Revealing the synergistic effect of Ni single atoms and adjacent 3d metal doped Ni nanoparticles in electrocatalytic CO₂ reduction

Abstract

Herein, we report the successful fabrication of a series of transition metal doped Ni nanoparticles (NPs) coordinated with Ni single atoms in nitrogen-doped carbon nanotubes (denoted as Ni_1+NPsM-NCNTs, M = Mn, Fe, Co, Cu and Zn; Ni₁ = Ni single atom). X-ray absorption fine structure reveals the coexistence of Ni single atoms with Ni–N₄ coordination and NiM NPs. When applied for electrocatalytic CO₂RR, the Ni_1+NPsM-NCNT compounds show the Faradaic efficiency of CO (FE_CO) with a volcano-like tendency of Mn < Fe ≈ Co < Zn < Cu, in which the Ni_1+NPsCu-NCNT exhibits the highest FE_CO of 96.92%, a current density of 171.25 mA cm⁻² and a sustainable stability over 24 hours at a current density of 100 mA cm⁻², outperforming most reported examples in the literature. Detailed experiments and theoretical calculations reveal that for Ni_1+NPsCu-NCNTs, the electron transfer from NiCu NPs to Ni single atoms strengthens the adsorption of *COOH intermediates. Moreover, the d-band center of Ni–N in Ni_1+NPsCu-NCNT is upshifted, providing stronger binding with the reaction intermediates of *COOH, whereas the NiCu NPs increase the Gibbs free energy change of the Volmer step, suppressing the competitive HER.

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Introduction

Due to the burning of fossil fuels and human activities, excessive emission of CO₂ has increased in the past decades, which quickens the pace of global warming and the rise of sea level.^1–4 Electrochemical CO₂RR is an efficient method to make use of the greenhouse gas CO₂ for its clean energy source and operational convenience.^5–7 Among multiple products in the CO₂RR, the two-electron product CO stands out as one of the most economically favorable products per unit of electrical energy input in the CO₂ reduction reaction (CO₂RR). It also serves as a crucial industrial feedstock in Fischer–Tropsch synthesis for producing multi-carbon chemicals. Despite its importance, the CO₂RR to CO faces several challenges, including the activation of CO₂ and slow hydrogenation kinetics, resulting in a high energy barrier for the overall electrocatalytic reaction. In addition, the overpotential of the CO₂RR is higher than that of the competing hydrogen evolution reaction (HER), which makes the selectivity of the CO₂RR hard to control.^8–12 Therefore, it's of vital importance to fabricate an efficient catalyst to promote CO₂RR to CO.

Single atom catalysts (SACs) with high utilization of atoms have emerged as a variety of promising materials in the CO₂RR and their distinct active sites are ascertained to be effective sites to produce CO.^13–15 In recent years, SACs containing Mn,¹⁶ Fe,^17–19 Co,^20,21 Ni,^22,23 Cu²⁴ and Zn^25,26 have been widely applied for the CO₂RR and been demonstrated as outstanding catalysts with high selectivity. However, during the CO₂RR, CO₂ and H₂O need to be simultaneously activated. SACs with uniform single designated active sites may not offer the optimized performance, which calls for new strategies for catalyst design.^27,28 Metal nanoparticles, such as Ni and Fe, are capable of promoting H₂O splitting, which can accelerate proton-feeding to generate the key intermediate *COOH.^29,30 Recently, Ren et al. reported that the high conductivity of Ni NPs can modulate the electronic structure of single sites, which would further promote the adsorption of *COOH intermediates.³¹ However, excessive Ni NPs would promote the HER, resulting in much more H₂ production and low CO selectivity. Hence, it's of great importance to promote the CO₂ activation and to restrain the side HER by rational design of catalysts in order to guarantee the kinetic balance and performance of CO₂RR to CO.^32,33

Herein, we report the pyrolysis of the mixture of NiMAl-LDH (M = Mn, Fe, Co, Cu and Zn) and melamine, which results in the formation of the co-existence of 3d transition metal doped Ni NPs and Ni single atoms that are encapsulated by N-doped carbon nanotubes (denoted as Ni_1+NPsM@NCNT, M = Mn, Fe, Co, Cu and Zn). The electron density on Ni single atoms in Ni_1+NPsM@NCNT can be successfully regulated by introducing 3d transition metals in Ni NPs. The Ni_1+NPsCu@NCNT exhibits the best FE_CO of 92.06% and a satisfying stability over 24 h. Moreover, when applied in a flow cell, the Ni_1+NPsCu@NCNT achieves a current density of 200 mA cm⁻² and a FE_CO above 90% over a wide range of 0.38 V to −1.08 V (vs. RHE). The DFT calculation proves that the NiM NPs can not only promote electron transfer to the active single Ni sites, but also upshift the 3d orbital of Ni, thus improving the intrinsic activity of Ni atoms.

Results and discussion

Synthesis and characterization

The Ni_1+NPsM@NCNT compounds are synthesized according to the literature with slight modification.³³ Typically, NiMAl-LDHs (M = Mn, Fe, Co, Cu, Zn) are synthesized by the coprecipitation method, whose X-ray diffraction (XRD) patterns show characteristic diffraction peaks corresponding to (00l) facets that match well with LDHs (Fig. S1†). Melamine is chosen as the source of carbon and nitrogen and then mixed with NiMAl-LDHs for the following calcination at 650 °C.³¹ Acid-leaching is applied to remove the acid soluble impurities of Ni_1+NPsM@NCNTs. The XRD pattern (Fig. S2†) shows diffraction peaks corresponding to (002) of graphite and (111) and (200) of Ni metal, which preliminarily prove the existence of Ni NPs and that structural modification caused by M doping is negligible. As presented in Fig. S3,† Raman spectra display a D band at about 1350 cm⁻¹ and a G band at 1600 cm⁻¹, which can be attributed to amorphous and ordered sp² carbon, respectively.^32,34 Intensity ratios between the two bands (I_D/I_G) can measure the defect degree of carbon-based materials. I_D/I_G values of Ni_1+NPsM@NCNT compounds are similar, indicating their unaffected graphitization degree. As shown in Fig. S4,† scanning electron microscopy (SEM) proves that Ni_1+NPsM@NCNT compounds are composed of carbon nanotubes. In addition, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) present NPs encapsulated in the carbon shell. As a result, the diameter of carbon nanotubes and the size of trapped NPs are measured to range from 10 nm to 20 nm (Fig. 1a, c and S5†). As shown in Fig. 1d, the lattice fringes of the NPs in Ni_1+NPsCu@NCNT are calculated to be 0.207 nm, which is slightly larger than that of pure Ni NPs (0.202 nm, Fig. 1b), matching well with NiCu.³⁵ Moreover, energy dispersive X-ray spectroscopy (EDX) elemental mapping images (Fig. 1e and S6†) reveal that C and N are uniformly distributed in the catalyst. Ni signals not only aggregate in the NPs, but also uniformly distribute throughout the carbon nanotube, demonstrating the uniform distribution of Ni single atoms. On the other hand, Cu can only be detected in the NP. In addition, the EDX line scan results in Fig. S7† reveal that both Cu and Ni can be detected in the NP but no Cu signal can be detected in NCNTs. These results preliminarily prove the basically non-existing Cu single sites compared to Ni single atoms and successfully synthesized NiCu NPs.

Fig. 1 TEM and High-resolution transmission electron microscopy (HRTEM) images of (a and b) Ni_1+NPs@NCNT and (c and d) Ni_1+NPsCu@NCNT; (e) EDX elemental mapping images of the Ni_1+NPsCu@NCNT.

X-ray absorption fine structure (XAFS) analysis is conducted to examine the electronic structures and chemical configurations of the metal elements. Ni K-edge X-ray absorption near-edge spectroscopy (XANES) spectra are displayed in Fig. 2a; the absorption edges of Ni_1+NPsCu@NCNT and Ni_1+NPs@NCNT are located nearly the same as that of Ni-foil, which proves that metallic Ni is the dominating species in NCNTs. Nevertheless, the oscillations of Ni_1+NPsCu@NCNT and Ni_1+NPs@NCNT in the E space are different from that of Ni-foil, suggesting that the coordination environments are not exactly alike. The extended XAFS (EXAFS) were Fourier transformed (FT) k³-weighted to further detect the fine structure of the samples (Fig. 2b). For Ni_1+NPsCu@NCNT and Ni_1+NPs@NCNT, both coordination shells of Ni–N (1.49 Å) and Ni–Ni (2.04 Å) can be observed, Furthermore, the coordination number (CN) of Ni–N is obtained from the EXFAS fitting results and the average values in Ni_1+NPsCu@NCNT and Ni_1+NPs@NCNT are determined to be 3.91 and 3.96, respectively (Fig. 2c and d). The EXAFS fitting results show that the first shell can be attributed to the Ni–N₄ coordination, which is in good agreement with the reported literature.^22,35 The EXAFS results demonstrate the co-existence of Ni NPs and single atoms, which is in good line with the TEM results. On the other hand, in Cu K-edge XANES, the pre-edge of Ni_1+NPsCu@NCNT performs basically the same as that of Cu foil (Fig. S9†), indicating that Cu mainly exist as metallic species. Such results agree well with EDX mapping results to affirm that Cu mainly exists in the NPs rather than single sites coordinated with N.

Fig. 2 (a) Ni K-edge XANES spectra and (b) Fourier transform EXAFS spectra in the R space of Ni_1+NPsCu@NCNT, Ni_1+NPs@NCNT and Ni foil; EXAFS fitting results of (c) Ni_1+NPs@NCNT; and (d) Ni_1+NPsCu@NCNT; (e) enlarged Ni 2p XPS spectra of Ni_1+NPs@NCNT, Ni_1+NPsMn@NCNT, Ni_1+NPsFe@NCNT and Ni_1+NPsCu@NCNT.

X-ray photoelectron spectroscopy (XPS) is then applied to investigate the elementary composition of Ni_1+NPsM@NCNT. As shown in Fig. S11a,† the N 1s spectra of Ni_1+NPs@NCNT and Ni_1+NPsM@NCNT (M = Mn, Fe, Cu) are located at similar binding energy and display identical shape features, which can be deconvoluted into five species: pyridinic N (398.9 eV), Ni–N (400.3 eV), pyrrolic N (400.4 eV), graphitic N (401.6 eV) and oxidized N (403.3 eV).³⁶ Meanwhile, the quantitative integral reveals that the contents of the N species are also similar in the four samples. In Fig. S11b† and 2e, the Ni 2p^2/3 spectra can be split into two peaks assigned to Ni^δ+ (0< δ < 2) and metallic Ni⁰, respectively. Notably, the Ni^δ+ peak shifts as follows: Ni_1+NPs@NCNT (855.06 eV) > Ni_1+NPsMn@NCNT (855.03 eV) > Ni_1+NPsFe@NCNT (854.94 eV) > Ni_1+NPsCu@NCNT (854.75 eV). These demonstrate that electrons are transferred from NiM NPs to the Ni^δ+ atoms and result in electron accumulation on the Ni–N₄ sites.

CO₂RR performance

The as-prepared Ni_1+NPsM@NCNT catalysts are then applied to electrochemical CO₂RR to evaluate their catalytic performance, which is first operated in an H-type cell. The linear sweep voltammetry (LSV) curves taken in 0.25 M KHCO₃ saturated with Ar or CO₂ are compared in Fig. S12;† all six samples show great enhancement in current density when CO₂ is injected into the electrolyte, indicating that the catalysts possess great capability for CO₂ reduction. FE_CO values of all catalysts under potentials ranging from −0.59 to −1.19 V (vs. RHE) are then measured and presented in Fig. S14a† and 3a. The gas chromatography (GC) and ¹H nuclear magnetic resonance (NMR) analyses are applied to detect the gaseous and liquid products, respectively (Fig. S13†). No other products except for CO and H₂ can be traced. All the Ni_1+NPsM@NCNT compounds show a higher FE_CO than that of Ni_1+NPs@NCNT and display a tendency of Mn < Fe ≈ Co < Zn < Cu. Among them, the Ni_1+NPsCu@NCNT outperforms others by a wide margin and reaches the highest FE_CO of 92.06% at −0.89 V (vs. RHE). The partial current density of CO is then calculated and shown in Fig. S14b† and 3b, where Ni_1+NPsCu@NCNT surpasses Ni_1+NPs@NCNT (17.5 mA cm⁻²) with an outstanding value of 21.8 mA cm⁻² at −0.89 (vs. RHE).

To meet the industrial demand for abundant CO production, the CO₂RR of Ni_1+NPsCu@NCNT is then tested in a flow cell equipped with gas diffusion electrodes (GDEs). The current density reaches up to 200 mA cm⁻² at −0.84 V (vs. RHE) in 1 M KOH with a CO₂ flow rate of 14 mL min⁻¹ (Fig. 3e). Moreover, a high CO selectivity of Ni_1+NPsCu@NCNT can also be extended to a wider potential window. As shown in Fig. 3f, a high FE_CO of above 90% is achieved under a wide range of applied potentials from −0.38 V to −1.08 V (vs. RHE), which reaches the maximum 96.92% at −0.78 V (vs. RHE). Such performance exhibits competitive superiority in the CO₂RR when compared with other state-of-the-art catalysts with Ni single atoms (Fig. 3g). The stability of Ni_1+NPsCu@NCNT is further tested to prove its practical significance for industrial application (Fig. 3h). After the galvanostatic measurement at 100 mA cm⁻² for 24 h, Ni_1+NPsCu@NCNT maintains a FE_CO of above 91% throughout the period and the applied potential only exhibits a slight increase from −0.61 V to −0.69 V (vs. RHE). XRD and TEM patterns of Ni_1+NPsCu@NCNT after the reaction display consistent diffraction peaks and morphology (Fig. S16†), illustrating the sustainable durability of Ni_1+NPsCu@NCNT.

Fig. 3 CO₂RR performance of Ni_1+NPs@NCNT, Ni_1+NPsMn@NCNT, Ni_1+NPsFe@NCNT and Ni_1+NPsCu@NCNT. (a) FE_CO and (b) J_CO of Ni_1+NPs@NCNT, Ni_1+NPsMn@NCNT, Ni_1+NPsFe@NCNT and Ni_1+NPsCu@NCNT; CO₂RR performance of Ni_1+NPsCu@NCNT in 1 M KOH flow cells. (c) LSV curves under Ar (black) and CO₂ (red) atmospheres; (d) FE_CO and FE_H2 under different potentials; (e) current density and FE_CO durability of Ni_1+NPsCu@NCNT at 100 mA cm⁻²; (f) CO₂RR performance of Ni_1+NPsCu@NCNT compared to other reported Ni-SACs; (g) the double-layer capacitances of Ni_1+NPs@NCNT, Ni_1+NPsMn@NCNT, Ni_1+NPsFe@NCNT and Ni_1+NPsCu@NCNT; (h) FE_CO of Ni_1+NPsCu@NCNT before and after 0.1 M KSCN treatment.

To elucidate the better CO₂RR performance of Ni_1+NPsCu@NCNT, electrochemical surface areas (ECSA) of Ni_1+NPsM@NCNT are investigated. ECSAs are obtained from the electrochemical double-layer capacitance (C_dl) by measuring cycle curves at scan speeds which increase equally (Fig. S15† and 3c). As a result, approximate values are obtained for Ni_1+NPs@NCNT, Ni_1+NPsMn@NCNT, Ni_1+NPsFe@NCNT and Ni_1+NPsCu@NCNT, indicating that the superior performance is more likely to originate from active species rather than the surface area available.³⁷ Thiocyanate ions (SCN⁻) are then added into the electrolyte as an inhibitor to block the metal-N sites, thus illustrating their activity for the CO₂RR.²⁵ As expected, the FE_CO exhibits a significant decline after the addition of SCN⁻ (Fig. 3d), confirming Ni–N₄ sites to be active centers for the CO₂RR. On the other hand, Ni_NPs@NCNT without Ni single atoms is in favor of the HER and the CO production is negligible, explaining that NPs play a role in *H generation for proton feeding.

DFT calculation

To shed light on the enhanced CO-produced CO₂RR performance of Ni_1+NPsCu@NCNT, the DFT calculation is further carried out. The free energy profile of the competitive HER is calculated first. All the Gibbs free energy barriers (ΔG_H*) of the Volmer step (H⁺ + e⁻ → H*) are increased after 3d transition metal doping, revealing the suppression of the HER process. The ΔG_H* increases in the following order: Ni (1.20 eV) < NiMn (1.27 eV) < NiFe (1.33 eV) < NiCu (1.50 eV), which is in good agreement with the sequence of FE_H2 decrease in Fig. 3, suggesting that ΔG_H* can be an effective indicator for the inhibition of the HER, and thus the Ni_1+NPsCu@NCNT with the highest H* energy barrier among these samples has a most favorable selectivity for CO.

Further electronic structure analysis is employed to understand the Cu doping from an atomistic perspective. The d-band center (ε_d) has been identified as a descriptor of intermediate binding in electrocatalytic systems.³⁸ The ε_d upshift of Ni 3d orbitals in Ni_1+NPsCu@NCNT (−1.68 eV) compared with Ni_1+NPs@NCNT (−1.58 eV) suggests the less occupancy of anti-bonding states, and thus stronger intermediate binding strength is promisingly obtained. The contour of electron density difference shows that the electron is transferred from Ni SAs to NPs (Fig. 4c). Then Mulliken charge analysis is used for further quantitative analysis.^39,40 It is found that the Mulliken charge of the Ni single site in Ni_1+NPsCu@NCNT (−0.22e) is more negative than that in Ni_1+NPs@NCNT (−0.2e), demonstrating more electrons are accumulated on Ni–N₄ in Ni_1+NPsCu@NCNT, which is consistent with the XPS data. Electron accumulation can also enhance the binding of the reaction site to form the key intermediates. The stronger affinity of the substrate and intermediate will be helpful for the sufficient utilization of active sites and reaction kinetics. Thus, the Gibbs free energy profile of the overall CO₂-to-CO reduction process is calculated to understand the function of Cu doping for the overall pathway. The Ni_1+NPsCu@NCNT shows a lower ΔG for the first CO₂ activation step, indicating CO₂ activation. For the formation step of key intermediate COOH* (CO₂* + H⁺ + e⁻ → COOH*), the free energy barrier decreases to 0.85 eV after Cu doping, which is obviously less than that of pristine Ni_1+NPs@NCNT (1.03 eV), suggesting a more thermodynamically favorable CO production. For the CO desorption process, the CO can be desorbed from the surface of Ni_1+NPsCu@NCNT with a lower energy barrier (0.77 eV) compared with Ni_1+NPs@NCNT (0.93 eV), which is in line with the optimal FE_CO and efficient CO production (Fig. 5).

Fig. 4 (a) Calculated Gibbs free energy diagrams of the HER on Ni_1+NPs@NCNT and Ni_1+NPsCu@NCNT; (b) calculated projected density of states (PDOS) of d orbitals on Ni_1+NPs@NCNT and Ni_1+NPsCu@NCNT; (c) the plots of electron density difference in Ni_1+NPs@NCNT and Ni_1+NPsCu@NCNT. Blue color indicates the loss of charge and yellow shows the gain of electrons. (d) Calculated Gibbs free energy diagrams of electrocatalytic CO₂RR to CO on Ni_1+NPs@NCNT and Ni_1+NPsCu@NCNT.

Fig. 5 The schematic illustration of enhanced CO₂RR performance on Ni_1+NPsM@NCNTs.

Conclusion

In summary, we make use of one-step pyrolysis to successfully fabricate a series of Ni_1+NPsM (M = Mn, Fe, Co, Cu, Zn) NPs and Ni single atoms that are encapsulated in NCNTs. The HRTEM images confirm the existence of NiM NPs with a size of 10–20 nm. EXAFS further demonstrates the fine structure of the as-prepared Ni_1+NPs@NCNTs: Ni single atoms are coordinated to N atoms with a similar Ni–N₄ structure. XPS proves the lower valence of Ni in Ni_1+NPsM@NCNT compounds than that in Ni_1+NPs@NCNT. The FE_CO values of the as-prepared Ni_1+NPsM@NCNT compounds display a volcanic tendency where Ni_1+NPsCu@NCNT reaches the highest value of 92.06% in an H-cell. When applied in a flow cell, the Ni_1+NPsCu@NCNT exhibits an FE_CO of 96.92% and a current density of 171.25 mA cm⁻² at −0.78 V (vs. RHE). During the galvanostatic test under 100 mA cm⁻², a long-lasting stability of 24 h is achieved. DFT calculation reveals that the NPs can induce electron accumulation on the Ni–N sites, thus promoting the activation of CO₂ and the stabilization of the *COOH intermediate. Meanwhile, the d-band center of Ni–N is also upshifted, offering stronger binding with reaction intermediates. In addition, NPs can also increase the Gibbs free energy change of the Volmer step, thus suppressing the competitive HER.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (22288102, 22178019, 22208013, and 22378012) and the Fundamental Research Funds for the Central Universities (XK1802-6, XK1803-05, and XK1902). We thank the 1W1B station in Beijing Synchrotron Radiation Facility (BSRF) and BL11B station in Shanghai Synchrotron Radiation Facility (SSRF) for XAFS measurement.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4na00167b

‡ These authors contributed equally to this work.

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