All chemicals used in this study were of analytical grade and used without further purification. Copper (II) sulfate pentahydrate (CuSO₄·5H₂O) and sodium hydroxide (NaOH) were purchased from DUKSAN. Lactic acid, Copper (I) sulfate (Cu₂SO₄), and sulfuric acid (H₂SO₄) were obtained from Sigma-Aldrich. Deionized water (18.2 MΩ cm) was used in all experiments.

GDEs were prepared using carbon paper as the substrate. The carbon paper was cut into 16 cm² pieces and cleaned by sequential ultrasonication in acetone, isopropyl alcohol (IPA), and deionized water for 5 minutes each to remove any impurities. The cleaned carbon paper was then dried at 70°C for 1 hour.

For the preparation of GDEs with a Cu interlayer, Cu was electrodeposited prior to the Cu₂O deposition. The electrolyte solution for Cu deposition contained 0.01 M Cu₂SO₄ and 0.1 M H₂SO₄. The electrodeposition was performed at a constant current density of –2.5 mA/cm² for 400 seconds. The electrodes were then rinsed with deionized water and dried under nitrogen gas before proceeding with Cu₂O deposition under the same conditions described above.

Cu₂O was electrodeposited onto the GDEs from an electrolyte solution containing 0.2 M CuSO₄·5H₂O and 3 M lactic acid. The pH of the solution was adjusted to 12 using NaOH. The electrodeposition was carried out in a three-electrode system with the GDE as the working electrode, an Ag/AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. A constant voltage of –0.6 V vs. Ag/AgCl was applied for 3 hours. After deposition, the electrodes were rinsed with deionized water and dried at 70°C for 1 hour.

The surface morphology of the electrodeposited catalysts was examined using Field Emission Scanning Electron Microscopy (FE-SEM, SU5000/Hitachi). Energy Dispersive X-ray Spectroscopy (EDS, SU5000/Hitachi) was utilized to analyze the elemental composition and distribution of the deposited layers. X-ray Diffraction (XRD, EMPYREAN/Malvern Panalytical) was employed to determine the crystalline structure of the catalysts. X-ray Photoelectron Spectroscopy (XPS, K-ALPHA+) was conducted to analyze the surface chemical states and composition of the catalysts. FE-SEM, EDS, and XRD analyses were conducted using equipment at Energy Convergence Core Facility in Chonnam National University.

The electrochemical reduction of CO₂ was carried out in a custom-made gas-tight electrochemical cell. The cell was equipped with separate compartments for the working electrode (WE), reference electrode (RE), and counter electrode (CE), each compartment separated by Nafion 117 membranes to prevent cross-contamination. The operation electrode partition was stably supplied to CO₂ gas continuously flowing through CO₂ gas continuously.

The prepared GDE with electrodeposited Cu₂O and Cu/Cu₂O catalysts served as the working electrodes. An Ag/AgCl (saturated KCl) electrode was used as the reference electrode, and a bare GDE was used as the counter electrode. The electrolyte used for CO₂ reduction was 1 N KOH solution, prepared using deionized water and purged with CO₂ gas for 30 minutes prior to the experiments to ensure saturation and a stable pH of approximately 14.

CO₂ gas (99.9999% purity) was continuously bubbled through the electrolyte at a flow rate of 20 sccm during the electrochemical measurements. Electrochemical experiments were conducted using a CH Instruments potentiostat (WPG100e/WonATech) in a three-electrode configuration.

Linear sweep voltammetry (LSV) was performed from –1.5 V to 1.0 V vs. RHE at a scan rate of 20 mV/s to evaluate the onset potential and current density for CO₂ reduction. And controlled-potential electrolysis was carried out at selected potentials (based on LSV results) for 1 hour. During electrolysis, the gas and liquid products were periodically sampled for analysis.

Gas products were analyzed using gas chromatography (GC, CN/7890B/Agilent Technologies) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Liquid products were analyzed using high-performance liquid chromatography (HPLC, 1260 infinity II/Agilent Technologies) and gas chromatography with a headspace sampler (Headspace-GC, 8890B/Agilent Technologies) to quantify the concentration of various reduction products. Faradaic efficiency for each product was calculated based on the charge passed and the amount of product formed.

EIS measurements were performed at open circuit potential and selected reduction potentials to study the charge transfer resistance and electrode kinetics. The frequency range for EIS was from 100 kHz to 0.1 Hz with an AC amplitude of 10 mV.

CV measurements were conducted in a non-faradaic region to estimate the electrochemical surface area (ECSA) of the catalysts. The scan rate was varied from 20 mV/s to 200 mV/s to establish the double-layer capacitance.

The measured potential (E_Ag/AgCl) was converted to the reversible hydrogen electrode (RHE, E_RHE) scale using Eq. 1.

Fig. 1a is a schematic diagram showing the microstructure of the GDE@Cu@Cu₂O electrode. GDE has a porous support structure, allowing CO₂ to be effectively transported to the catalyst surface. The Cu and Cu₂O layers deposited on GDE act as active sites for electrochemical CO₂ reduction reactions and are selectively converted to products such as carbon monoxide (CO), formic acid (HCOOH), and ethanol (C₂H₅OH). Fig 1b shows an X-ray diffraction (XRD) pattern. It provides information about the crystal structure of GDE@Cu₂O and GDE@Cu@Cu₂O electrodes. The XRD pattern of the GDE@Cu₂O electrode showed distinct peaks at 2θ values of about 29.6°, 36.4°, 42.3°, 61.4°, 73.6°, and 77.5° corresponding to the crystal phase of Cu₂O, indicating that (110), (111), (200), (220), (311), and (222) well-crystallized Cu₂O phase (JCPDS No. 05-0667) was formed in the plane [29,30]. In the case of the XRD pattern of the GDE@Cu@Cu₂O electrode, a pattern similar to that of the GDE@Cu₂O electrode was shown, but in addition, there were additional peaks suggesting the presence of the Cu interlayer. The Cu₂O peaks were present at the same 2θ values (29.6°, 36.4°, 42.3°, 61.4°, 73.6° and 77.5°), confirming the formation of the cuprous oxide phase on top of the Cu interlayer. The XRD pattern also exhibited peak at approximately 43.3o, corresponding to the (111) plane of metallic Cu, confirming the presence of the Cu interlayer (JCPDS No. 04-0836) [31,32].

The surface morphology of the GDE@Cu₂O electrode was investigated using FE-SEM at different magnifications, as shown in Fig. 1c and 1d. Fig. 1c presents the surface at ×2.00k magnification, while Fig. 1d shows a more detailed view at ×10.0k magnification. In comparison with the surface morphology of GDE@Cu@Cu₂O displayed in Fig. 1f and 1g, distinct differences can be observed. The GDE@Cu₂O surface exhibits tetrahedral Cu₂O crystals formed directly on the GDE substrate. In contrast, the GDE@Cu@Cu₂O surface features a leaf-like structure with smaller particles forming on these leaf-like surfaces. The morphological difference between the two electrodes is due to the presence of the Cu interlayer. The Cu interlayer affects nucleation and growth during the electrodeposition process of Cu₂O, preventing the formation of tetrahedral crystals and allowing them to exist in the form of spherical particles. Compared to the surface morphology of GDE@Cu, as shown in Supporting Information Fig. S1a and S1b, the structure of GDE@Cu@Cu₂O is extremely similar, suggesting that Cu₂O formed on the Cu interlayer.

The elemental composition and distribution of the GDE@Cu@Cu₂O electrode surface were analyzed using EDS mapping as shown in Fig. 1e. As can be seen from the mapping results, it shows a uniform distribution of Cu and O throughout the surface, confirming the presence of Cu₂O. This supports the structural information obtained from XRD analysis. The presence of carbon (C) is due to the electroplated Cu and the GDE substrate under Cu₂O.

XPS analysis was conducted to investigate the surface chemical states and composition of the GDE@Cu₂O and GDE@Cu@Cu₂O electrodes. The survey spectra, as well as core level spectra C 1s, O 1s, and Cu 2p spectra, were obtained for both samples this is shown in Fig 2. Fig. 2a is the XPS survey spectra of both GDE@Cu₂O and GDE@Cu@Cu₂O electrodes displayed characteristic peaks corresponding to carbon (C 1s), oxygen (O 1s), and copper (Cu 2p). The survey spectra for GDE@Cu@Cu₂O also showed a higher intensity for the Cu 2p peaks compared to GDE@Cu₂O, indicating the presence of the additional Cu interlayer [33]. Fig. 2b is the C 1s spectra for both electrodes showed a main peak at ~284.1 eV (C=C) with additional peaks at ~285.0 eV (C–OH) and ~278.9 eV (C=O), resulting from the presence of GDE [34]. Fig 2c is the O 1s spectrum displayed a dominant peak at ~530 eV, corresponding to metal oxides (Cu₂O). Fig. 2c shows a peak at 530 eV due to Cu₂O as an O1s spectrum. A peak at 531 eV, which is a higher binding energy, indicates the presence of adsorbed water or hydroxyl groups [35]. Fig. 2d is a Cu 2p spectrum characterized by two major peaks at 931.8 eV (Cu 2p_3/2) and 951.6 eV (Cu 2p_1/2) originating from Cu(I) species of Cu₂O [35–37]. The Cu 2p_3/2 peak at 931.8 eV was stronger in the GDE@Cu@Cu₂O sample compared to the GDE@Cu₂O sample. This is due to the presence of the Cu interlayer present in the GDE@Cu@Cu₂O sample. The Cu interlayer contributes to the additional metallic Cu, which is reflected by a stronger Cu 2p_3/2 peak at this binding energy [37,38]. A higher intensity at 931.8 eV means a higher concentration of metal Cu species in the GDE@Cu@Cu₂O sample. This indicates the presence of the Cu interlayer increasing the total metal Cu content [38,39]. In addition, the absence of satellite peaks suggests the presence of minimal Cu(II) species in the sample. However, for GDE@Cu@Cu₂O, the Cu 2p_3/2 and Cu 2p_1/2 peaks appear at similar binding energies, whereas the overall intensity is higher and there is a slight presence of satellite peaks, indicating a small amount of Cu(II) species due to slight oxidation between the Cu layers or formation of Cu(OH)₂ on the surface [40].

Fig. 3 shows a comparative analysis of the electrochemical performance of the GDE@Cu₂O and GDE@Cu@Cu₂O electrodes in a CO₂-saturated 1 M KOH aqueous solution, showing the effect of the Cu interlayer on the catalytic activity and the overall efficiency of the CO₂ reduction reaction. Fig. 3a shows linear sweep voltammetry (LSV) of the GDE@Cu₂O and GDE@Cu₂O electrodes. The GDE@Cu@Cu₂O electrode has a higher current density in a potential range compared to the GDE@Cu₂O electrode. This may have been due to the structural deformation induced by the presence of the Cu interlayer, which may more efficiently promote the interaction between the CO₂ molecule and the catalyst surface [41–43]. Fig. 3b shows the current density over time at a constant potential of –0.8 V (vs. RHE). For about 1 hour, the GDE@Cu@Cu₂O electrode maintained a very constant high current density, while the GDE@Cu₂O electrode showed a lower current density than the GDE@Cu@Cu₂O electrode, and after about 2500 seconds, the current density decreased sharply to less than half. Fig. 3c and 3d show the Faradaic efficiency at various voltages of the GDE@Cu₂O and GDE@Cu@Cu₂O electrodes, respectively. For GDE@Cu@Cu₂O electrodes, HCOOH is the main product, showing a significantly higher Faradaic efficiency compared to GDE@Cu₂O. This contrasts with the GDE@Cu₂O electrode, where H2 is the main product, which may mean that in the absence of the Cu interlayer, the electrode's ability to effectively reduce CO₂ is limited, resulting in higher hydrogen evolution. Therefore, the Cu interlayer is likely to stabilize intermediates such as CO* and HCOO* to promote subsequent reduction to CO and HCOOH, while inhibiting competitive hydrogen evolution reaction (HER) [44–47]. Stabilization of CO and HCOO intermediate at the Cu/Cu₂O interface is very important for improving the selectivity and efficiency of the CO₂ reduction reaction [48,49]. Similar Cu-based electrical catalysts have been extensively studied, and customized catalysts and surface composition have been found to optimize intermediate binding energy and reaction paths [10,50]. Fig. 3e is a Nyquist plot obtained by electrochemical impedance spectroscopy (EIS) at –0.8 V (vs. RHE), through which information on the charge transfer characteristics of the two electrodes can be obtained. The calculated Rs and Rct values are specified in Table S1 of the support information. Considering that the semicircle of GDE@Cu@Cu₂O in the Nyquist plot is significantly smaller than that of GDE@Cu₂O, the electron transfer resistance (Rct) of GDE@Cu@Cu₂O is smaller than that of GDE@Cu₂O. The decrease in Rct means that the Cu interlayer improves the electron transfer kinetics between the electrode and the electrolyte, which allows faster electron transfer to the CO₂ reduction reaction. This improvement in charge transfer allows electrons to reach the active site more easily, so CO₂ reduction can be achieved more efficiently [41,42,51]. Fig. 3f shows the Tafel plots of GDE@Cu₂O and GDE@Cu@Cu₂O. The Tafel slopes of GDE@Cu₂O and GDE@Cu@Cu₂O were measured to be –0.23 V/dec and –0.14 V/dec, respectively, indicating a distinct difference in catalytic performance between the two materials in the electrochemical reduction of CO₂ to formic acid. The low Tafel slopes of GDE@Cu@Cu₂O (–0.14 V/dec) compared to GDE@Cu₂O (–0.23 V/dec) suggest that GDE@Cu@Cu₂O will have much less overpotential in promoting a more efficient reaction pathway to achieve the same current density increase [52,53]. On the other hand, the high Tafel gradient of Cu₂O (–0.23 V/dec) indicates that the electron transfer process is less efficient and requires higher overpotential for the same level of catalytic activity [54,55]. This may be due to the intrinsic limitation of Cu₂O or to the absence of a metal Cu interlayer that could facilitate faster electron transport.

To calculate the electrochemically active surface area (ECSA), cyclic voltammetry (CV) measurements were performed on GDE@Cu₂O and GDE@Cu@Cu₂O electrodes at various scan rates increasing by 20 mV/s from 20 mV/s to 200 mV/s, and these are shown in Fig. 4a and 4b. The potential was converted to RHE contrast value using the Nernst equation (Eq. 1) [56,57].

As can be seen in Fig. 4a, the GDE@Cu₂O electrode exhibits typical capacitive behavior within the selected potential window (0.825 V to 1.25 V vs. RHE), with the current density increasing linearly with the scan rate. This behavior is indicative of the charging and discharging of the double layer, confirming that the measurements are conducted within the non-faradaic region [58,59]. Similarly, the CV curves for GDE@Cu@Cu₂O, displayed in Fig. 4b, also show linear increases in current density with scan rate, suggesting that the presence of the Cu interlayer does not significantly alter the capacitive behavior within the selected potential window. To further analyze the data, the current densities at the midpoint of the potential window were plotted against the scan rate for both electrodes. The difference between the anodic and cathodic current densities (ΔJ) was halved and plotted against the scan rate, as shown in Fig. 4c. This linear relationship between ΔJ/2 and the scan rate indicates that the measured current is primarily due to the charging of the double layer, and thus, the slope of this line can be used to calculate the double-layer capacitance (C_dl) for each electrode. Fig. 4d depicts the calculated ECSA derived from the CV data using Eq. 2 and 3 provided below [60].

The two main parameters, C_S and C_dl, are used to calculate ECSA. CS represents the non-capacitance of an ideal flat surface of an electrocatalyst, and for Cu₂O-based electrodes, the value of 0.035 mF/cm², which is commonly used in calculations, was used as reported in similar studies [61]. The double layer capacitance, C_dl, was measured experimentally and represents the charge storage capacity at the interface between the electrode surface and the electrolyte in the non-faradaic region. The higher the C_dl value, the larger the active surface area [62,63]. C_S refers to the standard electrical capacity per unit area and is the reference for the expected double layer electrical capacity of the known surface area under similar conditions. The C_dl to C_S ratio can be used to estimate the active surface area of the electrode, which provides information on the electrochemical performance of the produced electrode. The calculated results show that the GDE@Cu@Cu₂O electrode exhibits a larger ECSA than the GDE@Cu₂O, which is thought to be due to the increased surface roughness and improved active surface area due to the presence of the Cu interlayer as shown in the SEM image of Fig. 1. These results indicate that increasing the ECSA provides more active sites, thereby significantly enhancing the electrochemical performance of the electrode [64,65]. In summary, in GDE@Cu@Cu₂O catalysts, the Cu interlayer plays a key role in improving the CO₂ reduction performance. First, the Cu interlayer promotes electron transfer and increases reaction efficiency by reducing charge transfer resistance (Fig. 3e). Second, by changing the shape of the Cu₂O layer to form small spherical particles and increase the surface roughness, the ECSA is expanded (Fig. 1). Third, the Cu/Cu₂O interface stabilizes the CO and HCOO intermediates, increasing the selectivity of forming formic acid (HCOOH) and suppressing the hydrogen generation reaction (HER). Unlike alloying or doping, the Cu interlayer simultaneously enhances selectivity and efficiency through electron transfer and structural optimization while maintaining Cu-specific catalytic properties.