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. 2021 Dec 9;2(1):214–222. doi: 10.1021/jacsau.1c00487

Electrochemical CO2 Reduction Reaction over Cu Nanoparticles with Tunable Activity and Selectivity Mediated by Functional Groups in Polymeric Binder

Qiaowan Chang +, Ji Hoon Lee +,‡, Yumeng Liu +, Zhenhua Xie §, Sooyeon Hwang ∥, Nebojsa S Marinkovic +, Ah-Hyung Alissa Park +,⊥,#,*, Shyam Kattel 7,*, Jingguang G Chen +,§,#,*
PMCID: PMC8790809  PMID: 35098238

Abstract

graphic file with name au1c00487_0006.jpg

The electrochemical carbon dioxide reduction reaction (CO2RR) using copper (Cu)-based catalysts has received significant attention mainly because Cu is an element capable of producing hydrocarbons and oxygenates. One possible way to control the CO2RR performance at the electrode interface is by modifying catalysts with specific functional groups of different polymeric binders, which are necessary components in the process of electrode fabrication. However, the modification effect of the key functional groups on the CO2RR activity and selectivity is poorly understood over Cu-based catalysts. In this work, the role of functional groups (e.g., −COOH and −CF2 groups) in hydrophilic and hydrophobic polymeric binders on the CO2RR of Cu-based catalysts is investigated using a combination of electrochemical measurements, in situ characterization, and density functional theory (DFT) calculations. DFT results reveal that functional groups influence the binding energies of key intermediates involved in both CO2RR and the competing hydrogen evolution reaction, consistent with experimental observation of binder-dependent product distributions among formic acid, CO, CH4, and H2. This study provides a fundamental understanding that the selection of desired polymeric binders is a useful strategy for tuning the CO2RR activity and selectivity.

Keywords: polymeric binders, functional groups, copper, carbon dioxide reduction, density functional theory

1. Introduction

The electrochemical CO2 reduction reaction (CO2RR)1−7 has been the subject of many investigations in the past decades because this process can potentially provide an alternative way to upgrade CO2 into value-added chemicals using electrons from renewable energy resources. Recent efforts8−29 to search for different catalysts for CO2RR have identified copper (Cu) as an element that can produce hydrocarbons and oxygenates, including methane (CH4), ethylene (C2H4), carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), ethanol (C2H5OH), and acetone (C3H6O). The versatile CO2RR properties of Cu primarily originate from the optimal *CO binding energy on Cu surfaces,8,9 which allows the further hydrogenation of adsorbed *CO into hydrocarbon or oxygenate species without poisoning the catalyst surfaces and enables the C–C coupling between *CO and/or *CO/*CHO/*COH (* refers to adsorbed intermediates). However, due to the presence of multiple potential-dependent CO2RR reaction pathways and the competitive hydrogen evolution reaction (HER), achieving high selectivity toward a single product on Cu catalysts remains a challenge.

Therefore, most recent studies on Cu-based CO2RR electrocatalysts have been mainly focused on modulating product distribution. In an effort to modify the electronic state of Cu, controlling crystal facets10,11 (e.g., Cu(100) versus Cu(111)), enriching defective sites12,13 (e.g., point defect and grain boundary), adopting three-dimensional structures14 (e.g., inverse opals), isolating a Cu atom (Cu-single atom catalyst),15,30,31 and alloying with other metallic components16−21,32 have been investigated because these approaches can tune the binding energies of key reaction intermediates. In addition, the chemistry at the catalyst–electrolyte interface22 has also been explored. This approach includes the addition of halide-containing salts23 (i.e., KI, KBr, and KCl, etc.) in an electrolyte and the use of various alkali ions24,25,33 (i.e., AHCO3, A = Li+, Na+, K+, and Cs+), which lead to a different product distribution originating from modified metal–adsorbate interactions. Furthermore, it has been recently found that the partial pressure of CO2 can adjust product distribution of CO2RR over Cu-based catalysts because it can optimize the interactions between different reaction intermediates such as *CO2, *CO, and *H, etc. in an electrolyzer at a practical scale.26,27

In the current study, we demonstrate that the electrocatalytic CO2RR performance of carbon-supported Cu catalysts (Cu/C) can be influenced by the key functional groups of polymeric binders, which are an essential component for electrode fabrication. We adopted hydrophilic and hydrophobic binders with a similar chemical structure (poly(acrylic acid) (PAA, −COOH group) and polyvinylidene difluoride (PVDF, −CF2 group)) to evaluate the effect of the functional groups on the product distribution of the CO2RR over Cu/C. It was found that HCOOH was the main product (Faradaic efficiency (FE) ≈ 30%) at a potential of −1.4 V versus a reversible hydrogen electrode (VRHE) when the hydrophilic PAA was adopted. In contrast, when the hydrophobic PVDF was used, CH4 were favorably produced. Such a distinct dependence on the choice of polymeric binders primarily originates from their interactions with intermediates involved in both CO2RR and HER. Density functional theory (DFT) calculated free energy profiles for the formation of major products (H2, CO, HCOOH, and CH4) reveal that the interaction of functional groups with the Cu surface plays an important role in modulating the product distribution of CO2RR. The findings in this work suggest that a simple binder replacement can be a viable option in tuning the selectivity and activity of Cu-based electrocatalysts for CO2RR.

2. Experimental and Theoretical Methods

2.1. Electrochemical Measurements

All of the chemicals in this work were purchased from Sigma-Aldrich unless otherwise noted. All reagents were used without purification. Saturated calomel electrode (SCE) and graphite rod were used as the reference and counter electrodes, respectively. Each Cu/C electrode with a different binder was referred to as Cu–X, with X being the binder (i.e., Cu–Nafion, Cu–PAA, and Cu–PVDF). In the process of fabricating working electrodes, a binder content of 0.5 wt % was selected. The CO2-saturated 0.1 M potassium bicarbonate (KHCO3) aqueous solution was utilized as an electrolyte. The reference electrode was calibrated using a reversible hydrogen electrode (HydroFlex, ET-070, EDAQ). For CO2 electrolysis, a typical H-type cell consisting of two airtight compartments was used. The cathodic and anodic compartments were separated by a Nafion 117 membrane. The applied potential was controlled by iR-compensation (80%) and converted to the reversible hydrogen electrode (RHE). Here and afterward, all of the potentials (V) are with respect to an RHE potential unless otherwise noticed. See the Supporting Information for the detailed experimental conditions.

The CO2RR performance was evaluated using the chronoamperometry (CA) method at each constant potential. The gaseous and liquid products were quantified by using gas chromatography (GC, Agilent, Agilent 7890B) and high-performance liquid chromatography (HPLC, Agilent, 1260 Infinity II equipped with Hi-Plex H columns), respectively. The calculations of FE and partial current density (J) were the same as described in our previous work.17,34−37 The detailed procedure is provided in the Supporting Information.

2.2. In Situ X-ray Absorption Fine Structure (XAFS)

In situ XAFS measurements were conducted on the 1D beamline (KIST-PAL) at Pohang Light Sources (PLS) in Pohang Accelerating Laboratory (PAL) using fluorescent detection. Each potential was held for 12 min during XAFS measurements. The typical duration for a single spectrum was around 3 min, and four spectra were merged to improve the signal-to-noise ratio.

The obtained spectra were processed using the ATHENA and ARTEMIS software in the IFFEFIT package.38,39 The procedure described by Ravel et al.39 was followed during data processing. EXAFS analysis was conducted by using the ARTEMIS software. The k3-weighted EXAFS spectrum (χ(k)) was used to intensify the signal at a high k-regime. The Hanning window was utilized for the Fourier transform operation. All of the EXAFS fittings were done in the R space. The goodness of fit was evaluated based on the reliable factor (R-factor) and reduced chi-square (reduced χ2). The representative fitted EXAFS spectra for different catalysts at 0, −1.2, and −1.3 V are shown in Figure S9, and the fitting parameters are tabulated in Table S3.

2.3. DFT Calculations

Spin-polarized DFT calculations40,41 were performed at the generalized gradient approximation (GGA) level42 using the plane wave Vienna Ab-Initio Simulation Package (VASP) code.43,44 The core electrons were described using the projector augmented wave (PAW) potentials45 including the vdW correction with the DFT-D3 method of Grimme et al.46 using PW91 functionals.47 A kinetic energy cutoff of 400 eV and 3 × 3 × 1 k-point mesh were used in all structure-optimization calculations.

By considering that the main CO2RR product in this study is CH4, the thermodynamically most stable low index Cu(111) surface was selected as a DFT model surface.48 The Cu(111) surface was modeled using a four-layer 3 × 3 surface slab. Cu–Nafion was modeled as a combination of the −SO3H and −CF2 preadsorbed on Cu(111).49−51 Following the previous work of binders on Au(111),37 the COOH-containing binder on Cu (Cu–PAA in experiments) was represented by COOH preadsorbed on Cu(111), and F-containing binder on Cu (Cu–PVDF in experiments) was represented by CF2 preadsorbed on Cu(111). A vacuum layer of approximately 15 Å thick was added in the slab cell along the direction perpendicular to the surface to minimize the artificial interactions between the surface and its periodic images. Atoms in the bottom two layers were fixed, while all other atoms were allowed to relax during geometry optimization until the Hellmann–Feynman force on each ion was smaller than 0.02 eV/Å. The details on binding energy, activation energy, and Gibbs free energy calculations are described in the Supporting Information.

3. Results and Discussion

3.1. Characterization and Electrochemical Measurements

The impact of the surface functional groups of binder species on CO2RR was evaluated using different binders, which were Nafion, PAA, and PVDF. The resulting Cu/C electrodes were referred to as Cu–X with X being the binder, as shown in Figure S1. In the process of fabricating working electrodes, a binder content was selected to be 0.5 wt % because this binder loading exhibited the highest total current density for CO2RR compared with binder loadings of 0.05 wt and 5 wt % (Figure S2). High-resolution transmission electron microscopy (HR-TEM) analyses confirmed that the commercial Cu/C has an average Cu particle size of ∼50 nm with a well-defined crystalline feature (Figure S3). The electrochemical CO2RR performance of a series of Cu–X was examined by using the chronoamperometry (CA) method (Figure S4) in a high-purity CO2-saturated 0.1 M KHCO3 solution.

Parts A and B of Figure 1 display the FE profiles for various products over Cu–Nafion, Cu–PAA, and Cu–PVDF at −0.6 and −1.4 V, respectively. See Table S1 for detailed FE values. Cu–Nafion, the benchmarking electrode in the current study, produced a mixture of various products including H2, oxygenated C1 species (CO and HCOOH), and hydrocarbons (CH4 and C2H4), consistent with previous reports on Cu nanoparticles.52,53 At −0.6 V, HER surpassed CO2RR, as evidenced by higher FE (H2, 50.5%) over FE (CO, 12.6%) and FE (HCOOH, 19.8%). However, with increasing applied potential, FE of CO2RR increased as a result of the suppressed HER (e.g., 50.5% at −0.6 V → 26.1% at −1.4 V). Both FE(CO) and FE(HCOOH) increased up to 28.7% and 32.8% at −0.9 V and then gradually dropped down to 21.5% and 17.4% at −1.4 V, respectively. At the same time, FE(CH4) and FE(C2H4) increased up to 12.2 and 10.6% at −1.4 V, respectively, confirming that the reaction pathway for CO2RR products was greatly influenced by the applied potential.54−57 By considering that hydrocarbon production was favored at the expense of H2, CO, and HCOOH at negative potentials, it could be speculated that the reaction pathways for CH4 and C2H4 involved the hydrogenation step of either formate (*HCOO) or carbon monoxide (*CO) intermediates.

Figure 1.

Figure 1

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CO2RR performance of a series of Cu/C catalysts with different binders: (A) FE of HCOOH, CO, CH4, C2H4, and H2 at −0.6 V; (B) the same consideration at −1.4 V. Partial current density profiles of (C) HCOOH and (D) CH4.

Upon the replacement of Nafion with PAA or PVDF, Cu-based electrodes exhibited similar products detected in the Cu–Nafion electrode. When the applied potential was increased, CO2RR was generally enhanced while HER tended to be suppressed. However, their CO2RR product distribution was significantly modified, indicating that the CO2RR over the Cu surface was sensitively affected by the key functional groups of binders. For example, at −0.9 V, Cu–PVDF was efficient at producing CO and suppressing H2 evolution. Cu–PVDF exhibited the peak FE(CO) value of 37.4%, while Cu–Nafion and Cu–PAA showed FE(CO) of ∼29%, which might be also associated with its lowest FE (H2, 20.1%) of Cu–PVDF at the same potential. The product distribution over Cu–Nafion and Cu–PVDF at −1.2 and −1.4 V became generally similar to exhibiting better CH4 and C2H4 produciton compared to Cu–PAA, which could be understood by the similar key functional group (−CF2) in their binder structure.

It should be noticeable that HCOOH was most selectively produced over Cu–PAA with the FE(HCOOH) value of 37.6% at −1.2 V, which was 1.8 and 1.2 times higher than those of Cu–PVDF (21.1%) and Cu–Nafion (30.9%), respectively. Moreover, the high selectivity toward HCOOH over Cu–PAA was maintained with FE(HCOOH) of 29.8% at −1.4 V, which was 1.4–1.7 times higher compared to Cu–PVDF and Cu–Nafion. By considering the recent reports58,59 that HER is efficiently accelerated in hydrophilic environments, the high FE(HCOOH) values for Cu–PAA suggested that the protons present on the Cu–PAA surface were utilized as a reactant for HCOOH formation rather than for HER even at high overpotentials. Therefore, at −0.9 V and afterward, the surface H generated due to H2O dissociation on the hydrophilic Cu–PAA catalyst played an important role in tuning the favored CO2RR pathway.56 On the contrary, the selective hydrocarbon production was achieved on Cu–PVDF with its FE(CH4) being ∼1.6 times higher than that of Cu–PAA at −1.4 V, supporting again that the key functional groups effectively modified the reaction pathways. Such enhanced CH4 production on Cu–PVDF compared to Cu–PAA suggested that the hydrophobic environment near the electrode promoted hydrocarbon reaction pathway.28,37,60

Parts C and D of Figure 1 display the partial current density (J) of HCOOH and CH4, respectively. The partial density profiles of other products are shown in Figure S5. The different current densities of Cu–X electrodes may originate from the different wettabilities of binders in electrodes as well as their different selectivities toward each product.61−64 Consistent with the trend of FE values, Cu–PAA achieved higher J(HCOOH) over Cu–Nafion and Cu–PVDF over the entire potential ranges. In contrast, Cu–PVDF showed higher CH4 activity than Cu–PAA at high overpotentials (e.g., 1.9 versus 3.5 mA cm–2 at −1.4 V), originating from the surface hydrophobicity of the Cu–PVDF electrode. Overall, Cu–PVDF was more efficient at converting CO2 to hydrocarbon, while Cu–PAA was better at producing HCOOH throughout all of the potentials in this study.

All of the electrodes with different binders maintained their stable electrocatalytic activity for 2 h (Figure 2) at −1.4 V while exhibiting distinguished product distribution depending on the binder. It is interesting that FE(CH4) of Cu–PVDF increased from 12.0 to 39.0%, supporting again its high selectivity toward CH4. Thus, this result indicates that Cu–PVDF may require a longer time to reach a steady state. The J profiles of a series of Cu–X electrodes were found to be stable over the same period and exhibited the same trend with their FE profiles (Figure S6). It was also found that their CO2RR performance was also stable at low potentials such as −0.6 V (Table S2). Cu NPs remained the same after the stability tests regardless of the binder choice as shown in the ex situ TEM characterization of spent catalysts (Figure S7). Based on the observation that the electrodes remained firm without being detached from the carbon substrates on the completion of CO2RR, one can state that the functional groups in each binder would not be degraded (Figure S8).

Figure 2.

Figure 2

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Stability tests at −1.4 V for 2 h. FE profiles of (A) Cu–Nafion, (B) Cu–PAA, and (C) Cu–PVDF.

3.2. In Situ XAFS Characterization

To elucidate the active site of Cu/C catalysts modified by different binders, in situ XAFS analysis was performed at the Cu K-edge. On the basis of the X-ray absorption near edge structure (XANES) profiles (Figures 3A–C), the commercial Cu/C catalyst used in this study showed a mixed structure of cupric oxide (CuO) and metallic Cu at 0 V. However, with gradual electrochemical reduction reaction from 0 to −1.3 V, regardless of the binder choice, XANES profiles gradually shifted toward lower energy values and the intensity of white lines decreased, indicating that all of the electrodes mainly adopted the feature of metallic Cu as illustrated along the brown dotted lines. Furthermore, the extended-XAFS (EXAFS) profiles in Figure 3D–F reflected the same changes that the initial oxide feature (Cu–O bond at ∼1.5 Å) disappeared with an intensifying metallic Cu bond (Cu–Cu) at ∼2.2 Å. The coordination number profile as a function of applied potential also indicated that, with negatively increasing potentials, the Cu–O bond disappeared while the Cu–Cu bond formed (Figures 3G–I). In addition, the EXAFS fitting results supported the presence of metallic Cu at −1.2 and −1.3 V (Figure S9 and Table S3), suggesting that the metallic Cu surface serves as the catalytic active site for the CO2RR because all of the electrodes were fully reduced to metallic Cu during the LSV measurement prior to the actual CO2RR test. Therefore, the different CO2RR product distributions in this study mainly originated from the modification by the different binders rather than the oxidation state of Cu, allowing one to focus on the effect of the key functional group present in the binders in DFT calculations described below.

Figure 3.

Figure 3

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In situ XAFS analysis at Cu K-edge. XANES profiles for (A) Cu–Nafion, (B) Cu–PAA, and (C) Cu–PVDF. EXAFS profiles at 0 V (top) and −1.3 V (bottom) and corresponding contour map of EXAFS profiles from 0 to −1.3 V (middle) for (D) Cu–Nafion, (E) Cu–PAA, and (F) Cu–PVDF. Coordination number profiles for Cu–O (Cu2O, black) and Cu–Cu (Cu, red) for (G) Cu–Nafion, (H) Cu–PAA, and (I) Cu–PVDF.

3.3. DFT Calculations

DFT calculations40−47 were performed to gain insight into the role of PAA and PVDF binders on the selectivity of CO2RR on Cu(111), which represents the energetically most stable low index facet of metallic nanoparticles (∼50 nm) observed in the TEM and scanning TEM (STEM) images (Figures S3 and S7). The DFT calculations were carried out to determine the binding energies of reaction intermediates (Table S4) responsible for the conversion of CO2 to three C1 products, namely CO, HCOOH, and CH4. The DFT-calculated models were constructed by reflecting the choice of binders. PVDF and PAA binders were represented by −CF2 and −COOH groups on the Cu(111) surface, respectively, which thus led to −CF2 preadsorbed Cu(111) (CF2–Cu(111), Figure 4A) and COOH preadsorbed Cu(111) (COOH–Cu(111), Figure 4B). Our focus is to compare the effect of hydrophilic and hydrophobic functional groups. Because Nafion contains both hydrophilic (−SO3H) and hydrophobic (−CF2) groups, DFT of the Nafion/Cu interface does not allow us to conclusively differentiate the role of hydrophilicity on reaction pathways. The comparison with unmodified Cu should provide direct information regarding the effect of hydrophilicity and hydrophobicity on the binding energies of intermediates and the reaction pathways. For comparison, DFT calculations of the reaction network over SO3H–Cu(111) and CF2–Cu(111) are overlaid in Figure S10. Experimentally, the CO2RR activity and selectivity over Cu–Nafion are likely affected by the competing effects of the hydrophilic and hydrophobic groups.

Figure 4.

Figure 4

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DFT-optimized geometries: (A) CF2–Cu(111), (B) COOH–Cu(111), and (C–L) *H, *CO, *HOCO, *OH, *HCO, *H2CO, *H3CO, *HCOO, *HCOOH, and *H2COOH on COOH–Cu(111), respectively. Cu: blue, C: brown, F: green, O: red, and H: pink. *X represents species X adsorbed on the surface. Dashed lines in orange represents hydrogen bonding between the adsorbed COOH group and reaction intermediates.

In general, binding sites and configurations of the reaction intermediates were similar on Cu(111), CF2–Cu(111), and COOH–Cu(111) (Figure 4C–L). However, the reaction intermediates bonded more strongly on COOH–Cu(111) compared to Cu(111) and CF2–Cu(111), mainly due to hydrogen bonding between the reaction intermediates and the COOH functional group. Hydrogen bonding between intermediates and the CF2 functional group was not observed in CF2–Cu(111), most likely due to the hydrophobic nature of the −CF2 group. Such a difference in binding affinity of reaction intermediates should have a profound effect on the CO2RR selectivity as discussed below.

The DFT-calculated binding energies were used to determine the free energy change (ΔG) for the formation of CO, HCOOH, and CH4 at a potential (U) = 0 V as shown in Figure 5. Figure 5A shows the formation of CH4via the intermediate *HOCO and HCOOH via the intermediate *HCOO. The first (H+ + e–) transfer to CO2 leads to the formation of either *HOCO or *HCOO intermediate. The DFT-calculated results in Figure 5A show that *HCOO formation is energetically favorable over that of *HOCO on Cu(111), CF2–Cu(111), and COOH–Cu(111). The thermodynamically less favorable formation of *HOCO, a key intermediate for CO2RR to CO transformation, explains the lower FE(CO) observed in the experiments compared to the FE(HCOOH) at −0.6 V. Further (H+ + e–) transfer to *HCOO to form *HCOOH, which leaves the catalytic site as the HCOOH product, is uphill on Cu(111) and CF2–Cu(111). In contrast, this step is nearly thermoneutral on COOH–Cu(111). This suggests that HCOOH is preferentially formed on Cu–PAA at lower applied potential (U), consistent with the experimental observation of higher FE(HCOOH) and J(HCOOH) on Cu–PAA compared to Cu–PVDF (Figure 1). On COOH–Cu(111), the desorption of *HCOOH as a product is associated with the largest ΔG and thus is predicted to be the rate-determining step.

Figure 5.

Figure 5

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DFT-calculated free energy diagrams of CO2RR at a potential (U) = 0 V. (A) CO2 conversion to CO and CH4via the formation of *HOCO intermediate and to HCOOH via the formation of *HCOO intermediate. (B) CO2 conversion to CH4via the formation of *HCOO intermediate.

Because the first (H+ + e–) transfer to CO2 yields either *HCOO or *HOCO formation and the former is energetically preferred over the latter, the DFT calculation for the CH4 pathway is focused on the *HCOO intermediate. This consideration is also consistent with the experimental results that, as the potentials negatively increase, FE(HCOOH) and FE(H2) decrease while FE(CH4) increases (Figure 1). Figure 5B shows the free energy profile for CO2 to CH4 conversion via the *HCOO intermediate. Along this route, *HCOO is reduced to *HCOOH. Thus, the formed *HCOOH can undergo (H+ + e–) transfer to form *H2COOH or *HCO + H2O. As shown in Figure 5B, the formation of *H2COOH (see the dashed lines) is energetically more favorable than the formation of *HCO + H2O on Cu(111), CF2–Cu(111), and COOH–Cu(111). Thus, the DFT calculations predict that CO2 to CH4 conversion most likely occurs via the formation of *H2COOH intermediate. Additional (H+ + e–) transfer enables the C–O bond cleavage of *H2COOH to form *H2CO + H2O, with *H2CO then undergoing (H+ + e–) transfer reactions to form the final product CH4. On Cu(111) and CF2–Cu(111), the step of *HCOO + (H+ + e–) to *HCOOH has the largest positive change in free energy (∼0.44 and ∼0.48 eV, respectively) among all steps, is predicted to be the potential determining step. The limiting potential (UL), which is defined as a potential at which all the electrochemical steps along the reaction channel are thermodynamically downhill in energy, for the formation of CH4via the HCOO intermediate is predicted to be −0.48 V on CF2–Cu(111). Thus, the formation of CH4 is expected to be facile at high U. On the COOH–Cu(111), the step *HCOOH + (H+ + e–) to *H2COOH is predicted to be the potential determining step with a positive free energy change of ∼0.71 eV, and a significantly larger overpotential (compared to CF2–Cu(111)) is needed for CO2 to CH4 transformation on Cu–PAA. The UL value is determined on COOH–Cu(111) to be −0.71 V, which is significantly higher compared to the values on CF2–Cu(111). Overall, in agreement with the experimental findings, the DFT results predict that CF2–Cu(111) is more selective to CH4 during CO2RR at high potentials compared to COOH–Cu(111). Consistent with the experimental Faradaic efficiency in Figure 1, the DFT-predicted UL values for the formation of CH4 suggest the CH4 selectivity should be higher on CF2–Cu(111) than on COOH–Cu(111).

Hydrogen evolution reaction (HER) is a competing reaction in CO2RR. The DFT-calculated free energy diagrams of HER (Figure S11) show that the HER is favorable at intermediate U values on all surfaces considered in the present study. Therefore, H2 is predicted to be one of the major products of CO2RR on Cu(111), CF2–Cu(111), and COOH–Cu(111), consistent with the experimental observation in Figure 1. Overall, the DFT results are consistent with the experimental results and help explain the selectivity differences observed during CO2RR over different binders attached to the Cu catalysts.

4. Conclusions

Tuning the product selectivity of CO2RR on Cu electrodes remains a significant challenge. Here, we demonstrate that the selectivity to HCOOH and CH4 can be tuned by choosing polymeric binders in the electrode fabrication process. The combined experimental and DFT results show that Cu–PAA (functionalized by a −COOH group) is more selective to HCOOH production compared to Cu–PVDF (functionalized by −CF2 group) through the entire potential ranges while CH4 is favorably produced on Cu–PVDF. DFT calculations reveal that different binding energies of reaction intermediates account for the different product distribution on Cu–PAA and Cu–PVDF. Overall, the experimental observations are consistent with the DFT predictions. Results from the current study show that modification of Cu catalysts by key functional groups of polymeric binders, which are required in electrode fabrication, represents a relatively simple and promising methodology to tune the CO2RR product selectivity. Future studies can include a better understanding of interactions of electrocatalyst surfaces with both carbon chains and various functional groups in polymeric binders, which would further highlight the important role of binders in the CO2RR performance.

Acknowledgments

This research was supported by the U.S. National Science Foundation AccelNet Program (Grant No. 1927336). This work was also supported by the National Research Foundation of Korea (NRF-2021R1G1A1092280). This research used resources at the 1D beamline (KIST-PAL) of the Pohang Light Sources-II (PLS-II) in the Pohang Accelerating Laboratory (PAL). DFT calculations used resources of the Center for Functional Nanomaterials (CFN), which is a U.S. Department of Energy Office of Science Facility, and the Scientific Data and Computing Center, a component of the BNL Computational Science Initiative, as well as the technical support (electron microscopy), at Brookhaven National Laboratory (Contract No. DE-SC0012704); the Extreme Science and Engineering Discovery Environment (XSEDE) was also utilized, which is supported by the National Science Foundation (Grant No. ACI-1548562). Y.L. (No. 201806010243) acknowledges financial support from the China Scholarship Council.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.1c00487.

  • Methods, electrode pictures, additional electrochemical results, high-resolution TEM images, EXAFS fitting results, and additional DFT results (PDF)

Author Contributions

† Q.C., J.H.L., and Y.L. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

au1c00487_si_001.pdf (1.3MB, pdf)

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