Ni-doping optimized d-band center in bifunctional Fe₂O₃ modified by bamboo-like NCNTs as a cathode material for Zn–air batteries†

Yang Shi ^a, Songhan Hu ^b, Xinxin Xu *^a and Jin Chen ^b
aDepartment of Chemistry, College of Science, Northeastern University, Shenyang, 110819, P.R. China. E-mail: xuxx@mail.neu.edu.cn; Fax: +086-024-83684533; Tel: +086-024-83684533
^bKey Laboratory of Electromagnetic Processing of Materials, MOE, Northeastern University, Shenyang 110819, Liaoning, China

First published on 7th August 2024

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Introduction

Recently, driven by the global fossil energy and environmental pollution crisis, the exploration for clean, renewable, reliable energy conversion techniques and devices has become a significant challenge for researchers.^1,2 As a new generation of energy conversion devices, Zn–air batteries have gained broad attention owing to their great advantages such as environmental friendliness, excellent safety, high capacity and energy density.^3,4 For Zn–air batteries with alkaline electrolyte, discharge is achieved through ORR, which occurs at the cathode where O₂ is reduced to OH⁻.^5–7 On the contrary, during charging, OER takes place, converting OH⁻ back to O₂.^8–10 However, for most ORR and OER electrocatalysts, their inherently weak kinetic processes are too sluggish to achieve high performance in Zn–air batteries.^11,12 Pt/C and RuO₂ are promising ORR and OER electrocatalysts; however, their popularization in Zn–air batteries is still seriously restricted by their limited reserves, expensive price and poor stability.^13–16 Consequently, the exploration for stable and efficient bifunctional electrocatalysts with rich reserves and low price is pressing for further development of Zn–air battery.^17,18

In this aspect, transition metal oxides have attracted great interest owing to their special advantages.^19–21 First, transition metal oxides exhibit favorable corrosion resistance in alkaline electrolytes during the electrochemical process.^22,23 Secondly, due to the coexistence of multiple valence states, intensive interactions are generated in transition metal oxides, which improves electrochemical activity.^24–26 Considering the above-mentioned facts, cheap Fe₂O₃-based electrocatalysts appear to be ideal substitutes for Pt/C and RuO₂ in ORR/OER.^27–29 However, before entering into practical application as a cathode material for real Zn–air batteries, additional improvements are required for Fe₂O₃.^30–33 To improve ORR/OER activities, optimization of the interaction between electrocatalyst and relevant active oxygen species (such as O₂, OOH*, O*, OH* or OH⁻) is a feasible approach.^34–36 To achieve this goal, band structure adjustment through hetero component (such as transition metallic elements, Mn, Co, Cu, etc. and nonmetallic elements N, S, P, etc.) doping is a feasible strategy, which has achieved great success.^37–39

For Fe₂O₃, when employed as a bifunctional ORR and OER electrocatalyst in a Zn–air battery, poor conductivity is a conspicuous weakness, which weakens electrocatalytic activity greatly.^40,41 To resolve this problem, the combination of Fe₂O₃ and carbon materials, especially N-doped carbon materials can improve conductivity greatly.^42,43 This enhances ORR and OER activities simultaneously.^44–46 In the obtained composite electrocatalyst, carbon material plays an important role, which serves as a conductive carrier and active component.^47–50 Compared with carbon material in other forms, carbon nanotubes (CNTs), especially N-doped carbon nanotubes (NCNTs) seem more attractive because of their special characteristics, such as optimized pathway in charge transportation and short diffusion distance for relevant oxygen species in ORR/OER.^51–58 Furthermore, NCNTs also possess superiority in electrocatalysis. Therefore, the combination of Fe₂O₃ and NCNTs can obtain electrocatalysts with ideal ORR and OER activities.

As motivated by the above points, with MIL-88A, a MOF as a precursor, a bifunctional electrocatalyst (Ni/Fe-Fe₂O₃@NCNTs) was synthesized. In Ni/Fe-Fe₂O₃@NCNTs, Ni/Fe-Fe₂O₃ hexagonal nanorod is modified using bamboo-shaped NCNTs derived from dicyandiamide (DCDA) (Scheme 1). It shows good ORR activity. In alkaline electrolyte, its half-wave potential (E_1/2) achieves as high as 0.946 V with limiting current density (J_d) 5.35 mA cm⁻². In neutral electrolytes, the E_1/2 value still remains at 0.716 V. Its OER activity is also promising and the overpotential (η₁₀) is 388 mV with a Tafel slope of 89.1 mV dec⁻¹. Theoretical calculation demonstrates that Ni doping elevates the d-band center, which enhances adsorption towards relevant active oxygen species in ORR and OER. With Ni/Fe-Fe₂O₃@NCNTs as a cathode material, a Zn–air battery is constructed. In alkaline electrolyte, its peak density reaches 170.1 mW cm⁻². At 10 mA cm⁻², its specific capacity and energy density are 819.8 mA h g⁻¹ and 960.1 W h kg⁻¹, respectively. In neutral electrolytes, the discharge performance of the Zn–air battery is also well retained.

Experimental section

Synthesis of Ni-doped MIL-88A

At first, FeCl₃·6H₂O (6 mmol, 1.62 g) and Ni(NO₃)₂·6H₂O (0.5 mmol, 0.15 g) were dissolved in deionized water (30 mL). Then, fumaric acid (6 mmol, 0.7 g) was added. The obtained solution was stirred at room temperature for 30 min and transferred into a 50 mL stainless-steel autoclave. After heating at 70 °C for 16 h, the products were synthesized, which were washed with ethanol, and deionized water several times and dried in a vacuum oven at 80 °C for 12 h. To obtain neat MIL-88A, Ni(NO₃)₂·6H₂O was not added.

Synthesis of Ni/Fe-Fe₂O₃@NCNTs

DCDA (1.5 g, on the gas inlet side) and Ni-doped MIL-88A (0.1 g, on the gas outlet side) were dispersed on both ends of the porcelain boat. After heating at 800 °C under N₂ protection for 3 h at a rate of 5 °C min⁻¹, black powder of Ni/Fe-Fe₂O₃@NCNTs was obtained. The products were washed using ethanol and deionized water several times and dried in a vacuum oven at 80 °C for 12 h. When MIL-88A was used instead of Ni-doped MIL-88A, Fe-Fe₂O₃@NCNTs was synthesized.

Results and discussion

Structure and morphology of Ni/Fe-Fe₂O₃@NCNTs

The structure of the calcined product was characterized by powder X-ray diffraction (PXRD). The diffraction peaks at 23.8, 30.2, 35.6, 43.3, 57.4 and 62.9° correspond with (210), (220), (311), (400), (511) and (440) planes of Fe₂O₃, respectively (JCPDS No. 39-1346) (Fig. 1a). The left two peaks at 44.6 and 64.9^° originate from (110) and (200) planes of FeNi alloy (JCPDS No. 37-0474), respectively. Raman spectroscopy was also employed to study the structure. For Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs, two distinct peaks appear at 1350 and 1580 cm⁻¹, which correspond with D and G bands, respectively, (Fig. 1b) were observed.⁵⁹ In general, the D band reflects the content of carbon in the defect form, while the G band represents carbon in the graphic phase. The ratio between D and G bands (I_D/I_G) can be used to evaluate the degree of graphitization in carbon.⁶⁰ For Ni/Fe-Fe₂O₃@NCNTs, the I_D/I_G value is 0.96, which demonstrates that NCNTs exist in a high degree of graphitization. This improves conductivity and ORR/OER activities.

The composition and valence states of Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs were studied by X-ray photoelectron spectroscopy (XPS). For Ni/Fe-Fe₂O₃@NCNTs, C 1s, N 1s, O 1s, Fe 2p and Ni 2p peaks emerged in the survey spectrum (Fig. S1†). As for Fe-Fe₂O₃@NCNTs, Ni 2p peaks disappear completely. A high-resolution spectrum was used to analyze the valence states of the above five components. In the high-resolution Ni 2p spectrum, two doublet peaks emerge at 853.1, 870.6 and 855.5, 873.7 eV (Fig. 1c). They match well with Ni 2p_3/2 and Ni 2p_1/2 in Ni⁰ and Ni²⁺ states, respectively.⁶¹ The appearance of Ni²⁺ originates from the oxidation of FeNi alloy. The other two peaks at 861.8 and 880.2 eV can be ascribed to satellite peaks. In the high-resolution Fe 2p spectrum, the first pair of peaks located at 710.8 and 724.3 eV originate from Fe 2p_3/2 and Fe 2p_1/2, respectively, in the Fe²⁺ phase (Fig. 1d).⁶² The doublet peaks at 712.8 and 726.9 eV correspond with Fe 2p_3/2 and Fe 2p_1/2, respectively, of Fe³⁺. The other two are satellite peaks. Furthermore, compared with Fe-Fe₂O₃@NCNTs, Fe 2p peaks shift to a negative energy region, which demonstrates in Ni/Fe-Fe₂O₃@NCNTs, the electrons move from Ni to Fe₂O₃. In the high-resolution N 1s spectrum, the peaks at 398.8, 400.6 and 401.8 eV match with pyridinic N, pyrrolic N and graphitic N, respectively (Fig. 1e).⁶³ High-resolution O 1s spectrum reveals that lattice oxygen (O_α) and oxygen vacancies (O_β) coexist in Fe₂O₃ (Fig. 1f).⁶⁴ In the high-resolution C 1s spectrum, three peaks appear at 284.6, 285.3 and 286.3 eV (Fig. S2†). They correspond to C–C, C–N and CN, respectively⁶⁵

The morphology of Ni/Fe-Fe₂O₃@NCNTs was observed with scanning electron microscopy (SEM) at first. The SEM image illustrates that the Ni/Fe-Fe₂O₃@NCNTs hexagonal nanorod shows uniform size (Fig. S3†). The length of hexagonal nanorod ranges from 1.2 to 2 μm with a diameter of about 250 to 400 nm (Fig. 2a). It shows a similar appearance to MIL-88(A), besides surface-covered NCNTs derived from DCDA (Fig. S4†). The length of NCNTs is 120 to 200 nm with a diameter of 10 to 15 nm (Fig. 2b). Transmission electron microscopy (TEM) was also used to observe its morphology. TEM image reveals that hexagonal nanorod is composed of small Ni/Fe-Fe₂O₃ nanoparticles of about 20 to 40 nm size and NCNTs grow homogeneously on their surface (Fig. 2c). NCNTs exhibit a bamboo-shaped appearance with small particles residing in the channel (Fig. 2d). In NCNTs, Ni/Fe-Fe₂O₃ particles with the size about 10 to 15 nm reside homogeneously. With high-resolution transmission electron microscopy (HRTEM) images, more information is obtained. The lattice fringes of about 0.25 nm match with the (311) plane of Fe₂O₃ (Fig. 2e). Based on energy-dispersive X-ray fluorescence spectroscopy (EDX), C, N, O, Fe and Ni disperse evenly on Ni/Fe-Fe₂O₃@NCNTs (Fig. 2f). Furthermore, the EDX also confirms the content of Ni as 0.3%, which also proves that Ni was doped successfully.

ORR activity study in alkaline electrolyte

The ORR activity was studied with the rotating disk electrode (RDE) technique. In the 0.1 M KOH electrolyte saturated by O₂, Ni/Fe-Fe₂O₃@NCNTs exhibits more excellent ORR activity than Fe-Fe₂O₃@NCNTs and Pt/C. Based on linear sweep voltammetry (LSV) curves, its E_1/2 achieves 0.946 V, which is higher than that of Fe-Fe₂O₃@NCNTs (0.909 V) and Pt/C (0.896 V) (Fig. 3a). The J_d value of Ni/Fe-Fe₂O₃@NCNTs is 5.35 mA cm⁻² and this is larger than that of Pt/C (5.03 mA cm⁻²). Its merit over the other two competitors is also revealed by the Tafel slope. The slope of Ni/Fe-Fe₂O₃@NCNTs (68.79 mV dec⁻¹) is lower than that of Fe-Fe₂O₃@NCNTs (83.68 mV dec⁻¹) and Pt/C (76.8 mV dec⁻¹), which implies that Ni/Fe-Fe₂O₃@NCNTs can obtain high current density at low potential (Fig. 3b). ORR activity was also evaluated by cyclic voltammetry (CV). Reductive peaks of Ni/Fe-Fe₂O₃@NCNTs, Fe-Fe₂O₃@NCNTs and Pt/C are located at 0.897, 0.801 and 0.811 V, respectively (Fig. 3c). This also implies that Ni/Fe-Fe₂O₃@NCNTs shows better ORR activity than Fe-Fe₂O₃@NCNTs and Pt/C. The ORR activity of Ni/Fe-Fe₂O₃@NCNTs is comparable with those of materials reported recently (Table S1†).

To further explore the ORR activity of different electrocatalysts, electrochemically active surface area (ECSA) is an important parameter, which can be reflected by a double-layer capacitor (C_dl). For Ni/Fe-Fe₂O₃@NCNTs, its C_dl value reaches 12.56 mF cm⁻², which is higher obviously than Fe-Fe₂O₃@NCNTs (8.51 mF cm⁻²) (Fig. 3d and Fig. S5, S6†). The results indicate that Ni/Fe-Fe₂O₃@NCNTs can provide more active sites in ORR. In ORR, stability is also very important for Ni/Fe-Fe₂O₃@NCNTs. Its E_1/2 only shifts negatively about 10 mV after 5000 cycles LSV tests (Fig. 3e). The stability of Ni/Fe-Fe₂O₃@NCNTs can also be judged by chronoamperometric curves at a constant potential. After a continuous ORR test for 12 h, the current density still remains at 88.59% (Fig. 3f). All these facts confirm its good stability in ORR.

In ORR, electron transfer number (n) is a significant parameter, which can be measured using the rotating disk electrode (RDE). For Ni/Fe-Fe₂O₃@NCNTs, when rotating speed increases from 400 to 2500 rpm, current density also increases (Fig. 4a). Simulated from the above LSV curves, the Koutecky–Levich (K–L) plots at different potentials were obtained (Fig. 4b). At 0.5, 0.6 and 0.7 V (vs. RHE), its electron transfer numbers are 4.04, 4 and 3.92, respectively (Fig. 4c). This implies that Ni/Fe-Fe₂O₃@NCNTs shows 4e-character in ORR. Based on a similar method, the electron transfer numbers of Fe-Fe₂O₃@NCNTs are also calculated, which are 4.06, 3.96 and 3.94 at the above potentials (Fig. S7, S8 and S9,† respectively). This implies that Ni-doping does not affect electron transfer numbers. The ORR activities of Ni/Fe-Fe₂O₃@NCNTs and Fe₂O₃@NCNTs were further studied with a rotating ring disk electrode (RRDE). The disk current density of Ni/Fe-Fe₂O₃@NCNTs is 5.46 mA cm⁻², which is higher than that of Fe-Fe₂O₃@NCNTs (Fig. 4d and Fig. S10†). This further implies the superior ORR activity of Ni/Fe-Fe₂O₃@NCNTs over Fe-Fe₂O₃@NCNTs after Ni-doping. H₂O₂ selectivity of Ni/Fe-Fe₂O₃@NCNTs fluctuates between 1.93 and 11.2%, while the electron transfer number ranges from 3.78 to 3.96 (Fig. 4e and f). As for Fe-Fe₂O₃@NCNTs, it also shows low H₂O₂ selectivity from 1.31 to 5.75% with the electron transfer number increasing from 3.88 to 3.97 (Fig. S11 and S12†). Both RDE and RRDE results confirm the 4e-character of Ni/Fe-Fe₂O₃@NCNTs.

ORR activity study in neutral electrolyte

The ORR activity of Ni/Fe-Fe₂O₃@NCNTs was also studied in neutral electrolytes. As the CV curve illustrates it exhibits a distinct oxygen reduction peak in 0.1 M PBS (pH = 7.4) saturated by O₂ (Fig. S13†). The peak is located at 0.736 V (vs. RHE), which is obviously higher than that of Pt/C. This implies that Ni/Fe-Fe₂O₃@NCNTs shows more excellent ORR activity than Fe-Fe₂O₃@NCNTs under neutral conditions. In ORR, the advantage of Ni/Fe-Fe₂O₃@NCNTs over Fe-Fe₂O₃@NCNTs can be revealed by the LSV curve. The E_1/2 and J_d values of Ni/Fe-Fe₂O₃@NCNTs reach 0.716 V and 4.47 mA cm⁻², (Fig. 5a); As for Pt/C, the corresponding values are 0.616 V and 4.4 mA cm⁻², respectively. For OR electrocatalysts, the stability is also very important in practical applications. For Ni/Fe-Fe₂O₃@NCNTs, the E_1/2 value merely shifts negatively about 20 mV before and after 5000 cycles of the LSV tests (Fig. 5b). This illustrates that its excellent stability is well retained in neutral electrolytes.

The electron transfer number of Ni/Fe-Fe₂O₃@NCNTs was simulated based on the K–L equation with the RDE technique. The rotating speeds were 400, 625, 900, 1225, 1600, 2025 and 2500 rpm in the LSV tests (Fig. 5c). With increasing rotating speed, current density also increases. Simulated from the K–L equation, the linear relationship between J⁻¹ and ω^−1/2 is satisfactory (Fig. S14†). At 0.1, 0.2 and 0.3 V (vs. RHE), the electron transfer numbers are 3.86, 3.83 and 3.74, respectively (Fig. 5d). The RRDE test was also conducted to further evaluate the electron transfer number (Fig. 5e). The electron transfer number of Ni/Fe-Fe₂O₃@NCNTs ranges from 3.78 to 3.96 and H₂O₂ selectivity is in the scope of 1.93 and 11.2% (Fig. 5f and Fig. S15†). Both RDE and RRDE results imply that 4e⁻-ORR dominates the oxygen reduction process in neutral electrolytes. The stability of Ni/Fe-Fe₂O₃@NCNTs was studied using the chronoamperometry method. After the 12 h test, the current density still keeps as high as 90.45% (Fig. S16†). This confirms the ideal ORR stability of Ni/Fe-Fe₂O₃@NCNTs in neutral electrolytes.

OER activity study in alkaline electrolyte

In general, the performance of the Zn–air battery is also influenced greatly by the OER activity of the electrocatalyst. For Ni/Fe-Fe₂O₃@NCNTs, its OER activity was studied in 1 M KOH. At 10 mA cm⁻², its η₁₀ is only 388 mV, which is higher slightly than RuO₂ (Fig. 6a). As for Fe-Fe₂O₃@NCNTs, to obtain 10 mA cm⁻², it overpotential achieves 483 mV, which is higher than Ni/Fe-Fe₂O₃@NCNTs. This confirms that Ni-doping can improve OER activity effectively. For an electrocatalyst, the advantage in OER can also be illustrated by the Tafel slope. As for Ni/Fe-Fe₂O₃@NCNTs, its Tafel slope is 89.1 mV dec⁻¹ (Fig. 6b). This value is comparable to that of RuO₂ (83.9 mV dec⁻¹) and lower than that of Fe-Fe₂O₃@NCNTs (191.72 mV dec⁻¹). The stability in OER was also studied in 1 M KOH. The LSV curves before and after the 2000 cycles LSV test almost coincide, which indicates its promising stability (Fig. 6c). The OER activity is comparable with recently reported materials (Table S2†). To evaluate the applicability of bifunctional ORR/OER electrocatalyst, the gap (Δ) between E_1/2 and η₁₀ is a vital index. For Ni/Fe-Fe₂O₃@NCNTs, Δ value is 0.67 V (Fig. 6d). This demonstrates that it is an excellent bifunctional electrocatalyst for Zn–air batteries.

Mechanism study of ORR and OER

To analyze different electrocatalytic activities between Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs, density functional theory (DFT) calculation was employed. As for ORR, O₂ is adsorbed on the electrocatalyst, which is reduced to OOH*, O*, OH* and OH⁻ in turn (Table S3†).⁶⁶ For Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs, the free energy of each reductive step is thermodynamically favorable downhill as U = 0 V (Fig. 7a and b). After U increases to 1.23 V, the first and second steps (O₂ + H₂O + e → OOH* + OH⁻ and OOH* + e → O* + OH⁻) become uphill. It can be observed clearly that Ni/Fe-Fe₂O₃@NCNTs need lower energy than Fe-Fe₂O₃@NCNTs to complete the barrier. To evaluate ORR activity in theory, limiting potential (the highest potential to make all elementary reactions exothermic) is an important parameter, which is abbreviated as U_L(ORR). For Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs, the U_L(ORR) are 0.95 and 0.74 V. Higher U_L(ORR) of Ni/Fe-Fe₂O₃@NCNTs implies that it exhibits more excellent ORR activity than Fe-Fe₂O₃@NCNTs. In OER, as U = 0 V, the free energy of each step is uphill for Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs (Fig. 7c and d). At U = 1.23 V, the first two reactions (OH⁻ − e = OH*; OH* + OH⁻ − e = H₂O + O*) are still uphill and become the rate-determining steps. It is obvious that Ni/Fe-Fe₂O₃@NCNTs needs low energy to overcome the barrier. Furthermore, the U_L(OER) (limiting potential in OER) of Ni/Fe-Fe₂O₃@NCNTs is 1.51 V, which is lower than that of Fe-Fe₂O₃@NCNTs with U_L(OER) 1.78 V. This confirms superior OER activity of Ni/Fe-Fe₂O₃@NCNTs over Fe-Fe₂O₃@NCNTs.

In both ORR and OER, the rate-determining steps of Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs are the generation and reduction of OOH* and OH*. To accelerate these steps, intensive adsorption towards relevant oxygen species (O₂, OOH*, OH⁻ and OH*) becomes very significant. According to the d-band center theory, the adsorption towards oxygen species is determined by the position of the d-band center. If the d-band center approaches the Fermi energy level, adsorption is intensive. On the contrary, the adsorption becomes weak.⁶⁷ For Fe-Fe₂O₃@NCNTs, the d-band center is located at −2.69 eV based on the density of states (DOS). The d-band center of Ni/Fe-Fe₂O₃@NCNTs shifts to −2.16 eV (Fig. 7e). The position of the valence band can also be employed to measure the d-band center.⁶⁸ For Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs, the valence band is located at 1.31 and 1.57 eV, respectively, based on ultraviolet photoelectron spectrometer (UPS) (Fig. 7f). These imply that the d-band center of Ni/Fe-Fe₂O₃@NCNTs is closer to the Fermi energy level than that of Fe-Fe₂O₃@NCNTs, which reveals that Ni-doping intensifies adsorption towards oxygen species and improves ORR and OER activities.

The performance of a Zn–air battery

Due to excellent ORR and OER activities in an alkaline environment, a rechargeable Zn–air battery was constructed with Ni/Fe-Fe₂O₃@NCNTs as the cathode material. In this Zn–air battery, the Zn slice serves as an anode, while a mixed solution of KOH (6 M) and Zn(OAc)₂ (0.2 M) acting as an electrolyte. The open-circuit voltage of this battery reaches 1.46 V (Fig. 8a and b). This is obviously higher than that obtained for the Zn–air batteries employing Fe-Fe₂O₃@NCNTs and Pt/C + RuO₂ as cathode materials. Furthermore, the peak power density of this Zn–air battery achieves 170.1 mW cm⁻², which is superior to Zn–air batteries built by Fe-Fe₂O₃@NCNTs (137.48 mW cm⁻²) and Pt/C + RuO₂ (104.26 mW cm⁻²) (Fig. 8c). After linking two Zn–air batteries with series mode, green LED lights with rated voltage of about 2.5 V can be lit up and work continuously for more than 12 h (Fig. 8d). This battery also shows small gap between charge and discharge voltages (Fig. 8e). At 50 mA cm⁻², the voltage gap is only 1.12 V. This is comparable with most of recently reported Zn–air batteries.

The discharge performance of this Zn–air battery was studied with galvanostatic technique at 1, 5 and 10 mA cm⁻² (Fig. 8f). At the above current densities, the specific capacities are 863.9, 838.5, 819.8 mA h g⁻¹ and energy densities are 1105.8, 1006.2, 960.1 W h kg⁻¹, respectively. These values are adjacent to the theoretical parameters of the Zn–air battery. In addition, the specific capacities of Ni/Fe-Fe₂O₃@NCNTs and Fe-Fe₂O₃@NCNTs were studied at 5 mA cm⁻². The specific capacity of Ni/Fe-Fe₂O₃@NCNTs is obviously higher than that of Fe-Fe₂O₃@NCNTs (755.9 mA h g⁻¹) (Fig. S17†). Furthermore, the discharge performance of this Zn–air battery can be comparable with recently reported Zn–air batteries (Table S4†). For Zn–air battery, rate ability is also very important and this can be measured by galvanostatic technique at various current densities. Here, current densities were set at 1, 2, 4, 8 and 10 mA cm⁻² (Fig. 8g). Discharge voltage attenuates gradually with increasing current densities and the voltage plateaus are located at 1.32, 1.28, 1.24, 1.18 and 1.14 V. After 5 h of the discharge experiment, the current density returns to 1 mA cm⁻² and the voltage becomes 1.31 V with insignificant attenuation. This confirms the promising rate capacity of this Zn–air battery. The stability of the Zn–air battery was evaluated by long-time charge/discharge tests at 1 mA cm⁻² (Fig. 8h). After continuous tests lasting 80 h, the charge/discharge voltages still remained constant. This illustrates its ideal stability.

Due to excellent ORR activity in the neutral electrolyte, a new Zn–air battery was built up with Ni/Fe-Fe₂O₃@NCNTs as the cathode material, while a mixed solution of NH₄Cl (4 M) and KCl (1 M) serves as the electrolyte. For this new Zn–air battery, its open-circuit voltage reaches 1.45 V (Fig. 9a and b). When discharged at a similar voltage, it can provide a higher current density than a battery constructed by Pt/C and RuO₂. The peak power density of this Zn–air battery is 95.7 mW cm⁻² (Fig. 9c). This is obviously higher than the battery built by Pt/C and RuO₂ (41.9 mW cm⁻²). Two Zn–air batteries linked with series mode can also light up green LED lights with a rated voltage of about 2.5 V (Fig. 9d). Rate capacity of this battery was studied under different current densities. After the discharge at 0.5, 1, 2, 4, 8 and 10 mA cm⁻² for 6 h, the voltage can almost return to the original value (Fig. 9e). Discharge performance of this Zn–air battery was evaluated with galvanostatic technique at 5 and 10 mA cm⁻² (Fig. 9f). The specific capacities are 812.9 and 802.4 mA h g⁻¹ with energy density 646.3 and 579.4 W h kg⁻¹, respectively. The above facts imply that Ni/Fe-Fe₂O₃@NCNTs has excellent ORR and OER activities and is a qualified cathode material in both alkaline and neutral electrolytes.

Conclusions

In summary, Ni/Fe-Fe₂O₃@NCNTs, a bifunctional electrocatalyst was synthesized with Fe-MOF as a precursor. In Ni/Fe-Fe₂O₃@NCNTs, NCNTs grow on Ni/Fe-doped Fe₂O₃ hexagonal nanorod homogenously. In ORR, Ni/Fe-Fe₂O₃@NCNTs exhibit high E_1/2 and J_d in both alkaline and neutral electrolytes. In OER, it shows a low overpotential and Tafel slope. Theoretical calculation implies that Ni-doping promotes the d-band center in Fe₂O₃, which intensifies its interaction with relevant oxygen species during electrocatalysis. Using Ni/Fe-Fe₂O₃@NCNTs as a cathode material, the Zn–air battery is assembled in an alkaline electrolyte. It shows promising charge and discharge performance with a high specific capacity, energy density and power density. This battery also exhibits promising stability during charge and discharge. In neutral electrolytes, the discharge performance of the Zn–air battery is also well retained. We anticipate band structure adjustment by hetero component doping will be popularized to improve ORR and OER activities for Zn–air batteries.

Data availability

The data in this manuscript will be available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22171039); and Fundamental Research Funds for the Central University (N2209005).

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Footnote

† Electronic supplementary information (ESI) available. Materials and characterization; RRDE test in ORR; RDE test in ORR; OER test; Zn–air battery assemble; theoretical calculation details; comparison ORR, OER activities of Ni/Fe-Fe2O3@NCNTs with other materials; the reaction routes in ORR and OER; XPS survey spectra of Ni/Fe-Fe2O3@NCNTs and Fe-Fe2O3@NCNTs; XPS high resolution C 1s spectrum of Ni/Fe-Fe2O3@NCNTs; SEM image of Ni/Fe-Fe2O3@NCNTs; SEM image of MIL-88A; CV of Ni/Fe-Fe2O3@NCNTs at different scanning rates; CV curves of Fe-Fe2O3@NCNTs under different scanning rates; LSV curves of Fe-Fe2O3@NCNTs under different rotating rates; K–L equation simulation for Fe-Fe2O3@NCNTs; n values of Fe-Fe2O3@NCNTs under different potentials; RRDE voltammograms of Fe-Fe2O3@NCNTs; n of Fe-Fe2O3@NCNTs; H2O2 selectivity of Fe-Fe2O3@NCNTs; CV curves of Ni/Fe-Fe2O3@NCNTs and Pt/C in PBS (0.1 M PBS); K–L equation simulation for Fe-Fe2O3@NCNTs in PBS (0.1 M PBS); H2O2 selectivity of Fe-Fe2O3@NCNTs in PBS (0.1 M PBS); chronoamperometric curve of Ni/Fe-Fe2O3@NCNTs in PBS (0.1 M PBS). See DOI: https://doi.org/10.1039/d4dt01733a

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