Applied Science and Convergence Technology 2023; 32(2): 48-53
Published online March 30, 2023
Copyright © The Korean Vacuum Society.
Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea
†These authors contributed equally to this work.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Molybdenum disulfide (MoS2) is a promising material for energy and environmental applications. In this paper, we report the direct hydrothermal growth of MoS2 on a Mo support and its application in the rarely explored electrochemical CO2 reduction (EC CO2R) process. To investigate the effects of metal overlayers and the MoS2 support, Au, Ag, and Cu were sputter-deposited on a MoS2 electrode. Large amounts of CH4, C2−3 hydrocarbons, and formate were produced via EC CO2R on bare MoS2. The introduction of a Au overlayer on MoS2 enhanced the production of CO, methanol, and formate. Furthermore, the alkanes (CnH2n+2, n = 2, 3) to alkenes (CnH2n, n = 2, 3) ratio was dependent on the applied potential and overlayer metals. Notably, photoirradiation remarkably increased the CO and C2H4 concentrations by 28-fold and 10-fold, respectively. These findings provide valuable insights for the development of MoS2-based materials for CO2 recycling.
Keywords: Molybdenum disulfide, Electrochemical CO2 reduction, Photoirradiation, Formate, CO hydrocarbons
Molybdenum disulfide (MoS2), a well-known two-dimensional (2D)-layered transition metal dichalcogenide, has been extensively studied for various applications, including optoelectronic devices, energy storage, and catalysis [1–9]. In particular, MoS2 exhibits a high activity for photocatalyzed and electrocatalyzed hydrogen evolution reactions (HER) [4–8,10,11]. However, the MoS2-assisted catalytic CO2 reduction process has been rarely experimentally explored [12–27]. According to previous studies, CO is obtained as the major product of the photocatalytic reduction of CO2 over MoS2 (prepared by chemical vapor deposition) . Hybrid catalysts, such as Bi2S3/MoS2 (
MoS2 exhibits a low activity for electrochemical CO2 reduction (EC CO2R) because of its poor electron mobility . Density functional theory calculations show that surface modification through metal doping can suppress the HER and enhance the CO2 reduction activity and selectivity [10,16–18]. For example, theoretically, the catalytic activity of a single-atom Ni1/MoS2 catalyst for electrocatalytic CO2 reduction to produce methanol is higher than that of other metals . Moreover, ionic liquids are used in EC CO2R experiments [19–23]. Asadi
As mentioned earlier, the application of MoS2 in EC CO2R has been rarely studied, and its use as an electrode support remains hitherto unexplored, which motivated us to explore the potential of MoS2 as a support material for EC CO2. Additionally, we investigated the possible synergistic effects of using Au, Ag, and Cu as overlayer metals to improve CO2 reduction activity. Furthermore, the formation of an interface between the metal and MoS2 could provide new paths for CO2 reduction. Au and Ag were selected as preferred materials because of their high FE values for CO production during EC CO2R, as demonstrated in previous studies [28–32]. By contrast, Cu is a promising material for the production of C2+ (oxygenated) hydrocarbons [33–37]. These three metals served as excellent model overlayer metals for the EC CO2R experiments. In this study, MoS2 was directly grown on a Mo metal support and used as an electrode without applying further electrode preparation methods (such as mixing with a Nafion solution or pasting the sample on a carbon electrode), in stark contrast to the methods reported in previous studies [20–25]. The results of the EC CO2R tests, including the information on the reduction products, provide valuable insights into the development of MoS2 electrocatalysts.
To prepare the MoS2/Mo sheets, 30 mm × 50 mm × 0.1 mm Mo sheets were first cleaned by sonication in deionized water for 10 min and then subjected to ozone cleaning. The cleaned Mo sheets were dipped in a thiourea solution (0.84 g in 70 mL of deionized water) and tightly secured in a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was then heated to 180 °C and maintained at this temperature for 24 h. After the reaction, the autoclave was allowed to cool naturally, and the sheet was removed, gently cleaned using deionized water, and dried under an infrared lamp. This resulted in the growth of ultrathin MoS2 layers on the Mo metal sheet (MoS2/Mo electrode). To prepare the Au/MoS2/Mo, Cu/MoS2/M, and Ag/MoS2/Mo electrodes, Au, Cu, and Ag metals were deposited on the MoS2/Mo sheets, respectively, using an SPT-20 ion sputter coater (COXEM Co., Korea) for 600 s at 3 mA. For reference characterization, a MoS2 nanopowder sample was prepared by completely dissolving 1 mmol of (NH4)6Mo7O24·4 H2O (≥ 99.0 %, Sigma-Aldrich) and 10 mmol of thiourea (≥ 99.0 %, Sigma-Aldrich) in 30 mL of deionized water mixed with 5 mL of 1-methyl-2-pyrrolidinone (≥ 99.0 %, Sigma-Aldrich). The resulting solution was then placed in a Teflon-lined stainless-steel autoclave at 180 °C for 24 h. After the reaction, the resulting powder sample was completely cleaned using deionized water and ethanol, repeatedly centrifuged, and dried in an oven at 80 °C for 24 h for further characterization. The procedure for preparing the MoS2/Mo electrodes is illustrated in Fig. 1.
The surface morphology of the MoS2/Mo sheet was examined by scanning electron microscopy (SEM; S-4800, Hitachi, Ltd.) and compared with that of the MoS2 powder. High-resolution transmission electron microscopy (HRTEM; FEI Tecnai G2 F30 S-TWIN TEM) was performed at 300 kV. X-ray diffraction (XRD) patterns were obtained using a MiniFlex II X-ray diffractometer (Rigaku Corp., CNU Chemistry Core Facility center) with Cu Kα (40 kV and 30 mA) radiation to examine the crystal structures of the samples. Raman spectra were obtained using an ultraviolet (UV)–visible–near infrared Raman spectrometer (Horiba Jobin Yvon LabRAM HR-800, 514 nm laser, 100× objective, and 1800 grating). X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo Scientific K-alpha+ spectrometer (a hemispherical energy analyzer and monochromated Al Kα X-rays) to investigate the chemical composition and electronic structure of the samples.
For the EC CO2R reaction, a three-electrode system, comprising a Pt counter electrode, Ag/AgCl/3.0 M KCl reference electrode, and MoS2 working electrode, was used (Fig. 1). The dimensions of the working electrode were 5 mm × 30 mm × 0.1 mm. A potentiostat/galvanostat (WizECM-1200 Premium, WizMAC, Inc.) was used to control the potential, and 0.1 M KHCO3 (50 mL) in a 100 mL glass cell was used as the electrolyte. Prior to the EC CO2R, CO2 gas (99.999 %) was fully bubbled through the electrolyte. Further, amperometry experiments were conducted for 1 h at a fixed negative potential. Next, the photoelectrochemical (PEC) CO2R process was carried out under a 395-nm UV light source (6.54 mW·cm−2), and the corresponding experimental conditions were the same as those in the dark.
The gas and liquid products were analyzed after the EC CO2R and PEC CO2R reactions using gas chromatography (GC) and nuclear magnetic resonance (NMR) spectroscopy, respectively. For the GC analysis, the gas phase (0.5 mL) was injected into a YL 6500 gas chromatograph (Young In Chromass Co., Ltd.) equipped with a Ni catalyst methanizer assembly, thermal conductivity detector, and flame ionization detector. The gases were separated using 40/60 Carboxen-1000 and HP-Plot Q-PT columns. The liquid phase (0.5 mL) was analyzed using a 600 MHz FT-NMR spectrometer (AVANCE III, Bruker Corp.).
Figure 2(a) shows the linear sweep voltammetry (LSV) curves obtained from a MoS2 sheet immersed in N2 and CO2-saturated 0.1 M KHCO3 electrolytes. Evidently, the current density (CD) is slightly higher under the CO2-saturated condition possibly because of the CO2 reduction current. To investigate these observations in more detail, EC CO2R amperometry tests were performed at applied potentials of −2.0, −2.2, and −2.4 V (vs. Ag/AgCl); the corresponding CD values were determined to be in the range of 10–15 mA cm−2. Further, the corresponding GC profiles of the gaseous products are shown in Figs. 2(b) and 2(c). The NMR spectra shown in Fig. 2(d) indicate that formate is a liquid product, and Fig. 2(e) reveals that H2, with a concentration range of 40,000–100,000 ppm, is the dominant product. Other minor products include CO, C2H4, C2H6, and C3H8. Figure 2(f) shows that the FE of H2 is between 23 and 44 % and increases with the increasing applied potential. Further, the concentrations of the products CO, C2H4, C2H6, and C3H8 are in the ranges of 3.3–4.7, 1.7–2.7, 0.28–1.2, and 0.4–0.6 ppm, respectively [Fig. 2(g)]. In contrast, CH4 is produced in larger amounts, ranging from 17.5 to 23.8 ppm, which indicates that CH4 is the main CO2 reduction product. The FE values of CH4 and formate are the same (only 0.1 %) [Fig. 2(h)]. These results suggest that although the EC CO2R activity of MoS2 is extremely low, the minor products are crucial for understanding the CO2 adsorption and hydrogenation characteristics of the fabricated electrode system.
Subsequently, the PEC CO2R tests were performed to evaluate the feasibility of using the MoS2 sheets for CO2 reduction [7,8,38]. Figure 3(a) shows the results of the PEC CO2R amperometry test performed over the MoS2 sheet at −2.0 V (vs. Ag/AgCl) in a 0.1 M KHCO3 electrolyte; additionally, the sheet was irradiated by light of wavelength 395 nm for 1 h during the PEC test. The resulting GC profiles are shown in Figs. 3(b) and 3(c), and peaks corresponding to formate production appear in the NMR spectra in Fig. 3(d). Figures 3(e) and 3(f) indicate that the H2 signal intensity decreases from 34,873 ppm (FE = 23.2 %) to 23,191 ppm (FE = 14.7 %) upon photoirradiation, whereas the intensities of the CO and C2H4 signals drastically increase as shown in Fig. 3(g). Additionally, upon photoirradiation, the production of CH4 increases, whereas that of C2H6 and C3H8 slightly decreases. These results suggest that a decrease in the H2 production leads to a corresponding decrease in the saturated alkane production owing to the reduction in the number of hydrogen atoms on the electrode surface. Further, the CO concentration increases from 3.3 to 92.8 ppm, indicating a 28-fold production enhancement, consistent with previously reported results [39–41]. Similarly, the C2H4 and CH4 concentrations increase from 0.9 to 8.6 ppm and 19.2 to 27.5 ppm, indicating 9.6 and 1.4 times production enhancements, respectively. Figure 3(h) indicates that the FE of formate slightly decreases from 0.09 to 0.08 % upon photoirradiation. These results collectively validate the feasibility of using MoS2 in PEC CO2R reactions. Further- Figure 3. (a) Amperometry i–t curves of the MoS2/Mo electrode under dark conditions and a 395-nm light source, (b) and (c) GC profiles, (d) NMR spectra, (e) H2 concentration, (f) FE values of the EC products, (g) CO and hydrocarbon concentrations, and (h) FE values of CO, the hydrocarbons, and formate. more, the CO and C2H4 productions appear to be correlated, whereas the H2 production is correlated with that of the saturated alkanes.
The surface of MoS2 was modified by sputter-deposition of Au, Cu, and Ag for 600 s, and their EC CO2R performances were examined at an applied potential of −2.0 V (vs. Ag/AgCl) in a 0.1 M KHCO3 electrolyte. The gaseous and liquid products can be identified from the corresponding GC and NMR profiles shown in Figs. 4(a)–(c). The deposition of Au, Cu, and Ag decreases the amount (or FE) of H2 produced from 23.2 to 14.2, 15.6, and 13.5 %, respectively [Figs. 4(d) and 4(e)]. The CH4 concentration also decreases from 19 to 12–13 ppm upon the metal deposition [Fig. 4(f)]. By contrast, the produced concentration and FE of CO are enhanced by the deposited metals; the highest increase in CO concentration (12×) is observed upon Au deposition (from 3.3 to 39.7 ppm) [Fig. 4(f)]. Interestingly, the production of C2H4 and C2H6 on the Au/MoS2 electrode shows different behaviors. The GC profiles in the inset in Fig. 4(a) indicate that upon Au deposition, the concentration of C2H4 substantially increases from 0.9 to 3.5 ppm, while that of C2H6 decreases from 1.9 to 0.6 ppm. This concentration trend suggests different production pathways for these two compounds. Conversely, the deposition of Cu and Ag results in a slight decrease in the concentrations of the products C2H4 and C2H6. The amount of C3H8, which is extremely low, decreases upon the metal deposition. However, the hydrocarbons C3H6, C4H10, and C4H8 are detected after the Au deposition [Figs. 4(b) and 4(g)].
The NMR spectra shown in Fig. 4(c) reveal that compared to bare MoS2 (FE = 0.09 %), the Au- and Cu-deposited MoS2 sheet exhibits an enhanced formate production, and the corresponding FE values increase by 1.3 and 2.1 times, respectively [Fig. 4(h)]. In addition, methanol is detected on Au/MoS2, which is confirmed by the amperometry test results obtained at various applied potentials. Further, EC CO2R amperometry tests were conducted over the Au/MoS2 sheet in a 0.1 M KHCO3 electrolyte at applied potentials of −2.0, −2.2, and −2.4 V (vs. Ag/AgCl). The corresponding CD values were in the range of 8–14 mA cm−2. The gas and liquid products can be identified from the corresponding GC and NMR profiles shown in Figs. 5(a)–(c). Figures 5(d) and 5(e) show that the concentration of H2 decreases more at −2.2 V than at −2.0 V, whereas it substantially increases at −2.4 V. In contrast, Fig. 5(f) indicates that concentrations of CO and CH4 significantly increase by 8.1 and 2.1, respectively, compared to those (39.7 and 12.3 ppm) at −2.0 V, and are maximized at −2.2 V; the corresponding FE values are 0.29 and 0.09 %, respectively. At an applied potential of −2.4 V, 133.0 and 21.6 ppm of CO and CH4 are produced, respectively, indicating that the concentrations of CO and CH4 produced at −2.4 V are higher than those produced at −2.2 V. As shown in Fig. 5(g), the concentration of C2H4 decreases with the increasing potential, whereas those of C2H6 and C3H8increase with the increasing potential, implying that the amount of C2H4produced is inversely proportional to that of C2H6 and C3H8.
The resulting NMR spectra [Fig. 5(c)] and determined FE values of the liquid products [Fig. 5(h)] indicate that the maximum amount of formate is produced at −2.2 V with an FE of 0.49 %. The intensity of the NMR peak of methanol increases with the increasing potential, and the maximum FE of methanol (0.49 %) is achieved at −2.2 V. The FE values of CO, CH4, methanol, and formate maximize at −2.2 V, while that of H2 minimizes and that of C2H4 is the highest at −2.0 V. Conversely, the FE values of C2H6, C3H8, and C4H10 maximize at the highest potential of −2.4 V.
The selected MoS2/Mo and Au/MoS2/Mo electrodes were further characterized by XRD, Raman spectroscopy, SEM, and HRTEM. The XRD patterns of the MoS2/Mo sheet [Fig. 6(a)] show peaks at 2θ = 40.5 ° (vw), 58.6 ° (vs), and 73.6 ° (vs), which can be assigned to the (011), (002), and (112) crystal planes of the cubic-phase metallic Mo, respectively. These XRD signals can be mainly attributed to the Mo support. Further, no XRD signal of MoS2 is observed because of the extremely small thickness of the MoS2 layer on the Mo support. Thus, to examine the XRD peaks of the ultrathin MoS2 nanosheets, a bulk powder sample was synthesized. The corresponding XRD patterns [Fig. 6(a)] show two major broad peaks around 2θ = 32 and 57 °, which can be attributed to the (100) and (110) planes of MoS2, respectively [7,8,25]. The Raman spectrum of MoS2/Mo [Fig. 6(b)] shows two peaks at 378 and 405 cm−1, which can be ascribed to the E2g and A1g modes of MoS2, respectively [20,25,38].
The SEM image of the bare MoS2/Mo electrode [Figs. 6(c) and 6(d)] exhibits a uniform morphology of the ultrathin 2D nanosheets [7,8], indicating the successful and uniform growth of MoS2 nanosheets on the Mo support. Additionally, the SEM image [Fig. 6(e)] of the MoS2 powder sample shows a microstructure similar to that of the MoS2/Mo electrode and thus confirms the successful growth of ultrathin MoS2 nanosheets on the Mo support. The morphology of the MoS2/Mo electrode remains stable after the EC CO2R reaction at −2.0 V, indicating the stability of the electrode during EC CO2R [Fig. 6(f)]. The SEM image of Au/MoS2 [Fig. 6(g)] shows small clusters (sputter-deposited Au) that cover the nanosheets, indicating a well-formed interface between Au and MoS2. The morphology remains stable after the EC CO2R at −2.0 V [Fig. 6(h)]. The MoS2 nanosheet powder sample was further characterized by TEM [Figs. 6(i) and 6(j)], and the corresponding results reveal an ultrathin morphology with a clear lattice spacing [Fig. 6(j)]. The interlayer spacing of 0.65 nm corresponds to the (002) plane of MoS2 . The XRD, Raman, SEM, and TEM/HRTEM characterization results demonstrate the successful growth of ultrathin MoS2 nanosheets on the Mo support as well as the formation of a uniform interface between Au and MoS2.
Figure 7 presents the results of the XPS analysis, showing the oxidation states of the MoS2/Mo electrode and MoS2 powder sample. In Fig. 7(a), two major Mo 3
Figure 8 illustrates the reaction pathways of the products obtained in this study. H2, the dominant product, is generated through the adsorbed H (Had) process, which involves the initial formation of Had via H+ +
As previously elucidated, the MoS2 support demonstrated a weak synergistic effect on the EC CO2R process, resulting in an FE of < 2 % for the carbon products. In contrast, for the EC CO2R process over Au/TiC, the corresponding CO FE was higher than 40 % . Our results for the EC CO2R process and products obtained using the Au/MoS2 electrode provide valuable insights that can be implemented to realize highly efficient MoS2-based electrodes.
In summary, we successfully synthesized ultrathin MoS2 nanosheets on a Mo support using a hydrothermal method and investigated the feasibility of applying them in EC CO2R reactions. Under dark conditions, CH4 was the predominant CO2R product over MoS2, whereas CO production dominated under photoirradiation. Furthermore, UV light significantly enhanced CO desorption, resulting in a 28-fold increase in CO production and a 10-fold increase in C2H4 production. The deposition of Au also increased the production of CO. The alkanes (C
This research was supported by a research grant awarded by Chungnam National University (Project No. 2020-0690-01).
The authors declare no conflicts of interest.