The iron-chelating pyridine nitrogen-rich coordinated nanosheet (IPNCN) composites were fabricated through a convenient process, as schemed in Figure 1a-1b. First, the 1,10-phenanthroline-5,6-quinone were used as the raw materials to obtain the yellow powder products of the TP through the condensation of schiff-base reaction. Then, the nitrogen elements at both ends of the TP were coordinated with the iron ions from ferrous chloride to form the coordination polymers in the ethanol/water solution under the solvothermal conditions at 90 oC. The thus-formed coordination polymers were purified through washing and centrifugation, and then the targeted products of reddish-brown powders (the IPNCN precursors) were obtained. Finally, in order to achieve the better ORR catalytic performance, the IPNCN precursors were calcined at different temperatures (750-900 oC) in the nitrogen atmosphere. After cooling to room temperature, the calcined products were stirred in the 1 mol/L hydrochloric acid to remove the inactive and metal oxides produced in the heating process [13].
The structures of the TP were determined by the MALDI-TOF mass spectrum (Figure 1c), 1H NMR spectrum (Figure S1a), and 13C NMR spectrum (Figure S1b). A significant peak at 384.8 m/z was found in the MALDI-TOF mass spectrum (Figure 1c), which is consistent with the molecule weight of the TP. The 1H NMR spectrum (Figure S1a) in the CDCl3 (1 × 10−3 mol/L) at 25 oC displayed three highly resolved signals in the aromatic region, corresponding to the skeleton protons. In the solid-state 13C NMR spectrum (Figure S1b), the overlapping signals from 120 to 150 ppm belong to the aromatic carbons in the skeleton [14]. These results demonstrated that the novel organic ligands were successfully synthesized. The TP monomers contain up to 21.9% nitrogen element and both ends are pyridine nitrogen which can be coordinated with Fe2+, so that the TP and Fe2+ are alternately connected to form the coordination polymers with periodic repetitive structure units. The IPNCN precursors could be easily formed because of the strong coordination interactions between the nitrogen elements and the metal ions under high temperature and high pressure [15]. The TP and IPNCN composites were further characterized by Fourier transform infrared spectra (FT-IR) (Figure 1d). A new absorption band at 1022 cm−1 for the IPNCN (TP/Fe2+ =1/1) composites was observed to be assigned to the characteristic of Fe-N bonds, indicating that the coordination polymers are successfully formed [16]. Also, the literature [17] and the Gaussian calculations have showed that the main skeleton of the TP is planar. Also, the coordination form of Fe-N bonds can be further proven by Gaussian calculations. The details of the Gaussian computational method, cartesian coordinates, and electronic energies for all the complexes calculated in the present study were presented in “S2. Gaussian calculations” in the Supporting Information. In the process of coordination with Fe2+, which plays a key role in connecting points, the different angles can be formed and covered between the planes, resulting in a change in thickness and length of nanosheet.
To obtain the coordination polymers with different degree of reaction, the different mass ratios of TP to Fe2+ were adjusted among TP/Fe2+ = 2/1, TP/Fe2+ = 1/1, and TP/Fe2+ = 1/2, named as IPNCN-1, IPNCN-2, and IPNCN-3, respectively. The IPNCN samples burning at different calcinate temperatures were denoted as IPNCN-1-850, IPNCN-2-X (X = 750, 800, 850, and 900), and IPNCN-3-850, where the last character of X stands for the calcinate temperature. The TP-850 represents the TP samples which do not coordinate with metals and are calcined at 850 oC. Notably, the TP can also form the planar sheets due to the strong intermolecular large conjugated aromatic structures. The microstructures of the TP were characterized by the emission scanning electron microscopy (SEM) which shows the scale-like nanosheets (Figure 2a), indicating the consistency of the microstructures and the chemical structures. After coordinating with the metal ions through solvothermal method under high temperature and high pressure, the nanosheets still remain but the size was changed (Figure 2c) [18]. The reason is that the metal ions in the center can connect the planar flake structures and the both ends of the pyridine nitrogen from TP can coordinate with central metal ions at different angles in the three-dimensional space, resulting in the angle changes between the nanosheets [19]. The nanosheet morphologies of the TP were changed after calcination at 850 oC in the nitrogen atmosphere, and the flake morphologies would shrink and accumulate (Figure 2b). However, the morphologies of the IPNCN-2-800, IPNCN-2-850, and IPNCN-3-900 can be still maintained stable (Figure 2d, 2e, and 2f), and the sheet is still sustained, reflecting that the coordination interactions are beneficial to improving the thermal stability of the large conjugated aromatic structures.
The stability at different calcination temperatures (Figure 2) was mainly benefited from the fully conjugated rigid planes of the TP and the rigid polymers of the IPNCN-2 composites. The planar structures of the TP are also capable of acting as the templates for metals, which is favorable to coordination with metals and full contact with oxygen during the oxygen reduction reaction process [20]. The advantage of such design is the homogeneous distribution of the nitrogen and the uniformly dispersed metal catalytic active sites [21], which prevents the agglomeration of the metals in the calcination process. Therefore, it can be convinced from the above-mentioned results and discussion that the self-stable carbonization strategy has been discovered, which would provide a reference for the selection of the stable catalytic materials.
To examine the chemical constitutions of the as-prepared IPNCN composites, the X-ray photoelectron spectroscopy (XPS) measurements were further performed, as presented in Figure 3 and Figure S2-S3 in the Supporting Information. The existence of the C, N, O, and Fe elements in the IPNCN-2-850 can be convinced from Figure 3a, corresponding to the spectra peaks around 285, 399, 531, and 715 eV, respectively. The chemical elements of the IPNCN-2-X composites were counted and summarized in Table 1, indicating that the nitrogen atoms have been successfully doped into the IPNCN-2-X composites. The high-resolution C 1s spectra displayed four peaks at 284.58, 285.15, 286.12, and 288.56 eV (Figure 3b), assigning to the C-C, C-N, C-O, and C=O groups, respectively [22]. The N 1s peaks can be divided into 398.23, 400.24, 401.00, and 402.73 eV (Figure 3c), corresponding to the pyridinic-N, pyrrolic-N, graphitic-N, and N-oxide (N-O), respectively [23]. The contents of the different nitrogen species were also calculated and listed in Table 1. It can be seen that pyridinic-N and graphitic-N still have relatively high contents at high temperature, and the pyridinic-N and graphitic-N can serve as the efficient active sites during the oxygen reduction reaction process [24]. The total nitrogen content decreases from 11.14 at. % to 3.24 at. % with the increase of pyrolysis temperature from 750 to 900°C due to the decomposition of the N-C bonds at high temperature [25, 26]. However, the proportion of the pyridinic-N (27.8% in Table 1) in the IPNCN-2-850 composites could still strongly support the outstanding oxygen reduction reaction activity. The high-resolution Fe 2p XPS spectra of the IPNCN-2-850 composites in Figure 3d are divided into individual peaks [27]. The binding energy peak at 710.97, 714.08, 718.16, 723.30, and 725.76 eV corresponds to the 2p3/2 orbit of Fe2+, the 2p3/2 orbit of Fe3+, the satellite peak, the 2p1/2 orbit of Fe2+, and the 2p1/2 orbit of Fe3+, respectively. The Fe2+ groups act as the role of the active sites, promoting the adsorption of O2 and boosting the catalytic activity of the IPNCN composites [25, 28, 29].
Table 1
Summary of the nitrogen content and bonding state of the IPNCN-2 composite catalysts.
Sample | C (at. %) | N (at. %) | O (at. %) | Fe (at. %) | XPS (N content %) |
Pyridinic-N | Pyrrolic-N | Graphitic-N |
IPNCN-2-750 | 77.5 | 11.1 | 10.0 | 1.4 | 34.3 | 29.4 | 26.9 |
IPNCN-2-800 | 79.0 | 9.21 | 10.8 | 1.0 | 33.7 | 27.9 | 25.2 |
IPNCN-2-850 | 79.5 | 8.3 | 10.9 | 1.2 | 29.3 | 27.8 | 27.7 |
IPNCN-2-900 | 84.5 | 3.2 | 11.4 | 0.9 | 30.1 | 30.5 | 28.1 |
The surface areas and pore structures of the IPNCN-2 composite catalysts were determined by the nitrogen isothermal adsorption-desorption measurements through the Brunauer-Emmett-Teller (BET) (Figure 4). The IPNCN-2-X composite catalysts exhibit an obvious of type-H4 isotherm characteristics (Figure 4a) with an increase of nitrogen absorption at both low and high pressure [29, 30], which is the characteristic of the disordered micro-/meso-porous materials. The details of the BET surface areas and pore size distributions of the IPNCN-2 composite catalysts were listed in Table 2. The BET surface area of the IPNCN-2-750, IPNCN-2-800, IPNCN-2-850, and IPNCN-2-900 composite catalysts is 13.0, 14.1, 12.0, and 34.2 m2 g−1, respectively. The changing surface of the measured samples is attributed to the small organic molecules produced by the decomposition at relatively high temperature. The IPNCN-2-X composite catalysts have the mesopores with large sizes and the nanopores with small sizes that can be beneficial for the catalysis process and provide the more abundant active sites, respectively [31].
Table 2
The BET surface areas and pore size distributions of the IPNCN-2 composite catalysts.
Sample | SBET (m2 g−1) | Pore volume (mL g−1) |
Total | Micro | Meso |
IPNCN-2-750 | 13.000 | 0.030 | 0.011 | 0.021 |
IPNCN-2-800 | 14.100 | 0.040 | 0.013 | 0.029 |
IPNCN-2-850 | 12.000 | 0.040 | 0.005 | 0.037 |
IPNCN-2-900 | 34.223 | 0.091 | 0.026 | 0.064 |
To explore the ORR catalytic performances, the cyclic voltammetry (CV) measurements in the N2-saturated and O2-saturated 0.1 mol/L potassium hydroxide (KOH) solution for the IPNCN-2-850 composite catalysts were further performed on a rotating disk electrode (RDE) at the scan rate of 50 mV s−1 (Figure 5). Figure 5a depicts that the redox peak could not be found in the N2-saturated electrolyte, while a well-defined cathodic reduction peak at 0.80 V was clearly observed in the O2-saturated KOH electrolyte, suggesting an obvious intrinsic electrocatalytic activity for the ORR [32, 33]. For comparison, the apparent peak at 0.76, 0.78, and 0.80 V was observed for the IPNCN-2-750, IPNCN-2-800, and IPNCN-2-900 composite catalysts in the O2-saturated electrolyte, respectively (Figure S4a). The cathodic reduction current peak at about 0.81 V (vs. RHE) of the IPNCN-2-850 composite catalysts was higher than that of other samples (Figure 6a, Figure S4a, and Figure S4b), indicating that the IPNCN-2-850 composite catalysts have the best ORR activity among others. To further explore the electrocatalytic properties of the IPNCN-2-X composite catalysts, the linear sweep voltammogram (LSV) measurements were carried out by using the rotating disk electrode (RDE) in the O2-saturated 0.1 mol/L KOH solution (Figure 5b). The IPNCN-2-850 composite catalysts showed excellent onset potentials of 0.93 V and half-wave potentials of 0.84 V (vs. RHE), which can compare to the commercial Pt/C catalysts with onset potentials of 0.96 V and half-wave potentials of 0.85 V (vs. RHE). In addition, The ORR activity of the IPNCN-2-850 composite catalysts is more positive than the other IPNCN-2 composite catalysts at different temperatures (Figure 5b) and different mole ratios (Figure S4c), implying an excellent ORR performance for the stable IPNCN composite catalysts [34]. Furthermore, the limiting current density of the IPNCN-2-850 composite catalysts is also better than that of the Pt/C catalysts. Therefore, the most proper temperature for preparing the IPNCN-2 composite catalysts is 850 oC, which could create the useful mesoporous structures and the higher pyridinic-N content to offer the more active sites and facilitate the efficient mass transfer. In addition, the planar structures could further provide sufficient contact points for oxygen to promote the ORR catalytic performance [35].
The linear sweep voltammetry (LSV) curves of the IPNCN-2-850 composite catalysts varying from 400 to 2400 rpm in the O2-saturated 0.1 mol/L KOH solution were further presented in Figure S4d. The limiting current density obtained from the LSV curves shows a rapid increase as the rotation speed increases, which is due to the decrease of the diffusion distance under high speed condition. The corresponding Koutecky-Levich (K-L) plots of the IPNCN-2-850 composite catalysts at different potentials exhibit a great linearity and some are overlapped (Figure 5c), indicating a similar electron-transfer process. The electron transfer number (n) of the IPNCN-2-850 composite catalysts was calculated as 3.9 in average at various potentials from 0.2 to 0.6 V according the K-L plot (Figure 5d), indicating that the oxygen is reduced through a direct four-electron pathway.
For comparison, the TP catalysts were prepared via the same method for preparing the IPNCN-2-850 composite catalysts but without the addition of ferrous chloride, which were named as TP-850. The ORR activities of the TP-850 catalysts were observed as not good as the IPNCN-2-850 composite catalysts (Figure S4e and Figure S4f), suggesting the importance of the iron ions which are able to form the active sites. In order to prove the importance of the coordination interactions in the solvothermal process, the direct method of grinding TP and ferrous chloride was employed instead of the solvothermal method to prepare the Grind-TP-Fe-2-850 catalysts. The Grind-TP-Fe-2-850 catalysts also show the much lower onset potential and half-wave potential than the IPNCN-2-850 composite catalysts (Figure S4e and Figure S4f), indicating that the solvothermal method is conducive to form the coordination polymers and the uniform dispersion of the active sites [36–38].
In addition to the ORR activity, the electrochemical stability is another important aspect for the ORR catalysts in practical applications. The methanol resistance and electrochemical durability of the IPNCN-2-850 composite and Pt/C catalysts were characterized by the chronoamperometry measurements. After injecting the solution of methanol (1 mol/L), the Pt/C catalysts show an immediate drop in the current density. However, there is no obvious change in the current density for the IPNCN-2-850 composite catalysts during the ORR process (Figure 5e), revealing that the IPNCN-2-850 composite catalysts have the great tolerance to the methanol. Moreover, another significant parameter to detect the catalytic performance is the electrochemical durability, as illustrated in Figure 5f. The IPNCN-2-850 composite catalysts still keep 97.7% of the initial current while the Pt/C catalysts retain only 69% of the initial current after 40000 seconds (Figure 5f). It could be concluded that the as-prepared IPNCN-2-850 composite catalysts possess the much better methanol tolerance and electrochemical durability [39].