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In situ green architecture of the 3D FeZn–N–C based electrocatalyst for efficient oxygen reduction†

Kui Fu‡ ^a, Biao Ma‡^a, Jianling Liu^a, Meng Zhou^a, Yihai Xing^a, Xiangfeng Wei^a, Fancheng Meng^a and Jiehua Liu*^ab
aFuture Energy Laboratory, School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail: liujh@hfut.edu.cn
^bKey Laboratory of Advanced Functional Materials and Devices of Anhui Province, Engineering Research Center of High-Performance Copper Alloy Materials and Processing, Ministry of Education, Hefei 230009, China

First published on 22nd August 2024

The 1D nanostructure (nanotubes and nanofibers) has the geometrical characteristics of a high aspect ratio and large specific surface area, which can offer good electron conduction ability and load more catalytic sites. The root-like 1D structure can improve the ion-diffusion ability.⁶ In particular, the unique geometric structure of carbon nanotubes allows the inner and outer walls to fully expose the active sites, and the space in the tube is also conducive to ion transfer to accelerate the reaction.⁷ So far, a large number of 1D non-noble transition metal (Fe, Co, and Ni)-based carbon catalysts have been developed due to low cost, high activity, and quantity production.⁸ Different from 1D materials, the 2D graphene-based material not only has a large contact interface between the 2D sheets with improved conductivity, but also is a desired substrate with high surface area for loading other cocatalysts.⁹ For example, Co–Ru alloy was loaded on 2D nitrogen-doped carbon nanosheets, benefiting from the high surface area of 2D carbon-based nanosheets to ensure the full exposure of active sites, and exhibited an excellent oxygen reduction activity.¹⁰ However, aggregation is hardly avoided, which usually leads to a reduced active area in the number of exposed catalytic sites due to the high surface energy, electrostatic interaction, and π–π stacking.¹¹

Therefore, it is necessary to accurately combine the features of 1D and 2D structures through a green preparation strategy to overcome their shortcomings. In this work, MOF-based nanosheets were exfoliated from MOF-5 by a green hydrothermal method, while ferrous cyanide ions were adsorbed on its surface, and then mixed with melamine for pyrolysis. After annealing, the synthesized 3D FeZn–N–C electrocatalyst exhibits in situ growth of ultrafine 1D carbon nanotubes on microporous carbon nanosheets, which could expose the more catalytic sites and provide available channels for electron and oxygen transfer. FeZn–N–C-800 displayed the best ORR performance with a high half-wave potential of 0.87 V under alkaline conditions. ZABs exhibited a superior peak power density of 294 mW cm⁻² when assembled with FeZn–N–C obtained at 800 °C, 164% as high as that of the battery with the Pt/C catalyst.

Fig. 1a illustrates the green synthetic route in an aqueous system for in situ constructing the 3D electrocatalyst with 1D FeZn–N–C nanotubes and nanosheets derived from 2D Zn-MOF. The cubic MOF-5 was synthesized with a regular morphology and uniform size, and the average particle size is 2.91 μm (Fig. 1b and Fig. S1, ESI†), which is consistent with the previous literature.¹² Then, K₄[Fe(CN)₆]@MOF-5 nanosheets were in situ obtained by a hydrothermal method in aqueous solution with a mass ratio of 1:10 of K₄[Fe(CN)₆] and MOF-5. In the hydrothermal process, the water molecules insert into the MOF-5 surface, and break the coordinated Zn-OC band due to hydrolysing.¹³ The cubic MOF-5 is transformed into 2D nanosheets, as proven by scanning electron microscopy (SEM) (Fig. 1c). Meanwhile, [Fe(CN)₆]⁴⁻ ions are captured by Zn(II) on MOF-5 derived nanosheets due to low solubility of Zn₂[Fe(CN)₆]. Finally, the 3D hybrid was grown in situ on the two-dimensional porous carbon base at high temperature in N₂.

	Fig. 1 (a) The synthesis scheme of the FeZn–N–C-800 electrocatalysts. (b) SEM image, (c) TEM image, (d) HR-TEM lattice fringe images, and (e) EDS mappings of the FeZn–N–C-800.

X-ray diffraction (XRD) patterns of MOF-5-120 and K₄[Fe(CN)₆]@MOF-5-120 are provided in Fig. S2 (ESI†). The diffraction peaks of Prussian blue analogues such as K/Zn ferrocyanides were not observed in the XRD patterns of K₄[Fe(CN)₆]@MOF-5-120 that has typical peaks the same as MOF-5-120, indicating no crystalline ferrocyanides on the surface of the MOF-5-120 nanosheets. In Fig. S3 (ESI†), the stretching vibration peak of the cyanide ion was observed at 2096 cm⁻¹ in Fourier transform infrared spectroscopy (FTIR). The FTIR spectrum confirmed that ferrous cyanide ions were successfully adsorbed in the MOF-5-120 nanosheets. MOF-5 layered hydrolysis and ferrocyanide adsorption play a very important role in the in situ green synthesis of K₄[Fe(CN)₆]@MOF-5-120.

A field-emission scanning electron microscope (FESEM) was employed to observe the morphology of the electrocatalyst. As depicted in Fig. 1d, the carbon nanotubes grew in situ on the 2D mesoporous carbon-based nanosheets. The distinctive structure was obtained from the fact that ferrocyanide, which is originally uniformly adsorbed on the surface of the 2D MOF nanosheets, reacts with the pyrolysis products of melamine as both the carbon source and the nitrogen source to give rise to the in situ growth of nitrogen-doped carbon nanotubes during the pyrolysis process. The 3D carbon-based structure was formed by comprising 1D carbon nanotubes and 2D nanosheets. The N doping could notably enhance the electrocatalytic performance of oxygen reduction. Due to the lower electronegativity of carbon compared to that of nitrogen, the adjacent nitrogen atoms can offer unpaired electrons to expedite the adsorption of oxygen molecules.¹⁴ The N-doped carbon nanotubes are uniformly grown in situ on the surface of the nanosheets, which provides a valid space for the growth of N-doped carbon nanotubes and prevents agglomeration of large metal particles. The MOF sheets can be transformed into carbon nanosheets after pyrolysis, which can enhance the conductivity of the material. Simultaneously, hollow carbon nanotubes are beneficial for the transfer of electrons and the transfer of substances related to the reaction, reducing the transmission distance and increasing the specific surface area of the material.

A transmission electron microscope (TEM) was utilized to further analyze the morphology and structural information of FeZn–N–C-800. As illustrated in Fig. S4 (ESI†), the TEM image indicates that carbon nanotubes were in situ grown on the porous carbon nanosheets, encompassing coated metal nanoparticles. The porous structure of the carbon nanosheets is attributed to the volatilization of zinc during the pyrolysis.¹⁵ The porous carbon nanosheets and carbon nanotubes collectively constitute a 3D catalyst presenting a larger exposed area, enabling oxygen and ions to expedite diffusion via the pore microstructure.¹⁶ High-resolution TEM (HRTEM) was employed to observe the lattice spacings, as depicted in Fig. S5 (ESI†), which were 0.34 nm and 0.17 nm, respectively, corresponding to the (002) crystal plane of the outer layer of graphitized carbon and the (−421) crystal plane of the coated Fe₅C₂, maintaining the stable work function gap between the graphitized carbon layer and the metal particles. Elemental mappings reveal the distribution of the C, N, and Fe elements of FeZn–N–C-800 in Fig. 1e, with the contents of 79.79, 3.01, and 6.50 wt%, respectively. The N element is in the corresponding region of the carbon nanotubes, which demonstrates that the N element has been successfully doped into the carbon nanotubes. The mesoporous 2D carbon nanosheets can enhance the conductivity of the catalysts and oxygen absorption.

X-ray powder diffraction (XRD) was applied to analyze the crystal structure of FeZn–N–C-800. As depicted in Fig. 2a, the diffraction peak at 2θ ≈ 26° pertains to the (002) crystal plane of graphitized carbon, while the diffraction peak at 2θ ≈ 44° is deemed to pertain to Fe₅C₂ (PDF#20-0508).¹⁷ Owing to the evaporation of the metal zinc at high temperatures, the original FeZnC_0.5 (PDF#29-0741) was transformed into Fe₅C₂. Raman spectra were collected to assess the degree of graphitization of the obtained catalysts. In Fig. 2b, there are two distinct peaks at 1350 and 1580 cm⁻¹, corresponding to the D and G bands, respectively.¹⁸ When the pyrolysis temperature is 800 °C, the ratio of the I_D/I_G is higher than those of samples at 700 and 900 °C, indicating that the material has more defects, which may produce charge redistribution and thus improve the adsorption of oxygen intermediates.¹⁹

	Fig. 2 (a) XRD patterns, (b) Raman spectra, (c) N2 isotherm curves of samples at different pyrolysis temperatures, and (d) Fe 2p XPS spectra of the FeZn–N–C-800.

The N₂ adsorption–desorption curves were employed to analyze the specific surface area and pore size distribution of FeZn–N–C-800, FeZn–N–C-700, and FeZn–N–C-900. In Fig. 2c, FeZn–N–C-800 possesses a type IV N₂ isotherm and presents an evident hysteresis loop at P/P₀ = 0.4–1.0 for mesopores. As the temperature rises from 700 °C to 800 °C, the specific surface area of FeZn–N–C increases to 488 m² g⁻¹, which might be attributed to the decomposition of the original MOF skeleton, the volatilization of zinc, and the growth of carbon nanotubes. When the temperature escalates to 900 °C, the reduction in the specific surface area could be owing to the collapse of the original structure.²⁰ In Fig. S6 (ESI†), it can be observed from the pore size distribution that the pore structure of the FeZn–N–C-800 sample primarily consists of micropores and mesopores, which aids in fully exposing the catalytic active sites and the transport of reaction-related substances, and enhances the catalytic activity.

X-ray photoelectron spectroscopy (XPS) was used to detect the chemical composition and bonding state of the FeZn–N–C-800 surface. As shown in Fig. S7 (ESI†), the full XPS spectrum shows that the atomic percentages of C, N, O, Fe, and Zn were 72.76, 14.13, 10.03, 1.14, and 1.94 at%, respectively. The C 1s spectrum shows the presence of a O–CO bond (288.2 eV), C–O bond (286.2 eV), C–N bond (285.5 eV) and C–C (284.6 eV) (Fig. S8, ESI†). The high-resolution N 1s spectrum can be fitted into four peaks, corresponding to pyridine-N (398.8 eV), metal-N (399.4 eV), pyrrole-N (400.5 eV), and graphite-N (401.3 eV) (Fig. S9, ESI†), respectively.²¹ The metal-N indicates that nitrogen and iron coordinate to form a highly active Fe–N₄ site.²² Fe 2p is fitted into Fe(III) (711 and 723 eV), Fe(II) (713 and 725 eV), and a satellite peak (718 eV) (Fig. 2d),^2c which is consistent with the previous XRD and HRTEM analyses.

Cyclic voltammetry (CV) curve of FeZn–N–C-800 distributed in 0.1 M KOH solution saturated with oxygen and nitrogen (Fig. S10, ESI†). It can be found that in the case of oxygen saturation, FeZn–N–C-800 has a reduction peak at 0.87 V, while in the case of nitrogen saturation, there is no reduction peak, indicating that FeZn–N–C-800 has good electrocatalytic oxygen reduction ability. Furthermore, the linear sweep voltammetry (LSV) test was performed to evaluate its electrocatalytic performance. As shown in Fig. 3a, FeZn–N–C-800 exhibits the highest onset potential (E_onset) and half-wave potential (E_1/2), with distributions of 1.08 V and 0.87 V, respectively, more than FeZn–N–C-900 (E_onset = 0.97 V, E_1/2 = 0.84 V), FeZn–N–C-700 (E_onset = 0.91 V, E_1/2 = 0.68 V), MOF-5-800 (E_onset = 0.99 V, E_1/2 = 0.78 V) and Pt/C (E_onset = 0.98 V, E_1/2 = 0.81 V). Compared with other samples, FeZn–N–C-800 also has the largest limiting diffusion current (5.3 mA cm⁻²), indicating that its 3D carbon-based structure is conducive to improving the transport and conductivity of reaction products such as oxygen. It also has the largest kinetic current density (11.9 mA cm⁻²) at 0.8 V, which is 4.9 times that of Pt/C (Fig. S11, ESI†). In addition, the electrochemical kinetics of the electrocatalyst were also evaluated by the Tafel slope (Fig. 3b). The Tafel slope of FeZn–N–C-800 is the smallest (57 mV dec⁻¹), smaller than that of commercial Pt/C (83 mV dec⁻¹), indicating that it has excellent electron transport kinetics and excellent catalytic ability.

	Fig. 3 (a) ORR curves and (b) Tafel slopes for the ORR of the catalysts, and (c) and (d) LSV curves at 900–2500 rpm and the corresponding K–L plots at 0.5–0.7 V for FeZn–N–C-800.

By testing the LSV curves at different rotational speeds (Fig. 3c), it is found that FeZn–N–C-800 presents a gradient increase in the limiting current density and a stable onset potential. The ORR is also a significant aspect in evaluating electron transfer number. As illustrated in Fig. 3d, the electron transfer number (n) calculated based on the Koutecky–Levich (K–L) equation is 3.94, demonstrating a superior four-electron transfer mechanism. The durability of the material was assessed through chronoamperometry (Fig. S12, ESI†). At a constant voltage of 0.7 V, the current density of FeZn–N–C-800 can still maintain 80% after long-term testing for 30000 seconds, while that of commercial Pt/C is only 67%, indicating the good stability of FeZn–N–C-800. The double-layer capacitance (C_dl) and electrochemically active surface area (ECSA) were obtained by testing CV in the non-faradaic region (Fig. S13, ESI†). The C_dl value of FeZn–N–C-800 is calculated to be 17.8 mF cm⁻², which is much higher than those of the control samples (Fig. S14, ESI†). The open-circuit voltage (V_oc) of FeZn–N–C-800 could reach 1.42 V, which is higher than that of a commercial Pt/C assembled zinc–air battery (1.40 V) (Fig. S15, ESI†). The discharge rate performance of ZABs is further verified (Fig. 4a). The discharge voltage of FeZn–N–C-800 is 0.88 V, higher than that of Pt/C, when the discharge current is 150 mA cm⁻². The discharge polarization curve shows that the power density of FeZn–N–C-800 (294 mW cm⁻²) is higher than that of Pt/C (179 mW cm⁻²) (Fig. 4b). When the discharge current density is 50 mA cm⁻², the specific energy density of ZABs based on FeZn–N–C-800 (887 W h kg⁻¹) is also higher than that of ZABs based on Pt/C (704 W h kg⁻¹) (Fig. S16, ESI†). To test the long-term cycling stability of FeZn–N–C-800, a continuous cycle test with a current density of 10 mA cm⁻² and a cycle of 20 minutes was performed on the battery (Fig. S17, ESI†), and no obvious necrosis phenomenon was found after 240 h of ZAB cycling based on FeZn–N–C-800, which is far superior to the long-term cycling performance of zinc–air batteries assembled by Pt/C.

	Fig. 4 (a) Rate performances, and (b) discharge polarization curves and corresponding power density plots.

In summary, a 3D carbon-based catalyst featuring N-doped carbon nanotube riveted on 2D carbon nanosheets was successfully fabricated via the in situ growth of carbon nanotubes during the pyrolysis of potassium ferrocyanide by adsorbing on stripped MOF-5 nanosheets. The obtained catalyst possesses a high specific surface area and enriched catalytic sites, which attains higher electrocatalytic performance compared to the noble metal Pt/C in alkalic systems, with a power density of 294 mW cm⁻², specific energy density of 887 W h kg⁻¹, and good high-rate performance. It presents a potential approach to utilize low-cost transition metals instead of noble metal catalysts.

This research was supported by the National Natural Science Foundation of China (U1832136 and 21303038), the Natural Science Foundation of Anhui Province (305067828053), and the Fundamental Research Funds for the Central Universities (PA2022GDGP0029 and PA2024GDGP0042).

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Conflicts of interest

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