Theoretical Insights into Potential-Dependent C-C Bond Formation Mechanisms during CO2 Electroreduction into C2 Products on Cu(100) at Simulated Electrochemical Interfaces.

An improved CO coverage-dependent electrochemical interface model with an explicit solvent effect on Cu(100) is presented in this paper, by which theoretical insights into the potential-dependent C-C bond formation pathways occurring in CO2 electrochemical reduction to C2 products can be obtained. Our present studies indicate that CHO is a crucial intermediate toward C1 products on Cu(111), and dimer OCCO is found to not be a viable species for the production of C2 products on Cu(100). The reaction pathway of CHO with CO and CHO dimerization into dimers COCHO and CHOCHO may be C-C bond formation mechanisms at low overpotential. However, at medium overpotential, C-C bond coupling takes place preferentially through the reaction of COH with CO species and COH dimerization into dimers COCOH and COHCOH. The formed dimers COCHO, CHOCOH, and CHOCHO via reactions of CHO with CO, COH, and CHO species may lead to C2 products, which are regarded as C-C bond formation mechanisms at high overpotential. The difference of obtained adsorption isotherms of CO on Cu(100) with that of Cu(111) may be able to explain the effect of the crystal face of Cu on product selectivity. The excellent consistencies between our present obtained conclusions and the available experimental reports and partial theoretical studies validate the reasonability of the present employed methodology, which can be also used to systematically study potential-dependent CO2 electroreduction pathways toward C2 products on Cu(100) or other metal catalysts.

Introduction

Electroreduction of CO2 into hydrocarbons and alcohols on the Cu electrodes has been a remarkable topic of interest to both scientific and energy technology communities in recent years since it can be a promising candidate for a carbon-neutral and zero-emission energy cycle and thus mitigate the emission of CO2 in the atmosphere,[1−5] in which the formation of C2 products like C2H4 is particularly fascinating for energy conversion and storage in the chemical industry because of their higher energy densities than C1 products, such as CH4 and CH3OH. Although CO2 can be uniquely electrochemically reduced to hydrocarbons and alcohols on Cu electrodes, the required high overpotentials impede real application and spread of this technology.[6−10] Furthermore, the formation mechanism of C2 products on the Cu electrodes needs more experimental and theoretical evidence and is still a subject of ongoing discussion in recent decades. Understanding mechanisms of CO2 electrochemical reduction to C2 products can help design Cu-based alloy electrocatalysts that operated at relatively low overpotentials.

Cu(100) single-crystal surface had been experimentally reported to be especially selective toward C2 products.[11] However, C–C bond formation mechanisms on this facet and the effect of electrode potentials on the mechanisms have been still indistinct at present.[12−16] Recently, many experimental efforts have focused on C–C bond coupling mechanisms during CO2 electroreduction on Cu(100) using in situ spectroscopic technologies, in which the adsorbed CO as a key reaction intermediate has been consistently identified.[17−20] Beginning with the adsorbed CO intermediate, different C–C bond formation mechanisms have been proposed. For example, a CO dimerization mechanism was proposed by Gattrell et al. as a C–C coupling pathway on Cu(100) on the basis of both infrared and surface-enhanced Roman spectroscopy.[21] Similarly, using online electrochemical mass spectrometry, the CO dimerization mechanism was also speculated on Cu(100) by Schouten et al., in which C2 species like C2H4 is preferably produced without simultaneous production of C1 species, indicating that the electroreduction pathways of CO2 into C1 and C2 products must bifurcate during initial CO reduction.[7,22] Using Fourier transformation infrared spectroscopy, a hydrogenated dimer intermediate OCCOH was detected by Pérez-Gallent et al. at the potential of ca. −0.10 V (vs RHE) during CO electroreduction on the Cu(100) electrode, followed by proton-coupled electron transfer.[19] The experimental studies on formation paths of the C–C coupled intermediate, however, have been limited in that the spectroscopically detected species must be relatively stable. Moreover, observable intermediates may be only spectators during CO2 electroreduction.

Computationally, using various theoretical electrode/solution interface modeling, the facet dependence of products and C–C bond formation mechanisms on Cu electrodes were explored,[12,23−27] in which mechanistic understanding that is facile to be not obtained in experiments can be provided, and favored key intermediates can be identified toward the formation of C2 products. For example, Asthagiri et al. reported facet dependence of CO2 electrochemical reduction pathways on Cu electrodes using the linear kinetic barrier method based on Butler–Volmer theory with only one or two relaxed H2O molecules.[23] Thereinto, COH formation is favored, by which CH4 can be produced via CH2 species at high overpotentials, whereas CHO formation on Cu(100) is preferred and C2H4 can be produced via C–C coupling of two CHO intermediates at relatively lower overpotentials. Calle-Vallejo et al. suggested that CO dimer on Cu(100) via an Eley–Rideal mechanism can be preferably formed on the basis of the computational hydrogen electrode model,[12] which is the relevant precursor to the production of C2 species. However, this model to stabilize the CO dimer lacks the rigorous treatment of the electrode/solution interfaces. The computational studies from Nørskov et al. found that CO–CO coupling into the adsorbed OCCO intermediate is feasible on Cu(100) based on the like-capacitor model involving a single H2O layer and cation,[24,25] which can be stabilized by the charged H2O layer and play an important role during the production of C2 below Cu potential of zero charge. Using constant electrode potential and computational hydrogen electrode models in the presence of implicit solvent, Head-Gordon et al. revealed that C–C bond coupling occurs through a CO dimer at the low overpotential, whereas the adsorbed COCHO on Cu(100) was identified as a key intermediate for producing C2 products at high overpotentials, namely, CO reduction into adsorbed CHO species followed by reaction with adsorbed CO to form a COCHO intermediate, which is more favorable than CO dimerization.[26,27] Although extensive theoretical investigations on potential-dependent C–C coupling pathways on Cu(100), the exact C–C bond formation mechanisms toward C2 products are still debated, and many aspects of the atomic-level details of this mechanism are still elusive due to the complexity of electrochemical interfaces.

The experimental observations and theoretical calculations have showed that CO is a key intermediate during CO2 electroreduction on Cu electrodes since its further reduction can lead to similar product distribution as that of CO2.[6,15,21,28−31] Furthermore, it was felt that CO further electroreduction is the rate-determining step of the reaction due to high implied CO coverage, by which CO2 electroreduction kinetics can be determined on Cu electrode surfaces, thereby we can assume that pathway of CO2 electrochemical reduction into CO on Cu electrodes is much faster than CO further electroreduction steps toward production of hydrocarbons, such as CH4, C2H4, and alcohols, which may be able to lead to the abovementioned high implied CO coverage. Thus, we present a CO coverage-dependent electrochemical interface model on Cu(100) for the investigation of C–C coupling pathways taking place in production of C2 products that enables us obtain potential-dependent C–C bond formation mechanisms in this paper. Our present employed methodology for the simulations is differentiated from the previous studies on C–C bond formation. Our recent established explicit solvation model involving two relaxed H2O bilayer is employed to simulate the solvent effect,[32,33] which is of the essence in simulating electrochemical interfaces and allows us to better model the interactions among solvent, surface, and adsorbates. On the basis of this finding, this paper specifically focuses on the initial CO electroreduction and potential-dependent C–C bond formation pathways by applying a CO coverage-dependent interface model on the Cu surface, by which we can obtain a better mechanistic insight into the effect of Cu single-crystal surface on product selectivity and C–C coupling pathways toward C2 products.

CO Coverage–Dependent Equilibrium Potentials

CO adsorption configurations and sites with various coverage can be observed on the Cu(100) surface, as can be seen in Figure S1. The adsorbed CO prefers to occupy 4-fold hollow sites when CO coverage (θCO) is below 1/9 monolayer (ML). Simultaneous adsorption of CO at 4-fold hollow and 2-fold bridge sites is found when θCO is in the range of 1/9 to 1/3 ML. We can observe the simultaneous occupation of 4-fold hollow, 2-fold bridge, and 1-fold top sites when θCO is 4/9 ML. Simultaneous adsorption of CO at 2-fold bridge and 1-fold top sites is found when θCO is in the range of 4/9 to 1 ML. CO molecules are found to only adsorb at 1-fold top site when θCO is 1 ML in order to minimize repulsive interactions. The conditions of θCO above 1 ML are not further considered in this paper because CO dimers can be formed spontaneously when θCO are above 1 ML (see Figure S1). As shown in Figure S2a, a reasonable polynomial relationship of adsorption isotherms between differential adsorption energy of CO [ΔE(θ)] and θCO is exhibited on Cu(100), suggesting that CO adsorption on Cu(100) may abide by Langmuir adsorption isotherms. ΔE(θ) can be calculated by fitting of polynomial of the present ΔE(θ) ∼ θCO data at any θCO. Thus, according to the eq S4, the polynomial dependence of the computational equilibrium potentials (U) on θCO can be obtained on Cu(100), as can be seen in Figure S2b. The equilibrium potential of ca. 0.40 V (vs RHE) can be obtained when θCO is equal to zero, being comparable with a required value (ca. 0.17 V vs RHE) thermodynamically for CO2 electroreduction on Cu electrodes and thereby considered as the required equilibrium potential thermodynamically in the present paper, validating reasonability of our used interface model to some extent. The increasing θCO will make equilibrium potential be more and more negative. Interestingly, we note that the present obtained adsorption isotherms on Cu(100) is differentiated from the linear relationship between ΔE(θ) and θCO on Cu(111) that is proposed in our recent study,[33] in which the Termkin adsorption isotherms may be followed for adsorbed CO on Cu(111), suggesting that Cu single-crystal surface may be able to influence CO adsorption property. Thus, we can speculate that the difference of adsorption isotherms may be able to explain the effect of Cu single-crystal surface on product selectivity during CO2 electroreduction, which is not involved in early theoretical and experimental studies. In this paper, the electroreduction pathways containing proton and electron transfer are simulated through direct transfer of adsorbed H atoms to surface adsorbed species at the present simulated electrode/solution interface.[34,35]

Results and Discussion

Initial CO Reduction Pathways on Cu(111)

The early experimental reports performed by Hori et al. showed that single-crystal surface of Cu exhibits high selectivity for C1 and C2 products during CO2 electroreduction, namely, Cu(111) surface favors production of C1 product like CH4, and more open (100) facet of Cu exhibits excellent selectivity toward C2 products.[9,36] On the basis of our recent proposed CO coverage-dependent Cu(111)/H2O electrochemical interface model,[33] conditions of three electrode potentials are considered in order to obtain initial CO electroreduction pathways on Cu(111) and explain the effect of the crystal face on product selectivity at different applied potentials. The calculated equilibrium potential is ca. 0.10 V (vs RHE) when θCO is equal to 1/3 ML, corresponding to the condition of a low overpotential of ca. 0.17 V compared with the calculated thermodynamically required equilibrium potential of ca. 0.27 V when θCO is equal to zero on Cu(111). The equilibrium potential of ca. −0.15 V (vs RHE) is obtained when θCO is equal to 2/9 ML, which corresponds to the condition of a medium overpotential of ca. 0.42 V. When θCO is equal to 1/3 ML, the computational equilibrium potentials is ca. −0.50 V (vs RHE), corresponding to the conditions of high overpotentials of ca. 0.77 V. Three possible species involving the adsorbed CHO, COH, and dimer OCCO molecules may be able to be formed for initial CO reduction. As shown in Figure , at a low overpotential (θCO = 1/9 ML), the required activation barrier for the formation of CHO species is ca. 0.85 eV on Cu(111), being notably lower than that of COH formation (ca. 1.39 eV). Moreover, the formation pathway of COH species is strongly endothermic by 1.09 eV at the low overpotential, being significantly more positive than reaction free energy of the CHO formation pathway (ca. 0.65 eV). The barrier for the formation of the adsorbed dimer OCCO by CO dimerization is calculated as ca. 1.43 eV, being slightly higher than that of COH formation on Cu(111), and considerably more positive reaction free energy (ca. 1.38 eV) is also observed at the low overpotential, as shown in Figure . In the presence of a medium overpotential (θCO = 2/9 ML), the activation barrier and reaction free energy for CHO formation are almost unchanged, which are calculated as ca. 0.87 and 0.69 eV, respectively. However, formation of dimer OCCO has a relatively lower barrier than COH formation (ca. 1.30 vs 1.57 eV) at the medium overpotential, in which the formations of these both species are still strong endothermic processes (ca. 1.24 vs 1.10 eV), as shown in Figure . The almost identical structure images of initial, transition, and final states may lead to almost unchanged barriers and reaction free energies for the initial CO electroreduction into CHO pathways at the low and medium overpotentials, as shown in Figures S3–S8, in which C–H and C–O bond lengths of the final state CHO are ca. 1.11 and 1.26 Å on Cu(111), respectively.

Figure 1

Energy pathway diagram of initial CO reduction to CHO, COH, and dimer OCCO intermediates on Cu(111) at a low overpotential (θCO = 1/9 ML) (* represents surface adsorption of species).

Figure 2

Energy pathway diagram of initial CO reduction to CHO, COH, and dimer OCCO intermediates on Cu(111) at a medium overpotential (θCO = 2/9 ML).

The presence of the higher overpotential (θCO = 1/3 ML) can significantly decrease the barriers and reaction energies for initial CO reduction into the adsorbed CHO, COH, and dimer OCCO species, as shown in Figure . The corresponding barrier is ca. 0.51, 1.29, and 0.96 eV, respectively, in which the required formation barrier for dimer OCCO is still lower than that of COH formation at the higher overpotential. The corresponding structure images of initial, transition, and final states can be seen in Figures S9–S11. Although the higher overpotential can make barriers of COH and OCCO formations decrease, they are still significantly higher than that of CHO formation. Thus, we can speculate that initial CO reduction into the CHO intermediate is a more favorable pathway at various applied potentials based on energetics analysis. Simultaneously, we also note that the formed dimer OCCO by CO dimerization on Cu(111) is extremely unstable in conditions of three overpotentials, which is facile to back to adsorbed CO molecules with extremely low barriers of below 0.10 eV, as can be seen in Figures –4. In other words, the higher CO coverage on Cu(111) enables CHO formation to be more favorable. Thus, we can conclude that CHO is a key intermediate during initial CO reduction on Cu(111), which can finally lead to production of C1 products, explaining experimentally the observed effect of the Cu single-crystal surface on product selectivity.

Figure 3

Energy pathway diagram of initial CO reduction to CHO, COH, and dimer OCCO intermediate on Cu(111) at a high overpotential (θCO = 1/3 ML).

Figure 4

Energy pathway diagram of initial CO reduction to CHO, COH, and dimer OCCO intermediate on Cu(100) at a low overpotential (θCO = 1/9 ML).

At the high overpotential, the significantly decreased barrier for CHO formation may be attributed to change in CHO adsorption configurations, in which the mixed C–O coordination configurations at the low and medium overpotentials are formed, whereas the coordination pattern of a single C atom is obtained at the high overpotential, as shown in Figures S3, S6, and S8. Our present conclusion for initial CO reduction into CHO species is well consistent with the recent experimental investigation conducted by Koper et al. for CO2 reduction by using online electrochemical mass spectrometry,[7,22] in which the results indicated that CHO is the possibly crucial intermediate for production of the CH4 product on Cu(111).

C–C Bond Formation Mechanisms on Cu(100)

The Cu(100) single-crystal electrode had been reported experimentally to have unique selectivity for C2 products.[11] In this paper, three electrode potentials are considered to obtain potential-dependent C–C bond formation mechanisms on Cu(100). The calculated equilibrium potential is ca. 0.03 V (vs RHE) when θCO is 1/9 ML on Cu(100), which corresponds to the condition of a low overpotential of ca. 0.37 V compared with the present computational thermodynamically required equilibrium potentials when θCO is equal to zero. The equilibrium potential of ca. −0.08 V (vs RHE) can be obtained when θCO is equal to 2/9 ML, which corresponds to the condition of a medium overpotential of ca. 0.48 V. When θCO is equal to 1/3 ML, the calculated equilibrium potential is ca. −0.30 V (vs RHE), corresponding to the condition of a high overpotential of ca. 0.70 V. Thus, the initial CO reduction and C–C coupling pathways at different overpotentials can be calculated by employing the CO coverage-dependent electrochemical interface model on Cu(100).

The initial CO reduction into adsorbed CHO, COH, and dimer OCCO intermediate is first considered to determine whether CO dimerization mechanism leads to C–C coupling on the pathway to the production of C2 products on Cu(100). As can be seen in Figure , the barriers for the formations of CHO, COH, and dimer OCCO are calculated as ca. 0.82, 0.96, and 1.13 eV on Cu(100), respectively, indicating that the initial CO reduction prefers to form CHO species at the low overpotential, as observed on Cu(111). The increasing overpotentials change the initial CO reduction pathways, in which COH formation is the most favorable due to the lowest barrier of ca. 0.75 eV at the medium overpotential, as shown in Figure . The theoretical study from Asthagiri et al. also showed that the adsorbed COH species is more favorable than CHO on Cu(100) at relatively higher overpotentials,[23] confirming our present theoretical conclusion. However, CHO and COH species may be able to be simultaneously formed under the condition of high overpotential because of the almost equal and extremely low barriers of below 0.10 eV (see Figure ). The corresponding structure images of initial, transition and final states on Cu(100) for adsorbed intermediates CHO, COH, and dimer OCCO formations at various overpotentials are shown in Figures S12–S20.

Figure 5

Energy pathway diagram of initial CO reduction to CHO, COH, and dimer OCCO intermediates on Cu(100) at a medium overpotential (θCO = 2/9 ML).

Figure 6

Energy pathway diagram of initial CO reduction to CHO, COH, and dimer OCCO intermediates on Cu(100) at a high overpotential (θCO = 1/3 ML).

At the various applied potentials, we find that the formation of dimer OCCO by CO dimerization on Cu(100) is always the most difficult to occur due to significantly higher barriers and more positive reaction free energies than those of CHO and COH formations. Furthermore, the increasing overpotentials can lead to increasing barriers of CO dimerization, as observed values of ca. 1.13, 1.15, and 1.37 eV (see Figures –7), implying that the pathway of CO dimerization into C2 species will close at various applied potentials. Thus, it can be speculated that dimer OCCO species may not be a reliable intermediate during CO2 electroreduction to C2 products on Cu(100). The present obtained conclusion exhibits excellent consistency with a previous theoretical study on the C–C bond formation mechanism from Head-Gordon et al.(26,27) Using time-resolved attenuated total reflection surface-enhanced infrared absorption spectroscopy, however, recent experimental study from Hwang et al. suggested that C–C bond formation exclusively takes place by CO dimerization into adsorbed OCCO intermediate toward C2 products on prepared electrodeposited and sprayed power Cu electrodes, without the participation of adsorbed CHO intermediate, which can lead to productions of C1 products.[37] The inconsistencies of C–C coupling pathways may be able to be attributed to difference of employed crystal face of Cu surfaces, in which only fragmented Cu(100) single-crystal surface is used in our present theoretical work, whereas Cu electrodes with polycrystallinity may be prepared in the experimental investigation conducted by Hwang et al.

Figure 7

Energy pathway diagram of CHO further reduction into CH2O, CHOH, and dimers COCHO and CHOCHO intermediates on Cu(100) at a low overpotential (θCO = 1/9 ML).

To explore alternative C–C bond formation mechanisms at various applied potentials on Cu(100), the further reduction processes of C1 species formed by the initial CO reduction are carried out. Beginning with the most favorable CHO species at the low overpotential (θCO = 1/9 ML), four possibilities of CHO further reduction are considered, as shown in Figure . It is found that the required barriers for the formations of CH2O and CHOH species are ca. 0.35 and 0.70 eV, respectively, being significantly higher than those of dimers COCHO and CHOCHO formations (ca. 0.10 and 0.18 eV, respectively). Moreover, the reaction of the adsorbed CHO with CO and the adsorbed CHO dimerization to dimers COCHO and CHOCHO species are strongly exothermic by −0.75 and −1.89 eV, respectively. Thus, we can conclude that the reaction pathway of CHO with CO and CHO dimerization exhibit C–C bond formation mechanisms at the low overpotential, and the formed dimers COCHO and CHOCHO will result in the production of C2 products. The corresponding structure images of initial, transition, and final states at the low overpotential for CHO electroreduction into CH2O, CHOH, and dimers COCHO and CHOCHO species on Cu(100) are exhibited in Figures S21–S24. Our present study on CHO dimerization into CHOCHO species is well consistent with the previous theoretical work performed by Asthagiri et al. using the linear kinetic barrier method on the basis of Butler–Volmer theory, in which C–C coupling occurs by two adsorbed CHO intermediates at low overpotentials, leading to the production of C2 products like C2H4 on Cu(100).[23]

Under the condition of a medium overpotential (θCO = 2/9 ML), the energetics for COH further reduction is analyzed since initial CO reduction into COH is preferable. As shown in Figure , the activation barriers for formations of the adsorbed dimers COCOH and COHCOH species through the reaction of COH with CO and COH dimerization both are calculated as ca. 0.10 eV and surmountable at room temperature with a negative reaction free energy of ca. −0.72 and −1.76 eV, respectively, being notably lower than those of C along with H2O and CHOH formations on Cu(100). Therefore, we can speculate that C–C bond coupling occurs preferentially through the reaction of adsorbed COH with CO species and COH dimerization at the medium overpotential, and the formation of dimers COCOH and COHCOH will lead to the production of C2 products. The corresponding structure images of initial, transition, and final states for COH electroreduction into various possible species on Cu(100) at the medium overpotential can be seen in Figures S25–S28. In fact, the most recent experimental study from Pérez-Gallent et al. had shown that a hydrogenated dimer COCOH species is detected using Fourier transformation infrared spectroscopy at the potentials of ca. −0.10 V (vs RHE) in CO electroreduction on Cu(100),[19] confirming reasonability of our present used theoretical model and accuracy of conclusions.

Figure 8

Energy pathway diagram of COH further reduction into C along with H2O, CHOH, and dimers COCOH and COHCOH species on Cu(100) at a medium overpotential (θCO = 2/9 ML).

At the present simulated high overpotential (θCO = 1/3 ML), simultaneous formations of CHO and COH species can be concluded on Cu(100) based on the above energetics. Thus, CHO and COH further reduction processes and interactions of the adsorbed CHO with COH species are considered. As can be seen in Figure , it is found that the activation barrier for reactions of the adsorbed CHO with CO, COH, and CHO to form dimers COCHO, CHOCOH, and CHOCHO is ca. 0.25, 0.20, and 0.19 eV, respectively, being all lower than those of CH2O and CHOH formations. With the barriers of the adsorbed COH further reduction into C along with H2O formation, dimers COCOH and COHCOH are significantly increased to ca. 1.75, 0.80, and 0.56 eV at the high overpotential, respectively, whereas the CHOH formation barrier is almost unchanged (ca. 0.50 eV) in contrast with that at medium overpotential, as shown in Figure . The corresponding structure images of initial, transition, and final states for CHO and COH electroreduction are shown in Figures S29–S37. By comparing the adsorption configurations of initial states for dimers COCOH and COHCOH formation pathways at the medium and high overpotentials, it is observed that the distance between C atoms in the adsorbed CO and COH species increases from ca. 2.77 to 2.84 Å as the overpotentials increase, and the corresponding distance between the adsorbed COH species also increases from ca. 3.02 to 3.07 Å. Thus, we can conclude that the repulsive interactions between the co-adsorbed CO and COH species and between the co-adsorbed COH species on Cu(100) at the high overpotential may lead to difficult formations of dimers COCOH and COHCOH.

Figure 9

Energy pathway diagram of CHO further reduction into CH2O, CHOH, dimers COCHO, CHOCHO, and CHOCOH species on Cu(100) at a high overpotential (θCO = 1/3 ML).

Figure 10

Energy pathway diagram of COH further reduction into CHOH, C along with H2O formation, dimers COCOH and COHCOH species on Cu(100) at a high overpotential (θCO = 1/3 ML).

By comparing the barriers of CHO and COH further reduction, we find dimers COCHO, CHOCOH, and CHOCHO formations via the reactions of CHO with CO, COH, and CHO require significantly lower barriers than those of COH further reduction and can be overcome at room temperature on Cu(100) at the high overpotential. Therefore, it can be concluded that the formed dimers COCHO, CHOCOH, and CHOCHO may lead to production of C2 products at the high overpotential, which are regarded as the C–C coupling pathway. Formed COH species may be only able to react with CHO species during CO2 electroreduction on Cu(100) due to significant higher barriers for its further reduction into C along with H2O formation, dimers COCOH and COHCOH, which can result in CHOCOH formation. In earlier theoretical reports from Head-Gordon et al. on C–C bond formation mechanisms on Cu(100),[26,27] COCHO species was also identified as an important intermediate toward production of C2 products at high overpotentials, validating our present conclusion to some degree. The optimal C–C bond formation mechanisms on Cu(100) can be presented at the simulated electrode/solution interface by scrutinizing the above energy pathway diagram at various applied potentials, as shown in Figure .

Figure 11

Optimal C–C bond formation mechanism at the present simulated electrode/solution interface on Cu(100): (a) at the low overpotential; (b) at the medium overpotential; and (c) at the high overpotential.

Our present conclusions for C–C bond coupling mechanisms on Cu(100) are partially inconsistent with recent experimental observations. For instance, the experimental study by using in situ electrochemical attenuated total reflection Fourier transform infrared spectroscopy combined with density functional theory (DFT) calculations from Wang et al. showed that C–C coupling occurs by only adsorbed intermediate CHO dimerization during productions of C2 products on fluorine-modified Cu catalysts.[38] The crystalline structure and the morphology of electrodes may determine the difference of C–C coupling pathways, in which the high-resolution transmission electron microscopy results showed that Cu(111) facet could be formed, and it is also selected as a crystal face for CO2 reduction to C2 products in DFT calculations from Wang et al. The C–C coupling intermediate COCOH was identified experimentally by Fourier transform infrared spectroscopy studies combined with DFT calculations during photocatalytic CO2 reduction on CuO(111) surface conducted by Sheng et al.(39) Similarly, the difference of the selected crystal face may result in partially different C–C coupling pathways. Additionally, the experimental studies on formation mechanisms of the C–C coupled intermediate have been possibly limited in that the spectroscopically detected species must be relatively stable during CO2 electroreduction, which may also be able to lead to difference of C–C coupling mechanisms between our present theoretical calculations and recent experimental observations. In fact, cation effects, such as alkaline-metal adatoms Li, Na, K, and Cs, have a significant effect on the CO2 electrochemical reduction reaction rate and mechanism, in which electrostatic interactions play an important role in the alkaline-metal promotion effect. However, the cation effect is not involved in our present employed electrode/solution interface model, leading to no comparison between our calculated results and some existing experimental and theoretical data, which will be further considered in our next work.

Conclusions

In this paper, we present a CO coverage-dependent electrochemical interface model on Cu(100) with explicit solvent effect for studying C–C coupling pathways occurring during production of C2 products that enables us obtain theoretical insights into potential-dependent C–C bond formation mechanisms. Our results show that CHO is a crucial intermediate on Cu(111) toward the production of C1 products, and dimer OCCO may not be a reliable intermediate toward C2 products on Cu(100). At the low overpotential, the reaction pathway of CHO with CO and CHO dimerization exhibit C–C bond formation mechanisms, and the formed dimers COCHO and CHOCHO may result in production of C2 products. However, C–C coupling takes place preferentially through the reaction of COH with CO species and COH dimerization, and dimers COCOH and COHCOH formations on Cu(100) may result in production of C2 products at the medium overpotential. The formed dimers COCHO, CHOCOH, and CHOCHO through the reactions of CHO with CO, COH, and CHO species may lead to the production of C2 products, which are regarded as C–C bond formation mechanisms at the high overpotential. The formed COH species may be only able to react with CHO species on Cu(100), which can result in CHOCOH formation. The difference of obtained adsorption isotherms of CO on Cu(100) and Cu(111) may be able to explain the effect of Cu single-crystal surface on product selectivity during CO2 electroreduction. Our present conclusions exhibit excellent consistency with the available experimental reports and partial theoretical studies, confirming the reasonability of our present used electrode/solution interface model.

Model and Computational Details

Surface and solvation models, computational parameters, and determination methodologies for CO coverage-dependent equilibrium potentials on Cu(100) have been elaborated in the Supporting Information. Our present employed methodology for the simulations is different from the early theoretical studies on CO2 electroreduction mechanisms. For example, Calle-Vallejo et al. suggested that dimer OCCO on Cu(100) via an Eley–Rideal mechanism can preferably be formed based on the computational hydrogen electrode model,[12] which is the relevant precursor to the production of C2 species. However, this model to stabilize CO dimer lacks rigorous treatment of electrode/solution interfaces and cannot calculate potential-dependent kinetic barriers. Asthagiri et al. reported facet dependence of CO2 electroreduction pathways on Cu electrodes using the linear kinetic barrier method based on Butler–Volmer theory with only one or two relaxed H2O molecules,[23] in which CHO formation is preferred on Cu(100), and C2H4 can be produced via C–C coupling of two adsorbed CHO intermediates at relatively lower overpotentials. The previous used computational hydrogen electrode model and linear kinetic barrier method only considered the effect of the external electric field on electrocatalytic reaction mechanisms and neglected the effect of the internal electric field on the adsorption configuration of reaction species. In fact, the effect of the internal electric field should not be neglected in order to more accurately model electrocatalytic reaction mechanisms. In this manuscript, the electrode potential can be adjusted by changing CO coverage (θCO), and the increasing θCO will result in increasing negative equilibrium potential, in which internal electric field may be changed by changing θCO, thereby theoretical insights into the potential-dependent C–C bond formation pathways occurring during CO2 electroreduction into C2 products can be obtained by calculating reaction free energies and kinetic barriers.