CMPs are synthesized by various methods. Monomers containing N, S, Fe, Co, Cu and other atoms are introduced into the network structure of CMPs after polymerization (such as Sonogashira-Hagihara coupling, Buchwald-Hartwig coupling, Suzuki-Miyaura reaction). After carbonization, these heteroatoms are retained in the porous carbon^[46][47][48]. In N-doped CMPs, Li et al. Constructed N-doped CMPs network by Sonogashira-Hagihara coupling reaction with 2-amino-3,5-dibromopyridine as N-containing monomer. After pyrolysis, the maximum content of N element is 9.91 wt%, and the onset potential (E_o) reaches 1.15 V in 0.1 mol/L HClO₄ solution^[34]. In 2022, they prepared porphyrin-based CMPs by providing N and C sources from m-tetra (4-chlorophenyl) porphyrin monomer, and the limiting current density (J_d) of the sample pyrolyzed at 900 ° C reached 4.45 mA·cm^-2, which was higher than that of Pt/C catalyst under the same conditions^[36]. Interestingly, in this work, the catalytic performance of N-doped derived carbon was optimized only by controlling the morphology of CMPs precursor (from tubular to hollow spherical), which provided a design idea for the design of highly active ORR catalysts.

Lv et al. Pyrolyzed 1,3,5-triethynylbenzene self-polymerized CMPs in ammonia to prepare N-doped CMPs based derived carbon (N-HsGDY)^[18]. By X-ray absorption near edge structure (XANES) analysis of N K-edge combined with density functional theory (DFT) calculation, only pyridine-type N,J_d is present in the N-HsGDY-900 ℃ sample with a value of 6.2 mA·cm^-2, which is the highest value reported so far. To better understand the catalytic activity, DFT was used to compare the calculated ORR free energy diagrams of N-HsGDY and pyridine N-graphene at zero electrode potential (U = 0 V vs. NHE) and equilibrium potential (U = 0.455 V vs. NHE). It was found that the C atom of the ethynyl group adjacent to the benzene ring in the N-HsGDY structure is the active site for OOH^*, O^*, and OH^* with a higher positive charge density (0.694 vs. 0.392). In alkaline medium, different charge distributions lead to different rate-determining steps. The adsorption of OOH^* is the controlling step when N-HsGDY is used as a catalyst, while the desorption of OH^* is the rate-determining step in pyridinic N-graphene. Comparing the free energy change of the control step of the two catalysts, the ORR diffusion resistance of N-HsGDY is significantly smaller than that of N-graphene at 0.455 V (0.87 eV vs. 0.94 eV) and 0 V (0.42 eV vs. 0.49 eV), which further verifies the excellent ORR activity of N-HsGDY.

N and S co-doped CMPs based derived carbon also showed better ORR performance, especially the N atom has higher electronegativity (3.04 vs. 2.55), which often acts as an electron donor in the molecular structure and endows the N-doped CMPs materials with redox activity. Bhosale et al. Introduced 2,4,6-tris (5-bromothiophen-2-yl) -1,3,5-triazine as N and S element donor monomer into the structure of CMPs through Sonogashira-Hagihara coupling reaction, and produced N and S double-doped ORR catalyst after pyrolysis^[49]. Ren et al. Constructed new bis-heterocyclic CMPs as catalyst precursors through condensation cyclization between aldehyde group and thiocarboxamide group^[43]. In these works, although the electronegativity of S is similar to that of C, the synergistic effect between S and N atoms and the difference of electron spin density of S atoms lead to the improvement of the overall ORR activity of carbon materials.

Porphyrin is a macrocyclic conjugated compound with 18π electrons. Porphyrin has a hole in the center of the ring, which can combine with a variety of metal ions (such as Fe, Co, Zn, Cu, etc.) To form metal complexes^[50,51]. On the one hand, the network constructed by these conjugated macrocycles and aromatic groups has a porous and π-conjugated structure, which can ensure the exposure of active sites and electron transfer during electrocatalysis; On the other hand, the N₄ coordination site in the porphyrin group structure can form the M-N₄ active site. The introduction of metalloporphyrin building blocks into the network structure of CMPs by chemical bonding provides an effective way to prepare CMPs-based ORR catalysts doped with transition metals and N atoms. In addition, the advantage of using metalloporphyrins is that after carbonization, metal nanoparticles can usually be anchored on the nitrogen-doped carbon skeleton, which effectively avoids the agglomeration of metal nanoparticles. Li et al. Synthesized different metal (Co, Cu, Fe) porphyrin-based CMPs by Schiff reaction, and obtained monometal-doped ORR catalysts by pyrolysis at 700 ~ 900 ℃^[38]. In the Co-doped porous carbon, the high content of Co (1. 1%) and N (4. 26%) synergistically improved the overall catalytic activity of the material, and the E_o and E_1/2 were 0. 80 V and 0. 7 V, respectively, in the 0.5 mol/L H₂SO₄ test solution, which were close to those of the Pt/C catalyst. M Müllen et al. Prepared Fe, Co bimetallic doped carbon-based ORR catalyst (Fe/Co-CMP-800) by carbonized metalloporphyrin-based CMPs^[41]. In this work, Fe and Co atoms were derived from two metalloporphyrin monomers, respectively, and simultaneously entered the structure of CMPs through Suzuki coupling reaction, and two active sites of FeN₄ and CoN₄ were formed after carbonization treatment (Fig. 5). The presence of CoN₄ can inhibit the formation of low activity nanoparticles and increase the proportion of active FeN₄ sites. In addition, the vicinal position of the CoN₄ and FeN₄ sites compensates for the poor catalytic selectivity of CoN₄, promoting higher electron density for FeN₄. Therefore, Fe/Co-CMP-800 combines the advantages of the physical characteristics of cobalt-based catalysts (discontinuous, ribbon structure) and the electrochemical characteristics of iron-based catalysts (high E_o value, low H₂O₂ evolution). Fe/Co-CMP-800 exhibited ORR performance close to that of Pt/C in acidic medium (half-wave potential (E_1/2) was only 0.08 V lower) compared with the single transition metal doped ORR catalyst. Dou et al. Used CMPs doped with B, N, Co and Fe obtained by Sonogashira-Hagihara coupling reaction as precursors to prepare ORR catalysts. Through KSCN poisoning experiment and acid leaching experiment combined with XPS characterization analysis, it was considered that Co₃Fe₇ and Fe/Co-N were the active sites of the system^[31].

In the post-doping design strategy based on CMPs, CMPs can act as either a carbon source or a heteroatom carrier. Among them, FeCl₃ is a commonly used Fe source in the design of ORR catalyst. Sun et al. Prepared porphyrin-based CMPs, and physically ground and blended FeCl₃ to form Fe-N-C structure after carbonization, which effectively improved the ORR activity of the material^[35]. Dai et al. Used 1,2,4,5-benzenetetramine as N source to react with hexanone cyclohexane under acid catalysis to synthesize N-CMPs, which were then blended with phytic acid (N source) and carbonized at 900 ℃ and 1000 ℃ to prepare N, P-doped CMPs derived carbon (N, P-CMP-900 and N, P-CMP-1000)^[45]. Affected by the monomer, this CMPs system contains a large number of O atoms, and the presence of these O atoms makes the carbonization process beneficial to the construction of micropores in the porous network of the catalyst, which acts like a C atom "etchant", and the BET surface area increases from 27 m²·g^-1 to 171 m²·g^-1. It showed good ORR activity in 0.1 mol/L KOH, 0.1 mol/L HClO₄ and PBS solutions. In addition, this work is the first to use N and P co-doped electrocatalyst as a biosensor to detect dissolved oxygen in tears. Jiao et al. Used ionic liquids containing N and S as dopants and activators. S plays two roles in the material system: ① Polymer fragments containing S will decompose into small molecular substances under high temperature, which will escape from the system and form micropores, similar to the physical activation process, thus improving the specific surface area of the material^[33]. (2) S in the liquid will interact with the carbon skeleton to form thiophene-S, which can change the electron distribution on the surface of porous carbon and cooperate with N to improve the catalytic activity of the material. It should be noted that the specific surface area and the amount of microporosity are not the only factors affecting the performance of ORR. In this system, C-CMPs-1 NP has the highest E_o and E_1/2 values, but its J_d value is the lowest, which is due to the high sulfur content and partial mesopores in C-CMPs-1 NP, which is beneficial to current transport. Li et al. Prepared Fe, Co and N ternary doped graphene composite catalyst by introducing three heteroatoms of Fe, Co and N into CMPs through monomers containing iron porphyrin and cobalt porphyrin, and then blending and pyrolyzing with graphene oxide^[39]. In this system, CMPs act as both carbon source and heteroatom dopant.

In metal-doped carbon-based catalysts, the agglomeration of metal particles is an important reason for the decrease of ORR performance of materials^[52]. To overcome this problem, Hu et al. Mixed triazine-based CMPs with CoO and ZnCl₂ by ball milling blending method, and prepared CoO, ZnO, N ternary doped CMPs-based porous carbon (CoO/ZnO @ N-PC) by assisted molten salt activation method (Fig. 6)^[44]. In the reaction system, the triazine-based CMPs provide a N source and a C source, and the ZnCl₂ can be used as a template agent and an activator to prevent pore collapse and promote the generation of a porous structure at high temperature; On the other hand, the generated ZnO can be used as a "dispersant" to effectively inhibit the agglomeration of CoO particles. Compared with the triazine-based CMPs direct carbonization materials, CoO/ZnO @ N-PC exhibited higher E_o(0.91 V) and E_1/2 values (0.85 V). Wang et al. Prepared Fe-N-C carbon-based catalyst (BPCMP-Fe-800) based on the coordination of bipyridine N atom and Fe³⁺ in the unit structure of CMPs, and the doping amount of Fe element was more than 10 wt%^[32]. Compared with the control sample (BBCMP-Fe-800) prepared by conventional blending method, the V vs. Of BPCMP-Fe-800 was improved to nearly 20% (0. 97 V vs. 0. 81 V).

A variety of metal and N-doped COFs-based derived carbons can be synthesized by using the strategy that pyridine and its derivatives can form stable complexes with transition metal ions such as Fe and Co. Park et al. Synthesized pyridine-linked triazine COFs (PTCOF) by nucleophilic substitution reaction using cyanuric chloride and diaminopyridine as monomers^[56]. Compared with conventional COFs, the crystallinity of the synthesized PTCOF is lower, while the amorphous or low-crystalline carbon structure formed after pyrolysis is more conducive to ion migration and mass transport at the solid-liquid interface^[57]. In order to adjust the electronic structure of PTCOF, Co-doped COFs-based porous carbon (CoNP-PTCOF) was prepared by blending PTCOF with Co(NO₃)₂ and carbonizing at 800 ℃. Due to the coordination effect, the Co²⁺ is fixed at the pyridine and triazine groups, so the Co nanoparticles are uniformly dispersed on the surface of the carbon skeleton after carbonization. In addition, due to the partial charge transfer from Co to the C framework after Co doping, the p-band center of the C active site in CoNP-PTCOF is downshifted compared with that in pristine PTCOF, which is more favorable for the oxygen intermediate to adsorb and desorb on the pyridine carbon active site, and the measured E_1/2 is 0.87 V, which is close to that of Pt/C (0.85 V).

Among COFs-based M-N-C catalysts, single-atom catalysts reported in recent years have become the research frontier. Due to their special structure, M-N-C single-atom catalysts tend to exhibit higher atom utilization and significant catalytic activity^[58]. At present, the mainstream synthesis method is to use wet chemistry to load metal ions onto COFs carriers for pyrolysis treatment^[59,60]. Yang et al. Synthesized COFs precursors using 1,3,6,8-tetrakis (4-formylphenyl) pyrene and 2,2 '- (1,4-phenylene) diacetonitrile as monomers, and then Fe²⁺ ions (derived from iron porphyrin) were adsorbed on the surface and pores of COFs to form Fe-COF, which was finally pyrolyzed at high temperature to synthesize nanorod-derived carbon materials^[60]. Fe in the material was found to have a Fe-N₄ and Fe-Fe structure by XANES spectroscopy, and the Fe-Fe distance (2. 5252 Å) was much lower than the graphite interlayer spacing (~ 3.33 Å).Combined with high-angle annular dark-field scanning transmission electron microscopy, it was inferred that two Fe atoms coordinated two adjacent N atoms to form an approximately monoatomic structure. Using the synthetic strategy of "COF adsorption-pyrolysis", Zhang et al. First prepared COFs nanospheres by solution method using 1,3,5-tris (4-aminophenyl) benzene and 1,3,5-benzaldehyde as monomers. Secondly, COFs were used to adsorb Co, Fe and Pt ions in aqueous solution. Finally, the Pt, Fe, Co/N-C ternary monatomic catalyst is obtained through high-temperature pyrolysis^[59]. On the one hand, the introduction of Fe and Co elements avoids the clustering or aggregation of metal atoms through the chelation with N element, thereby improving the stability of the catalyst, and the current loss is only 6%; On the other hand, it can optimize the central local coordination environment and electron distribution of Pt atom, weaken the adsorption of oxygen intermediate, and improve the ORR catalytic activity, with a E_1/2 value of 0. 845 V in 0. 1 mol/L potassium hydroxide solution.

Direct pyrolysis of COFs often leads to unexpected structural collapse and pore structure changes, which can be effectively avoided by using templates or compositing with substrate materials. Zhao et al. Pyrolyzed Fe, N-doped COFs-based mesoporous carbon (mC-TpBpy-Fe) using p-toluenesulfonic acid-assisted mechanochemical synthesis with trialdehydotriphenol and 2,2 '-bipyridine-5,5' -diamine as raw materials and SiO₂ beads as hard templates^[61]. The SiO₂ sphere template plays an important role in promoting the formation of mesoporous structure, thereby improving the mass transfer and the utilization rate of active sites. At the same time, the bipyridine in the structure of COFs is coordinated with Fe³⁺, so that the Fe atoms after pyrolysis can be uniformly dispersed in the carbon skeleton, forming Fe-N active sites to improve the ORR performance. Chen et al. Used a similar synthetic strategy to introduce Co²⁺ into TpBpy-COFs^[62]. The difference is that the surface of the SiO₂ bead template is modified by amino groups in advance to increase the compatibility between the surface and the monomer molecules. After pyrolysis, the Co²⁺ was converted into Co subnanoclusters rather than Co-N_x bonds. Due to the strong synergistic effect between Co subnanoclusters and N/C, they proposed that the decomposition and adsorption of oxygen occurred simultaneously on the surface of the catalyst in this system, and OH^* was directly generated without the intermediate step of OOH^*.

Zhang et al. Reported a COFs-based derived carbon with core-shell structure (GC @ COF-NCT) for ORR^[63]. ZIF-67 was first carbonized, and the organic linker in the ZIF-67 crystal was carbonized to form a graphitic carbon (GC) core during pyrolysis, while the Co²⁺ was converted to Co/CoO nanoparticles, which were transferred from within the ZIF- 67 structure to the surface of the carbon skeleton. Then a fibrous COFs layer (DAAQ-TFP-COF) was formed on the GC surface by Schiff reaction before further carbonization. The strong interfacial interaction between DAAQ-TFP-COF and ZIF-derived graphitic carbon provides a significant confinement effect, allowing the inner core GC layer to abut against the inner surface of COFs during the following pyrolysis process, resulting in a uniform hollow microporous or mesoporous structure. The ORR performance was affected by the pyrolysis temperature and the monomer concentration of DAAQ-TFP-COF, and the sample with the monomer concentration of 0. 04 mmol/L and carbonized at 800 ℃ had the best ORR performance with E_o~0.923 V,E_1/2~0.841 V, which was close to that of Pt/C. Liu et al. Also used a similar synthetic strategy to prepare COF layer (TP-BPY-COF) -coated ZIF-derived carbon (Fig. 7)^[64]. Different from the previous work, in this work, the ZIF was not carbonized in advance, and the COF layer was directly grown on the surface of ZIF-67. Compared with the ZIF-67 direct carbonization product (ZIF800), the core-shell COF @ ZIF800 has increased pyridine N and Co-N contents, and the E_o and E_1/2 are higher than those of ZIF800 and Pt/C. They believed that the pyrolysis product of TP-BPY-COF had a stronger structural framework than that of ZIF-67, and the thin COFs shell not only inhibited the structural collapse of the overall structure of COF @ ZIF due to heat and promoted the formation of mesopores, but also effectively avoided the agglomeration of Co particles, which provided a reference for the design of composite COFs-based derived carbon ORR catalysts. Liu et al. Prepared COFs-derived carbon materials with flower-like morphology, and the carbon nanosheets stacked by a large number of graphite layers provided abundant edge sites for the catalytic reaction, which could expose more N atoms, thus showing excellent ORR catalytic activity. The ORR catalytic activity of COF800 in alkaline environment was comparable to that of commercial Pt/C catalyst, and the E_o, E_1/2 and J_d reached 0.86 V, 0.79 V and 5.0 mA·cm^-2, respectively^[65].

Zhu et al. Prepared carbazole-based HCPs with carbazole as monomer and FDA as external crosslinking agent^[73]. By optimizing the monomer/catalyst/crosslinker ratio and pyrolysis temperature, the best ORR performance (E_o~0.89 V,E_1/2~0.78 V) of TSP-HCP-900 was obtained, although there is still a certain distance between this value and Pt/C, which provides an idea for the preparation of N-containing monomers required for N-doped HCPs. In addition, doping transition metal elements such as Fe and Co into the structure of carbazole-based HCPs is also an effective strategy to improve the electrocatalytic performance. Under the same test conditions, the E_o and E_1/2 values of Co-TSP-HCP-900 were increased by 10 mV and 20 mV, respectively. Chen et al. Used a similar design idea to synthesize carbazole-based HCPs, using bis- (N-carbazolyl) -1,2,4,5-tetrazine as a monomer and AlCl₃ as a catalyst to first synthesize carbazole-based HCPs (HCTCz @ PA), adsorbing phytic acid (P source), then introducing Fe³⁺ and Co²⁺ through coordination, and pyrolyzing to obtain Fe, Co, P, N-doped porous carbon (FeCoP/NPC)^[79]. They systematically compared the effects of Fe, Co, and phytic acid on the catalytic activity of the materials, and found that in the FeCoP/NPC structure, Fe and Co mainly exist in the form of FeP and CoP, rather than the form of M-N_x reported in the general literature. The E_o and E_1/2 of FeCoP/NPC are 0.948 V and 0.855 V, respectively. Although they are slightly lower than those of Pt/C, their lower Tafel slope (58 mV·dec^-1vs.77 mV·dec^-1) indicates that FeCoP/NPC has a faster reaction kinetic rate during ORR.

Pitch is a by-product of petroleum and coal chemical industry. Yang et al. Synthesized pitch-based HCPs by Friedel-Crafts reaction with FDA as crosslinking agent, and prepared N-doped HCP-based ORR catalyst (N-PHCP-x) by high temperature NH₃ activation process^[76]. During pyrolysis at high temperature, ammonia acts as an etchant, and the surface area of N-PHCP-900 is up to 992 m²·g^-1, which increases the exposure of active sites. In addition, it can be seen from the pore size distribution that there are a large number of micropores and continuous mesopores in the sample, which is beneficial to the mass transfer and electrolyte diffusion in the ORR, so that the E_1/2 value of N-PHCP-900 shows a positive shift of 10 mV compared with that of the commercial Pt/C catalyst, reaching 0. 883 V.

Compared with metalloporphyrin-based MOFs or CMPs, the construction process of porphyrin-based HCPs is simpler. Tan et al. Prepared metal (Fe, Co) porphyrin-based HCPs by external crosslinker braiding method, and prepared carbon-based porous materials (MPH-Fe/C and MPH-Co/C) with high specific surface area after KOH activation; the specific surface area of MPH-Fe/C was as high as 2768 m²·g^-1,Fe, and the Co content was as high as 9.97 wt% and 10.35 wt%, respectively, showing ORR performance comparable to that of Pt/C^[74]. Zhu et al. Prepared porphyrin-based HCPs (TPP-HCPs) using 5,10,15,20-4 '-chlorophenyl porphyrin as a monomer, and obtained TPP-HCP-based derived carbon after one-step pyrolysis^[78]. They found that when the FeCl₃ and FDA dosage were increased, the catalyst could obtain higher specific surface area at the same pyrolysis temperature, thus optimizing the ORR performance.

Most of the HCPs-based derived carbons are amorphous or granular, while there are relatively few reports on special morphologies. Qian et al. Prepared hollow tubular HCPs (HCP-NT) with hexakis (benzylthio) benzene and thiophene as monomers, chloroform as crosslinking agent, and FeCl₃ as catalyst by adjusting the amount of crosslinking agent^[80]. The hollow tubular structure remains intact after carbonization. In the preparation of HCP-based derived carbon, the researchers used three methods, as shown in Figure 8: ① Direct inert gas pyrolysis to obtain S-doped material (HCP-NT-Ar-800). Pyrolysis under ②NH₃ atmosphere, S atoms were almost replaced by N atoms, and the specific surface area of HCP-NT-NH₃-800 was greatly increased due to the etching effect of NH₃, forming C-N active sites. Due to the difference of electron density caused by doping atoms and the different porous structure of the samples, the E_o and E_1/2 of the HCP-NT-NH₃-800 are higher than those of HCP-NT-Ar-800, reaching 1. 01 V and 0. 85 V, respectively. (3) The surface of HCP-NT-Ar-800 was modified by Ar Plasma to obtain S and O co-doped HCP-NT-Plasma, which not only increased the oxygen content in the structure, but also introduced additional defect sites. Unlike the previous two samples, the 2e^-ORR selectivity of HCP-NT-Plasma is more than 80%, which is attributed to the increase of C = O and COOH species in the structure after Plasma treatment.

Compared with other POPs materials, another advantage of HCPs-based derived carbon ORR catalyst is that in this system, the FeCl₃ is not only the catalyst for building HCPs network, but also acts as metal dopant and pore-forming agent. Zhang et al. Systematically compared the performance of Fe, N and S heteroatoms on HCPs-derived carbon-based ORR catalysts^[71]. Supercrosslinked polypyrrole and polythiophene were synthesized by Friedel-Crafts reaction using similar pyrrole and thiophene as monomer molecules, and Fe-N-based, Fe-S-based and Fe-N-S-based catalysts were obtained after pyrolysis at different temperatures. It was found that Fe in the PPFeC-800 structure prepared with pyrrole as a monomer mainly exists in the form of Fe₃O₄, while Fe_1-xS nanocrystals mainly exist in the PTFeC-800 structure prepared with thiophene as a monomer. They suggested that Fe element is favorable for N doping, and the stronger electronegativity of N atom changes the electronic properties of the neighboring C atom or transition metal atom, which contributes to O₂ adsorption, so N/Fe₃O₄ is the catalytic active site of PPFeC-800. In the PTFeC-800 structure, S/Fe_1-xS reduces the catalytic activity of the material for ORR, but S doping can change the spin density distribution near the Fe center and reduce the activation barrier on Fe/N/C, so the Fe-N-S based catalyst (MixFeC-800) shows the highest E_o value (0.983 V) and E_1/2 value (0.844 V) among the catalysts with three compositions.

The CTF framework structure has high nitrogen content, which can provide abundant nitrogen coordination sites to anchor the filter metal at a fixed position while changing the electron distribution of the carbon material, so as to construct the M-N-C type ORR catalyst, realize the uniform dispersion of transition metal atoms and improve the utilization rate. Zhu et al. Designed a CTFs with abundant N₄structural units based on o-phenanthroline monomer to anchor Fe³⁺,After pyrolysis treatment, the Fe element in the sample is uniformly distributed and maintains the FeN₄ configuration (there is C-FeN₄(—O) or C-FeN₄(—OH)), but with the increase of carbonization temperature, the non-planarity of FeN₄/C increases significantly, which makes the 2e^- selectivity decrease gradually^[98]. In the free energy diagram (Fig. 9), the intensity of ^*OOH adsorption increases obviously with the introduction of Fe. At the same time, with the increase of non-planarity of Fe/N/C structure, the adsorption of (2-C-FeN₄4-C-FeN₄6-C-FeN₄),^*OOH on C site becomes stronger, and the reaction pathway switches from 2e^- to 4e^-. This work shows that the electronic structure of the catalyst can be adjusted by controlling the pyrolysis temperature to optimize the reaction path, and also explains the reason for the improvement of the ORR performance of the catalyst by Fe doping from the structural change.

Zhu et al. Developed a series of S-doped Fe-N-C catalysts with six different aromatic nitriles as building blocks^[94]. Different from the traditional ZnCl₂ molten salt method, the researchers used the molten mixture of FeCl₃ and S to prepare CTFs. Molten FeCl₃ can not only be used as a strong Lewis acid to catalyze the polymerization of CTFs, but also can be mixed with CTFs at the molecular level to become the Fe source of Fe-N/C catalyst, and FeCl₃ can be used as a pore-forming agent in the pyrolysis process to optimize the pore structure of carbon materials. In the reaction process, S element can alleviate the decomposition of Fe-N_x active sites at high temperature and improve the ORR performance of Fe-N-C catalyst. They found that the number ratio of nitrile group to benzene ring (Nnitrile/Nbenzene) in the structural module had a great influence on the ORR activity of the catalyst. DCBP-Fe-N/S/C has the highest measured N/C ratio, but its catalytic performance is the worst. However, DCP-Fe-N/S/C has the lowest measured N/C ratio, but shows moderate ORR performance. The smaller the Nnitrile/Nbenzene, the fewer the crosslinking nodes (triazines) and the more benzene rings in CTFs, resulting in an increase in the degree of graphitization of the final catalyst during pyrolysis. Therefore, the aromatic ring is a necessary condition for Fe-N-C catalyst to obtain high ORR activity in thermal treatment.

Dun et al. Constructed a CTF-derived Fe/Co bimetallic single-atom electrocatalyst (Fe/Co-N-C, Fig. 10) using isophthalonitrile as monomer and CoCl₂ and FeCl₃ as metal sources by sequential chemical doping and complexation adsorption strategy^[99]. The porous structure and abundant N atoms of the CTFs precursor can provide suitable sites for the anchoring of Fe and Co. Compared with the mono-metal catalyst, the atomically dispersed binary metal catalyst can enhance its ORR activity by optimizing and modifying the local geometric and electronic structure, and the V vs. And E_1/2 reach 1. 02 V and 0. 878 V, respectively, showing a stable open circuit voltage (1. 52 V vs. 1. 47 V) superior to that of the commercial Pt/C catalyst in Zn-air battery.

Zheng et al. Prepared hollow nanoflower CTFs-based carbon materials with high N-doping content through a simple synthesis strategy^[89]. The key of this synthesis strategy is to attach the CTFs monomer (cyanuric chloride) to the melamine-cyanuric acid (MCA) substrate through covalent bonds, which serves as an anchor point for the subsequent growth of CTFs on the MCA surface to avoid random growth in the liquid reaction medium. During the pyrolysis process, due to the structural stability of CTFs, the hollow nanoflower morphology is retained after the MCA crystal is decomposed into organic vapor. In the same year, they rapidly synthesized N, P, F tri-doped carbon nanospheres (NPF-CNSs) with spherical morphologies by ultrasound-assisted precipitation polycondensation based on the self-templated carbonization strategy^[97]. According to XPS analysis, the content of heteroatoms in the material is also higher than that in other traditional synthesis methods (F ~ 0.57 at%, P ~ 1.21 at%, N ~ 3.89 at%). At the same time, this work is also the first time that F element is introduced into the carbon structure derived from CTFs, on the one hand, the surface hydrophilicity of the bulk material is improved, on the other hand, the fluorination treatment of the carbon matrix induces the formation of ionic C — F bonds (684. 9 eV) and semi-ionic C — F bonds (687. 8 eV).The increased adsorption of oxygen molecules, which in turn stimulates the cleavage of O — O bonds, enhances the electrocatalytic performance of NPF-CNS, with E_o and E_1/2 values of 0.93 V and 0.81 V, respectively.