FeCo nanoalloys embedded in nitrogen-doped carbon nanosheets/bamboo-like carbon nanotubes for the oxygen reduction reaction

Abstract

Herein, FeCo bimetallic organic frameworks (MOFs) with different compositions were fabricated by controlling the initial molar ratio of Fe³⁺/Co²⁺ ions. Through a one-step carbonization treatment of FeCo MOFs and the melamine mixture, a series of FeCo nanoalloys encapsulated in bamboo-like N-doped carbon nanotubes (CNTs), entangled with N-doped carbon nanosheet nanocomposites (Fe_xCo_y–N–C), were prepared. The Fe_xCo_y–N–C materials presented an excellent ORR performance in alkaline medium. X-ray photoelectron spectroscopy (XPS) studies revealed the existence of ORR-active centers, such as Co–N_x, pyridinic-N and graphitic-N, which were responsible for the enhanced ORR activity. Besides, the synergistic coupling between FeCo alloy and N-doped carbon nanotubes favors the enhancement of the electrocatalytic performance. Among Fe_xCo_y–N–C electrocatalysts, the optimized Fe₁Co₃–N–C and Fe₃Co₁–N–C catalysts showed higher ORR performance. In addition, both Fe₁Co₃–N–C and Fe₃Co₁–N–C showed superior stability compared to the benchmark commercial Pt/C catalyst.

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1. Introduction

With population growth and rapid economic development, more and more fossil fuels are being used, leading to increased global warming and environmental pollution. Fuel cells (FCs) have been considered as an alternative to achieve the utilization of clean energy due to their high energy conversion efficiency and “zero” pollutant emission.^1–4 One technical limitation requiring efficient electrocatalysts is the sluggish kinetically controlled ORR at the cathode.^5–7 To date, Pt-based materials have been employed as commercial catalysts for the ORR.^8,9 However, noble metal-based catalysts suffer from high cost, inferior stability, and intolerance to the fuel crossover phenomenon, which hinder the large-scale commercialization of fuel cells.^10,11 Therefore, intensive efforts have been devoted to exploring highly efficient and stable ORR electrocatalysts based on non-precious metal centers.^12–14 In particular, metal/N-doped carbon materials, e.g., M–N–C, M = Fe, Co, are recognized as one of the most promising non-precious catalysts owing to their low cost and desirable catalytic activity.^15,16 It was reported that Fe–N–C materials possess superior ORR activity compared to rather stable Co–N–C catalysts.^17,18 With respect to activity and stability, the synergistic effect between the Fe and Co centers favors better ORR performance than their monometallic counterparts.^18–27 The enhancement of the electrocatalytic ORR performance is induced by various types of active sites present in the metallic Fe–Co–N–C catalysts. These active sites are metal–nitrogen moieties embedded in carbon substrates (labeled as Fe–N_x and Co–N_x),^17,28–33 bimetallic alloy species encapsulated in a nitrogen doped carbon matrix (labeled as FeCo@C),^6,34–46 and nitrogen–carbon moieties (labeled as N_xC_y).⁴⁷ With regard to the interfacial electrocatalytic reactions, an efficient transport of reactants and electrons to the active sites is another crucial factor for fabricating high-performing electrocatalysts. For example, various 3D-structural electrocatalysts consisting of 0D nanoalloys, 1D nanotubes and 2D nanosheets, such as Fe/Fe₃C@NGL–NCNTs,⁴⁸ Fe@C–NG/NCNTs,⁴⁹ CoFe/N–GCT,⁴¹ FeNi@N–CNT/NCSs,⁵⁰ and CoFe/Co@NCNT/NG,²⁶ have been prepared with an increased number of active sites and large surface areas to improve the ORR electrocatalytic performance.

Metal–organic frameworks (MOFs), with well-tunable physico-chemical properties, such as a large surface area, functional modification, and adjustable porosity, have served as self-sacrificing templates to prepare transition metal nanoparticles/carbon composites, especially Fe(Co)–N/C materials.³ Those from the carbonization of MOFs possibly display the following qualities: (i) high distribution of M–N active moieties; (ii) various types of nitrogen species; (iii) large surface area and improved accessibility of active sites; (iv) porous structure; and (v) enhanced electrical conductivity as a result of graphitized carbon. Recently, Chen et al.⁵¹ reported a host–guest strategy using Fe_mIm nanocluster (NC) in a zeolite imidazole framework-8 (ZIF-8) to obtain well-configured Fe–N/C electrocatalysts. Due to the host–guest effect, the Fe–N/C catalysts produced various types of Fe–N_x models, including N–Fe–N₄, Fe–N₄, and Fe–N₂, as a result of which 5% Fe–N/C catalyst with abundant N–Fe–N₄ sites showed an outstanding ORR activity in acidic media. Zhong et al.⁵² reported cobalt nanoparticles interacting with N-doped carbon nanotubes (Co/CoN_x/N–CNT/C) produced by in situ pyrolysis of Co-mela-BDC (Co MOF, composed of Co ions, 1,4-dicarboxybenzene ligands (BDC) and melamine) at 800 °C under a N₂ atmosphere. The Co/CoN_x/N–CNT/C showed high density of NCNTs and N content resulting in excellent ORR activity (E_onset = 0.90 V, E_1/2 = 0.80 V vs. RHE), close to Pt/C values (E_onset = 0.93 V, E_1/2 = 0.80 V) in alkaline media.

Inspired by these facts, a series of FeCo nanoalloys embedded in bamboo-like N-doped carbon nanotubes (CNTs), entangled with N-doped carbon nanosheet (Fe_xCo_y–N–C) nanocomposites with varied ratios of Fe/Co, were successfully prepared by pyrolysis of FeCo bimetallic MOFs and melamine precursors (x and y refer to the feeding molar ratio of Fe³⁺/Co²⁺). Interestingly, the initial molar ratio of Fe³⁺/Co²⁺ for the Fe_xCo_y–N–C nanocomposite plays a key role in the ORR electrochemical performance. In particular, the Fe₁Co₃–N–C and Fe₃Co₁–N–C catalysts showed higher ORR activity and stability than Fe₁Co₂–N–C, Fe₁Co₁–N–C and Fe₂Co₁–N–C.

2. Experimental section

2.1 Synthesis of the bimetallic Fe_xCo_y MOFs

Typically, 3 mmol FeCl₃·6H₂O, 1 mmol Co(OOCCH₃)₂·4H₂O and 6.3 mmol 1,4-dicarboxybenzene (BDC) were dissolved in mixed solvents of N,N-dimethylformamide (DMF, 40 mL) and absolute ethanol (20 mL). The resulting mixture was treated by ultrasound for 30 min to obtain a homogeneous solution and then sealed in a Teflon-lined stainless steel autoclave at 150 °C for 12 h. After cooling down, the precipitate was collected by centrifugation and washed thoroughly with DMF, and dried at 60 °C overnight. The product was denoted as the Fe₃Co₁ MOF with a Fe³⁺/Co²⁺ molar ratio of 3 : 1.

2.2 Synthesis of Fe_xCo_y–N–C catalysts

A mixture with a ratio of Fe_xCo_y MOFs to melamine powder of 4/3 was mechanically mixed by grinding. The mixture was subjected to heat treatment under a N₂ atmosphere at 250 °C with a ramp rate of 5 °C min⁻¹ for 2 h, and then the temperature was increased up to 800 °C with the same heating rate for another 4 h. After calcination, the black powder was immersed in 10 wt% HNO₃ for 3 h, washed with ultra-pure water until the pH value ≈7 was attained, and then dried at 60 °C under air. The pyrolyzed products were labeled as Fe_xCo_y–N–C (i/y = 1 : 3, 1 : 2, 1 : 1, 2 : 1, and 3 : 1), respectively.

2.3 Physicochemical characterization

The crystalline phases of products were characterized by X-ray diffraction (XRD) analysis using a Rigaku UItima III diffractometer with a scanning rate of 10° (2θ) min⁻¹ using Cu-Kα (λ = 0.15406 nm) radiation at 40 kV and 40 mA. The SEM and HRTEM images were individually examined using a ZEISS Supra 55 scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) on a Tecnai G2 F20 field emission transmission electron microscope (FEI) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed using an Axis Supra X-ray photoelectron spectrometer equipped with an Al/Mg double anode. Low-temperature nitrogen adsorption–desorption experiments were performed using a Gemini VII 2390 series surface area analyzer. The specific surface area was calculated according to the Brunauer–Emmett–Teller (BET) method based on the adsorption isotherm. The pore volume and pore size distribution were calculated using the Barrett–Joyner–Halenda (BJH) method. Raman spectra were recorded using a LabRAM Aramis Raman spectrometer with a visible laser beam of 532 nm.

2.4 Electrochemical measurements

The electrochemical measurements were conducted in a standard three-electrode glass cell with 0.1 M KOH as the electrolyte at 25 °C. A carbon plate and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All measured potentials vs. SCE were converted with respect to the RHE based on the formula: E(RHE) = E(SCE) + 1.009 V. A glassy carbon disk electrode with a geometric area of 0.07 cm² was used as the working electrode, which was polished with γ-alumina powder and successively ultrasonicated in water and ethanol for 1 min prior to any measurement. The catalyst ink was prepared by dispersing 4.0 mg of catalyst in 700 μL deionized water, 250 μL isopropanol and 50 μL Nafion® mixed by ultrasound for 2 h. 4.0 μL of the ink was dropped on the working electrode surface, and dried in air at 300 rpm. The mass loading was 0.23 mg cm⁻² for the Fe_xCo_y–N–C catalyst. The cyclic voltammograms (CVs) were recorded at a scan rate of 50 mV s⁻¹ in an Ar- or O₂-saturated electrolyte. In an O₂-saturated electrolyte, linear sweep voltammetry (LSV) curves were recorded at a scan rate of 5 mV s⁻¹ with different rotating speeds from 625 to 2500 rpm. The ORR kinetics was analyzed using the Koutecky–Levich (K–L) equation.where j is the current density (mA cm⁻²); j_k is the kinetic current density (mA cm⁻²); j_L is the limiting current density (mA cm⁻²); n is the number of electrons transferred per oxygen molecule; F (96 485 C mol⁻¹) is the Faraday constant; D (1.9 × 10⁻⁵ cm s⁻¹) is the diffusion coefficient of O₂ in 0.1 M KOH; ν (0.01 cm² s⁻¹) is the kinetic viscosity of the solution; C₀ (1.2 × 10⁻⁶ mol cm⁻³) is the concentration of O₂ in the electrolyte; and ω is the rotating rate (rad s⁻¹).⁵³

The number of electrons transferred (n) and the peroxide yield (H₂O₂%) were obtained from the rotating ring-disk electrode (RRDE) tests by using the following equations:

where I_d is the disk current (mA), I_r is the ring current (mA), and N, the current collection coefficient, of the Pt ring is 0.424 under our test conditions.

The electrochemically active surface area (ECSA) measurements were performed between 0.31 and 0.41 V vs. RHE at the scan rates from 20 to 120 mV s⁻¹ in 0.1 M N₂-saturated KOH. The accelerated stability test was conducted in 0.1 M O₂ saturated KOH solution at a scan rate of 100 mV⁻¹. The durability of the catalyst was tested by chronoamperometric measurement at 1600 rpm at 0.7 V in 0.1 M O₂ saturated KOH solution for 9 h.

3. Results and discussion

Fig. 1a shows the X-ray diffraction (XRD) patterns of bimetallic Fe_xCo_y-MOFs with various molar ratios of Fe³⁺/Co²⁺ (x/y = 1 : 3, 1 : 2, 1 : 1, 2 : 1, 3 : 1). Their XRD patterns display several diffraction peaks centered at 9.1°, 16.7°, 18.5°, 20.6° and 25.4°/2θ, indicating a similar crystalline structure. One can observe that the diffraction peaks at around 11°/2θ are slightly shifted to a lower angle (2θ) with the molar ratio of Fe³⁺/Co²⁺, demonstrating structural differences among Fe_xCo_y-MOFs as a result of the different coordinated configurations between the Fe/Co and BDC organic ligand. The morphologies of Fe_xCo_y-MOFs were characterized using a scanning electron microscope (SEM), cf. Fig. 1b–f. The Fe₁Co₃, Fe₁Co₂ and Fe₁Co₁ MOFs show irregular morphological features, whereas the Fe₂Co₁-MOF and Fe₃Co₁-MOF with a higher relative amount of Fe³⁺ present irregular spindle-shaped nanorods. Notably, the particle size and regularity of the Fe_xCo_y-MOF are strongly associated with the concentration of Fe³⁺. The Fe₃Co₁–MOF sample displays a more uniform and well-dispersed structure with an equatorial diameter of ca. 120 nm and a length of ca. 450 nm, Fig. 1f. This phenomenon suggests that Fe³⁺ has an important impact on the morphology of the bimetallic Fe_xCo_y-MOF.^54,55 Moreover, the morphologies of these bimetallic Fe_xCo_y-MOFs are quite different from the Fe-MOF and Co-MOF using the same BDC organic ligand as a linker,^52,56 thus revealing that Fe and Co ions are involved in the formation process of bimetallic Fe_xCo_y-MOFs.

Fig. 1 (a) XRD patterns of bimetallic Fe_xCo_y MOFs and SEM images of (b) Fe₁Co₃ MOF, (c) Fe₁Co₂ MOF, (d) Fe₁Co₁ MOF, (e) Fe₂Co₁ MOF and (f) Fe₃Co₁ MOF.

The bimetallic Fe_xCo_y MOFs with different compositions were further pyrolyzed at 800 °C under a N₂ atmosphere for 4 h, and then subjected to acid etching to produce Fe₁Co₃–N–C, Fe₁Co₂–N–C, Fe₁Co₁–N–C, Fe₂Co₁–N–C and Fe₃Co₁–N–C samples. ICP-AES results clarify that the Fe content substantially increases from 1.6 to 6.1 wt% with the Fe³⁺/Co²⁺ molar ratio from 1/3 to 3/1, while Co content gradually decreases from 5.0 to 1.9 wt%, Fig. S1, ESI.†Fig. 2a depicts the XRD patterns of all Fe_xCo_y–N–C in the interval of 20–80°/2θ. The relatively strong diffraction peak at 26.3°/2θ corresponds to the (002) plane of graphitic carbon (JCPDS No. 41-1487), while the two peaks at 44.6° and 64.9°/2θ match well with the (110) and (200) planes of the FeCo alloy (JCPDS No. 48-1816), respectively. One notices that the diffraction peak position of the (110) plane gradually shifts to a lower angle with increasing Fe/Co molar ratio of the FeCo alloy, attributable to the lattice contraction in FeCo.⁵⁷ In addition, along with increasing the initial Fe/Co molar ratio, the diffraction density of the FeCo alloy becomes stronger, suggesting a larger crystal size and higher crystallinity. The SEM images of Fe_xCo_y–N–C samples show the appearance of carbon nanotubes after the calcination of the FeCo MOFs and melamine mixture, suggesting that Co and Fe metals could facilitate the formation and growth of carbon nanotubes, Fig. 2b–f. Interestingly, the abundant interconnected 3D networks of NCNTs are obviously observed on the Fe₁Co₃–N–C and Fe₃Co₁–N–C samples, especially the latter, coming from the Fe₃Co₁ MOF precursor with a regular spindle-shape and well-dispersed structure. Here, the reaction between the Fe_xCo_y MOF template and melamine induces the following: (i) the metal nano-catalysts derived from MOFs catalyze the carbonization and formation of NCNTs; (ii) the confinement effect of the MOF matrix aids to improve the length and dispersion of NCNTs; and (iii) melamine serves as basic units of NCNTs and acts as a pore former as well as the N source.

Fig. 2 (a) XRD patterns of Fe_xCo_y–N–C materials and SEM images of Fe₁Co₃–N–C (b), Fe₁Co₂–N–C (c), Fe₁Co₁–N–C (d), Fe₂Co₁–N–C (e) and Fe₃Co₁–N–C (f).

In order to obtain further compositional information, Rietveld analysis (RF) was performed on all Fe_xCo_y–N–C samples, as shown in Fig. 3(a) with a blue line.^33,58 One notes that the predominant phases in all samples are carbon and the FeCo alloy, whereas the presence of the Fe_0.28Co_0.72 phase (PDF#51-0740) is revealed in Fe₁Co₃–N–C (24 wt% Fe nominal). The latter was previously reported for its similarity to α-Mn,⁵⁹ which agrees with the nominal mass fraction of Fe, in the Fe₁Co₃–N–C. On the other hand, a third phase is observed in the Fe₂Co₁–N–C sample, namely, CoFe₂O₄ (PDF#79-1744). This phenomenon is not evident when the nominal mass fraction of Fe increases from 66 wt% to 74 wt%. The structural properties of Fe–Co nanoalloys are summarized in Table 1, which shows that the crystallite size is in the range of 11 to 170.1 nm for the Fe-based samples. As discussed above, in the Fe₁Co₃–N–C and Fe₂Co₁–N–C samples, the predominant phases are Fe_0.28Co_0.72 and CoFe₂O₄, respectively. Fig. 3(b) presents the relationship of the lattice parameter (a) as a function of the nominal mass fraction of Fe present in the samples. It clearly shows that Vegard's law could be described with a mass fraction less than 20 wt% Fe, where the stable phase at 100 wt% of Co is the Co (Fm3̄m) phase based on the phase Fe–Co diagram.⁶⁰ Sánchez-De Jesús et al.⁶¹ found that at a concentration of 10 wt% Fe, the reticular parameter is 2.897 Å. This value is very close to the calculated value of the solid solution of FeCo, cf. Table 1. In particular, at 20 wt% of Fe, the predominant phase is Fe_0.28Co_0.72 (I4̄3m) with a certain amount of FeCo phase (Pm3̄m).⁵⁹ If each α-Mn atom in the cubic phase is substituted by Fe_0.28Co_0.72, leading to a conversion to the FeCo alloy as a function of the nominal percentage of Fe in the Co_xFe_y phase. This rearrangement of the Fe_0.28Co_0.72 structure indicates that one Fe atom is centered in the body of the cubic structure with a cobalt-shell-nucleus-like building block. This formation tendency is congruent with the phase diagram of FeCo, when comparing the cubic structure of pure Fe (PDF#6-0696).⁶⁰ Particularly, such a FeCo cobalt-shell-nucleus-like structure remains the same with the increase of the mass fraction of Fe, in (2) – (5) samples, cf. Fig. 3(b). The addition of Fe into Co-based catalysts could not only modify the electronic structures but also afford stronger affinity for the adsorption of oxygen species.⁶² The presence of the CoFe₂O₄ phase possibly means another transformation zone towards the Fe₂O₃ phase (Pm3̄m).

Fig. 3 (a) Rietveld refinement analyses of XRD patterns of samples: Fe_xCo_y–N–C in the angle interval of 20 to 80°. The fitting parameters are on average R_wp% = 33.4, and GoF = 1.1; (b) experimental reticular parameter (a) as a function of the nominal mass fraction of Fe in Fe_xCo_y–N–C samples, (1)–(5) are Fe₁Co₃–N–C, Fe₁Co₂–N–C, Fe₁Co₁–N–C, Fe₂Co₁–N–C and Fe₃Co₁–N–C, respectively. Fe₁₀Co₉₀ is taken from a reference.⁶¹

Table 1 Structural properties, mass percentage, and crystallite size of Fe-based samples

Samples	x wt Fe	Phases	Reticular parameter a/Å	Reticular parameter c/Å	Mass percentage phase/wt%	Crystallite size 〈d〉/nm
Fe1Co3–N–C	0.24	FeCo	2.8572 (0.0009)	n. a.	37.99 (0.0)	34.1 (6.6)
		Fe0.28Co0.72	8.7329 (0.0085)	n. a.	62.01 (10.8)	11.1 (1.3)
Fe1Co2–N–C	0.32	FeCo	2.8702 (0.0007)	n. a.	100	21.1 (1.2)
Fe1Co1–N–C	0.49	FeCo	2.8635 (0.0004)	n. a.	100	23.5 (0.7)
Fe2Co1–N–C	0.67	FeCo	2.8752 (0.0002)	n. a.	77.16 (5.5)	79.9 (3.6)
		CoFe2O4	5.9640 (0.0074)	14.6097 (0.0352)	22.84 (2.2)	81.7 (23.4)
Fe3Co1–N–C	0.74	FeCo	2.8758 (0.00017)	n. a.	100	170.1 (17.1)

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The microstructure and the morphology of Fe_xCo_y–N–C samples were further studied using a high-resolution transmission electron microscope (HRTEM), cf. Fig. 4 and Fig. S2–S5.† The HRTEM images evidence that the porous carbon nanosheets (NSs) entangled with bamboo-like NCNTs emerge on a Fe₃Co₁–N–C sample, while the black dots correspond to the FeCo alloy nanoparticles embedded in the carbon support. Fig. 4c reveals that the dark core shows a lattice fringe d-spacing of 0.20 nm ascribed to the (110) plane of FeCo nanoalloys, while CNTs composed of layered graphitic carbon with a d-spacing of 0.34 nm are observed. Moreover, the selected area electron diffraction (SAED) pattern of the Fe₃Co₁–N–C sample shows the rings and polycrystalline diffraction scattering dots, which are assigned to the diffraction of the graphitic carbon shell and the FeCo alloys, Fig. 4d. This fact confirms again that the FeCo nanoalloys are encapsulated in the graphitic carbon layer.

Fig. 4 (a–c) HRTEM images and (d) SAED pattern of the Fe₃Co₁–N–C sample.

Fig. 5a shows the N₂ adsorption–desorption isotherms of all Fe_xCo_y–N–C samples, indicating a type-IV characteristic with a H2-type hysteresis loop in the relative pressure (P/P₀) range of 0.45–1.0, which is related to the interconnected and uniform channel-like mesoporous structure, the so-called “ink-bottle pores”.⁶³ The BJH pore size distribution curves also manifest the presence of mesopores (3–4 nm) on Fe_xCo_y–N–C samples, as shown in Fig. S6.† The pore size distribution is mainly below 4 nm due to the random and loose stacking of tiny graphitic layers in the NCNTs.⁶⁴ The calculated Brunauer–Emmett–Teller (BET) surface area is 282.31, 277.69, 226.83, 185.39 and 308.95 m² g⁻¹ for Fe₁Co₃–N–C, Fe₁Co₂–N–C, Fe₁Co₁–N–C, Fe₂Co₁–N–C and Fe₃Co₁–N–C samples, respectively. The mesoporous structure and large surface area of the Fe₃Co₁–N–C electrocatalyst favor the transport of mass/electrons certainly improving the accessibility of ORR active sites during the electrocatalytic reaction process.

Fig. 5 (a) N₂ adsorption–desorption isotherms and (b) Raman spectra of the Fe_x/Co_y–N–C samples.

Raman spectroscopy was carried out to investigate the chemical modifications on the CN/CNT surface. Fig. 5b contrasts the Raman spectra of all Fe_xCo_y–N–C samples at the energy interval of 800–2000 cm⁻¹. After deconvolution, the Raman spectra display five overlapping bands, namely, I band (∼1200 cm⁻¹), D band (∼1345 cm⁻¹), D′′ band (∼1500 cm⁻¹), G band (∼1585 cm⁻¹), and D′ band (∼1617 cm⁻¹).⁶⁵ The D band is related to the symmetry breakdown of sp³ carbon atoms, structural disorders and defects, while the D′′ band is relevant to the amorphous sp² phase. The dominant D band and the presence of D′′ band infer that all Fe_xCo_y–N–C samples have a highly disordered degree of graphitic carbon. The G band arises from the in-plane tangential stretching vibration mode of sp² carbon atoms. The I_D/I_G ratio, derived from the integrated intensity of each band, is used as an indicator of the defect fraction and disordered degree in the carbonaceous materials. The I_D/I_G ratio of Fe₁Co₃–N–C and Fe₃Co₁–N–C samples is higher than those of the Fe₁Co₂–N–C, Fe₁Co₁–N–C and Fe₂Co₁–N–C, caused by the higher N-dopant content.⁶⁶ An increase in the number of defects on Fe₁Co₃–N–C and Fe₃Co₁–N–C samples may result from the lattice distortion of sp² hybrid carbon when N atoms intercalate into the carbon matrix. Besides, as summarized in Table 2, the G-band position of the Fe₃Co₁–N–C sample is centered at a higher wavenumber with respect to other Fe_xCo_y–N–C samples, suggesting that Fe₃Co₁–N–C is highly disordered.⁶⁷ The relation of the in-plane crystallite size (L_a) and I_D/I_G ratio is given by eqn (5):

where λ is the laser wavelength (532 nm). As summarized in Table 2, it can be noticed that the in-plane crystallite size of Fe₃Co₁–N–C is the smallest one (16.13 nm), probably because of the grafting of more N-containing groups on the basal-plane surface of carbon in Fe₃Co₁–N–C.

Table 2 Pore diameter and BET surface area of all Fe_x/Co_y–N–C samples

Samples	Fe1/Co3–N–C	Fe1/Co2–N–C	Fe1/Co1–N–C	Fe2/Co1–N–C	Fe3/Co1–N–C
BET surface area (m^2 g^−1)	282.31	277.69	226.83	185.39	308.95
Pore diameter (nm)	3.47	3.51	3.59	3.57	3.59
G-band position (cm^−1)	1588	1583	1583	1581	1590
L a (nm)	16.27	16.41	19.20	19.79	16.13

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To probe the elemental composition and surface valence state of Fe_xCo_y–N–C samples, X-ray photoelectron spectroscopy (XPS) measurements were performed and are shown in Fig. 6 and S7.† The XPS survey spectra of all Fe_xCo_y–N–C samples reveal the presence of Co 2p, Fe 2p, N 1s, C 1s, and O 1s, cf. Fig. S7a.† As shown in Fig. 6a, the high-resolution Co 2p spectra present a spin–orbit coupling of Co 2p_3/2 and Co 2p_1/2, and their corresponding satellite peaks. After deconvolution, three signals centered at ∼778.6 eV, ∼780.5 eV and ∼783.6 eV are attributed to the metallic Co, CoO_x and CoN_x species, respectively.^52,68 The CoN_x species are possibly the result of coordination between the surrounding Co atoms, in the cubic structure with a cobalt-shell-nucleus-like building block, and N moieties. Such synergistic coupling between FeCo alloy and N-doped carbon nanotubes is proposed to enhance the electrocatalytic performance.⁶⁹ One notes that the relative proportion of Co–N_x, known as highly ORR-active centers, decreases along with the increase of the Fe/Co ratio of Fe_xCo_y–N–C samples, Table 3.⁵³ The Fe 2p_3/2 spectra can be identified as Fe⁰ (∼707.0 eV), Fe²⁺ (∼708.5 eV), and Fe³⁺ (∼711.0 eV), respectively. Most of the Fe²⁺ and Fe³⁺ ions correspond to the Fe–N_x configuration, while a fraction of them comes from the surface oxidation of CoFe alloy nanoparticles in air.⁵¹ Fe⁰ originated from the FeCo alloy and metallic Fe could provide extra electrons to neighboring Fe–N_x, and increase the HOMO energy of Fe–N_x to overlap with the LUMO of triplet O₂ molecules, which leads to the enhancement of ORR activity.^70,71 It can be found that the Fe–N_x (Fe²⁺ and Fe³⁺) content decreases with the increasing Fe/Co ratio in the MOF precursors from 1/3 to 3/1. In contrast, the relative percentage of Fe⁰ increases. The deconvoluted XPS spectra of N 1s show five types of N configuration, namely, pyridinic N (398.6 eV), Co(Fe)–N_x (399.6 eV), pyrrolic N (401.1 eV), graphitic N (402.7 eV), and oxidized N (405.9 eV), respectively (Fig. 6c).^38,43,68 It indicates the disruption of π conjugation on the outer layers of the carbon lattice due to nitrogen doping. Such an N-doping effect stems from the pyrolysis process of melamine at high temperatures, accompanied by the release of NH₃ gas. All types of nitrogen dopants are believed to play vital roles in the improvement of ORR performance, except for the oxidized N moieties.⁷² Different types of N active species are reported to have different influences on the ORR performance; for example, M–N_x, pyridinic N as well as pyrrolic N contribute to improve the onset potential (E_onset), while graphitic-N could enhance the limiting diffusion current density (j_L). In particular, previous literature studies reported that the enhanced ORR activity in the N-doped carbon materials containing pyridinic-type nitrogen functionalities is ascribed to the conjugation effect of the nitrogen lone pair electrons on the nitrogen and graphene π-systems.^73–75 The presence of Co(Fe)–N_x species further supports that FeCo nanoparticles strongly interact with the N-doped carbon support, favoring to facilitate electron transfer from the metal atoms to CNTs, thereby accelerating the formation of the OOH species.⁶⁴ Besides, in Fig. S7b,† the total content of N on Fe₁Co₃–N–C and Fe₃Co₁–N–C samples is higher than those of the Fe₁Co₂–N–C, Fe₁Co₁–N–C and Fe₂Co₁–N–C, in agreement with the Raman results. Such nitrogen enrichment might provide a higher density of catalytically active centers with low stereohindrance for binding redox species, followed by the complete reduction of oxygen to water via a four-electron reaction path.

Fig. 6 XPS spectra of (a) Co 2p, (b) Fe 2p, and (c) N 1s for Fe_xCo_y–N–C materials.

Table 3 Analysis of the Fe 2p and Co 2p photoemission lines of all Fe_x/Co_y–N–C samples

Samples	Fe1/Co3–N–C (at%)	Fe1/Co2–N–C (at%)	Fe1/Co1–N–C (at%)	Fe2/Co1–N–C (at%)	Fe3/Co1–N–C (at%)
Co^0	16.5	15.8	18.6	18.2	18.4
CoOx	35.7	40.5	45.2	53.0	56.0
CoNx	47.8	43.7	36.2	28.8	25.6
Fe^0	8.5	15.1	17.2	22.4	21.2
Fe^2+	24.6	11.3	33.2	12.5	37.4
Fe^3+	66.9	73.6	49.5	67.1	41.3

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3.1. ORR catalytic activity evaluation

The ORR electrocatalytic activity of Fe_xCo_y–N–C catalysts was evaluated in 0.1 M KOH with 20 wt% commercial Pt/C catalyst as the benchmark. Fig. 7a shows the cyclic voltammetry (CV) curves of Fe_xCo_y–N–C and Pt/C electrodes in O₂ (solid line)- and Ar (dashed line)-saturated electrolytes, respectively. In contrast to Pt/C, all Fe_xCo_y–N–C catalysts display featureless quasi-rectangular voltammograms in an Ar-saturated solution, whereas the well-defined ORR cathodic peaks can be observed at ca. 0.80 V in the O₂-saturated solution. The reduction peak potential (E_redox) of the Fe₁Co₃–N–C and Fe₃Co₁–N–C catalysts is 0.82 V higher than those values of Fe₁Co₂–N–C (0.81 V), Fe₁Co₁–N–C (0.79 V) and Fe₂Co₁–N–C (0.80 V) catalysts. The linear sweep voltammetry (LSV) curves of all Fe_xCo_y–N–C at various rotating speeds show increased diffusion current density along with increasing rotating speeds, in Fig. S8–12.† Furthermore, the Koutecky–Levich (K–L) plots at different electrode potentials display linear and parallel lines indicating an ORR first-order reaction kinetics on Fe_xCo_y–N–C electrodes, with respect to the concentration of dissolved O₂.

Fig. 7 (a) Cyclic voltammetry curves at a scan rate of 50 mV s⁻¹ in Ar (dashed line) and 0.1 M O₂-saturated (solid line) KOH; (b) linear sweep voltammograms of electrocatalysts at 5 mV s⁻¹ and 1600 rpm in 0.1 M O₂-saturated KOH; (c) electron transfer number (n) and peroxide (H₂O₂) yield of Fe₃Co₁–N–C; (d) ORR Tafel plots derived from the K–L equation; (e) the C_dl calculation and (f) chronoamperometry (i–t) curves at 0.70 V vs. RHE of all Fe_xCo_y–N–C and Pt/C catalysts.

Fig. 7b depicts the linear sweep voltammetry (LSV) curves recorded at 1600 rpm of all Fe_xCo_y–N–C and Pt/C electrocatalysts. The onset potentials, E_onset, of Fe_xCo_y–N–C catalysts are nearly the same, but are negatively related to that of Pt/C. In terms of the half-wave potential (E_1/2) and kinetic current density at 0.8 V, a series of Fe_xCo_y–N–C catalysts show no significant difference, cf. Table 4, which might be the result of the same cubic structure. The RRDE measurement was conducted to further analyze the electron transfer number (n) and the H₂O₂ yield. It is found that the H₂O₂ yield of Fe₁/Co₃–N–C and Fe₃Co₁–N–C catalysts is around 15% in the potential range of 0.2 to 0.7 V, lower than those of other Fe_xCo_y–N–C catalysts, Fig. 7c and S8–11.† The calculated electron transfer number (n) is >3.7, which suggests that the ORR on both the Fe₁/Co₃–N–C and Fe₃Co₁–N–C proceeds, via a nearly 4-electron pathway. Moreover, the Tafel slopes at the low overpotential region, in Fig. 7d, of Fe₁/Co₃–N–C and Fe₃Co₁–N–C catalysts display small values of 65.62 and 59.93 mV dec⁻¹ with high ORR kinetics current density, respectively, lower than those of Fe₁Co₂–N–C (74.85 mV dec⁻¹), Fe₁Co₁–N–C (77.62 mV dec⁻¹), Fe₂Co₁–N–C (68.48 mV dec⁻¹). It is worth noting that the ORR performance of Fe₁Co₃–N–C and Fe₃Co₁–N–C is comparable to that of some reported FeCo supported on N-doped carbon materials, for instance, CoFe/N–GCT,⁴¹ N–C–CoFe,⁴² Fe–Co–N–C,²³ and CoFe/NC–0.2-900,⁴⁵ see Table S1.†

Table 4 ORR activity parameters of all Fe_x/Co_y–N–C and Pt/C catalysts

Samples	E onset/V	E 1/2/V	j k@0.8 V/mA cm^−2	Tafel slope/mV dec^−1
Fe1/Co3–N–C	0.87	0.81	−6.98	67
Fe1/Co2–N–C	0.87	0.81	−6.39	75
Fe1/Co1–N–C	0.87	0.80	−5.18	78
Fe2/Co1–N–C	0.85	0.80	−5.06	68
Fe3/Co1–N–C	0.87	0.82	−6.81	60
Pt/C	0.90	0.84	−17.02	67

---

The electrochemically active surface area (ECSA) of the catalyst is strongly associated with the site density and the double layer capacitance (C_dl). The C_dl values of all Fe_xCo_y–N–C samples were evaluated by the double-layer charging/discharging curves at different scan rates in the non-faradaic potential range of 0.31–0.41 V, Fig. S13.† The plots of the current density Δj = (j_a − j_c)/2 at 0.36 V (j_a and j_c are the anodic and cathodic current density, respectively) against scan rates show a linear relationship and the corresponding slope value is related to C_dl, see Fig. 7e. One notes that Fe₃Co₁–N–C has the largest C_dl of 12.6 mF cm⁻¹ among all Fe_xCo_y–N–C catalysts, thus confirming again that the high ORR electrochemical activity occurs on the Fe₃Co₁–N–C catalyst. This fact signifies that the Fe₃Co₁–N–C exposes more electro-active sites due to its abundant interconnected NCNT networks, which is consistent with the results of the BET surface area. Besides, in Fig. 7f, the high remaining current density over 9 h of continuous operation at 0.7 V suggests that the Fe_xCo_y–N–C catalysts display better durability for the ORR compared with the commercial Pt/C catalyst. Importantly, the Fe₃Co₁–N–C shows a near-90% current retention, higher than those for other Fe_xCo_y–N–C samples. Only a slight decrease (12 mV) in E_1/2 is observed after 5000 cycles (Fig. S14†), which suggest a good stability of the Fe₃Co₁–N–C catalyst.

The improved ORR performance (activity, durability and stability) of Fe₁Co₃–N–C and Fe₃Co₁–N–C complex composites can be ascribed to the following reasons: (i) the abundance of M–N_x ORR active sites (CoN_x) on the Fe₁Co₃–N–C catalyst is responsible for the enhanced ORR activity, and the strong interaction between the metal centers and N-doped carbon support, favoring to facilitate electron transfer from N-doped carbon to the Fermi level of FeCo alloys, and hence leading to a higher electronic density and efficient ORR kinetics; (ii) the synergistic effect between the FeCo alloy and N-doped carbon could enhance the electrocatalytic performance; (iii) the nitrogen-doped carbon not only effectively participates in the ORR process in terms of various N moieties like pyridinic-N, graphitic-N and pyrrolic-N, but also can protect the FeCo nanoalloys, preventing them from being leached out under the oxidative alkaline conditions with respect to the long-term stability/durability; and (iv) particularly, in Fe₃Co₁–N–C, large surface area, porous structure and abundant NCNTs greatly aid to expose more active sites and improve availability of catalytic centers.

4. Conclusion

In summary, we reported an in situ pyrolysis strategy to fabricate high-performance FeCo bimetallic MOF-derived Fe_xCo_y–N–C catalysts for the ORR in alkaline medium. The Fe_xCo_y–N–C materials with tunable morphology and chemical composition as well as pore structure mainly depend on the molar ratio of Fe³⁺/Co²⁺. Due to the confinement effect of MOFs, melamine can be effectively transformed into uniformly bamboo-like N-doped CNTs tangled with N-doped carbon nanosheets, while the obtained FeCo nanoalloys are encapsulated in the porous carbon matrix. The optimized Fe₁Co₃–N–C and Fe₃Co₁–N–C catalysts exhibit excellent ORR activity comparable to the commercial Pt/C catalyst but superior long-term stability. The formation of M–N_x (Fe–N_x and Co–N_x) species, N-doped carbon and varied N moieties (pyridinic-N, graphitic-N and pyrrolic-N), as well as the synergistic effect between FeCo alloy and N-doped carbon, is responsible for the high ORR performance. This work opens a new avenue to develop promising electrocatalysts for the ORR in alkaline medium.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China, the Programme XU Guangqi (Code Projet: 45644RC), and the Fundamental Research Funds for the Central Universities (JD1716, 12060093063). H. H. Zhong specially acknowledges the financial support from the China Scholarship Council. Xiao-Wei Song thanks the University Natural Science Research Project of Anhui Province (grant number KJ2019A0549) and the talent introduction fund of Anqing Normal University (grant number 150002019) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Physicochemical characterization details for all Fe_xCo_y–N–C presented herein including, but not limited to, ICP-AES, HRTEM, XPS, pore diameter size distribution curves, ORR–LSV curves, peroxide yields, electron transfer numbers, electrochemical double-layer capacitance, and accelerated stability tests. See DOI: 10.1039/d0qi01037e

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