Carbon dioxide (CO
2) is a major contributor to global warming [
1,
2], which causes serious environmental issues such as ocean acidification and wildfires [
3,
4]. Electrocatalytic CO
2 reduction reaction (CO
2RR) driven by renewable energy is a potential approach to reduce CO
2 emissions [
5–
8]. The conversion of CO
2 into important fuels and chemicals by electrocatalytic reduction with great significance, which is conducive to the development of industrial decarbonization. Although the CO
2RR can produce a variety of products such as carbon monoxide (CO), formic acid, methane (CH
4), ethylene (C
2H
4), ethanol, and acetic acid, it is still extremely difficult to achieve the high efficiency and selectivity towards high value C
2 products with high energy density [
9–
14]. Cuprous oxide (Cu
2O) is one of the most promising catalysts for electrochemical conversion of CO
2 into value-added C
2 products [
15–
18]. Among these C
2 products, C
2H
4 is extremely attractive as an important raw material in the chemical industry [
19]. We therefore urgently need to develop an optimal catalyst to increase the selectivity of C
2H
4 production. The rate-determining step (RDS) for the formation of C
2H
4 involves C–C coupling between adsorbed *CO or coupling of *CO hydrogenated derivatives [
20–
22]. However, the symmetric binding of atop-type *CO (*CO
atop) occurs on conventional low-index crystal planes of Cu
2O due to weak *CO adsorption, leading to high-energy barrier C–C coupling reactions and hindering the CO
2-to-C
2 conversion [
23].
By adjusting the crystal planes orientation of Cu
2O, the surface energy and affinity of reaction intermediates can be effectively altered, thereby influencing the adsorption and coupling of *CO species and reduction pathway [
24–
26]. In particular, the design of high-index crystal planes with significant amounts of atomic steps, edges, and unsaturated coordinated sites, provides more opportunities for the preparation of catalysts with high activity and selectivity [
27]. High-index crystal planes, such as (311) or (522) [
28], typically exhibit higher *CO binding energies because of the higher surface energy and coordination saturation [
29,
30]. This results in the formation of more stable adsorbed *CO species compared to *CO
atop thereby improving the C
2 products selectivity [
31]. For instance, Han
et al. synthesized with a controllable Cu
2O (311) crystal plane and its C
2 selectivity was about 37.5% at −1.4 V
vs. RHE, which was doubled compared with 16% for the low-index crystal planes [
27]. The highly chemically active high index crystal plane Cu
2O (311) is energetically favorable to reduce and simultaneous evolve Cu
2O fine grains, which benefits charge transfer and increases the local pH by confining OH
−, resulting in high selectivity towards C
2 products.
A conventional approach to tailoring crystal planes involves the introduction of extrinsic additives, such as surfactants and reductants, during catalyst synthesis [
32,
33]. However, the removal of residual surfactant requires complex procedures and may result in catalyst poisoning [
34]. Although use of a reductant,
e.g., NH
2OH·HCl, can address the adverse effects of residual surfactant and expose the desired crystal planes [
35], the design of such synthetic protocols remains complex and requires massive trials. Recently, a self-selective strategy involving the introduction of reactants during catalyst synthesis was proposed [
36]. Compared with the introduction of extrinsic additives, the proposed self-selective strategy can address the issues of additive residue and need for massive trials. For example, using CO
2 as an inducer to stabilize CO
2 favored crystal planes during self-selective catalyst preparation, thereby promoting the CO
2RR reaction [
36,
37]. Inspired by this strategy, we believe that the self-selective CO-favored crystal planes can be induced by introducing CO during the synthesis of Cu
2O. This self-selective strategy tends to stabilize the planes that adsorb *CO intermediates most strongly, since the adsorption of the CO molecules reduces the relative surface energies of these planes. This aims to form other types of with stronger adsorbed *CO species besides atop-type, overcoming the limitations of symmetric coupling based on *CO
atop to promote asymmetric C–C coupling.
Here, we developed a novel Cu2O electrocatalyst with high-index (311) crystal planes (named CO–Cu2O) synthesized by a facile self-selective CO-induced strategy using CO as an inducer. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and density functional theory (DFT) calculations revealed that both atop-type and hollow-type adsorption (*COhollow) of *CO species occurred on CO–Cu2O. The asymmetric C–C coupling between *COatop and *COhollow is kinetically favorable, resulting in a lower energy barrier for CO2RR compared to the symmetric coupling of *COatop. Consequently, the Faradaic efficiency of the C2 products (FEC2) generated with CO–Cu2O was increased by 100% than that of pristine Cu2O.
In typical synthesis, the pristine Cu
2O nanoparticles were synthesized under alkaline conditions using ascorbic acid as a reductant [
24], while CO–Cu
2O nanoparticles were prepared following the same procedure under a CO atmosphere. Further details on the experimental section, including materials source and preparation method, can be found in Supporting information. Scanning electron microscopy (SEM) images revealed that the prepared Cu
2O and CO–Cu
2O nanoparticles exhibited cubic structural morphology with a diameter of about 600 nm (Figs. S1 and S2 in Supporting information). Based on the results of high-resolution transmission electron microscopy (HR-TEM) images, the interplanar spacings have been reduced from 0.215 nm in pristine Cu
2O to 0.130 nm in CO–Cu
2O (
Figs. 1a and
b). This indicates that a significant number of (311) crystal planes appear in CO–Cu
2O through the self-selective CO-induced strategy. Atomic force microscopy (AFM) images (
Figs. 1c and
d, Fig. S3 in Supporting information) revealed that the roughness of CO–Cu
2O was significantly higher than that of pristine Cu
2O because the (311) crystal planes possessing abundant step- and kink- sites [
38]. The line profiles (
Figs. 1e and
f) demonstrated an increase in the height variation on the CO–Cu
2O surface compared to that on the Cu
2O surface. The average heights of the CO–Cu
2O and Cu
2O surfaces, determined
via line profile extraction analysis, were approximately 6.11 nm and a mere 0.76 nm, respectively. These particulate edges represented undercoordinated sites associated with crystal microfacets, which are considered to play a crucial role in structure-activity relationship [
39,
40].
As shown in Fig. S4 (Supporting information), the X-ray diffraction (XRD) spectra demonstrated that the peaks attributed to Cu
2O conformed to those of standard Cu
2O (PDF #99–0041). After self-selective induction, the diffraction peaks of CO–Cu
2O are similar to Cu
2O, confirming that no apparent difference between Cu
2O and CO–Cu
2O catalysts in the crystal structure. Besides, the grazing incidence XRD analysis indicated that the (311)/(100) crystal plane intensity ratio in CO–Cu
2O (0.68) was increased by 51% compared to that in pristine Cu
2O with only 0.45 (Fig. S5 and Table S1 in Supporting information), indicating the increase of the preferential orientation of the crystal planes after the induction. The X-ray photoelectron spectroscopy (XPS) spectra of Cu
2O and CO–Cu
2O exhibited obvious peaks at 932.1 and 952.0 eV (Figs. S6 and S7 in Supporting information), being attributed to the Cu 2p
1/2 and Cu 2p
3/2 of Cu
+, respectively. Additionally, XPS was employed to assess the differences in the O 1s spectra of various Cu
2O catalysts (Fig. S8 in Supporting information). The O 1s spectra were decomposed into peaks at 529.6 and 531.4 eV, corresponding to surface-absorbed oxygen (O
ads) and crystal lattice oxygen species (O
latt), respectively. The peak ratio between O
latt and O
ads served as an indicator of the quantity of surface oxygen vacancies (O
v) [
41]. Compared to Cu
2O, CO–Cu
2O enriched with O
v could stabilize Cu
+ and thereby strengthen *CO adsorption to promote C–C coupling [
42].
To further elucidate the chemical state and coordination environment of Cu in Cu
2O and CO–Cu
2O at the atomic level, the X-ray near edge spectroscopy (XANES) profiles at the Cu K-edge of the prepared Cu
2O and CO–Cu
2O catalysts were compared with those of commercial Cu foil, Cu
2O, and CuO as reference materials (
Fig. 2a). The energy positions of the prepared Cu
2O and CO–Cu
2O catalysts were similar to that of reference Cu
2O, confirming that the valence state of Cu was mainly +1. The peak observed at 1.4 Å (without phase correction) in the
R-space (
k3-weighted) extended X-ray fine structure (EXAFS) spectra could be attributed to coordination between Cu and O atoms (
Fig. 2b). By contrast, the peak at 2.6 Å represented Cu-Cu interactions. Similarly, wavelet transform (WT)-EXAFS was employed to further visualize the coexistence of Cu-O and Cu-Cu coordination in Cu
2O and CO–Cu
2O (
Figs. 2c and
d). The fitting results (
Figs. 2e and
f, Fig. S9 in Supporting information) showed that the Cu-Cu coordination number in CO–Cu
2O were about 8.96, which was higher than that of Cu
2O at 8.15 (Table S2 in Supporting information). The CO–Cu
2O shows a higher mean Cu-Cu bond length of approximately 3.05 Å than Cu
2O (2.99 Å), which is due to the tensile strain effect induced by the (311) crystal plane. Notably, the ratio of Cu-O to Cu-Cu coordination strengths in CO–Cu
2O was slightly greater than that in commercial Cu
2O, leading to increased structural disorder and potentially more undercoordinated sites [
43].
To determine the effect of adsorbed *CO species on the high-index crystal planes of CO–Cu
2O,
in situ ATR-SEIRAS was carried out (
Figs. 3a and
b), and peaks were classified as the corresponding intermediates according to reported studies (
Fig. 3c). The spectra were collected in CO
2-saturated 0.5 mol/L KHCO
3 solution at different potentials from 0 V to −1.0 V
vs. RHE. The peak intensity is expressed in terms of absorbance and the positive peak represents product formation. In particular, two peaks appeared at approximately 2050–1820 cm
−1 for CO–Cu
2O, which were associated with *CO
atop and *CO
hollow on the surface, respectively [
44]. By comparison, the spectrum of Cu
2O mainly exhibited the *CO
atop peak in the same potential region. Two *CO
atop peaks were clearly recognized in the spectra of CO–Cu
2O at potentials above −0.4 V
vs. RHE. The *CO
atop peaks could be classified into high-frequency binding *CO (CO–HFB) and low-frequency binding *CO (CO-LFB) [
45,
46]. Prior research reported that CO-LFB is associated with C–C coupling and C
2H
4 production, while CO–HFB is typically related to the production of gaseous CO [
47,
48]. Notably, as the potential shifted to more negative values, the red shift observed for the CO stretching bands in the CO–Cu
2O spectra may have been attributed to the Stark effect [
49]. This finding suggested an increase in the adsorption of *CO
atop on the surface, potentially fostering further protonation or C–C coupling reactions [
50]. Hence, we plotted the integrated *CO
atop peak area against the applied potentials (
Figs. 3d and
e).
As the applied potential increased, the integrated area of CO–HFB on the Cu
2O surface decreased, and no CO-LFB appeared within the applied potential range. Meanwhile, CO-LFB was observed on CO–Cu
2O when the applied potential exceeds −0.4 V
vs. RHE and the integration area reached a maximum at −0.7 V
vs. RHE. Compared with single CO–HFB on Cu
2O, CO-LFB newly appeared on CO–Cu
2O with a more negative potential. Notably, the absence of CO-LFB signals was detected due to low C–C coupling activity at lower potentials. At higher potentials, the integrated area for CO–Cu
2O dropped owing to *CO consumption for C–C coupling. In addition, new bands emerged on the CO–Cu
2O surface at 1835–1780 cm
−1 when the applied potential was above −0.4 V
vs. RHE, corresponding to *CO
hollow species [
44]. These results suggested that the electronic structure of the catalyst was changed
via CO induction, which triggered more *CO adsorption configurations. As the potential was further shifted to more negative values, the quantity of adsorbed *CO
hollow progressively augmented. Benefiting from the (311) crystal plane in CO–Cu
2O with stronger affinity for *CO, thus CO–Cu
2O catalyst has higher *CO
atop and *CO
hollow coverage at high overpotentials, further promoting asymmetric C–C coupling. Besides, the peak at 1720 cm
−1 can be assigned to *CHO (Fig. S10 in Supporting information), which is well accepted as the starting adsorption mode of intermediate for C–C coupling [
14,
51]. However, no significant *CHO adsorption peak was detected for Cu
2O at all potentials, indicating a high protonation energy barrier affecting subsequent C–C coupling. Meanwhile, an enhanced water (*H-O-H) peak at approximately 1620 cm
−1 was observed on Cu
2O, which may have led to a blocking effect, facilitating substitution of *H for *CO [
52]. Notably, the *H coverage on Cu
2O is opposite to that on CO–Cu
2O, which is associated with higher *CO coverage, resulting in *CO occupying more active sites. In addition, we then also investigated the CO adsorption ability of Cu
2O and CO–Cu
2O
via electrochemical CO stripping voltammetry [
53,
54]. With the activation process going on, the intensity of oxidic peak corresponding to CO adsorbed on CO–Cu
2O was significantly increased compared to that for Cu
2O (
Fig. 3f), indicating stronger CO binding capacity and higher CO coverage that attributed to the increased of electrochemical adsorption of CO [
49], which was consistent with the IR spectral analysis. The above results demonstrated that CO–Cu
2O possessed an asymmetric *CO coupling mode and higher *CO coverage than Cu
2O, promoting asymmetric C–C coupling on nearby sites under electrochemical reduction conditions.
To verify the above results, we performed CO
2RR for Cu
2O and CO–Cu
2O. The electrochemical CO
2RR was performed using a flow cell with 0.5 mol/L KHCO
3 as electrolyte. The gas chromatography and
1H nuclear magnetic resonance spectroscopy (
1H NMR) were employed to quantify the products in the gas and liquid phases, respectively. The specific relationships among different products determined from the NMR spectra are presented in Fig. S11 (Supporting information). Linear sweep voltammetry (LSV) images of the Cu
2O and CO–Cu
2O electrodes were acquired in CO
2 or N
2-saturated atmospheres electrolyte (
Fig. 4a). Under CO
2 and N
2 atmospheres, Cu
2O catalysts exhibited relatively lower current density (j) values at the same applied potentials than CO–Cu
2O. The results suggested that the increment in high-index crystal planes substantially enhanced the CO
2RR activity of CO–Cu
2O.
Figs. 4b and
c present the selectivity of CO–Cu
2O under various imposed potentials, where the FE
C2 (62%) of CO–Cu
2O was nearly double that of Cu
2O (31%) at −1.2 V
vs. RHE. For pristine Cu
2O, the main product was H
2 with nearly 30% FE, while the C
2H
4 selectivity promoted from 16.2% to 41.1% in CO–Cu
2O, whose value was 150% than pristine Cu
2O. Meanwhile, the partial current density of C
2 products generated with CO–Cu
2O reached 66 mA/cm
2 compared to 25 mA/cm
2 for pristine Cu
2O (
Fig. 4d). Additionally, the electrochemical active surface areas (ECSA) of Cu
2O and CO–Cu
2O were measured (Fig. S12 in Supporting information). The ECSA of CO–Cu
2O was 65.8 cm
2, which was larger than that of Cu
2O (50.7 cm
2). This finding indicated that CO–Cu
2O could provide more active sites than Cu
2O. The intrinsic activity of the catalysts was obtained by normalizing the current density to the ECSA [
55]. The modified current densities indicated that CO–Cu
2O exhibited greater activity than Cu
2O for C
2 products formation and inhibited H
2 generation (
Figs. 4e and
f).
As shown in
Fig. 4g, CO–Cu
2O achieved a 2.35-fold increase in the FE
C2/FE
C1 ratio at −1.2 V
vs. RHE compared to pristine Cu
2O. Moreover, CO–Cu
2O exhibited a lower FE
H2 than pristine Cu
2O at all potentials, and the FE
C1 of CO–Cu
2O did not significantly increase compared to that of pristine Cu
2O. This suggests that the hydrogen evolution reaction (HER) in CO–Cu
2O was effectively suppressed, and the increased in the amount of C
2 products could be attributed to the inhibition of *H conversion. This is consistent with
in situ ATR-SEIRAS spectra observations of higher *CO and lower *H coverage, where elevated *CO is involved in the C–C coupling process to form C
2 products. Additionally, the long-term durability performance of CO–Cu
2O catalyst reveal that it maintained a relatively stable current density and C
2H
4 selectivity during the whole operation period (
Fig. 4h). Combined with CO
2RR performance, the study findings indicated that *CO
hollow became dominant at higher cathodic potential, contributing to asymmetric *CO
atop and *CO
hollow coupling that enhanced C
2 products formation [
50]. As shown in Fig. S13 (Supporting information), electrochemical impedance spectroscopy (EIS) revealed a lower charge transfer resistance (
Rct) of CO–Cu
2O at the open circuit potential compared to Cu
2O, which improved the charge transfer kinetics of the developed catalyst compared to pristine Cu
2O. However, CO
2 reduction typically occurs at much more negative applied potentials, and the arcs in the Nyquist diagrams substantially decreased from Cu
2O to CO–Cu
2O, further demonstrating the reduced the transfer resistance of CO–Cu
2O compared to pristine Cu
2O [
49]. Hence, it could be concluded that the presence of high-index crystal plane also had an improvement in electron transfer performance to boost the CO
2RR performance.
Stabilizing the Cu
+ content during the CO
2RR process is crucial for generating the corresponding *CO intermediate and final C
2 products [
56], whereas presence of Cu
+can promote *CO to *CHO and the adsorption of OH species, causing C
2 intermediates preferably to form the sp
3 hybrid C atom and undergo asymmetric hydrogenation, leading to the enhancement of C–C coupling [
57]. The SEM images revealed that the surface morphology of CO–Cu
2O after CO
2RR for 1 h at −1.2 V
vs. RHE had better stability than Cu
2O (Fig. S14 in Supporting information). In addition, the XRD spectra indicated the generation of reduced metal Cu (PDF #04–0836) species on Cu
2O and CO–Cu
2O (
Fig. 5a). The peak intensity of Cu (111) and Cu (200) on Cu
2O was significantly higher than that on CO–Cu
2O after the reaction, indicating that more Cu
+ was reduced on Cu
2O. Similarly, the
ex-situ TEM images and selected area electron diffraction patterns revealed the presence of metallic Cu and Cu
+ in Cu
2O and CO–Cu
2O after the reaction (Fig. S15 in Supporting information), indicating that Cu
+ underwent reduction. Moreover, the stable presence of the (311) plane of CO–Cu
2O is still observed after the reaction, indicating that (311) plane contributes to the formation of stable atomic clusters or structures during electroreduction and that this reconfiguration contributes to the fixation of the Cu
+ valence state. Based on the XRD spectra, the Cu LMM spectra of Cu
2O and CO–Cu
2O after 1 h CO
2RR are examined to show the changing valence state of Cu. The Cu LMM Auger spectra (
Figs. 5b and
c) clarified that Cu
+ (916.6 eV) and metallic Cu (918.5 eV) species co-existed on the Cu
2O and CO–Cu
2O catalysts. The results indicated that Cu
+ species on CO–Cu
2O remained well stabilized, with only 3% conversion to metallic Cu, whereas approximately 23% of Cu
+ in Cu
2O was converted to metallic Cu upon electroreduction. The above results indicate that the CO–Cu
2O catalyst can stabilize the surface morphology and Cu
+ species, which is conducive to maintaining the activity of the electroreduction reaction.
DFT calculations are conducted to verify the possible asymmetric C–C coupling mechanism on Cu
2O (311) crystal plane (
Fig. 6a and Fig. S16 in Supporting information). As shown in
Fig. 6a, the CO
2 molecule initially adsorbs on the Cu site, undergoing hydrogenation to form the *COOH intermediate. This is then reduced to the *CO intermediate, which combines with protons to yield the *CHO intermediate. Subsequently, the adsorbed *CHO undergoes a similar process to form 2*CHO intermediate on the surface. This 2*CHO intermediate then undergoes dimerization to form the *OHCCHO intermediate, which is finally hydrogenated to produce C
2H
4. Intriguingly, the (311) crystal plane boosts the activation of Cu site and reaches the hollow-type adsorption of *CO, which is consistent with the monitoring results from
in situ spectroscopies. Consequently, the energy barrier of their asymmetric coupling of *CO
atop and *CO
hollow decreases with 47.8% compared to the conventional symmetric coupling of *CO
atop.
The two critical steps in the CO
2RR pathway to C
2H
4 are the hydrogenation of *CO to form the *CHO intermediate and the coupling of *CHO [
58]. In the former *CO → *CHO step, the Gibbs free energy change on the Cu
2O (311) surface (Δ
G = 0.49 eV) is slightly lower than on the Cu
2O (100) surface (Δ
G = 0.58 eV). This suggests that CO–Cu
2O with (311) is more easily protonated, which is consistent with the weaker *CHO that is observed in the infrared spectra. For the *CHO dimerization step, the uphill reaction energy for the formation of 2*CHO from *CHO is an RDS for the entire reaction pathway. In addition, *CO is the pivotal intermediate in the conversion of CO
2 to C
2 products [
59,
60], with the adsorption energy of *CO playing an important role in their reaction rates [
61]. During the activation process, the decomposition of *COOH on Cu
2O (311) has a higher free energy compared to that on Cu
2O (100), indicating that the formation of *CO is more favorable on Cu
2O (311). As shown in Figs. S17 and S18 (Supporting information), the hollow site of the (311) crystal plane exhibited the most robust CO adsorption (Δ
E = −1.57 eV), and strong *CO adsorption ensured adequate *CO coverage on its surface [
62,
63]. In this way, *CO occupied the catalytic active sites on the (311) crystal plane, which strengthened asymmetric C–C coupling and CO
2 to C
2 production.
To better visualize the incorporation of the *CO intermediate with the catalysts, we calculated the differential charge density (DCD) of Cu
2O (100) and Cu
2O (311) crystal planes after *CO adsorption (
Figs. 6b and
c). Compared with the (100) crystal plane, the charge distributed around the Cu atom on the (311) crystal plane moved more toward the C atom side, indicating enhanced binding energy between *CO and the CO–Cu
2O surface. As shown in the electron localization function (ELF) topographical analysis in
Fig. 6d, the (311) crystal plane displayed higher electron delocalization around the formed *CO adsorption sites than that of (100) crystal plane. This suggests that *CO adsorbed on the (311) surface favors the formation of a more stable adsorption configuration, resulting in asymmetric C–C coupling. Work function (WF) can serve as a characteristic quantity to evaluate the catalytic activity [
64]. Correspondingly, the detailed WF revealed that the WF value in the (311) crystal plane was lower than that of the (100) crystal plane (
Figs. 6e and
f). The lower the WF value, the more easily electrons can leave the metal, leading to more efficient electron transfer. To summarize, Cu
2O (311) surface conducive to the electron transfer to the direction of CO
2 electroreduction, thus enhancing the activity and selectivity for the CO
2RR performance.
In summary, the asymmetric coupling of atop-type and hollow-type adsorbed *CO species occurring on the constructed high-index crystal planes was crucial for enhancing the CO2-to-C2 conversion. An efficient catalyst CO–Cu2O with Cu2O (311) crystal planes was synthesized via the facile self-selective CO-induced strategy proposed in this work. The energy barrier of their asymmetric coupling of *COatop and *COhollow decreases with 47.8% compared to the conventional symmetric coupling of *COatop. Benefiting from the asymmetric coupling mechanism, the FEC2 generated with CO–Cu2O was increased by nearly 100% at −1.2 V vs. RHE compared to that with pristine Cu2O. This study elucidates the mechanism of asymmetric C–C coupling facilitated on high-index crystal planes and offers a method to design efficient catalysts for CO2-to-C2 conversion.
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