C Hybrid for Enhanced Nitrogen Reduction

Subscriber access provided by University of Winnipeg Library

Article

Rational Design of Fe-N/C Hybrid for Enhanced Nitrogen Reduction Electrocatalysis under Ambient Conditions in Aqueous Solution Ying Wang, Xiaoqiang Cui, Jingxiang Zhao, Guangri Jia, Lin Gu, Qinghua Zhang, Lingkun Meng, Zhan Shi, Lirong Zheng, Chunyu Wang, Ziwei Zhang, and Weitao Zheng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03802 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Rational Design of Fe-N/C Hybrid for Enhanced Nitrogen Reduction Electrocatalysis under Ambient Conditions in Aqueous Solution Ying Wang,1 Xiaoqiang Cui,1,* Jingxiang Zhao,2 Guangri Jia,1 Lin Gu,3 Qinghua Zhang,3 Lingkun Meng,4 Zhan Shi,4 Lirong Zheng,5 Chunyu Wang,6 Ziwei Zhang,1 and Weitao Zheng 1,* 1.

State Key Laboratory of Automotive Simulation and Control, School of Materials Science and

Engineering, and Key Laboratory of Automobile Materials of MOE, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China 2.

Key Laboratory of Photonic and Electronic Bandgap Materials, Ministry of Education, and

College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin, 150025, China 3.

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, P. R. China 4.

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P. R. China 5.

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100190, P. R. China 6.

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, 130012, P. R. China

1 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT: Developing efficient noble-metal-free catalysts for the electrochemical N2 reduction reaction (NRR) under ambient conditions shows promise in fertilizer production and hydrogen storage. Here, as a proof-of-concept prototype, we design and implement an Fe-N/Ccarbon nanotubes (CNTs) catalyst derived from a metal-organic framework and carbon nanotubes-based composite with built-in Fe-N3 active sites. This catalyst exhibits enhanced NRR activity with NH3 production (34.83 μg·h−1·mg−1cat.), faradaic efficiency (9.28% at −0.2 V vs. RHE), selectivity, and stability in 0.1 M KOH aqueous media under mild conditions. Experimental and theoretical results both reveal that Fe-N3 species are the primary catalytically active centers for the NRR. This work provides insight into precise construction of more efficient and stable NRR electrocatalysts and further expands the possibilities of transition metalnitrogen-carbon (M-N-C)-based nanomaterials in NRR fields.

KEYWORDS: Electrocatalysis; Nitrogen fixation; Ammonia synthesis; Noble-metal-free; FeN/C catalysts; Fe-N3 species


Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1. INTRODUCTION With a growing world population and increased energy demands, ammonia (NH3) is not only widely used in fertilizer production but also regarded as a promising transport carrier and ideal chemical storage material for hydrogen energy.1 However, due to the chemical inertness of N2 and high energy barrier for N≡N bond cleavage, synthesis of NH3 mainly relies on the traditional Haber-Bosch process at high temperature and pressure from atmospheric nitrogen and fossil fuels, which leads to intensive energy consumption and greenhouse gas emissions.2-3 Hence, the quest for sustainable and effective approaches to producing ammonia under a mild condition continues to be a great impetus.4-6 Considerable research efforts have devoted to N2 fixation, such as biological nitrogenase,

7-8

homogeneous9-11 or heterogeneous6,

12-14

electrocatalytic and photocatalytic approaches.15-16 Among these processes, electrocatalytic ammonia production is regarded as an environmentally benign solution since it can be powered by renewable electrical energy without massive fossil fuel consumption and carbon dioxide emissions.17-18 This strategy directly transforms N2 into NH3 under moderate conditions utilizing water as alternative hydrogen (H)-atom source instead of high-purity hydrogen (H2).19-20 To date, the theoretical and experimental investigations on the electrochemical N2 reduction reaction (NRR) by heterogeneous catalysts were mainly focused on noble metals (Au,21-23 Ru,2425

Rh,26 and Pd,27-28), transition metal oxide/nitride/carbide/chalcogenide,29-32 and metal-free

catalysts.33-35 In particular, Fe is of high crustal abundance and serves as a key element for the active sites in nitrogenase for the biological N2 fixation process.7 Several works so far have been reported on homogeneous Fe-dinitrogen complexes11-12 and heterogeneous iron-based catalysts for the NRR (Table S1).36-38 Nevertheless, there is still a grand challenge but blossoming interest to achieve the high-efficiency electrochemical NRR in aqueous media.39 In addition, the close


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

potential region between the NRR and the reduction of water to hydrogen causes the hydrogen evolution reaction (HER) to be the major competing reaction and results in relatively low faradaic efficiency (FE) towards the NRR.3, 20, 40 Therefore, it was an urgent need to develop state-of-the-art heterogeneous catalysts with high activity and selectivity for the NRR. The theoretical prediction and recent reports showed that the metal atom-nitrogen/carbon (M-N-C) are promising materials for the NRR under ambient conditions.24, 41-42 More significantly, Li et al. theoretically proposed that FeN3-embedded graphene was a potential catalyst for the NRR at ambient temperatures, in which the FeN3 centers with high-spin polarization were recognized as effective active sites to adsorb and activate N2.43 Huang et al. and Choi et al. also reported the similar computational study of Fe atom supported on the defective graphene for NRR.44 However, this promising paradigm for effective artificial N2 fixation has not been implemented and verified by experiments thus far. Consequently, considering the abovementioned characteristics, we conclude that iron-nitrogen/carbon (Fe-N/C)-based nanomaterials, which are generally considered the most promising non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR)45-46 and CO2 reduction reaction (CO2RR),47-48 are potential candidates to access new possibilities in the NRR. On this basis, herein, as a proof-concept experiment, we designed and synthesized the FeN/C electrocatalysts with built-in Fe-N3 sites from the carbonizing Fe-doped zeolitic-imidazolate framework (ZIF)-carbon nanotubes (CNTs) templates. The hierarchical porous architecture, large electrochemically active surface area, positively charged surface, weak ferromagnetism and strong nitrogen chemisorption all facilitated enormous potential of Fe-N/C-CNTs for the electrochemical NRR. Compared with CNTs or N/C-CNTs alone, the introduction of Fe species into CNTs or N/C-CNTs enhanced the NRR performance under an ambient atmosphere.


Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Experimental results and theoretical calculations further revealed that Fe-N3 species played a primary role in effective electrochemical synthesis of ammonia and proceeded preferentially via the distal pathway. 2. RESULTS AND DISCUSSION 2.1 Morphology and Structure Characterization of Fe-N/C-CNTs An illustration of the synthetic procedures of Fe-N/C-CNTs is shown in Figure 1a. Detailed information is added in the Experimental Section of the Supporting Information. As shown in Figure 1b, the external surface of CNTs was fully and uniformly covered by Fe-ZIF-8 crystals. After pyrolysis at 1000 °C under N2 atmosphere, Fe-ZIF-8-CNTs were facilely transformed into the Fe-N/C-CNTs 3D network hybrid with porous carbon interlinked by CNTs (Figure 1c). The introduction of CNTs played a role in avoiding agglomeration of ZIF-8 and increasing the active surface areas as confirmed by optimization (Figure S2-S4). The X-ray diffraction (XRD) patterns of the Fe-ZIF-8-CNTs precursors displayed in Figure 1d agreed well with that of simulated ZIF8, indicating that the introduction of Fe2+ and CNTs did not alter the ZIF-8 framework units. Transmission electron microscopy (TEM) images show that nanoporous Fe-N/C layers derived from wrapped Fe-ZIF-8 were uniformly attached on the CNT surface and stacked by oriented multilayer graphene layers (Figure 1e and Figure S5). The XRD of Fe-N/C-CNTs exhibited four peaks at 26.2°, 42.9°, 54.3°, and 77.7°, corresponding to the (002), (100), (004), and (110) plane of the graphitic carbon, respectively. There were no obvious peaks related to the crystalline iron phase. These results were consistent with the selected-area electron diffraction (SAED) patterns (the inset of Figure 1e). High-angle annular dark-field (HAADF) imaging and the corresponding scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS)


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

elemental mapping demonstrated that the distributions of C, N, and Fe species were highly uniform (Figure 1f). The Fe contents in the Fe-N/C-CNTs were as high as 0.50 wt% based on inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis.

Figure 1. (a) Scheme of the synthesis of Fe-N/C-CNTs. Characterization of the morphology and structure of Fe-N/C-CNTs catalysts. Scanning electron microscopy (SEM) images of Fe-N/CCNTs before (b) and after (c) thermal conversion. (d) XRD pattern. (e) Magnified TEM image of the interface between amorphous porous carbon (green) and the CNTs surface (yellow). Inset: Corresponding SAED pattern. (f) HAADF imaging and corresponding STEM-EDS elemental mapping of Fe-N/C-CNTs, C (red), N (green), and Fe (blue). The detailed elemental composition and states of the Fe-N/C-CNTs were explored by X-ray photoelectron spectroscopy (XPS), as shown in Figure 2a-2b and Figure S6. Compared with its


Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

precursor (Figure S7), the absence of Zn 2p peaks in the XPS spectra verified that the Zn species had been completely removed under high temperature and acid washing conditions. The highresolution N 1s spectrum was deconvoluted into pyridinic (398.4 eV), pyrrolic (399.7 eV), graphitic (400.9 eV), and pyridine-oxide (402.1 eV) nitrogen species. The pyridinic nitrogen species may also be ascribed to anchor points for iron atoms because of the small difference between the binding energies of them and N-Fe.45, 49 XPS Fe 2p results showed two major peaks at binding energies (BEs) of 711.4 eV and 724.5 eV accompanied by their satellite peaks (715.8 and 729.6 eV) in accord with the characteristic peaks of Fe 2p3/2 and Fe 2p1/2 orbit levels, respectively, indicating that iron was in an oxidized state.46, 48 More structural insight and the coordination environment of iron in the Fe-N/C-CNTs catalyst and its precursor of Fe-ZIF-8-CNTs were further identified by X-ray absorption fine structure (XAFS) analysis. As shown in the Fe K-edge X-ray absorption near-edge structure (XANES) (Figure 2c), the absorption edge of the precursor suggested that the Fe species in FeZIF-8-CNTs was in a high oxidation state similar to that of Fe2O3 reference. Fe2+ was oxidized into Fe3+ during the preparation process of Fe-ZIF-8-CNTs under ambient condition.50-51 In contrast, the high-temperature procedure could cause the reduction of the oxidation state of iron in Fe-N/C-CNTs. It was further confirmed that the near-edge adsorption energy of the Fe-N/CCNTs was situated between a standard iron foil and Fe2O3, indicating that the bulk average valence states of the Fe species in Fe-N/C-CNTs were between Fe0 and Fe3+.45, 52 Moreover, the intensity of shoulder peak for the Fe-N/C-CNTs was relatively reduced compared to that of iron phthalocyanine (FePc) featured at 7115 eV, which is the typical characteristic of the squareplanar D4 h local symmetry of Fe-N4 moieties, indicating a weaker symmetry and different coordination environment between Fe and N in the as-prepared samples than that in FePc.53-55


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

The Fourier transforms of extended X-ray absorption fine structure (FT-EXAFS) spectra of the Fe-ZIF-8-CNTs exhibited a main peak at about 1.50 Å ascribed to the Fe-N(O) scattering path in reference to that of FePc (Figure 2d). Notably, this FT amplitude at about 1.50 Å was also found in Fe-N/C-CNTs and another peak at approximately 2.40 Å assigned to the Fe-C distance.56 Also, the intensity of the Fe-N(O) peak of the sample was lower than that of FePc and Fe-ZIF-8-CNTs, reflecting a decrease in the surrounding coordination number of the Fe center.50 Taking into consideration of the fact that Fe-N bond was longer than Fe-O bond and the Fe-O coordination introduced by the ex-situ XAS would be smalled than 1,57 the least-squares EXAFS fitting results attributed the peak at 1.50 Å to Fe-Nx configurations and the corresponding Fe-N coordination numbers within the Fe-ZIF-8-CNTs, FePc, and Fe-N/C-CNTs were of 4.4, 4.0, and 3.0, respectively (Figure S8-S9 and Table S2). The corresponding bonding lengths of Fe-N were also successively shortened. In addition, a weak Fe-Fe peak at approximately 2.08 Å could be observed, implying that trace metallic iron species may exist in the Fe-N/C-CNTs, which was verified by TEM of the Fe3C or Fe species (Figure S10),56 and their influence on the NRR will be explored next in detail. Raman spectra (Figure S11a) showed only two dominant band characteristics of D-band (1357 cm-1) and G-band (1580 cm-1) that were corresponding to the disordered carbon and the well-ordered sp2-hybridized graphitic carbon, respectively.58 The ID/IG value is a qualitative standard for evaluating the carbon disorder degree of carbon material.59 The ID/IG of 0.846 for Fe-N/C-CNTs indicated the relatively high degree of graphitization, which was beneficial to enhance the electrical conductivity in the NRR. The N2 adsorption/desorption isotherms (Figure S11b) with a type IV isotherm, the corresponding pore size distribution (Figure S11c) and the TEM images (Figure S11d) suggested that the existence of a hierarchical porous architecture


Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

provided a more effective surface area and channels for electrolyte and nitrogen accessibility. The magnetic hysteresis (MH) curves (Figure 2e) displayed weak ferromagnetism for Fe-N/CCNTs, which played an important role for the catalysts on activating N2, as proved by the previous report and subsequently theoretical calculation.43-44 In addition, the positive charge of the Fe-N/C-CNTs would electrostatically repel positively charged H+ cations and contributed to the adsorption of electroneutral nitrogen in electrochemical NRR competing with H2 evolution on the electrode surface (Figure S12a).22 As shown in the temperature-programmed desorption (TPD) results of Figure 2f, a peak at approximately 92 °C indicated the physical adsorption of nitrogen. Afterwards, the main desorption peak centered at 280 °C was nitrogen chemisorption on the carbon surface with built-in FeN3 sites of the Fe-N/C-CNTs.34, 60-61 The broad area of this peak suggested a large adsorption capacity with multiple adsorption forms of the gas on Fe-N/CCNTs, including above-mentioned characteristic of ferromagnetism, electrostatic attraction interactions between nitrogen and catalysts, and the chemisorption of nitrogen on the bonding sites of catalysts. A much weaker physical adsorption intensity and negligible chemisorption signals was obtained in the TPD of He inert gas (Figure S12b), confirming that the signal in N2TPD was not from Fe-N/C-CNTs itself. Along with these unique characteristics, we expected that Fe-N/C-CNTs provide potential applications in the electrochemical reduction of N2.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

Figure 2. High-resolution XPS spectra of (a) N 1s, and (b) Fe 2p for the Fe-N/C-CNTs catalysts. (c) The normalized XANES spectra of different samples at the Fe K-edge; the red area highlights the pre-edge XANES spectra. (d) The k3-weighted Fourier transform (FT) spectra at the Fe Kedge of different samples. (e) Magnetization versus magnetic field curves for Fe-N/C-CNTs at 300 K after deducting the diamagnetic signals; the inset is a closer view of the hysteresis. (f) N2TPD profiles of the Fe-ZIF-8-CNTs (red) and Fe-N/C-CNTs catalysts (green). 2.2 NRR Properties of Fe-N/C-CNTs The NRR performance was first investigated by linear sweep voltammetry (LSV) measurements displayed in Figure S13a, which showed different tendencies under N2 or Ar saturation (an Ag/AgCl/saturated KCl reference electrode was calibrated to the reversible hydrogen electrode (RHE), as shown in Figure S14). Since the H2 evolution could be a major


Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

equilibrium reaction competing with the NRR, a sequence of potentials was applied to achieve the optimum condition for NH3 formation, as plotted in Figure 3a and listed in Table S3. The highest average yield of NH3 was 34.83 μg·h−1·mg−1cat., and the corresponding FE of 9.28% was obtained at −0.2 V versus RHE (Figure S15-S17). Various control and complementary experiments were carried out to evaluate the potential ammonia contamination in the ambient atmosphere and the other possible ammonia contribution from the reaction system.62, 36-37, 63 The ambient ammonia contamination was determined to be trace amount of 8.72~9.39 μg·m−3 that was much lower than the indoor air quality (IAQ) standard of 200 μg·m−3 (GB/T18883-2002, China). The day to day variability was relatively low with a relative standard deviation (RSD) of 3.83% (Table S5). The corresponding control experiments showed that there were no ammonia contribution from the N2 gas, carbon paper cathode, catalysts, and Nafion probably due to the limited detection sensitivity of the spectrophotometric and flow injection analysis (FIA) method that we used (Figure S18 and Table S4). The background was measured throughout the work and the controls were performed consistently that showed the RSD% values of < 3% (Figure S19a). A series of control experiments for each repetition, each catalyst preparation, and each electrode preparation all showed RSD% values less than 5% (Figure S19b). To exclude the intrinsic N atoms from Fe-N/C-CNTs, preliminary control experiments were performed in an Ar atmosphere. The result showed slight fluctuation compared with the blank sample (Figure S13b), which could only be detected by FIA method of 29.0 nmol ammonia compared with the blank sample. This tiny amount of ammonia was from the above-mentioned inevitable ammonia contamination, which had been evaluated in detail and was much less than the total yield of ammonia (1.964 μmol).64 We also performed the 15N isotopic measurements to verify the origination of the nitrogen in the ammonia product (Figure S20a). The 1H nuclear


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

magnetic resonance (NMR) signals (Figure S20b) showed distinguishable chemical shift of a doublet pattern with the coupling constant of 1JN-H = 72 Hz for 15NH4+ NRR samples and a triplet coupling with the 1JN-H = 52 Hz for 14NH4+ samples, which were correspond with the 15NH4+ and 14NH + 4

standard, respectively.28,

65

The

14NH + 4

samples was also analyzed quantitatively for

evaluating the consistency degree of results between the spectrophotometric and NMR spectral methods (Figure S20c-S20d).37, 66 In addition, no observed signals were obtained when Ar was employed as the feeding gas, suggesting that there was negligible ammonia contamination contribution consistent with the control experiment results. These results reconfirmed the product of ammonia did generate from the nitrogen fixation. Our material showed similar NRR properties as those of noble metal and Fe-based electrocatalysts under ambient conditions in the alkaline aqueous medium and was comparable to those of some catalysts under harsh temperatures and pressures (Table S6-S7). No by-product of hydrazine was detected, suggesting the good selectivity (versus N2H4·H2O formation) of Fe-N/CCNTs towards ammonia synthesis (Figure S21a). The corresponding chronoamperometric curves revealed good durability under long-term electrolysis (Figure 3b). The current density was maintained without an obvious decrease for 48 h at −0.2 V vs. RHE (Figure S22a). During the 5th consecutive cycle, both the ammonia production rate and faradaic efficiency exhibited no obvious decline, indicating excellent stability of the Fe-N/C-CNTs for the electrochemical NRR (Figure 3c and Figure S22b). N2 reduction efficiency increased exponentially with temperature (Figure 3d), from which the activation energy (Ea) was calculated to be of 5.56 kJ·mol−1 from the Arrhenius plot (Figure S21b). Notably, Ea in this work was much lower than the conventional industrial Haber-Bosch process (≈ 335 kJ·mol−1) and the recently reported NRR catalysts (Table


Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

S8), indicating that N2 fixation based on Fe-N/C-CNTs could achieve at room temperature and atmospheric pressure.

Figure 3. Electrocatalytic NRR on Fe-N/C-CNTs catalysts in 0.1 M KOH electrolyte. (a) The yield rate of NH3 (blue) and the Faradaic efficiency (red) at each given potential. (b) Chronoamperometric results at the corresponding potentials. (c) The yield rate of NH3 at a potential of −0.2 V vs. RHE during five consecutive cycles. (d) The yield of ammonia and Faradic efficiency versus the catalytic temperature under atmospheric pressure at −0.2 V vs. RHE. To shed light on the advantages of Fe-N/C-CNTs with built-in Fe-N3 sites, we studied several derived catalysts by the chronoamperometric NRR and the corresponding UV-vis absorption spectra tests under similar conditions and potential (−0.2 V vs. RHE). The single substrate of CNTs or N-doped porous carbon on CNTs (NC-CNTs) showed a relatively low N2 fixation yield (Figure 4a and Figure S23). As displayed in Figure S24a, with the increase in Fe precursor dosage, the N2 reduction yields were not always promoted. The morphology of FeZIF-8 on the CNT surface was dependent on the concentration of Fe, changing from small uniform particles into the isolated stacks.50, 67 The BET surface area raised from 563.27 m2·g−1 at 2.5 % to 816.25 m2·g−1 at 5.0% Fe because of the increasing amount of Fe-ZIF-8, and almost


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

reached a plateau at 10% Fe-N/C-CNTs. The pore size distribution curves depicted that the FeN/C-CNTs had typical micropores with extra increased meso/macropores due to the spaces caused by the accumulation of Fe-ZIF-8 (Figure S25 and Table S9). XPS spectra of Fe-N/C-CNTs showed that Fe 2p peaks of 5% Fe were positively shifted 0.9 eV than that of 2.5% Fe. In contrast, the N 1s peaks of 5% Fe were shifted to lower BEs of 0.4 eV (Figure S26). These results suggested that there was strong interaction between Fe and N atoms.55, 68 However, non-negligible metallic Fe, iron oxide or FexC phases were revealed in the XRD patterns, HRTEM images, and XPS analysis of 10% Fe-N/C-CNTs (Figure S27). Also, the Fe 2p peaks of 10% Fe exhibited a relatively small positive shift (0.7 eV), and the N 1s peaks also presented 0.3 eV shift to low energy region, which was attributed to the appearance of other iron species and the interference of forming more Fe-Nx sites.56 A relation between NRR activity and different N or Fe species by XPS analysis was concluded in Figure 4b and Table S10. We found that the NH3 yield correlated with the content of pyridinic N rather than the Fe content. The highest pyridinic N in 5% Fe-N/C-CNTs was due to the most Fe-ZIF-8 precursor homogeneously distributed on the CNT surface. The pyridinic N decreased in 10% Fe-N/C-CNTs was attributed to the oversized Fe-ZIF-8 particles isolated on the CNTs, which was easily agglomerated to the other metallic Fe phase based on the above characterizations.50 The NRR properties possessed the similar tendency to the content of pyridinic N, indicating that pyridinic N provided the coordination anchoring sites for the Fe-N3 species as a primary NRR active site.67, 69 Considering that the Fe-N3 active center could be blocked by SCN− in acidic media,70-71 and we performed a KSCN poisoning experiment to confirm whether the NRR active sites were Fe-N3 moieties. As shown in Figure 4c and Figure S28a of the Supporting Information, the remarkable decline in NRR reduction yields could be


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

ascribed to blocking of the Fe-N3 active sites by SCN−. The SCN−-poisoned NRR active sites could be recovered in alkaline media owing to the dissociation of SCN− on the Fe-N3 active sites (Figure 4d and Figure S28b).45 These results demonstrated that the built-in Fe-N3 species were the primary catalytically active centers for the NRR.

Figure 4. (a) Yield rate of NH3 at −0.2 V vs. RHE at room temperature and ambient pressure with different catalysts. (b) The correlation between the yield rate of NH3 and different N contents with catalysts with different amounts of Fe. The different UV-vis absorption spectra when the Fe-N/C-CNTs catalysts were poisoned with 10 mM KSCN. (c) The 0.1 M HCl electrolyte stained with indophenol indicator and (d) 0.1 M KOH electrolyte stained with Nessler’s reagent indicator. Insets in (c) and (d) show the corresponding schematic illustrations. 2.3 DFT Calculation We performed a DFT calculation to elucidate the catalytic active centers and reaction mechanism of the NRR. As displayed in Figure 5a and Figure S29, N2 could be spontaneously chemisorbed on the Fe-N3 species with free energy (ΔG, eV) of −0.75 eV. In contrast, positive ΔG indicated weak interactions for the chemisorption of N2 on Fe-N4 species, which could be


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

clarified by the recent study on Fe-N4 sites with an unsatisfactory performance for NRR.72 Similarly, the Fe-N5 site was also used to adsorb N2, but it was ruled out due to the weak interactions with a adsorption energy of 0.16 eV. The N* adsorption energy is a widely used descriptor to compare with the competing reactions and was about −5.47 eV in our system, which was stronger than that of OH−, H+ and H2O of −4.34 eV, −0.53 eV and −0.21 eV, respectively.73 Furthermore, the Fe-N3 active sites were anchored in amorphous carbon (Figure 1e-1f and Figure S5b), so it was most likely that the adventitious carbon (C*) could exist around the Fe-N3 active sites, which played a strong interaction with N2 and suppressed the completion reactions, especially for oxygen reduction reactions (ORR).74 All those results demonstrated the adsorption stability of N2 on proposed Fe-N3 active sites. According to previous theoretical studies, the spin properties could also play an important role for the catalysts on activating N2.43-44 Our results demonstrated that the Fe-N3 species exhibited a large spin moment of 3.16 μB, which was well consistent with its high chemical reactivity towards N2 adsorption. In particular, the spin moment of Fe-N3 species mainly located on the central Fe site (about 96%), indicating that Fe site was the active site. In contrast, the spin moment of the Fe-N4 species was smaller by 1.18 μB than that of Fe-N3 one, which agreed well with its weaker adsorption strength with N2. The computed free-energy diagram of the most favorable pathway of N2 electroreduction on Fe-N3 active sites was displayed in Figure 5b-5c. These results demonstrated that N2 electroreduction preferred to proceed via the distal pathway on Fe-N3 active sites, in which hydrogenation of the adsorbed N2 molecule to N2H species was the potential-limiting step with a free-energy barrier of 0.84 eV. The influence of trace Fe/Fe3C nanocrystals on the NRR was also explored by DFT calculations. The sites with two adjacent Fe on the Fe@Fe-N3/C-CNTs showed that an alternating pathway was energetically favorable for


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

the NRR (Figure S30). The potential-determining step lied at the reaction of NH2* → NH3 (g) with a free-energy barrier of 1.66 eV, which was much larger than that on Fe-N3 active sites (0.84 eV), demonstrating the feasibility of NRR on our proposed Fe-N3 moiety.

Figure 5. (a) The different free-energy diagrams of the N2 adsorption on the Fe-N4 and Fe-N3 structures. (b) The free-energy diagram for the NRR on Fe-N3/C-CNTs at zero and the applied potential through the distal pathway. (c) Optimized geometric structures of various intermediates along the reaction path of the NRR on Fe-N3/C-CNTs through distal mechanisms. 3. CONCLUSION In summary, we prepared a noble-metal-free Fe-N/C-CNTs catalyst with enhanced intrinsic activity for the electrochemical N2 reduction reaction in aqueous media under ambient conditions. More importantly, we demonstrated by experimental results and theoretical calculations that built-in Fe-N3 moieties as a valid catalytically active site elevated the NRR activity effectively. This study not only serves as a typical example of precisely designing catalysts for N2 fixation but also offers a promising application for M-N-C materials in the electrochemical reduction of nitrogen.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

ASSOCIATED CONTENT The following files are available free of charge: Chemicals and materials, characterization, procedures for the synthesis of catalysts, electrochemical measurements, determination of ammonia or hydrazine, the control experiments for the investigations of ammonia contamination, the calculation of performance evaluation parameters, and DFT computational methods; sixteen figures and three tables showing SEM images, XRD patterns, XPS surveys, EXAFS fitting curves, Raman spectra, N2 sorption isotherms, pore size distribution, zeta potential, and He-TPD profile of MWCNTs, Fe-ZIF-8-CNTs, or Fe-N/C-CNTs; fourteen figures and seven tables showing CV, LSV, UV-vis absorption, i-t curves, control experiments, 1H NMR spectra, comparison of NRR catalytic activities, and DFT calculations of Fe-N/C-CNTs.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X. Q. Cui) * E-mail: [email protected] (W. T. Zheng) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Program of China (2016YFA0200401), the National Natural Science Foundation of China (51571100), the Program for JLU Science and Technology Innovative Research Team 18 ACS Paragon Plus Environment

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(JLUSTIRT, 2017TD-09) and the Fundamental Research Funds for the Central Universities. We would like to thank the Beijing Synchrotron Radiation Facility (BSRF) for providing beam time on beamline 1 W1B for the XAS measurements. We express our sincere thanks to Xue’s Group and Jilin University Little Swan Instruments Co., Ltd. for their help. REFERENCES (1) Service, R. F. Liquid Sunshine. Science 2018, 361, 120-123. (2) Chen, J. G.; Crooks, R. M.; Seefeldt, L. C.; Bren, K. L.; Bullock, R. M.; Darensbourg, M. Y.; Holland, P. L.; Hoffman, B.; Janik, M. J.; Jones, A. K.; Kanatzidis, M. G.; King, P.; Lancaster, K. M.; Lymar, S. V.; Pfromm, P.; Schneider, W. F.; Schrock, R. R. Beyond Fossil Fuel-driven Nitrogen Transformations. Science 2018, 360. (3) van 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, 5183-5191. (4) Wang, L.; Xia, M. K.; Wang, H.; Huang, K. F.; Qian, C. X.; Maravelias, C. T.; Ozin, G. A. Greening Ammonia toward the Solar Ammonia Refinery. Joule 2018, 2, 1055-1074. (5) Service, R. F. New Recipe Produces Ammonia from Air, Water, and Sunlight. Science 2014, 345, 610-610. (6) Guo, C. X.; Ran, J. R.; Vasileff, A.; Qiao, S. Z. Rational Design of Electrocatalysts and Photo(electro)catalysts for Nitrogen Reduction to Ammonia (NH3) under Ambient Conditions. Energ. Environ. Sci. 2018, 11, 45-56. (7) Hoffman, B. M.; Lukoyanov, D.; Yang, Z. Y.; Dean, D. R.; Seefeldt, L. C. Mechanism of Nitrogen Fixation by Nitrogenase: The Next Stage. Chem. Rev. 2014, 114, 4041-4062. (8) Milton, R. D.; Abdellaoui, S.; Khadka, N.; Dean, D. R.; Leech, D.; Seefeldt, L. C.; Minteer, S. D. Nitrogenase Bioelectrocatalysis: Heterogeneous Ammonia and Hydrogen Production by MoFe Protein. Energ. Environ. Sci. 2016, 9, 2550-2554. (9) Čorić, I.; Mercado, B. Q.; Bill, E.; Vinyard, D. J.; Holland, P. L. Binding of Dinitrogen to an Iron-Sulfur-Carbon Site. Nature 2015, 526, 96.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

(10) Jia, H. P.; Quadrelli, E. A. Mechanistic Aspects of Dinitrogen Cleavage and Hydrogenation to Produce Ammonia in Catalysis and Organometallic Chemistry: Relevance of Metal Hydride Bonds and Dihydrogen. Chem. Soc. Rev. 2014, 43, 547-564. (11) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501, 84. (12) Liu, J. C.; Ma, X. L.; Li, Y.; Wang, Y. G.; Xiao, H.; Li, J. Heterogeneous Fe3 Single-Cluster Catalyst for Ammonia Synthesis via an Associative Mechanism. Nat. Commun. 2018, 9, 1610. (13) Jin, H. Y.; Guo, C. X.; Liu, X.; Liu, J. L.; Vasileff, A.; Jiao, Y.; Zheng, Y.; Qiao, S. Z. Emerging Two-Dimensional Nanomaterials for Electrocatalysis. Chem. Rev. 2018, 118, 63376408. (14) Liu, J. L.; Zhu, D. D.; Zheng, Y.; Vasileff, A.; Qiao, S. Z. Self-Supported Earth-Abundant Nanoarrays as Efficient and Robust Electrocatalysts for Energy-Related Reactions. ACS Catal. 2018, 8, 6707-6732. (15) Foster, 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, 490-500. (16) Yan, D. F.; Li, H.; Chen, C.; Zou, Y. Q.; Wang, S. Y. Defect Engineering Strategies for Nitrogen Reduction Reactions under Ambient Conditions. Small Methods 2018, DOI: 10.1002/smtd.201800331. (17) Deng, J.; Iñiguez, J. A.; Liu, C. Electrocatalytic Nitrogen Reduction at Low Temperature. Joule 2018, 2, 846-856. (18) Kyriakou, V.; Garagounis, I.; Vasileiou, E.; Vourros, A.; Stoukides, M. Progress in the Electrochemical Synthesis of Ammonia. Catal. Today 2017, 286, 2-13. (19) Fukuzumi, S. Production of Liquid Solar Fuels and Their Use in Fuel Cells. Joule 2017, 1, 689-738. (20) Cui, X. Y.; Tang, C.; Zhang, Q. A Review of Electrocatalytic Reduction of Dinitrogen to Ammonia under Ambient Conditions. Adv. Energy Mater. 2018, 8, 1800369. (21) Li, S. J.; Bao, D.; Shi, M. M.; Wulan, B. R.; Yan, J. M.; Jiang, Q. Amorphizing of Au Nanoparticles by CeOx-RGO Hybrid Support towards Highly Efficient Electrocatalyst for N2 Reduction under Ambient Conditions. Adv. Mater. 2017, 29, 1700001.


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(22) Shi, M. M.; Bao, D.; Wulan, B. R.; Li, Y. H.; Zhang, Y. F.; Yan, J. M.; Jiang, Q. Au SubNanoclusters on TiO2 toward Highly Efficient and Selective Electrocatalyst for N2 Conversion to NH3 at Ambient Conditions. Adv. Mater. 2017, 29, 1606550. (23) Bao, D.; Zhang, Q.; Meng, F. L.; Zhong, H. X.; Shi, M. M.; Zhang, Y.; Yan, J. M.; Jiang, Q.; Zhang, X. B. Electrochemical Reduction of N2 under Ambient Conditions for Artificial N2 Fixation and Renewable Energy Storage Using N2/NH3 Cycle. Adv. Mater. 2017, 29, 1604799. (24) Geng, Z. G.; Liu, Y.; Kong, X. D.; Li, P.; Li, K.; Liu, Z. Y.; Du, J. J.; Shu, M.; Si, R.; Zeng, J. Achieving a Record-High Yield Rate of 120.9 µgNH3 mgcat.-1 h-1 for N2 Electrochemical Reduction over Ru Single-Atom Catalysts. Adv. Mater. 2018, 30, 1803498. (25) Wang, D. B.; Azofra, L. M.; Harb, M.; Cavallo, L.; Zhang, X. Y.; Suryanto, B. H. R.; MacFarlane, D. R. Energy Efficient Nitrogen Reduction to Ammonia at Low Overpotential in Aqueous Electrolyte under Ambient Conditions. ChemSusChem 2018, 11, 3416-3422. (26) Liu, H. M.; Han, S. H.; Zhao, Y.; Zhu, Y. Y.; Tian, X. L.; Zeng, J. H.; Jiang, J. X.; Xia, B. Y.; Chen, Y. Surfactant-Free Atomically Ultrathin Rhodium Nanosheet Nanoassemblies for Efficient Nitrogen Electroreduction. J. Mater. Chem. A 2018, 6, 3211-3217. (27) Shi, M. M.; Bao, D.; Li, S. J.; Wulan, B. R.; Yan, J. M.; Jiang, Q. Anchoring PdCu Amorphous Nanocluster on Graphene for Electrochemical Reduction of N2 to NH3 under Ambient Conditions in Aqueous Solution. Adv. Energy Mater. 2018, 8, 1800124. (28) Wang, J.; Yu, L.; Hu, L.; Chen, G.; Xin, H. L.; Feng, X. F. Ambient Ammonia Synthesis via Palladium-Catalyzed Electrohydrogenation of Dinitrogen at Low Overpotential. Nat. Commun. 2018, 9, 1795. (29) Zhang, Y.; Qiu, W. B.; Ma, Y. J.; Luo, Y. L.; Tian, Z. Q.; Cui, G. W.; Xie, F. Y.; Chen, L.; Li, T. S.; Sun, X. P. High-Performance Electrohydrogenation of N2 to NH3 Catalyzed by Multishelled Hollow Cr2O3 Microspheres under Ambient Conditions. ACS Catal. 2018, 8, 85408544. (30) Han, J. R.; Liu, Z. C.; Ma, Y. J.; Cui, G. W.; Xie, F. Y.; Wang, F. X.; Wu, Y. P.; Gao, S. Y.; Xu, Y. H.; Sun, X. P. Ambient N2 Fixation to NH3 at Ambient Conditions: Using Nb2O5 Nanofiber as a High-Performance Electrocatalyst. Nano Energy 2018, 52, 264-270. (31) Zhang, X. P.; Kong, R. M.; Du, H. T.; Xia, L.; Qu, F. L. Highly Efficient Electrochemical Ammonia Synthesis via Nitrogen Reduction Reactions on a VN Nanowire Array under Ambient Conditions. Chem. Commun. 2018, 54, 5323-5325.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(32) Zhang, L.; Ji, X. Q.; Ren, X.; Ma, Y. J.; Shi, X. F.; Tian, Z. Q.; Asiri, A. M.; Chen, L.; Tang, B.; Sun, X. P. Electrochemical Ammonia Synthesis via Nitrogen Reduction Reaction on a MoS2 Catalyst: Theoretical and Experimental Studies. Adv. Mater. 2018, 30, 1800191. (33) Lv, C. D.; Qian, Y. M.; Yan, C. S.; Ding, Y.; Liu, Y. Y.; Chen, G.; Yu, G. H. Defect Engineering Metal-Free Polymeric Carbon Nitride Electrocatalyst for Effective Nitrogen Fixation under Ambient Conditions. Angew. Chem. Int. Ed. 2018, 57, 10246-10250. (34) Yu, X. M.; Han, P.; Wei, Z. X.; Huang, L. S.; Gu, Z. X.; Peng, S. J.; Ma, J. M.; Zheng, G. F. Boron-Doped Graphene for Electrocatalytic N2 Reduction. Joule 2018, 2, 1610-1622. (35) Qiu, W. B.; Xie, X. Y.; Qiu, J. D.; Fang, W. H.; Liang, R. P.; Ren, X.; Ji, X. Q.; Cui, G. W.; Asiri, A. M.; Cui, G. L.; Tang, B.; Sun, X. P. High-Performance Artificial Nitrogen Fixation at Ambient Conditions using a Metal-Free Electrocatalyst. Nat. Commun. 2018, 9, 3485. (36) Suryanto, B. H. R.; Kang, C. S. M.; Wang, D. b.; Xiao, C. L.; Zhou, F. L.; Azofra, L. M.; Cavallo, L.; Zhang, X. Y.; MacFarlane, D. R. Rational Electrode-Electrolyte Design for Efficient Ammonia Electrosynthesis under Ambient Conditions. ACS Energy Lett. 2018, 3, 1219-1224. (37) Zhou, F. L.; Azofra, L. M.; Ali, M.; Kar, M.; Simonov, A. N.; McDonnell-Worth, C.; Sun, C. H.; Zhang, X. Y.; MacFarlane, D. R. Electro-Synthesis of Ammonia from Nitrogen at Ambient Temperature and Pressure in Ionic Liquids. Energ. Environ. Sci. 2017, 10, 2516-2520. (38) Hu, L.; Khaniya, A.; Wang, J.; Chen, G.; Kaden, W. E.; Feng, X. F. Ambient Electrochemical Ammonia Synthesis with High Selectivity on Fe/Fe-Oxide Catalyst. ACS Catal. 2018, 8, 9312-9319. (39) Cui, X. Y.; Tang, C.; Liu, X. M.; Wang, C.; Ma, W. J.; Zhang, Q. Highly Selective Electrochemical Reduction of Dinitrogen to Ammonia at Ambient Temperature and Pressure over Iron Oxide Catalysts. Chem. Eur. J. 2018, 24, 1-9. (40) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355. (41) Choi, C.; Back, S.; Kim, N. Y.; Lim, J.; Kim, Y. H.; Jung, Y. Suppression of Hydrogen Evolution Reaction in Electrochemical N2 Reduction Using Single-Atom Catalysts: A Computational Guideline. ACS Catal. 2018, 8, 7517-7525.


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(42) Ling, C. Y.; Bai, X. W.; Ouyang, Y.; Du, A. J.; Wang, J. L. Single Molybdenum Atom Anchored on N-Doped Carbon as a Promising Electrocatalyst for Nitrogen Reduction into Ammonia at Ambient Conditions. J. Phys. Chem. C 2018, 122, 16842-16847. (43) Li, X. F.; Li, Q. K.; Cheng, J.; Liu, L. L.; Yan, Q.; Wu, Y. C.; Zhang, X. H.; Wang, Z. Y.; Qiu, Q.; Luo, Y. Conversion of Dinitrogen to Ammonia by FeN3-Embedded Graphene. J. Am. Chem. Soc. 2016, 138, 8706-8709. (44) Guo, X. Y.; Huang, S. P. Tuning Nitrogen Reduction Reaction Activity via Controllable Fe Magnetic Moment: A Computational Study of Single Fe Atom Supported on Defective Graphene. Electrochim. Acta 2018, 284, 392-399. (45) Jun, C. Y.; Fang, J. S.; Gang, W. Y.; Cai, D. J.; Xing, C. W.; Zhi, L.; An, S. R.; Rong, Z. L.; Bin, Z. Z.; Sheng, W. D.; Dong, L. Y. Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chem. 2017, 129, 7041-7045. (46) Satoshi, Y.; Atom, F.; Yosuke, U.; Jeheon, K.; Kei, M. Iron-Nitrogen-Doped Vertically Aligned Carbon Nanotube Electrocatalyst for the Oxygen Reduction Reaction. Adv. Funct. Mater. 2016, 26, 738-744. (47) Hu, X. M.; Hval, H. H.; Bjerglund, E. T.; Dalgaard, K. J.; Madsen, M. R.; Pohl, M.-M.; Welter, E.; Lamagni, P.; Buhl, K. B.; Bremholm, M.; Beller, M.; Pedersen, S. U.; Skrydstrup, T.; Daasbjerg, K. Selective CO2 Reduction to CO in Water using Earth-Abundant Metal and Nitrogen-Doped Carbon Electrocatalysts. ACS Catal. 2018, 8, 6255-6264. (48) Ye, Y. F.; Cai, F.; Li, H. B.; Wu, H. H.; Wang, G. X.; Li, Y. S.; Miao, S.; Xie, S. H.; Si, R.; Wang, J.; Bao, X. H. Surface Functionalization of ZIF-8 with Ammonium Ferric Citrate toward High Exposure of Fe-N Active Sites for Efficient Oxygen and Carbon Dioxide Electroreduction. Nano Energy 2017, 38, 281-289. (49) Fei, H. L.; Dong, J. C.; Arellano-Jiménez, M. J.; Ye, G. L.; Dong Kim, N.; Samuel, E. L. G.; Peng, Z. W.; Zhu, Z.; Qin, F.; Bao, J. M.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M. Atomic Cobalt on Nitrogen-Doped Graphene for Hydrogen Generation. Nat. Commun. 2015, 6, 8668. (50) Lai, Q. X.; Zheng, L. R.; Liang, Y. Y.; He, J. P.; Zhao, J. X.; Chen, J. H. Metal-OrganicFramework-Derived Fe-N/C Electrocatalyst with Five-Coordinated Fe-Nx Sites for Advanced Oxygen Reduction in Acid Media. ACS Catal. 2017, 7, 1655-1663.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

(51) Wang, X. J.; Zhang, H. G.; Lin, H. H.; Gupta, S.; Wang, C.; Tao, Z. X.; Fu, H.; Wang, T.; Zheng, J.; Wu, G.; Li, X. G. Directly Converting Fe-Doped Metal-Organic Frameworks into Highly Active and Stable Fe-N-C Catalysts for Oxygen Reduction in Acid. Nano Energy 2016, 25, 110-119. (52) Jiao, L.; Wan, G.; Zhang, R.; Zhou, H.; Yu, S. H.; Jiang, H. L. From Metal-Organic Frameworks to Single-Atom Fe Implanted N-doped Porous Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic Media. Angew. Chem. 2018, 130, 8661-8665. (53) Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Identification of Catalytic Sites for Oxygen Reduction in Iron- and Nitrogen-Doped Graphene Materials. Nat. Mater. 2015, 14, 937. (54) Li, T. F.; Peng, Y. X.; Li, K.; Zhang, R.; Zheng, L. R.; Xia, D. G.; Zuo, X. Enhanced Activity and Stability of Binuclear Iron(III) Phthalocyanine on Graphene Nanosheets for Electrocatalytic Oxygen Reduction in Acid. J. Power Sources 2015, 293, 511-518. (55) Cao, R. G.; Thapa, R.; Kim, H.; Xu, X. D.; Gyu Kim, M.; Li, Q.; Park, N.; Liu, M. L.; Cho, J. Promotion of Oxygen Reduction by a Bio-Inspired Tethered Iron Phthalocyanine Carbon Nanotube-Based Catalyst. Nat. Commun. 2013, 4, 2076. (56) Jiang, W. J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L. J.; Wang, J. Q.; Hu, J. S.; Wei, Z. D.; Wan, L. J. Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx. J. Am. Chem. Soc. 2016, 138, 3570-3578. (57) Miao, Z. P.; Wang, X. M.; Tsai, M. C.; Jin, Q. Q.; Liang, J. S.; Ma, F.; Wang, T. Y.; Zheng, S. J.; Hwang, B. J.; Huang, Y. H.; Guo, S. J.; Li, Q. Atomically Dispersed Fe-Nx/C Electrocatalyst Boosts Oxygen Catalysis via a New Metal-Organic Polymer Supramolecule Strategy. Adv. Energy Mater. 2018, 8, 1801226. (58) Zhu, Q. L.; Xia, W.; Zheng, L. R.; Zou, R. Q.; Liu, Z.; Xu, Q. Atomically Dispersed Fe/NDoped Hierarchical Carbon Architectures Derived from a Metal-Organic Framework Composite for Extremely Efficient Electrocatalysis. ACS Energy Lett. 2017, 2, 504-511. (59) Choi, C. H.; Park, S. H.; Woo, S. I. Binary and Ternary Doping of Nitrogen, Boron, and Phosphorus into Carbon for Enhancing Electrochemical Oxygen Reduction Activity. ACS Nano 2012, 6, 7084-7091.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(60) Liu, Q. X.; Ai, L. H.; Jiang, J. MXene-Derived TiO2@C/g-C3N4 Heterojunctions for Highly Efficient Nitrogen Photofixation. J. Mater. Chem. A 2018, 6, 4102-4110. (61) Cheng, L. C.; Tuo, W.; Jian, Z. Z.; Min, Y. W.; Feng, L. J.; Ang, L.; Lin, Y. Z.; A., O. G.; Long, G. J. Promoted Fixation of Molecular Nitrogen with Surface Oxygen Vacancies on Plasmon-Enhanced TiO2 Photoelectrodes. Angew. Chem. Int. Ed. 2018, 57, 5278-5282. (62) Greenlee, L. F.; Renner, J. N.; Foster, S. L. The Use of Controls for Consistent and Accurate Measurements of Electrocatalytic Ammonia Synthesis from Dinitrogen. ACS Catal. 2018, 8, 7820-7827. (63) Boucher, D. L.; Davies, J. A.; Edwards, J. G.; Mennad, A. An Investigation of the Putative Photosynthesis of Ammonia on Iron-Doped Titania and Other Metal Oxides. J. Photochem. Photobiol. A 1995, 88, 53-64. (64) Lee, H. K.; Koh, C. S. L.; Lee, Y. H.; Liu, C.; Phang, I. Y.; Han, X. M.; Tsung, C.-K.; Ling, X. Y. Favoring the Unfavored: Selective Electrochemical Nitrogen Fixation using a Reticular Chemistry Approach. Sci. Adv. 2018, 4. (65) Liu, J.; Kelley, M. S.; Wu, W.; Banerjee, A.; Douvalis, A. P.; Wu, J.; Zhang, Y.; Schatz, G. C.; Kanatzidis, M. G. Nitrogenase-Mimic Iron-Containing Chalcogels for Photochemical Reduction of Dinitrogen to Ammonia. Proc. Natl. Acad. Sci. USA 2016, 113, 5530-5535. (66) Chen, G. F.; Ren, S. Y.; Zhang, L. L.; Cheng, H.; Luo, Y. R.; Zhu, K. H.; Ding, L. X.; Wang, H. H. Advances in Electrocatalytic N2 Reduction—Strategies to Tackle the Selectivity Challenge. Small Methods 2018, DOI:10.1002/smtd.201800337. (67) Zhang, H. G.; Hwang, S.; Wang, M. Y.; Feng, Z. X.; Karakalos, S.; Luo, L. L.; Qiao, Z.; Xie, X. H.; Wang, C. M.; Su, D.; Shao, Y. Y.; Wu, G. Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation. J. Am. Chem. Soc. 2017, 139, 14143-14149. (68) Zhang, Z. P.; Gao, X. J.; Dou, M. L.; Ji, J.; Wang, F. Biomass Derived N-Doped Porous Carbon Supported Single Fe Atoms as Superior Electrocatalysts for Oxygen Reduction. Small 2017, 13, 1604290. (69) Liu, W. G.; Zhang, L. L.; Liu, X.; Liu, X. Y.; Yang, X. F.; Miao, S.; Wang, W. T.; Wang, A. Q.; Zhang, T. Discriminating Catalytically Active FeNx Species of Atomically Dispersed Fe-N-C Catalyst for Selective Oxidation of the C-H Bond. J. Am. Chem. Soc. 2017, 139, 10790-10798.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

(70) Thorum, M. S.; Hankett, J. M.; Gewirth, A. A. Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity. J. Phys. Chem. Lett. 2011, 2, 295-298. (71) Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You, Y.; Liu, J. G.; Wu, X. L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S. G. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882-10885. (72) Mukherjee, S.; Cullen, D. A.; Karakalos, S.; Liu, K. X.; Zhang, H.; Zhao, S.; Xu, H.; More, K. L.; Wang, G. F.; Wu, G. Metal-Organic Framework-Derived Nitrogen-Doped Highly Disordered Carbon for Electrochemical Ammonia Synthesis using N2 and H2O in Alkaline Electrolytes. Nano Energy 2018, 48, 217-226. (73) Montoya, J. H.; Tsai, C.; Vojvodic, A.; Nørskov, J. K. The Challenge of Electrochemical Ammonia Synthesis: A New Perspective on the Role of Nitrogen Scaling Relations. ChemSusChem 2015, 8, 2180-2186. (74) Comer, B. M.; Liu, Y. H.; Dixit, M. B.; Hatzell, K. B.; Ye, Y. F.; Crumlin, E. J.; Hatzell, M. C.; Medford, A. J. The Role of Adventitious Carbon in Photo-Catalytic Nitrogen Fixation by Titania. J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b08464.


Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Graphical Abstract

The Noble-Metal-Free Fe-N/C-CNTs Catalysts with potentially active Fe-N3 sites are built from carbonizing Fe-doped ZIF-8-CNTs templates and exhibit enhanced nitrogen reduction performance in alkaline media under mild conditions.


C Hybrid for Enhanced Nitrogen Reduction

Recommend Documents