Boron Nanosheet: An Elemental 2D Material for Ambient

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Letter

Boron Nanosheet: An Elemental 2D Material for Ambient Electrocatalytic N2-to-NH3 Fixation in Neutral Media Xiaoxue Zhang, Tongwei Wu, Huanbo Wang, Runbo Zhao, Hongyu Chen, Ting Wang, Peipei Wei, Yonglan Luo, Yanning Zhang, and Xuping Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05134 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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Boron Nanosheet: An Elemental 2D Material for Ambient Electrocatalytic N2-to-NH3 Fixation in Neutral Media Xiaoxue Zhang,†,# Tongwei Wu, ⊥ ,# Huanbo Wang,§ Runbo Zhao, ⊥ Hongyu Chen,†, ⊥ Ting Wang,†, ⊥ Peipei Wei,⊥ Yonglan Luo*,† Yanning Zhang,⊥ and Xuping Sun*, †, ⊥ †Chemical

Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, Sichuan, China, §School of Environment and Resource, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China, and ⊥ Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China ABSTRACT: The Haber–Bosch process for industrial NH3 production suffers from harsh reaction conditions and serious CO2 emission. Electrochemical N2 reduction offers a carbon-neutral alternative for more energy-saving NH3 synthesis but needs active electrocatalysts for the N2 reduction reaction (NRR). In this Letter, boron nanosheet (BNS) is proposed as an elemental 2D material to effectively catalyze the NRR toward NH3 synthesis with excellent selectivity. When tested in 0.1 M Na2SO4, such BNS catalyst attains a high Faradaic efficiency of 4.04% and a large NH3 yield of 13.22 µg h−1 mg−1cat. at –0.80 V vs. reversible hydrogen electrode, with strong electrochemical durability. Density functional theory calculations suggest that the B atoms of both oxidized and H-deactivated BNS can more effectively catalyze the NRR than clean BNS, and the rate-determination step is the desorption process of the second NH3 gas.

KEYWORDS: boron nanosheet, elemental 2D material, N2 reduction reaction, neutral electrolyte, density functional theory calculations

INTRODUCTION NH3 is an activated nitrogen building block for fertilizer, medicament, resin, dye, and explosive, etc.1–3 It is also a promising carbon-neutral energy carrier with high energy density.4 Currently, industrial-scale NH3 production is mainy produced by the Haber−Bosch process (N2 + 3H2 →2NH3) operating at high temperature (300–500°C) and pressure (150– 250 atm) conditions.5–7 However, this process suffers from intensive energy consumption with serious CO2 emission, which significantly stimulates the exploration for sustainable and less energy-intensive approaches for NH3 production. Electrochemical N2 reduction is emerging as an environment-benign process for energy-saving NH3 synthesis.8,9 However, running the N2 reduction reaction (NRR) at ambient conditions is extremely challenging because of the high bond energy of diatomic N2 and the absence of the permanent dipole of the triple bond,10 and its efficiency strongly depends on the identification of catalysts with high activity for NRR.8,9,11,12 Noble-metal catalysts perform efficiently but suffer from the scarcity and high cost.13–17 Great recent attention has thus focused on non-noble-metal alternatives.18−32 Such catalysts however may release metal ions causing the issue of environmental pollution. In this regard, it is highly attractive to identify metal-free materials for efficient N2 reduction electrocatalysis.

Due to the unique properties, 2D materials hold great promises for technological applications and fundamental sciences.33,34 In recent years, elemental 2D materials as the chemically simplest cases have emerged as strong contenders in the realm of 2D materials.35,36 2D materials are reported as efficient NRR electrocatalysts, including B-doped graphene,37 polymeric carbon nitride sheet,38 B4C nanosheet,39 and black phosphorus.40 All these catalysts however perform in acids causing corrosion issue. Different from other well-known 2D materials, 2D boron is electron-deficiency,41 offering great benifit to N2 activation,42 but its use for NRR electrolysis remains unexplored before. Here, we report the first experimental observation that boron nanosheet (BNS) acts as a metal-free elemental 2D material toward artificial NH3 synthesis with excellent selectivity at neutral pH. In 0.1 M Na2SO4, it achieves a high Faradaic efficiency (FE) of 4.04% and a large NH3 yield of 13.22 µg h−1 mg−1cat. at –0.80 V vs. reversible hydrogen electrode (RHE), with strong electrochemical durability. Density functional theory (DFT) calculations suggest that the B atoms of both oxidized and Hdeactivated BNS could be more effectively catalyze NRR than clean BNS, and the rate-determination step is the desorption process of the second NH3 gas. RESULTS AND DISCUSSION Figure 1a shows X-ray diffraction (XRD) patterns for bulk B powder and its exfoliated product (see Supporting

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Information for preparative details). Bulk B shows diffraction peaks at 11.1°, 16.2°, 17.5°, 19.0°, 19.7°, 20.1°, 26.6°, 36.3°, 37.6°, and 38.8° that are indexed to the (003), (110), (104), (021), (113), (202), (205), (217), (119), and (042) planes for B (JCPDS No. 11-0618), respectively. BNS also shows characteristic peaks of bulk B, indicating BNS retains the crystallinity of the original bulk B. After liquid exfoliation of bulk B, the presence of nanosheet can be clearly seen from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure 1b,c. As revealed by high-resolution TEM (HRTEM) images (Figure 1d), the spaces of the lattice fringes are 0.506 nm, 0.795 nm, and 0.466 nm, which are consistent with the (104), (003), and (021) planes of B, respectively. The atomic force microscope (AFM) image and the corresponding height profile (Figure S1) show that the average thickness of BNS is about 3.0 nm. N2 adsorption–desorption isotherms of BNS and bulk B (Figure S2) suggest that BNS has a larger Brunauer-Emmett-Teller surface area (16.38 m2 g–1) than that of bulk B (7.27 m2 g–1). Figure 1e exhibits X-ray photoelectron spectroscopy (XPS) spectrum for bulk B in B 1s region. The peak with binding energy (BE) at 187.7 eV corresponds to B−B bond.43 The peaks at 188.9 and 192.1 eV are ascribed to oxidized B. Note that B 1s peak at 187.4 eV of BNS (Figure 1f) is slightly

In NRR experiment, BNS droped on carbon paper (BNS/CP, BNS loading: 0.1 mg cm–2) serves as the working electrode. All potentials were calibrated at a RHE scale. Figure S3 displays linear sweep voltammetry curves for BNS/CP in 0.1 M Na2SO4 solution. As shown, the higher current density was obtained in N2-saturated electrolyte, suggesting BNS/CP is catalytically active for N2 reduction. Figure 2a displays time-dependent current density curves for BNS/CP under different potentials in Na2SO4 solution. The produced NH3 was detected by the method of spectrophotometry with indophenol blue.46 The possible byproduct of N2H4 was examined with Watt and Chrisp’s method.47 Figure S4 and S5 present corresponding calibration curves. Figure 2b displays UV-Vis absorption spectra for electrolyte stained with indophenol indicator under different potentials, suggesting NRR experiment can operate at potentials from –0.75 to –0.95 V. The calculated results of NH3 yields and FEs are plotted in Figure 2c. Obviously, BNS/CP achieves the largest NH3 yield of 13.22 μg h–1 mg– 1 , larger than those of Pd/C (4.5 μg h−1 cm−2),16 Fe O -CNT cat. 2 3 (0.22 μg h−1 cm−2),23 and PEBCD/C (1.58 µg h−1 cm−2)48 in neutral electrolytes. It also attains the highest FE of 4.04% at the same potential, and this value also compares favourably to the behaviours of MoS2/CC (1.17%),21 Fe2O3-CNT (0.15%),23 PEBCD/C (2.85%),48 Fe3O4/Ti (2.6%),49 TiO2-rGO (3.3%),50 Mn3O4 nanocube (3.0%),51 and SnO2 (2.17%).24 A more detailed comparison is listed in Table S1. Furthmore, we prolonged reaction time of BNS/CP at various potentials in 0.1 M Na2SO4 for 4 h (Figure S6), indicating BNS/CP is an excellent electrocatalyst for NH3 synthesis. When the applied potential is below –0.80 V, hydrogen evolution reaction (HER) becomes the primary process resulting in decreased NH3 yields and FEs.39

shifted compared to bulk B. This phenomenon is in agreement with some reported 2D B nanosheets.44,45 Moreover, B 1s peak at 192.1 eV of bulk B disappears after liquid exfoliation on account of interactions between the B sheets and solvent. The peak with BE at 190.4 eV of BNS possibly is attributed to B−N bond due to air contamination.

Figure 2. (a) Time-dependent current density curves of BNS/CP under various potentials in 0.1 M Na2SO4. (b) UV-Vis absorption spectra of the electrolytes stained with indophenol indicator after NRR electrolysis. (c) NH3 yields and FEs for BNS/CP under the corresponding potentials. (d) The mass of produced NH3 with different electrodes at –0.80 V after 2 h electrolysis.

Figure 1. (a) XRD patterns of bulk B and BNS. (b) SEM image for bulk B. (c) TEM and (d) HRTEM images for BNS. XPS spectra for (e) bulk B and (f) BNS in the B 1s region.

To verify that NH3 is produced through NRR process of BNS/CP, we separately carried out electrolysis in N2-saturated solution under open circuit potential and Ar-saturated solution at –0.80 V for 2 h. As expected, Figure S7 reflects that almost

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no NH3 was detected for both cases. The experiments of BNS/CP were performed in N2- or Ar-bubbled electrolyte (0.1 M Na2SO4) with no electrochemistry reaction and N2- or Arbubbled electrolyte at –0.80 V for 2 h (Figure S8). After eliminating background of NH3, BNS still achieves a high FE of 4.04% and a large NH3 yield of 13.22 µg h−1 mg−1cat.. Of note, we also failed to detect N2H4 in N2-saturated solution after 2 h electrolysis at –0.80 V (Figure S9), reflecting BNS is an excellent NRR electrocatalyst with good selectivity for NH3 formation. As shown in Figure S10, the values of UV-Vis absorption spectra for blank CP and bulk B/CP are very small. We further quantified the NH3 produced on blank CP, bulk B/CP, and BNS/CP after 2-h electrolysis (Figure 2d). Obviously, both blank CP and bulk B/CP achieve a very small amount of NH3 (0.13 and 0.17 µg, respectively). In sharp contrast, the amount of produced NH3 for BNS/CP is 2.64 µg,

suggesting BNS is highly active for NRR. Such superior NRR performance for BNS/CP can be attributed to the following two reasons. (1) 2D nature for BNS favours the exposure of more active sites (Figure S11), which are advantage for the NRR process. (2) The electrochemical impedance spectroscopy data (Figure S12) display that BNS/CP possesses a smaller radius comparing to bulk B/CP, suggesting that BNS/CP catalyst has a lower charge transfer resistance.52 Figure 3. (a) Recycling tests for BNS/CP during NRR at –0.80 V. (b) Time-dependent current density curve of BNS/CP at –0.80 V. (c) NH3 yields and corresponding FEs for BNS/CP with the intervals of 2 h cycles in N2- and Ar-saturated electrolytes. (d) The mass of produced NH3 vs. time recorded at –0.80 V.

Moreover, stability is a key parameter to evaluate catalyst performance. Recycling experiment was tested in Na2SO4 solution and both NH3 yields and FEs reveal a slight fluctuation (Figure 3a), indicating BNS/CP possesses stable NRR performance. As shown in Figure 3b, the current density shows almost no fluctuation at −0.80 V for 24 h electrolysis. After 24 h electrolysis, the NRR performance of the BNS/CP was tested. The UV-Vis absorption spectrum for fresh electrolyte after 2-h electrolysis (Figure S13) shows the similar intensity at −0.80 V as that shown in Figure 2b, indicating BNS/CP is still highly active for N2 reduction. Moreover, UV-Vis absorption spectrum of electrolyte after 24 h electrolysis was also provided in Figure S14, suggesting BNS/CP with strong electrochemical stability. After long-term

NRR electrolysis, XRD analysis confirms that this catalyst is still boron in nature (Figure S15) and TEM image shows that this catalyst still maintains its nanosheet morphology (Figure S16). There is no obvious change in XPS spectrum of BNS in the B 1s region after stability test, as shown in Figure S17. Figure S18 shows a small amount of N for BNS before and after long-term NRR electrolysis. We performed a total of 12 h cycles with the intervals of 2 h in N2- and Ar-saturated electrolytes at −0.80 V and found that NH3 was just formed in N2-saturated electrolyte (Figure 3c). It can be observed the mass of NH3 nearly linearly increases with increased electrolysis time, as shown in Figure 3d. In addition, we carried out electrocatalytic NRR performance of the BNS/CP in 0.1 M HCl and 0.1 M KOH solution. Results suggest that the NRR performance of BNS under both of these conditions is inferior to that of in 0.1 M Na2SO4 solution (Figure S19 and S20). To verify N source of generated NH3, we also performed the 15N isotopic labeling experiment. Figure S21 displays 1H nuclear magnetic resonance spectra. 15NH4+ signals were detected when 15N2 was bubbled into the cathode.

Figure 4. (a) The optimal structure of N2 adsorption with end-on coordination on 3B and 3B(H) sites. (b) The electron density difference after N2 adsorption on 3B and 3B(H) sites, yellow and blue colors represent electron accumulation and depletion, respectively, and the isosurface level is 0.01e/Bohr3. (c,d) The corresponding free energy diagram of NRR on B (104) surface at U= –0.80 V. (e) The DOS of 3B(H) site before and after N2 adsorption. B, pink; N, blue; H, white.

DFT calculations on the active sites and the corresponding electronic structure properties were performed to gain further insight into the NRR mechanism at an atomic scale. Based on the result of HRTEM images, we considered (104), (003), and (021) surfaces for the following NRR. The B (104) surface possesses eight kinds of terminations, and the surface structure (Figure S22a) has the lowest surface energy (Ef =0.273 eV) and thus was used for the following calculations. The spin electronic structure of optimal B (104) surface (Figure S23) shows that surface B atom could fall into three categories, namely, 1B, 2B, and 3B, and these B atoms should experience spin polarization with the magnetic moment of 0.13, 0.25, and 0 μB, respectively. We calculated the density of states (DOS) of 1B, 2B, and 3B atoms before N2 adsorption (named as 1B before, 2B before, and 3B before) and its electronic state appears around Fermi energy, as shown in Figure S24. The 2B site is selected to examine the effect of slab thickness on N2 adsorption, and calculation results show that the adsorption

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energy of N2 molecule is unchanged since it adsorbs on the different slab thickness from 9.98 to 14.85 Å, indicating 9.98 Å is enough, as shown in Figure S25. Subsequently, the N2 molecular adsorption on 1B, 2B, and 3B sites with end-on coordination and side-on coordination between 1B and 2B are considered, as shown in Figure 4a (up panel) and Figure S26a,b,c. The calculation results show that N2 molecule prefers to adsorb on edge 3B atom by end-on coordination with ∆G (3B) = –0.66 eV in Figure 4a (up panel) and the bond elongation of N2 molecule is changed from 1.112 to 1.132 Å. The calculated electron density difference (∆ρ = ρN2/3B (104) - ρN2 – ρ3B (104)) suggests that the strong electron accumulations between N2 and 3B atom in Figure 4b (up panel). Bader charge analysis indicates that 0.45e- is injected into the N2 molecule. The DOS of 1B, 2B and 3B atom sites after N2 adsorption (named as 1B after, 2B after, and 3B after) show that there is an obvious hybridization between the B-2p orbital and *N2-2p orbital below the Fermi energy, which correspond to  donation and  back-bonding, respectively, as shown in Figure S24, similar to Légaré's work.53 As well known, hydrogenation of N2 is carried out by adding H atoms one by one to the adsorbed species with a distal or alternating mechanism. We considered 1B, 2B, and 3B active sites for NRR and calculation results show that the rate-determining step of NRR on 1B, 2 B, and 3B active sites is desorption of first or second NH3 gas in free energy (2.21 ~ 2.54 eV), indicating that the formed NH3 will be difficult to remove, as shown in Figure 4c and S27a,b. The atom configurations of NRR process on B (104) surface are presented in Figure S28. However, the desorption of NH3 gas with higher energy demand is difficult to occur at room temperature. Therefore, we consider to increase NH3 coverage from original 1/6 ML to 1/3 ML on B (104) surface, and then results show that desorption energies of first and second NH3 gas are decreased to the range of 0.49 to 1.11 eV importantly. These values could occur thermally at room temperature. It suggests that under experiment condition, the NRR process on catalyst surface has a concentration effect and it could promote NH3 gas desorption obviously, as shown in Figure S29. For HER on B (104) surface, the 3B (104) active site prefers to proceed HER with ΔG (*H) = 0.23 eV and other boron sites (1B and 2B) have higher free energy (-1.67 ~ -1.86 eV), indicating 1B and 2B sites may be deactivated by H atom, as shown in Figure S30. Therefore, we also considered the B (104) surface in which the 1B and 2B sites are deactivated by H atom (named as B(104) H surface) in Figure S26d. Then the NRR on 3B(H) active site is explored, in Figure 4d. The calculation results show that free energy change of N2 adsorption (∆G (3B, H)) is –0.69 eV and the following NRR only experiences distal mechanism on 3B(H) site, in which the hydrogenation of N2 is downhill pathway until hydrogenation process of *NH2 to *NH3 and the remaining hydrogenation processes of *NH2 to *NH3 and *NH3 to * are uphill pathway. Surprisely, the free energy of rate-determining step is decreased to 1.80 eV and HER becomes -1.17 eV. It suggests that H-deactivated B (104) surface could effectively catalyze NRR and suppress HER, and 3B(H) atom site serves as the main active site for the NRR process. The atom configurations of NRR process on 3B (104) H surface are presented in Figure S31. Subsequently, we also discussed the electronic structure

before and after N2 adsorption, and there is an obvious hybridization between the B-2p orbital and *N2-2p orbital below the Fermi energy. This hybridization mainly originates from B(2pz)-*N2(2pz) at -11.5 ~ -12.5 eV and B(2px,2py)*N2(2px,2py) at -6.0 ~ -7.0 eV, which correspond to  donation and  back-bonding, respectively, as shown in Figure 4e. The calculated electron density difference (∆ρ = ρN2/3B (104) H - ρN2 – ρ3B (104) H) also suggests that the strong electron accumulations between N2 and 3B(H) atom in Figure 4b (down panel). Bader charge analysis indicates that 0.38e- is injected into the N2 molecule. The bond elongation of N2 molecule is changed from 1.112 to 1.129 Å in Figure 4a (down panel). On the other hand, XPS shows that BNS is partially oxidized to form B-O bond. Thus, the oxidized B (104) surface is considered, as shown in Figure S32a,b,c. The calculation results show that 3B site prefers to be oxidized (in Figure S32c), and then is quickly saturated by H atom to form O-H bond (Figure S32d) rather than B-H bond (Figure S32e). Subsequently, NRR on oxidized B (104) surface is calculated and results show that N2 molecule is effectively activated because bond length is elongated to 1.127 Å and charge transfer is 0.33e-, as shown in Figure S33 inset. The desorption of second NH3 gas is rate-determining step with an uphill ∆G of 1.85 eV, as shown in Figure S33. The atom configurations of NRR process on oxidized B (104) surface are presented in Figure S34. The oxidized BNS prefers to proceed NRR than clean BNS and closes to H-deactivated BNS. The oxidized BNS also shows a poor HER performance due to higher free energy change with -1.26 eV, as shown in Figure S30. For (021) surface, the possible adsorption sites (1B and 2B) are considered, in Figure S35. The calculations results show that the rate-determining step is desorption of second NH3 gas with ΔG = 1.98eV in Figure S27c,d, and the atom configurations of NRR process on 1B (021) and 2B (021) sites are presented in Figure S36. The (003) surface B atom should be deactivated by *NH2 in Figure S37 and HER is suppressed due to ∆G (*H) = -1.40 eV in Figure S30. Finally, we compared the electronic structures between bulk B and BNS, and the DOS shows that the former is semiconductor with a band gap of 1.21 eV and the latter is conductor, as shown in Figure S38. It suggests that the conductor is more advantage to NRR than semiconductor in this work. CONCLUSION In summary, BNS is experimentally proved as an efficient metal-free electrocatalyst for ambient N2–to-NH3 fixation with excellent selectivity. It achieves a NH3 yield of 13.22 µg h–1 mg–1cat. and a FE of 4.04% at –0.80 V in 0.1 M Na2SO4, with strong electrochemical stability. DFT calculations suggest that the B atoms of both oxidized and H-deactivated BNS more prefer to proceed NRR than clean BNS, and the ratedetermination step is the desorption process of second NH3 gas. It is the first demonstration of 2D B material for electrocatalytic NH3 synthesis in neutral media and would open up an exciting new avenue to the rational design and development of B-based nanocatalysts for artificial N2 fixation.

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Supporting Information Experimental details; AFM; N2 adsorption–desorption isotherms; Linear sweep voltammetry curves; UV-Vis absorption spectra and calibration curves; Time-dependent current density curves; NH3 yields and FEs; Nyquist plots; XRD pattern; TEM image; XPS spectra; 1H NMR spectra; Surface terminations; Spin electronic structure; DOS; Thickness test; Adsorption sites; Free energy diagram; NH3 gas desorption; Oxidized B; Atom configurations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.L); [email protected] (X.S.) #X.Z.

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and T.W. contributed equally to this work (20)

Notes The authors declare no competing financial interest.

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Boron nanosheet (BNS) is verified as a metal-free elemental 2D material for ambient N2-to-NH3 conversion with excellent selectivity. In 0.1 M Na2SO4, this BNS attains a high Faradaic efficiency of 4.04% and a large NH3 yield of 13.22 µg h−1 mg−1cat. at –0.80 V vs. reversible hydrogen electrode. Density functional theory calculations suggest that the B atoms of both oxidized and H-deactivated BNS could be more effectively catalyze NRR than clean BNS.

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