Fe/N/C Catalysts with Hierarchical Porous Structure Derived from Fe-doped ZIF-8 for Accelerated ORR Activity

Fe/N/C Catalysts with Hierarchical Porous Structure Derived from Fe-doped ZIF-8 for Accelerated ORR Activity | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Fe/N/C Catalysts with Hierarchical Porous Structure Derived from Fe-doped ZIF-8 for Accelerated ORR Activity Kaixiang Li, Jinyu Zhao, Ruipeng Yuan, Jiajun Chen, Huijun Li, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5221463/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract Fe-N-C is considered to be the most promising candidate for catalyzing oxygen reduction reaction (ORR), and its large-scale development is crucial to reducing the cost of proton exchange membrane fuel cells (PEMFCs). However, its simple and efficient synthesis still faces great challenges, and the microstructure changes in the pyrolysis process are not clear. Herein, we report a high-performance Fe-N-C catalyst, which is produced from the high temperature pyrolysis of Fe-doped ZIF-8 precursor. The effect of pyrolysis temperature on the specific surface area, porous structure and graphitization level of Fe-N-C catalyst is systematically studied. Eminently, Fe-N-C 1000, which was obtained via pyrolysis of Fe-ZIF-8 at 1000 °C, possesses highly dispersed Fe-N 4 active sites on the high surface area polyhedral, ensuring the high intrinsic activity. The simultaneous hierarchically ordered porous architecture provides a wealth of mass transfer channels to improve dynamic performance. It exhibits an outstanding ORR activity in acidic solution (E 1/2 of 0.791 V). High graphitization also enhances its corrosion resistance, showing superior stability (only change 20 mV after 5000 cycles in 0.5M H 2 SO 4 ). This work well demonstrates the importance of establishing the structural equilibrium of the catalyst under pyrolysis conditions for efficient ORR. Oxygen reduction reaction metal-organic framework Fe-N-C electrocatalyst hierarchical porous structure Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction As clean and efficient energy-conversion device, proton-exchange memlbrane fuel cells (PEMFCs) have attracted substantial attention [1–3] . However, Oxygen reduction reaction (ORR) at cathodes is always restricted to slow kinetics progress [4–6] . Therefore, it is essential to develop high-performance electrocatalysts to improve the kinetics of oxygen reduction reactions. Currently, platinum-based catalysts are the most efficient and commercially available ORR electrocatalysts [7] . However, platinum-based catalysts have serious drawbacks such as high cost, limited reserves, and declines in activity and stability due to the dissolution and aggregation of platinum particles in the process of ORR [8, 9] . Fe-N-C material has the characteristics of low cost and high active site density in oxygen reduction catalytic process, exhabiting excellent potential to replace Pt catalyst [10, 11] . Studies on Fe-N-C catalysts show that most of them are synthesized by high-temperature pyrolysis of iron salts, nitrogen and carbon precursors [12–14] .The high intrinsic activity mainly comes from the nitrogen coordination iron atom embedded in the carbon material with high specific surface area [15–17] . However, most of Fe and N are embedded in the carbon substrate in an uncontrolled form during high-temperature pyrolysis, resulting in agglomeration rather than the formation of Fe-N x active sites [18, 19] . Moreover, it is very difficult to obtain a suitable porous structure since the morphology of the precursor will be damaged after high-temperature pyrolysis [20] . In this regard, metal-organic frameworks (MOFs), as precursors to form such catalysts, stand out due to their well-defined metal location, high specific surface area, and adjustable porous structure [21, 22] . Specifically, MOF-derived carbon-based non-precious metal catalysts exhibit the following outstanding advantages: metal species can evenly disperse in the carbon-based materials [23] ; the porosity of the carbon-based materials can be adjusted to further expose the active sites of the electrocatalytic reaction and reducing the mass transport resistance of O 2 [24–26] ; multi-species (i.e., monatomic metals and heteroatoms coordinated with them) can be encapsulated into the pores of carbon-based materials and distributed more evenly [27, 28] . Nevertheless, the relationship between pyrolysis temperature and microstructure evolution of MOF-derived catalysts remains unclear and needs further investigation. In this work, we synthesized a series of Fe–N–C catalysts via pyrolysis of Fe-ZIF-8 precursor at difficult temperature, and the effect of pyrolysis temperature on specific surface area, porous structure and graphitization degree was systematically studied. The Fe-N-C material obtained by the pyrolysis of Fe-ZIF-8 presented excellent oxygen reduction catalytic activity in acidic electrolytes. This is mainly due to the iron present in a single atom or amorphous form, which promotes the formation of Fe-N x active sites. In addition, the hierarchical porous structure facilitates the mass transfer process of O 2 . The electrochemical test results well demonstrated that the pore distribution of the material can be effectively controlled by temperature effect and finally contribute to the excellent ORR catalytic activity. The as-prepared Fe–N–C-1000 catalyst exhibited a promising ORR activity in the acid electrolyte (E 1/2 of 0.791 V), as well as superior stability (half-wave potential only changed 20 mV after 5000 cycles in 0.5M H 2 SO 4 ). 2 Experimental 2.1 The preparation of Fe-N-C catalyst Zn(NO) 3 ∙6H 2 O (1.762 g) and Fe(NO) 3 ∙6H 2 O (60 mg) were dissolved in 120 ml of methanol, followed by addition of 120ml of methanol solution containing 4.039 g dimethylimidazole. After stirring at room temperature for 6h, the yellowy crystal was isolated by centrifugation, which was washed three times with methanol solution and dried overnight at 80°C, and the product was collected and named Fe-ZIF-8. Then Fe-ZIF-8 powder was heat-tearted in Ar atomosphere at different temperature (700℃, 800℃, 900℃, 1000℃, 1100℃) for 1h with a heat rate of 5℃ min -1 and naturally cooled to room temperature. Finally, the catalysts annealed at different temperatures were collected. The catalysts treated at different annealing temperatures were collected and named as Fe-N-C 700, Fe-N-C 800, Fe-N-C 900, Fe-N-C 1000, and Fe-N-C 1100. 2.2 Material structure characterization The crystal structure and phase composition of the materials (ZIF-8, Fe-ZIF-8, Fe-N-C) were characterized by X-ray diffraction with Cu-Kα radiation source (XRD, Rigaku Rint-2000, 5°-90°,10° min − 1 ); the morphology and microstructure of the materials were characterized by scanning electron mi croscopy (SEM, TESCAN Czech Republic); the elemental distribution of the materials was characterized by energy dispersive spectroscopy (EDS, TESCAN Czech Republic); and the elemental distribution of the materials was characterized by thermogravimetric analysis (TGA, Netzsch, Ar, 30°C-1000°C ,10°C min − 1 . Energy dispersive spectroscopy (EDS, TESCAN Czech Republic) to characterize the elemental distribution of the materials; thermogravimetric analysis (TGA, Netzsch, Ar, 30°C-1000°C ,10°C min − 1 ) to characterize the pyrolysis temperature of Fe-ZIF-8; Raman spectroscopy (Raman) to characterize the degree of graphitization of the materials; BET ratio (BET, Micromeritics) to characterize the specific surface area and pore size distribution of the material. 2.3 Electrochemical measurements The electrochemical performances measurements were conducted using electrochemical workstation (CHI760E) in a conventional three-electrode cell. Rotating disk electrode(RDE) and rotating-rinf disk electrode(RRDE) was served as the working electrode, Ag/AgCl were used as the reference electrode and platinum foil electrode were used as the counter electrode. The electrolyte is 0.5 M H 2 SO 4 . Electrochemical tests were detailedly described in Supporting Information. The 5 mg catalyst power was dispersed in a mixture (1 mL) of 980 µL ethanol and 20 µL Nafion (5 wt%), and the ultrasonically dispersed ink was formed after 1h. The ink is then applied in even drops to the RRDE with a load of 0.5 mg cm − 2 and dried at room temperature. 3 Results and Discussion 3.1 Composition and structure analysis Figure 1 a illustrates the synthetic route of Fe–N–C catalysts, which were constructed via the pyrolysis of Fe-ZIF-8 with a microporous structure. Zn(NO) 3 ∙6H 2 O, Fe(NO) 3 ∙ 6H 2 O and 2-methylimidazole were used as precursors for the synthesis of Fe-ZIF-8 in methanol solution, in which Fe 3+ substitute for partial Zn 2+ owing to the electronegativity and the radius between Fe 3+ and Zn 2+ is similar. Subsequently, the Fe-ZIF-8 precursors were subjected to a series of high-temperature annealing treatments to carbonize and then transform into a porous Fe-N-C catalyst. Figure 1 (a) shows the XRD patterns of the obtained Fe-ZIF-8 and ZIF-8. It is reported that the characteristic peaks at 2θ = 7.4°, 10.4°, 12.8°, 16.5° and 18.1° in the XRD spectrum correspond to the crystal planes of (011), (002), (112), (022), (013) and (222) of ZIF-8 respectively [29] . The positions of the characteristic peaks of the Fe-ZIF-8 largely coincide with the positions of ZIF-8, corresponding to a lattice constant a = b = c = 1.699 nm. This result indicates that the doping of Fe (Zn/Fe = 40:1, molar ratio) does not affect the crystal structure of ZIF-8 compared to pure ZIF-8 [30] . The broader characteristic peaks at 2θ = 26° and 43° in Fig. 1 (b) correspond to the (002) and (101) crystallographic planes of graphitic carbon, respectively, indicating that Fe-ZIF-8 underwent carbonization after pyrolysis at high temperatures [30] . The ICP test shows the content of Fe in Fe-N-C-1000 is about 0.28%. It is also noteworthy that the absence of any characteristic peaks associated with crystalline Fe, such as metallic Fe, FeO x , or Fe 3 C, in the XRD patterns of a series of Fe-N-C samples implies that Fe may exist in monoatomic or amorphous form, which in turn forms the active site of Fe-N x . To further visualize the state of single Ni atoms on NG, the high-angle annular field-scanning transmission electron microscopy (HAADF-STEM) was utilized to understand the Fe-N-C 1000 (Fig. R3). A large amount of bright dots (≈ 2–3 Å) are randomly distributed on carbon, corresponding to single Fe atoms on account of atomic number contrast in the image. The morphological changes and elemental distribution of Fe-ZIF-8 precursors and Fe-N-C catalyst were characterized via SEM and EDS. The Fe-ZIF-8 still maintains the dodecahedron structure, which is consistent with ZIF-8. Figure 2 (a) shows that the doping of Fe did not destroy the morphology and crystal structure of ZIF-8, which is in agreement with the XRD characterization results. Figure 2 .(b-f) show the SEM images of Fe-N-C catalyst derived from Fe-ZIF-8 by a series of high-temperature annealing treatments. It can be seen that particles retain their polyhedral shape after high-temperature pyrolysis. Zn 2+ evaporates from the Fe-ZIF-8 precursor during the pyrolysis, resulting in the formation of lots of micropores, and the micropores increased with the increase of pyrolysis temperature. It can also be seen that the size of the Fe-N-C product gradually decreases with the increase in temperature. EDS mapping of Figure S1 shows the uniform distribution of Fe, N, C. After heat treatment, the surface roughness of the Fe-N-C catalyst is improved compared to the smooth surface of the precursor Fe-ZIF-8. But the morphology of the crystal remains substantially unchanged, the frame structure of the material does not collapse which ensures that the porous structure of the material is not destroyed, thereby ensuring full exposure of the active site [31] . The absence of metal particles or aggregates proves that the dispersion of Fe in Fe-ZIF-8 and Fe-N-C samples may be at the atomic level. Raman spectra of Fe-N-C catalyst are assumed in Fig. 3 (a). The diffraction peaks at 1353 cm -1 and 1588 cm -1 are attributed to the D and G peaks, respectively [32] . Where the D peak originates from the structural defects of the carbon and the G peak mainly derived from the synergistic in-plane vibrations of the sp 2 hybridized carbon inside the graphite layer, representing the degree of graphitization of the material [33] . It was reported that the disordered carbon represents the defects in the carbon structure such as dislocations, vacancies, and edges [34] , which can be used as catalytically active sites for ORR, at the same time the presence of graphitic carbon gives the material outstanding electrical conductivity [35] . The smaller the intensity ratio of the D-peak to the G-peak of the resulting product represents the higher degree of graphitization and better electrical conductivity of the material [36, 37] . The I D /I G of, Fe-N-C 800, Fe-N-C 900, Fe-N-C 1000, Fe-N-C 1100(Fig. 3 (b)) were calculated to be 1.08, 1.02, 0.98 and 0.98, respectively. Therefore, the Fe-N-C 1000 has the highest degree of graphitization and the best electrical conductivity, which means that this catalyst has the most active sites and its catalytic performance is optimal. The thermogravimetric curves in Fig. 3 (c) indicate that the pyrolysis of Fe-ZIF-8 precursors can be divided into three main stages. The first stage mainly occurs below 550°C, and the mass loss of Fe-ZIF-8 is tiny, which is caused by the evaporation of crystalline water inside the precursor. The second stage occurs after the temperature reaches 550°C with an obvious mass loss and heat absorption, which is speculated that the mass loss is due to the decomposition of Fe-ZIF-8 crystal structure. The third stage eventuates above 600°C, in which the Fe-ZIF-8 precursor undergoes carbonization, and the Fe-N-C catalyst was formed finally as the pyrolysis temperature increases. According to reports, the high surface area and porous structure of the catalysts facilitate the exposure of active sites and mass transport [38] . To analyze the pore structure and specific surface area of catalysts, an analysis was carried out using ASAP 2460. As shown in Table S1 , Brunauer-Emmett-Teller (BET) specific surface area increases dramatically from 44.9 m 2 g -1 for Fe-N-C 700 to 1162.2 m 2 g -1 for Fe-N-C 1000. As shown in Fig. 3 (d) and Fig. S2, the adsorption/desorption isotherms of Fe-N-C 1100 catalyst conformed to the type IV isotherm with an obvious hysteresis loop, indicating that the obtained series of Fe-N-C catalysts annealed at high temperatures all have mesoporous properties. The obvious hysteresis loop throughout the entire pressure range. Moreover, the adsorption increases sharply at low relative pressure, revealing that micropores exist in the catalyst materials [39] . Iron atom plays a crucial role in the mesopore formation of Fe-N-C materials by catalyzing the decomposition or dehydrogenation of ZIF-8 organic ligands, which can induce the formation of the mesoporous structure of the materials [40] . Therefore, it is speculated that the mesoporous properties of the obtained series of Fe-N-C catalysts are related to the introduction of Fe source in ZIF-8. Combining the specific surface area and the percentage of pore size distribution counted (Table S1 ), it can be obtained that the specific surface area of Fe-N-C catalysts increases with the increase of annealing temperature. According to the calculation of DFT, the percentage of micropores (0.5-2.0 nm) and mesopores (2.0-50.4 nm) of a series of Fe-N-C catalysts can beware increased with the increase of temperature while the percentage of macropores (> 100 nm) decreases with the increase of temperature using the model of lamellar pores, which is also the reason for the increase of the specific surface area of catalysts. It has been reported that the active sites of heteroatom-doped materials tend to be located in or around the micropores, so the abundance of micropores in Fe-N-C facilitates the accommodation of more active sites [41] . The Fe-N-C 1000 has the smallest microporous pore size distributed at 0.85 nm and 1.13 nm respectively and the highest microporous ratio, exhibiting optimal catalytic performance. In addition, the plentiful mesopores provide abundant open channels for the reactant O 2 , which in turn accelerate the transport of substances and improve the ORR reaction efficiency. It can be concluded that the ORR activity of the Fe-N-C catalyst improves significantly with the increase in the percentage of micropores and mesopores. It is an effective strategy to improve the ORR activity of the catalyst by optimizing the pyrolysis temperature to construct a hierarchical porous structure with micropores and mesopores. Figure S3 shows the XPS spectra of N 1s and Fe 2p. It can be seen that the N 1s can be divided into the Graphitic N, Pyrrolic N, Pyridinic N and Fe-N bond, verfing the Fe-N-C structure. 3.2 Electrochemical measurements The ORR activity of Fe-N-C catalysts obtained by pyrolysis at different temperatures was performed and compared in O 2 saturated 0.5M H 2 SO 4 electrolyte with a rotating disk electrode. As shown in Fig. 4 (a), the Fe-N-C 1000 and Fe-N-C 1100 catalysts in O 2 -saturated 0.5 M H 2 SO 4 electrolyte showed a cathodic reduction peak, indicating that they have ORR catalytic activity [42] . Moreover, Fe-N-C 1000 catalyst has higher cathodic oxygen reduction potential, indicating that Fe-N-C 1000 has higher oxygen reduction activity [43] . To further investigate the ORR activity of the prepared Fe-N-C catalysts, linear scanning voltammetry (LSV) tests were performed in O 2 -saturated 0.5 M H 2 SO 4 electrolyte at 1600 rpm with a sweep rate of 10 mv s -1 . As clearly visualized in Fig. 4 (b) and Fig. 4 (d), the half-wave potential E 1/2 (0.791 V vs. RHE) of the Fe-N-C 1000 catalyst is similar to the commercial Pt/C (20%) catalyst (0.786 V vs. RHE), which is significantly higher than the Fe-N-C 700, Fe-N-C800, Fe-N-C900 and Fe-N-C 1100. In addition, the limiting diffusion currents of Fe-N-C 1000 catalyst were significantly higher than those of other catalysts. In addition, the Tafel slopes of Fe-N-C 1000 catalysts are similar to those of Pt/C catalysts, indicating that Fe-N-C 1000 catalysts also have similar O 2 transport efficiency in acidic media. The electrochemical performance is higher than the other catalysts (Table S2). As the pyrolysis temperature increased from 700 ℃ to 1000 ℃, the volume of micropores and mesopores increased significantly at the same time. When the pyrolysis temperature increases from 1000 ℃ to 1100 ℃, the volume of the micropores continues to increase, but the volume of the mesopores decreases. Therefore, the efficiency of the oxygen mass transfer process decreases, and finally the catalyst activity decreases. With the increase of pyrolysis temperature, the graphitization degree of Fe-N-C catalysts was gradually increased. There is a disordered and amorphous structure in Fe-N-C 1100, which greatly promoted the ORR reaction. Combined with the XRD characterization, it can be seen that there are no characteristic peaks of Fe or Fe 3 C in the Fe-N-C catalyst after pyrolysis. Iron atoms may exist mainly in monoatomic or amorphous forms, forming a large number of Fe-N x active sites which improve the catalytic activity of ORR. It has been reported that the active sites may be located in or around micropores in the carbon structure [44] . Therefore, with the increase of the pyrolysis temperature, the micropores and the specific surface area of Fe-N-C catalysts increase significantly, which is conducive to accommodating the high density of active sites. It is also responsible for enhanced catalytic activity. In addition, the mesopores of Fe-N-C catalysts increased with the increase of pyrolysis temperature from 700℃ to 1000℃, while the macropore decreased with the increase of pyrolysis temperature. This result indicates that the stacking between particles is dense and the structure and channel for Oxygen transfer is exoteric, which would facilitate the transport process of reactants O 2 and reaction products H 2 O and further promote the ORR reaction. It has been reported that the kinetic current density (J K ) can characterize the intrinsic activity of the catalyst material, and the catalytic efficiency improves significantly with the increase of kinetic current density [45] . The kinetic current densities (J K @0.78V) of Fe-N-C 1000, Fe-N-C 11000 and Pt/C were 4.24 mA cm -2 , 2.63 mA cm -2 , and 5.82 mA cm -2 respectively.The ORR reaction path, electron transfer number (n) and H 2 O 2 yield (%) were investigated by RRDE. The results in Fig. 5 (b) showed that the electron transfer numbers of both Fe-N-C 1000) catalysts and Pt/C catalysts were close to 4 at a range of 0.2 V to 0.8 V, which was consistent with the K-L curve fitting results. Indicating that the ORR processes of both Fe-N-C 1000 catalysts and Pt/C catalysts in 0.5 M H 2 SO 4 were 4e - path. And the H 2 O 2 yield of Fe-N-C 1000, Fe-N-C 1100 and Pt/C was kept at a very low level. The Fe–N–C 1000 also shows excellent durability and methanol tolerance in acidic electrolytes. As shown in Fig. 5 (d), after the acceleration dynamic test for 5000 cycles, the E 1/2 changed slightly (negative shift of merely 20 mV). The morphology and structure were basically unchanged before and after stability test (Fig. S4). Moreover, the stability of Fe − N−C 1100 and Pt/C was evaluated by i − t chronoamperometric measurement. As shown in Fig. 5 (e), the relative current of Fe–N–C 1000 keeps 93% retention after 20000s continuous operation, much higher than the 74.8% of Pt/C. Methanol tolerance is also a key performance indicator of catalysts. It can be seen in Fig. 5 (f), the Fe–N–C 1000 showed excellent methanol tolerance in alcidic electrolyte, there was a sightly change after methanol injection. There is an obvious drop in Pt/C caused by the methanol oxidation reaction. Furthermore, Fig. S5 shows the LSV curves of FeNC-1000 and Pt/C catalysts in 0.1 M HClO4. The onset potential and half-wave potential of FeNC-1000 and Pt/C are 1.210/1.198 V and 0.928/0.886 V. Moreover, it is proved that the catalytic performance of FeNC-1000 is superior to that of Pt/C. 4 Conclusions In summary, a series of Fe-N-C materials with high-performance ORR activity was synthesized through pyrolysis of Fe-ZIF-8 precursor. The different porous structures, specific surface area and graphitization levels can be easily modified by adjusting the pyrolysis temperature. The as-prepared Fe − N−C 1100 catalyst with the micro-/mesoporous forms, high specific surface area, and graphitization level presented an excellent ORR activity. This is due to those special structures conducive to transporting matter rapidly, ionic diffusion distance, and exposure to more active sites. Its half-wave potential is 0.791V, which is higher than the commercial Pt/C catalyst by 5 mV. The E onset and limited current density are higher than those of commercial Pt/C. The Fe-N-C 1100 whose electron transfer number is ≈ 4 between 0.2 and 0.8 V presented a four-electron route. At the same time, the Fe-N-C 1100 possesses excellent stability. Its half-wave potential changed slightly after 5000 cycles in 0.5M H 2 SO 4 , and the relative current of Fe–N–C 1000 keep 93% retention after 20000s continuous operation, much higher than the 74.8% of Pt/C. Declarations Author Contribution Kaixiang Li: Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis, Data curation, Conceptualization. Jinyu Zhao: Writing – review & editing, Methodology, Validation.Ruipeng Yuan: Validation. Jiajun Chen: Validation.Huijun Li: Validation. Xiaomin Wang: Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Data curation, Conceptualization. Acknowledgements This work has been supported by the National Natural Science Foundation of China (52072256, 52301282), Shanxi Province Science and Technology Program Unveiled Bidding Program (20201101016). References YANG L, SHAO Z. 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Multifunctional transitional metal-based phosphide nanoparticles towards improved polysulfide confinement and redox kinetics for highly stable lithium-sulfur batteries [J]. Chemical Engineering Journal, 2022, 450. SHANG L, YU H, HUANG X, et al. Well-Dispersed ZIF-Derived Co,N-Co-doped Carbon Nanoframes through Mesoporous-Silica-Protected Calcination as Efficient Oxygen Reduction Electrocatalysts [J]. Adv Mater, 2016, 28(8): 1668-74. LI G, LI J, CUI Q, et al. Using a Fe-doping MOFs strategy to effectively improve the electrochemical activity of N-doped C materials for oxygen reduction reaction in alkaline medium [J]. Journal of Solid State Electrochemistry, 2020, 24(10): 2427-39. ZHANG K, ZHAO Z, WANG X. Ni2P/rGO as a highly efficient sulfur host toward enhancing the polysulfides redox for lithium-sulfur batteries [J]. Journal of Alloys and Compounds, 2022, 906: 164376. YAO P, LI T, QIU Y, et al. N-doped hierarchical porous carbon derived from bismuth salts decorated ZIF8 as a highly efficient electrocatalyst for CO2 reduction [J]. Journal of Materials Chemistry A, 2021, 9(1): 320-6. ZHAO Z, YI Z, LI H, et al. Understanding the modulation effect and surface chemistry in a heteroatom incorporated graphene-like matrix toward high-rate lithium-sulfur batteries [J]. Nanoscale, 2021, 13(35): 14777-84. XU X, XIA Z, ZHANG X, et al. Atomically dispersed Fe-N-C derived from dual metal-organic frameworks as efficient oxygen reduction electrocatalysts in direct methanol fuel cells [J]. Applied Catalysis B: Environmental, 2019, 259. ZHAO Z, YI Z, LI H, et al. Synergetic effect of spatially separated dual co-catalyst for accelerating multiple conversion reaction in advanced lithium sulfur batteries [J]. Nano Energy, 2021, 81. SHI W, WANG Y-C, CHEN C, et al. A mesoporous Fe/N/C ORR catalyst for polymer electrolyte membrane fuel cells [J]. Chinese Journal of Catalysis, 2016, 37(7): 1103-8. DENG Y, TIAN X, CHI B, et al. Hierarchically open-porous carbon networks enriched with exclusive Fe–Nx active sites as efficient oxygen reduction catalysts towards acidic H2–O2 PEM fuel cell and alkaline Zn–air battery [J]. Chemical Engineering Journal, 2020, 390. CHEN X-L, MA L-S, SU W-Y, et al. ZIF-derived bifunctional Cu@Cu–N–C composite electrocatalysts towards efficient electroreduction of oxygen and carbon dioxide [J]. Electrochimica Acta, 2020, 331. ZHAO Z, LI H, CHENG X, et al. Multifunctional FeP/Spongy Carbon Modified Separator with Enhanced Polysulfide Immobilization and Conversion for Flame‐Retardant Lithium‐Sulfur Batteries [J]. ChemistrySelect, 2021, 6(28): 7098-102. WANG X, LI Q, PAN H, et al. Size-controlled large-diameter and few-walled carbon nanotube catalysts for oxygen reduction [J]. Nanoscale, 2015, 7(47): 20290-8. HUANG J-W, CHENG Q-Q, HUANG Y-C, et al. Highly Efficient Fe–N–C Electrocatalyst for Oxygen Reduction Derived from Core–Shell-Structured Fe(OH)3@Zeolitic Imidazolate Framework [J]. ACS Applied Energy Materials, 2019, 2(5): 3194-203. Additional Declarations No competing interests reported. Supplementary Files supportinginformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 10 Nov, 2024 Reviews received at journal 17 Oct, 2024 Reviews received at journal 15 Oct, 2024 Reviewers agreed at journal 14 Oct, 2024 Reviewers agreed at journal 13 Oct, 2024 Reviews received at journal 10 Oct, 2024 Reviewers agreed at journal 09 Oct, 2024 Reviewers agreed at journal 09 Oct, 2024 Reviewers invited by journal 09 Oct, 2024 Editor assigned by journal 08 Oct, 2024 Submission checks completed at journal 08 Oct, 2024 First submitted to journal 07 Oct, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5221463","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":376242115,"identity":"a858ff2c-4d27-49b3-b092-3ad095b362cf","order_by":0,"name":"Kaixiang Li","email":"","orcid":"","institution":"GAC AION NEW ENERGY AUTOMOBILE Co. Ltd","correspondingAuthor":false,"prefix":"","firstName":"Kaixiang","middleName":"","lastName":"Li","suffix":""},{"id":376242116,"identity":"ca2fb3af-feac-48a7-9151-ad54d8933f15","order_by":1,"name":"Jinyu Zhao","email":"","orcid":"","institution":"Taiyuan University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Jinyu","middleName":"","lastName":"Zhao","suffix":""},{"id":376242117,"identity":"94367b47-4b75-4546-a888-54f9bd64bc71","order_by":2,"name":"Ruipeng Yuan","email":"","orcid":"","institution":"Taiyuan University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Ruipeng","middleName":"","lastName":"Yuan","suffix":""},{"id":376242118,"identity":"06828cff-e1aa-46bb-a6c6-330fb1b74d54","order_by":3,"name":"Jiajun Chen","email":"","orcid":"","institution":"Taiyuan University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Jiajun","middleName":"","lastName":"Chen","suffix":""},{"id":376242119,"identity":"495eeb99-d844-4850-9698-05ab93a16cc2","order_by":4,"name":"Huijun Li","email":"","orcid":"","institution":"Taiyuan University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Huijun","middleName":"","lastName":"Li","suffix":""},{"id":376242120,"identity":"f457ba9a-461d-4522-97e4-b519d6dd6395","order_by":5,"name":"Xiaomin Wang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA80lEQVRIiWNgGAWjYNACAyBmZmB8gMQlTguzwQEGBgkitUAAmwRRWgyOnz38mqfATk63nfdY9ce2w3UM7M3bJBhq7uDWciYvzXKGQbKx2WG+tBsH2w5LMPAcK5NgOPYMpxazAzlmBh8MmBO3HeYxg2iRyDGTYGw4jFvL+TdmBgkG9WAtBWAt8m8IaLmRY/zgg8FhsBYGiC08+LXY33hjxjjD4DjQLzzGEmfOpUu28aQVWyQcw61Fsj/H+DPPn2o5s/NnDD9UlFnz87Mf3njjQw1uLQyg6IAzGdmAXBAjAZ8GYLR/QLD/4Fc6CkbBKBgFIxMAAIbjU5Xdf6OkAAAAAElFTkSuQmCC","orcid":"","institution":"Taiyuan University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Xiaomin","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2024-10-08 03:08:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5221463/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5221463/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":69993102,"identity":"54e57b9f-800a-434c-b0e5-208572ae95ee","added_by":"auto","created_at":"2024-11-27 09:40:29","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":236102,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) The synthetic route of Fe-N-C, (b) XRD patterns of ZIF-8 and Fe-ZIF-8, (c) Raman spectra of Fe-N-C-700, Fe-N-C-800, Fe-N-C-900, Fe-N-C-1000, Fe-N-C-1100, (d) STEM of Fe-N-C 1000\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/cbdd0620a0745828ebc9848a.png"},{"id":69993109,"identity":"91f44103-3b2f-4405-9964-e652fb160c53","added_by":"auto","created_at":"2024-11-27 09:40:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1056292,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSEM images of (a)Fe-ZIF-8, (b)Fe-N-C-700, (c)Fe-N-C-800, (d)Fe-N-C-900, (e)Fe-N-C-1000, (f)Fe-N-C-1100.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/ed9dce39d332bda0547f3e36.png"},{"id":69991458,"identity":"4a5bfccf-091a-4bf8-b5c3-3041baf7a16b","added_by":"auto","created_at":"2024-11-27 09:32:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":151943,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) Raman spectra and (b) (I\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eD\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/I\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eG\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) intensity of Fe-N-C 800, Fe-N-C 900, Fe-N-C 1000 and Fe-N-C 1100, (c) Thermogravimetric curves of Fe-ZIF-8, (d) N\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e adsorption/desorption isotherms of Fe-N-C 1000 and\u003c/strong\u003e \u003cstrong\u003eembedded figure is pore size distribution of Fe-N-C 1000.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/f1b05feefc2dc2421e91eab3.png"},{"id":69991460,"identity":"67ec6e98-2c2c-4cd6-82b1-ca2dc877d9ae","added_by":"auto","created_at":"2024-11-27 09:32:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":121993,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) CV curves in\u003c/strong\u003e \u003cstrong\u003eN\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2 \u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eand O\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e saturated 0.5 M H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e solution at a scan rate of 0.05 V s\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e, (b) LSV curves at a rotating speed of 1600 rpm with a scan rate of 10 mV s\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e in O\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e saturated 0.5 M H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eSO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e solution,\u003c/strong\u003e \u003cstrong\u003e(c) Tafel plots, (d) half-wave potential (E\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e1/2)\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and\u003c/strong\u003e \u003cstrong\u003eonset potential (E\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eonset\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) (e) LSV curve of Fe-N-C 1000 at different rotating speeds, (f)\u003c/strong\u003e \u003cstrong\u003ethe corresponding Koutecky–Levich plots under various potentials.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/5950dcde860452cae9922ba9.png"},{"id":69991456,"identity":"2ce32a70-56ab-4e4b-ac00-90a4243d88c4","added_by":"auto","created_at":"2024-11-27 09:32:29","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":202077,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e\u0026nbsp;(a) Comparison of kinetic current, (b) RRDE voltammograms of different catalysts, (c) H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e productivity and corresponding electron transfer number (n), (d) The ORR LSV curves of Fe–N–C-1000 before and after 5000 cycles between 0.6 v and 1.1 V (vs.RHE) with a scan rate of 100 mV s\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e-1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e, (e) Normalized chronoamperometry curves, (f) Methanol tolerance tests.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/67b6ec8b65fae5ab0152cf30.png"},{"id":69993374,"identity":"16af0269-82bf-485d-aac0-3e6c1e8e1f1c","added_by":"auto","created_at":"2024-11-27 09:48:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2661840,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/3132ae2c-7d02-4693-9dcb-936f2cbc9509.pdf"},{"id":69993103,"identity":"cd8d63e2-7be1-47c2-8639-025330b0098b","added_by":"auto","created_at":"2024-11-27 09:40:30","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1806607,"visible":true,"origin":"","legend":"","description":"","filename":"supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5221463/v1/209d1303e32d6630450b3ec1.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fe/N/C Catalysts with Hierarchical Porous Structure Derived from Fe-doped ZIF-8 for Accelerated ORR Activity","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eAs clean and efficient energy-conversion device, proton-exchange memlbrane fuel cells (PEMFCs) have attracted substantial attention \u003csup\u003e[1\u0026ndash;3]\u003c/sup\u003e. However, Oxygen reduction reaction (ORR) at cathodes is always restricted to slow kinetics progress\u003csup\u003e[4\u0026ndash;6]\u003c/sup\u003e. Therefore, it is essential to develop high-performance electrocatalysts to improve the kinetics of oxygen reduction reactions. Currently, platinum-based catalysts are the most efficient and commercially available ORR electrocatalysts\u003csup\u003e[7]\u003c/sup\u003e. However, platinum-based catalysts have serious drawbacks such as high cost, limited reserves, and declines in activity and stability due to the dissolution and aggregation of platinum particles in the process of ORR\u003csup\u003e[8, 9]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFe-N-C material has the characteristics of low cost and high active site density in oxygen reduction catalytic process, exhabiting excellent potential to replace Pt catalyst\u003csup\u003e[10, 11]\u003c/sup\u003e. Studies on Fe-N-C catalysts show that most of them are synthesized by high-temperature pyrolysis of iron salts, nitrogen and carbon precursors\u003csup\u003e[12\u0026ndash;14]\u003c/sup\u003e.The high intrinsic activity mainly comes from the nitrogen coordination iron atom embedded in the carbon material with high specific surface area \u003csup\u003e[15\u0026ndash;17]\u003c/sup\u003e. However, most of Fe and N are embedded in the carbon substrate in an uncontrolled form during high-temperature pyrolysis, resulting in agglomeration rather than the formation of Fe-N\u003csub\u003ex\u003c/sub\u003e active sites\u003csup\u003e[18, 19]\u003c/sup\u003e. Moreover, it is very difficult to obtain a suitable porous structure since the morphology of the precursor will be damaged after high-temperature pyrolysis\u003csup\u003e[20]\u003c/sup\u003e. In this regard, metal-organic frameworks (MOFs), as precursors to form such catalysts, stand out due to their well-defined metal location, high specific surface area, and adjustable porous structure\u003csup\u003e[21, 22]\u003c/sup\u003e. Specifically, MOF-derived carbon-based non-precious metal catalysts exhibit the following outstanding advantages: metal species can evenly disperse in the carbon-based materials\u003csup\u003e[23]\u003c/sup\u003e; the porosity of the carbon-based materials can be adjusted to further expose the active sites of the electrocatalytic reaction and reducing the mass transport resistance of O\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e[24\u0026ndash;26]\u003c/sup\u003e; multi-species (i.e., monatomic metals and heteroatoms coordinated with them) can be encapsulated into the pores of carbon-based materials and distributed more evenly\u003csup\u003e[27, 28]\u003c/sup\u003e. Nevertheless, the relationship between pyrolysis temperature and microstructure evolution of MOF-derived catalysts remains unclear and needs further investigation.\u003c/p\u003e \u003cp\u003eIn this work, we synthesized a series of Fe\u0026ndash;N\u0026ndash;C catalysts via pyrolysis of Fe-ZIF-8 precursor at difficult temperature, and the effect of pyrolysis temperature on specific surface area, porous structure and graphitization degree was systematically studied. The Fe-N-C material obtained by the pyrolysis of Fe-ZIF-8 presented excellent oxygen reduction catalytic activity in acidic electrolytes. This is mainly due to the iron present in a single atom or amorphous form, which promotes the formation of Fe-N\u003csub\u003ex\u003c/sub\u003e active sites. In addition, the hierarchical porous structure facilitates the mass transfer process of O\u003csub\u003e2\u003c/sub\u003e. The electrochemical test results well demonstrated that the pore distribution of the material can be effectively controlled by temperature effect and finally contribute to the excellent ORR catalytic activity. The as-prepared Fe\u0026ndash;N\u0026ndash;C-1000 catalyst exhibited a promising ORR activity in the acid electrolyte (E\u003csub\u003e1/2\u003c/sub\u003e of 0.791 V), as well as superior stability (half-wave potential only changed 20 mV after 5000 cycles in 0.5M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e).\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 The preparation of Fe-N-C catalyst\u003c/h2\u003e \u003cp\u003eZn(NO)\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO (1.762 g) and Fe(NO)\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO (60 mg) were dissolved in 120 ml of methanol, followed by addition of 120ml of methanol solution containing 4.039 g dimethylimidazole. After stirring at room temperature for 6h, the yellowy crystal was isolated by centrifugation, which was washed three times with methanol solution and dried overnight at 80\u0026deg;C, and the product was collected and named Fe-ZIF-8. Then Fe-ZIF-8 powder was heat-tearted in Ar atomosphere at different temperature (700℃, 800℃, 900℃, 1000℃, 1100℃) for 1h with a heat rate of 5℃ min\u003csup\u003e-1\u003c/sup\u003e and naturally cooled to room temperature. Finally, the catalysts annealed at different temperatures were collected. The catalysts treated at different annealing temperatures were collected and named as Fe-N-C 700, Fe-N-C 800, Fe-N-C 900, Fe-N-C 1000, and Fe-N-C 1100.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Material structure characterization\u003c/h2\u003e \u003cp\u003eThe crystal structure and phase composition of the materials (ZIF-8, Fe-ZIF-8, Fe-N-C) were characterized by X-ray diffraction with Cu-Kα radiation source (XRD, Rigaku Rint-2000, 5\u0026deg;-90\u0026deg;,10\u0026deg; min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e); the morphology and microstructure of the materials were characterized by scanning electron mi croscopy (SEM, TESCAN Czech Republic); the elemental distribution of the materials was characterized by energy dispersive spectroscopy (EDS, TESCAN Czech Republic); and the elemental distribution of the materials was characterized by thermogravimetric analysis (TGA, Netzsch, Ar, 30\u0026deg;C-1000\u0026deg;C ,10\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Energy dispersive spectroscopy (EDS, TESCAN Czech Republic) to characterize the elemental distribution of the materials; thermogravimetric analysis (TGA, Netzsch, Ar, 30\u0026deg;C-1000\u0026deg;C ,10\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) to characterize the pyrolysis temperature of Fe-ZIF-8; Raman spectroscopy (Raman) to characterize the degree of graphitization of the materials; BET ratio (BET, Micromeritics) to characterize the specific surface area and pore size distribution of the material.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Electrochemical measurements\u003c/h2\u003e \u003cp\u003eThe electrochemical performances measurements were conducted using electrochemical workstation (CHI760E) in a conventional three-electrode cell. Rotating disk electrode(RDE) and rotating-rinf disk electrode(RRDE) was served as the working electrode, Ag/AgCl were used as the reference electrode and platinum foil electrode were used as the counter electrode. The electrolyte is 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. Electrochemical tests were detailedly described in Supporting Information. The 5 mg catalyst power was dispersed in a mixture (1 mL) of 980 \u0026micro;L ethanol and 20 \u0026micro;L Nafion (5 wt%), and the ultrasonically dispersed ink was formed after 1h. The ink is then applied in even drops to the RRDE with a load of 0.5 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and dried at room temperature.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Composition and structure analysis\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea illustrates the synthetic route of Fe\u0026ndash;N\u0026ndash;C catalysts, which were constructed via the pyrolysis of Fe-ZIF-8 with a microporous structure. Zn(NO)\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO, Fe(NO)\u003csub\u003e3\u003c/sub\u003e\u003cb\u003e∙\u003c/b\u003e6H\u003csub\u003e2\u003c/sub\u003eO and 2-methylimidazole were used as precursors for the synthesis of Fe-ZIF-8 in methanol solution, in which Fe\u003csup\u003e3+\u003c/sup\u003e substitute for partial Zn\u003csup\u003e2+\u003c/sup\u003e owing to the electronegativity and the radius between Fe\u003csup\u003e3+\u003c/sup\u003e and Zn\u003csup\u003e2+\u003c/sup\u003e is similar. Subsequently, the Fe-ZIF-8 precursors were subjected to a series of high-temperature annealing treatments to carbonize and then transform into a porous Fe-N-C catalyst.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a) shows the XRD patterns of the obtained Fe-ZIF-8 and ZIF-8. It is reported that the characteristic peaks at 2θ\u0026thinsp;=\u0026thinsp;7.4\u0026deg;, 10.4\u0026deg;, 12.8\u0026deg;, 16.5\u0026deg; and 18.1\u0026deg; in the XRD spectrum correspond to the crystal planes of (011), (002), (112), (022), (013) and (222) of ZIF-8 respectively\u003csup\u003e[29]\u003c/sup\u003e. The positions of the characteristic peaks of the Fe-ZIF-8 largely coincide with the positions of ZIF-8, corresponding to a lattice constant a\u0026thinsp;=\u0026thinsp;b\u0026thinsp;=\u0026thinsp;c\u0026thinsp;=\u0026thinsp;1.699 nm. This result indicates that the doping of Fe (Zn/Fe\u0026thinsp;=\u0026thinsp;40:1, molar ratio) does not affect the crystal structure of ZIF-8 compared to pure ZIF-8\u003csup\u003e[30]\u003c/sup\u003e. The broader characteristic peaks at 2θ\u0026thinsp;=\u0026thinsp;26\u0026deg; and 43\u0026deg; in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (b) correspond to the (002) and (101) crystallographic planes of graphitic carbon, respectively, indicating that Fe-ZIF-8 underwent carbonization after pyrolysis at high temperatures\u003csup\u003e[30]\u003c/sup\u003e. The ICP test shows the content of Fe in Fe-N-C-1000 is about 0.28%. It is also noteworthy that the absence of any characteristic peaks associated with crystalline Fe, such as metallic Fe, FeO\u003csub\u003ex\u003c/sub\u003e, or Fe\u003csub\u003e3\u003c/sub\u003eC, in the XRD patterns of a series of Fe-N-C samples implies that Fe may exist in monoatomic or amorphous form, which in turn forms the active site of Fe-N\u003csub\u003ex\u003c/sub\u003e. To further visualize the state of single Ni atoms on NG, the high-angle annular field-scanning transmission electron microscopy (HAADF-STEM) was utilized to understand the Fe-N-C 1000 (Fig. R3). A large amount of bright dots (\u0026asymp;\u0026thinsp;2\u0026ndash;3 \u0026Aring;) are randomly distributed on carbon, corresponding to single Fe atoms on account of atomic number contrast in the image.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe morphological changes and elemental distribution of Fe-ZIF-8 precursors and Fe-N-C catalyst were characterized via SEM and EDS. The Fe-ZIF-8 still maintains the dodecahedron structure, which is consistent with ZIF-8. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows that the doping of Fe did not destroy the morphology and crystal structure of ZIF-8, which is in agreement with the XRD characterization results. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.(b-f) show the SEM images of Fe-N-C catalyst derived from Fe-ZIF-8 by a series of high-temperature annealing treatments. It can be seen that particles retain their polyhedral shape after high-temperature pyrolysis. Zn\u003csup\u003e2+\u003c/sup\u003e evaporates from the Fe-ZIF-8 precursor during the pyrolysis, resulting in the formation of lots of micropores, and the micropores increased with the increase of pyrolysis temperature. It can also be seen that the size of the Fe-N-C product gradually decreases with the increase in temperature. EDS mapping of Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e shows the uniform distribution of Fe, N, C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter heat treatment, the surface roughness of the Fe-N-C catalyst is improved compared to the smooth surface of the precursor Fe-ZIF-8. But the morphology of the crystal remains substantially unchanged, the frame structure of the material does not collapse which ensures that the porous structure of the material is not destroyed, thereby ensuring full exposure of the active site\u003csup\u003e[31]\u003c/sup\u003e. The absence of metal particles or aggregates proves that the dispersion of Fe in Fe-ZIF-8 and Fe-N-C samples may be at the atomic level.\u003c/p\u003e \u003cp\u003eRaman spectra of Fe-N-C catalyst are assumed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a). The diffraction peaks at 1353 cm\u003csup\u003e-1\u003c/sup\u003e and 1588 cm\u003csup\u003e-1\u003c/sup\u003e are attributed to the D and G peaks, respectively\u003csup\u003e[32]\u003c/sup\u003e. Where the D peak originates from the structural defects of the carbon and the G peak mainly derived from the synergistic in-plane vibrations of the sp\u003csup\u003e2\u003c/sup\u003e hybridized carbon inside the graphite layer, representing the degree of graphitization of the material\u003csup\u003e[33]\u003c/sup\u003e. It was reported that the disordered carbon represents the defects in the carbon structure such as dislocations, vacancies, and edges\u003csup\u003e[34]\u003c/sup\u003e, which can be used as catalytically active sites for ORR, at the same time the presence of graphitic carbon gives the material outstanding electrical conductivity\u003csup\u003e[35]\u003c/sup\u003e. The smaller the intensity ratio of the D-peak to the G-peak of the resulting product represents the higher degree of graphitization and better electrical conductivity of the material\u003csup\u003e[36, 37]\u003c/sup\u003e. The I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e of, Fe-N-C 800, Fe-N-C 900, Fe-N-C 1000, Fe-N-C 1100(Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)) were calculated to be 1.08, 1.02, 0.98 and 0.98, respectively. Therefore, the Fe-N-C 1000 has the highest degree of graphitization and the best electrical conductivity, which means that this catalyst has the most active sites and its catalytic performance is optimal.\u003c/p\u003e \u003cp\u003eThe thermogravimetric curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c) indicate that the pyrolysis of Fe-ZIF-8 precursors can be divided into three main stages. The first stage mainly occurs below 550\u0026deg;C, and the mass loss of Fe-ZIF-8 is tiny, which is caused by the evaporation of crystalline water inside the precursor. The second stage occurs after the temperature reaches 550\u0026deg;C with an obvious mass loss and heat absorption, which is speculated that the mass loss is due to the decomposition of Fe-ZIF-8 crystal structure. The third stage eventuates above 600\u0026deg;C, in which the Fe-ZIF-8 precursor undergoes carbonization, and the Fe-N-C catalyst was formed finally as the pyrolysis temperature increases.\u003c/p\u003e \u003cp\u003eAccording to reports, the high surface area and porous structure of the catalysts facilitate the exposure of active sites and mass transport\u003csup\u003e[38]\u003c/sup\u003e. To analyze the pore structure and specific surface area of catalysts, an analysis was carried out using ASAP 2460. As shown in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, Brunauer-Emmett-Teller (BET) specific surface area increases dramatically from 44.9 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for Fe-N-C 700 to 1162.2 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for Fe-N-C 1000. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d) and Fig. S2, the adsorption/desorption isotherms of Fe-N-C 1100 catalyst conformed to the type IV isotherm with an obvious hysteresis loop, indicating that the obtained series of Fe-N-C catalysts annealed at high temperatures all have mesoporous properties. The obvious hysteresis loop throughout the entire pressure range. Moreover, the adsorption increases sharply at low relative pressure, revealing that micropores exist in the catalyst materials\u003csup\u003e[39]\u003c/sup\u003e. Iron atom plays a crucial role in the mesopore formation of Fe-N-C materials by catalyzing the decomposition or dehydrogenation of ZIF-8 organic ligands, which can induce the formation of the mesoporous structure of the materials\u003csup\u003e[40]\u003c/sup\u003e. Therefore, it is speculated that the mesoporous properties of the obtained series of Fe-N-C catalysts are related to the introduction of Fe source in ZIF-8.\u003c/p\u003e \u003cp\u003eCombining the specific surface area and the percentage of pore size distribution counted (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), it can be obtained that the specific surface area of Fe-N-C catalysts increases with the increase of annealing temperature. According to the calculation of DFT, the percentage of micropores (0.5-2.0 nm) and mesopores (2.0-50.4 nm) of a series of Fe-N-C catalysts can beware increased with the increase of temperature while the percentage of macropores (\u0026gt;\u0026thinsp;100 nm) decreases with the increase of temperature using the model of lamellar pores, which is also the reason for the increase of the specific surface area of catalysts. It has been reported that the active sites of heteroatom-doped materials tend to be located in or around the micropores, so the abundance of micropores in Fe-N-C facilitates the accommodation of more active sites\u003csup\u003e[41]\u003c/sup\u003e. The Fe-N-C 1000 has the smallest microporous pore size distributed at 0.85 nm and 1.13 nm respectively and the highest microporous ratio, exhibiting optimal catalytic performance. In addition, the plentiful mesopores provide abundant open channels for the reactant O\u003csub\u003e2\u003c/sub\u003e, which in turn accelerate the transport of substances and improve the ORR reaction efficiency. It can be concluded that the ORR activity of the Fe-N-C catalyst improves significantly with the increase in the percentage of micropores and mesopores. It is an effective strategy to improve the ORR activity of the catalyst by optimizing the pyrolysis temperature to construct a hierarchical porous structure with micropores and mesopores. Figure S3 shows the XPS spectra of N 1s and Fe 2p. It can be seen that the N 1s can be divided into the Graphitic N, Pyrrolic N, Pyridinic N and Fe-N bond, verfing the Fe-N-C structure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Electrochemical measurements\u003c/h2\u003e \u003cp\u003eThe ORR activity of Fe-N-C catalysts obtained by pyrolysis at different temperatures was performed and compared in O\u003csub\u003e2\u003c/sub\u003e saturated 0.5M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte with a rotating disk electrode. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), the Fe-N-C 1000 and Fe-N-C 1100 catalysts in O\u003csub\u003e2\u003c/sub\u003e-saturated 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte showed a cathodic reduction peak, indicating that they have ORR catalytic activity\u003csup\u003e[42]\u003c/sup\u003e. Moreover, Fe-N-C 1000 catalyst has higher cathodic oxygen reduction potential, indicating that Fe-N-C 1000 has higher oxygen reduction activity\u003csup\u003e[43]\u003c/sup\u003e. To further investigate the ORR activity of the prepared Fe-N-C catalysts, linear scanning voltammetry (LSV) tests were performed in O\u003csub\u003e2\u003c/sub\u003e-saturated 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte at 1600 rpm with a sweep rate of 10 mv s\u003csup\u003e-1\u003c/sup\u003e. As clearly visualized in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d), the half-wave potential E\u003csub\u003e1/2\u003c/sub\u003e (0.791 V vs. RHE) of the Fe-N-C 1000 catalyst is similar to the commercial Pt/C (20%) catalyst (0.786 V vs. RHE), which is significantly higher than the Fe-N-C 700, Fe-N-C800, Fe-N-C900 and Fe-N-C 1100. In addition, the limiting diffusion currents of Fe-N-C 1000 catalyst were significantly higher than those of other catalysts. In addition, the Tafel slopes of Fe-N-C 1000 catalysts are similar to those of Pt/C catalysts, indicating that Fe-N-C 1000 catalysts also have similar O\u003csub\u003e2\u003c/sub\u003e transport efficiency in acidic media. The electrochemical performance is higher than the other catalysts (Table S2).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs the pyrolysis temperature increased from 700 ℃ to 1000 ℃, the volume of micropores and mesopores increased significantly at the same time. When the pyrolysis temperature increases from 1000 ℃ to 1100 ℃, the volume of the micropores continues to increase, but the volume of the mesopores decreases. Therefore, the efficiency of the oxygen mass transfer process decreases, and finally the catalyst activity decreases. With the increase of pyrolysis temperature, the graphitization degree of Fe-N-C catalysts was gradually increased. There is a disordered and amorphous structure in Fe-N-C 1100, which greatly promoted the ORR reaction.\u003c/p\u003e \u003cp\u003eCombined with the XRD characterization, it can be seen that there are no characteristic peaks of Fe or Fe\u003csub\u003e3\u003c/sub\u003eC in the Fe-N-C catalyst after pyrolysis. Iron atoms may exist mainly in monoatomic or amorphous forms, forming a large number of Fe-N\u003csub\u003ex\u003c/sub\u003e active sites which improve the catalytic activity of ORR. It has been reported that the active sites may be located in or around micropores in the carbon structure\u003csup\u003e[44]\u003c/sup\u003e. Therefore, with the increase of the pyrolysis temperature, the micropores and the specific surface area of Fe-N-C catalysts increase significantly, which is conducive to accommodating the high density of active sites. It is also responsible for enhanced catalytic activity. In addition, the mesopores of Fe-N-C catalysts increased with the increase of pyrolysis temperature from 700℃ to 1000℃, while the macropore decreased with the increase of pyrolysis temperature. This result indicates that the stacking between particles is dense and the structure and channel for Oxygen transfer is exoteric, which would facilitate the transport process of reactants O\u003csub\u003e2\u003c/sub\u003e and reaction products H\u003csub\u003e2\u003c/sub\u003eO and further promote the ORR reaction.\u003c/p\u003e \u003cp\u003eIt has been reported that the kinetic current density (J\u003csub\u003eK\u003c/sub\u003e) can characterize the intrinsic activity of the catalyst material, and the catalytic efficiency improves significantly with the increase of kinetic current density \u003csup\u003e[45]\u003c/sup\u003e. The kinetic current densities (J\u003csub\u003eK\u003c/sub\[email protected]) of Fe-N-C 1000, Fe-N-C 11000 and Pt/C were 4.24 mA cm\u003csup\u003e-2\u003c/sup\u003e, 2.63 mA cm\u003csup\u003e-2\u003c/sup\u003e, and 5.82 mA cm\u003csup\u003e-2\u003c/sup\u003e respectively.The ORR reaction path, electron transfer number (n) and H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yield (%) were investigated by RRDE. The results in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) showed that the electron transfer numbers of both Fe-N-C 1000) catalysts and Pt/C catalysts were close to 4 at a range of 0.2 V to 0.8 V, which was consistent with the K-L curve fitting results. Indicating that the ORR processes of both Fe-N-C 1000 catalysts and Pt/C catalysts in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e were 4e\u003csup\u003e-\u003c/sup\u003e path. And the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e yield of Fe-N-C 1000, Fe-N-C 1100 and Pt/C was kept at a very low level.\u003c/p\u003e \u003cp\u003eThe Fe\u0026ndash;N\u0026ndash;C 1000 also shows excellent durability and methanol tolerance in acidic electrolytes. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d), after the acceleration dynamic test for 5000 cycles, the E\u003csub\u003e1/2\u003c/sub\u003e changed slightly (negative shift of merely 20 mV). The morphology and structure were basically unchanged before and after stability test (Fig. S4). Moreover, the stability of Fe\u0026thinsp;\u0026minus;\u0026thinsp;N\u0026minus;C 1100 and Pt/C was evaluated by i\u0026thinsp;\u0026minus;\u0026thinsp;t chronoamperometric measurement. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(e), the relative current of Fe\u0026ndash;N\u0026ndash;C 1000 keeps 93% retention after 20000s continuous operation, much higher than the 74.8% of Pt/C. Methanol tolerance is also a key performance indicator of catalysts. It can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(f), the Fe\u0026ndash;N\u0026ndash;C 1000 showed excellent methanol tolerance in alcidic electrolyte, there was a sightly change after methanol injection. There is an obvious drop in Pt/C caused by the methanol oxidation reaction. Furthermore, Fig. S5 shows the LSV curves of FeNC-1000 and Pt/C catalysts in 0.1 M HClO4. The onset potential and half-wave potential of FeNC-1000 and Pt/C are 1.210/1.198 V and 0.928/0.886 V. Moreover, it is proved that the catalytic performance of FeNC-1000 is superior to that of Pt/C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cp\u003eIn summary, a series of Fe-N-C materials with high-performance ORR activity was synthesized through pyrolysis of Fe-ZIF-8 precursor. The different porous structures, specific surface area and graphitization levels can be easily modified by adjusting the pyrolysis temperature. The as-prepared Fe\u0026thinsp;\u0026minus;\u0026thinsp;N\u0026minus;C 1100 catalyst with the micro-/mesoporous forms, high specific surface area, and graphitization level presented an excellent ORR activity. This is due to those special structures conducive to transporting matter rapidly, ionic diffusion distance, and exposure to more active sites. Its half-wave potential is 0.791V, which is higher than the commercial Pt/C catalyst by 5 mV. The E\u003csub\u003eonset\u003c/sub\u003e and limited current density are higher than those of commercial Pt/C. The Fe-N-C 1100 whose electron transfer number is \u0026asymp;\u0026thinsp;4 between 0.2 and 0.8 V presented a four-electron route. At the same time, the Fe-N-C 1100 possesses excellent stability. Its half-wave potential changed slightly after 5000 cycles in 0.5M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and the relative current of Fe\u0026ndash;N\u0026ndash;C 1000 keep 93% retention after 20000s continuous operation, much higher than the 74.8% of Pt/C.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eKaixiang Li: Writing \u0026ndash; review \u0026amp; editing, Writing \u0026ndash; original draft, Visualization, Investigation, Formal analysis, Data curation, Conceptualization. Jinyu Zhao: Writing \u0026ndash; review \u0026amp; editing, Methodology, Validation.Ruipeng Yuan: Validation. Jiajun Chen: Validation.Huijun Li: Validation. Xiaomin Wang: Writing \u0026ndash; review \u0026amp; editing, Validation, Supervision, Project administration, Funding acquisition, Data curation, Conceptualization.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work has been supported by the National Natural Science Foundation of China (52072256, 52301282), Shanxi Province Science and Technology Program Unveiled Bidding Program (20201101016).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eYANG L, SHAO Z. Tunable and convenient synthesis of highly dispersed Fe\u0026ndash;Nx catalysts from graphene-supported Zn\u0026ndash;Fe-ZIF for efficient oxygen reduction in acidic media [J]. RSC Advances, 2019, 9(72): 42236-44.\u003c/li\u003e\n \u003cli\u003eZHANG H, HWANG S, WANG M, et al. Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation [J]. 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Size-controlled large-diameter and few-walled carbon nanotube catalysts for oxygen reduction [J]. Nanoscale, 2015, 7(47): 20290-8.\u003c/li\u003e\n \u003cli\u003eHUANG J-W, CHENG Q-Q, HUANG Y-C, et al. Highly Efficient Fe\u0026ndash;N\u0026ndash;C Electrocatalyst for Oxygen Reduction Derived from Core\u0026ndash;Shell-Structured Fe(OH)3@Zeolitic Imidazolate Framework [J]. ACS Applied Energy Materials, 2019, 2(5): 3194-203.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Oxygen reduction reaction, metal-organic framework, Fe-N-C electrocatalyst, hierarchical porous structure","lastPublishedDoi":"10.21203/rs.3.rs-5221463/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5221463/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFe-N-C is considered to be the most promising candidate for catalyzing oxygen reduction reaction (ORR), and its large-scale development is crucial to reducing the cost of proton exchange membrane fuel cells (PEMFCs). However, its simple and efficient synthesis still faces great challenges, and the microstructure changes in the pyrolysis process are not clear. Herein, we report a high-performance Fe-N-C catalyst, which is produced from the high temperature pyrolysis of Fe-doped ZIF-8 precursor. The effect of pyrolysis temperature on the specific surface area, porous structure and graphitization level of Fe-N-C catalyst is systematically studied. Eminently, Fe-N-C 1000, which was obtained via pyrolysis of Fe-ZIF-8 at 1000 °C, possesses highly dispersed Fe-N\u003csub\u003e4\u003c/sub\u003e active sites on the high surface area polyhedral, ensuring the high intrinsic activity. The simultaneous hierarchically ordered porous architecture provides a wealth of mass transfer channels to improve dynamic performance. It exhibits an outstanding ORR activity in acidic solution (E\u003csub\u003e1/2\u003c/sub\u003e of 0.791 V). High graphitization also enhances its corrosion resistance, showing superior stability (only change 20 mV after 5000 cycles in 0.5M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e). This work well demonstrates the importance of establishing the structural equilibrium of the catalyst under pyrolysis conditions for efficient ORR.\u003c/p\u003e","manuscriptTitle":"Fe/N/C Catalysts with Hierarchical Porous Structure Derived from Fe-doped ZIF-8 for Accelerated ORR Activity","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-27 09:32:25","doi":"10.21203/rs.3.rs-5221463/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-11-10T13:02:43+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-18T02:25:51+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-16T02:56:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"267050297123920253818055955659524338864","date":"2024-10-14T22:40:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"61799860293454667990717298841773249751","date":"2024-10-14T03:20:14+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-11T00:41:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"203093136554080681585865391668865444177","date":"2024-10-09T23:49:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"98845077159705463928372644647490406246","date":"2024-10-09T23:34:18+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-10-09T22:13:29+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-10-09T01:29:09+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-10-09T01:28:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2024-10-08T03:03:21+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d02bde39-d472-44ee-80da-de3e6eb3a454","owner":[],"postedDate":"November 27th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-11-30T10:38:22+00:00","versionOfRecord":[],"versionCreatedAt":"2024-11-27 09:32:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5221463","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5221463","identity":"rs-5221463","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}