Electronically Coupled SnO2 Quantum Dots and Graphene for

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Electronically Coupled SnO2 Quantum Dots and Graphene for Efficient Nitrogen Reduction Reaction Ke Chu, Ya-ping Liu, Yu-biao Li, Jing Wang, and Hu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08055 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Electronically Coupled SnO2 Quantum Dots and Graphene for Efficient Nitrogen Reduction Reaction Ke Chu 1*, Ya-ping Liu 1, Yu-biao Li 1, Jing Wang1, Hu Zhang 2 1

School of Materials Science and Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China 2 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China *Corresponding author. E-mail address: [email protected] (K. Chu)

Abstract Electrocatalytic N2 reduction reaction (NRR) provides an effective and renewable approach for artificial NH3 production, but still remains a grand challenge because of the low NH3 yield and Faradaic efficiency (FE). Herein, we reported that the SnO2 quantum dots (QDs) supported on reduced graphene oxide (RGO) could efficiently and stably catalyze NRR at ambient conditions. The NRR performance of resulting SnO2/RGO was studied by both experimental techniques and density functional theory (DFT) calculations. It was found that the ultrasmall SnO2 QDs (2 nm) grown on RGO could provide abundant sites for efficient N2 adsorption. Significantly, the strongly electronically coupled SnO2 QDs and RGO brought about the enhanced conductivity and the decreased work function, which led to a considerably lowered energy barrier of *N2 → *N2H that was the rate-determining step of NRR process. Meanwhile, the SnO2/RGO exhibited an inferior HER activity. As a result, the SnO2/RGO delivered a high NH3 yield of 25.6 μg h−1 mg−1 (5.1 μg cm-2 h-1) and an FE of 7.1% in 0.1 M Na2SO4 at −0.5 V (vs. RHE), together with the outstanding selectivity and stability, endowing it a promising electrocatalyst for N2 fixation. KEYWORDS: Electrocatalytic N2 reduction reaction; SnO2 quantum dots; Graphene hybrid; Density functional theory; N2 activation.

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INTRODUCTION The ever-growing global energy demands and the excessive depletion of fossil fuels have stimulated scientists to search for clean and sustainable energy sources. Ammonia (NH3), with high energy density and renewability, is a very promising candidate to replace fossil fuels1. Currently, the commercial NH3 production is still heavily dominated by the Haber-Bosch process under harsh conditions, giving rise to the intensive energy consumption and serious CO2 emissions2. It is therefore highly imperative to explore efficient, energy-saving and environmentally friendly routes enabling NH3 production at milder conditions. Electrocatalytic N2 reduction reaction (NRR) offers a simple and renewable approach for artificial N2 fixation at ambient conditions3-4. However, the kinetics of NRR is very sluggish due to the extremely low N2 solubility in aqueous solution, poor adsorption of N2 on catalyst surface and large energy barrier for the cleavage of strong N-N triple bond5. In addition, when catalyzing NRR, the competing hydrogen evolution reaction (HER) takes place synchronously6-7, giving rise to the low NRR selectivity and faradaic efficiency (FE). To circumvent these problems, intensive efforts have been undertaken lately to develop efficient electrocatalysts to boost the NRR kinetics while concurrently minimizing the adverse HER. Precious-metal-based catalysts, including Au8 and Ru9 have been proved to be the efficient electrocatalysts to catalyze NRR, but the scarce reserves of precious-metals impede their wide and practical applications. Accordingly, various earth-abundant transition-metal-based oxides10-17, nitrides18-21, sulfides22-25 and carbides26-28 have been developed, and some of them, such as Mo2C nanorod28 and single-atom Fe on N-doped carbon29, delivered the exceptional NRR performances comparable to or even higher than those of precious-metal-based catalysts. The high NRR activity of transition-metal-based catalysts originates from the availability of d-orbital electrons for π-back-donation of N-N30, promoting the adsorption and activation of N2. Nevertheless, the d-orbital electrons of transition metals also favor the adsorption of *H to form strong metal-H bonds31, resulting in the enhanced HER 2


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and thus compromised FE of NRR. In this context, the researchers recently begin to focus on new types of catalysts coming from the main group metals (Bi, Sn) with weak hydrogen adsorption32-35. The Bi-based catalysts, including mosaic Bi nanosheets32, Bi nanocrystals33 and flower-like β-Bi2O334 have been developed to show fascinating NRR performances, particularly high FE. From the other side, Sn-based materials, especially Sn oxides (i.e., SnO2), have been widely employed for the

electrocatalytic

applications

of

lithium-ion

batteries36,

fuel

cells37,

supercapacitors38 and electrochemical sensing39. The recent report also demonstrates the prominent performance of SnO2 for electrocatalytic CO2 reduction reaction (CRR)40. Given that Sn possesses an analogous electronic structure with Bi, and NRR has a lot in common with CRR, it is believed that the Sn-based electrocatalysts may hold great promise for achieving desired NRR activity. However, to date only one work has reported the Sn-related catalyst (SnO2 sub-micron particles35) toward NRR, where the obtained NRR yield (4.03 μg h−1 mg−1) and FE (2.17%) are relatively low. Such low NRR performance is due possibly to the limited active sites and low conductivity of SnO2 sub-micron particles. Herein, we reported that the SnO2 quantum dots (QDs) supported on reduced graphene oxide (SnO2/RGO) could efficiently and stably catalyze NRR at ambient conditions. It was demonstrated that the electronically coupled SnO2 QDs and RGO could lead to the fascinating NRR activity, that was, NH3 yield of 25.6 μg h−1 mg−1 (5.1 μg cm-2 h-1) and FE of 7.1% in 0.1 M Na2SO4 at −0.5 V (vs. RHE), outperforming the most recently reported NRR electrocatalysts. Density functional theory (DFT) calculations were conducted to further reveal the underlying NRR mechanism of SnO2/RGO.

RESULTS AND DISCUSSION SnO2/RGO was synthesized by a self-propagating combustion (SPC) method16, 41, as displayed in Figure S1 (Supporting Information). The X-ray diffraction (XRD) pattern (Figure 1a) reveals characteristic diffraction peaks matching well with the tetragonal SnO2 phase. The lack of graphene phase is due primarily to the overlap of 3



SnO2 (110) peak and graphene (002) peak and the low diffraction intensity of graphene42-43. The mass percentage of SnO2 QDs in SnO2/RGO is determined to be 41.6 wt% by thermogravimetric (TG) measurement (Figure S2). The transmission electron microscopy (TEM) image of SnO2/RGO (Figure 1b) presents the typically wrinkled structure of graphene sheets, and the magnified image (Figure 1c) discloses the amounts of SnO2 QDs that are densely and homogeneously distributed on RGO. The statistical size distribution (Figure 1c, inset) shows an average size of 2 nm for SnO2 QDs.

Figure 1. Characterizations of SnO2/RGO: (a) XRD pattern. (b, c) TEM images (inset of c: SnO2 QD size distribution). (d) HRTEM image. (e) STEM element mapping images.

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The selected area electron diffraction (SAED) pattern (Figure S3) imaged from the SnO2 QDs displays several diffraction rings, which are assigned to the tetragonal structure of SnO2, in concert with the XRD data (Figure 1a). The high-resolution transmission electron microscopy (HRTEM) image reveals an interplanar spacing of 0.33 nm (Figure 1d), which corresponds to (110) lattice plane of tetragonal SnO2. From the scanning transmission electron microscopy (STEM) elemental mapping images, a homogeneous distribution of C, O, and Sn can be evidenced, suggesting once again the even distribution of SnO2 QDs on RGO. The RGO plays a critical role in strongly anchoring SnO2 QDs and preventing their aggregation. Without RGO support, the bare SnO2 QDs are found to be heavily agglomerated (Figure S4), leading to the much reduced specific surface area (Figure S5) and thus limited active sites, which are unfavourable for electrocatalytic activity.

0

OKLL

Sn3p3/2

Sn3p1/2

(b)

200

400

600

800

1000

Binding energy (eV)

(d) Intensity (a.u.)

(c)

Sn3d5/2 Sn3d3/2 O1s

C1s

Intensity (a.u.)

(a)

Sn4d

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


ID/IG=0.96

SnO2/RGO RGO

ID/IG=1.05

GO

ID/IG=0.88

1000

1500

-1

2000

Raman shift (cm )

Figure 2. (a) XPS survey spectrum of SnO2/RGO. (b) XPS C1s spectra of SnO2/RGO. (c) XPS Sn3d spectra of SnO2/RGO and bare SnO2 QDs. (d) Raman spectra of GO, RGO and SnO2/RGO.

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The chemical valence state of elements in SnO2/RGO is studied by X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum (Figure 2a) shows the presence of C1s, O1s, Sn4d, Sn3d and Sn3p peaks, with no evidence of impurities. The C1s spectra (Figure 2b) can be divided into three peaks of C=C/C-C, C-O-C/C-O and O=C-O/C=O. In comparison with the deconvoluted C1s spectra of pristine GO (Figure S6a), the sharp decrease in the peak intensity of C-O-C/C-O and O=C-O/C=O for SnO2/RGO means the efficient GO reduction during the SPC process. The O1s spectra (Figure S6b) show three deconvoluted peaks of Sn-O bond (530.9 eV), C−O (531.8 eV) and C−O−C (533.4 eV), in which the latter two indicate the small amount of residual oxygen functionalities left on RGO. Furthermore, the Sn3d can be deconvoluted into two peaks (Figure 2c), located at 487.7 and 496.2 eV, which are assigned to Sn3d5/2 and Sn3d3/2, respectively. Notably, compared to Sn3d spectra of bare SnO2 QDs, there is a positive shift of 0.58 eV for Sn3d spectra of SnO2/RGO. Likewise, a negative shift of 0.34 eV can be found in the C1s spectra of SnO2/RGO relative to RGO alone (Figure S7). These indicate that the SnO2 QDs and RGO are electronically coupled in SnO2/RGO. The strong interaction between RGO and SnO2 QDs can be further corroborated by the almost unchanged fraction of SnO2 QDs on RGO after a long ultrasonication treatment of SnO2/RGO (Figure S8). Raman spectroscopy is employed to examine the graphene defect level of the samples, which is defined as the intensity ratio of D-band to G-band (ID/IG)44. It is shown in Figure 2d that the ID/IG of RGO (1.05) is much larger than that of GO (0.88), which is ascribed to the more exposed defect sites on RGO after elimination of most oxygen functional groups45-46. However, the ID/IG of SnO2/RGO (0.96) is reduced compared to that of RGO (1.05), due primarily to the growth of SnO2 QDs at the defect sites of RGO, which suppresses the D-band vibration and leads to a reduced ID/IG47. These collective characterizations demonstrate the successful synthesis of SnO2/RGO comprised of uniformly-distributed ultrafine SnO2 QDs (2 nm) that are strongly anchored on the defect sites of RGO.

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Figure 3. (a) Time-dependent current densities of SnO2/RGO at various potentials. (b) UV-Vis absorption spectra of the electrolytes after 2 h electrolysis on SnO2/RGO at various potentials, and (c) corresponding NH3 yields (columns) and FEs (stars). (d) NH3 yields (columns) and FEs (stars) of SnO2/RGO, bare SnO2 QDs, RGO and SnO2+RGO at -0.5 V. The ambient NRR performance of SnO2/RGO (0.2 mg/cm2) attached on carbon paper (CC) is explored in 0.1 M Na2SO4, utilizing an electrochemical H-cell (Figure S9). As shown in Figure S10, the linear sweep voltammetry (LSV) curves reveal that the SnO2/RGO shows an enhanced current density in N2 solution relative to Ar solution within the potential range of −0.4 to −0.9 V, implying that the SnO2/RGO possesses the prominent NRR activity. On the basis of LSV curves, the potential window is selected in the range of −0.4 ~ −0.9 V to further examine the NRR performance of SnO2/RGO. After NRR electrolysis for 2 h at various potentials (Figure 3a), the produced NH3 and possible N2H4 are spectrophotometrically analyzed by the indophenol blue method (Figure S11) and the Watt and Chrisp method (Figure S12), respectively. No byproduct of N2H4 can be detected (Figure S13), suggesting the good NRR selectivity of SnO2/RGO. As depicted in Figure 3a, the time-dependent

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current densities at various potentials show the stable current density over 2 h electrolysis. Based on the UV-vis absorption spectra of the electrolytes after catalyzing on SnO2/RGO at various potentials (Figure 3b), the NH3 yields and FEs can be obtained, as shown in Figure 3c. With the potential reaching -0.5 V, the NH3 yield and FE of SnO2/RGO both achieve the maximum of 25.6 μg h−1 mg−1 (5.1 μg cm-2 h-1) and 7.1%, respectively. It is important to point out that the NRR performance of SnO2/RGO is comparable to or higher than that of most recently reported NRR catalysts as shown in Table S1, suggesting the favorable NRR activity of SnO2/RGO. However, when the potential exceeds −0.5 V, the FE and NH3 yield of SnO2/RGO both reduce dramatically, which can be ascribed to the enhanced HER that prohibits the N2 adsorption on the active sites (Figure S14)48. To further explore the NRR activity of SnO2/RGO, the controlled samples including SnO2 QDs and RGO alone, and physically mixed SnO2+RGO are prepared and their NRR properties are studies under the same conditions. It is seen in Figure 3d that the SnO2 QDs and RGO alone both show much reduced NH3 yield and FE compared to those of SnO2/RGO. Also, the NRR performance of SnO2+RGO is much inferior to that of SnO2/RGO. Additionally, the SnO2/RGO requires a relatively lower potential of -0.5 V to reach the optimum performance compared to -0.7 V for bare SnO2 QDs and -0.6 V for SnO2+RGO, indicating the reduced NRR kinetic barrier and improved conversion efficiency of SnO2/RGO. To uncover the coupling effect of SnO2 QDs and RGO in enhancing the NRR activity, the electrochemical active surface area (ECSA) and electrochemical impedance spectroscopy (EIS) measurements are performed. ECSA results (Figure S15) indicate that the SnO2/RGO displays the highest double-layer capacitance (Cdl) value of 17.6 mF cm-2, followed by SnO2+RGO (8.9 mF cm-2), bare SnO2 QDs (6.3 mF cm-2) and RGO (1.8 mF cm-2). The rather small Cdl value of RGO indicates the very limited active sites of RGO, accounting for its poor NRR performance, in consistence with the literature results26,

49.

Nevertheless, RGO as a support can

effectively avoid the self-aggregation of SnO2 QDs, enabling SnO2 QDs to expose more active sites than bare SnO2 QDs that are heavily aggregated (Figure S4). The 8


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local aggregation of SnO2 QDs still exists in SnO2+RGO mixture (Figure S16), rationalizing its moderate Cdl value. This discloses that the in-situ growth of SnO2 QDs on RGO effectively restricts the aggregation and detaching of SnO2 QDs, leading to the better SnO2 QD distribution and exposure of more active sites with respect to the physically mixed SnO2+RGO. Temperature-programmed desorption (TPD) is conducted to further investigate the N2 adsorption behavior, as detailed in Figure S17. Interestingly, the N2 chemisorption ability on the different samples follows the same order as ECSA, that is, SnO2/RGO > SnO2+RGO > SnO2 QDs > RGO, suggesting that the more exposed sites are favorable for the better N2 adsorption. Thus, the well-distributed and ultrasmall SnO2 QDs grown on RGO can provide abundant active sites for efficient N2 adsorption on SnO2/RGO. EIS is conducted to study the electrode kinetics of various catalysts. The Nyquist plots (Figure S18) show that the SnO2/RGO delivers the lower electron transfer resistance than bare SnO2 QDs, indicating that the RGO support can largely enhance the conductivity of SnO2 QDs. In addition, the charge transfer resistance of SnO2/RGO is also smaller than that of SnO2+RGO, suggesting that the strongly coupled SnO2/RGO derived from the in-situ growth of SnO2 QDs on RGO can greatly accelerate the electrode charge transfer. The enhanced charge transport kinetics of SnO2/RGO enables the attainment of electrocatalytic NRR at relatively low overpotential (Figure 3d). Therefore, the strongly electronically coupled SnO2 QDs and RGO largely promote the NRR activity of SnO2/RGO by not only affording more active sites for N2 adsorption, but also enhancing the electron transfer for accelerating the NRR kinetics. To confirm whether the source of NH3 comes from the NRR, the controlled experiments are firstly performed by UV-Vis analysis at the optimum potential of -0.5 V. As shown in Figure 4a, compared with the conspicuous NH3 yield in N2-saturated solution, almost no NH3 yield can be detected after electrolysis in Ar-saturated solution, or in N2-saturated solution at open-circuit, or in N2-saturated solution on bare CC as working electrode. The origination of N source is further examined by the isotopic labelling measurements26,

30.

It is shown in Figure 4b that a distinguished 9



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triplet (or doublet) coupling representing 14NH4+ (or 15NH4+) can be found when using 14N

2

(or

15N

2)

as feeding gas, but no

14NH + 4

(15NH4+) signal can be detected when

using Ar as feeding gas. Furthermore, the alternating cycling test (Figure S19) by switching electrolysis between Ar-saturated and N2-saturated solutions indicates that the pronounced NH3 yield is only detected in N2-saturated solution but not in Ar-saturated solution50. Moreover, the time-dependent test (Figure S20) shows that the NH3 formation is linear with electrolysis time, indicating that the NH3 is generated continuously by the NRR. These results surely prove that the detected NH3 stems solely from the NRR on SnO2/RGO.

Figure 4. (a) NH3 yields derived from the UV-Vis absorption spectra (inset) of the electrolytes after 2 h electrolysis in various controlled conditions. (b) 1H NMR spectra of 14NH4+ and 15NH4+ standard samples, and the electrolytes after NRR catalyzing on SnO2/RGO for 2 h using 14N2, or 15N2 or Ar as feeding gases. (c) Chronoamperometry test (4 h) of SnO2/RGO at various potentials. (d) Cycling test of SnO2/RGO and corresponding NH3 yield (column) and FE (star) of SnO2/RGO after each cycle. The stability is also an important index to evaluate the performance of electrocatalysts

for

practical

applications.

As

shown

10


in

Figure

4c,

the

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chronoamperometry test of SnO2/RGO indicates that the current density keeps stable at each given potential after 4 h (14400 s) continuous electrolysis. The obtained NH3 yield and FE do not show an obvious change when compared with those after 2 h electrolysis (Figure S21). The SnO2/RGO even displays a table current density after 10 h continuous electrolysis (Figure S22), with 94.1 % retention of its initial NH3 yield (Figure S23), demonstrating its excellent long-term stability. In addition, as shown in Figure 4d, during seven cycles of chronoamperometric runs (Figure S24), no obvious change in NH3 yield/FE can be observed, implying the good cycling stability. Furthermore, the characterizations of SnO2/RGO after NRR stability test reveal well-matched SnO2 phase from XRD (Figure S25), well-retained morphology from TEM (Figure S26), and well-maintained chemical state from XPS (Figure S27), proving the excellent structural stability of SnO2/RGO. These results corroborate the outstanding stability of SnO2/RGO, which is attributed to the strongly anchored SnO2 QDs on RGO, as evidenced by the previous XPS (Figure 2c) and TG analysis (Figure S8). Taken together, all above results demonstrate that the SnO2/RGO exhibits a favorable NRR performance with high NH3 yield/FE achieved at relatively low potential, as well as the outstanding selectivity and stability, endowing it a promising efficient NRR electrocatalyst. To get insight into the NRR mechanism of SnO2/RGO, DFT calculations are conducted to study the electronic structures and catalytic reactions. To simplify the computational process, SnO2 QD is modeled as the SnO2 cluster composed of three SnO2 units. Two types of graphene substrate are considered, namely defect-free graphene (G) and defective graphene (DG) with monovacancy. The optimized structures, including SnO2 cluster, DG, SnO2/G and SnO2/DG are all shown in Figure S28.

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(a)

SnO2/G

(b)

(c)

SnO2/G

(d)

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SnO2/DG

SnO2/DG

Interface

Interface

(f)

(e)

Figure 5. (a, b) Charge density distributions of (a) SnO2/G and (b) SnO2/DG. Yellow and cyan regions correspond to the election depletion and accumulation, respectively. (c. d) Contour charge maps of (c) SnO2/G and (d) SnO2/DG. Red and blue regions correspond to the election accumulation and depletion, respectively. (e) DOS of SnO2, SnO2/G and SnO2/DG. (f) Work function values of SnO2, SnO2/G and SnO2/DG. Figure 5a and b display the charge density distributions of SnO2/G and SnO2/DG systems. It is apparent that the intense charge interaction takes place between SnO2 and DG in SnO2/DG (Figure 5b), whereas the weak charge interaction occurs in SnO2/G (Figure 5a). The strong electron interaction of SnO2/DG can be further revealed by the charge density contour map (Figure 5c and d), in which the electronic density accumulates more prominently at SnO2-DG interface (Figure 5c) than at

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SnO2-G interface (Figure 5d). Thus, the graphene defects help to firmly anchor SnO2 on graphene surface, in good accordance with the Raman results (Figure 2d). This can be explained that the defects can trap metastable electrons that are liable to transfer into the valence band of anchored SnO251, promoting the immobilization of SnO2 on DG. The calculated density of states (DOS, Figure 5e) reveals that the electronically coupled SnO2 and DG (SnO2/DG) can positively shift the valence band of SnO2 toward the Fermi level, leading to the narrowed bandgap and enhanced conductivity. Furthermore, the calculated work function (Figure 5f and Figure S29) indicates that the SnO2/DG exhibits a smaller work function (Ф) of 4.831 eV than SnO2 (6.128 eV) and SnO2/G (5.108 eV), manifesting that the SnO2/DG delivers an increased Fermi level energy that facilitates the donation of more valence electrons to the absorbed N2 molecular52. Therefore, the strongly electronically coupled SnO2/DG brings about the enhanced conductivity and decreased work function, which are expected to enhance the NRR activity by accelerating the reaction kinetics and promoting the N2 absorption and activation. For NRR, the catalytic activity of a catalyst is mainly determined by the initial N2 adsorption and following binding energies of reactive intermediates3. Specially, the first hydrogenation step (*N2 → *N2H) resulting in the *N2H formation is commonly regarded as the most difficult and critical step for the whole NRR process5, 53.

As shown in Figure 6a, for NRR on SnO2/DG, the DFT results reveal that the N-N

bond is elongated from 1.103 Å to 1.125 Å for absorbed *N2 and further to 1.283 Å for *N2H. These elongated bond values are much larger than those achieved on SnO2/G (1.119 Å, 1.198 Å, Figure 6b) and SnO2 (1.107 Å, 1.162 Å, Figure 6c). To explain the enhanced *N2H binding on SnO2/DG, the projected p-orbital DOS (PDOS) of *N2H is analyzed by examining the position of the highest pelectronic states (Ep) in the valence band of *N2H. It is well established that the Ep position closer to the Fermi level (Ef) can make the anti-bonding states (σ*) of *N2H move higher with a lower occupancy, resulting in the stronger binding of *N2H on catalyst surface, and vice versa49, 54-55. Figure 6d shows the PDOS of *N2H on various catalysts. Noticeably, the Ep of *N2H on SnO2/DG is closer to the Fermi level than that on SnO2 and 13



SnO2/DG, which rationalizes the enhanced *N2H binding on SnO2/DG. As a result, the calculated formation Gibbs free energies (ΔG) of *N2 and, in particular, *N2H on SnO2/DG are both lower than those on SnO2 and SnO2/DG (Figure 6e), suggesting that the SnO2/DG is very beneficial for N-N bond cleavage and activating the NRR. The above results elucidate that the SnO2 alone and weakly coupled SnO2/G possess poor performances toward NRR, whereas the strongly electronically coupled SnO2/DG can effectively activate the NRR by lowering the energy barriers for N2 absorption and *N2H formation.

Figure 6. (a-c) Adsorption of *N2 and *N2H on (a) SnO2/DG, (b) SnO2/G and (c) SnO2. (d) PDOS of the *N2H intermediate on SnO2, SnO2/G and SnO2/DG. (e) Gibbs free energy diagrams of *N2 and *N2H formations on SnO2, SnO2/G and SnO2/DG.

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To understand the whole NRR process of SnO2/DG, the reaction pathway is further examined by calculating the ΔG values of all the possible intermediates (Figure S30). Figure 7a shows the NRR free energy diagrams of SnO2/DG with alternating and distal pathways at zero energy (U = 0 V). For both pathways, it is seen that the first hydrogenation step of *N2 →*NH2 showing the highest energy barrier of 0.93 eV, is the rate-determining step (RDS). After first hydrogenation step, the alternating pathway needs to overcome the additional high energy barrier of 0.63 eV, whereas the rest of distal reaction steps are thermodynamically downhill. Thus, the NRR of SnO2/DG prefers to proceed through the distal pathway. When applying the potential of U = −0.93 V (Figure S31), the RDS energy barrier (*N2 →*NH2) can be overcome and the NRR process is energetically downhill.

(b)

(a) Alternating Distal *NHNH *NNH 0

N2

*NNH2

*N2 RDS

*NHNH2

*NNH3 -2

*NH2NH2 *NH2

*NH

NH3

2

SnO2 Free energy (eV)

2

Free energy (eV)

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


+

0

-1

Reaction coordinate

SnO2/DG

1

-

H +e

Pt

1/2 H2

Reaction coordinate

Figure 7. (a) Gibbs free energy diagrams of NRR process for SnO2/DG (U = 0 V). (b) Gibbs free energy diagrams of *H (ΔGH*) on SnO2, SnO2/DG and reported Pt56. Data in (b) is reproduced with permission from ref (56). Copyright 2005 American Chemical Society. Aside form the high NRR activity, the weak HER activity is also critical for an efficient NRR catalyst to attain a favorable FE. For HER performance, the adsorption free energy of H* (ΔGH*) is demonstrated to be a key indicator to the catalyst activity31, and an efficient HER catalyst should possess a modest H* adsorption with a near-zero ΔGH*. As showed in Figure 7b, the predicted ΔGH* of SnO2/DG is only slightly lower than that of SnO2, which remains significantly larger than that of benchmark Pt catalyst, suggesting that the SnO2/DG possesses a poor HER activity.

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This is due likely to the difficulty in the formation of Sn-H bond causing the inferior HER. The weak HER performance of SnO2/RGO can also be experimentally confirmed, as detailed in Figure S32. Since the RGO intrinsically possesses a high level of defects (Figure S33)57-59, the SnO2/RGO can be well resembled to SnO2/DG. Therefore, the SnO2/RGO is theoretically and experimentally proved to be an efficient NRR catalyst owing to its favorable NRR activity and inferior HER activity.

CONCLUSIONS In summary, SnO2/RGO with ultrasmall SnO2 QDs (2 nm) strongly anchored on RGO was successfully synthesized. The SnO2/RGO showed a high NH3 yield of 25.6 μg h−1 mg−1 (5.1 μg cm-2 h-1) and an FE of 7.1% at −0.5 V in 0.1 M Na2SO4, far outperforming the bare SnO2 QDs and SnO2+RGO mixture, and comparing favorably to the most reported NRR catalysts. The SnO2/RGO also showed excellent selectivity and stability. Experiments indicated that the well-distributed and ultrasmall SnO2 QDs on RGO offered abundant sites for effective N2 adsorption. DFT calculations revealed that the strongly electronically coupled SnO2 QDs and RGO could enhance the conductivity and reduce the work function, which promoted the NRR activation by lowering the energy barrier of *N2 → *N2H that was the rate-determining step of whole NRR process. Meanwhile, the SnO2/RGO exhibited an inferior HER activity. Our experimental and mechanistic findings suggest that the rationally designed SnO2/RGO represents a promising direction for exploring high-efficiency Sn-based catalysts for electrocatalytic N2 fixation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details. Determination of NH3 concentration and N2H4 concentration. N2-TPD measurements. Determination of electrochemical double layer capacitances. Structures of SnO2/RGO after stability test. Additional DFT information.

AUTHOR INFORMATION 16


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Corresponding Authors *E-mail address: [email protected] Notes The authors declare no competing financial interest.

Acknowledgement This work is supported by National Natural Science Foundation of China (51761024, 51671022), Tianjin University-Lanzhou Jiaotong University Joint Innovation Fund Project, "Feitian Scholar" Program of Gansu Province, CAS "Light of West China" Program, and Foundation of A Hundred Youth Talents Training Program of Lanzhou Jiaotong University.

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