2D and 3D phthalocyanine covalent organic frameworks for electrocatalytic carbon dioxide reduction
Qi Zhang , Bin Han , Yucheng Jin , Mingrun Li , Enhui Zhang , Jianzhuang Jiang
Covalent organic frameworks (COFs), as an emerging kind of crystalline organic polymers constructed from organic building units through strong covalent bonds, have received increasing attention for diverse application, such as adsorption [1-4], molecular separation [5-7], sensing [8-11], catalysis [12-16], and optoelectronics [17-19] due to their well-defined structures, tailor-made functionality, ample porosity, and good stability. COFs have steadily shifted from initially structural design to the functional design involved in large numbers of availably functional building blocks, linked units, and linkages [20-24]. Compared with the dominant two-dimensional (2D) COFs, the finitely available building blocks and the difficulties in their crystallization and structure determination processes lead to synthetic challenge of three-dimensional (3D) COFs [25-28]. However, the efficiency of active sites utilization in 2D COFs is relatively low arising from the inaccessible active centers in the buried layers, which in some degree limits their functionality, in particular in photo/electron/chemical catalysis. Nevertheless, porous 3D COFs with dispersed conjugated modules can augment the exposed active centers and promote catalytic performance [29-31]. Obviously, the dimensionality of COFs influences on the electronic properties and significantly differentiate their functional properties, but rarely be explored in this field.
On the other hand, with rapidly increasing combustion of fossil fuels, excessive carbon dioxide emission has caused many environmental issues, such as global warming and sea level rise, which is endangering people's living environment [32-35]. Many efforts have been made to utilize and convert CO2 to fuels and chemicals by means of diversified photo/electro/chemical-catalysts [36-42]. Among them, electrocatalytic carbon dioxide reduction reaction (CO2RR) has been considered as one of the promising strategy to address this issue [43-48]. Significantly, the development of suitable electrocatalysts is essential for the effective execution of this electrocatalytic process. COFs with high conductivity, good stability, and uniformly distributed active units are expected to act as promising electrocatalysts for promoting this catalytic process [49-53]. In particular, 2D COFs with metal phthalocyanine functional building blocks have been revealed to efficiently convert CO2 to CO with high Faradaic efficiency and large current density [54-56]. Notably, 3D phthalocyanine COFs were rarely investigated on the electrocatalytic reactions due to the huge synthetic challenge and relatively low conductivity compared with their 2D counterparts. Our previous work has demonstrated that ultrahigh electroactive sites of 3D COFs is crucial in the electrocatalysis [57]. Moreover, the dimensionality can affect the electrocatalytic capacity induced by their different electronic properties and porous structures. As a consequence, the investigate on the correlation between dimensionality and electrocatalytic activity of COFs is important and significant.
Herein, one new 2D phthalocyanine COF, namely 2D-NiPc-COF, and one new 3D phthalocyanine COF, namely 3D-NiPc-COF, were fabricated through the solvothermal reaction between tetraanhydrides of 2, 3, 9, 10, 16, 17, 23, 24-octacarboxyphthalocyaninato nickel(Ⅱ) (Ni(TAPc)) with [2, 2-bipyridine]−5, 5-diamine (Bpyd) and tetrakis-(4-aminophenyl) methane (TAPM), respectively. Both COFs have good crystallinity revealed by the powder X-ray diffraction (PXRD) analysis and high resolution transmission electron microscope (HRTEM) photos. 3D-NiPc-COF shows the high utilization efficiency of Ni(TAPc) electroactive sites, amounting to 26.8%, almost 2 times higher than the in-plane stacking 2D-NiPc-COF. As a result, 3D-NiPc-COF electrode displays high CO2 to CO Faradaic efficiency (FECO) over 90% in a wide potential window from −0.7 V to −0.9 V (vs. reversible hydrogen electrode (RHE)). Moreover, 3D-NiPc-COF has superior electrocatalytic capacity with the larger partial CO current density (jCO) of −13.97 mA/cm2 at −0.9 V (vs. RHE), and higher turnover number (TON) and turnover frequency (TOF) with the values of 5741.6 and 0.18 s-1 at −0.8 V during the 8 h lasting test.
C4 symmetrical Ni(TAPc) was employed to prepare 2D-NiPc-COF and 3D-NiPc-COF according to the imide reaction with C2 symmetrical Bpyd and Td symmetrical TAPM in the mixed solvent of 1-butanol/N-methylpyrrolidone (NMP)/isoquinoline (0.5 mL/0.5 mL/0.1 mL, v/v/v) in 180 ℃ for 5 days (Fig. 1a). The structural information was firstly analyzed according to the fourier transform infrared (FT-IR) spectra. As shown in Figs. S1 and S2 (Supporting information), the disappeared amino bands of Bpyd and TAPM around 3300 cm-1 and C=O bands of Ni(TAPc) at ca. 1847 and 1775 cm-1 indicate the inexistence of raw materials. In addition, the new peaks at about 1773 and 1714 cm-1 can be attributed to the C=O bonds in the imide moiety. The typical stretching vibration of C—N-C bonds at 1363 cm-1 further demonstrated the successful formation of two polyimide COFs. The Ni contents in 2D-NiPc-COF and 3D-NiPc-COF were measured as 4.86 and 4.93 wt%, respectively, on the basis of inductively coupled plasma optical emission spectrometer measurement.
PXRD analysis combined with computational simulation were carried out to investigate the crystalline structures of these two COFs. For 2D-NiPc-COF, AA-stacking model with P4/mmm space groups was established (Fig. 1b and Fig. S3 in Supporting information). With the assistance of pawley refinement, the unit cell parameters were obtained to be a = b = 27.56 Å, c = 3.44 Å, and α = β = γ = 90° accompanied by Rp = 0.80% and Rwp = 1.30%. The four intense peaks at ca. 3.69°, 6.75°, 10.14°, and 26.48° correspond to the (100), (200), (300), and (001) planes (Fig. 1c). For 3D-NiPC—COF, pts topology structure model was built refer to the isostructural CoPc-PI-COF-3 (Fig. 1d) [57]. The unit cell parameters were determined to be a = b = 24.22 Å, c = 32.84 Å, and α = β = γ = 90° with Rp = 0.30% and Rwp = 0.57% by the means of the pawley refinement. The four obvious diffraction signals in the PXRD pattern at 2θ ≈ 4.10°, 6.86°, 11.3°, and 26.87° can be attributed to the (100), (200), (300), and (001) reflections (Fig. 1e). The negligible difference between the experimental PXRD patterns and simulated ones derived from their structural models indicates the validity of the established models.
Irregular polyhedral morphology can be observed for both COFs from the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figs. S4-S7 in Supporting information). The well-aligned lattice fringes disclosed in the HRTEM images indicate the good crystallinity of 2D-NiPc-COF and 3D-NiPc-COF (Figs. 1f and g). The space distance of 0.33 nm for 2D-NiPc-COF matches well with the simulated model. The element energy dispersive spectroscopy (EDS) mapping unveils the homogeneous distribution of carbon, nitrogen, oxygen, and nickel elements in the 2D-NiPc-COF and 3D-NiPc-COF skeleton (Figs. 1h and i). The thermogravimetric analysis measurement under N2 atmosphere reveals that both COFs can keep undecomposed over 320 ℃, inferring their good thermal stability (Figs. S8 and S9 in Supporting information). Furthermore, 2D-NiPc-COF and 3D-NiPc-COF samples were soaked in deionized water, strong HCl, Dimethylformamide, 0.5 mol/L KHCO3, NMP, tetrahydrofuran, and ethanol, respectively, for ten days to investigate their chemical stability. Fortunately, the PXRD patterns of these treated COF samples agreed well with their fresh samples, inferring the retained crystalline structures and their good chemical stability (Figs. S10 and S11 in Supporting information).
The structural information of both COFs were further analyzed by N2 sorption experiment. As shown in Figs. S12 and S13 (Supporting information), 2D-NiPc-COF and 3D-NiPc-COF have the moderate Brunauer-Emmett-Teller (BET) surface areas of 213.9 and 184.4 m2/g, respectively. Furthermore, the pore sizes distribution is concentrated around 2.53 and 1.74 nm for these two COFs (Figs. S14 and S15 in Supporting information). Moreover, the CO2 uptake capacity was obtained as 23.8 and 38.4 cm3/g at 298 K/1.0 bar and 273 K/1.0 bar for 3D-NiPc-COF, much higher than those of 2D-NiPc-COF (18.1 and 26.7 cm3/g at 298 K/1.0 bar and 273 K/1.0 bar), implying the more favorable CO2 affinity of 3D-NiPc-COF (Figs. S16 and S17 in Supporting information).
X-ray photoelectron spectroscopy (XPS) measurement revealed the divalent nickel nature in 2D-NiPc-COF and 3D-NiPc-COF with the binding energies sitting at ca. 873.1 (Ni 2p1/2) and 855.1 eV (Ni 2p3/2) for the former one and 874.1 (Ni 2p1/2) and 855.1 eV (Ni 2p3/2) for the latter one (Figs. S18 and S19 in Supporting information). Additional evidence for standing this point can be acquired from the X-ray absorption fine structure spectroscopy (XAFS) investigation. The Ni K-edge X-ray absorption near-edge structure spectroscopy (XANES) studies announce the basically consistent absorption edge of two COF samples with commercial NiPc rather than Ni foil, further indicating the divalent nickel nature in both COFs (Fig. 2a and Fig. S20 in Supporting information). Moreover, the Ni K-edge extended X-ray absorption fine structure (EXAFS) curves have profiles more akin to the NiPc reference (Fig. 2b and Fig. S21 in Supporting information), suggesting that the Ni atoms in two COFs are located in a N4 square-planar coordination geometry. Besides, the quantitative EXAFS fitting curves of two COFs are well agreed with the corresponding curves derived from their structural models, giving the Ni-N distance of ca. 1.7 Å for both COFs (Fig. 2c, Fig. S22 and Table S1 in Supporting information). Additionally, only one intensity maximum at ca. 4.7 Å-1 can be observed from the wavelet transformation (WT) profile of Ni K-edge for 2D-NiPc-COF and 3D-NiPc-COF, in line with the NiPc (Figs. 2d-f and Fig. S23 in Supporting information). These results unveil the Ni atom coordination structure as same as NiPc.
The solid UV–vis absorption spectra disclose the typical absorption of soret bands at about 306 nm and Q bands at about 675 nm of phthalocyanine species for 3D-NiPc-COF. However, the absorption of soret bands and Q bands of 2D-NiPc-COF significantly red-shift to 317 nm and 680 nm, respectively, inferring the extended conjugation of 2D-NiPc-COF than 3D-NiPc-COF (Fig. 3a). Additional evidence can be got from the direct optical bands gaps (Eg) obtained from tauc plots. The Eg was determined to be 1.53 eV for 2D-NiPc-COF, smaller than that of 3D-NiPc-COF (1.58 eV), further indicating the extended conjugation of 2D-NiPc-COF (Figs. 3b and c). Additionally, Current-voltage curves reveal that 2D-NiPc-COF exhibits a higher conductivity of 7.47 × 10–4 S/m than that of 3D-NiPc-COF (7.22 × 10–5 S/m) (Fig. 3d). These results disclose the extended conjugation system of planar 2D-NiPc-COF than 3D-NiPc-COF with dispersed conjugated modules due to the aggregation effect.
Gas chromatograph (GC) revealed that only CO and H2 existed in the gaseous products. There were no liquid products dissolved in the electrolyte according to 1H NMR spectroscopic measurement (Fig. S24 in Supporting information). As shown in the linear sweep voltammetry (LSV) curves, the current densities measured in CO2 saturated electrolyte are much larger than those in Ar saturated electrolyte for both COF samples, manifesting the electrocatalytic CO2RR activity of two COFs (Fig. 4a and Fig. S25 in Supporting information). In addition, negligible CO product existed in the system when the working electrode was substituted with only carbon black and nafion, inferring the electrocatalytic nature of two COF samples (Figs. S26 and S27). Additionally, there was no CO generated during this electrocatalytic process in Ar saturated electrolyte, inferring the carbon source comes from carbon dioxide.
3D-NiPc-COF electrode exhibits the FECO of 67.9%, 87.8%, 94.4%, 97.0%, 90.0% and 73.6% at −0.5, −0.6, −0.7, −0.8, −0.9, and −1.0 V, higher than that of 62.8%, 81.8%, 94.5%, 95.5%, 86.3%, and 63.9% for 2D-NiPc-COF at the same potentials (Fig. 4b), inferring the higher electroactivity of 3D-NiPc-COF. Moreover, 3D-NiPc-COF shows much larger jCO of −0.14, −0.51, −2.99, −8.25, −13.97, and −19.77 mA/cm2 at −0.5, −0.6, −0.7, −0.8, −0.9, and −1.0 V, higher than those of −0.13, −0.32, −1.32, −4.15, −8.68, and −14.30 mA/cm2 at the same potentials for 2D-NiPc-COF (Fig. 4c, Figs. S28 and S29 in Supporting information). These results indicate that 3D-NiPc-COF has superior electrocatalytic performance than 2D-NiPc-COF. Especially, the FECO, jCO, and TOF of 3D-NiPc-COF were determined as 97.0%, −8.25 mA/cm2, and 0.18 s-1 at −0.8 V, superior to 2D-NiPc-COF and most other reticular materials reported thus far under similar experimental conditions (Fig. 4f and Table S2 in Supporting information).
For the sake of investigating the influence of dimensionality on their electrocatalytic capacity, the electrochemical impedance spectroscopy (EIS) experiments were executed. As shown in Fig. S30 (Supporting information), 3D-NiPc-COF displayed slightly larger transfer resistance than that of 2D-NiPc-COF, might due to the smaller conjugate of the former one than the latter one. Another evidence to support this point comes from the much higher conductivity of 2D-NiPc-COF than that of 3D-NiPc-COF as mentioned above. However, the surface concentration of active NiPc in the electrode was determined as 225 nmol/cm2 for 3D-NiPc-COF, indicating 26.8% of the total NiPc act as active sites, much larger than that of 2D-NiPc-COF (124 nmol/cm2 and 14.9%) on the basis of the integration of the reduction wave at their CV curves measured in Ar saturated electrolyte (Figs. S31 and S32 in Supporting information). This result indicates 3D-NiPc-COF with dispersed conjugated modules exposes more metal active sites during the electrocatalytic process, which is benefit to promote the electrocatalytic performance and well explain the better electrocatalytic performance of 3D-NiPc-COF than 2D-NiPc-COF. In addition, the Tafel plots of 2D-NiPc-COF and 3D-NiPc-COF electrodes were determined to be 232 and 148 mV/dec, respectively (Fig. 4d), inferring the faster CO2RR kinetics of 3D-NiPc-COF than 2D-NiPc-COF.
The long-term electrocatalytic measurement was carried out at −0.8 V (vs. RHE) using an H-cell for evaluating the stability of two COF samples. As shown in Fig. 4e, the FECO of the two COFs electrodes keep almost unattenuated and retain above 90% during 8 h lasting test. The FT-IR spectra and PXRD patterns of two COFs after long time test are well agreed with the as-synthesized samples (Figs. S33-S36 in Supporting information). Besides, negligible nickel content was detected in the electrolyte for 2D-NiPc-COF and 3D-NiPc-COF. Additionally, as shown in Figs. S31 and S32 (Supporting information), the TON and TOF on the basis of the active NiPc sites were calculated to be 5141.6 and 0.18 s-1 for 3D-NiPc-COF, 4230.5 and 0.16 s-1 for 2D-NiPc-COF during the 8 h lasting test.
In summary, 2D-NiPc-COF and 3D-NiPc-COF were synthesized according to the imide reaction to investigate the correlation between dimensionality and electrochemical properties for 2D and 3D phthalocyanine COFs. Compared with planar 2D-NiPc-COF, the 3D-NiPc-COF with dispersed conjugated modules has higher ratio of electroactive sites, resulting in the better electrocatalytic performance than 2D-NiPc-COF. These results not only provide two efficient electrocatalysts for CO2RR, but also unveil the influence of dimensionality on functionality.
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.
Qi Zhang: Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. Bin Han: Writing – review & editing, Writing – original draft, Project administration, Methodology, Funding acquisition, Conceptualization. Yucheng Jin: Validation, Investigation, Formal analysis. Mingrun Li: Validation, Supervision, Investigation. Enhui Zhang: Supervision, Software. Jianzhuang Jiang: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Formal analysis, Conceptualization.
Financial support from the Natural Science Foundation (NSF) of China (Nos. 22205015, 22175020, and 22235001), the National Postdoctoral Program for Innovative Talents (No. BX20220032), the China Postdoctoral Science Foundation Funded Project (No. 2022BG013), the Fundamental Research Funds for the Central Universities (Nos. 00007709 and 00007770), University of Science and Technology Beijing is gratefully acknowledged.
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a) Schematic synthesis of two NiPc based COFs. Simulated packing structures of (b) 2D-NiPc-COF and (d) 3D-NiPc-COF. Experimental PXRD pattern (black), refined profile (red), simulation pattern (blue), and difference (gray) of (c) 2D-NiPc-COF and (e) 3D-NiPc-COF. HRTEM and EDS mapping photos of (f, h) 2D-NiPc-COF and (g, i) 3D-NiPc-COF.
Figure 2 (a) XANES spectra, (b) Fourier-transformed (FT) EXAFS spectra, and (c) EXAFS fitting curves of 3D-NiPc-COF. The WT-EXAFS of (d) 3D-NiPc-COF, (e) Ni foil, and (f) NiPc samples.
Figure 3 (a) Solid UV–Vis absorption spectra of 2D-NiPc-COF and 3D-NiPc-COF. Tauc plots of (b) 2D-NiPc-COF and (c) 3D-NiPc-COF. (d) Current-voltage curves of 2D-NiPc-COF and 3D-NiPc-COF.
Figure 4 (a) LSV curves of 2D-NiPc-COF. (b) Faradaic efficiency, (c) partial CO current density, (d) Tafel plots, and (e) electrochemical stability test of 2D-NiPc-COF and 3D-NiPc-COF. (f) Comparison of CO partial current densities at different potentials between 3D-NiPc-COF with partial porphyrin/phthalocyanine based COFs electrocatalysts.
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