English

• The electrochemical reduction reaction (CO₂RR) is a promising method to convert CO₂ into valuable chemicals and fuels under mild operating conditions [1], which utilizes renewable intermittent energy as the driving force. Among the products of CO₂RR, CO is one of the products that can be integrated into existing downstream reactions within the pharmaceutical, chemical, and metallurgical industries [2]. Therefore, it is highly desirable to design and synthesize electrocatalysts with excellent performance for CO₂ conversion.

  At present, single-atom catalysts (SACs) have shown great prospects in the application of CO₂RR. The typical transition metals coordinated with nitrogen (M-N_x) moieties are generally considered as the CO₂RR active sites. The Ni-N-C SACs have obtained more attention because of their excellent CO Faradaic efficiency. However, most of studies have shown that the charge-symmetric Ni-N₄ moieties are the active sites of Ni-N-C SACs, which is not conducive to regulating the electron distribution and improving the intrinsic catalytic performance [3]. The different coordination numbers were be studied [4-6], demonstrating that the broken symmetry electron density of the central atom can improve the catalytic performance. It is of great significance to optimize the CO₂RR performance by adjusting the coordination environment of the center single-metal active site, such as breaking the symmetrical electronic structure of Ni-N₄ by introducing other higher electronegativity atom. Many studies have demonstrated that a coordinated O atom could promote the CO₂RR activity higher than that of the nitrogen atom, due to the electronegativity of O atom is higher than that of N atom [7]. However, one O atom instead of a planar N atom to form Ni-N₃O moieties is liable to change, and the oxygen atom will be removed under high temperatures due to the weaker Ni-O interaction, then the Ni-N₃O SAC turns into a vacancy-defect Ni-N₃-V [8]. However, the axial O atom coordinated with four planar N atoms to form Ni-N_4-O species is relatively stable. Atomically dispersed Ni sites coordinated with four N atoms and one axial O atom (Ni-N_4-O/C) was fabricated by an axial strategy method, the maximum FE_CO (close to 100%) was obtained at −0.9 V vs. RHE [9]. The asymmetrically coordinated Ni-N_4-O catalyst was synthesized through a template-sacrificing strategy, FE_CO over 90% in a wide potential window [3]. Typically, synthesizing the penta-coordinated single atom catalysts require the N- and O- rich precursors. Mostly, the O atoms are vanished or only incorporated into a carbon matrix, without the elaborate construction of precursors, making it difficult to form the axial O-coordinated SACs.

  Electrochemical CO₂ reduction reaction is a typical heterogeneous reaction, and the mass and electronic transport properties of catalysts play a critical role in its efficiency. Consequently, various porous materials have been synthesized to enhance the electrochemical performance of SACs by improving their mass and electronic transport properties [8,10-14]. The Ni@CC-T catalyst was synthesized via a molten salt approach, with Ni-N₄ on curved surfaces, it obtained reasonable selectivity for CO conversion (−0.6 V to −1.0 V vs. RHE) [13]. The Ni SAs@3D-INCT with 3D interconnected N-doped carbon tubes was prepared with 3D-AAO templates, with the Ni-N₄ moiety as active site, which was reported to have high total current density (−70 mA/cm², −1.0 V vs. RHE) with relatively low selectivity (~80%) within the applied potential window (−0.7 V to −1.0 V vs. RHE), corresponding to a C_dl of 28.3 mF/cm² [14]. However, most of porous materials were synthesized with complex methods. Therefore, a strategy that not only can construct axial O-coordinated Ni-N₄ active site with regulated electronic structure, but also with hierarchically porous structures corresponding to a high specific surface area to facilitate mass and electronic transfer, is of high significance. On the basis of the above considerations, we designed a nickel single-atom anchored N-doped carbon material catalyst (Ni SA/CNs) by a facile pyrolysis method with the assistance of NaNO₃ pore-forming agent (Scheme 1). The abundant hierarchically porous structured Ni SA/CNs with axial O-coordinated Ni-N₄ unique site moieties as active sites were constructed, corresponding nearly 100% CO selectivity within the applied potential range of −0.6 V to −1.0 V vs. RHE. An ultra-high specific surface area (1477 m²/g) facilitates the exposure of more Ni single-atoms, corresponding a high ECSA value (757 cm²), enhancing the adsorption of key intermediates (^*COOH), improving the mass and electron transfer abilities during the CO₂RR reaction, and consequently suppressing the side reaction of hydrogen evolution.

  Scheme 1

  

  Scheme 1.  Scheme of the formation of the Ni SA/CNs catalyst.

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  As indicated by the scanning electron microscope (SEM) images, the Ni SA/CNs (Fig. 1a), C (Figs. S1a and b in Supporting information), and CNs (Figs. S1c and d in Supporting information) have similar morphological structures. Conversely, the obtained Ni NP/CNs (Figs. S1e and f in Supporting information) has a compact structure. The Ni SA/CNs has a few-layer graphene-like structure (Fig. 1b), it was also proved by the curved onion-like lattice fringes in the high-resolution transmission electron microscope (HRTEM) image (Fig. 1c) and Raman spectra (Fig. S2 in Supporting information). The scanning transmission electron microscopy (STEM) and corresponding energy-dispersive X-ray spectroscopy (EDS) analysis images of Ni SA/CNs are shown in Fig. 1d, demonstrating that the C, N, and Ni elements all were dispersed uniformly in the whole carbon material. Additionally, the aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC HAADF-STEM) image (Fig. 1e) also shows the atomic distribution of Ni atoms in the Ni SA/CNs, as a number of isolated bright points (with yellow circles) show. The HRTEM image of Ni NP/CNs (Fig. S3 in Supporting information) shows the clear lattice fringes, with a distance of 0.32, 0.20, and 0.18 nm corresponding to the disordered carbon [15], the (111), and (200) interplane spacing of NiO [16,17]. These observations indicate that nickel atoms are agglomerating within the bulk N-doped carbon material, due to the lack of pore structures that promoting an insufficient abundance of anchoring sites. Accordingly, the NaNO₃ pore-forming agent plays a key role in forming the abundant pore structure to anchor more Ni single atoms during the pyrolysis process.

  Figure 1

  

  Figure 1.  (a) SEM, (b) TEM, (c) HRTEM, (d) STEM and corresponding elemental mapping images of C, N, and Ni, and (e) HADDF-STEM image of Ni SA/CNs.

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  The N₂ adsorption and desorption curves show the morphology of obtained materials, corresponding to the details shown in the Table S1 (Supporting information). As shown in Fig. 2a, the isotherm plot of Ni SA/CNs exhibits a type I isotherm with a type-H2 hysteresis loop. The insert image displays micropores with clear peaks at 1.9 nm, 3.8 nm, along with a minor peak corresponding to mesopores at 14.5 nm. Obviously, the micropores are the predominant structures in Ni SA/CNs. The C and CNs materials have the similar hierarchical pore structures (Figs. S4 and S5 in Supporting information). However, the isotherm plot of Ni NP/CNs shows the remarkably different structural profile (Fig. S6 in Supporting information) with a small amount of crack at 38.1 nm and 75.6 nm. As the Table S1 shows, the Ni SA/CNs exhibits a high specific surface area (1477 m²/g) due to its hierarchical pore structures. Both C and CNs exhibit a similar specific surface area to Ni SA/CNs, corresponding 1024 m²/g and 1599 m²/g, respectively. The Ni NP/CNs catalyst with a considerably small specific surface area (4.86 m²/g) and pore volume (0.01 m³/g). The large specific surface area effectively anchors and exposes more available active sites and consequently enhance electrochemical reduction activity [14,18]. This is consistent with the inductively coupled plasma optical emission spectrometer (ICP-OES) results, which show that the Ni SA/CNs contains 1.91 wt% of Ni atoms, significantly more than the 1.05 wt% in the Ni NP/CNs. Therefore, the NaNO₃ pore-forming agent plays a key role in the formation process of hierarchical pore structures. The decomposition of NaNO₃ could release a significant amount of gas in a short time, increasing the internal pressure within the bulk materials and the abundance pore structures that were formed simultaneously. The powder X-ray diffraction (XRD)profiles show in Fig. S7 (Supporting information), C, CNs, and Ni SA/CNs only exhibit two peaks at 24.2° and 44.0°, which represent carbon (002) and (004) diffraction peaks, respectively [19]. The Ni NP/CNs exhibits the diffraction peaks of Ni NPs (ICSD No. 96–210–2279), and the carbon diffraction peaks turn out to be stronger and sharper than those in the Ni SA/CNs, indicating that the aggregation of Ni nanoparticles existed due to its inability to anchor well on a smaller special surface area [20].

  Figure 2

  

  Figure 2.  (a) N₂ adsorption-desorption isotherm curves of Ni SA/CNs, with the inserted image displaying the pore distribution. (b) C K-edge, (c) N K-edge, and (d) O K-edge of Ni SA/CNs and Ni NP/CNs. (e) The normalized Ni K-edge XANES curves and (f) the Fourier transformed Ni K-edge EXAFS spectra of Ni SA/CNs and references.

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  Throughout the pyrolysis process, the rapid decomposition of all precursors unleashed a surge of gas, generating an instantaneous spike in internal pressure. Simultaneously, a unique coordination environment was formed. The coordination environment of the metal sites was verified through high-resolution full-spectrum X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption spectroscopy (XAS). The full XPS spectra (Fig. S8a in Supporting information) reveal the presence of all elements in the samples. The binding energy of the Ni 2p_3/2 peak is located between 856.0 eV and 853.5 eV (Fig. S8b in Supporting information), indicating that the valence of Ni atoms in Ni-containing catalysts lies between 0 and +2 [21].

  The chemical environments of the modulated catalysts were further examined by the X-ray absorption spectroscopy. The C K-edge spectra (Fig. 2b) display three peaks attributed to C=C (286.5 eV), C—N (289 eV), and C—C ring (293.5 eV), respectively [22,23]. N K-edge spectra (Fig. 2c) show the four special peaks attributed to pyridinic N (399.5 eV), Ni-N-C (401 eV), pyrrolic N (402.5 eV), and graphitic N (408.5 eV), respectively [22,24]. Notably, the Ni-N-C peak was observed in Ni SA/CNs but absent in Ni NP/CNs [15]. Additionally, high-resolution XPS N 1s for Ni NP/CNs and Ni SA/CNs, demonstrate similar phenomena (Fig. S9 in Supporting information). And the pre-peaks range 530–535 eV in the O K-edge spectra (Fig. 2d) reflect the structure of the empty electronic states on oxygen p-orbitals that strongly hybridized with transition metal 3d-orbitals [25,26], which indicate that O atoms were successfully coordinated with Ni atom. Therefore, the Ni atom is not only bound with O atoms but also with N atoms in Ni SA/CNs, and the Ni atom coordinates with the O atom in Ni NP/CNs. As the Fig. 2e shows, the Ni K-edge near-edge absorption energy spectra (XANES) of Ni SA/CNs is located between Ni foil and NiO, indicating that the valence state of Ni in Ni SA/CNs is between 0 and +2, consistent with the XPS results. Furthermore, the peaks patterns and intensities of Ni K-edge of Ni SA/CNs are different from the typical Ni-N₄ structure in stand nickel phthalocyanine (NiPc) [18,27], implying the different coordination environments. The intensity of A peak of Ni SA/CNs shows higher than it in NiPc, implying it has broken D4h symmetry resulting from 3d and 4p orbital hybridization of the Ni central atoms [28]. And the existed axial oxygen atom coordination breaks the symmetry [29]. The intensity of B peak of Ni SA/CNs lower than it in NiPc also conforms the distorted D4h symmetry of the Ni central atoms, concluded from the origin peak B in NiPc with square-planar Ni-N₄ moieties [30].

  To further analyze the possible coordination environment around Ni single atoms, as the k²-weighted Fourier transforms of the Ni K-edge extended X-ray absorption fine structure (EXAFS) show (Fig. 2f), the main peak at 1.88 Å of Ni SA/CNs can be assigned to the Ni-N/O scattering path, the Ni-Ni bonding is not displayed. This result demonstrates the absence of metallic and oxide crystals in the Ni SA/CNs. The Ni-N/O coordination numbers are (~5). This indicates that most of the single Ni atoms in Ni SA/CN are bonded with four nitrogen atoms and one oxygen atom in the first shell. Combining the above characterization analysis, Ni atoms in Ni SA/CNs is coordinated with in-plane four nitrogen atoms and an axial oxygen atom.

  The electrochemical CO₂RR activities of obtained samples were preliminarily tested by the linear sweep voltammetry (LSV) method. As shown in Fig. 3a, under CO₂-saturated electrolyte, all of them show that current density increased with the applied potential being more negative. Notably, the Ni SA/CNs catalyst exhibited the highest total current density of −50.0 mA/cm² at −1.1 V vs. RHE. In contrast, the current densities of Ni NP/CNs were significantly lower, which is similar to the current density of C and CNs. The current densities of all of prepared catalysts in N₂-saturated electrolyte were lower than those in CO₂ atmosphere (Fig. S10 in Supporting information). And the current densities of Ni SA/CNs in CO₂-saturated electrolyte is significantly higher than it in N₂-saturated electrolyte, indicating the effective electrochemical CO₂ reduction activity of Ni SA/CNs. For all of synthesized samples, only CO and H₂ gas products were be tested. As shown in Fig. 3b, Ni SA/NCs has the highest FE_CO value of 98% at −0.68 V vs. RHE, and the FE_CO kept well above 80% in the applied potential window of −0.48 to −1.10 V vs. RHE. This almost completely surpasses the same category of electrochemical CO₂RR catalysts [3,31-39]. The Ni NP/CNs displayed poor electrochemical CO₂RR activity in the applied potential window, the maximum FE_CO of Ni NP/CNs is only 51% at −0.78 V vs. RHE. As shown in Fig. S11 (Supporting information), the corresponding FE_H2 of the Ni SA/NCs almost cannot be tested within the range of −0.6 V to −1.0 V vs. RHE, the FE_H2 of Ni NP/CNs almost reach to 80% in all applied potentials, and the C, and CNs show the similar FE_H2 values. The CO partial current density (j_CO) of prepared materials is shown in Fig. 3c. For Ni SA/CNs, the clearly high j_CO value of −31.13 mA/cm² at −0.98 V vs. RHE. However, the j_CO values of C, CNs, and Ni NP/CNs are almost similar and significantly lower than the j_CO of Ni SA/CNs, and an enlarged image was inserted.

  Figure 3

  

  Figure 3.  (a) LSV curves, (b) FE_CO curves, (c) j_CO curves, (d) Tafel slope, (e) Nyquist plots in the CO₂-saturated electrolyte, and (f) stability test results.

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  To analyze catalysts' reaction kinetics, the Tafel slopes of Ni SA/CNs and Ni NP/CNs were checked (Fig. 3d). The Tafel slopes of Ni SA/CNs and Ni NP/CNs were 104.7 mV/dec and 315.6 mV/dec, respectively. The Tafel slope of Ni SA/CNs smaller than that of Ni NP/CNs, indicating the rate-determining step is the first proton-coupled electron transfer to form the key intermediate (COOH^*) and the fast rate-determining step due to the highly active Ni-N_4-O moiety [40]. During the CO₂RR process, the charge-transfer resistance was analyzed by the electrochemical impedance spectroscopies (EIS) [41]. The diameter of the semicircular Nyquist plots is usually used to calculate the charge-transfer resistance. The Nyquist plots, shows in Fig. 3e, were fitted and demonstrate that Ni SA/CNs exhibits a smaller charge transfer resistance compared to Ni NP/CNs. This is attributed to the hierarchic pore structures that enable faster electron transfer from the unique Ni-N_4-O moiety to CO₂, facilitating the favorable formation of the CO₂^•− radical anion intermediate [42,43]. To assess the intrinsic activity of the prepared catalysts, the TOF of Ni SA/CNs and Ni NP/CNs was calculated (Fig. S12 in Supporting information). The TOF value of Ni SA/CNs reached to 34,081 h⁻¹ at −0.98 V vs. RHE, which is 101.5 times higher than that of Ni NP/CNs (336 h⁻¹), where the highest TOF value for Ni NP/CNs was only 722 h⁻¹ at −0.78 V vs. RHE. The electrochemical active surface area (ECSA) of the catalysts were evaluated to further understand the electrochemical performance. The slope of double-layer capacitance (C_dl) was calculated from the CV data at different scan rate (Fig. S13 in Supporting information). The ECSA values of Ni SA/CNs and Ni NP/CNs are 757 cm⁻² and 17.25 cm⁻², respectively. The ECSA value of Ni SA/CNs is around 44 times higher than that of Ni NP/CNs. The substantially higher ECSA value of the Ni SA/CNs can be attributed to its high specific area, which is favorable for the exposure of active sites [44]. Furthermore, to evaluate the stability of the Ni SA/CNs, the long-term activity was be tested over 12 h at −0.88 V vs. RHE. As shown in Fig. 3f, there is no obvious decay in both the FE_CO and current density. As a consequence, the hierarchical pore structure could provide a high specific area, facilitating the highly dispersed distribution of Ni single atom within the materials. Consequently, the unique Ni-N_4-O species were successfully formed, promoting the excellent electroreduction CO₂-to-CO ability.

  To gain insight into the mechanism, in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements were operated to monitor reaction intermediates during the electrochemical reduction at various applied potentials in a CO₂-satuated electrolyte. As shown in Fig. 4a, the signal intensity of Ni SA/CNs changed with increasing applied biased potential. The peak centered around 1930 cm⁻¹ belong to linearly bonded CO (CO_L) at Ni single atoms from CO₂ [18,45]. The peaks at around 1410, 1550, and 1620 cm⁻¹ can be assigned to the C—O stretching of ^*COOH, the COO stretching of CO₃²⁻, and the OH⁻ of absorbed H₂O, respectively [46-48]. The intensity of peak at 1410 cm⁻¹ increased as the applied potential became more negative, indicating an increased coverage of ^*COOH, consistent with the j_CO. On the other hand, the in-situ ATR-SEIRAS spectra of Ni NP/CNs in Fig. 4b showed lower intensities for all peaks compared to Ni SA/CNs. Weak peaks of ^*COOH and absorbed H₂O were observed, indicating lower adsorption and activation abilities of intermediates. The peaks in 1800-1900 cm⁻¹ region, weakly adsorbed, belong to the bridge-bonded CO (CO_B), suggesting the presence of Ni nanoparticles [39,40]. They observations correspond to the lower electrochemical performance of Ni NP/CNs. In summary, the hierarchic pore structures not only provide a high specific area to anchor specific Ni-N_4-O active sites, promoting the adsorption of key intermediates (such as ^*COOH), but also promote mass and electronic transfer. This leads to the depression of hydrogen evolution and enhancement of electrochemical reduction ability.

  Figure 4

  

  Figure 4.  In-situ ATR-SEIRAS plots of (a) Ni SA/CNs, and (b) Ni NP/CNs.

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  In summary, the Ni SA/CNs catalyst, featuring unique Ni-N_4-O active sites, was successfully developed with the assistance of NaNO₃ pore-forming agent. The formation of hierarchical pore structures occurred during the decomposition of precursors and pore-forming agent, releasing a significant amount of gas and increasing the inner pressure within the bulk materials. Simultaneously, the formation of unique Ni-N_4-O sites was facilitated. These factors collectively contribute to the catalyst's outstanding performance, achieving nearly 100% FE_CO within the applied potential window (−0.4 V to −1.0 V vs. RHE) and demonstrating excellent stability for CO₂RR. The in-situ ATR-IR analysis provided insights into the reaction process over Ni SA/CNs. This study demonstrates how hierarchical pore structures with unique Ni-N₄—O activity sites play a crucial role in promoting catalytic activity, providing an effective catalytic system for CO₂ utilization to produce CO.

  CRediT authorship contribution statement

  Xiaoxu Duan: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation. Junli Xu: Data curation. Jiwei Li: Visualization. Congcong Du: Resources. Kai Chen: Software. Teng Xu: Data curation. Yifei Sun: Writing – review & editing, Supervision. Haifeng Xiong: Supervision.

  Acknowledgments

  The work was financially supported by National High-Level Talent Fund and National Natural Science Foundation of China (Nos. 22372138, 22461160253, 22121001, and 22072118). We also thank financial support from State Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University and Shenzhen Science and Technology Program (No. JCYJ20220530143401002). Part fund was supported by Science and Technology Projects of Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM) (No. HRTP-[2022]-3) and the Fundamental Research Funds for the Central Universities (No. 20720220008).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110340.

  
  
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• 

  Scheme 1  Scheme of the formation of the Ni SA/CNs catalyst.

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  Figure 1  (a) SEM, (b) TEM, (c) HRTEM, (d) STEM and corresponding elemental mapping images of C, N, and Ni, and (e) HADDF-STEM image of Ni SA/CNs.

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  Figure 2  (a) N₂ adsorption-desorption isotherm curves of Ni SA/CNs, with the inserted image displaying the pore distribution. (b) C K-edge, (c) N K-edge, and (d) O K-edge of Ni SA/CNs and Ni NP/CNs. (e) The normalized Ni K-edge XANES curves and (f) the Fourier transformed Ni K-edge EXAFS spectra of Ni SA/CNs and references.

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  Figure 3  (a) LSV curves, (b) FE_CO curves, (c) j_CO curves, (d) Tafel slope, (e) Nyquist plots in the CO₂-saturated electrolyte, and (f) stability test results.

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  Figure 4  In-situ ATR-SEIRAS plots of (a) Ni SA/CNs, and (b) Ni NP/CNs.

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