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

Ethylene is an ideal CO2 product in an electrocatalytic CO2 reduction reaction (CO2RR) with high economic value. This paper synthesised Al-doped octahedral Cu2O (Al–Cu2O) nanocrystal by a simple wet chemical method. The selectivity of CO2RR products was improved by doping Al onto the surface of octahedral Cu2O. 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 transfer from the Al atom to the Cu atom occurs after Al doping in Cu2O, optimizing the electronic structure and facilitating CO2RR to ethylene production. The DFT calculation showed that the Al–Cu2O catalyst could 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 due 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.


Introduction
The increased CO 2 emissions in the atmosphere result in a serious greenhouse effect and the elevated sea level [1,2].The electrochemical CO 2 reduction reaction (CO 2 RR) is a promising strategy to mitigate the global warming and energy crisis while transforming CO 2 into fuels and chemical feedstocks [3][4][5].It can use clean electric energy generated by renewable solar and wind energy to drive the conversion of CO 2 under mild conditions [6,7].Electroreduction of CO 2 in molten salts, also called molten salt carbon capture and electrochemical transformation (MSCC-ET), is an advanced method which can capture carbon dioxide from the atmosphere or flue gases [8][9][10][11].The reduction products of CO 2 RR include CO [12][13][14], HCOOH [15][16][17][18], alcohols [19][20][21], and various hydrocarbons [22][23][24].Among them, C 2 H 4 has attracted more and more attention due to its high energy density.Furthermore, C 2 H 4 is essential in producing various plastics, solvents, and cosmetics in the chemical industry [25].
Up to now, Cu-based materials can electrocatalytic convert CO 2 into C2/C2+ products.Among reported Cu-based catalysts, Cu 2 O nanocrystal has attracted much attention due to their electrocatalytic activity and high selectivity toward C 2 H 4 .To improve the CO 2 RR performance of Cu 2 O, 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 [26].Cu 2 O nanoparticles (Cu 2 O NPs) exhibit good performance for CO 2 RR, possibly because the low coordination Cu + on the surface contributes to C-C coupling, thus promoting the production of C 2 H 4 [27].Other strategies, including crystal facet controlling [28], defect engineering [29,30], alloying [31], valence adjustment [32], and surface molecular modification [24], have been employed to improve the electrocatalytic performance of CO 2 RR to produce C 2 H 4 .For example, Shang et al. [33] have designed a core-shell Cu@Cu 2 O catalyst on which a thin layer of natural oxide grows on the surface under environmental conditions.The synergistic effect between Cu + and Cu 0 on the Cu@Cu 2 O surface helps to improve its efficiency and selectivity for C2 products.Ning et al. [34] reported the preparation of Cu 2 O/nitrogen-doped carbon shell (Cu 2 O/NCS) composite and its application in CO 2 electroreduction to selective formation of C 2 H 4 .However, copper-based catalysts still face many problems, such as inevitable competitive hydrogen evolution reaction (HER), complex reaction mechanisms diversification of products, and low selectivity of target products [35].Therefore, it is of huge challenge to design CO 2 RR electrocatalysts with high activity, selectivity, and satisfied stability.
This work proposed an effective strategy to improve the CO 2 RR activity by doping Al on the surface of octahedron Cu 2 O nanocrystals.Al-doped Cu 2 O (Al-Cu 2 O) was used as an effective electrocatalyst for selective ethylene production by CO 2 RR.Al-Cu 2 O 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-Cu 2 O catalyst can significantly reduce the free energy and promote the formation of C 2 H 4 .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 Cu 2 O, thus improving the activity and selectivity of CO 2 conversion to ethylene.

Morphology and Structure Analysis
As shown in Figure 1, Al-Cu 2 O-X nanocrystals were prepared by a simple one-step method (Experimental section for details).To further characterize Al-Cu 2 O-X catalysts, the XRD pattern was used to study Cu 2 O, Al-Cu 2 O, and Al-Cu 2 O-2 crystal structure.From the XRD pattern in Figure 2a, the peaks at 29      S2a).Due to the doping effect of Al, the Al-Cu 2 O nanocrystal presented an octahedral shape with a more rough surface and formed a defect structure (Figure 2b), which may provide abundant active sites for CO 2 RR [36].When the concentration of Al 3+ increased from 0.02 M to 0.03 M, Al-Cu 2 O-2 catalyst exhibits a cube shape (Figure S2b).However, it was reported that the resulting cube Al-Cu 2 O catalyst is not conducive to forming C 2 H 4 [27].The better-performing Al-Cu 2 O with a homo-octahedral shape was observed by TEM (Figure 2c), which was consistent with the SEM image.The 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 Cu 2 O.The HAADF-STEM image (Figure 2e) also exhibited an octahedral shape.The composition of Al-Cu 2 O was reconfirmed by elemental mapping (Figure 2f).The Al (red), Cu (blue), and O (green) elements are uniformly distributed over the Al-Cu 2 O nanocrystals.The surface composition and valence of Cu2O and Al-Cu2O nanocrystals were characterized by X-ray photoelectron spectroscopy (XPS).As shown in Figure 3a,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 Cu 2+ .For Al-Cu2O, the 932.89 and 952.73 eV peaks 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 Cu 2+ .The above results showed that the existence of Cu 0 may be due to the partial reduction of Cu2O in the CO2RR process [37].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 [38].When octahedral Cu2O nanocrystals were doped with Al, the peaks of Cu 2p3/2, Cu 2p1/2, and Cu 2+ of Cu2O or Cu were shifted positively.The surface composition and valence of Cu2O and Al-Cu2O nanocrystals were characterized by X-ray photoelectron spectroscopy (XPS).As shown in Figure 3a,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 Cu 2+ .For Al-Cu2O, the 932.89 and 952.73 eV peaks 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 Cu 2+ .The above results showed that the existence of Cu 0 may be due to the partial reduction of Cu2O in the CO2RR process [37].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 [38].When octahedral Cu2O nanocrystals were doped with Al, the peaks of Cu 2p3/2, Cu 2p1/2, and Cu 2+ of Cu2O or Cu were shifted positively.These results may be attributed to 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

Electrocatalytic CO2RR Performances
To further analyze the electrochemical performance of the catalyst, the linear sweep voltammetry (LSV) of Cu2O and Al-Cu2O-X in saturated CO2 electrolyte and saturated N2 electrolyte were tested.The analysis of Figure 4a shows 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 to CO2RR.The LSV curve was measured in a CO2-saturated 0.1 M KHCO3 electrolyte (Figure S3a).The current density of the Al-Cu2O catalyst in CO2 saturated electrolyte was higher than that of Cu2O and Al-Cu2O-2 catalysts, indicating that the Al-Cu2O catalyst had better electrocatalytic activity to CO2RR. Figure S3b shows the potentiostatic electrolysis of CO2 at various potentials.The almost constant current signal indicated 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 had a higher current density for ethylene formation than that of Cu2O and the Al-Cu2O-2 catalysts in a wide potential range.The partial current density of 16.7 mA cm −2 was achieved at −1.38 V (vs.RHE).The above results showed that the Al-Cu2O catalyst was more conducive to producing ethylene as the main product and has a better inhibitory effect on competition for hydrogen formation.

Electrocatalytic CO 2 RR Performances
To further analyze the electrochemical performance of the catalyst, the linear sweep voltammetry (LSV) of Cu 2 O and Al-Cu 2 O-X in saturated CO 2 electrolyte and saturated N 2 electrolyte were tested.The analysis of Figure 4a shows that the current density of Al-Cu 2 O catalyst in CO 2 saturated electrolyte is higher than that in N 2 , indicating that Al-Cu 2 O catalyst had higher activity to CO 2 RR.The LSV curve was measured in a CO 2 -saturated 0.1 M KHCO 3 electrolyte (Figure S3a).The current density of the Al-Cu 2 O catalyst in CO 2 saturated electrolyte was higher than that of Cu 2 O and Al-Cu 2 O-2 catalysts, indicating that the Al-Cu 2 O catalyst had better electrocatalytic activity to CO 2 RR. Figure S3b shows the potentiostatic electrolysis of CO 2 at various potentials.The almost constant current signal indicated that the Al-Cu 2 O catalyst exhibited good electrochemical stability during the CO 2 RR process.In Figure 4b, the formation rates of three kinds of catalysts were presented for ethylene products.The Al-Cu 2 O catalyst had a higher current density for ethylene formation than that of Cu 2 O and the Al-Cu 2 O-2 catalysts in a wide potential range.The partial current density of 16.7 mA cm −2 was achieved at −1.38 V (vs.RHE).The above results showed that the Al-Cu 2 O catalyst was more conducive to producing ethylene as the main product and has a better inhibitory effect on competition for hydrogen formation.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).From Figures 4c and 5, the CO2RR products by Cu2O, and Al-Cu2O-X catalysts were C2H4, HCOO − , CO, CH4 and by-product H2. Figure 5a shows the FE of the electrochemical CO2RR product catalyzed by an octahedral Cu2O nanocrystal catalyst without the Al doping.The octahedral Cu2O nanocrystal catalyst had a good effect on inhibiting HER at low potential, and the FEC2H4 was 26.1%.As shown in Figure 4b, with the potential increase, the FE value of H2 decreases from 35.1% to 22.1%.
On the contrary, the FE value of C2H4 increases to 44.9% at −1.23 V (vs.RHE).The results showed the catalystʹs good selectivity for ethylene production and inhibition effect on HER.If an appropriate amount of Al (0.02 M) was introduced into the octahedral Cu2O nanocrystal (Figure 4c), the selectivity of the Al-Cu2O catalyst was improved.If more Al 3+ was added to the reaction, The result showed that the FE of C2H4 was 32.8%, indicating that the catalyst had a good selectivity for ethylene (Figure 5b).we also studied the effects of catalysts synthetic conditions, including reaction time (Figure 5c,d) and NaOH concentration (Figure 5e,f), on the selectivity of the CO2RR product.The results indicated that optimising reaction time and NaOH concentration could give the catalyst a certain selectivity.Figure 4d compares the selectivity of three kinds of catalysts (Cu2O, Al-Cu2O, and Al-Cu2O-2) for ethylene products.Under different applied potentials, the efficiency of the Al-Cu2O catalyst for CO2RR to C2H4 was higher than that of the other two catalysts.This result suggested that the Al introduced into the catalyst affected the selectivity of the catalyst.This may be because 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.It was worth noting that the Faradaic efficiencies sometimes do not reach 100%.A small number of liquid products may still be produced in the electrocatalysis process.To determine the CO 2 RR selectivity of the Al-Cu 2 O 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).From Figures 4c and 5 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 On the contrary, the FE value of C 2 H 4 increases to 44.9% at −1.23 V (vs.RHE).The results showed the catalyst's good selectivity for ethylene production and inhibition effect on HER.If an appropriate amount of Al (0.02 M) was introduced into the octahedral Cu 2 O nanocrystal (Figure 4c), the selectivity of the Al-Cu 2 O catalyst was improved.If more Al 3+ was added to the reaction, The result showed that the FE of C 2 H 4 was 32.8%, indicating that the catalyst had a good selectivity for ethylene (Figure 5b).we also studied the effects of catalysts synthetic conditions, including reaction time (Figure 5c,d) and NaOH concentration (Figure 5e,f), on the selectivity of the CO 2 RR product.The results indicated that optimising reaction time and NaOH concentration could give the catalyst a certain selectivity.Figure 4d compares the selectivity of three kinds of catalysts (Cu 2 O, Al-Cu 2 O, and Al-Cu 2 O-2) for ethylene products.Under different applied potentials, the efficiency of the Al-Cu 2 O catalyst for CO 2 RR to C 2 H 4 was higher than that of the other two catalysts.This result suggested that the Al introduced into the catalyst affected the selectivity of the catalyst.This may be because Al-doped Cu 2 O 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 CO 2 RR process and enhancing the selectivity of the reaction to the products.It was worth noting that the Faradaic efficiencies sometimes do not reach 100%.A small number of liquid products may still be produced in the electrocatalysis process.
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 the cyclic voltammograms (Figure S4a-c S5).The EIS arc of the Al-Cu 2 O catalyst was smaller than that of the Cu 2 O catalyst.The results indicate that interface charges can be rapidly transferred during the reaction process, and catalytic activity can be improved.To better understand the activity and kinetics of Al-Cu 2 O materials on CO 2 RR, the Tafel slope analysis of the local current density of the catalyst product was carried out.As shown in Figure 4f, the Tafel slope of the Al-Cu 2 O catalyst (74.3 mV dec −1 ) was lower than that of Cu 2 O (85.9 mV dec −1 ) and the Al-Cu 2 O-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 from the surface of Al-Cu 2 O catalyst.
The stability of the Al-Cu 2 O catalyst was investigated in the CO 2 RR process.As seen in Figure 6a, the current density of the Al-Cu 2 O catalyst can be kept stable, and the FE of ethylene can be kept above 40% in the first 3600 s.With the change in reaction time, the current density increases gradually.However, the selectivity of the catalyst to ethylene decreased obviously after two hours of electrolysis.This may be because of the shedding of the catalyst in the long-term electrolysis process, resulting in a decrease in the FE of the catalyst.The stability of copper-based catalysts is poor.Therefore, other strategies must be used to improve the stability of copper-based catalysts for a long time [40].The XRD pattern after long-term electrolysis showed that the Al-Cu 2 O showed good structure stability (Figure 6b) in the whole CO 2 RR test.After the electrolysis of the Al-Cu 2 O catalyst for 10 min, 20 min, 30 min and 7 h (Figure S6a-d), the morphology of the octahedron remains unchanged.With the increase of electrolysis time, some small pores appear on the catalyst's surface.The appearance of these pores may provide more active sites, resulting in an increase in current density in the electrolysis process.However, it yielded a decrease in the FE of ethylene.The above results show that the catalyst can maintain stability under long-term electrolysis.
remains unchanged.With the increase of electrolysis time, some small pores app the catalyst's surface.The appearance of these pores may provide more active s sulting in an increase in current density in the electrolysis process.However, it yi decrease in the FE of ethylene.The above results show that the catalyst can maint bility under long-term electrolysis.

DFT Computations
We used Density functional theory (DFT) to further calculate, simulate and co the CO2RR reaction path on the surface of Al-Cu2O and Cu2O catalysts to understa path from CO2 to C2H4. Figure 7 shows the spatial structure (Figure 7a) and ener tribution of Al-Cu2O and Cu2O. Figure 7b shows the energy distribution of ethyle duction and by-product H2 of Cu2O and Al-Cu2O catalysts.The Gibbs free ener each intermediate along ethylene on Cu2O and Al-Cu2O catalysts *CHCOH, *CCH, *CCH2, *CHCH2 (intermediates for ethylene production) and *H (intermediates have been calculated.Because the Gibbs free energy of the Al-Cu2O catalyst wa than that of the Cu2O catalyst in each reaction step, the path of ethylene produc CO2RR was easier to occur.It can be seen that the strategy of doping Al to octahedra was beneficial in improving the selectivity of product C2H4 [41].At the same time, th ysis of Figure 7c showed that the Al-Cu2O catalyst with Al doping enhanced the a tion of intermediate *H and further departed from the ideal hydrogen adsorption v eV).It makes the competitive reaction of HER more disadvantageous.To further a the potential reason for the selective improvement of this product, the density o (DOS) of d orbitals on Cu2O (001) and Al-Cu2O (001) surfaces before CHCOH ads was compared (Figure 7d,e).Since the electronic states near the Fermi level are contributed by the d electrons of Cu atoms, it is observed that the reaction is mainly by the interaction between Cu and C, and the d band center of octahedron Cu2O (0 −2.087 eV.The −2.027 eV of the Al-Cu2O (001) surface was closer to the Fermi level and the d-band shifts upward on the Abscissa, which makes the center of the d-band to the Fermi level and increases the density of electronic states.So, the adsorption atoms through d electrons and intermediates was facilitated, thus promoting the process and improving the selectivity of the catalyst for the C2H4 product.

DFT Computations
We used Density functional theory (DFT) to further calculate, simulate and compare the CO 2 RR reaction path on the surface of Al-Cu 2 O and Cu 2 O catalysts to understand the path from CO 2 to C 2 H 4 .Figure 7 shows the spatial structure (Figure 7a) and energy distribution of Al-Cu 2 O and Cu 2 O. Figure 7b

Preparation of Al-Cu2O Nanocrystals
The Al-Cu2O nanocrystals were synthesized with an improved method according to the literature [42].The specific step was as follows: 10 mL of 0.6 M NaOH aqueous solution was first added to the sample bottle.Subsequently, a certain amount of 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 were 0.10 M, 0.02 M, and 0.07 M, respectively.After continuous agitation for 5 min, the sample bottle was placed in a 70 °C water bath and vigorously stirred for 4 min.The precipitation obtained by centrifugal collection was rinsed with deionized water and dried under vacuum at room temperature for 12 h to obtain an Al-Cu2O catalyst.At the same time, the effects of the amount of Al 3+ , NaOH concentration, and the sythiestic 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.

Preparation of Al-Cu 2 O Nanocrystals
The Al-Cu 2 O nanocrystals were synthesized with an improved method according to the literature [42].The specific step was as follows: 10 mL of 0.6 M NaOH aqueous solution was first added to the sample bottle.Subsequently, a certain amount of CuCl 2 •2H 2 O, Al(NO 3 ) 3 •9H 2 O and glucose were added to the sample bottle successively.The concentrations of CuCl 2 •2H 2 O, Al(NO 3 ) 3 •9H 2 O, and glucose were 0.10 M, 0.02 M, and 0.07 M, respectively.After continuous agitation for 5 min, the sample bottle was placed in a 70 • C water bath and vigorously stirred for 4 min.The precipitation obtained by centrifugal collection was rinsed with deionized water and dried under vacuum at room temperature for 12 h to obtain an Al-Cu 2 O catalyst.At the same time, the effects of the amount of Al 3+ , NaOH concentration, and the sythiestic 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.

Preparation of Al-Cu 2 O Coated Carbon Paper Electrode
5 mg of the prepared catalyst was added to 25 µL of Nafion.Then, 300 µL of distilled water and 175 µL of ethanol to prepare 500 µL of reagent was added and mixed by sonication for 2 h.Subsequently, 100 µL was uniformly applied with a pipette to a carbon paper (type 060) with a total surface area of 1 cm 2 .The loading on the carbon paper was calculated to be 1 mg cm −2 and dried in a vacuum oven to obtain the Al-Cu 2 O electrode for the next test.

Electrochemical Measurements
The electrocatalytic CO 2 RR was carried out in an H-type electrolytic cell with a proton exchange membrane (Nafion 117) separation.A platinum sheet (1 cm 2 ) as the counter electrode and Ag/AgCl (saturated KCl) as the reference electrode, respectively.Before conducting the experimental test, CO 2 (99.999% purity) or N 2 gas was introduced into the electrolytic cell, which was saturated with 0.1 M KHCO 3 (pH = 6.8) electrolyte after approximately 30 min.In this work, all electrochemical performance was measured on the electrochemical workstation (CHI760E, Shanghai Chenhua, Shanghai, China).All electrode potentials were converted into electrode potentials relative to RHE through the Nernst equation: E (vs.RHE) = E (vs.Ag/AgCl) + 0.0591 × pH + 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 KHCO 3 electrolyte saturated with CO 2 for 10 min at different measurement potentials and then analyzed using gas chromatography (8890, Agilent, Santa Clara, CA, USA).The liquid products of the four catalysts were collected by electrolysis in an aqueous 0.1 M KHCO 3 solution saturated with CO 2 for 30 min at each measurement potential, followed by qualitative and quantitative analysis using ion chromatography (AS-DV, Thermo Scientific, Waltham, MA, USA).

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 eCO 2 RR by-product H 2 .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 C 2 H 4 , H 2 and CO was calculated as follows: where v is the CO 2 flow rate (v = 20 mL min −1 ), n is the total molar fraction of C 2 H 4 , H 2 or CO of the gas measured in the GC, N is the number of electrons required to form a molecule of H 2 or CO (N = 2), F is Faraday's constant (96,485 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 CO 2 was electro-catalyzed by the Coulomb method using a controlled potential, and the electrolytic reduction product was analyzed and calculated after 0.5 h.The CO 2 flow rate during electrolysis was controlled at 20 mL min −1, and the liquid product was determined by ion chromatography (AS-DV, Thermo Scientific, Waltham, MA, USA).The FE of the liquid phase product was calculated as follows: where N is the number of electrons transferred, n is the amount of formate in the cathode chamber, F is Faraday's constant (96,485 C mol −1 ) and Q is the total charge passing through the electrode.

Conclusions
In summary, the Al-doped octahedral Cu 2 O nanocrystal was successfully prepared and used as an efficient CO 2 RR electrocatalyst.The Al-Cu 2 O exhibited high activity and selectivity for ethylene production.The Al-Cu 2 O catalyst demonstrates a high % faradaic efficiency of 44.9% at −1.23 V (vs.RHE) for C 2 H 4 production.The high catalytic activity for CO 2 electrochemical reduction is due to the optimized electronic state by Al doping in octahedral Cu 2 O nanocrystals.The DFT simulation suggested the C-C coupling mechanism in the electrochemical CO 2 RR process.The Al-Cu 2 O doped Cu 2 O octahedron can significantly reduce the free energy in the coupling process of *CHCOH intermediate, promote the formation of C 2 H 4 , and inhibit the occurrence of HER side effect.Furthermore, our work demonstrates a simple doping strategy for preparing copper-based catalysts, which can be extended to the design and study of other highly efficient electrocatalysts.

Figure 2 .
Figure 2. Characterization of Al-Cu 2 O: (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).The surface composition and valence of Cu 2 O and Al-Cu 2 O nanocrystals were characterized by X-ray photoelectron spectroscopy (XPS).As shown in Figure 3a,b, four peaks were observed in Cu 2p spectrum for both Cu 2 O and Al-Cu 2 O samples.For Cu 2 O, 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 Cu 2 O or Cu, respectively.The binding energies at 935.28 eV and 944.48 eV were ascribed to the peaks of Cu 2+ .For Al-Cu 2 O, the 932.89 and 952.73 eV peaks corresponded to the Cu 2p 3/2 and Cu2 p 1/2 of Cu 2 O or Cu, respectively.The binding energy of 935.26 and 944.46 eV belonged to the peak of Cu 2+ .The above results showed that the existence of Cu 0 may be due to the partial reduction of Cu 2 O in the CO 2 RR process [37].The existence of trace CuO may be mainly due to the oxidation of a small amount of Cu 2 O catalyst to CuO in the air after the synthesis of Cu 2 O [38].When octahedral Cu 2 O nanocrystals were doped with Al, the peaks of Cu 2p 3/2 , Cu 2p 1/2 , and Cu 2+ of Cu 2 O or Cu were shifted positively.These results may be attributed to the introduction of Al, which can induce charge transfer from Al atoms to Cu atoms, thus modulating the electronic structure

Figure 4 .
Figure 4. (a) LSV curves of Cu 2 O, Al-Cu 2 O and Al-Cu 2 O-2 catalysts in 0.1 M KHCO 3 aqueous solutions saturated CO 2 , (b) partial current density of Cu 2 O, Al-Cu 2 O and Al-Cu 2 O-2 catalysts, sweeping speed of 5 mV s −1 , (c) FE values of Al-Cu 2 O catalyst in 0.1 M KHCO 3 aqueous solutions with saturated CO 2 , (d) The FE C2H4 values of Cu 2 O, Al-Cu 2 O and Al-Cu 2 O-2 catalysts, (e) The linear relationship between ∆J and scanning rates, (f) Tafel plots of Cu 2 O, Al-Cu 2 O and Al-Cu 2 O-2.
) of Cu 2 O, Al-Cu 2 O, and Al-Cu 2 O-2 catalysts at different scanning rates (20, 40, 60, 80,100, 120 mV s −1 ), the capacitance values of Cu 2 O, Al-Cu 2 O, and Al-Cu 2 O-2 catalysts were 0.109, 0.122, and 0.076 mF cm −2 , respectively, as shown in Figure 4e.The largest C dl of the Al-Cu 2 O electrocatalyst suggested the high electrochemical activity surface area of the Al-Cu 2 O-2 catalyst.This high ECSA can offer a lot of catalytic active sites for improving the electrocatalytic performance of CO 2 RR, which was consistent with the previous research conclusion.The impedance of Cu 2 O and Al-Cu 2 O catalysts under open-circuit voltage was obtained (Figure

Figure 6 .
Figure 6.Al-Cu2O catalyst in 0.1 M KHCO3 electrolyte (a) electrochemical stability test patt (b) the XRD of Al-Cu2O catalyst after long-term stability test.

Figure 6 .
Figure 6.Al-Cu 2 O catalyst in 0.1 M KHCO 3 electrolyte (a) electrochemical stability test pattern and (b) the XRD of Al-Cu 2 O catalyst after long-term stability test.
shows the energy distribution of ethylene production and by-product H 2 of Cu 2 O and Al-Cu 2 O catalysts.The Gibbs free energies of each intermediate along ethylene on Cu 2 O and Al-Cu 2 O catalysts *CHCOH, *CCH, *CCH, *CCH 2 , *CHCH 2 (intermediates for ethylene production) and *H (intermediates to H 2 ) have been calculated.Because the Gibbs free energy of the Al-Cu 2 O catalyst was lower than that of the Cu 2 O catalyst in each reaction step, the path of ethylene production of CO 2 RR was easier to occur.It can be seen that the strategy of doping Al to octahedral Cu 2 O was beneficial in improving the selectivity of product C 2 H 4 [41].At the same time, the analysis of Figure 7c showed that the Al-Cu 2 O catalyst with Al doping enhanced the adsorption of intermediate *H and further departed from the ideal hydrogen adsorption value (0 eV).It makes the competitive reaction of HER more disadvantageous.To further analyze the potential reason for the selective improvement of this product, the density of states (DOS) of d orbitals on Cu 2 O (001) and Al-Cu 2 O (001) surfaces before CHCOH adsorption was compared (Figure 7d,e).Since the electronic states near the Fermi level are mainly contributed by the d electrons of Cu atoms, it is observed that the reaction is mainly caused by the interaction between Cu and C, and the d band center of octahedron Cu 2 O (001) was −2.087 eV.The −2.027 eV of the Al-Cu 2 O (001) surface was closer to the Fermi level (0 eV), 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.So, the adsorption of Cu atoms through d electrons and intermediates was facilitated, thus promoting the CO 2 RR process and improving the selectivity of the catalyst for the C 2 H 4 product.

Funding:
This work was financially supported by the National Natural Science Foundation of China (No. 22262027, 22132003), Ningxia leading scientific and technological innovation talents projects (No.KJT2018002) and Ningxia Natural Science Foundation (No. 2022AAC03103).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.