Development of Cu₃P/SnS₂ Composite and Its High Efficiency Electrocatalytic Reduction of Carbon Dioxide

Abstract

With the increase of CO₂ emissions caused by human activities, the development of efficient CO₂ reduction technology is crucial to help address the energy crisis and mitigate climate change. In this study, a series of Cu₃P/SnS₂ composites with varying Cu/Sn molar ratios were synthesized using a hydrothermal method to improve the activity and selectivity of the electrocatalytic reduction of CO₂ (CO₂RR). The successful synthesis and structural advantages of the composite were verified via XRD, XPS, SEM, TEM, and BET. Cu₃P/SnS₂-3 (Cu/Sn = 2:1) had the largest specific surface area (78.01 m² g⁻¹) and abundant active sites. The electrochemical performance test showed that in 0.1 M KHCO₃ electrolyte saturated with CO₂, the Faraday efficiency of Cu₃P/SnS₂-3 to CO reached 87% at −1.0 V potential, which was 29 times and 1.78 times higher than that of Cu₃P (3%) and SnS₂ (48.88%). In addition, the catalyst maintained a CO Faraday efficiency of more than 75% in a 5 h stability test. The mechanism study shows that the low Tafel slope, low charge transfer resistance, and high electrochemically active area of the composite significantly promote the CO₂RR kinetics.

1. Introduction

The increasing CO₂ released by human activities has gradually broken the balance between the production and consumption of carbon dioxide on Earth [1], causing energy crises and climate change, and the problem of global warming is imminent [2,3]. Concerns about the increasing concentration of carbon dioxide in the atmosphere have accelerated the development of various CO₂ reduction and conversion technologies, including chemical [4,5], photochemical [6,7], electrochemical, and photoelectrochemical methods [8,9,10]. The hope is to convert CO₂ into value-added chemical products and fuels and contribute to a sustainable carbon cycle [11,12]. Direct electrocatalytic reduction of CO₂ (CO₂RR) into fuels and useful chemical products is a viable way to rebalance the carbon cycle. The electricity in the CO₂RR process comes from a variety of renewable energy sources, such as solar, wind, hydrothermal, and tidal energy [13], and the electrolytes can be recycled. Therefore, CO₂RR is considered as an ideal means of CO₂ reduction. This method can not only effectively alleviate the over-dependence on fossil energy but also provide valuable resources for the industrial sector [14,15,16].

Electrochemical catalytic removal of CO₂ from the atmosphere has great development prospects [17]. However, traditional electrocatalysts have problems, such as poor selectivity, low energy efficiency, and high onset potential, which greatly limit the electrocatalytic efficiency [18]. In the long-term development process of electrocatalytic technology, a series of improvement methods have been summarized to enhance the electrocatalytic activity. In terms of the improvement of electrocatalysts, it can be achieved by constructing bimetallic materials, metal oxides, preparing nitrogen-doped carbon materials (NCs), constructing metal–organic framework materials (MOFs), etc. [19,20,21,22,23,24,25]. Due to the advantages of rich reserves and low price of metal copper, scholars have carried out a large number of studies on its morphology, crystal phase, exposed surface, defect sites, oxidation state, alloying, surface modification strategies, etc., aiming to improve its electrocatalytic selectivity and activity [26]. For example, Peng et al. successfully synthesized the Cu₃P/C nanocomposite using the copper–organic framework as a precursor. The research shows that this nanocomposite has high activity and good selectivity for the CO₂ reduction reaction. The onset potential is about −0.25 V, and when the potential is −0.3 V, the Faraday efficiency of CO reaches 47%. In addition, metal Sn shows selectivity for the formation of formic acid in the process of electrocatalytic reduction of CO₂, and it has non-toxic and environmentally friendly characteristics. Therefore, as an efficient electrochemical catalyst for CO₂, it has received extensive research and attention in the academic community [27,28]. He et al. successfully prepared the catalyst SnS₂ for electrocatalytic reduction of CO₂. The research shows that in terms of formic acid production, this catalyst has a high Faraday efficiency of 94% and excellent stability. This is because the thin atomic layer of the catalyst SnS₂ is beneficial to the key initial step of generating the HCOO* intermediate, and its electron-rich surface further promotes the subsequent proton–electron transfer, thus promoting the electrocatalytic CO₂ reduction reaction [29]. In addition, combining with another metal to form a bimetallic catalyst can also effectively enhance the catalytic performance [30,31]. Compared with the selectivity exhibited by copper, the bimetallic catalyst of Cu shows further improvement in the catalytic performance of CO₂RR. For instance, in the study by Yang et al., Sn-doped copper-based bimetallic catalysts were synthesized, demonstrating a Faraday efficiency of 98% for CO and long-term stability, which was attributed to the weakened adsorption capacity of the Sn-doped Cu surface for the intermediate *CO [32].

In this work, cuprous phosphide and tin disulfide were combined to improve the catalytic activity and selectivity by adjusting the morphology and surface-active site of the catalyst (Figure 1a). Cuprous phosphide tin disulfide composite is mainly obtained by a simple hydrothermal reaction. In this work, the effect of the molar ratio of Cu to Sn on the electrocatalytic carbon dioxide reduction as well as the effect of different potentials, different electrolytes and catalyst loads on the electrocatalytic performance were studied.

2. Results and Discussion

2.1. Synthesis and Characterization of Cu₃P/SnS₂ Catalysts

In order to determine the crystal structure characteristics of Cu₃P, SnS₂, and Cu₃P/SnS₂ composites, XRD tests were conducted on the prepared samples. As can be seen from Figure 1b, the diffraction angles of Cu₃P samples at 36.0°, 39.1°, 45.1°, 46.2°, and 47.3° correspond to the elemental phase of Cu₃P (PDF#71-2261), corresponding to the crystal faces of Cu₃P (112), (202), (300), (113), and (212), and there are fewer hybrid peaks. This phenomenon indirectly indicates that the prepared Cu₃P sample has a high purity. It can also be seen from the figure that the diffraction peaks of SnS₂ at 28.2°, 32.1°, 41.9°, 50.0°, 52.5°, 55.0°, and 67.2° are completely consistent with the SnS₂ standard card (PDF#23-0677). Corresponding to the (100), (101), (102), (110), (111), (103), and (202) crystal planes of SnS₂, indicating the successful synthesis of SnS₂. In the composite Cu₃P/ SnS₂-1-4, the diffraction peaks of both Cu₃P and SnS₂ can be observed, and with the increase in the molar ratio of Cu₃P/SnS₂, the diffraction peaks of Cu₃P can be seen to gradually increase, which proves the successful synthesis of the composite Cu₃P/SnS₂. In order to investigate the elemental composition and chemical state of Cu₃P/SnS₂-3 composites, XPS was used to characterize them. Figure S1a shows the full spectrum of XPS. Characteristic peaks of Cu, O, Sn, C, S, and P are detected in the range of 0–1200 eV, indicating that the sample contains these six elements. Figure 1c shows the high-resolution XPS spectrum of Cu 2p. Strong peaks at 932.6 eV and 952.6 eV indicate the presence of Cu(0) or Cu(I), and weak peaks at 934.3 eV and 954.1 eV indicate the presence of a small amount of Cu(II). The satellite peaks at 944.2 eV and 963.0 eV may be related to surface oxidation. Combined with XRD results, the peaks at 932.6 eV and 952.6 eV are likely to belong to Cu 2p_3/2 and Cu 2p_1/2 of Cu₃P. The quantitative analysis showed that Cu(I) accounted for 82.2% and Cu(II) accounted for 17.8%, among which Cu(I) was the catalytic active center, which was conducive to the electrocatalytic reduction of CO₂. Figure 1e shows the high-resolution XPS spectra of P 2p. The peaks at 129.1eV and 130.1 eV correspond to the peaks at P 2p_3/2 and P 2p_1/2, 133.6 eV in Cu₃P, respectively, indicating the presence of P-O structure, which may be caused by surface oxidation of Cu₃P. Figure 1d shows the Sn 3d fine scan. The peaks at 486.6 eV and 495.0 eV belong to Sn⁴⁺ 3d_5/2 and Sn⁴⁺ 3d_3/4, respectively. Figure 1f shows the S 2p high-resolution XPS spectrum, and the peaks at 161.3 eV and 162.5 eV are attributed to S 2p_3/2 and S 2p_1/2 of SnS₂, respectively. Figure S2 shows the nitrogen absorption and desorption curves of pure Cu₃P, SnS₂, and Cu₃P/SnS₂-3. Cu₃P/SnS₂-3 has the largest BET specific surface area (78.01 m² g⁻¹), and the total pore volume is 0.0875 cm³ g⁻¹. The BET specific surface area of Cu₃P was 1.43 m² g⁻¹, and the total pore volume of Cu₃P was 0.0014 cm³ g⁻¹. The BET specific surface area of pure SnS₂ is only 0.6079 m² g⁻¹, and the total pore volume of a single point is 0.0007 cm³ g⁻¹. Compared with Cu₃P and SnS₂, Cu₃P/SnS₂-3 has a larger specific surface area, which is conducive to improving the catalytic performance of the material and has a better potential in electrocatalytic reduction of CO₂. Cu₃P usually has a close-packed crystal structure, and the specific surface area of the SnS₂ is limited if the stacking spacing is insufficient or the open porous structure is not formed. Cu₃P/SnS₂-3 may form a new micropore or mesoporous structure through its unique interface and structural reconstruction, and Cu₃P particles can be embedded into the SnS₂ structure to form an interpenetrating network, increasing the porosity and channel and greatly increasing the specific surface area. In addition, the hydrothermal method during synthesis optimizes the nanostructure, promotes particle dispersion and interface contact, and also helps to increase its specific surface area.

In order to characterize the Cu₃P/SnS₂ composite, scanning electron microscope (SEM) scanning was used to investigate the Cu₃P/SnS₂ 1-4 composite and the pure sample Cu₃P and SnS₂. According to Figure 2a, Cu₃P appears to be particles or patches of different sizes, with particles gathering together and being densely arranged, and the Spaces between particles are small. Figure 2b shows that SnS₂ has a flower-like structure composed of thin sheets, which makes it have a large surface area and provides more spots for the loaded Cu₃P. Figure 2c–f show the SEM diagram of Cu₃P/SnS₂ 1-4. It can be observed that Cu₃P is loaded on the SnS₂ surface. With the increase in Cu/Sn molar ratio, the number of particles in Cu₃ increased, and the structure of SnS₂ gradually changed from flake to fluffy flake ball and eventually to stacked flake structure, which caused the void and specific surface area of the composite to change correspondingly. In Figure 2e, Cu₃P was more evenly loaded and denser, and the fluffy flake flower ball SnS₂ had a large specific surface area, indicating a potential optimal molar ratio Cu/Sn of 2:1. The results above confirm that Cu₃P is successfully loaded on the SnS₂ surface, so Cu₃P/SnS₂ composite is successfully synthesized. In addition, transmission electron microscopy (TEM) was used to investigate the morphology, composition, and structure of the catalyst. According to Figure 2g–i, Cu₃P/SnS₂-3 was composed of a crisscrow-sided sheet structure and an agnate granular structure. The granular structure was evenly loaded on the sheet structure, and a great voidage formed in the structure. According to Figure 2j, the lattice spacing was about 0.249 nm, which was consistent with the theoretical value of the (112) crystal face spacing of Cu₃P. In addition, the 0.316 nm crystal face spacing was also analyzed, which was consistent with the SnS₂ (100) crystal face. EDS tests were carried out to clarify element composition, as shown in Figure 2k–n, where red represents P element, green represents S element, blue represents Cu element, and purple represents Bi element. It can be observed that the sample is mainly composed of Cu, P, Sn, and S elements, and these elements are evenly distributed in the sample. The results of XRD and SEM indicate that Cu₃P/SnS₂-3 composite has been synthesized successfully.

2.2. Electrochemical Performance of Cu₃P/SnS₂ Catalysts

In order to evaluate the electrocatalytic performance of Cu₃P/SnS₂-3 composites on CO₂RR, in a standard three-electrode system of H-type electrolytic cell, Linear sweep voltammetry (LSV) was used to measure the current densities of pure Cu₃P, SnS₂, and composite Cu₃P/SnS₂-3 in the range of −1.15–0 V vs. RHE. As can be seen from Figure S3a–c, the current density of pure Cu₃P, SnS₂, and composite Cu₃P/SnS₂-3 in CO₂-saturated 0.1 M KHCO₃ solution is generally higher than that in N₂ atmosphere in the entire potential range, indicating that these three materials have the ability of electrocatalytic reduction of CO₂. As shown in Figure 3a, the current density of Cu₃P/SnS₂-3 is higher than that of pure Cu₃P and SnS₂ in CO₂ atmosphere, and the current density in CO₂-saturated electrolyte increases sharply when the potential is lower than −0.7 V, indicating that the CO₂ electrocatalytic reduction performance of Cu₃P/SnS₂-3 is better than that of pure Cu₃P and SnS₂. As shown in Figure S4, gas chromatography (GC) and nuclear magnetic resonance spectroscopy (NMR) were used for quantitative analysis of gaseous and liquid products, respectively. As shown in Figure S4a, peaks of CO, CH₄, and CO₂ can be seen in the FID column of gas chromatography (GC), and peaks of H₂ and N₂ can be seen in the TCD column of Figure S4b. Among them, CO₂ and N₂ are the gases entered in the experiment, and CO, CH₄, and H₂ belong to gaseous products. As shown in Figure S4c, the peak of internal standard DMSO and HCOOH can be seen, indicating that the liquid phase product has HCOOH, but the yield is small, indicating that the catalyst has poor selectivity for HCOOH. The electrocatalytic reduction of CO₂ from pure Cu₃P, SnS₂, and Cu₃P/SnS₂-3 composites was tested in CO₂-saturated 0.1 M KHCO₃ electrolyte at potentials ranging from −0.8 V to −1.1 V, respectively. As shown in Figure 3b,c,f, Faraday efficiencies of pure Cu₃P, SnS₂, and composite Cu₃P/SnS₂-3 at different potentials are shown. Specifically, for pure Cu₃P, the CO Faraday efficiency decreases first and then increases in the range of −0.8 V to −1.1 V potential. On the contrary, the CO Faraday efficiency of SnS₂ and Cu₃P/SnS₂-3 increases first and then decreases in this potential range. When the potential is −1.0 V, the Faraday efficiency of Cu₃P/SnS₂-3 electrocatalytic CO₂ production of CO is 87%, which is about 29 times that of pure Cu₃P and 1.78 times that of pure SnS₂. Compared with pure Cu₃P and SnS₂, the selectivity of Cu₃P/SnS₂-3 to CO is higher, indicating that the combination of Cu₃P and SnS₂ is beneficial to the electrocatalytic reduction of CO₂ to produce CO. In order to test the effect of different molar ratios of Cu/Sn on the electrocatalytic reduction performance of CO₂, Cu₃P/SnS₂ composites with different molar ratios were prepared in the experiment. In a CO₂-saturated 0.1 M KHCO₃ electrolyte with a potential range of −0.8 V to −1.1 V, the electrocatalytic CO₂ reduction experiments were carried out on each catalyst, and the products were tested using GC and NMR. Figure 3d–h are the Faraday efficiency diagrams of the composite material. It can be seen that the CO Faraday efficiency of Cu₃P/SnS₂-1, Cu₃P/SnS₂-2, Cu₃P/SnS₂-3, and Cu₃P/SnS₂-4 shows a trend of first rising and then decreasing within the potential range of −0.8 V to −1.1 V. When the potential is −1.0 V, the Faraday efficiency of CO is the highest, and the Faraday efficiencies of the composite at this potential are 65.8%, 83%, 87%, and 68.3%, respectively. Among them, Cu₃P/SnS₂-3 has better CO selectivity and Faraday efficiency is 87%. It can be seen that the best potential in this experiment is −1.0 V, the best catalyst is Cu₃P/SnS₂-3, and the best FE of CO is 87%, about 29 times that of pure Cu₃P (3%) and 1.78 times that of pure SnS₂ (49%). The possible reason is that the large electrochemical active area of the composite Cu₃P/SnS₂-3 provides more active sites for it. These active sites can adsorb more CO₂, thus promoting the reaction. When the potential reaches −1.0 V, there is more energy to overcome the reaction energy barrier of the RDS step, making the CO FE of Cu₃P/SnS₂-3 higher at −1.0 V.

After the optimum catalyst Cu₃P/SnS₂-3 was determined, the electrocatalytic reduction of CO₂ by Cu₃P/SnS₂-3 in different electrolytic solutions was investigated. As shown in Figure 4a, it is the Faraday efficiency of CO₂RR products in different electrolytes of the catalyst Cu₃P/SnS₂-3 at the −1.0 V potential. By comparing the electrocatalytic performance of Cu₃P/SnS₂-3 in different electrolytes, 0.1 M KHCO₃, 0.1 M NaHCO₃, and 0.1 M KCl, it was found that the Faraday efficiency of CO first decreased and then increased. By comparing different concentrations of electrolyte 0.1 M KHCO₃ and 0.5 M KHCO₃ and 0.1 M NaHCO₃ and 0.5 M NaHCO₃, it was found that the Faraday efficiency of CO showed a decreasing trend. Through comparison, it was found that Cu₃P/SnS₂-3 in the 0.1 M KHCO₃ electrolyte showed a good electrocatalytic performance. The possible reason is that the KHCO₃ electrolyte buffers pH changes, while K⁺ promotes CO₂RR and inhibits HER. When the electrolyte concentration is increased, HCO₃⁻ may act as a proton donor and promote HER, thereby inhibiting CO₂RR, and thus exhibit the best electrocatalytic performance in the 0.1 M KHCO₃ electrolyte. In previous experiments, it was found that the presence of K⁺ can inhibit the hydrogen evolution process and contribute to the selective electrocatalytic reduction of CO₂ under strongly acidic conditions (pH < 1), and with the increase in the K⁺ concentration, its contribution to the formation of multi-carbon (C₂₊) products gradually increases. Relevant studies have shown that bicarbonate aqueous solution is the most commonly used electrolyte in CO₂RR, which can not only act as pH buffer or proton donor but also increase local CO₂ concentration through rapid equilibrium with CO₂(aq), thus promoting electrocatalytic CO₂ reduction [33]. As shown in Figure 4b, the electrocatalytic performance of Cu₃P/SnS₂-3 catalysts with different loads was tested in 0.1 M KHCO₃ electrolyte at −1.0 V potential, and the Faraday efficiency of the products was calculated, respectively. It can be seen that with the increase in the catalyst load, the Faraday efficiency of CO goes through a change process of first rising and then decreasing. When the loading rate is 0.37 mg cm⁻², it shows better electrocatalytic performance. Later, with the increase in loading rate, the CO₂RR performance begins to decrease. The possible reason is that too much catalyst loading will lead to a too-thick catalyst on the carbon paper, which is not favorable for the reaction between CO₂ and the inner catalyst. As shown in Figure 4c, the electrocatalytic reduction CO₂ electrolysis experiment of Cu₃P/SnS₂-3 catalyst was carried out continuously for 5 h in KHCO₃ electrolyte saturated with CO₂ at −1.0 V, so as to explore the stability of the catalyst. It was found that the current density of Cu₃P/SnS₂-3 remained within a certain range without obvious fluctuation, and the CO Faraday efficiency in the electrocatalytic CO₂ reduction reaction was always above 75%, indicating that the catalyst Cu₃P/SnS₂-3 had good stability. The stability of the Cu₃P/SnS₂-3 catalyst decreased slightly after 4 h, which may be due to the prolonged stability test leading to phase separation of the originally uniformly dispersed Cu₃P and SnS₂, reducing the synergistic effect between the two phases, thus affecting the overall properties of the material.

2.3. Mechanism Research of Cu₃P/SnS₂ Catalysts

As shown in Figure 5a, the Tafel slopes of Cu₃P, SnS₂, Cu₃P/SnS₂-1, Cu₃P/SnS₂-2, Cu₃P/SnS₂-3, and Cu₃P/SnS₂-4 catalysts are 267 mV dec⁻¹, 478 mV dec⁻¹, 125 mV dec⁻¹, 138 mV dec⁻¹, 120 mV dec⁻¹, and 296 mV dec⁻¹, respectively. It can be seen that the catalyst Cu₃P/SnS₂-3 has the lowest Tafel slope, so the catalyst Cu₃P/SnS₂-3 has faster CO₂RR kinetics than other materials. It is shown that the final step of Cu₃P/SnS₂-3 electrocatalytic CO₂ reduction process is to obtain an electron on the catalyst surface and activate CO₂ molecules into CO₂^·− free radicals. In order to further investigate the electrocatalytic activity of Cu₃P and Cu₃P/SnS₂-1-4, electrochemical impedance tests were performed in the experiment. Figure 5b shows the Nyquist diagram of the electrocatalyst, which is mainly composed of semi-arcs and curves. The semi-arc belongs to the high-frequency region, which represents the electron transfer during the reaction, the diameter of the semi-arc represents the electrochemical impedance value, and the part of the curve behind belongs to the low-frequency region, which represents the material diffusion. From the figure, we can see that the electrochemical impedance (Rct) of pure Cu₃P is about 650 Ω, Rct of pure SnS₂ is 500 Ω, and the Rcts of the materials Cu₃P/SnS₂-1-4 are 120 Ω, 100 Ω, 70 Ω, and 800 Ω, respectively. Compared with Cu₃P and SnS₂, the resistance of Cu₃P/SnS₂-1-3 decreases, while the resistance of Cu₃P/SnS₂-4 increases. The material Cu₃P/SnS₂-3 has the smallest diameter in the Nyquist diagram, with a charge transfer resistance of about 70 Ω. The results show that the Cu₃P/SnS₂-3 catalyst has a smaller surface electron transfer resistance and faster charge transfer rate, which promotes the charge transfer between the reactive substance and the catalyst in the electrolyte, and has a better catalytic performance of CO₂RR. As shown in Figure S5a–f, they are the cyclic voltammetry curves of Cu₃P, SnS₂ and the composite material Cu₃P/SnS₂ 1-4 at different scanning speeds in 0.1 M KHCO₃ electrolyte, with scanning speeds ranging from 20 to 120 mV s⁻¹. As shown in Figure 5c, the double-layer capacitance (Cdl) of pure Cu₃P, SnS₂, and composite Cu₃P/SnS₂-1-4 are obtained by calculation, which are 0.037 mF cm⁻², 0.061 mF cm⁻², 0.233 mF cm⁻², and 0.309 mF cm⁻², respectively. Compared with pure Cu₃P, SnS₂, and other composites, it can be seen that the double-layer capacitance value of Cu₃P/SnS₂-3 is the most prominent. In general, double-layer capacitors (Cdl) are used to qualitatively evaluate the electrochemically active area (ECSA). The larger the Cdl, the larger the corresponding ECSA, indicating that there are more catalytic active sites, which means that the composite Cu₃P/SnS₂-3 has better CO₂RR catalytic activity.

3. Experimental Procedures

3.1. Materials

All the reagents were analytical grade and used without further purification. Copper chloride dihydrate (CuCl₂·2H₂O), red phosphorus, tin tetrachloride (IV) pentahydrate (SnCl₄·5H₂O), ethanol, thiourea, potassium bicarbonate (KHCO₃), sodium bicarbonate (NaHCO₃), and potassium chloride (KCl) were purchased from Shanghai Yi En Chemical Technology Co., Ltd., Shanghai, China. High-purity carbon dioxide and high-purity nitrogen were purchased from Huzhou First Gas Co., Ltd., Zhejiang, China. Nafion was acquired from DuPont, Wilmington, DE, USA.

3.2. Synthesis of Cu₃P, SnS₂, and Cu₃P/SnS₂

Synthesis of Cu₃P: Mix 0.511 g of CuCl₂·2H₂O and 0.3097 g of red phosphorus, then put the mixture in a beaker and add 60 mL of deionized water, stir continuously for 30 min, then transfer it to a 100 mL reaction kettle, and put it into a 180 °C oven for continuous heating for 12 h. After the reaction was finished, the sample was naturally cooled down to room temperature, then taken out for centrifugation, and washed with ethanol and deionized water for several times alternately. After the reaction was completed, the sample was centrifuged, washed with ethanol and deionized water alternately for several times, and finally the mixture was dried at 70 °C for 4 h.

Synthesis of SnS₂: In total, 0.3506 g of SnCl₄·5H₂O and 0.2284 g of thiourea were weighed and added to 40 mL of distilled water successively and continuously stirred in a magnetic stirrer for 30 min. The mixture was then transferred to a high-pressure reactor with a capacity of 100 mL, and a hydrothermal reaction was carried out at 180 °C for up to 12 h. After the reaction, the samples were cooled to room temperature, centrifuged, and then washed alternately with deionized water and anhydrous ethanol. Finally, the cleaned sample was placed in the oven and dried at 70 °C for 10 h to obtain SnS₂ material.

Synthesis of Cu₃P/SnS₂: In total, 0.3506 g of SnCl₄·5H₂O and 0.2284 g of thiourea were weighed and added into 40 mL of distilled water successively, stirring with a strong magnetic flux for 30 min. Then, the synthesized Cu₃P powder is prepared according to a certain molar ratio (Cu₃P:SnS₂ = 1:2; 1:1; 2:1; 3:1) Add the mixture to the above solution, stir continuously for 20 min, and then transfer the mixture into a 100 mL reactor after mixing evenly. After the hydrothermal reaction at 180 °C for 12 h, centrifuge after the reaction is finished and cooled to room temperature, and then wash with deionized water and anhydrous ethanol alternately for several times. Finally, the samples were dried at 70 °C for 5 h to obtain Cu₃P/SnS₂ composites, which were successively named Cu₃P/SnS₂-1, Cu₃P/SnS₂-2, Cu₃P/SnS₂-3, and Cu₃P/SnS₂-4.

3.3. Characterization

In this paper, the model of an X-ray diffractometer Bruker D8 Advance is used, the radiation source is Cu-Kα, λ = 0.15406 nm, the scanning speed is 1.2° min⁻¹, and the scanning angle 2θ = 5°–90°.

In this experiment, a K-Alpha X-ray photoelectron spectrometer was used, and Al-K-α radiation was selected as the excitation light source. In the course of the experiment, the chemical element composition of the catalyst was qualitatively identified and quantitatively determined according to the position and relative strength of the characteristic spectral lines in the obtained spectra.

In this experiment, a Micromeritics ASAP 2020 (Micromeritics Instrument Corporation, Norcross, GA, USA) equipment was used to carry out the test work. The samples were first placed in the instrument and degassed for 10.0 h at 110.0 °C to fully remove the residual gas. Subsequently, the same Micromeritics ASAP 2020 instrument was used to continue the rigorous test program to accurately determine key physical parameters, such as the specific surface area and pore size of the catalyst.

The scanning electron microscope used in this experiment is S-4800 (Hitachi, Ltd., Tokyo, Japan), and the acceleration voltage condition used is 15 kV or 10 kV. In this experiment, Zeiss LIBRA 200 FEG (Oberkochen, Germany) transmission electron microscopy was used to analyze the morphology, structure, and size of the catalyst.

3.4. Electrochemical Measurements

Electrode preparation: In order to prepare a uniform catalyst suspension, the 5 mg catalyst powder was first accurately weighed, and then 375 μL of deionized water, 125 μL of ethanol, and 40 μL of Nafion solution with a concentration of 5 wt.% were added in sequence. The mixture was placed in the ultrasonic dispersion equipment for 30 min, and finally, the uniformly dispersed catalyst suspension was obtained. Then, the catalyst suspension of 40 μL was uniformly coated on the carbon paper with the size of 2 cm × 1 cm, and the catalyst load on the carbon paper was 0.37 mg cm⁻². Finally, the carbon paper uniformly coated with the catalyst was placed in a constant temperature oven at 60 °C for drying treatment, and the duration was set to 30 min. After the drying process is over, take out the treated carbon paper to obtain the required working electrode.

The electrocatalytic experiments in this study were all carried out at room temperature, using H-type electrolytic cell, and the electrolytes without special instructions were 0.1 M KHCO₃ solution. Before the start of the formal experiment, CO₂ gas should be continuously passed into the electrolyte in the cathode chamber, the ventilation time should be set to 30 min, in order to fully expel the residual air inside the electrolyte, make CO₂ saturated in the electrolyte of 0.1 M KHCO3, and then carry out a series of ECR experiments. The reaction stirring rate was 400 rpm, and no iR compensation was carried out.

The gas phase product was analyzed using a gas chromatograph model Agilent 7890A (Santa Clara, CA, USA). Liquid phase product detection: After each electrolytic experiment, collect the electrolyte, use the pipette to take 500 μL electrolyte, and then take 0.1 μL of dimethyl sulfoxide (DMSO) into the electrolyte to be measured. Before the test, the standard curve of the liquid product was prepared. The concentration of the standard solution was taken as the horizontal coordinate, and the ratio between the peak area of the liquid product and the peak area of DMSO measured by nuclear magnetic resonance spectroscopy was taken as the vertical coordinate to establish the standard concentration curve.

3.5. Computational Methods

The Faraday efficiency of gas products is calculated as follows:

$$F{E}_g={\displaystyle \frac{Q_g}{Q_{total}}}\times 100\%={\displaystyle \frac{1.67\times {10}^{-8}\times F\times x_g\times v_{C{O}_2}\times K}{i_{total}\times V_m}}\times 100\%$$

where F represents Faraday’s constant, F = 96,485 C·mol⁻¹; x_g is the target product concentration, unit ppm; ν_CO2 is the CO₂ flow rate at the outlet of the electrolyzer, in mL·min⁻¹; K is the number of transferred electrons corresponding to different products; itotal is the total current in the reaction, expressed in mA; V_m represents the molar volume of the gas, V_m = 24.5 L·mol⁻¹ at normal temperature and pressure.

The Faraday efficiency of liquid products is calculated as follows:

$$FE={\displaystyle \frac{m\times n\times F}{I\times t}}$$

where m represents the number of moles of the product, n represents the number of electrons transferred by the CO₂ reduction product, F is the Faraday constant (96,485 C·mol⁻¹), I is the current of the electrolytic process, and t is the duration of the electrolytic reaction.

4. Conclusions

In this paper, a new composite material Cu₃P/SnS₂ was prepared using the hydrothermal method and applied to CO₂RR. According to the regulation of different molar ratios of Cu and Sn, the microstructure and electrocatalytic performance of Cu₃P/SnS₂ catalyst were optimized. The catalyst was characterized by XRD, XPS, BET, SEM, and TEM, which proved the successful recombination of Cu₃P and SnS₂. The LSV curves of Cu₃P/SnS₂-3 indicate that the current density of Cu₃P/SnS₂-3 is high, and the combination of Cu₃P and SnS₂ is beneficial to improve the electrocatalytic performance of Cu₃P/SnS₂-3. The i–t test was carried out to calculate the Faraday efficiency of the products. It was found that the main gaseous products were CO, CH₄, and H₂. After comparing the Faraday efficiency of different catalysts, it was determined that the CO Faraday efficiency of Cu₃P/SnS₂-3 was the highest (87%) in CO₂ saturated 0.1 M KHCO₃ electrolyte and −1.0 V potential in the H-type electrolyte, which was about 29 times that of pure Cu₃P (3%) and 1.78 times that of SnS₂ (49%). The results showed that the selectivity of CO₂RR to CO was improved by the combination of Cu₃P and SnS₂, and the performance of CO₂RR could be effectively regulated by adjusting the molar ratio of Cu to Sn. The experimental results show that the optimal potential and electrolyte for Cu₃P/SnS₂-3 electrocatalytic reduction of CO₂ are the −1.0 V and 0.1 M KHCO₃ electrolytes, and the optimal load of catalyst supported on carbon paper is 0.37 mg cm⁻². In the stability test, the prepared Cu₃P/SnS₂-3 was tested for CO₂RR for five consecutive hours, and its Faraday efficiency was only reduced from the initial 87% to 77%, which strongly confirmed the excellent stability of the catalyst. Finally, the excellent performance of Cu₃P/SnS₂-3 is proven by the tests of Tafel slope, electrochemical impedance, and electrochemical active area. This study provides a new method for producing CO from CO₂RR.