1. Introduction
2. Experimental Procedure
2.1. Chemicals
2.2. Synthesis of WCu-C/N composite
2.3. Electrochemical measurement
3. Results and Discussion
3.1. Characterization
3.2. Electrochemical performance
3.3. Reaction mechanism of WCu-C/N for CO2 reduction to CO
4. Conclusion
1. Introduction
To reduce the concentration of CO2 in the atmosphere, domestic and foreign researchers have proposed many solutions, principally involving reducing CO2 emissions at source, capturing CO2 and converting CO2 into compounds with an application value.1) In contrast, the electrocatalytic reduction reaction of CO2 (CO2RR) not only reduces CO2 to fuels such as CO and organic small molecules, such as formates, alcohols, hydrocarbons, alkanes, etc., but also is an efficient method of storing renewable energy. In this way, an artificial carbon cycle is expected to be realised under moderate and controlled conditions.2) Among all possible products, CO2RR to CO is considered as a more accessible and utilizable option, because CO can readily be used as a feedstock for value-added chemicals and fuels in existing downstream thermochemical reactions. And it has the advantages of high activity, high selectivity and excellent stability.3)
In the last few decades, researchers began from the starting point of the high energetic nature of electrocatalytic reduction to search for catalytic materials with a high selectivity, stability, and high current density at low potentials of CO2. And have studied numerous metals and alloys and screened gold, silver, copper, tin, tin oxides, copper alloys, and nitrogen-doped carbon, all of which are considered to be promising materials.4) Although many studies have been carried out on electrocatalytic reduction of CO2 to CO, they still face technical obstacles such as high overpotential, low Faraday efficiency (FE) and weak product selectivity.5) Among the materials used for the catalytic reduction of CO2, copper-based materials show excellent ability in adsorption and conversion of CO2, firmly adsorbing CO∙ intermediates to produce CO.6) However, most of the copper-based catalysts suffer from a series of structure-dependent problems-prone to collapsing and resulting in higher overpotentials, deterioration of CO2RR selectivity and lifetime Cu(I)-based catalysts are susceptible to a range of phenomena including easy reduction during CO2 electroreduction.7) This may be attributed to the fact that metal Cu is very active in electrochemistry. In addition W/C/N is also a hotly studied MXenes (metal carbide and nitride) material, due to the nano carbon (nitrogen) layer can be well confined to the metal tungsten to exert its reduction performance. W/C is can be used as an alternative catalyst to the noble metal Pt. In Levy group’s study, the electron distribution in the crystal lattice of W due to the emergence of C undergoes a special change, which makes the W/C have an electronic conductivity similar to that of Pt.8) The electrochemical performance test showed that the Pd-MXene catalyst could catalytically reduce CO2 to CH3OH, with a 67.8 % Faraday current efficiency at -0.5 V. Similarly, Kannan et al.9) successfully synthesized a ZnO-Fe-MXene nanocomposite with a high current density of 18.75 mA ‧ cm-2 under a CO2 atmosphere. Inspired by this, we envisioned to combine the bimetallic sites of copper and tungsten and to immobilise metallic copper for the purpose of electrocatalytic CO2 reduction by a nanocarbon (nitrogen) layer that can bind the copper ions well, and then efficiently produce CO during electrocatalytic reduction of CO2. However, considering that the two-dimensional materials prepared by conventional high-temperature calcination still lack abundant active sites, which hinders the further application of electrocatalytic reduction of CO2.10) Therefore, we turn our attention to metal-organic framework (MOF) materials. Since the first use of copper ruby acid MOF for CO2RR by Hinogami et al.11) in 2012, MOF has received increasing attention as a CO2RR catalyst. According to relevant experiments,12) high-temperature calcination of MOF under vacuum conditions will retain certain pore structures, which broaden the structure of the original materials and can be used as a means to optimise the electronic structure of metals near the Fermi energy level (EF).13) Haiyan Jin’s group have successfully optimized the electronic features near the EF of WC by simple annealing of CoW-based MOF precursors in an inert atmosphere. At the same time, after high-temperature treatment, it has an original morphology and porous structure, and the metal in the metal-organic precursor can form an M-N site,14) which is favorable for CO2RR. The two-dimensional material MXenes cannot further improve the FE of CO2RR,15) while the new approach can provide more abundant active sites for the layered metal carbon nitride to capture CO2 from two-dimensional flakes to three-dimensional agglomerates for the rapid reduction reaction.
Inspired by the above, we investigated catalysts made by a new method with the dual purpose of improving the selectivity of CO2RR reduction products and lowering the overpotential of the reaction. We synthesized nanomaterials with good selectivity and stability of W and Cu bimetals dispersed in C/N (WCu-C/N) for the conversion of CO2RR to CO. The catalysts were produced by hyperthermal decomposition of an organic small molecule [2-Methylimidazole (2-MIM, C4H6N2)] with W and Cu salts [ammonium metatungstate (AMT, H8N2O4W) and Copper nitrate trihydrate (Cu(NO3)2 ‧ 3H2O)]. Through adjustments to the ratio of W/Cu, the active sites and conductivity of the material were improved, the WCu-C/N electrocatalyst eventually showed excellent electrochemical properties. After a series of comparative experiments, WCu-C/N (W:Cu = 3:1) exhibits optimal electroreductive properties among all samples. It could operate stably for 12 h in the electrocatalytic reduction of CO2 to CO system, and the FE of CO is as high as 94 %.
2. Experimental Procedure
2.1. Chemicals
Potassium bicarbonate (KHCO3), AMT, Cu(NO3)2 ‧ 3H2O, 2-MIM, C4H6N2, and Nafion (5 %) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). All chemical reagents used without further purified. And all solutions were prepared with 18.2 MΩ cm ultrapure water output from a TS-S20 deionized water processor.
2.2. Synthesis of WCu-C/N composite
This sampling method could be divided into two steps (Fig. 1). In the first step, the precursor was synthesized with the W/Cu bimetal, the weighing balance was used to accurately weigh 2.9563 g of AMT (1 mmol) and 0.9663 g Cu(NO3)2 ‧ 3H2O (4 mmol); the molar ratio of the two metal elements was 3:1. The weighed agent was placed in the same beaker and dissolved using 120 mL of deionized water; then, the washed magnetic mixer was put into a beaker and put on a magnetic mixing table. Subsequently, the fully dissolved 9.8 g 2-MIM solution slowly added to the mixture of Cu(NO3)2 ‧ 3H2O and AMT, and the reaction was stirred at 750 rpm for 36 h at 30 °C. At the end of the first reaction, the solution was centrifuged and the product obtained by centrifugation was washed three times alternately with ultrapure water and ethanol, and then dried in a vacuum oven at 70 °C overnight. In the second step, the dried MOF precursor was ground in a mortar, evenly placed in a quartz boat, and calcined at 800 °C in a tubular furnace with argon gas for 2 h. After slow cooling to room temperature under vacuum, the calcined material was ground to obtain WCu-C/N (W/Cu molar ratio 3:1, and C/N represents the carbon and nitrogen elements remaining in the sample after high temperature calcination). Through changes to the molar ratio of AMT to Cu(NO3)2 ‧ 3H2O, taking a similar approach as before, six materials with different W/Cu mole ratios were prepared: 1:0, 2:1, 1:3, 1:5, 1:9, and 0:1. The final seven samples were named WCu-C/N (1:0), WCu-C/N (2:1), WCu-C/N (1:3), WCu-C/N (1:5), WCu-C/N (1:9), WCu-C/N (0:1), and WCu-C/N (3:1), respectively.
2.3. Electrochemical measurement
The working electrode was prepared on a super-clean table. First, the hydrophobic carbon paper was cut into a matrix electrode of 1 × 1 cm size with a cutting plate, and the treated carbon paper was placed in a petri dish to ensure that it was not contaminated. After that, 5 mg of catalyst was accurately weighed and added into the mixture containing 950 µL of ethanol and 50 µL of 5 wt% Nafiong117, and sonicated for 15 min to fully dissolve the catalyst. A pipette gun was used to accurately take 200µL of the mixture and evenly drop it on the carbon paper; then, the drop was dried with an infrared lamp for 5 min, so that the catalyst was closely supported on the carbon paper, and to ensure that 1 mg of the electrocatalyst was supported on the carbon paper per unit area.
The CO2RR performance was tested through Shanghai Chenhua electrochemical workstation (CHI 660C) in 0.5 mol L-1 KHCO3 solution as electrolyte.16) In this paper, all electrochemical tests were performed using the traditional three-electrode system (Fig. 2) if not otherwise specified: a platinum sheet electrode (1 × 1 cm) as the counter electrode, an Ag/AgCl electrode as the reference electrode, and a carbon paper (1 × 1 cm) with WCu-C/N loading as the working electrode. The requirement for the electrolyte is that any air that may be present in the 0.5 M KHCO3 solution or in the upper portion of the cathode chamber must be excluded with Ar, followed by the introduction of carbon dioxide and ensuring that only CO2, a reducible gas, is present in the electrolyte. All electrochemical impedance spectroscopy (EIS) test frequencies in this study were performed upon 10-2~106 Hz with an amplitude of 5 mV, and the test potential was the initial potential when the CO2RR occurred, and data obtained were not corrected according to Ohmic drop resistance. Linear sweep voltammetry (LSV) were performed at a sweep speed of 1 mV ‧ s-1, the blank electrolyte was a 0.1 mol L-1 KOH solution filled with Ar, the electrolyte of CO2 reduction was a KHCO3 solution as mentioned above, and the scanning potential was 0~-0.9 V [V vs. reversible hydrogen electrode (RHE)]. In addition, cyclic sweep voltammetry (CV) from 10 mV s-1 to 50 mV s-1 were tested in the above KHCO3 solution at an open circuit voltage.
To further study the catalytic performance of different catalysts in the electrocatalytic reduction of CO2, five representative proportional catalysts were selected for a FE test. During the test process, it was ensured that the experimental conditions with different catalysts were the same, and five overpotentials were selected: -0.4 V, -0.5 V, -0.6 V, -0.7 V, and -0.8 V (V vs. RHE). The gaseous products were collected at each test potential at hourly intervals and then injected into the GC for detection.
3. Results and Discussion
3.1. Characterization
The morphology of WCu-C/N (3:1) was mainly observed using scanning electron microscope (SEM) [Fig. 3(a-c)]. As shown in Fig. 3(a, a’), the smoothness of the Cu-C/N surface without the support of W elements is detrimental to the catalytic reaction. On the other hand, as shown in Fig. 3(b, c), after doping with W elements, the composite WCu-C/N (3:1) exhibits a dense and abundant state, and the association of W and Cu produces a large number of small spheres and attaches to the surface of the carbon layer, which makes them tightly bonded together. Meanwhile, we also explored the morphological changes of different doping ratios, as shown in Fig. 3 (d, e), when changing the doping ratio of W to Cu, we found that it has a larger effect on the overall morphology. It is evident that the material morphology changes with the introduction of each of the two metals due to the cluster effect. It can be clearly seen that the simple W-C/N decreases in size and forms more microscopic spheres with Cu doping, which also suggests that the two metals, W and Cu, play a role in reorganizing the structural configuration.
Consistent with the SEM analysis results, the transmission electron microscope (TEM) atlas can provide more direct observations that the MOF material forms irregular nanoscale spherical particles loaded onto the carbon layer after calcination, and the average size of the irregular spheres is 2.5 nm. So it can be inferred that this design promoted electron transfer between the catalytic centre and the electrode and growed more active sites on the smooth surface. TEM observation [Fig. 4(a, b)] showed that the lattice spacings of WCu-C/N (3:1) were 0.221 nm (121), 0.236 nm (111) and 0.258 nm (101), respectively. The elemental distribution map (Fig. 4) further shows that N atoms (green), W atoms (purple) and Cu atoms (blue) are uniformly distributed in C (yellow) in the composite. The crystal structure of WCu-C/N (W:Cu = 3:1) was determined using X-ray diffraction spectroscopy (XRD) spectroscopy. In Fig. 5, the main diffraction peaks of the composite are at 34.5°, 38.1°, 39.6°, 40.3°, 58.3°, and 40.3°, corresponding to the W2C (PDF#35-0776), (110), (200), and (211) faces of Cu0.4W0.6 (PDF#50-1451), respectively. In addition, the diffraction peaks which appear at 43.3°, 50.4° correspond to copper (PDF#04-0836). On the other hand, the appearance of Cu0.4W0.6 indicates that there is an interaction between Cu and W, and the Cu atoms are successfully doped. Through the EM atlas, it can be more intuitively observed that after calcination of metal-organic materials, regular nano-sized square particles are formed on the carbon layer, and the average size of irregular spheres is 2.5 nm. It can also be found that the C element is tightly wrapped around the other three elements, as shown in Fig. 4, which further confirms that the nanoparticles after metal-organic calcination are evenly distributed on the N-doped carbon. In previous reports, it was demonstrated that the increase in defects in carbon materials is caused by the etching effect of the metal and nitrogen in the precursor during heat treatment, which enhances the electrocatalytic activity of the materials.17)
In addition, as shown in Fig. 5, to determine the optimal proportion of samples, the X-ray diffraction peak test of three types of doped materials is performed (W:Cu = 3:1; 2:1; 1:3), and it is discovered that the main peak position remains basically unchanged, which proves that the composition of the materials is generally consistent. However, the intensity of the peak varies significantly with the difference in the ratio. When the tungsten-copper ratio reaches 1:3, it is discovered that the peak of WC is near 37.65°, which indicates that, when the doping amount of copper is excessive, it will affect the combination of tungsten and carbon, thus showing the weak peak of tungsten carbide.
As WCu-C/N with W:Cu = 3:1 is the best in terms of morphology, structure, and electrochemical performance, the valence states of elements in it are explored. The surface chemical composition of WCu-C/N (W:Cu = 3:1) was investigated using X-ray photoelectron spectroscopy (XPS) analysis. It can be clearly seen from Fig. 6(b) that the material shows signal peaks of W, Cu, N, C, and O, which further confirms that the material contains W, Cu, C, N, and O elements. In order to further understand the valence states of elements in the material, the energy spectrum analysis of four elements of the material was also performed, as shown in Fig. 6, where Fig. 6(d) is the energy spectrum of W 4f. It can be found that four peaks appear at 29.7 eV, 31.7 eV, 33.2 eV, and 35.2 eV of the material. The characteristic peaks at the binding energies of 29.7 eV and 31.7 eV are W-C bonds, where the two characteristic peaks are typical peaks of tungsten carbide, and the two characteristic peaks at the higher binding energies are 35.7 eV and 37.8 eV, which are caused by the appearance of tungsten oxide after the surface of the material is in contact with air following synthesis. The presence of tungsten oxide also matches the characteristic peaks of O 1s in the full spectrum. In Fig. 6(d), the XPS energy spectrum of Cu 2p is shown. The signal peaks at 929.6 eV and 949.9 eV belong to the two main peaks of Cu 2p3/2 and Cu 2p2/1, respectively, and the binding energy difference between the two peaks is about 20 eV, which further confirms the existence of Cu in the material. The satellite peak appearing at the binding energy of 932.6 eV and 952.6 eV is the typical characteristic peak of Cu (I), corresponding to the Cu+, and the peak appearing at the binding energy of 940.4 eV and 960.7 eV corresponds to the Cu (II), which indicates that there are CuO and Cu2O substances in the material. This is the same as W due to Its oxidation in contact with air. A further analysis of the Cu 2p pattern does not correlate with the appearance of Cu-C, which indicates that the catalytic effect of the catalyst originates from itself, and is unrelated to the carbon paper supported by the catalyst. At the same time, combined with the XPS pattern analysis of Cu 2p and W 4f, it can be found that, after the addition of WCu (W:Cu = 3:1), the signal peaks of W and Cu shift to the low binding energy, which indicates that the association of Cu with W makes a significant effect on the electronic structure. Combining the previous SEM with the results in Fig. 6(e, f) clearly shows that different morphologies can be formed by doping different ratios of W to Cu, and the appropriate doping ratio will improve the specific surface area and morphology of the material.
3.2. Electrochemical performance
Before the electrochemical test, we loaded each catalyst onto 1 × 1 cm hydrophobic carbon paper. To investigate whether WCu-C/N is catalytically active, we compared the current densities of seven catalysts in a 0.5 M potassium bicarbonate solution saturated with Ar and carbon dioxide using LSV, as shown in Fig. 7(a).
To facilitate calculations and to exclude the effect of factors such as pH, RHE was served as the standard potential in this study. It can be clearly seen that, in the 0.5M potassium bicarbonate solution saturated with Ar and carbon dioxide, only 3:1 and 2:1 ratios of the catalyst have a high current density. Compared with the current density of other proportional composites in the 0.5 M potassium bicarbonate solution, the current density increases significantly as the test electrolyte is inflated with carbon dioxide. In addition, the rational control of the ratio of W and Cu content enhances the electron transfer ability, so that the initial reduction potential of WCu-C/N (3:1) is much lower than that of other catalysts. These results suggest that WCu-C/N (3:1) composites have good carbon dioxide reduction properties. Mono-metallic with only Cu or W loads are inactive for the CO2RR because their ability to activate carbon dioxide is negligible. Analysing the Faraday efficiencies of all the catalysts shown in Fig. 7(b). only the five catalysts shown in the figure have the function of reducing carbon dioxide. Among them, the FE of WCu-C/N (3:1) WCu-C/N (2:1) is second, while WCu-C/N (1:0) and WCu-C/N (0:1) start to reduce at -0.5V to produce a small amount of CO. In addition, WCu-C/N (1:3) only produces a small amount of CO at -0.7V and -0.8V, because the reduction in the tungsten content weakens, so it cannot exert its unique catalytic properties, and there are fewer active sites of tungsten and copper in the form of M-N. The above results are consistent with LSV, indicating that WCu-C/N (3:1) exhibits the best electrocatalytic reduction performance.
The product selectivity of CO2RR was determined with electrolysis at a constant potential for 12 h. During the test process, the oxygen evolution reaction occurred in the anode chamber. A large number of bubbles were found on the platinum electrode, and the same situation was also produced from the working electrode. The total FE values for the products H2 and CO were approximately 100 % over the entire potential range, and no other gaseous or liquid products were detected by gas chromatography and fully digital nuclear magnetic resonance (NMR) spectroscopy. Here, the hydrogen is a by-product, produced via a hydrogen evolution reaction. It is important to note that the electrolyte in the cathode chamber and the anode chamber were measured using an all-digital nuclear magnetic resonance spectrometer (AVANCEIII400MHZ). The results show no liquid product is obtained. It can be seen that WCu-C/N has a typical volcanic selectivity for CO generation in the range of application potential. Initially, for most of the WCu-C/N catalysts, the FE of CO increased gradually with increasing potential, especially reaching their maximum value at -0.6 V (V vs. RHE). Meanwhile, the FE of CO was also found to be the highest for WCu-C/N (3:1).
Apparently, except for WCu-C/N (1:3), the FE of CO was much lower at -0.8 V (V vs. RHE), while the FE of H2 was much elevated, and the FE of hydrogen reached up to 24.4 %, indicating a serious hydrogen evolution reaction. Then, the current density of the WCu-C/N (3:1, 2:1) catalysts contribute to CO, which effectively inhibits the side reaction of hydrogen evolution, as shown in Fig. 7(b). These results suggest that the WCu-modified C/N can improve the selectivity of CO and inhibit the generation of hydrogen during the electrochemical reduction of CO2. And Fig. 8 shows that WCu-C/N (3:1) can better inhibit the hydrogen precipitation reaction at -0.6 V. In order to observe the durability of the WCu-C/N (3:1) electrocatalyst, the reaction was carried out continuously for 12 h at a fixed potential of -0.6V. The stability test in Fig. 9(f) revealed that WCu-C/N (3:1) achieved a stable current density of 7.9 mA cm-2 and 90.4 % FE of CO. Over a 12-h period, there was only a slight decay in both the FE and current density.
Significantly, two essential criteria for evaluating electrocatalytic CO2 reduction for CO production are the higher current density achieved at a lower reduction potential and the highest FE achievable at a lower reduction potential.18)Table 1 lists some of the recently reported novel catalysts for the CO2RR to CO. In comparison, WCu-C/N (3:1) for the CO2RR to CO at a lower overpotential, and it compared favorably with the results of the latest reports.
For the pH value in Eq. (1), we did some tests. The pH of 0.5 M potassium bicarbonate solution saturated with Ar is 8.75 and the pH of 0.5 M potassium bicarbonate solution saturated with carbon dioxide is 7.52.
Table 1.
The comparison WCu-C/N (3:1) catalyst with reported high-efficiency CO2RR catalyst.
Catalyst | FECO (%) | Potential vs. RHE (V) | Medium |
Fe-TPP-Dimer19) | 90 | -0.88 | 0.1 M KHCO3 |
Ultra-Small Size ZIF-820) | 91 | -1.8 | 0.5 M KHCO3 |
Ligand-Free Silver Nanoparticles21) | 23.5 | -1.2 | 0.1 M KHCO3 |
Asymmetric Push-Pull Type Co(II) Porphyrin22) | 95 | -0.7 | 0.5 M KHCO3 |
Tri-Ag-NPs23) | 96.8 | -0.856 | 0.1 M KHCO3 |
PorZn24) | 23.5 | -1.4 | 0.1 M TBAPF6 in DMF/H2O |
PcZn-O8-Cu/CNT25) | 8.8 | -0.7 | 0.1M KHCO3 |
Ag@Al-PMOF26) | 55.8 | -1.1 | 0.1 M KHCO3 |
Fe-MOF-52527) | 50 | -2 | 1 M TBAPF6/DMF |
NiNPs-NC28) | 90 | -1.6 | 1 M KOH |
Ni SAs/NCNTs29) | 91 | -0.8 | 0.5 M KHCO3 |
N,O-Ni/CMK330) | 89 | -1.0 | 0.5 M KHCO3 |
WCu-C/N (3:1) | 94 | -0.6 | 0.5 M KHCO3 |
3.3. Reaction mechanism of WCu-C/N for CO2 reduction to CO
Small organic molecules consisting of some transition metal salts and C/N-rich organic molecules can be merged at high temperatures to form more tightly linked composites, which are suitable to act as catalysts in electrochemical carbon dioxide reduction processes.31) Carbonization at a specific temperature can better maintain the morphology and structure of MOF, and can form a porous carbon-nitrogen matrix so that the W and Cu active metal sites can be well dispersed on it, and this design is conducive to increasing its active area, stabilizing its metal sites,32) and better exploiting the advantages of the bimetallic so as to comprehensively improve the performance of its electroreduction to CO.
Table 2 shows that the conversion of CO2RR to CO is mainly divided into two possible basic reaction pathways;33) steps (1)-(3) indicate that the CO2 molecule is dissolved in the electrolyte and adsorbed around the electrode, and then the CO2 acquires an electron (e-) to produce CO2∙−, which in turn reacts with HCO3- in an alkaline solution to give CO; whereas, step (4) involves the absorption of two electrons and the generation of CO with the intervention of H+. There is an obvious electron transfer between HCOO∙ and WCu-C/N, resulting in electron enrichment near HCOO∙. WCu-C/N (3:1) is more enriched and dispersed than the other proportions of substances, and therefore has a stronger interaction.
Table 2.
Reaction pathway table for CO2RR to CO.
CO2 + e- ⟶ CO2∙- | (1) |
CO2∙- + HCO3- ⟶ HCOO∙ + CO32- | (2) |
HCOO∙ + e- ⟶ CO + OH- | (3) |
CO2 + 2H+ + 2e- = CO + H2O | (4) |
The larger the electrochemically active surface area (ECSA) is, the more catalytic active sites are provided by the CO2 electrocatalyst, and the Cdl value of the catalyst maintains a positive correlation with ECSA. This means that the Cdl of the catalyst can be used to evaluate the catalytic active site of the catalyst. The ECSA is linearly fitted with electrical double-layer capacitor (Cdl) based on the CV curves.34) After CV tests of different catalysts, the Cdl values of different materials, as shown in Fig. 10(b), were obtained. As illustrated in Fig. 10(b), the Cdl fitting data of WCu-C/N (3:1), WCu-C/N (2:1), WCu-C/N (1:0), WCu-C/N (1:3), and WCu-C/N (0:1) were 12.10 mF cm-2, 6.40 mF cm-2, 2.10 mF cm-2, 0.60 mF cm-2 and 0.43 mF cm-2, respectively. The catalysts made by WCu-C/N (3:1) have the highest Cdl values, which indicates that the catalyst has a greater electrocatalytic active area than other catalysts, which dramatically improves its catalytic performance. The differences in the properties of all the materials here are consistent with the results of other previous experiments on catalysis. The increase in electrochemically active area is traceable to the fact that choosing the proper ratio of W to Cu in WCu-C/N catalysts improves the electronic structure of the raw material, forming nanoscale particles on N-containing carbon and providing more exposed catalytically active sites for the catalysts. Thereby facilitating the process of electrochemical CO2RR. The agglutination effect caused the decrease in catalytic active sites, when the ratio of W and Cu is inappropriate.
In Fig. 10(a), there is an equivalent fitting circuit made by fitting analysis of Z-view (3.0) software. The data can be obtained through multiple fitting: the equivalent circuit consists of solution resistance (Rs), interface transfer resistance (Rp), normal phase angle element (CPE) and Warburg impedance (W1, mixed ion battery conductor).35,36) This indicates that the process of CO2RR is initially controlled by charge transfer, and then by diffusion at the mass transfer interface of the entire system, followed by the transfer of electrolyte liquid ions to electrons in the catalyst to achieve the whole reduction process. The EIS pattern generally consists of a semi-curved region at high frequencies and a slash region at low frequencies. The size of the curvature diameter of the semi-curved region at high frequencies characterizes the size of the resistance to charge transfer between the working electrode and the electrolyte, and the larger its diameter, the greater the charge transfer resistance at the working electrode interface. Obviously WCu-C/N (3:1) possesses the lowest transfer resistance.
4. Conclusion
In summary, MOF precursors containing W and Cu bimetals were synthesized at room temperature and calcined at a high temperature under the protection of an inert gas to prepare the WCu-C/N catalyst. Through adjustments of the molar ratio of W/Cu during the precursor synthesis phase, metal nanospheres with different contents were loaded onto the nitrogen-rich carbon (chemistry) clusters. Compared with all series of WCu-C/N, the WCu-C/N (3:1) composite has the highest current density and CO2 reduction activity of FE-CO in 0.5 M KHCO3 solution, especially at -0.6V. The chemical coupling between W and Cu makes it more durable. The excellent performance of WCu-C/N is due to the effective regulation of the surface properties of W-C/N through continuous cation and ligand exchange during the reduction of Cu2+ to Cu0, whereas the role of C/N is to prevent the spontaneous aggregation of W/Cu, the role of C/N is to avoid the spontaneous aggregation of W/Cu to achieve the exposure of more active sites. The kinetic experiments indicated that WCu-C/N (3:1) nanoclusters could enhance the capacity of adsorbing carbon dioxide and CO2∙-, increase the initial transfer rate of electrons quickly to break the C=O bond in carbon dioxide, reduce the interfacial charge transfer resistance, and effectively inhibit the occurrence of side reactions generated by hydrogen. In this paper, the synthesis strategy is emphasized, it is illustrated that the purpose of the second metal doping is to modify the electronic structure, and a effective method of creating a high performance catalyst is provided. Therefore, it can be considered a novel catalyst with potential applications in the electrocatalytic reduction of CO.