Electrochemical ammonia synthesis: Mechanistic understanding and catalyst design

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