Alignment of active sites on Ag–Ni catalysts for highly selective CO₂ reduction to CO

Abstract

Arranging the active ingredients on the support surface at the molecular or atomic scale to create high-selectivity sites for the electrocatalytic CO₂ reduction reaction (CO₂RR) holds great promise, which is highly challenging. In this study, we report a strategy for constructing a bimetallic Ag–Ni electrocatalyst with Ni nanoparticles for H₂O dissociation, as well as Ni atoms in the carbon skeleton and Ag nanoparticles on the surface for the CO₂RR. These sites synergistically enable high selectivity for improving the conversion of CO₂ to CO. Specifically, the FE_CO reaches 99.3% on Ag–Ni/CB at −0.8 V vs. RHE, and its FE_CO can be maintained over 95.8% in the potential range from −0.8 to −1.1 V vs. RHE. Our research presents an approach for alignment of bifunctional two-component active sites on the surface of a carbon support.

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1. Introduction

The electrocatalytic CO₂ reduction reaction (CO₂RR) paves a promising avenue for converting anthropogenic CO₂ into value-added fuels/chemicals and storing intermittent solar and wind energy in the form of chemical bonds.^1–5 The complex mechanisms of the CO₂RR involve multiple proton and electron transfer processes,^6–10 providing a wide variety of chemicals, such as CO, hydrocarbons and oxygenates.^11–15 The CO production from the CO₂RR has been proven to be particularly appealing through a two-electron transfer process (CO₂ + 2H⁺ + 2e⁻ → CO + H₂O) since CO can be utilized as a feedstock for the Fischer–Tropsch reactions.^16,17 Nevertheless, high-selectivity CO₂ reduction towards CO with a high faradaic efficiency (above 95%) is a highly challenging task owing to the subtle differences in reaction potentials in terms of different reduction products.¹⁸

To circumvent this challenge, the development of effective electrocatalysts is necessary for industrial purposes.^19–22 To date, metallic Ag and Ni–nitrogen-doped carbon materials are the most potential candidates among the reported catalysts for electrochemical CO₂-to-CO conversion in view of their superior catalytic performance.^23–30 For instance, an Ag catalyst with rich defects presents a nearly 100% faradaic efficiency of CO and excellent stability for 120 h, attributable to the enhancement of intermediate COOH adsorption and the optimization of reaction routes.³¹ Ag nanoparticles supported on ultrathin CoAl-layered double hydroxide nanosheets display a CO selectivity of 94.5% under light irradiation.³² Moreover, hydrophobic Ag₂₅ nanoclusters with an organic shell exhibit improved CO₂RR activity in H-type cell and membrane electrode equipment,³³ and a triazole modified Ag crystal breaks the adsorbate linear scaling relationship to achieve a 98% faradaic efficiency for CO production.³⁴ Additionally, Ni–N–C catalysts with NiN_x sites intrinsically enhance the catalytic performance for CO₂-to-CO with an industrial current density of 726 mA cm⁻² in a flow cell.¹² More interestingly, single atom Ni on N-doped carbon with weak cation sensitivity can directly convert the captured CO₂ in a membrane electrode device.³⁵ Furthermore, Ag and Ni are combined to improve highly efficient CO₂-to-CO conversion. The synergistic effects of the coordinated Ni–Ag pairs can decrease the formation energy barrier of the crucial *COOH intermediate, thereby achieving high CO selectivity.³⁶

Regarding the intrinsic activity of metallic Ag and Ni–nitrogen-doped carbon materials for CO₂-to-CO, herein, we constructed a bimetallic Ag–Ni/CB electrocatalyst aiming to improve the conversion of CO₂ to CO. This architecture creates synergistically catalytic sites for the CO₂RR. The FE_CO reaches 99.3% on the Ag–Ni/CB catalyst at −0.8 V vs. RHE. Also, its FE_CO can be maintained over 95.8% in the potential range from −0.8 to −1.1 V vs. RHE. In situ ATR-SEIRAS demonstrates that the dissociation of H₂O on the surface of the Ag–Ni/CB catalyst leads to the formation of adsorbed hydrogen, which is crucial for the production of the key intermediate *COOH. The Ni atoms within the carbon skeleton and the Ag nanoparticles on the surface act as two types of active sites for the CO₂RR to CO.

2. Experimental section

2.1. Chemicals and materials

Silver nitrate (AgNO₃, AR), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, AR), nitric acid (HNO₃, 68.0%) and absolute ethanol (C₂H₅OH, 99.7%) were purchased from Tianjin Fengchuan Scientific Co., Ltd. 2-Methylimidazole (2-MeIm) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Carbon black (CB) was acquired from Rhawn Reagent of Shanghai Yien Chemical Technology Co., Ltd. Potassium bicarbonate (KHCO₃, 99.5%) was purchased from Xinbote Chemical Co., Ltd. High purity CO₂ (99.999%) and N₂ (99.999%) were obtained from Beijing Beiyang Special Gas Institute. All chemicals and materials in this work were used as received without further purification.

2.2. Catalyst preparation

A combination of wetness impregnation and acid leaching was utilized to prepare the Ag–Ni/CB catalyst with slight modifications to a previous report.³⁷ In a typical synthesis, two precursors, namely, 0.17 g of AgNO₃ and 0.29 g of Ni(NO₃)₂·6H₂O were added into 15 mL of deionized water and ethanol mixture containing 0.10 g of carbon black, followed by ultrasonic treatment for 20 min. 1.64 g of 2-MeIm was dissolved in 15 mL of deionized water and stirred for 5 min. Thereafter, the aqueous solution of 2-MeIm was added dropwise into the above mixture, and then vigorously stirred for 4 h at room temperature. The suspension was collected by centrifugation and rinsed three times with deionized water. Subsequently, the precipitation was dried at 80 °C overnight. The obtained solid was pyrolyzed under a flowing nitrogen atmosphere in a tube furnace, which was heated from room temperature to 700 °C with a ramping rate of 5 °C min⁻¹ and maintained at 700 °C for 2 h. After naturally cooling down to room temperature, the solid powder was immersed in 0.1 M HNO₃ and stirred at 60 °C for 2 h, and then thoroughly washed with deionized water. Finally, the resulting product was dried at 80 °C overnight in an oven to obtain the Ag–Ni/CB catalyst. In addition, Ag/CB and Ni/CB as two comparison samples were synthesized through the same synthetic route. Moreover, the acid leaching solution was modified when preparing the Ni/CB, Ag/CB and Ag–Ni/CB catalysts for a comparative study.

2.3. Catalyst characterization methods

The contents of Ni and Ag were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5110). The morphology, microstructure and elemental distribution of the as-prepared catalysts were acquired using a field emission scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, FEI Tecnai G2 F20) equipped with a scanning transmission electron microscopy energy-dispersive X-ray elemental mapping analysis system (STEM-EDS). The crystalline structure was obtained by X-ray diffraction (XRD) on a PANalytical X'pert PRO diffractometer with Cu Kα radiation (λ = 1.5406 Å). The supplemental structural information was gained through a HORIBA LabRAM HR Evolution spectrometer at an excitation wavelength of 514 nm. The textural properties of the catalysts were measured on a Micromeritics ASAP 2020 adsorption apparatus. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were applied to determine the specific surface areas and the distribution of pore sizes, respectively. The chemical compositions and states were analyzed via X-ray photoelectron spectroscopy (XPS) on a Thermo ESCALAB 250XI instrument with monochromatic Al Kα (hv = 1486.6 eV). The reaction intermediates on the catalyst surface under open circuit potential and reactivity potentials were detected through in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) on a Nicolet iS50 spectrometer (Thermo Scientific).

2.4. Electrochemical performance measurements

The electrochemical CO₂ reduction experiments were conducted on a three-electrode electrochemical system (CHI 660E, CH Instruments, China). A gas-tight H-type reactor was segregated into the anode and cathode compartments with a Nafion 117 proton-exchange membrane (DuPont, USA). The Ag/AgCl and Pt foil electrodes served as the reference and counter electrodes, respectively, and the electrolyte was 0.1 M KHCO₃ solution. To prepare the working electrode, 5 mg of catalyst was dispersed in 40 μL of 5 wt% Nafion solution and 960 μL of isopropanol. After being sonicated for 30 min, the catalyst ink was uniformly diffused onto the carbon paper surface (1.0 cm × 1.0 cm). High purity CO₂ was fully bubbled in the electrolyte at a flow rate of 20 mL min⁻¹ during the experiments. All potentials were recalculated into the reversible hydrogen electrode (RHE) using eqn (1).²³

V(vs. RHE) = V(vs. Ag/AgCl) + 0.197 + 0.059 × pH (1)

The gaseous products from CO₂ electrolysis were monitored using an online gas chromatograph (GC2014, Shimadzu) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The liquid products were collected and quantified by nuclear magnetic resonance (¹H NMR). The faradaic efficiency (FE) of the gas products was calculated according to eqn (2).^38,39

FE% = (αnF × 100%)/Q (2)

where α is the transferred electron number for the product (α = 2 for CO and H₂), n is the amount of the target product, F is the Faraday constant (96 485 C mol⁻¹), and Q is the total charge consumed amount during the electrolysis process.

3. Results

3.1. Structural properties of the as-synthesized catalysts

The fabrication process of the Ag–Ni/CB catalyst is illustrated in Fig. 1a. Briefly, the Ag–Ni/CB catalyst was synthesized by wetness impregnation combined with acid leaching methods. The acid leaching was utilized to remove the unstable active species on the catalysts prior to the electrochemical performance measurements. The morphological features of carbon black, Ag/CB, Ni/CB and Ag–Ni/CB were characterized by the SEM technique (Fig. S1†). The purchased carbon black exhibits a granular morphology structure with a diameter of 50–100 nm. After loading the active components, the Ag/CB, Ni/CB and Ag–Ni/CB catalysts have a similar morphology to the carbon black. The main difference with the original support resides in the increased roughness and particle sizes. The distribution of active ingredients was observed in the TEM images of the Ag–Ni/CB catalyst (Fig. 1b–d). The average particle sizes are approximately 11.0 ± 1.2 nm (Fig. 1b). The lattice fringes with interplanar spacings of 0.24 and 0.21 nm indicate that the exposed facets on Ag–Ni/CB are Ag (111) and Ni (111) in the HRTEM image (Fig. 1e),^40,41 respectively, confirming the existence of Ag and Ni nanoparticles, which is further verified by the XRD results, vide infra. Additionally, the contact interface between Ag and Ni is identified in Fig. 1e, signifying the presence of interaction between the two metals in the Ag–Ni/CB catalyst. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. 1f), combined with the EDS elemental mapping of the Ag–Ni/CB catalyst (Fig. 1g–j), reveals that the Ni element is present in two forms through a combination method of wetness impregnation and acid leaching, including the Ni nanoparticles on the surface and the highly dispersed Ni atoms in the carbon matrix.

Fig. 1 (a) Schematic illustration of the synthesis process of the Ag–Ni/CB catalyst. (b–e) TEM and HRTEM images at different magnifications of Ag–Ni/CB and the corresponding particle size distribution histogram. (f) HAADF-STEM and (g–j) EDS elemental mapping images of Ag–Ni/CB.

Powder X-ray diffraction patterns were collected to study the crystalline structure of the samples. For the CB support, the broad diffraction peaks at 25° and 43° can be observed in Fig. 2a, corresponding to the representative (002) and (100) planes of graphite carbon with low crystallization, respectively.^42–44 In addition, the diffraction peaks of Ni/CB at 44.3°, 51.7° and 76.3° are indexed to the (111), (200) and (220) crystal planes of Ni (PDF#04-0850), respectively.⁴¹ The diffraction peaks of Ag/CB at 38.1°, 44.3°, 64.4° and 77.4° are assigned to the (111), (200), (220) and (311) crystal facets of Ag (PDF#04-0783).^40,45,46 In the case of Ag–Ni/CB, both Ni and Ag characteristic peaks are detected in the XRD pattern, while the NiO or Ag₂O phase is absent in the Ni/CB, Ag/CB and Ag–Ni/CB catalysts. As expected, the introduction of Ag and Ni in the Ag–Ni/CB catalyst has almost no effect on the phase structure of graphite carbon.

Fig. 2 (a) XRD patterns and (b) Raman spectra of CB, Ni/CB, Ag/CB and Ag–Ni/CB.

Raman spectroscopy was applied to detect the vibration signal of surface chemical bonds aiming to quantitatively analyze carbon defects of the as-prepared samples. Two peaks of carbon with a prominent disordered D band at 1350 cm⁻¹ and a graphitic G band at 1585 cm⁻¹ are observed in the Raman spectra (Fig. 2b).^47–49 For the Ni/CB, Ag/CB and Ag–Ni/CB catalysts, the I_D/I_G values are much greater than that of the black carbon (1.05), signifying the presence of abundant defects in the carbon skeleton of the as-prepared catalysts.⁵⁰ Note that the Ni/CB and Ag–Ni/CB catalysts exhibit higher I_D/I_G in comparison with the Ag/CB catalyst. This phenomenon implies that a portion of nickel atoms is inserted into the carbon matrix, consistent with the morphological analyses above.

Subsequent N₂ physisorption measurements were conducted to examine the porous properties of the samples. As shown in Fig. S2,† the samples exhibit type IV N₂ adsorption–desorption isotherms in the relative pressure (P/P₀) range of 0.1–1.0, in accordance with the characteristics of a mesopore structure. Additionally, the average pore sizes of Ni/CB and Ag–Ni/CB distinctly decrease after loading the active components (Table S1†). Correspondingly, the BET surface area and pore volume increase for these two catalysts, which is attributed to that the Ni nanoparticles can break the C–C bond on the CB to generate more pores or incorporate into the carbon matrix.⁵¹ The BET surface areas of Ni/CB and Ag–Ni/CB are 133.9 and 103.3 m² g⁻¹, respectively, higher than that of the commercial CB (51.5 m² g⁻¹), while that of Ag/CB is only 17.6 m² g⁻¹. This demonstrates that a higher number of pores are produced during the wetness impregnation and acid leaching process in the Ni/CB and Ag–Ni/CB catalysts. In contrast, the Ni atoms easily migrate into the carbon skeleton to create the pores than the Ag nanoparticles based on the analyses of N₂ adsorption–desorption results, in agreement with the morphology and Raman characterization.

3.2. Elemental compositions and valence states of the catalysts

XPS measurements were conducted to analyze the surface compositions and chemical valence states of the as-synthesized catalysts. The full scan survey spectra of the Ni/CB, Ag/CB and Ag–Ni/CB catalysts in Fig. 3a signify the presence of elemental signals of Ni, O, N, Ag and C. The high-resolution Ag 3d XPS spectrum (Fig. 3b) of Ag/CB is composed of two individual peaks at 374.6 and 368.5 eV, which are assigned to Ag 3d_3/2 and Ag 3d_5/2 of metal Ag, respectively.⁵² In the case of the Ag–Ni/CB catalyst, the binding energy of Ag 3d at 368.5 eV negatively shifts by about 0.1 eV compared to that of Ag/CB. For the Ni 2p XPS spectra, two peaks centered at 872.9 and 855.6 eV for the Ni/CB and Ag–Ni/CB catalysts (Fig. 3c) correspond to Ni 2p_1/2 and Ni 2p_3/2, respectively, suggesting that Ni²⁺ is the main valence state on the catalyst surface (Fig. 3d and Table S2†).⁵³ Note that Ni characteristic peak is observed and no NiO phase is detected in the XRD characterization. The high proportion of Ni²⁺ on the surface of the Ni/CB and Ag–Ni/CB catalysts is attributed to the high valence state of Ni species in the carbon skeleton and the surface oxidation of Ni nanoparticles upon exposure to air. Correspondingly, the Ni 2p peaks of Ag–Ni/CB present a shift to higher binding energy in comparison with those of Ni/CB. The above results confirm the electron transfer from Ni to Ag on the Ag–Ni/CB catalyst owing to the difference in electronegativity between Ag (1.93) and Ni (1.91).^54,55 As such, the presence of Ag regulates the local electron densities of Ni 3d orbitals, and Ni species with high positive charge densities on the surface easily adsorb the reaction intermediates during the CO₂RR.³⁷ As exhibited in Fig. 3d and Table S2,† the Ag–Ni/CB catalyst displayed a similar proportion of Ni²⁺ to the Ni/CB catalyst, suggesting that the incorporation of Ag did not impact the Ni²⁺/Ni⁰ ratio.

Fig. 3 High-resolution XPS spectra of the Ni/CB, Ag/CB and Ag–Ni/CB catalysts. (a) Survey, (b) Ag 3d, (c) Ni 2p, (e) N 1s, and (d) valence ratios of Ni species on the Ag–Ni/CB and Ni/CB surface. (f) The proportion of different N species.

The high-resolution N 1s XPS spectra of the catalysts (Fig. 3e) can be deconvoluted into four components at 401.2, 400.2, 399.4 and 398.5 eV, which are attributed to graphitic N, pyrrolic N, Ni bonded N and pyridinic N, respectively.^53,56 The proportions of Ni bonded N are 42.7% on the Ni/CB catalyst and 45.3% on the Ag–Ni/CB catalyst (Fig. 3f and Table S3†), demonstrating the incorporation of Ni and N elements into the carbon skeleton,⁵⁶ which is consistent with the Raman and XPS measurements. Additionally, the C 1s XPS spectra of the Ni/CB, Ag/CB and Ag–Ni/CB catalysts are divided into three carbon species centered at 289.2, 285.8 and 284.8 eV (Fig. S3†), corresponding to C–O, C–N and C–C/C=C, respectively.^53,57 The presence of C–N corroborates the fact that N species are incorporated into the carbon matrix.⁵⁸

3.3. Electrochemical reduction of CO₂ to CO

The electrochemical CO₂RR performances of the as-synthesized catalysts were evaluated under stirring to minimize mass transfer limitation in a gas-tight H-type cell with CO₂-saturated or Ar-saturated 0.1 M KHCO₃ electrolyte. As exhibited in the linear sweep voltammetry (LSV) curves (Fig. 4a), the Ag–Ni/CB catalyst in CO₂-saturated electrolyte has much larger current density than in Ar-saturated solution, suggesting its activity for the electrocatalytic CO₂RR. Additionally, the Ag–Ni/CB catalyst displays the largest current density in CO₂-saturated electrolyte among the tested catalysts. The gaseous and liquid products from CO₂ electrolysis were detected through online gas chromatography and ¹H NMR spectroscopy, respectively. The results signify that CO is the dominant reduction product (Fig. 4b and c and S4†). Fig. 4b and c show the faradaic efficiency (FE_CO) and partial current density for CO (j_CO) over the catalysts in an applied potential range from −0.7 to −1.2 V vs. RHE. Remarkably, the FE_CO reaches 99.3% on the Ag–Ni/CB catalyst at −0.8 V vs. RHE. Moreover, its FE_CO can be maintained over 95.8% in the potential range from −0.8 to −1.1 V vs. RHE. Meanwhile, the Ag–Ni/CB catalyst presents the highest j_CO between −0.7 and −1.2 V vs. RHE (Fig. 4c). Note that the FE_CO of Ag–Ni/CB after acid leaching is higher than that of its counterpart before acid treatment (Fig. S5†). When comparing the electrochemical CO₂RR performances of the as-synthesized catalysts using different acid leaching solutions (Fig. S6 and S7†), it is found that the Ag–Ni/CB catalyst under the preparation conditions of 0.1 M HNO₃ acid leaching presents the optimal catalytic performance. Moreover, the electrochemical performance of physically mixed Ag/CB and Ni/CB was measured, and the results show that the FE_CO of Ag–Ni/CB is superior to that of the mixed sample (Fig. S8†), attributable to the interaction between Ag and Ni in the synthesized catalyst.

Fig. 4 Electrochemical CO₂RR performances. (a) LSV curves, (b) faradaic efficiencies of CO, (c) partial current density of CO (j_CO) and (d) the double-layer capacitance calculated based on the CV curves of Ni/CB, Ag/CB and Ag–Ni/CB in CO₂-saturated 0.1 M KHCO₃ electrolyte.

To probe the intrinsic activity of the electrocatalytic CO₂ reduction over the Ag/CB, Ni/CB and Ag–Ni/CB catalysts, the electrochemically active surface areas (ECSAs) were analyzed from the double-layer capacitance (C_dl) based on cyclic voltammogram curves at various scan rates (Fig. S9†). As shown in Fig. 4d, the C_dl values of Ag/CB, Ni/CB and Ag–Ni/CB were determined to be 4.0, 18.9 and 30.1 mF cm⁻², respectively. Notably, the C_dl of Ag–Ni/CB is about 8 times as high as that of Ag/CB, manifesting the high intrinsic activity of Ag–Ni/CB among the as-prepared catalysts. Additionally, Tafel slope analysis was performed to investigate the reaction kinetics over the Ag/CB, Ni/CB and Ag–Ni/CB catalysts. The Tafel slope of Ag–Ni/CB (Fig. S10†) is 152 mV dec⁻¹, which is the smallest among the three catalysts, revealing that the construction of Ag–Ni/CB can accelerate the kinetics of CO₂ reduction to CO. Furthermore, the charge-transfer ability of the Ag/CB, Ni/CB and Ag–Ni/CB catalysts was compared by electrochemical impedance spectroscopy (EIS). As exhibited in Fig. 5a, the Ag–Ni/CB catalyst has a smaller semicircle diameter in comparison with the Ag/CB and Ni/CB catalysts, confirming its smallest interfacial resistance and best interfacial charge-transfer properties.⁵⁹

Fig. 5 (a) Nyquist plots of Ni/CB, Ag/CB and Ag–Ni/CB. (b) Stability of the Ag–Ni/CB catalyst at a constant potential of −0.8 V vs. RHE. (c) Comparison of CO₂RR performances of this work with other reported CO₂-to-CO electrocatalysts.

The durability of the optimal Ag–Ni/CB catalyst was measured at a constant potential of −0.8 V vs. RHE during CO₂-to-CO electrocatalysis. Note that the Ag–Ni/CB catalyst exhibits good stability for 11 h with a FE_CO of nearly 99.0% owing to its unique composition and structure, revealing its good CO selectivity in a H-type reactor during the CO₂RR. The electrocatalytic CO₂-to-CO performance of the synthesized Ag–Ni/CB was compared with those of recently reported catalysts. In short, the Ag–Ni/CB catalyst in this work shows comparable catalytic performance in Table S4† and Fig. 5c, demonstrating possible prospects of future applications.

3.4. Intermediate species during CO₂ reduction to CO

To experimentally trace the reaction intermediate species adsorbed on the surface of the Ag–Ni/CB catalyst, in situ ATR-SEIRAS experiments were performed under open circuit potential and reactivity potentials in a CO₂-saturated 0.1 M KHCO₃ solution during the electrochemical reaction. As exhibited in Fig. 6a and b, the broad bands from 3900 to 3300 cm⁻¹ are related to the O–H stretching modes of H₂O,^59,60 and its frequencies depend on the reactivity potentials. Two weaker bands located at 2330 and 2360 cm⁻¹ are ascribed to characteristic peaks of CO₂.⁶¹ Additionally, the feature bands at 1922, 1867 and 1747 cm⁻¹ (Fig. 6c) correspond to the *CO intermediate on the Ag–Ni/CB catalyst, and the signals of *CO gradually enhance as the reactivity potentials increase. The bands at about 1646 and 1560 cm⁻¹ in Fig. 6c and d are assigned to the *COO⁻ asymmetric stretching.⁶² Moreover, the bands at 1395, 1340 and 1172 cm⁻¹ (Fig. 6d) belong to the *COOH intermediate. In analogy to the *CO case, the intensity of the *COOH intermediate increases from −0.6 to −1.2 V vs. RHE, implying that *COOH and *CO are critical intermediates for electrochemical reduction of CO₂ to CO.

Fig. 6 In situ ATR-SEIRAS spectra of the Ag–Ni/CB catalyst under open circuit potential and reactivity potentials during the CO₂RR.

4. Discussion

Altogether, the as-synthesized Ag–Ni/CB catalyst presents excellent CO₂-to-CO catalytic performance for the CO₂RR in a gas-tight H-type cell. The morphological characterization demonstrates that the Ag and Ni nanoparticles are distributed on the surface of carbon black (Fig. 7), and the contact interface between Ag and Ni is identified in Fig. 1e. XRD analyses corroborate the presence of Ni and Ag characteristic peaks in the Ag–Ni/CB catalyst. Raman results reveal that the abundant defects in the carbon skeleton are efficiently created after introducing the Ni active ingredient (Fig. 7), implying that a portion of nickel atoms is inserted into the carbon matrix, consistent with the morphological analyses.

Fig. 7 Schematic illustration of CO₂-to-CO conversion on the Ag–Ni/CB catalyst.

XPS measurements manifest that the valence state of Ag is zero and those of Ni are Ni⁰ and Ni²⁺ on the Ag–Ni/CB catalyst (Fig. 3d and Table S2†). Concurrently, the shifts of binding energy of Ag 3d and Ni 2p peaks prove the electron transfer from Ni to Ag on the Ag–Ni/CB surface due to the difference in electronegativity between Ag and Ni (Fig. 7). The existence of Ni bonded N and C–N in the high-resolution N 1s and C 1s XPS spectra of Ag–Ni/CB corroborates the fact that N species are incorporated into the carbon matrix.

Electrochemical CO₂RR performance tests verify that Ag–Ni/CB is an effective catalyst for CO₂-to-CO conversion. Specifically, the FE_CO reaches 99.3% on the Ag–Ni/CB catalyst at −0.8 V vs. RHE. Also, its FE_CO can be maintained over 95.8% in the potential range from −0.8 to −1.1 V vs. RHE. The ECSA, Tafel slope and EIS electrochemical characterization provide the evidence as to why the Ag–Ni/CB catalyst exhibits superior intrinsic activity in the electrocatalytic CO₂ reduction. The stability and comparable catalytic performance of Ag–Ni/CB with those of recently reported catalysts in Table S4† and Fig. 5c suggest its possible prospects of industrial application for the electrocatalytic reduction of CO₂ to CO.

In situ ATR-SEIRAS studies on the Ag–Ni/CB catalyst reveal the presence of the *CO₂⁻ intermediate from the initial one-electron reduction as well as the critical *COOH and *CO intermediates for electrochemical reduction of CO₂ to CO. Thereinto, the Ni nanoparticles promote the dissociation of H₂O to generate the adsorbed hydrogen for *COOH formation,⁷ while the Ni atoms in the carbon skeleton serve as the active sites for the CO₂RR to CO. Additionally, the Ag nanoparticles also act as the active components for CO₂ to CO conversion. The adjacent Ni nanoparticles offer electrons and the adsorbed hydrogen for the protonation process. That is, the Ni nanoparticles promote the dissociation of H₂O to provide the adsorbed hydrogen for producing the critical intermediate *COOH, and the Ni atoms in the carbon skeleton and the Ag nanoparticles on the surface are two-type active sites over the as-synthesized Ag–Ni/CB for the electrocatalytic reduction of CO₂ to CO.

5. Conclusions

To conclude, we developed a bimetallic Ag–Ni/CB electrocatalyst aiming to improve the conversion of CO₂ to CO. In this regard, the FE_CO reaches 99.3% on the Ag–Ni/CB catalyst at −0.8 V vs. RHE. Also, its FE_CO can be maintained over 95.8% in the potential range from −0.8 to −1.1 V vs. RHE. Combining ex situ characterization and in situ ATR-SEIRAS experiments, it is validated that the Ni nanoparticles promote the dissociation of H₂O to provide the adsorbed hydrogen for producing the critical intermediate *COOH, and the Ni atoms in the carbon skeleton and the Ag nanoparticles on the surface are two-type active sites for the CO₂RR to CO. The stability and comparable catalytic performance of Ag–Ni/CB with those of recently reported catalysts suggest its possible prospects of industrial application. We envisage that this versatile strategy will pave the way for designing efficient CO₂-to-CO catalysts.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Huangdong Wang: conceptualization, data curation, formal analysis, methodology, resources, software, validation, visualization, writing – original draft, writing – review & editing. Zhihua Guo: data curation, formal analysis. Heng Zhang: formal analysis, validation. Lin Jia: formal analysis, writing – review & editing. Min Sun: validation. Lifeng Han: validation. Haorun Li: validation. Yan Guo: funding acquisition, writing – review & editing. Shanghong Zeng: conceptualization, funding acquisition, resources, supervision, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22468034, 21968020 and 22261040) and the Natural Science Foundation of Inner Mongolia (2022MS02011). The authors thank Dr Dan He (Analysis and Testing Center of Large Instrumentation in Inner Mongolia University) for her help with ATR-SEIRAS measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy01149j

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