Ag/Cu foam catalyst for selective reduction of CO₂ to CH₃OH at low potential

Abstract

Electrocatalytic selective reduction of CO₂ to liquid phase products, particularly methanol, is a promising technique for CO₂ utilization. However, the challenge is daunting because the reduction of carbon dioxide to CH₃OH requires six electron transfer reactions and four protons derived from water. In this study, a complex plating method was employed to modify the surface of Cu foam catalyst with Ag. The electrolyte solution used for CO₂ reduction at a low potential (−0.1 V vs. RHE) was 0.1 M Na₂SO₄. This is the minimum voltage reported for methanol production, with a remarkable Faraday efficiency (FE) of 81.33%. The reduction products of CO₂ in this system included CH₃OH, CH₄, CO and H₂, the impact of silver plating duration on the electrochemical performance of Ag/Cu foam catalyst was examined and a reduction pathway of Ag/Cu foam catalyst in selective electrocatalytic CO₂ was proposed.

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1. Introduction

The increasing atmospheric CO₂ levels resulting from fossil fuel combustion have raised significant environmental concerns. Electrochemical reduction of CO₂ (eCO₂R) presents a promising solution to address the challenges of energy and environmental sustainability.¹ However, the eCO₂R process encounters numerous challenges due to the inherent stability of CO₂, leading to sluggish reaction kinetics. Additionally, the low solubility of CO₂ in the electrolyte and competing reactions like the hydrogen evolution reaction (HER) further restrict overall reaction efficiency.² To overcome these obstacles, extensive efforts have been dedicated towards enhancing CO₂ reduction reactivity, reducing overpotential, and enhancing selectivity for desired reduction products. One effective strategy involves harnessing efficient energy utilization processes for capturing, storing or transforming CO₂ into valuable carbon compounds using renewable electricity sources such as solar and wind power along with water as a proton source. This approach enables conversion of CO₂ into a diverse range of carbon compounds including carbon monoxide, formate, methanol, methane, ethanol and ethylene which can also serve as potential energy sources.

Highly active electrocatalysts are crucial during the eCO₂R process. Among the various metal catalysts investigated for CO₂ reduction, Cu has been demonstrated as the most promising candidate.³ Previous studies have demonstrated that Cu is the only transition metal electrocatalyst capable of converting CO₂ to multi-carbon hydrocarbons and oxygenates, albeit with low efficiency and selectivity towards products.^4,5 This can be attributed to its strongest interaction with reactant intermediates during the CO₂ reduction process, which helps capture and stabilize these intermediates while preventing further dissociation or oxidation. Furthermore, Cu possesses an appropriate electronic structure characterized by advantages in band filling and energy level distribution, enabling efficient electron transfer and regulated reactivity.⁶ Additionally, Cu exhibits a high adsorption capacity for both CO₂ and its intermediates on its surface, thereby promoting catalytic activity. Notably, it is worth mentioning that Cu is unique in having negative adsorption energy^7–10 for *CO, while displaying positive adsorption energy for *H. This characteristic facilitates the adsorption of the carbo-oxygen group (*CO) intermediate during eCO₂R due to its proper binding strength with *CO and the carbo-oxygen group (*CHO). As a result of these factors, a prolonged average residence time of *CO on the surface active sites and increased surface coverage are achieved.

Recently, Cu-based bimetallic catalysts have emerged as a promising candidate for enhancing the selectivity of CO₂ electroreduction towards desired products. This can be achieved by precisely regulating the composition, structure, or morphology of these components to further modulate the adsorption behavior of crucial intermediates on the catalyst surface. For example, the Cu–Ag surface exhibits enhanced activity and selectivity for CO, while the compressively strained bimetallic Cu–Ag selectively produces multi/carbon oxygenates. The presence of Ag tends to suppress the competitive hydrogen evolution reaction,¹¹ thereby promoting the formation of hydrocarbon–oxygen products such as alcohols, aldehydes, and acids through the dimerization of *CO followed by its reaction with water molecules. Copper-based materials exhibit selectivity towards hydrocarbon–oxygen products due to their ability to further reduce carbohydrates.¹² Moreover, silver demonstrates low overpotential and high current density, facilitating the selective catalysis of CO₂ to CO which serves as an intermediate for subsequent conversion, thereby enhancing the eCO₂R.

Studies suggest that Ag/Cu bimetallic catalysts exhibit the potential to generate a diverse range of reduction products, including methane, methanol, and formic acid, during CO₂ electrocatalysis.¹³ However, attaining high selectivity remains a formidable challenge, particularly in practical applications necessitating specific products. The performance of Ag/Cu bimetallic catalysts in eCO₂R is still limited by their activity and stability constraints. Despite their favorable electron transfer properties and catalytic activity, long-term durability continues to pose a significant challenge. High current density and prolonged operation time can lead to issues such as deactivation and corrosion, presenting a demanding task for enhancing the stability and durability of Cu–Ag catalysts.

However, simultaneously achieving low potential, high Faraday efficiency, and high selectivity for the electrochemical conversion of CO₂ to methanol remains a formidable challenge. The reduction of CO₂ to hydrocarbons, aldehydes and alcohols exhibits a slightly positive equilibrium potential of reversible hydrogen electrode (RHE), but its direct electrochemical production still necessitates substantial energy input. Moreover, eCO₂R processes require significant overpotential, resulting in operation at higher potential that reflect the minimum energy requirement for a given product. Consequently, these factors pose challenges not only from a technical standpoint but also from an economic perspective as they influence the feasibility and viability of large-scale implementation. In this study, Ag-modified copper foam was employed as a highly efficient electrocatalyst for the reduction of CO₂ to C₁ products. The primary products obtained included CH₄, CO and CH₃OH, exhibiting an impressive Faraday efficiency of 81.33% at a low potential (−0.1 V vs. RHE) on Ag/Cu foam.

2. Experimental

2.1. Catalyst synthesis

The 2 mm thick copper foam purchased from Suzhou Kesen was cut into pieces measuring 1 cm × 1.5 cm. The pretreatment process involved sequential ultrasonic treatment in acetone, 10% hydrochloric acid, anhydrous ethanol, and deionized water for a duration of 15 min each. Subsequently, the samples were placed in a vacuum drying oven for 3 hours before proceeding to composite electroplating. For this purpose, the copper foam was immersed in a mixed solution containing 1 mmol of silver nitrate and 10 mmol of trisodium citrate followed by electroless plating at a temperature of 40 °C under constant stirring at a speed of 200 rpm for a period of 60 min. Finally, the sample underwent vacuum drying at a temperature of 60 °C for 6 h. The control factors were adjusted accordingly for other catalysts used in this study following the aforementioned steps. All chemicals utilized were procured from Chengdu Colon Chemical Co., Ltd., and possessed analytical purity.

2.2. Electrochemical measurements

The CO₂ reduction performance of the Ag/Cu foam working electrode was evaluated using a three-electrode H-type cell in 0.1 M Na₂SO₄ solution, with Pt foil and Ag/AgCl electrode (saturated KCl filling solution) employed as the counter and reference electrodes, respectively. Prior to testing, N₂ or CO₂ gas was continuously introduced into the cathode compartment to create N₂-saturated 0.1 M Na₂SO₄ (pH 8.45) or CO₂-saturated 0.1 M Na₂SO₄ (pH 4.68) solutions. Initially, polarization curves of the Ag/Cu foam electrodes were measured under N₂- and CO₂-saturated atmospheres, followed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests conducted in a CO₂-saturated atmosphere. The EIS data were analyzed using Zview software. The potentials provided herein have been IR-corrected and referenced to the reversible hydrogen electrode (RHE), employing eqn (S1).†

2.3. CO₂ reduction experiments and product analysis

The eCO₂R experiment was conducted in a 0.1 M Na₂SO₄ solution saturated with CO₂, where electrolysis was carried out within the potential range of −0.05 V (V vs. RHE) to −0.5 V (V vs. RHE), with an increment of 0.05 V. Prior to the eCO₂R process, a continuous supply of CO₂ into the cathode chamber for 40 min ensured sufficient availability, followed by a closed electrolysis period of 2 h. Gas phase product composition analysis was carried out using gas chromatography (GC, Techcomp, GC-7980). The gaseous sample was prepared using N₂ (99.9999%) and passed through a Plot-Q capillary column packed in series. H₂ detection utilized TCD, while CO and CH₄ were detected by FID. The FE of the gas phase products (CO, H₂, and CH₄) can be calculated using eqn (1).where z denotes the number of electrons required for electrocatalytic reduction of CO₂ to CO, H₂, and CH₄. F represents Faraday's constant with a value of 96 485 C mol⁻¹. n is the number of moles of the produced H₂, CO, and CH₄. Q refers to the total charge during eCO₂R for gas chromatograms and standard curves of gases shown in Fig. S7–S9.† The low concentration of methanol was determined using meteorological chromatography (Techcomp, GC-7980) and the standard curve was constructed (Fig. S10†). The FE calculation (eqn (2)) can be used to determine the efficiency of gas phase products such as CH₃OH.where 6 represents the number of electrons required for electrocatalytic reduction of CO₂ to CH₃OH. F denotes Faraday's constant (96 485 C mol⁻¹). n is the number of moles of the produced CH₃OH, and Q indicates the total charge during eCO₂R. The higher concentration of CH₃OH was determined using an ultraviolet spectrophotometer (UV-1780, Shimadzu), and a color rendering method based on a standard curve (Fig. S11†) was employed to assess its concentration.

2.4. Computational modeling and DFT calculations

The DFT calculations were performed using the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient exchange approximation (GGA) correlational functional in the CASTEP module.¹⁴ A 3 × 3 × 1 supercell was employed for the calculation process. Free energy calculations were performed using the Dmol3 module of Materials Studio, with geometry optimization using the PBE function of GGA. The dual numerical additive polarization (DNP) basis set was employed, and van der Waals interactions were adjusted using the DFT-D method with Grimme parameters. To account for layer-to-layer interactions, a vacuum space of 15 Å was applied along the z-direction. The energy convergence accuracy was set at 1 × 10⁻⁵ Ha, with maximum stress and displacement limits of 4 × 10⁻³ Ha Å⁻¹ and 5 × 10⁻³ Ha, respectively. The Gibbs free energy change (ΔG) was calculated using eqn (S2).†

3. Results and discussion

3.1. Characterization of Ag/Cu foam electrocatalysts

The morphology of Ag/Cu foam was examined via SEM analysis. As depicted in Fig. 1, the pore size and distribution of Ag on the copper foam were observed. It is evident that Ag/Cu foam contains a significantly higher amount of granular material compared to Cu foam. At higher magnification, the dendritic shape with intricate branches and leaves becomes clearly discernible. This hierarchical structure resembles that of trees, with sizes ranging from 0.3 μm to 2.5 μm. Under citrate conditions, dendritic structures were formed due to non-uniform growth and deposition processes on the surface of the Ag material.

Fig. 1 SEM images of (a) Cu foam and (b) Ag/Cu foam. (c) Enlargement of the red box in (b).

The crystal structure of Ag/Cu foam catalyst was analyzed using XRD patterns, as shown in Fig. 2. Ag/Cu foam-60 min refers to Cu foam silver plating for 60 min, and so forth. The XRD patterns of the prepared Cu catalysts exhibit distinct characteristic peaks corresponding to the microcrystalline structures of Cu (PDF card no. 04-0836) and CuO (PDF card no. 78-0428), indicating the formation of a composition consisting of both Cu and CuO phases. The formation of CuO is observed when Ag/Cu foam is crushed into particles. This can be attributed to the oxidation of the surface of Cu powder upon contact with air due to the presence of loaded Ag particles on the copper foam matrix, as confirmed by Ag (PDF card no. 04-0783). The XRD patterns obtained from bimetallic electrodes reveal that diffraction peaks corresponding to Cu (111) and Cu (200) shift towards higher angles, while those related to Ag (200) and Ag (220) shift towards lower angles. These observations suggest minimal alloying between Cu and Ag, with a more pronounced peak shift observed as complexation time increases. In particular, in the case of complex silver plating for 30 min on Ag/Cu foam, predominantly exposed crystal surfaces are identified as being composed solely by Ag (111), without any other visible crystalline surfaces evident in the XRD pattern analysis results. Furthermore, migration tendencies are more prominent for crystal surfaces represented by peaks related to Ag (200) and Ag (220), particularly noticeable when comparing samples plated for either 90 or 60 min on Ag/Cu foam. Through characterization after electrolysis, it was observed that the CuO peak was absent from all samples. This result can be attributed to the reached reduction potential of CuO during eCO₂R, resulting in the reduction of CuO, and thus its disappearance on the electrode. The main peaks associated with Ag are present, indicating that the attached Ag is stable.

Fig. 2 The XRD patterns of various samples, including Cu foam, Ag/Cu foam-30 min, Ag/Cu foam-30 min after electrolysis, Ag/Cu foam-60 min, Ag/Cu foam-60 min after electrolysis, Ag/Cu foam-90 min, and Ag/Cu foam-90 min after electrolysis.

The XPS technique was employed to investigate the surface chemical state and electronic structure. As depicted in Fig. 3, the Ag/Cu foam-60 min sample exhibited identical peaks for Ag 3d_3/2 (374.2 eV) and Ag 3d_5/2 (368.2 eV), as shown in Fig. 3(a), indicating a zero valence chemical state (Ag⁰). This observation is consistent with the spin–orbit splitting characteristic of the Ag 3d peak. Analysis of the Cu 2p XPS spectrum of Ag/Cu foam-60 min revealed characteristic peaks corresponding to Cu⁰'s Cu 2p_3/2 and Cu 2p_1/2 levels. Additionally, a prominent peak at 948 eV was observed in the Cu 2p region map, positioned on the high binding energy side of the main Cu 2p_3/2 peak. Typically, satellite peaks associated with Cu²⁺ appear around 943 eV. However, considering the presence of CuO in the XRD spectrum, we attribute this shift towards higher binding energy to electron energy state modification induced by introduced Ag affecting the outermost copper orbitals.

Fig. 3 (a) Ag 3d and (b) Cu 2p XPS spectra of Ag/Cu foam-60 min.

3.2. Evaluation of electrochemical CO₂ reduction performance of Ag/Cu foam

3.2.1 Effect of duration of Ag plating on the electrochemical reduction of CO₂ using Ag/Cu foam. We evaluated the performance of Ag/Cu foam in various electrolytes and identified 0.1 M Na₂SO₄ as the optimal electrolyte for electrocatalytic reduction of CO₂ on Ag/Cu foam (Fig. S1†). Prior to conducting eCO₂R, the surface of the Ag/Cu foam electrode was activated through cyclic voltammetry (CV) measurements. The CV curves were obtained by performing 20 cycles from 0 V to −1.5 V_RHE at a scanning rate of 10 mV s⁻¹ in a N₂ atmosphere with 0.1 M Na₂SO₄ electrolyte. The experiments were carried out using an H-type electrolytic cell as shown in Fig. S2.† Notably, both copper foam and Ag/Cu foam exhibited distinct color changes (see Fig. S3†). By comparing the linear sweep voltammetry (LSV) curves obtained under different atmospheres, as illustrated in Fig. 4(a), we preliminarily assessed the CO₂ reduction performance of the Ag/Cu foam-60 min electrocatalyst. We propose that eCO₂R is primarily driven by changes in current density resulting from alterations in atmosphere conditions, while Cu foam is influenced by relative rates of j_CO2 and j_N2 under N₂ atmosphere conditions. To account for these effects, our results were normalized to a simple hydrogen evolution reaction. In CO₂-saturated atmospheres, current densities not only drive hydrogen evolution reactions but also facilitate concurrent eCO₂R. The current density exhibits a sharp increase with an inflection point occurring at approximately −0.5 V_RHE. After subjecting the Ag/Cu foam to identical conditions for 60 min, this inflection point occurs earlier at around −0.1 V_RHE.

Fig. 4 Electrochemical testing diagrams: (a) LSV diagrams of Cu foam and Ag/Cu foam-60 min, (b) Tafel diagrams of Cu foam with different silver plating times, (c) Cu foam and C_dl diagrams of Cu foam with different silver plating times, (d) EIS diagrams of Cu foam with different silver plating times.

Based on Fig. 4(a), it can be inferred that Ag/Cu foam exhibits electrochemical reduction of CO₂, enabling the investigation of the influence of silver plating time on eCO₂R, as presented in Fig. S4.† The response of Ag/Cu foam-x min to eCO₂R can be attributed to its higher current density in CO₂-saturated electrolyte compared with N₂-saturated electrolyte, further confirming the enhanced performance of Ag growth on copper foam over blank Cu foam. Furthermore, valuable insights into the eCO₂R dynamics can be obtained from the Tafel curve analysis. As depicted in Fig. 4(b), the linear segment of the Tafel curve is fitted using eqn (3):

η = a + b log j (3)

where j represents current density and b represents Tafel slope. The Tafel slope for Ag/Cu foam-60 min is determined to be 81.52 mV dec⁻¹.

The electrochemical experiment confirms that Ag/Cu foam-60 min exhibits the lowest Tafel slope among all tested samples of Ag/Cu foam-x min. Systems with lower Tafel slopes typically exhibit higher catalyst activity, indicating enhanced promotion of the reaction, and provide more efficient reaction sites at a reduced potential. A higher electron transfer rate facilitates faster progress in the reaction while achieving a greater current density response at a lower potential, as demonstrated in Fig. 4(a). Compared to Cu foam, Ag/Cu foam-60 min exhibited higher j_CO2 than j_N2 under different gas atmospheres and displayed an earlier inflection point. Furthermore, bifurcation of current density was observed for Ag/Cu foam-60 min at −0.1 V under two gas atmospheres, as shown in Fig. S4.† Inflection points were observed for Ag/Cu foam-30 min and Ag/Cu foam-90 min at −0.43 V_RHE and −0.3 V_RHE, respectively, indicating that Ag/Cu foam-60 min possesses comparatively lower potential for promoting eCO₂R.

Additionally, to gain a more profound comprehension of the origin of Ag/Cu foam's enhanced activity, we conducted an analysis of the electrochemically active surface area by measuring double layer capacitance (C_dl). The electrode's C_dl was determined through CV analysis, which involved scanning rates ranging from 10 to 100 mV s⁻¹ in CO₂-saturated Na₂SO₄ electrolyte (0.1 M). The voltage range for the selected CV curve was between 0 V (V vs. RHE) and 0.05 V (V vs. RHE). Fig. 4(c) illustrates that Ag/Cu foam-60 min exhibits a maximum capacitance value of 10.2 mF cm⁻². Complete CV diagrams can be found in Fig. S5.† EIS measurements were conducted under constant overpotential, as depicted in Fig. 4(d). EIS measurements provide valuable insights into catalysts' charge-transfer resistance with the semicircle observed in the Nyquist plot attributed to R_ct representing charge transfer resistance where lower R_ct indicates faster reaction rates. As expected, Ag/Cu foam-60 min displays significantly lower R_ct compared to other catalysts and blank samples. To evaluate charge transfer behavior, EIS analyses were conducted, revealing excellent conductivity and faster reaction kinetics as demonstrated by impedance calculations described in Table S1.† Notably, during the complex silver plating process, Ag/Cu foam-90 min exhibited a relatively superior electrochemical performance, while Fig. S6† shows that the instability of IT prevented it from being chosen as an optimal catalyst evidenced by a sharp drop in current density during testing. The reduction of the Ag peak on the surface of Ag/Cu foam-90 min is observed in Fig. 2, following a complex plating process that lasted for 60 min. The complete exposure of Ag leads to the accumulation of tree-like morphologies, which are clearly visible in SEM images. This phenomenon can be attributed to the coupling between citric acid anions and metal ions, which affects nucleation and growth locations. Consequently, an excessive accumulation of dendritic Ag on the surface of copper foam occurs as a result. Notably, during the experiment, a gray flocculent structure was observed precipitating into the H-type electrolytic cell when high voltage was applied, which is consistent with previous XRD test findings.

3.2.2 Analysis of products derived from the electrocatalytic reduction of CO₂ using Ag/Cu foam. As depicted in Fig. 5(a), Cu foam predominantly generates hydrogen, while CO is produced only after −0.6 V_RHE during eCO₂R. Furthermore, as illustrated in Fig. 5(b), the addition of Ag inhibits hydrogen evolution by approximately 30%, resulting in the formation of CH₃OH and CH₄ at low potential levels. Cu foam and Ag/Cu foam-60 min exhibit distinguishable color in Fig. S3.† The inhibition of hydrogen evolution by Ag is attributed to its surface adsorption characteristics and electrochemical reaction dynamics:¹⁵ the strong hydrogen adsorption capacity of the Ag surface hinders the adsorption of hydrogen molecules and subsequent hydrogen evolution reaction. According to research, the Ag surface exhibits a high capacity for hydrogen adsorption, resulting in the formation of an adsorbed hydrogen state (H*), which impedes further hydrogen molecule adsorption and participation in the hydrogen evolution reaction.¹⁶ This property of adsorption serves to suppress the occurrence of said reaction. From a dynamic perspective, the presence of adsorbed hydrogen atoms on the Ag surface can alter reaction kinetics at the electrochemical interface. These adsorbed hydrogen atoms elevate the overpotential required for hydrogen electrochemical precipitation, resulting in an increased potential needed for the hydrogen evolution reaction. Consequently, this raises the energy barrier and decreases the rate of the hydrogen evolution reaction, ultimately impeding its progress. From a free energy standpoint, alterations in free energy are pivotal factors in determining whether an electrochemical reaction occurs. The adsorption of hydrogen on the Ag surface can modify the associated free energy change during the hydrogen evolution reaction. By virtue of its ability to adsorb hydrogen atoms, Ag can mitigate water dissociation and restrict the generation of hydrogen ions, thereby decreasing overall free energy change during hydrogen evolution.¹⁷ This leads to an increase in the free energy of the hydrogen evolution reaction, thereby hindering its progress. Interestingly, the activity of the Cu phase on the Ag/Cu electrode remains unaffected by near-surface composition, indicating that Ag can function as a surface accelerator for Cu. The stronger interaction between *CO and Cu compared to Ag is believed to be the driving force behind segregation of the Cu phase onto the Ag phase surface in the initial Cu foam.^18,19

Fig. 5 (a) Products obtained from Cu foam electrocatalytic reduction of CO₂, (b) products obtained from Ag/Cu foam-60 min electrocatalytic reduction of CO₂, (c) long-term test results of Ag/Cu foam at −0.1 V (V vs. RHE).

In order to further investigate the distribution of reduction products and narrow down the voltage gap between them, potentiostatic electrolysis experiments were conducted on Ag/Cu foam-60 min at various applied potentials. Additionally, potential-dependent electrolysis experiments with CO₂-saturated 0.1 M Na₂SO₄ electrolyte (duration 2 h) were performed. The gas and liquid products were analyzed using gas chromatography and ultraviolet spectrophotometry, as detailed in Fig. S7–S11.† The results of the electrolysis experiments are presented in Fig. 5(b), indicating that eCO₂R can be achieved at a remarkably low potential of Ag/Cu foam-60 min, while CH₃OH formation occurs at −0.05 V_RHE. At −0.1 V_RHE, the Faraday efficiency of CH₃OH reaches 81.33%, demonstrating that CO₂ reduction to CH₃OH is predominantly driven by Ag inhibition of the hydrogen evolution reaction. It was observed that an increase in voltage at −0.2 V_RHE resulted in the appearance of CH₄ as a product; however, its Faraday efficiency was low due to the requirement for more electrons for its reduction compared to generating CH₃OH which requires only 6 electrons. Insufficient energy output at a lower potential hindered the enhancement of CH₄ yield. In this work, we successfully regulated the Faraday efficiency of the product by adjusting the voltage: high selectivity for methanol was achieved when controlling the voltage at −0.1 V_RHE, whereas methanol appeared when increasing the voltage level. As higher voltages were used, both Faraday efficiency and selectivity towards methanol decreased while methane production increased. Moreover, long-term electrolysis was performed for 100 h at a stationary cathode potential of −0.1 V_RHE to test the stability of Ag/Cu foam (Fig. 5(c)), which was another major concern for eCO₂R performance evaluation. The superior performance described in this study can be compared with other relevant studies as summarized in Table 1.

Table 1 A summary of eCO₂R to methanol

Electrocatalyst	Electrolyte	Potential (V)	FE (%)	Ref.
Cu63.9Au36.1/NCF	0.5 M KHCO3	−1.1 (V vs. RHE)	15.9	20
FeS2/NiS	0.5 M KHCO3	−0.6 (V vs. RHE)	64	21
Pd/SnO2	0.1 M NaHCO3	−0.24 (V vs. RHE)	54.8	22
Oxide-derived Cu/C	0.1 M KHCO3	−0.3 (V vs. RHE)	43.2	23
Cu2O/MWCNTs	0.5 M NaHCO3	−0.8 (V vs. RHE)	38	24
Ag/Cu foam	0.1 M Na2SO4	−0.10 (V vs. RHE)	81.33	This work
PO-5 nm Co/SL-NG	0.1 M NaHCO3	−1.0 (V vs. SCE)	23.2	25
Cu2O nanoparticles	KHCO3	−2.0 (V vs. SHE)	47.5	26
BP	0.1 M KHCO3	−0.5 (V vs. RHE)	92.0	27
Zn/Ag	0.1 M KHCO3	−1.38 (V vs. RHE)	10.5	28
Pd83Cu17 aerogel	[Bmim][BF4] : H2O (1 : 3)	−2.1 (V vs. Ag/Ag^+)	80	29
Ag–Co3O4–CeO2-LGC	0.1 M KHCO3	−0.85 (V vs. RHE)	23.4	30
Cu1.63Se(1/3)	30% [Bmim][PF6]/MeCN/H2O (5%)	−2.1 (V vs. Ag/Ag^+)	77.6	31
Fe2P2S6	0.5 M NaHCO3	−0.20 (V vs. RHE)	65.2	32
C–Py–Sn–Zn	0.01 mol L^−1 4-aminopyridine and 0.1 mol L^−1 LiClO4 ethanol	−0.5 (V vs. RHE)	59.9	33
[PYD]@Pd	0.5 M KCl	−0.04 (V vs. RHE)	35	34
Co(CO3)0.5(OH)·0.11H2O	0.1 M KHCO3	−0.34 (V vs. RHE)	97	35

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The Faraday efficiency of H₂ observed on the bimetallic electrode is significantly lower compared to that observed on the Cu foam. In contrast, the Cu–Ag bimetallic electrode effectively utilizes most of this current for CH₃OH production, achieving an average FE exceeding 20%. However, a fraction of the current is also utilized for synthesizing CO and CH₄ on bare Cu foam at −0.6 V_RHE with no CH₄ generation. Remarkably, the bimetallic electrode exhibits exceptional selectivity towards carbon-containing oxygen compounds, reaching a maximum Faraday efficiency of 81.33%, which cannot be achieved using bare Cu foam.

3.3. High selectivity mechanism of Ag/Cu foam for electrocatalytic reduction of CO₂ to CH₃OH

The eCO₂R reaction occurs at the interface between the electrode and the electrolyte, where the former serves as a solid electrocatalyst while the latter is an aqueous solution containing CO₂.³³ Electrocatalytic reactions typically occur at this interface between the active site of the material and the electrolyte, involving several steps:³⁵ (1) CO₂ molecules are adsorbed onto the electrode surface to form a partially charged *CO₂^δ− species through interaction with surface metal atoms. It should be noted that carbon dioxide can be physically adsorbed as a linear molecule during electrolysis and further activated by electrode surface defects and solvent action.^36,37 (2) Electron transfer and protonation lead to the cleavage of C=O bonds, forming new C–H and C–O bonds. (3) Rearrangement of products is followed by desorption from active sites and diffusion into bulk electrolyte solutions. Carbon dioxide molecules possess linearity with two σ bonds and two π⁴₃ bonds between their carbon and oxygen atoms, resulting in partial triple bond properties for their double bonds that shorten their length significantly.

Based on previous experimental studies and DFT calculations,^38–42 the eCO₂R pathway^43,44 in this catalyst is illustrated in Fig. 6. The selected crystal faces are Cu (111) and Ag (111), which correspond to the predominant crystal facets determined by XRD analysis. CO₂ molecules adsorb onto the surface of Cu–Ag, forming partially charged *CO₂^δ− species that interact with Ag atoms for adsorption, subsequently generating *COOH groups at low voltage conditions. Free *H in water combines to produce H₂ as a by-product. A fraction of *COOH transforms into *CO intermediates, which exhibit high instability and eventually lead to CO overflow in the electrolyte solution. Another portion of *CO reacts with available *H in solution to form *CH₂O intermediates; notably, Cu exhibits positive adsorption energy towards *H, facilitating CH₃OH formation through subsequent combination reactions with available hydrogen atoms. As voltage increases, some of the *H within the solvated *CH₂O species undergoes dehydrogenation and forms intermediate species containing C double bonds (*CH₂). These intermediates then combine with two additional hydrogen atoms (*H) to yield CH₄ production, further confirming that Cu (111) promotes CH₄ formation. This proposed mechanism aligns well with experimental observations where CH₃OH and CO are predominantly produced at low voltage while an increase in voltage leads to enhanced generation of electrons, resulting in increased CH₄ production.

Fig. 6 The path diagram of electrocatalytic CO₂ reduction on Ag/Cu foam.

During the Materials Studio modeling process, the unit cells of Cu and Ag were imported. Subsequently, the crystal planes of Cu (111) and Ag (111) were cut, followed by stacking and optimization of the two supercell structures to obtain a minimum energy unit configuration consisting of two layers of Cu and one layer of Ag, as shown in Fig. 7. This structure will serve as our model for Ag/Cu catalysts, on which we will conduct a series of calculations. In this work, CH₄ is produced at −0.2 V potential. Based on DFT calculations performed under two different potential conditions for all reduction products, it was determined that *COOH formation represents the rate-determining step with respect to the first carbon intermediate's energy barrier. In terms of hydrocarbon formation, *CO plays a crucial role; previous research conducted by Sargent et al.'s group⁴⁵ on M-doped Cu systems revealed that Ag-doped Cu exhibits the lowest activation energy for C₁–C₁ and C₁–C₂ coupling reactions. These findings indicate that Ag-doped Cu serves as an efficient catalyst for generating C₊ products from *CO intermediates, a conclusion consistent with our calculations.

Fig. 7 (a) The D-band center diagram of Cu foam and Ag/Cu foam-60 min, (b) the figure of orbital overlap area of Cu foam and Ag/Cu foam-60 min, (c) free energy diagrams and reaction pathway of eCO₂R with Ag/Cu foam-60 min under −0.1 V_RHE, (d) free energy diagrams and reaction pathway of eCO₂R with Ag/Cu foam-60 min under −0.2 V_RHE.

Bader charge analysis was conducted to investigate the impact of electrons on the bonding strength of intermediates and the electronic structure of catalysts. Following Ag plating, the D-band center of Ag/Cu foam-60 min (−1.933 eV) catalyst was observed to be lower than that of Cu foam (−1.537 eV), indicating a doping effect and local structural deformation. It is reasonable to hypothesize that strong *COOH adsorption leads to the entrapment of intermediates on the Cu surface. In both doped catalysts, there are more electrons in the Cu active center near the heteroatom in Ag/Cu foam-60 min compared to Cu foam near the Fermi level (Fig. 7(a)). These disparities suggest that Ag/Cu foam-60 min exhibits a mild form of *COOH bonding, where orbital overlap area and anti-bonding orbitals co-regulate interactions, thereby facilitating subsequent methanol formation (Fig. 7(b)).

The voltage difference between the two is only 0.1 V, and the energy barrier does not result in a significant disparity (ΔG is 0.933 eV at −0.1 V_RHE, and 0.833 eV at −0.2 V_RHE). The relatively low free energy barrier that needs to be overcome at these voltages also accounts for the high FE% of eCO₂R observed in this study. At −0.1 V_RHE, the process from *CO hydrogenation to *COH necessitates overcoming an additional free energy barrier ΔG = 0.379 eV, indicating that electrochemical eCO₂R at the Cu–Ag interface constructed here is both thermodynamically and kinetically feasible. The crucial intermediate for subsequent steps involves either hydrogenating *CH₃O to CH₃OH or dehydrating it to form *CH₂; thus, our focus will be on its selectivity analysis. At −0.1 V_RHE, ΔG for CH₃OH formation is −0.6 eV while ΔG for *CH₂ formation it is 0.566 eV; clearly demonstrating that under this voltage condition, CH₃OH formation occurs more favorably than *CH₂ formation does due to lower activation barriers associated with it in thermodynamic and kinetic aspects, respectively. At −0.2 V_RHE, we can intuitively observe a decrease in the ΔG value of *CH₂ which drops to −0.034 eV; further hydrogenation of *CH₃ through two consecutive steps leads directly towards methane generation as predicted by favorable thermodynamic and dynamic conditions, explaining why CH₄ production predominates at −0.2 V_RHE. As depicted in Fig. 7(c) and (d), regardless of potential conditions considered, the pathway leading towards CH₄ production must involve a dehydration step following the hydrogen evolution reaction involving *CH₃O, whereas the selective determination step governing CH₃OH production must involve direct hydrogenation of *CH₃O as supported by DFT calculations presented in Fig. S11–S13.†

According to the aforementioned calculation results, it is evident that the intermediates involved in product formation can be effectively regulated by manipulating the voltage, thereby influencing the pathway of product generation. Specifically, at −0.1 V, *CH₃O intermediates assume a pivotal role as they exhibit a higher propensity to react with *H and form methanol rather than eliminating water molecules to generate *CH₂. This phenomenon substantiates our ability to achieve remarkably selective reduction of methanol under low voltage conditions.

4. Conclusion

We demonstrate that dual doping of Ag/Cu foam with appropriate components represents a promising strategy for enhancing the eCO₂R to methanol. By adjusting the voltage, Ag/Cu foam exhibits improved selectivity towards CH₃OH, achieving an FE% as high as 81.33%. Furthermore, this catalyst demonstrates excellent stability over a period of 100 h and promotes CH₄ production. Our DFT calculations reveal that low voltages favor CH₃OH generation, while high voltages lead to CH₄ formation, consistent with experimental observations. The introduction of Ag effectively suppresses the hydrogen evolution reaction and facilitates the generation of *CO species, which can be readily protonated into *CHO intermediates and subsequently converted into either CH₄ or CH₃OH. These findings highlight the potential application of our efficient and stable catalyst in eCO₂R for methanol synthesis.

Data availability

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

Conflicts of interest

The authors hereby disclose that they have no known financial or personal relationships that could be perceived as potential conflicts of interest and may have influenced the findings presented in this paper.

Acknowledgements

This work was financially supported by the Production-Education Integration Demonstration Project of the Province “Photovoltaic Industry Production-Education Integration Comprehensive Demonstration Base of Sichuan Province (Sichuan Financial Education [2022] No. 106)”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy01056f

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