# Fine-tuning electronic structure of N-doped graphitic carbon-supported Co- and Fe-incorporated Mo2C to achieve ultrahigh electrochemical water oxidation activity

Md. Selim Arif Sher Shah, Hyeonjung Jung, Vinod K. Paidi, Kug-Seung Lee, Jeong Woo Han, Jong Hyeok Park

► To cite this version:

Md. Selim Arif Sher Shah, Hyeonjung Jung, Vinod K. Paidi, Kug-Seung Lee, Jeong Woo Han, et al.. Fine-tuning electronic structure of N-doped graphitic carbon-supported Co- and Fe-incorporated Mo2C to achieve ultrahigh electrochemical water oxidation activity. Carbon Energy, 2024, 6 (7), pp.e488. 10.1002/cey2.488. hal-05118234

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# Fine-tuning electronic structure of N-doped graphitic carbon-supported Co- and Fe-incorporated Mo₂C to achieve ultrahigh electrochemical water oxidation activity

Md. Selim Arif Sher Shah<fnref n="1" />,<fnref n="2" /> ID | Hyeonjung Jung<fnref n="3" /> ID | Vinod K. Paidi<fnref n="4" />,<fnref n="5" /> ID |

Kug-Seung Lee<fnref n="4" /> | Jeong Woo Han<fnref n="3" /> ID | Jong Hyeok Park<fnref n="1" /> ID

## Correspondence

Jong Hyeok Park, Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea.

Email: lutts@yonsei.ac.kr

Kug-Seung Lee, Pohang Accelerator Laboratory, Beamline Division, 80 Jigok-ro 127 beon-gil, 37673 Pohang, Republic of Korea.

Email: lks3006@postech.ac.kr

Jeong Woo Han, Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea.

Email: jwhan98@snu.ac.kr

## Funding information

National Research Foundation of Korea, Grant/Award Numbers: RS-2023-00302697, RS-2023-00268523; SERB-SURE under file number, Grant/Award Number: SUR/2022/003129

## Abstract

Mo₂C is an excellent electrocatalyst for hydrogen evolution reaction (HER). However, Mo₂C is a poor electrocatalyst for oxygen evolution reaction (OER). Herein, two different elements, namely Co and Fe, are incorporated in Mo₂C that, therefore, has a finely tuned electronic structure, which is not achievable by incorporation of any one of the metals. Consequently, the resulting electrocatalyst Co₀.₈Fe₀.₂-Mo₂C-80 displayed excellent OER catalytic performance, which is evidenced by a low overpotential of 214.0 (and 246.5) mV to attain a current density of 10 (and 50) mA cm⁻<fnref n="2" />, an ultralow Tafel slope of 38.4 mV dec⁻<fnref n="1" />, and long-term stability in alkaline medium. Theoretical data demonstrates that Co₀.₈Fe₀.₂-Mo₂C-80 requires the lowest overpotential (1.00 V) for OER and Co centers to be the active sites. The ultrahigh catalytic performance of the electrocatalyst is attributed to the excellent intrinsic catalytic activity due to high Brunauer–Emmett–Teller specific surface area, large electrochemically active surface area, small Tafel slope, and low charge-transfer resistance.

## KEYWORDS

fine-tuning electronic structures, heteronanostructures, Mo₂C, multimetal (Co/Fe), oxygen evolution reaction

<footnote n="1">¹Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, Republic of Korea</footnote>

<footnote n="2">²Department of Chemistry, University of North Bengal, Raja Rammohunpur, Darjeeling, West Bengal, India</footnote>

<footnote n="3">³Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul, Republic of Korea</footnote>

<footnote n="4">⁴Pohang Accelerator Laboratory, Beamline Division, Pohang, Republic of Korea</footnote>

<footnote n="5">⁵European Synchrotron Radiation Facility, France</footnote>
# 1 INTRODUCTION

Electrochemical water splitting (EWS) has been demonstrated to be one of the most promising technologies that has the potential to meet the increasing energy demand of the growing population of the world by supplying clean and sustainable hydrogen energy.¹⁻³ The EWS is the combination of two half-cell reactions that occur separately but simultaneously at the two electrodes—cathode and anode. At the cathode, hydrogen evolution reaction (HER) occurs, while at the anode, oxygen evolution reaction (OER) proceeds. Consequently, the ultimate target is to increase the efficiency of the HER for a sustainable supply of hydrogen energy from the EWS. Unfortunately, it is not possible to increase the HER performance of the EWS without enhancing the efficiency of the OER, which has been regarded to be inherently sluggish and, consequently, decreases the overall efficiency of the EWS.⁴ Therefore, it is required to look for efficient and cost-effective OER electrocatalysts that would overcome the slow OER kinetics and increase the HER efficiency. To surmount the OER kinetics barrier, Ru and Ir oxide-based electrocatalysts have been employed in commercial electrolyzers.⁵⁻⁷ Nevertheless, poor stability, especially at large current densities, and high cost limit their large-scale and long-term applications.⁸,⁹ Therefore, the development of earth-abundant and low-cost electrocatalysts is urgently required.

Recently, transition metal-based materials, electrocatalysts, such as carbides, oxides, nitrides, phosphides, and so forth, have been drawn significant attention for the EWS.⁶,¹⁰⁻¹⁹ Among them, Mo₂C has been paid special attention because of its excellent HER performance originating from similar d-band electronic structures with Pt, which is the state-of-the-art HER electrocatalyst with high electrical conductivity and low cost.²⁰⁻²³ However, the OER performance of Mo₂C is negligible due to the high overpotential that is required to overcome the sluggish OER kinetics, under which the stability of Mo₂C is a serious concern.²⁴ To deal with this shortcoming, significant efforts have been made to increase the OER activity of Mo₂C at low overpotentials.

In this endeavor, Regmi et al.²⁵ engaged significant attention to using crystalline Mo₂C for OER. In their work, the best electrocatalyst displayed an overpotential of approximately 420 mV to achieve a current density of 10 mA cm⁻² (hereafter denoted as η₁₀). Successively, much effort has been exercised employing several approaches, such as different nanostructures of MoₓC,²⁶ hybridization with other materials,¹³,¹⁴,²⁷,²⁸ manipulation of electronic structures,²⁹,³⁰ and so forth to decrease its OER overpotentials. Among these techniques,

electronic structure manipulation is one of the most attractive and promising, therefore extensively studied by many researchers.³¹⁻³³

Although many works were carried out in manipulating the electronic structure of MoₓC to increase its OER performance, these works demonstrated incorporation of a single metal, mainly Co.⁴,¹³,²⁴,³⁴ Incorporation of other metals and multiple metals in Mo₂C is rare. Nevertheless, incorporation of a second metal could lead to the formation of different electronic structures in the resultant electrocatalyst that would be favorable for OER. Therefore, it is absolutely fascinating to fabricate Mo₂C-based electrocatalysts incorporating two metals and to explore their electronic structures and OER properties. Keeping this in mind, herein, we systematically proceed to incorporate two metals in Mo₂C. First, different elements are incorporated in Mo₂C to check the best-performing metal, followed by its partial replacement by other metal to find the best combination. It is observed that the incorporation of Co and its partial replacement by Fe led to the best-performing OER electrocatalyst. Data proved that the electronic structure of Mo₂C is manipulated by incorporation of Co, which is further finely tuned by partial replacement of Co by Fe. The manipulation of the electronic structures is not achievable by incorporation of any one metal. Consequently, the resultant electrocatalysts display significantly enhanced OER activity that is not achievable by the incorporation of any one metal. It is proved that electron transfer occurred from Co to Mo and from Fe to Co and Mo. Theoretical data provide evidence for weakly binding intermediates on Mo₂C containing Co that results in more favorable O₂ production.

# 2 EXPERIMENTAL SECTION

## 2.1 Materials

Ammonium molybdate tetrahydrate (AMT; ACS reagent), Co(NO₃)₂·6H₂O (98%), Fe(NO₃)₃·9H₂O, dopamine hydrochloride, KOH pellets, cobalt acetate tetrahydrate, iron acetate, and 2-aminoterephthalic acid (2ATA) were purchased from Sigma-Aldrich. Ammonia solution (28%-30%), ethanol, methanol, and N,N-dimethylformamide (DMF) were obtained from Samsung Chemical Company. Deionized (DI) water with a resistance of ≥18.2 M Ohm was used whenever required.

## 2.2 Synthesis of Mo polydopamine (MoPDA)

Typically, 450 mg AMT was dissolved in 15 mL DI H₂O, followed by the addition of 5 mL EtOH and stirring for
10 min. To it, a solution of 200 mg dopamine hydrochloride in 5 mL DI H₂O and 5 mL EtOH was added and stirred for 10 min, followed by the addition of 0.45 mL ammonia solution dropwise. The reaction mixture was stirred for 6 h, followed by washing (with DI H₂O and EtOH) and drying at 70°C overnight.

## 2.3 | Synthesis of Coₓ@MoPDA and CoₓFe₁₋ₓ@MoPDA

In a typical experiment, 80 mg MoPDA was dispersed in 25 mL methanol by ultrasonication for 30 min in a cleaning bath ultrasonicator. Then, 420 µL ammonia was added, followed by the addition of Co(NO₃)₂·6H₂O (0.5, 1.0, and 1.5 mmol) in 10 mL methanol. Water (5 mL) was added to the reaction mixture which was then subjected to a continuous vigorous stirring stirring for 30 min. The product was harvested after centrifuging and washing the reaction mixture with methanol once, followed by drying at 70°C overnight. The products were designated as Coₓ@MoPDA-80, where x is Co²⁺ ions in mmol. Similarly, CoₓFe₁₋ₓ@MoPDA was synthesized with the required amount of Co and Fe salt.

## 2.4 | Synthesis of Co-MOF, Fe-MOF, and CoₓFe₁₋ₓ-MOF

Typically, 2 mmol Co(OAc)₂·4H₂O or Fe(OAc)₂ in 20 mL DMF was rapidly added to a solution of 2 mmol 2ATA in 10 mL DMF, followed by vigorous stirring for 1.5 h. The product was collected after washing with DMF twice and EtOH thrice, followed by drying at 70°C overnight. Similarly, CoₓFe₁₋ₓ-MOF was synthesized with the required amount of Co and Fe salt.

## 2.5 | Carbonization

Carbonization was carried out at 700°C, if not mentioned otherwise, under an Ar atmosphere in a tube furnace at a heating ramp of 3°C min⁻¹ and maintained at that temperature for 2 h, followed by cooling at 3°C min⁻¹. The products were denoted as CoₓFe₁₋ₓ-Mo₂C-80.

## 2.6 | Characterization

Powder X-ray diffraction (XRD) patterns were obtained with a Siemens D500/5000 diffractometer and Cu Kα radiation (λ = 1.5418 Å) over the 2θ range from 2° to 80° with a scan rate of 3° min⁻¹. The accelerating voltage and

applied current were 40 kV and 40 mA, respectively. Scanning electron microscopy images were collected with a field-emission scanning electron microscope (FESEM; JSM-7600F). Energy dispersive X-ray spectrum (EDX) spectroscopy was acquired with an accessory attached to the scanning electron microscope. Transmission electron microscopy (TEM) (JEM F200; JEOL) was carried out with an acceleration voltage of 300 kV. High-angle annular dark-field scanning TEM (HAADF-STEM) images and elemental maps were collected using the same instrument equipped with an EDX spectroscopy accessory. The Raman spectra were acquired using a Micro-Raman spectrometer system (Bruker FRA 160S). The sample was loaded on a silicon wafer and focused with a ×50 objective. Spectra were collected in the range of 1 cm. The Brunauer–Emmett–Teller (BET) specific surface area and porosity of the samples were evaluated on the basis of nitrogen adsorption isotherms measured at –196°C using a gas adsorption apparatus (ASAP 2020; Micromeritics). All the samples were degassed at 120°C before the nitrogen adsorption measurements. The BET-specific surface area was determined using multipoint adsorption data over the relative pressure (P/P₀) ranging from 0.06 to 0.2. The pore volume and pore size distribution of the samples were determined from the adsorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) characterization was performed in an instrument (Thermo VG) with an Al Kα X-ray (1486.6 eV) source. All the binding energy values were corrected by calibrating the C 1s peak at 284.6 eV. High-resolution peaks were deconvoluted using Gaussian–Lorentzian functions with identical full width at half maxima after a Shirley background subtraction.

## 2.7 | Electrochemical measurements

All electrochemical measurements were carried out in a three-electrode system wherein Hg/HgO (1 M NaOH) was used as reference electrode and Pt mesh served as counter electrode using a CHI potentiostat (CHI 660; CH Instrument). A glassy carbon electrode (GCE, diameter: 3 mm) with catalyst ink coating was used as the working electrode. Catalyst ink was prepared by dispersing 5 mg catalyst in a mixture of 0.7 mL DI H₂O, 0.27 mL EtOH, and 30 µL Nafion (5 wt%) by ultrasonication for 30 min. The ink (5 µL) was dropped on the GCE which was then dried. Catalyst loading was ~0.4 mg cm⁻². Linear sweep voltammetry (LSV) was measured in a N₂-saturated 1.0 M KOH solution (pH 13.8) in the potential range from 0.90 to ~1.6 V for OER and 0.2 to −0.3 for the HER (vs. reversible hydrogen electrode [RHE]) at 5 mV s⁻¹ scan rate.
Potentials were iR-compensated using the inbuilt program in the instrument used for measurements. The formula used to convert the potentials was $E_{RHE} = E_{Hg/HgO} + 0.14 + 0.059 \times \text{pH}$. The overpotential was calculated by using the equation $\eta = E_{RHE} - 1.23 \text{ V}$ for the OER. The electrical double-layer capacitance ($C_{dl}$) was calculated from the CV curves recorded over the potential window from 0.2 to 0.3 V (vs. Hg/HgO) at scan rates from 20 to 200 mV s$^{-1}$. Electrochemical impedance spectra (EIS; CHI608C model) were measured in the frequency range of 50 mHz to 100 kHz with 5 mV of amplitude.

## 2.8 | X-ray absorption fine structure (XAFS) measurement

XAFS was measured at the 8C nanoprobe XAFS beam-line (BL8C) of Pohang Light Source-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 the higher-order harmonics. The X-ray beam was then delivered to a secondary source aperture where the beam size was adjusted to be 0.5 mm (v) × 1 mm (h). The XAFS spectra were collected at room temperature in transmission mode. The detectors were ionization chamber based. For the Co and Mo K-edge measurements, the monochromator energies were calibrated using Co and Mo foils, respectively. All the obtained spectra were processed using the Demeter package. The reference materials used for the linear combination fitting (LCF) analysis of the Mo spectra were Mo foil (0), Mo$_2$C (+2), MoO$_2$ (+4), and MoO$_3$ (+6), where the numbers in the parenthesis are their corresponding oxidation states. Similarly, the reference materials used for the LCF analysis of the Co spectra were Co metal (0), CoO (+2), and Co$_2$O$_3$ (+3). The extended X-ray absorption fine structure (EXAFS) spectra were modeled over a Fourier transform range from ~3 to 12 Å$^{-1}$ with a Hanning window applied between ~1.2 and 3.2 Å, depending on the data. The amplitude reduction factor ($S_o^2$) was obtained by fitting the Mo metal. The scattering paths used for the EXAFS analysis were guided by the XRD results. The Nyquist criterion was considered to avoid overparameterization of the EXAFS fits.

## 2.9 | Density functional theory (DFT) calculations

DFT calculations were performed using the Vienna ab initio Simulation Package.35,36 The Perdew-Burke-Ernzerhof exchange-correlation functional37 and projector

augmented-wave38 pseudopotential were adopted with spin polarization. Plane-wave cutoff energy was set to 400 eV for geometry optimization and 520 eV for electronic structure calculation. During geometric optimization, the convergence criterion of total energy was set to $10^{-4}$ eV and the atoms were relaxed until the force acting on each atom was less than 0.03 eV Å$^{-1}$. Brillouin-zone sampling was sampled with 12 × 12 × 12, 12 × 12 × 12, and 8 × 8 × 6 Monkhorst-Pack grid39 for Co bulk, Fe bulk, and Mo$_2$C bulk, respectively. Modeling grain boundary heterostructure consisting of α-Mo$_2$C and metal bulk, we aligned α-Mo$_2$C(010), fcc Co(111), and bcc Fe(110) planes on the same side as exposed surfaces. Brillouin-zone sampling was sampled with 3 × 1 × 3, 2 × 1 × 2, 3 × 1 × 1, and 2 × 1 × 1 Monkhorst-Pack grid for Co$_1$-Mo$_2$C-80 bulk, Fe$_1$-Mo$_2$C-80 bulk, Co$_1$-Mo$_2$C-80 slab, and Fe$_1$-Mo$_2$C-80 slab models, respectively. Each slab model was separated from its neighbors by a sufficiently thick vacuum layer (~10 Å) to diminish the virtual interaction between periodic cells. Gibbs free energies were calculated by correcting the DFT energy (E) with entropy (S) energy at room temperature ($T = 298.15$ K), and zero-point energy (ZPE) using the following equation.

$$ \Delta G = \Delta E - T\Delta S + \Delta ZPE. $$

The computational hydrogen electrode method was applied to calculate the energy of proton and electron.

# 3 | RESULTS AND DISCUSSION

## 3.1 | Construction of Co$_x$Fe$_{1-x}$-Mo$_2$C-80 heteronanostructures

To begin with, the precursor MoPDA microspheres were synthesized following our previous report.40 Co$^{2+}$ and Fe$^{3+}$ ions were immobilized on MoPDA with the help of an aqueous ammonia solution, followed by carbonization that led to the formation of the desired product, as shown in the schematic (Figure 1A).

## 3.2 | Characterization of the Co$_x$Fe$_{1-x}$-Mo$_2$C-80 heteronanostructures

Crystal structure of the heteronanostructures of Co$_{0.8}$Fe$_{0.2}$-Mo$_2$C-80 was identified with powder XRD, which confirmed (red circles) the presence of face-centered cubic phases of Co (PDF #00-015-0806)41,42 and body-centered cubic Fe (PDF #00-001-1267)43 and α-Mo$_2$C (PDF #01-035-0787)20,44 (Figure 1B). The Rietveld refinement (solid black line) of the XRD pattern of Co$_{0.8}$Fe$_{0.2}$-Mo$_2$C-80 proved the presence of Mo$_2$C, Co,
<figure><img src="image_2.png" /><figcaption>FIGURE 1 (A) Schematic followed to synthesize the materials, where M1 and M2 are two different metals. (B) X-ray diffraction pattern of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 and Rietveld refinement. Red circles are observed data (Obs) and solid black line is calculated data (Calc). The gray line at the bottom depicts the difference (Obs – Calc) between data and fit. (C) N<sub>2</sub> adsorption-desorption isotherms of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 and its pore size distribution (inset).</figcaption></figure>

and Fe. The difference (Obs – Calc) between the data and Rietveld refinement is shown below by a solid gray line. Lattice parameters, estimates of crystallite size, and phase fractions are summarized in Table S1. Notably, no peak characteristics of metal nitrides were detected, confirming the absence of metal nitride. XRD patterns of controls are depicted in Figure S1 and explained therein. Notably, the diffraction pattern of Co<sub>0.8</sub>Fe<sub>0.2</sub>-NC showed a peak at approximately 26°, which confirmed the presence of carbon.<sup>45,46</sup>

Nitrogen adsorption-desorption isotherms (Figures 1C and S2) were of typical type IV, which confirmed the presence of micropores and mesopores in the synthesized materials.<sup>47</sup> The heteronanostructure Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 possessed high BET specific surface area (84 m<sup>2</sup> g<sup>-1</sup>) determined from the N<sub>2</sub> sorption data (Table S2). The BJH desorption pore volume and pore diameter (inset in Figure 1C) of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 were 0.24 cm<sup>3</sup> g<sup>-1</sup> and 3.8–7.3 nm, respectively. High specific surface area, presence of micropores and mesopores, and large pore volume could benefit exposure of the catalytic active sites and mass transfer during the O<sub>2</sub> evolution processes.<sup>47</sup> Consequently, high

OER performance of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 heteronanostructures was anticipated.<sup>48</sup> The N<sub>2</sub> sorption isotherms, pore diameter, and pore volume of different materials are shown in Figure S2, and the data are assembled in Table S2.

The morphologies of MoPDA, Co<sub>0.8</sub>Fe<sub>0.2</sub>-MoPDA-80, and Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 were characterized with FESEM and TEM. FESEM image of MoPDA (Figure 2A) and the derived Mo<sub>2</sub>C (Figure S3) displayed three-dimensional (3D) microflower-like morphology with size of approximately 1 µm. After the incorporation of Co<sup>2+</sup> or Co<sup>2+</sup> and Fe<sup>3+</sup>, the MoPDA microflowers were destroyed partly because of the acidic nature of the Co and Fe salts (Figure 2B). Nevertheless, the FESEM micrographs (Figures 2C, S3, and S4) showed that the majority of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 particles were present as 3D microflowers and their size was approximately 1 µm.

TEM micrograph (Figure 2D) confirmed the size of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 particles, validating the FESEM data. High-resolution TEM (HRTEM) micrograph (Figure 2E) revealed interplanar spacings of 2.02, 2.08, and 2.33 Å, which were identified as Co(111),<sup>13,49</sup> Fe(202),<sup>50</sup> and Mo<sub>2</sub>C(002)<sup>51</sup> planes, respectively. In addition, the line spacing of 3.5 Å
<figure><img src="image_3.png" /><figcaption>FIGURE 2 FESEM of (A) MoPDA, (B) Co<sub>0.8</sub>Fe<sub>0.2</sub>-MoPDA-80, and (C) Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 heteronanostructure. (D) TEM, (E) HRTEM, and (F) HAADF-STEM images of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 heteronanostructure. (G) Elemental mapping, (H) atomic composition, and (I) EDX spectrum of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 heteronanostructure.</figcaption></figure>

(Figure 2E) proved the presence of (002) planes of graphitic carbon (Figure S1C–E). Pores are also evident in the HRTEM image. The HAADF-STEM image (Figure 2F) was consistent with the FESEM and TEM micrographs. The HAADF-STEM micrograph further confirmed the porous structure of the heteronanostructures of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 that would be beneficial for effective mass transport for the OER. Elemental mapping (Figure 2G) shows a uniform distribution of Co, Fe, Mo, C, O, and N throughout the heteronanostructures. Figure 2H describes the atomic composition of these elements obtained from EDX (from HAADF-STEM), which is consistent with the Rietveld refinement (Table S1) for metallic constituents. The EDX spectrum (Figure 2I) further ascertained the presence of the elements mentioned above.

The surface electronic properties of the synthesized materials were characterized by XPS. Survey XPS spectrum (Figure S5) of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 demonstrated the presence of Mo, Co, Fe, C, N, and O, the concentrations of which are assembled in Table S3. Notably, the concentrations were consistent with the inductively coupled plasma optical emission spectroscopy data. The high-resolution XPS (HRXPS) spectrum of Co 2p (Figure S6) contained a doublet at 778.3 and 793.3 eV due to, respectively, 2p<sub>3/2</sub> and 2p<sub>1/2</sub> of metallic Co.<sup>4,19,42</sup> Additionally, the doublet centered at 781.2 and 797.0 eV was arose from, respectively, 2p<sub>3/2</sub> and 2p<sub>1/2</sub> of Co(+2).<sup>4,13,19</sup> Comparison of the peak intensities proved that most of the cobalt was present in the metallic state as observed in the XRD pattern. A doublet for satellite
peaks appeared at 787.3 and 803.0 eV.¹⁹,⁵² The HRXPS spectrum of Fe 2p (Figure S7) possessed a peak centered at 706.8 eV, which was assigned to metallic Fe (see XRD).⁵³

The HRXPS spectrum of Mo 3d (Figure 3A) displayed a doublet centered at 232.3 and 235.6 eV due to the +6 state of Mo 3d<sub>5/2</sub> and 3d<sub>3/2</sub>, respectively.<sup>40,54</sup> The peak at 231.4 eV was due to Mo(+4).<sup>55</sup> Presence of Mo(+4/+6) species was ascribed to the inevitable surface oxidation of Mo<sub>2</sub>C.<sup>44,54</sup> The doublet at 228.6 and 231.4 eV was assigned, respectively, to the 3d<sub>5/2</sub> and 3d<sub>3/2</sub> of Mo(+2) from Mo<sub>2</sub>C.<sup>44</sup> The peak at 228.3 was due to metallic Mo. Therefore, the XPS results were consistent with the XRD data. The C 1s XPS spectrum (Figure 3B) showed a major peak centered at 284.6 eV due to the C=C bonds, demonstrating the presence of graphitic carbon as the main carbon species that was observed in HRTEM.<sup>56</sup>

Moreover, Figure 3B proved the presence of Mo-C (283.5 eV)<sup>47,56</sup> and C-N/C=N (285.7 eV).<sup>57</sup> In the meantime, Raman spectrum (Figure S8) of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 showed two peaks at ~1348 and 1576 cm<sup>-1</sup>, respectively, due to the defect or disorder-induced D band and the first-order graphite G band.<sup>58</sup> Notably, the intensity of the G band (I<sub>G</sub>) was stronger than that of the D band (I<sub>D</sub>), proving the high degree of graphitization, which is clear from the I<sub>D</sub>/I<sub>G</sub> ratio that was calculated to be 0.73. Such a high degree of graphitization was due to the metal nanoparticles generated during the synthesis that crystallized the carbon at a temperature of 700°C (also see Figure S1C-E).<sup>59</sup> The N 1s XPS spectrum (Figure S9) showed the presence of pyridinic and graphitic N centered at ~398.3 and 401.4 eV, respectively.<sup>15,57</sup> Notably, no peak was observed for metal nitride (see XRD data).<sup>50</sup> Furthermore, Figure S9 exhibited a peak at

<figure><img src="image_4.png" /><figcaption>FIGURE 3 High resolution XPS spectrum of (A) Mo 3d and (B) C 1s in Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80. (C) XPS spectra of Mo 3d in Mo<sub>2</sub>C, Co<sub>1</sub>-Mo<sub>2</sub>C-80, and Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80. (D) XPS spectra of Co 2p in Co<sub>1</sub>-Mo<sub>2</sub>C-80 and Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 (inset highlights the difference between the peaks).</figcaption></figure>
~394.5 eV due to Mo 3p.⁶⁰ O 1s XPS spectrum (Figure S10) proved the presence of Mo–O bond.⁶¹

Comparison of Mo 3d XPS spectra of different materials (Figure 3C) expressed that the Mo 3d peak at ~232.5 and 229.2 eV in Mo₂C was shifted to lower energy in Co₁–Mo₂C-80, which was attributed to the electron transfer from Co to Mo. Notably, after partial replacement of Co in Co₁–Mo₂C-80 by Fe, these peaks (in Co₀.₈Fe₀.₂–Mo₂C-80) remained almost at the same positions, which indicated a similar electronic environment around Mo (compared to Co₁–Mo₂C-80). However, in Co₀.₈Fe₀.₂–Mo₂C-80, there is a lesser amount of Co (compared to Co₁–Mo₂C-80) and an added amount of Fe. Therefore, Fe donated electrons to Mo or Co, from which extra charge was transferred to Mo. These data proved that the electronic structures of Mo₂C were manipulated by Co (in Co₁–Mo₂C-80), and partial replacement of Co by Fe finely tuned the electronic structures of Mo in Co₀.₈Fe₀.₂–Mo₂C-80 heteronanostructures. Comparison of Co 2p XPS spectra (Figure 3D) revealed that partial replacement of Co by Fe shifted the peak at ~782 eV (in Co₁–Mo₂C-80) to lower energy (in Co₀.₈Fe₀.₂Mo₂C-80) by ~0.33 eV (inset), due to electron flow from Fe to Co. Thus, Fe tuned the electronic structures of Co and Mo in Co₀.₈Fe₀.₂–Mo₂C-80 heteronanostructures.

The synthesized materials were further characterized with X-ray absorption near-edge structure (XANES) and EXAFS spectroscopy. The XANES spectra of Mo K-edge (Figures 4A and S11) showed that the pre-edge intensity of Co₁–Mo₂C-80 decreased in comparison with that of Mo₂C, which confirmed the decreased concentration of higher valent Mo species (+4 and/or +6) in the former material.⁴⁰ This was due to electron transfer from Co to Mo. Similarly, the pre-edges of all CoₓFe₁₋ₓ–Mo₂C-80 heteronanostructures (where x = 0.7, 0.8, and 0.9) exhibited diminished intensity (of variable degrees; Figure S11) in comparison to Mo₂C. Therefore, in these materials, Mo functioned as electron acceptor, whereas Co and Fe (see XPS analysis) acted as electron donors. Notably, among all the CoₓFe₁₋ₓ–Mo₂C-80 heteronanostructures, Co₀.₈Fe₀.₂–Mo₂C-80 was characterized with the highest pre-edge intensity (Figure S11) that established the highest concentration (61 ± 3%) of Mo(+6) (Table 1). Therefore, there were optimum concentrations of Co and Fe that determined the contents of different species of Mo.

Notably, we compared the oxidation state obtained through edge interpolation in the heteronanostructures and cross-referenced it with the results of linear combination analysis. Specifically, for the interpolation analysis, we focused on the edge region rather than the pre-edge region. According to the edge region, the energies corresponding to the standards Mo(0) foil,

MoO₂(+4), and MoO₃(+6) are 20011.0, 20019.2, and 20020.2 eV, respectively (Figure 4B). Consequently, employing the interpolation method, the determined oxidation state of Mo in Co₀.₈Fe₀.₂–Mo₂C-80 heteronanostructures was 4 (Figure 4C). Conversely, by using linear combination analysis (Table 1), the oxidation state was calculated to be 4.6 (calculated as [(0.41 × 0) + (0.24 × 4) + (0.61 × 6)]). The slight difference in the oxidation states can be attributed to the variations in the local geometry.

Moreover, EXAFS spectra of Mo in R-space (Figure S12) demonstrated that the local structure of Mo₂C was significantly different from that of the heteronanostructures containing Co and Co with Fe. Notably, all the heteronanostructures of CoₓFe₁₋ₓ–Mo₂C-80 demonstrated similar local environment of Mo. However, Co₀.₈Fe₀.₂–Mo₂C-80 displayed increased Mo–C/O shell intensity, which proved increased coordination intensity of Mo, consistent with the Mo K-edge XANES data. These data proved that the electronic structure of Mo₂C was manipulated by the presence of Co, which in turn was finely tuned by partial replacement of Co by Fe.

The XANES spectra of Co K-edge of different materials are presented in Figure 4C, which demonstrated that the pre-edge intensity and position (~7712 eV) of the synthesized materials were similar in nature. However, the white line position (~7727 eV) and intensity are different. Figure 4D demonstrates that the white lines for Co-NC and Co₁–Mo₂C-80 were similar, although they were significantly different from that of Co₀.₈Fe₀.₂-NC, for which this line shifted to higher energy. Similarly, for the CoₓFe₁₋ₓ–Mo₂C-80 heteronanostructures, with the increasing amount of Fe (or decreasing amount of Co), the white line shifted to the higher energy, which indicated an increased fraction of Co existing in the higher oxidation state (valence state; Table 1).⁴² Table 1 clearly shows that Co₀.₈Fe₀.₂–Mo₂C-80 contained a maximum concentration (13 ± 1%) of Co(+2) species. These results were attributed to charge transfer from Co. On the other hand, EXAFS spectra of Co in R-space (Figure S13) of CoₓFe₁₋ₓ–Mo₂C-80 exhibited a strong peak centered at ~2.10 Å due to Co–Co single scattering, which indicated that the local structure of Co was dominantly metallic (see XRD, XPS, and XANES data). In addition, comparison of the EXAFS spectra of CoₓFe₁₋ₓ–Mo₂C-80 revealed that Co₀.₈Fe₀.₂–Mo₂C-80 displayed lowest intensity, demonstrating a lower fraction of Co engaged in Co–Co interaction, which was ascribed to the increased interaction of Co with Mo in the heteronanostructure.⁴² Therefore, it was clear that the XANES and EXAFS data are in mutual agreement with each other, which was consistent with the XRD and XPS analyses. In the meantime, Bader charge analysis proved
<figure><img src="image_5.png" /><figcaption>FIGURE 4 Mo K-edge (A) XANES spectra of (1) Mo(0) foil, (2) Mo(+4), (3) Mo(+6), (4) Mo₂C, (5) Co₁–Mo₂C-80, (6) Co₀.₉Fe₀.₁–Mo₂C-80, (7) Co₀.₈Fe₀.₂–Mo₂C-80, and (8) Co₀.₇Fe₀.₃–Mo₂C-80. (B) Normalized Mo K-edge XANES of Co₀.₈Fe₀.₂–Mo₂C-80, along with the experimental standards corresponding to Mo(0) foil, MoO₂(+4), and MoO₃(+6). Edge region was chosen for the interpolation analysis, and increase in oxidation state shifts the energy (edge) to higher values (indicated by the arrow). (C) Mo oxidation states of various standards were plotted as a function of the edge position. Mo(0) corresponds to Mo foil, Mo(+4) corresponds to MoO₂, and Mo(+6) corresponds to MoO₃ standards. The three standards were fit to a straight line and the Co₀.₈Fe₀.₂–Mo₂C-80 catalysts oxidation state was estimated from the interpolation analysis. (D) Co K-edge XANES spectra of (1) Co foil, (2) Co₁–Mo₂C-80, (3) Co-NC, (4) Co–Fe₂C, (5) Co₀.₉Fe₀.₁–Mo₂C-80, (6) Co₀.₈Fe₀.₂–Mo₂C-80, and (7) Co₀.₇Fe₀.₃–Mo₂C-80.</figcaption></figure>

<table><caption>TABLE 1 Normalized concentrations (atom %) of different species of Mo, Co, and Fe obtained from the XANES data.</caption><thead><tr><th>Material<sup>a</sup></th><th>Mo(0)</th><th>Mo(+4)</th><th>Mo(+6)</th><th>Co(0)</th><th>Co(+2)</th><th>Co(+3)</th><th>Fe(0)</th><th>Fe(+2/3)</th></tr></thead><tbody><tr><td>Mo₂C</td><td>-</td><td>21 ± 1</td><td>79 ± 2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Co-NC</td><td>-</td><td>-</td><td>-</td><td>-</td><td>93 ± 1</td><td>7 ± 1</td><td>-</td><td>-</td></tr><tr><td>Co₁–Mo₂C-80</td><td>21 ± 2</td><td>20 ± 1</td><td>59 ± 3</td><td>88 ± 1</td><td>12 ± 1</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Co₀.₈Fe₀.₂–NC</td><td>-</td><td>-</td><td>-</td><td>70 ± 1</td><td>7 ± 1</td><td>23 ± 1</td><td>54 ± 1</td><td>46 ± 3</td></tr><tr><td>Co₀.₉Fe₀.₁–Mo₂C-80</td><td>17 ± 2</td><td>28 ± 2</td><td>54 ± 3</td><td>91 ± 1</td><td>9 ± 1</td><td>-</td><td>77 ± 2</td><td>23 ± 3</td></tr><tr><td>Co₀.₈Fe₀.₂–Mo₂C-80</td><td>14 ± 2</td><td>24 ± 2</td><td>61 ± 3</td><td>87 ± 1</td><td>13 ± 1</td><td>-</td><td>81 ± 1</td><td>19 ± 3</td></tr><tr><td>Co₀.₇Fe₀.₃–Mo₂C-80</td><td>22 ± 2</td><td>28 ± 2</td><td>51 ± 3</td><td>65 ± 2</td><td>9 ± 1</td><td>27 ± 2</td><td>89 ± 1</td><td>11 ± 1</td></tr></tbody></table>

Abbreviations: NC, N-doped carbon; XANES, X-ray absorption near-edge structure.

<footnote>ᵃConcentration of each metal was normalized to 100% to maintain simplicity and avoid fraction.</footnote>
charge transfer from Co to Mo (Figure S32) and from Fe to Mo (Figure S33), supporting the experimental data.

## 3.3 | Electrochemical OER performance

The electrocatalysts were designed by systematic analyses of the OER performances of Mo₂C that incorporated different metals and, subsequently, suffered from partial substitution by partial substitution (Figures S16–S20) to find the best catalyst. The onset overpotentials (at which the current density was 1.0 mA cm⁻²) of different electrocatalysts are shown in Figure S21. Figures 5A, S19, and S21 depicted that Mo₂C containing Co (Co₁–Mo₂C-80) and Fe (Fe₁–Mo₂C-80) showed better OER performance than pristine Mo₂C, although the effect was more pronounced for Co. Consequently, Co₁–Mo₂C-80 was taken as the base material, wherein Co was partially replaced by Fe. It was observed that partial replacements of Co by Fe further improved the OER catalytic activity. The heteronanostructures Co₀.₈Fe₀.₂–Mo₂C-80 was found to be the best electrocatalyst for OER that displayed an overpotential of 214.0 mV as η₁₀ (against geometrical surface area of the electrode). However, Co₀.₉Fe₀.₁–Mo₂C-80 and Co₀.₇Fe₀.₃–Mo₂C-80 demonstrated, respectively, 236.4 and 240.2 mV overpotential as η₁₀. Studies of the electrochemical performance of the heteronanostructures (synthesized at different temperatures) revealed that the electrocatalyst synthesized at 700°C (Figure S22) showed the lowest η₁₀ and η₅₀ (overpotential to achieve 50 mA cm⁻² current density). Therefore, the temperature 700°C was chosen as the synthesis temperature. Moreover, the best catalytic activity of the electrocatalyst was observed when the weight of MoPDA was 80 mg (Figure S23).

The bar diagram in Figure 5B represented that the electrocatalyst Co₀.₈Fe₀.₂–Mo₂C-80 displayed an η₅₀ of 246.5 mV, which was the lowest overpotential among all the catalysts discussed in this work. These data confirmed that Co₀.₈Fe₀.₂–Mo₂C-80 was not only the best electrocatalyst in this work but also one of the best among carbide and noncarbide-based electrocatalysts, such as layered double hydroxide (LDH), alloy, nanoparticles, and so forth reported in literature as evidenced in Table 2.⁶²⁻⁶⁶ However, high electrical conductivity and excellent HER activity of Mo₂C offered added advantages over LDH.²⁰,²¹,⁵⁷

Kinetic information of the OER processes was collected from the Tafel slope. Figures 5C and S24 confirmed that Co₀.₈Fe₀.₂–Mo₂C-80 exhibited the lowest Tafel slope of 38.4 mV dec⁻¹, which demonstrated that a small increase in applied potential could cause a fast

increment of O₂ generation rate. Therefore, the electrocatalyst Co₀.₈Fe₀.₂–Mo₂C-80 was characterized with the fastest OER kinetics. The value of the Tafel slope is one of the lowest in the literature, indicating that Co₀.₈Fe₀.₂–Mo₂C-80 was one of the best electrocatalysts (Table 2). Contrarily, Co₀.₉Fe₀.₁–Mo₂C-80 and Co₀.₇Fe₀.₃–Mo₂C-80 exhibited Tafel slopes of 43.6 and 50.7 mV dec⁻¹, respectively. Tafel slope was demonstrated to be a measure of the intrinsic catalytic activity of an electrocatalyst. Therefore, Co₀.₈Fe₀.₂–Mo₂C-80 possessed the highest intrinsic catalytic activity.

Electrochemically active surface area (ECSA) was measured from the C₉₁ obtained from the CVs measured at different scan rates in the non-Faradaic region of the electrochemical potential window (Figures S25 and S26). The plot of scan rate against the difference of (anodic and cathodic) current density (Figure 5D) resulted in straight lines, the slope of which is twice of C₉₁.⁶⁷ Figure 5E delineated that the ECSA of the electrocatalyst Co₀.₈Fe₀.₂–Mo₂C-80 was 781 cm²ECSA², which was greater than that of Co₀.₉Fe₀.₁–Mo₂C-80 (696 cm²ECSA²) and Co₀.₇Fe₀.₃–Mo₂C-80 (497 cm²ECSA²). As the number of catalytic active sites is proportional to the ECSA, these data proved that Co₀.₈Fe₀.₂–Mo₂C-80 possessed the highest number of catalytic active sites among all CoₓFe₁⁻ₓ–Mo₂C-80 electrocatalysts. Figure 5F displayed ECSA normalized OER activity (jECSA) that determined intrinsic catalytic activity. The data illustrated that at 250 mV overpotential, the electrocatalyst Co₀.₈Fe₀.₂–Mo₂C-80 achieved the highest current density of 5.0 mA cm²ECSA². In contrast, Co₀.₉Fe₀.₁–Mo₂C-80 and Co₀.₇Fe₀.₃–Mo₂C-80 displayed jECSA of 1.9 and 2.2 mA cm²ECSA², respectively, at the same overpotential. Notably, Mo₂C and Co₁–Mo₂C-80 exhibited jECSA of, respectively, 0.01 and 1.1 mA cm²ECSA² at 250 mV overpotential. These data confirmed the effectiveness of the strategy to partially replace Co by Fe to generate highly efficient OER electrocatalysts with high intrinsic catalytic activities.

Mass activity (jₘ, geometrical current density, in A cm⁻², per unit mass of the catalyst loading in gram)⁶⁸ was plotted in Figure 5G, which established that the Co₀.₈Fe₀.₂–Mo₂C-80 heteronanostructures experienced a mass normalized current density of 295.4 A cm⁻² g⁻¹ at 220 mV overpotential. Contrarily, Co₀.₉Fe₀.₁–Mo₂C-80 and Co₀.₇Fe₀.₃–Mo₂C-80 demonstrated jₘ of, respectively, 80.6 and 78.9 A cm⁻² g⁻¹ at the same overpotential. Meanwhile, Co₁–Mo₂C-80 exhibited jₘ of 99.9 A cm⁻² g⁻¹. Therefore, jₘ of the electrocatalyst Co₀.₈Fe₀.₂–Mo₂C-80 was ~3 times that of Co₁–Mo₂C-80, which proved that partial replacement of Co by Fe had great potential to design high-performance electrocatalyst for OER. These data confirmed exceptional intrinsic catalytic activity of the electrocatalyst Co₀.₈Fe₀.₂–Mo₂C-80.
<figure><img src="image_6.png" /><figcaption>FIGURE 5 (A) LSV curves at a scan rate of 5 mV s⁻¹ in the potential window of 0.7 V and (B) bar plot of different materials. Overpotentials to reach 10 and 50 mA cm⁻² current densities were marked with two dashed lines. (C) Tafel slopes, (D) pseudocapacitance, and (E) ECSA of different electrocatalysts. (F) ECSA- and (G) mass-normalized current densities of various catalysts. (H) EIS results of different catalysts. Insets are high-magnification EIS spectra and equivalent circuit of the spectra. (I) LSVs of Co₀.₈Fe₀.₂-Mo₂C-80 for pristine electrode and after 5,000th and 10,000th CV cycles, at 5 mV s⁻¹ scan rate. CVs were collected at 40 mV s⁻¹ scan rate in a potential window of 0.7 V.</figcaption></figure>

EIS spectra (Figures 5H, S27, and Table S4) confirmed the lowest charge-transfer resistance ($R_{ct}$) of 1.10 Ω for Co₀.₈Fe₀.₂-Mo₂C-80 against 1.65 and 1.75 Ω for Co₀.₇Fe₀.₃-Mo₂C-80 and Co₀.₉Fe₀.₁-Mo₂C-80, respectively. In addition, the Warburg impedance ($Z_W$) of Co₀.₈Fe₀.₂-Mo₂C-80 was the smallest (12.67 Ω), confirming the fastest mass transport to/from the electrode. Notably, Co₀.₇Fe₀.₃-Mo₂C-80 and Co₀.₉Fe₀.₁-Mo₂C-80 displayed $Z_W$ of, respectively, 23.21 and 13.57 Ω. An equivalent circuit was depicted in the inset. Consequently, the heteronanostructures Co₀.₈Fe₀.₂-Mo₂C-80 was one of the most efficient electrocatalysts for OER (Table 2) due to fast reaction kinetics, large ECSA, high intrinsic catalytic activity, and low $R_{ct}$.

Long-term stability, an important characteristic of electrocatalysts, of Co₀.₈Fe₀.₂-Mo₂C-80 was measured by cycling the electrode in the potential window at 40 mV s⁻¹, followed by collecting LSVs. Notably, the cycling of the electrodes lasted for over 97 h. The result illustrated that the electrocatalyst experienced similar i-V characteristics after 5,000 and 10,000 CVs (Figure 5I) with the pristine electrode, except a minute increase in potential of, respectively, 9 and 27 mV at 50 mA cm⁻² (7 and 13 mV at 10 mA cm⁻²) current density, demonstrating excellent stability of the electrocatalyst. Therefore, the electrode was stable for at least 97 h. Moreover, the durability of the electrode was tested by chronoamperometric experiment (Figure S28) at 230 mV
<table><caption>TABLE 2 Comparison of OER activity of Co<sub>0.8</sub>Fe<sub>0.2</sub>Mo<sub>2</sub>C-80 electrocatalyst with literature.</caption><thead><tr><th>Electrocatalyst (units)<sup>a</sup></th><th>Mass loading (mg cm<sup>-2</sup>)</th><th>Electrolyte</th><th>η<sub>10</sub> (mV)<sup>a</sup></th><th>Tafel slope (mV dec<sup>-1</sup>)</th><th>Reference</th></tr></thead><tbody><tr><td>.Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80</td><td>0.4</td><td>1 M KOHb</td><td>214</td><td>38.4</td><td>This work</td></tr><tr><td>Mo/Mo<sub>2</sub>C@CoO<br>@NC</td><td>–</td><td>1 M KOH</td><td>215</td><td>151.7</td><td>39</td></tr><tr><td>Ti<sub>3</sub>C<sub>2</sub>-Co-TiO<sub>2</sub></td><td>0.2</td><td>1 M KOH</td><td>276</td><td>58.3</td><td>44</td></tr><tr><td>NiCoAlO-P</td><td>0.3</td><td>1 M KOH</td><td>300</td><td>86</td><td>57</td></tr><tr><td>Co<sub>2.5</sub>Fe<sub>0.5</sub>Mo<sub>3</sub>N</td><td>2.0</td><td>1 M KOH</td><td>218</td><td>41</td><td>7</td></tr><tr><td>Ni<sub>3</sub>Fe-NC/NF</td><td>4.3</td><td>1 M KOH</td><td>203</td><td>57.2</td><td>15</td></tr><tr><td>Spinel Co<sub>3-x</sub>Cr<sub>x</sub>O<sub>4</sub></td><td>0.84</td><td>1 M NaOH</td><td>350</td><td>60</td><td>58</td></tr><tr><td>Co-Mo<sub>2</sub>C-CN<sub>x</sub>-2</td><td>1.05</td><td>1 M KOH</td><td>338</td><td>70</td><td>13</td></tr><tr><td>CeO<sub>2</sub>/Co<sub>4</sub>N</td><td>0.28</td><td>1 M KOH</td><td>318</td><td>81.1</td><td>19</td></tr><tr><td>Co/Mo<sub>2</sub>C@C</td><td>0.34</td><td>1 M KOH</td><td>254</td><td>123</td><td>24</td></tr><tr><td>NP Au/CoMoN<sub>x</sub></td><td>–</td><td>1 M KOH</td><td>230</td><td>46</td><td>59</td></tr><tr><td>Co(OH)<sub>2</sub>-CoO nanosheets</td><td>–</td><td>1 M KOH</td><td>278</td><td>78</td><td>8</td></tr><tr><td>CoFe LDH</td><td>0.1</td><td>0.1 M KOH</td><td>404</td><td>–</td><td>60</td></tr><tr><td>Co<sub>3</sub>Fe alloy<br>nanoparticles/S-N-C</td><td>0.5</td><td>0.1 M KOH</td><td>358</td><td>259</td><td>61</td></tr></tbody></table>

Abbreviations: LDH, layered double hydroxide; NC, N-doped carbon; NF, nickel foam; OER, oxygen evolution reaction; S-N-C, S, N-codoped carbon nanotubes.

<sup>a</sup>η<sub>10</sub>: overpotential to reach 10 mA cm<sup>-2</sup> current density.

<sup>b</sup>S-N-C: S, N-codoped carbon nanotubes.

overpotential. Data showed that the electrode lost 15% current density after 115 h. Analysis of the electrode after the stability test revealed that the electrode lost its active materials, which was evidenced by elemental mapping, EDX, and elemental composition (Figure S29) obtained from HAADF-STEM.

## 3.4 | DFT analysis

DFT computation was performed to estimate the Gibbs free energy and overpotential (see Supporting Information for calculation) for different OER steps based on the 4e transfer mechanism, according to which oxygen-containing intermediates (OCIs: *OH, *O, and *OOH) were bound to the catalytic active sites through oxygen via single bonds.⁶⁹ For the alkaline OER on Co<sub>1</sub>-Mo<sub>2</sub>C-80 and Fe<sub>1</sub>-Mo<sub>2</sub>C-80, OCIs are bonded to the Co and Fe sites, respectively, of the electrocatalyst (Figure S30). Notably, all electrocatalysts Co<sub>x</sub>Fe<sub>1-x</sub>-Mo<sub>2</sub>C-80 displayed better catalytic activity than Co<sub>1</sub>-Mo<sub>2</sub>C-80 or Fe<sub>1</sub>-Mo<sub>2</sub>C-80. Catalytic activities of Co<sub>x</sub>Fe<sub>1-x</sub>-Mo<sub>2</sub>C-80 depended on both Co and Fe content, although the effect was more pronounced for Co. These results confirmed that the Co

sites are catalytically more active than the Fe sites and the former can be identified as the main catalytic species.

OER energy profile (Figure 6A) displayed that formation of *OOH was the rate-determining step (RDS) for Mo<sub>2</sub>C, Co<sub>1</sub>-Mo<sub>2</sub>C-80, and Fe<sub>1</sub>-Mo<sub>2</sub>C-80 and the overpotentials for these catalysts were, respectively, 1.92, 1.06, and 1.11 V. Notably, Co<sub>1</sub>-Mo<sub>2</sub>C-80 showed the lowest overpotential among these electrocatalysts. It was further proved by the density of states data (Figure S31).²⁴ These data proved the effectiveness of incorporation of Co and Fe in Mo<sub>2</sub>C. To understand the effect of partial replacement of Co with Fe further experiment was carried out wherein, under the limitation of the periodic boundary condition, one Co atom in the Co<sub>1</sub>-Mo<sub>2</sub>C-80 model was replaced with an Fe atom to represent the Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 system. This substitution was performed in two ways: placing the Fe atom adjacent to the Co-active site to consider the indirect effect and positioning it directly at the active site itself to capture its direct impact on the OER activity (Figure S32). For both *OH and *OOH, we observed a strengthening of adsorption in Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 compared to Co-Mo<sub>2</sub>C, regardless of the sites of Fe addition (Figure 6A). Interestingly, in the case of *O adsorption, the adsorption strength exhibited the following order: Co<sub>1</sub>-Mo<sub>2</sub>C-80≈Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80
<figure><img src="image_7.png" /><figcaption>FIGURE 6 (A) Gibbs free energy profile at different OER steps for different materials. (B) Energy involved in the RDS and adsorption and decomposition (to produce O₂) energy of *OOH.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIGURE 7 (A) LSV curves, (B) bar diagram showing overpotential to achieve 10 mA cm⁻² current density, and (C) Tafel plots of different electrocatalysts.</figcaption></figure>

(Co site) < Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 (Fe site) < Fe<sub>1</sub>-Mo<sub>2</sub>C-80. These data demonstrated that the inclusion of Fe leads to enhanced *O* adsorption. The distinctive trend of *O* binding energy compared to *OH* and *OOH* binding energies has been explained with their bonding properties: *O* requires two electrons from the surface to satisfy the octet rule, while *OH* and *OOH* need one electron.⁷⁰ Consequently, the Co site within the Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 system demands the lowest energy requirement for the third step (Figure 6A, red line), as anticipated. The RDS was shifted from the third step (*O* → *OOH*) to the fourth step (*OOH* → O<sub>2</sub>), resulting in the lowest OER overpotential (1.00 V) among the studied systems. Moreover, *OOH* weakly adsorbed and decomposed more easily on the Co site than on the Fe site of Co<sub>0.8</sub>Fe<sub>0.2</sub>-Mo<sub>2</sub>C-80 (Figure 6B and S33) (adsorption energy 2.69 and 2.63 eV and decomposition energy 2.23 and 2.29 eV, respectively). Notably, direct utilization of Fe as an active site did not lead to a decrease in the OER

overpotential of Co-Mo₂C (1.06 V). Therefore, addition of a small amount of Fe to Co₁-Mo₂C-80 could optimize the formation and decomposition of *OOH* for more favorable water oxidation activity, as observed experimentally. Consequently, in the catalyst system, Mo₂C acted as a support.

Therefore, both experimental and theoretical data proved that partial substitution of Co with Fe in Co<sub>1</sub>-Mo<sub>2</sub>C-80 resulted in the manipulation of the electronic structures of the latter, which led to the development of efficient water oxidation electrocatalysts.

## 3.5 HER activity

Figure S34 delineates electrochemical HER performance of different electrocatalysts discussed in this contribution. It shows that the electrocatalyst Co<sub>0.7</sub>Fe<sub>0.3</sub>-Mo<sub>2</sub>C-80
displayed 235 mV as η₁₀. We then measured the effect of the concentration of Mo₂C on the HER performance (Figure 7A). It was observed that with the increasing amount of Mo₂C, the overpotential decreased and reached a minimum value of 168.3 mV (Figure 7B). Further increase in the amount of Mo₂C did not produce a significant effect on overpotential, especially at low current densities. Therefore, the electrocatalyst Co₀.₇Fe₀.₃–Mo₂C-110 was the best for the electrochemical HER. It exhibited 285.5 mV overpotential for η₅₀. On the contrary, Co₀.₇Fe₀.₃–Mo₂C-60, Co₀.₇Fe₀.₃–Mo₂C-80, and Co₀.₇Fe₀.₃–Mo₂C-140 displayed overpotentials of, respectively, 318.2, 352.0, and 300.0 mV for η₅₀. Figure 7C displayed Tafel plots, which showed that the electrocatalyst Co₀.₇Fe₀.₃–Mo₂C-110 experienced a Tafel slope of 95.2 mV dec⁻¹. On the contrary, the electrocatalysts Co₀.₇Fe₀.₃–Mo₂C-60, Co₀.₇Fe₀.₃–Mo₂C-80, and Co₀.₇Fe₀.₃–Mo₂C-140 displayed Tafel slopes of 130.3, 134.0, and 129.1 mV dec⁻¹, respectively. Therefore, Co₀.₇Fe₀.₃–Mo₂C-110 exhibited the fastest HER kinetics among all electrocatalysts discussed herein.

## 4 | CONCLUSION

To conclude, an electrocatalys, wherein Mo₂C was incorporated with Co which was partially replaced with Fe, was designed. The incorporation of both Co and Fe finely tuned the electronic structure of Mo₂C, which was not achievable with a single element. Experimental data and Bader charge analysis proved that in these electrocatalysts, Co and Fe acted as electron donors whereas Mo₂C (and Co) functioned as electron acceptor. The best electrocatalyst showed excellent catalytic activity for electrochemical OER in an alkaline medium that was evidenced by a low overpotential and an ultralow Tafel slope. The electrocatalytic OER performance depended on the amount of both Co and Fe, although the effect was more prominent for the former. Theoretical data proved that partial replacement of Co with Fe resulted in the lowest OER overpotential, making Co₀.₈Fe₀.₂–Mo₂C-80 (Co site) the best electrocatalyst. Moreover, on the Co site of Co₀.₈Fe₀.₂–Mo₂C-80, *OOH adsorbed more weakly and subsequently decomposed easily, making the Co sites as active sites. We anticipate that the strategy used herein could be applied to develop an excellent Mo₂C-based low-cost electrocatalyst for energy conversion and storage applications.

## ACKNOWLEDGMENTS

Md. Selim Arif Sher Shah acknowledges financial support from the SERB-SURE under file number of SUR/2022/003129. Jong Hyeok Park acknowledges the

support of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (RS-2023-00302697, RS-2023-00268523).

## CONFLICT OF INTEREST STATEMENT

The authors declare that there are no conflicts of interests.

## ORCID

Md. Selim Arif Sher Shah https://orcid.org/0000-0003-4767-5075

Hyeonjung Jung http://orcid.org/0000-0003-4954-890X

Vinod K. Paidi http://orcid.org/0000-0002-1085-4988

Jeong Woo Han http://orcid.org/0000-0001-5676-5844

Jong Hyeok Park http://orcid.org/0000-0002-6629-3147

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** Sher Shah MSA, Jung H, Paidi VK, Lee K-S, Han JW, Park JH. Fine-tuning electronic structure of N-doped graphitic carbon-supported Co- and Fe-incorporated Mo₂C to achieve ultrahigh electrochemical water oxidation activity. *Carbon Energy.* 2024;6:e488.
doi:10.1002/cey2.488