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. 2026 Jul 17;6(5):923–932. doi: 10.1021/acsnanoscienceau.6c00082

In Situ Growth of Copper Phenylacetylide Nanobelts on Copper Foil for Enhanced Electroreduction of CO2 to Multi-Carbon Products

Shijin Yang †, Qi Fang †, Jing Tang †,‡,*, Jianping Ge †,‡,*
PMCID: PMC13693223  PMID: 42921419

Abstract

Copper-based metal–organic composites are promising catalysts for electrochemical CO2 reduction reaction (eCO2RR). However, it is difficult to prepare stable Cu+ active sites in a controllable way and maintain them in the catalytic reduction. In this work, a (CuPA-Ag)/Cu electrode was prepared by the ion sputtering of Ag NPs on the copper foil, followed by the in situ growth of Ag NP-grafted CuPA NBs on its surface. The electrode possessed a large amount of Cu+ active sites due to the strong σ-donation and π-backdonation interactions between the phenylacetylene ligand and Cu+, which significantly improved the catalytic performances. Using the optimized electrode and reaction conditions, the eCO2RR achieved Faraday efficiencies of 44.5% for C2H4 and 56.5% for C2+ products in the H cell. With periodic supplementation of the CO2 source, the eCO2RR maintained an FEC2H4 above 40% for 11 h. Its superior catalytic performance originated from the densely stacked nanobelt structures with a large electrochemical surface area and enhanced electron transfer, the synergistic effect between Ag and Cu+ to promote the CO2 absorption and C–C coupling, and the hydrophobicity of CuPA to inhibit the hydrogen evolution reaction.

Keywords: eCO2RR, copper phenylacetylide (CuPA), nanobelts, multicarbon products, ethylene

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1. Introduction

Atmospheric carbon dioxide concentrations keep climbing, making the greenhouse effect an ever more pressing concern. Cutting carbon emissions and advancing resource recycling have now reached a worldwide consensus. , Among all the processes, electrochemical CO2 reduction reaction (eCO2RR) enables the conversion of carbon dioxide into high-value chemicals and fuels, including carbon monoxide, ethanol, and ethylene, at mild operating conditions powered by renewable electricity. This technology facilitates closed-loop carbon cycling to alleviate the greenhouse effect while serving as an effective carrier for the storage of intermittent renewable energy. As a pivotal technique bridging carbon mitigation and energy transition, eCO2RR brings concurrent environmental and economic merits and bears profound implications for progressing global sustainable development. , With the progress of eCO2RR research, more attention has been paid to the ethylene, ethanol, and other C2+ products as they are more valuable than C1 products for modern chemical and energy industries. − Copper-based materials stand as an important class capable of efficiently facilitating C–C coupling to selectively produce multicarbon products, which affords them unique and irreplaceable advantages. ,

Among diverse copper-based composite catalysts, “copper–organic composite catalysts” exhibit exceptional performance. Benefiting from coordination between organic ligands and metallic Cu centers, such materials possess abundant porous architectures, which enable efficient CO2 enrichment and directional mass transport of CO2 at catalytic interfaces. Meanwhile, ligand-induced electronic modulation tunes the electronic configuration of Cu active sites, thereby optimizing adsorption energy barriers for pivotal reaction intermediates, including *COOH and *CO. − For example, Zhang et al. developed a layered Cu–Ag bimetallic covalent organic framework (Cu–Ag MCOF) to achieve an FEC2H4 of 51.5% in eCO2RR. By rationally regulating the spatial distribution of metal sites, the *CO intermediate was favorably generated on Ag sites and directionally migrated to Cu sites to enhance the activity and selectivity. Han et al. synthesized an alkyne-protected Cu-MOF catalyst to realize an FECH4 of 83.6%. The MOF structure not only stabilizes the Cu+ active sites but also greatly accelerates the reaction kinetics of CO2 conversion to CH4. Zhu et al. introduced phosphorus and nitrogen atoms to construct a dual-copper site molecular catalyst (Cu2-AC) with an asymmetric coordination structure. This structure distorts and localizes the electron cloud around Cu sites, optimizes the adsorption properties of key reaction intermediates, and achieves a Faraday efficiency (FE) of 75.4% for C2 products. Tang et al. synthesized a C3-symmetric alkynyl-protected superatomic nanocluster, which provided favorable adjacent sites and surface curvature for C–C coupling, and finally achieved an FE of 56.3% for C2+ products.

Although copper-organic composite catalysts with Cu+ active sites can effectively reduce the energy barrier of C–C coupling and promote C2+ products, they also face challenges in practical eCO2RR. Generally, the Cu+ valence state is difficult to stabilize and susceptible to irreversible transformation under reductive potentials, which leads to decreased catalytic activity or even complete deactivation. Meanwhile, the copper-organic composite catalysts usually possessed poor electrical conductivity and easy detachment from the electrode under long-term reactions. Therefore, it is urgent to develop better catalysts with more stable Cu+ active sites and improved performance in this field. ,

In this work, we reported a (CuPA-Ag)/Cu catalytic electrode synthesized by the ion sputtering of Ag NPs on the copper foil, followed by the in situ growth of Ag NP-grafted CuPA NBs on its surface. This catalyst possessed a large amount of stable Cu+ active sites due to the strong σ-donation and π-backdonation interactions between the phenylacetylene ligand and Cu+. Meanwhile, the in situ growth of CuPA from the Cu foil enables the firm attachment of the active materials and fast electron transfer from the collector to the catalyst surface. These factors relieve the activity and stability concerns for the traditional copper-organic catalyst, which rendered the (CuPA-Ag)/Cu electrode FEs of 44.5% for C2H4 and 56.5% for C2+ products and 11 h stable conversion in the H-cell. In the following discussion, we introduced the synthesis control and structural characterization of the (CuPA-Ag)/Cu electrode. Its superior catalytic performance compared to the Cu, Ag/Cu, and CuPA/Cu electrodes would be explained in the aspects of the nanostructures of the electrode, the synergistic effect between Ag and Cu+, and the hydrophobicity effect of CuPA. Finally, a possible reaction mechanism for the formation of C2+ product was proposed according to the in situ IR spectra.

2. Experimental Section

2.1. Materials

Cu foil (high-purity, oxygen-free, 100 × 100 × 0.3 mm), potassium bicarbonate (KHCO3, 99%), phenylacetylene (C8H6, ≥97%), acetonitrile (C2H3N, ≥99.9%, Macklin), isopropanol (IPA, 99%), deuterium oxide (D2O), and dimethyl sulfoxide (DMSO, 99%) were purchased from Aladdin Co., Ltd. Carbon dioxide (CO2, 99.999%), argon (Ar, 99.999%), and nitrogen (N2, 99.999%) were obtained from PuJiang Special Gas Co., Ltd. in Shanghai Chemistry Industrial Zone. Gaoss Union platinum foil (10 × 10 × 0.2 mm), RE-2BP saturated calomel electrode (SCE), and Dupont N211 proton exchange membrane (PEM) were purchased from local dealers.

2.2. Preparation of Cu and Ag/Cu Electrodes

High-purity oxygen-free copper foil was first cut into appropriate dimensions (15 × 10 × 0.3 mm) to fit the electrochemical cell. Then, the copper foil was successively sonicated in anhydrous ethanol, deionized (DI) water, sulfuric acid (0.1 M), and DI water for 5, 5, 15, and 5 min, respectively, which removed the organic contaminants and copper oxides on the surface. After drying with high-purity nitrogen (N2) gas, the copper foil was polished with 800-grit sandpaper to produce the final Cu electrode for eCO2RR. The above Cu electrode was further used to prepare an Ag/Cu electrode through the ion sputtering of Ag nanoparticles, where the sputtering current and duration were set as 5 mA and 20 s, respectively.

2.3. Preparation of (CuPA-Ag)/Cu and CuPA/Cu Electrodes

To prepare the (CuPA-Ag)/Cu electrode, the aforementioned Ag/Cu electrode was immersed in the acetonitrile solution of phenylacetylene (PA) and placed in an oven for 12 h to produce a thin layer of CuPA-Ag nanobelts on the surface. During the growth of nanobelts, the PA concentration was set from 10% to 50%, and the reaction temperature was controlled at 20 °C, 40 °C, 60 °C, and 80 °C to form different composite electrodes. After the reaction, the (CuPA-Ag)/Cu electrode dried in air at 20 °C was ready for the following eCO2RR. The CuPA/Cu electrode was prepared in a similar way, where the Ag/Cu electrode was replaced with the Cu electrode in the growth of the CuPA nanobelt layer.

2.4. Electrochemical Reduction of CO2 and Measurement of FE

The electrochemical reduction of CO2 and the evaluation of the catalytic performance were conducted in an H-type electrochemical cell, which utilized the Cu, Ag/Cu, (CuPA-Ag)/Cu, and CuPA/Cu plates (15 × 10 × 0.3 mm) as the working electrodes, platinum foil (10 × 10 × 0.2 mm) as the counter electrode, and a saturated calomel electrode (SCE, RE-2BP, BAS) as the reference electrode. The cathodic and the anodic compartments were both filled with an aqueous solution of KHCO3 (0.1 M) and separated by a Dupont N211 proton exchange membrane (PEM). In a typical eCO2RR, the CO2 gas was first bubbled into the cathodic compartment with a flow rate of 15 mL·min–1 for 30 min to saturate the electrolyte solution with CO2 before the initiation of the electrocatalytic reaction. During the eCO2RR, the CO2 gas was continuously bubbled at the same rate to maintain an adequate supply of CO2 for the reduction, and the gaseous products could be collected by a gas sampling bag simultaneously. The liquid phase products are left in the cathodic compartment. The electrochemical signals during the reduction are collected by the electrochemical workstation.

After 30 min of reduction reaction, the gaseous products in the sampling bag were analyzed by a Shimadzu 2014C gas chromatography (GC) equipped with TCD and FID detectors, which provided the molar amounts of each product, such as H2, CH4, C2H4, etc. The Faraday efficiencies (FE) of gaseous products were calculated by eq , where k is the calibration constant, A is the area in the chromatogram, f is the flow rate, N A is Avogadro’s constant, e is the elementary charge, n e is the number of electrons, I total is the total current, and V mol is the molar volume. The liquid products in the electrolyte solution were analyzed by a Bruker Avance III HD500 nuclear magnetic resonance (NMR) spectrometer, where DMSO with a chemical shift of 2.6 ppm was used as the internal standard for the measured 1H spectra. The FE of the liquid products was calculated by eq , where V total is the total volume of electrolyte in the electrolytic cell, n H is the ion exchange membrane separation correction factor for H-type electrolytic cell, F is the Faraday constant, C is the total charge, V elec is the sample volume for detection, and the rest of the constants and variables are the same as those in eq .

FE=k×Aproductf×NA×e×neItotal×Vmol 1
FE=k×AproductVtotal×nenH×F×C×Velec 2

The catalyst stability test was stopped at 11 h because the multiple resets of the H-cell and repeated connection to the (CuPA-Ag)/Cu with an electrode clamp inevitably caused the slight scratching and peeling of the CuPA-Ag nanobelts. After 11 h reaction and 3 times of reset, the collective peeling of CuPA-Ag cannot be ignored anymore, which shall lead to a decreased current density if calculated by the original area of the active electrode. However, such a decrease in current density does not suggest the deactivation of the catalyst. To avoid misleading and present practical stability, the test was stopped at 11 h.

2.5. Characterization

The nanostructure of the electrodes was observed by a S-4800 scanning electron microscope equipped with an X-Max energy-dispersive spectrometer (EDS) and a Phenom Nano desktop scanning electron microscope. The morphology of the CuPA-Ag nanobelts was characterized by a JEM-2100plus transmission electron microscope working at 200 kV with an electron gun of LaB6 filament. X-ray photoelectron spectra (XPS) were collected on an Axis Supra spectrometer with monochromated Al Kα, where the raw spectra were calibrated with the C1s peak at 285 eV. X-ray diffraction (XRD) patterns were recorded on an Ultima IV X-ray diffractometer with Cu Kα as a source and D/teX-Ultra as a detector. IR spectra were measured by a NEXUS 670 Fourier transform infrared spectrometer. The elementary analysis was achieved by a PerkinElmer Optima 8300 inductively coupled plasma optical emission spectrometer. The electrochemical properties of the electrodes were measured by a ChenHua CHI-630e electrochemical workstation.

3. Results and Discussion

The (CuPA-Ag)/Cu composite electrode was prepared by the ion sputtering of silver nanoparticles (Ag NPs) on cleaned and polished copper foil, followed by the in situ growth of Ag NP-modified copper phenylacetylide nanobelts (CuPA-Ag NBs) on its surface (Figure a). In a typical process, an Ag/Cu electrode was first fabricated by depositing Ag NPs onto the copper foil through ion sputtering in a vacuum environment, where the Cu foil was cleaned by anhydrous ethanol and sulfuric acid and polished by sandpaper. SEM images suggested that the Ag NPs are uniformly distributed on the surface of the Cu substrate with an average particle size around 10–20 nm (Figure S1). Subsequently, the Ag/Cu electrode was immersed in an acetonitrile solution of phenylacetylene (PA) to produce a thin layer of yellow-green CuPA-Ag NBs on the surface through the in situ reaction between Cu and PA. Here, Cu is oxidized by the trace amount of oxygen dissolved in acetonitrile, which further reacts with PA to generate CuPA and water. This reaction continuously takes place along one-dimensional direction to form nanobelts because of the intrinsic structure of CuPA, where the CuPA molecules were arranged and connected by the Cu–Cu metallic bond in one dimension. The CuPA NBs also grew on Cu foil without Ag deposition, but the deposited Ag did promote the growth of CuPA NBs under the same conditions (Figures S2 and S3).

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(a) Scheme for the preparation of (CuPA-Ag)/Cu electrode and the crystal structure of CuPA nanobelts; (b,c) SEM and (d) TEM images of the CuPA-Ag NBs; (e–g) EDS mapping of C, Cu, and Ag elements in the CuPA-Ag NBs; (h,i) XRD patterns of the Cu foil, the Ag/Cu electrode, and the (CuPA-Ag)/Cu electrode; and (j) FTIR spectrum of the CuPA-Ag NBs.

SEM images (Figure b,c) suggested the production of CuPA-Ag NBs with a disordered interwoven network structure. A high-magnification SEM image further revealed that Ag NPs with a diameter of 10–20 nm were embedded on the surface of CuPA NBs with an average width of 100 nm. The TEM image (Figure d) suggested that the sputtered Ag NPs migrate to the surface of nanobelts during their growth, which eventually forms the CuPA-Ag composite NBs. EDS mapping (Figure e–g) further confirmed the composite structure according to the overlap nanobelt-like distribution of C, Cu, and Ag signals.

In addition to the morphological characterization, the compositional and structural changes of the electrode were also verified by the XRD patterns, the FTIR spectrum, the XPS spectra, and the AES spectra of the electrodes during the synthesis (Figures h–j and ). For the step of ion sputtering, both the Cu and Ag/Cu electrodes only showed characteristic diffraction peaks of Cu in their XRD patterns. No Ag peaks were observed for the Ag/Cu electrode because of the low amount and small size of Ag NPs on the Cu foil. However, the appearance of the Ag3d XPS signal did indicate Ag deposition on the Cu surface. The Ag deposition enhanced the adsorption of CO2 from the air, as proven by an emerging C1s peak at 289 eV. According to the Cu2p XPS and Cu LMM AES spectra, it also increased the surface Cu2+/Cu0 ratio due to the larger negativity of Ag compared to Cu.

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(a) Stabilization of Cu+ by the σ-donation and π-backdonation interactions between the phenylacetylene ligand and Cu+; (b) C1s, (c) Ag3d, (d) Cu2p XPS spectra, and (e) Cu LMM AES spectra for Cu, Ag/Cu, and (CuPA-Ag)/Cu electrodes.

For the step of nanobelt growth, three peaks at 6.26°, 17.66°, and 24.66° in the XRD pattern of (CuPA-Ag)/Cu were well consistent with the standard peaks of CuPA crystals. Meanwhile, the C1s XPS peaks of the (CuPA-Ag)/Cu at 284.8, 286.3, and 288.2 eV could be attributed to the sp2-hybridized aromatic C–C bond and adventitious carbon (C–H) from ambient air, the CC–Cu bond, and the O–CO bond, respectively. Both the Cu2p XPS and Cu-LMM AES spectra show the characteristic peak of Cu+, which matches the Cu valence state in CuPA. Furthermore, in the FTIR spectrum of (CuPA-Ag)/Cu, multiple peaks around 1500 cm–1, 750 cm–1, and 680 cm–1 could be attributed to the vibration of the aromatic ring, and the peak around 515 cm–1 could be attributed to the stretching of the alkyne bond. All these spectral changes directly proved the formation of CuPA NBs.

The as-prepared (CuPA-Ag)/Cu electrode had stable Cu+ active sites, which were the key to its high performance in the eCO2RR. Literature reports suggested that the copper atom existed in the form of Cu+ in CuPA, which is stabilized by strong σ-donation and π-backdonation interactions (Figure a) between the phenylacetylene ligand and Cu+. Specifically, the benzene ring and alkynyl group donate electrons to the empty s orbital of Cu+ to form π-s σ bonds, while the d orbitals of Cu+ back-donate electrons to the π antibonding orbitals of the alkynyl to form a d-p π-back-bond. Therefore, the XRD pattern, which confirms the formation of CuPA from the aspect of crystal structure, is a strong support for the presence of Cu+, as it is an inevitable consequence of the coordination of PA to Cu. Second, the presence of Cu+ can also be supported by the change of Cu 2p XPS spectra before and after the production of CuPA NBs (Figure d). As shown in these spectra, the copper species for Cu and Ag/Cu electrodes were mainly attributed to Cu0 and Cu2+, where Cu2+ originated from the surface oxidation with different degrees. However, the XPS peak of Cu0 at 932.9 eV shifted to a higher binding energy at 933.2 eV after the growth of CuPA NBs, which corresponded to the characteristic peak of Cu+. Third, the presence of Cu+ is confirmed by the change of Cu-LMM AES spectra (Figures e and S4). According to the literature reports, the auger electron kinetic energy of the Cu and Ag/Cu electrode reveals the coexistence of Cu0 and Cu2+, while that of the (CuPA-Ag)/Cu electrode clearly indicates the presence of Cu+ and the drastic decrease of Cu0 and Cu2+ signals. All the above results proved that the stable coordination structure effectively suppresses the conversion from Cu+ to Cu0 or Cu2+, which provides stable active sites for CO2 reduction.

The CuPA loading on the electrode surface could be controlled by the PA concentration and reaction temperature during the growth of CuPA NBs (Figure ). Here, different (CuPA-Ag)/Cu electrodes were prepared with PA concentrations of 10%, 20%, 30%, 40%, and 50% at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C, respectively. The CuPA loading was characterized by the XRD peak intensities of CuPA, the extent of color change from red-brown (Cu) to yellowish-green (CuPA), and the mass increase. Control experiments suggested that the CuPA loading at the same temperature increases with the increase of PA concentration, as the growth of CuPA was enhanced by high-concentration precursors. As the XRD peak intensities enhanced with the PA concentration, the electrode surface gradually exhibits the color of CuPA, and the total mass of the electrode increases accordingly. The weight analysis indicated that the influence of PA concentration is particularly evident at temperatures above 60 °C. Similarly, the CuPA loading at the same PA concentration increases with the increase of reaction temperature, as the in situ growth of CuPA NBs was also promoted by a higher temperature. One can find similar evolution in XRD intensities, color change, and mass increase of the electrodes. The weight analysis indicated that the effect of reaction temperature becomes evident at temperatures above 40 °C and high PA concentrations. In a brief word, the CuPA loading monotonically increased with the PA concentration and the reaction temperature.

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(a-d) XRD patterns, (e) digital photos, and (f) the mass of deposited CuPA NBs for the (CuPA-Ag)/Cu electrodes prepared at 20 °C, 40 °C, 60 °C, and 80 °C with a PA concentration ranging from 10% to 50% in its ACN solution.

Based on the control of CuPA loading, we tried to determine the optimal composition of the (CuPA-Ag)/Cu electrode and its synthetic conditions according to the eCO2RR results with these electrodes. Here, the eCO2RR was performed at a potential of −1.3 V vs RHE after control experiments in an H-type electrolytic cell with CO2-saturated 0.1 M KHCO3 as an electrolyte (Figures S5 and S6). Then, the Faraday efficiencies of C2H4 (FEC2H4 ) and multicarbon products (FEC2+ ) were used as the major criteria to evaluate the performance of the electrode (Figure ). For the electrodes synthesized at the same temperature but different PA concentrations, their catalytic performance gradually rose as PA Conc. increased from 10% to 40%, and then it declined as PA Conc. further increased to 50%. For the electrodes synthesized at the same PA concentration but different temperatures, their catalytic performance rose from 20 to 40 °C and declined from 40 to 80 °C in most cases. The volcano distribution of FEC2H4 and FEC2+ with synthetic conditions suggested that the catalytic performance was not solely determined by CuPA loading. The optimal (CuPA-Ag)/Cu electrode was prepared at 40 °C with a PA Conc. of 40%, where the loading of CuPA-Ag NBs was 0.167 mg/cm2, and the mass ratio of CuPA-Ag was 50:3. Using this electrode, the FEC2H4 and FEC2+ of the eCO2RR reaches 44.5% and 56.5%, and the selectivity of the H2 was inhibited below 17.7%. The electrode with a moderate loading of CuPA shows the best catalytic performance because CuPA loading had multiple influences on the physicochemical properties of the electrode. On one hand, an increasing loading of CuPA provided a greater number of Cu+ active sites to enhance the adsorption of CO2 and conversion of various intermediates. On the other hand, excessive CuPA loading might block the active sites of previously deposited CuPA NBs and cause low efficiency of electron transportation and mass transfer, which conversely reduces the Faradaic efficiencies.

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Product distributions of the eCO2RR evaluated by the Faraday efficiencies of each product for the (CuPA-Ag)/Cu electrodes prepared at (a) 20 °C, (b) 40 °C, (c) 60 °C, and (d) 80 °C with PA concentration ranging from 10% to 50% in its ACN solution.

We are particularly interested in the superior catalytic performance of (CuPA-Ag)/Cu electrode in the eCO2RR. In the following discussions, we tried to find the reason through the analysis of catalytic activity/stability and different physicochemical properties for the Cu, Ag/Cu, CuPA/Cu, and (CuPA-Ag)/Cu electrodes. The synthesis of the four electrodes was introduced in the experimental section, where the CuPA NBs were grown at 40 °C with a PA concentration of 40%. Similar to the above experiments, the catalytic activity and stability were evaluated by the Faraday efficiencies and product distribution of the eCO2RR performed at −1.3 V vs RHE in a H-type electrolytic cell with CO2-saturated 0.1 M KHCO3 as an electrolyte (Figure a,b). The catalytic results suggested that a cleaned and polished Cu electrode yields C2+, C2H4 products, and H2 with FEs of 17.2%, 11.5%, and 33.3%, respectively. The ion sputtering of Ag NPs on the Cu electrode increased the FEC2+ and FEC2H4 to 39.6% and 26.2%, while decreasing the FEH2 to 15.3%. On the other hand, the growth of CuPA NBs on the Cu electrode significantly increased the FEC2+ and FEC2H4 to 54.2% and 40.1%, while decreasing the FEH2 to 25.0%. These experiments suggested that the introduction of Ag and CuPA both promoted the multicarbon products and suppressed the H2 evolution. Relatively, Ag plays a dominant role in the inhibition of H2, and CuPA enhanced the multicarbon production. When Ag and CuPA were both deposited, the (CuPA-Ag)/Cu electrode showed the best catalytic performance, which increased the FEC2+ and FEC2H4 to 56.5% and 44.5%, while decreasing the FEH2 to 17.7%. Although the Ag content did not promote the multicarbon products compared to the CuPA/Cu electrode, it did inhibit H2 production and improve the total selectivity of all carbon products.

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(a) H-type electrolytic cell equipped with Cu, Ag/Cu, CuPA/Cu, and (CuPA-Ag)/Cu electrodes as working electrodes for eCO2RR; (b) activity of the electrodes evaluated by the Faraday efficiencies of each product; and (c) stability of the electrodes evaluated by the time evolution of current density and FE of C2H4.

In addition to the activity and selectivity for carbon products, the (CuPA-Ag)/Cu electrode also showed better stability compared to the other three electrodes. During the potentiostatic eCO2RR, FEC2H4 and current density (j) for the (CuPA-Ag)/Cu electrode stabilized at averagely 40% and −15 mA·cm–2 in the first 5 h. In contrast, the FEC2H4 and j decreased earlier for the CuPA/Cu, Ag/Cu, and Cu electrodes (Figure c). Here, the activity decay arises from the CO2 mass-transport limitation in a H-cell but not the catalyst degradation. During long-term electrolysis, massive H2, CH4, and C2H4 bubbles generate, accumulate, and tightly attach to the porous CuPA nanobelt surface, forming a dense gas film that isolates the catalyst from the electrolyte. Such a limitation in mass transportation leads to severe local CO2 depletion at the electrode–electrolyte interface even when the CO2 gas is continuously bubbled, which caused the decay of FE and j in eCO2RR. While timely reset of the H-cell removes accumulated bubbles and restores an adequate local CO2 concentration for the reaction. Therefore, the electrolytic cell was periodically reset to replenish the carbon source, which indeed extended a stable FEC2H4 above 40% and j above −20 mA·cm–2 for 11 h (Figure a). The SEM image and EDS mapping of the (CuPA-Ag)/Cu electrode after catalysis suggested that the nanobelt morphology was preserved and Ag NPs were well dispersed on the nanobelts (Figure S7). Cu-LMM AES spectra showed the same Cu+ peak around 915 eV, which proved the chemical stability of the Cu+ active sites during long-term electrolysis (Figure b). Raman spectra showed a slight weakening but stabilized vibration peaks of the CC bond, Cu–C bond, and benzene ring-related bonds during the eCO2RR (Figure c). All the above characterizations suggested that the (CuPA-Ag)/Cu electrode is stable in the catalytic reaction. A flow cell design may overcome the bottleneck of CO2 supply and render the electrode a longer working period in practical applications.

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(a) Time evolution of current density and FE of C2H4. In an eCO2RR catalyzed by the (CuPA-Ag)/Cu electrode, where the electrolytic cell was periodically reset to achieve the saturation absorption of CO2 in solution; (b) Cu-LMM AES spectra for the (CuPA-Ag)/Cu electrode before and after the reaction; and (c) Raman spectra of the (CuPA-Ag)/Cu electrode at different times during the eCO2RR.

Electrochemical characterizations further confirmed the superior activity of the (CuPA-Ag)/Cu electrode in electrochemical processes. All the characterizations were performed with the electrodes placed in a CO2-saturated KHCO3 solution with a concentration of 0.1 M. The LSV curves (Figure a) showed that the onset potential of eCO2RR for the (CuPA-Ag)/Cu electrode exhibited a significant positive shift compared to the other electrodes, and its current density at the same potential is higher than that of the Cu and CuPA/Cu electrodes, both of which indicated its superior electrochemical activity and reaction kinetics. The Tafel slope (Figures b and S8) of the (CuPA-Ag)/Cu electrode was measured to be 311 mV·dec–1, which was lower than those of the Cu (397 mV·dec–1), the Ag/Cu (326 mV·dec–1), and the CuPA/Cu electrodes (333 mV·dec–1). This usually suggests a lower energy barrier for the rate-determining step and faster reaction kinetics in eCO2RR. The EIS Nyquist plots (Figure c) revealed that the charge-transfer resistance (R ct) of the Ag/Cu (105 Ω) and the (CuPA-Ag)/Cu (179 Ω) electrodes was smaller than that of the Cu (129 Ω) and CuPA/Cu (223 Ω) electrodes, respectively. It suggested that the introduction of Ag effectively improved the electron transfer within the electrodes. Finally, ECSA measurements (Figures d and S9) were performed to evaluate the double-layer capacitance (C dl) and quantify the number of active sites. The C dl of the (CuPA-Ag)/Cu electrode (330 μF·cm–2) is slightly lower than that of the Ag/Cu electrode (386 μF·cm–2) but significantly higher than those of the Cu (192 μF·cm–2) and CuPA/Cu (159 μF·cm–2) electrodes. It suggests that the sputtering of Ag NPs introduces more active sites to the electrode, and the (CuPA-Ag)/Cu electrode retains most of these active sites as the Ag NPs migrate to the surface of CuPA NBs during their in situ growth.

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(a) LSV curve in a CO2 atmosphere, (b) Tafel slopes, (c) EIS curves, and (d) ECSA curves and C dl for the Cu, Ag/Cu, CuPA/Cu, and (CuPA-Ag)/Cu electrodes.

The superior catalytic performance of the (CuPA-Ag)/Cu composite electrode first originated from the catalytically active one-dimensional nanobelt structures in situ grown from the Cu substrate. On one hand, the grown CuPA-Ag NBs are stacked on the Cu substrate to form a porous but conductive fibrous film with a high surface area. Such a nanostructure exposed a large number of Ag and Cu+ active sites, which was proven by the large ESCA and C dl for (CuPA-Ag)/Cu. On the other hand, the CuPA-Ag NBs were grown from and firmly attached to the Cu substrate, which reduced the interfacial resistance between the collector and the active materials. The Ag NPs grafted on the surface of the nanobelt further helped the electron transfer from the current collector to the catalytic sites along the less-conductive CuPA NB. Both of the above influences accelerated electron transfer and reaction kinetics, as proven by the large current density and low Tafel slope of the (CuPA-Ag)/Cu electrode.

Second, the superior performance of (CuPA-Ag)/Cu might be attributed to the synergistic effect between CuPA NBs and Ag NPs, which promoted the CO2 absorption and C–C coupling to form multicarbon (C2+) products. Here, the Ag NPs on the surface of CuPA NB first enhanced the absorption of CO2, which could be supported by the appearance of the O–CO signal in the C1s XPS spectra of Ag/Cu and (CuPA-Ag)/Cu, and the larger current density for electrodes containing Ag NPs in their LSV curves measured in CO2. The Ag NPs also suppressed the H+ adsorption and reduced H2 generation, which could be proved by the smaller current density for electrodes containing Ag NPs in their LSV curves measured in Ar. Since the *CO intermediates transformed from CO2 were weakly adsorbed on Ag, they quickly desorbed and formed a high local concentration of *CO at the Ag/Cu interfaces. According to the σ-donation and π-backdonation interactions between the phenylacetylene ligand and Cu+ and the clear Cu+ signal in the Cu2p XPS spectrum of (CuPA-Ag)/Cu, we believe the enriched *CO had great chances to be adsorbed by the stable Cu+ active sites and coupled to form C2+ products. In a word, the synergistic effect of Ag and Cu+ was critical to the selective formation of multicarbon products.

Third, the superior performance of (CuPA-Ag)/Cu was related to the hydrophobicity of the CuPA, which inhibited the hydrogen evolution reaction (HER) and promoted the reduction of CO2. Due to the hydrophobic property of the PA ligand, the growth of CuPA NBs significantly enhanced the surface hydrophobicity of the electrode. This could be verified by the measurements of water contact angles (CA) on four electrodes, where the CAs of (CuPA-Ag)/Cu (145°) and CuPA/Cu (150°) were much larger than those of Ag/Cu (45°) and CuPA/Cu (80°) (Figure S10). When the electrochemical activity for these four electrodes was examined in an Ar-saturated electrolyte, their LSV curves indicated that the current density for (CuPA-Ag)/Cu and CuPA/Cu electrodes was much lower than that of Ag/Cu and Cu electrodes, respectively (Figure S11). Since no CO2 was introduced to the electrolyte, the current density here could be used to indicate the HER activity. Combined with the contact angle measurement, it was clear that the hydrophobic CuPA NBs inhibited the adsorption of water molecules on the electrode surface and thereby suppressed HER and promoted the eCO2RR. − Furthermore, the hydrophobic CuPA NBs also acted as a protective layer, which prevented the erosion of Cu+ by water and the peeling of Ag NPs. −

To determine the reaction pathway for eCO2RR, the in situ FTIR spectra of the carbon paper-supported CuPA-Ag NBs were measured under increasing potentials in Ar and CO2 atmospheres, respectively (Figure S12). In the Ar atmosphere, the IR spectra did not show any characteristic peaks of the carbon species (Figure a). However, in the CO2 atmosphere, the IR spectra exhibited a series of absorption peaks around 2110 cm–1, 1590 cm–1, 1440 cm–1, 1290 cm–1, and 1130 cm–1, which were attributed to the absorption of *CO, *OCCO, *CHO, *OCCOH, and *COH, respectively. , (Figure b) The stable *CO peak from the open circuit potential (OCP) to −1.6 V vs RHE indicated the formation of this key intermediate in eCO2RR. Strong absorption peaks of *OCCO and *OCCOH suggested the presence of C2 intermediates and the effective C–C coupling on the CuPA-Ag NBs. Meanwhile, the weak absorption peaks of *CO, *CHO, and *COH might suggest that the *CO was quickly converted to the C2 intermediate, which significantly inhibits the formation of C1 intermediates and products.

8.

8

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In situ FTIR spectra of the (CuPA-Ag)/Cu electrode under increasing potentials in (a) Ar and (b) CO2 atmospheres.

Based on the above analysis, we proposed the following eCO2RR pathway to selectively produce multicarbon products on the (CuPA-Ag)/Cu electrode. , (Figure ) First of all, a large amount of CO2 molecules was adsorbed on the electrode due to its porous nanostructure and large surface area, low energy barrier for CO2 adsorption by Ag, , and inhibition of competitive water adsorption by the hydrophobic surface. The Cu+ active sites stabilized by the PA ligand and neighboring Ag (with higher negativity) interacted with the lone-pair electrons of CO2, which further strengthens the adsorption of CO2. The adsorbed CO2 gained one electron from the electrode to form the negatively charged *CO2 – intermediate, which combined with protons to form the *COOH intermediate. Then, the *COOH further combined with an electron and a proton to form the *CO intermediate after the leaving of one molecule of water. At the most critical moment, two *CO adsorbed on the neighboring Cu+ site coupled to form the key *CO–COH intermediate through the addition of another electron and proton from the electrode and solution. Through further combination with electrons and protons, the *CO–COH eventually turned to ethanol, ethylene, and other multicarbon products.

9.

9

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Reaction mechanism for the electrochemical reduction of CO2 to form C2+ products on the (CuPA-Ag)/Cu electrode.

The Cu+ active sites were critical to multistep hydrogenation in the successive reduction of CO2 to C2 products. The formation of C1 product was suppressed probably because the fast coupling of *CO intermediates on Cu+ inhibited the hydrogenation on the single *CO to form the *CHO intermediate and the final CH4 molecules. On the pathway from the *CO–COH intermediate to various C2 products, the hydrogenation does not stop at earlier stages to form liquid C2 (ethanol and acetic acid) also because the Cu+ sites are highly active to drive fast hydrogenation. Furthermore, the produced liquid C2 is difficult to detach from the electrode compared to the gaseous ethylene, which promotes its further reduction by protons. Ethane is not found in the products probably because of the inadequate reduction power and kinetics in the current reaction conditions, so that ethylene becomes the main C2 product.

4. Conclusion

In summary, a (CuPA-Ag)/Cu electrode for eCO2RR was prepared by the ion sputtering of Ag NPs on polished copper foil, followed by the in situ growth of Ag NP-grafted CuPA NBs on its surface. The synthetic route as well as the intermediate/final products were confirmed by the SEM, TEM, EDS, XRD, FTIR, and XPS characterizations. Such a synthetic route was particularly suitable to prepare a Cu-based catalytic electrode with fast electron transfer from the current collector and firmly attached active nanocatalyst structures. It might be extended to the synthesis of similar copper-organic composites with Au, Pd, and Cu as a metal modifier and phthalic acid or pyridine carboxylic acid as an organic ligand. The CuPA loading on the electrode was controlled by the PA concentration and reaction temperature during the growth of CuPA NBs. Combined with the catalytic results, the optimal (CuPA-Ag)/Cu electrode was prepared at 40 °C with a PA Conc. of 40%, where the loading of CuPA-Ag NBs was 0.167 mg/cm2 and the mass ratio of CuPA-Ag was 50:3. Using this electrode in a H-type electrolytic cell, the eCO2RR achieved an FE of 44.5% for C2H4 and 56.5% for C2+ products, and the selectivity of the H2 was inhibited below 17.7%. With a periodic reset of the H-cell to replenish the carbon source, the eCO2RR also maintained an FEC2H4 above 40% for 11 h.

Compared to the Cu, Ag/Cu, and CuPA/Cu electrodes, the (CuPA-Ag)/Cu electrode possessed improved activity and stability, which was confirmed by various electrochemical properties, such as onset potential, current density, Tafel slope, R ct, and ECSA. The superior catalytic performance of the (CuPA-Ag)/Cu electrode was believed to originate from the in situ growing nanobelt structures with a large surface area and enhanced electron transfer, the synergistic effect between Ag and Cu+ to promote the CO2 absorption and C–C coupling, and the hydrophobicity of CuPA to inhibit the hydrogen evolution reaction. Eventually, a reaction pathway was proposed for the electrochemical reduction of CO2, where a large number of stable Cu+ sites on the (CuPA-Ag)/Cu electrode were the key to promoting the C2+ products.

Supplementary Material

ng6c00082_si_001.pdf (990.2KB, pdf)

Acknowledgments

J.G. thanks the funds from the National Natural Science Foundation of China (22472061, 22172054). J.T. thanks the funds from the Natural Science Foundation of Shanghai (24ZR1419100).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.6c00082.

  • SEM of Cu and Ag/Cu electrodes, SEM and EDS of CuPA/Cu electrode, comparison of CuPA growth on Cu and Ag/Cu, optimal working potential for the (CuPA-Ag)/Cu electrode, H cell for eCO2RR, Tafel curves, CV curves, water contact angles, and LSV­(Ar) curves for Cu, Ag/Cu, CuPA/Cu, and (CuPA-Ag)/Cu electrodes, and device for in situ FTIR spectra (PDF)

Shijin Yang: Data curation, Methodology, Investigation, Visualization, Validation, Formal analysis, Writingoriginal draft; Qi Fang: Data curation, Formal analysis; Jing Tang: Funding acquisition, Supervision; Jianping Ge: Funding acquisition, Project administration, Supervision, Writingreview and editing. The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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Supplementary Materials

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