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Article

Preparation of High-Efficiency Fe/N-Doped Carbon Catalysts Derived from Graphite Phase Carbon Nitride for Reduction of Oxygen

by
Yan Wang
1,2,3,
Wuxin Liu
1,2,3,
Rongzhe Wang
1,2,3,
Qing Wang
1,2,3,4,*,
Shaohua Luo
1,2,3,4,*,
Pengqing Hou
3,4,5,
Yahui Zhang
1,2,3,4,
Shengxue Yan
1,2,3,4,
Xin Liu
1,2,3,4 and
Jing Guo
1,2,3,4
1
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
2
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
3
Hebei Key Laboratory of Dielectric and Electrolyte Functional Material, Qinhuangdao 066004, China
4
School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
5
School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(4), 279; https://doi.org/10.3390/catal14040279
Submission received: 26 March 2024 / Revised: 13 April 2024 / Accepted: 17 April 2024 / Published: 19 April 2024
(This article belongs to the Section Electrocatalysis)
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Review Reports Versions Notes

Abstract

:
Fe/N-doped carbon (Fe-NC) is an excellent base-metal catalyst for use in an electrocatalytic oxygen reduction reaction (ORR) with high activity. In this paper, graphite phase carbon nitride (g-C3N4) was first obtained from the pyrolyzing of melamine, and then different proportions of FeCl3 were separately doped into g-C3N4 to further prepare the Fe-NC catalyst. The Fe-NC catalyst was applied in an ORR reaction, and the results show that the Fe-NC catalyst doped with 0.5 mmol FeCl3 possesses exceptional electrocatalytic performance, with an onset potential of 0.96 V and a half-wave potential of 0.81 V, which approaches that of a Pt/C catalyst. Meanwhile, the Fe-NC catalyst displays high stability and methanol resistance. The results supply a new way to prepare efficient ORR electrocatalysts.
Keywords:
Fe-N-C; g-C3N4; doping; ORR

1. Introduction

With the increasingly prominent fossil resource crisis and global environmental issues, the development of new renewable green alternative energy has become an important measure for coping with global energy shortages and climate change [1,2,3]. In recent years, metal-air batteries have attracted a lot of attention in the field of electrochemistry because of the simplicity of operation, sustainability, environmental friendliness, and high efficiency. Among the existing metal-air batteries, the rechargeable zinc-air battery (ZABs) is widely regarded as one of the energy storage and conversion systems with the most potential, owing to its high energy density, reliability, environmental friendliness, and superior safety [4]. Oxygen reduction reaction (ORR) and oxygen extraction reaction (OER) are the two basic electrode reactions involved in ZABs [5]. However, the sluggish kinetics of ORRs on air cathodes leads to the low energy conversion efficiency output performance and long-cycle stability of zinc-air batteries; therefore, it is imperative to rationally design OER and ORR electrocatalysts with high activity [6,7]. Currently, the commonly used Pt/C electrocatalyst exhibits advantages in electrochemical performance, but poor OER activity due to the oxidation of Pt at high overpotentials, the shortage of precious metal resources, and the increase in cost still cannot completely solve the problem of cathodes for zinc-air batteries [2,8,9,10]. For these reasons, the large-scale commercial construction and application of zinc-air batteries have always been seriously hindered. Therefore, developing efficient and stable base-metal catalysts is of great significance for improving the performance of zinc-air batteries [11].
A M-N-C catalyst formed by nitrogen-doped carbon-supported transition metal atoms (Fe, Co, etc.) is a class of highly efficient oxygen electrocatalysts [2,12,13,14,15]. Among them, the Fe-NC catalyst has attracted huge attention because of the activity of ORRs, such as a Pt-based catalyst [16,17,18,19]. Due to the uniform distribution of electron cloud density between C-C bonds, the ORR electrocatalytic activity and stability of the carbon materials are weakened. The N and Fe co-doping of carbon-based materials is beneficial as it can chang the electron cloud density around C atoms, polarize the C atoms near the heteroatoms, and improve the electrocatalytic activity of ORR catalysts [20]. Meanwhile, N atoms have a higher electronegativity than C atoms, resulting in easier coordination between Fe and N when forming the catalytic active center FeNx [21].
The high-temperature cracking of precursors containing metal Fe, N, and C is a traditional method for preparing Fe-NC [22,23,24,25]. In general, a high nitrogen content in the precursor is required in order to retain more nitrogen during the pyrolysis process and form highly nitrogen-doped carbon. Graphite phase carbon nitride(g-C3N4) is a typical carbon–nitrogen compound with a graphene-like lamellar structure. The poor electrical conductivity of g-C3N4 limits its direct application in the field of electrocatalysis. However, owing to its extremely high nitrogen content of up to 57.1 at%, g-C3N4 is an excellent precursor for synthesizing nitrogen-doped carbon materials. By doping iron salts, g-C3N4 can be converted into Fe and N co-doped electrocatalytic carbon-based materials.
In this paper, low-cost melamine is used to produce g-C3N4, followed by the addition of iron salts. The mixture is pyrolyzed at a high temperature in order to produce Fe-NC materials. Finally, Fe-NC is used as an electrocatalyst in the ORR reaction. The results show that Fe-NC exhibits outstanding electrocatalytic performance, approaching that of the commercial Pt/C catalyst. Meanwhile, Fe-NC also possesses an exceptional methanol resistance and long-term stability.

2. Results and Discussion

Figure 1 shows the preparation diagram of the Fe-NC catalyst. Melamine was first heated at 550 °C and converted into g-C3N4. Subsequently, the as-obtained g-C3N4 was mixed with different proportions of FeCl3·6H2O. Finally, the mixture was heated at 800 °C to obtain the Fe-NC catalyst. The Fe-NC catalysts prepared by adding different amounts of iron salts were marked as Fe-NC-x (x indicates the amount of iron salt added; for example, Fe-NC-0.1 indicates the addition of 0.1 mmol FeCl3·6H2O).
The SEM and TEM of g-C3N4 are analyzed and shown in Figure 2a,b, from which it can be observed that the g-C3N4 formed via the heat treatment of melamine exhibits a blocky structure, which consists of irregular two-dimensional lamellae stacked on top of each other. The elemental composition and content of g-C3N4 were investigated using XPS and the results are shown in Figure 2c. From the figure, it can be discovered that g-C3N4 has a N content of up to 44%, close to the content of C. The rich nitrogen content of the precursor is provides benefits when forming active centers. To better observe the morphology of nitrogen-doped carbon matrix-loaded Fe atoms, the SEM characterization of Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5 was performed, as shown in Figure 2d–f. From the figures, it can be seen that the lamellar g-C3N4 was transformed into elongated nanotubes carbon under the catalytic effect of Fe atoms [26], which is favorable for the improvement of the active site and mass transfer kinetics [27]. With the increase in the Fe content, the tubes gradually become clear and elongated. However, when the Fe doping amount reaches 1.5 mmol, the tubes gradually become blurred and decrease in number, which might be ascribed to the agglomeration triggered by the excessive Fe atoms, making the density of Fe single atoms decrease, and the catalytic performance weaken.
Figure 3a–c show the TEM characterization of Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5, respectively. From the images, it can be seen that the different doping ratios all form a folded tubular morphology. The Fe-NC-0.1 and Fe-NC-0.5 samples have a clear tubular morphology with clear walls, but the walls of the Fe-NC-1.5 sample become blurred and lack a clear boundary. In addition, black Fe nanoparticles are observed in all Fe-NC samples, some of which are encapsulated in intertwined tubes, and some are located in the outer tube walls. The size of the nanoparticles becomes progressively larger as the amount of iron doping increases, indicating increasingly severe Fe agglomeration. In addition, the holes in the tube are irregular and almost invisible. To determine the material structure and composition of the Fe-NC catalyst, the Fe-NC-0.5 sample was characterized using HRTEM and fast Fourier inverse transform (FFT). As can be seen in Figure 3d,e, the largest layer spacing of 0.346 nm should correspond to the (002) facets of graphite, the layer spacing of 0.201 nm is attached to the Fe3C substance, and the smallest layer spacing of 0.193 nm reflects the (110) facets of the α-Fe.
The elemental distribution scanning mapping of Fe-NC-0.5 (Figure 3f) reveals that both C and N are uniformly distributed throughout the sample, indicating the generation of N-doped carbon. Iron atoms are distributed inside nitrogen-doped carbon nanotubes, suggesting that the iron is doped in the carbon.
The XRD patterns of all the samples are shown in Figure 4a. From the XRD pattern of g-C3N4, it can be seen that there are two diffraction peaks at 13.2° and 27.4°, which belong to the (100) and (002) facets of the g-C3N4, respectively. After the addition of FeCl3·6H2O and the second calcination at 800 °C, a serial of diffraction peaks at 44.7°, 43.8°, 45.8°, and 26.2° appeared in the XRD spectrum. The strong peak at 44.7° represents diffraction from the Fe3C substance. The peak at 43.8° is generated by the diffraction of the (111) facet of hexagonal ε-Fe3N. The weak and broad peak at 26.2° might be the (002) crystal facet of carbon, and the peak at 45.8° should be the (110) crystal facet of Fe. In addition, some small weak peaks might be attributed to Fe3C, but it is debatable whether they are catalytically active or not [28]. According to the literature [21,29], through high-temperature treatment, FeCxNy or FeNx might also be formed due to the interactions between Fe, N, and C. In ORRs, FeCxNy or FeNx molecules with α-Fe and ε-Fe3N constitute the electrocatalytic active center [30,31].
The Raman spectra of pure g-C3N4 and Fe-NC catalysts are displayed in Figure 4b. From the figure, it can be seen that g-C3N4 shows only a high intensity noise line. All Fe-NC materials exhibit two strong characteristic peaks near 1350 cm−1 and 1580 cm−1, belonging to the disordered carbon bands (D-band) and graphitic carbon bands (G-band), respectively [22]. This indicates that g-C3N4 is converted to carbon under pyrolysis at 800 °C. The intensity area ratio (ID/IG) of the two bands reflects the disorder degree in the carbon structure [32]. The high ID/IG ratio implies a high degree of disorder in carbon. As the amount of Fe added increases, the ID/IG value of the Fe-NC material decreases from 1.06 to 0.92, indicating an increase in the graphitization degree of carbon. The reason for this is that the heterogeneous catalytic effect of Fe reduces the activation energy for the transition from the amorphous to the graphite phase, thus promoting an increase in the graphitization. In addition, higher ID/IG values indicate the presence of a large amount of defects in the samples caused by N doping, and the defects can then become the catalytic active center [33,34].
The specific surface area and pore structures of the three samples were detected utilizing a nitrogen adsorption/desorption technique. Figure 5 reveals the nitrogen absorption and desorption curves of the three samples and the corresponding pore diameter distribution. It can be seen from the figures that the specific surface areas of Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5 are 52.9, 89.2, and 41.6 m2 g−1, respectively. The high surface area can supply more active sites. In addition, the figure shows that the three samples make up the hierarchical distribution of micropores and mesopores, providing a favorable material transport channel for ORR reactions.
XPS was used to analyze the surface chemical elements and binding states of all samples (Figure 6). As displayed in Figure 6a, the total XPS spectra reveal the existence of C, N, Fe, and O elements in all of the samples, where the presence of O could be attributed to the exogenous oxygenated compounds adsorbed on the surface. As can be seen in Figure 6b, the Fe-NC-0.5 sample has the highest N and O content among all the samples. The C1s spectra of the Fe-NC samples in Figure 6c,f,i can be divided into four peaks, representing the telescopic vibrations of C=C/C-C (284.5 eV), C=N (285.3 eV), C-O (286.5 eV), and C=O (289.0 eV), respectively [22,31]. The relatively strong peak intensity corresponding to C=N suggests more N doping in the carbon. From Figure 6d,g,j, the N1s spectra can be divided into four peaks located at 398.2, 400.3, 401.3, and 404.0 eV. Among them, the peaks at 400.3, 401.3, and 404.0 eV represent pyrrole N, graphitized N, and quaternary N, respectively, whereas the peak situated at 398.2 eV represents the pyridine N and Fe-N [35,36,37]. According to the literature [38], the N group-modified conductive surfaces are able to provide binding sites because the active sites are generated on the surfaces via metal–nitrogen coordination. In addition, the ORR activity of N-doped catalysts is also affected by the content of pyridine and graphitized N. It is generally considered that pyridine N and graphitized N might promote oxygen-selective reduction through a four-electron mechanism [37,39,40]. Table 1 displays the nitrogen contents of all three catalysts. The contents of pyridine N and graphitized N in Fe-NC-0.5 accounted for 84.7% of the total peak content, slightly higher than those found in Fe-NC-0.1 and Fe-NC-1.5, which was conducive to the enhancement of active sites and electrical conductivity, thus promoting the ORR reaction. Figure 6e,h,k show the detailed valence states of Fe elements in Fe-NC compounds. The peaks situated at 706.8 eV and 707.3 eV belong to Fe3N and metallic Fe, respectively [29]. The peaks at 720.3 eV and 710.1 eV belong to the Fe2+ states pertaining to Fe 2p1/2 and Fe 2p3/2 [23]. The peak situated at 723.4 eV represents the Fe3+2p1/2, and another peak located at 712.3 eV indicates the Fe3+2p3/2 [41]. Since no iron oxides were discovered in XRD, the Fe2+ and Fe3+ should stem from the existence of FeCxNy and FeNx [42].
To analyze the ORR performances of all catalysts, cyclic voltammetry (CV) curves were operated in a three-electrode system using the rotating electrode method in an oxygen-free/oxygen-saturated 0.1 M KOH alkaline electrolyte, as displayed in Figure 7. In the oxygen-saturated electrolyte, all the fabricated electrocatalysts except g-C3N4 show obvious ORR peaks. Among them, the oxygen reduction peak of Fe-NC-0.5 is the most obvious, and the oxidation peak appears in the oxygen-saturated CV curve at about 0.9 V, indicating that the redox reactions occurred between Fe(II) and Fe(III) in the existence of O2. No ORR peaks were found in the oxygen-free saturated electrolyte, which also proved the catalyst’s capability of oxygen reduction. In addition, both the Fe-NC-0.5 and Fe-NC-1.5 samples show reversible and featureless wide dual-layer charging current profile characteristics under an oxygen-free medium, thus implying good electronic conductivity and charge diffusion [43].
The ORR performance of Fe-NC catalysts with different Fe doping levels in the oxygen-saturated 0.1 M KOH solution was further investigated using linear scanning voltammetry (LSV) on an RDE at 1600 rpm, and the electrochemical parameters related to the ORR performance are shown in Figure 8 and Table 2. As can be observed from the data in Table 2, Fe-NC-0.5 has the best electrocatalytic property, with an onset potential (Eonset) of 0.96 V, a half-wave potential (E1/2) of 0.81 V, and a limiting current density (Jd) of 5.97 mA cm−2, which is close to that of Pt/C. In the same conditions, the catalytic activities of Fe-NC-0.1 (Eonset = 0.92 V, E1/2 = 0.76 V, and Jd = 4.6 mA cm−2) and Fe-NC-1.5 (Eonset = 0.88 V, E1/2 = 0.77 V, and Jd = 4.14 mA cm−2) are relatively low. The results might prove the key role of metal Fe in the catalyst. Among these catalysts, F-NC-0.5 has the lowest surface nitrogen content, but shows the best ORR performance, probably because Fe plays a more critical catalytic role. The doping of Fe provides single-atom active sites, but the excessive doping of Fe might result in severe agglomeration, forming large-sized particles, which leads to a decrease in the density of Fe single atoms [44].
The LSV curves and K-L plots of the Fe-NC catalysts at different rotational speeds were conducted and displayed in Figure 9. From Figure 9a,c,e, it can be discovered that the current density enhances significantly when the rotational speed increases. The charge transfer numbers were calculated from the slope of the j−1 vs. ω−1 curves according to the Koutecky–Levich (K-L) equation. As revealed in Table 2, the charge transfer numbers of Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5 catalysts are 3.81, 3.90, and 3.89, respectively. The results indicate that these catalysts undergo four-electron reactions during oxygen reduction. Among them, Fe-NC-0.5 has the largest number of electrons transferred and the fastest four-electron reaction rate.
The reaction kinetics were further explored via the analysis of the polarization curves of all catalysts at 1600 rpm through the Tafel equation. As shown in Figure 10a,b, the Tafel slopes of Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5 are 93, 68, and 94 mV dec−1, respectively. The smaller the Tafel slope, the faster the current density can increase as the change in overpotential decreases. The smallest slope for Fe-NC-0.5 indicates its outstanding electrocatalytic kinetics. The stability of the Fe-NC-0.5 catalyst was detected in an oxygen-saturated 0.1 M KOH electrolyte (0.4 V at 1600 rpm). From Figure 10c, it can be seen that there is no significant change in the polarization curve of Fe-NC-0.5 after 5000 cycles, which confirms the outstanding stability of the prepared Fe-NC-0.5. To test the methanol resistance of the catalyst, the LSV curves of Fe-NC-0.5 in the solution with or without methanol are compared and displayed in Figure 10d. It can be seen that no significant differences occurred between the two LSV curves, thus indicating the excellent methanol tolerance of the Fe-NC-0.5 catalyst.
The electrochemical dual layer capacitance (Cdl) can be used to calculate the effective surface area involved in an electrochemical reaction, called the electrochemical active area (ECSA). In electrochemical reactions, a larger active surface area indicates the higher utilization of active sites. The linear fitting of the electrochemical double-layer capacitance is conducted via measuring the non-Faraday interval double-layer capacitance currents at different scan rates. The central potential used for the study is 0.1 V, with a potential range of 50 mV and scanning rates of 10 mV s−1, 20 mV s−1, 30 mV s−1, and 50 mV s−1. By plotting the charging current density difference (ΔJ = Janode − Jcathode) against the scan rate, the corresponding slope is twice that of Cdl, meaning the electrochemically active area can be represented by the corresponding slope. Figure 11a–d shows the CV curves of the four catalysts at different sweep speeds, which were linearly fitted to obtain Figure 11e. From Figure 11e, the slopes of Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5 are 1.35, 4.51, and 0.19 mF cm−2, respectively. The largest slope for Fe-NC-0.5 indicate a larger electrochemically active area, improving its catalytic activity.
The excellent ORR performance of the Fe-NC-0.5 catalyst could be ascribed to several reasons. Firstly, the N doping in the catalyst derived from g-C3N4 provides more surface defects and catalytically active sites, thus promoting the ORR reaction. In addition, by changing the electron cloud density, N activates the adjacent C atoms, increasing the electron transfer from metal atoms to carbon atoms and generating high catalytic activity. Secondly, the catalytic effect of Fe increases the graphitic degree of carbon materials, which is conducive to the improvement of electronic conductivity. Thirdly, the nanotube structures of carbon are beneficial for improving the mass transfer (such as O2, H2O, and intermediate products), which then enhances the reaction kinetics in the ORR reaction. Fourthly, the relatively high specific surface area also increases the exposure of the active site and thus enhances the catalytic activity. Finally, the formation of α-Fe, Fe3N, FeCxNy, or FeNx from the interactions between Fe, N, and C further enhances the active sites of the ORR reaction. In summary, the synergistic effect of multiple factors leads to excellent electrochemical performance of Fe-NC-0.5 catalysts.

3. Materials and Methods

3.1. Chemicals and Materials

Melamine (C3H6N6) was obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Ferric chloride hexahydrate (FeCl3·6H2O) and potassium hydroxide (KOH) were gained from Aladdin Bio-technology Co., Ltd., Shanghai, China. The Nafion solution (C9HF17O5S.5%) was purchased from DuPont, Wilmington, DE, USA, and the platinum carbon (Pt/C, 20%) was purchased from Johnson Matthey Company (London, UK). All chemical reagents utilized in this experiment were of an analytical grade. The entire experiment was carried out using deionized water.

3.2. Preparation of Fe-NC Catalysts

Firstly, melamine was heated to 550 °C under an argon atmosphere at an elevated rate of 5 °C/min and kept for 4 h; in turn, a light-yellow g-C3N4 powder was gained. Furthermore, 2 g g-C3N4 was mixed with 0.1, 0.5, and 1.5 mmol of FeCl3·6H2O, respectively, and then immersed in a beaker containing 80 mL of deionized water and 20 mL of ethanol solution, followed by drying at 80 °C. The mixture was heated at 800 °C in a tube furnace (Henan Fryer Instrument Co., Luoyang, China), and then held for 2 h to obtain the Fe-NC catalyst. The Fe-NC catalysts with different Fe doping amounts were named Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5, respectively.

3.3. Characterization of Materials

The microstructure and morphology of the samples were studied using scanning electron microscopy (SEM, SUPRA-55, Carl Zeiss AG, Oberkochen, German) and transmission electron microscopy (TEM, FEI-Tecnai G2 F20, Hillsboro, OR, USA). The phase structure was investigated via an X-ray powder diffraction (XRD, model: Smartlab X-ray diffractometer (Rigaku, Akishima, Japan) with Cu-Ka radiation, wavelength: λ = 0.15406 nm, scanning: angle 2θ ranging from 10 to 80°) technique. The phase structures of the samples were further detected via the use of Raman spectroscopy (HORIBA Scientific Lab RAM HR Evolution, Kyoto, Japan). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, ThermoFischer, Waltham, MA, USA) was utilized to investigate the elemental composition and valence states of the sample surfaces, and the measured data were analyzed using Thermo Avantage 5.9931.

3.4. Electrochemical Measurements

The electrochemical property of the catalyst in the ORR was tested on a CHI760E electrochemical workstation (CH Instruments, Bee Cave, TX, USA) with a three-electrode system. Graphite is a counter electrode, with reference electrodes of a modified glassy carbon disk electrode (RDE, 0.19625 cm−2) and a saturated calomel electrode (SCE). The working electrode was in argon- and oxygen-saturated 0.1 M KOH electrolyte. The mixture of the 2 mg Fe-NC catalyst, 100 μL deionized water, 95 μL anhydrous ethanol, and 5 μL of Nafion solution (5 wt%) was sonicated for 2 h. Subsequently, 10 μL of the mixture (at a concentration of about 0.5 mg cm−2) was dropped onto the working electrode. The electrolyte was filled with oxygen or argon for 30 min before testing. Cyclic voltammetry (scanning speed of 50 mV s−1, voltage range from −1.0 V to 0.2 V) and linear scanning voltammetry (scanning speed of 10 mV s−1, rotational speed from 400 to 1600 rpm) were then tested on the RDE. The ORR performance of commercial 20% Pt/C electrodes was also tested in order to compare with the performance of the Fe-NC catalysts. The LSV curves were compared before and after 5000 cycles to assess the durability of the catalyst (voltage 0.4 V, speed 1600 rpm). LSV was performed in 0.1 M KOH + 3 M methanol solution to assess the tolerance of the catalyst to methanol. The charge transfer number of the Fe-NC catalyst can be calculated using the Koutecky–Levich equation [29,45]:
1 J = 1 J L + 1 J K = 1 B ω 1 / 2 + 1 J K
B = 0.62 n F C 0 D 0 2 / 3 υ − 1 / 6
where J represents the current density, JL denotes the diffusion current density, JK stands for the kinetic current density, ω represents the rotation rate of the RDE, F is Faraday’s constant (96,485 C mol−1), C0 is the volumetric concentration of O2 in oxygen-saturated 0.1 M KOH, D0 is the diffusion coefficient of O2, υ denotes the kinetic viscosity, and n is the number of charge transfers that can be calculated based on the slope of the K-L plot.

4. Conclusions

In this paper, g-C3N4 was first synthesized with pyrolyzing melamine and then converted to a series of Fe-NC catalysts through mixing with FeCl3·6H2O and a calcination process. Among them, the Fe-NC-0.5 catalyst exhibits outstanding performance with Eonset potential of 0.96 V and E1/2 potential of 0.81 V, respectively, which are significantly higher than those of the Fe-NC-0.1 and Fe-NC-1.5 catalysts. In addition, Fe-NC-0.5 exhibits high catalytic stability and an excellent methanol tolerance. These results express that the Fe-NC-0.5 catalyst is a prospective candidate material for ORR electrocatalysis. The conspicuous ORR property of Fe-NC-0.5 is ascribed to its rich surface defects derived from N doping, nanotube structures, the increased degree of graphitization, and the formation of multiple active centers, such as α-Fe, ε-Fe3N, FeCxNy, or FeNx.

Author Contributions

Conceptualization, W.L., Q.W. and S.L.; Methodology, Y.Z.; Validation, R.W.; Formal analysis, P.H.; Investigation, S.Y.; Writing—original draft, Y.W. and W.L.; data curation, W.L.; Writing—review & editing, Q.W.; Resources, X.L. and J.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 52104291, 52274295 and 52304325), the Natural Science Foundation of Hebei Province (No. E2021501007, E2021501029, E2022501029, E2022501028, and E2023501016), the Natural Science Foundation-Steel, the Iron Foundation of Hebei Province (No. E2022501030), the Science and Technology Project of Hebei Education Department (No. ZD2022158 and QN2024238), the Central Guided Local Science and Technology Development Fund Project of Hebei Province (No. 226Z4401G), the Fundamental Research Funds for the Central Universities (N2223010, N2223009 and N2323013), Performance subsidy fund for Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province (No. 22567627H). Qinhuangdao Science and Technology Research and Development Plan Project (No. 202302B046 and 202302B006).

Data Availability Statement

Data available on request due to restrictions, e.g., privacy or ethical. The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Catalysts 14 00279 g001
Figure 1. The preparation diagram of the Fe-NC catalyst.
Figure 1. The preparation diagram of the Fe-NC catalyst.
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Figure 2. (a) SEM and (b) TEM image of g-C3N4. (c) XPS elemental content diagram of g-C3N4. (d–f) SEM image of Fe-NC.
Figure 2. (a) SEM and (b) TEM image of g-C3N4. (c) XPS elemental content diagram of g-C3N4. (d–f) SEM image of Fe-NC.
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Figure 3. (a–c) TEM image of the Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5, respectively. (d) HRTEM of the Fe-NC-0.5. (e) Line scan of the FFT. (f) TEM image of the Fe-NC-0.5 and homologous elemental maps of C, N, and Fe, respectively.
Figure 3. (a–c) TEM image of the Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5, respectively. (d) HRTEM of the Fe-NC-0.5. (e) Line scan of the FFT. (f) TEM image of the Fe-NC-0.5 and homologous elemental maps of C, N, and Fe, respectively.
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Figure 4. (a) XRD patterns, (b) Raman spectra of the samples.
Figure 4. (a) XRD patterns, (b) Raman spectra of the samples.
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Figure 5. (a–c) BET images of the Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5, respectively.
Figure 5. (a–c) BET images of the Fe-NC-0.1, Fe-NC-0.5, and Fe-NC-1.5, respectively.
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Figure 6. (a) Full XPS spectra of three catalysts and g-C3N4. (b) Elemental change diagrams of three catalysts. (c,f,i) C1s, (d,g,j) N1s, and (e,h,k) Fe 2p XPS spectra of three catalysts.
Figure 6. (a) Full XPS spectra of three catalysts and g-C3N4. (b) Elemental change diagrams of three catalysts. (c,f,i) C1s, (d,g,j) N1s, and (e,h,k) Fe 2p XPS spectra of three catalysts.
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Figure 7. (a) CV curves of g-C3N4. (b–d) CV curves of Fe-NC-0.1, Fe-NC-0.5, Fe-NC-1.5, respectively.
Figure 7. (a) CV curves of g-C3N4. (b–d) CV curves of Fe-NC-0.1, Fe-NC-0.5, Fe-NC-1.5, respectively.
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Figure 8. LSV curves of five catalysts.
Figure 8. LSV curves of five catalysts.
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Figure 9. (a,c,e) LSV curves of three F-NC catalysts measured from 400–1600 rpm. (b,d,f) Charge transfer numbers and K-L plots.
Figure 9. (a,c,e) LSV curves of three F-NC catalysts measured from 400–1600 rpm. (b,d,f) Charge transfer numbers and K-L plots.
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Figure 10. (a) Tafel plots of three F-NC catalysts and Pt/C obtained from the LSV at 1600 rpm. (b) Tafel slope histogram of three F-NC catalysts and Pt/C. (c) LSV curves of the first cycle and 5000 cycles later. (d) LSV curves of the F-NC-0.5 both with and without methanol.
Figure 10. (a) Tafel plots of three F-NC catalysts and Pt/C obtained from the LSV at 1600 rpm. (b) Tafel slope histogram of three F-NC catalysts and Pt/C. (c) LSV curves of the first cycle and 5000 cycles later. (d) LSV curves of the F-NC-0.5 both with and without methanol.
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Figure 11. (a–d) ECSA curve comparisons of four catalysts. (e) The pattern is based on the difference in the charging current density and scanning rate.
Figure 11. (a–d) ECSA curve comparisons of four catalysts. (e) The pattern is based on the difference in the charging current density and scanning rate.
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Table 1. Different peak contents of C1s by XPS.
Table 1. Different peak contents of C1s by XPS.
SamplesQuarternary-N (%)Graphitic-N (%)Pyrrolic-N (%)Pyridinic-N (%)
0.1 Fe7.9722.5913.6955.74
0.5 Fe8.4228.276.8856.43
1.5 Fe6.7127.3711.7254.21
Table 2. ORR properties of different catalysts.
Table 2. ORR properties of different catalysts.
SamplesOnset Potential
(Eonset V)		Half-Wave Potential
(E1/2 V)		Limiting Current Density
(mA cm−2)		Electron Transfer Number
(n)
0.1 Fe0.920.764.63.81
0.5 Fe0.960.815.973.90
1.5 Fe0.880.774.143.89
Pt/C0.980.845.48—
g-C3N40.69—1.4—
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Wang, Y.; Liu, W.; Wang, R.; Wang, Q.; Luo, S.; Hou, P.; Zhang, Y.; Yan, S.; Liu, X.; Guo, J. Preparation of High-Efficiency Fe/N-Doped Carbon Catalysts Derived from Graphite Phase Carbon Nitride for Reduction of Oxygen. Catalysts 2024, 14, 279. https://doi.org/10.3390/catal14040279

AMA Style

Wang Y, Liu W, Wang R, Wang Q, Luo S, Hou P, Zhang Y, Yan S, Liu X, Guo J. Preparation of High-Efficiency Fe/N-Doped Carbon Catalysts Derived from Graphite Phase Carbon Nitride for Reduction of Oxygen. Catalysts. 2024; 14(4):279. https://doi.org/10.3390/catal14040279

Chicago/Turabian Style

Wang, Yan, Wuxin Liu, Rongzhe Wang, Qing Wang, Shaohua Luo, Pengqing Hou, Yahui Zhang, Shengxue Yan, Xin Liu, and Jing Guo. 2024. "Preparation of High-Efficiency Fe/N-Doped Carbon Catalysts Derived from Graphite Phase Carbon Nitride for Reduction of Oxygen" Catalysts 14, no. 4: 279. https://doi.org/10.3390/catal14040279

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