Carbon-nanotube wall nanoengineering strategy to stabilize FeNi nanoparticles and Fe single atoms for rechargeable Zn–air batteries

Abstract

The great interest in rechargeable Zn–air batteries (ZABs) stimulates extensive research on efficient and robust electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a novel ORR/OER bifunctional catalyst is developed using carbon-nanotube wall nanoengineering. In this design, FeNi nanoparticles are inserted into the wall via a carbothermic reaction to enhance the OER, while isolated Fe atoms in iron-phthalocyanine anchored on the wall via π–π coupling interaction are used to catalyze the ORR. Accordingly, the resulting electrocatalyst exhibits outstanding ORR and OER activities such as a small potential difference of 0.67 V. In situ Raman spectroscopy measurements verify the presence of reconstruction transformation from an alloy phase to a high-activity spinel phase during the OER process. When used in ZABs, high peak power densities of 208.5 mW cm⁻² under a liquid-state electrolyte and 150.1 mW cm⁻² in a solid-state electrolyte are demonstrated. Furthermore, outstanding battery durability is illustrated by a small and stable charge–discharge voltage gap of 0.78 V at 10 mA cm⁻² after 1400 cycles. This study offers a novel method to fabricate bifunctional ORR/OER electrocatalysts and possibly extends to multi-site catalysts.

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Introduction

With the increasing severity of environmental problems arising from rapid utilization of fossil fuels, clean energy (such as solar energy and wind energy) has received more and more attention.¹ Owing to the intermittent characteristic of these clean energies, it is quite necessary to develop energy storage and conversion technologies for their effective utilization.² As a type of energy storage and conversion technology, Zn–air batteries (ZABs) have attracted great interest because of their advantages of high energy density, environmental friendliness, low cost and intrinsic safety.^3–5 However, the main challenge for the practical application of ZABs lies in realizing high power density and long-life charge–discharge cycles, which are dependent on the harsh and slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on air electrodes.^6–8 In this case, numerous studies have been devoted to developing low-cost, high-efficiency and high-stability electrocatalyst to boost ORR/OER processes.^9–11

There are two main types of low-cost ORR/OER bifunctional electrocatalysts.^12–14 One type comprises metal-free heteroatom-doped carbon electrocatalysts, in which N-doped carbon materials are the most typical form of electrocatalysts.¹⁵ Another type comprises carbon-based noble–metal-free catalysts.¹⁶ Compared with metal-free electrocatalysts, noble–metal-free catalysts generally show better electrocatalytic performance and therefore attract more attention.^17–19 Significant progress has been recently made in developing high-performance noble–metal-free catalysts. Among these, iron-group species-modified carbon materials represent the most popular research subjects owing to their outstanding electrocatalytic ORR/OER performance.^20–22 Specifically, atomic Fe–N₄ moieties anchored on the carbon plane represent the highest number of ORR active sites but suffer from poor OER activity, while bimetallic iron-group compounds, especially FeNi species, show excellent OER performance.^23–25 Thus, stabilizing both Fe–N₄ moieties and FeNi species on/in a carbon substrate can simultaneously realize outstanding ORR and OER activities.²⁶

Recently, some methods have been proposed to prepare such electrocatalytic materials enriched with single-atom and nanosized metal active parts.^27–29 High-temperature pyrolysis of precursors containing iron-group metal sources is a common approach. Notably, there is a concentration trade-off between metal single atoms and nanoparticles because of inevitable Ostwald ripening and migration-coalescence of metal nanoparticles during the pyrolysis process.³⁰ Under such circumstances, dual-phasic carbon electrocatalysts, showing excellent activity and stability, derived from bicomponent precursors have been developed to solve this dilemma.³¹ Nevertheless, the well-designed precursors are extremely complex and hard to control in terms of the preparation process.

Herein we report a simple carbon-nanotube (CNT) wall nanoengineering strategy to realize the bifunctional electrocatalysts enriched with atomic Fe–N₄ moieties and FeNi nanoparticles. Specifically, FeNi nanoparticles are partially inserted into the wall of CNTs through a carbothermic reaction to enhance the OER, while atomic Fe–N₄ moieties in iron phthalocyanine (FePc) anchored on the wall by π–π coupling interaction are used to catalyse the ORR. As a result, the resulting material (denoted as FeNi/CNT–FePc) shows outstanding ORR activity with a positive half-wave potential of 0.90 V as well as excellent OER activity with a small overpotential of 0.34 V at 10 mA cm⁻². When used in ZABs, excellent battery output performance and outstanding battery durability are demonstrated. Furthermore, electrocatalytic mechanisms are discussed based on the results obtained by in situ Raman spectroscopy measurements.

Result and conclusion

Fig. 1a shows the synthetic process of FeNi/CNT–FePc. First, a CNT sample, subjected to oxidation treatment under a mixture solution of concentrated nitric acid and concentrated sulfuric acid, was used as the support for the growth of FeNi hydroxide/oxyhydroxide (FeNi–HO) by means of a coprecipitation reaction (Fig. S1†). Then, the resulting FeNi–HO/CNT was subjected to thermal treatment at 500 °C under Ar atmosphere. In this case, FeNi–HO species were decomposed into FeNi oxides and then the resulting FeNi oxides reacted with the carbon wall of CNTs to perform the reduction (Fig. S2†) through a carbothermic reaction.³² This carbothermic reaction could result in the FeNi particles partially inserted in the wall of CNTs (FeNi/CNT) and therefore enhance the stability. Finally, the FeNi/CNT sample was dispersed in FePc solution under ultrasonic treatment.³³ Owing to strong π–π coupling interaction between the carbon wall of FeNi/CNT and FePc, the hybrid material of FeNi/CNT–FePc was obtained.

Fig. 1 (a) Schematic showing the synthetic process for FeNi/CNT–FePc. (b) High-resolution TEM image and (c) the corresponding FFT pattern of FeNi/CNT–FePc. (d) HAADF-STEM images of FeNi/CNT–FePc. (e) Atomic-resolution HAADF-STEM image of FeNi/CNT–FePc. (f) STEM-EDS images of FeNi/CNT–FePc.

Transmission electron microscopy (TEM) was performed to reveal the structural information of FeNi/CNT–FePc. Some large-size metal nanoparticles (>5 nm) on the wall of CNTs are observed during the TEM characterization but the density seems to be low (Fig. S3†). The reason for this phenomenon can be attributed to the relatively higher crystallinity of large-size metal nanoparticles and therefore more pronounced contrast. A high-resolution TEM image in Fig. 1b shows a metal nanoparticle with good crystallinity that is partially inserted in the wall of a CNT in FeNi/CNT–FePc. Through fast Fourier transformation (FFT) analysis (Fig. 1c), the nanoparticle, showing (11−1), (020) and (−111) lattice planes and the corresponding [−10−1] zone axis, is identified as FeNi₃. The X-ray diffraction pattern displays that obvious characteristic peaks of FeNi₃ are present in FeNi/CNT–FePc (Fig. S4†), which is in good agreement with the TEM result. Besides, there are characteristic peaks of the FeO, NiFe₂O₄ and FePc phases. Compared with single FePc and CNT samples, the coupling hybrids, including FeNi/CNT–FePc and CNT–FePc, exhibit two relatively obvious additional characteristic peaks at 39° and 41° (Fig. S5†) attributed to strong π–π coupling interactions.³⁴ These results illustrate that metal nanoparticles such as FeNi₃, FeO and NiFe₂O₄ are anchored on/in the CNT walls in FeNi/CNT–FePc while FePc molecules are coupled with the CNT walls.

To further reveal the structural information of FeNi/CNT–FePc at the atomic level, aberration-corrected scanning TEM (STEM) imaging was carried out. Similar low-density large-size metal nanoparticles anchored on CNTs were also observed during the bright-field STEM imaging (Fig. S6†). Nevertheless, under the high-angle annular dark-field STEM (HAADF-STEM) imaging, abundant metal nanoparticles with relatively low crystallinity are observed on CNTs (Fig. 1d and Fig. S7†). Statistical analysis shows that the average particle size is about 4.2 nm (Fig. S8†). Together with energy-dispersive X-ray spectroscopy (EDS) analysis, the metal nanoparticles were identified as FeNi oxides (Fig. S9†). Fig. 1e shows a high-magnification HAADF-STEM image of FeNi/CNT–FePc. Numerous isolated bright spots attributed to single Fe atoms, were observed to be anchored on the CNT substrate, illustrating that FePc was successfully coupled to the CNT wall and well consistent with the XRD result. In addition, HAADF-STEM imaging together with EDS elemental mapping, confirmed that N, O, Fe and Ni are uniformly distributed on/in the CNT substrate (Fig. 1f), suggesting FeNi nanoparticles and FePc molecules are uniformly stabilized in/on the walls of CNTs.

The chemical composition was further explored by X-ray photoelectron spectroscopy (XPS), which also confirmed the presence of C, N, O, Fe and Ni in FeNi/CNT–FePc (Fig. S10†). The Fe 2p XPS spectrum of FeNi/CNT–FePc (Fig. 2a) shows that the peaks near 708.9 eV, 711.4 eV and 713.5 eV are respectively attributed to the Fe⁰, Fe²⁺ and Fe³⁺ 2p_3/2, while the peaks near 722.0 eV, 724.4 eV and 726.5 eV are derived from Fe⁰, Fe²⁺ and Fe³⁺ Fe 2p_1/2, respectively.³⁵ Analogously, Ni⁰ (853.2 eV for Ni 2p_3/2 and 871.3 eV for Ni 2p_1/2), Ni²⁺ (856.4 eV for Ni 2p_3/2 and 874.2 eV for Ni 2p_1/2) and Ni³⁺ (859.2 eV for Ni 2p_3/2 and 877.2 eV for Ni 2p_1/2) are also confirmed in the Ni 2p XPS spectrum of FeNi/CNT–FePc (Fig. 2b). Among these, the Fe⁰ and Ni⁰ species are due to FeNi₃ alloy, while the high valence states of Fe and Ni originate from FePc, FeO, NiFe₂O₄, and even surface oxidation states of FeNi₃ alloy. The N 1s XPS spectrum that can be deconvoluted into two peaks at 398.5 eV for pyridinic N and 400.6 eV for pyrrolic N in FePc of FeNi/CNT–FePc (Fig. S11†).³⁶Fig. 2c shows the Fourier transform infrared (FTIR) spectra of FeNi/CNT–FePc and reference samples (Fig. 2c). Obvious characteristic peaks of FePc are present in FeNi/CNT–FePc such as 725, 1075, 1118, 1285 and 1330 cm⁻¹.³⁷ Likewise, the Raman spectrum also demonstrates that some characteristic peaks of FePc are also detected in FeNi/CNT–FePc (Fig. S12†).³⁸ These spectroscopic evidences undoubtedly confirm that FePc is successfully coupled on the CNT wall, in good agreement with the TEM result.

Fig. 2 (a) Fe 2p and (b) Ni 2p XPS spectra of FeNi/CNT–FePc. (c) FTIR spectra of FeNi/CNT–FePc and reference samples. (d) Fe K-edge XANES spectra of FeNi/CNT–FePc and reference samples. (e) Fe average valence state in FeNi/CNT–FePc. (f) FT-EXAFS k-weighted spectra of FeNi/CNT–FePc and reference samples. WT plots of (g) FeNi/CNT–FePc, (h) FePc and (i) Fe₂O₃.

Synchrotron-based X-ray absorption spectroscopy measurement was performed to further clarify the valence and coordination environments. Fig. 2d exhibits the K-edge X-ray near-edge structure (XANES) spectra of FeNi/CNT–FePc and the reference samples. The pre-edge peak of FeNi/CNT–FePc (7114.6 eV) lies between those of Fe₂O₃ (7113.1 eV) and FePc (7117.4 eV), illustrating that Fe-based oxides and FePc can coexist in FeNi/CNT–FePc. Besides, the absorption edge energy of FeNi/CNT–FePc is located between those of Fe foil and Fe₂O₃, which is in good agreement with the XPS result that multiple valence states of Fe species are present in FeNi/CNT–FePc. The calculated average valence state of Fe species is +2.71 (Fig. 2e), suggesting that most of the Fe species in FeNi/CNT–FePc are high valence oxidation states and this is corresponding to the TEM observation that only a few of FeNi₃ alloy nanoparticles are present, while abundant FeNi oxides co-exist in FeNi/CNT–FePc. Fig. 2f exhibits Fourier-transform extended X-ray absorption fine structure (FT-EXAFS) spectra. There are two prominent scattering peaks at ∼1.5 Å and ∼2.4 Å for FeNi/CNT–FePc. The first peak is well aligned with the first-shell Fe–N scattering peak of FePc and Fe–O scattering peak of Fe₂O₃, while the second peak lies between the first-shell Fe–Fe scattering peak of Fe foil (∼2.2 Å) and the second-shell Fe–Fe peak of Fe₂O₃ (∼2.5 Å) and Fe–C scattering peak of FePc.³⁹ That is, the first peak is attributed to the first-shell Fe–N scattering of FePc and Fe–O scattering of FeNi oxides in FeNi/CNT–FePc, while the second peak can originate from the first-shell Fe–Fe/Ni scattering of FeNi₃ alloy, the second-shell Fe–Fe/Ni peak of FeNi oxides (∼2.5 Å) and even the second-shell Fe–C scattering peak of FePc. The wavelet transform (WT) contour plot of FeNi/CNT–FePc (Fig. 2g) shows two maxima. One WT maximum of FeNi/CNT–FePc at 4.1 Å in k-space and 1.5 Å in R-space corresponds well to those of FePc (Fig. 2h) and Fe₂O₃ (Fig. 2i), which can be attributed to Fe–N/O contributions. The other WT maximum of FeNi/CNT–FePc is centered at 6.0 Å in k-space and 2.4 Å in R-space, showing a peak shift compared with that of Fe₂O₃ (7.5 Å in k-space and 2.5 Å in R-space), due to the presence of metallic Fe species.⁴⁰ These results further demonstrate that atomic Fe–N_x species, Fe-based oxides and alloys exist in FeNi/CNT–FePc.

The electrocatalytic performance of FeNi/CNT–FePc was assessed in 0.1 M KOH using rotating disk electrode (RDE) technique. Commercial Pt/C (20 wt% of Pt) and Ir/C catalysts (20 wt% of Ir) were used as benchmarks for ORR and OER, respectively. Notably, FeNi/CNT–FePc represents an optimized bifunctional electrocatalyst through modulating the synthesis parameters such as active components (Fig. S13†) and carbothermic reaction temperatures (Fig. S14†). Fig. 3a displays the RDE polarization curves of FeNi/CNT–FePc and Pt/C for ORR. FeNi/CNT–FePc shows a positive half-wave potential of 0.90 V, which is clearly superior to that of Pt/C (0.85 V), illustrating that FeNi/CNT–FePc has excellent ORR activity. Tafel plots (Fig. S15†), derived from mass transport correction, reveal that FeNi/CNT–FePc has significantly lower Tafel slope value (32 mV dec⁻¹) than that of Pt/C (72 mV dec⁻¹), indicating superior ORR kinetics. Furthermore, the kinetic current density of FeNi/CNT–FePc arrives 4.11 mA cm⁻² at 0.9 V (Fig. 3b), which is notably higher than that of the commercial Pt/C catalyst at 0.98 mA cm⁻². The HO₂⁻ yields and electron transfer numbers of FeNi/CNT–FePc and Pt/C, derived from recorded disk and ring currents, are shown in Fig. 3c. The HO₂⁻ yield of FeNi/CNT–FePc is below 2.2% at all potentials, and its matching average electron transfer number is 3.99. This indicates that FeNi/CNT–FePc holds good selectivity, following a highly efficient 4e⁻ transfer process. In addition, the ORR stability of FeNi/CNT–FePc was evaluated by potential cycling, with the polarization curve after 5000^th potential cycle almost overlaps that of the initial state (Fig. 3d), which demonstrates its excellent ORR stability.

Fig. 3 (a) ORR polarization curves, (b) kinetic current density curves, and (c) peroxide yields and electron transfer numbers of FeNi/CNT–FePc and Pt/C. (d) ORR stability evaluation curves of FeNi/CNT–FePc. (e) OER polarization curves of FeNi/CNT–FePc and Ir/C. (f) OER stability evaluation curves of FeNi/CNT–FePc. (g) In situ Raman spectra collected on FeNi/CNT–FePc during the OER process.

Fig. 3e exhibits the OER polarization curves of FeNi/CNT–FePc and Ir/C. FeNi/CNT–FePc has an OER potential of 1.57 V at 10 mA cm⁻², which is better than that of Ir/C (1.61 V) and verifies the outstanding OER performance of FeNi/CNT–FePc. The overall oxygen electrode performance is evaluated by the value difference between OER potential at 10 mA cm⁻² and ORR half-wave potential. The potential difference of FeNi/CNT–FePc reaches as small as 0.67 V, representing one of the best oxygen reaction electrocatalysts (Table S1†). Tafel plots (Fig. S16†) show that the Tafel slope of FeNi/CNT–FePc is 65 mV dec⁻¹, almost as small as that of Ir/C (67 mV dec⁻¹), suggesting its excellent OER kinetics. Furthermore, the OER stability assessment was performed by potential cycling. As shown in Fig. 3f, the OER polarization curve of FeNi/CNT–FePc recorded after 5000^th potential cycle almost overlaps that of the initial state, verifying its exceptional OER stability. Besides, a stable chronoamperometric response is also observed, which further demonstrates the excellent OER stability of FeNi/CNT–FePc (Fig. S17†). FTIR spectra of FeNi/CNT–FePc (Fig. S18†) before and after OER cycling show that FePc characteristic peaks at 731, 879, 1053, 1093, 1383 cm⁻¹ are still retained after the OER stability test, illustrating the presence of an unchanged FePc coupling structure.

Previous reports gave explicit evidence to reveal the nature of high-activity FePc-based coupling hybrids for ORR, including in situ experiments and theoretical simulations.^41–43 Likewise, the atomic Fe–N₄ moiety on the graphene plane is the active site of FeNi/CNT–FePc for ORR. This conclusion is further demonstrated by contrast experiments that both FeNi/CNT and FePc show poor ORR performance while another coupling hybrid of CNT–FePc displays similar ORR activity to FeNi/CNT–FePc (Fig. S19†). For the OER process, it is complex due to the presence of an in situ reconstruction process from metallic species to corresponding spinel oxides, hydroxides, and/or oxyhydroxides.^44–46 To understand the catalytic OER mechanism of FeNi/CNT–FePc, in situ Raman spectroscopy measurements were performed. As shown in Fig. 3g, a broad and weak peak around 461 cm⁻², attributed to the asymmetric stretch of Fe/Ni–O in NiFe₂O₄ spinel oxide, is detected.⁴⁷ When the applied potentials are set at 0.96–1.36 V, there is no significant peak change. Upon increasing the applied potentials at 1.46 V and beyond, the asymmetric stretch mode intensity seems to be relatively stronger. Besides, a new peak at ∼625 cm⁻², originating from the symmetric stretch of oxygen atoms along Fe/Ni–O bonds in NiFe₂O₄, appears and becomes stronger with the increase of applied potential.⁴⁷ These results demonstrate that the reconstruction process from the FeNi₃ alloy along with FeO to NiFe₂O₄ and the practical active sites for the OER process can be the NiFe₂O₄ spinel oxides. XRD analysis further demonstrates that the reconstruction of metal nanoparticles generates the NiFe₂O₄ species (Fig. S20†).

To evaluate the practical applications, liquid-state ZABs were assembled using FeNi/CNT–FePc or Pt/C + Ir/C coated carbon cloths as the air cathodes. Compared with the Pt/C + Ir/C battery (∼1.41 V) (Fig. S21†), FeNi/CNT–FePc has a higher open circuit voltage (OCV) of 1.49 V (Fig. S22†), suggesting its better ORR electrocatalytic activity and being in good agreement with the RDE result. Fig. 4a shows the power density curves of the FeNi/CNT–FePc and Pt/C + Ir/C cathodes. FeNi/CNT–FePc exhibits a peak power density of 208.5 mW cm⁻², which is superior to that of Pt/C + Ir/C (147.2 mW cm⁻²) and represents the leading position in recently advanced noble–metal-free electrocatalyst-based ZABs (Table S2†). In an oxygen atmosphere, FeNi/CNT–FePc exhibits a peak power density of 384.7 mW cm⁻², which is also superior to that of Pt/C + Ir/C (332.2 mW cm⁻²) (Fig. S23†). Besides, the battery specific capacities and rate performances were measured respectively by galvanostatic discharging at 10 mA cm⁻² to consume the anodic Zn and galvanostatic discharging at different current densities. Based on the Zn consumption, the derived specific capacity of FeNi/CNT–FePc is 751.1 mA h g⁻¹ (Fig. 4b), obviously higher than that of Pt/C + Ir/C (716.1 mA h g⁻¹). Fig. 4c exhibits the discharge curves at different current densities of FeNi/CNT–FePc and Pt/C + Ir/C. FeNi/CNT–FePc holds conspicuously better output voltages compared to Pt/C + Ir/C at the same discharge current densities ranging from 1 mA cm⁻² to 10 mA cm⁻². When the discharge current densities return from 10 mA cm⁻² to 1 mA cm⁻², FeNi/CNT–FePc shows smaller voltage drops, illustrating its better rate performance. The charge–discharge cycling stability, an important performance to evaluate ZABs in practical applications, was also measured. As shown in Fig. 4d, the FeNi/CNT–FePc battery displays a very stable charge–discharge voltage gap of ∼0.78 V without any obvious decay during the long-time cycling process (1400 cycles) at a current density of 10 mA cm⁻². By contrast, the Pt/C + Ir/C ZAB seems to present serious degradation after only 200 cycles. These above results demonstrate that the FeNi/CNT–FePc battery holds excellent output performance and stability, showing great application prospects.

Fig. 4 (a) Power density curves, (b) discharge curves at 10 mA cm⁻², (c) rate performance, and (d) galvanostatic charge–discharge curves of liquid-state ZABs based on FeNi/CNT–FePc and Pt/C + Ir/C. (e) Power density curves of solid-state ZABs based on FeNi/CNT–FePc and Pt/C + Ir/C. (f) OCV evolution of solid-state FeNi/CNT–FePc ZAB under different bending conditions. (g) Photograph showing a mobile phone being charged, powered by solid-state FeNi/CNT–FePc ZABs.

Besides, solid-state ZABs were studied. FeNi/CNT–FePc-based solid-state ZAB shows an OCV value of 1.41 V (Fig. S24†), higher than that of Pt/C + Ir/C (∼1.38 V) (Fig. S25†). The peak power density of the FeNi/CNT–FePc battery reaches as high as 150.1 mW cm⁻² (Fig. 4e), better than the Pt/C + Ir/C one (118.7 mW cm⁻²). Fig. 4f shows the OCV curve of FeNi/CNT–FePc-based solid-state ZAB under bending conditions. Clearly, the OCV value (∼1.41 V) maintains very stable at different bending angles, demonstrating excellent flexibility of FeNi/CNT–FePc-based solid-state ZAB. The battery stability was also evaluated by charge–discharge cycling at 2 mA cm⁻². After the 300 cycles, the charge–discharge voltage platforms of the FeNi/CNT–FePc battery remain almost unchanged (Fig. S26†), indicating that FeNi/CNT–FePc-based solid-state ZAB has good cycling stability. Finally, the FeNi/CNT–FePc-based solid-state ZABs were integrated into a series circuit to power electronic devices. For example, four batteries in series can charge a smartphone (Fig. 4g) and brighten a string of colored lights (Fig. S27†), indicating good practicability of FeNi/CNT–FePc based solid-state ZABs.

Conclusions

In conclusion, a simple CNT wall nanoengineering strategy is developed to construct a high-efficient and robust oxygen electrode catalyst based on FeNi/CNT–FePc, including atomic Fe–N₄ sites anchored on the wall by π–π coupling interaction and FeNi nanoparticles partially inserted in the wall by carbothermic reaction. Among these, the atomic Fe–N₄ sites are used to boost the ORR, while the FeNi nanoparticles serve as active parts to accelerate the OER. As a result, the resulting FeNi/CNT–FePc electrocatalyst has both outstanding ORR and OER activities, showing a small potential difference of 0.67 V. Furthermore, in situ Raman spectroscopy is performed to reveal the electrocatalytic mechanism of the FeNi active parts, which demonstrates that reconstruction transformation from the alloying phase to a high-activity spinel counterpart takes place during the OER process. Finally, FeNi/CNT–FePc was used to assemble rechargeable ZABs. Large peak power densities of 208.5 mW cm⁻² under liquid-state electrolyte and 150.1 mW cm⁻² in solid-state electrolyte are demonstrated. Besides, outstanding battery durability is also verified by the small and stable battery charge–discharge voltage gap of 0.78 V at 10 mA cm⁻² after 1400 cycling. This study offers a novel method to fabricate bifunctional ORR/OER electrocatalysts and possibly extends to multi-site catalysts.

Author contributions

Y. Y., P. D., T. L., and J. L. conceived the carbon-nanotube wall nanoengineering strategy to stabilize FeNi nanoparticles and Fe single atoms. Y. Y. performed the synthesis of FeNi/CNT–FePc and its ZABs, including fabrication and test. Y. Y., L. H., P. X., and H. Q. carried out the RDE, XRD, TEM, and XPS experiments. T. L. and L. F. performed the XAS measurements and analysis. Y. Y., L. H., P. X., D. F., Y. X., and J. L. conducted the characterization/measurement data interpretation and construction of figures. Y. Y., Y. X., and J. L. co-wrote the manuscript. All the authors commented on the manuscript.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52372049) and Yunnan Fundamental Research Projects (202301AW070016).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi03361b

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