Single-atomic-Ni electrocatalyst derived from phthalocyanine-modified MOF for convoying CO₂ intelligent utilization

Synthesis of Ni-N-C-l

Firstly, 2.30 g of 2-methylimidazole and 0.025 g of IM₄NiPc were dissolved in 30 mL of methanol using ultrasound for 5 min, which was subsequently added into 30 mL of methanol containing 1.04 g of Zn(NO₃)₂·6H₂O under vigorous stirring. The obtained solution was stirred for another 24 h. The precipitate, namely ZIF-8-(IM₄NiPc), was collected by centrifugation and washed with methanol for several times and dried in vacuum at 70 °C for 12 h.

Secondly, the above ZIF-8-(IM₄NiPc) powder was heat-treated at 1,000 °C for two hours under Ar atmosphere to produce the Ni-N-C-l catalyst.

Synthesis of Ni-N-C-a

First, 0.05 g of N-C derived from ZIF-8 (carbonized at 1,000 °C) was dispersed in 50 mL of methanol via sonication for 30 min. Next, 30 mL of methanol containing 0.025 g of IM₄NiPc was added to the above solution. After being stirred for 24 h, the Ni-N-C-a catalyst was obtained using a similar collection method.

Polymer engineering

Tris(4-iodophenyl) amine (TIA) (62.1 mg, 0.1 mmol), palladium acetate (10 mg), piperazine or 2-methy piperazine (0.15 mmol), and bis(2-diphenylphosphinophenyl)ether (DPEphos, 10 mg) were added in 8 mL of toluene solution. The mixed solution was heated to 80 °C while continuously pumping in the produced gas from eCO₂RR. After reaction for 12 h, the obtained polymer materials were washed with toluene and ethanol and then dried under vacuum at 60 °C.

The additional detailed experimental data, such as reagents, characterizations, and electrochemical measurements, are provided in the supporting information.

Synthesis and characterization of catalysts

Distinguishing traditional introduction of node-evolved or cavity-confined nickel sites, we innovatively designed and synthesized a novel IM₄NiPc molecule with four 2-methylimidazole groups as the additive ligands to introduce the active Ni sites into ZIF-8 (denoted as ZIF-8-(IM₄NiPc), as shown in Supplementary Figure 1 and Figure 1A. Importantly, the feasibility of this strategy is mainly dependent on the IM₄NiPc having good methanolic solubility, well-defined structure [Figure 1B and C]^[44], and unique coordination ability with Zn²⁺ [Supplementary Figure 2]; other detailed data are shown in the supporting information and Supplementary Figures 3-5. Moreover, Figure 1A further depicts a schematic representation of the synthesis procedure for ZIF-8-(IM₄NiPc) derived Ni-N-C-l catalyst. Meanwhile, for comparison, IM₄NiPc was also immobilized onto congeneric pyrolytic N-C to form the counterpart, labeled as Ni-N-C-a.

Figure 1. Synthesis illustration of (A) IM₄NiPc, ZIF-8-(IM₄NiPc), Ni-N-C-l and Ni-N-C-a, (B) optical photograph of IM₄NiPc in methanol, and (C) high-resolution mass spectrum of IM₄NiPc.

Fundamentally, the Ni-N-C-l/a electrocatalysts were controlled to load nearly identical Ni contents to shut out the influence of nickel contents^[45,46], as verified by inductively coupled plasma atomic emission spectrometry (ICP-AES) tests [Supplementary Figure 6]. The powder X-ray diffraction (PXRD) patterns in Supplementary Figure 7 and Figure 2A present that ZIF-8-(IM₄NiPc) displays similar characteristic patterns with pristine ZIF-8, indicating the introduction of the IM₄NiPc molecules does not disturb the ZIF-8 framework. Subsequently, the designed two catalysts have been successfully derived from the parent materials, exhibiting similar carbon characteristics but without any diffraction peaks associated with Ni nanoparticles. This indicates the highly dispersed configuration of Ni species. Furthermore, the Raman spectra of two catalysts reveal low-intensity ratios of the D band to G band [Figure 2B]. Particularly, the designed Ni-N-C-l exhibits a lower ratio value, indicating a higher degree of graphitization due to the participation of IM₄NiPc. It is well-known that pore engineering in catalysts is closely related to the exposure of active sites and continuous mass transfer. Accordingly, representative N₂ sorption isotherms disclose the specific surface areas and pore size distribution of the selected catalysts [Figure 2C and D]. Interestingly, compared to Ni-N-C-a catalyst, the Ni-N-C-l reveals a higher specific surface area of 1,342 m² g^-1, indicating that designing a metallomacrocycle-assisted route is an efficient tactic to construct the accessible single-atom sites. Moreover, the surface compositions and chemical states of the Ni-N-C-l/a were further investigated by X-ray photoelectron spectroscopy (XPS)^[47], as depicted in Figure 2E and F. Generally, the N 1s spectra of the selected catalysts can be fitted into five typical peaks corresponding to pyridinic N, metal-N species, pyrrolic N, graphitic N, and oxidized N, respectively^[48-50]. Meanwhile, we prudently assigned N species of IM₄NiPc molecules in the N 1s spectrum of the Ni-N-C-a catalyst. In fact, the peak area of graphite N in Ni-N-C-l is larger than that of Ni-N-C-a, which is in agreement with the outcomes of Raman spectra, illustrating the potential higher conductivity of Ni-N-C-l. Impressively, in comparison with Ni²⁺ (855.8 eV) in Ni-N-C-a, the Ni 2p spectrum of pyrolytic Ni-N-C-l indicates the oxidation state of Ni to be between Ni⁰ and Ni²⁺. This finding is in line with previous studies^[51-53], further supporting the existence of single Ni atoms.

Figure 2. (A) PXRD patterns, (B) Raman spectra, (C) N₂ sorption isotherms, (D) pore size distributions, (E) XPS N 1s and (F) Ni 2p spectra of the designed catalysts.

We next used scanning electron microscopy (SEM) to identify the morphologies of the designed catalysts. Compared with protogenetic ZIF-8, the particle size of ZIF-8-(IM₄NiPc) was significantly reduced after introducing molecular nickel source [Supplementary Figure 8]. After pyrolysis, the Ni-N-C-l material exhibits a shrunk morphology as compared to ZIF-8-(IM₄NiPc) [Supplementary Figure 9]. To further visualize morphologies of the Ni-N-C-l/a, we employed transmission electron microscopy (TEM). As can be observed in Figure 3, Ni-N-C-l displays a fluffed nanoparticle feature consistent with the SEM result, with an absence of metal particles or clusters. Furthermore, the energy-dispersive X-ray (EDX) element mapping images show that Ni, N, and C are uniformly dispersed throughout the sample of Ni-N-C-l [Figure 3B]. This is also observed in the Ni-N-C-a sample [Supplementary Figure 10], which likewise suggests the high dispersity of nickel species at the same scale. More importantly, the aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) image clearly shows that Ni-N-C-l consists of atomically dispersed Ni sites, as evidenced by the distinct white dots in Figure 3C. The X-ray absorption near-edge structure (XANES) spectrum of Ni-N-C-l [Figure 3D], obtained from the Ni K-edge, displays the Ni edge energy of Ni-N-C-l that lies between those of metal foil and NiPc references. This observation suggests that the Ni species in Ni-N-C-l may exist within the valence state of 0 to +2, which agrees with the XPS results. Moreover, the signal of the D_4h symmetry structure with 1s to 4p_z transition in Ni-N-C-l is lower than that of NiPc, indicating that the symmetric structure of square-planar Ni-N₄ was modulated during pyrolysis. Additionally, the analysis of the Fourier transformed extended X-ray absorption fine structure (EXAFS) supports the existence of atomically dispersed Ni sites with Ni-N coordination in Ni-N-C-l [Figure 3E], in accordance with the reported works^[54-56]. In particular, the fitting analysis is carried out to verify the atomic configuration of Ni-N-C-l; that is, the single Ni atom is coordinated with four N atoms [Figure 3F, Supplementary Figure 11 and Supplementary Table 1].

Figure 3. (A) TEM, (B) EDX elemental mapping and (C) AC-HAADF-STEM images of Ni-N-C-l, (D) Ni k-edge XANES and (E) Fourier transformed EXAFS spectra of Ni foil, NiO, NiPc and Ni-N-C-l, (F) EXAFS R-space fitting curve of Ni-N-C-l.

eCO₂RR performance

Based on the analysis of structures and morphologies mentioned above, the electrocatalytic activities of CO₂ conversion over the comparable catalysts have been evaluated in a typical H-type cell with CO₂-saturated 0.5 M KHCO₃ solution. Linear sweep voltammetry (LSV) curves indicate that Ni-N-C-l exhibits an exceptional current density of nearly 70 mA cm^-2 at -1.1 V, greatly surpassing that of Ni-N-C-a [Figure 4A]. Intriguingly, the LSV curves in Ar-saturated solutions further indicate the contribution of CO₂ electroconversion to current densities [Supplementary Figure 12]. Generally, the reduction products from electrochemical CO₂ conversion were analyzed by off-line ¹H nuclear magnetic resonance (NMR) spectrometer and online gas chromatograph (GC), respectively. By combining the obtained ¹H NMR and GC spectra of Ni-N-C-l operating at -0.78 V [Supplementary Figures 13 and 14], indeed, the only reduction product of CO₂ electroconversion is CO. As shown in Figure 4B and Supplementary Figures 15 and 16, the Ni-N-C-l catalyst exhibits higher Faradic efficiencies of CO (FE_CO) within a wider potential window compared to other counterparts. Concretely, the FE_CO and current densities of Ni-N-C-l were optimized by controlling the amount of IM₄NiPc (related to active sites) and the carbonization temperature (related to conductivity). Moreover, as observed in Figure 4C and Supplementary Figure 17, the CO partial current densities (j_CO) of Ni-N-C-l are better than those of Ni-N-C-a, reaching a j_CO of 57.5 mA cm^-2 at -0.98 V, which is 4.75 times higher than that of Ni-N-C-a. To further emphasize the superiority of Ni-N-C-l in CO₂ electroreduction, it is evident that it exhibits higher turnover frequency (TOF) values [Figure 4D]. Undeniably, the stability is another essential reference for the practical application of catalysts. Figure 4E demonstrates the exceptional long-time durability of Ni-N-C-l, as both j and FE_CO almost remain unchanged for continuous electrolysis of 20 h at -0.78 V. This stands in sharp contrast to the Ni-N-C-a counterpart and reported catalysts [Supplementary Table 2], showcasing the outstanding eCO₂RR performance of structure-endowed Ni-N-C-l from multiple perspectives.

Figure 4. (A) LSV curves, (B) FE_CO, (C) j_CO, (D) TOFs and (E) long-time durability of Ni-N-C-a and Ni-N-C-l.

To gain a deep understanding of the eCO₂RR kinetics of the investigated catalysts in this system, a fitted Tafel slope of Ni-N-C-l in Supplementary Figure 18 indicates that the first electron transfer to form site-adsorbed intermediates may be the rate-determining step^[24,57]. Meanwhile, the electrochemical active areas of the two catalysts were compared by evaluating the electrochemical double-layer capacitances, and the results show that Ni-N-C-l has a larger active surface area [Supplementary Figure 19]^[58]. In addition, the fast interfacial charge-transfer capacity of Ni-N-C-l during the eCO₂RR is verified by the Nyquist plot in Supplementary Figure 20, as compared to Ni-N-C-a. These findings undeniably demonstrate the inherent high activity of Ni-N-C-l towards CO₂ electroconversion, with a traceable mechanism [Supplementary Figure 21].

Integrated CO₂ electroreduction to polymer engineering

To tackle the issue of economic and efficient product purification, we proposed, for the first time, the direct downstream conversion protocol of CO through polymer engineering. As can be seen in Supplementary Figure 22, we constructed the cascade design by integrating electrocatalysis and thermocatalysis. Interestingly, the two proof-of-concept covalent organic polymers with different functional groups (simplified as COP and COP-Me) were correspondingly obtained by carbonyl insertion under vent gas from CO₂ electroreduction [Figure 5A]. By analyzing XRD patterns and SEM images, we can preliminarily determine the synthesis of amorphous polymers with irregular morphologies [Supplementary Figures 23 and 24]. In order to further confirm the synthesis of target materials, infrared (IR), ultraviolet (UV) and ¹H NMR characterization were performed. As shown in Figure 5B, we can obviously observe the vibration peaks of amide groups at 1,600~1,700 cm^-1 in the COP and COP-Me, in comparison with tris(4-iodophenyl)amine (TIA). UV-Vis spectra clearly indicate the generation of new skeleton structures with differentiated absorption [Figure 5C]. ¹H NMR spectra in Supplementary Figure 25 further reveal the anisotropic C-H environments of polymers compared to the monomer. In addition, thermogravimetric (TG) analysis shows that COP and COP-Me have similar thermal stability [Figure 5D]. In prospect, these industrial CO₂-derived polymer materials may be used in wastewater treatment, precious metal recovery, and other fields [Figure 5E]^[59,60]. By means of this cascade strategy, the integration of electrochemical CO₂-to-polymer engineering is achieved, meaningfully, offering a potential avenue for future operation.

Figure 5. (A) Schematic diagram of designed COP and COP-Me based on CO₂-to-CO steered carbonyl insertion, (B) IR and (C) UV spectra of COP, COP-Me and TIA, (D) TG curves of COP and COP-Me, (E) application diagram of polymer materials from CO₂ conversion.