ARTICLE

Ambient ammonia synthesis via palladium-catalyzed electrohydrogenation of dinitrogen at lowoverpotentialJun Wang1, Liang Yu2, Lin Hu3, Gang Chen4, Hongliang Xin2 & Xiaofeng Feng 1

Electrochemical reduction of N2 to NH3 provides an alternative to the Haber−Bosch process

for sustainable, distributed production of NH3 when powered by renewable electricity.

However, the development of such process has been impeded by the lack of efficient elec-

trocatalysts for N2 reduction. Here we report efficient electroreduction of N2 to NH3 on

palladium nanoparticles in phosphate buffer solution under ambient conditions, which exhi-

bits high activity and selectivity with an NH3 yield rate of ~4.5 μgmg−1Pd h−1 and a Faradaic

efficiency of 8.2% at 0.1 V vs. the reversible hydrogen electrode (corresponding to a low

overpotential of 56mV), outperforming other catalysts including gold and platinum. Density

functional theory calculations suggest that the unique activity of palladium originates from its

balanced hydrogen evolution activity and the Grotthuss-like hydride transfer mechanism on

α-palladium hydride that lowers the free energy barrier of N2 hydrogenation to *N2H, the

rate-limiting step for NH3 electrosynthesis.

DOI: 10.1038/s41467-018-04213-9 OPEN

1 Department of Physics, University of Central Florida, Orlando, FL 32816, USA. 2Department of Chemical Engineering, Virginia Polytechnic Institute and StateUniversity, Blacksburg, VA 24061, USA. 3 Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816, USA.4Department of Chemistry, University of Central Florida, Orlando, FL 32816, USA. Correspondence and requests for materials should be addressed toX.F. (email: Xiaofeng.Feng@ucf.edu) or to H.X. (email: hxin@vt.edu)

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Due to the limited supply of fossil fuels, there is a criticaldemand to use renewable energy to drive the chemicalprocesses that have heavily relied on the consumption of

fossil fuels1. One such energy-intensive chemical process is theHaber−Bosch process2, 3, which produces NH3 from N2 and H2

using iron-based catalysts under high temperature (350−550 °C)and high pressure (150−350 atm). NH3 is one of the most highlyproduced inorganic chemicals in the world, because of its vastneed in fertilizer production, pharmaceutical production, andmany other industrial processes4, 5. In 2015, around 146 milliontons of NH3 was produced globally through the Haber−Boschprocess6, which consumes 3−5% of the annual natural gas pro-duction worldwide, approximating to 1−2% of the global annualenergy supply2, 5. This industrial process is also responsible for>1% of the global CO2 emissions2. Therefore, it is highly desirableto develop an alternative, efficient process for NH3 synthesisusing renewable energy7–11, which can simultaneously reduce theCO2 emissions.

One alternative approach to the Haber−Bosch process is to useelectrical energy to drive the NH3 synthesis reaction underambient conditions12–15, which can reduce the need for hightemperature and pressure, and, thereby lower the energy demand.When powered by electricity from renewable energy sources suchas solar and wind, electrochemical synthesis of NH3 from N2 andH2O can facilitate sustainable, distributed production of NH3, aswell as the storage of renewable energy in NH3 as a carbon-neutral liquid fuel16–18, owing to its high energy density (4.32kWh L−1), high hydrogen content (17.6 wt%), and facile liqui-dation (boiling point: −33.3 °C at 1 atm). However, the devel-opment of the process has been impeded by the lack of efficientelectrocatalysts for N2 reduction reaction (N2RR). Althoughelectrochemical synthesis of NH3 has been demonstrated onvarious materials including Ru, Pt, Au, Fe, and Ni19–31, most ofthem showed low activity and selectivity (typically, Faradaicefficiency <1%) for NH3 production26–31. Therefore, majorimprovement in N2RR catalysts is essential for the developmentof low-temperature NH3 electrolyzers, which necessitates a betterunderstanding and control of the catalytic materials and thereaction kinetics.

There are two major challenges associated with electrochemicalNH3 synthesis in aqueous media32, 33. From the thermodynamicpoint of view, the splitting of the strong N≡N bond requires areduction potential where the hydrogen evolution reaction (HER)readily occurs, leading to an extremely low Faradaic efficiency forN2RR under ambient conditions26–31. Therefore, it would beoptimal to find a catalytic system that can promote N2RR at lowoverpotentials and suppress the competing HER. From the kineticperspective, it is suggested that the rate-determining step for N2RRis the formation of *N2H through a proton-coupled electrontransfer process (H+ + e−+ *+N2 → *N2H)33, 34, where * sig-nifies an active site on the catalyst surface. It involves a protonfrom the electrolyte, an electron transferred from the electrode,and a N2 molecule in the solution. The strong solvent reorgani-zation required for the endergonic charge transfer steps has a lowprobability of occurrence, leading to the sluggish kinetics. Instead,if the atomic *H species can be formed on a catalyst surface anddirectly react with N2, it may largely accelerate the kinetics to form*N2H: *H+N2 → *N2H. Indeed, there have been several investi-gations of metal hydride complexes for N2 reduction in a homo-genous medium35, 36. Similarly, NH3 synthesis has been achievedat a low temperature by LiH-mediated N2 transfer and hydro-genation on the transition metals37. Therefore, it is imperative toexamine such a hydrogenation pathway for the electrochemicalN2RR in aqueous electrolyte systems under ambient conditions.

Here we report an ambient electrochemical reduction of N2 toNH3 on carbon black-supported Pd nanoparticles (Pd/C), which

can form Pd hydrides under certain potentials and promotesurface hydrogenation reactions. Operating in a N2-saturatedphosphate buffer solution (PBS) electrolyte, the Pd/C catalystenables NH3 production with a yield rate of around4.5 μg mg−1

Pd h−1 and a high Faradaic efficiency of 8.2% at 0.1 Vvs. the reversible hydrogen electrode (RHE), which correspondsto a low overpotential of 56 mV. This catalytic performance isenabled by an effective suppression of the HER activity in theneutral PBS electrolyte and the Grotthuss-like hydride transfermechanism on α-PdH for N2 hydrogenation. All potentialsreported in this study are with respect to the RHE scale.

ResultsSynthesis and characterization of Pd/C catalyst. The Pd/Ccatalyst was prepared using polyol reduction method (seeMethods section for experimental details). Figure 1a shows arepresentative transmission electron microscopy (TEM) image ofthe obtained Pd/C catalyst, which suggests that the Pd nano-particles are homogeneously dispersed on the carbon black. Thenanoparticle sizes have a narrow distribution between 4 and 9nm, with an average size of around 6 nm (see the inset of Fig. 1a).A high-resolution TEM image in Fig. 1b shows the atomic latticefringes of the particles with lattice plane spacings determined tobe 0.225 nm, corresponding to the (111) lattice spacing of Pd. AnX-ray diffraction (XRD) pattern of the Pd/C catalyst is shown inFig. 1c, in which the peaks with 2θ values of 40.1o, 46.6o, 68.1o,82.1o, and 86.6o can be indexed to the diffraction from the (111),(200), (220), (311), and (222) lattice planes of Pd, respectively(PDF#65-2867). X-ray photoelectron spectroscopy (XPS) wasused to examine the elemental composition of the Pd/C catalyst.As shown in Fig. 1d, only Pd, C, and O were observed in thesurvey spectrum, where the binding energies of 335.4 and 340.9eV correspond to the 3d5/2 and 3d3/2 levels of metallic Pd0. Ofnote, no N species were observed within the detection limit of theXPS (~0.1 atomic percent), as shown in Supplementary Fig. 1.

Electroreduction of N2 to NH3 on the Pd/C catalyst. Theelectrochemical measurements were performed using a gas-tighttwo-compartment electrochemical cell separated by a piece ofNafion 115 membrane (Supplementary Fig. 2). A piece of Ptgauze and Ag/AgCl electrode (filled with saturated KCl solution)were used as counter electrode and reference electrode, respec-tively. The working electrode was prepared by dispersing the Pd/C catalyst on a carbon paper or a glassy carbon substrate, asspecified below. N2 gas was delivered into the cathodic com-partment by N2 gas bubbling. The N2RR activities of the elec-trodes were evaluated using controlled potential electrolysis withN2-saturated electrolyte for 3 h. All potentials were iR-compensated and converted to the RHE scale via calibration(Supplementary Fig. 3). The gas-phase product (H2) was quan-tified by periodic gas chromatography of the headspace. Theproduced NH3 in the solution phase was quantified at the end ofeach electrolysis using the calibration curves established by theindophenol blue method38 (Supplementary Fig. 4). Anotherpossible solution-phase product, N2H4, was also determinedusing a spectrophotometric method developed by Watt andChrisp39 (Supplementary Fig. 5), whereas no N2H4 was detectedin our studies within the detection limit of the method.

To boost the selectivity for N2RR, we need to find an electrolytethat can effectively suppress the competing HER. We comparedthe HER activities of the Pd/C catalyst in three Ar-saturatedelectrolytes: 0.05 M H2SO4 (pH = 1.2), 0.1 M PBS (pH = 7.2), and0.1 M NaOH (pH = 12.9). Linear sweep voltammograms (LSV) ofthe Pd/C catalyst in the three electrolytes show that the currentdensities measured in H2SO4 and NaOH are both several times

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higher than those in PBS in a wide potential range (see Fig. 2a andSupplementary Fig. 6a), indicating an effective suppression of theHER activity in the neutral PBS electrolyte. The less favorablekinetics of HER in PBS is because of its higher barrier for mass-and charge-transfer40, as evidenced by the electrochemicalimpedance spectra in Supplementary Fig. 6b. Similarly, thecontrolled potential electrolysis in N2-saturated PBS at −0.05 Vvs. RHE produces a current density of about 0.3 mA cm−2

(Fig. 2b), which is much lower than that in H2SO4 (~8 mA cm−2) and NaOH (~3.5 mA cm−2). However, the NH3 yield rate inPBS reaches 4.9 μg mg−1

Pd h−1 (Fig. 2c), which is around twotimes of that in H2SO4 (2.5 μg mg−1

Pd h−1) and that in NaOH(2.1 μg mg−1

Pd h−1). More strikingly, a Faradaic efficiency of2.4% is achieved in PBS, whereas both H2SO4 and NaOHelectrolytes give rise to a Faradaic efficiency lower than 0.1%.These results clearly indicate that PBS is a promising electrolytefor electrochemical N2RR due to its effective suppression of theHER activity. Therefore, all the following N2RR experiments wereperformed with 0.1 M PBS electrolyte.

Subsequently, the activities of the Pd/C catalyst for N2RR weresystematically investigated in N2-saturated PBS at variouspotentials with separately prepared electrodes. As shown inFig. 2d, the total current density increases from ~0.05 to morethan 1.2 mA cm−2, as the potential shifts from 0.1 to −0.2 V.Interestingly, the NH3 yield rate remains similar within thispotential range, fluctuating around 4.5 μg mg−1

Pd h−1 (Fig. 2e).Strikingly, the Faradaic efficiency for NH3 production reaches amaximum value of 8.2% at 0.1 V, which corresponds to a lowoverpotential of 56 mV, given the equilibrium potential of 0.156V for N2 reduction to NH3 under our experimental conditions(see Methods section for the calculations). The Faradaic efficiency

decreases gradually at more negative potentials, which is mainlycaused by the rapid rising of the HER activity (SupplementaryFig. 7). To the best of our knowledge, the Pd/C catalyst achievesan NH3 yield rate and Faradaic efficiency that are comparable tothe recently reported catalysts for N2RR under ambient condi-tions (Supplementary Table 1), but uses an overpotential lower byat least 300 mV, making it one of the most active and selectiveelectrocatalysts for ambient NH3 synthesis.

In addition, we have carefully examined the N source of theproduced NH3. First, control experiments with Ar-saturatedelectrolyte or without Pd catalyst were performed. As shown inFig. 2f, no apparent NH3 was detected using the indophenol bluemethod when the bubbled N2 gas was replaced by Ar or when acarbon paper electrode without Pd was used, indicating that theNH3 was produced by N2 reduction in the presence of Pd catalyst.Furthermore, 15N isotopic labeling experiment was performed asan alternative method to verify the N source of the produced NH3

in 0.1 M PBS electrolyte. A triplet coupling for 14NH4+ and a

doublet coupling for 15NH4+ in the 1H nuclear magnetic

resonance (1H NMR) spectra are used to distinguish them25.As shown in Supplementary Fig. 8, only 15NH4

+ was observed inthe electrolyte when 15N2 was supplied as the feeding gas, and noNH4

+ was detected when Ar was supplied, which are consistentwith the control experiments and confirm that the NH3 wasproduced by Pd-catalyzed electroreduction of N2.

The stability of the Pd/C catalyst for electrochemical N2RR wasassessed by consecutive recycling electrolysis at −0.05 V vs. RHE.After five consecutive cycles, only a slight decline in the totalcurrent density was observed, as shown in Supplementary Fig. 9.However, the NH3 yield rate and Faradaic efficiency decreased to2.4 μg mg−1

Pd h−1 and 1.2% after five cycles, indicating a loss of

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Fig. 1 Structural and compositional characterizations of the Pd/C catalyst. a TEM and b high-resolution TEM images of the Pd/C catalyst. Inset a: particlesize distribution. Scale bars, a 50 nm; b 2 nm. c XRD pattern and d XPS survey spectrum of the Pd/C catalyst. Inset d: high-resolution XPS spectrum of Pd3d peaks

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the N2RR activity by 50% after the 15 h operation. The decreasein the N2RR activity is due to the loss of active Pd surface areacaused by the aggregation of Pd nanoparticles on the carbonsupport, as evidenced by the TEM images of the Pd/C catalystafter the recycling test (Supplementary Fig. 9). Further improve-ment in the dispersion of Pd nanoparticles on the support andinteraction between them will be beneficial for the long-termdurability of the catalyst.

DiscussionThe Pd/C catalyst exhibits high activity and selectivity for theN2RR at a low overpotential of 56 mV. To explore the underlyingmechanism and to see whether it is exclusive to Pd, we preparedAu/C and Pt/C catalysts with identical metal loading using thesame method, and compared their N2RR catalytic performancewith that of the Pd/C catalyst (Fig. 3). The structural and com-positional characterizations of the Au/C and Pt/C catalysts,including TEM, XRD, and XPS (Supplementary Fig. 10), confirmthe successful synthesis of the metal nanoparticles with similarsizes as that of the Pd/C catalyst. At −0.05 V vs. RHE, the Au/Ccatalyst exhibits a current density (<0.04 mA cm−2) much lowerthan that of the Pd/C catalyst, whereas the Pt/C catalyst shows aslightly higher current density (see inset of Fig. 3). Both the Au/Cand Pt/C catalysts produce NH3 at a rate of about 0.3 μg mg−1

metal h−1, which is lower than that of the Pd/C catalyst by morethan one order of magnitude. The Au/C catalyst achieves a Far-adaic efficiency of 1.2% for N2RR, due to its low activity for theHER. In contrast, the Faradaic efficiency for N2RR on the Pt/Ccatalyst is only 0.2%, much lower than that of the Au/C catalyst,because Pt is intrinsically the most active catalyst for HER

(Supplementary Fig. 11). In comparison, such a significant dif-ference in both N2RR activity and selectivity clearly indicates thatPd is a unique catalyst for N2RR. Actually, Pd can readily absorbH atoms in its lattice, forming Pd hydrides under operatingconditions41. The cathodic current observed at 0.10 and 0.05 V issimilar to the data in a previous study42, and the unaccountedcurrent at the two potentials (see Supplementary Fig. 7b) may besimilarly attributed to the dynamic hydrogen adsorption andabsorption on Pd42, in addition to the capacitance of the carbon

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Fig. 3 Comparison of the Pd/C catalyst with Au/C and Pt/C catalysts forN2RR. NH3 yield rates and Faradaic efficiencies of the Au/C, Pd/C, and Pt/C catalysts on the carbon-paper support measured in N2-saturated 0.1 MPBS electrolyte at −0.05 V vs. RHE. The error bars correspond to thestandard deviations of measurements over three separately preparedsamples under the same conditions. Inset: chronoamperometric curvesobtained for each catalyst

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Fig. 2 Electroreduction of N2 to NH3 on the Pd/C catalyst. a Linear sweep voltammetric curves of the Pd/C catalyst on the glassy-carbon support measuredin the three Ar-saturated electrolytes. b Chronoamperometric curves of the Pd/C catalyst on the carbon-paper support measured at −0.05 V vs. RHE inthe three N2-saturated electrolytes, and c corresponding NH3 yield rates and Faradaic efficiencies in the three electrolytes. d Chronoamperometric curvesof the Pd/C catalyst on the carbon-paper support in N2-saturated 0.1 M PBS at various potentials, and e corresponding NH3 yield rates and Faradaicefficiencies at the potentials. The error bars correspond to the standard deviations of measurements over three separately prepared samples under thesame conditions. f UV-vis absorption spectra of the electrolytes after electrolysis at −0.05 V vs. RHE for 3 h under different conditions. No apparent NH3

was detected for the control experiments with Ar-saturated electrolyte (CP-Pd/C-Ar) or without Pd catalyst (CP-N2), indicating that NH3 was produced byPd-catalyzed electroreduction of N2

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support. It has been reported that Pd catalyzes the electro-reduction of CO2 to formate with high activity and selectivity atlow overpotentials43, 44, and both experimental and computa-tional studies have confirmed that it is attributed to a hydro-genation mechanism through in situ formed PdHx phase45.Interestingly, a recent study of N2RR on commercial Pd/C cata-lysts in acidic and alkaline electrolytes showed Faradaic effi-ciencies lower than 0.1% at −0.2 V vs. RHE31, which areconsistent with our results under similar conditions (Fig. 2c), andhighlight the critical roles of the HER suppression in PBS and thehydrogenation via hydride transfer pathway at low overpotentials(vide infra).

To understand the unique activity of N2RR on Pd/C, we per-formed density functional theory (DFT) calculations for theenergetics of HER and N2RR steps on the (211) surfaces of Au, Pt,and Pd hydride (see Supplementary Note 1 for the computationaldetails). As employed previously, the undercoordinated stepatoms are assumed to be the catalytic site for activating the N≡Nbond46. For Pd, we have two subsurface *H (*Hsub) at the octa-hedral sites underneath the Pd edge atoms to simulate the α-phase Pd hydride (α-PdH), which is the stable phase underoperating potentials45. According to the differential adsorptionfree energies of *H on the (211) surfaces (Supplementary Fig. 12),we adopted models with 2/3 monolayer of *H for α-PdH and Pt,on which the step/terrace sites are occupied by *H and thebottom-of-the-edge sites are free, while a clean surface for Au wasused (geometric structures shown in Fig. 4). In the inset of Fig. 4,we can see that the HER on Pt is facile with the free formationenergy of *H close to zero47. In contrast, the process is limited by*H desorption on α-PdH and by *H adsorption on Au. Inter-estingly, the free energy cost in creating a *H vacancy (*H-v)necessary for N2 collision at step sites is much lower onα-PdH (0.18 eV) than that on Pt (0.41 eV). Furthermore, the

hydrogenation of N2 by a surface *H to form *N2H on α-PdH(1.18 eV) is thermodynamically more favorable than that on Pt(1.37 eV) and Au (2.21 eV). On α-PdH, the surface hydrogenationis accompanied by the transfer of *Hsub to the surface site, ana-logous to the Grotthuss-like proton-hopping mechanism in awater network48. The reaction-free energy of N2 hydrogenationon Pd without the hydride transfer is less favorable by 0.3 eV (seeSupplementary Fig. 13 for a direct comparison). The nature ofthis rate-determining step, i.e., surface chemical hydrogenationinstead of the proton-coupled electron transfer, supports theobserved weak potential dependence of the NH3 yield rate at lowoverpotentials. DFT-calculated free energies of hydrogenation ofN2 vs. hydrogen evolution across metal surfaces (Au, Pt, andα-PdH) rationalize the activity and selectivity trends in Fig. 3.

In summary, we have discovered an efficient electro-hydrogenation of N2 to NH3 on Pd/C catalyst at an overpotentialas low as 56 mV for the electrochemical NH3 synthesis underambient conditions. The Pd/C catalyst exhibits high activity andselectivity for N2RR in a PBS electrolyte, achieving an NH3 yieldrate of about 4.5 μg mg−1

Pd h−1 and a Faradaic efficiency of 8.2%at 0.1 V vs. RHE. Comparative experiments indicate an effectivesuppression of the HER in the neutral PBS electrolyte, and asignificantly higher N2RR activity of Pd than other catalystsincluding Au and Pt. The DFT calculations suggest that the in situformed α-PdH allows the activation of N2 via a Grotthuss-likehydride transfer pathway that is thermodynamically morefavorable than direct surface hydrogenation or proton-coupledelectron transfer steps. Our findings open up an avenue todevelop efficient electrocatalysts for not only the electroreductionof N2 to NH3, but also other challenging electrocatalytic reactionsfor renewable energy conversions.

MethodsSynthesis of carbon black-supported metal catalysts. The Pd/C catalyst wassynthesized using polyol reduction method. First, the Pd precursor (K2PdCl4)solution was prepared by dissolving PdCl2 in water in the presence of KCl.Typically, 70 mg of carbon black was dispersed in 100 mL of ethylene glycol, fol-lowed by sonication for 30 min. Then, 5 mL of K2PdCl4 solution (Pd 6 mgmL−1)was added into the mixture. After stirring for another 30 min, the mixture washeated to 130 °C and kept at this temperature for 2 h. The catalyst slurry wasfiltered and washed with water. The resulting Pd/C catalyst was dried at 60 °Covernight, with a Pd loading of 30 wt%. Comparative samples of Au/C and Pt/Ccatalysts with a metal loading of 30 wt% were prepared using the same procedure,except with different metal precursors.

Physical characterizations. TEM images were acquired using a FEI Tecnai F30Transmission Electron Microscope, with a field emission gun operated at 200 kV.The XRD pattern was collected using a PANalytical Empyrean diffractometer, witha 1.8 KW copper X-ray tube. XPS was performed using a Physical Electronics 5400ESCA photoelectron spectrometer. Gas products were analyzed by a gas chroma-tograph (SRI Multiple Gas Analyzer #5) equipped with molecular sieve 5 A andHayeSep D columns, with Ar as the carrier gas.

Preparations of the working electrodes. First, 8 mg of carbon black-supportedmetal catalyst was dispersed in diluted Nafion alcohol solution containing 1.5 mLethanol and 60 μL Nafion, which formed a homogeneous suspension after soni-cation for 1 h. Two types of substrates were used to prepare electrodes in this study:one was carbon paper, and the other was glassy carbon. The carbon-paper elec-trodes were prepared by drop-casting the suspension on a piece of carbon paper(1 × 1 cm2), with a total mass loading of 1 mg (of which 30 wt% is Pd), which wereused for all controlled potential electrolyses. The glassy-carbon electrodes wereprepared by drop-casting the suspension on a round glassy carbon (diameter = 3mm), with a total mass loading of 0.435 mg cm−2 (of which 30 wt% is Pd), whichwere used for all linear sweep voltammetry measurements, except SupplementaryFig. 6a.

Electrochemical measurements. Prior to N2RR tests, Nafion 115 membraneswere heat-treated in 5% H2O2, 0.5 M H2SO4, and water for 1 h, respectively. Afterbeing rinsed in water thoroughly, the membranes were immersed in deionizedwater for future use. Electrochemical measurements were performed using a CHInstruments 760E Potentiostat, with a gas-tight two-compartment electrochemicalcell separated by a piece of Nafion 115 membrane at room temperature

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Fig. 4 Computational studies. DFT-calculated free energy pathways of HER(inset) and the relevant steps of N2RR on the (211) surfaces of Au, Pt, andα-PdH at surface potential of 0 V vs. RHE under 298.15 K (atomicstructures shown in the insets). The first step denotes the formation of *Hon the Au surface and *H vacancy (*H-v) on Pt and α-PdH step sites. Thesecond step is the direct surface hydrogenation of N2(g) forming *N2H onAu and Pt, and the Grotthuss-like hydride transfer pathway on α-PdH

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(Supplementary Fig. 2). A piece of Pt gauze and Ag/AgCl/sat. KCl were used ascounter electrode and reference electrode, respectively. The linear sweep voltam-metry was scanned at a rate of 5 mV s−1. The N2RR activity of an electrode wasevaluated using controlled potential electrolysis in an electrolyte for 3 h at roomtemperature (~293 K). Prior to each electrolysis, the electrolyte was presaturatedwith N2 by N2 gas bubbling for 30 min. During each electrolysis, the electrolyte wascontinuously bubbled with N2 at a flow rate of 10 sccm, and was agitated with astirring bar at a stirring rate of ~800 rpm. No in-line acid trap was used to captureNH3 that might escape from the electrolyte in our study, as no apparent NH3 wasdetected in the acid trap under our experimental conditions. The applied potentialswere iR-compensated, and the reported current densities were normalized togeometric surface areas.

Calibration of the reference electrodes. All potentials in this study were con-verted to the RHE scale via calibration (Supplementary Fig. 3). The calibration wasperformed using Pt gauze as both working electrode and counter electrode in H2-saturated electrolyte. Cyclic voltammograms were acquired at a scan rate of 1 mV s−1. The two potentials at which the current equaled zero were averaged and used asthe thermodynamic potential for the hydrogen electrode reactions.

Ammonia quantification. The produced NH3 was quantitatively determined usingthe indophenol blue method38. Typically, 2 mL of the sample solution was firstpipetted from the post-electrolysis electrolyte. Afterwards, 2 mL of a 1M NaOHsolution containing salicylic acid (5 wt%) and sodium citrate (5 wt%) was added,and 1 mL of NaClO solution (0.05 M) and 0.2 mL of sodium nitroferricyanidesolution (1 wt%) were added subsequently. After 2 h, the absorption spectra of theresulting solution were acquired with an ultraviolet-visible (UV-vis) spectro-photometer (BioTek Synergy H1 Hybrid Multi-Mode Reader). The formed indo-phenol blue was measured by absorbance at λ = 653 nm. In order to quantify theproduced NH3, the calibration curves were built using standard NH4Cl solutions inthe presence of 0.05 M H2SO4, 0.1 M PBS, and 0.1 M NaOH, respectively (Sup-plementary Fig. 4), to take into account the possible influence of different pHvalues38. The measurements with the background solutions (no NH3) were per-formed for all experiments, and the background peak was subtracted from themeasured peaks of N2RR experiments to calculate the NH3 concentrations and theFaradaic efficiencies.

Hydrazine quantification. The yellow color developed upon the addition of p-dimethylaminobenzaldehyde (PDAB) to solutions of N2H4 in dilute hydrochloricacid solution was used as the basis for the spectrophotometric method to quantifythe N2H4 concentration39. Typically, 5 mL of the electrolyte solution was taken outand then mixed with 5 mL of the coloring solution (4 g of PDAB dissolved in 20mL of concentrated hydrochloric acid and 200 mL of ethanol). After 15 min, theabsorption spectra of the resulting solution were acquired using a UV-vis spec-trophotometer (BioTek Synergy H1 Hybrid Multi-Mode Reader). The solutions ofN2H4 with known concentrations in 0.1 M PBS were used as calibration standards,and the absorbance at λ = 458 nm was used to plot the calibration curves (Sup-plementary Fig. 5).

Calculation of the equilibrium potential. The standard potential for the halfreaction of N2 reduction to NH4OH was calculated according to the standardmolar Gibbs energy of formation at 298.15 K49. The equilibrium potential underour experimental conditions is calculated using the Nernst equation, assuming 1atm of N2 and a NH4OH concentration of 0.01 mM in the solution.

N2 gð Þ þ 2H2Oþ 6Hþ þ 6 e� ! 2NH4OH aqð Þ ΔG� ¼ �33:8 kJmol�1 ð1Þ

E° =−ΔG°/nF = 0.058 V, where n = 6 is the number of electrons transferred in thereaction and F is the Faraday constant.

The equilibrium potential under our experimental conditions is calculated usingthe Nernst equation, assuming 1 atm of N2 and a 0.01 mM concentration ofNH4OH in the solution.

E ¼ E� � RT6F

lnNH4OH½ �2

Hþ½ �6 !

þ 0:059V ´ pH ¼ 0:156V vs:RHE ð2Þ

Calculation of the Faradaic efficiency and the yield rate. The Faradaic efficiencywas estimated from the charge consumed for NH3 production and the total chargepassed through the electrode:

Faradaic efficiency ¼ ð3F ´ cNH3´VÞ=Q ð3Þ

The yield rate of NH3 can be calculated as follows:

Yield rate ¼ð17cNH3´VÞ=ðt ´mÞ ð4Þ

where F is the Faraday constant (96,485 Cmol−1), cNH3is the measured NH3

concentration, V is the volume of the electrolyte, Q is the total charge passed

through the electrode, t is the electrolysis time (3 h), and m is the metal mass of thecatalyst (typically 0.3 mg). The reported NH3 yield rate, Faradaic efficiency, anderror bars were determined based on the measurements of three separately pre-pared samples under the same conditions.

15N isotope labeling experiment. The isotopic labeling experiment was carriedout using 15N2 as the feeding gas (Sigma-Aldrich, 98 atom % 15N) with 0.1 M PBSelectrolyte. After electrolysis at −0.05 V vs. RHE for 10 h, 10 mL of the electrolytewas taken out and acidized to pH ~3 by adding 0.5 M H2SO4, and then con-centrated to 2 mL by heating at 70 °C. Afterwards, 0.9 mL of the resulting solutionwas taken out and mixed with 0.1 mL D2O containing 100 ppm dimethyl sulph-oxide (Sigma-Aldrich, 99.99%) as an internal standard for 1H nuclear magneticresonance measurement (1H NMR, Bruker Avance III 400MHz).

Computational studies. DFT calculations were performed using the plane-wave-based PWSCF (Quantum-ESPRESSO) program and the Atomic SimulationEnvironment (ASE). The ultrasoft Vanderbilt pseudopotential method withPerdew–Burke–Ernzerhof (PBE) exchange-correlation functional was adopted.More calculation details and relevant references are provided in the SupplementaryNote 1, Supplementary Table 2, and Supplementary References.

Data availability. The data that support the findings of this study are availablewithin the paper and its Supplementary Information file or are available from thecorresponding authors upon reasonable request.

Received: 20 December 2017 Accepted: 11 April 2018

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AcknowledgementsThis work is supported by the Startup Fund from the University of Central Florida(UCF). X.F. is a member of the Energy Conversion and Propulsion Cluster at UCF. J.W.gratefully acknowledges the Preeminent Postdoctoral Program (P3) award from UCF. L.Y. and H.X. acknowledge the financial support from the American Chemical SocietyPetroleum Research Fund (ACS PRF 55581-DNI5) and the NSF CBET Catalysis Pro-gram (CBET-1604984).

Author contributionsJ.W. and X.F. conceived and designed the experiments. J.W. synthesized the materialsand carried out the experimental work. L.Y. and H.X. performed the computationalwork. L.H. assisted in the experimental work. G.C. contributed to NH3 quantification. X.F. and H.X. co-wrote the manuscript. All authors discussed the results and commentedon the manuscript.

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