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Nitrogen doped MoS2 and nitrogen doped carbon dots composite catalyst for electroreduction CO2 to CO with high Faradaic efficiency
Nano Energy 63 (2019) 103834
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Nitrogen doped MoS2 and nitrogen doped carbon dots composite catalyst for electroreduction CO2 to CO with high Faradaic efficiency
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Kuilin Lv, Weiqun Suo, Mingda Shao, Ying Zhu, Xingpu Wang, Jingjing Feng, Mingwei Fang, Ying Zhu∗ Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Advanced Innovation Center for Biomedical Engineering, School of Chemistry, Beihang University, Beijing, 100191, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 electroreduction Nitrogen doped MoS2 NCDs High FE DFT
CO2 utilization by direct electroreduction offers an attractive route for preparing valuable chemicals or alternative liquid fuels, and mitigating the hazardous effects of global warming. Unfortunately, the electroreduction CO2 currently suffers from poor efficiency, low produce selectivity, and large overpotential is required to initiate CO2 reduction due to the thermodynamic stability of CO2. Therefore, the development of low-cost transitionmetal chalcogenides electrocatalysts for high selectivity electroreduction CO2 under considerably low overpotentials are still challenge, due to its high catalytic activity of hydrogen evolution reaction and poor electronic conductivity. Herein, for the first time, N-doped MoS2 nanosheets and N-doped carbon nanodots (N-MoS2@ NCDs) composite was successfully prepared by solvothermal method in the presence of E-MoS2 and DMF solvent. During solvothermal process, N atoms were introduced into MoS2 framework and N-doped CDs were formed on the surface of MoS2 nanosheets at the same time. The optimized N-MoS2@NCDs-180 for electroreduction CO2 displayed a high CO Faradaic efficiency up to 90.2% and a low onset overpotential requirement of 130 mV for CO formation, which were significantly superior to those of the E-MoS2 and other previously reported transitionmetal sulfides electrocatalysts. The experiment verified that the N-doped CDs on the surface MoS2 provided a good electrical conductivity, which accelerated electron transport. Moreover, DFT theoretical calculation demonstrated that the N doping into MoS2 could decrease the energy barrier of the COOH* intermediate formation and create more electrons on the edge Mo of N-MoS2, thereby enhancing catalytic activity of CO2 electroreduction to CO.
1. Introduction Electrochemical CO2 reduction reaction (CO2RR) is considered as the promising strategy for conversion CO2 to value-added chemicals and alternative fuels, which delivers significant environmental and economic benefits [1,2]. However, CO2RR involves multiple protoncoupled electron transfer processes that are kinetically sluggish and in need of high potentials for CO2 reduction, thus leading to a poor product selectivity and low conversion efficiency [3]. To address these issues, therefore, various electrocatalysts, including noble metals [4], carbon materials [5], metal oxides [6], and transition-metal chalcogenides [7] have been developed to drive the CO2RR with high Faradaic efficiency (FE), and product selectivity. Among these electrocatalysts, the 2D transition-metal chalcogenides (MX2, M = Nb, Mo, W; X = S, Se), particularly MoS2, as catalysts for electroreduction CO2 have attracted much attention due to their low-cost and unique sandwich
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structure that provides more active sites for electrocatalysis [8]. However, the practical applications of MX2 in electroreduction CO2 has been hampered in aqueous electrolyte, due to their inherent limitations, including the high catalytic activity of hydrogen evolution reaction (HER) and the intrinsically poor electronic conductivity [9]. So the alloying, metal doping and surface modification of MoS2 were found to be efficient ways to tune their electronic structures and interface properties, thus leading to a significant improvement in CO2RR performance and a suppression of HER. For instance, Han et al. [10] developed Mo-Bi bimetallic chalcogenide by direct thermal decomposition of ammonium tetrathiomolybdate and bismuth nitrate, which was used as electrocatalyst for reduction CO2 to methanol with a high methanol FE of 71.2%, onset overpotential of 360 mV for methanol formation, and a current density of 12.1 mA cm−2 at −0.7 V (vs. SHE). Xie et al. [11] fabricated MoSeS alloy monolayers by high temperature doping Se on oil/water interface, which could catalyze the conversion of CO2 into CO
Corresponding author. E-mail address: [email protected] (Y. Zhu).
https://doi.org/10.1016/j.nanoen.2019.06.030 Received 7 May 2019; Received in revised form 8 June 2019; Accepted 13 June 2019 Available online 19 June 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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doped CDs (NCDs) can graciously catalyze the electrochemical reduction of CO2 with high-selectivity and catalytic activity. In the work, N-MoS2@NCDs were prepared by one-pot solvothermal reaction in the presence of the exfoliated MoS2 (E-MoS2) and N, NDimethylformamide (DMF) solvent. E-MoS2 nanosheets were fabricated by wet ball-milling in the large scale [13]. During the solvothermal process, the N elements were doped into MoS2 framework, simultaneously NCDs derived from DMF were formed and homogeneously dispersed on the surface of N-MoS2 nanosheets [18a]. Among them, the as-prepared N-MoS2@NCDs-180 possessed a high N doping content of 8.35 at%, which exhibited high catalytic activity toward electroreduction CO2 to CO with a FE of 90.2% for CO production at −0.9 V (vs. RHE) and an low onset overpotential for CO formation (130 mV), which were significantly superior to those of the origin E-MoS2 (41.2%, 260 mV) and previously reported analogues [9–13,19], as demonstrated in Fig. 4B and Table S3. It was reported that the NCDs decorated on the surface of MoS2 could improve conductivity and lower the onset overpotential, thereby enhancing CO2RR performance. In addition, according to DFT calculation, the N element doping into MoS2 nanosheets could lower the energy barrier of the formation of COOH* intermediates (0.76 eV) on N-MoS2 nanosheet, typically smaller than that on MoS2 nanosheets (0.87 eV), thus facilitating the rate-limiting COOH* intermediate formation step. In addition, according to the bader charge analysis, moreover, the doped N atom can also tailor electronic properties of Mo atom that will create more electrons on the Mo atom edge, and then bring more opportunity for electron participating in reducing CO2 to CO. The doping heteroatoms into transitionmetal chalcogenides may provide an effective strategy to provide costeffective, robust electrocatalysts for CO2RR reduction with a low onset overpotential and high efficiency.
with FE of 45.2% by regulating off-centre charge around the Mo atoms, much larger than that of MoS2 (16.6%) and MoSe2 monolayers (30.5%), and achieved a onset overpotential (430 mV) for CO formation. Abbasi et al. [12] synthesized the vertically aligned niobium (Nb)-doped MoS2 (VA-Mo0.95Nb0.05S2) catalysts by using chemical vapor deposition (CVD), which was used as an electrocatalyst for converting CO2 to CO with a high FE of 82% for CO formation and low onset overpotential (90 mV). The excellent activity was attributed to the embed Nb atom into MoS2 atomic structure, which could modify electronic properties of MoS2. In addition, our recent work [13] reported that the fluorosilane decorated of E-MoS2 nanosheets was able to electroreduce CO2 to CO with a FE of 82% and an onset overpotential of 190 mV, which could be attributed that the F atoms could tune electronic structures of the edge Mo atoms for decreasing free energy of CO2 conversion and provided hydrophobic surface for effectively depressing HER. Although there have been significant advances in CO2RR, developing affordable MoS2based electrocatalyst for CO2RR with a high FE and low overpotential is still a big challenge from a practical point of view. N-doped carbon materials has been demonstrated to be the promising electrocatalysts for oxygen reduction reaction (ORR) [14a,b], Zn-air battery [14c], electrochemical water splitting [14d], and CO2RR [5], which are attributed to the higher electronegativity of N with respect C that can tailor the charge distribution of carbon adjacent to N dopants, thus expediting the electron transport [15]. Inspired by the superior catalytic performance of N-doped carbon material, it is expected that N doping into transition-metal chalcogenides may provide the enhanced CO2RR performance. Recently, a little theoretical and experimental studies have been proved that N doped MoS2 can acted as electrocatalysys for ORR and lithium ion batteries [16,17] For instance, Hao et al. [16] reported that the N-doped MoS2/carbon (N-MoS2/C) for ORR were produced by using the aqueous evaporation-induced selfassembly process. N-MoS2/C showed a maximum power density of 0.815 W m−2, which was far higher than that of Pt/C (0.520 W m−2), and only had a decline of 1.23% after 1800 h operation. The outstanding ORR performances of N-MoS2/C were accounted for by generating abundant defects (pyridinic N, graphitic N and Mo-N), as well as enhancing electric conductivity. Liu et al. [17] reported for the first time that the N-doped MoS2 as anode materials of lithium ion batteries, displayed a higher discharge specific capacity (1130.8 mAh g−1) than bulk MoS2 (898 mAh g−1). Electrochemical measurements and density functional theory (DFT) calculations demonstrated that N doping could enhance the electron conductivity of MoS2, thereby creating the fast transportation of electrons and ions. To the best of our knowledge, nevertheless, doping N element into MoS2 to improve its CO2RR performance has not been reported. Furthermore, the electrochemical performance of the doped MoS2 is still far from ideal, and it still suffers from severe activity fading because of its poor electronic conductivity, structural destruction during cycling. Hybridization of MoS2 with other highly conductive carbon to form hierarchical nanostructures is an effective approach to enhance its conductivity, thus achieving optimal catalytic activity. Carbon dots (CDs) are desirable for an promising component of CO2RR electrocatalysts owing to their large number of exposed edge sites and excellent electronic properties [18]. For instance, Ajayan et al. [18a] synthesized N-doped graphene quantum dots (NGQDs) as electrocatalysis for CO2RR by solvothermal process in the presence of DMF and graphene oxide, which showed a highest total FE of 45% for C2H4 and CH3CH2OH at −0.7 V, due to the high conceration of terminal active sites caused by solvothermal process and the N elements doping. Recently, Liu et al. [18b] found that the NGQDs decorcted-Bi2O3 (Bi2O3-NGQDs) by a solvothermal method in polyol solution exhibited an excellent catalytic activity for CO2 electroreduction into HCOOH with up to 90% FE from −0.9 to −1.2 V. The experiments and DFT calculations demonstrated that NGQDs could increase adsorption capacity of CO2 (ads) and COOH* intermediate, thereby leading to the enhanced the activity of Bi2O3 nanosheets. Accordingly, we expected that combination of N-doped MoS2 nanosheets and N-
2. Experimental section 2.1. Materials Molybdenum (IV) sulfide powder (MoS2, CAS: 1317-33-5) was supplied by Sigma-Aldrich. N, N-Dimethylformamide (DMF, CAS: 6812-2) was purchased from Alfa Aesar. 1-Ethyl-3-methylimidazolium Tetrafluoroborate (EMIM-BF4, CAS: 143314-16-3) was purchased from Shanghai Cheng Jie Chemical Co. LTD. All reagents were used without further treatment.
2.2. Sample preparation The exfoliated MoS2 (E-MoS2) nanosheets was prepared according to our previous report [13]. N-MoS2@NCDs-X was synthesized by solvothermal approach in the presence of E-MoS2 nanosheets in dimethylformamide (DMF) solvent. The typical synthesis was given as follows, 300 mg E-MoS2 was dispersed in 40 mL DMF, and sonicated in ultrasonicator for 60 min to form a homogeneous dispersion. Afterwards, the E-MoS2 suspension was transferred to a 50 mL PTFE liner. The N-MoS2@NCDs-180 was formed in a hydrothermally analogous process at 180 °C for 20 h. In this process, E-MoS2 was further exfoliated and exposed more terminal active sites, and simultaneously doped by N into the carbon lattice and MoS2 with N source from DMF. After cooling naturally, the resultant was washed with deionized water and ethanol for three times and dried at 70 °C for 12 h, yielding the N-MoS2@NCDs180. And N-MoS2@C-200 and N-MoS2@C-220 were synthesized in a PTFE-lined autoclave at 200 and 220 °C, respectively. For comparision, the H-MoS2 was prepared in a similar way, except using water as the solvent. And N-MoS2 was obtained by heating N-MoS2@NCDs-180 in the muffle furnace at 300 °C for 12 h, NCDs could be removed during the heating process.
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MoS2@NCDs-180 (6.65 × 10−3 S/cm) is higher than that of E-MoS2 (2.04 × 10−3 S/cm). The higher magnification HAADF image (Fig. 1C) showed that Mo atoms were the outmost terminating atoms on the edges of the MoS2 nanosheets, as marked by red arrow. According to the previous result reported by Asadi [22], Mo-terminated edges have the lowest formation energy that allows more active edge sites, thus enhancing the performance of CO2RR. The average length of N-MoS2@ NCDs-180 was about 166 nm, as statistically counted by 583 flakes (Figure S2B). TEM image and its corresponding elemental mapping of the N-MoS2@NCDs-180 (Fig. 1D) clearly showed a thin nanosheet with a length of 423 nm, and the homogeneous distribution of Mo, S, C and N atoms, which demonstrated that N elements were successfully introduced into MoS2 nanosheets and carbon dots. As presented in AFM image, N-MoS2@NCDs-180 nanosheets had thickness between 3.58 nm and 4.81 nm, corresponding to 3–5 atomic layers of MoS2 (Figure S3) [13]. Compared to our previous work [13], N-MoS2@NCDs-180 has a smaller size than that of E-MoS2 obtained by direct liquid exfoliation, which was attributed to solvothermal treatment. It is reasonable to assume that a smaller size of N-MoS2@NCDs-180 may expose more Mo edges for the enhanced performance of CO2RR [23]. The molecular structures of the N-MoS2@NCDs-X were analyzed by the X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2A, XRD spectra of N-MoS2@NCDs-X all showed the peaks located at 14.3°, 32.6°, 39.5°, 49.8° and 60.1°, corresponding to the (002), (100), (103), (105) and (008) diffraction reflections of bulk MoS2 (indexed by JCPDS NO. 37–1492), respectively. Compared with E-MoS2, the d-spacing of N-MoS2@NCDs-180 increased to 0.6115 from 0.6019 nm of E-MoS2 (Figure S4A), indicating that N atoms were successfully introduced into the MoS2 framework [24]. And it could be seen that N-MoS2@NCDs-180 had the largest d-spacing among N-MoS2@NCDs-X samples (Fig. 2A), which may deduce that more N atoms were successfully doped into N-MoS2@NCDs-180 during the hydrothermal process [24]. Figure S4B showed the enlarged Raman spectra of E-MoS2 and N-MoS2@NCDs-180 measured by using the laser excitation wavelength of 532 nm. For E-MoS2 sample, the two Raman characteristic bands at 375.3 and 402.1 cm−1 corresponded to the E12g and A1g vibration modes, which had a peak frequency difference of 26.8 cm−1. For N-MoS2@NCDs-180, however, the two the E12g and A1g modes located at 377.1 and 402.2 cm−1 with frequency difference of 25.1 cm−1 that was small than that of E-MoS2, indicating that the MoS2 nanosheets were further cut into smaller pieces by solvothermal method [24]. Raman spectra of N-MoS2@NCDs-200 and N-MoS2@NCDs-220 were shown in Fig. 2B, the peak frequency differences were also calculated to be 25.3 and 25.6 cm−1, respectively, indicating that the size of N-MoS2@NCDs-X nanosheets decreased with increasing the treatment temperature [23]. XPS spectra were conducted to analyze the chemical compositions of Mo and S in N-MoS2@NCDs-X and E-MoS2 samples. As showed in the XPS spectra (Figure S5 A and B), the Mo, S elements were found in all the samples, but the N and C elements were only found in N-MoS2@NCDs-X samples, indicating that N doping and NCDs coating were realized successfully via a facile solvothermal treatment. As shown in Table S1, the element contents of N-MoS2@ NCDs-180 were measured to be 28.23 at% for Mo, 53.99 at% for S, 8.35 at% for N, and 7.41 at% for C. Similarly, the N element contents of N-MoS2@C-200 and N-MoS2@C-220 were calculated to be 6.15 at% and 5.71 at%, which were smaller than that of N-MoS2@NCDs-180. We deduced that the doping contents of N element could be tuned by adjusting solvothermal temperature, and appropriate solvothermal temperature was beneficial to high level of N doping. The high resolution Mo 3d XPS spectrum of E-MoS2 clearly displayed two peaks at 228.82 and 232.15 eV (Fig. 2C), being attributed to the doublet Mo 3d5/2 and Mo 3d3/2 spin doublets, respectively [13]. While high-resolution S 2p XPS spectrum of E-MoS2 (Fig. 2D) showed two the peaks at 161.66 and 162.86 eV related to S 2p3/2 and S 2p1/2 binding energies, respectively, being consistent with the oxidation state of S2− in E-MoS2 [13]. After N doping, however, two peaks of Mo 3d5/2 and Mo 3d3/2 of N-MoS2@
2.3. Characterizations The surface morphology analysis of sample was observed by Field Emission Scanning Electron Microscope (FESEM, JEOL, JSM-7500F). The High-resolution images and lattice structure of samples were observed by HRTEM (JEOL, JEM-2100F) and Scanning Transmission Electron Microscopy (STEM, FEI Titan 80–300, Japan). The thickness of sample was obtained by Atomic force microscopy (AFM, VT STM/ AFM). X-ray diffraction (XRD) patterns were collected on a Shimadzu Xray Diffractometer (XRD-6000) with Cu-Ka1 source (λ = 1.5406 Å). Xray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi with Al Kα radiation. Raman spectra of these samples were analyzed by LabRAM HR800 with a 532 nm laser excitation. 2.4. Computational details The calculations were carried out using density functional theory (DFT) with PBE + U (the effective U is 4.0 eV for d orbital of Mo atom). The Vienna ab-initio simulation package (VASP) was employed. The plane wave energy cutoff was set as 400 eV. The Fermi scheme was employed for electron occupancy with an energy smearing of 0.1 eV. The first Brillouin zone was sampled in the Monkhorst−Pack grid. The 3 × 3 × 1 k-point meshes for the surface calculation. The energy (converged to 1.0 × 10−6 eV/atom) and force (converged to 0.01 eV/ Å) were set as the convergence criterion for geometry optimization. The spin polarization was considered in all calculation. The MoS2 (001) surface was obtained by cutting the MoS2 bulk along the {001} direction. The thickness of surface slab was chosen to be one slab. A vacuum layer as large as 15 Å was used along the c direction normal to the surface to avoid periodic interactions. For the edge of the MoS2 (001), a vacuum layer (∼12 Å) was chosen along the {010} of the MoS2 (001). For N doped configuration, one of the S is replaced by N atom. The free energy (ΔGi) of elementary reaction is defined as follows:
ΔG = ΔEDFT + ΔEZPE − T ΔS where ΔEDFT is the reaction energy of elementary reaction obtained by DFT calculation, i.e. the energy difference between the final and initial states, ΔEZPE is the difference in zero-point energy between the final and initial states, while TΔS is the entropy change of the elementary reaction. 3. Results and discussion 3.1. Preparation and characterization of N-MoS2@NCDs-X N-MoS2@NCDs-X (X represents reaction temperature) were designed and fabricated by a simple, low cost solvothermal approach in the presence of E-MoS2 nanosheets and DMF solvent. E-MoS2 nanosheets have been prepared by our previous work [13]. As the typical electrocatalysts, the morphologies of E-MoS2 and N-MoS2@NCDs-180 were characterized by scanning electron microscopy (SEM), high-resolution TEM (HRTEM), high-angle annular dark-field (HADDF) TEM and atomic force microscopy (AFM). HRTEM images of E-MoS2 (Fig. 1A) displayed a typical fringe of MoS2 crystallite, which was in agreement with the previous report [13]. Compared to E-MoS2, NCDs with the approximate diameter of 6 nm, were grown uniformly on the surface of N-MoS2 nanosheets after solvothermal treatment (Fig. 1B). As presented in Figure S1, two clearly distinct structural areas could be observed. One was a typical lamellar structure of MoS2 (Figure S1B), which was confirmed by the clear lattice fringe with a spacing of 0.63 nm, being assigned to the (002) crystal plane of MoS2 [8,20]. The other was observed for NCDs with a lattice spacing of 0.21 nm (Figure S1B), which corresponds to the (100) facet of graphite [21], and also confirmed by the Fast Fourier Transform analyses (inset of Fig. 1B). After solvothermal treatment, as Table S2 shown, the conductivity of N3
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Fig. 1. (A) HRTEM image of E-MoS2. (B) HRTEM image of N-MoS2@NCDs-180, the blue frame represents NCD and the red frame represents N-MoS2. (C) Higher magnification HAADF image of N-MoS2@NCDs180 shows clearly distinct atomic configuration. (D) TEM image and corresponding elemental mappings of the N-MoS2@C-180 nanosheets.
Fig. 2. (A) XRD patterns and (B) Raman spectra of N-MoS2@NCDs-180, N-MoS2@ NCDs-200 and N-MoS2@NCDs −220, respectively. (C, D) Mo 3d and S 2p XPS spectra of for N-MoS2@NCDs-180 and E-MoS2. 4
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−0.9 V, indicating that N-MoS2@NCDs-180 has an obvious catalytic activity for CO2RR. As shown in Fig. 3A and Figure S8, the onset potential on the N-MoS2@NCDs-180 electrode for CO2 electroreduction was −0.24 V, which was more positive than those of the N-MoS2@C200 electrode (−0.26 V), N-MoS2@C-220 electrode (−0.29 V) and EMoS2 electrode (−0.30 V), thus suggesting a very low onset overpotential (130 mV) for CO formation on N-MoS2@NCDs-180 electrode. Importantly, the N-MoS2@NCDs-180 electrode exhibited a significantly high current density (59 mA cm−2) at −1.1 V, which was higher than that of E-MoS2 nanosheets (48 mA cm−2). The performance of CO2 electroreduction was operated at different applied potentials between −0.7 and −1.05 V in CO2-saturated EMIM-BF4 solutions (94 mol% water). Fig. 3B showed the Faradaic efficiency (FE) for CO production on the E-MoS2, N-MoS2@NCDs-X electrodes at different applied potentials. Under the reported reaction conditions, the main gas products were CO and H2 determined by gas chromatography (GC). And no liquid product could be detected from 1H NMR analysis, as shown in Figure S9. N-MoS2@NCDs-180 with 8.35 at% N content showed that the FE of CO production increased with decreasing the applied potentials and reached a maximum of 90.2% at −0.9 V, larger than those of CO production on N-MoS2@NCDs-200 (82.9%) and N-MoS2@NCDs220 (81.5%) electrodes, showing an outstanding CO product selectivity. While the maximum FE of E-MoS2 electrode for CO formation was only 41.7% at −0.95 V, and the reduction potential of the highest CO production was −0.95 V, more negative than that of N-MoS2@NCDs-180 (−0.9 V). In addition, the CO FE at low applied potentials on E-MoS2, N-MoS2@NCDs-180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 electrode were shown in Fig. 4A. We also studied the onset FE for CO formation to obtain a better perspective on catalytic performance of NMoS2@NCDs-X (Fig. 4A), and the onset FE for CO formation on the N-
NCDs-180 (Fig. 2C) shifted positively to 229.12 and 232.39 eV. Simultaneously, the two peaks of S 2p3/2 and S 2p1/2 of N-MoS2@NCDs180 (Fig. 2D) also moved positively to 161.9 and 163.12 eV, respectively. It was clear that the peaks of Mo 3d5/2 and S 2p3/2 shifted to higher binding energy side, indicating that N element was successfully doped into E-MoS2 [23], which agreed with the EDS and XRD results of the N-MoS2@NCDs-180. Furthermore, as shown in Figure S5C, there was a hump on the side of Mo 3p3/2 peak that originates from N 1s, thus indicating the existence of Mo-N bond (401.7 eV) [25]. The specific N configuration could also be divided into pyridinic N (398.1 eV) and pyrrolic N (399.9 eV), as indicated by the deconvolution N1s peak (Figure S5C), which further confirmed the presence of C-N bonds in the N-MoS2@NCDs-180 [26]. 3.2. CO2 electroreduction on N-MoS2@NCDs-X catalysts The electrocatalytic activity and selectivity of N-MoS2@NCDs-X toward CO2RR were evaluated in a flow double electrolytic cell, in which N-MoS2@NCDs-X were loaded on a 1 × 2 cm2 area of glass carbon (GC) without using any binder. The linear sweep voltammetry (LSV) were conducted to evaluate the catalytic performance in N2 or CO2 saturated 6 mol% EMIM-BF4 and 94 mol% water solutions (pH∼4.24), as demonstrated by our previous work [13]. It was reported that the EMIM+ with CO2 can form stable [EMIM-CO2]+ intermediate to promote the generation of CO in the EMIM-BF4 aqueous electrolyte [22,27]. As given in curves of Figure S7A, the N-MoS2@ NCDs-180 in N2-saturated solution delivered around 25 mA cm−2 current density at −0.9 V (vs. RHE; all potentials are reported with respect to RHE), that was attributed to HER. Notably, in CO2-saturated electrolyte, while a current density of about 36.2 mA cm−2 was obtained at
Fig. 3. (A) LSV curves for E-MoS2, N-MoS2@NCDs-X electrodes at a sweep rate of 0.02 Vs-1 in CO2-saturated EMIM-BF4 solutions (94 mol% water). (B) The CO FE at different applied potentials for E-MoS2, N-MoS2@NCDs-X (X: 180, 200, 220) electrodes in EMIM-BF4 solutions (94 mol% water). (C) Tafel plots for CO production on N-MoS2@NCDs-X electrodes in EMIM-BF4 solutions (94 mol% water). (D) The FE ratio of CO and H2 on N-MoS2@NCDs-180; Partial CO current density plots of EMoS2 and N-MoS2@NCDs-180 at different applied potentials. 5
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Fig. 4. (A) The FEs for CO formation at low applied potentials on E-MoS2, N-MoS2@NCDs-180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 electrodes. (B) The CO FEs and onset overpotentials for CO formation at the applied potentials on different doped transition-metal chalcogenides catalysts. (The numbers represent relevant literature reports; the star symbol represents our work).
The XRD and Raman spectra of N-MoS2 have been measured, as shown in Figure S6B and C. It was easy to found that the d-spacing and the peak frequency difference of N-MoS2 were similar with those of NMoS2@NCDs-180, indicting that the crystal structures of N-MoS2 was not affected by the existence of NCDs. As shown in Fig. 5B, the highest CO FE of 88.2% on N-MoS2 was closed to that of N-MoS2@NCDs-180 (90.2%) at −0.9 V, but the current density of N-MoS2 (26.7 mA cm−2) was obvious inferior to N-MoS2@NCDs-180 (36.2 mA cm−2) at −0.9 V, which indicated the NCDs obviously enhance the reduction current density due to its fast transportation of electrons (Table S2). In addition, the onset potential for CO formation on the N-MoS2 elelctrode was −0.45 V, which was more negative than that of on the N-MoS2@NCDs180 elelctrode (−0.24 V). Hence, NCDs on the surface of N-MoS2 could not only enhance materials conductivity, but also lower the onset overpotential. It is therefore clear that N doping transition-metal chalcogenide with compositing NCDs is a promising strategy to improve their catalytic activity of CO2RR.
MoS2@NCDs-180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 were 4.2% (−0.24 V), 2.8% (−0.26 V) and 3.6% (−0.29 V), respectively. This result indicated that CO formed at a lower applied potential on NMoS2@NCDs-X electrode, which was also consistent with the LSV results (Fig. 3A). What's more, the high FE (90.2%) and low onset potentials (130 mV) on N-MoS2@NCDs-180 were significantly superior to those of the vast majority of reported the doped transition-metal chalcogenides [9–13,19], as demonstrated in Fig. 4B and Table S3. The Tafel slopes were employed to determine the CO2 reduction kinetics (Fig. 3C), N-MoS2@NCDs-X all exhibited similar linear responses with respect to overpotentials. The Tafel slopes of N-MoS2@NCDs-180, NMoS2@NCDs-200 and N-MoS2@NCDs-220 were measured to be 101.9, 108.6, and 126.8 mV dec−1, respectively. The Tafel slope of 118 mV dec−1 is a theoretical value, which indicates that the chemical ratedetermining step is an initial single-electron transfer step [28,29]. Therefore, N-MoS2@NCDs-180 exhibited a much lower Tafel slope than that of N-MoS2@NCDs-200 and N-MoS2@NCDs-220, evidencing the improved kinetic activity for electroreduction CO2 to CO over N-MoS2@ NCDs-180. As shown in Fig. 3D, the FE ratios of CO/H2 indicated that competitive HER took place at low potentials from −0.7 to −0.85 V, but CO2 electroreduction was predominantly prone to occur from −0.9 to −1.1 V on the N-MoS2@NCDs-180 electrode. And the partial CO current density of N-MoS2@NCDs-180 was approximately 4 times larger than that of E-MoS2 at applied potentials of −0.9 V. Moreover, the durability is another important criterion to evaluate the suitability of electrocatalysts for practical applications. The chronoamperometric responses were thus performed 10 h in a CO2-saturated electrolyte. As shown in Figure S7B, the total current densities on the N-MoS2@NCDs180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 electrodes all showed no attenuation activity of CO2RR during the reduction process of 10 h, displaying a good long-time stability due to the existence of NCDs on the surface of N-MoS2. To illuminate the role of N doping on MoS2, H-MoS2 was prepared by hydrothermal treatment of E-MoS2 at 180 °C in the absence of DMF solvent. As shown in Fig. 5A, the highest CO FE of 67% on H-MoS2 was clearly inferior to that of N-MoS2@NCDs-180 (90.2%), but higher than that of E-MoS2 (41.7%) at −0.95 V. These results may be attributed to the fact that the N doping into MoS2 framework could tune its electron structures and produced many terminal active sites, thus leading to the enhanced electrocatalytic activity for reduction CO2 to CO [18a]. Moreover, to prove the effect of NCDs, N-MoS2@NCDs-180 was heated in a muffle furnace at 300 °C for 12 h to remove NCDs, which was denoted as N-MoS2. As showed in Figure S6A and Table S1, the C element content in N-MoS2 sample almost disappeared after the heat treating, and the N element content also reduced from 8.35 at.% to 5.96 at.%.
3.3. DFT calculations and reaction mechanism of CO2RR To deeply discuss the activity of N-MoS2@NCDs-X for electroreduction CO2 to CO, the first principle calculation based on density functional theory (DFT) of has been employed to investigate the reaction mechanism of CO2RR. Due to the role of NCDs in improving CO2RR performance, therefore, the N-MoS2 was chosen as model molecule for calculation. DFT results showed that the adsorption of COOH* and CO* on MoS2 and N-MoS2 electrodes were exothermic, as shown in Fig. 6A and Table S4. Compared with the MoS2, the N-MoS2 monolayers had the obviously lower formation energy of COOH* intermediate than MoS2 monolayers, thereby indicating that the formation of COOH* is more favorable on N-MoS2 surface. In addition, desorption of CO* intermediate had ΔG of 0.49 eV on N-MoS2 which was slightly lower than ΔG of 0.53 eV on MoS2, weakening the adsorption strength of CO* with the Mo atom on N-MoS2 was another factor to promote the catalytic conversion CO2 to CO. Taken together, DFT theoretical calculations confirmed that N doping into MoS2 is beneficial for CO2 electroreduction to CO. To further explain these results, the bader charge analysis has been employed. The results indicated that the electrons on the MoS2 were redistributed, after N atom was doped into MoS2. The valence electron of Mo in N-MoS2 was simulated to be 5.02 e, which is higher than that of pure MoS2 (the valence electron of Mo: 4.87 e), more electron will be deposited on the edge Mo of N-MoS2, which led to a biased electron density of Mo atom in the N-MoS2. It is known that the adsorbed intermediate of COOH* on the single Mo atom would shift to the charge accumulation region [12], which will bring more 6
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Fig. 5. (A) The CO FEs at different applied potentials for H-MoS2, N-MoS2 and N-MoS2@NCDs-180 electrodes. (B) LSV curves for H-MoS2, N-MoS2 and N-MoS2@ NCDs-180 electrodes from 0.2 to −1.1 V under CO2-saturated electrolyte.
opportunity for electrons participating in the catalytic reaction [30], therefore the CO2RR can be improved on N-MoS2. As shown in Figure S10, the COOH*-Mo bonds of N-MoS2 were 2.180 Å, shorter than that of MoS2 (2.204 Å), which declared that COOH* desorption on the N-MoS2 surface is harder than that on the MoS2 surface, thus facilitating the subsequent reaction for COOH* on the N-MoS2. According to experiment and calculation results, we proposed the possible mechanism for CO2 electroreduction to CO on N-MoS2 electrode (Fig. 6B). First, the CO2 was combined with a proton-electron pair (H+ + e–) and adsorbed on the single Mo atom, forming the adsorbed COOH*. Because the ΔG of COOH* formation (0.49 eV) on N-MoS2 was lower than that on MoS2 electrode (0.53 eV), thereby generating more COOH* intermediates on the neighbouring Mo atoms. Afterwards, the COOH* automatically reached a more stable configuration on N-MoS2, in which COOH* bonded with the two neighbouring Mo atoms. And then, CO* combined with another proton-electron pair (H+ + e–) to release H2O molecule, which could easier desorb from the N-MoS2 surface than that from MoS2 to form free CO molecule. We deducted, therefore, that the improved catalytic activity toward CO2RR can be achieved through reducing the reaction energy barrier of COOH* intermediate and a weak binding of CO* intermediate with active sites.
framework and NCDs were uniformly distributed on the surface of nanosheets at the same time. In particular, N-MoS2@NCDs-180 with the high N content (8.35 at.%) exhibited excellence catalytic activity toward electroreduction CO2 to CO, as evidenced by a high current density (36.2 mA cm−2) at −0.9 V, a low onset overpotential (130 mV), and a high FE (90.2%) for CO formation. The experiment demonstrated that NCDs increases the conductivity, thus promoting a fast electron transfer and high reduction current density. DFT calculations showed that the N doping in the MoS2 decreased the energy barrier of the COOH* intermediate formation and weakened the adsorption strength of CO*, thus promoting electroreduction CO2 to CO. In addition, the bader charge analysis also found that N doping lead to more electron deposition on the edge Mo of N-MoS2, which would make more opportunity for electrons participating in the electrocatalytic process. The excellent electrocatalytic performances of N-MoS2@NCDs-180 were attributed to the changes in the electronic structure of Mo atoms by the high doped N level, the enhanced conductivity by NCDs. Our results provide a facile and cost-effective strategy to fabricate hybrids of Ndoped transition-metal chalcogenides in combination with carbon dots for CO2 electroreduction, which show high product selectivity, low overpotential and high current density.
4. Conclusions Notes In summary, N-MoS2@NCDs-X has been fabricated by one-step cost effective solvothermal method in the presence of E-MoS2 nanosheets and NMP solvent, in which the N elements were introduced into E-MoS2
The authors declare no competing financial interest.
Fig. 6. (A) Calculated free energy diagrams for CO2 electroreduction to CO on MoS2 and N-MoS2 electrode. (B) Schematic representation of CO formation mechanism on the N-MoS2 monolayer. 7
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Kuilin Lv received his B. S. in applied chemistry from Huaqiao University in 2013 and M. S in Polymer Materials from Beijing University Of Chemical Technology in 2016. He is currently pursuing his Ph.D. under the supervision of Professor Ying Zhu at Beihang University, China. His research interests focus on preparation and characterization of sulfide and metal-free carbon materials for carbon dioxide electroreduction.
Acknowledgements The work was supported by the National Natural Science Foundation of China (51672019, 51872013), the National Key Research and Development Program of China (2017YFA0206902),and the 111 Project (B14009). Appendix A. Supplementary data Supporting characterization and Figures, Numerical simulation. Electrode Calibration. Energy conversion efficiency. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.06.030.
Weiqun Suo majors in chemistry at Beihang University from 2015 to now. He is currently a candidate in Prof. Ying Zhu's group at Beihang University (BUAA). His research interests focus on preparation and characterization of sulfide and metal-free carbon materials for carbon dioxide electroreduction.
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Mingda Shao majors in chemistry at Beihang University from 2015 to now. He is currently a candidate in Prof. Ying Zhu's group at Beihang University (BUAA). His research interests focus on preparation of electrocatalysts for carbon dioxide electroreduction.
Ying Zhu received her B. S. in applied chemistry from Yantai University in 2015. She is currently a Ph. D candidate in Prof. Ying Zhu's group at School of Chemistry, Beihang University. Her research is focused on preparation and characterization of nitrogen doped catalysts for carbon dioxide electroreduction.
Xingpu Wang received his B. S. (2015) and M. S. (2018) in Chemical Engineering and Technology from Northwest Normal University (NWNU), China. He is currently pursuing his Ph.D. under the supervision of Professor Ying Zhu at Beihang University, China. His research interests focus on preparation and characterization of non-precious metal materials and metal-free carbon materials for carbon dioxide electroreduction
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K. Lv, et al. Jingjing Feng received her B. E. in Polymer Material and Engineering from Hebei University of Technology in 2017. She is currently a graduate student in Beihang University. Her research is focused on preparation and characterization of ion exchange membrane for osmotic energy conversion.
Ying Zhu Doctoral Supervisor, Vice dean of School of Chemistry, Beihang University. She has undertaken more than 10 significant programs, including the National 863 Program, National 973 Program and National Science Foundation, and so on. She has already authorized 14 national invention patents, and 6 national and international invention patents are in publicity. She has been awarded as National Natural Science Award of China, Second Prize (2014); Beijing Science and Technology Award, First Prize (2012), New Century Excellent Talents in University, Ministry of Education (2008), K. C. Wong Post-doctoral Research Award Funding, Chinese Academy Sciences (2006).
Mingwei Fang received his B. E. in Chemical engineering and technology from China University of mining and technology, Beijing (CUMTB) in 2018. He is currently a M. E. candidate in Prof. Ying Zhu's group at Beihang University (BUAA). His research interest is focused on carbon dioxide electroreduction.
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Full paper
Nitrogen doped MoS2 and nitrogen doped carbon dots composite catalyst for electroreduction CO2 to CO with high Faradaic efficiency
T
Kuilin Lv, Weiqun Suo, Mingda Shao, Ying Zhu, Xingpu Wang, Jingjing Feng, Mingwei Fang, Ying Zhu∗ Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Advanced Innovation Center for Biomedical Engineering, School of Chemistry, Beihang University, Beijing, 100191, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 electroreduction Nitrogen doped MoS2 NCDs High FE DFT
CO2 utilization by direct electroreduction offers an attractive route for preparing valuable chemicals or alternative liquid fuels, and mitigating the hazardous effects of global warming. Unfortunately, the electroreduction CO2 currently suffers from poor efficiency, low produce selectivity, and large overpotential is required to initiate CO2 reduction due to the thermodynamic stability of CO2. Therefore, the development of low-cost transitionmetal chalcogenides electrocatalysts for high selectivity electroreduction CO2 under considerably low overpotentials are still challenge, due to its high catalytic activity of hydrogen evolution reaction and poor electronic conductivity. Herein, for the first time, N-doped MoS2 nanosheets and N-doped carbon nanodots (N-MoS2@ NCDs) composite was successfully prepared by solvothermal method in the presence of E-MoS2 and DMF solvent. During solvothermal process, N atoms were introduced into MoS2 framework and N-doped CDs were formed on the surface of MoS2 nanosheets at the same time. The optimized N-MoS2@NCDs-180 for electroreduction CO2 displayed a high CO Faradaic efficiency up to 90.2% and a low onset overpotential requirement of 130 mV for CO formation, which were significantly superior to those of the E-MoS2 and other previously reported transitionmetal sulfides electrocatalysts. The experiment verified that the N-doped CDs on the surface MoS2 provided a good electrical conductivity, which accelerated electron transport. Moreover, DFT theoretical calculation demonstrated that the N doping into MoS2 could decrease the energy barrier of the COOH* intermediate formation and create more electrons on the edge Mo of N-MoS2, thereby enhancing catalytic activity of CO2 electroreduction to CO.
1. Introduction Electrochemical CO2 reduction reaction (CO2RR) is considered as the promising strategy for conversion CO2 to value-added chemicals and alternative fuels, which delivers significant environmental and economic benefits [1,2]. However, CO2RR involves multiple protoncoupled electron transfer processes that are kinetically sluggish and in need of high potentials for CO2 reduction, thus leading to a poor product selectivity and low conversion efficiency [3]. To address these issues, therefore, various electrocatalysts, including noble metals [4], carbon materials [5], metal oxides [6], and transition-metal chalcogenides [7] have been developed to drive the CO2RR with high Faradaic efficiency (FE), and product selectivity. Among these electrocatalysts, the 2D transition-metal chalcogenides (MX2, M = Nb, Mo, W; X = S, Se), particularly MoS2, as catalysts for electroreduction CO2 have attracted much attention due to their low-cost and unique sandwich
∗
structure that provides more active sites for electrocatalysis [8]. However, the practical applications of MX2 in electroreduction CO2 has been hampered in aqueous electrolyte, due to their inherent limitations, including the high catalytic activity of hydrogen evolution reaction (HER) and the intrinsically poor electronic conductivity [9]. So the alloying, metal doping and surface modification of MoS2 were found to be efficient ways to tune their electronic structures and interface properties, thus leading to a significant improvement in CO2RR performance and a suppression of HER. For instance, Han et al. [10] developed Mo-Bi bimetallic chalcogenide by direct thermal decomposition of ammonium tetrathiomolybdate and bismuth nitrate, which was used as electrocatalyst for reduction CO2 to methanol with a high methanol FE of 71.2%, onset overpotential of 360 mV for methanol formation, and a current density of 12.1 mA cm−2 at −0.7 V (vs. SHE). Xie et al. [11] fabricated MoSeS alloy monolayers by high temperature doping Se on oil/water interface, which could catalyze the conversion of CO2 into CO
Corresponding author. E-mail address: [email protected] (Y. Zhu).
https://doi.org/10.1016/j.nanoen.2019.06.030 Received 7 May 2019; Received in revised form 8 June 2019; Accepted 13 June 2019 Available online 19 June 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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doped CDs (NCDs) can graciously catalyze the electrochemical reduction of CO2 with high-selectivity and catalytic activity. In the work, N-MoS2@NCDs were prepared by one-pot solvothermal reaction in the presence of the exfoliated MoS2 (E-MoS2) and N, NDimethylformamide (DMF) solvent. E-MoS2 nanosheets were fabricated by wet ball-milling in the large scale [13]. During the solvothermal process, the N elements were doped into MoS2 framework, simultaneously NCDs derived from DMF were formed and homogeneously dispersed on the surface of N-MoS2 nanosheets [18a]. Among them, the as-prepared N-MoS2@NCDs-180 possessed a high N doping content of 8.35 at%, which exhibited high catalytic activity toward electroreduction CO2 to CO with a FE of 90.2% for CO production at −0.9 V (vs. RHE) and an low onset overpotential for CO formation (130 mV), which were significantly superior to those of the origin E-MoS2 (41.2%, 260 mV) and previously reported analogues [9–13,19], as demonstrated in Fig. 4B and Table S3. It was reported that the NCDs decorated on the surface of MoS2 could improve conductivity and lower the onset overpotential, thereby enhancing CO2RR performance. In addition, according to DFT calculation, the N element doping into MoS2 nanosheets could lower the energy barrier of the formation of COOH* intermediates (0.76 eV) on N-MoS2 nanosheet, typically smaller than that on MoS2 nanosheets (0.87 eV), thus facilitating the rate-limiting COOH* intermediate formation step. In addition, according to the bader charge analysis, moreover, the doped N atom can also tailor electronic properties of Mo atom that will create more electrons on the Mo atom edge, and then bring more opportunity for electron participating in reducing CO2 to CO. The doping heteroatoms into transitionmetal chalcogenides may provide an effective strategy to provide costeffective, robust electrocatalysts for CO2RR reduction with a low onset overpotential and high efficiency.
with FE of 45.2% by regulating off-centre charge around the Mo atoms, much larger than that of MoS2 (16.6%) and MoSe2 monolayers (30.5%), and achieved a onset overpotential (430 mV) for CO formation. Abbasi et al. [12] synthesized the vertically aligned niobium (Nb)-doped MoS2 (VA-Mo0.95Nb0.05S2) catalysts by using chemical vapor deposition (CVD), which was used as an electrocatalyst for converting CO2 to CO with a high FE of 82% for CO formation and low onset overpotential (90 mV). The excellent activity was attributed to the embed Nb atom into MoS2 atomic structure, which could modify electronic properties of MoS2. In addition, our recent work [13] reported that the fluorosilane decorated of E-MoS2 nanosheets was able to electroreduce CO2 to CO with a FE of 82% and an onset overpotential of 190 mV, which could be attributed that the F atoms could tune electronic structures of the edge Mo atoms for decreasing free energy of CO2 conversion and provided hydrophobic surface for effectively depressing HER. Although there have been significant advances in CO2RR, developing affordable MoS2based electrocatalyst for CO2RR with a high FE and low overpotential is still a big challenge from a practical point of view. N-doped carbon materials has been demonstrated to be the promising electrocatalysts for oxygen reduction reaction (ORR) [14a,b], Zn-air battery [14c], electrochemical water splitting [14d], and CO2RR [5], which are attributed to the higher electronegativity of N with respect C that can tailor the charge distribution of carbon adjacent to N dopants, thus expediting the electron transport [15]. Inspired by the superior catalytic performance of N-doped carbon material, it is expected that N doping into transition-metal chalcogenides may provide the enhanced CO2RR performance. Recently, a little theoretical and experimental studies have been proved that N doped MoS2 can acted as electrocatalysys for ORR and lithium ion batteries [16,17] For instance, Hao et al. [16] reported that the N-doped MoS2/carbon (N-MoS2/C) for ORR were produced by using the aqueous evaporation-induced selfassembly process. N-MoS2/C showed a maximum power density of 0.815 W m−2, which was far higher than that of Pt/C (0.520 W m−2), and only had a decline of 1.23% after 1800 h operation. The outstanding ORR performances of N-MoS2/C were accounted for by generating abundant defects (pyridinic N, graphitic N and Mo-N), as well as enhancing electric conductivity. Liu et al. [17] reported for the first time that the N-doped MoS2 as anode materials of lithium ion batteries, displayed a higher discharge specific capacity (1130.8 mAh g−1) than bulk MoS2 (898 mAh g−1). Electrochemical measurements and density functional theory (DFT) calculations demonstrated that N doping could enhance the electron conductivity of MoS2, thereby creating the fast transportation of electrons and ions. To the best of our knowledge, nevertheless, doping N element into MoS2 to improve its CO2RR performance has not been reported. Furthermore, the electrochemical performance of the doped MoS2 is still far from ideal, and it still suffers from severe activity fading because of its poor electronic conductivity, structural destruction during cycling. Hybridization of MoS2 with other highly conductive carbon to form hierarchical nanostructures is an effective approach to enhance its conductivity, thus achieving optimal catalytic activity. Carbon dots (CDs) are desirable for an promising component of CO2RR electrocatalysts owing to their large number of exposed edge sites and excellent electronic properties [18]. For instance, Ajayan et al. [18a] synthesized N-doped graphene quantum dots (NGQDs) as electrocatalysis for CO2RR by solvothermal process in the presence of DMF and graphene oxide, which showed a highest total FE of 45% for C2H4 and CH3CH2OH at −0.7 V, due to the high conceration of terminal active sites caused by solvothermal process and the N elements doping. Recently, Liu et al. [18b] found that the NGQDs decorcted-Bi2O3 (Bi2O3-NGQDs) by a solvothermal method in polyol solution exhibited an excellent catalytic activity for CO2 electroreduction into HCOOH with up to 90% FE from −0.9 to −1.2 V. The experiments and DFT calculations demonstrated that NGQDs could increase adsorption capacity of CO2 (ads) and COOH* intermediate, thereby leading to the enhanced the activity of Bi2O3 nanosheets. Accordingly, we expected that combination of N-doped MoS2 nanosheets and N-
2. Experimental section 2.1. Materials Molybdenum (IV) sulfide powder (MoS2, CAS: 1317-33-5) was supplied by Sigma-Aldrich. N, N-Dimethylformamide (DMF, CAS: 6812-2) was purchased from Alfa Aesar. 1-Ethyl-3-methylimidazolium Tetrafluoroborate (EMIM-BF4, CAS: 143314-16-3) was purchased from Shanghai Cheng Jie Chemical Co. LTD. All reagents were used without further treatment.
2.2. Sample preparation The exfoliated MoS2 (E-MoS2) nanosheets was prepared according to our previous report [13]. N-MoS2@NCDs-X was synthesized by solvothermal approach in the presence of E-MoS2 nanosheets in dimethylformamide (DMF) solvent. The typical synthesis was given as follows, 300 mg E-MoS2 was dispersed in 40 mL DMF, and sonicated in ultrasonicator for 60 min to form a homogeneous dispersion. Afterwards, the E-MoS2 suspension was transferred to a 50 mL PTFE liner. The N-MoS2@NCDs-180 was formed in a hydrothermally analogous process at 180 °C for 20 h. In this process, E-MoS2 was further exfoliated and exposed more terminal active sites, and simultaneously doped by N into the carbon lattice and MoS2 with N source from DMF. After cooling naturally, the resultant was washed with deionized water and ethanol for three times and dried at 70 °C for 12 h, yielding the N-MoS2@NCDs180. And N-MoS2@C-200 and N-MoS2@C-220 were synthesized in a PTFE-lined autoclave at 200 and 220 °C, respectively. For comparision, the H-MoS2 was prepared in a similar way, except using water as the solvent. And N-MoS2 was obtained by heating N-MoS2@NCDs-180 in the muffle furnace at 300 °C for 12 h, NCDs could be removed during the heating process.
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MoS2@NCDs-180 (6.65 × 10−3 S/cm) is higher than that of E-MoS2 (2.04 × 10−3 S/cm). The higher magnification HAADF image (Fig. 1C) showed that Mo atoms were the outmost terminating atoms on the edges of the MoS2 nanosheets, as marked by red arrow. According to the previous result reported by Asadi [22], Mo-terminated edges have the lowest formation energy that allows more active edge sites, thus enhancing the performance of CO2RR. The average length of N-MoS2@ NCDs-180 was about 166 nm, as statistically counted by 583 flakes (Figure S2B). TEM image and its corresponding elemental mapping of the N-MoS2@NCDs-180 (Fig. 1D) clearly showed a thin nanosheet with a length of 423 nm, and the homogeneous distribution of Mo, S, C and N atoms, which demonstrated that N elements were successfully introduced into MoS2 nanosheets and carbon dots. As presented in AFM image, N-MoS2@NCDs-180 nanosheets had thickness between 3.58 nm and 4.81 nm, corresponding to 3–5 atomic layers of MoS2 (Figure S3) [13]. Compared to our previous work [13], N-MoS2@NCDs-180 has a smaller size than that of E-MoS2 obtained by direct liquid exfoliation, which was attributed to solvothermal treatment. It is reasonable to assume that a smaller size of N-MoS2@NCDs-180 may expose more Mo edges for the enhanced performance of CO2RR [23]. The molecular structures of the N-MoS2@NCDs-X were analyzed by the X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2A, XRD spectra of N-MoS2@NCDs-X all showed the peaks located at 14.3°, 32.6°, 39.5°, 49.8° and 60.1°, corresponding to the (002), (100), (103), (105) and (008) diffraction reflections of bulk MoS2 (indexed by JCPDS NO. 37–1492), respectively. Compared with E-MoS2, the d-spacing of N-MoS2@NCDs-180 increased to 0.6115 from 0.6019 nm of E-MoS2 (Figure S4A), indicating that N atoms were successfully introduced into the MoS2 framework [24]. And it could be seen that N-MoS2@NCDs-180 had the largest d-spacing among N-MoS2@NCDs-X samples (Fig. 2A), which may deduce that more N atoms were successfully doped into N-MoS2@NCDs-180 during the hydrothermal process [24]. Figure S4B showed the enlarged Raman spectra of E-MoS2 and N-MoS2@NCDs-180 measured by using the laser excitation wavelength of 532 nm. For E-MoS2 sample, the two Raman characteristic bands at 375.3 and 402.1 cm−1 corresponded to the E12g and A1g vibration modes, which had a peak frequency difference of 26.8 cm−1. For N-MoS2@NCDs-180, however, the two the E12g and A1g modes located at 377.1 and 402.2 cm−1 with frequency difference of 25.1 cm−1 that was small than that of E-MoS2, indicating that the MoS2 nanosheets were further cut into smaller pieces by solvothermal method [24]. Raman spectra of N-MoS2@NCDs-200 and N-MoS2@NCDs-220 were shown in Fig. 2B, the peak frequency differences were also calculated to be 25.3 and 25.6 cm−1, respectively, indicating that the size of N-MoS2@NCDs-X nanosheets decreased with increasing the treatment temperature [23]. XPS spectra were conducted to analyze the chemical compositions of Mo and S in N-MoS2@NCDs-X and E-MoS2 samples. As showed in the XPS spectra (Figure S5 A and B), the Mo, S elements were found in all the samples, but the N and C elements were only found in N-MoS2@NCDs-X samples, indicating that N doping and NCDs coating were realized successfully via a facile solvothermal treatment. As shown in Table S1, the element contents of N-MoS2@ NCDs-180 were measured to be 28.23 at% for Mo, 53.99 at% for S, 8.35 at% for N, and 7.41 at% for C. Similarly, the N element contents of N-MoS2@C-200 and N-MoS2@C-220 were calculated to be 6.15 at% and 5.71 at%, which were smaller than that of N-MoS2@NCDs-180. We deduced that the doping contents of N element could be tuned by adjusting solvothermal temperature, and appropriate solvothermal temperature was beneficial to high level of N doping. The high resolution Mo 3d XPS spectrum of E-MoS2 clearly displayed two peaks at 228.82 and 232.15 eV (Fig. 2C), being attributed to the doublet Mo 3d5/2 and Mo 3d3/2 spin doublets, respectively [13]. While high-resolution S 2p XPS spectrum of E-MoS2 (Fig. 2D) showed two the peaks at 161.66 and 162.86 eV related to S 2p3/2 and S 2p1/2 binding energies, respectively, being consistent with the oxidation state of S2− in E-MoS2 [13]. After N doping, however, two peaks of Mo 3d5/2 and Mo 3d3/2 of N-MoS2@
2.3. Characterizations The surface morphology analysis of sample was observed by Field Emission Scanning Electron Microscope (FESEM, JEOL, JSM-7500F). The High-resolution images and lattice structure of samples were observed by HRTEM (JEOL, JEM-2100F) and Scanning Transmission Electron Microscopy (STEM, FEI Titan 80–300, Japan). The thickness of sample was obtained by Atomic force microscopy (AFM, VT STM/ AFM). X-ray diffraction (XRD) patterns were collected on a Shimadzu Xray Diffractometer (XRD-6000) with Cu-Ka1 source (λ = 1.5406 Å). Xray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi with Al Kα radiation. Raman spectra of these samples were analyzed by LabRAM HR800 with a 532 nm laser excitation. 2.4. Computational details The calculations were carried out using density functional theory (DFT) with PBE + U (the effective U is 4.0 eV for d orbital of Mo atom). The Vienna ab-initio simulation package (VASP) was employed. The plane wave energy cutoff was set as 400 eV. The Fermi scheme was employed for electron occupancy with an energy smearing of 0.1 eV. The first Brillouin zone was sampled in the Monkhorst−Pack grid. The 3 × 3 × 1 k-point meshes for the surface calculation. The energy (converged to 1.0 × 10−6 eV/atom) and force (converged to 0.01 eV/ Å) were set as the convergence criterion for geometry optimization. The spin polarization was considered in all calculation. The MoS2 (001) surface was obtained by cutting the MoS2 bulk along the {001} direction. The thickness of surface slab was chosen to be one slab. A vacuum layer as large as 15 Å was used along the c direction normal to the surface to avoid periodic interactions. For the edge of the MoS2 (001), a vacuum layer (∼12 Å) was chosen along the {010} of the MoS2 (001). For N doped configuration, one of the S is replaced by N atom. The free energy (ΔGi) of elementary reaction is defined as follows:
ΔG = ΔEDFT + ΔEZPE − T ΔS where ΔEDFT is the reaction energy of elementary reaction obtained by DFT calculation, i.e. the energy difference between the final and initial states, ΔEZPE is the difference in zero-point energy between the final and initial states, while TΔS is the entropy change of the elementary reaction. 3. Results and discussion 3.1. Preparation and characterization of N-MoS2@NCDs-X N-MoS2@NCDs-X (X represents reaction temperature) were designed and fabricated by a simple, low cost solvothermal approach in the presence of E-MoS2 nanosheets and DMF solvent. E-MoS2 nanosheets have been prepared by our previous work [13]. As the typical electrocatalysts, the morphologies of E-MoS2 and N-MoS2@NCDs-180 were characterized by scanning electron microscopy (SEM), high-resolution TEM (HRTEM), high-angle annular dark-field (HADDF) TEM and atomic force microscopy (AFM). HRTEM images of E-MoS2 (Fig. 1A) displayed a typical fringe of MoS2 crystallite, which was in agreement with the previous report [13]. Compared to E-MoS2, NCDs with the approximate diameter of 6 nm, were grown uniformly on the surface of N-MoS2 nanosheets after solvothermal treatment (Fig. 1B). As presented in Figure S1, two clearly distinct structural areas could be observed. One was a typical lamellar structure of MoS2 (Figure S1B), which was confirmed by the clear lattice fringe with a spacing of 0.63 nm, being assigned to the (002) crystal plane of MoS2 [8,20]. The other was observed for NCDs with a lattice spacing of 0.21 nm (Figure S1B), which corresponds to the (100) facet of graphite [21], and also confirmed by the Fast Fourier Transform analyses (inset of Fig. 1B). After solvothermal treatment, as Table S2 shown, the conductivity of N3
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Fig. 1. (A) HRTEM image of E-MoS2. (B) HRTEM image of N-MoS2@NCDs-180, the blue frame represents NCD and the red frame represents N-MoS2. (C) Higher magnification HAADF image of N-MoS2@NCDs180 shows clearly distinct atomic configuration. (D) TEM image and corresponding elemental mappings of the N-MoS2@C-180 nanosheets.
Fig. 2. (A) XRD patterns and (B) Raman spectra of N-MoS2@NCDs-180, N-MoS2@ NCDs-200 and N-MoS2@NCDs −220, respectively. (C, D) Mo 3d and S 2p XPS spectra of for N-MoS2@NCDs-180 and E-MoS2. 4
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−0.9 V, indicating that N-MoS2@NCDs-180 has an obvious catalytic activity for CO2RR. As shown in Fig. 3A and Figure S8, the onset potential on the N-MoS2@NCDs-180 electrode for CO2 electroreduction was −0.24 V, which was more positive than those of the N-MoS2@C200 electrode (−0.26 V), N-MoS2@C-220 electrode (−0.29 V) and EMoS2 electrode (−0.30 V), thus suggesting a very low onset overpotential (130 mV) for CO formation on N-MoS2@NCDs-180 electrode. Importantly, the N-MoS2@NCDs-180 electrode exhibited a significantly high current density (59 mA cm−2) at −1.1 V, which was higher than that of E-MoS2 nanosheets (48 mA cm−2). The performance of CO2 electroreduction was operated at different applied potentials between −0.7 and −1.05 V in CO2-saturated EMIM-BF4 solutions (94 mol% water). Fig. 3B showed the Faradaic efficiency (FE) for CO production on the E-MoS2, N-MoS2@NCDs-X electrodes at different applied potentials. Under the reported reaction conditions, the main gas products were CO and H2 determined by gas chromatography (GC). And no liquid product could be detected from 1H NMR analysis, as shown in Figure S9. N-MoS2@NCDs-180 with 8.35 at% N content showed that the FE of CO production increased with decreasing the applied potentials and reached a maximum of 90.2% at −0.9 V, larger than those of CO production on N-MoS2@NCDs-200 (82.9%) and N-MoS2@NCDs220 (81.5%) electrodes, showing an outstanding CO product selectivity. While the maximum FE of E-MoS2 electrode for CO formation was only 41.7% at −0.95 V, and the reduction potential of the highest CO production was −0.95 V, more negative than that of N-MoS2@NCDs-180 (−0.9 V). In addition, the CO FE at low applied potentials on E-MoS2, N-MoS2@NCDs-180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 electrode were shown in Fig. 4A. We also studied the onset FE for CO formation to obtain a better perspective on catalytic performance of NMoS2@NCDs-X (Fig. 4A), and the onset FE for CO formation on the N-
NCDs-180 (Fig. 2C) shifted positively to 229.12 and 232.39 eV. Simultaneously, the two peaks of S 2p3/2 and S 2p1/2 of N-MoS2@NCDs180 (Fig. 2D) also moved positively to 161.9 and 163.12 eV, respectively. It was clear that the peaks of Mo 3d5/2 and S 2p3/2 shifted to higher binding energy side, indicating that N element was successfully doped into E-MoS2 [23], which agreed with the EDS and XRD results of the N-MoS2@NCDs-180. Furthermore, as shown in Figure S5C, there was a hump on the side of Mo 3p3/2 peak that originates from N 1s, thus indicating the existence of Mo-N bond (401.7 eV) [25]. The specific N configuration could also be divided into pyridinic N (398.1 eV) and pyrrolic N (399.9 eV), as indicated by the deconvolution N1s peak (Figure S5C), which further confirmed the presence of C-N bonds in the N-MoS2@NCDs-180 [26]. 3.2. CO2 electroreduction on N-MoS2@NCDs-X catalysts The electrocatalytic activity and selectivity of N-MoS2@NCDs-X toward CO2RR were evaluated in a flow double electrolytic cell, in which N-MoS2@NCDs-X were loaded on a 1 × 2 cm2 area of glass carbon (GC) without using any binder. The linear sweep voltammetry (LSV) were conducted to evaluate the catalytic performance in N2 or CO2 saturated 6 mol% EMIM-BF4 and 94 mol% water solutions (pH∼4.24), as demonstrated by our previous work [13]. It was reported that the EMIM+ with CO2 can form stable [EMIM-CO2]+ intermediate to promote the generation of CO in the EMIM-BF4 aqueous electrolyte [22,27]. As given in curves of Figure S7A, the N-MoS2@ NCDs-180 in N2-saturated solution delivered around 25 mA cm−2 current density at −0.9 V (vs. RHE; all potentials are reported with respect to RHE), that was attributed to HER. Notably, in CO2-saturated electrolyte, while a current density of about 36.2 mA cm−2 was obtained at
Fig. 3. (A) LSV curves for E-MoS2, N-MoS2@NCDs-X electrodes at a sweep rate of 0.02 Vs-1 in CO2-saturated EMIM-BF4 solutions (94 mol% water). (B) The CO FE at different applied potentials for E-MoS2, N-MoS2@NCDs-X (X: 180, 200, 220) electrodes in EMIM-BF4 solutions (94 mol% water). (C) Tafel plots for CO production on N-MoS2@NCDs-X electrodes in EMIM-BF4 solutions (94 mol% water). (D) The FE ratio of CO and H2 on N-MoS2@NCDs-180; Partial CO current density plots of EMoS2 and N-MoS2@NCDs-180 at different applied potentials. 5
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Fig. 4. (A) The FEs for CO formation at low applied potentials on E-MoS2, N-MoS2@NCDs-180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 electrodes. (B) The CO FEs and onset overpotentials for CO formation at the applied potentials on different doped transition-metal chalcogenides catalysts. (The numbers represent relevant literature reports; the star symbol represents our work).
The XRD and Raman spectra of N-MoS2 have been measured, as shown in Figure S6B and C. It was easy to found that the d-spacing and the peak frequency difference of N-MoS2 were similar with those of NMoS2@NCDs-180, indicting that the crystal structures of N-MoS2 was not affected by the existence of NCDs. As shown in Fig. 5B, the highest CO FE of 88.2% on N-MoS2 was closed to that of N-MoS2@NCDs-180 (90.2%) at −0.9 V, but the current density of N-MoS2 (26.7 mA cm−2) was obvious inferior to N-MoS2@NCDs-180 (36.2 mA cm−2) at −0.9 V, which indicated the NCDs obviously enhance the reduction current density due to its fast transportation of electrons (Table S2). In addition, the onset potential for CO formation on the N-MoS2 elelctrode was −0.45 V, which was more negative than that of on the N-MoS2@NCDs180 elelctrode (−0.24 V). Hence, NCDs on the surface of N-MoS2 could not only enhance materials conductivity, but also lower the onset overpotential. It is therefore clear that N doping transition-metal chalcogenide with compositing NCDs is a promising strategy to improve their catalytic activity of CO2RR.
MoS2@NCDs-180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 were 4.2% (−0.24 V), 2.8% (−0.26 V) and 3.6% (−0.29 V), respectively. This result indicated that CO formed at a lower applied potential on NMoS2@NCDs-X electrode, which was also consistent with the LSV results (Fig. 3A). What's more, the high FE (90.2%) and low onset potentials (130 mV) on N-MoS2@NCDs-180 were significantly superior to those of the vast majority of reported the doped transition-metal chalcogenides [9–13,19], as demonstrated in Fig. 4B and Table S3. The Tafel slopes were employed to determine the CO2 reduction kinetics (Fig. 3C), N-MoS2@NCDs-X all exhibited similar linear responses with respect to overpotentials. The Tafel slopes of N-MoS2@NCDs-180, NMoS2@NCDs-200 and N-MoS2@NCDs-220 were measured to be 101.9, 108.6, and 126.8 mV dec−1, respectively. The Tafel slope of 118 mV dec−1 is a theoretical value, which indicates that the chemical ratedetermining step is an initial single-electron transfer step [28,29]. Therefore, N-MoS2@NCDs-180 exhibited a much lower Tafel slope than that of N-MoS2@NCDs-200 and N-MoS2@NCDs-220, evidencing the improved kinetic activity for electroreduction CO2 to CO over N-MoS2@ NCDs-180. As shown in Fig. 3D, the FE ratios of CO/H2 indicated that competitive HER took place at low potentials from −0.7 to −0.85 V, but CO2 electroreduction was predominantly prone to occur from −0.9 to −1.1 V on the N-MoS2@NCDs-180 electrode. And the partial CO current density of N-MoS2@NCDs-180 was approximately 4 times larger than that of E-MoS2 at applied potentials of −0.9 V. Moreover, the durability is another important criterion to evaluate the suitability of electrocatalysts for practical applications. The chronoamperometric responses were thus performed 10 h in a CO2-saturated electrolyte. As shown in Figure S7B, the total current densities on the N-MoS2@NCDs180, N-MoS2@NCDs-200 and N-MoS2@NCDs-220 electrodes all showed no attenuation activity of CO2RR during the reduction process of 10 h, displaying a good long-time stability due to the existence of NCDs on the surface of N-MoS2. To illuminate the role of N doping on MoS2, H-MoS2 was prepared by hydrothermal treatment of E-MoS2 at 180 °C in the absence of DMF solvent. As shown in Fig. 5A, the highest CO FE of 67% on H-MoS2 was clearly inferior to that of N-MoS2@NCDs-180 (90.2%), but higher than that of E-MoS2 (41.7%) at −0.95 V. These results may be attributed to the fact that the N doping into MoS2 framework could tune its electron structures and produced many terminal active sites, thus leading to the enhanced electrocatalytic activity for reduction CO2 to CO [18a]. Moreover, to prove the effect of NCDs, N-MoS2@NCDs-180 was heated in a muffle furnace at 300 °C for 12 h to remove NCDs, which was denoted as N-MoS2. As showed in Figure S6A and Table S1, the C element content in N-MoS2 sample almost disappeared after the heat treating, and the N element content also reduced from 8.35 at.% to 5.96 at.%.
3.3. DFT calculations and reaction mechanism of CO2RR To deeply discuss the activity of N-MoS2@NCDs-X for electroreduction CO2 to CO, the first principle calculation based on density functional theory (DFT) of has been employed to investigate the reaction mechanism of CO2RR. Due to the role of NCDs in improving CO2RR performance, therefore, the N-MoS2 was chosen as model molecule for calculation. DFT results showed that the adsorption of COOH* and CO* on MoS2 and N-MoS2 electrodes were exothermic, as shown in Fig. 6A and Table S4. Compared with the MoS2, the N-MoS2 monolayers had the obviously lower formation energy of COOH* intermediate than MoS2 monolayers, thereby indicating that the formation of COOH* is more favorable on N-MoS2 surface. In addition, desorption of CO* intermediate had ΔG of 0.49 eV on N-MoS2 which was slightly lower than ΔG of 0.53 eV on MoS2, weakening the adsorption strength of CO* with the Mo atom on N-MoS2 was another factor to promote the catalytic conversion CO2 to CO. Taken together, DFT theoretical calculations confirmed that N doping into MoS2 is beneficial for CO2 electroreduction to CO. To further explain these results, the bader charge analysis has been employed. The results indicated that the electrons on the MoS2 were redistributed, after N atom was doped into MoS2. The valence electron of Mo in N-MoS2 was simulated to be 5.02 e, which is higher than that of pure MoS2 (the valence electron of Mo: 4.87 e), more electron will be deposited on the edge Mo of N-MoS2, which led to a biased electron density of Mo atom in the N-MoS2. It is known that the adsorbed intermediate of COOH* on the single Mo atom would shift to the charge accumulation region [12], which will bring more 6
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Fig. 5. (A) The CO FEs at different applied potentials for H-MoS2, N-MoS2 and N-MoS2@NCDs-180 electrodes. (B) LSV curves for H-MoS2, N-MoS2 and N-MoS2@ NCDs-180 electrodes from 0.2 to −1.1 V under CO2-saturated electrolyte.
opportunity for electrons participating in the catalytic reaction [30], therefore the CO2RR can be improved on N-MoS2. As shown in Figure S10, the COOH*-Mo bonds of N-MoS2 were 2.180 Å, shorter than that of MoS2 (2.204 Å), which declared that COOH* desorption on the N-MoS2 surface is harder than that on the MoS2 surface, thus facilitating the subsequent reaction for COOH* on the N-MoS2. According to experiment and calculation results, we proposed the possible mechanism for CO2 electroreduction to CO on N-MoS2 electrode (Fig. 6B). First, the CO2 was combined with a proton-electron pair (H+ + e–) and adsorbed on the single Mo atom, forming the adsorbed COOH*. Because the ΔG of COOH* formation (0.49 eV) on N-MoS2 was lower than that on MoS2 electrode (0.53 eV), thereby generating more COOH* intermediates on the neighbouring Mo atoms. Afterwards, the COOH* automatically reached a more stable configuration on N-MoS2, in which COOH* bonded with the two neighbouring Mo atoms. And then, CO* combined with another proton-electron pair (H+ + e–) to release H2O molecule, which could easier desorb from the N-MoS2 surface than that from MoS2 to form free CO molecule. We deducted, therefore, that the improved catalytic activity toward CO2RR can be achieved through reducing the reaction energy barrier of COOH* intermediate and a weak binding of CO* intermediate with active sites.
framework and NCDs were uniformly distributed on the surface of nanosheets at the same time. In particular, N-MoS2@NCDs-180 with the high N content (8.35 at.%) exhibited excellence catalytic activity toward electroreduction CO2 to CO, as evidenced by a high current density (36.2 mA cm−2) at −0.9 V, a low onset overpotential (130 mV), and a high FE (90.2%) for CO formation. The experiment demonstrated that NCDs increases the conductivity, thus promoting a fast electron transfer and high reduction current density. DFT calculations showed that the N doping in the MoS2 decreased the energy barrier of the COOH* intermediate formation and weakened the adsorption strength of CO*, thus promoting electroreduction CO2 to CO. In addition, the bader charge analysis also found that N doping lead to more electron deposition on the edge Mo of N-MoS2, which would make more opportunity for electrons participating in the electrocatalytic process. The excellent electrocatalytic performances of N-MoS2@NCDs-180 were attributed to the changes in the electronic structure of Mo atoms by the high doped N level, the enhanced conductivity by NCDs. Our results provide a facile and cost-effective strategy to fabricate hybrids of Ndoped transition-metal chalcogenides in combination with carbon dots for CO2 electroreduction, which show high product selectivity, low overpotential and high current density.
4. Conclusions Notes In summary, N-MoS2@NCDs-X has been fabricated by one-step cost effective solvothermal method in the presence of E-MoS2 nanosheets and NMP solvent, in which the N elements were introduced into E-MoS2
The authors declare no competing financial interest.
Fig. 6. (A) Calculated free energy diagrams for CO2 electroreduction to CO on MoS2 and N-MoS2 electrode. (B) Schematic representation of CO formation mechanism on the N-MoS2 monolayer. 7
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Kuilin Lv received his B. S. in applied chemistry from Huaqiao University in 2013 and M. S in Polymer Materials from Beijing University Of Chemical Technology in 2016. He is currently pursuing his Ph.D. under the supervision of Professor Ying Zhu at Beihang University, China. His research interests focus on preparation and characterization of sulfide and metal-free carbon materials for carbon dioxide electroreduction.
Acknowledgements The work was supported by the National Natural Science Foundation of China (51672019, 51872013), the National Key Research and Development Program of China (2017YFA0206902),and the 111 Project (B14009). Appendix A. Supplementary data Supporting characterization and Figures, Numerical simulation. Electrode Calibration. Energy conversion efficiency. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.06.030.
Weiqun Suo majors in chemistry at Beihang University from 2015 to now. He is currently a candidate in Prof. Ying Zhu's group at Beihang University (BUAA). His research interests focus on preparation and characterization of sulfide and metal-free carbon materials for carbon dioxide electroreduction.
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Mingda Shao majors in chemistry at Beihang University from 2015 to now. He is currently a candidate in Prof. Ying Zhu's group at Beihang University (BUAA). His research interests focus on preparation of electrocatalysts for carbon dioxide electroreduction.
Ying Zhu received her B. S. in applied chemistry from Yantai University in 2015. She is currently a Ph. D candidate in Prof. Ying Zhu's group at School of Chemistry, Beihang University. Her research is focused on preparation and characterization of nitrogen doped catalysts for carbon dioxide electroreduction.
Xingpu Wang received his B. S. (2015) and M. S. (2018) in Chemical Engineering and Technology from Northwest Normal University (NWNU), China. He is currently pursuing his Ph.D. under the supervision of Professor Ying Zhu at Beihang University, China. His research interests focus on preparation and characterization of non-precious metal materials and metal-free carbon materials for carbon dioxide electroreduction
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Nano Energy 63 (2019) 103834
K. Lv, et al. Jingjing Feng received her B. E. in Polymer Material and Engineering from Hebei University of Technology in 2017. She is currently a graduate student in Beihang University. Her research is focused on preparation and characterization of ion exchange membrane for osmotic energy conversion.
Ying Zhu Doctoral Supervisor, Vice dean of School of Chemistry, Beihang University. She has undertaken more than 10 significant programs, including the National 863 Program, National 973 Program and National Science Foundation, and so on. She has already authorized 14 national invention patents, and 6 national and international invention patents are in publicity. She has been awarded as National Natural Science Award of China, Second Prize (2014); Beijing Science and Technology Award, First Prize (2012), New Century Excellent Talents in University, Ministry of Education (2008), K. C. Wong Post-doctoral Research Award Funding, Chinese Academy Sciences (2006).
Mingwei Fang received his B. E. in Chemical engineering and technology from China University of mining and technology, Beijing (CUMTB) in 2018. He is currently a M. E. candidate in Prof. Ying Zhu's group at Beihang University (BUAA). His research interest is focused on carbon dioxide electroreduction.
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