Al-Doped Octahedral Cu2O Nanocrystal for Electrocatalytic CO2 Reduction to Produce Ethylene

1. Introduction

The increased CO₂ emissions in atmosphere results in serious greenhouse effect and the elevated sea level [1,2]. The electrochemical CO₂ reduction reaction (CO₂RR) is a promising strategy to mitigate the global warming and energy crisis, while transforming CO₂ into fules and chemical feedstocks [3, 4, 5]. It can use clean electric energy generated by renewable solar and wind energy to drive the conversion of CO₂ under mild conditions [6, 7]. The reduction products of CO₂RR include CO [8,9,10], HCOOH [11,12,13,14], alcohols [15,16,17], and various hydrocarbons [18,19,20]. Among them, C₂H₄ has attracted more and more attention due to its high energy density. Furthermore, C₂H₄ is regarded as an important component in the production of various plastics, solvents, and cosmetics in the chemical industry [21].

Up to now, Cu-based materials can electrocatalytic convert CO₂ into C2/C2+ products. Among reported Cu-based catalysts, Cu₂O nanocrystal has attracted much attention due to their electrocatalytic activity and high selectivity toward C₂H_4. To improve the CO₂RR performance of Cu₂O, great efforts have been made on its structural design. Metal ions can be used as structure-guiding agents to optimize the micro-structure of nanocrystals [22].Cu₂O nanoparticles (Cu₂O NPs) exhibit good performance for CO₂RR, possibly because the low coordination Cu⁺ on the surface contributes to C-C coupling, thus promoting the production of C₂H₄ [23]. Other strategies including crystal facet controlling [24], defect engineering [25,26], alloying [27], valence adjustment [28], and surface molecular modification [29] have been employed to improve the electrocatalytic performance of CO₂RR to produce C₂H₄. For example, Shang et al. [30] have designed a core-shell Cu@Cu₂O catalyst, on which a thin layer of natural oxide grows on the surface under environmental conditions. The synergistic effect between Cu⁺ and Cu⁰ on the Cu@Cu₂O surface helps to improve its efficiency and selectivity for C2 products. Ning et al. [31] reported the preparation of Cu₂O/nitrogen-doped carbon shell (Cu₂O/NCS) composite and its application in CO₂ electroreduction to selective formation of C₂H₄. However, copper catalysts still face many problems, such as inevitable competitive hydrogen evolution reaction, complex reaction mechanisms diversification of products, and low selectivity of target products [32] Therefore, it is of huge challenge to design CO₂RR electrocatalysts with high activity, selectivity and satisfied stability.

In this work, an effective strategy was proposed to improve the CO₂RR product by doping Al on the surface of octahedron Cu₂O nanocrystals. Al doped Cu₂O (Al–Cu₂O) was used as an effective electrocatalyst for selective ethylene production by CO₂RR. Al–Cu₂O exhibits a high Faraday efficiency (FE_C2H4) of 44.9% at -1.23 V (vs. RHE). During the coupling process of * CHCOH intermediate, the Al–Cu₂O catalyst can significantly reduce the free energy and promote the formation of C₂H₄. It can also inhibit the occurrence of HER side reaction. Therefore, the doping strategy is beneficial for the adsorption of intermediates to reconstruct the internal stable state of electronic structure of Cu₂O, thus improving the activity and selectivity of CO₂ conversion to ethylene.

2. Results

2.1. Morphology and structure analysis

As shown in Figure 1, Al–Cu₂O–X catalyst was prepared by a simple one-step method (Experimental section for details). In order to further characterize Al–Cu₂O–X catalysts, XRD pattern was used to study the crystal structure of Cu₂O, Al–Cu₂O, and Al–Cu₂O–2. From the XRD pattern in Figure 2a, the peaks at 29°, 36°, 42°, 61°, 73°, and 77° correspond to the (110), (111), (200), (220), (311) ,and (222) planes of Cu₂O, respectively, which agree well with the octahedral Cu₂O (PDF#75–1535). The XRD patterns of Al–Cu₂O–X (3-6) were shown in Figure S1. During the preparation process, the catalyst synthesized by adjusting the amount of Al³⁺, the concentration of NaOH, and the reaction time. Al–Cu₂O–X (3-6) were all single-phase Cu₂O nanocrystals. The morphologies of the Cu₂O, Al–Cu₂O, and Al–Cu₂O–2 were monitored by scanning electron microscopy (SEM). Cu₂O nanocrystal without Al doping showed a octahedron shape with smooth surface (Figure S2a). Due to the doping effect of Al, the Al–Cu₂O nanocrystal presented a octahedral shape with a more rough surface and formed a defect structure (Figure 2b), which may provide abundant active sites for CO₂RR [33]. When the concentration of Al³⁺ increased from 0.02 M to 0.03 M, Al–Cu₂O–2 catalyst exhibits a cube shape (Figure S2b). However, it was reported that the resulting cube Al–Cu₂O catalyst is not conducive to the formation of C₂H₄ [23]. The better-performing Al-Cu₂O with a homo-octahedral shape was observed by TEM (Figure 2c), which was consistent with SEM image. High resolution transmission electron microscopy (HRTEM) image in Figure 2d presented that the lattice stripe spacing d marked was 0.304 nm, corresponding to the (110) crystal plane of Cu₂O. The HAADF-STEM image (Figure 2e) also exhibited an octahedral shape. The composition of Al-Cu₂O was reconfirmed by elemental mapping (Figure 2f). The Al (red), Cu (blue), and O (green) elements uniformly distributed over the Al-Cu₂O nanocrystals.

The surface composition and valence of Cu₂O and Al–Cu₂O nanoscrystals were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3a and b, four peaks were observed in Cu 2p spectrum for both Cu₂O and Al–Cu₂O samples. For Cu₂O, the peaks at 932.78 and 952.62 eV corresponded to the binding energies of Cu 2p_3/2 and Cu 2p_1/2 of Cu₂O or Cu, respectively. The binding energies at 935.28 eV and 944.48 eV were ascribed to the peaks of Cu²⁺. For Al–Cu₂O, the peaks at 932.89 and 952.73 eV corresponded to the Cu 2p_3/2 and Cu2 p_1/2 of Cu₂O or Cu, respectively. The binding energy of 935.26 and 944.46 eV belonged to the peak of Cu²⁺. The above results showed that the existence of Cu⁰ may be due to the partial reduction of Cu₂O in the CO₂RR process [34]. The existence of trace CuO may be mainly due to the oxidation of a small amount of Cu₂O catalyst to CuO in the air after the synthesis of Cu₂O [35]. When octahedral Cu₂O nanocrystals were doped with Al, the peaks of Cu 2p_3/2, Cu 2p_1/2, and Cu²⁺ of Cu₂O or Cu were shifted positively. This results may be attributed the introduction of Al, which can induce charge transfer from Al atoms to Cu atoms, thus modulating the electronic structure of Al–Cu₂O. The existence of Cu₂O was also confirmed in the O 1s XPS spectra of Cu₂O and Al-Cu₂O (figure 3c-d). There were three XPS peaks in both catalysts, of which the peak at 530.5 eV corresponds to the Cu-O bond, and 532.11 and 532.77 eV correspond to Olat and C=O, respectively [36]. In the high-resolution spectrum of Al 2p (figure 3e), the peaks at 74.55 and 77.35 eV correspond to the Al 2p_1/2 and Al 2p_3/2 of metal Al, respectively. The Al atom was 0.41% by XPS analysis, indicating that the Al–Cu₂O catalyst has been successfully prepared.

2.2. Electrocatalytic CO₂RR perfomances

To further analyze the electrochemical performance of the catalyst, the linear sweep voltammetry (LSV) of Cu₂O and Al–Cu₂O–X catalysts in saturated CO₂ electrolyte and saturated N₂ electrolyte were tested. The analysis of Figure 4a shown that the current density of Al–Cu₂O catalyst in CO₂ saturated electrolyte is higher than that in N₂, indicating that Al–Cu₂O catalyst had higher activity. The LSV curve was measured in CO2-saturated 0.1 M KHCO₃ electrolyte (figure S3a). The current density of the Al–Cu₂O catalyst in CO₂ saturated atmosphere was higher than that of Cu₂O and Al–Cu₂O–2 catalysts, indicating that the Al–Cu₂O catalyst had better electrocatalytic activity for CO₂RR. Figure S3b shows the potentiostatic electrolysis of CO₂ at various potentials. The almost constant current signal indicates that the Al-Cu₂O catalyst exhibited good electrochemical stability during the CO₂RR process. In Figure 4b, the formation rates of three kinds of catalysts were presented for ethylene products. The Al–Cu₂O catalyst has higher current density for ethylene formation than that of Cu₂O and the Al–Cu₂O–2 catalysts in a wide potential range, and reaches the partial current density of 16.7 mA cm^-2 at -1.38 V (vs. RHE). The above results showd that the Al–Cu₂O catalyst was more conducive to the production of ethylene as the main product and has a better inhibitory effect on compete hydrogen formation.

In order to determine the CO₂RR selectivity of the Al–Cu₂O catalyst, the reduction products were qualitatively and quantitatively analyzed. In this study, the reduction products of each catalyst were determined in the wide potential range from-0.98 V to-1.38 V (vs. RHE). The analysis of Figure 4c and S4 shows that the products of Cu₂O ,and Al–Cu₂O–X catalysts were C₂H₄, HCOO^-, CO, CH₄ and by-product H₂. When Al³⁺ was not introduced in the reaction, the octahedral Cu₂O nanocrystal catalyst was synthesized, and Figure S4a shows the FE of the catalyst to the CO₂RR product. The result shown that the catalyst had a good effect on inhibiting by product H₂ at low potential, and the highest FE_C2H4 was 26.1%. If an appropriate amount of Al³⁺ was introduced into octahedral Cu₂O nanocrystal (0.02 M), the selectivity of Al–Cu₂O catalyst was improved. As shown in Figure 4b, with the increase of catalyst potential, the FE value of H₂ decreases from 35.1% to 22.1%. On the contrary, the FE value of C₂H₄ increases from 12.9% at-0.98 V (vs. RHE) to 44.9% at-1.23 V (vs. RHE). The result shown that the catalyst had good selectivity for ethylene and good inhibition effect on HER. If more Al³⁺ was added to the reaction (0.03 M), the FE of the prepared Al–Cu₂O–2 catalyst for the CO₂RR product was shown in Figure S4b. The result has shown that the FE of C₂H₄ was 32.8%, indicating that the catalyst had a good selectivity for ethylene. It was worth noting that we also studied the effects of reaction time (S4cpender d) and NaOH concentration (S4ePowerf) on the selectivity of the catalyst. The result showed that the FE of the four catalysts C₂H₄ was 40.1%, 38.6%, 41.0%,and 30.8%, respectively, indicating that the optimization of reaction time and NaOH concentration can make the catalyst have a certain selectivity, but compared with the quantity conditions of introducing Al³⁺. The effect of improving the selectivity of the product is weaker, mainly because the addition of different amounts of Al³⁺ will form different morphology of the catalyst, resulting in different selectivity of the catalyst to the product. Figure 4d compares the selectivity of three kinds of catalysts Cu₂O, Al–Cu₂O, and Al–Cu₂O–2 for ethylene products. The result showed that under different applied voltages, the efficiency of the Al–Cu₂O catalyst for reducing CO₂ to C₂H₄ in 0.1 M KHCO₃ electrolyte was higher than that of the other two catalysts, indicating that the amount of Al³⁺ introduced into the catalyst effected the selectivity of the catalyst. This may be due to the fact that Al -doped Cu₂O will cause changes in the electronic structure and the morphology of the catalyst, thus reducing the adsorption energy of the catalyst for ethylene intermediates in the CO₂RR process and enhancing the selectivity of the reaction to the products.

The electrochemical surface area (ECSA) is also a key point for the electrocatalyst. According to the formula for calculating ECSA, it is known that this parameter is related to the C_dl and C_ds values of their catalysts, because the catalysts are coated on hydrophobic carbon paper (model 060). Therefore, the Cds of the three catalysts are the same, and only the C_dl value of the catalyst can be calculated to determine the ECSA of the catalyst. According to the Cu₂O, Al–Cu₂O, and Al–Cu₂O–2 catalysts, the cyclic voltammograms (Figure S5a-c) of 0.47 V~0.57 V (vs. RHE) at different scanning rates (20, 40, 60, 80,100, 120 mV s^-1). It can be seen from Figure 4e that the capacitance values of Cu₂O, Al–Cu₂O, and Al–Cu₂O–2 catalysts were 0.109, 0.122, and 0.076 mF cm^-2, respectively. The largest C_dl of the Al–Cu₂O electrocatalyst suggested that the high electrochemical activity surface area of the Al–Cu₂O–2 catalyst. This high ECSA can offer a lot of catalytic active sites for improving the electrocatalytic performance of CO₂RR, thus improving the selectivity of the catalyst, which was consistent with the previous research conclusion.

The impedance of several different catalysts under open-circuit voltage was obtained (Figure S6a). Compared with the octahedral Cu₂O catalyst without Al doping, the EIS arc of the Al–Cu₂O catalyst was smaller than that of the octahedral Al–Cu₂O catalyst. The results indicate that during the reaction process, interface charges can be rapidly transferred and catalytic activity can be improved. In order to better understand the activity and kinetics of Al-Cu₂O materials on CO₂RR, the Tafel slope analysis of the local current density of the catalyst product is carried out. As shown in Figure 4f, the Tafel slope of the Al–Cu₂O catalyst (74.3 mV dec^-1) was lower than that of Cu₂O (85.9 mV dec^-1) and the Al–Cu₂O–2 (110.4 mV dec^-1), indicating that the electron transfer rate of the catalyst is faster, which was beneficial to the rapid adsorption and desorption of the important intermediate of ethylene from the surface of Al–Cu₂O catalyst. To speed up the reaction, therefore, the introduction of Al into octahedral Cu₂O catalyst may help to reduce the activation energy of various intermediates in the CO₂RR reaction process, thus making the catalyst show better selectivity and activity for the products.

We further studied the stability of the CO₂RR material. As seen in Figure 5a, the Al–Cu₂O catalyst was electrolyzed at-1.23V (vs. RHE) for about 25000s. The analysis chart shown that the current density of the Al–Cu₂O catalyst can be kept stable and the FE of ethylene can be kept above 40% in the first 3600s. With the change in reaction time, the current density increases gradually. By observing the stable SEM of the Al–Cu₂O catalyst, we can see that there are some small pores on the surface of the catalyst, which may provide more active sites, thus increasing the current density. However, the selectivity of the catalyst to ethylene, the main product, began to decrease obviously after two hours of testing, probably because the catalyst was dripped on hydrophobic carbon paper during the test, which may lead to the shedding of the catalyst in the long-term electrolysis process, resulting in a decrease in the FE of the catalyst. At the same time, in the current research, the stability of copper-based catalysts is poor, in later research, other strategies need to be used to improve the stability of copper-based catalysts for a long time [37]. Through the XRD spectrum after long-term electrolysis, it was found that the composition of the electrode was consistent with that before the reaction, indicating that the Al–Cu₂O showed good electrochemical stability (Figure 5b) in the whole CO₂RR test. It was worth noting that after the electrolysis of Al–Cu₂O catalyst for 10min, 20min, 30min and 7h (Figure S7a-d), the result showed that the morphology of the octahedron remains unchanged. With the increase of electrolysis time, some small pores appear on the surface of the catalyst. The appearance of these pores may provide more active sites, resulting in an increase in current density in the electrolysis process, but a decrease in the FE of ethylene. The result shown that the increase of these active sites was not conducive to improving the selectivity of the main products, and the above results shown that the catalyst can maintain good stability under long-term electrolysis.

2.3. DFT Computations

In order to further explore the catalytic reaction mechanism, we used Density functional theory (DFT) to calculate, simulate and compare the CO₂RR reaction path on the surface of Al-Cu₂O and Cu₂O catalysts to further understand the path from CO₂ to C₂H₄. Figure 6 shows the spatial structure (Figure 6a) and energy distribution of Al–Cu₂O and Cu₂O. Figure 6b shown the energy distribution of ethylene production and by-product H₂ of Cu₂O and Al–Cu₂O catalysts. The Gibbs free energies of each intermediate along ethylene on Cu₂O and Al-Cu₂O catalysts *CHCOH, *CCH, *CCH, *CCH₂, *CHCH₂ (intermediates for ethylene production) and *H (intermediates to H₂) have been calculated. Because the Gibbs free energy of Al–Cu₂O catalyst was lower than that of Cu₂O catalyst in each reaction step, the path of ethylene production of CO₂RR was easier to occur. It can be seen that the strategy of doping Al to octahedral Cu₂O was beneficial to improve the selectivity of product C₂H₄ [38]. At the same time, the analysis of figure 6c shows that the Al-Cu₂O catalyst doped with Al enhances the adsorption of intermediate * H and was further away from the ideal hydrogen adsorption value (0 eV). It makes the competitive reaction of HER more disadvantageous, thus inhibiting the occurrence of side effects.To further analyze the potential reason for the selective improvement of this product, the density of states (DOS) of d orbitals on Cu₂O (001) and the Al–Cu₂O (001) surfaces before CHCOH adsorption is compared (Figure 6d,e). Since the electronic states near the Fermi level are mainly contributed by the d electrons of Cu atoms, it is indicated that the reaction is mainly caused by the interaction between Cu and C, and the d band center of undoped Al octahedron Cu₂O (001) was -2.08 7eV. The d-2.027 eV of the Al-doped Al–Cu₂O (001) surface was closer to the Fermi level (0eV), and the d-band shifts upward on the Abscissa, which makes the center of the d-band closer to the Fermi level and increases the density of electronic states, which is beneficial to the adsorption of Cu atoms through d electrons and intermediates, thus promoting the CO₂RR process and improving the selectivity of the catalyst for C₂H₄ product.

3. Materials and Methods

3.1. Preparation of Al-Cu₂O nanocrystals

The Al-Cu₂O nanocrystals was synthesized with a improved method according to the literature [39]. The specific step was as follows: 10 mL of 0.6 M NaOH aqueous solution was first added to the sample bottle. Subsequently, CuCl₂·2H₂O、Al(NO₃)₃·9H₂O and glucose were added to the sample bottle successively. The concentrations of CuCl₂·2H₂O, Al(NO₃)₃·9H₂O,and glucose in the solution were 0.10 M, 0.02 M, and 0.07 M, respectively. After continuous agitation for 5min, the sample bottle was placed in a 70℃ water bath and vigorously stirred for 4min. The precipitation obtained by centrifugal collection was rinsed with deionized water, and finally dried under vacuum at room temperature for 12 h to obtain Al–Cu₂O catalyst. At the same time, the effects of the amount of Al³⁺, the concentration of NaOH and the reaction time on ethylene products were also investigated in this chapter, and the optimum preparation conditions were obtained, as shown in the following table S1-1.

3.2. Preparation of Al-Cu₂O coated carbon paper electrode

5 mg of the prepared catalyst was taken and added to 25 μL of Nafion, followed by 300 μL of distilled water and 175 μL of ethanol to prepare 500 μL of reagent, which was mixed by sonication for 2 hours and then 100 μL was uniformly applied with a pipette to a carbon paper type 060 with a total surface area of 1 cm². The loading on the carbon paper was calculated to be 1 mg cm^-2 and subsequently dried in a vacuum oven to obtain the Al-Cu₂O electrode for the next test.

3.3. Electrochemical measurements

The electrocatalytic CO₂RR was carried out in an H-type electrolytic cell with a proton exchange membrane (Nafion 117) separating the electrolyte. Working electrodes were prepared, with a platinum sheet (1 cm²) as the counter electrode and Ag/AgCl (saturated KCl) as the reference electrode. Before conducting the experimental test, CO₂ (99.999% purity) or N₂ gas was introduced into the electrolytic cell, which was saturated with 0.1 M KHCO₃ (pH=6.8) electrolyte after approximately 30 min. In this work, all electrochemical performance was measured on the electrochemical workstation (CHI760E, Shanghai Chenhua). Convert all electrode potentials into electrode potentials relative to RHE through Nernst equation:$\mathrm{E}(\mathrm{v}\mathrm{s}.\mathrm{R}\mathrm{H}\mathrm{E})=\mathrm{E}(\mathrm{v}\mathrm{s}.\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l})+0.0591\times \mathrm{p}\mathrm{H}+0.197\mathrm{V}$. The electrochemical active surface area was tested by the cyclic voltammetry curves of the bilayer capacitance values at different scanning rates (20, 40, 60, 80, 100 and 120 mV s^-1). The gaseous products were collected by electrolysis of the four catalysts in a 0.1 M KHCO₃ electrolyte saturated with CO₂ for 10 min at different measurement potentials and then analyzed by using gas chromatography (8890, Agilent). The liquid products of the four catalysts were collected by electrolysis in an aqueous 0.1 M KHCO₃ solution saturated with CO₂ for 30 min at each measurement potential, followed by qualitative and quantitative analysis using ion chromatography (AS-DV, Thermo Scientific, America).

3.4. Product analysis

The gas products are detected by gas chromatography (GC, Agilent 8890) directly from the gas outlet. The carbonaceous gas products from the cathode chamber are analyzed by a methane reformer and flame ionization detector (FID). A thermal conductivity detector (TCD) was used to detect the eCO₂RR by-product H₂. When the current stabilizes, the gas product is detected. Quantification of the gaseous products was determined by comparison with the standard curve. the Faraday efficiency (FE) of C₂H₄, H₂ and CO was calculated as follows:

$$\mathrm{F}\mathrm{E}=\frac{N\times n\times v\times F}{V_m\times j}\times 100\%$$

where $v$ is the CO₂ flow rate (v= 20 mL min^-1), n is the total molar fraction of C₂H₄, H₂ or CO of the gas measured in the GC, N is the number of electrons required to form a molecule of H₂ or CO (N=2), F is Faraday's constant (96485 C mol^-1), and V_m is the molar volume of the gas at 298 K and j is current at each potential (A).

Liquid products Faraday efficiency test method:A saturated solution of electrocatalytic CO₂ was electrocatalyzed by the Coulomb method using a controlled potential, and the electrolytic reduction product was analyzed and calculated after 0.5 h. The CO₂ flow rate during electrolysis was controlled at 20 mL min^-1 and the liquid product was determined by ion chromatography (AS-DV, Thermo Scientific, America). The FE of the liquid phase product was calculated as follows:

$$\mathrm{F}\mathrm{E}=\frac{NnF}{Q}\times 100\%$$

where N is the number of electrons transferred, n is the amount of formate in the cathode chamber, F is Faraday's constant (96485 C mol^-1) and Q is the total charge passing through the electrode.

4. Conclusions

In summary, the Al-doped octahedral Cu₂O nanocrystal was successfully prepared and used as an efficient CO₂RR electrocatalyst. The Al–Cu₂O exhibited high activity and selectivity for ethylene production. The Al–Cu₂O catalyst demonstrate a high faradaic efficiency 44.9% at -1.23V (vs. RHE) for C₂H₄ production. The high catalytic activity for CO₂ electrochemical reduction is due to optimized electronic state by Al doping in octahedral Cu₂O nanocrystal. The DFT simulation suggested the C–C coupling mechanism, and proved that the catalyst Al–Cu₂O doped Cu₂O octahedron can greatly reduce the free energy in the coupling process of *CHCOH intermediate, promote the formation of C₂H₄, and inhibit the occurrence of HER side effect. Therefore, the active degree of electronic states increases, which was conducive to the adsorption of Cu atoms and intermediates, improving the selectivity of product C₂H₄. Furthermore, our work demonstrates a simple doping strategy for the preparation of novel copper-based catalysts, which can be extended to the design and study of other types of highly efficient electrocatalysts.