# Instantaneous Thermal Energy for Swift Synthesis of Single-Atom Catalysts for Unparalleled Performance in Metal-Air Batteries and Fuel Cells

Injoon Jang, Sehyun Lee, Dong-Gun Kim, Vinod K. Paidi, Sujin Lee, Nam

Dong Kim, Jae Young Jung, Kug-Seung Lee, Hyung-Kyu Lim, Pil Kim, et al.

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## ► To cite this version:

Injoon Jang, Sehyun Lee, Dong-Gun Kim, Vinod K. Paidi, Sujin Lee, et al., Instantaneous Thermal Energy for Swift Synthesis of Single-Atom Catalysts for Unparalleled Performance in Metal-Air Batteries and Fuel Cells. Advanced Materials, 2024, 36 (32), pp.2403273. 10.1002/adma.202403273. hal-05098739

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# Instantaneous Thermal Energy for Swift Synthesis of Single-Atom Catalysts for Unparalleled Performance in Metal-Air Batteries and Fuel Cells

Injoon Jang, Sehyun Lee, Dong-gun Kim, Vinod K. Paidi, Sujin Lee, Nam Dong Kim, Jae Young Jung, Kug-Seung Lee, Hyung-Kyu Lim,* Pil Kim,* and Sung Jong Yoo*

Based on experimental and computational evidence, phthalocyanine (Pc) compounds in the form of quaternary-bound metal-nitrogen (N) atoms are the most effective catalysts for oxygen reduction reaction (ORR). However, the heat treatment process used in their synthesis may compromise the ideal structure, causing the agglomeration of transition metals. To overcome this issue, a novel method is developed for synthesizing iron (Fe) single-atom catalysts with ideal structures supported by thermally exfoliated graphene oxide (GO). This is achieved through a short heat treatment of only 2.5 min involving FePc and N, N-dimethylformamide in the presence of GO. According to the synthesis mechanism revealed by this study, carbon monoxide acts as a strong linker between the single Fe atoms and graphene. It facilitates the formation of a structure containing oxygen species between FeN₄ and graphene, which provides high activity and stability for the ORR. These catalysts possess an enormous number of active sites and exhibit enhanced activity toward the alkaline ORR. They demonstrate excellent performance when applied to real electrochemical devices, such as zinc-air batteries and anion exchange membrane fuel cells. It is expected that the instantaneous heat treatment method developed in this study will aid in the development of high-performing single-atom catalysts.

## 1. Introduction

Fuel cells and batteries are expected to play key roles in meeting the demands of various human activities for a sustainable future. The oxygen reduction reaction (ORR), which occurs at the cathode, is considered to be the rate-determining step in fuel cells and batteries because of its sluggish kinetics.[1,2] Precious metals, such as platinum (Pt), exhibit excellent ORR activity; however, their high material cost and limited reserves hinder their practical applicability in fuel cells and batteries. As alternatives to precious metals, 3d transition metal (TM)-based electrocatalysts have attracted significant attention.[3] However, volcano plots of their oxygen (O) binding energies (BEO) versus ORR activities revealed that relatively inexpensive TMs tend to strongly bind to O, causing incomplete ORR.[4,5] To tailor the BEO of TM for exhibiting efficient ORR, TM/nitrogen (N)-doped carbon (C) materials, known as M-N-C materials, can be used as the cathode catalysts.

I. Jang, S. J. Yoo
Hydrogen-Fuel Cell Research Center
Korea Institute of Science and Technology (KIST)
5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
E-mail: ysj@kist.re.kr

I. Jang
Department of Chemical Engineering
Chungbuk National University
Cheongju, Chungbuk 28644, Republic of Korea

S. Lee
Department of Environment and Energy Engineering
Sungshin Women's University
Seoul 01133, Republic of Korea

D.-gun Kim, S. Lee, P. Kim
School of Chemical Engineering
School of Semiconductor and Chemical Engineering
Clean Energy Research Center
Jeonbuk National University
Jeonju 54896, Republic of Korea
E-mail: kimpil1@jbnu.ac.kr

V. K. Paidi
European Synchrotron Radiation Facility
Grenoble 38043 Cedex 9, France

N. D. Kim
Functional Composite Materials Research Center
Korea Institute of Science and Technology (KIST)
Jeollabuk-do 55324, Republic of Korea

J. Y. Jung
Fuel Cell Research and Demonstration Center
Hydrogen Energy Institute
Korea Institute of Energy Research (KIER)
Joellabuk-do 56332, Republic of Korea

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202403273

© 2024 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adma.202403273
Notably, iron (Fe)–N–C emerges as one of the most promising candidates for enhancing ORR performance.[6,7] As Fe undergoes quaternary bonding to N in the FeN₄ chemical structure, changes in the electronic structure by equatorial and axial ligand effects can be used to fine-tune the BEO.[8,9] Furthermore, resolving the scaling relation is anticipated by adjusting the change of Gibbs free energy (ΔG) of the ORR intermediates (ΔG<sub>OOH</sub> = ΔG<sub>OH</sub> + 3.2 eV).

Phthalocyanine (Pc) is a planar compound comprising a metal atom symmetrically surrounded by four N atoms (MN₄ structure). This structural uniqueness not only facilitates the realization of an ideal M-N-C configuration but also grants insolubility in water and alkali media.[10] Over the past few decades, Pc-based molecules have undergone extensive scrutiny due to their immense potential as ORR electrocatalysts.[11–16] Despite boasting an ideal structure and great potential, the utilization of Pc as an ORR catalyst presents big challenges owing to its low electrical conductivity. Four electrons are required to complete the ORR, regardless of the pathway; therefore, low electrical conductivity can be a critical limitation for an ORR electrocatalyst because of the resulting sluggish kinetics. To obtain sufficient electrical conductivity of Pc while maintaining the M–N₄ structure, the incorporation of highly conductive carbon materials emerges as a favorable strategy.

Among the various conducting C candidates, graphene[17,18] stands out as a rational choice for preparing nanocomposites with Pc because of its 2D structure, featuring a planar basal plane that resembles the structures of Pc molecules.[19] Heat treatment at a temperature greater than 700 °C is generally conducted to ensure robust catalyst-support interactions in Pc-graphene nanocomposites.[20] However, owing to the low thermal stability of the Pc moiety, it is difficult to circumvent the decomposition of the Pc structure at ≈400 °C, similar to most small organic molecules.[21] The conventional heat treatment process involves a continuous supply of thermal energy for an extended period of time until the target temperature is reached. The instability of the Pc molecules under the heat-treatment conditions leads to agglomeration of the TMs in Pc and the decomposition of Pc. Consequently, this process gives rise to the formation of metal clusters and nanoparticles.

For more efficient heat treatment, previous studies have demonstrated “instantaneous heat treatment”—heating the sample over a preheated furnace for a short period of time—as an alternative to gradually increasing the temperature.[22–25] No-

tably, according to previous research, a very short heat treatment time appears to be efficient for the synthesis of small-molecule-incorporated nanocomposites. H. Fei et al. successfully demonstrated the ultrafast heating effects to synthesize single-atom catalysts via microwave.[26–29] This rapid heating process prevented metal agglomeration, which can lead to decent graphene-supported single-atom catalysts. However, few studies have successfully demonstrated nanocomposite fabrication by instantaneous heat treatment in furnace systems; therefore, the formation mechanism of nanocomposites through this method is not yet fully understood.

In this study, we introduce a novel and highly efficient method for synthesizing single-atom catalysts using an instantaneous heat treatment process. Building on previous findings,[30] our technique employs iron phthalocyanine (FePc) and graphene oxide (GO) to swiftly generate FeN₄ catalysts with an optimal structure for the alkaline ORR. Central to our innovation is the proposed formation mechanism for nanocomposites, which features an oxygen-bridged FeN₄–O/NGO structure.[31] This structure has been demonstrated to enhance ORR activity in line with leading Fe-N-C catalysts. Crucially, we highlight the transformative role of CO, generated from the decomposition of N, N-dimethylformamide (DMF), in healing vacancy defects within GO.[32,33] This mechanism not only preserves the integrity of the graphene framework but also facilitates the integration of active FeN₄ sites, significantly boosting the catalytic performance. Our prepared catalysts exhibited impressive peak power densities of 217 and 881 mW cm⁻² when used in zinc (Zn)–air batteries and anion exchange membrane fuel cells (AEMFCs), respectively, confirming the effectiveness of our catalyst and synthesis method. In addition, using advanced analytical methods, we elucidated the synthesis mechanism. Overall, this study provides valuable insights for the future development of high-performance single-atom catalysts based on various Pc-based complexes.

## 2. Results and Discussion

### 2.1. Synthesis and Structural Characterization of Catalysts

**Figure 1a** shows a schematic of the instantaneous heat-treatment method for catalyst synthesis. FePc dispersed in a N, N-dimethylformamide (DMF) solution was filtered and dried to obtain DMF-adsorbed FePc, which was then subjected to heat treatment in an ammonia (NH₃) atmosphere using a preheated furnace at 950 °C. Heat treatment was conducted for 0.0, 1.0, 2.5, and 5.0 min, and the samples were accordingly named FeN₄/NGO_0.0m, FeN₄/NGO_1.0m, FeN₄/NGO_2.5m, and FeN₄/NGO_5.0m, respectively. Figure 1b shows the X-ray diffraction (XRD) patterns for the samples as a function of the heat treatment time. Red inverted triangles, black diamonds, and black dots represent the XRD patterns of FePc, carbon (002), and carbon (102), respectively. The XRD pattern of the FeN₄/NGO_0.0m sample, which did not undergo heat treatment, clearly showed peaks corresponding to FePc and C, indicating that FePc was distributed on the GO surface in the bulk state, and maintained its crystallinity. Meanwhile, even though FeN₄/NGO_1.0m underwent a short heat treatment of only 1 min, the FePc peak disappeared, indicating that FePc lost

K.-S. Lee
Pohang Accelerator Laboratory
Pohang University of Science and Technology
Pohang 37673, Republic of Korea

H.-K. Lim
Division of Chemical and Bioengineering
Kangwon National University
Chuncheon 24341, Republic of Korea
E-mail: hklim@kangwon.ac.kr

S. J. Yoo
Division of Energy & Environmental Technology
KIST school
University of Science and Technology (UST)
Daejeon 34113, Republic of Korea
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Instantaneous heat treatment and physical characterization of the prepared catalysts. a) Simplified diagram of an instantaneous heat treatment process for catalyst synthesis. b) XRD patterns of the prepared catalysts. c–f) HAADF-STEM images of catalysts heat-treated for different times. g,h) Fe K-edge XANES spectra (g) and Fourier transforms of the EXAFS spectra (h) for FeN<sub>4</sub>/NGO<sub>t</sub> (t = 0.0, 1.0, 2.5, and 5.0 m). i) Coordination number for each bond, as obtained by EXAFS fitting.</figcaption></figure>

its crystallinity owing to its fragmentation at the molecular level during the heat treatment at 950 °C. The results were consistent for all samples with heat treatment times of at least 60 s, suggesting that at least 60 s of heat treatment is required for FePc to decompose and disperse into single atoms. Figure 1c–f and Figure S1 (Supporting Information) show the scanning transmission electron microscopy (STEM) images and the low-magnification transmission electron microscopy (TEM) images of each sample, respectively, revealing the morphologies of the catalysts. As can be seen in Figure 1c, for FeN<sub>4</sub>/NGO<sub>0.0m</sub>, it was difficult to ob-

serve single Fe atoms on the GO. This is because, as shown in Figure S1a (Supporting Information), FePc was dispersed on GO in bulk, with a size of several hundred nanometers. Figure 1d and Figure S1b (Supporting Information) show the STEM and TEM images, respectively, of FeN<sub>4</sub>/NGO<sub>1.0m</sub>; they revealed that particles ranging in size from single atoms to several nanometers were dispersed on the GO surface. Figure 1e and Figure S1c (Supporting Information) show STEM and TEM images, respectively, of FeN<sub>4</sub>/NGO<sub>2.5m</sub>; it can be observed that the nanoparticles and sub-nano clusters disappeared from the GO, and mostly
only single atoms remained. Figure 1f and Figure S1d (Supporting Information) show STEM and TEM images, respectively, of FeN<sub>4</sub>/NGO_5.0m, which revealed that nanoparticles ≈10 nm in size were distributed on the GO. Therefore, it can be inferred that the particles underwent a transformation between 0.0 and 5.0 min of heat treatment. Up to 2.5 min, FePc gradually decomposed into single atoms, after which the single atoms appeared to aggregate to form nanoparticles. Thus, it can be inferred that the synthesis of single-atom catalysts can be achieved with only ≈150 s of heat treatment, without needing extended heat or subsequent acid treatments, as previously required for conventional heat treatment. The elemental distribution of the catalyst was determined using TEM-energy dispersive X-ray spectroscopy (EDS) spectra and mapping images (Figure S2, Supporting Information). Fe and N were found to be evenly dispersed on GO in both FeN<sub>4</sub>/NGO_1.0m and FeN<sub>4</sub>/NGO_2.5m. However, in FeN<sub>4</sub>/NGO_5.0m, FeO<sub>x</sub> nanoparticles were clearly visible on the GO. Table S2 (Supporting Information) presents the elemental contents of the samples, as found using inductively coupled plasma spectroscopy, elemental analysis (EA), and X-ray photoelectron spectroscopy (XPS). All the samples contained ≈2.5 wt.% of Fe. As the heat treatment progressed, the N content increased, which could be attributed to the N-doping effect of NH<sub>3</sub> gas. However, after 5 min of heat treatment, the N content decreased, possibly because of the decomposition of NGO by the prolonged high-temperature treatment.[34] Instantaneous heat treatment may therefore be advantageous for the catalytic activity, as it ensures a high N content, which is known to favor ORR catalytic activity.[35]

Figure 1g,h shows the X-ray absorption fine structure (XAFS) results, which were obtained to analyze the structure of the catalyst and the environment of the ligand coordination of Fe. Four characteristic peaks were observed in the Fe K-edge X-ray absorption near-edge structure (XANES) spectra, as shown in Figure 1g. Peak-A (7114 eV) and Peak-B (7118 eV) peaks in the pre-edge region provide information about the molecular structural symmetry of FePc, whereas Peak-C (7132 eV) and Peak-D (7139 eV) in the post-edge region provide information about the coordination environment of Fe. Peak-A and Peak-B originated from the square pyramidal and square planar symmetries of FePc, respectively.[36] The two peaks of the four samples exhibited opposite trends to those of FePc in Figure S3 (Supporting Information), indicating that FePc acquired distorted symmetry during dispersion with DMF and the heat-treatment process. In addition, the increase in the intensity of Peak-C and the decrease in the intensity of Peak-D became progressively more pronounced as the heat-treatment time of the samples increased. These contrasting changes in the intensities of Peak-C and Peak-D were caused by the central Fe atom moving away from the Fe-N<sub>4</sub> plane in response to changes in the ligand environment, creating distorted Fe-N<sub>4</sub> sites for catalytic activity.[37] We have estimated the valence states of the pyrolyzed samples using derivative spectra from Fe K-edge XANES, with the fitting results presented in Figure S4 (Supporting Information). Our analysis indicates that the valence states of the samples increased post-heat treatment compared to the original iron phthalocyanine (FePc). Specifically, valence states were measured at +2.58 for FeN<sub>4</sub>/NGO_1m, +2.46 for FeN<sub>4</sub>/NGO_2.5m, and +3.18 for FeN<sub>4</sub>/NGO_5m. These increases in valence states suggest the formation of Fe-O bonds, indicative of changes in

the chemical environment surrounding the iron.[31] Notably, the valence state exceeding +3 observed for FeN<sub>4</sub>/NGO_5m may also imply the formation of FeO<sub>x</sub> nanoparticles, which is consistent with our TEM findings. Moreover, the slight variation in valence states between FeN<sub>4</sub>/NGO_1m and FeN<sub>4</sub>/NGO_2.5m could be attributed to structural distortions within the FeN<sub>4</sub> complex or alterations in the local coordination environment of iron. In summary, the XANES characterization suggests that FePc undergoes a change in ligand coordination and subsequent structural distortion during synthesis.

Further structural information regarding the local environment of Fe was obtained from Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra (Figure 1h). Also, Figure S5 (Supporting Information) shows the Fourier transforms of the k<sup>2</sup>-weighted EXAFS spectra (circle) and their best fits (solid line), with the window range applied for the spectra fits indicated; the fitting results are shown in Figure 1i and Table S3 (Supporting Information). The EXAFS spectra exhibited three characteristic peaks at ≈1–2, 2–2.5 Å, which are attributed to Fe-N/O, and Fe-Fe/C respectively. It was found that the local structure of Fe is strongly affected by heating time. Without heat treatment, FeN<sub>4</sub>/NGO_0.0m contained only Fe-N bonds in the first coordination shell. However, the use of Fe-N and Fe-O scattering paths led to a better agreement with the experimental results for FeN<sub>4</sub>/NGO_1.0m and FeN<sub>4</sub>/NGO_2.5m samples. Therefore, as evidenced by the other spectra, heating for 1, and 2.5 resulted in the formation of additional Fe-O bonds to FeN<sub>4</sub>. When oxygen forms a new bond with iron while maintaining the FeN<sub>4</sub> structure, it would be highly unusual for all four elements of the Fe-N<sub>4</sub>-O complex to lie in the same plane. Therefore, it seems plausible that when oxygen bonds to the already planar FeN<sub>4</sub>, it does so in a manner that deviates from this plane. Consequently, the oxygen in FeN<sub>4</sub>-O is likely involved in axial coordination with iron. This suggests that the FeN<sub>4</sub>-O<sub>a</sub> (O<sub>a</sub>: axially coordinated oxygen) structure forms as a result of heat treatment for 2.5 min. The spectra clearly show that with an increase in heating time, the Fe-C bond contributions were reduced. The EXAFS fitting indicates that after five minutes of heat treatment, Fe primarily forms Fe-O or Fe-C bonds. Additionally, the presence of Fe-Fe bonds at 3.08 Å, based on Fe-O, suggests that prolonged heat treatment promotes the clustering of FeO<sub>x</sub> particles. The absence of distinct peaks for FeO<sub>x</sub> and FeC<sub>x</sub> in XRD can be attributed to these particles being in a low crystallinity state, transitioning to oxides or carbides. Supporting this, the large Debye-Waller factor values in the EXAFS fitting also indicate a transition state of the particles. The special fit results are consistent with the observations from the TEM images. To further confirm the spectral fitting results, a wavelet transform analysis was performed, as shown in Figure S6 (Supporting Information). Wavelet transform analysis clearly and visually demonstrates the weakening of the Fe-C second shell bonds in FePc.

EXAFS analysis has yielded four notable results: 1) preservation of the Fe-N<sub>4</sub> structure in samples treated for less than five minutes, 2) deformation of the external structure of FePc, 3) formation of FeO<sub>x</sub> and FeC<sub>x</sub> nanoparticles in samples treated for five minutes or more, and 4) formation of FeN<sub>4</sub>-O<sub>a</sub> bonds in FeN<sub>4</sub>/NGO_1.0m and FeN<sub>4</sub>/NGO_2.5m. As the Fe-N<sub>4</sub> structure is the active site that ensures high ORR activity, maintaining the Fe-N<sub>4</sub> structure is essential for high-performance
catalysts. However, the existing synthesis methods fail to maintain the Fe—N₄ structure owing to the excessive decomposition of FePc. In contrast, the instantaneous heat treatment method developed in this study is excellent for maintaining the Fe—N₄ structure. In addition, the disappearance of Fe—C bonds in the second shell indicates the rearrangement of carbons located outside the FeN₄ structure within the FePc. During this process, interactions may have occurred between the external regions of the FePc and graphene, which are anticipated to contribute to the stabilization of the overall structure. The formation of Fe—O bonds immediately after heating is likely to originate from the O in DMF or GO, given the heat-treatment conditions in the NH₃ atmosphere. To clarify this, the binding energies of various functional groups containing O that could interact with FePc were obtained through density functional theory (DFT) calculations; the structures and binding energies for the selected O species are shown in Figure S7 (Supporting Information). DFT calculations revealed that the O in DMF (O<sub>DMF</sub>) had the most distinctive intrinsic binding affinity to Fe than that in any oxygen functional group present in GO. Numerous studies have demonstrated that DMF is an excellent solvent for dissolving FePc.[38,39] Given the importance of the attractive interaction between solvent and solute for high solubility, the high affinity of Fe—O<sub>DMF</sub> could be a major rationale for the high solubility of FePc in DMF. Therefore, it is likely that prior to the heat treatment, FePc and the O<sub>DMF</sub> were attractively interacting, and as the heat treatment was applied, stronger bonding of Fe with DMF-derived oxygen species resulted in the formation of vertically bonded Fe with O (Fe—O).

The behavior of FePc and DMF during instantaneous heat treatment was further clarified by thermogravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FT-IR). TGA was performed in an argon (Ar) atmosphere at a ramp rate of 10 °C min⁻¹ to 900 °C. For FeN₄/NGO_1.0m, weight loss first occurred at temperatures below 200 °C owing to the decomposition or evaporation of DMF,[40] and a second weight loss occurred at 400 °C owing to the thermal decomposition of FePc.[41] However, neither of the two weight losses occurred for FeN₄/NGO_2.5m as they did for FeN₄/NGO_1.0m, indicating that heating for more than 2.5 min resulted in the complete removal of DMF and the collapse of the FePc structure. In addition, from the absence of a significant mass loss in the TGA analysis of pure GO shown in Figure S8 (Supporting Information), it was confirmed that the aforementioned results did not originate from changes such as the decomposition of GO. This trend is also evident from the FT-IR analysis, where the bands at ≈721 and ≈1720 cm⁻¹ in the FT-IR spectra (Figure 2b) are assigned to the stretching vibrations of C=O for DMF (triangles) and C—H for FePc (circles), respectively.[42,43] Similar to the TGA results, the vibrational bands disappeared for FeN₄/NGO_2.5m, indicating the decomposition of DMF and the FePc structure. Therefore, TGA and FT-IR analyses suggested that an instantaneous heat treatment strategy lasting at least 2.5 min was required to complete the synthesis reaction.

The bonding of Fe was further elucidated by investigating its electronic structure using XPS. Figure 2c shows the Fe2p XPS spectra of FePc, FeN₄/NGO_0.0m, FeN₄/NGO_1.0m, and FeN₄/NGO_2.5m, which are composed of the doublets 2p1/2

and 2p3/2. The Fe2p spectrum of FeN₄/NGO_0.0m is almost identical to that of FePc, indicating that FeN₄/NGO_0.0m is a simple mixture of intact FePc, DMF, and GO. It can also be seen that the FeN₄—O<sub>DMF</sub> was a weak attractive interaction that did not affect the Fe electronic structure. In contrast, the Fe2p peaks in the FeN₄/NGO_1.0m and FeN₄/NGO_2.5m spectra were shifted toward higher binding energies compared to those in the FePc and FeN₄/NGO_0.0m spectra. This is commonly observed when Fe, the central metal in FePc, binds to O,[44] further supporting the formation of Fe—O bonds. Figure 2d shows the deconvoluted XPS spectra of FeN₄/NGO_1.0m and FeN₄/NGO_2.5m. As the heat treatment progressed, the peaks for Fe2p, Fe²⁺, and Fe³⁺ shifted to lower binding energies. This is due to the difference in Fe electronic structure between the structure of the catalyst at the end of the reaction and that of the reaction intermediate. To determine the structure of FeN₄/NGO_2.5m, Fe2p binding energies for possible catalytic structures during and at the end of the reaction were obtained from DFT calculations and are shown in Figure S9 (Supporting Information). Among the three structures, FeN₄—O—Gr had the lowest binding energy. Given that FeN₄/NGO_2.5m has a lower Fe2p binding energy, as observed from the XPS results, it is reasonable to conclude that FeN₄—O<sub>DMF</sub> and FeN₄—O were completely converted to the FeN₄—O—Gr structure. In summary, due to the attractive interaction between Fe and DMF, FePc was effectively dispersed in DMF (FeN₄—O<sub>DMF</sub>), followed by the instantaneous heat treatment to the FeN₄—O structure and finally synthesized into a catalyst with the FeN₄—O—Gr structure.

## 2.2. Healing of Graphene Defects by CO

Further efforts were made to elucidate the formation mechanism of FeN₄—O—Gr. Previous analyses have repeatedly demonstrated that DMF decomposes as heat treatment progresses. As DMF decomposes into carbon monoxide (CO) and dimethylamine at temperatures above its boiling point (Figure 3a),[45] it is plausible that CO was formed in this synthesis due to the decomposition of DMF as the heat treatment was conducted above the boiling point of DMF. However, at higher temperatures like 950 °C, this behavior might change, possibly resulting in additional byproducts. Although there are no precise reports on DMF's decomposition at such high temperatures, it's reasonable to assume that further decomposition of dimethylamine could occur. Despite the potential for minor traces of other compounds at extreme temperatures, CO remains the primary product in our synthesis. The effect of this generated CO on the formation of the FeN₄—O—Gr structure was investigated through DFT calculations and experiments, the results of which are shown in Figure 3b and Figure S8 (Supporting Information). Nine of the most common graphene structures with defects were selected for the DFT calculations, the models of which are shown in Figure S10 (Supporting Information). Representative monovacancy (V₁) and double-vacancy (V₂) defects were formed on the graphene surface, and V₂ was categorized as V₂.₁ and V₂.₂ according to its structure. Because O and N can replace C in graphene, a structure reflecting this was also added. The chemical interaction of CO with these structures was investigated, and it was found that
<figure><img src="image_5.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Electro-structural characterization of synthesized catalysts. a) TGA results of FeN4/NGO_1.0m and FeN4/NGO_2.5m. The triangles and circles represent the thermal decomposition of DMF and FePc, respectively. b) FT-IR spectra of FePc, FePc + DMF, FeN4/NGO_1.0m, and FeN4/NGO_2.5m. The triangles and circles indicate the stretching vibrational bands of C=O and C-H, respectively. The yellow rectangles with a red dotted border show the sources of the C-H and C=O bonds. c,d) Fe 2p XPS spectra of the synthesized catalyst and FePc (c), deconvoluted XPS spectra of FeN4/NGO_1.0m and FeN4/NGO_2.5m and the binding energy shifts of each peak obtained from them (d).</figcaption></figure>

CO was bound very strongly to most of the defects present in the graphene (Figure 3b). For most structures, except V$_{2}$$_{.}$$_{2}$ and V$_{2}$$_{.}$$_{2}$-N, binding energies exceed -1.4 eV, implying strong chemical interactions corresponding to covalent bonds. However, because carbon networks of V$_{2}$$_{.}$$_{2}$ and V$_{2}$$_{.}$$_{2}$-N defects are inherently stable, CO cannot easily bind to it; however, if O substitutes the C in graphene, the interaction between CO and graphene becomes much stronger. More notably, from the graphene structure before and after CO binding, the defective graphene metrics are recovered by the CO binding to the defects, specifically in a C-head manner. This implies that graphene can become more crystalline by exposure to CO, which is in agreement with the previous report.[46] This healing of graphene defects by CO is a key aspect of the synthesis mechanism that must be experimentally demonstrated, not just simulated computationally. Therefore, the changes in the surface defects of GO, CO-treated GO (denoted as GO_CO), and boiling DMF-treated GO (denoted as GO_DMF) were investigated. For detailed experimental methods, see the supporting information. Vacancy- and sp³-type defects are the most common defects in graphene, caused by the loss of C and the binding of O species to the graphene surface, respectively.

The structural changes in graphene predicted by DFT calculations upon the binding of graphene to CO were the disappearance of vacancy-type defects and the formation of sp³-type defects (Figure 3d). To identify the type of defects in graphene, the I$_{D}$/I$_{D}$' ratios obtained from the Raman analysis were compared; if only vacancy-type or sp³-type defects are present on the graphene surface, the ratio would be 7 or 13, respectively.[47] Figure S11 (Supporting Information) and Figure 3d show the Raman spectra of GO, GO_CO, and GO_DMF and the I$_{D}$/I$_{D}$' ratios obtained from them. From the significant intensities of the D peaks in the Raman spectra, it appears that the GO used in this study has many defects compared to the elaborately synthesized graphene. Furthermore, the I$_{D}$/I$_{D}$' ratio of GO was ≈8, indicating that vacancy- and sp³-type defects coexisted; however, vacancy-type defects were relatively prevalent. The I$_{D}$/I$_{D}$' ratio of GO_CO and GO_DMF increased to ≈11, indicating that vacancy-type defects were removed from graphene by the CO and DMF treatments. Thus, it was demonstrated through DFT calculations and experiments that the CO generated from the decomposition of DMF binds to and heals vacancy-type defects in graphene.
<figure><img src="image_9.png" /><figcaption>**Figure 3.** The healing of defects in GO by CO. a) Scheme of DMF decomposition at temperatures above boiling point. b) Structures of the mono-defect (V1) and double-defects (V2-1 and V2-2) that can exist in graphene. c) The binding energy of CO to the defect for each structure. d) Illustration of the defect healing process of CO-induced conversion of graphene with vacancy-type defects to graphene with sp3-type defects. e, Ratios of D-band and D"-band intensities for the three samples. The two dash lines represent ID/ID" values of 7 and 13, respectively.</figcaption></figure>

**2.3. Mechanistic Insights into the Activity and Stability Enhancement**

**Figure 4a** illustrates the synthesis mechanism that occurred during the instantaneous heat treatment process, as revealed by the

preceding discussion. The synthesis mechanism comprises the following steps: 1) FePc, DMF, and GO are mixed, and the Fe in FePc and the O in DMF attractively interact to form $FeN_4-O_{DMF}$; 2) at the beginning of the heat treatment, DMF decomposes, forming $FeN_4-CO$; 3) $FeN_4-CO$ then binds to the vacancy
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

**Figure 4.** Synthesis mechanism and prediction of ORR activity and durability. a) Schematic representation of the synthesis mechanism that occurs during instantaneous heat treatment. b) Mechanism illustration of the oxygen reduction reaction through four consecutive proton bond electron transfers. c) Equilibrium potentials (E°) for ORR at Fe single-atom sites generated from various defects, as predicted by DFT calculations. Volcano trend in theoretical ORR potential with respect to *OOH binding free energy. As oxophilicity decreases, the rate-determining step shifts from the 4th to the 1st Proton-Coupled Electron Transfer (PCET). d) Comparison of the energy required for demetallization in the structures of the FeN4—O—Gr groups. e) Illustration of demetallization.
defects in graphene, completing the synthesis of FeN₄/NGO with an FeN₄—O—Gr structure. In this process, the carbon from CO enters the defect site in graphene, forming new bonds with the surrounding carbon atoms in graphene and reconfiguring the bonds to achieve the most stable structure. Therefore, the structural outcome of O—Gr varies depending on the specific defect configuration in the graphene. Thus far, the analysis has demonstrated that CO from the decomposition of DMF plays a crucial role as a bridging agent in the synthesis of graphene-supported Fe single-atom catalysts. The C and O in CO form very stable and strong bonds with graphene and FeN₄, respectively, facilitating the even dispersion of FeN₄ on the graphene surface throughout the heat treatment process.

Notably, the FeN₄/NGO catalysts synthesized via instantaneous heat treatment contained O species between FeN₄ and GO. The binding of the ligand to the central metal of the M—N₄ structure results in the modulation of its electronic structure, which is considered a strategic means to improve ORR activity as it optimizes the binding energy with O.[31,48] Therefore, the FeN₄/NGO_2.5m catalysts synthesized by instantaneous heat treatment are also expected to exhibit enhanced ORR activity, and further investigation was carried out. For this purpose, the FeN₄—O—Gr structures derived from the nine defect structures were first determined by DFT calculations and shown in Figure S12 (Supporting Information). The ORR reactivity of these selected structures and the FeN₄—Gr structure without O bridging was investigated. In general, the oxygen reduction reaction is achieved through four consecutive proton-coupled electron transfers (PCETs) as illustrated in Figure 4b.[49] The equilibrium potential (E₀) for the ORR was determined for each structure, and the results are shown in Figure 4c. The FeN₄—O—Gr structures arising from the seven defects other than V₁—N and V₂,₂—O exhibited higher E₀ values than FeN₄, indicating increased ORR activity. Furthermore, the decline in the ORR activity was negligible, whereas the improvement in the activity was significant. Further analysis of the effect of electronic structure modulation on the Fe active site reveals that the spin density of the *d*₂₂ orbital decreases due to strong CO coordination in the axial direction. This results in a general trend of weakening oxophilicity (Figure S13, Supporting Information). Considering the E₀ trend in relation to ΔG<sub>B,OOH</sub>), a clear volcano behavior appears as oxophilicity diminishes. This weakening in oxophilicity brings about a balance in the reaction energy at each PCET step during the ORR process, thus optimizing activity in V₂,₁—N and V₂,₂—N models. Additionally, the catalyst degradation caused by demetallization, that is, the central metal loss in M—N₄ catalysts is a significant concern. DFT calculations can provide the energy required for demetallization; the higher the energy, the more stable the catalyst. As shown in Figure 4d, all the FeN₄—O—Gr structures formed from the nine defect models required more energy to remove Fe than FeN₄—Gr without O species ligands, indicating that FeN₄/NGO_2.5m is likely to have higher catalytic stability because the FeN₄—O—Gr structure has a higher energy barrier to demetallization, as depicted in Figure 4e. Eventually, the instantaneous heat treatment promotes strong bonding between the FePc and graphene through the effective bridging agent of CO. This leads to the uniform distribution of FeN₄ on the graphene and the dominant formation of the FeN₄—O—Gr structure, which gives the catalyst high activity and stability toward the ORR.

## 2.4. Identification and Electrochemical Characterization of Active Sites

To fully understand the electrochemical properties of a synthesized catalyst, it is crucial to accurately determine its electrochemically active site density. An effective method for the evaluation of the active sites of non-precious metal-based single-atom catalysts is CO temperature-programmed desorption (TPD).[41] This technique involves measuring the temperature-dependent desorption of CO adsorbed on a catalyst to quantify the density at a particular site; this process is illustrated in Figure 5a. The catalyst was heated to a temperature greater than 600 °C to eliminate any O species present on its surface. After that, the catalyst was placed under a CO atmosphere at a temperature of -30 °C to enable CO adsorption onto the catalyst. The temperature of the sample was then increased, and the amount of CO released was measured (see the Experimental Section for a detailed description of the analysis and calculations). The CO-TPD results are shown in Figure 5b, where the conventional method represents the catalyst synthesized by increasing the temperature to 950 °C at a rate of 10 °C min⁻¹. At temperatures below 150 °C, all four CO-TPD curves exhibited peaks corresponding to CO desorption from Fe.[50,51] The site densities of single Fe atoms can be estimated from the integrated areas under these desorption peaks via conversion, as shown in Figure 5c. FeN₄/NGO_2.5m had the highest site density, as it had the highest number of single Fe atoms distributed on graphene. FeN₄/NGO_1.0m had a relatively low site density because unreacted FePc was distributed on the graphene, as observed in the TEM (Figure 1d). FeN₄/NGO_5.0m had a further reduced site density owing to the formation of Fe clusters and nanoparticles by the aggregation of single atoms. In other words, the site density value was consistent with that from the previous analysis. Such results are inconsistent with the BET surface area analysis shown in Figure S14 and Table S4 (Supporting Information). The BET analysis indicates that the surface area increases with longer heat treatment times, which can be attributed to the etching effect of NH₃ gas on the carbon support during the heat treatment process. This discrepancy between the two analyses highlights the limitations of BET analysis in estimating the ECSA of single-atom catalysts from the transition metal series and suggests that more advanced techniques such as CO-TPD are better suited for estimating the intrinsic activity of catalysts. Not surprisingly, the sample prepared from the conventional method had the lowest site density, with a value that was 8.3-times lower than that of FeN₄/NGO_2.5m. demonstrating that instantaneous heat treatment is excellent for synthesizing catalysts with significantly higher electrochemical active areas compared to those fabricated using conventional synthesis methods.

Electrochemical analysis was performed using a triple-electrode system containing a Pt counter electrode and an Ag/AgCl reference electrode in a 0.1 M potassium hydroxide (KOH) electrolyte. Cyclic voltammetry (CV) curves of the four samples were obtained at a scan rate of 100 mV s⁻¹ over a voltage range of 0.05–1.1 V<sub>RHE</sub> (Figure 5d). Two characteristic peaks were observed in the forward and backward scans, respectively, for FeN₄/NGO_0.0m and FeN₄/NGO_1.0m, which are attributed to the redox of the central metal, Fe, and are typical for FePc.[52] This indicates that unreacted FePc was still present on
<figure><img src="image_15.png" /><figcaption>**Figure 5.** Fe site density derivation and electrochemical characterization. a) Illustration of the CO pulse chemisorption and CO-TPD processes. The black, red, blue, and yellow spheres represent the elements C, O, N, and Fe, respectively. b,c) CO-TPD spectra (b) and calculated site densities (c) for FeN<sub>4</sub>/NGO<sub>1.0m</sub>, 2.5m, and 5.0m. The Fe catalyst synthesized using the conventional method is shown for comparison. d) CVs measured over a voltage range of 0.05–1.05 VRHE in 0.1 M KOH (N2 sat’d) at a scan rate of 100 mV s<sup>-1</sup>. e) Steady-state and iR-free ORR polarization curves. Results were obtained in 0.1 M KOH (O2 sat’d) at a scan rate of 10 mV s<sup>-1</sup> over a voltage range of 0.05–1.05 VRHE. The rotational speed of the rotating disk electrode (RDE) was 1600 rpm and the amounts of catalysts used for the measurements were 100 ug<sub>Cat</sub> cm<sup>-2</sup> and 12 ug<sub>Pt</sub> cm<sup>-2</sup>. f) Comparison of the change in half-wave potential before and after ADT (10 000 repetitive cycles over a voltage range of 0.65–1.05 VRHE).</figcaption></figure>

the graphene. Conversely, the samples that were heat-treated for more than 2.5 min did not have a Fe redox characteristic peak, indicating that all the FePc was converted to Fe single atoms or nanoparticles, which is consistent with the previous discussion on the behavior of FePc.

ORR activity was also evaluated using linear sweep voltammetry (LSV) under the same conditions as CV. A rotating disk electrode (RDE) was used as the working electrode and was rotated at 1600 rpm during the measurement (Figure 5e). In a previous study, it was reported that FePc inherently exhibits good ORR activity, which was further confirmed by the FeN<sub>4</sub>/GO<sub>0.0m</sub> samples exhibiting similar or higher ORR activities than that of Pt/C, even before heat treatment.<sup>[53]</sup> The ORR activity of the FeN<sub>4</sub>/NGO<sub>5.0m</sub> sample sharply decreased, which was attributed to the collapse of the FeN<sub>4</sub> structure and the formation of Fe clusters and nanoparticles. In contrast, heat treatment for 1.0 and 2.5 min dramatically enhanced the ORR activity of the catalyst, which can be attributed to the maximization of the electrochemically active sites caused by the distribution of FeN<sub>4</sub> and the ligand effect. Interestingly, FeN<sub>4</sub>/NGO<sub>1.0m</sub> had the highest onset and half-wave potential, followed by FeN<sub>4</sub>/NGO<sub>2.5m</sub>,

FeN<sub>4</sub>/NGO<sub>0.0m</sub>, Pt/C, and FeN<sub>4</sub>/NGO<sub>5.0m</sub>. The higher ORR activity of FeN<sub>4</sub>/NGO<sub>1.0m</sub> compared to FeN<sub>4</sub>/NGO<sub>2.5m</sub> can be attributed to the change in the N structure contained in the graphene support. In N-doped carbon support-based non-precious metal catalysts, the N structure is a major determinant of catalytic activity; a trend of increasing ORR activity in the order of graphitic N, pyrrolic N, and pyridinic N has been reported.<sup>[54]</sup> The results of XPS analysis to identify the N species in FeN<sub>4</sub>/NGO<sub>1.0m</sub> and FeN<sub>4</sub>/NGO<sub>2.5m</sub> are shown in Table S5 (Supporting Information), and the increase in heat treatment time caused a decrease in pyridinic N of ≈19%. The decrease in N species favorable to ORR would have eventually caused the decrease in ORR activity of FeN<sub>4</sub>/NGO<sub>2.5m</sub>. However, despite its excellent activity, FeN<sub>4</sub>/NGO<sub>1.0m</sub> cannot be considered a practical electrochemical catalyst. This is because, as supported by the Fe redox peaks in the CV curve, the FeN<sub>4</sub>/NGO<sub>1.0m</sub> sample contained unreacted FePc and DMF, with FeN<sub>4</sub>—O<sub>DMF</sub> unlikely to stably bind to graphene. This is particularly evident from the electrochemical stability assessments. Figure S15 (Supporting Information) shows the LSV curves before and after 10 000 repetitive CV measurements at a rate of 100 mV s<sup>-1</sup> over a
voltage range of 0.6–1.0 V$_{RHE}$ for each sample. All the LSV curves shifted in the negative direction after the accelerated durability test (ADT), indicating a decrease in the ORR activity; however, the magnitude of the decrease in activity varied from sample to sample. Therefore, for a clear comparison, the retention of the half-wave potential was derived, as shown in Figure 5f. The four samples—Pt/C, FeN$_4$/NGO_0.0m, FeN$_4$/NGO_1.0m, and FeN$_4$/NGO_2.5m—exhibited a decrease in their half-wave potentials by 5.8, 5.1, 1.5, and 0.7%, respectively. This clearly shows that the electrochemical stability of FePc increased rapidly with increasing heat-treatment time, indicating that unreacted FePc is unsuitable as a catalyst, even if it exhibits ORR activity. Considering both the ORR activity and electrochemical stability, FeN$_4$/NGO_2.5m was the optimal catalyst among the heat-treated samples and proved to be superior to commercial Pt/C catalysts with a clearly higher ORR activity and durability.

To confirm the generality of the synthesis mechanism proposed in this work, catalysts were synthesized on different precursors, and the analytical results of the synthesized catalysts are shown in Figure S16 (Supporting Information). The different precursors used were FeCl$_3$, CoPc, and CuPc. As a result, the synthesis of CoN4/NGO catalysts using CoPc precursor was successfully achieved and was found to exhibit compliant ORR activity, adding to the promise of generalized application of this synthesis strategy. For a detailed discussion, please refer to the Supporting Information.

## 2.5. Fuel Cell and Battery Performance

To investigate whether a catalyst synthesized by instantaneous heat treatment could function as an effective ORR catalyst in practical electrochemical energy conversion systems, FeN$_4$/NGO_2.5m was applied to two representative devices that use ORR catalysts in an alkaline atmosphere: a Zn-air battery and an AEMFC. Figure 6a shows the discharge and power density curves for Zn-air batteries using commercial Pt/C and FeN$_4$/NGO_2.5m as the cathode catalysts. The battery with FeN$_4$/NGO_2.5m exhibited a high current density of 217 mW cm$^{-2}$, which is ≈40% higher than that of the battery with Pt/C at the same voltage and peak power density (PPD). A comparison of the PPD of the Zn-air battery using FeN$_4$/NGO_2.5m with those reported for Zn-air batteries over the last five years is shown in Figure 6b and Table S6 (Supporting Information). The 35 Zn-air battery systems selected for comparison were limited to those using transition metal (Fe, Co, and Ni) catalysts. The FeN$_4$/NGO_2.5m system not only ranks in the top 11% of all the compared systems but also has the highest PPD values when compared to single-atom or Fe-based catalysts. Meanwhile, Figure 6c shows galvanostatic discharge curves of the batteries at a current density of 5.0 mA cm$^{-2}$; the Zn-air battery catalyzed by FeN$_4$/NGO_2.5m delivered a stable discharge voltage of ≈1.2 V without undergoing significant degradation. Furthermore, the specific capacity at 5.0 mA cm$^{-2}$ was calculated as 750 mAh g$^{-1}$, which is slightly higher than that of a typical Zn-air battery with Pt/C (740 mAh g$^{-1}$).[55] To evaluate the rechargeability and cycling stability of the air cathode, the battery was cycled at a current density of 10 mA cm$^{-2}$ with each cycle for 100 h (Figure 6d). FeN$_4$/NGO_2.5m exhibited voltaic efficiencies of 51 and 50% at

the beginning and end of the cycle, respectively, with no degradation owing to long-term cycling. In sharp contrast, Pt/C underwent significant degradation, with the voltaic efficiency decreasing from 45% at the beginning of the cycle to 16% at the end of the cycle. These results suggest with great emphasis that FeN$_4$/NGO_2.5m can significantly enhance the efficiency and durability of Zn-air batteries when used as a bifunctional catalyst. The prepared catalyst was applied as the cathode in an AEMFC. Figure 6e shows the current-voltage and power density curves of the AEMFC with FeN$_4$/NGO_2.5m. At a voltage of 0.65 V, it exhibited a current density of 1040 mA cm$^{-2}$, demonstrating outstanding single-cell performance that exceeded the DOE 2022 milestone. In addition, a PPD of ≈881 mW cm$^{-2}$ was achieved, which is in the top 1% of 70 previously reported performances (Figure 6f, Table S7, Supporting Information). In particular, the graph of PPD versus half-wave potential shows that FeN$_4$/NGO_2.5m is the catalyst with the best activity and performance for both half- and single cells. Zn-air batteries and AEMFCs using FeN$_4$/NGO_2.5m exhibited better performance and stability than conventional batteries, which is likely due to the structure of the O sandwiched between FeN$_4$ and GO, that is, FeN$_4$—O—Gr. Thus, it has been demonstrated that instantaneous heat treatment is an effective and promising strategy for synthesizing high-performance single-atom catalysts for energy conversion devices.

## 3. Conclusion

We successfully developed a novel and efficient approach for synthesizing single-atom catalysts via instantaneous heat treatment. By eliminating the need for conventional long-duration heat treatments and additional processes, our method provides a more reliable and streamlined approach for the production of single-atom catalysts. Through advanced analytical methods, we uncovered the mechanism behind the CO-mediated synthesis of the catalyst, demonstrating the formation of a unique structure with O species sandwiched between FeN$_4$ and graphene, resulting in high activity and excellent long-term stability for the ORR. Furthermore, we validated the performance of our Fe single-atom catalysts in actual energy conversion devices, specifically Zn-air batteries and AEMFCs, showing significant improvements in their performance. Overall, our study provides a significant contribution to the development of superior single-atom catalysts. We believe that our findings will inspire further research and advancements in this field.

## 4. Experimental Section

**Catalyst Preparation:** Graphene oxide (GO): Five grams of graphite was ball-milled using 50 g of stainless steel grinding balls for 1 h, following which it was filtered with 500 mL of 1 M HCl and washed with more than 1 L of distilled water to remove any Fe powder generated during the ball milling process. The graphite was then dried in an oven at 100 °C. Subsequently, 2 g of the ball-milled graphite was oxidized by adding steam at 300 °C for 12 h, and the sample was collected after the reaction. The oxidized graphite was subjected to the Hummer's method, which involved adding 1.5 g of the sample, 9 g of KMnO$_4$, 180 mL of H$_2$SO$_4$, and 20 mL of H$_3$PO$_4$ to a three-necked flask and stirring at 50 °C for 12 h. After adding 200 mL of ice-cold distilled water and 4 mL of H$_2$O$_2$ and centrifuging, the resulting sample was dried and recovered. Finally, thermal exfoliation was
<figure><img src="image_16.png" /><figcaption>**Figure 6.** Zn-air battery and AEMFC performance measurements. a) Discharge polarization and power density curves of Zn-air batteries with FeN4/NGO_2.5m and commercial Pt/C as catalysts. The prepared FeN4/NGO_2.5m catalyst had a loading of 2 mg cm⁻², while the commercial Pt/C had a loading of 1 mg cm⁻². b) Comparison with recently reported peak power densities of Zn-air batteries. Yellow and green bars indicate Fe-based and single-atom catalysts, respectively. c) Discharge profile for FeN4/NGO_2.5m at 10 mA cm⁻². d) Galvanostatic discharge-charge curves for FeN4/NGO_2.5m at 10 mA cm⁻². e) Polarization curves and power densities of a single AEMFC fabricated with commercial PtRu/C as the anode catalyst and FeN4/NGO_2.5m as the cathode catalyst (2 mg cm⁻²). f) Half-wave potential versus peak power density of 69 previously reported non-precious metal catalysts for AEMFC.</figcaption></figure>

performed on the recovered sample by heating it to 600 °C under a nitrogen atmosphere for 1.5 min. FeN₄/NGO catalysts: First, 16 mg of FePc was dissolved in 100 ml of DMF and stirred. GO (50 mg) was then added to the solution and stirred for 10 min, followed by sonication for 10 min. The resulting mixture was stirred and allowed to adsorb at room temperature for 12 h. The adsorbed mixture was washed three times with acetone in a centrifuge at 1200 rpm for 10 min, with the resulting FePc/GO oven-dried at 60 °C to obtain the desired catalyst. To synthesize FeN₄/NGO catalysts, the FePc/GO samples were subjected to hourly heat treatment for 1, 2.5, and 5 min each in a furnace pre-heated to 950 °C. Four catalysts were synthesized, including the sample without heat treatment, and were named FeN₄/NGO_0.0m, FeN₄/NGO_1.0m, FeN₄/NGO_2.5m, and FeN₄/NGO_5.0m based on the heat-treatment duration.

**Physical Characterization:** Synchrotron XRD measurements were performed with a PANalytical device using Cu Kα radiation (2θ = 30°–130°), scanning a 2θ range of 20°–90° at 2° min⁻¹. XPS was performed using a Multilab ESCA 2000 system (VG Microtech, East Grinstead, West Sussex, UK) with Al Kα as the X-ray source. Before performing the measurements, the binding-energy scale was calibrated using the corresponding Ag 3d 5/2 photoelectron lines at 368.21 eV. TGA was performed using an SDT Q600 (TA instruments) at a ramp rate of 10 °C min⁻¹ from room temperature to 900 °C and under an inert Ar atmosphere. FT-IR spectra of the samples were recorded using a Thermo Scientific NICOLET iS10 FT-IR spectrometer with a potassium bromide plate. Raman spectroscopy was performed using an InVia Raman microscope (Renishaw) with an Nd: Yad laser source with a wavelength of 532 nm. TEM was performed using
an FEI Titan 80-300 microscope (FEI Company, Hillsboro, Oregon, USA) operating at 300 kV. EA was conducted using TEM EDS (Talos F200X, FEI Company, Hillsboro, Oregon, USA). XAFS measurements were performed at the 8C nanobope XAFS beamline (BL8C) of the Pohang Light Source (PLS-II) in the 3.0-GeV storage ring with a ring current of 250 mA. The X-ray beam was monochromated by a Si(111) double crystal, and the beam intensity was reduced by 30% to eliminate higher-order harmonics. The X-ray beam was then delivered to a secondary source aperture, where the beam size was adjusted to 0.5 mm (v) × 1 mm (h). The XAFS spectra were collected in transmission mode. The obtained spectra were processed using the Demeter package. EXAFS spectra were fitted in a Fourier-transform range of 3–12 Å⁻¹ with a Hanning window applied between 1.2 and 2.7 Å. The amplitude reduction factor (S₀²) was obtained by fitting Fe metal. Wavelet transform analysis was performed using the EvAX software; R₀ was set to 2 for all systems. XRD and TEM were measured in the Future Energy Convergence Core Center (FECC) and the Center for University-Wide Research Facilities (CURF) at Jeonbuk National University, respectively.

**Electrochemical Characterization:** An Autolab PGSTAT302N potential-stat controlled by a computer was used to conduct the electrochemical experiments in a standard three-compartment glass cell at room temperature. The working electrode was a thin layer of Nafion-impregnated catalyst on a glassy carbon disc held in a Teflon cylinder using a RDE setup with a geometric surface area of 0.196 cm². A graphite rod and a saturated calomel electrode were used as the counter and reference electrodes, respectively. The working electrode was intended to have a synthesized-catalyst loading of 100 ugCat cm⁻². The planned amount for the Pt/C catalyst was 12 ugPt cm⁻². Catalyst slurry inks were prepared by mixing catalyst powder in 5 wt.% Nafion solution as a binder and 2-propanol as a solvent, and subsequently sonicating the mixture to obtain a finely dispersed slurry. After loading 5 µL of the ink onto the glassy carbon electrode and drying it, steamed deionized (DI) water was flowed into the RDE equipment to impart the working electrode with surface wettability. All potentials were reported with respect to the reversible hydrogen electrode (RHE). Prior to obtaining the ORR polarization curves, multiple CV measurements were performed in a deaerated 0.1 M KOH solution at a scan rate of 100 mV s⁻¹ in a potential window of 0.05–1.05 V versus RHE at room temperature. The ORR activity was evaluated using the rotating disk electrode technique in an O₂-saturated 0.1 M KOH solution, with a sweep rate of 10 mV s⁻¹ at 1600 rpm at ≈25 °C. ADTs were conducted in an O₂-saturated KOH electrolyte without rotation in a potential range of 0.65–1.05 V at a scan rate of 100 mV s⁻¹ for 10 000 cycles.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2021M3H4A1A02042948, 2020M3H4A3106316), the New & Renewable Energy Core Technology Program of KETEP (20203020030010), the “Regional Innovation Strategy (RIS)” through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE) (2023RIS-008) and the KIST Institutional Program. This research was supported by Chungcheongbuk-do Province and the Convergence and Open Sharing System (COSS) Development Project (2023).

## Conflict of Interest

The authors declare no conflict of interest.

## Author Contributions

I.J., S.L., and D.-G.K. contributed equally to this work. I.J. and S.L. were the primary writers of the manuscript. D.-G.K., S.L., N.D.K., J.Y.J., and P.K. de-

signed catalyst synthesis and performed the electrochemical experiments, characterized the catalyst, and analyzed the data. H.-K.L. designed and performed DFT calculations. V.K.P. and K.-S.L. together designed and performed X-ray absorption spectroscopy and data analysis. S.J.Y. supervised the execution of the overall project.

## Data Availability Statement

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

## Keywords

atomic dispersion, fuel cells, instantaneous heat-treatment, M-N-C catalyst, oxygen reduction reaction, Zn-air batteries

Received: March 4, 2024

Revised: May 5, 2024

Published online: May 28, 2024

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