Efficient Electrochemical N2 Reduction to NH3 on MoN Nanosheets

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Efficient Electrochemical N2 Reduction to NH3 on MoN Nanosheets Array under Ambient Conditions Ling Zhang, Xuqiang Ji, Xiang Ren, Yonglan Luo, Xifeng Shi, Abdullah M. Asiri, Baozhan Zheng, and Xuping Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01438 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Sustainable Chemistry & Engineering

Efficient Electrochemical N2 Reduction to NH3 on MoN Nanosheets Array under Ambient Conditions Ling Zhang,†,‡ Xuqiang Ji,† Xiang Ren,† Yonglan Luo,† Xifeng Shi,§ Abdullah M. Asiri,ʃ Baozhan Zheng,‡,* and Xuping Sun†,* †

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China, ‡College of Chemistry, Sichuan University, Chengdu 610064, China, §College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China, and ʃChemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. * E-mail: [email protected] (X.S.); [email protected] (B.Z.) ABSTRACT: Electrochemical N2 reduction reaction (NRR) under ambient conditions offers us an environmentally friendly route for artificial synthesis of NH3. However, up till now, few noble-metal-free electrocatalysts with satisfactory catalytic activities have been explored. In this Communication, we demonstrate that MoN nanosheets array on carbon cloth (MoN NA/CC) acts as a highperformance NRR electrocatalyst toward NH3 electrosynthesis in 0.1 M HCl under ambient conditions. This catalyst achieves a large NH3 yield of 3.01 × 10–10 mo1 s–1 cm–2 and a Faradaic efficiency of 1.15% at –0.3 V vs. reversible hydrogen electrode with strong electrochemical durability and selectivity. Density functional theory calculations reveal that MoN NA/CC catalyzes NRR via the Mars-van Krevelen mechanism. KEYWORDS: MoN nanosheets array, Non-noble-metal catalyst, Electrocatalysis, N2 reduction reaction, Ambient conditions NH3 has triggered numerous interests as a potential energy carrier, a transportation fuel and an indispensable chemical for fertilizer synthesis.1–4 Up to now, the dominant route for artificial synthesis of NH3 is energy-consuming and capitalintensive Haber-Bosch (HB) process.5 However, it operates under harsh conditions (400-550 °C and 150-250 bar) and the necessary H2 feedstock and energy input are mainly originated from fossil fuels, leading to substantial CO2 emission.5,6 Worse still, because HB process is exothermic, relatively high temperature shifts the equilibrium towards reactants, resulting in lower conversion rate.7 Therefore, a more effective and economical route for synthesis of NH3 is urgently desired. So far, considerable efforts have been devoted to synthesis of NH3 including biological strategies,8,9 plasma-induced method,10 photo-11 and electro-chemical catalysis.12,13 Among these methods, electrocatalytic N2 reduction reaction (NRR) proceeding under benign conditions appears as a promising approach for electrochemical synthesis of NH3. Noble-metal catalysts are favorable for NRR but suffer from scarcity and high cost. 14–17 Cost-effective catalysts have thus been designed and developed as promising alternatives, including Fe2O3CNT,18 Bi4V2O11/CeO2,19 Mo nanofilm,20 PEBCD/C,21 MoS2/CC,22 and MoO3,23 etc. Although impressive NRR performances have been achieved, there is still room in developing new non-noble-metal catalysts. Recently, transition metal nitrides (TMNs) with high electronic conductivity and reliable chemical stability, have been theoretically investigated as potential candidates for NRR under ambient conditions.24,25 Moreover, nanoarray catalysts can expose abundant active sites and facilitate diffusion of electrolyte and evolution of gas.26–31 Therefore, we expect that TMN

nanoarray catalyst is highly active toward NRR, which, however, has not been reported before. In this work, we describe our recent finding that selfsupported MoN nanosheets array on carbon cloth (MoN NA/CC) acts as a high-performance catalyst for electrochemical conversion of N2 to NH3 at ambient conditions. It achieves a large NH3 yield of 3.01 × 10–10 mol s–1 cm–2 and a Faradaic efficiency (FE) of 1.15% at –0.3 V vs. reversible hydrogen electrode (RHE) in 0.1 M HCl, outperforming most reported NRR electrocatalysts under mild conditions. Notably, MoN NA/CC also shows excellent selectivity to NH3, with no formation of N2H4, while maintaining strong electrochemical stability for at least 27 h. Density functional theory (DFT) calculations were also performed to gain further insight into the catalytic mechanism involved. Figure 1a shows the X-ray diffraction (XRD) pattern of MoN NA/CC. Peaks at 32.01°, 36.23°, 49.06°, 65.21°, 74.52°, and 78.39° can be well indexed to characteristic (002), (200), (022), (220), (222), (105), and (204) planes of MoN phase (JCPDS No. 77–1999), respectively. Scanning electron microscopy (SEM) images of MoN NA/CC with different magnifications (Figure 1b and 1c) suggest that densely MoN nanosheets are uniformly coated on CC (Figure S1) and transmission electron microscopy (TEM) observation (Figure 1d) confirms its sheet-like feature. High-resolution TEM (HRTEM) taken from the nanosheet (Figure 1e) shows the lattice fringe with a distance of 0.24 nm indexed to the (200) facet of MoN. Energy-dispersive X-ray (EDX) elemental mapping images (Figure 1f) confirm the presence of Mo and N in the nanoarray. The peaks of Mo and N mainly dominate the X-ray photoelectron spectroscopy (XPS) survey spectrum of

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MoN (Figure 1g). In Mo 3d region (Figure 1h), peaks at binding energies (BEs) of 229.5 and 233.1 eV are assigned to the Mo 3d5/2 and Mo 3d3/2, respectively, which are corresponding to Mo in MoN.32 The peak at 236.2 eV is ascribed to Mo6+ in MoO3, which implys the existence of surface oxidation of MoN owing to exposure to air.33 Peaks located at 398.2 and 394.9 eV in N 1s spectrum (Figure 1i) correspond to N 1s and Mo 3p, respectively.33 All these observations strongly confirm the formation of MoN NA/CC.

and FE suggests that NRR is independent of the gas-solid interface.

Figure 2. (a) A schematic diagram for electrocatalytic NRR. (b) LSV curves of MoN NA/CC recorded in N2-saturated (green line) and Ar-saturated (orange line) 0.1 M HCl. (c) NH3 yields and FEs at a series of potentials. (d) NH3 yields and FEs under different N2 ﬂow rates at –0.3 V. All experiments were conducted at room temperature and ambient pressure.

Figure 1. (a) XRD pattern for MoN NA/CC. (b,c) SEM images for MoN NA/CC. (d) TEM and (e) HRTEM images for MoN nonosheet. (f) SEM and corresponding EDX elemental mapping images of Mo and N for MoN NA/CC. (g) XPS survey spectrum for MoN. XPS spectra of MoN in (h) Mo 3d and (i) N 1s regions.

For N2 electroreduction measurement, a H-type cell separated by proton exchange membrane was employed. N2 gas was bubbled to surface of cathode. Protons from electrolyte (0.1 M HCl, pH=1) react with N on the surface of MoN NA/CC to produce NH3, and dissolved N2 later regenerates with the catalyst (Figure 2a). We measured linear sweep voltammetry (LSV) curves under N2- and Ar-saturated 0.1 M HCl solutions at ambient conditions (Figure 2b). Note that the potentials reported in this work were converted to scale. all potentials were reported on a RHE scale. A distinct current density enhancement under N2 atmosphere reveals that MoN NA/CC has catalytic activity for NRR. Concentrations of produced NH3 and N2H4 are estimated by indophenol blue method34 and Watt and Chrisp method,35 respectively. Corresponding calibration curves are shown in Figure S2. The effect of electrode potentials on NH3 yields and corresponding FEs were investigated (Figure 2c). The highest NH3 yield of 3.01 × 10–10 mol s–1 cm–2 and FE of 1.15 % were obtained at – 0.3 V. When applied potential is below –0.3 V, competing H2 evolution reaction greatly reduces the NH3 yield rate and FE,15 as demonstrated by the LSV curves (Figure 2b). MoN NA/CC shows larger NH3 yield and/or FE than most reported data at ambient conditions. A more detailed comparison is shown in Table S1. Notably, MoN NA/CC shows high selectivity for NRR with no N2H4 formation (Figure S3). NRR catalytic activities of MoN NA/CC at different N2 ﬂow rates (Figure 2d) were also measured. Unnoticeable change in NH3 yield

Figure 3. (a) Cycling test of MoN NA/CC at –0.3 V. (b) Chronoamperometry results at different potentials.

Stability of catalysts for NRR is critical for practical applications. In Figure 3a, ignorable variation is observed in the NH3 yield and FE during successive 7 times cycling electrolysis. MoN NA/CC also exhibits superior stability under long-term NRR electrolysis at a series of potentials (Figure 3b). 27-h electrolysis at potential of –0.3 V only leads to a slight decrease in current density (Figure S4a), Corresponding NH3 yield and FE (Figure S4b) decrease moderately compared with those initial values, suggesting that MoN NA/CC still effectively catalyzes the generation of NH3 after long-term NRR. SEM images suggests that MoN NA/CC still keeps its initial morphology after stability test (Figure S5a). XRD analysis confirms the MoN nature of catalyst (Figure S5b). Signals of Mo and N elements are also observed in the XPS survey spectrum of post-NRR MoN (Figure S6a). Figure S6b and S6c demonstrate that Mo 3d and N 1s peaks in MoN are well maintained after stability test. To validate that detected NH3 originated from the electrocatalytic N2 reduction by MoN NA/CC, two control experiments were conducted using CC as working electrode in N2 gas at −0.3 V for 3 h (Figure S7) and employing MoN NA/CC as working electrode in N2 gas at open-circuit potential for 3 h (Figure S8). Corresponding UV-Vis absorption spectra demonstrate that there is almost no NH3


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formation in either case. Ar gas flow was also fed at −0.3 V for 3 h when MoN NA/CC was employed as working electrode. Corresponding UV-Vis absorption spectrum and FE (Figure S9) reveal that NH3 is produced under Ar atmosphere. We further performed a long-term electrolysis in Ar-saturated 0.1 M HCl. Corresponding chronoamperometry curve and NH3 yield are shown in Figure S10. As observed, current density and NH3 yield decrease gradually with test time increasing to about 62 h and then maintain stable. XPS and EDX spectra suggest the absence of N in MoN after electrolysis under Ar atmosphere (Figure S11 and S12a) while the maintaince of N in MoN under N2 atmosphere (Figure S6 and S12b). XRD pattern (Figure S13) indicates the formation of metallic Mo after long term electrolysis under Ar.36 We performed 15N isotopic labeling experiment to verify the N source of the produced NH3. As shown in Figure S14, 1H nuclear magnetic resonance spectra (1H NMR) show a triplet coupling for 14 NH4+ and a doublet coupling for 15NH4+ standard samples, and the use of 15N2 as the feeding gas yields both 14NH4+ and 15 NH4+. These results strongly confirm that detected NH3 is produced from the electrocatalytic conversion of N2 by MoN NA/CC and it catalyzes NRR via the Mars-van Krevelen (MvK) mechanism.25 According to MvK mechanism, a surface N atom on MoN is reduced to form NH3 and subsequently the resulting N-vacancy is replenished by a adsorbed N2 molecule. However, when operated in Ar, the resulting N-vacancy on the surface migrates to the bulk of the MoN, which is replaced by another N from MoN. This process can proceed repeatedly until the depletion of all N atoms in MoN37 and that is why no N signal was detected in MoN after electrolysis under Ar atmosphere.

energy barrier of replenishment of N vacancies is below 0.5 eV, indicating a significant rate of N2 splitting as well as feasible reconstruction of the surface under ambient condition (Figure 4b). To sum up, MoN nanosheets array has been experimentally proven as a non-noble-metal N2 reduction electrocatalyst with high catalytic activity and excellent selectivity (without N2H4 formation) under mild conditions. Such nanoarray achieves a high NH3 yield of 3.01 × 10–10 mo1 s–1 cm–2 with a FE of 1.15%, rivaling the performances of most reported aqueous-based NRR electrocatalysts. Notably, it also shows strong electrochemical stability. DFT calculations reveal that MoN NA/CC catalyzes NRR via the MvK mechanism. This study not only offers us an attractive earth-abundant electrocatalyst an attractive catalyst material for NH3 electrosynthesis but opens an exciting new direction toward the rational design of molybdenum nitride nanocatalysts for NH3 electrosynthesis.39

ASSOCIATED CONTENT Supporting Information Experimental section; SEM images; UV-Vis absorption, XPS, EDX and 1H NMR spectra; chronoamperometry curves; NRR data; XRD patterns; simulation box; Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (X.S.); [email protected] (B.Z.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21575137). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

REFERENCES Figure 4. Free energy profiles of NRR pathways on MoN(200) surface. (b) Geometries of the key intermediates of surface-N removing steps on MoN. Color code: H, white; N, blue or purple for highlight; Mo, cyan; S, yellow. We further employed DFT calculations to investigate the impressive performance of MoN NA/CC for NRR. The NH3 formation on TMN usually goes through MvK mechanism.25 A surface N is reduced to NH3 firstly and N vacancy is replenished by adsorbed N2 subsequently. Compared with widely studied associative/dissociative Heyrovsky mechanisms38 (Supporting Information), splitting inert N-N triple bond with activated surface at the end of the reaction via MvK mechanism is thermodynamically more favorable. Following the proposed MvK mechanism, calculated free energy profiles are shown in Figure 4a. The potential determining step is the second protonation of the surface N. Without considering external potential, the highest energy barriers are 0.59 eV for the generation of the first NH3, and 0.75 eV for the second one. Herein, the newly generated vacancy slightly weakens the reactivity of its neighbor surface N. These barriers are close to the calculated barriers of other TMNs, normally ranging from 0.5 to 0.8 eV.25 Moreover the kinetic

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MoN nanosheets array on carbon cloth (MoN NA/CC) behaves as a highly active and selective electrocatalyst for N2 reduction reaction under ambient conditions. A rate of NH3 yield of 3.01 × 10–10 mol s–1 cm–2 can be achieved at –0.3 V with Faradic efficiency of 1.15 % in 0.1 M HCl. Notably, this catalyst electrode shows strong electrochemical stability.

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