Electronic State Coupling for Cu⁺ Stabilization to Boost Highly Efficient Transformation of CO₂ to C2 Products

ABSTRACT

Cu‐based nanomaterials are attracting great attention for the electroreduction of CO₂ to valuable C2 products. However, the optimization of physicochemical properties of Cu‐based active species, especially the electronic valence and orbital states, remains one of the greatest challenges in achieving desirable C2 products synthesis performance under harsh reductive electrolysis conditions. To tackle this obstacle, we propose a pulse‐enabled Cu valence‐state regulation strategy by integrating carbon quantum dots (CQDs). Under pulsed electrolysis, Cu_2‐xSe/CQDs delivers a Faradaic efficiency (FE) of up to 85.3% for C2 products and maintains FE_C2>70% over an ultra‐wide potential window of 1.6 V. Multiple in situ spectroscopic characterizations and theoretical simulations clarify that the strong electronic state coupling between Cu_2‐xSe and CQDs, together with pulsed electrolysis, maintains monovalent Cu species (Cu⁺), guaranteeing the outstanding C2 products synthesis efficiency. This work presents an innovative and universal protocol to guide the rational design of catalysts requiring precise oxidation‐state control.

A Cu valence‐state regulation strategy is developed by integrating carbon quantum dots (CQDs) with Cu_2‐xSe. The strong electronic state coupling, assisted by pulsed electrolysis, synergistically stabilizes the active Cu⁺ species, delivering an 85.3% Faradaic efficiency (FE) for C2 products and an ultra‐wide 1.6 V window (FE_C2>70%). This work provides a universal blueprint for precise oxidation‐state control.

1. Introduction

CO₂ electroreduction reaction (CO₂ER) to C2 products (mainly ethylene and ethanol) powered by renewable energy sources possesses an integrated function of greenhouse gas CO₂ elimination and valuable chemicals supply, which has been recognized as one of the most ideal solutions to realize the goal of sustainable development [1, 2, 3]. The C2 products synthesis efficiency of CO₂ER heavily relies on the rational design of highly efficient catalytic materials [4]. Cu‐based chalcogenides, especially copper selenide, stand out among various candidates owing to the optimized key intermediate evolution behaviors for C2 products (mainly ethylene and ethanol) synthesis, due to the exceptional capability in activating inert CO₂ molecules and promoting C─C bond coupling [5, 6, 7, 8, 9, 10]. Notably, the electronic valence and orbital states of Cu‐based active species have been recognized as the most important factor in affecting the performance of CO₂ER to valuable C2 products.

However, the identity of the “true active” Cu valence state that governs C2 products formation remains the subject of vigorous and unresolved debate [11, 12, 13, 14]. Depending on the applied potential, local pH, electrolyte composition, and catalyst morphology, distinct valence states‐including Cu⁰, Cu⁺, Cu²⁺, and even dynamically evolving mixed‐valence ensembles‐have all been reported as potentially active for C2 products synthesis [15, 16, 17, 18, 19]. For example, Yang and Zhang et al. showed strong in‐operando evidence that the active sites were fully metallic Cu for C2 product synthesis [11, 17]. Other operando and mechanistic studies have revealed that the Cu⁺ sites played an indispensable role in steering C2 product pathways [7, 8, 9, 10, 20]. This diversity underscores that the catalytic “active state” of Cu species is not a single universal entity but rather a condition‐dependent outcome shaped by the complex interfacial chemistry of CO₂ reduction [21, 22]. Consensus remains elusive regarding which valence state of Cu‐based active sites is beneficial for the synthesis of C2 products [23, 24, 25]. Nonetheless, the development of more efficient catalysts for CO₂ER to C2 and the in‐depth understanding of the reaction mechanism are extremely urgent and should always be ongoing [5, 26, 27, 28, 29]. Inspired by previous reported work, flexibly adjusting the valence state of the Cu‐based active sites that accommodate the C─O activation and C─C coupling behaviors is the most essential issue in boosting the CO₂ER to C2 performance [30, 31, 32, 33, 34, 35]. Therefore, it is a priority to develop a facile and reliable protocol for achieving on‐demand valence state tailoring of Cu‐based species [36, 37, 38].

Carbon‐based materials with extended π‐conjugation (e.g., fullerenes, carbon nanotubes, and graphene) have been well‐established as excellent electron acceptors [39, 40]. Among these candidates, carbon quantum dots (CQDs) stand out as a particularly promising electron‐regulation medium owing to their highly tunable energy levels induced by quantum confinement [41, 42]. Such features endow CQDs with the ability to modulate the local electronic configuration of adjacent metal centers, offering a powerful handle for dynamically regulating the valence states of Cu‐based catalysts under CO₂ER conditions [43]. In parallel, pulsed electrolysis has recently emerged as an advanced operational strategy capable of reshaping the local reaction microenvironment by periodically switching between distinct potentials [44, 45]. Unlike conventional static‐potential electrolysis, pulsed operation introduces temporal control over electron flux, surface charge density, and interfacial reaction kinetics, thereby enabling more flexible modulation of catalyst valence states and intermediate evolution [46, 47]. The synergistic integration of electronically adaptive CQDs with the pulsed electrolysis may provide a new conceptual avenue for dynamically tailoring the oxidation state distribution of Cu sites during CO₂ER, potentially overcoming the long‐standing challenge of maintaining catalytically favorable valence states under harsh reducing conditions.

Herein, we propose a Cu valence state regulation strategy, which integrates CQDs modification with the pulsed electrolysis strategy. Under pulsed electrolysis, Cu_2‐xSe/CQDs delivers a Faradaic efficiency (FE) of up to 85.3% for C2 products and maintains FE_C2>70% over an ultra‐wide potential window of 1.6 V. Additionally, it demonstrates exceptional durability with stable C2 products generation over 36 h of operation. Multiple in situ spectroscopic characterizations and theoretical simulations clarify that the strong electronic state coupling between Cu_2‐xSe and CQDs, together with pulsed electrolysis, effectively stabilizes Cu⁺ species, thereby ensuring the outstanding efficiency of C2 product synthesis. This work provides insights for the ongoing academic discussion on the “true active” Cu valence state and presents an innovative protocol for stabilizing valence states of Cu‐based active species, offering a promising blueprint for the rational design of other catalysts requiring appropriate oxidation‐state active sites to enhance the catalytic performance.

2. Results and Discussion

2.1. Rational Design of Cu‐Based Electrocatalyst

Briefly, the CQDs were synthesized via a hydrothermal treatment of citric acid and ethylenediamine, followed by dialysis and lyophilization. As depicted in Figure S1a, the prepared CQDs displayed uniform spherical morphology with an average diameter of ca. 5 nm. Obvious lattice fringes were observed, corresponding to the abundant sp² lattice carbon skeleton of CQDs (Figure S1b). The highly delocalized and unsaturated π‐electron structure endowed CQDs with excellent capabilities to rapidly transport electrons [41, 43]. Cu_2‐xSe nanosheets decorated with CQDs were synthesized via a one‐pot hydrothermal method. Specifically, copper nitrate trihydrate (Cu(NO₃)₂·3H₂O) and sodium selenite (Na₂SeO₃) were dissolved in a mixed solvent of diethylenetriamine (DETA) and deionized water. A pre‐determined mass of the as‐prepared CQDs was added to this mixture. After hydrothermal treatment, the resulting precipitate was collected, washed, rapidly cooled with liquid nitrogen, and freeze‐dried to obtain the final Cu_2‐xSe/CQDs catalyst (Figure 1a) [48]. The transmission electron microscopy (TEM) images revealed that Cu_2‐xSe possessed a sheet‐like ultrathin structure (Figures 1b and S2), which facilitates CQDs loading and exposes more active sites. Notably, CQDs were uniformly dispersed on the surface of Cu_2‐xSe, contrasting sharply with the smooth surface of pristine Cu_2‐xSe (Figure S3). The x‐ray diffraction (XRD) pattern confirmed that the decoration of CQDs preserved the crystal structure and phase purity of host Cu_2‐xSe (Figure S4).

FIGURE 1.

(a) The schematic diagram for elucidating the synthesis of Cu_2‐xSe/CQDs. (b) HRTEM image of the Cu_2‐xSe/CQDs sample. The dots in the white circle are CQDs. The default Cu_2‐xSe/CQDs without a suffix refers to samples with a CQDs loading of 6 mg. (c) Normalized Cu K‐edge XANES spectra of Cu_2‐xSe and Cu_2‐xSe/CQDs. (d) Fourier‐transformed Cu K‐edge EXAFS spectra. (e) Morlet WT of the k³‐weighted EXAFS data for the Cu_2‐xSe and Cu_2‐xSe/CQDs. (f) Time‐resolved photoluminescence spectra of CQDs, Cu_2‐xSe, and Cu_2‐xSe/CQDs. (g) Planar average potential for Cu_2‐xSe/CQDs. The yellow and blue regions represent the accumulation and depletion of charges, respectively.

2.2. Identification of Electron Transfer Pathway Between Cu_2‐xSe and CQDs

Cu K‐edge x‐ray absorption near‐edge structure (XANES) measurements were performed to evaluate the electron transfer between Cu_2‐xSe and CQDs. As shown in Figure 1c, Cu species in both Cu‐based catalysts existed dominantly in Cu⁺ form, and electrons transferred from Cu sites to CQDs. Critically, extended x‐ray absorption fine structure (EXAFS) analysis revealed that direct chemical bonding arose from the intimate interfacial contact between Cu_2‐xSe and CQDs, namely, Cu atoms in Cu_2‐xSe coordinated with light atoms (C/N/O) in the CQDs (Figures 1d, S5, S6, and Table S1). Such bonding provided efficient electronic state coupling, thereby enabling rapid interfacial electron transfer [49]. This direct chemical bond was further visualized in the Morlet wavelet transform (WT) of the k³‐weighted EXAFS signals (Figure 1e). Compared to the traditional combination of electrostatic adsorption, the direct bonding significantly reduced electron transfer resistance. The x‐ray photoelectron spectroscopy (XPS) results further reinforced this mechanism (Figures S7–S9). Electron redistribution from Cu_2‐xSe toward CQDs decreased the electron density at Cu sites, leading to a relative shift toward a more oxidized Cu state. As shown in Figure S10, fluorescence spectra revealed a static quenching process arising from the formation of interfacial complexes due to the intimate contact between Cu_2‐xSe and CQDs, which is related to the direct chemical bonding identified above. Importantly, this photophysical static quenching reflected strong electronic state coupling rather than static charge confinement during catalysis. Such strong interfacial bonding established the essential highly conductive framework required for the rapid, dynamic electron transfer under applied electrochemical bias. Time‐resolved photoluminescence (TRPL) measurements were further conducted to probe the carrier dynamics directly (Figure 1f). The TRPL spectra revealed that the fast decay time (τ ₁) of Cu_2‐xSe/CQDs was markedly prolonged to 11.1 ns compared with that of pristine Cu_2‐xSe (7.5 ns). This extension in lifetime could be ascribed to the efficient transfer of photo‐excited electrons from Cu_2‐xSe to CQDs, which suppresses electron–hole recombination and slows down the relaxation process. Such behavior was consistent with the strong interfacial electronic state coupling and static quenching effect, highlighting the crucial role of CQDs in facilitating interfacial charge separation and modulating electron dynamics. The DFT calculations further demonstrated that Cu_2‐xSe possessed both a higher Fermi level and lower work function than CQDs, offering the thermodynamic feasibility of the electron transfer from Cu_2‐xSe to CQDs (Figure S11). The differential charge density directly visualized this electron redistribution between the two components (Figure 1g), correlating with elevated Cu species oxidation state and d‐band center (Figure S12). Collectively, integrated multi‐modal analyses confirmed a direct, low‐resistance electron‐transfer pathway at the Cu_2‐xSe/CQDs interface, enabling CQDs to directly modulate the chemical state of Cu_2‐xSe.

2.3. CO₂ER Performance Evaluation

The catalysts (Cu_2‐xSe and Cu_2‐xSe/CQDs) were evaluated for the CO₂ER under both static and pulsed electrolysis in a flow cell with a 1 M KOH aqueous electrolyte (Figures 2a and S13). Static electrolysis was first implemented. As shown in Figure S14, the pristine Cu_2‐xSe catalyst primarily produced H₂ under static electrolysis, and the highest FE_C2 was only 28.6% at −1.6 V versus RHE. Subsequent tests on Cu_2‐xSe/CQDs catalysts with different CQDs loadings (Figure 2b) revealed that the incorporation of CQDs effectively increased the selectivity of C2 products. Specifically, the Cu_2‐xSe/CQDs‐6 mg catalyst achieved a higher FE_C2 of 45.5% at −1.6 V versus RHE under static electrolysis. This improvement demonstrated the favorable modulation of the Cu_2‐xSe valence state enabled by the CQDs. Subsequently, pulsed electrolysis was implemented. The CO₂ER performance results with different electrolysis time of reduction/oxidation steps and different pulsed potentials demonstrated that the appropriate pulsed electrolysis parameters (anodic potential E _r = 0.6 V vs. RHE, anodic duration t _r = 5 s; cathodic potential E _c, cathodic duration t _c = 50 s) were essential for boosting C2 product selectivity (Figure S15). As shown in Figure S16, the FE_C2 of pristine Cu_2‐xSe increased from 28.6% to 45.7% under pulsed electrolysis. When tested under identical pulsed conditions, all Cu_2‐xSe/CQDs catalysts exhibited significantly increased FE_C2 and decreased FE_CO compared to static electrolysis (Figure 2c). Crucially, Cu_2‐xSe/CQDs‐6 mg exhibited the highest FE_C2, reaching up to 85.3% at −1.6 V versus RHE, which is comparable to representative Cu‐based CO₂ER catalysts reported in the literature (Figures 2d and S17). Ethanol was one of the main C2 products, with an FE of up to 43.7%. Some ethylene (FE = 24.7%) and acetic acid (FE = 16.9%) were also generated. Notably, as shown in Figure S18, analysis of the products in the counter electrode compartment revealed the presence of a small amount of H₂. In stark contrast, when a constant potential equivalent to the anodic pulse (0.6 V vs. RHE) was directly applied, almost no H₂ evolution was observed. Furthermore, the negligible CO signal indicated that C─C coupling kinetically outcompeted CO desorption, with nearly all CO intermediates being consumed toward C2 product synthesis. Notably, even at a severely negative potential of −2.4 V versus RHE, hydrogen evolution remained suppressed, and FE_C2 retained an exceptional level of 77.4%. In addition, the products of pure CQDs were mainly concentrated in H₂ and HCOOH, indicating that the excellent C2 product selectivity of Cu_2‐xSe/CQDs did not originate from the intrinsic reactivity of the CQDs alone (Figure S19). More importantly, across the optimal CQDs loading range (4–8 mg), pulsed electrolysis enabled an ultra‐wide operating window of 1.6 V (FE_C2> 70%) (Table S2). A wide potential window significantly enhances the process resilience of the electrocatalytic system, ensuring that the catalyst maintains high C2 product selectivity and stable performance against potential fluctuations caused by electrode aging or electrolyte variations in practical operating environments. As shown in Figure S20, the Cu_2‐xSe/CQDs‐6 mg catalyst achieved higher partial current density for C2 products and a peak ratio (FE_C2/FE_C1) of 12.0 under pulsed conditions, underscoring its superior C2 selectivity. Moreover, under pulsed electrolysis, Cu_2‐xSe/CQDs achieved a higher C2 partial current density relative to pristine Cu_2‐xSe (Figure S21). This comparison was intended to highlight the benefit of the pulsed strategy within the present Cu_2‐xSe system, rather than to imply a universal energy advantage over state‐of‐the‐art Cu catalysts operated in advanced flow‐cell configurations. These results collectively demonstrated a pronounced synergistic effect between CQDs decoration and the pulsed‐electrolysis protocol, enabling substantially higher C2 selectivity than either strategy could achieve on its own (Figure 2e). To quantitatively assess the individual contributions of two factors to performance enhancement, deconvolution analysis is shown in Figure 2f. The results revealed that pulsed operation contributed dominantly to FE_C2 enhancement compared to CQDs loading. More importantly, the maximum C2 selectivity could be achieved only through the synergistic integration of CQDs decoration and pulsed electrolysis strategy.

FIGURE 2.

(a) Schematic diagram of the potential curve under static and pulsed electrolysis. (b, c) FE of C2 products on the Cu_2‐xSe/CQDs catalysts with different CQDs loadings under static electrolysis (b) and pulsed electrolysis (c) at different applied potentials. (d) The comparison of performance for CO₂ER to C2 products on Cu_2‐xSe/CQDs with some representative Cu‐based catalysts. Detailed comparison data are summarized in the Supporting Information. (e) Synergistic enhancement of C2 selectivity by CQDs loading and pulsed electrolysis. (f) The contribution ratio of CQDs loading and pulsed electrolysis to the improvement of FE_C2 with different CQDs loadings. (g) Time‐dependent total current density and FE_C2 under static electrolysis and pulsed electrolysis for 36 h.

Furthermore, complementary electrochemical characterization, including Tafel analysis, open‐circuit voltage (OCV), electrochemical impedance spectroscopy (EIS), and electrochemically active surface area (ECSA) measurements (Figures S22, S23, and Table S3), confirmed that CQDs enhanced the intrinsic activity of Cu_2‐xSe toward C2 production. As shown in Figure 2g, long‐term stability tests confirmed that there was no obvious decrease in C2 products for Cu_2‐xSe/CQDs catalyst under pulsed electrolysis even after 36 h. In contrast, pristine Cu_2‐xSe lost the production capability of C2 products within 10 h under static electrolysis. Although pulsed operation marginally extended the lifetime of pristine Cu_2‐xSe, it failed to maintain high C2 selectivity (FE>70%). The further post‐reaction characterization of the spent Cu_2‐xSe/CQDs catalysts was conducted. The TEM images confirmed that the nanosheet morphology and CQDs distribution were well‐preserved in the Cu_2‐xSe/CQDs after CO₂ER (Figure S24). In addition, the inductively coupled plasma (ICP) and elemental analysis (EA) results demonstrated that the elemental composition of the catalyst showed only modest changes in the recovered bulk composition, which might reflect possible minor Cu atom leaching (Table S4). More importantly, as shown in Figure S25, the Cu 2p and Cu LMM Auger XPS results revealed the retention of Cu⁺ species, which may play a vital role in boosting C2 products synthesis. This aspect will be systematically elucidated in the following section through detailed electrochemical measurements and comprehensive operando spectroscopic characterizations. These results collectively established that the Cu_2‐xSe/CQDs catalyst under pulsed electrolysis achieved exceptional selectivity and operation stability for CO₂ER to C2 products.

To further probe the unique role of CQDs, we replaced CQDs with carbon nanotubes (CNTs) or graphene. As shown in Figure S26, both Cu_2‐xSe/CNTs and Cu_2‐xSe/graphene exhibited markedly lower selectivity toward C2 products compared with the CQDs‐modified counterparts. This performance loss can be attributed to the larger lateral dimensions of CNTs and graphene, which hinders the formation of intimate chemical contact and effective interfacial bonding with Cu_2‐xSe. As a result, these extended carbon scaffolds were unable to form the low‐resistance electron‐transfer pathways and effectively regulate the chemical state of Cu_2‐xSe. To verify the universality of the synergistic effect between CQDs decoration and the pulsed‐electrolysis protocol, we further implemented the strategy on other Cu‐based catalysts, such as Cu₃N and Cu₂O (Figure S27). Under pulsed‐potential operation, both Cu₃N/CQDs and Cu₂O/CQDs displayed substantially enhanced C2 selectivity. Post‐electrolysis XRD and XPS analyses confirmed that both Cu₃N/CQDs and Cu₂O/CQDs robustly retained their original phases and Cu⁺ states with no detectable metallic Cu° formation, providing additional evidence that this synergism is a broadly applicable strategy to promote C─C coupling and boost C2 product synthesis across diverse Cu‐based systems.

2.4. Evidence and Origin of Synergism Effect

The cyclic voltammetry (CV) test served as a primary diagnostic tool to validate the role of CQDs for valence state regulation of Cu based catalysts. To probe the intrinsic redox characteristics, CV tests were performed in a dimethylformamide (DMF) electrolyte containing 0.1 M (Bu)₄NBF₄ (Figure S28). Notably, the CV profile of Cu_2‐xSe/CQDs mirrored CQDs instead of Cu_2‐xSe, which suggests that the CQDs impose a strong electronic modulation on the Cu_2‐xSe and effectively reshape its redox behavior. This phenomenon persisted under catalytically relevant aqueous conditions, as shown in Figures 3a and S29. While pristine Cu_2‐xSe showed irreversible Cu⁺/Cu⁰ redox waves, Cu_2‐xSe/CQDs exhibited CV profiles similar to pure CQDs, implying that strong electronic state coupling might effectively stabilize Cu⁺ species by suppressing their over‐reduction. Subsequent linear sweep voltammetry (LSV) in a flow electrolyzer further revealed the unique role of CQDs. As shown in Figure 3b, under a CO₂ atmosphere, Cu_2‐xSe/CQDs achieved almost twice the current density as much as the Cu_2‐xSe. Critically, pristine Cu_2‐xSe exhibited a pronounced Cu⁺ reduction signal, whereas this feature was significantly mitigated for Cu_2‐xSe/CQDs. Furthermore, the intrinsic redox waves of the CQDs themselves became indistinguishable, which might be attributed to strong solvation effects in the aqueous electrolyte and dynamic surface structural changes under CO₂ reduction conditions (Figure S30). Upon switching to an Ar atmosphere, Cu_2‐xSe/CQDs immediately triggered a strong electron‐absorption peak of the CQDs, reflecting the intrinsic capability for charge accommodation. In contrast, the pristine Cu_2‐xSe continued to display the Cu⁺ reduction signal. These observations suggested that the strong electronic state coupling enabled the CQDs to effectively intercept the transient electron flux, thereby shielding the vulnerable Cu⁺ species in Cu_2‐xSe from irreversible over‐reduction. Nonetheless, the Cu_2‐xSe/CQDs catalyst achieved only a modest improvement in C2 selectivity under conventional static electrolysis, falling short of the desired performance.

FIGURE 3.

(a) CV curves of Cu_2‐xSe, Cu_2‐xSe/CQDs, and CQDs samples with respect to Ag/AgCl at a scan rate of 50 mV/s in the 0.5 M KHCO₃ electrolyte. (b) LSV curves of Cu_2‐xSe and Cu_2‐xSe/CQDs catalysts with respect to RHE under the CO₂ or Ar atmospheres in 1 M KOH electrolyte. (c) Current density of Cu_2‐xSe and Cu_2‐xSe/CQDs in the pulsed electrolysis. The potentials in the legend are referenced to the RHE scale. (d) Quantification of electron transfer of Cu_2‐xSe with different CQDs loadings. (e) Cycling performance of full cells at 0.1 A g⁻¹ for Cu_2‐xSe/CQDs with different CQDs loadings at 0.1 A g⁻¹ in 3 M ZnSO₄ electrolytes. The illustration shows a digital photo of the AZIBs.

To elucidate the pivotal role of the pulsed‐electrolysis protocol, further electrochemical investigations were conducted. Current density under pulsed potentials revealed that Cu_2‐xSe/CQDs exhibited a significantly slow current decay, while pristine Cu_2‐xSe showed a rapid decay (Figure 3c). Within the electronic state coupling framework, this sluggish transient response likely reflected the continuous controlled transfer of dynamically delocalized electrons from the CQDs. Subsequent electrochemical analyses further confirmed this phenomenon (Figures S31–S33). It is reasonable to infer that there was a pronounced synergistic effect between CQDs and pulse electrolysis, which may serve as the key factor underpinning the outstanding C2 products synthesis performance. The pulsed current curves with various loading amounts of CQDs at different reduction times were also tested, as shown in Figures 3d and S34. Quantitative evaluation was achieved by integrating the pulsed‐current profiles of Cu_2‐xSe/CQDs and subtracting that of pristine Cu_2‐xSe. It can be observed that in the loading range of 4–10 mg, the magnitude of charge transfer showed a linear increasing trend, highlighting the tunability of this interfacial electronic state coupling. Furthermore, quantitative integration of the i‐t curves during stability tests delivered an average transferred charge of N _pulse ≈ 8.7×10¹⁶ electrons per cycle as shown in Figure S35. This result implied that the delocalized electrons were extracted in a controlled and reversible manner during the anodic pulse.

In addition, we extended Cu_2‐xSe/CQDs application to rechargeable aqueous zinc ion batteries (AZIBs)—an ideal validation platform given their safety, eco‐compatibility, and high theoretical capacity (Figure S36) [50, 51]. As cathode material, Cu_2‐xSe/CQDs exhibited pronounced charge/discharge plateaus as shown in Figure S37. The pristine Cu_2‐xSe electrode delivered a discharge specific capacity of only 28.1 mA h g⁻¹ at 0.1 A g⁻¹. Notably, the Cu_2‐xSe/CQDs showed progressively higher capacities with increasing CQDs loadings. The average capacity increase per mg of CQDs reached up to 14.4 mA h g⁻¹, directly attributed to the abundant accessible electronic states and rapid charge transfer capabilities of CQDs. Crucially, Cu_2‐xSe/CQDs electrodes also demonstrated superior cycling stability (Figure 3e), which can maintain a high level of capacity after 100 cycles at 0.1 A g⁻¹. The π‐conjugated framework of CQDs enabled extensive electron delocalization, facilitating rapid charge transport and stabilization during cycling, which parallels how CQDs preserve Cu⁺ states in CO₂ER.

2.5. Time‐Dependent Operando Characterization

To further elucidate the intrinsic origin of the superior C2 selectivity, the operando Raman spectra were preliminarily employed to monitor the Cu oxidation state through the Cu─Se bond vibration at 259 cm⁻¹(Figure S38) [8]. Remarkably, sustained Cu⁺ stability was observed exclusively upon combining CQDs decoration with pulsed electrolysis, which is consistent with the electrochemical results (Figures S39–S41). Under other operating conditions, the signal of Cu⁺ would eventually disappear. Moreover, operando XRD analysis revealed that Cu_2‐xSe/CQDs underwent rapid reconstruction under pulsed electrolysis to form a Cu₂Se (PDF#27‐1131)/CuSe (PDF#27‐0184) mixed‐phase rich in high‐valence Cu species (Figure 4a), without an obvious Cu⁰ signal observed. Critically, these Cu⁺ sites remained stable under harsh reductive conditions, preventing reduction to Cu⁰ by high‐energy electrons. On the contrary, the Cu⁺ species in the pristine Cu_2‐xSe catalyst would eventually collapse into Cu⁰.

FIGURE 4.

(a) In situ XRD patterns of Cu_2‐xSe under static potential (top image) and Cu_2‐xSe/CQDs under pulsed electrolysis (bottom image), plotted on a logarithmic vertical axis. The color transition from blue to red represents the reaction time increasing from 0 to 120 min. (b, c) In situ Cu K‐edge XANES (b) and EXAFS (c) spectra of Cu_2‐xSe under static potential (blue curves) and Cu_2‐xSe/CQDs under pulsed electrolysis (red curves). The color transition from deep to light represents the reaction time increasing from 0 to 60 min. (d) Highly time‐resolved in situ Cu K‐edge XANES spectra during pulsed CO₂ER. (e) Highly time‐resolved Raman spectra monitoring the evolution of the characteristic peaks of CQDs under pulsed electrolysis, using a 785 nm excitation laser and Na₂SO₄ electrolyte to minimize interference from surface intermediates. (f) In situ Raman spectra of Cu_2‐xSe/CQDs as a function of reaction time under pulsed and static electrolysis. The ν represents the stretching vibration, while ν_as represents the asymmetric stretching vibration. Electrolytic parameters: E _c = −1.6 V versus RHE, E _r = 0.6 V versus RHE, t _c = 50 s, and t _r = 5 s for pulsed electrolysis; E _c = −1.6 V versus RHE for static electrolysis.

Operando XAS measurements provided compelling evidence for the distinct structural evolution of the pristine and CQDs‐modified catalysts under electrochemical conditions. For the unmodified Cu_2‐xSe catalyst, both XANES and EXAFS analyses revealed a progressive reduction of Cu species during static electrolysis (Figure 4b,c). The EXAFS fitting results showed a gradual attenuation of the Cu–Se scattering contribution at ≈ 2.0 Å, accompanied by the emergence and growth of a Cu─Cu coordination path near ≈ 2.2 Å, consistent with the stepwise transformation of Cu_2‐xSe into metallic Cu (Figures 4c, S42, and Table S5). In striking contrast, upon applying CQDs modification coupled with pulsed electrolysis, the XANES spectra demonstrated that the Cu⁺ species were effectively preserved throughout the reaction, preventing the reduction to Cu⁰. The corresponding EXAFS profiles further confirmed that the Cu–Se coordination remained intact and structurally stable over the entire electrolysis period, with no observable Cu–Cu coordination signals (Figure S43 and Table S6). Notably, a persistent coordination signature associated with the intimate interfacial contact between Cu_2‐xSe and CQDs was detected, indicating that the CQDs remained chemically anchored to the Cu_2‐xSe surface under operational conditions. Furthermore, the Debye–Waller factors remained relatively stable without exhibiting any order‐of‐magnitude fluctuations, implying that the lattice structural integrity might be preserved within the experimental resolution. The highly time‐resolved in situ Cu K‐edge XANES was conducted to investigate the potential dynamic formation of Cu⁰ [52]. At the current resolution limit, the results showed that the Cu‐edge position remained essentially unchanged during both cathodic and anodic pulse steps, with no detectable Cu‐foil‐like signature (Figure 4d). Meanwhile, EXAFS and stable Debye–Waller factors results indicated a stable Cu–Se coordination peak without an obvious Cu–Cu scattering (Figures S44, S45, and Table S7). These results indicated that Cu remains predominantly in a Cu⁺ environment during pulsed CO₂ER, indicating that rapid, large‐scale Cu⁰/Cu⁺ interconversion might not serve as the dominant process.

Furthermore, highly time‐resolved in situ Raman spectroscopy was conducted using 785 nm excitation in a 0.1 M Na₂SO₄ electrolyte to directly probe the working mechanism of CQDs during pulsed electrolysis (Figure 4e). During the cathodic pulse, the red‐shift G‐band of the CQDs (E _2g mode) might be consistent with phonon softening induced by transient electron injection into the antibonding π* orbitals (LUMO‐derived states) [53, 54, 55]. Upon switching to the anodic pulse, the G‐band reversibly blue‐shifted back to its initial position, probably corresponding to the controlled extraction of the delocalized electrons. Collectively, these results implied that strong electronic state coupling provided a highly conductive structural framework that enables the rapid, reversible interfacial charge redistribution required to prevent Cu⁺ over‐reduction.

To elucidate the reaction mechanism of CO₂ER toward C2 products, in situ Raman spectra were collected to characterize the reaction intermediates and identify possible reaction pathways on Cu_2‐xSe/CQDs catalyst. As shown in Figures 4f and S46, the adsorption peak near 780 cm⁻¹ was attributed to the Cu‐OH_ad stretching mode due to hydroxyl adsorption from KOH electrolyte onto Cu_2‐xSe [37]. A weak Raman signal peak detected around 1050 cm⁻¹ corresponded to νCO₃ ²⁻ from the reaction of CO₂ with the electrolyte [56]. The peaks near 1100 and 1550 cm⁻¹ could correspond to νCOO⁻ and ν_asCOO⁻, respectively, carboxylate species from CO₂ adsorption [57]. The Raman peaks at 1350, 1600, and 2100 cm⁻¹ were caused by adsorbed *CO on the catalyst surface, attributed to νC─O, νC═O, and νC≡O, respectively [58]. Critically, a distinct peak near 2650 cm⁻¹ corresponding to the *CHOCO intermediate was detected, confirming efficient C2 production via the *CHO‐*CO coupling pathway [10]. Afterward, the static potential was employed. A clear progressive attenuation in the intensity of all peaks was observed over time except the persistent Cu‐OH_ad signal, indicating a gradual loss of CO₂ER activity [59]. This may arise directly from the Cu⁺ species destabilization under long‐term static conditions. Furthermore, in situ Raman spectroscopy under suboptimal pulsed potentials (Figure S47) revealed that insufficient or excessive pulsed potential diminished key intermediates like *CO and *CHOCO. These results confirmed that under appropriate pulsed electrolysis, Cu_2‐xSe/CQDs leveraged these synergistic intermediates to achieve high FE_C2, providing direct spectroscopic validation of the electronic state coupling role in enabling excellent CO₂ER performance.

2.6. Theoretical Simulations

To verify the critical link between the exceptional C2 selectivity and Cu⁺ stability observed in electrochemical and operando spectroscopic characterizations, DFT calculations preliminarily compared Gibbs free energies of two fundamental C─C coupling routes: symmetric (*CO+*CO→*COCO) and asymmetric (*CO+*CHO→*COCHO) pathways on Cu⁰, Cu⁺, and Cu²⁺ sites. As shown in Figure S48, the asymmetric C─C coupling exhibited a lower energy barrier than its symmetric counterpart. Notably, the Cu⁺ sites demonstrated the most favorable thermodynamics, achieving the lowest C─C coupling barrier (0.23 eV) and the highest HER barrier (0.28 eV), which is consistent with the above results. Subsequently, the complete reaction pathways were constructed based on the key intermediates that were detected by in situ Raman spectra (Figure S49). Figures 5a and S50 compared the Gibbs free energy diagrams for key reaction steps on Cu_2‐xSe/CQDs, Cu_2‐xSe, and metallic Cu (Cu⁰). These analyses clearly identified *CO→*CHO protonation and the asymmetric *CO+*CHO→*COCHO coupling reaction steps as the primary potential‐determining steps. Crucially, Cu_2‐xSe/CQDs exhibited the lowest thermodynamic energy barrier for both steps. Furthermore, the step of *CH₂CH₂OH intermediate tended to be protonated to *C₂H₅OH, instead of dehydrating to the precursor of ethylene (*CH₂CH₂). Therefore, ethanol was identified as the most thermodynamically favorable C2 products, being consistent with the experimental product distribution. In contrast, Cu⁰ presented the highest energy barrier for C─C coupling, explaining its inherent inertness in generating C2 products. Furthermore, the stronger *CO adsorption energy on Cu_2‐xSe/CQDs facilitated deep reduction of *CO intermediate, thereby promoting C2 products formation (Figure S51).

FIGURE 5.

DFT calculations on the catalytic mechanism and electron transfer analysis. (a) Gibbs free energy profiles for CO₂ electroreduction to C2 products on the Cu_2‐xSe, Cu_2‐xSe/CQDs, and Cu models. (b) Differential charge density plots for Cu_2‐xSe/CQDs under electric fields with different intensities ranging from 2.0 to −2.0 eV/Å. (c) Quantitative analysis of the number of transferred charges between Cu_2‐xSe and CQDs. (d) Gibbs free energy of C─C coupling and *CO protonation to *CHO under various electric fields. (e) Energy level diagram of Cu_2‐xSe/CQDs.

To explicitly probe electric field‐driven electron transfer between Cu_2‐xSe and CQDs, the differential charge density under applied electric fields was calculated as depicted in Figure 5b. Under positive potentials (electric field vector directed outward normal to the surface plane), increasing field strength progressively reduced the yellow charge accumulation regions at Cu⁺ sites, which signifies electron transfer from CQDs to Cu_2‐xSe. Conversely, under negative potentials, charge density at Cu⁺ sites increased with field strength, indicating electron transfer from Cu_2‐xSe to CQDs. We further quantified the directional electron transfer under varying field strengths (Figure 5c). At zero electric field strength, electrostatic interactions drove partial electron transfer from Cu_2‐xSe to CQDs, being consistent with XANES and XPS results. Crucially, the degree of electron transfer exhibited a dependency on electric field strength.

Furthermore, the effect of electric fields on key reaction energetics was evaluated (Figure 5d). Under increasingly negative electric fields, the electronic state coupling enabled the Cu_2‐xSe/CQDs to consistently maintain lower energy barriers for C─C bond coupling. Conversely, pristine Cu_2‐xSe exhibited a sharp increase in C─C coupling energy barriers under negative fields, severely limiting C2 products formation. Notably, the thermodynamic barrier for *CO → *CHO protonation of Cu_2‐xSe/CQDs was significantly lower than that on pristine Cu_2‐xSe and decreased with negative field strength. This demonstrated the superior capability of Cu_2‐xSe/CQDs catalyst to generate *CHO intermediates for C─C bond coupling, indicating that *CO protonation ceased to be a potential‐determining step under operational potentials.

2.7. Electronic State Coupling Hypothesis

Integrating the experimental observations with theoretical simulations, we proposed a working hypothesis centered on the electronic state coupling between Cu_2‐xSe and CQDs, which may account for the suppression of the over‐reduction of Cu⁺ species. Induced by direct interfacial chemical bonding, this strong coupling provided an efficient pathway for rapid electron redistribution and modulated the local electron density around Cu sites under operating conditions. Furthermore, assisted by a pulsed protocol that periodically mitigates sustained electron accumulation at Cu sites, the intimately integrated Cu_2‐xSe/CQDs catalyst effectively stabilized the Cu⁺ species, ultimately achieving outstanding C2 products selectivity.

To rationalize this hypothesis, detailed electron structure and energy level analyses were conducted (Figures S52 and S53). As described in Figure 5e, the energy levels for generating ethanol and ethylene were only ca. 0.1 V versus NHE, indicating that CO₂ER was thermodynamically favorable and consumed the majority of injected electrons. The essence of Cu⁺ reduction to metallic Cu⁰ lies in the injection of excessive electrons from the external circuit into the Cu sites. Crucially, without CQDs modification, electrons that exceeded the Cu⁺/Cu⁰ reduction potential (−0.37 V vs. NHE) would inevitably reduce catalytic Cu⁺ species, consistent with the results of Pourbaix diagram (Figure S54). However, after CQDs incorporation, the strong electronic state coupling between Cu_2‐xSe and CQDs established an alternative pathway for rapid interfacial electron redistribution. Due to the precise energy level matching between the conduction band of Cu2‐xSe (−0.40 V vs. NHE) and the lowest unoccupied molecular orbital (LUMO) of CQDs (−0.31 V vs. NHE), excess electrons can be preferentially redistributed toward the CQD‐coupled interfacial region rather than continuously localized on Cu⁺ sites. In this context, the moderate 90 meV energy offset might precisely control the electron density of Cu sites without suppressing the CO₂ reduction performance. Under pulsed electrolysis, the mild anodic pulse (0.6 V vs. RHE) might further mitigate interfacial electron accumulation, but is insufficient to drive the substantial hole injection into Cu_2‐xSe or lattice oxidation of Cu⁺ species. Assisted by pulsed electrolysis, the strong electronic state coupling helped suppress the persistent over‐reduction of Cu⁺ species, thereby contributing to the long‐term maintenance of active Cu⁺ sites and high C2 selectivity (Figure S55). Although the current experimental observations supported this interpretation, more advanced and direct characterizations would still be required in future work to further verify and deepen the mechanistic understanding of this system.

3. Conclusion

A Cu valence state regulation strategy that integrates CQDs with the pulsed electrolysis was proposed to boost C2 product synthesis for CO₂ER. Cu_2‐xSe/CQDs delivered a FE of up to 85.3% for C2 products under pulsed electrolysis, along with an ultra‐wide potential window exceeding 1.6 V, where FE of C2 remained above 70%. The in situ spectroscopy and detailed electrochemical analyses demonstrated that the active Cu⁺ sites with high CO₂ER activity were stabilized. The synergistic integration of this innovative material design and pulsed electrochemical technology effectively addressed the critical challenge of maintaining the active Cu valence state in Cu‐based CO₂ER catalysts under harsh electroreduction conditions. This work sheds new light on the active‐valence‐state requirements for C2 product formation in CO₂ER and provides a feasible protocol for the rational design of other catalytic sites with appropriate oxidation state requirements.

Author Contributions

Xinze Bi: writing – original draft, visualization, and investigation. Yifan Yan: visualization, investigation, and validation. Hongzhi Wang: conceptualization, methodology, writing – review and editing. Jiatao Zhang: writing – review and editing. Yuezhu Zhao: validation and visualization. Wenhang Wang: writing – review and editing. Licheng Liu: writing – review and editing. Yang Wang: methodology, writing – review and editing, supervision, and funding acquisition. Mingbo Wu: methodology, writing – review and editing, supervision, and funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72678‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.