Tuning the selectivity of CO2 conversion to CO on partially reduced Cu2O/ZnO heterogeneous interface

The development of stable and efficient low‐cost electrocatalysts is conducive to the industrialization of CO2. The synergy effect between the heterogeneous interface of metal/oxide can promote the conversion of CO2. In this work, Cu2O/ZnO heterostructures with partially reduced metal/oxide heterointerfaces in Zn plates (CZZ) have been synthesized for CO2 electroreduction in different cationic solutions (K+ and Cs+). Physical characterizations were used to demonstrate the heterojunction of Cu2O/ZnO and the heterointerfaces of metal/oxide; electrochemical tests were used to illustrate the enhancement of the selectivity of CO2 to CO in different cationic solutions. Faraday efficiency for CO with CZZ as catalyst reaches 70.9% in K+ solution (current density for CO −3.77 mA cm−2 and stability 24 h), and the Faraday efficiency for CO is 55.2% in Cs+ solution (−2.47 mA cm−2 and 21 h). In addition, in situ techniques are used to elucidate possible reaction mechanisms for the conversion of CO2 to CO in K+ and Cs+ solutions.


| INTRODUCTION
The use of fossil fuels produces large amounts of CO 2 , creating environmental problems such as the greenhouse effect.[6] As an important chemical raw material, CO is widely used in the synthesis of fine chemicals and the preparation of other fuels and chemicals. [7,8] Electrocatalytic CO 2 reduction to prepare CO is a promising and green approach. [11,12][15] Therefore, design and development of efficient and stable catalysts for CO 2 reduction is the key to research.
[21] To increase the activity and selectivity of CO 2 reduction reaction (CO 2 RR) for CO, surface electronic structure manipulation is usually employed. [22,23]Currently, some progress has been made in CO 2 reduction by electronic structure modulation to form heterostructure. Heterostructures possess different semiconductor properties while being able to induce built-in fields at interfaces.The built-in fields could modulate local electronic properties and improve charge transfer, thus providing additional driving force.It can also modulate the primordial dband centers of the semiconductors, optimizing the binding energy of the intermediates and thus functioning to modulate the catalytic behavior of catalysts. [24]s a unique d-block metal catalyst, Cu is an ideal value-added product among various CO 2 reduction products. [25]But among all product types, it is very attractive to produce high-purity CO through CO 2 RR.The CO 2 -to-CO consists of two electron transfer steps: CO 2 + * + H + + e − → *COOH; *COOH + H + + e − → *CO + H 2 O; and one CO desorption step from active sites: *CO → CO + *.Although the *COOH formation energy barrier is large, the desorption of CO is usually the ratedetermining step from CO 2 -to-CO.Therefore, the key problem of CO 2 -to-CO is to weaken the *CO binding energy on active sites. [26]Recently, research shows that CuCo diatomic site catalyst (DASC) exhibits excellent activity of CO 2 RR, and the Faraday efficiency (FE) can reach 99.1% at a partial current density of 483 mA cm −2 . [27]u-based catalysts with CO as the product are still attractive in recent research.
Porous C-coated Sn-ZnO heterojunction shows a good performance with CO FE (FE CO ) up to 96% at −0.63 V versus reversible hydrogen electrode (RHE), current density of 15 mA cm −2 . [28]Heterogeneous structures can enhance adsorption through interfacial synergistic effects.It has been reported that Ag/Cr 2 O 3 achieved 99.6% FE CO at −0.8 V versus RHE and 25 h stability in alkaline conditions, which is superior to Ag and Cr 2 O 3 . [29]One-dimensional Pd/PdO heterostructure as catalysts for CO 2 reduction reached 94% FE CO at −0.8 V versus RHE in 1 M KOH. [30]Despite some progress, the design of a CO 2 RR catalyst with high performance and low overpotential is still demanding.Notably, the Cu 2 O-based catalyst undergoes reconfiguration during electrocatalytic CO 2 reduction, enhancing the adsorption of key intermediates and providing an opportunity for the conversion of CO 2 to CO. [31] Long-term studies prove that a metal/oxide interface formed between metal and carrier, which resulted in a strong influence on the catalytic activity of catalysts. [32]Therefore, there is a need to construct metal/oxide interfaces as CO 2 RR catalysts.Recent studies have shown that Cu/In 2 O 3 results in an enhanced effect on the binding of *COOH on the catalyst surface, thus contributing to CO generation. [33]herefore, attempts can be made to promote CO 2 conversion and *COOH formation through the synergistic interaction of heterojunctions and metal/oxide interfaces.
When electrocatalytic CO 2 RR is carried out in strong alkaline environment, the catalysts show poor stability. [34]nder neutral conditions, alkali metals have been used in most studies.A Number of theories have been proposed to explain through which molecular mechanism the cation affects the reduction of CO 2 . [35]Studies showed that the CO 2 concentration at the interface decreases sequentially in the order Li + > Na + > K + ≈ Rb + ≈ Cs + , and the catalytic activity of CO 2 RR is higher in K + , Rb + , and Cs + (compared to Li + and Na + ). [36]Ni monoatomic as a catalyst with FE CO of 91% in 1 M KCl electrolyte. [8]owever, the main role of metal ions has not been well explained.
In this work, Cu 2 O/ZnO heterostructures with partially reduced metal/oxide heterointerfaces in Zn plates (CZZ) have been synthesized for CO 2 electroreduction in neutral electrolytes.Unlike pure ZnO, CZZ with multiple and variable active sites is an effective catalyst for CO 2 RR.TEM results illustrate the existence of a significant heterogeneous interface between ZnO and Cu 2 O.In situ measurements show that *COOH is significantly enhanced at the K + solution, confirming the conversion of CO 2 to CO occurring on the CZZ surface.The intensity of CO

| Structural and compositional analysis
In Figure 1A, the schematic illustrates the synthesis process of CZZ.Cu 2 O/ZnO heterojunction was prepared by the impregnation-annealing method, and metal/oxide heterointerface was prepared by in situ partial electroreduction, which can be illustrated by the increase in the peak intensity of Zn in Supporting Information S1: The elemental surface scanning map confirms that Cu, Zn, and O are uniformly distributed on the surface of the material (Figure 1C). Figure 1D further shows that the elemental composition of the CZZ is dominated by Zn, and Cu is involved in electronic structure modulation as a guest metal to form heterojunctions.The X-ray photoelectron spectra (XPS) in Figure 1E and Supporting Information S1: Figure S3 show the presence of Zn(0) and Cu(0) metal phases along with Zn(II) and Cu(I), which indicates that the Cu 2 O/ZnO heterojunction is partially reduced. [37]The Raman spectrum of CZZ shows two characteristic peaks in the range of 300-1000 cm −1 : one is the Zn-O peak, and the other is the Cu-O peak, which indicates the presence of Cu 2 O and ZnO on CZZ (Figure 1F). [38,39]The presence of Cu 2 O/ZnO heterojunctions and metal/oxide heterointerfaces can be proved by the above-mentioned X-ray diffraction, transmission electron microscopy, and XPS results.Notably, electron spin resonance (ESR) further confirmed the increase of O vacancies after the construction of heterojunctions in CZZs, which can provide more additional active sites for electrode reactions (Figure 1G). [40,41]2 | Electrocatalytic performance The electrocatalytic CO 2 RR performance was investigated in a K + solution, and 1 M KCl was used as an electrolyte for the tests. [42]To compare the electrochemical activity after constructing a heterojunction, comparisons were made using ZnO/Zn (ZZ).The CO 2 reduction activity of CZZ was first investigated by linear scanning voltammetry (LSV).Tests were performed between −0.65 and −1.65 V versus RHE.It can be seen that the onset potential of CZZ is close to −0.7 V versus RHE, which is lower than ZZ (−1.0 V vs. RHE) (Figure 2A).This suggests that the material is capable of reacting at higher potentials after constructing a heterojunction. [43]Under N 2 , the current density reached −7 mA cm −2 at −1.6 V versus RHE, which was caused by a hydrogen precipitation reaction.A larger current response solution was observed in the CO 2 -saturated solution compared to the current response in N 2 , which reached −20 mA cm −2 at −1.6 V versus RHE.LSV results showed that CZZ had a higher current response and higher onset potential under CO 2 than under N 2 conditions, suggesting that CO 2 reduction occurs at the cathode. [44,45]o further analyze the reduction products, controlled potential electrolysis of CO 2 was carried out at different potentials.The CO 2 reduction products were detected by gas chromatograph, and the liquid-phase products were detected by nuclear magnetic resonance hydrogen spectroscopy ( 1 H NMR). The results suggest that the products are predominantly gas-phase products, and the liquid-phase products are negligible.The CO yields at different potentials can be obtained shown in Figure 2B.The CO yield increases with increasing voltage, reaching 125 μmol at −1.05 V versus RHE, and decreases in a volcano curve with the voltage increasing. [46]At −0.95 V versus RHE, the Faraday efficiency (FE) for CO is 70.9% (Figure 2C).ZZ was tested under the same conditions, showing that FE for CO is only 25.6%.To continue exploring the CO yield, we analyzed the FE ratio of CO/H 2 (Figure 2D).The selectivity of CO is clearly demonstrated at lower voltages, especially at the optimum potential. [27]The ratio is 2.7, which is higher than ZZ (the ratio is 0.6).Furthermore, to illustrate the contribution of current to CO 2 generation, in Figure 2E, the CO partial current density of the CZZ reaches a maximum value of −6.5 mA cm −2 at −1.05 V versus RHE, which is higher than that of ZZ (−0.6 mA cm −2 ). [47]Its reaction kinetics can be further characterized by impedance spectroscopy, where the semicircle in the high-frequency region of the CZZ is much smaller than that of the ZZ, suggesting a faster charge transfer to the CO 2 RR and thus an excellent catalytic activity for the conversion of CO 2 to CO (Figure 2F). [48]Further durability tests in 1 M KCl showed that CZZ could be operated stably at a constant voltage of −0.95 V versus RHE for 24 h with no significant decrease in CO selectivity (Figure 2G).
To test the CO 2 RR performance at Cs + solution, 2.5 mM Cs 2 SO 4 was used. [49]LSV test between −1.25 and −2.25 V versus RHE still showed a slight increase in current density compared to CZZ in N 2 and ZZ in CO 2 , but the enhancement of the CO 2 reaction was less effective (Figure 3A).Its lower current density in the N 2 environment indicates that its electrode activity is also lower.The enhancement of CO by CZZ was more pronounced as the potential increased, suggesting that the increase in potential made the CO more selective.In Figure 3B, the CO yield at different potentials can be obtained.The CO yield increases with increasing voltage, reaching 46 μmol at −1.75 V versus RHE, which is higher than ZZ (10 μmol).FE for CO is 55.2% at −1.75 V versus RHE.ZZ was tested under the same conditions and was shown to have an FE of only 24.3% for CO; the selectivity was not significant by ZZ (Figure 3C).To continue exploring the yield of CO, we analyzed the FE ratio of CO/H 2 (Figure 3D).The results show that the CZZ increases with potential, and its ratio is 2.4 at −1.75 V versus RHE, which is much higher than ZZ (0.6).Furthermore, to illustrate the contribution of current to CO generation, as shown in Figure 3E, the fractional current density of CO for CZZ reaches a maximum value of −2.5 mA cm −2 , which is higher than that of ZZ (−0.6 mA cm −2 ).Its reaction kinetics can be further characterized by impedance spectroscopy, where the semicircle of the CZZ in the high-frequency region is smaller than that of ZZ at Cs + solution, indicating a faster charge transfer to the CO 2 RR and hence excellent catalytic activity in converting CO 2 to CO (Figure 3F).Further durability tests in 2.5 mM Cs 2 SO 4 solution showed that the CZZ could be operated stably at a constant voltage of −1.75 V versus RHE for 21 h without a significant decrease in CO selectivity (Figure 3G).
Electrochemical active surface area is a key parameter to evaluate the catalytic activity of electrode materials.It can be calculated by electric double-layer capacitance calculation (C dl ) and tested by cyclic voltammetry.In Supporting Information S1: Table S2, the C dl of CZZ at K + solution is 1.1 mF cm −2 , which is 4 times higher than that of ZZ (0.25 mF cm −2 ).Meanwhile, in Supporting Information S1: Table S3, the C dl of CZZ is 0.06 mF cm −2 at Cs + solution, which is 10 times higher than that of ZZ (0.006 mF cm −2 ).The increase of C dl shows stronger capacitance, which makes CZZ easier to store and release charges than ZZ.The heterostructure of CZZ may be the reason for the capacitance enhancement. [50]

| Mechanism on the enhanced performance
To deeply investigate the electrocatalytic CO 2 reduction process and reaction intermediates, in situ attenuated total reflection surface-enhanced infrared absorption spectrum (ATR-SEIRAS) measurements of CZZ were carried out at K + F I G U R E 4 In situ attenuated total reflection surface-enhanced infrared absorption spectrum measurements of Cu 2 O/ZnO heterostructure with partially reduced matal/oxide heterostructure in zinc plate (CZZ) at (A) K + and (B) Cs + solution.X-ray photoelectron spectra O 1s spectra of CZZ at (C) K + and (D) Cs + solution.
and Cs + solution.In situ ATR-SEIRAS measurements at K + solution in Figure 4A show that *COOH increases with increasing voltage, indicating that the reaction intermediate product accumulates as the reaction proceeds, which is a piece of strong evidence for the conversion of CO 2 to CO. [51] Based on the above results, it is assumed that the reaction pathway involves the formation of CO 2 radicals from CO 2 , followed by the conversion to *COOH intermediates, which is a common step in the reduction of CO 2 to CO. [52] In Figure 4B, the in situ ATR-SEIRAS measurements at the Cs + solution show that the CO 3 2− peak gradually decreases with the increase of voltage, which indicates that the Cs + solution carbonates to CO 2 and improves the CO 2 utilization rate. [53]n Figure 4C,D, the O 1s XPS spectrum of CZZ consists of two peaks attributed to metal-oxygen bonded, surfaceoxygen species. [54]The binding energies are generally low in the K + solution and high in the Cs + solution.This is due to an increase in the electron density around O atoms in the K + solution, which in turn leads to a decrease in the lattice oxygen XPS binding energy.On the contrary, an increase in the Cs + concentration leads to a decrease in the electron density around O atoms. [55]In short, the high performance of CZZ stems from the construction of a heterogeneous structure that promotes the adsorption of CO 2 to form COOH* at K + solution and inhibits the formation of carbonates from CO 2 at Cs + solution.
Figure S2.The physical characterization was carried out to examine the synthesis of CZZ.In Figure 1B, high-resolution transmission electron microscopy results show the interfacial structure of the heterojunction, confirming the existence of different crystalline surfaces in the heterojunction structure.By analyzing the lattice fringes, the three crystallographic planes correspond to (111) crystallographic plane of Cu 2 O, (100) crystallographic plane of ZnO, and (102) crystallographic plane of Zn, indicating that the synthesis of Cu 2 O/ZnO heterojunctions has been successfully achieved with the metal/oxide heterointerfaces obtained after partial reduction.
CO 2 electroreduction at 1 M KCl.(A) Linear scanning voltammetry (LSV) curves of Cu 2 O/ZnO heterostructure with partially reduced matal/oxide heterostructure in zinc plate (CZZ) under N 2 and ZnO/Zn (ZZ), CZZ under CO 2 , (B) CO yield, and (C) Faraday efficiency (FE) of CZZ and ZZ, (D) FE CO/H 2 , and (E) CO fractional current densities of CZZ and ZZ at different potentials.(F) Nyquist plots of CZZ and ZZ and (G) long-term stability test of CZZ at −0.95 V versus reversible hydrogen electrode.

F
I G R E 3 CO 2 electroreduction at 2.5 mM Cs 2 SO 4 .(A) Linear scanning voltammetry (LSV) curves of Cu 2 O/ZnO heterostructure with partially reduced matal/oxide heterostructure in zinc plate (CZZ) under CO 2 and N 2 , and ZnO/Zn (ZZ) under N 2 , (B) CO yield, (C) FE of CZZ and ZZ, (D) FE CO/H 2 , and (E) CO fractional current densities of CZZ and ZZ at different potentials.(F) Nyquist plots of CZZ and ZZ and (G) long-term stability tests of CZZ at −0.95 V versus reversible hydrogen electrode.

3 |
CONCLUSIONCu 2 O/ZnO heterojunction with metal/oxide heterointerface on Zn plates was successfully synthesized.Compared with ZZ, the CZZ catalyst showed excellent catalytic activity and selectivity in the electrochemical process of electrocatalytic CO 2 reduction to CO under different cationic solutions.The FE and current density of CO reached 70.9% and −3.77 mA cm −2 , respectively, in the K + solution, and the maximum FE also reached 55.2% with a current density of −2.47 mA cm −2 in the Cs + solution.In situ characterization shows that the Cu 2 O/ZnO heterojunction acts synergistically with the metal/oxide interface to promote CO 2 conversion and *COOH formation during electrolysis.In conclusion, this study developed a very promising catalyst for the efficient electroreduction of CO 2 to CO at different cationic solutions for practical applications.