Enhancing Cu–ligand interaction for efficient CO₂ reduction towards multi-carbon products

Abstract

Electrochemical CO₂ reduction (CO₂R) to valuable products provides a promising strategy to enable CO₂ utilization sustainably. Here, we report the strategy of using Cu-DAT (3,5-diamino-1,2,4-triazole) as a catalyst precursor for efficient CO₂ reduction, demonstrating over 80% selectivity towards multicarbon products at 400 mA cm⁻², with intrinsic activity over 19 times higher than that of Cu nanoparticles. The catalyst’s active phase is determined to be metallic copper wrapped with the DAT ligand. We attribute this enhanced CO₂R performance to the accelerated steps of *CO adsorption and C–C coupling induced by the closely cooperated DAT ligand.

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Electrochemical CO₂ reduction (CO₂R) has gained increasing attention as a promising approach to tackle the challenges of CO₂ emissions, owing to its ability of converting CO₂ into value-added chemicals when powered by renewable electricity.^1,2 Among numerous metal-based electrocatalysts,³ Cu-based materials possess moderate intermediate CO binding strength, favouring the formation of multi-carbon (C₂₊) products (i.e., ethylene, ethanol).^4,5 Several strategies have been employed to further promote the formation of C₂₊ products, such as organic modification,^6–9 alloying^10,11 and introducing a dopant,^12,13etc. Recently, gas diffusion electrode (GDE)^14,15 based CO₂R has largely alleviated the severe CO₂ diffusion limitations in conventional H-cells, enabling CO₂R at an industrially relevant current density.^16–19 However, a highly alkaline environment is typically necessary for achieving high selectivity towards C₂₊ products in GDE reactors.²⁰ Consequently, severe CO₂ loss, carbonate salt formation and catalyst structural damage can occur, leading to impeded CO₂ diffusion and deteriorated activity.^21–23 In contrast, neutral electrolytes (i.e., KHCO₃, KCl) can preserve the catalyst integrity and mitigate salt precipitation problems, allowing stable operation of CO₂R.²⁴ Herein, we demonstrate the optimization of the microenvironment for CO₂R in neutral electrolytes through the introduction of an amino group-rich ligand, resulting in substantially enhanced activity towards C₂₊ products.

We first conducted CO₂R measurements based on the physical mixture of commercial copper nanoparticles (CuNPs) and 3,5-diamino-1,2,4-triazole (DAT), a ligand rich in amino groups, in a typical flow cell in 1 M KHCO₃ (Fig. 1a). As shown in Fig. 1b and Fig. S1 (ESI†), the C₂₊ products selectivity increased significantly (i.e., from <47% to >65%) as the DAT to CuNP ratio increases, indicating the profound promotion effect attributed to the DAT ligand. Notably, the overpotential at 300 mA cm⁻² also decreased by as much as ∼100 mV after the introduction of DAT, indicating the enhanced intrinsic activity towards C₂₊ products (all electrode potentials mentioned here are vs. RHE, unless otherwise defined). Energy-dispersive X-ray spectroscopy (EDS) (Fig. S2, ESI†) suggests the even dispersion of N and Cu elements after CO₂R, indicating that the ligand effect is originated from the intimate contact between the DAT ligand and the Cu active-sites. However, it is obvious that the interface between the DAT ligand and Cu particles is not optimized (Fig. S3, ESI†). Hence, to further promote the CO₂R performance, particularly the activity and selectivity towards C₂₊ products, we speculate that it would be beneficial to employ the Cu(II)-DAT-based complex as a catalyst precursor for CO₂R, so that maximized contact between Cu active-sites and the DAT ligand can be achieved.

Fig. 1 (a) Schematic of CO₂R in a flow cell using a physical mixture of CuNPs and DAT ligand. (b) C₂₊ selectivity and cathodic potential at 300 mA cm⁻² with different ratios of DAT to CuNPs. Increasing ligand content leads to enhanced C₂₊ selectivity and reduced overpotential. (c) Synthetic approach and chemical structure of CuDAT. (d) TEM images, (e) EDS mapping and (f) N 1s XPS of the as-synthesized CuDAT. (g) Cu K edge FT-EXAFS of as-prepared CuDAT and Cu foil.

We then prepared an organometallic complex using Cu²⁺ (CuSO₄) as the metal center and DAT as the chelating ligand based on a modified procedure,²⁵ and the obtained complex is denoted as CuDAT (Fig. 1c).^26,27 Transmission electron microscopy (TEM) presents the morphology of the CuDAT particles (Fig. 1d), with a diameter of ∼100 nm and lattice spacing of ∼0.636 nm. The crystallinity is further elucidated by XRD (Fig. S4, ESI†). Besides, the EDS mapping (Fig. 1e) confirms the uniform dispersion of Cu and N. X-ray photoelectron spectroscopy (XPS) suggests that the Cu centers remain as Cu²⁺, with Cu 2p_1/2 and 2p_3/2 located at 953.6 eV and 934.6 eV, respectively (Fig. S5, ESI†).^28,29 Additionally, N species from the DAT ligand (–NH₂ and the triazole heterocycle) and the Cu–N bond can be clearly deconvoluted (Fig. 1f), in agreement with the proposed structure.^30,31 Moreover, Fourier transformation of the extended X-ray absorption fine structure (FT-EXAFS) consolidates the Cu–N bond around R = 1.5 Å (Fig. 1g),³² suggesting the coordination between Cu²⁺ and DAT. The complex assembly is also manifested by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Fig. S6, ESI†), exhibiting the characteristic peaks of –NH₂ stretching at 3400 cm⁻¹, –NH stretching at 3300 cm⁻¹, –NH₂ bending at 1640 cm⁻¹ and C–NH₂ stretching at 1050 cm⁻¹, in line with the characteristic signals of the DAT ligand.³³

The CO₂R performance of CuDAT was assessed in a flow cell (Fig. 2a). Notably, CuDAT exhibited high faradaic efficiency (FE) of >80% towards C₂₊ products at 400 mA cm⁻², which is comparable, if not higher, compared to the state-of-the-art CO₂R electrocatalysts under neutral conditions.³⁴ In contrast, CuNPs only delivered <60% FE (C₂₊) under the same conditions (Fig. S7, ESI†). Additionally, CuDAT exhibited much lower overpotential compared to CuNPs at the same C₂₊ current density (Fig. 2b). To assess the intrinsic activity of CuDAT, we estimated the electrochemical active surface area (ECSA) of both CuDAT and CuNPs (Fig. S8, ESI†).³⁵ Strikingly, at a modest overpotential of −0.86 V vs. RHE, the ECSA-normalized j_C2+ of CuDAT is over 19 times higher than that of CuNPs (Fig. S9, ESI†). Moreover, CuDAT also demonstrated promising CO₂R stability at 200 mA cm⁻² (Fig. S10, ESI†). Overall, we demonstrate that applying an organometallic complex derived catalyst could be an effective strategy in enhancing the intrinsic activity and selectivity of Cu for C₂₊ products.

Fig. 2 (a) CO₂R performance on CuDAT in 1 M KHCO₃. (b) j_C2+ on the function of the overpotential for CuDAT and CuNPs. (c) Operando Cu K-edge XANES for CuDAT during CO₂R in a flow cell. (d) Oxidation state, deduced from the rising edge energy of Cu K edge XANES in CuDAT during CO₂R at different overpotentials. (e) N 1s XPS of the post-CO₂R CuDAT and (f) atomic percentage of N species for as prepared and post-reaction CuDAT. Triazole N and amino N were largely reserved.

To explore the changes in the chemical state of Cu in CuDAT during CO₂R, we performed operando X-ray absorption fine structure (XAFS) experiments using the same flow cell. As shown in Fig. 2c, the rising edge energy of the Cu K-edge X-ray absorption near edge structure (XANES) decreases gradually from open circuit voltage (OCV) to −0.94 V, reflecting the transition of oxidized Cu to its metallic state (Fig. 2d). This is further evidenced by the SEM and TEM images (Fig. S11, ESI†), in which drastic morphological changes occurred after CO₂R. Crucially, after CO₂R, the Cu (111) lattice is identified and forms aggregated particles ∼5 nm (Fig. S12, ESI†). Additionally, the post-reaction CuDAT sample exhibits a discernible Cu (111) peak on the XRD spectrum, providing further confirmation of the transition from Cu²⁺ to metallic Cu (Fig. S13, ESI†). The increased peak width is indicative of the small size of the Cu nanoparticles based on the Scherrer equation.³⁶ In comparison with the XRD pattern of as-prepared CuDAT, the original crystallinity is disrupted, underscoring the structural reconstruction that occurs during CO₂R (Fig. S14, ESI†). Additionally, cross-section EDS mapping reveals the uniform dispersion of N and Cu elements, indicating that N-containing ligands are retained (Fig. S15, ESI†). Moreover, we found the existence of triazole N at 399.1 eV, amino N at 398.6 eV and N coordinated with Cu at 399.4 eV (Fig. 2e and f). Furthermore, FTIR spectra exhibit characteristic peaks of –NH₂ stretching at 3400 cm⁻¹, –NH₂ bending at 1640 cm⁻¹, and C–NH₂ stretching at 1050 cm⁻¹ (Fig. S16, ESI†).³⁷ Taken together, we conclude that during CO₂R, the Cu centers were reduced and formed small Cu clusters/particles, while the DAT ligand remained intact and maintained intimate interaction with the Cu active sites.

In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was carried out to probe the reaction intermediates (Fig. 3a, Fig. S17, ESI†). Absorption bands located at ∼2050 cm⁻¹ were found corresponding to the C≡O stretching mode of *CO_L (linearly bonded CO).³⁸ After emerging at −0.5 V, the band intensity quickly decreases from −0.6 V to −1.1 V, indicating the rapid consumption of the *CO intermediate due to the C–C coupling towards C₂₊ products. Moreover, the *CO band position undergoes blue-shift first followed by red-shift along with the increase of overpotential,³⁹ which is caused by both the changes in *CO coverage and the Stark shift effect.^40,41 Besides, the *CO_L absorbance on CuNPs increases gradually from −0.5 V to −0.8 V (Fig. 3b). In contrast, *CO_L on CuDAT shows a sharp increase and quickly reaches its maximum at −0.65 V, indicating the facilitated *CO adsorption on CuDAT compared to that of CuNPs. Passing the *CO_L absorbance maximum, CuDAT exhibits a more rapid decrease of the *CO band intensity than CuNPs, suggesting the accelerated rate for producing multi-carbon products.^42,43 Moreover, the *CO band on CuDAT appears at lower wavenumber, consolidating its favoured *CO adsorption (Fig. 3c).⁴⁴

Fig. 3 (a) In situ ATR-SEIRAS of CuDAT. (b) Relative absorbance of the *CO_L band for CuDAT and CuNPs. (c) *CO_L band in in situ ATR-SEIRAS at −0.7 V vs. RHE.

The preferred *CO adsorption is further confirmed through CO stripping experiments (Fig. S18, ESI†). In comparison to CuNPs, CuDAT displays a CO stripping peak at more positive overpotential, denoting a stronger *CO adsorption on the Cu sites. To underscore the influence of the DAT ligand, we studied in situ ATR-SEIRAS on the physical mixture of CuNPs and DAT ligand as the catalyst composite during CO₂R (Fig. S19a, ESI†). Upon the introduction of the DAT ligand, the *CO_L absorbance band undergoes more rapid emergence and decrease than that of CuNPs as the overpotential increases. This implies the acceleration of both *CO_L adsorption and C–C coupling (Fig. S19b, ESI†). Additionally, the *CO_L absorbance band undergoes a redshift with the ligand addition at −0.7 V, further confirming the favoured *CO adsorption after the introduction of the DAT ligand (Fig. S19c, ESI†).

To conclude, we attribute the enhanced C₂₊ activity of CuDAT to the optimized binding strength of surface *CO, achieved through intimate Cu/ligand contact. This underscores the effectiveness of establishing such close interactions to maximize the promotion effect.

It is worthwhile to develop cost-effective approaches that can directly convert diluted CO₂ streams into valuable chemicals and fuels.^45,46 However, significant concentration overpotential is anticipated when using a low-concentration CO₂ stream, especially at high current densities. Nevertheless, we evaluated CuDAT for CO₂R using low-concentration CO₂ streams (Fig. 4a, b and Fig. S20, ESI†), as amino-functionalized materials have demonstrated prominent capabilities for local CO₂ enrichment.⁴⁷ At 300 mA cm⁻², it is encouraging to observe that CuDAT exhibits both higher CO₂R selectivity and significantly reduced overpotential compared to CuNPs across all CO₂ partial pressures (p_CO2). Specifically, CuDAT maintains high CO₂R selectivity (>80%) at 0.2 atm of CO₂ (Fig. 4c). Additionally, compared with CuNPs, CuDAT shows reduced overpotential at 300 mA cm⁻² by >30 mV at each p_CO2 (Fig. S21, ESI†). Moreover, CuDAT maintains high C₂₊ selectivity (over 70%) p_CO2 of 0.3 atm (Fig. 4d), significantly higher than those of CuNPs. These stark contrasts underscore the notable CO₂ enrichment capability of CuDAT. It is worth noting that the C₂₊ formation during CO₂R initially increased and then began to decrease at p_CO2 around 0.4 atm for both catalysts (Fig. S22, ESI†). We attribute this phenomenon to potential-dependent changes in selectivity,⁴⁸ as the higher overpotentials (increased concentration overpotential) are favoured for C₂₊ formation.

Fig. 4 (a) CO₂R on CuDAT with different CO₂ concentrations at 300 mA cm⁻². (b) CO₂R performance comparison between CuDAT and CuNPs. (c) CO₂R selectivity and (d) FE(C₂₊) of CuDAT and CuNPs at different CO₂ partial pressures.

To summarize, we present a strategy to enhance CO₂R towards C₂₊ products by constructing catalyst precursors through complexation of Cu with organic ligands possessing desirable properties. Owing to the intimate interactions between the Cu active sites and the organic ligands, CuDAT exhibits outstanding CO₂R performance towards C₂₊ products. Based on detailed mechanistic investigations, the enhanced C₂₊ production on CuDAT is attributed to the facilitated *CO adsorption and accelerated C–C coupling rate, induced by the closely cooperated DAT ligands. Moreover, CuDAT also demonstrates great potential for low-concentration CO₂ reduction.

We acknowledge A*STAR (Agency for Science, Technology and Research) under its LCERFI program (award no. U2102d2002). We would also like to acknowledge the support of the National Research Foundation (NRF) Singapore, under the NRF Fellowship (NRF-NRFF13-2021-0007).

Conflicts of interest

There are no conflicts to declare.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cc05972c

‡ Jingyi Chen and Lei Fan have contributed equally to this work.

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