Electrocatalytic CO reduction to multi-carbon alcohols on functional ionic liquid-incorporated Cu-based electrodes

INTRODUCTION

Rising anthropogenic CO₂ emissions demand sustainable carbon conversion technologies^[1]. While electrochemical CO₂ reduction reaction (CO₂RR) on copper (Cu)-based catalysts provides a promising pathway for synthesizing valuable multi-carbon (C₂₊) products, its selectivity is hindered by unstable oxygenated intermediates and parasitic carbonate formation^[2]. Alternatively, electrochemical CO reduction reaction (CORR) avoids CO₂ loss and suppresses C₁ products, yet directing high selectivity toward C₂₊ alcohols remains challenging^[3]. Morphology engineering and surface modification have been explored to enhance product selectivity, with confined structures shown to increase *CO coverage and promote C–C coupling^[4,5]. Moreover, Cu⁺ species facilitate *CO dimerization but are intrinsically unstable and readily reduced to Cu⁰ under reaction conditions^[6]. Consequently, recent efforts, including heteroatom incorporation (e.g., iodine or boron) and functional molecular modification, have demonstrated that stabilizing Cu⁺ species and tuning the local electronic structure are imperative for improving the selectivity toward C₂₊ alcohols^[7,8].

Building on these insights, we report an ionic liquid (IL)-assisted strategy for the rational design of a flower-like Cu₂O/CuO-IL catalyst with enhanced selectivity toward C₂₊ alcohols. The imidazolium-based 1-octyl-3-methylimidazolium tetrafluoroborate ([Omim][BF₄]) exhibits dual functionality. Specifically, the [Omim]⁺ cation directs the formation of hierarchical flower-like microspheres assembled by interconnected nanosheets, while the [BF₄]^- anion acts as a boron source to generate CuBO₂ in situ during electroreduction. This bifunctional role effectively stabilizes the active Cu⁺ species and enhances *CO coverage, thereby accelerating C–C coupling. Thus, the Cu₂O/CuO-IL catalyst achieves an impressive Faradaic efficiency of 52.7% for C₂₊ alcohols (FE_{C2+ alcohols}) at -0.77 V vs. reversible hydrogen electrode (RHE) (FE_ethanol: 25.6%, and FE_n-propanol: 27.1%). This work provides a versatile approach to regulate catalyst reconstruction and preserve active sites by employing IL as structure-directing agent and a heteroatom source.

RESULTS AND DISCUSSION

A series of Cu-based catalysts were synthesized via a one-pot solvothermal method with varying [Omim][BF₄] concentration [Supplementary Figures 1 and 2]. Morphological characterization revealed an IL-concentration-dependent evolution from irregular pristine Cu nanoparticles to flower-like architectures composed of interconnected nanosheets at optimal IL loading [Figure 1A and B, Supplementary Figure 3]. This structural transformation was directed by the surfactant-like [Omim]⁺ cation, which acted as soft template to construct hierarchical structures, thereby maximizing the active surface area and facilitating mass transport. The high-resolution transmission electron microscope (HRTEM) image of Cu₂O/CuO-IL clearly shows the Cu₂O(111) and CuO(002) facets [Figure 1C]. Powder X-ray diffraction (PXRD) analysis revealed that modulating the [Omim][BF₄] content (0-2.0 mol) drove a phase evolution from metallic Cu to Cu₂O, Cu₂O/CuO, and CuO, while simultaneously diminishing the overall crystallinity [Figure 1D]. Fourier transform infrared (FT-IR) spectra of the [Omim][BF₄]-modified catalysts exhibited characteristic C–H and B–F vibrations, indicating the successful IL incorporation [Figure 1E]. Thermal gravimetric analysis (TGA) curves quantified the grafting amounts of IL [Figure 1F and Supplementary Table 1]. X-ray photoelectron spectroscopy (XPS) revealed the coexisting Cu⁺ and Cu²⁺ species in Cu₂O/CuO-IL [Figure 1G], consistent with the lattice fringes observed in the HRTEM image. Conversely, the bare Cu catalyst only possessed a metallic Cu⁰ phase [Supplementary Figure 4]. Moreover, the presence of the imidazolium ring and B in [Omim][BF₄] was further supported by N 1s and B 1s spectra, respectively [Figure 1H and I]. Elemental mapping images demonstrated the homogeneous distribution of [Omim][BF₄] throughout the catalyst, indicating the uniform IL dispersion on Cu₂O/CuO-IL [Supplementary Figure 5]. These results collectively confirm the successful integration of [Omim][BF₄], which simultaneously modulates the surface microenvironment and electronic structure of the Cu-based catalysts.

Figure 1. Structural characterization of IL-modified Cu-based catalyst. (A) SEM, (B) TEM, and (C) HRTEM images of Cu₂O/CuO-IL catalyst; (D) PXRD patterns of Cu-based catalysts with and without [Omim][BF₄] modification; (E) FT-IR spectra of [Omim][BF₄] and [Omim][BF₄]-modified Cu-based catalysts; (F) TGA curves; (G) Cu 2p, (H) N 1s, and (I) B 1s XPS spectra of Cu-based catalysts with and without [Omim][BF₄] modification. IL: Ionic liquid; SEM: scanning electron microscope; TEM: transmission electron microscope; HRTEM: high-resolution transmission electron microscope; PXRD: powder X-ray diffraction; [Omim][BF₄]: 1-octyl-3-methylimidazolium tetrafluoroborate; FT-IR: Fourier transform infrared; TGA: thermal gravimetric analysis; XPS: X-ray photoelectron spectroscopy.

The CORR performance of both pristine and IL-modified Cu-based catalysts was then systematically evaluated in a flow cell to determine the influence of the [Omim][BF₄] modifier [Supplementary Figure 6]. Compared with pristine Cu, [Omim][BF₄]-modified catalysts exhibited significantly enhanced current density and selectivity toward C₂₊ alcohols [Figure 2A-C]. Notably, the Cu₂O/CuO-IL catalyst achieved a maximum FE_{C2+ alcohol} of 52.7% at -0.77 V vs. RHE (FE_ethanol: 25.6% and FE_n-propanol: 27.1%), with a partial current density of ~52 mA·cm^-2 [Figure 2B and C, Supplementary Figure 7]. In contrast, the pristine Cu showed negligible n-propanol production under identical conditions. The enhanced CORR performance was further verified by comprehensive electrochemical measurements. [Omim][BF₄]-modified catalysts exhibited more positive onset potentials, lower charge-transfer resistance, and larger electrochemically active surface areas than pristine Cu, collectively reflecting improved intrinsic activity and greater accessibility of active sites [Figure 2D and E, Supplementary Figure 8]. Evaluation of the stability of Cu₂O/CuO-IL revealed that the FE_{C2+ alcohols} remained above 30% over 10 h of continuous operation [Figure 2F]. These results clearly demonstrate that the introduction of [Omim][BF₄] on Cu-based catalysts enables efficient C₂₊ alcohol production.

Figure 2. CO electroreduction performance. (A) Linear sweep voltammetry curves; (B) FEs of all CORR products at different potentials over pure Cu and Cu₂O/CuO-IL catalysts [Error bars represent SD from three independent replicates (n = 3 per group; rectangle filled with lines: Cu catalyst; solid-filled rectangle: Cu₂O/CuO-IL catalyst)]; (C) FEs_{C2+ alcohols} and partial current densities for C₂₊ alcohols; (D) C_dl plots; (E) Nyquist plots of Cu-based catalysts with and without IL modification; (F) Stability for CORR over the Cu₂O/CuO-IL catalyst. FEs: Faradaic efficiencies; CORR: CO reduction reaction; IL: ionic liquid; SD: standard deviation; RHE: reversible hydrogen electrode.

Structural characterization was further performed on the post-reaction catalysts to elucidate the reasons driving the product distributions. PXRD pattern, XPS spectrum, and HRTEM image revealed that Cu₂O/CuO-IL underwent pronounced electrochemical reconstruction to form small Cu/CuBO₂ particles [Figure 3A and B, Supplementary Figures 9 and 10]. Furthermore, the XPS spectrum presented a noticeable negative shift in B 1s binding energy after electrolysis, attributed to the weaker electron-withdrawing property of O in CuBO₂ compared with F in [Omim][BF₄] [Figure 3C]. This structural transformation originated from the hydrolysis of [BF₄]^- and the subsequent reaction with Cu⁺ species to form CuBO₂.

Figure 3. Investigation of reaction mechanism. (A) PXRD patterns of Cu-based catalysts with and without IL modification after CORR; (B) Comparison of Cu LMM spectra of Cu₂O/CuO-IL before and after CORR; (C) Comparison of B 1s spectra of IL-modified Cu-based catalysts after CORR; In situ Raman spectra of (D) Cu and (E) Cu₂O/CuO-IL during CORR at various potentials; (F) Time-resolved in situ Raman spectra of Cu₂O/CuO-IL during CORR; (G) Optimized models of Cu(111) and CuBO₂(001); (H) Reaction free energies for the formation of C₂₊ alcohols on Cu and CuBO₂. PXRD: Powder X-ray diffraction; IL: ionic liquid; CORR: CO reduction reaction; LMM: one type of Auger electron peak; OCP: open circuit potential.

To clarify the enhanced C₂₊ alcohol selectivity on [Omim][BF₄]-modified Cu catalyst, in situ Raman spectroscopy was employed to detect the active species on the catalyst surface during CORR. Both Cu and Cu₂O/CuO-IL exhibited characteristic bands at ~293 and 2,120 cm^-1, assigned to the Cu–CO and C≡O stretching vibrations of atop-bound *CO intermediate (*CO_atop)^[9,10], respectively [Figure 3D and E]. The substantially higher peak intensities for Cu₂O/CuO-IL than the bare Cu indicate an elevated *CO surface coverage, driven by the intermediate confinement effect of the imidazolium alkyl chains. Furthermore, time-resolved Raman spectra of Cu₂O/CuO-IL presented progressively intensified peaks at 220 and 499 cm^-1, corresponding to active Cu⁺ species and Cu–O–B bonds^[11,12], respectively [Figure 3F]. These observations demonstrate that B retains oxygen to stabilize Cu⁺ active sites. Consequently, the synergy of elevated *CO coverage and robust Cu⁺ stabilization explains the superior C₂₊ alcohol generation on Cu₂O/CuO-IL compared to bare Cu.

Density functional theory (DFT) calculations on CuBO₂(001) and Cu(111) models further elucidate the enhanced C₂₊ alcohol selectivity of IL-derived CuBO₂ during CORR [Figure 3G]. *CO adsorption is thermodynamically favored on CuBO₂ (-1.38 eV) compared to bare Cu (-0.67 eV). This enhanced affinity originates from electron-deficient B atoms (empty p-orbitals) that suppress lattice oxygen loss and electronically stabilize Cu⁺ active sites, directly corroborating in situ Raman observations [Figure 3H, Supplementary Figures 11 and 12]. Furthermore, the barrier for the rate-determining *CO dimerization (*OCCO formation) on CuBO₂ is merely 0.30 eV, substantially lower than on Cu (0.73 eV). By simultaneously elevating local *CO coverage and kinetically facilitating C–C coupling, the B-modulated Cu⁺ species seamlessly overcome the C₂ formation bottleneck. This drives subsequent hydrogenation to ethanol or *C₂–*C₁ coupling toward n-propanol, whereas the weak *CO affinity and high coupling barrier on bare Cu severely hinder C₂₊ alcohol synthesis.

CONCLUSION

In summary, we report a one-pot method to synthesize the flower-like Cu-based nanocatalysts utilizing [Omim][BF₄] as a versatile modifier. The Cu₂O/CuO-IL catalyst with the optimal IL content achieves a FE_{C2+ alcohols} of 52.7% at -0.77 V vs. RHE (including FE_ethanol of 25.6% and FE_n-propanol of 27.1%), which is superior to the pristine Cu catalyst. Structure characterization and in-situ Raman results confirm that IL modification stabilizes the active Cu⁺ species and enhances the *CO coverage, facilitating selective generation of C₂₊ alcohols. This work not only provides valuable guidance for elucidating the formation mechanism of high-value-added C₂₊ alcohols, but also offers a strategy for designing high-performance catalysts via IL surface modification in accordance with green chemistry principles.