Filling the nitrogen vacancies with sulphur dopants in graphitic C3N4 for efficient and robust electrocatalytic nitrogen reduction

Journal Pre-proof Filling the Nitrogen Vacancies with Sulphur Dopants in Graphitic C3 N4 for Efficient and Robust Electrocatalytic Nitrogen Reduction Ke Chu, Qing-qing Li, Ya-ping Liu, Jing Wang, Yong-hua Cheng

PII:

S0926-3373(20)30108-9

DOI:

https://doi.org/10.1016/j.apcatb.2020.118693

Reference:

APCATB 118693

To appear in:

Applied Catalysis B: Environmental

Received Date:

27 November 2019

Revised Date:

20 January 2020

Accepted Date:

25 January 2020

Please cite this article as: Chu K, Li Q-qing, Liu Y-ping, Wang J, Cheng Y-hua, Filling the Nitrogen Vacancies with Sulphur Dopants in Graphitic C3 N4 for Efficient and Robust Electrocatalytic Nitrogen Reduction, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118693

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Filling the Nitrogen Vacancies with Sulphur Dopants in Graphitic C3N4 for Efficient and Robust Electrocatalytic Nitrogen Reduction Ke Chu*, Qing-qing Li, Ya-ping Liu, Jing Wang, Yong-hua Cheng

School of Materials Science and Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China

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*Corresponding author. E-mail address: [email protected] (K. Chu)

Highlights

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Graphical Abstract

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 Graphitic C3N4 is modulated by filling the nitrogen vacancies (NVs) with S dopants, and resultant S-NV-C3N4 could serve as an active and durable

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metal-free catalyst for electrochemical N2 reduction reaction (NRR) in neutral solution.

 S-NV-C3N4 exhibited a fascinating NRR performance with an NH3 yield of 32.7 μg h−1 mg−1 and a Faradaic efficiency of 14.1% at −0.4 V (vs. RHE), together with the outstanding durability for at least 20 h, greatly outperforming NV-containing C3N4 without S-filling.  DFT calculations revealed that filled S dopants could break the *N2H-*NH2 1

scaling relation to result in more optimized adsorption of NRR intermediates and a significantly reduced energy barrier.  This study demonstrates that filling vacancy with heteroatoms may open a new avenue to design powerful NRR catalysts for efficient electrocatalytic N2 fixation to NH3.

Abstract

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Exploring active and durable metal-free electrocatalysts represents a promising direction for electrocatalytic N2 reduction reaction (NRR). Herein, by filling the

nitrogen vacancies (NVs) with S dopants in graphitic C3N4, we showed that the

resultant S-NV-C3N4 could be an efficient and robust metal-free NRR catalyst in

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neutral solution. S-NV-C3N4 with a high S concentration of 5.2 at.% exhibited a fascinating NRR performance with an NH3 yield of 32.7 μg h−1 mg−1 and a Faradaic

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efficiency of 14.1% at −0.4 V (vs. RHE), considerably outperforming pristine C3N4 and NV-containing C3N4. S-NV-C3N4 also showed exceptional durability for at least

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20 h, in stark contrast to the inferior stability of NV-containing C3N4. Density functional theory calculations disclosed that the filled S dopants could break the

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scaling relation to effectively stabilize *N2H and destabilize *NH2 on S-NV-C3N4, leading to more optimized adsorption of NRR intermediates and a significantly reduced energy barrier. Meanwhile, the competitive HER can be effectively suppressed

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on S-NV-C3N4 due to the high energy barrier for water dissociation. Keywords: Electrocatalytic nitrogen fixation; Metal-free catalysts; Nitrogen vacancy;

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Sulphur doping; Density functional theory.

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1. Introduction Ammonia (NH3) is not only a crucial feedstock for industrial chemicals, but also a carbon-free power source in the future hydrogen economy[1]. However, current industrial-scale NH3 production heavily depends on energy- and capital-intensive Haber−Bosch process, which involves a high emission of CO2 and serious environmental concerns[2]. Therefore, it is urgently required to explore more clean and energy-saving alternative methods for artificial NH3 synthesis. Electrochemical N2 reduction reaction (NRR) using renewable electricity

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represents a green and sustainable alternative for NH3 production under ambient

conditions[3-5]. Nevertheless, the NRR process is significantly hindered by the sluggish reaction kinetics arising from poor N2 adsorption and high activation barrier,

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together with the unsatisfied Faradaic efficiency (FE) as a result of the competition with the hydrogen evolution reaction (HER)[6-8]. To overcome these obstacles, great

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efforts have been devoted to exploring efficient electrocatalysts to promote the NRR reaction kinetic while suppressing the HER[9-11]. Till now, a large number of

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metal-based catalysts have been emphasized as promising candidates for NRR, including noble metals[12-14], transition metal-based compounds[15-18] and single-metal-atom catalysts[19-21] Their performances can be further improved by

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phase manipulation[22], defect engineering[23], hetero-elemental doping[24] and heterostructure coupling[25]. Nonetheless, most metal-based catalysts have the

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inherent shortcomings of corrosion susceptibility to acidic/basic media and detrimental environmental impact[26]. To this end, a variety of metal-free materials

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have been recently developed as the promising alternatives to metal-based catalysts for NRR[27], including B4C nanosheets[28], BN nanostructures[29, 30], black phosphorus[31], boron nanosheets[32], boron-rich covalent organic frameworks (COFs)[33], and heteroatom-doped/defective carbon materials[34-37]. Some of them, such as B4C nanosheets[28] and boron-rich COFs[33], exhibit the brilliant NRR activity comparable or even superior to those of most top-performing metal-based catalysts.

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Graphitic carbon nitride (C3N4) and related materials hold great potential for exploring advanced metal-free catalysts in photocatalytic and electrocatalytic applications due to their unique 2D morphology, tunable electronic structure, rich pyridinic-N content, high durability, low cost and environmental benignity[38-40]. C3N4 has a more negative conduction position relative to reduction potential of N2/NH3, which favors the activation of adsorbed N2 to expedite the photocatalytic N2 fixation under visible-light irradiation[38]. In addition, the nitrogen vacancies (NVs) can be readily created on C3N4, which can promote the electrocatalytic N2 fixation by

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inducing the localized electrons for π back-donation of the N2 molecule [38, 41]. As reported by Yu and co-workers[41], C3N4 via engineering NVs delivered a maximum

NH3 yield of 8.09 μg h−1 mg−1, which was over tenfold enhancement with respect to unmodified C3N4. From the other side, the stability of the NVs remains to be

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confirmed. Owing to the high binding energy of N atom on a vacancy, NVs can be

easily vanish as a result of N filling[15, 17], especially under the N2-supply condition

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of NRR process, resulting in NV concentration decay and thus reduced NRR activity/stability. It has been demonstrated that NVs on a VN0.7O0.45 catalyst can be

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deactivated during the NRR process, making VN phase inactive[17]. The vacancy stability problem has also been encountered in oxygen vacancy (OV) of metal oxides

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under the oxidizing conditions of oxygen reduction reaction (OER) process[42]. In tackling OV stability problem, Xiao et al. proposed a strategy to fill OVs with P dopants in Co3O4[43], and P-modulated Co3O4 showed excellent electrocatalytic

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activity and stability for HER and OER, dramatically superior to OV-containing Co3O4. By incorporating N dopants into OVs, Cui et al. found that the filled N

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dopants could stabilize the OVs and currently boost the lithium-ion diffusion and charge transfer[44], which are advantages to effective and robust lithium-ion storage. Enlightened by these pioneering works, it is anticipated that filling NVs with heteroatoms may offer a feasible method to realize both high NRR activity and stability of the C3N4 catalyst. In this study, C3N4 was modulated by filling S dopants into the NVs, and resulting S-NV-C3N4 could serve as an efficient and durable metal-free NRR 4

electrocatalyst in neutral solution. S-NV-C3N4 with a high S concentration of 5.2 at.% exhibited a fascinating NRR performance with an NH3 yield of 32.7 μg h−1 mg−1 and an FE of 14.1% at −0.4 V (vs. RHE) in 0.5 M LiClO4, together with the outstanding durability for at least 20 h, greatly outperforming NV-C3N4 without S-filling. Density functional theory (DFT) calculations further revealed that the filled S-dopants played an essential role in boosting the NRR activity by breaking the *N2H-*NH2 scaling relation.

2. Results and discussion

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The synthetic route of S-NV-C3N4 is illustrated in Fig. 1. First, C3N4 nanosheets

were prepared by calcination of a mixture of melamine and urea in air followed by liquid exfoliation. The as-prepared C3N4 was then annealed in Ar to create NVs on

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C3N4 (NV-C3N4). Subsequently, S-NV-C3N4 was obtained by directly annealing of

NV-C3N4 in sulfur vapour, in which the NVs were in-situ occupied by S dopants. The

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synthesis details are provided in the Experimental Section (Supporting Information). According to the element analysis, N/C ratio is decreased remarkably from 1.83 (C3N4)

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to 1.51 (NV-C3N4), indicating the generation of plentiful NVs on NV-C3N4. Further sulfuration process leads to a successful incorporation of S dopants into S-NV-C3N4

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with a high S concentration of 5.2 at.%.

Fig. 1. Schematic of the synthesis process of S-NV-C3N4.

The transmission electron microscopy (TEM) images (Fig. 2a-c & Fig. S1) show that all C3N4, NV-C3N4 and S-NV-C3N4 display a crumpled sheet-like morphology[45,

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46]. Careful inspection reveals that NV-C3N4 (Fig. 2b) exhibits more wrinkled structure in comparison with pristine C3N4 (Fig. 2a), ascribed to the damaged long-range order in atomic arrangements of C3N4 after NV introduction[47, 48]. S-NV-C3N4 (Fig. 2c) shows nearly the same morphology as NV-C3N4, suggesting that S-doping process does not change the morphology of NV-C3N4. As shown in the X-ray diffraction (XRD) pattern (Fig. S2), all the samples shows a main (002) diffraction peak at around 27.5°, corresponding to the interlayer reflection of the graphitic structure[49-51]. Compared to pristine C3N4, the (002) peaks of NV-C3N4

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and S-NV-C3N4 are apparently broadened, indicating the construction of disordered structures in the atomic arrangements of NV-C3N4 and S-NV-C3N4[41], in accordance with the TEM observations (Fig. 2a-c). Furthermore, as shown in Fig. S3, NV-C3N4

(38.5 m2 g-1) and S-NV-C3N4 (34.3 m2 g-1) present a higher surface area than pristine

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C3N4 (22.7 m2 g-1), presumably arising from their more wrinkled structure (Fig. 2b, c) caused by the introduction of NVs or S-dopants. The high surface area is generally

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beneficial for the electrocatalytic activity by exposing more geometrically active sites and facilitating the accessibility of electrolyte/reactants[52].

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X-ray photoelectron spectroscopy (XPS) was used to examine the chemical bonding states of the samples. As presented in Fig. 2d, the deconvolution of N1s

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spectra showed three well-resolved peaks of C-N=C (398.7 eV), N-(C)3 (399.8 eV) and C-N-H (401.2 eV). The quantitative analysis indicates that the percent ratio of C-N=C/N-(C)3 reduces from 4.8 (C3N4) to 2.5 (NV-C3N4), revealing that N-C2

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(two-coordinated-N) vacancies are predominantly formed on NV-C3N4 rather than N-C3 (three-coordinated-N) vacancies, as illustrated in Fig. 2e. The C-N=C/N-(C)3

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ratio, however, remains almost unchanged for S-NV-C3N4 (2.3) relative to NV-C3N4 (2.5), which indicates that introduced S dopants mainly interact with C atoms rather than N atoms. This can also be confirmed by the deconvoluted C1s (Fig. 2f) and S2p (Fig. 2g) spectra, in both of which the noticeable C-S bond can be observed in S-NV-C3N4 while such C-S bond is absent in NV-C3N4.

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Fig. 2. (a-c) TEM images of (a) C3N4, (b) NV-C3N4 and (c) S-NV-C3N4. (d) XPS N1s spectra of C3N4, NV-C3N4 and S-NV-C3N4. (e) Atomic structure of NV-C3N4. (f) XPS C1s and (g) S2p spectra of NV-C3N4 and S-NV-C3N4. (h) Four possible S-doping sites in S-NV-C3N4, and corresponding formation energies (Ef). (i) EPR spectra of C3N4, NV-C3N4 and S-NV-C3N4.

DFT calculations were conducted to determine the formation energies (Ef) of S-NV-C3N4 with four possible S-doping configurations, including direct filling of the

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NV by a S atom (S1 position) and substitution of one N atom (either in S2, S3 or S4 positions) by a S atom, as illustrated in Fig. 2h and detailed in Fig. S4. The

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computational results show that S-filled configuration (S1: Ef = -6.29 eV) is much more energetically stable than S-substituted configurations (Ef = 2.77, 3.58 and 1.96 eV for S2, S3 and S4, respectively), suggesting that S dopants can be easily incorporated into the NVs of C3N4 with the formation of strong C-S bond. The electron paramagnetic resonance (EPR) measurement was performed to further probe the evolution of NVs, as displayed in Fig. 2i. In the EPR spectra, a paramagnetic absorption signal at g = 2.0035 corresponds to the unpaired electrons trapped by NVs 7

[41]. Compared with NV-C3N4, the intensity of EPR signal in S-NV-C3N4 is much declined, which even approaches the level of pristine C3N4. Such considerable decrease in EPR signal indicates the loss of NVs as a result of S-filling. Therefore, all the XPS, DFT and EPR results prove the successful filling of NVs by S-dopants. The ambient NRR performance of S-NV-C3N4 coated on CC (0.2 mg cm−2) was assessed in 0.5 M LiClO4 using a two-compartment cell[53, 54], as illustrated in Fig. S5. An absorber was set at the end of cell to avoid the loss of produced NH3 by N2 flow during the NRR test[55]. The RHE calibration was experimentally determined in

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the high-purity hydrogen saturated solution by cyclic voltammetry curves (Fig. S6). After each NRR electrolysis, the indophenol blue method (Fig. S7) was used to

quantitatively determine the NH3 yield and FE of produced NH3[56]. The possible

N2H4 as a byproduct was measured by the approach of Watt and Chrisp (Fig. S8) [57].

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As shown in Fig. S9, no N2H4 is detectable, confirming a high selectivity of

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S-NV-C3N4 for N2-to-NH3 conversion.

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Fig. 3. (a) 1H NMR spectra of 15NH4+ standard samples, and the electrolytes after 2 h of NRR electrolysis on S-NV-C3N4 using Ar and 15N2 as feed gases. (b) UV-vis absorption spectra of different working electrolytes after 2 h of electrolysis on S-NV-C3N4 in N2-saturated solution, Ar-saturated solutions, N2-saturated solution at open circuit, N2-saturated solution on pristine CC and blank reaction. (c) chronoamperometry tests of S-NV-C3N4 for 2 h of NRR electrolysis at various potentials, and corresponding (d) UV−vis absorption spectra of resultant electrolytes (stained with indophenol indicator) and (e) calculated NH3 yields and FEs. (f) NRR performances of C3N4, NV-C3N4 and S-NV-C3N4 under identical NRR tests at -0.4 V.

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Because

the

S-NV-C3N4

itself and

its

synthesis route

involve

the

nitrogen-containing chemicals, a series of control experiments were carried out to determine whether the nitrogen contaminant exerted influence on the NRR results[58, 59]. 1H NMR measurements were firstly performed to validate the origin of the generated NH3. After NRR electrolysis using 15N2 as feed gas, as shown in Fig. 3a, a characteristic

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NH4+ doublet peak can be found, while such doublet peak is

undetected when using Ar as feed gas. The NMR measurement was also utilized to quantitatively determine the concentration of NRR-derived NH3, as detailed in Fig.

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S10[60, 61]. Obviously, the NMR result corresponds well to that achieved by the indophenol blue method. In addition, the UV–vis tests (Fig. 3b) show that the noticeable NH3 can only be detected in N2-saturated electrolyte, while there is no

product of NH3 in the measurements using Ar-saturated electrolyte or at open circuit

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potential or blank reactions. Moreover, the time-dependent test (Fig. S11) shows a

linear relationship between the generated NH3 and electrolysis time, suggesting the

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continuously produced NH3 during the NRR process[62]. Therefore, all these results verify the reliability of our NRR results.

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Linear scan voltammogram (LSV) was employed to preliminarily evaluate the NRR activity of S-NV-C3N4. As shown in Fig. S12, S-NV-C3N4 presents a higher

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current density in N2-saturated solution with respect to Ar-saturated solution, demonstrating the feasibility of S-NV-C3N4 for NRR. To quantitatively determine the NRR performance of S-NV-C3N4, the combined chronoamperometry (Fig. 3c) and

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UV−vis tests (Fig. 3d) were performed at various potentials, and the corresponding data of NH3 yield and FE are calculated and presented in Fig. 3e. Obviously, the

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optimum NRR activity of S-NV-C3N4 is achieved at -0.4 V (RHE), corresponding to an NH3 yield of 32.7 μg h−1 mg−1 and an FE of 14.1%. Such NRR performance exceeds those of most previously reported metal-based (Table S1) and metal-free (Table S2) catalysts, demonstrating the fascinating NRR activity of S-NV-C3N4 for N2 fixation. However, beyond the optimum potential of -0.4 V, the NRR activity declines significantly, due mainly to the enhanced competing HER at elevated potentials[6]. For comparison, the NRR performances of pristine C3N4 and NV-C3N4 were also 9

evaluated under the identical conditions in terms of NH3 production rate. It is presented in Fig. 3f (Fig. S13) that the pristine C3N4 is nearly inactive for NRR with a considerably low NH3 yield, consistent with the literature report[41]. In contrast, NV-C3N4 and S-NV-C3N4 both present much enhanced NRR activity, which can be partially attributed to the improved N2 chemisorption on NV-C3N4 and S-NV-C3N4 over pristine C3N4, as evidenced by dinitrogen temperature-programmed desorption (N2-TPD) analysis (Fig. S14). Impressively, S-NV-C3N4 also remarkably outperforms NV-C3N4 with NH3 yield being twice that of NV-C3N4, indicating that S dopants are

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superior to NVs in promoting the NRR performance of C3N4. Electrochemical double-layer capacitance (Cdl) and electrochemical impedance spectroscopy (EIS) measurements were conducted to further investigate the electrocatalytic behaviors of

the catalysts, as shown in Fig. S15 & 16. Compared to NV-C3N4, S-NV-C3N4 exhibits

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comparable Cdl (Fig. S15) and charge-transfer resistance (Fig. S16), but a far better

NRR performance, corroborating that the enhanced NRR activity of S-NV-C3N4 over

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NV-C3N4 stems primarily from the S-dopant-induced electronic modulation, rather than other factors of electrochemically active surface area and charge transfer

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efficiency.

On the other hand, one may raise a question: why not directly conduct S-doping

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on pristine C3N4 rather than fill the NVs by S dopants on NV-C3N4? To answer this question, we perform the experiment to prepare S-doped C3N4 (denoted as S-C3N4) in the absence of intermediate step for preparing NV-C3N4. The XPS analysis (Fig. S17a)

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indicates that the S-doping level of S-C3N4 is as low as 0.55 at.%, which is nearly one-tenth of that for S-NV-C3N4 (5.2 at.%), implying the incorporation of negligible

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S-dopants in S-C3N4. This is consistent with our DFT result (Fig. S4) that the direct substitution of N atoms by S dopants leads to much positive formation energies (Ef=1.96~3.58 eV), and thus is thermodynamically unfavorable. On account of extremely low S dopant content, S-C3N4 exhibits a much inferior NRR activity than S-NV-C3N4 (Fig. S17b, c). Hence, a high S-doping level is highly necessary for C3N4 to obtain the desired NRR properties. In the present work, we show that S-filling of NVs indeed offers an efficient strategy to reduce the doping energy and achieve a 10

high S doping concentration, significantly beneficial for the NRR activity of S-NV-C3N4. Stability

is

also

crucial

for

a

catalyst

for

practical

applications.

Chronopotentiometric test (Fig. 4a) shows that S-NV-C3N4 can retain a stable current density for at least 20 h of continuous electrolysis. After 20 h of electrolysis, S-NV-C3N4 presents no pronounced degradation in NH3 yield relative to that after 2 h of electrolysis (Fig. 4b), proving the excellent long-term stability of S-NV-C3N4. After stability test, the resulting S-NV-C3N4 was collected and characterized by TEM,

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XPS and EPR. It is found that S-NV-C3N4 almost maintains its initial morphology (Fig. S18), chemical bonding state (Fig. S19) and NV concentration (Fig. S20),

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verifying the robust structural stability.

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Fig. 4. (a) Chronoamperometry test of S-NV-C3N4 for 20 h of NRR electrolysis. (b, c) NH3 yield of (b) S-NV-C3N4 and (c) NV-C3N4 before and after 20 h of NRR electrolysis. (d) Seven cycles of chronoamperometric runs (each for 2 h of NRR electrolysis, and corresponding (e) UV−vis absorption spectra of resultant electrolytes and (f) obtained NH3 yields and FEs. All the NRR tests are at -0.4 V.

For comparison, we also evaluate the stability of NV-C3N4 by the same

chronopotentiometric measurement. Disappointingly, NV-C3N4 not only displays a continuous decay in current density (Fig. S21), but also loses more than 41.8% of its initial NH3 yield (Fig. 4c) after 20 h of electrolysis, indicating the much inferior NRR stability of NV-C3N4. As shown in Fig. S22, in comparison with the initial state, 11

NV-C3N4 after 20 h of electrolysis exhibits a noticeably increased C-N=C/N-(C)3 ratio (Fig. S22a) and decreased EPR signal intensity (Fig. S22b), which are even comparable to those of pristine C3N4. This manifests that most created NVs are vanish during the NRR process, thereby leading to the poor stability of NV-C3N4. We further test the cycling stability of S-NV-C3N4. As displayed in Fig. 4d-f, when conducting seven cycles of chronoamperometric runs (Fig. 4d), no considerable change can be found in the UV−vis spectra of the resulting electrolytes (Fig. 4e), nor in the calculated NH3 yields and FEs (Fig. 4f), suggesting the favorable cycling stability. In

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view of above, it clearly turns out that filling of NVs by S dopants can be an efficacious approach to enable the effective and robust NRR for C3N4-based materials.

DFT calculations were conducted to further elucidate the crucial role of filled S

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dopants in boosting the NRR activity of S-NV-C3N4. To start, we evaluate the N2

adsorption behaviors of all the considered C3N4, NV-C3N4 and S-NV-C3N4 catalysts,

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as N2 adsorption on catalyst surface is the first and critical step for NRR[63, 64]. As shown in the differential charge density maps (Fig. 5a-c), there is a negligible electron

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transfer between C3N4 and *N2 (Fig. 5a), indicating that N2 molecule can not be adsorbed on the pristine C3N4. In stark contrast, both NVs (Fig. 5b) and S dopants

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(Fig. 5c) can effectively tune the electronic structures of C3N4 to enable the intensified charge exchange between NV-C3N4 (S-NV-C3N4) and *N2, which can weaken the inert N-N triple bond to polarize N2, agreeing well with the N2-TPD analysis (Fig.

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S14). The efficient N2 adsorption of NV-C3N4 (S-NV-C3N4) can also be confirmed by the projected density of states (PDOS) analysis (Fig. S23), in which NV-C3N4

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(S-NV-C3N4) displays the increased electronic state of *N2 DOS at the Fermi level, while near-zero electronic state can be observed at the Fermi level in *N2 DOS of pristine C3N4. The increased electronic state of *N2 DOS means the efficient back-donation of electrons from NV-C3N4 (S-NV-C3N4) into unoccupied 2π* orbitals of N2 to activate the N2 molecules[65].

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Fig. 5. (a-c) Differential charge density maps of (a) C3N4, (b) NV-C3N4 and (c) S-NV-C3N4 upon N2 adsorption. Yellow and cyan regions represent the election accumulation and depletion, respectively. (d) Free energy diagrams of distal NRR pathway on NV-C3N4 and S-NV-C3N4 at U = -0.4 V and pH = 7. (e) PDS energy barriers of C3N4, NV-C3N4 and S-NV-C3N4. (f, g) PDOS of active C atoms on NV-C3N4 and S-NV-C3N4 upon (f) *N2H adsorption and (g) *NH2 adsorption.

The free energy profiles of catalyst reaction pathways are further evaluated

through a distal associative mechanism. To fit the electrochemical experiment conditions, we adopt the applied potential of U = -0.4 V and pH = 7. All the optimized structures of key NRR intermediates are presented in Fig. S24-26. As shown in Fig. S24a, because of the weak adsorption of N2 on pristine C3N4, the first

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hydrogenation step from *N2 to *N2H is greatly restricted with a substantially high energy barrier of 1.88 eV, consequently rendering pristine C3N4 unfavorable for NRR. Notably, as depicted in Fig. 5d, the presence of NVs can make the free energies of *N2H on NV-C3N4 even negative (-0.53 eV) and the corresponding energy barrier of *N2 →*N2H is dramatically reduced to −0.69 eV, manifesting that the NV introduction can promote *N2H stabilization conspicuously. Nevertheless, the following reaction steps of NV-C3N4 involve a high energy barrier of 1.13 eV for the last step of *NH2 →NH3 (potential determining step (PDS)), which hinders the

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effective NH3 formation and desorption, eventually resulting in a compromised NRR performance. This stems from the intrinsic limitation of scaling relation between

*N2H and *NH2[66, 67], which prohibits the NV-C3N4 from reaching the region of optimal NRR activity. Encouragingly, once the S dopants are introduced to fill the

-p

NVs, S-NV-C3N4 exhibits a much reduced energy barrier of 0.18 eV for *NH2→NH3

due to the much weakened adsorption of *NH2, while still keeping *N2→*N2H

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energy-favorable with 0.56 eV uphill, even lower than that on NV-C3N4 (0.69 eV). The PDS of S-NV-C3N4 turns into the fourth step of *N2H2 → *N with only 0.67 eV,

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considerably lower than those of C3N4 and NV-C3N4 (Fig. 5e). These results imply that S doping has the potential to break the *N2H-*NH2 scaling relation on

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S-NV-C3N4, leading to more optimized adsorption of NRR intermediates and thus a significantly improved NRR activity. To uncover the underlying mechanism of S-dopant induced breaking of

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*N2H-*NH scaling relation, the PDOS of active C atoms on NV-C3N4 and S-NV-C3N4 upon adsorption of *N2H and *N2H are analyzed, as shown in Fig. 5f, g.

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Generally, the position of Ep (highest peak of active center below the Fermi level) is a good descriptor to evaluate the binding strength of intermediates on catalyst surface[68], and a higher location of Ep makes the anti-bonding states move higher with a lower occupancy, resulting in stronger intermediate binding on the catalyst, and vice versa[37, 69]. Upon *N2H adsorption (Fig. 5f), the Ep position of active C atom (C-Ep) between NV-C3N4 and S-NV-C3N4 is comparable, suggesting they have the similar binding strength to stabilize *N2H. The prominent deviation, however, can be 14

found upon *NH2 adsorption (Fig. 5g), where S-NV-C3N4 presents a much lower C-Ep than NV-C3N4, indicating the weakened adsorption of *N2H on S-NV-C3N4, in line with the energy profile results (Fig. 5d). The similar trend can also found in PDOS of *N2H and *NH2 themselves (Fig. S27). Upon *NH2 adsorption, Mullikan charge analysis indicates that S dopant grabs 0.13 |e| from active C atom, and this causes a decreased electron exchange between active C atom and *NH2, consequently weakening the *N2H binding on S-NV-C3N4. Therefore, with the particular role of S-dopant, S-NV-C3N4 possesses the capability to break the scaling relation to

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effectively stabilize *N2H and destabilize *NH2, which are substantially favorable for the NRR activity.

As the dominant competing reaction for the NRR, the HER behavior of the catalysts is further examined. In neutral or alkaline electrolyte, HER involves a

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two-step process: water dissociation and then hydrogen evolution[70]. As shown in

Fig. 6, albeit both NV-C3N4 and S-NV-C3N4 exhibit a low adsorption free energy of

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H* (ΔGH*) being beneficial for hydrogen evolution, their free energies for water dissociation (ΔGH2O) are considerably high, especially for S-NV-C3N4. The high

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ΔGH2O retards the splitting water into H*, and the deficient H* in turn slows down the following hydrogen evolution, leading to the sluggish HER kinetics of entire process.

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Therefore, the competitive HER can be effectively suppressed on S-NV-C3N4 in

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neutral media, enabling the high selectivity toward the N2-to-NH3 conversion.

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Fig. 6. Free energy diagram of HER on NV-C3N4 and S-NV-C3N4 at U = -0.4 V and pH = 7.

3. Conclusions In summary, we have demonstrated a successful engineering of C3N4 by filling the NVs with S dopants. Compared to NV-C3N4, S-NV-C3N4 presented a conspicuously improved NRR activity and durability in neutral media. As disclosed by the DFT calculations, the filled S dopants could break the *N2H-*NH2 scaling

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relation to result in more optimized adsorption of NRR intermediates and a significantly reduced reaction barrier. Meanwhile, the competitive HER could be

suppressed on S-NV-C3N4 because of the high energy barrier for water dissociation. Therefore, filling vacancy with heteroatoms may open a new avenue to design

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powerful NRR catalysts for efficient electrocatalytic N2 fixation to NH3.

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Declaration of Interest Statement

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Vacancy engineering and heteroatom doping are two of the most popular approaches to tailor the electronic structures of the electrocatalysts for enhanced electrocatalytic activity. Herein, these two approaches were rationally combined to

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modulate the graphitic C3N4 by filling the nitrogen vacancies (NVs) with S dopants, and resulting S-NV-C3N4 were confirmed to be a highly efficient and durable

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metal-free catalyst for electrochemical N2 reduction reaction (NRR). The theoretical calculations disclosed that filled S dopants could break the *N2H-*NH2 scaling

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relation to result in more optimized adsorption of NRR intermediates and a significantly reduced reaction energy barrier. These findings demonstrate that filling vacancy with heteroatoms may offer a new strategy for exploring novel high-performance NRR electrocatalysts.

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Conflicts of interest There are no conflicts of interest to declare.

Credit author statement Ke Chu conceived the project and designed the experiments. Qing-qing Li performed the DFT calculations. Ya-ping Liu prepared and characterized the catalysts. Jing Wang and Yong-hua Cheng conducted the electrochemical measurements. All authors discussed the results and commented on the manuscript. Ke Chu and

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Qing-qing Li wrote the manuscript.

Acknowledgement

This work is supported by National Natural Science Foundation of China

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(51761024), and Foundation of A Hundred Youth Talents Training Program of

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Lanzhou Jiaotong University.

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