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Self-supported NbSe2 nanosheet arrays for highly efficient ammonia electrosynthesis under ambient conditions
Journal of Catalysis 381 (2020) 78–83
Contents lists available at ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Priority Communication
Self-supported NbSe2 nanosheet arrays for highly efficient ammonia electrosynthesis under ambient conditions Yong Wang a,1, Anran Chen a,1, Shuhua Lai b, Xianyun Peng b, Shunzheng Zhao c, Guangzhi Hu a,⇑, Yuan Qiu b, Junqiang Ren d, Xijun Liu b,⇑, Jun Luo b a
School of Chemical Science and Technology, School of Energy, Yunnan University, Kunming 650091, China Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China c Department of Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China d State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China b
a r t i c l e
i n f o
Article history: Received 5 September 2019 Revised 22 October 2019 Accepted 23 October 2019
Keywords: NbSe2 nanosheets 3D nanoarchitecture Self-supported N2 electroreduction NH3 electrosynthesis
a b s t r a c t As a promising alternative to the Haber–Bosch process for producing NH3, the electrocatalytic nitrogen reduction reaction (NRR) in the aqueous electrolyte has attracted much attention. However, the presence of sluggish reaction kinetics and competitive hydrogen evolution could result in poor activity and unsatisfactory selectivity. Herein, self-supported NbSe2 nanosheet arrays have been prepared and tested as electrocatalysts for NH3 electrosynthesis with a Faradaic efficiency of 13.9 ± 1.0% at 0.4 V versus the reversible hydrogen electrode (vs RHE) and a yield rate of 89.5 ± 6.0 lg h1 mg1 cat. at 0.45 V vs RHE in 0.1 M Na2SO4 under ambient conditions. Moreover, this electrocatalyst showed excellent durability during the 60-h electrolysis (no stable decay of Faradaic efficiencies and NH3 yield rates). Furthermore, density functional theory calculations disclosed that NbSe2 can effectively catalyze the dissociation of the adsorbed N2 molecule and thus promote the NRR process. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Ammonia (NH3) synthesis from molecular nitrogen and hydrogen is the most attractive and important process in the chemical industry. Up to now, for the most part, NH3 is produced through the Haber–Bosch process, which consumes 2% of the anthropogenic energy supply and releases over 40 million metric tons of CO2 annually [1–3]. Therefore, the search of energy-efficient and CO2-neutral alternatives for NH3 synthesis is of particular interest for the scientific community. The electrocatalytic N2 reduction reaction (NRR) conducted under ambient conditions is recognized as a sustainable and green route for NH3 synthesis, showing an intriguing prospect [1–7]. However, efficient NRR has proven extremely challenging to achieve in practice, underscoring the fact that the high energy barrier for dissociating N2 molecules and the competing hydrogen evolution reaction (HER) [1,2]. In this regard, intensive efforts have been devoted to designing various kind of electrocatalysts, including transition metal-based electrocatalysts [8–14], noble-metal⇑ Corresponding authors. 1
E-mail addresses: [email protected] (G. Hu), [email protected] (X. Liu). These authors contributed equally to this work.
https://doi.org/10.1016/j.jcat.2019.10.029 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
based electrocatalysts [15–18], and metal-free electrocatalysts [19,20]. Most of these electrocatalysts, however, still suffer from limited selectivity (known as Faradaic efficiency, FE) and activity (that is, the NH3 yield rate). Recently, niobium (Nb) -based compounds have been theoretically predicted to be capable of catalyzing NRR [21]. However, to date only three experimental works have demonstrated that Nb-based electrodes, such as NbO2, Nb2O5 and Nb3O7(OH), can be served as active electrocatalysts for NH3 producing [22–24]. These significant breakthroughs guide the researchers to develop efficient electrocatalysts for achieving high FE and NH3 yield rates in aqueous solution under ambient conditions. Here, we report that three-dimensional (3D) network of NbSe2 nanosheet arrays (namely NbSe2 NSA) on Ni foam is capable of converting N2 into NH3 under ambient conditions in a neutral aqueous electrolyte. The maximum FE and NH3 yield rate using the NbSe2 NSA electrocatalyst in 0.1 M Na2SO4 was measured to be 13.9 ± 1.0% (at 0.4 V vs RHE) and 89.5 ± 6.0 lg h1 mg1 cat. (at 0.45 V vs RHE), respectively. Clearly, the NbSe2 NSA shows a competitive superiority as compared with the state-of-the-art NRR electrocatalysts under comparable conditions (Table S1). Moreover, NbSe2 NSA displayed a stable activity during the 60-h electrolysis. This work provides a promising lead for the design of active and robust Nb-based catalysts for the electrocatalytic NRR.
Y. Wang et al. / Journal of Catalysis 381 (2020) 78–83
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Fig. 1. Synthesis and characterization of NbSe2 NSA. (a) Schematic illustration of the synthetic procedure. (b) XRD pattern. (c) SEM and (d) TEM images. (e) HRTEM image. The inset in (e) is the corresponding FFT pattern.
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2. Results and discussion NbSe2 NSA was prepared by a simple oil-phase synthetic approach in conformity to the literature (see details in Experimental section) [25], as illustrated in Fig. 1a. The X-ray diffraction (XRD) pattern is all well-indexed to the hexagonal NbSe2 phase (JCPDS no. 65-3484, Fig. 1b). The morphology of NbSe2 NSA has been investigated by scanning electron microscopy (SEM). As shown in Fig. 1c, the NbSe2 nanosheets were uniformly coated on the ligaments of Ni foam, retaining its typical 3D structure (Fig. S1). This feature favors active sites exposure and mass diffusion during NRR. Fig. 1d displays the transmission electron microscopy (TEM) image of NbSe2 nanosheets decomposed ultrasonically from Ni foam. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping indicated that Nb and Se elements are highly dispersive in the NbSe2 nanosheets (Fig. S2). Corresponding the fast Fourier transform (FFT) pattern (inset of Fig. 1e), the highresolution TEM (HRTEM) image exhibited two interplanar spaces of 0.29 and 0.24 nm (Fig. 1e), which can be indexed to the (1 0 0)
and (0 1 3) planes of NbSe2, respectively. The magnified HRTEM image and its corresponding FFT result in Fig. 1e showed the NbSe2 nanosheet oriented along the [0 3 1] zone axis, which is further verified by by the electron diffraction simulation (Fig. S3). In addition, atomic force microscopy (AFM) measurements revealed that the thickness of NbSe2 nanosheets was ~3 nm (Fig. S4). Further, the chemical states of the as-synthesized NbSe2 nanosheets were examined by X-ray photoelectron spectroscopy
(XPS) analysis. As for high-resolution Nb 3d XPS spectrum (Fig. S5a), apart from the peaks of Nb4+ (203.4 and 206.5 eV), the peaks at 207.2 and 209.9 eV belong to Nb5+ were observed as well [25]. Besides, the peaks associated with Se2 3d5/2 and 3d3/2 levels were located at 53.4 and 54.2 eV, respectively (Fig. S5b). Additional two peaks at 54.8 and 55.7 eV could be assigned to the Se4+ species [26]. The surface oxidation of NbSe2 also has been reported in the literature [25]. The electrocatalytic NRR performances of NbSe2 NSA were evaluated in a three-electrode H-cell electrochemical system (see details in Experimental section). NbSe2 powder was prepared and spin-coated on Ni foam for comparison (Figs. S6 and S7). Prior to each NRR test, the used N2 (14N2 or 15N2) and Ar were purified by 2 M NaOH, 0.1 M FeSO4 and 5 mM H2SO4 solutions to exclude the possible interferences of NH3 and NOx in the feeding gases [8,9]. All reported potentials here are referenced to the reversible hydrogen electrode (RHE). The produced NH3 amount was quantified by the spectrophotometrical indophenol blue method (Figs. S8 and S9) [8,9]. The bare Ni foam showed a negligible NRR activity within the potential range (Fig. S10). Linear sweep voltammetry (LSV) curves were recorded on a NbSe2 NSA electrode in both Ar- and N2-saturated electrolytes (Fig. 2a). As seen, in N2-saturated electrolyte, NbSe2 NSA showed a lower onset potential and much higher reduction current density compared to that in Ar-saturated solution. This comparison indicates the possibility of electrocatalytic N2 reduction taking place on the NbSe2 NSA electrode.
Fig. 2. NRR performances at room temperature and atmospheric pressure. (a) LSV curves in Ar- and N2-saturated 0.1 M Na2SO4 solution. (b) Calculated FEs and yield rates of NH3 at different potentials. (c) FEs at 0.40 V vs RHE during the 60-h electrolysis. (d) NH3 yield rates at 0.45 V vs RHE during the 60-h electrolysis. The error bars correspond to the standard deviations of six measurements.
Y. Wang et al. / Journal of Catalysis 381 (2020) 78–83
After electrolysis for 2 h at the potential range from 0.25 to 0.5 V vs RHE, NbSe2 NSA achieved a high FE (13.9 ± 1.0%) at 0.4 V vs RHE and NH3 yield rate (89.5 ± 6.0 lg h1 mg1 cat.) at 0.45 V vs RHE (Fig. 2b). It should be noted that the obtained NH3 yield rate value is higher than that of ever reported Nb-based electrocatalysts except for the Nb3O7(OH)/CFC catalyst (Table S1). Over 0.45 V vs RHE, the NH3 yield rate of NbSe2 NSA (Fig. S11) would be suppressed due to the intensive HER competition [5,6]. By contrast, the NH3 yield rate and FE of NbSe2 powder only reached 15.6 ± 3 lg h1 mg1 cat. and 5.8 ± 0.8% at 0.40 V vs RHE, respectively (Fig. S12), which is much lower than those of NbSe2 NSA. When the NbSe2 nanosheets peeled off from the Ni foam, the NH3 yield rate and FE could be declined from 89.5 ± 6.0 lg h1 mg1 cat. and 13.9 ± 1.0% to 24.8 ± 3.0 lg h1 mg1 cat. and 8.6 ± 0.4% (Fig. S13). The obtained NH3 yield rate values have been re-checked by the independent ion chromatography method [7], confirming their validity (Fig. S14). Meantime, no N2H4 was detected (Fig. S15), indicative of the good selectivity of NbSe2 NSA in the NRR process. Electrochemical durability is an important criterion to estimate the NRR activity of the as-prepared NbSe2 NSA. As depicted in Fig. 2c,d and Fig. S16, no apparent decrease in FE and NH3 yield rate could be observed during the long-term electrolysis at 0.4 and 0.5 V vs RHE, respectively, which demonstrated the good stability of NbSe2 NSA. Additionally, the reduction current density of NbSe2 NSA remained unchanged. Besides, as a demonstration shown in
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Figs. S17 and S18, the morphology and crystalline structure of the nanosheet arrays still preserved after the electrolysis. Also, NbSe2 NSA has stable electrocatalytic kinetics through the 60-h stability test (Fig. S19). The efficient NRR performance of NbSe2 NSA could be attributed to various factors: (1) The NbSe2 nanosheets uniformly grown on Ni foam contributed to a large surface area, thus causing an increase in the accessible surface between the electrocatalysts and electrolyte. Meantime, the high surface area promoted the exposure of active sites [27]. To confirm this hypothesis, it is essential to evaluate the electrochemical active surface area (ECSA) of the samples from the double-layer capacitance (Cdl), which is directly proportional to the ECSA (Fig. S20). As seen, NbSe2 NSA exhibited a larger ECSA (37 mF cm2) compared with NbSe2 powder (0.4 mF cm2). (2) The 3D nanoarchitecture provided hierarchical porous channels that are valuable to the fast penetration of reactants as a result of structural interconnectivity [28]. (3) The direct growth of electroactive materials on conductive substrate avoided the use of polymer binder, which reduces the dead volume in NRR [27,29]. More importantly, the in situ-formed interface Ni/NbSe2 interaction resulted in improved charge transfer kinetics, as verified by the electrochemical impedance spectroscopy (EIS) analyses (Fig. S21). A series of control experiments were performed according to the well-established rigorous guideline of detecting contamina-
Fig. 3. Control experiments and DFT calculations. (a) UV–vis absorption spectra of the electrolytes after electrolysis at 0.40 V vs RHE for 2 h fed by Ar gas, without NbSe2 NSA (carbon paper), and at an open circuit. (b) 15N isotope labeling experiment. (c) Charge density difference of the *N2 on NbSe2 (1 0 4). The value of iso-surface is 0.0012 e Å3. (d) Free energy profiles of NbSe2 for NRR. Note that the asterisk (*) represents the adsorption site. The reason to choose (1 0 4) is that Fig. 1b implies the preferential orientation of the NbSe2 nanosheets to be h1 0 4i.
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tions [1,30]. As presented in Fig. 3a, very little or no NH3 was detected by the indophenol blue method in (1) Ar-saturated electrolyte electrolysed for 2 h with NbSe2 NSA at 0.40 V vs RHE, (2) N2-saturated electrolyte electrolysed for 2 h without NbSe2 NSA at 0.40 V vs RHE, or (3) N2-saturated electrolyte electrolyzed for 2 h with NbSe2 NSA at open circuit. Further, 15N isotope labeling experiments were performed. Clearly, only doublet peaks corresponding to 15NH+4 were detected in the 1H nuclear magnetic resonance (1H NMR) spectra of the electrolyte, while no triple coupling indicating 14NH+4 could be observed (Fig. 3b). It was worth noting that the obtained 15NH3 concentration at 0.45 V vs RHE for 2 h (260.9 lM) quantified from the NMR analysis (Fig. S22) is close to the value determined by spectrophotometric method (264.4 lM). All the above results confirmed that NH3 was generated from the electroreduction of gaseous N2 over NbSe2 NSA. To gain further insight into the reaction mechanisms of NRR on NbSe2 (1 0 4) at the atomic level, density functional theory (DFT) calculations were conducted. As displayed in Fig. 3c, the N2 molecule was tilted on the exposed Nb site of the NbSe2 (1 0 4) in an end-on manner with an elongated N„N bond length of 1.133 Å. Besides, a notable charger transfer from the Nb site to the adjacent N2 molecule was observed (Fig. 3c), implying that NbSe2 can effectively catalyze the dissociation of the N2 molecule. The different NRR reaction pathways on NbSe2 were further examined (Fig. 3d), and the results indicated that the first hydrogenation step (NN* + H+ + e ? *NNH) is the potential-determining step, in line with our previous work [9]. Additionally, the alternating pathway was more favorable than the distal one, since the small freeenergy change required for the hydrogenation. 3. Conclusions To summarize, through a simple oil-phase synthetic approach, we have prepared NbSe2 nanosheet arrays on a 3D Ni foam for efficient electrocatalytic NRR in aqueous solution at ambient conditions. The 3D nanoarchitecture remarkably promoted the exposure of active sites and facilitated the mass diffusion, while the in situ-formed interface metal/metal selenide interaction significantly accelerated the charge transfer procedure. Theoretical calculations further revealed that NbSe2 can effectively stabilize and activate N2 molecules and thus facilitate the overall NRR. Consequently, the obtained electrocatalyst exhibited an FE of 13.9 ± 1.0% at 0.4 V vs RHE and an NH3 yield rate of 89.5 ± 6.0 lg h1 mg1 cat. at 0.45 V vs RHE, both of which are comparable to those of the best noble metal-based electrocatalysts under similar conditions. Moreover, this electrocatalyst displayed relatively stable NRR activity during the 60-h electrolysis (no obvious decay of FE and NH3 yield rate). Our work not only represents an attempt to electrochemical reduction of N2 to produce NH3 with both high selectivity and yield rate in aqueous electrolyte but also pave the way for developing advanced 3D Nb-based materials for N2 fixation or other electrocatalytic reactions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by National Key R&D Program of China (2017YFA0700104), National Natural Science Foundation of China (21601136, 21677171 and 51761165012). The authors also acknowledge National Supercomputing Center in
Shenzhen for providing the computational resources and materials studio (version 7.0, Dmol3). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.10.029. References [1] G.-F. Chen, S. Ren, L. Zhang, H. Cheng, Y. Luo, K. Zhu, L.-X. Ding, H. Wang, Advances in electrocatalytic N2 reduction-strategies to tackle the selectivity challenge, Small Methods 6 (2018) 1800337. [2] Z.W. Chen, J.-M. Yan, Q. Jiang, Single or double: which is the altar of atomic catalysts for nitrogen reduction reaction?, Small Methods 6 (2018) 1800291. [3] C. Guo, J. Ran, A. Vasileff, S.-Z. Qiao, Rational design of electrocatalysts and photo(electro)catalysts for nitrogen reduction to ammonia (NH3) under ambient conditions, Energy Environ. Sci. 11 (2018) 45–56. [4] V. Rosca, M. Duca, M.T. de Groot, M.T.M. Koper, Nitrogen cycle electrocatalysis, Chem. Rev. 109 (2009) 2209–2244. [5] Y.-C. Hao, Y. Guo, L.-W. Chen, M. Shu, X.-Y. Wang, T.-A. 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Zhang, X. Wang, J. Wang, J. Ge, X. Hong, Y. Li, Y. Wu, Y. Li, Atomically dispersed Au1 catalyst towards efficient electrochemical synthesis of ammonia, Sci. Bull. 63 (2018) 1246–1253. [19] W. Qiu, X.-Y. Xie, J. Qiu, W.-H. Fang, R. Liang, X. Ren, X. Ji, G. Cui, A.M. Asiri, G. Cui, B. Tang, X. Sun, High-performance artificial nitrogen fixation at ambient conditions using a metal-free electrocatalyst, Nat. Commun. 9 (2018) 3485. [20] X. Yu, P. Han, Z. Wei, L. Huang, Z. Gu, S. Peng, J. Ma, G. Zheng, Boron-doped graphene for electrocatalytic N2 reduction, Joule 2 (2018) 1610–1622. [21] Á.B. Höskuldsson, Y. Abghoui, A.B. Gunnarsdóttir, E. Skúlason, Computational screening of rutile oxides for electrochemical ammonia Formation, ACS Sustainable Chem. Eng. 5 (2017) 10327–10333. [22] L. Huang, J. Wu, P. Han, A.M. Al-Enizi, T.M. Almutairi, L. Zhang, G. Zheng, NbO2 Electrocatalyst toward 32% faradaic efficiency for N2 fixation, Small Methods 6 (2018) 1800386. [23] J. Han, Z. Liu, Y. Ma, G. 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Contents lists available at ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Priority Communication
Self-supported NbSe2 nanosheet arrays for highly efficient ammonia electrosynthesis under ambient conditions Yong Wang a,1, Anran Chen a,1, Shuhua Lai b, Xianyun Peng b, Shunzheng Zhao c, Guangzhi Hu a,⇑, Yuan Qiu b, Junqiang Ren d, Xijun Liu b,⇑, Jun Luo b a
School of Chemical Science and Technology, School of Energy, Yunnan University, Kunming 650091, China Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China c Department of Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China d State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China b
a r t i c l e
i n f o
Article history: Received 5 September 2019 Revised 22 October 2019 Accepted 23 October 2019
Keywords: NbSe2 nanosheets 3D nanoarchitecture Self-supported N2 electroreduction NH3 electrosynthesis
a b s t r a c t As a promising alternative to the Haber–Bosch process for producing NH3, the electrocatalytic nitrogen reduction reaction (NRR) in the aqueous electrolyte has attracted much attention. However, the presence of sluggish reaction kinetics and competitive hydrogen evolution could result in poor activity and unsatisfactory selectivity. Herein, self-supported NbSe2 nanosheet arrays have been prepared and tested as electrocatalysts for NH3 electrosynthesis with a Faradaic efficiency of 13.9 ± 1.0% at 0.4 V versus the reversible hydrogen electrode (vs RHE) and a yield rate of 89.5 ± 6.0 lg h1 mg1 cat. at 0.45 V vs RHE in 0.1 M Na2SO4 under ambient conditions. Moreover, this electrocatalyst showed excellent durability during the 60-h electrolysis (no stable decay of Faradaic efficiencies and NH3 yield rates). Furthermore, density functional theory calculations disclosed that NbSe2 can effectively catalyze the dissociation of the adsorbed N2 molecule and thus promote the NRR process. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Ammonia (NH3) synthesis from molecular nitrogen and hydrogen is the most attractive and important process in the chemical industry. Up to now, for the most part, NH3 is produced through the Haber–Bosch process, which consumes 2% of the anthropogenic energy supply and releases over 40 million metric tons of CO2 annually [1–3]. Therefore, the search of energy-efficient and CO2-neutral alternatives for NH3 synthesis is of particular interest for the scientific community. The electrocatalytic N2 reduction reaction (NRR) conducted under ambient conditions is recognized as a sustainable and green route for NH3 synthesis, showing an intriguing prospect [1–7]. However, efficient NRR has proven extremely challenging to achieve in practice, underscoring the fact that the high energy barrier for dissociating N2 molecules and the competing hydrogen evolution reaction (HER) [1,2]. In this regard, intensive efforts have been devoted to designing various kind of electrocatalysts, including transition metal-based electrocatalysts [8–14], noble-metal⇑ Corresponding authors. 1
E-mail addresses: [email protected] (G. Hu), [email protected] (X. Liu). These authors contributed equally to this work.
https://doi.org/10.1016/j.jcat.2019.10.029 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
based electrocatalysts [15–18], and metal-free electrocatalysts [19,20]. Most of these electrocatalysts, however, still suffer from limited selectivity (known as Faradaic efficiency, FE) and activity (that is, the NH3 yield rate). Recently, niobium (Nb) -based compounds have been theoretically predicted to be capable of catalyzing NRR [21]. However, to date only three experimental works have demonstrated that Nb-based electrodes, such as NbO2, Nb2O5 and Nb3O7(OH), can be served as active electrocatalysts for NH3 producing [22–24]. These significant breakthroughs guide the researchers to develop efficient electrocatalysts for achieving high FE and NH3 yield rates in aqueous solution under ambient conditions. Here, we report that three-dimensional (3D) network of NbSe2 nanosheet arrays (namely NbSe2 NSA) on Ni foam is capable of converting N2 into NH3 under ambient conditions in a neutral aqueous electrolyte. The maximum FE and NH3 yield rate using the NbSe2 NSA electrocatalyst in 0.1 M Na2SO4 was measured to be 13.9 ± 1.0% (at 0.4 V vs RHE) and 89.5 ± 6.0 lg h1 mg1 cat. (at 0.45 V vs RHE), respectively. Clearly, the NbSe2 NSA shows a competitive superiority as compared with the state-of-the-art NRR electrocatalysts under comparable conditions (Table S1). Moreover, NbSe2 NSA displayed a stable activity during the 60-h electrolysis. This work provides a promising lead for the design of active and robust Nb-based catalysts for the electrocatalytic NRR.
Y. Wang et al. / Journal of Catalysis 381 (2020) 78–83
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Fig. 1. Synthesis and characterization of NbSe2 NSA. (a) Schematic illustration of the synthetic procedure. (b) XRD pattern. (c) SEM and (d) TEM images. (e) HRTEM image. The inset in (e) is the corresponding FFT pattern.
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2. Results and discussion NbSe2 NSA was prepared by a simple oil-phase synthetic approach in conformity to the literature (see details in Experimental section) [25], as illustrated in Fig. 1a. The X-ray diffraction (XRD) pattern is all well-indexed to the hexagonal NbSe2 phase (JCPDS no. 65-3484, Fig. 1b). The morphology of NbSe2 NSA has been investigated by scanning electron microscopy (SEM). As shown in Fig. 1c, the NbSe2 nanosheets were uniformly coated on the ligaments of Ni foam, retaining its typical 3D structure (Fig. S1). This feature favors active sites exposure and mass diffusion during NRR. Fig. 1d displays the transmission electron microscopy (TEM) image of NbSe2 nanosheets decomposed ultrasonically from Ni foam. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping indicated that Nb and Se elements are highly dispersive in the NbSe2 nanosheets (Fig. S2). Corresponding the fast Fourier transform (FFT) pattern (inset of Fig. 1e), the highresolution TEM (HRTEM) image exhibited two interplanar spaces of 0.29 and 0.24 nm (Fig. 1e), which can be indexed to the (1 0 0)
and (0 1 3) planes of NbSe2, respectively. The magnified HRTEM image and its corresponding FFT result in Fig. 1e showed the NbSe2 nanosheet oriented along the [0 3 1] zone axis, which is further verified by by the electron diffraction simulation (Fig. S3). In addition, atomic force microscopy (AFM) measurements revealed that the thickness of NbSe2 nanosheets was ~3 nm (Fig. S4). Further, the chemical states of the as-synthesized NbSe2 nanosheets were examined by X-ray photoelectron spectroscopy
(XPS) analysis. As for high-resolution Nb 3d XPS spectrum (Fig. S5a), apart from the peaks of Nb4+ (203.4 and 206.5 eV), the peaks at 207.2 and 209.9 eV belong to Nb5+ were observed as well [25]. Besides, the peaks associated with Se2 3d5/2 and 3d3/2 levels were located at 53.4 and 54.2 eV, respectively (Fig. S5b). Additional two peaks at 54.8 and 55.7 eV could be assigned to the Se4+ species [26]. The surface oxidation of NbSe2 also has been reported in the literature [25]. The electrocatalytic NRR performances of NbSe2 NSA were evaluated in a three-electrode H-cell electrochemical system (see details in Experimental section). NbSe2 powder was prepared and spin-coated on Ni foam for comparison (Figs. S6 and S7). Prior to each NRR test, the used N2 (14N2 or 15N2) and Ar were purified by 2 M NaOH, 0.1 M FeSO4 and 5 mM H2SO4 solutions to exclude the possible interferences of NH3 and NOx in the feeding gases [8,9]. All reported potentials here are referenced to the reversible hydrogen electrode (RHE). The produced NH3 amount was quantified by the spectrophotometrical indophenol blue method (Figs. S8 and S9) [8,9]. The bare Ni foam showed a negligible NRR activity within the potential range (Fig. S10). Linear sweep voltammetry (LSV) curves were recorded on a NbSe2 NSA electrode in both Ar- and N2-saturated electrolytes (Fig. 2a). As seen, in N2-saturated electrolyte, NbSe2 NSA showed a lower onset potential and much higher reduction current density compared to that in Ar-saturated solution. This comparison indicates the possibility of electrocatalytic N2 reduction taking place on the NbSe2 NSA electrode.
Fig. 2. NRR performances at room temperature and atmospheric pressure. (a) LSV curves in Ar- and N2-saturated 0.1 M Na2SO4 solution. (b) Calculated FEs and yield rates of NH3 at different potentials. (c) FEs at 0.40 V vs RHE during the 60-h electrolysis. (d) NH3 yield rates at 0.45 V vs RHE during the 60-h electrolysis. The error bars correspond to the standard deviations of six measurements.
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After electrolysis for 2 h at the potential range from 0.25 to 0.5 V vs RHE, NbSe2 NSA achieved a high FE (13.9 ± 1.0%) at 0.4 V vs RHE and NH3 yield rate (89.5 ± 6.0 lg h1 mg1 cat.) at 0.45 V vs RHE (Fig. 2b). It should be noted that the obtained NH3 yield rate value is higher than that of ever reported Nb-based electrocatalysts except for the Nb3O7(OH)/CFC catalyst (Table S1). Over 0.45 V vs RHE, the NH3 yield rate of NbSe2 NSA (Fig. S11) would be suppressed due to the intensive HER competition [5,6]. By contrast, the NH3 yield rate and FE of NbSe2 powder only reached 15.6 ± 3 lg h1 mg1 cat. and 5.8 ± 0.8% at 0.40 V vs RHE, respectively (Fig. S12), which is much lower than those of NbSe2 NSA. When the NbSe2 nanosheets peeled off from the Ni foam, the NH3 yield rate and FE could be declined from 89.5 ± 6.0 lg h1 mg1 cat. and 13.9 ± 1.0% to 24.8 ± 3.0 lg h1 mg1 cat. and 8.6 ± 0.4% (Fig. S13). The obtained NH3 yield rate values have been re-checked by the independent ion chromatography method [7], confirming their validity (Fig. S14). Meantime, no N2H4 was detected (Fig. S15), indicative of the good selectivity of NbSe2 NSA in the NRR process. Electrochemical durability is an important criterion to estimate the NRR activity of the as-prepared NbSe2 NSA. As depicted in Fig. 2c,d and Fig. S16, no apparent decrease in FE and NH3 yield rate could be observed during the long-term electrolysis at 0.4 and 0.5 V vs RHE, respectively, which demonstrated the good stability of NbSe2 NSA. Additionally, the reduction current density of NbSe2 NSA remained unchanged. Besides, as a demonstration shown in
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Figs. S17 and S18, the morphology and crystalline structure of the nanosheet arrays still preserved after the electrolysis. Also, NbSe2 NSA has stable electrocatalytic kinetics through the 60-h stability test (Fig. S19). The efficient NRR performance of NbSe2 NSA could be attributed to various factors: (1) The NbSe2 nanosheets uniformly grown on Ni foam contributed to a large surface area, thus causing an increase in the accessible surface between the electrocatalysts and electrolyte. Meantime, the high surface area promoted the exposure of active sites [27]. To confirm this hypothesis, it is essential to evaluate the electrochemical active surface area (ECSA) of the samples from the double-layer capacitance (Cdl), which is directly proportional to the ECSA (Fig. S20). As seen, NbSe2 NSA exhibited a larger ECSA (37 mF cm2) compared with NbSe2 powder (0.4 mF cm2). (2) The 3D nanoarchitecture provided hierarchical porous channels that are valuable to the fast penetration of reactants as a result of structural interconnectivity [28]. (3) The direct growth of electroactive materials on conductive substrate avoided the use of polymer binder, which reduces the dead volume in NRR [27,29]. More importantly, the in situ-formed interface Ni/NbSe2 interaction resulted in improved charge transfer kinetics, as verified by the electrochemical impedance spectroscopy (EIS) analyses (Fig. S21). A series of control experiments were performed according to the well-established rigorous guideline of detecting contamina-
Fig. 3. Control experiments and DFT calculations. (a) UV–vis absorption spectra of the electrolytes after electrolysis at 0.40 V vs RHE for 2 h fed by Ar gas, without NbSe2 NSA (carbon paper), and at an open circuit. (b) 15N isotope labeling experiment. (c) Charge density difference of the *N2 on NbSe2 (1 0 4). The value of iso-surface is 0.0012 e Å3. (d) Free energy profiles of NbSe2 for NRR. Note that the asterisk (*) represents the adsorption site. The reason to choose (1 0 4) is that Fig. 1b implies the preferential orientation of the NbSe2 nanosheets to be h1 0 4i.
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tions [1,30]. As presented in Fig. 3a, very little or no NH3 was detected by the indophenol blue method in (1) Ar-saturated electrolyte electrolysed for 2 h with NbSe2 NSA at 0.40 V vs RHE, (2) N2-saturated electrolyte electrolysed for 2 h without NbSe2 NSA at 0.40 V vs RHE, or (3) N2-saturated electrolyte electrolyzed for 2 h with NbSe2 NSA at open circuit. Further, 15N isotope labeling experiments were performed. Clearly, only doublet peaks corresponding to 15NH+4 were detected in the 1H nuclear magnetic resonance (1H NMR) spectra of the electrolyte, while no triple coupling indicating 14NH+4 could be observed (Fig. 3b). It was worth noting that the obtained 15NH3 concentration at 0.45 V vs RHE for 2 h (260.9 lM) quantified from the NMR analysis (Fig. S22) is close to the value determined by spectrophotometric method (264.4 lM). All the above results confirmed that NH3 was generated from the electroreduction of gaseous N2 over NbSe2 NSA. To gain further insight into the reaction mechanisms of NRR on NbSe2 (1 0 4) at the atomic level, density functional theory (DFT) calculations were conducted. As displayed in Fig. 3c, the N2 molecule was tilted on the exposed Nb site of the NbSe2 (1 0 4) in an end-on manner with an elongated N„N bond length of 1.133 Å. Besides, a notable charger transfer from the Nb site to the adjacent N2 molecule was observed (Fig. 3c), implying that NbSe2 can effectively catalyze the dissociation of the N2 molecule. The different NRR reaction pathways on NbSe2 were further examined (Fig. 3d), and the results indicated that the first hydrogenation step (NN* + H+ + e ? *NNH) is the potential-determining step, in line with our previous work [9]. Additionally, the alternating pathway was more favorable than the distal one, since the small freeenergy change required for the hydrogenation. 3. Conclusions To summarize, through a simple oil-phase synthetic approach, we have prepared NbSe2 nanosheet arrays on a 3D Ni foam for efficient electrocatalytic NRR in aqueous solution at ambient conditions. The 3D nanoarchitecture remarkably promoted the exposure of active sites and facilitated the mass diffusion, while the in situ-formed interface metal/metal selenide interaction significantly accelerated the charge transfer procedure. Theoretical calculations further revealed that NbSe2 can effectively stabilize and activate N2 molecules and thus facilitate the overall NRR. Consequently, the obtained electrocatalyst exhibited an FE of 13.9 ± 1.0% at 0.4 V vs RHE and an NH3 yield rate of 89.5 ± 6.0 lg h1 mg1 cat. at 0.45 V vs RHE, both of which are comparable to those of the best noble metal-based electrocatalysts under similar conditions. Moreover, this electrocatalyst displayed relatively stable NRR activity during the 60-h electrolysis (no obvious decay of FE and NH3 yield rate). Our work not only represents an attempt to electrochemical reduction of N2 to produce NH3 with both high selectivity and yield rate in aqueous electrolyte but also pave the way for developing advanced 3D Nb-based materials for N2 fixation or other electrocatalytic reactions. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by National Key R&D Program of China (2017YFA0700104), National Natural Science Foundation of China (21601136, 21677171 and 51761165012). The authors also acknowledge National Supercomputing Center in
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