Boron‐Doped Carbon Coatings Stabilize Cu⁺/Cu⁰ Interfaces to Promote Multicarbon Formation in CO₂ Electroreduction

Abstract

The construction of a stable Cu⁺/Cu⁰ reaction interface is a promising yet challenging strategy for the electrosynthesis of multicarbon (C₂₊) products from CO₂ reduction. Herein, we report a dual‐phase Cu₂O/Cu catalyst encapsulated by boron (B)‐doped carbon coatings (BC), denoted as Cu₂O(Cu)@BC. In a flow cell system with 1 M KOH electrolyte, this catalyst delivers a maximum C₂₊ Faradaic efficiency of 83.4% at −0.7 V versus the reversible hydrogen electrode (RHE) and a partial C₂₊ current density of 460 mA cm⁻² at −0.9 V versus RHE, while maintaining stable operation for over 42 h. This performance surpasses that of its counterpart without the BC coatings and other Cu‐based catalyst systems. Comprehensive experimental investigations reveal that the BC coatings facilitate the formation of Cu⁺ species with higher oxidation states and stabilize the Cu⁺/Cu⁰ interface under reductive conditions. The resulting abundant and stable interfacial sites significantly increase *CO intermediate surface coverage, facilitating carbon–carbon coupling and promoting the formation of the key *OCH₂CH₃ intermediate for C₂₊ production. This work offers an effective strategy to maintain a high density of Cu⁺/Cu⁰ interfacial active sites and provides deeper insights into the stabilization and degradation mechanisms of Cu‐based catalysts for efficient electrochemical CO₂‐to‐C₂₊ conversion.

The boron‐doped carbon coating of a dual‐phase Cu₂O(Cu) catalyst stabilizes abundant Cu⁺/Cu⁰ interfaces during CO₂ electrolysis, enhancing *CO coverage and promoting C₂₊ formation. The catalyst achieves a C₂₊ Faradaic efficiency of 83.4%, a partial current density of 460 mA cm⁻², and operates stably for at least 42 h. This work highlights carbon coating engineering for efficient C₂₊ electrosynthesis.

1. Introduction

The electrosynthesis of multicarbon (C₂₊) products via the electrochemical CO₂ reduction reaction (eCO₂RR), powered by renewable electricity, represents a promising and sustainable route for storing intermittent energy (e.g., solar energy and wind energy) into value‐added chemicals [1]. Among various catalysts, metallic copper (Cu) serves as the most prominent candidate for the electrosynthesis of C₂₊ products owing to its moderate binding affinity toward *CO, a key intermediate in carbon–carbon (C–C) coupling, although other metals also exhibit significant potential for C₂₊ production [2, 3, 4]. However, Cu also binds protons favorably, and the close thermodynamic equilibrium potential between the hydrogen evolution reaction (HER) and CO₂ reduction leads to severe competition, resulting in limited selectivity toward C₂₊ products [2]. To address this challenge, extensive efforts have been devoted to modulating Cu‐based catalysts through strategies such as valence‐state engineering [5], facet engineering [6], and doping engineering [7], aiming to enhance *CO coverage and promote C–C coupling.

Previous studies have demonstrated that Cu^{δ +} species play a crucial role in promoting the formation of C₂₊ products by enhancing the adsorption of *CO intermediates and facilitating the dimerization of *CO and its derivatives (e.g., *CHO and *COH) [8]. However, during the CO₂ reduction process, metastable Cu^{δ +} species are inevitably reduced to Cu⁰ under negative potentials, leading to a significant decline in selectivity toward C₂₊ products [7]. Therefore, the construction and stabilization of Cu^{δ +} species are necessary for promoting C₂₊ production over Cu‐based catalysts. Introducing electronegative heteroatoms provides a feasible strategy to stabilize Cu^{δ +} by regulating the electronic properties of Cu. For instance, Yang et al. reported a F‐stabilized Cu⁺/Cu⁰ interface model catalyst synthesized via one‐step pulsed potential conversion. The catalyst achieves a C₂₊ Faradaic efficiency (FE_C2+) of 80.2%, which is 18.5‐fold higher than that of pristine Cu catalysts [9]. Sargent et al. reported a boron (B)‐doped Cu catalyst that achieves a FE_C2+ of 79% ± 2%, attributed to the B dopants stabilizing Cu⁺ species [10].

Carbon coating engineering provides an alternative structural strategy by creating a confined and protective environment around Cu‐based catalysts via catalyst–support interactions and nanoconfinement effects, thereby mitigating the irreversible reduction of Cu^{δ +} species during the eCO₂RR process [11, 12]. Furthermore, the nanoconfinement effect of carbon coatings promotes the enrichment of *CO intermediates at the reaction interface, thereby enhancing the C–C coupling process and increasing C₂₊ product selectivity [13]. For example, Bao et al. reported a carbon‐wrapped CuOx catalyst, in which the carbon shells stabilize Cu⁺ species and ultimately enable a FE_C2H5OH of 46% [14]. However, a fully continuous carbon shell may hinder reactant access to the catalytic interface, thereby reducing the number of accessible active sites [15]. To address this limitation, heteroatom incorporation into carbon coatings has been proposed as a promising strategy. On one hand, it preserves the nanoconfinement effect while inducing defective carbon structures that favor CO₂ adsorption. On the other hand, heteroatom doping modulates the electronic structure of the catalyst, thereby influencing its electroactivity. For instance, Zhang et al. reported a core–shell catalyst with an N‐doped carbon shell, where the selectivity between CH₄ and C₂H₄ can be tuned by adjusting the type and concentration of nitrogen species [16]. Notably, both theoretical and experimental studies have demonstrated that electron‐withdrawing B atoms can effectively stabilize Cu⁺ species, while their strong affinity toward oxygen contributes to the stabilization of oxygen‐containing intermediates [10, 17, 18].

Herein, a dual‐phase Cu₂O/Cu catalyst encapsulated with a BC coating was synthesized via a facile solvothermal–pyrolysis method, which generates abundant Cu⁺/Cu⁰ active sites. Importantly, the Cu⁺/Cu⁰ interfacial structure can be effectively preserved under negative potentials, owing to the strong metal–support interaction provided by the carbon coatings. Systematic in situ Raman spectroscopy and electrochemical analyses confirm that these stable Cu⁺/Cu⁰ active sites can significantly increase the surface coverage of *CO intermediates, thereby facilitating the C–C coupling and promoting the formation of *OCH₂CH₃ intermediates toward C₂₊ products. As a result, the Cu₂O(Cu)@BC achieves a maximum FE_C2+ of 83.4% at −0.7 V versus the reversible hydrogen electrode (RHE, all potentials mentioned hereafter are referenced to RHE, unless otherwise specified), along with a high C₂₊ partial current density (j _C2+) of 460 mA cm⁻² at −0.9 V, while maintaining superior long‐term stability for over 42 h. This work presents a feasible strategy to stabilize Cu⁺ species under reducing conditions and provides profound insights into the degradation mechanism of Cu‐based catalysts for efficient C₂₊ production.

2. Results and Discussion

2.1. Synthesis and Characterization of Catalysts

The Cu₂O(Cu)@BC and Cu₂O(Cu) catalysts were synthesized via a facile solvothermal–pyrolysis method (Figure 1a). Specifically, the Cu₂O(Cu)@BC was obtained by pyrolysis in the presence of phenylboronic acid, which served as both the carbon and boron source, whereas Cu₂O(Cu) was prepared by direct pyrolysis of the Cu precursor in the absence of phenylboronic acid. Details are provided in the Supporting Information (SI). Powder X‐ray diffraction (PXRD) was employed to analyze the phase structures of the as‐synthesized catalysts. As shown in Figure 1b, both Cu₂O(Cu)@BC and Cu₂O(Cu) display characteristic diffraction peaks corresponding to Cu₂O (PDF#89‐2529) and metallic Cu (PDF#99‐0034). No diffraction peaks associated with CuO are observed, indicating the absence of CuO species. These results confirm that both catalysts consist of mixed Cu₂O and Cu phases [19]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out to examine the morphological features of the catalysts. SEM images reveal that both Cu₂O(Cu)@BC and Cu₂O(Cu) exhibit similar branch‐like morphologies (Figure S1), indicating that the introduction of phenylboronic acid has a negligible impact on the overall morphology. Consequently, the influence of morphology on CO₂ reduction performance can be reasonably excluded, allowing the observed catalytic differences to be primarily attributed to compositional variations and interfacial structures. High‐resolution TEM images further demonstrate that both catalysts exhibit lattice spacings of 0.25 and 0.21 nm, corresponding to the (111) crystal plane of Cu₂O and Cu, respectively (Figures 1c and S2) [20]. This observation confirms the presence of well‐defined Cu⁺/Cu⁰ interfacial structures. Notably, in contrast to Cu₂O(Cu), the Cu₂O and Cu phases in Cu₂O(Cu)@BC are encapsulated by amorphous carbon coatings with an average thickness of ≈4 nm, indicating the successful formation of carbon coatings on the catalyst surface. Raman spectroscopy further corroborates the presence of amorphous carbon. Cu₂O(Cu)@BC exhibits two characteristic peaks at 1286 and 1607 cm⁻¹, corresponding to the D band and G band of carbon coatings, whereas these features are absent in Cu₂O(Cu) (Figure S3). In addition, the stronger D band relative to G band suggests that the carbon coatings possess abundant structural defects. These defects are expected to provide pathways for electrolyte penetration and maintain the accessibility of active sites, rather than forming a fully blocking layer.

FIGURE 1.

Synthesis and characterization of catalysts. (a) Schematic diagram of catalysts preparation. (b) PXRD patterns of catalysts. (c) High‐resolution TEM image of Cu₂O(Cu)@BC. (d) Cu K‐edge XANES spectra of catalysts. (e) Average oxidation state of Cu in catalysts.

To verify the incorporation of B and elucidate the chemical valence states of Cu species, X‐ray photoelectron spectroscopy (XPS) was performed. In the Cu₂O(Cu)@BC catalyst, the survey XPS spectrum exhibits a distinct B 1s signal at 192.0 eV, whereas this signal is absent in Cu₂O(Cu) (Figure S4a) [10]. The fine B 1s spectra further reveal the presence of B—C bonds (Figure S4b), confirming that B dopants are successfully incorporated into the carbon matrix. Fine Cu 2p spectra demonstrate that both Cu₂O(Cu)@BC and Cu₂O(Cu) display prominent peaks at 932.4 and 952.1 eV, corresponding to Cu⁺/Cu⁰ species, along with weaker peaks at 934.7 and 954.8 eV assigned to Cu²⁺ species (Figure S4c) [21]. The presence of Cu²⁺ is attributed to surface oxidation of Cu⁺/Cu⁰ upon air exposure. Notably, compared with Cu₂O(Cu), Cu₂O(Cu)@BC exhibits a markedly lower Cu²⁺ peak intensity, indicating that the carbon coatings effectively suppress oxidation and stabilize Cu⁺/Cu⁰ species. To further distinguish between Cu⁺ and Cu⁰ species, Cu LMM Auger electron spectroscopy was further performed (Figure S4d). A dominant peak centered at around 916.9 eV, characteristic of Cu⁺ [22], corroborates the predominance of Cu⁺ species in the catalysts. X‐ray absorption spectroscopy was carried out to precisely probe the oxidized valence state of Cu, with Cu foil and standard Cu₂O as reference standards. As shown in Figure 1d, the Cu K‐edge X‐ray absorption near‐edge structure (XANES) spectra of both catalysts exhibit white‐line peaks located between those of Cu foil and Cu₂O, indicating an intermediate valence state between Cu⁰ and Cu⁺. To quantitatively evaluate the average oxidation state of Cu, first‐derivative XANES spectra were analyzed (Figure S5). The results show that Cu₂O(Cu)@BC possesses a higher average Cu oxidation state of 0.88 compared to that of 0.6 in Cu₂O(Cu) (Figure 1e). This difference is primarily attributed to the electron‐withdrawing effect of B dopants, which induce electron transfer from Cu to the BC coatings.

2.2. Evaluation of eCO₂RR Performances

The eCO₂RR performances of the catalysts were evaluated in a flow‐cell configuration using 1 M KOH electrolyte (Figure S6). Linear sweep voltammetry measurements show that both Cu₂O(Cu)@BC and Cu₂O(Cu) exhibit significantly higher cathodic current densities under CO₂ compared to an Ar atmosphere within the potential range from 0 to −1.0 V, indicating their favorable activity toward CO₂ reduction (Figure S7). Under CO₂ conditions, Cu₂O(Cu)@BC achieves a maximum current density of 840 mA cm⁻² at −1.0 V, which is ≈1.5‐fold higher than that of 512 mA cm⁻² in Cu₂O(Cu). This enhanced performance is attributed to improved electron transfer enabled by the conductive BC coatings [23]. Consistently, electrochemical impedance spectroscopy reveals that Cu₂O(Cu)@BC exhibits a lower charge transfer resistance of 6.8 Ω compared to 7.9 Ω in Cu₂O(Cu) (Figure S8). The potential‐dependent product distributions of the catalysts were analyzed over the potential range from −0.4 to −1.0 V via constant‐potential electrolysis. The resulting gaseous and liquid products were quantified by online gas chromatography and proton nuclear magnetic resonance (¹H NMR), respectively (Figure S9). As shown in Figure S9a, CO and C₂H₄ are the dominant gaseous products for both catalysts, accompanied by H₂ as a byproduct. Notably, the ¹H NMR spectrum for Cu₂O(Cu)@BC reveals the formation of n‐propanol and acetate, whereas these liquid products are absent in Cu₂O(Cu) (Figure S9b). These results suggest that the BC coatings promote the adsorption and stabilization of key intermediates, thereby facilitating the formation of more highly hydrogenated products.

The total FEs for eCO₂RR products as a function of applied potential were evaluated. As shown in Figures 2a and S10, the total FEs for all catalysts approach ≈100%, excluding the formation of undetected products. For Cu₂O(Cu)@BC, the FEs of C₂H₅OH and C₂H₄ increase with increasingly negative potentials, reaching maximum values of 42.5% at −0.7 V and 29.0% at −0.6 V, respectively (Figure 2a). At more cathodic potentials, the FEs decrease due to the enhanced formation of competing products such as H₂ and HCOO^‐. A similar trend is observed for Cu₂O(Cu), with peak FEs of 14.5% for C₂H₅OH and 36.1% for C₂H₄ at −0.7 V (Figure S10). To further evaluate C–C coupling performance, the FEs and partial current densities of C₂₊ products were plotted as a function of applied potential (Figure 2b). Among them, Cu₂O(Cu)@BC achieves a maximum FE_C2+ of 83.4% at −0.7 V, significantly higher than that of 53.6% in Cu₂O(Cu) under identical conditions. Moreover, Cu₂O(Cu)@BC delivers a peak j _C2+ of 460 mA cm⁻² at −0.9 V, which is more than three times higher than that of 144 mA cm⁻² at −0.7 V in Cu₂O(Cu). The selectivity toward C₂H₅OH is also markedly improved, as evidenced by a maximum C₂H₅OH/C₂H₄ ratio of 1.7 at −0.8 V, nearly threefold greater than the 0.6 observed in Cu₂O(Cu) at −0.9 V (Figure 2c). These results demonstrate that the introduction of BC coatings in Cu₂O(Cu)@BC significantly promotes the formation of C₂₊ products while improving C₂H₅OH selectivity. The durability of catalysts was evaluated by long‐term electrolysis at −0.7 V. The Cu₂O(Cu)@BC catalyst maintains stable FEs of ≈40% for C₂H₅OH and 20% for C₂H₄ without noticeable decay for over 42 h, indicating excellent stability (Figure S11). Meanwhile, postelectrolysis TEM image further reveals that the carbon coating remains intact, along with the presence of Cu⁺/Cu⁰ interfaces, indicating the structural stability of the catalyst (Figure S12). In contrast, Cu₂O(Cu) shows a rapid decline in FEs after only 4 h, which can be attributed to the depletion of oxidized Cu species. Overall, these results indicate that the BC coatings not only significantly enhance the electrocatalytic activity of the catalysts by accelerating electron transfer between Cu and BC coatings, but also strengthen the stability by maintaining stable Cu⁺/Cu⁰ active sites through a carbon confinement effect. Consequently, Cu₂O(Cu)@BC exhibits C₂₊ activity comparable to that of other state‐of‐the‐art Cu‐based catalysts (Figure 2d) [9, 10, 22, 24, 25, 26, 27].

FIGURE 2.

Evaluation of eCO₂RR performance. (a) Potential‐dependent product distributions of Cu₂O(Cu)@BC over the potential range from −0.4 to −1.0 V. (b) FE_C2+ and j _C2+ in all catalysts as a function of the applied potential. (c) The C₂H₅OH/C₂H₄ ratio in all catalysts as a function of applied potential. (d) Comparison of eCO₂RR performance between Cu₂O(Cu)@BC and other electrocatalysts.

2.3. Dynamic Evolution of Cu⁺ Species

To investigate the evolution of Cu⁺ species during the CO₂ reduction process, in situ Raman spectroscopy was performed in a commercial electrocatalytic cell using 1 M KOH as the electrolyte. This technique enables real‐time monitoring of structural and chemical changes in the catalysts under operating conditions. Cu₂O possesses several well‐defined Raman features at 146, 221, 412, 528, and 625 cm⁻¹ [22], providing distinct spectral fingerprints of Cu⁺ species. Therefore, Raman spectroscopy provides an effective approach to quantitatively track the dynamic evolution of Cu⁺ species under varying potentials and reaction times. At open circuit potential (OCP), both Cu₂O(Cu)@BC and Cu₂O(Cu) display three prominent Raman peaks centered at ≈146, 221, and 625 cm⁻¹, corresponding to characteristic vibrational modes of Cu₂O (Figure 3a,b). Among these, the peak located at 625 cm⁻¹ shows the highest intensity and is identified as the primary fingerprint signal of Cu₂O. Accordingly, this peak was selected for quantitative analysis of Cu⁺ species, and its integrated area at OCP was used as the reference baseline to evaluate the relative variation of Cu⁺ content during the reaction. To further examine the structural evolution under eCO₂RR conditions, time‐dependent in situ Raman spectra were collected at the optimal potential of −0.7 V over a reaction period ranging from 1 to 120 min (Figure 3a,b). The relative residual Cu⁺ content was then determined based on the integrated area of the 625 cm⁻¹ peak and plotted as a function of reaction time (Figure 3d). For Cu₂O(Cu), the intensity of the Cu₂O Raman signal continually decreases with increasing reaction time and eventually vanishes after ≈60 min, indicating that the Cu₂O phase is almost completely reduced to metallic Cu under negative potential conditions. As a result, the progressive depletion of Cu⁺ species results in the loss of Cu⁺/Cu⁰ interfacial sites, which are essential for C–C coupling, thereby impairing C₂₊ activity. This result also explains a significant drop in FE_C2+ for Cu₂O(Cu) after electrolysis 4 h.

FIGURE 3.

Time‐dependent in situ Raman spectra of (a) Cu₂O(Cu)@BC and (b) Cu₂O(Cu) collected at −0.7 V (the optimal potential for eCO₂RR performance) as a function of electrolysis time. (c) Time‐dependent in situ Raman spectra of Cu₂O(Cu)@BC collected −1.0 V as a function of electrolysis time. (d) Residual Cu⁺ content of all catalysts as a function of electrolysis time.

In contrast, the Raman evolution behavior of Cu₂O(Cu)@BC is markedly different and can be clearly divided into two stages. In the initial stage (<30 min), the Cu₂O characteristic Raman signal gradually decreases with electrolysis time, which can be attributed to the reduction of unstable surface Cu⁺ species. In the subsequent stage (>30 min), the Cu⁺ content exhibits only a slight decline and eventually stabilizes at approximate 60% of its initial value. This behavior differs significantly from the continuous decrease observed in the Cu₂O(Cu) sample. Therefore, the minor decline of Cu⁺ content during the second stage can be reasonably attributed to signal attenuation by gas bubble accumulation and the adsorption of reaction intermediates during electrolysis rather than the further reduction of Cu⁺ species. Importantly, the stable FE_C2+ during long‐term electrolysis further supports the structural and catalytic stability of catalyst under operating conditions. These results indicate that the Cu₂O phase in Cu₂O(Cu)@BC undergoes significantly higher thermodynamic/kinetic stability compared to bare Cu₂O(Cu) due to the presence of BC coatings, which act as a protective barrier for Cu₂O. Consequently, a considerable amount of Cu⁺ species and Cu⁺/Cu⁰ interfacial sites can be preserved during the reaction. To verify whether the carbon‐coated Cu species serve as accessible active sites, additional time‐dependent in situ Raman measurements were conducted at a more negative potential of −1.0 V (Figure 3c). Compared to −0.7 V, the Cu⁺ content decreases to ≈20% at −1.0 V under stead‐state conditions, indicating that carbon‐coated Cu₂O species can be further reduced at more negative potentials (Figure 3d). Since this process involves coupled electron and ion transfer to maintain charge neutrality, it suggests that the Cu₂O phase is not fully isolated by the carbon coating, but remains electrochemically accessible and can participate in the catalytic process. These results demonstrate that a relatively stable population of Cu⁺/Cu⁰ interfacial sites is maintained under moderate potentials as a result of the BC coatings, thereby facilitating C–C coupling and promoting the formation of C₂₊ products.

2.4. Mechanism Investigation

The identification of key intermediates (e.g., *CO) is crucial for elucidating the mechanistic origin of C₂₊ product formation during eCO₂RR. In particular, the evolution of carbon‐containing intermediates largely determines whether the reaction pathway proceeds toward C–C coupling or terminates at C₁ products. To gain deeper insight into the dynamic evolution of surface intermediates during the catalytic process, potential‐dependent in situ Raman spectroscopy was conducted. Raman spectra were collected under OCP and at applied potentials ranging from −0.4 to −1.0 V with an interval of 0.1 V, enabling systematic monitoring of surface species as the electrochemical environment gradually shifts toward stronger reduction conditions. The wavenumber range from 1700 to 2200 cm⁻¹ corresponds to the characteristic fingerprint region of adsorbed *CO species [28]. Under OCP, both Cu₂O(Cu)@BC and Cu₂O(Cu) show no detectable Raman signals (Figure 4a,b), indicating the absence of observable adsorbed intermediates prior to electrochemical activation. This observation excludes interference from background signals or extraneous contributions and confirms that the Raman features observed under applied potentials originate exclusively from intermediates generated during the eCO₂RR process. Upon applying negative potentials, a distinct set of characteristic peaks gradually emerges in this region, indicating the formation of adsorbed *CO species on the catalyst surfaces.

FIGURE 4.

Identification of reaction intermediates and proposed reaction mechanism. Potential‐dependent in situ Raman spectra of (a) Cu₂O(Cu) and (b) Cu₂O(Cu)@BC, with all spectra baseline‐corrected using OCP spectra. (c) Schematic illustration of the C₂₊ formation mechanism on Cu⁺/Cu⁰ interfaces.

Notably, the Raman signal of *CO‐related species is broad and weak, which can be attributed to the coexistence of multiple adsorption configurations (e.g., bridge‐adsorbed *CO, linearly adsorbed *CO, and free CO molecules), together with signal attenuation caused by gas bubble accumulation and dynamic adsorption/desorption processes. This coexistence of multiple *CO adsorption configurations is consistent with previous studies, which indicate that the Cu^{δ +}/Cu⁰ interfacial structures facilitate asymmetric C–C coupling, thereby promoting the formation of oxygenated products such as C₂H₅OH [29, 30, 31]. In addition, the Raman spectra of both sample under OCP conditions show no discernible differences and thus were used as the baseline reference for background subtraction (Figure S13). Therefore, the integrated area of the *CO‐related spectral region is used as a quantitative descriptor to analysis the evolution of *CO‐related species (Figure S14). Compared with Cu₂O(Cu), the Raman peak area corresponding to *CO‐related species is significantly enhanced for Cu₂O(Cu)@BC. The intensified peak area of *CO‐related species indicates a substantially higher surface coverage of *CO intermediates on Cu₂O(Cu)@BC under reductive conditions. Since C–C coupling is widely regarded as the key elementary step for C₂₊ products formation, the enriched *CO coverage on Cu₂O(Cu)@BC is expected to significantly increase the probability of the dimerization of *CO and its derivatives, thereby promoting the formation of C₂ intermediates. Consistently, an additional Raman band emerges at ≈2930 cm⁻¹ under negative potentials, which can be assigned to the ʋ_CHx vibrational mode of the *OCH₂CH₃ intermediate. This species is widely recognized as a product selectivity‐determining intermediate governing the formation of C₂H₅OH and C₂H₆ [32]. Similar to the *CO‐related signals, the ʋ_CHx Raman signal for Cu₂O(Cu)@BC increases more prominently than that for Cu₂O(Cu) with increasingly negative potentials, indicating a higher accumulation of *OCH₂CH₃ intermediates. This enhanced *OC₂H₅ adsorption can be attributed to the presence of strong oxygen affinity of B dopants, which can be evidenced by both theoretical calculations and experimental observations [18, 24, 33]. These results suggest that the BC‐modified catalyst surface promotes the stabilization and subsequent transformation of C₂ intermediates toward oxygenated products.

Based on the above in situ spectroscopic observations, a plausible reaction mechanism for the enhanced C₂₊ production over Cu₂O(Cu)@BC is proposed (Figure 4c). The BC coatings play a critical role in stabilizing the Cu⁺/Cu⁰ mixed‐valence active sites under reductive electrochemical conditions. Meanwhile, the coexistence of Cu⁺ and Cu⁰ sites can create synergistic catalytic interfaces that enhance CO₂ activation and intermediate adsorption. Consequently, the stabilized Cu⁺/Cu⁰ interface increases the surface coverage of *CO intermediates and promotes their asymmetric coupling into C₂ intermediates. These intermediates subsequently evolve into *OCH₂CH₃‐related species and ultimately lead to the formation of C₂H₅OH. Overall, these results demonstrate that the introduction of the BC coatings effectively regulates the surface electronic structure and adsorption behavior of key intermediates. By maintaining a high density of Cu⁺/Cu⁰ active sites and increasing the surface coverage of the key intermediates such as *CO and *OCH₂CH₃, the Cu₂O(Cu)@BC catalyst achieves enhanced C₂₊ activity and improved C₂H₅OH selectivity.

3. Conclusions

In conclusion, we developed a Cu₂O(Cu)@BC catalyst characterized by abundant and stable Cu⁺/Cu⁰ active sites under eCO₂RR conditions. In situ Raman spectroscopy combined with electrochemical investigations reveals that the BC coatings facilitate the formation of high‐valence Cu^{δ +} species and stabilize the abundant Cu⁺/Cu⁰ interfacial sites under reductive conditions. The stabilized interfaces increase the surface coverage of *CO intermediates, thereby promoting C–C coupling and facilitating the formation of key *OCH₂CH₃ intermediate toward C₂₊ products. As a result, the Cu₂O(Cu)@BC catalyst achieves a maximum FE_C2+ of 83.4% at −0.7 V and a high j _C2+ of 460 mA cm⁻² at −0.9 V, while maintaining stable operation for over 42 h. This work provides a feasible strategy for stabilizing Cu⁺/Cu⁰ interfacial active sites and offers valuable insights for the rational design of efficient Cu‐based catalysts for CO₂ electroreduction to C₂₊ products.

Supporting Information

Additional SI can be found online in the Supporting Information section.

Author Contributions

Haoming Yu: conceptualization, investigation, formal analysis, methodology, writing – original draft. Zhengyu Hua: investigation, validation. Lei Liu: investigation, formal analysis. Qiang Zhu: software, visualization. Fangqi Yang: visualization, validation. Tonglin Yang: formal analysis, investigation. Jun Wang: supervision, funding acquisition, writing – review & editing, Jie Zhang: supervision, funding acquisition, writing – review & editing.

Funding

This work was supported by Australian Research Council (CE230100017), Foundation for Innovative Research Groups of the National Natural Science Foundation of China (22368034), and Natural Science Foundation of Jiangxi Province (20224BAB203021).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.