Electrochemical ammonia synthesis: Mechanistic understanding and catalyst design

Abstract

Ammonia, the second largest synthetic chemical commercialized worldwide, is widely used as a fertilizer and is a key intermediate for production of all nitrogen-atom-containing chemicals. It could also be employed for fueling applications. Electrochemical N2 reduction reaction (NRR) offers a renewable and distributed route for NH3 production. Heightened research efforts have focused on the design and development of advanced electrocatalysts to enhance the efficiency of NRR to make it competitive against the Haber-Bosch process from the economic and ecological viewpoints. We describe the latest advances in the NRR from both theoretical and experimental aspects and provide a guide on how electrocatalysis of NRR could be improved. We discuss the roles of emerging in situ and operando methods in elucidating the dynamic catalyst structure and other reaction parameters. The possible reaction pathways and the major challenges in improving the NRR are also highlighted. NH3 production is dependent on the century-old Haber-Bosch process, which is energy and capital intensive and relies on H2 from steam reforming, hence, contributing to greenhouse gas emissions. Electrochemical NH3 synthesis can be realized by reaction of N2 and a proton source under mild conditions powered by renewable electricity, which offers a promising carbon-neutral and sustainable strategy. However, N2 has remarkable thermodynamic stability and requires high energy to be activated. Implementation of this “clean” NH3 synthesis route therefore still requires significant enhancement in energy efficiency, conversion rate, and durability, which is only achievable through the design of efficient electrocatalysts. This article provides a timely theoretical and experimental overview of recent advances in the electrocatalytic conversion of N2 to NH3 underlining the development of novel electrocatalysts. Advances of in situ and operando studies for mechanistic understanding of the reaction and the main challenges and strategies for improving electrocatalytic N2 reduction are highlighted. NH3 production is dependent on the century-old Haber-Bosch process, which is energy and capital intensive and relies on H2 from steam reforming, hence, contributing to greenhouse gas emissions. Electrochemical NH3 synthesis can be realized by reaction of N2 and a proton source under mild conditions powered by renewable electricity, which offers a promising carbon-neutral and sustainable strategy. However, N2 has remarkable thermodynamic stability and requires high energy to be activated. Implementation of this “clean” NH3 synthesis route therefore still requires significant enhancement in energy efficiency, conversion rate, and durability, which is only achievable through the design of efficient electrocatalysts. This article provides a timely theoretical and experimental overview of recent advances in the electrocatalytic conversion of N2 to NH3 underlining the development of novel electrocatalysts. Advances of in situ and operando studies for mechanistic understanding of the reaction and the main challenges and strategies for improving electrocatalytic N2 reduction are highlighted. Ammonia plays a key role in sustaining life and the global chemical economy with an annual production exceeding 200 million tons.1Andersen S.Z. Čolić V. Yang S. Schwalbe J.A. Nielander A.C. McEnaney J.M. Enemark-Rasmussen K. Baker J.G. Singh A.R. Rohr B.A. et al.A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements.Nature. 2019; 570: 504-508Crossref PubMed Scopus (363) Google Scholar The bulk of industrial NH3 is primarily used to make fertilizers in agriculture (~80%) and to produce explosives, pharmaceuticals, refrigerants, and cleaning products (~20%). NH3 is also being reckoned to be a potential fuel as well as ideal hydrogen carrier with a high gravimetric hydrogen content (~17.6 wt %) and large volumetric hydrogen energy density (10.7 kg H2/100 L), in addition to advantages of easy liquefaction for handling, storage, and transportation. NH3 fuel produces zero CO2 and low overall emissions. N2 is regenerated at the point of use and released into the atmosphere in a closed-cycle process. Currently, about 90% of the NH3 produced worldwide still relies on the century-old, fossil-fuel-powered Haber-Bosch process, which entails thermocatalytic conversion of N2 and H2 (N2 + 3H2 → 2NH3 with a standard enthalpy of formation ΔHf0 = −45.9 kJ mol−1 and standard Gibbs free energy ΔGf0 = −16.48 kJ mol−1) at high temperature (>300°C) and intense pressure (>15 MPa), over Fe- or Ru-based catalysts (utilized in the Kellog, Brown, and Root [KBR] advanced ammonia process [KAAP]) with promoters (such as Al2O3 and K). Recently, a nickel-loaded LaN catalyst was reported to be capable of accelerating the dissociation of N2 (the kinetically determining step), comparable with ruthenium-based catalysts.2Ye T.N. Park S.W. Lu Y. Li J. Sasase M. Kitano M. Tada T. Hosono H. Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst.Nature. 2020; 583: 391-395Crossref PubMed Scopus (37) Google Scholar The Haber-Bosch process is among the top largest industrial chemical processes. The reaction has been claimed to be one of the greatest inventions of the 20th century and has been the subject of three chemistry Nobel prizes. However, it is capital intensive, requiring large and centralized plant infrastructure, and energetically demanding, requiring an energy input of ~485 kJ mol−1 (responsible for over 1% of the world’s annual energy consumption). The process combines N2 from the air with pure H2 derived from endothermic steam-methane reforming (i.e., CH4 + H2O → CO + 3H2), consuming 3%–5% of global natural gas or other fossil resources (e.g., coal), which emits huge quantities of the greenhouse gas CO2 into the atmosphere (from water gas shift, i.e., CO + H2O → CO2 + H2, the global average is ~2.86 tons of CO2 released per ton of NH3 and 1.6 tons of CO2 released per ton of NH3 in the most efficient plants).3Soloveichik G. Electrochemical synthesis of ammonia as a potential alternative to the Haber–Bosch process.Nat. Catal. 2019; 2: 377-380Crossref Scopus (90) Google Scholar The Haber-Bosch process also has a drawback of low energy efficiency with an NH3 conversion of less than 15% per cycle (limited by thermodynamics). Demand for NH3 continues to increase to support the growing global population. Hence, a high-efficiency, mild (avoiding unfavorable equilibrium issues), sustainable, and eco-friendly alternative approach to manufacturing NH3 is of significant importance for both scientific research and industrial applications. From these scenarios, three major clean routes for ammonia synthesis involving biocatalysis, photocatalysis, and electrocatalysis have sparked increasing research interest in recent years, as illustrated in Figure 1. Biological nitrogen fixation in nature is attained under mild conditions (<40°C, atmospheric pressure) by metalloenzyme nitrogenases that are composed of FeMo, FeV, or FeFe cofactor as active sites with FeMo being the most active and abundant enzyme for N2 reduction.4Liu T. Gau M.R. Tomson N.C. Mimicking the constrained geometry of a nitrogen-fixation intermediate.J. Am. Chem. Soc. 2020; 142: 8142-8146Crossref PubMed Scopus (6) Google Scholar A minimum of 16 moles of adenosine triphosphate (ATP) is necessary to reduce one mole of N2 (N2 + 8H+ + 8e− + 16ATP → 2NH3 + H2 + 16ADP + 16PO43− where ADP is adenosine diphosphate) with concomitant formation of one mole of H2 and a corresponding transfer of 8(e−/H+), not 6 (which could result in dissipative hydrolysis of 4 ATP).5Foster S.L. Bakovic S.I.P. Duda R.D. Maheshwari S. Milton R.D. Minteer S.D. Janik M.J. Renner J.N. Greenlee L.F. Catalysts for nitrogen reduction to ammonia.Nat. Catal. 2018; 1: 490-500Crossref Scopus (381) Google Scholar As a consequence, production of one NH3 consumes 8 ATP, requiring an energy input of 244 kJ mol−1. However, biological nitrogen fixation occurs only in a select group of microorganisms, and the nitrogenases are susceptible to deactivation by oxygen.6Lancaster K.M. Roemelt M. Ettenhuber P. Hu Y. Ribbe M.W. Neese F. Bergmann U. DeBeer S. X-ray emission spectroscopy evidences a central carbon in the nitrogenase iron-molybdenum cofactor.Science. 2011; 334: 974-977Crossref PubMed Scopus (552) Google Scholar In addition, biological conversion of nitrogen into NH3 has a low space-time yield. Noteworthy, NH3 production via natural processes cannot meet the current and future NH3 needs. Photocatalytic NH3 synthesis only requires solar energy, water, and N2, encompassing two coupled redox half reactions, i.e., oxidization of water by photogenerated holes (3H2O [l] + 6h+ → 6H+ [aq.] + 3/2O2 [g]) in the valence band (VB) and reduction of N2 via photogenerated electrons (N2 [g] + 6H+ [aq.] + 6 e− → 2NH3 [g]) in the conduction band (CB). Nonetheless, the overall solar-to-chemical conversion efficiency is far from satisfactory owing to poor light utilization, low density of active sites, and rapid recombination of photoexcited electron-hole pairs. Electrochemical synthesis of NH3 was first demonstrated by Humphrey Davy in 1,807,7Davy H. I. The Bakerian Lecture, on some chemical agencies of electricity.Phil. Trans. R. Soc. 1807; 97: 1-56Crossref Google Scholar while a relevant patent was provided in 1908. Reliable quantification of NH3 was achieved late in 1922 by Fichter and Suter.8Fichter F. Suter R. Zur Frage der kathodischen Reduktion Des elementaren Stickstoffs.Helv. Chim. Acta. 1922; 5: 246-255Crossref Scopus (4) Google Scholar Electrochemical synthesis of NH3 via N2 reduction is attractive because of (1) potentially higher energy efficiency than the Haber-Bosch process,5Foster S.L. Bakovic S.I.P. Duda R.D. Maheshwari S. Milton R.D. Minteer S.D. Janik M.J. Renner J.N. Greenlee L.F. Catalysts for nitrogen reduction to ammonia.Nat. Catal. 2018; 1: 490-500Crossref Scopus (381) Google Scholar (2) environmental compatibility through coupling with carbon-free renewable energy resources (solar, tidal, and wind), (3) elimination of fossil fuels as H2 sources whereby the required protons (H+) are generated in situ from water oxidation, (4) flexible control of the reactions by adjusting external parameters (such as electrochemical voltage), being conducive to modular and small-scale operation, and (5) scalability and on-demand, on-site NH3 production. This direct N2 reduction process is supposed to be less capital intensive than a combined water electrolysis with Haber-Bosch process, which is only about 40% energy efficient.9MacLaughlin C. Role for standardization in electrocatalytic ammonia synthesis: a conversation with leo liu, Lauren Greenlee, and Douglas macfarlane.ACS Energy Lett. 2019; 4: 1432-1436Crossref Scopus (12) Google Scholar Electrochemical N2 reduction reaction (NRR) to generate NH3 was mostly undertaken at high temperatures (above 500°C) using proton-conductive solid electrolytes before 2000.10Marnellos G. Stoukides M. Ammonia synthesis at atmospheric pressure.Science. 1998; 282: 98-100Crossref PubMed Scopus (347) Google Scholar However, this high-temperature route suffers from bottlenecks of low electronic and/or ionic conductivity in the electrolyte and NH3 decomposition, in addition to the parasitic hydrogen evolution reaction (HER). To facilitate electrochemical NH3 synthesis at lower temperatures (100°C–500°C), molten electrolytes with higher ion conductivity such as eutectic-based systems were attempted. Unfortunately, operations at such intermediate temperatures usually require large overpotentials, thus, decreasing energy efficiency at high current densities. Poor durability in the long-term operation is another issue of concern. The synthesis of NH3 below 100°C using aqueous electrolytes has been the focus of interest since 2000, which was motivated by Nørskov and coworkers to mimic the FeMo cofactor in the nitrogenase enzyme.11Rod T.H. Logadottir A. Nørskov J.K. Ammonia synthesis at low temperatures.J. Chem. Phys. 2000; 112: 5343-5347Crossref Scopus (156) Google Scholar This low-temperature synthetic process significantly reduces equipment and operational costs and increases stability of NH3 produced for distributed deployment. However, binding and activation of N2 under ambient conditions remains a grand challenge because the molecule is thermodynamically very stable and kinetically inert. Homogeneous (e.g., nitrogenase enzymes and molecular catalysts) and heterogeneous catalysts have been applied to accelerate this up-hill reduction reaction. Although high turnover number is attained in homogeneous catalysis, most homogeneous systems have high cost, toxicity, poor stability, and involve complex post-separation steps, but they are less likely the case in heterogeneous catalysis, thereby limiting their prospects for industrial application. Therefore, major endeavors have been devoted to developing heterogeneous electrocatalysts based on rational design approaches. However, despite recent advances achieved in this effort (Figure 2; Table S1), breakthrough progress is still hampered with (1) low faradic efficiency (FE) (typically not more than 15% due to the overwhelming HER catalyzed at similar or even lower overpotentials), (2) large overpotential (or low energetic efficiency), (3) slow kinetics resulting in small exchange current density, and (4) deactivation of electrodes in less than 100 h, restricting practical use and technological commercialization. High current efficiency (the fraction of electric charge that is used for the formation of NH3) is usually obtained at the expense of a low NH3 production rate, compromising the overall economic viability of the process. The majority of N2 reduction electrocatalysts reported thus far operate below 20 mA cm−2, which is, however, far less than that required for commercial electrolyzers. Under these circumstances, heightened research efforts have focused on the design and development of advanced electrocatalysts, toward lower energy cost to compete with the Haber-Bosch process and attainment of higher current density to minimize capital costs. Because of the high interest and rapid advances pertaining to this reaction, this article aims to provide a comprehensive and up-to-date review of nanostructured heterogeneous catalysts for the electrochemical NRR, with emphasis on relationships between structures and properties. A summary of the most important developments in catalyst design, and analytic tools and techniques for electrocatalytic N2 reduction, as well as possible reaction mechanisms and pathways are presented. Next, we discuss the strategies available to tune the electronic structure of electrocatalysts and other relevant parameters in order to further enhance the performance and efficiency of electrochemical N2 reduction. We also introduce emerging in situ and operando methods for investigations of the dynamic catalyst structure and other reaction parameters during the NRR. Finally, future challenges and opportunities of NH3 synthesis by NRR are outlined. Electrochemical reduction of N2 takes place at electrode-electrolyte interfaces and requires protons transferred from the electrolyte, electrons transferred from the electrode (catalyst), and optimized sites on the electrode surface onto which N2 molecules can be adsorbed and subsequently activated. Three major elementary steps should be taken into account when modeling the fundamental processes: (1) diffusion and chemical adsorption of N2 molecules and protons onto the cathode surface; (2) activation of N2 and reductive addition of hydrogen atoms; and (3) rearrangement and desorption of the product, NH3, or other products (e.g., hydrazine [N2H4] and diazene [N2H2]) from the electrode surface and migration into the electrolyte. The presence of oxygen in the cathodic compartment deteriorates NRR performance, resulting in higher overpotentials. Moreover, aqueous electrocatalytic NRR is plagued by concomitant production of molecular hydrogen, similar to the cases in biological N2 fixation and electrochemical CO2 reduction.12Sun Z. Ma T. Tao H. Fan Q. Han B. Fundamentals and challenges of electrochemical CO2 reduction using two-dimensional materials.Chem. 2017; 3: 560-587Abstract Full Text Full Text PDF Scopus (359) Google Scholar This leads to low selectivity toward NH3 formation and loss of efficiency, especially because the HER has intrinsically much faster kinetics compared with the NRR. For all metals, the HER proceeds at a more positive potential than for NH3 synthesis.13Singh A.R. Rohr B.A. Statt M.J. Schwalbe J.A. Cargnello M. Nørskov J.K. Strategies toward selective electrochemical ammonia synthesis.ACS Catal. 2019; 9: 8316-8324Crossref Scopus (46) Google Scholar Moreover, the rate of hydrogen production was modeled to be the first order in the electron and proton concentrations, while the rate of NH3 production was zeroth order in both.14Singh A.R. Rohr B.A. Schwalbe J.A. Cargnello M. Chan K. Jaramillo T.F. Chorkendorff I. Nørskov J.K. Electrochemical ammonia synthesis—the selectivity challenge.ACS Catal. 2017; 7: 706-709Crossref Scopus (298) Google Scholar Hence, to boost the NRR, the availability and activity of protons at the electrochemical interface, properties of the electrode, reaction conditions, and nature of the electrolyte should be manipulated to minimize the undesirable HER. Mitigating hydrogen evolution can be addressed by (1) selection of an electrolyte with reduced proton donor activity,15Zhou F. Azofra L.M. Ali M. Kar M. Simonov A.N. McDonnell-Worth C. Sun C. Zhang X. MacFarlane D.R. Electro-synthesis of ammonia from nitrogen at ambient temperature and pressure in ionic liquids.Energy Environ. Sci. 2017; 10: 2516-2520Crossref Google Scholar (2) addition of soluble coordination complexes to facilitate N–H bond formation by mediating net H atom transfers,16Chalkley M.J. Del Castillo T.J. Matson B.D. Peters J.C. Fe-mediated nitrogen fixation with a metallocene mediator: exploring pKa effects and demonstrating electrocatalysis.J. Am. Chem. Soc. 2018; 140: 6122-6129Crossref PubMed Scopus (70) Google Scholar (3) optimization of reaction conditions (pH, applied potential, and reactor configuration),17Hao Y.C. Guo Y. Chen L.W. Shu M. Wang X.Y. Bu T.A. Gao W.Y. Zhang N. Su X. Feng X. et al.Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water.Nat. Catal. 2019; 2: 448-456Crossref Scopus (237) Google Scholar (4) Li+ association to decouple N2 fixation and NH3 evolution,18Ma J.L. Bao D. Shi M.M. Yan J.M. Zhang X.B. Reversible nitrogen fixation based on a rechargeable lithium-nitrogen battery for energy storage.Chem. 2017; 2: 525-532Abstract Full Text Full Text PDF Scopus (82) Google Scholar (5) engineering of electrode surface and electrode-electrolyte interface to regulate hydrophobicity (Figure 3A),19Koh C.S.L. Lee H.K. Fan Sim H.Y. Han X. Phan-Quang G.C. Ling X.Y. Turning water from a hindrance to the promotor of preferential electrochemical nitrogen reduction.Chem. Mater. 2020; 32: 1674-1683Crossref Scopus (4) Google Scholar and (6) tuning of electrocatalysts to favor adsorption and binding of nitrogen instead of protons.20Wang J. Yu L. Hu L. Chen G. Xin H. Feng X. Ambient ammonia synthesis via palladium-catalyzed electrohydrogenation of dinitrogen at low overpotential.Nat. Commun. 2018; 9: 1795Crossref PubMed Scopus (352) Google Scholar Figure 3B illustrates that diminishing proton concentrations by using an aprotic (or very alkaline) solvent offers an effective means to impede proton transfer thermodynamically. Figure 3C shows an alternative route to kinetically inhibit proton transfer by creating an aprotic and hydrophobic protection layers. Another issue in NRR is the extremely low solubility of N2 in aqueous electrolytes (water: ~0.00061 M at 25°C and P = 1 atm) because of its nonpolar nature, strong triple bond, and low polarizability, dramatically limiting the amount of N2 available for reaction. To enhance N2 dissolution, several strategies can be adopted: (1) employing low operating temperatures to reduce the Henry constant, (2) increasing N2 feed gas pressure, (3) using non-aqueous electrolytes (such as aprotic ionic liquids), and (4) designing a hydrophobic mesoporous structure with high gas sorptivity that can adsorb N2 but weaken interactions of water and the electrode surface. However, lowering the reaction temperature markedly restricts N2 diffusion, and increasing operating pressure would add technological complexity and cost. Conversely, a flow rate of N2 into catholyte below 10 sccm affects reaction kinetics, meanwhile, a higher N2 flow rate should in principle result in larger quantitative rates and FEs, which, however, may level off above a flow rate of 20 sccm.21Han Z. Choi C. Hong S. Wu T.S. Soo Y.L. Jung Y. Qiu J. Sun Z. Activated TiO2 with tuned vacancy for efficient electrochemical nitrogen reduction.Appl. Catal. B. 2019; 257: 117896Crossref Scopus (51) Google Scholar At higher flows, no effect of the gas flow rate and the position of the gas inlet was observed on the NRR results.22Jaecheol C. Hoang-Long D. Manjunath C. Bryan H.R. Alexandr S. Douglas M. Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water.https://chemrxiv.org/articles/preprint/Promoting_Nitrogen_Electroreduction_to_Ammonia_with_Bismuth_Nanocrystals_and_Potassium_Cations_in_Water/11768814/1Date: 2020Google Scholar Coupling electrolytes with a high N2 solubility and engineered electrocatalysts with a high density of N2 adsorption sites appears to be a method of choice for enhanced NRR performance. In contrast, the low solubility and slow transport of N2 can be overcome by design and use of gas diffusion electrodes to allow intimate contact between the gas, electrolyte, and catalyst.23Lazouski N. Chung M. Williams K. Gala M.L. Manthiram K. Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen.Nat. Catal. 2020; 3: 463-469Crossref Scopus (21) Google Scholar Parameters that may be useful in mechanistic analysis and benchmarking of electrocatalysts for the NRR include the following: (1) NH3 production rate (mgNH3 h−1 cm−2 or mg h−1 mgcat.−1); (2) FE (FE = 3nF/Q, where 3 is the number of electrons transferred per NH3 molecule, n is the number of moles for NH3 produced, F is Faraday’s constant (96,485 C mol−1), and Q is all the charge passed during the electrolysis process); (3) overpotential (η, defined as the difference between the thermodynamic potential of NRR and the applied potential required to achieve a desired current density); (4) NRR current density at a specific electrode potential of cell voltage; (5) energy efficiency (EE% = ΔGm0/E = (1000 × FE × 339.2)/[3 × F × (1.23 − η)] assuming an ideal nonpolarizable anodic oxygen evolution reaction with no overpotential and kinetic limitation, where ΔGm0 represents the standard Gibbs free energy of NH3 formation and E is the average mole energy input (kJ mol−1)); and (6) turnover frequency (TOF, s−1), a measure of per-site activity of catalysts (io [A cm−2] × FE/active site density (sites cm−2) × [1.602 × 10−19 (C/e−1) × 6e−1/N2], where io refers to exchange current density). For the sake of accurate comparison between different materials, a combined figure of merit is preferred. In some cases, a single metrics may fail to accurately represent the catalytic property. FE is commonly used as a measure of the amount of charge that is effectively used for a specific faradic reaction relative to the total charge that flows. However, only comparing FE is unlikely to give a complete picture of catalyst performance. Note that an improvement in FE may be not necessarily accompanied with an increase in yield rate. The latter is linked to product partial current density. A production rate normalized to electrochemical surface area (ECSA) provides information for intrinsic performance of a catalyst. While normalization based on geometric surface area is fundamental from the viewpoint of cost of a practical NRR cell. Additionally, high area-normalized NH3 yield rate does not always mean large mass-normalized NH3 yield rate. Reporting of both mass- and area-normalized NH3 production is reasonable to compare N2 fixation among different catalysts. To attain an efficient N2 electrolyzer, it is indispensable to maximize NH3 generation rate per unit of energy input, NH3 partial current density at the highest energy efficiency as well as the energy efficiency at the largest partial current density. The overall cell voltage required for NRR involves potentials for both the anode and cathode processes (Ecell = Eanode – Ecathode), where water oxidation (3 H2O ⇌ 6 H+ + 3/2 O2 + 6e–, E0[298 K] = +1.229 V [versus standard hydrogen electrode, SHE]) is recognized as the default anode process. Coupling this to cathodic NRR generates a minimum of several hundred millivolt overpotential for real-world electrocatalytic NRR. The role of the anode in NRR should not be neglected because it consumes almost half of the electrical input. Lowing oxygen evolution reaction overpotential and improving the anode efficiency can reduce the total energy cost of electrochemical NH3 synthesis, thus, promoting the prospects of its practical implementation. An N2 molecule comprises two nitrogen atoms bound by a disproportionately strong homonuclear triple bond. Each atom possesses a pair of electrons in the 2s orbital with opposite spin direction and three lone-pair electrons dispersed in the 2p orbitals with the same spin direction. Hybridization of the s-p atomic orbitals leads to formation of four bonding orbitals (two σ and two π orbitals) and four antibonding orbitals (two σ∗ and two π∗ orbitals), with the shared electrons in the π and 2σ orbitals forming an N≡N bond (Figure 4A).24Kitano M. Inoue Y. Yamazaki Y. Hayashi F. Kanbara S. Matsuishi S. Yokoyama T. Kim S.W. Hara M. Hosono H. Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store.Nat. Chem. 2012; 4: 934-940Crossref PubMed Scopus (607) Google Scholar From a thermodynamic perspective, NRR is feasible with overall negative Gibbs free energy. Nevertheless, activating N2 at ambient conditions is a formidable challenge because of (1) large energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of N2 (10.82 eV), impeding electron transfer; (2) high enthalpy of the first H atom addition to form N2H+ (ΔH0 = +37.6 kJ mol−1) before breakage of the N–N bond; (3) extreme stability and inertness of N2 with high cleavage energy (945 kJ mol−1) and first-bond breaking energy (410 kJ mol−1), non-polarity (absence of permanent dipole), and large triplet state energy (6.17 eV); and (4) negative electron affinity (−1.9 eV), low proton affinity (5.12 eV), and large ionization potential (15.85 eV). Electrochemical NRR proceeds through coupled or sequential proton/electron transfer processes, as summarized in Table 1. The former process seems to occur more favorably than the latter one with lower energetic barriers, while the disruptive two-electron HER is also more severe at similar potentials in aqueous electrolytes (Equations 4 versus 6; Equations 7 versus 8). Multiple intermediates (such as N2H4 and N2H2) may be involved during the concerted proton-electron transfer steps.25Cui X. Tang C. Zhang Q. A review of electrocatalytic reduction of dinitrogen to ammonia under ambient conditions.Adv. Energy Mater. 2018; 81800369Crossref Scopus (426) Google Scholar,26van der Ham C.J.M. Koper M.T.M. Hetterscheid D.G.H. Challenges in reduction of dinitrogen by proton and electron transfer.Chem. Soc. Rev. 2014; 43: 5183-5191Crossref PubMed Google Scholar The addition of the first H atom to form N2H (Equation 9) demands a rather negative equilibrium potential (−3.2 V versus reversible hydrogen electrode, RHE), while an even more negative potential is needed for the first electron transfer to yield N2− (Equation 17). At a high pH of 14, Equation 17 may compete with Equation 9 provided a weak affinity for ∗N2H (∗ represents an active site on a catalyst) but a stabilizing interaction of the catalyst with the N2−. The preferred pathway between concerted proton-electron transfer and sequential proton-electron transfer was recently reported to be pH dependent. However, the majority of theoretical calculations of NRR thus far did not consider the pH impact. Hence, further exploration in this regard is necessary.Table 1HER and electrochemical NRR processes with corresponding equilibrium potentialsEquationReactionE0 (V)25Cui X. Tang C. Zhang Q. A review of electrocatalytic reduction of dinitrogen to ammonia under ambient conditions.Adv. Energy Mater. 2018; 81800369Crossref Scopus (426) Google Scholar,26van der Ham C.J.M. Koper M.T.M. Hetterscheid D.G.H. Challenges in reduction of dinitrogen by proton and electron transfer.Chem. Soc. Rev. 2014; 43: 5183-5191Crossref PubMed Google Scholar1N2 (g) + 6H+ (aq.) + 6e− ⇌ 2NH3 (g)0.0577 (versus SHE)2N2 (g) + 8H+ (aq.) + 6e− ⇌ 2NH4+ (aq.)+0.274 (versus SHE)3N2 (g) + 8HBase+ (MeCNaAcetonitrile.) + 6e− ⇌ 2NH4+ (MeCN) + 8Base+0.361 – 0.079 pKa (versus Fc+/0)4N2 (g) + 2H2O (l) + 6H+ (aq.) + 6e− ⇌ 2NH3·H2O (aq.)+0.092 (versus SHE) or +0.23 (versus RHE)bThermodynamic equilibrium potential is calculated based on Nernst equation.5N2 (g) + 6HBase+ (MeCN) + 6e− ⇌ 2NH3 (MeCN) + 6Base+0.035 – 0.059 pKa (versus Fc+/0)62H+ (aq.) + 2e− ⇌ H2 (g)0 (versus SHE)7N2 + 6H2O (l) + 6e− ⇌ 2NH3 (g) + 6OH− (aq.)−0.736 (versus RHE, pH 14)82H2O (l) + 2e− ⇌ H2 (g) + 2OH− (aq.)−0.828 (versus normal hydrogen electrode, NHE, pH 14)9N2 (g) + H+ (aq.) + e− ⇌ N2H (g)−3.2 (versus RHE)10N2 (g) + 2H+ (aq.) + 2e− ⇌ N2H2 (g)−1.10 (versus RHE)11N2 (g) + 2HBase+