Zeolitic imidazolate frameworks (ZIFs), as a subclass of MOFs composed of transition metal cations and 2-methylimidazole, have atomically dispersed metal sites and are well suited as precursors for the preparation of M-N-C catalysts^{[25⇓⇓⇓~29]}. In 2022, Sun et al. Synthesized Hemin @ ZIF-8 precursor by using Hemin molecules containing carboxyl and Fe as iron source, mixing with Zn(NO₃)₂·6H₂O and then mixing with methanol solution containing 2-methylimidazole, and successfully prepared an oxygen-containing Fe-N-C catalyst after pyrolysis at 900 ° C in N₂ atmosphere (Figure 1A)^[30]. This catalyst exhibits a Tafel slope （74 mV·dec^-1） in acidic solution comparable to the commercial Pt/C catalyst （76 mV·dec^-1） and reaches power densities of 0.88 W·cm^-2 and 1.24 W·cm^-2 in proton exchange membrane fuel cells and anion exchange membrane fuel cells, respectively. In Fe-N-C catalysts, besides the FeN₄ sites with good 4 e selectivity, some non-metallic N-C sites show high 2 e selectivity, which has a negative impact on the overall stability of the catalyst. The key to improve the stability of Fe-N-C catalyst is to select the appropriate preparation method to transform and eliminate these active sites for side reactions. Xia et al. Synthesized Fe @ ZIF-8 precursor with similar materials and steps using Fe（NO₃）₃·9H₂O as iron source, and obtained five kinds of Fe-N-C catalysts (FeNC-900, FeNC-1000, FeNC-1100, Fe NC-1200, Fe NC-1300) by pyrolysis at high temperature (900 ~ 1300 ℃), which had not been paid attention to before. After 30 H of constant voltage (0.5 V) test, the current density of the membrane electrode loaded with FeNC-900 decreased by 83.5%, while the current density of the membrane electrode loaded with^[31]; After 50 H, the current density of the membrane electrode loaded with FeNC-1300 decreased by only 7.9%. It is found that the improvement of stability is due to the removal of non-coordination sites of nitrogen atoms with low activity, which are prone to produce harmful H₂O₂ by-products, at high temperature, and the transformation of the less stable D1 site （FeN₄C₁₂） into the more stable D2 structural site （FeN₄C₁₀）（ in this temperature range^[31]. Transition metal oxides may be a better choice for precursors than traditional transition metal salts. Liu et al. Pyrolyzed the prepared Fe₂O₃@ZIF-8 precursor at 800 ° C under Ar atmosphere using Fe₂O₃ as the iron source, and due to the coordination of Fe ions gradually released from Fe₂O₃ during pyrolysis with N ligands, this catalyst has a higher density and more dispersed active sites than the Fe-N-C catalyst prepared by direct pyrolysis of Fe @ ZIF-8^[32]. When mixed with NH₄Cl and then heat treated at 1100 ° C in Ar atmosphere, the NH₄Cl decomposed into NH₃ and HCl, which was used to form a large number of micropores and defects on the carbon basal plane, and in the following chemical vapor deposition treatment, a thin carbon layer doped with nitrogen was covered on the carbon basal plane to repair the defects. After the above treatment, a part of the pyrrole nitrogen coordinated with the central atom Fe was converted into pyridine nitrogen with better stability, thus improving the overall stability of the catalyst. The results show that in the fuel cell durability test, the voltage at a current density of 0.8 A·cm^-2 only decreased by 30 mV after 30 000 voltage cycles (0.6 – 1.0 V), reaching the target of a decrease of less than or equal to 30 mV before 2025, and the cell performance of this catalyst at the end of the durability test is comparable to that of commercial Pt/C^[33].

In the preparation of catalysts, increasing the pyrolysis temperature can improve the stability of the carbon skeleton, thereby improving the stability of the catalyst, but the control space of a single pyrolysis is relatively limited. Through multiple pyrolysis, not only the stability of the carbon support can be further improved, but also the addition sequence of metal salts and precursors can be flexibly adjusted, and the pore structure of the carbon support can be adjusted by post-treatment, which is a new design direction for Fe-N-C catalysts with high stability. Peng et al. Reported a porous FeN₄-O-NCR electrocatalyst loaded with catalytically accessible FeN₄-O sites on N-doped carbon nanorods^[34]. During the synthesis process, the carbon nanorods formed by the first pyrolysis inherited the high aspect ratio of the MOF-74-Rod precursor, but all the sizes were significantly reduced; During the second pyrolysis, KOH and NH₃ are added, O element and N element are successfully introduced, and a porous structure is formed; Subsequently, Fe-Phen was added and combined with microwave-assisted pyrolysis to synthesize the axially coordinated O FeN₄-O-NC catalyst (Fig. 1C), and the half-wave voltage only decreased by 5 mV after 5000 voltage cycles (0.6 ~ 1.0 V) under alkaline conditions. Liu et al. Synthesized a Fe-N-C catalyst with a multi-dimensional concave structure by using a carboxylate molecular tailoring method. This molecular tailoring strategy endows the Fe @ MNC-OAc catalyst with dense accessible active sites, multi-dimensional mass transfer paths, and a hierarchical porous structure^[35]. Firstly, 2-methylimidazole and Zn（NO₃）₂·6H₂O were mixed with methanol solution, and then Fe salt and NaOAc were added to obtain the precursor Fe @ ZIF8-OAc, which was heated from room temperature to 900 ℃ at a heating rate of 5 ℃/min and kept for 3 H. After the sample was cooled to room temperature, the unstable Fe element was washed away with 0.5 mol/L H₂SO₄, and the temperature was raised to 700 ℃ again to repair the carbon skeleton. After 5000 voltage cycles (0.6 – 1.0 V) under acidic conditions, the half-wave voltage decreased by only 5 mV. As a bottom-up synthesis method, the steric confinement method can anchor single atoms by using pre-constructed confined space such as nanocages/pores/defects. The advantage of this method is that it can stabilize the isolated metal atoms on the support, further inhibit the migration and aggregation of metal atoms, and produce high-density isolated metal sites^[36]. However, the randomness of one-step pyrolysis makes it difficult to ensure the uniformity of active sites, which reduces the atom utilization of the catalyst^[37]. Moreover, the improvement of the stability of the carbon skeleton by one-step pyrolysis is relatively limited, and multiple pyrolysis combined with pickling can ensure a higher atom utilization rate while improving the stability.

The obtained carbon skeleton is filled with disordered micropores during pyrolysis at high temperature, and the embedding of a large number of monatomic active sites in the disordered micropores usually leads to the aggregation of iron species and the generation of metal nanoparticles due to steric hindrance and mass transfer resistance, which has a non-negligible effect on the catalytic performance. The template-assisted strategy can be used to adjust the morphological and structural properties of the catalyst during pyrolysis or etching, and then synthesize Fe-N-C electrocatalysts with high iron loading and fully exposed active sites, which is of great significance to further improve the ORR activity.

The template method promotes the formation of ordered porous structures by introducing a template agent that can be removed during the synthesis process. The control of pore structure will directly affect the mass transfer problem of the catalyst and the formation of three-phase interface, and then control the stability. Niu et al. Formed FeO (OH) nanoparticles on polymer substrates by hydrothermal treatment of FeCl₃ and 2-fluoroaniline, and the template FeO (OH) was decomposed into Fe³⁺ and gaseous water when pyrolyzed at 800 ° C^[38]. After the above reaction process, highly ordered mesopores were successfully left on the carbon support, and the active site density of the catalyst was improved. Cyclic voltammetry results showed that the synthesized catalyst exhibited a higher peak current density （1.51 mA·cm^-2） than that of the Pt/C catalyst （1.20 mA·cm^-2）, and the current density retention rate was 93. 3% after continuous operation in 0. 7 V potassium hydroxide solution for 10 000 s, which was much higher than that of the Pt/C catalyst (50. 4%).

When the template method is used to synthesize the catalyst, the microscopic morphology and size of the catalyst are determined by the template. Gong et al. Successfully prepared atomically Fe and N co-doped hierarchical porous carbon nanorods (Fe/N-CNR) catalyst by in situ polymerization^[39]. In this method, the 1D Fe₂O₃ obtained by high temperature oxidation of Fe-MIL-88 B was used as a template to induce the polymerization of 1D rod-like structures. Moreover, the residual Fe₂O₃ after polymerization is beneficial to maintaining the original high porosity of the material and promoting the formation of highly active atomic-scale Fe-N₄ sites during carbonization (Fig. 1D). The final Fe/N-CNR catalyst showed excellent ORR activity and stability, and the half-wave voltage decreased by 12. 5 mV after 5000 voltage cycles (0. 6 ~ 1.0 V) under alkaline conditions. In addition to this self-sacrificial template, SiO₂ has also become a widely used template because it can restrict the free migration of iron. Han et al. Impregnated an ordered mesoporous SiO₂ template with a pore size of 8.4 nm with a methanol solution containing Fe (II) -Phen complex and 2-methylimidazole, and pyrolyzed the resulting sample under Ar atmosphere to convert the precursor into a nitrogen-doped ordered mesoporous carbon framework^[40]. A three-dimensional N-doped ordered mesoporous carbon supported Fe atom catalyst (Fe-N-C/NOMC) was prepared by heat treatment of the residue under Ar atmosphere after removing the SiO₂ template with HF, as shown in Figure 1E. The catalyst exhibits a half-wave voltage of 0.92 V under alkaline conditions, which is better than that of Pt/C catalyst. After 5000 voltage cycles (0.6 ~ 1.0 V), the half-wave voltage drops by only 5 mV, demonstrating excellent stability. In the synthesis process, the acid washing step can remove unstable metal particles and oxides, reduce the pollution to the electrolyte, and improve the stability of the catalyst. At present, the use of multiple pyrolysis is still less, but from the results of some literatures, multiple pyrolysis can improve the atomic dispersion, thereby improving the stability, and can also improve the activity^[34,35,39]. In addition, the addition of Fe species after pyrolysis to form the precursor is also a way to improve the stability. Table 1 summarizes the Fe-N-C catalysts with better stability in the field in recent years^{[30⇓~32,35,41⇓⇓⇓⇓⇓⇓⇓~49]}. The template method is also a bottom-up synthesis method, which can not only effectively restrict and stabilize the single metal sites, but also significantly change the pore structure, such as specific surface area and pore volume, during pyrolysis^[50]. The template used in the synthesis of nanomaterials can be divided into soft template and hard template, and the template used in the synthesis of Fe-N-C catalyst is mostly hard template.Compared with the soft template, it has the advantages of high stability and good confinement, and can strictly control the size and morphology of the catalyst, but the disadvantage is that the structure is relatively single, and the morphology of the nanomaterials prepared by the hard template is less^[51].

In addition to the host-guest chemical strategy, other synthesis methods are also widely used, such as co-precipitation, chemical vapor deposition, ball milling and so on^[52][53][54]. Generally speaking, the single-atom Fe-N-C catalysts prepared by multiple pyrolysis and wet chemical methods have better stability. However, it should be noted that the stability of the catalyst is also related to the operating conditions and environment in practical applications, so it needs to be verified in specific application scenarios.

For the convenience of operation, the potential cycle in solution is usually carried out under acidic or alkaline conditions, which is called half-cell test, which can reflect the activity and stability of the catalyst under fuel cell operating conditions to a certain extent. Researchers usually select a triangular wave or square wave voltage cycle in the range of 0.6 ~ 1.0 V as the Accelerated Stress Test (AST), and compare the linear sweep voltammetric curve at the initial time with that at the end state to evaluate the deactivation of the catalyst. In addition to this, constant voltage/constant current tests are also frequently used to evaluate the stability of fuel cell catalysts. For non-precious metal catalysts, although many researchers have proved the good stability of the catalyst by the experimental results of Rotating Disk Electrode (RDE) test in liquid environment, this practice is not appropriate in some cases. Zhao et al. Synthesized the FeIM-ZIF-8 catalyst by pyrolysis of ZIF-8, and when the RDE test was conducted in 0.1 M HClO₄, the activity did not decrease significantly no matter 10 500 voltage cycles under Ar atmosphere (Fig. 2a) or 2000 voltage cycles under O₂ atmosphere (Fig. 2b)^[55]. However, subsequent fuel cell tests came to the opposite conclusion, with the activity decaying rapidly from the initial moment, and only 25% of the initial current density remained after the reaction lasted for 100 H (Fig. 2C). Similarly, this phenomenon still exists for noble metal catalysts (fig. 3)^[56]. This shows that in order to accurately evaluate the stability of PEMFC cathode non-precious metal catalyst, the stability test of single cell should be carried out, which can better reflect the real situation.

The single cell test means that the catalyst slurry is coated on the proton exchange Membrane to form a Membrane Electrode Assembly (MEA), which is loaded into the fuel cell test system for stability test. According to the fuel cell test protocol, there are currently four main test standards for precious metal catalysts and membrane electrodes for fuel cells^[55].

(1) Catalyst stability: The surface of the catalyst was oxidized and reduced by continuous scanning of triangular wave potential (0.6 ~ 1.0 V) to accelerate its attenuation. The triangular wave potential range used is close to the voltage variation range of a real vehicle fuel cell, and at the same time, the decay of the catalyst itself is accelerated to the greatest extent on the premise of minimizing the influence of carrier corrosion. This standard is only applicable to the evaluation of platinum-based carbon-supported catalysts. (2) Carbon support stability: The electrochemical oxidation of the carbon support is accelerated by rapid triangular wave potential cycling (1.0 ~ 1.5 V), while the degradation of the catalyst is minimized. The used potential range is close to the start/stop state of the real vehicle fuel cell system. (3) Chemical stability of MEA: It accelerates the generation of free radicals by maintaining the open circuit potential, resulting in the attenuation of membranes and other MEA components. (4) Mechanical stability of MEA: Cyclic regulation of humidity can cause swelling and contraction of the membrane, thereby accelerating the breakage (cracks and holes) of the membrane. In 2022, Los Alamos National Laboratory (LANL) released a "tailor-made" test standard for non-precious metal catalysts, which is different from platinum-based catalysts mainly in the upper limit of cycling voltage and the change of cathode gas from N₂ to O₂^[57]. This is due to the fact that the potential cycling of single-atom M-N-C catalysts in an inert gas atmosphere tends to result in a relatively small performance loss, masking the actual durability of this type of ORR catalyst. When this type of catalyst is subjected to potential cycling in the presence of O₂, which is closer to the actual PEMFC operating conditions, more significant performance degradation occurs^[58].

The traditional way to test the durability of non-precious metal catalysts electrochemically is to accelerate the stress test and apply a constant voltage or current for a certain period of time, generally until 80% of the performance is lost. This method only superficially indicates the rate of catalyst degradation, but does not provide any information about the cause and mechanism of catalyst degradation. Electrochemical impedance spectroscopy (EIS) can be used to analyze the interfacial charge transfer and reactant diffusion impedance of ORR reaction, which provides a new idea for decoupling the deactivation mechanism of Fe-N-C catalyst. Liu et al. Used the relaxation time distribution technique (DRT) to analyze the EIS results under fuel cell operating conditions for 60 hours, so that the changes of O₂ molecular diffusion, O₂ molecular adsorption-desorption and charge transfer at the electrode interface, and proton conduction resistance with time corresponding to low, medium, and high frequencies, respectively, could be determined^[59][59]. This advanced electrochemical characterization technique provides a new perspective to resolve the rapid deactivation of Fe-N-C catalysts in fuel cells.

Due to the different experimental devices and test environments of RDE and MEA, many catalysts with good performance in solution conditions have a "cliff-like" performance decline after being loaded into MEA. The main reasons can be attributed to the following two points: (1) oxygen solubility difference; (2) Difference of catalyst dosage.

In the RDE test, in order to control the mass transfer rate of the reactant and the effective diffusion layer thickness, a certain rotating speed is applied to the electrode, but the current density obtained in the experiment is often small due to the low solubility of the O₂. In the MEA test, in order to achieve the power density required by PEMFC, the device needs to provide a large current, which can not be reflected by the data obtained by RDE. Mason et al. Constructed microporous nanocrystalline silicalite-1-NCs with hydrophobic inner surface and hydrophilic outer surface, which greatly improved the dissolution of O₂ in the electrolyte and increased the ORR current density in RDE by 3.8 ~ 3.9 times^[60]; Considering the low intrinsic activity of non-noble metal catalyst, a large loading is needed to achieve the activity matching with Pt catalyst in the MEA test, and a thick catalyst layer will increase the mass transfer resistance of reactants, which is not conducive to the diffusion of O₂ and the timely discharge of H₂O. In addition, many active sites buried in the catalyst layer have no chance to contact with the reactants, resulting in low catalyst utilization and accessibility, which does not exist for the rotating disk electrode with a thin catalyst layer.

In addition to the above two main reasons, the unique high working temperature (60 ~ 120 ℃) of MEA and the damage of reactive oxygen species (ROS) induced by H₂O₂ to the membrane electrode will also widen the gap between the two test results. Therefore, in order to correctly evaluate the performance of non-precious metal catalysts in practical application scenarios, it is necessary to optimize the design of catalysts and build a reasonable and efficient MEA.

For Fe-N-C catalysts, the active center is usually a Metal-nitrogen-carbon (M-N-C) structure formed by iron (Fe), nitrogen (N) and carbon (C) atoms, in which Fe atoms play a key role in catalysis. However, due to the agglomeration effect during pyrolysis, there are inevitably nano-oxide or carbide particles formed by the polymerization of metal atoms in the synthesized catalysts. In acidic environment, these iron-containing species will dissolve, which will affect the activity and stability of the catalyst.

In 2015, Choi et al. Used scanning flow cell (SFC) coupled with inductively coupled plasma mass spectrometry (ICP-MS) to detect the amount of Fe dissolved from Fe-N-C catalyst in real time during the voltage change process, and the results showed that when the voltage decreased to 0.77 V, the dissolved Fe species began to be detected in the solution (Figure 4A), but the ORR activity did not decrease at this time, which was due to the fact that the detected Fe²⁺ originated from Fe nanoparticles with poor catalytic activity^[61].

Although the metal center atom in the Fe-N-C structure is more resistant to dissolution, the generated active oxygen species such as · OH will cause strong oxidative corrosion to the carbon support and proton exchange membrane due to the Fenton reaction between Fe²⁺ and H₂O₂ generated in the 2e process, which will greatly shorten the catalyst life and irreversibly reduce the cell performance. In addition, some studies have shown that there is H₂O₂ one-electron reduction of （H₂O₂electrochemical reduction） in the ORR process in acidic system, and the rate of hydroxyl radical formation is significantly higher than that of Fenton (-like) reaction^[62]. Wan et al. Deeply explored the deactivation mechanism of molecular catalyst FePc with similar FeN₄structure to Fe-N-C catalyst, and formed a good linear relationship between the · OH content determined by coumarin as a fluorescent probe and the degree of kinetic current decay before and after the stability test.Similar X-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman spectroscopy (SERS) signals were observed for the · OH-treated FePc catalyst and the control experiment after ORR reaction, which means that · OH is the main reason for the deactivation of the molecular catalyst FePc^[63]. In subsequent experiments, they extended the deactivation mechanism to the Fe-N-C catalyst synthesized by pyrolysis.

In 2022, Qiu et al. Used the in-situ electrochemical probe method to detect the H₂O₂ concentration of the cathode catalyst layer of PEMFC in real time, thus revealing its dynamic variation (Figure 4E)^[64]. It is found that the local H₂O₂ concentration in the cathode catalyst layer can reach up to 17 mmol/L under PEMFC operation, which further reveals the causes of deactivation of non-precious metal catalysts and provides a direction for the life extension strategy of catalysts. To improve the solubility resistance of the FeN₄ site, strengthening the bond energy of the Fe — N bond is the most direct and effective strategy. Gao et al. Constructed Fe-N-C catalyst loaded with Metallic particles^[65]. The linear relationships of Fe dissolved amount, oxidation state, and Fe-N bond energy were established by in situ ICP-MS, X-ray absorption spectroscopy (XAS), and DFT calculations. Therefore, a universal mechanism for metal particles to inhibit the dissolution of Fe monoatomic sites is proposed: as electron donors, metal particles increase the electron density of FeN₄ sites and reduce the oxidation state of Fe, which strengthens the bond energy of Fe — N bond and inhibits the dissolution of iron. The experimental results and theoretical calculation show that Pt particles can suppress the demetallization effect to the greatest extent compared with other metal elements. In fact, in this kind of composite catalyst, the Fe-N-C structure can also be used as a support to regulate the electronic structure of Pt particles/clusters and weaken the ^*O adsorption strength on Pt particles/clusters, thus inhibiting the formation of Pt oxides and reducing the dissolution rate of Pt in acidic and alkaline electrolytes^[66]. To sum up, this particle-assisted single-atom strategy can improve the solubility resistance of metal particles and Fe single-atom sites at the same time by using the interaction between the two, thus promoting the overall stability of the catalyst.

Carbon oxidation corrosion is another main reason for the deactivation of non-precious metal catalysts. Unlike precious metal catalysts (such as Pt, Pd), the carbon skeleton in M-N-C catalysts not only acts as a catalyst support to support active sites, but also participates in coordination bonding, and its oxidation and corrosion will directly affect the activity of the catalyst. Therefore, it is particularly important to study the mechanism of carbon oxidation corrosion in non-precious metal catalysts.

Generally, carbon oxidation corrosion is divided into electrochemical oxidation and chemical oxidation. Electrochemical oxidation generally occurs under extreme operating conditions of the PEMFC, such as start-stop, rapid load change, and gas transport channel blockage by impurities. The hydrogen supply at the anode side is insufficient to meet the electron demand of the fuel cell system. At this time, the fuel cell will obtain additional electrons through abnormal reactions, such as OER/COR. At high voltage (> 0.9 V), the electrochemical oxidation of carbon will become very severe. According to Choi et al., when the voltage reaches 0.9 V and 1.2 V, carbon is gradually oxidized to CO₂ and CO^[61]. In addition, after 5000 times of stability tests at 1.2 ~ 1.5 V, the two-dimensional images of carbon materials show 5% ~ 15% area reduction (Fig. 4 B, C), which means that different degrees of pore collapse and shedding have occurred.

Chemical oxidation means that during the oxygen reduction reaction, the active sites on the surface of the catalyst may be attacked by reactive oxygen species (such as · OH, HO₂·）), which react with the carbon support, and the resulting chemical oxidation leads to the formation of surface oxygen-containing groups, such as hydroxyl and epoxy groups, which reduce the conversion frequency (TOF) of FeN₄ sites by weakening the binding with O₂. Unlike electrochemical corrosion, chemical corrosion is independent of temperature and potential. In 2020, Kumar et al. Designed three different voltage cycling intervals (0.3 ~ 0.7 V, 0.5 ~ 0.9 V, 0.6 ~ 1.0 V) at 80 ℃ for LSV test, and there was no significant difference in ORR activity^[58]. It is worth noting that the corrosion of carbon materials can also lead to the shedding of metal atoms in the Fe-N-C structure, which accelerates the demetallization process. It can be seen that there is a coupling relationship between demetallization and carbon corrosion, and the bridge of this synergistic effect is the attack of reactive oxygen species (Figure 4D)^[67].

Carbon oxidation corrosion not only directly leads to the decrease of catalyst activity, but also may lead to the change of catalyst morphology, the increase of hydrophilicity and the decrease of surface area, which indirectly accelerates the deactivation of catalyst. Therefore, slowing down carbon corrosion is one of the important ways to improve the long-term stability of Fe-N-C catalyst.

Liu et al. First attributed the loss of PEMFC activity under acidic conditions to the protonation of pyridine nitrogen. The lone electron pair on pyridine nitrogen may combine with the proton in the acidic medium of PEMFC, resulting in the inactivation of pyridine nitrogen in ORR, because the loss of lone electron pair will weaken the adsorption of oxygen molecules^[68]. Herranz et al. Proposed another protonation mechanism, that is, the surface basic N group is protonated and then combined with the anion by electrostatic interaction, which can be restored by removing the anion adsorbed on the basic nitrogen group in an acidic environment by heating or chemical methods^[69]. According to the quantitative calculation study, the bare Fe-N₄ site is difficult to demetalate due to the stable Fe — N bond. When two ^*OH are bound to the metal center on the same side, the Fe center will form an unstable ^*Fe（OH）₂ intermediate and interact with the protonated N, which will further lead to demetalation and reduced catalytic activity^[70]. It is still controversial whether the mechanism of protonation is the main reason for the decrease of catalytic performance.

In PEMFC, the ORR process of the cathode catalyst layer will produce more water, and the Fe-N-C catalyst is prepared from porous materials, when the pore absorbs water through capillary force, it will lead to flooding of the catalyst pore. According to the research of Jaouen et al., the activity decline of the catalyst becomes more and more serious with the increase of the proportion of micropores, and the active sites of FeN₄ are mainly distributed in the micropores, so they concluded that the flooding phenomenon of micropores is an important factor causing the deactivation of the catalyst^[71]. However, because Jaouen et al. Did not establish a direct link between the degree of micropore flooding and the degree of deactivation, this inference is not rigorous. Choi et al. Conducted electrochemical performance tests under 60% and 100% relative humidity conditions respectively, and observed similar double layer capacitance changes and polarization curves, which indicated that most of the micropores in the Fe-N-C catalyst were wet in the initial stage, and micropore flooding was not the main reason for the rapid deactivation of PEMFC in the initial stage^[72]. The hydrophilicity of micropores is mainly caused by — OH and — OOH adsorbed on the carbon basal plane after carbon oxidation corrosion, but because the transport of ions requires the presence of water, the proper wetting of micropores is essential for good performance, and the construction of micropore walls with too strong hydrophobicity will inhibit the occurrence of ORR process^[73]. Therefore, it is very important to rationally regulate the hydrophilicity/hydrophobicity of carbon carriers.

Demetallization and protonation reflect the deactivation behavior at the atom/active site scale, while micropore flooding explains the deactivation phenomenon induced by the accessibility of the catalyst active site from a more macroscopic perspective of the mass transfer of oxygen and water molecules. On the one hand, carbon oxidation corrosion will lead to the loss of metal centers due to the defects of carbon skeleton, on the other hand, the regulation of oxygen functional groups on the hydrophilicity of the interface will also affect the transport behavior of reactant/product molecules and induce deactivation. It can be seen that due to the special structure of Fe-N-C catalyst, the oxidation/corrosion phenomenon of carbon support has a more significant effect on the stability of Fe-N-C catalyst.