eCO2RR performance in an MEA reactorAs-prepared samples were employed as cathode electrocatalysts in the 5 cm2 MEA reactor (Fig. 1a) to evaluate the effects of ACB on eCO2RR, including product distributions and current densities. The eCO2RR activity of commercial Cu NPs served as the baseline for comparison with other electrocatalysts (Supplementary Fig. 1). Major products such as H2, CO, and C2H4 were produced on the Cu NPs, with minimal ethanol detected. The maximum FE obtained was 32.9% ± 1.8 for C2H4 at ~100 mA cm−2 (Supplementary Fig. 1). However, at higher cathodic potentials, the dominant HER suppressed the C2+ reaction pathways, thereby limiting the partial current density of C2+ products. To tackle this challenge, ACB with different functional groups was introduced to regulate the interfacial microenvironment of Cu NPs in the MEA reactor. Among all the tested catalysts, F–Cu and OH–Cu exhibited high partial current densities of CO at low cell potentials (Supplementary Fig. 2a), directly influencing the subsequent C–C coupling pathway toward C2H4. Comparison of the partial current densities of H2 and C2H4 indicated that the increased selectivity resulted from suppression of HER at high cell potentials (Supplementary Fig. 2b vs Supplementary Fig. 2c). Therefore, at −3.7 V, OH–Cu achieved a FE of 42.1% ± 2.7 for C2H4 at a current density of 356.6 mA cm−2 (Fig. 1b), while the F–Cu sample demonstrated an FE of 55.6% ± 2.8 for C2H4 at a current density of 316 mA cm−2 (Fig. 1c). These experimental results indicate that introducing ACB is a promising strategy to enhance the eCO2RR performance and highlight the significant advancements in the eCO2RR-to-C2H4 production, with F–Cu and OH–Cu showing excellent performance compared to most Cu-based electrocatalysts (Supplementary Table 1).Fig. 1: eCO2RR performance of as-prepared catalysts.a Schematic of the MEA reactor. b, c eCO2RR performance of b OH–Cu and c F–Cu catalysts in the 5 cm2 MEA reactor. d The long-term stability test for F–Cu at a current density of 300 mA cm−2 (1.5 A). All applied potentials were not iR corrected (R = 0.25 ± 0.01 Ω). Error bars represent the standard deviations calculated from three independent measurements.To determine whether the catalyst’s activity correlates directly with the electrochemical surface area (ECSA), we compared the double-layer capacitance (Cdl) of all samples. Based on the Cdl values, we observed that introducing ACB significantly increased the ECSA of the catalysts, thereby enhancing eCO2RR activity (Supplementary Fig. 3). However, the ECSA of F–Cu was notably lower than that of OH–Cu. The higher activity of F–Cu indicated that ECSA cannot solely determine the enhancement of eCO2RR activity. The fluorination promoted eCO2RR activity by a different mechanism, such as water activation. Additionally, we varied the F–CB loading to investigate the effect of the interfacial microenvironment of Cu NPs on eCO2RR activity (Supplementary Fig. 4). A low-loading F–CB substantially enhanced the intrinsic activity of Cu NPs, as evidenced by a higher C2H4 FE (42.5% ± 3.7 at a current density of 269.6 mA cm−2) at −3.6 V (Supplementary Fig. 4a) compared to Cu NPs (Supplementary Fig. 1). However, excessive F–CB loading increased the thickness of the gas diffusion electrode (GDE), thereby hindering the transport of CO2 to the catalysts surface. Consequently, the increase in C2H4 FE (33% ± 4.3) at −3.5 V was lower than that achieved with lower F–CB loading, although the current density (~200 mA cm−2) (Supplementary Fig. 4b) was twice that of Cu NPs at −3.3 V (Supplementary Fig. 1). This indicates that the modulation of the interfacial microenvironment follows a volcano-shaped curve (Supplementary Fig. 4c), necessitating the identification of an optimal loading to effectively enhance the catalyst’s performance.Long-term testing was conducted to assess the stability of the F–Cu catalyst in the 5 cm2 MEA reactor (Fig. 1d). At a current density of 300 mA cm−2, the catalyst operated continuously for 35 h with C2H4 selectivity decreasing only from 50% initially to 42%. This highlights the potential of F–Cu for industrial-relevant eCO2RR conversion.Ex situ characterization of electrocatalystsEx situ scanning electron microscopy (SEM) images were taken to investigate the dynamic morphological and structural evolution of the catalysts. Cu NPs were sprayed onto GDEs, and their original morphology remained unchanged before the reaction (Fig. 2a and Supplementary Fig. 5a). After the MEA test, some nanowhiskers grew in situ on the GDEs, forming bundles and stacking on each other (Supplementary Fig. 5b, c). They had lengths of tens of micrometers and consisted of Cu, C, and O elements (Supplementary Fig. 5d–f). Similar morphological evolution and elemental distribution were observed in OH–Cu on GDEs (Supplementary Fig. 6). This indicates that carbon is involved in the structural evolution of the nanowhiskers, possibly originating from the GDEs, ACB, or CO32− from KHCO3 electrolyte. Compared to Cu NPs, the coverage of nanowhsikers on the GDEs in the OH–Cu sample was significantly higher, and their growth was more uniform without stacking (Supplementary Fig. 5b, c vs Supplementary Fig. 6b, c). Both samples were tested under identical experimental conditions. This suggests that the carbon originates from the ACB, triggering structural evolution. This assumption is further confirmed by the F–Cu sample (Supplementary Fig. 7). Compared to OH–Cu (Supplementary Fig. 6a–c), the longer lengths (>50= μm) of nanowhiskers on the F–Cu sample indicate that the fluorination significantly enhances structural transformation (Fig. 2b, c and Supplementary Fig. 7), highlighting the major impact of functional groups on the surface of ACB on the structural evolution of nanowhiskers.Fig. 2: Characterizations of fresh and used electrocatalysts.a–c SEM images of a fresh Cu NPs, b fresh F–Cu on GDEs, and c used F–Cu on GDEs. d TEM images and e corresponding high-resolution TEM images of used F–Cu catalysts; f, g HAADF-STEM images and h–k corresponding EDS elemental mapping images of used F–Cu catalyst.High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images provided deeper insights into the in situ reconstruction of Cu NPs at the nanometer scale. For example, in used F–Cu, 60–80 nm Cu NPs underwent potential-induced fragmentation, forming Cu nanoclusters with diameters of 3–6 nm after the eCO2RR test in the MEA (Fig. 2d, e and Supplementary Fig. 8a–d). Subsequently, these nanoclusters oxidized upon exposure to air during ex situ characterization, resulting in the formation of CuO NPs (Fig. 2f and Supplementary Fig. 8c, d). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed a homogeneous distribution of Cu, O, F, and C atoms on the used F–Cu (Fig. 2g–k and Supplementary Fig. 9a–e). F–CB served as the substrate for the deposition of CuO NPs. Moreover, the morphological evolution of F–Cu was captured by ex situ SEM images under various cathodic potentials in the MEA. The 60–80 nm Cu NPs were held at −3.4 V and then underwent significant particle migration at −3.5 V (Fig. 3a). At this stage, the presence of nanowhiskers ~2.5 μm long indicated that the Cu NPs experienced initial particle reconstruction on the catalyst layer and subsequently grew into nanowhiskers with high length-to-diameter ratios (with 10 μm in length). Finally, raising the potentials promoted the growth of nanowhiskers (up to 50 μm), resulting in a uniform distribution of nanowhiskers on the surface of GDEs (Fig. 3a). Based on ex situ characterization analyses, we conclude that during eCO2RR in the MEA reactor, the fragmented nanoparticles detach from the Cu matrix and rapidly evolve into well-defined nanoclusters attached to the carbon support. Subsequently, the potential-induced migration of nanoclusters leads to their structural transformation into nanowhiskers23 (Fig. 3a, b).Fig. 3: The structural evolution process of the catalyst during eCO2RR in the MEA reactor.a Ex situ SEM images of structural evolution of F–Cu catalysts at negative potentials of −3.4 V, −3.5 V, and −3.6 V in the MEA reactor. b A schematic structural evolution mechanism for Cu NPs during eCO2RR in the MEA reactor. All applied potentials were not iR corrected (R = 0.25 ± 0.01 Ω).Grazing incidence X-ray diffraction (GIXRD) was performed to investigate the changes in composition and structural features of samples before and after the reaction. The characteristic peaks at 43.2°, 50.4°, and 74.1° correspond to the (111), (200), and (220) reflections of Cu, respectively (Supplementary Fig. 10a)24. The Cu2O peaks at (111) and (220) in all fresh samples indicated the oxidation of Cu25. GIXRD analysis revealed that before the reaction, the sample consisted of a mixture of Cu and Cu2O (Supplementary Fig. 10a). However, after the eCO2RR, when the sample was removed from the MEA and exposed to air for ex situ characterization, the unstable and active Cuδ+ species generated during eCO2RR further oxidized to CuO. Due to the in situ growth of nanowhiskers on the GDEs, which underwent hydrophobic treatment with PTFE, and the CuO was encapsulated by the graphene-like ACB carrier formed during morphology reconstruction, it became challenging to obtain accurate phase information of the post-reaction sample (Supplementary Fig. 10b). However, conclusions drawn from STEM and TEM images in Fig. 2f, g and Supplementary Fig. 8c, d indicated that the intrinsic composition of the post-reaction sample was CuO loaded onto the ACB support.X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface compositions and chemical states of both fresh and used catalysts. The Cu 2p3/2 spectra of all fresh samples exhibited peaks at 933.4 and 935.2 eV, corresponding to Cu+ or Cu0 and Cu2+, respectively (Supplementary Fig. 11a)26. The presence of Cu2+ species in all fresh samples was attributed to surface oxidation in air. The ACB significantly enhanced the intensity of the C = O bond at 532.4 eV, making it the dominant species in the O 1s spectra of F–Cu and OH–Cu, contrasting with the predominant lattice oxygen at 530.0 eV and the adsorbed O2 in η1(O) configuration (O2*) at 531.4 eV observed in Cu NPs (Supplementary Fig. 11b, c)27. After eCO2RR, the Cu+ or Cu0 species were absent, and the Cu 2p peaks shifted to higher binding energy at ~935.3–936.1 eV (Fig. 4a). Theoretically, Cu species follow the binding energy trend: Cu+ ions < Cu2+ ions < Cu(II) bound to extra-framework oxygen28. Since no Cu–F bond was observed at 684.4 eV in the F 1s spectra29 (Supplementary Fig. 12a, b), the high binding energy in the Cu 2p spectra indicated that Cu species were bound to extra-framework oxygen atoms (Cu–OH). In contrast, both OH–Cu and F–Cu exhibited depletion of O2* species (531.4 eV) and lattice oxygen (530 eV) after the MEA test (Fig. 4b vs Supplementary Fig. 11b). This revealed that lattice oxygen did not convert into O2 during structural transformation, as evidenced by the decrease in the O2* peak of used Cu NPs (Fig. 4b). Therefore, lattice oxygen converted to OH radicals after accepting electrons from Cu atoms, leading to a shift in the Cu 2p binding energy to higher values (Fig. 4a vs Supplementary Fig. 11a). Unlike used OH–Cu, the shift of the C=O bond peak to higher energy in used F–Cu highlighted the influence of fluorination (Fig. 4b). Additionally, owing to the numerous surface vacancies and hydrogen bonds, the water adsorbed at surface oxygen vacancy sites (H2O*@Ovac at 533.2 eV) induced lattice rearrangement27, resulting in structural transformation (Fig. 4b).Fig. 4: Ex situ X-ray spectroscopic analysis of electrocatalysts.a Cu 2p XPS spectra and b O 1s XPS spectra of used Cu NPs, OH–Cu, and F–Cu catalysts; c Cu K-edge XANES spectra and d Cu K-edge EXAFS experimental and fitting spectra of fresh and used Cu NPs, OH–Cu, and F–Cu samples and corresponding references (Cu foil, Cu2O, and CuO).Cu K-edge X-ray absorption Near-Edge Structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) were collected to enhance understanding of the electronic structure and coordination environment of copper species in fresh and used samples (Fig. 4c, d). Compared to Cu foil, XANES spectra in Fig. 4c indicated that the Cu sites in all fresh samples were in an oxidized state (Cuδ+ species, 0 < δ < 1). For Cu NPs, copper oxidation may be attributed to exposure to air during ex situ characterization. With OH–CB and F–CB, the absorption edge of copper shifted to higher energy, indicating an interaction between the functional groups (OH and F) of ACB and the active copper sites. However, the oxidation state of copper in all used samples transitioned to a divalent state (Fig. 4c). This indicated that the eCO2RR altered the overall structure and coordination environment of active copper sites in fresh samples, causing additional oxidation of unstable and active copper species during eCO2RR to a divalent state upon exposed to air. Furthermore, regardless of whether samples are fresh or used, copper oxidation follows this sequence: fluorination effect > hydroxylation effect > no carbon black, highlighting the impact of functional groups on the copper structure. Notably, the valence state of Cu for Cu NPs shifted to divalent after MEA testing, even in the absence of ACB. This indicated that the change in the valence state was influenced by the eCO2RR itself, rather than by the functional groups. The functional groups (F and OH) served solely as catalysts in promoting the valence evolution. Additionally, the EXAFS fitting data of fresh Cu NPs, OH–Cu, and F–Cu NPs revealed a Cu-Cu scattering path at ~2.52 Å (Fig. 4d, Supplementary Fig. 13a–c and Supplementary Table 2), and characteristic of metallic Cu. However, in fresh F–Cu, the Cu–Cu bonds transformed into Cu–O bonds after eCO2RR testing in the MEA, reducing the coordination number from 10.3 (0.6) (Cu-Cu) before the eCO2RR to 5.8 (0.5) (Cu–O) after eCO2RR in the MEA (Fig. 4d, Supplementary Fig. 13d–f and Supplementary Table 2). Similar changes were observed in OH–Cu and Cu NPs, suggesting a uniform coordination environment among all tested samples. The oxidative species produced during eCO2RR in the MEA destabilized active Cu sites, leading to changes in the valence state and coordination environment of Cu NPs. The functional groups from ACB (F and OH) exclusively facilitated the structural transformation of nanowhiskers.In situ multimodal characterizations of electrocatalysts for eCO2RR in neutral mediaTo enhance our understanding of eCO2RR in an MEA reactor, in situ characterization techniques were crucial for monitoring the structural evolution of Cu species and investigating the formation of nanowhiskers. In situ Cu K-edge XANES spectra were recorded in CO2-saturated 0.1 M KHCO3 electrolyte while varying the cathodic potentials from −1 V to −3.5 V (Fig. 5a, b and Supplementary Figs. 14–17). Switching from cyclic voltammetry (CV) to open circuit potential (OCP) caused the oxidation state of Cu sites in Cu NPs to approach the Cu(I) state (Cuδ+, 0 < δ < 1) (Supplementary Fig. 16a). This indicated that highly oxidative radicals generated in situ in the pH-neutral electrolyte facilitated the rapid reoxidation of active Cu sites. A similar phenomenon was observed in OH–Cu (Supplementary Fig. 17a) and F–Cu (Fig. 5a). In situ potential-dependent XANES spectra were conducted to examine the stability of Cuδ+ species during eCO2RR. Despite the reduction of Cuδ+ species in all samples with increasing negative potentials, dynamically generated Cuδ+ (0 < δ < 1) species persisted as non-oxide-derived Cu sites during eCO2RR (Fig. 5a, Supplementary Figs. 16a and 17a). Specifically, the Cuδ+ species in OH–Cu after eCO2RR underwent a reduction and re-oxidation process in the neutral electrolyte over time (Supplementary Fig. 17a), indicating the persistence of strongly oxidative radicals in the pH-neutral electrolyte, which induced the spontaneous oxidation of Cu sites. Although Cu NPs were predominantly in their metallic state, the Cu nanoclusters formed during structural transformation were highly unstable. When exposed to air, H2O adsorbed on surface defect sites and atmospheric oxygen can penetrate the structural network of nanowhiskers, accelerating the oxidation of Cuδ+ species to Cu(II) state. These findings are consistent with the observations from ex situ XANES spectra (Fig. 4c). Additionally, in situ Cu K-edge EXAFS spectra of Cu NPs showed mixed metallic Cu and oxidized Cuδ+ species at low cathodic potentials during eCO2RR (Supplementary Fig. 16c). The strong reducing effect led to the reduction of Cuδ+ species, with only metallic Cu observed at high negative potentials (Supplementary Fig. 16c). As the negative potentials increased, the Cu–Cu bond length in Cu NPs gradually increased, indicating fragmentation and migration of some Cu atoms from the Cu matrix under negative bias, contributing to structural reconstruction from nanoparticles to nanowhiskers. In contrast, OH–Cu (Supplementary Fig. 17c) and F–Cu (Fig. 5b) exhibited stabilized Cuδ+ species and metallic Cu after switching to OCP. Comparing the SEM images of the structural reconstruction of Cu NPs, OH–Cu, and F–Cu (Supplementary Figs. 5–7) revealed that the structural reconstruction of Cu NPs was primarily driven by the tensile strain effect on some metallic Cu sites resulting from the electric field and OH radicals. Hydroxylated Cuδ+ species play a crucial role in the large-scale and regular structural evolution of nanowhiskers in OH–Cu and F–Cu. The Cu–Cu bond length for Cu NPs was close to the initial value after eCO2RR (Supplementary Fig. 16c), suggesting non-continuous evolution in Cu NPs. F–Cu (Fig. 5b) and OH–Cu (Supplementary Fig. 17c) still maintained oxidized Cuδ+ species after eCO2RR, indicating continuous structural evolution in both samples, even without an electric field. Therefore, the mechanisms behind the structural evolution of Cu NPs differ from those of OH–Cu and F–Cu. For Cu NPs, the tensile strain effect induced by hydroxyl radicals detaches Cu atoms from the surface, contributing to structural transformation under negative bias. The involvement of functional groups in OH–Cu and F–Cu enhances the surface oxidative microenvironment of active Cu sites, increasing the production of oxidative radicals and stabilizing Cuδ+ species, thus inducing the uniform structural evolution of nanowhiskers under electric potentials.Fig. 5: In situ multimodal characterizations of electrocatalysts during eCO2RR in neutral media.a In situ Cu K-edge XANES spectra and b in situ Cu K-edge EXAFS spectra of F–Cu at different cathodic potentials (OCP, −1 V, −1.5 V, −2 V, −2.5 V, −3 V, −3.5 V, and after reaction). (R = 212 ± 5 Ω). c In situ time-dependent normalized EPR spectra of F–Cu in CO2-saturated 0.1 M KHCO3 electrolyte at −1.5 V, followed by switching to OCP for different periods (R = 212 ± 5 Ω). EPR signals marked by asterisk is from the sample tube. d In situ Raman spectra of F–Cu in CO2-saturated 0.1 M KHCO3 electrolyte at different potentials (R = 181 ± 3 Ω). e In situ real-time ATR FT-IR spectra of F–Cu sample under different applied potentials (R = 156 ± 2 Ω). All applied potentials were not iR corrected.In situ time-dependent electron paramagnetic resonance (EPR) spectra were conducted to confirm the presence of strongly oxidative radicals during eCO2RR (Fig. 5c and Supplementary Fig. 18). The F–Cu catalyst underwent a 20-min eCO2RR at −1.5 V before electrolyte collection to mitigate the impact of residual oxidative species in the CO2-saturated 0.1 M KHCO3 electrolyte containing 10 mM 5,5-dimethyl-1-pyrroline N-oxide (DMPO). OH radicals (AN = 1.50 mT, AH = 1.48 mT) rather than hydrogen radicals, were produced during eCO2RR, differing from previous studies30. The in situ-generated strongly oxidative OH radicals can induce the oxidation of active Cu species during eCO2RR. This indicated that a locally oxidative microenvironment on the catalyst surface facilitated the C–C coupling for C2H4 production, even in a neutral electrolyte. Upon switching to OCP, the OH radicals in the electrolyte did not diminish, and their persistent presence after the eCO2RR reaction stabilized active Cuδ+ species, further inducing the structural evolution of catalysts. This finding agrees with in situ XAS spectra results (Fig. 5a, b and Supplementary Fig. 17).The presence of an oxygenated Cu surface during eCO2RR was further confirmed by the in situ Raman spectra (Supplementary Fig. 19). Ex situ SEM images tracked the morphological changes during eCO2RR in the Raman cell, correlating with MEA reactor conditions. Similar structural evolution was confirmed (Supplementary Fig. 20). Initially, Cuδ+ species clustered in islands near GDEs cracks. Subsequently, structural evolution began at the center and edges of the island. Accumulated Cuδ+ species from the bulk phase promoted rapid nanowhisker growth, covering the entire area. This morphological transformation highlights the relevance of our in situ Raman characterization to actual eCO2RR in the MEA reactor. In the Raman spectra of F–Cu (Fig. 5d and Supplementary Fig. 21a), peaks at 565.2 cm−1 (Cu2O)30, 364 cm−1 (Cu–CO)10, 1073.5 cm−1 (CO32−)30, and 1522 cm−1 (C=C stretching from CO2)31 emerged at −0.5 V. The absence of Cuδ+–OH species indicated no morphology reconstruction at low potentials. However, surface Cuδ+–OH species at 532 cm−110 became predominant at −3.5 V (Fig. 5d), suggesting the onset of structural evolution. At −3.6 V, a Cu2O peak reappeared (Fig. 5d), further driving the structural transformation of nanowhiskers. After eCO2RR, these oxidized Cuδ+ species (Cu–OH, Cu2O) underwent additional oxidation to form CuO (Supplementary Fig. 21b), consistent with observations from HRTEM, HADDF-STEM, and ex situ XAS spectra (Fig. 2f, Fig. 4c, d and Supplementary Fig. 8c, d). Furthermore, the resistance of Cuδ+ (Cu–OH, Cu2O) species to reduction under high cathodic potentials indicates that the stabilized Cuδ+ species are the actual active sites during eCO2RR, while changes in morphology enhance eCO2RR-to-C2H4 conversion. Additionally, prior studies have shown that F doping can regulate the electronic structure of the active site to optimize the eCO2RR performance32,33. Therefore, we investigated the impact of F-CB by analyzing the in situ Raman spectra of Cu NPs (Supplementary Fig. 21c). We found that while Cu NPs showed Raman peaks similar to F–Cu (Cu–CO and Cu2O) at low potentials, Cu–OH species were absent at high potentials. Comparison of the structural evolution of Cu NPs and F–Cu (Supplementary Fig. 5 vs Supplementary Fig. 7) indicated that F–CB altered the interfacial microenvironment of Cu NPs, enhancing the structural transformation with Cuδ+–OH species playing a critical role. F-functional groups can activate water, enriching strongly oxidative OH radicals on the Cu surface, thereby stabilizing Cuδ+ species. We conclude that Cuδ+–OH species are not involved in reconstruction under low electric fields, resulting in a reversible process, despite the detection of Cuδ+ (Cu2O and Cu-CO) species during eCO2RR. Conversely, under high electric fields, Cuδ+ (Cuδ+–OH) species are generated; surface defects and functional groups from ACB act as kinetically trapped sites, facilitating the migration of Cuδ+ species from bulk to the support surface. In situ-generated, strongly oxidative OH radicals stabilize oxidized Cuδ+ species in nanowhiskers, ultimately leading to irreversible structural reconstruction under high bias.Real-time ATR FT-IR spectra were employed to monitor intermediate adsorption processes at the electrode-electrolyte interface. Spectra were collected every two minutes during potential application and after potential cessation in a CO2-staturated 0.1 M KHCO3 solution (Fig. 5e and Supplementary Figs. 22 and 23). Referring to in situ ATR FT-IR spectra of Cu NPs (Supplementary Fig. 23a), only a few intermediate peaks related to eCO2RR were detected, likely due to multiple reaction pathways and low intermediate concentrations. In contrast, the F–Cu sample showed more prominent adsorption bands at 1003 cm-1 and 1148 cm-1, attributed to the *CHO and the C–O bond vibration mode34,35, respectively. These are recognized as key intermediates in the C–C coupling during eCO2RR to C2H4 (Fig. 5e). Furthermore, increasing cathodic potentials resulted in higher intensity of intermediate adsorption on the F–Cu surface, indicating the potential-dependent nature of eCO2RR reaction rates. Additionally, a broad adsorption band related to H-bonded hydroxyl groups appeared in the 2500–3600 cm−1 range, accompanied by adsorbed water (δ(H–O–H)) at 1630 cm−1 (Fig. 5e)36. These bands are indicative of perturbed water molecules under negative potentials. However, extra-ligand OH exhibited stretching vibration modes of ν(OH) at 3721 cm−1 and δ(OH) at 1252 cm−13637, on the external surfaces of F–Cu (Fig. 5e), primarily due to the contribution of framework modes. This indicated the presence of a metal ionic site (e.g., Cuδ+ species). Thus, the bands at 890 cm−1 and 3650 cm−1 were considered characteristic features of Cuδ+–OH species stabilized in the ν(OH) and δ(OH) modes38,39, respectively. Again, this underscored the crucial role of hydroxyl species as oxidants for Cuδ+ species (Fig. 5e). Although Cu NPs also exhibited infrared peaks corresponding to Cuδ+–OH species, there was no disturbance observed in the vibrations of water molecules at 1620 cm−1 (Supplementary Fig. 23a). This further indicated that the introduction of F-functional groups enhanced the activation and splitting of water molecules, facilitating Cu binding with OH species.Intermediate adsorption processes were recorded to elucidate the origin of hydroxyl species (Fig. 5e and Supplementary Fig. 23b–f). Bands showing significant changes were marked with dotted lines and arrows. With increasing applied potentials, the vibrational modes of bicarbonate at 1360 cm−1 gradually shifted toward a broader band centered on the asymmetric C–O stretch of carbonate at 1390 cm−140. This was explained by the increase in OH radicals at the cathodic electrode interface during water reduction, which reduced adsorbed bicarbonate and increased carbonate under more negative potentials. This hypothesis was further confirmed by water molecule perturbations. Increasing cathodic potentials shifted δ(H–O–H) from 1630 cm−1 to 1575 cm−1 with decreased intensity, indicating that water dissociation produced more OH radicals to stabilize Cuδ+ species, particularly under high negative potentials. Moreover, the higher intensity of Cuδ+–OH and extra-ligand OH species with increasing potentials revealed changes in the surface structure. Additionally, the mechanism of depopulation of intermediates was investigated (Supplementary Fig. 23b–f). At a low cathodic potential of −1.5 V, the bicarbonate band at 1359 cm−1 remained unchanged, while the δ(H–O–H) band slightly shifted. These peaks gradually disappeared after the potentials were turned off, but the in situ-generated eCO2RR intermediates disappeared immediately. Further increasing the potential resulted in a greater shift for bicarbonate and stronger perturbations of water molecules, but they gradually returned to the initial stage when the potentials were removed. Furthermore, extra-ligand OH species were unlikely to stabilize on active Cuδ+ sites after eCO2RR under high negative potentials. Instead, water molecules coordinated with Cuδ+ species to form aqua complexes and maintained a specific configuration. When exposed to air, it gradually oxidized to CuO.Based on the above analysis, OH radicals can create a locally oxidative microenvironment on the Cu surface during eCO2RR, stabilizing Cuδ+ species and enhancing C2H4 selectivity. Although in situ Raman spectroscopy, EPR spectra, ATR FT-IR spectroscopy, and XAS spectra can directly detect Cuδ+-OH species, the significant difference in current densities between in situ electrochemical cell and MEA reactor precludes accurate simulation of catalyst evolution in an MEA. This is because the rapid kinetics of the eCO2RR at high current densities introduce gas bubbles that critically affect data acquisition. Furthermore, in the MEA reactor, the catalyst layer on the GDEs is closely attached to the AEM, preventing direct monitoring of surface structural changes of the catalyst using Raman, infrared, and EPR spectroscopy. XAS spectroscopy could be a feasible method for monitoring changes in the oxidation state and coordination environment of copper during the MEA testing. However, to gain more structural insights, future endeavors should focus on rationally designing in situ MEA cells and conducting coordinated multimodal characterization studies to elucidate the evolution of active sites.
