than Ru, the high cost and the lower performance of Rh would limit its applications. Few studies have used Pt-based catalysts for electrochemical NRR, because they are usually more active for the competing HER.53 Negligible NH3 formation was detected on Pt-based catalysts in most studies.121 In a SEIRAS study, negligible or almost no peaks assigned to intermediates N2Hy were detected on Pt films during NRR at potentials below 0 V vs. RHE.53 Although some studies reported an NH3 formation rate of 9.37 × 10-6 mol m-2 s-1 and a Faradaic efficiency of 0.83% on a Pt/C anode at 80 oC in a Li+/H+/NH4+ mixed conducting electrolyte under 1.2 V,75,139 these results are questionable as the contribution of converting NH4+ in the mixed conducting electrolyte to NH3 was not excluded.

3.2.1.6 Li-based catalysts Li can react with N at room temperatures to form Li3N, and NH3 may be generated by the protonolysis of Li3N. Although it is still under debates on whether Li is a mediator or a catalyst, the reaction cycle displayed in Figure 2c has been proposed to explain the activation of N2 on Li in electrochemical NRR.65 Sakata et al. reported the electrochemical NRR on Li, and a Faradaic efficiency of 8% toward NH3 production was found in a solution of LiClO4 (0.2 M) + ethanol (0.18 M) in tetrahydrofuran under ambient conditions.140 The Faradaic efficiency could be manipulated by tuning the pressure


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of N2, the nature and the amount of proton sources, such as alcohol, carboxylic acid, or H2O, and electrode materials.140 To suppress the competing HER, Nørskov et al. used a stepwise approach to physically separate NRR from the subsequent protonation to NH3.40 First, a voltage was applied to reduce LiOH in electrolyte to highly active Li and deposited them on electrodes in a proton-free environment. Next, Li was exposed to N2, and N2 was activated to generate Li3N. Afterward, the voltage was removed and protons were added to hydrolysize Li3N, which yielded NH3 and reproduced LiOH.40 Isotopic labelling studies with

15N

2

as reactant suggested that N2 was the

origin for the produced ammonia. This method yielded an initial Faradaic efficiency of 88.5% toward NH3 production.40 DFT calculations suggested that this process was not only suitable to Li-based materials, but also could be extended to other materials.40 O atoms in polymeric catalysts were found to be competent of facilitating H2 adsorption and accelerating HER.141 Incorporating Li+ into polymers is another method to suppress the competing HER. For example, Wang et al. incorporated Li+ into poly(N-ethyl-benzene-1,2,4,5tetracarboxylic diimide) (PEBCD) to form O-Li+ sites, which block the O atoms for the Tafel (2Hads → H2) or Heyrovsky (Hads+H2O+e− → H2+OH−) steps, thus suppressed HER (see Figure 10a).76 The obstruction of HER led to enhanced selectivity to NH3. Li+ incorporated PEBCD covered by carbon cloth achieved a Faradaic efficiency of 2.85% (Figure 10b) and an NH3 production rate of 1.58 μg·h-1·cm-2 (Figure 10c).76


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(a)

(b)

(c)

Figure 10 (a) Schematic illustration of Li+ association with O sites. (b) Faraday efficiency, and (c) NH3 production rate of Li+ incorporated PEBCD/C in electrochemical NRR at various potentials under 25 °C. Reproduced with permission from reference.76 Many efforts have been made in applying Li-based catalysts for electrochemical NRR. In particular, high selectivity to NH3 was achieved by using the stepwise approach to suppress the competing HER. More research efforts are still required to clarify the exact roles of Li salts.

3.2.2 Nitride, sulphide, carbide and black phosphorus catalysts 3.2.2.1 Transition metal nitrides Transition metal nitrides have been found to outperform pure transition metals in electrochemical NRR, which has been credited to their distinct MvK reaction mechanism.61 N atoms incorporated in the outmost surface layer of transition metal nitrides can be readily reduced to NH3 and leave N vacancies on the surface, which are then replenished by adsorbing gaseous N2 molecules to form a complete catalytic cycle.139 Compared with the dissociative and associative N2 activation mechanisms on pure transition metals, the MvK mechanism requires lower overpotentials and less energy inputs, which facilitates more efficient NH3 production.


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Several DFT studies have been carried out to search for potential catalysts. Skulason et al. identified VN, ZrN, NbN, and CrN as promising catalysts, which would enable electrochemical NRR to NH3 with a low bias and good selectivity.38 They also unraveled that transition metal nitrides with different exposed facets have distinct catalytic performance. For example, for (111) facets of transition metal nitrides with NaCl-type structures, TiN, VN, CrN, MnN, ZrN, NbN, MoN, HfN, WN, and ReN were more active and selective for NRR, while ScN, YN, and TaN were more selective towards HER.142 For (110) facets of transition metal nitrides with zincblende structures, RuN, CrN, and WN were predicted to be more stable and active for NRR.143 For (100) facets of transition metal nitrides with rock-salt structures, VN and ZrN are more promising.144,145 The crystalline structure of metal nitrides also strongly influences their catalytic performances, especially their catalytic stability. Polycrystalline structured ZrN, NbN, and CrN would decompose and result in stability deterioration, while single crystal surfaces of these materials were found to be stable and the decomposition could be avoided.38 Defects on metal nitride surfaces can also affect their catalytic performance. A DFT study by Catlow et al. tried to elucidate the possibility of the two N2 activation pathways for NH3 synthesis on surface defective Co3Mo3N, the dissociative pathway (Langmuir-Hinshelwood mechanism, the schematic illustration in Figure 11a) and the associative pathway (Eley-Rideal/MvK mechanism, the schematic illustration shown in Figure 11b).146 It was found that N2 was more easily activated on surface defective sites via the Eley-Rideal/MvK mechanism, where H then reacted directly with the activated N to form NH3 under considerably milder conditions, with hydrazine and diazane as important reaction intermediates.146


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(a)

(b)

(c)

Figure 11 (a) Langmuir-Hinshelwood mechanism (dissociative pathway), (b) Eley-Rideal/MvK mechanism (associative pathway) for NH3 synthesis on defective Co3Mo3N, and (c) the relative energy diagram of the two mechanisms. Reproduced with permission from reference.146

Two dimensional (2D) materials are atomic thin layers with unique electronic, optical, thermal, chemical and physical properties.147,148 Recently, many studies have explored 2D metal nitrides as catalysts for various reactions. The possibility of applying 2D metal nitrides for electrochemical NRR has been investigated by several DFT studies. 2D MoN2 nanosheets, which showed activity for several other electrochemical reactions, were found not an ideal catalyst for 39 ACS Paragon Plus Environment

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NRR because it would require a high overpotential of 1.75 eV. Fe doping of MoN2 could reduce the required overpotential to 0.47 V, suggesting that Fe/MoN2 might be a promising catalyst for electrochemical NRR.149 Similarly, single metal atoms supported on 2D C2N were found to be active for electrochemical NRR. Mo/C2N was predicted to be an active catalyst, and the computed onset potential was only 0.17 V, even lower than that required for Ru (0001) surface (0.43 V).150 Experiential studies have also demonstrated the catalytic activity of 2D metal nitrides. MoN nanosheet arrays supported on carbon cloth (MoN/C) delivered an NH3 production yield of 3.01 × 10-10 mo1 s-1 cm-2, a Faradaic efficiency of 1.15% at -0.3 V vs. RHE, and high durability and selectivity to NH3.151 Similarly, VN nanowire arrays supported on carbon cloth showed an NH3 production yield of 2.48 × 10-10 mol s-1 cm-2 at -0.3 V vs. RHE in 0.1 M HCl, a Faradaic efficiency of 3.58%, high selectivity (no N2H4 was formed), and good durability.152 Co3Mo3N was theoretically predicted to be active for electrochemical NRR,146 and its performance was further confirmed by several experimental studies. Using a Co3Mo3N-Ag composite as a cathode, a Ag-Pd alloy as an anode, and a LiAlO2-(Li/Na/K)2CO3 composite as an electrolyte, an NH3 formation rate of 3.27 × 10-10 mol s-1 cm-2 was recorded at 450 °C under at 0.8 V.153 The NH3 formation rate of Co3Mo3N-Ag electrocatalyst was further promoted by 48% when a K-β”-Al2O3 solid electrolyte was used and the ratio of K+ in the electrolyte per mole of Co3Mo3N was kept around 1%.154

3.2.2.2 Nitrides without transition metals Metal free catalysts have been studied for their potential advantages of being environmentally friendly and good resistance to corrosions.155,156 For example, boron nitride nanotubes (BNNTs) 40 ACS Paragon Plus Environment

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were investigated in a DFT study for electrochemical NRR. It was discovered that B-antisite sites (BAS) in BNNTs are catalytically active.157 N2 would be preferably adsorbed on BAS via a Schrock adsorption mode and activated via the distal mechanism with the first protonation as the rate determining step, and hydrazine would be a major byproduct.157 This theoretical finding opens the possibility of exploring nitrides without transition metals for electrochemical NRR.

3.2.2.3 Sulfides The studies of sulfides for electrochemical NRR can be dated back to 1990, when Furuya et al. screened a series of sulfides. It was reported that the performance of sulfides depended strongly on the metal species and followed the order of ZnS > NiS > CdS > CuS > Bi2S3> FeS > MoS > Sb2S3 > SnS > MnS > PdS > PbS > AgS > CoS. ZnS had the highest faradic efficiency of 0.964% at -1.0 V vs. RHE.158 Recently, MoS2 has also been found to be active in electrochemical NRR. Sun et al. reported that MoS2 had a Faradaic efficiency of 1.17% and an NH3 production yield of 8.08 × 10-11 mol s-1 cm-1 at -0.5 V vs. RHE in 0.1 M Na2SO4.159 The edge sites of MoS2 were theoretically predicted as active sites. A recent DFT study suggested that the rate determining step on defect-rich MoS2 would require a lower overpotential of 0.60 eV compared to 0.68 eV required by defect-free MoS2.160 Experimental studies showed that the defect-rich MoS2 delivered a Faradaic efficiency of 8.34%, an NH3 production yield of 29.28 μg h-1 mg-1 catalyst at -0.40 V vs. RHE in 0.1 M Na2SO4 , high selectivity to NH3, and good stability, which was better than the defect-free MoS2 with a Faradaic efficiency of 2.18% and an NH3 production yield of 13.41 μg h-1 mg-1 catalyst.160


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The crystalline structure of MoS2 has also been proposed to determine its activity. For example, 2H-MoS2 was predicted to be inactive in electrochemical NRR.161

3.2.2.4 Carbides Carbides have also been found to be active for electrochemical NRR. Sun et al. theoretically predicted that 2D transition metal carbides (V3C2 and Nb3C2), also called “MXenes”, would be active in N2 capture, activation and conversion into NH3.162 As shown in Figure 12a, carbides possessed strong affinity to N2 adsorption, and the adsorbed N2 was spontaneously activated via the diazene-hydrazine-ammonia pathway. The Gibbs free energy profile calculated at the DFT+D3 computational level (Figure 12b) suggested that the first proton-electron transfer was the limiting reaction step. The required activation energy was 0.64 eV vs. SHE for V3C2 or 0.85 eV vs. SHE for Nb3C2, suggesting that V3C2 and Nb3C2 would be potential catalysts for electrochemical NRR.162 Similarly, Cr3C2, Mo3C2, Ta3C2 were also theoretically predicted to be active catalysts for electrochemical NRR.162


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(a)

(b)

Figure 12 (a) A proposed mechanism for NRR on transition metal carbides, (b) The minimum energy path for NRR toward NH3 on V3C2 (top) and Nb3C2 (bottom) using DFT+D3 calculations. Reproduced with permission from reference.162

So far, most of exploring of transition metal carbides for electrochemical NRR reaction are mainly limited to theoretical studies. As a metal-free catalyst, boron carbide (B4C) nanosheet exhibited good performance in electrochemical NRR under ambient conditions, achieving an ammonia yield of 26.57 μg h–1 mg–1 cat. and a Faradaic efficiency of 15.95% at -0.75 V versus RHE.163 DFT calculations suggest that the *NH2–*NH2→*NH2–*NH3 reaction is the rate limiting 43 ACS Paragon Plus Environment

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step.163 SiC, a metal free carbide, has recently been studied experimentally, which delivered a Faradaic efficiency of 0.09% for NH3 production at -0.4 V vs. RHE.158

3.2.3 N- or B-doped catalysts Because surface atoms play critical roles in catalytic reactions,135 it has been speculated that covering or doping N atoms on or in the outmost surface layer of transition metals may endow them with similar catalytic properties of metal nitrides. Vegge et al. used DFT calculations to show that N covered Mo nanoparticles was active for electrochemical NRR with a low onset potential and a high faradic efficacy toward NH3.164 It was also found that N2 activation on N covered Mo nanoparticles follows two possible reaction pathways. In the first one, gaseous N2 would be adsorbed on surface N vacancy sites, and then protonated to generate NH3. In the second, N atoms from the surface layer of Mo nanoparticles would be directly protonated to produce NH3, leaving the N vacancy sites to be refilled by gaseous N2.164 N doped carbon materials are also active for electrochemical NRR. For example, N doped porous carbon (NPC) derived from the pyrolysis of ZIF delivered an NH3 production rate of 1.40 mmol g-1 h-1 at -0.9 V vs. RHE. Pyridinic and pyrrolic N on NPC were identified as the active sites.165 The morphology and structure of carbon substrates also affect the catalytic activity. For example, N doped carbon nanospikes (CNS) (Figure 13a) showed an NH3 production rate of 11.56 ± 0.85% at -1.19 V vs. RHE in 0.25 M LiClO4 electrolyte (Figure 13b and Figure 13c).166 It was proposed that a dehydrated cation layer with high electric field was formed at the tips of the sharp carbon spikes, which would facilitate the access of N2 and promote NRR.166


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(a)

(b)

(c)

Figure 13 (a) An SEM image of N doped carbon nanospikes. (b) The partial current densities and NH3 formation rate under the potential range of -1.29 ~ -0.79 V in 0.25 M LiClO4 electrolyte. (c) The Faradaic efficiency at various potentials. Reproduced with permission from reference.166

Du et al. predicted that the singly doped B atoms at BN edge (B@BN) are active for electrochemical NRR under ambient conditions. N2 could be activated over B@BN via the distal mechanism requiring an overpotential as low as 0.13 V.35 Cheng et al. synthesized B-doped graphene, and the 6.2% B-doped graphene delivered an NH3 production rate of 9.8 μg·hr-1·cm-2 and a Faradaic efficiency of 10.8% at -0.5 V vs. RHE in aqueous solutions.56 It was proposed that B doping would redistribute the electron density in the graphene framework. The electron-deficient B sites in a BC3 structure could provide enhanced adsorption capability for N2 molecules and enable a low activation barrier for NRR.56 3.2.4 Black phosphorus and metal phosphide catalysts Wang et al. reported that the exfoliated black phosphorus nanosheets was a superior nonmetallic catalyst for electrochemical NRR, recording an ammonia yield of 31.37 mg h-1 mg1cat

under ambient conditions.167 The zig-zag and diff-zigzag type edges on black phosphorus

nanosheets were predicted active for N2 activation and NH3 was prospected to be formed via the 45 ACS Paragon Plus Environment

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alternating hydrogenation pathway.167 This work proves the feasibility of black phosphorus as a nonmetallic catalyst for N2 reduction to NH3. Metal phosphide is also active for electrochemical NRR to synthesize NH3. Xu et al. prepared a 3D hierarchical nanoparticle-nanosheet-nanocage CoP catalyst with a metal-organic framework as precursor.168 It was discovered that, the CoP catalyst exhibited good performance in electrochemical NRR with a Faraday efficiency of 7.36% at 0 V vs RHE, an ammonia yield rate reached 10.78 μg h-1 mg-1cat at -0.4 V vs RHE, and good selectivity (no hydrazine production) under ambient conditions.168 Zhang et al. theoretically investigated the feasibility of singly dispersed transition metal atom (including Ti, V, Cr, Mn, Fe, Co, Ni, Ru, Rh and Pd) on MoP surfaces as electrocatalysts for NRR.169 The results revealed that Mn-MoP was a potential catalyst to realize electrochemical NRR, because on one hand, Mn-MoP exhibited an energy change value of 0.95 eV, while on the other hand, Mn-MoP can effectively reduce the competing HER by suppressing the adsorption of the adsorbed *H species.169

3.2.5 Hybrid catalysts 3.2.5.1 Bimetallic catalysts Fe is active (the initial Faradaic efficiency could be as high as 41%) catalyst for electrochemical NRR, however, Fe is unstable, and the Faradaic efficiency would decrease to a single digit value within a few hours. In contrast, Ni demonstrates a low Faradaic efficiency initially, but its stability is good. Thus, an FeNi bimetallic catalyst was tested for electrochemical NRR.170 Compared with monometallic Fe and Ni, the FeNi catalyst could combine the merits of


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both Fe and Ni, and its catalyst efficiency and stability were found to be even better than that of Pt/C catalysts.170 RuPt alloy dispersed on a carbon substrate (Vulcan XC-72) (RuPt/C) was reported to be an active catalyst for electrochemical NRR.171 Compared with monometallic Ru/C and Pt/C catalysts, RuPt/C exhibited higher activity with an NH3 formation rate of 5.1 × 10-9 gNH3 s-1 cm-2 at 0.123 V vs. RHE, a Faradaic efficiency of 13.2%, and high selectivity toward NH3 (no hydrazine was detected). The good performance was attributed to the synergistic effect between Ru and Pt, where Ru served as active sites for N2 adsorption and activation and Pt activated H2O to intermediate Pt-H to provide H for NH3 synthesis, following the three equations (equations 13 - 15). 2Ru + N2 → 2Ru-Nad

(13)

6Pt + 6H2O + 6e- → 6Pt-Had + 6OH-

(14)

Ru-Nad + 3Pt-Had → RuPt + NH3

(15)

3.2.5.2 Other hybrid catalysts An oxide-oxide hybrid, PbO-TiO2, as well as metal-oxide hybrids, such as Sb-SnO2 and SnIn2O3, have been reported to be active for electrochemical NRR. Their Faradaic efficiency was reported to be higher than that of Fe2O3.158 However, a detailed mechanism on how these components work synergistically is currently still missing.

3.2.6 Other heterogeneous catalysts 3.2.6.1 Perovskite catalysts 47 ACS Paragon Plus Environment

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Perovskite-type oxides have a general formula of ABO3, in which A is a rare-earth or alkaline earth element, and B is a transition metal element. Perovskite-type oxides are usually used as electrolytes in NRR. However, several studies have used them as catalysts in electrochemical NRR. ABO3 with the A or B sites substituted by foreign elements was often studied as catalysts because the substitution could enhance catalytic activities. For example, Sm0.5Sr0.5CoO3-δ,172 Ba0.5Sr0.5Co0.8Fe0.2O3-δ,173 and La0.6Sr0.4Co0.2Fe0.8O3-δ121 showed high NH3 formation rates when used as cathodes for electrochemical NRR. Nevertheless, these Co-based perovskites suffered from high cost, easy reduction and evaporation of Co at high temperatures.174 It is desirable to explore Co-free perovskite oxides. For example, La0.6Sr0.4Fe0.8Cu0.2O3-δ delivered an NH3 production rate of 5.39 × 10-9 mol s-1 cm-2 in a Ce0.8Sm0.2O2-δ-(Li/Na/K)2CO3 composite electrolyte at 450 °C and 0.8 V.175 SmFe0.7Cu0.1Ni0.2O3-δ exhibited an NH3 production rate of 1.13 × 10-8 mol cm-2 s-1 and a Faradaic efficiency of 90.4% at 80 oC in Nafion electrolyte, with Ni-doped Ce0.8Sm0.2O2-δ as the anode and Ag-Pt as the current collector.176 Few computational studies have been carried out on perovskite-type oxides, thus the reaction mechanism is less clear.

3.2.6.2 Spinel and pyrochlore catalysts Spinels have the formula of AB2O4, and pyrochlores have the structure of A2B2O7. They have been extensively investigated as catalysts for various electrochemical reactions due to their unique electrical, mechanical, magnetic and optical properties as well as their chemical and thermal stablility.177 They have also been explored as both electrolytes and catalysts for electrochemical NRR. For example, spinel CoFe2O4 prepared by a sol-gel method presented an NH3 production rate of 6.5 × 10-11 mol s-1 cm-2 at 400 oC and 1.6 V,178 while CoFe2O4 synthesized via a coprecipitation method gave an NH3 production rate of 2.32 × 10-10 mol s-1 cm-2 at 400 °C and 0.8 48 ACS Paragon Plus Environment

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V.179 Pyrochlore type La1.95Ca0.05Zr2O7-d and La1.95Ca0.05Ce2O7-d presented the NH3 production rates of 2.0 × 10-9 mol s-1 cm-2 and 1.3 × 10-9 mol s-1 cm-2, respectively.180 Overall, the activity for electrochemical NRR based on spinel and pyrochlore catalysts reported so far is still low, and mechanism studies are needed for providing theoretical guidance to improve their performance further.

3.2.6.3 Ion liquid based catalysts Ionic liquids are a category of salts existing as a liquid under ambient conditions. They possess some unique properties, such as high thermal and chemical stability, low volatility, electric conductivity, and wide electrochemical windows.181 A recent study used Cp2TiCl2 supported ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate for electrochemical NRR, and delivered an NH3 production yield of 27% per Cp2TiCl2 and a Faradaic efficiency of 0.2% in a solid polymer electrolyte.182 However, it is difficult to discriminate the exact contribution of ionic liquids in this study because the Cp2TiCl2 support was also an active catalyst for electrochemical NRR.182

3.2.6.4 Other heterogeneous catalysts Except for Fe, metallic states other than oxides have been considered as active sites of metal catalysts for NRR. Furuya et al. reported some metal oxides, such as ZnO, SnO2, Fe2O3 and TiO2, could also serve as catalysts for electrochemical NRR.158 It was found that both metals and their oxidation states affect the activity of these metal oxide catalysts. Their Faradaic efficiency followed the trend of ZnO > SnO2 > Fe2O3 > TiO.158 Skulason et al. used DFT calculations to 49 ACS Paragon Plus Environment

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show that rutile structured transition metal oxides could be active for electrochemical NRR.183 The (110) facets of NbO2, ReO2 and TaO2 were found to be promising, because they had relatively low onset potentials of -0.57 V, -1.07 V and -1.21 V vs. SHE and also suppressed the competing HER.183 IrO2 had a lower onset potential of -0.36 V for electrochemical NRR, however its surface favored H2 adsorption, and it might be easily poisoned by adsorbed H atoms.183

3.2.7 Summary of heterogeneous catalysts Heterogeneous catalysts can be easily separated from reactants/products/electrolyte mixtures, for example by filtration method. Thus, expensive catalysts could be effectively recycled, this makes them attractive for potential commercialization. As reviewed in this session, various heterogeneous catalysts have been studied for electrochemical NRR, especially transition metals, metal nitrides, and several others have shown promising performance. Some representatives of the widely explored Fe-, Ru-, Mo-, Au-based catalysts, and nitride catalysts are summarized, with their reaction conditions and performance being listed in Table 3. It is clear that enormous achievements have been achieved for electrochemical NRR using heterogeneous catalysts. Despite of this progress, many challenges remain to be solved. For example, heterogeneous catalysts are in a solid phase, while electrolytes are in liquid phase. The overall performance of heterogeneous catalysts for NRR is limited by the mass transfer of reactants from a liquid phase to active sites in the solid phase, which often leads to high overpotentials, poor NH3 selectivity, low NH3 yield, and faradic efficiency. Further, geometric/chemical structures and surface chemistry of active sites on heterogeneous catalysts are usually non-uniform, which renders complicity in understanding their reaction mechanism and achieving good reaction selectivity.


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Table 3. A summary of representative heterogeneous catalysts for electrochemical NRR. Catalysts

Fe plate

Reaction conditions

NH3 production rate

H-type cell, 6.0 m KOH

2.8×10-3 μg h-1 cm-2, at -1.07 V

Faradaic efficiency

Reference

(%)

s

1

184

37

vs. SCE Fe2O3-CNT

PEM-type cell, Pt as counter

0.22 μg h-1 cm-2, at -2.0 V vs.

0.15 at -1.0 V vs.

electrode, Ag/AgCl as reference

Ag/AgCl

Ag/AgCl

Back-to-back cell with anion-

0.95 μg h-1 cm-2, at 1.6 V cell

0.044

108

exchange-membrane,

voltage

0.28

77

0.217

185

electrode, 2.0 M NaHCO3 γ-Fe2O3

Ag/AgCl

as reference electrode, 0.50 M KOH Ru/C

PEM-type cell, Ag/AgCl as

0.21 μg h-1 cm-2, at -1.10 V vs.

reference electrode, 2.0 M KOH,

Ag/AgCl

20 oC Ru nanosheets

H-type cell, 0.10 M KOH

23.88 μg h-1 mg-1cat at -0.2 V vs. RHE

Mo film

Pt plate as counter electrode,

1.89 μg h-1 cm-2 at -0.49 V vs.

Ag/AgCl as reference electrode,

RHE

0.72 at 0.14 V

0.01 M H2SO4 electrolyte Au/TiO2

H-type cell, Pt foil as counter

21.4 μg h-1 mg-1cat at -0.2 V vs.


RHE

8.11

134

10.1

107

electrode, 0.10 m KOH Au-CeOx/RGO


8.3 μg h-1 mg-1cat at -0.2 V vs.


RHE

electrode, 0.10 m KOH


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VN on carbon cloth

Graphite

rod

as

counter


15.2 μg h-1 cm-2 at -0.3 V vs.

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3.58

186

1.42

165

RHE

electrode, 0.1 M HCl electrolyte N-doped carbon


2.38×104 μg h-1 g-1cat at -0.9 V


vs. RHE

electrode, 0.05 M H2SO4

3.3 Biological catalysts Enzymatic catalysts responsible for biological fixation of N2 to NH3 are called nitrogenases.187,188 Nitrogenases can only be found in a small group of microorganisms. They can activate N2 at ambient conditions by using the energy from the hydrolysis of adenosine 5’triphosphate (MgATP) to adenosine 5’-diphosphate (MgADP) according to the equation (16) N2 + 8e- + 16MgATP + 8H+ → 2NH3 + H2 + 16MgADP + 16Pi

(16)

NRR (a six-electron reaction) requires 8 electrons from a nitrogenase and the hydrolysis of 16 molecules of ATP to ADP and inorganic phosphate (Pi).83,188


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Figure 14 The structure, reactants, and cofactors of a Fe protein/MoFe protein nitrogenase complex. Reproduced with permission from reference.83

The structure of typical nitrogenases consists of two proteins. One serves as binding sites for ATP, and the other contains an active metal site. There are three different types of metal-containing proteins according to their active metals, including Mo, V or Fe. The structure of a nitrogenase containing a Fe protein and a MoFe protein is displayed in Figure 14. The Fe protein houses the binding sites for ATP and a single [4Fe4S] redox cluster, while the MoFe protein has a [Fe7S9MoChomocitrate] cofactor (FeMo-cofactor), which is the active site for N2 reduction, and a [Fe8S7] Pcluster, which mediates the electron transfer from the Fe protein to the FeMo-cofactor. The transformation of N2 to NH3 has been proposed to follow a general kinetic scheme as depicted in Scheme 2. The catalytic intermediates were denoted as E0 to E8, in which the subscripts 0 - 8 represented the number of electrons/protons delivered to the FeMo-cofactor. The transfer of 53 ACS Paragon Plus Environment

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electrons/protons was accomplished by the binding and hydrolysis of ATP within the Fe protein. At stage 4 (E4), one H2 molecule was eliminated leaving a highly activated doubly reduced site for N2 binding, which was subsequently hydrogenated to form NH3.189,190

Scheme 2 Kinetic scheme of the sequential transfer of electrons/protons to the FeMo-cofactor, the futile H2-production cycles (in red), the reversible binding of N2, and the release of NH3. Reproduced with permission from reference.83

To date, few nitrogenases have been directly used as catalysts for electrochemical NRR toward the formation of NH3, because most nitrogenases would lose their activities when they are out of their original plants.191 Some research efforts have been devoted to isolating active components from nitrogenases for electrocatalytic applications, which provide strong evidence for the capability of adopting nitrogenase out of their original plants for electrochemical NRR. For example, Newton et al. isolated a FeMo-cofactor from a Mo-Fe protein of Azotobacter vinelandii nitrogenase and found that it could undergo electrochemical redox reactions on a glassy carbon electrode in TV-methylformamide.192 Minteer et al. constructed an interface between enzymes and electrodes as bioelectrochemical devices.193 They reported that a non-physiological electron donor methyl viologen (N,N’-dimethyl-4,4’-bipyridinium) could shuttle electrons to nitrogenases and 54 ACS Paragon Plus Environment

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sustain NRR by H2, delivering an NH3 production rate of 286 nmol NH3 mg-1 MoFe protein and a faradic efficiency of 26.4%.193 Further, FeMo-cofactors have been successfully applied as electrocatalyst for reducing protons to dihydrogen, azide to NH3, and nitrite to NH3.194 Nitrogenase FeMo-cofactors have a composition of MoFe(6-8)S(4-9). Some compounds have been synthesized to mimic the structure of FeMo-cofactors in order to create active catalysts. For example, MacFarlane et al. theoretically predicted that MoS2 sheets supported Fe catalysts could be active for NH3 production.161 Ohki et al. synthesized a Mo-Ti-S [Mo3S4Ti] cluster, which imitated FeMo-cofactors’ structures, and achieved the conversion of N2 into NH3 and N2H4 in the presence of a reducing reagent.103 Overall, nitrogenases exhibit higher activity and better selectivity towards NH3 production when they function in plants or microorganisms.194 However, they are highly sensitive to pH and temperature, and most of their activity would be lost when they are out of their native environments.191 Some research efforts have been devoted to developing catalysts that have similar structures as those of nitrogenases with the aim to mimic or reproduce the performance of nitrogenases. However, the performance of these catalysts is still unsatisfactory and requires substantial improvements.

4. Summary and perspectives NH3 is important in modern society. The current NH3 production relies on the Haber-Bosch process, which has various issues. An emerging method is to synthesize NH3 via electrochemical NRR at ambient conditions. The required electricity can be obtained from renewable energy sources, such as solar and wind. Significant research efforts have been devoted to NH3 synthesis 55 ACS Paragon Plus Environment

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via electrochemical NRR, including different designs of reactors, electrolytes, and electrocatalysts. In this review, we summarized various catalysts that can be used for electrochemical NRR at ambient conditions. These catalysts were categorized into homogeneous, heterogeneous, and biological catalysts. Homogeneous catalysts are ideal for carrying out in-depth mechanistic studies because of their well-defined structures, and they usually exhibit high activity due to efficient mass transfer of reactants, products, and catalysts in the same liquid phase. However, the difficulty in separating and recovering homogeneous catalysts limits their practical applications because it is desirable to recycle and reuse catalysts. Biological catalysts, e.g., nitrogenases or their analogs, exhibit higher activity and better selectivity towards NH3 production when they are in plants or microorganisms. Nevertheless, much more efforts are required to retain their high activities when they are separated from their native environments especially under the conditions outside of their optimum pH- and temperature windows. Despite structure complicity of heterogeneous catalysts, they are currently the most widely studied category of catalysts for electrochemical NRR owing to their easy separation properties and relatively high catalytic activities, which makes them attractive for potential commercialization. The current process of heterogeneous catalyst studies can be found in the following several aspects. First, even though the N ≡ N triple bond is one of the most stable bonds in chemistry, theoretical studies showed that the electrochemical NRR at ambient temperatures is feasible based on thermodynamic data. Several types of catalysts were found to be active for electrochemical NRR. Transition metal catalysts can activate N2 via the direct N2 dissociation or associative N2 dissociation pathways. Among studied transition metal catalysts, (1) Fe-based catalysts are 56 ACS Paragon Plus Environment

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currently the most widely used non-noble-metal catalysts for electrochemical NRR due to their high activity for dissociating the N≡N triple bond. The valence, crystalline structure of Fe compounds as well as the synergistic effects between Fe species and catalyst substrates strongly affect the performance of Fe-based catalysts in NRR. (2) Ru catalysts could drive electrochemical NRR at relatively low reaction temperatures. B5 sites on Ru catalysts are the active center for N2 activation. Therefore, catalyst morphology, including the size and defect sites of Ru nanoparticles, which determine the amount of exposed B5 sites, affects their catalytic activities. (3) Mo-based catalysts are also active in electrochemical NRR, and current studies are primarily based on small Mo clusters and individually dispersed Mo atoms. (4) The crystalline structure together with the size of Au nanoparticles plays important roles in determining the activities of Au-based catalysts. However, the reaction mechanism of NRR on Au-based catalysts are unclear and require further explorations. (5) Rh and Pt may not be promising for electrochemical NRR due to their high cost and unsatisfied catalytic performances. Compared to transition metal catalysts, nitride catalysts are more active toward electrochemical NRR, because N2 activation and NH3 formation on nitrides via the MvK mechanism require relatively lower overpotentials. Sulphide-, carbide-based catalysts, as well as these non-metal-element doped catalysts, are also potential candidates for electrochemical NRR, whereas these studies are still in their infancy stage, more exploration is needed to understand how these catalysts activate N2. Hybrid catalysts, consisting of two or more active components could exhibit performances with the merits of all components. Li salts, perovskites, spinels, pyrochlores, as well as ionic liquids, have potential in electrochemical NRR. Nevertheless, more research efforts are required to clarify their exact roles, as catalysts or as mediators.


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HER is an inevitable reaction competing with NRR. Several strategies have been proposed to increase the selectivity to NH3. (1) Some catalysts with active sites that are more efficient for N2 adsorption and activation rather than proton adsorption may increase the selectivity to NH3. For example, computational results showed that early transition metals, such as Sc, Y, Ti, and Zr, are more

selective

to

NRR

than

HER.59

(2)

Catalysts

with

specified

structures/configurations/morphologies may also suppress HER. For example, confining Fe2O3 on activated charcoal inhibits HER.112 (3) Limiting proton concentration in electrolytes can suppress HER. For example, a high pH solution can suppress the adsorption of protons on catalyst surfaces. (4) Reaction conditions can be regulated to accelerate NRR other than HER. For example, different reaction temperatures can tune the overpotentials of NRR and HER.2 Despite substantial research, key challenges remain unsolved in finding efficient catalysts for practical NRR applications. Most of the current studied catalysts show low N2 conversion rates, low turnover frequencies, and larger overpotentials. The NH3 production rates are generally in the magnitude of mg hour-1 cm-2 of catalyst, which is far away from the requirement of practical industrial applications. The Faradaic efficiency for NH3 production is usually lower than 10%, and many are below 1%. We envision that the following research topics are important to move this research field forward. First, N2 activation is often the rate-determining step in electrochemical NRR. Therefore, catalysts that could significantly reduce the activation barrier for N2 activation is critical to achieving higher NH3 production rates. One potential strategy is to shift the determining step from N2 activation to other elementary steps. For example, Hosono et al. showed that Ru catalysts on a strong electron donator 12CaO∙7Al2O3 shifted the rate determining step to the formation of N-Hn species and achieved an NH3 production rate an order of magnitude higher than previous Ru catalysts.129 Second, the reaction mechanisms, especially active sites for N2/proton 58 ACS Paragon Plus Environment

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adsorption/activation, are still poorly understood. In this aspect, it is urgent to develop advanced approaches for more deepened theoretical studies, targeting at gaining a better understanding of electrochemical N2 reduction mechanism at the atomic level, which will then provide guidance for material screening and catalyst structure design. On one hand, considering that the complexity of the catalyst surface generally results in a complicated series of adsorbed intermediates, it is required to develop computational methods and models that could accurately describe the catalyst surfaces for electrochemical N2 reduction. On the other hand, in addition to the theoretical studies, advanced in situ/operando characterization techniques, such as in situ X-ray absorption near edge structure, in situ extended X-ray absorption fine structure and in situ X-ray photoelectron spectroscopy, are critical in investigating the reaction intermediates and the actual active sites for NRR reaction and HER reaction under operating conditions. The determination of the active sites could guide the rational design of catalysts that could selectively produce the target product at a reasonable conversion rate. Together with the theoretical studies and in situ/operando characterizations, it is believed that catalysts with low overpotential, high energy efficiency and good scalability/modularity for electrochemical N2 fixation are prospected in the near future. Third, it may be feasible to regulate the selectivity to NH3 or other byproducts by careful mass transport management. Controlling the transfer rates of protons and/or N2 to catalyst surfaces may enable selective transfer of N2 molecules to active catalytic sites. For example, coating catalysts with a hydrophobic surrounding layer or sterically large proton donors with poor proton transfer kinetics may allow selective N2 penetration and hinder proton transfer. Forth, the stability of catalysts determine their lifetime. The current studies of catalyst stability for NRR are still limited. It is important to understand catalyst deactivation theoretically and experimentally, which would guide the rational design of effective catalysts with long lifetime. Fifth, the current synthesis and 59 ACS Paragon Plus Environment

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evaluation of NRR catalysts are often carried out in research labs with a small amount of samples. It is important to investigate the scalable and economical synthesis of promising catalyst candidates. The trade-off between performance and cost often determine the chance of catalysts being applied for practical industrial applications.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Z.-j. Wang), [email protected] (D. He), [email protected] (Y. Chen) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support for this work from Australia Research Council Discovery Early Career Researcher Award (DE180100523), Future Fellowships scheme (FT160100107), Discovery Project (DP180102210) and Natural Science Foundation of China (No. 21573120, No. 21776007, and No. 21811530293), the Fundamental Research Funds for the Central Universities (XK1802-1) and Scholarship by China Scholarship Council (CSC) (No. 201706885023).

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