Strategic Design and Synthesis of Ferrocene Linked Porous Organic Frameworks toward Tunable CO2 Capture and Energy Storage

This work focuses on porous organic polymers (POPs), which have gained significant global attention for their potential in energy storage and carbon dioxide (CO2) capture. The study introduces the development of two novel porous organic polymers, namely FEC-Mel and FEC-PBDT POPs, constructed using a simple method based on the ferrocene unit (FEC) combined with melamine (Mel) and 6,6′-(1,4-phenylene)bis(1,3,5-triazine-2,4-diamine) (PBDT). The synthesis involved the condensation reaction between ferrocenecarboxaldehyde monomer (FEC-CHO) and the respective aryl amines. Several analytical methods were employed to investigate the physical characteristics, chemical structure, morphology, and potential applications of these porous materials. Through thermogravimetric analysis (TGA), it was observed that both FEC-Mel and FEC-PBDT POPs exhibited exceptional thermal stability. FEC-Mel POP displayed a higher surface area and porosity, measuring 556 m2 g−1 and 1.26 cm3 g−1, respectively. These FEC-POPs possess large surface areas, making them promising materials for applications such as supercapacitor (SC) electrodes and gas adsorption. With 82 F g−1 of specific capacitance at 0.5 A g−1, the FEC-PBDT POP electrode has exceptional electrochemical characteristics. In addition, the FEC-Mel POP showed remarkable CO2 absorption capabilities, with 1.34 and 1.75 mmol g−1 (determined at 298 and 273 K; respectively). The potential of the FEC-POPs created in this work for CO2 capacity and electrical testing are highlighted by these results.


Introduction
Numerous energy-collecting techniques have been developed as a result of the rising need for energy that emits no greenhouse gases. Although they are essential, renewable energy sources cannot provide all of our daily energy needs. Investigating reasonable and cost-effective methods of energy collection and storage is therefore important. Due to their excellent qualities including high energy densities, quick charge/discharge rates, and extended lifespan, supercapacitors (SCs) have received a lot of interest in this respect [1][2][3]. SCs are appropriate for a variety of applications, including biological defibrillators and wind turbines, thanks to their advantageous characteristics [4][5][6]. A number of factors, such as (i) reactions occurring at the surfaces of the electrode materials, (ii) physical charge separation across the EDLC surfaces, frequently using porous carbons as electrodes, and (iii) faradaic reactions involving organic molecules with redox activity and electrolytes, all have an impact on the ability of SCs to store electrical charge. The characteristics of the

Synthesis and Molecular Characterization of FEC-CHO, PBDT, and FEC-POPs
Two porous organic polymers (POPs) incorporating a ferrocene moiety were synthesized using a polycondensation reaction. The building unit, FEC-CHO, was reacted with Mel and PBDT in DMSO at 180 • C to yield FEC-Mel and FEC-PBDT POPs, respectively (see Figure 1a,b). To obtain the FEC-CHO compound, FEC was reacted with POCl 3 in DMF as a solvent for 16 h, resulting in crimson crystals (Scheme S1a). The FTIR spectrum of FEC-CHO exhibited peaks at 1243 and 1034 cm −1 , corresponding to the presence of cyclopentadiene rings [ Figure S1]. The 1 H NMR spectrum of FEC-CHO displayed signals at 4.80, 4.67, 4.28, and 9.90 ppm, which were attributed to the cyclopentadiene rings and the aldehyde unit's carbonyl group, respectively [ Figure S2]. The 13 C NMR analysis also identified signals at 194.32 ppm for the C=O group and at 69.77 and 73.53 ppm for the carbons in the FEC moiety [ Figure S3]. For the synthesis of the PBDT monomer, GD-2CN was refluxed with BZ-2CN and KOH in DMF at 130 • C, resulting in a white powder (Scheme S1b). The FTIR spectrum exhibited peaks at 3301 and 3125 cm −1 due to the NH 2 group in PBDT [ Figure S4]. The proton signals at 8.35 ppm and 6.90 ppm in the 1 H NMR spectrum [ Figure S5a] corresponded to the aromatic protons and the amino group in the PBDT structure, respectively. Moreover, the 13 C NMR analysis of PBDT [ Figure S5b] revealed signals at 170.91, 168.33, 140.24, and 128.20 ppm, indicating the presence of the carbonyl group, triazine unit, and aromatic ring, respectively. The successful synthesis of the FEC-CHO and PBDT monomers was confirmed through FTIR, 1 H NMR, and 13 C NMR analyses. The produced FEC-POPs exhibited remarkable chemical stability and were insoluble in commonly used organic solvents such as MeOH, DCM, acetone, DMF, and EtOH, indicating a high degree of polymerization.
The structures of the obtained FEC POPs (FEC-Mel and FEC-PBDT POPs) were confirmed through solid-state 13 C NMR spectra and FT-IR measurements. The FT-IR spectra of FEC POPs displayed absorption peaks at 1023 cm −1 , which corresponded to the C=C stretching of the ferrocene unit. Notably, absorptions at around 1082, 1554, and 3418 cm −1 were attributed to C-N, C=N and NH units, respectively. Moreover, distinct vibration bands for the aliphatic C-H group originating from the ferrocene unit were observed at 2927 cm −1 (Figure 2a). In the 13 C NMR profiles of FEC-Mel and FEC-PBDT POPs, signals between 116 and 146 ppm were assigned to carbon atoms in the aromatic rings. Additionally, a signal at 165 ppm indicated the presence of the C=N group in FEC-POPs. Signals ranging from 63 to 76 ppm were assigned to FEC moieties (Figure 2b). Thermogravimetric analysis (TGA) measurements conducted under a nitrogen environment were used to evaluate the thermal characteristics of FEC-POPs (Figure 2c). The TGA profiles of FEC-Mel and FEC-PBDT POPs demonstrated their chemical stability, with decomposition tem-peratures of 281 and 198 • C, respectively, at 5 wt%, and 353 and 278 • C, respectively, at 10 wt%. Furthermore, the residual weights of FEC-Mel and FEC-PBDT POPs at 800 • C were 52% and 55%, respectively. These results confirmed the high thermal stability of the FEC-POPs. The elemental compositions of the FEC-POPs were determined through X-ray photoelectron spectroscopy (XPS) (Figure 2d). The XPS spectra exhibited signals at 284, 400, and 532 eV, corresponding to the presence of C, N, and O atoms in the structures of the FEC-POPs. Additionally, the peak of the Fe element from the FEC unit appeared at a binding energy of 710 eV [61], validating the successful incorporation of the ferrocene unit into the FEC-POPs networks. Figure S6 illustrates the thermal stability analysis of the FEC-Mel POP sample, conducted via FTIR analysis, across a temperature range from 25 to 200 • C. The results demonstrated that all absorption peaks corresponding to NH, aliphatic C-H, and C=N functionalities remained unchanged. This finding strongly suggests that the FEC-Mel POP sample exhibited excellent thermal stability, even at elevated temperatures. The structures of the obtained FEC POPs (FEC-Mel and FEC-PBDT POPs) were confirmed through solid-state 13 C NMR spectra and FT-IR measurements. The FT-IR spectra of FEC POPs displayed absorption peaks at 1023 cm −1 , which corresponded to the C=C stretching of the ferrocene unit. Notably, absorptions at around 1082, 1554, and 3418 cm −1 were attributed to C-N, C=N and NH units, respectively. Moreover, distinct vibration bands for the aliphatic C-H group originating from the ferrocene unit were observed at 2927 cm −1 (Figure 2a). In the 13 C NMR profiles of FEC-Mel and FEC-PBDT POPs, signals between 116 and 146 ppm were assigned to carbon atoms in the aromatic rings. Additionally, a signal at 165 ppm indicated the presence of the C=N group in FEC-POPs. Signals ranging from 63 to 76 ppm were assigned to FEC moieties ( Figure 2b). Thermogravimetric analysis (TGA) measurements conducted under a nitrogen environment were used to evaluate the thermal characteristics of FEC-POPs (Figure 2c). The TGA profiles of FEC-Mel and FEC-PBDT POPs demonstrated their chemical stability, with decomposition temperatures of 281 and 198 °C, respectively, at 5 wt%, and 353 and 278 °C, respectively, at 10 wt%. Furthermore, the residual weights of FEC-Mel and FEC-PBDT POPs at 800 °C were 52% and 55%, respectively. These results confirmed the high thermal stability of the FEC-POPs. The elemental compositions of the FEC-POPs were determined through X-ray photoelectron spectroscopy (XPS) (Figure 2d). The XPS spectra       The SEM-EDS mapping confirmed the presence of carbon (C), nitrogen (N), and oxygen (O) atoms in FEC POPs (Figure 5a-h). Overall, the FEC-Mel and FEC-PBDT POPs exhibited N-heteroatom structures, high surface areas, meso-and microporous characteristics, and significant pore volumes. These findings suggest that these materials hold promise as potential candidates for applications in energy storage and gas capture [63]. XRD analysis ( Figure S7) indicated the presence of semi-crystalline peaks and broad diffraction peaks in the XRD profiles, suggesting the structural characteristics of the materials [59].

CO 2 Uptake of FEC-Mel and FEC-PBDT POPs
To evaluate the CO 2 uptake capabilities of FEC-Mel and FEC-PBDT POPs, CO 2 isotherm measurements were conducted at temperatures of 298 K and 273 K, respectively. At 298 K, FEC-Mel and FEC-PBDT POPs displayed a CO 2 capacity of 1.34 and 0. 51 mmol g −1 , respectively. At 273 K, the CO 2 uptake capacities increased to 1.57 mmol g −1 for FEC-Mel POP and 1.53 mmol g −1 for FEC-PBDT POP ( Figure 6). The superior CO 2 uptake performance of FEC-Mel POP can be explained by its high S BET surface area and total pore volume. exhibited N-heteroatom structures, high surface areas, meso-and micropor characteristics, and significant pore volumes. These findings suggest that these mater hold promise as potential candidates for applications in energy storage and gas cap [63]. XRD analysis ( Figure S7) indicated the presence of semi-crystalline peaks and br diffraction peaks in the XRD profiles, suggesting the structural characteristics of materials [59].  To evaluate the CO2 uptake capabilities of FEC-Mel and FEC-PBDT POPs, CO2 isotherm measurements were conducted at temperatures of 298 K and 273 K, respectively. At 298 K, FEC-Mel and FEC-PBDT POPs displayed a CO2 capacity of 1.34 and 0. 51 mmol g −1 , respectively. At 273 K, the CO2 uptake capacities increased to 1.57 mmol g −1 for FEC-Mel POP and 1.53 mmol g −1 for FEC-PBDT POP ( Figure 6). The superior CO2 uptake performance of FEC-Mel POP can be explained by its high SBET surface area and total pore volume.

Electrochemical Analysis of FEC-POPs
The electrochemical performance of FEC-Mel and FEC-PBDT POPs warrants investigation owing to their considerable BET surface areas and incorporation of triazine moieties, as illustrated in Figure 1. In this study, we examined the cyclic voltammetry (CV) profiles of FEC-Mel and FEC-PBDT POPs at different scan rates ranging from 5 to 200 mV s −1 . Additionally, the potential window spanning from −1 to 0 V versus Hg/HgO was explored for both materials. These measurements were conducted using a three-electrode system with 6 M KOH serving as the electrolyte. The obtained results are presented in Figure 7. The CV curves depicted in Figure 7a,b exhibit rectangular shapes without any redox peak for both FEC-Mel and FEC-PBDT POPs, indicating that the electrochemical properties of these materials primarily resemble those of electrical double-layer capacitors (EDLCs). Moreover, our findings demonstrate that the CV behavior of the FEC-Mel and FEC-PBDT POPs samples remains stable and reversible without any distortion across a scanning rate range of 5 to 200 mV s −1 . This data signifies that these two FEC-POPs materials possess favorable electron transfer rates and ion exchange capabilities. In addition, the capacitance performance of both FEC-Mel and FEC-PBDT POPs samples was assessed using galvanostatic charge-discharge (GCD) measurements at different current

Electrochemical Analysis of FEC-POPs
The electrochemical performance of FEC-Mel and FEC-PBDT POPs warrants investigation owing to their considerable BET surface areas and incorporation of triazine moieties, as illustrated in Figure 1. In this study, we examined the cyclic voltammetry (CV) profiles of FEC-Mel and FEC-PBDT POPs at different scan rates ranging from 5 to 200 mV s −1 . Additionally, the potential window spanning from −1 to 0 V versus Hg/HgO was explored for both materials. These measurements were conducted using a three-electrode system with 6 M KOH serving as the electrolyte. The obtained results are presented in Figure 7. The CV curves depicted in Figure 7a,b exhibit rectangular shapes without any redox peak for both FEC-Mel and FEC-PBDT POPs, indicating that the electrochemical properties of these materials primarily resemble those of electrical double-layer capacitors (EDLCs). Moreover, our findings demonstrate that the CV behavior of the FEC-Mel and FEC-PBDT POPs samples remains stable and reversible without any distortion across a scanning rate range of 5 to 200 mV s −1 . This data signifies that these two FEC-POPs materials possess favorable electron transfer rates and ion exchange capabilities. In addition, the capacitance performance of both FEC-Mel and FEC-PBDT POPs samples was assessed using galvanostatic charge-discharge (GCD) measurements at different current densities (ranging from 0.5 to 20 A g −1 ) and all CGD profiles are approximately in an isosceles triangle shape. The results, depicted in Figure 7c,d, clearly demonstrate that the FEC-PBDT POP sample exhibits higher specific capacitance values compared to the FEC-Mel POP sample.

Materials
Ferrocene (FEC), 1,4-dicyanobenzene (BZ-2CN), dimethyl sulfoxide (DMSO), potassium hydroxide (KOH), chloroform, dimethyl formamide (DMF), phosphoryl chloride (POCl3), sodium acetate, hexane, acetone, NaOH, 2-cyanoguanidine (GD-2CN), tetrahydrofuran (THF), ethanol (EtOH), and methanol (MeOH) were obtained via various trade resources, such as Sigma-Aldrich (Darmstadt, Germany), and Alfa Aesar  Py-PDT POP 76 28 F g −1 /0.5 A g −1 [21] The Ragone plot shown in Figure 8c compares the energy density of the electrode materials FEC-Mel and FEC-PBDT POPs. The FEC-PBDT material exhibits an energy density of 7.42 Wh kg −1 , while the FEC-PBDT POPs material has a higher energy density of 11.36 Wh kg −1 . These values correspond to a power density of 250 W kg −1 . Notably, both FEC-Mel and FEC-PBDT POPs materials outperform other N-doped porous carbon materials, with an energy density of 7.11 Wh kg −1 [75], and a porous graphene carbon material, with an energy density of 2.4 Wh kg −1 [76]. This highlights the superior performance of FEC-Mel and FEC-PBDT POPs as highly efficient N-doped porous carbon materials. To comprehend the ion diffusion process and electrical resistance of the electrodes, electrochemical impedance spectroscopy (EIS) was employed. By analyzing Figure 8d, depicting the Nyquist plot, we can assess the resistances exhibited by FEC-Mel POP and FEC-PBDT POP electrodes. Our primary focus was investigating the ohmic resistances of these electrodes, which were determined as 18.50 and 4.20, respectively. In EIS measurements, ohmic resistance pertains to the resistance encountered by the electric current as it flows between the bulk electrolyte and the electrode-electrolyte interface. This resistance comprises several components, such as electrolyte resistance, resistance at the electrodeelectrolyte interface, and any other resistances present within the system. In Figure S8, the chemical structure stability of both the FEC-Mel POP and FEC-PBDT POP samples was examined through FTIR analysis after undergoing electrochemical measurements. The results revealed that all absorption peaks associated with NH, aliphatic C-H, and C=N functionalities remained unaltered. This outcome strongly indicates that the FEC-Mel POP sample displayed outstanding chemical stability, even after the electrochemical experiment.

Synthesis of Ferrocenecarboxaldehyde (FEC-CHO)
POCl 3 (37 mL, 0.4 mol) was then progressively added after DMF (75 mL, 0.98 mol) had been chilled in an ice bath. A 15 min break was followed by the addition of 100 mL of CHCl 3 to thin the liquid. FEC (25 g, 0.14 mol) was then included in the mixture. The resultant mixture was agitated at 60 • C (for 16 h) and had a dark amber appearance. Upon completion of the reaction, the mixture was allowed to cool down. Ice water (500 mL) was then added gradually, followed by the slow addition of 35 g of NaOH and 107 g of sodium acetate. The product was extracted using chloroform (500 mL) and subsequently washed three times with water. To purify the product, it was passed through a silica column, and any remaining impurities were removed by elution with a hexane/acetone mixture (ratio of 5/1). Finally, crimson crystals of the desired product were obtained (17 g, yield: 61%, Scheme S1a). FTIR: 1243, 1034 cm −1 ( Figure S1). 1  A reaction was carried out using the following procedure: In a flask, BZ-2CN (0.386 g, 2.3 mmol) was added into DMF (10 mL). Separately, a mixture of KOH (0.281 g, 5 mmol) and GD-2CN (1.012 g, 12 mmol) was added into DMF (40 mL). The KOH/GD-2CN solution was then added to the BZ-2CN solution. About 20 h were spent stirring and refluxing the resultant mixture at 130 • C under a N 2 . Once the reaction was complete, the product was subjected to thorough washing with EtOH and MeOH to obtain PBDT (Scheme S1b). FTIR ( Figure S4

Synthesis of FEC-Mel POP
Mel (0.5 g, 3.96 mmol) and FEC-CHO (1.5 g, 7.01 mmol) in DMSO (20 mL) were produced and charged into a Schlenk flask. Three cycles of freezing and thawing were performed on the flask. The reaction flask was then heated to 180 • C and held at this temperature while stirring for 3 days. Following the reaction, the flask was allowed to cool to room temperature. After filtering, the finished product was washed with acetone, MeOH, and THF. The resulting black powder is known as FEC-Mel POP (Figure 1a). To prepare FEC-PBDT POP, FEC-CHO (0.87 g, 4.06 mmol) and PBDT (0.5 g, 1.69 mmol) were mixed in 20 mL of DMSO at 180 • C for 3 days. After the reaction period, the flask was allowed to cool down. The resulting product was filtered and washed sequentially with THF, MeOH, and acetone. The obtained solid material, which appeared black, was further dried at 100 • C. As a result, FEC-PBDT POP was obtained as a black powder (Figure 1b).

Conclusions
In summary, we developed two types of porous organic polymers (FEC-POPs) by incorporating the FEC unit with different aryl amines through a polycondensation reaction. The resulting polymers were named FEC-Mel and FEC-PBDT POPs. Both the FEC-Mel and FEC-PBDT POPs exhibited remarkable thermal stability, with a T d10 value of up to 353 • C and a char yield of approximately 54.55 wt% at 800 • C, as determined using thermal gravimetric analysis (TGA). For the potential applications for FEC-POPs, we revealed that the FEC-PBDT POP electrode exhibited an exceptional capacitance of 82 F g −1 , confirming its suitability for application in supercapacitors. Furthermore, the FEC-Mel POP sample demonstrated impressive CO 2 capture capacities, measuring 1.34 and 1.57 mmol g −1 at 298 and 273 K, respectively. These findings highlight the potential of the synthesized FEC-POPs for utilization in energy storage and gas adsorption devices.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.