A tandem nanoreactor constructed by coating Cu₂O on the surface of single-atom catalyst Ni-NC₃ for electroreduction of CO₂ to C₂ products

Abstract

Electroreduction of CO₂ to C₂ products is an extremely significant and challenging undertaking. Herein, by electrodepositing Cu₂O to encapsulate the single-atom catalyst Ni-NC₃, we fabricated a core–shell tandem nanoreactor, designated as Ni-NC₃@Cu₂O, for electrocatalytic reduction of CO₂ to C₂ products. Notably, under the potential of −1.4 V vs. RHE and strongly alkaline conditions, Ni-NC₃@Cu₂O manifested a high Faradaic efficiency (C₂) of 59% and industrial partial current density of 448 mA cm⁻² in the electroreduction of CO₂ to C₂ products.

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10th anniversary statement

Time flies, Inorganic Chemistry Frontiers has been launched for ten years! Since the very beginning of the journal, I was honored to be invited to participate in the Advisory Board, and invited to submit our manuscripts for this emerging journal. Whereafter, my co-workers and I published, from time to time, our research papers in this prestigious journal. In fact, I experienced very pleasant interactions with the highly professional and efficient editorial board and reviewers, and witnessed and enjoyed the rapid growth and great success of this journal during the past ten years. Therefore, I am strongly confident that it has become an internationally leading journal in the field of Inorganic Chemistry. On the occasion of the 10th anniversary of the journal, I warmly congratulate the success of the journal, and sincerely wish the journal has continuous progress and greater contributions to the chemistry community.

The electrochemical carbon dioxide reduction reaction (eCO₂RR) represents a highly promising strategy, capable of mitigating artificial CO₂ emissions and easing the energy crisis.^1–4 By virtue of different types of electrocatalysts, eCO₂RR can produce a diverse array of value-added chemicals, such as methanol,⁵ methane,⁶ formic acid,⁷ and ethylene.⁸ Among all the reduction products, C₂ products like ethylene and ethanol possess higher energy density and additional economic value in comparison with C₁ products.⁹ Nevertheless, achieving efficient acquisition of C₂ products via eCO₂RR remains an arduous challenge, as the generation pathway encompasses complex steps such as the C–C coupling of two *CO intermediates and multi-electron proton transfer processes.¹⁰

Among numerous transition metals, copper distinguishes itself by featuring a negative adsorption energy for *CO and a positive adsorption energy for *H.¹¹ This is of paramount significance for the effective generation of C₂ products and it is currently regarded as the sole metal capable of catalysing the formation of C₂ products from CO₂.¹⁰ Nevertheless, mono-component copper-based catalysts encounter the issue of inadequate surface coverage by *CO, which gives rise to sluggish C–C coupling reactions and ultimately manifests in the low rate and low selectivity of C₂ product generation.¹² Liu et al. simulated polycrystalline copper with abundant grain boundaries (GB), where disordered Cu atoms near the GB interface enhance the bonding strength of *CO and surface coverage compared to Cu(100) surfaces.¹³ Owing to the high overpotential and low Faradaic efficiency (FE) of mono-component copper-based catalysts,^14,15 researchers have developed various types of tandem catalytic systems through approaches such as using bimetallic catalysts,¹⁶ mechanically mixed two catalysts,¹⁷ and employing tandem reactors,¹⁸ thereby significantly enhancing the FEs and current densities of C₂ products. However, the reports on tandem catalysts are still limited,¹⁹ and the underlying tandem catalytic mechanism is not fully understood. Moreover, the performance of tandem catalysts used for yielding C₂ products still has room for improvement.²⁰ It is necessary to develop more suitable tandem catalytic systems for eCO₂RR to yield C₂ products.

Single-atom catalysts (SACs) typically demonstrate very high catalytic activity in electroreduction of CO₂ into CO,^21,22 while nano-Cu₂O catalyst exhibits considerable superiority in the electrocatalytic reduction of CO to yield C₂ products.²³ Composing a tandem catalytic system by integrating SACs and nano-Cu₂O into composite catalysts is likely to provide an opportunity to achieve efficient eCO₂RR to generate C₂ products.¹⁹ Previous literature has likewise demonstrated that decoupling catalytic processes involving multiple elementary reactions with tandem catalysts significantly boosts reaction efficiency.^24,25 Based on this strategy, within the tandem catalytic system, rational integration of the catalyst accountable for eCO₂RR to CO and the relay catalyst for electrochemical CO reduction reaction (eCORR) to C₂ is anticipated to improve the overall catalytic performance.²⁶ Prior investigations have indicated that tandem nanoreactors formed by partially encapsulating a catalyst generating CO with a relay catalyst perform better compared to other combinations such as mechanical mixing.^27,28 Nevertheless, tandem nanoreactors formed by Cu₂O encapsulating SACs have not been studied for such catalytic performance. We recently reported that a single-atom catalyst, Ni-NC₃ possesses superior electrocatalytic performance in reduction of CO₂ to CO compared to other SACs.²⁹ Herein, we fabricated a new tandem nanoreactor by partially coating Ni-NC₃ with nano-Cu₂O into a new composite tandem catalyst, and investigated its performance of eCO₂RR to yield C₂ products, as well as the underlying mechanisms.

Firstly, a metal-azolate framework (MAF), namely MAF-5 (Fig. S1†), was synthesized according to the literature,³⁰ followed by carbonization and Ni single atom anchoring to prepare Ni-NC₃ (Fig. S2†). The powder of Ni-NC₃ was coated on the carbon paper with Nafion binder to prepare the working electrode. The Ni-NC₃@Cu₂O tandem catalyst was prepared by electrodepositing Cu₂O onto the surface of Ni-NC₃ particles in CuSO₄ solution (pH = 9), with lactic acid serving as the complexing agent (see ESI† for details).²³ Different electrodeposition times of 10 min and 30 min yielded partially and fully coated samples, respectively designated as Ni-NC₃@Cu₂O-10 and Ni-NC₃@Cu₂O-30 (Scheme 1).

Scheme 1 Synthesis of Ni-NC₃@Cu₂O-10 and Ni-NC₃@Cu₂O-30.

The samples of Ni-NC₃@Cu₂O-10 and Ni-NC₃@Cu₂O-30 were characterized by the powder X-ray diffraction (PXRD) pattern, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) (Fig. 1, S3 and Table S1†). X-ray photoelectron spectroscopy (XPS) revealed that valence states of Ni in both samples were in the range of 0 to +2, while those of Cu were both +1 (Fig. S4–8†). Scanning electron microscopy (SEM) images showed that in Ni-NC₃@Cu₂O-10, Ni-NC₃ was partially coated by Cu₂O, whereas in Ni-NC₃@Cu₂O-30, Ni-NC₃ was fully encapsulated by Cu₂O (Fig. 2). SEM images revealed that Ni-NC₃@Cu₂O particles were nanospherical with a diameter of 4.04 ± 0.95 μm (Fig. S9†). The energy-dispersive X-ray (EDX) mapping images demonstrated that, in the uncoated region, the intensity of C is the most prominent, while in the coated region, those of Cu and O are the most prominent, with the ratio of the Cu to O signal intensities being close to 2 : 1 (Fig. S10–S13 and Tables S2, S3†). These results indicate that Ni-NC₃@Cu₂O particles have spheric core–shell structures, with Ni-NC₃ as the cores and Cu₂O as the outer shell.

Fig. 1 (a) PXRD patterns of Ni-NC₃@Cu₂O-10 and Ni-NC₃@Cu₂O-30. (b) Cu K-edge XANES and (c) EXAFS spectra of Ni-NC₃@Cu₂O-10 and Cu-based references. (d) The R′ space fitting of Cu₂O in Ni-NC₃@Cu₂O-10.

Fig. 2 SEM images of (a and b) Ni-NC₃@Cu₂O-10 and (c and d) Ni-NC₃@Cu₂O-30.

The eCO₂RR performance was measured in 1 M KOH aqueous solution using a flow cell device with two compartments, while CO₂ was supplied directly to the catalyst. Firstly, linear scanning voltammetry (LSV) curves showed that Ni-NC₃@Cu₂O-10 has a larger response current to CO₂ reduction compared to Ni-NC₃@Cu₂O-30, suggesting a higher activity for eCO₂RR (Fig. 3a). Online gas chromatography (GC) and off-line ¹H nuclear magnetic resonance (¹H NMR) spectroscopy were used to analyze the gas-phase and liquid-phase products (Fig. S14 and 15†), respectively. Interestingly, for Ni-NC₃@Cu₂O-10, at −1.0/−1.4 V vs. Reversible Hydrogen Electrode (RHE), the FE(C₂) values are as high as 55.5%/60.6% (C₂H₄: 27.03%/19.10%, ethanol: 24.68%/32.72%, acetate: 3.83%/7.23%) with the high C₂ partial current densities of 154/448 mA cm⁻² (Fig. 3a, b and S16, 17†). This performance surpasses those of many reported copper-based catalysts (Fig. S18 and Table S4†).^{8,19,20,25,27,28,31–35} Additionally, the performance remains nearly unchanged over 32 hours of continuous operation at −1.2 V vs. RHE (Fig. 3c), indicating excellent stability of Ni-NC₃@Cu₂O-10. Furthermore, the electrolyte collected after 30 minutes of electrocatalysis at −1.4 V vs. RHE for Ni-NC₃@Cu₂O-10 and Ni-NC₃ was checked with inductively coupled plasma atomic emission spectrometer (ICP-AES), and anodic stripping voltammetry was performed on Ni-NC₃ (Fig. S19 and Table S5†). All test results indicated that there is no leakage of metal ions from the catalyst, and the composition and valence state of SAC Ni-NC₃ and its Cu₂O shell remained unchanged during electrocatalysis. In contrast, for Ni-NC₃@Cu₂O-30, at −1.0/−1.4 V vs. RHE, the FE(C₂) values are just 39.09%/19.29% (C₂H₄: 37.60%/18.78%, ethanol: 1.49%/0%, acetate: 0%/0.51%) with the low C₂ partial current densities of 53.06/85.42 mA cm⁻² (Fig. 3a, b and S16†), which are much lower than those of Ni-NC₃@Cu₂O-10. These results indicate that, when completely encapsulated, Ni-NC₃ almost cannot contact with CO₂ to produce CO for the subsequent redcution catalyzed by Cu₂O, thereby resulting in deteriorated the FE and current density of C₂ production. These results indicate that when fully encapsulated, Ni-NC₃ is unable to come into contact with CO₂ to produce CO for subsequent reduction, leading to deterioration of the FE and current density of C₂ production.

Fig. 3 (a) LSV curves and (b) FEs of Ni-NC₃@Cu₂O-10 and Ni-NC₃@Cu₂O-30 in CO₂-saturated 1 M KOH solution. (c) Long-term stability test of Ni-NC₃@Cu₂O-10 operated at −1.2 V vs. RHE. (d) The operando ATR-FTIR spectra of Ni-NC₃@Cu₂O-10 measured at −1.4 V vs. RHE (see ESI† for details).

To further investigate the catalytic mechanism, several control experiments were conducted. Firstly, using the same synthetic method, we replaced Ni-NC₃ with another SAC Ni-N₄ (Fig. S20 and 21†) and CB, which have poor electrocatalytic performance for eCO₂RR to CO,²⁹ to fabricate the other two tandem catalysts, respectively designated as Ni-N₄@Cu₂O-10 and CB@Cu₂O-10. The performance tests show that at the potential of −1.4 V vs. RHE, the FE(C₂) and partial current density (FE(C₂) = 31.7%/6.7%, j(C₂) = 227.3/30.4 mA cm⁻²) of Ni-N₄@Cu₂O-10 and CB@Cu₂O-10 are much lower than those of Ni-NC₃@Cu₂O-10 (FE(C₂) = 60.6%, j(C₂) = 448 mA cm⁻²) (Fig. S22 and 23†). This phenomenon may be attributed to the fact that, in comparison to Ni-NC₃, both Ni-N₄ and CB exhibit inferior CO production performance, leading to a reduced supply of CO to Cu₂O. Consequently, the CO coverage on Cu₂O is diminished, which is detrimental to C–C coupling. In addition, we also mechanically mixed nano-Cu₂O with Ni-NC₃ to prepare a composite tandem catalyst Cu₂O/Ni-NC₃. The performance test shows that at −1.4 V vs. RHE, although the performance (FE(C₂) = 21.8%, j(C₂) = 96.3 mA cm⁻²) of Cu₂O/Ni-NC₃ is better than that of pure Cu₂O (FE(C₂) = 1.67%, j(C₂) = 3.97 mA cm⁻²) (Fig. S24 and 25†), it is much worse than that of Ni-N₄@Cu₂O-10. This result indicates that homogeneous and tight integration of Ni-NC₃ and Cu₂O in a partial encapsulation fashion is more helpful than that of a simple mechanical mixing in facilitating the migration of CO species to the Cu₂O surface for the C–C coupling.

In order to identify the reaction intermediates, operando attenuated total reflectance infrared (ATR-FTIR) spectra of Ni-NC₃@Cu₂O-10 was measured (Fig. 3d). It can be preliminarily observed that the signal peak of the characteristic intermediate *COOH of CO₂ → CO emerged at 1253 cm⁻¹, suggesting that Ni-NC₃, as an outstanding CO-donor catalyst, is capable of generating CO effectively. The peaks of *H₂C=, *CHO and *COCHO, which are the key intermediates for C₂H₄ generation, were observed at 894, 1060 and 1631 cm⁻¹, respectively. It is hypothesized that *CO adsorbed on the two close Cu sites undergoes C−C coupling to form *COCHO, followed by multiple hydrogenation steps and the removal of two O atoms to form *HC=CH₂ and finally C₂H₄.

Conclusions

In summary, we employed an electrochemical deposition technique to encapsulate the single-atom catalyst Ni-NC₃ with Cu₂O and fabricated two core–shell structured materials with varied degrees of encapsulation by manipulating the electrodeposition duration. Through comparative experiments, on the one hand, we verified that the synergistic tandem effect between Cu₂O and Ni-NC₃ can enable high-performance eCO₂RR to yield C₂ products. On the other hand, we demonstrated that the integration fashion of partially encapsulation of Ni-NC₃ with Cu₂O is more conducive to the tandem reaction for yielding of C₂ products compared to the mechanical mixing method. These experimental outcomes offer new clues for designing high-performance of eCO₂RR to yield high-value products.

Author contributions

P.-Q. L. and X.-M. C. conceived and designed the research. C.-P. L. performed the syntheses and measurements. C.-P. L., J.-R. H., P.-Q. L. and X.-M. C. wrote the manuscript, and all authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There is no conflict of interest to report.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2021YFA1500401), NSFC (21821003, 22371304, and 223B2123), Fundamental Research Funds for the Central Universities, Sun Yat-Sen University (24lgzy006), the Special Fund Project for Science and Technology Innovation Strategy of Guangdong Province (STKJ2023078), and the Guangzhou Science and Technology Program (SL2023A04J01767).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02713b

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