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

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10 July 2023

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11 July 2023

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Abstract
Ethylene is an ideal CO2 product in an electrocatalytic CO2 reduction reaction (CO2RR), which has high economic value. In this paper, Al–doped octahedral Cu2O (Al–Cu2O) catalyst was synthesized by a simple wet chemical method. The selectivity of CO2RR products was improved by doping Al onto the surface of octahedral Cu2O by regulating the Al content. The Al–Cu2O was used as an efficient electrocatalyst for CO2RR with selective ethylene production. The Al–Cu2O exhibited a high Faradic efficiency (FEC2H4) of 44.9% at -1.23 V (vs. RHE) in CO2 saturated 0.1 M KHCO3 electrolyte. Charge transferring from Al atom to Cu atom take place after Al doping in Cu2O, thereby optimizing the electronic structure, which facilitates CO2RR to ethylene production. The DFT calculation showed that the Al–Cu2O catalyst can effectively reduce the adsorption energy of the *CHCOH intermediate and promote the mass transfer of charges, thus improving the FEC2H4. After Al doping into Cu2O, the center of d orbitals shift positively, which makes the d–band closer to the Fermi level. Furthermore, the density of electronic states increases, which was conducted to the interaction between Cu atoms and intermediates, thus accelerating the electrochemical CO2 reduction process. This work proved that the metal doping strategy can effectively improve the catalytic properties of Cu2O, thus providing a useful way for CO2 cycling and green production of C2H4.
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Subject: Chemistry and Materials Science  -   Electrochemistry

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 C2H4. 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 (FEC2H4) 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 2p3/2 and Cu 2p1/2 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 2p3/2 and Cu2 p1/2 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 2p3/2, Cu 2p1/2, 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 2p1/2 and Al 2p3/2 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 FEC2H4 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 Cdl and Cds 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 Cdl 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 Cdl 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: E ( v s . R H E ) = E ( v s . A g / A g C l ) + 0.0591 × p H + 0.197 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:
F E = N × n × v × F V m × j × 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 Vm 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:
F E = N n F Q × 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.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 22262027). We also acknowledged the support of Ningxia leading scientific and technological innovation talents projects (No.KJT2018002), and Ningxia Natural Science Foundation (No.2022AAC03103).

Conflicts of Interest

The authors declared that there was no competing financial interest.

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Figure 1. Schematic illustration of the fabrication process for Al–Cu2O–X (X=2、3、4、5、6).
Figure 1. Schematic illustration of the fabrication process for Al–Cu2O–X (X=2、3、4、5、6).
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Figure 2. Characterization of Al–Cu2O: (a) XRD, (b) SEM, (c) TEM, (d) HRTEM, (e) HAADF–TEM, and (f) elemental mapping (blue, green and red represents Cu, O and Al element, respectively).
Figure 2. Characterization of Al–Cu2O: (a) XRD, (b) SEM, (c) TEM, (d) HRTEM, (e) HAADF–TEM, and (f) elemental mapping (blue, green and red represents Cu, O and Al element, respectively).
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Figure 3. XPS spectrum of Cu 2p: (a) Cu2O and (b)Al–Cu2O, O 1s spectrum of (c) Cu2O and (d) Al–Cu2O, (e) Al 2p spectrum of the Al–Cu2O.
Figure 3. XPS spectrum of Cu 2p: (a) Cu2O and (b)Al–Cu2O, O 1s spectrum of (c) Cu2O and (d) Al–Cu2O, (e) Al 2p spectrum of the Al–Cu2O.
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Figure 4. (a) the polarization curves of Cu2O, Al–Cu2O and Al–Cu2O–2 catalysts in 0.1 M KHCO3 aqueous solutions with saturated CO2, (b) partial current density of Cu2O, Al–Cu2O and Al–Cu2O–2 catalysts, sweeping speed of 5 mV s-1, (c) FE values of Al-Cu2O catalyst in 0.1 M KHCO3 aqueous solutions with saturated CO2, (d) The FEC2H4 values of Cu2O, Al–Cu2O and Al–Cu2O–2 catalysts, (e) The linear relationship between △J and scanning rates , (f) Tafel plots of Cu2O, Al–Cu2O and Al-Cu2O–2.
Figure 4. (a) the polarization curves of Cu2O, Al–Cu2O and Al–Cu2O–2 catalysts in 0.1 M KHCO3 aqueous solutions with saturated CO2, (b) partial current density of Cu2O, Al–Cu2O and Al–Cu2O–2 catalysts, sweeping speed of 5 mV s-1, (c) FE values of Al-Cu2O catalyst in 0.1 M KHCO3 aqueous solutions with saturated CO2, (d) The FEC2H4 values of Cu2O, Al–Cu2O and Al–Cu2O–2 catalysts, (e) The linear relationship between △J and scanning rates , (f) Tafel plots of Cu2O, Al–Cu2O and Al-Cu2O–2.
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Figure 5. Al–Cu2O catalyst in 0.1 M KHCO3 electrolyte (a) electrochemical stability test pattern and (b) the XRD of Al–Cu2O catalyst after long–term stability test.
Figure 5. Al–Cu2O catalyst in 0.1 M KHCO3 electrolyte (a) electrochemical stability test pattern and (b) the XRD of Al–Cu2O catalyst after long–term stability test.
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Figure 6. Free energy diagram of ethylene and hydrogen produced by CO2RR on the surface of (a) side and top views of Cu2O (001) and Al–Cu2O (001) configurations, (b) Cu2O (001) and (c) Al-Cu2O (001) catalysts, DOS of d orbitals on (d) Cu2O (001) and (e) Al–Cu2O (001) surfaces before *CHCOH adsorption.
Figure 6. Free energy diagram of ethylene and hydrogen produced by CO2RR on the surface of (a) side and top views of Cu2O (001) and Al–Cu2O (001) configurations, (b) Cu2O (001) and (c) Al-Cu2O (001) catalysts, DOS of d orbitals on (d) Cu2O (001) and (e) Al–Cu2O (001) surfaces before *CHCOH adsorption.
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