# Effect of Posttreatment on the Catalytic Performances of Fe-N-C for Oxygen Reduction Reactions

Dong-Gun Kim, Injoon Jang, Yeonsun Sohn, Eunhye Joo, Chanil Jung, Nam

Dong Kim, Jae Young Jung, Vinod K. Paidi, Kug-Seung Lee, Pil Kim, et al.

► To cite this version:

Dong-Gun Kim, Injoon Jang, Yeonsun Sohn, Eunhye Joo, Chanil Jung, et al. Effect of Posttreatment on the Catalytic Performances of Fe-N-C for Oxygen Reduction Reactions. International Journal of Energy Research, 2023, 2023, pp.1-12. 10.1155/2023/3265915. hal-05087937

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Distributed under a Creative Commons Attribution 4.0 International License
Research Article

Effect of Posttreatment on the Catalytic Performances of Fe-N-
C for Oxygen Reduction Reactions

Dong-gun Kim,<fnref n="1" /> Injoon Jang,<fnref n="2" /> Yeonsun Sohn,<fnref n="1" /> Eunhye Joo,<fnref n="1" /> Chanil Jung,<fnref n="1" />
Nam Dong Kim ID,<fnref n="3" /> Jae Young Jung,<fnref n="3" /> Vinod K. Paidi ID,<fnref n="4" /> Kug-Seung Lee ID,<fnref n="5" /> Pil Kim ID,<fnref n="1" />
and Sung Jong Yoo ID<fnref n="2" />,<fnref n="6" />,<fnref n="7" />

Correspondence should be addressed to Kug-Seung Lee; lks3006@postech.ac.kr, Pil Kim; kimpil1@jbnu.ac.kr, and Sung Jong Yoo; ysj@kist.re.kr

Received 30 November 2022; Revised 9 January 2023; Accepted 3 March 2023; Published 30 March 2023

Academic Editor: Denis Osinkin

Copyright © 2023 Dong-gun Kim et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.

Herein, we report the significance of posttreatment in the design of high-performance Fe-N-C-type catalysts for oxygen reduction reaction (ORR). Enhancing the catalytic performance of Fe-N-C requires a postprocess for removing the aggregated iron species formed during high-temperature pyrolysis since they have a negative effect on ORR. We were able to obtain a catalyst precursor for the Fe-N-C reaction via pyrolysis with iron-adsorbed 1,8-diaminonaphthalene (FeDN), which was then treated with a concentrated HCl solution for the removal of the aggregated iron species produced. The HCl-treated FeDN was subsequently annealed at various temperatures to investigate the effect of the annealing process on the physical properties and ORR performance. The annealing temperature was a critical factor affecting the residual contents of impurities, as well as the active component and the electronic state of nitrogen. We also examined the influence of different types of acid (HCl, H₂SO₄, and HNO₃) on the catalytic performance. The type of acid used for removing aggregated iron species had an impact on both the Fe contents and the relative nitrogen species content. Among the catalysts tested, the catalyst prepared by annealing the HCl-treated FeDN at 700°C had the best ORR performance. Although its initial ORR performance in acidic conditions was lower than that of a commercial Pt/C, it was more durable. In alkaline conditions, it delivered a comparable initial ORR performance to Pt/C and also exhibited higher durability.

**1. Introduction**

The oxygen reduction reaction (ORR), which occurs at the
cathode of electrochemical energy conversion devices, such
as fuel cells, is known to be a kinetically sluggish reaction
and one of the critical steps determining the overall perfor-
mance of such devices [1-4]. Therefore, using an electrode

catalyst that can increase the ORR rate is crucial for improv-
ing energy conversion device performance. Although Pt-
based catalysts are currently employed for ORR because of
their high activity and durability, their high cost and rarity
hinder the widespread applications of fuel cell systems
[5-8]. As a result, research efforts have been made to
develop inexpensive nonprecious metal-based catalysts that

<footnote n="1">¹School of Chemical Engineering, School of Semiconductor and Chemical Engineering, Jeonbuk National University, Clean Energy Research Center, Jeonju 54896, Republic of Korea</footnote>

<footnote n="2">²Hydrogen · Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea</footnote>

<footnote n="3">³Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Wanju 55324, Republic of Korea</footnote>

<footnote n="4">⁴EuropeanSynchrotron Radiation Facility, Grenoble 38043, Cedex 9, France</footnote>

<footnote n="5">⁵Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea</footnote>

<footnote n="6">⁶Graduate School of Energy & Environment, Korea University, Seoul 02841, Republic of Korea</footnote>

<footnote n="7">⁷KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul 02447, Republic of Korea</footnote>
function similarly to Pt-based catalysts in terms of ORR. Among the nonprecious catalysts studied, base metals such as Fe and Co-coordinated nitrogen and carbon composites (MNC) have been reported to have high ORR performances that can potentially enable the replacement of Pt-based catalysts [9–15].

Most MNC catalysts were prepared by pyrolysis with catalyst precursors containing metal, nitrogen, and carbon components [16–19]. During pyrolysis, the catalyst precursors are carbonized into carbon to enhance electrical conductivity, and the interaction between metal and nitrogen is simultaneously enhanced, resulting in the generation of various types of active sites [20, 21]. In addition to being active components themselves, metallic species of catalyst precursors are crucial for producing highly active sites during high-temperature pyrolysis [22–24]. For this reason, the catalyst precursors with excessive metal contents are usually preferred for increasing the concentration of highly active sites in the resultant MNC catalysts. It should be noted that metal aggregation occurs easily during high-temperature pyrolysis, which has a negative impact on the catalytic performance by decreasing the uniformity of the active site and the interfacial concentration of metal–nitrogen bonding [25–27]. In this case, an etching process with highly concentrated acids would be required for the removal of metal aggregation to improve the catalytic performance [28, 29]. It should also be noted that the interaction between metal and nitrogen would be loosened, and the active components are deteriorated during the etching process, resulting in poor ORR performance. The additional activation processes were reported to be effective in improving the ORR performance of acid-etched MNC catalysts [30, 31]. For example, MNC catalysts prepared by pyrolysis with Fe and Co-supported aniline exhibited enhanced ORR performance when agglomerated metal particles were etched from the pyrolyzed samples and further activated at high temperatures [25–27]. Although such activation can enhance the ORR performance of MNC catalysts, there has not been a systematic investigation into why the acid-etched samples exhibit inferior activities and how their catalytic performance can be enhanced by a second activation.

In this work, we investigated the effect of a posttreatment, which includes acid etching and a second heat treatment, on the ORR performance of MNC catalysts. The catalyst precursor was simply prepared by reacting 1,8-diaminonaphthalene (DN) with Fe ions. The pyrolyzed samples were treated with acids before additional heating. Several types of acids were examined to determine the effect of acid properties on the physical properties and ORR performance of the prepared catalysts. In particular, different temperatures were applied to the second activation to better understand how it affected the structure of active sites that were crucial to the catalytic performance.

## 2. Experimental

**2.1. Preparation of FeDN-Derived Catalysts.** The catalyst precursor, FeDN, was prepared by pyrolysis with a Fe-DN composite at 700°C under an N₂ atmosphere for 1 h. For the

preparation of the Fe-DN composite, 1.12 g of DN (Alfa Aesar) was dissolved in 150 ml of ethanol (Samchun, 96%). The DN-dissolved solution was combined with 0.16 M of iron (II) chloride solution (Sigma-Aldrich). The temperature of the solution was increased to 50°C and maintained for 15 h. To produce the Fe-DN composite, a brown solid material was obtained by filtering and washing with ethanol before drying at 80°C for 12 h. To produce FeDN-AT, FeDN was treated in 5 M HCl at 90°C for 12 h before being filtered and washed with copious water. To produce FeDN-AT-X (X stands for annealing temperature: 300, 500, and 700°C), FeDN-AT was annealed at a certain temperature for 1 h under an N₂ stream. In addition, FeDN-H₂SO₄-700 and FeDN-HNO₃-700 were prepared using the same procedure as for FeDN-AT-700, with the exception that concentrated H₂SO₄ and HNO₃, respectively, were employed.

**2.2. Characterizations.** The crystal structure was confirmed by powder X-ray diffraction (XRD, MAX-2500). Contents of metal and nitrogen were measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Perkin Elmer) and element analysis (EA, FlashSmart). The configuration of carbon was confirmed by Raman spectroscopy (Nanophoton RAMANTouch). The morphology was analyzed by transmission electron microscopy (TEM, Hitachi H-7650). The electronic state of the element was analyzed by X-ray photoelectron spectroscopy (XPS) (Nexsa XPS system, Thermo Fisher Scientific).

**2.3. Electrochemical Characterizations.** The electrochemical behavior of the prepared catalyst was measured using a three-electrode system. The carbon rod was used as a counter electrode, while Ag/AgCl (3 M $Cl^{-}$ saturated) was used as a reference electrode. The working electrode was prepared by coating a catalyst ink on a rotating ring disk electrode (RRDE, Pine Research Instrumentation Inc.). The catalyst ink was prepared by dispersing 10 mg of the catalyst in a mixed solution containing water, Nafion ionomer (Sigma-Aldrich), and isopropyl alcohol (Samchun). The redox behavior of the catalyst was examined by cyclic voltammetry (CV), which was conducted in an N₂-saturated HClO₄ or KOH solution. To evaluate the ORR performance, linear sweep voltammetry (LSV) was carried out in an O₂-saturated HClO₄ or KOH solution at a certain rotation speed of the electrode. The durability of the catalyst was examined by evaluating the ORR performance after conducting an accelerated degradation test (ADT). For the ADT, 5000- and 10000-potential cycles were carried out in an O₂-saturated HClO₄ and KOH solution, respectively.

## 3. Results and Discussion

We first examined the effect of the second activation temperature on the structure and catalytic performance of the prepared catalysts. The XRD patterns of FeDN, FeDN-AT, and FeDN-AT-X are shown in Figure 1. The peak at 26°, which corresponds to the (002) basal plane of graphite, was present in all samples [32]. This was probably due to the catalytic action of iron species that were formed during the heat
<figure><img src="image_2.png" /><figcaption>FIGURE 1: XRD patterns of FeDN, FeDN-AT, and FeDN-AT-X.</figcaption></figure>

treatment, which accelerated the graphitization of the carbon precursor. In addition to the graphitic carbon peak, other peaks were observed in the samples. For FeDN, the diffraction of metallic Fe occurred at 44.5°, indicating that Fe ions coordinated to DAN were reduced to metallic Fe. The additional peaks at 42.8°, 43.6°, 45.7°, and 49° could be attributed to the diffractions of Fe₃C [32, 33]. After HCl etching, the intensities of characteristic peaks for metallic Fe and Fe₃C dramatically decreased owing to the dissolution of most of these species. The diffraction patterns of FeDN-AT-X were similar to those of FeDN-AT, though the peak intensities for the iron species were slightly higher. This implies that the crystalline properties of the samples remained unchanged after the second activation.

Figure 2 shows the TEM images of the prepared samples. FeDN displays a carbon with a spherical shell-shaped structure that is dispersed with both small and large irregular-shaped particles. As confirmed by XRD, these particles are most likely to be metallic Fe and Fe₃C. For FeDN-HT, most of the small particles dispersed on FeDN were unnoticed owing to the dissolution under acidic conditions, and only a few large particles encapsulated in carbon were observed. This is because the small particles exposed on the surface were easily removed by acid etching, while some of the large particles were shielded from acid leaching by carbon shells. The morphology of FeDN-AT-X was similar to that of FeDN-AT, indicating that no appreciable change in the morphology of carbon and iron species was induced as a result of the second activation. As can be seen from the HR-STEM in Figure 2(f), in the FeDN-AT-700 sample, along with iron-species nanoparticles, Fe single atoms are also uniformly distributed on the carbon surface. In addition, in the HR-TEM image, (Figure 2(h)) of the enlarged nanoparticles in Figure 2(g), lattice fringes according to the regular arrangement of atoms are clearly shown. Moreover, an inverse fast Fourier transform (FFT) image of the FFT

patterns (inset in Figure 2(g)) is shown in Figure 2(i), which reveals the arrangement of atoms more clearly. A total of three lattice spacings are observed, with values of 2.40 Å, 2.03 Å, and 1.95 Å, respectively. Through the reference XRD pattern of Fe₃C (PDF# 65-2411), it was confirmed that these three values are similar to the interspacing values of the (121), (220), and (112) planes of Fe₃C, respectively, which demonstrates that the nanoparticles contained in FeDN-AT-700 are Fe₃C. The confirmation of Fe₃C nanoparticles through HR-TEM is consistent with the XRD results, and it is expected that other catalysts prepared through the same synthesis process also contain Fe₃C nanoparticles. In addition, several carbon layers surrounding the Fe₃C nanoparticle can be clearly observed through the HR-TEM image in Figure S1. As a result, TEM analysis indicates that Fe₃C nanoparticles surrounded by a carbon layer and Fe single atoms coexist in the synthesized catalyst.

The degree of defect present in the samples was confirmed by Raman spectroscopy. As shown in Figure S2, all the samples exhibited two peaks at 1569.4 cm⁻¹ (G-band) and 1332.7 cm⁻¹ (D-band), which resulted from the sp₂ and sp₃ configurations of carbon [34, 35]. A significant change in the two bands was observed in FeDN-AT; the D-band became much broader than that of FeDN. This is probably due to the change in carbon configuration induced by the dissolution of the iron species. The intensity ratio of the D-band to the G-band was a little different among the FeDN-AT-X samples, indicating that the defect sites in these samples have similar contents.

The surface areas and pore volumes of FeDN, FeDN-AT, and FeDN-AT-X are listed in Table S1. The surface area exhibited a similar trend to the pore volume. When compared to FeDN, FeDN-AT, and FeDN-AT-X have higher surface areas owing to the removal of aggregated Fe species. The reactivation significantly affected the surface area. The drastic change in the surface area occurred when FeDN-AT was reheated. The increased surface area of FeDN-AT-X was due to the removal of Cl⁻ adsorbed on FeDN. The surface area of FeDN-AT-X slightly decreased as the reheating temperature increased.

The oxidation states of the surface species were analyzed by XPS. Figure 3(a) shows the N1s photoelectron peaks of the FeDN catalysts. All the examined catalysts had two peaks that could be divided into various nitrogen species with different oxidation states. They included pyridinic N (at around 398.3 eV), Fe-N (at around 399.3 eV), pyrrolic N (at around 400.5 eV), graphitic N (at around 401.5 eV), and oxidized N (at around 402.4 eV). The binding energy and relative intensity of each N species are summarized in Table 1. FeDN was deconvoluted to have the highest relative intensities for the graphitic N and Fe-N species as a result of the pyrolysis of the Fe-DAN composite. For FeDN-AT, the intensities of the N species were largely attenuated owing to the destruction of the Fe-N bond by acid treatment, and those of pyridinic, pyrrolic, and oxidized N species increased. During acid treatment, electronegative impurities, such as oxygen and chlorine, are prone to adsorb, leading to the formation of oxidized N. As observed on FeDN-AT-X, the intensity of oxidized N gradually decreased as the reheating
<figure><img src="image_3.png" /><figcaption>FIGURE 2: TEM images of (a) FeDN, (b) FeDN-AT, (c) FeDN-AT-300, (d) FeDN-AT-500, and (e) FeDN-AT-700, (f-h) HR-TEM images of FeDN-AT-700, (i) IFFT image based on the FFT pattern of the insert in (g).</figcaption></figure>

temperature increased. This indicates that the adsorbed electronegative elements were gradually desorbed as the reheating temperature increased. The intensities of Fe-N, pyridinic, and pyrrolic N species increased as the reheating temperature increased, indicating that the interaction between Fe and nitrogen was enhanced as a result of the reheating process.

Figure 3(b) shows the Cl 2p photoelectron spectra of FeDN-AT and FeDN-AT-X. FeDN-AT had peaked at 198.1 eV, 200.2 eV, and 201.9 eV, which could be assigned to Cl 2p<sub>3/2</sub> (Cl<sup>-</sup>), Cl 2p<sub>3/2</sub> (C-Cl<sup>-</sup>), and Cl 2p<sub>1/2</sub> (Cl<sup>-</sup>), respectively. FeDN was treated with high concentrations of HCl to dissolve the aggregated Fe species and produce FeDN-AT. Therefore, the Cl species on FeDN-AT probably resulted from the adsorption of chloride during the acid treatment. The intensity of the Cl species gradually decreased as the reheating temperature increased. The Cl species were removed when the reheating temperature was above 500°C, as observed in FeDN-AT-500 and FeDN-AT-700.

The XPS peaks of Fe 2p are presented in Figure 3(c). When compared to other samples, FeDN exhibited the highest intensity for the Fe 2p photoelectron peak, representing the highest concentration of Fe, as shown in Table S2. Interestingly, although the FeDN-AT and FeDN-AT-X samples had a similar concentration of Fe species, the peak intensity of the Fe species for FeDN-AT was hardly observable, whereas the FeDN-AT-X samples exhibited Fe 2p photoelectron peaks (Table S2). In the case of FeDN-AT, the large portion of Fe species on the surface of FeDN was effectively eliminated during the acid treatment, and residual Fe species were possibly encapsulated by impurities, such as Cl, suppressing the photoelectron peaks for Fe species. As the reheating temperature increased, the surface concentration of the Cl species decreased, exposing the Fe species on the surface of FeDN-AT-X. This describes the Fe 2p intensity trend in the FeDN-AT-X sample data.
<figure><img src="image_4.png" /><figcaption>FIGURE 3: (a) N1s XPS spectra, (b) Cl 2p, and (c) Fe 2p of FeDN catalysts.</figcaption></figure>

<table><caption>TABLE 1: N1s XPS data of FeDN catalysts.</caption><thead><tr><th></th><th></th><th>Pyridinic-N</th><th>Fe-N</th><th>Pyrrolic-N</th><th>Graphitic N</th><th>Oxidized N</th></tr></thead><tbody><tr><td rowspan="2">FeDN</td><td>Binding energy (eV)</td><td>398.3</td><td>399.3</td><td>400.7</td><td>401.5</td><td>402.2</td></tr><tr><td>Relative intensity (%)</td><td>23.7</td><td>14.0</td><td>27.2</td><td>21.9</td><td>13.2</td></tr><tr><td rowspan="2">FeDN-AT</td><td>Binding energy (eV)</td><td>389.4</td><td>399.3</td><td>400.6</td><td>401.5</td><td>402.6</td></tr><tr><td>Relative intensity (%)</td><td>25.0</td><td>4.8</td><td>38.8</td><td>5.0</td><td>26.4</td></tr><tr><td rowspan="2">FeDN-AT -300</td><td>Binding energy (eV)</td><td>398.3</td><td>399.5</td><td>400.5</td><td>401.1</td><td>402.5</td></tr><tr><td>Relative intensity (%)</td><td>31.7</td><td>9.3</td><td>29.5</td><td>12.8</td><td>16.7</td></tr><tr><td rowspan="2">FeDN-AT -500</td><td>Binding energy (eV)</td><td>398.2</td><td>399.3</td><td>400.5</td><td>401.1</td><td>402.4</td></tr><tr><td>Relative intensity (%)</td><td>33.1</td><td>12.4</td><td>27.0</td><td>17.3</td><td>10.2</td></tr><tr><td rowspan="2">FeDN-AT -700</td><td>Binding energy (eV)</td><td>398.2</td><td>399.1</td><td>400.5</td><td>401.0</td><td>402.4</td></tr><tr><td>Relative intensity (%)</td><td>32.9</td><td>12.3</td><td>25.0</td><td>20.2</td><td>9.6</td></tr></tbody></table>

Figure 4(a) shows the CVs obtained using FeDN-AT and FeDN-AT-X. The electrochemical behavior of all the samples exhibited a similar trend, with oxidation peaking at 0.845 V in the anodic sweep and reduction peaking at

around 0.76 V in the cathodic sweep. These were assigned to the conversion of Fe²⁺ to Fe³⁺ and Fe³⁺ to Fe²⁺, respectively [36, 37]. The CV area corresponding to the capacitance varied with the reheating temperature. FeDN-AT had
<figure><img src="image_5.png" /><figcaption>FIGURE 4: (a) CV, (b) ORR LSV, (c) H₂O₂ yield, and (d) electron transfer number of FeDNHT-AT-700HT catalysts. The electrochemical behavior of the prepared catalyst was measured using a three-electrode system. The carbon rod was used as a counter electrode, while Ag/AgCl (3 M Cl⁻ saturated) was used as a reference electrode. CV and LSV were conducted in an N₂-saturated KOH and an O₂-saturated HClO₄ solution, respectively.</figcaption></figure>

the lowest capacitance. The capacitance of FeDN-AT-X decreased as the reheating temperature increased. The capacitance trend was similar to that of the surface area (Table. S1), as the capacitive charge was proportional to the surface area.

LSV measurements in an HClO₄ solution that was O₂-saturated were used for the evaluation of the ORR performance of the prepared catalysts. As shown in Figure 4(b), the half-wave potential and the limiting current varied with reheating temperature. FeDN-AT and FeDN-AT-300 exhibited lower limiting currents than the other samples, indicating poor ORR activity. When compared to FeDN-AT, FeDN-AT-300 exhibited higher performance in terms of half-wave potential. The FeDN-AT-500 and FeDN-AT-700 delivered limiting currents comparable to a commercial Pt/

C, although they have lower onset and half-wave potentials. Among the catalysts examined, FeDN-AT-700 exhibited the highest ORR performance in terms of limiting and half-wave potentials. The catalytic performance of FeDN-AT and FeDN-AT-X was further evaluated by calculating the amount of hydrogen peroxide (H₂O₂ yield) produced during the ORR. Since H₂O₂ is produced via two electron transfers, a high yield of H₂O₂ indicates poor ORR performance. As shown in Figure 4(c), the H₂O₂ yield increased in the following order: FeDN-AT-700 < FeDN-AT-500 < FeDN-AT-300 < FeDN-AT, and this was consistent with the ORR performance trend described in terms of half-wave potential and limiting current.

As discussed above, all the catalysts prepared in this study contain Fe₃C nanoparticles surrounded by carbon
<figure><img src="image_6.png" /><figcaption>FIGURE 5: XANES spectra of acid-treated and reheated samples.</figcaption></figure>

layers and Fe single atoms. In the case of these catalysts, it has been demonstrated by many studies that both Fe species can act as active sites for ORR [38–41]. Therefore, the ORR performance trend of prepared catalysts having the same active site can be attributed to the contents and oxidation states of active components (i.e., nitrogen and iron species). As confirmed by EA (Table S2), all samples had similar nitrogen contents, although that of FeDN-AT-700 slightly decreased. Since the ORR performance of FeDN-AT-700 was the highest among the catalysts examined, it can be deduced that the total concentration of nitrogen is not a critical factor determining the ORR performance of FeDN-AT and FeDN-AT-X. On the other hand, the oxidation state of nitrogen is a crucial factor that influences ORR performance. Pyridinic N and Fe-N species, as characterized by XPS, are known to be more active than other N species [42, 43]. In addition, oxidized N species formed by the adsorption of Cl species were almost inactive for ORR [44, 45]. As confirmed by XPS, FeDN-AT had the highest contents of oxidized N and the lowest contents of pyridinic nitrogen, resulting in the worst ORR performance among the catalysts examined. The Cl contents and the oxidized N species could be largely reduced by reheating, as in the case of FeDN-AT-X. However, FeDN-AT-300 still had residual Cl species and oxidized N species, though their relative intensities were very weak when compared to those of

FeDN-AT. In the case of FeDN-AT-500 and FeDN-AT-700, the Cl species were observed to be almost completely removed, and the relative intensity of oxidized N species was negligible. Their contents of pyridinic N species were also higher than those of FeDN-AT-300. This was manifested by larger limiting currents on FeDN-AT-500 and FeDN-AT-700. FeDN-AT-700 exhibited higher relative contents of pyridinic N and Fe-N species than FeDN-AT-500, leading to slightly better ORR performance. For the FeDN-AT-700, LSV was measured at varied rpms to confirm the number of electrons (n) transferred during the ORR. As shown in Figure 4(d), the limiting current increased as the rotation speed of the electrode increased, and the *n* value was calculated to be almost 4, according to the Koutech–Levich (K–L) plot (inset of Figure 4(d)), demonstrating a low yield of H₂O₂ on FeDN-AT-700.

We investigated the effect of the acid type (for dissolving iron species in FeDN) on the properties and catalytic performance of the resultant catalyst. Two different acids (H₂SO₄ and HNO₃) were examined, in which the reheating temperature was the same as for FeDN-AT-700. The diffraction patterns of FeDN-H₂SO₄-700 and FeDN-AT-700 were similar, while that of FeDN-HNO₃-700 was different (Figure S3). Both FeDN-H₂SO₄ and FeDN-AT-700 had diffraction peaks that were iron species-related, but no such diffraction was observed in FeDN-HNO₃-700. Since
<figure><img src="image_7.png" /><figcaption>FIGURE 6: Fourier-transforms of k²-weighted EXAFS spectra (circle) and their best fits (solid line in red); (a) FeDN-AT, (b) FeDN-AT-700, (c) FeDN-H₂SO₄, (d) FeDN-H₂SO₄-700, (e) FeDN-HNO₃, and (f) FeDN-HNO₃-700.</figcaption></figure>

HNO₃ is highly oxidative, the dissolution of iron species in FeDN was more facilitated, and the partial corrosion of carbon took place during the acid process. This was further confirmed by ICP-OES analysis (Table S3). FeDN-AT-700 and FeDN-H₂SO₄-700 contained similar contents of iron, while FeDN-HNO₃-700, which uses HNO₃ to probe more extensive iron dissolution, had a relatively low iron content. The adsorbed anion species during acid treatment

were completely removed, and this was confirmed by the absence of the S photoelectron peaks on FeDN-H₂SO₄-700 and nitrate N on FeDN-HNO₃-700 (Figures S4 and S5). Therefore, it was assumed that the anion species adsorbed can be effectively removed by reheating the sample at 700°C, regardless of the type of acid used. The relative intensity of nitrogen species was obtained by separating the N1s spectra, as summarized in Table S4. FeDN-H₂SO₄-700
<figure><img src="image_8.png" /><figcaption>FIGURE 7: Electrocatalytic performance of FeDN-AT-700 catalysts. (a) ORR LSV in acidic condition, (b) 5000 cycles durability test in acidic condition, (c) LSV curve at different rotating speeds in alkaline condition, and (d) 10000 cycles durability test in alkaline condition.</figcaption></figure>

had similar trends to FeDN-AT-700 in terms of the relative intensity of nitrogen species. When compared to these two catalysts, FeDN-HNO₃-700 had a higher content of oxidized N and a lower content of pyridinic N.

An X-ray absorption fine structure (XAFS) analysis was performed to compare the oxidation state and local structure of the samples. X-ray absorption near edge structure (XANES) spectra are displayed in Figure S6. The rising edges of acid-treated samples are located between those of Fe foil and Fe₂O₃ references. The oxidation state of FeDN-H₂SO₄ is almost the same as that of FePc, while those of FeDN-HNO₃ and FeDN-AT are slightly lower and higher, respectively. Therefore, the acid-treated samples have oxidation states

similar to those of FePc. After reheating, the acid-treated catalysts were partially reduced, as shown in Figure 5. Fourier transforms (FTs) of extended X-ray absorption fine structure (EXAFS) and the results of their best fits are shown in Figure 6 and Table S5. The acid-treated samples have first shells of light elements, such as carbon, nitrogen, and/or oxygen, corresponding to Fe₃C, FeNC, and oxygen adsorbate, respectively. The second and third shells comprised Fe-Fe scattering from Fe metal clusters and Fe-C scattering from FeNC structure, respectively. Fe-O and Fe-Fe from Fe oxide and Fe-Cl from Fe chloride were found in some samples as well. After the reheating process, Fe-Fe from Fe metal clusters appeared, and the
size of the Fe cluster increased. For the HCl-treated sample, the residual chloride was eliminated after the reheating process. For reference, the FT-EXAFS of Fe foil, $Fe_2O_3$, and FePc are shown in Figure S7. Fe-Fe scattering from Fe metal at ~2.2 Å and Fe-O/Fe-N scattering from $Fe_2O_3$/FePc at ~1.5 Å can be compared with those in the acid-treated and reheated samples [19, 46].

Figure 7(a) shows LSVs obtained in an $O_2$-saturated aqueous $HClO_4$ solution using FeDN-AT-700, FeDN-HNO$_3$-700, and FeDN-H$_2$SO$_4$-700. FeDN-HNO$_3$-700 exhibited the worst ORR performance in terms of half-wave potential and limiting current. This is because FeDN-HNO$_3$-700 had the lowest contents of active nitrogen species, such as pyridinic N, and iron species when compared to those of FeDN-AT-700 and FeDN-H$_2$SO$_4$-700. FeDN-H$_2$SO$_4$-700 exhibited slightly lower ORR activity than FeDN-AT-700. The kinetic current at 0.8 V was calculated to be 0.18 mA and 0.24 mA for FeDN-H$_2$SO$_4$-700 and FeDN-AT-700, respectively. The difference in ORR activity between these two catalysts is related to their electronic states of nitrogen. Compared to FeDN-H$_2$SO$_4$-700, FeDN-AT-700 had higher amounts of active nitrogen (pyridinic N and Fe-N species), resulting in better ORR performance.

For FeDN-AT-700, which had the best performance, an accelerated durability test was conducted under acidic conditions. As shown in Figure 7(b), FeDN-AT-700 maintained its ORR performance after a potential 5000 cycles, which is comparable durability to a commercial Pt/C. The ORR performance of FeDN-AT-700 was also studied in alkaline conditions. Figure 7(c) shows LSVs measured in $O_2$-saturated aqueous KOH solution with a varied rotation speed of the electrode, with which a K-L plot was created (inset of Figure 7(c)). The value of n was calculated to be 4, indicating high selectivity for the reduction of $O_2$ into $H_2O$. Figure 7(d) displays LSVs before and after 10000 potential cycles in $O_2$-saturated KOH solution using FeDN-AT-700 and commercial Pt/C. The initial performance of the FeDN-AT-700 was comparable to that of the Pt/C. Moreover, FeDN-AT-700 exhibited a negligible loss in ORR performance after 10000 potential cycles, whereas Pt/C exhibited a significant performance loss. These results suggest that the understanding and proper application of posttreatment would be important for designing nonprecious metal catalysts with enhanced ORR activity and durability.

## 4. Conclusions

We demonstrated that acid treatment and the annealing process, as posttreatments, were crucial for enhancing the ORR performance of Fe-N-C catalysts. The acid treatment was effective for the elimination of aggregated iron species on FeDN, and it resulted in the formation of adsorbed impurity species such as counter anion. As a result, the electronic state of nitrogen was more oxidative. The annealing temperature was crucial for effectively removing adsorbed impurities and increasing the contents of highly active nitrogen such as pyridinic N and Fe-N species. This manifested the trend in the ORR performance of the FeDN-AT-X catalysts. An important factor for achieving high ORR performance

was the acid type, which affected the concentration of iron species and relative contents of nitrogen species on the catalyst. Among the catalysts examined, FeDN-AT-700 showed the best ORR performance. In acidic conditions, it was more durable, although its initial performance was lower than that of Pt/C. Furthermore, it demonstrated superior durability to Pt/C in addition to an initial ORR performance that was comparable to that of Pt/C.

## Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## 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.

## Authors' Contributions

Dong-gun Kim and Injoon Jang contributed equally to this work.

## Acknowledgments

This work was supported by the NRF of Korea grant (2020M3H4A3106313, 2021M3H4A1A02042948). We acknowledge the support of high-quality TEM analysis from the center of University-Wide Research Facilities (CURF) at Jeonbuk National University.

## Supplementary Materials

Figure S1: HR-TEM image of FeDN-AT-700. Figure S2: Raman spectra of FeDN catalysts. Figure S3: XRD pattern of $H_2SO_4$ and HNO$_3$-treated FeDN catalysts. Figure S4: S 2p XPS spectra of FeDN-H$_2$SO$_4$. Figure S5: N 1s XPS spectra of FeDN-HNO$_3$. Figure S6: XANES spectra of acid-treated and reference samples. Figure S7: Fourier-transforms of k$^2$-weighted EXAFS spectra of reference samples. Table S1: N$_2$ adsorption-desorption data of FeDN catalysts. Table S2: N and Fe contents in the catalysts. Table S3: Fe contents in catalysts measured by ICP-OES. Table S4: N 1s XPS data of acid-treated FeDN catalysts. Table S5: Summary of EXAFS fitting. (Supplementary Materials)

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