Spinel nickel ferrite nanoparticles strongly cross-linked with multiwalled carbon nanotubes as a bi-efficient electrocatalyst for oxygen reduction and oxygen evolution

Abstract

It is of great concern to explore new electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, spinel NiFe₂O₄ nanoparticles cross-linked with the outer walls of multiwalled carbon nanotubes (MWCNTs) were successfully prepared by a simple, scalable hydrothermal method. The as-synthesized NiFe₂O₄/MWCNT nanohybrid shows not only a better ORR catalytic activity than pure NiFe₂O₄ and MWCNTs, but also a close four-electron reaction pathway. Meanwhile, the NiFe₂O₄/MWCNT nanohybrid exhibits a much higher OER catalytic activity when compared to NiFe₂O₄, MWCNTs and commercial Pt/C in terms of the onset potential and current density. Moreover, the NiFe₂O₄/MWCNT nanohybrid demonstrates the preeminent long-term durability measured by the current–time chronoamperometric test for both the ORR and OER, which evidently outperforms commercial Pt/C. The excellent bi-functional electrocatalytic activities of the NiFe₂O₄/MWCNT nanohybrid are attributed to the strong coupling between the NiFe₂O₄ nanoparticles and the MWCNTs as well as the network structure.

---

Introduction

Nowadays, rechargeable metal–air batteries have attracted more and more immediate attention due to their probable advantages like low-cost, environmental benignity and high energy density.¹ Nevertheless, the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been the most critical issue that affects the cycling efficiency, thus impeding the development of metal–air batteries.² Pt-based materials are usually used as the most reliable electrocatalysts to enhance the kinetics of the ORR and OER.³ However, they are very precious and have insufficient availability for extensive commercial production and application.⁴ Therefore, it is highly necessary to develop non-noble electrocatalysts as substitutes for the ORR and OER.

Recently, researchers have made great efforts to search for non-precious metal catalysts as substitutes for Pt-catalysts, such as metal oxides and metal-free carbon materials.^3c,5 Yeo et al. reported that gold-supported Co₃O₄ exhibited enhanced catalytic activities for the OER.⁶ Single crystalline NiO nanoflakes have been demonstrated to show significant ORR catalytic activities for metal–air batteries.⁷ In addition, N-doped carbon nanotubes as efficient and durable metal-free cathodic catalysts for the ORR have been reported by Feng and his co-workers.⁸ Among these non-precious catalysts, spinel oxides exhibit good catalytic activities for the ORR and OER, have low cost and have environmental friendliness. However, due to their poor electron conductivity, they are usually supported on a conducting carbon matrix aimed at speeding up electron transport. For example, Lou et al. reported the synthesis of NiCo₂O₄/reduced graphene oxide (rGO) hybrid nanosheets by a simple polyol process followed by thermal annealing treatment in air. The resulting nanocomposite exhibits a remarkable electrocatalytic activity and favorable kinetics for the ORR in alkaline electrolyte.⁹ The spinel ZnCo₂O₄/N-doped carbon nanotube composite prepared via a hydrothermal strategy demonstrates an estimable electrocatalytic activity for the ORR, attributed to the synergistic effect of ZnCo₂O₄ and the N-doped carbon nanotube.¹⁰ Dai and his co-workers studied the electrocatalytic performance of Ni–Fe layered double hydroxide on carbon nanotubes for water oxidation.^5b Lee et al. have reported mesoporous NiCo₂O₄ nanoplatelets anchored on graphene as a bi-functional electrocatalyst for the ORR and OER.¹¹ Prabu et al. synthesized CoMn₂O₄ nanoparticles anchored on nitrogen-doped graphene nanosheets as a bi-functional electrocatalyst for both the ORR and OER.¹² Multiwalled carbon nanotubes (MWCNTs) possess many topological defects in the tube walls so that they have more active sites on the surface than other graphite carbon materials.¹³ To the best of our knowledge, NiFe₂O₄/MWCNT nanocomposites have been reported previously in the application of gas sensing,¹⁴ for the determination of the voltammetric behaviour of dopamine,¹⁵ and in magnetic hyperthermia,¹⁶ but their electrocatalytic activities for both the ORR and OER have been seldom reported.

In this paper, we exhibit spinel NiFe₂O₄ nanoparticles cross-linked by multiwalled carbon nanotubes (MWCNTs) via a simple, scalable hydrothermal method. The electrocatalytic activities of the NiFe₂O₄/MWCNT nanohybrid in alkaline solution were systematically investigated by using a rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE). During the oxygen reduction reaction (ORR) process, the NiFe₂O₄/MWCNT nanohybrid shows a more positive onset potential and higher current density than pure NiFe₂O₄ and MWCNTs. For the oxygen evolution reaction (OER), the NiFe₂O₄/MWCNT nanohybrid exhibits a better catalytic activity than NiFe₂O₄, MWCNTs and commercial Pt/C, in view of the onset potential and current density. Moreover, the NiFe₂O₄/MWCNT nanohybrid demonstrates superior durability on the basis of the current–time chronoamperometric test for both the ORR and OER.

Experimental section

Oxidative treatment of MWCNTs

To acquire water-soluble MWCNTs, pristine MWCNTs were subjected to oxidative treatment according to the reported method.¹⁷ Briefly, 1 g of MWCNTs was suspended in 80 mL of a 3 : 1 mixture of concentrated H₂SO₄/HNO₃, sonicated for 1 h and then transferred into a Teflon-lined stainless autoclave and heated to 100 °C for 12 h. After cooling, the resulting product was collected by filtration, washed with water and ethanol three times, and then dried at 100 °C overnight.

Preparation of the NiFe₂O₄/MWCNT nanohybrid

The NiFe₂O₄/MWCNT nanohybrid was synthesized via a general hydrothermal approach. In a typical synthesis procedure, 0.04 g of MWCNTs was dissolved in a mixture of H₂O (40 mL) and EtOH (20 mL) and subjected to ultrasonication for 120 min to form a homogenous solution. Then, 0.582 g of Ni(NO₃)₂·6H₂O, 1.616 g of Fe(NO₃)₃·9H₂O (the molar ratio of Ni/Fe = 1/2) and 0.4 g of polyvinylpyrrolidone (PVP) were dissolved under magnetic stirring for 1 h. Subsequently, 6 mL NH₄OH was added drop by drop under drastic stirring for 1 h, which was used as a precipitation reagent. The mixed solution was transferred into a Teflon-lined stainless autoclave and heated to 180 °C for 12 h. After the black product was collected by filtering, washed with ethanol and deionized water, and dried under vacuum, it was then annealed at 400 °C in nitrogen for 4 h to dislodge the residuary PVP molecules in the nanohybrid. The pure NiFe₂O₄ nanoparticles were prepared using the same procedure in the absence of MWCNTs.

Characterization

The crystal structures of the as-synthesized samples were investigated using an X-ray powder diffraction (XRD) diffractometer (D8 ADVANCE) with Cu Kα radiation (40 kV and 200 mA). Transmission electron microscopy (TEM) images were obtained on a JEM-2100F transmission electron microscope. Thermogravimetric (TG) analysis was performed on a thermal analysis instrument (STA449C) in an air flow within a temperature range of 30 to 800 °C at a heating rate of 10 °C min⁻¹, aimed at confirming the amount of MWCNTs in the NiFe₂O₄/MWCNT nanohybrid. Surface analysis of the samples was performed with an X-ray photoelectron spectroscopy spectrometer (XPS, ESCALAB 250 X-ray photoelectron spectrometer) with Al Kα radiation. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific Co., USA) with a 532 nm excitation length.

Electrode preparation and electrochemical measurements

The electrochemical activities of the materials were measured on a PINE instrument using a conventional three electrode system with 0.1 M KOH aqueous solution via rotating disk electrode (RDE) voltammetry, rotating ring-disk electrode (RRDE) voltammetry and cyclic voltammetry (CV) for the ORR. The Ag/AgCl electrode and Pt flake electrode were used as the reference and counter electrodes, respectively. The glassy carbon electrode (GCE, diameter = 5 mm) was used as the working electrode. The RRDE electrode consists of a GC disk (0.2475 cm² of geometric surface area), surrounded by a Pt ring (0.1866 cm² of geometric surface area). The active sample (5 mg) was dispersed in 1 mL of 1 : 1 (v/v) water/ethanol mixed solution containing 30 μL of Nafion solution (5 wt%) by sonication for about 2 h to form a homogeneous ink. Then, 20 μL of the ink was pipetted onto a GCE, yielding a catalyst level of 0.64 mg cm⁻². The electrode with the catalyst was dried at 50 °C, and was used as the working electrode for further electrochemical measurements. Commercial 20 wt% Pt/C (Johnson Matthey) was used for comparison.

For the ORR test, the electrolyte (0.1 M KOH) was purged with high-purity O₂ gas to ensure O₂ saturation. The CV measurement was performed by sweeping the potential from 0 V to −0.9 V at a scan rate of 50 mV s⁻¹. The LSV tests were carried out by sweeping the potential from 0 V to −0.9 V at a scan rate of 10 mV s⁻¹, with the electrode rotating at 400, 625, 900, 1225, 1600, and 2025 rpm. The kinetic parameters of the ORR tests could be determined by the Koutecky–Levich equation (eqn (1)):

where j, j_k and j_d correspond to the measured disk current density, and the kinetics and diffusion limiting current densities, respectively; ω is the electrode rotation speed; B can be analysed by the following equation (eqn (2)):

B = 0.2nFC_O2D_O2^2/3υ^−1/6 (2)

in which n is the transferred electron number of the ORR pathway, F is the Faraday constant (96 485 C mol⁻¹), D_O2 is the diffusion coefficient of oxygen (D_O2 = 1.86 × 10⁻⁵ cm² s⁻¹), ν is the kinetics viscosity of the solution (ν = 0.01 cm⁻² s⁻¹), C_O2 is the bulk concentration of O₂ dissolved in the electrolyte (C_O2 = 1.21 × 10⁻⁶ mol cm⁻³).

For the RRDE test, the ring potential was held at 0.2 V (vs. Ag/AgCl) at a scanning rate of 10 mV s⁻¹. The percentage of H₂O₂ and the transferred electron number during the ORR process can be calculated via the following equations (eqn (3) and (4)):

in which I_d is the disk current, I_r is the ring current, and N is the current collection efficiency of the Pt ring (N = 0.37).

For the OER test, the electrolyte was purged with high-purity N₂ gas to spin off the O₂ and ensure N₂ saturation during the voltammetry testing. The LSV measurements were performed at a scan rate of 10 mV s⁻¹ and a rotating speed of 1600 rpm.

Results and discussion

TG measurements were performed to confirm the content of the MWCNTs in the NiFe₂O₄/MWCNT nanohybrid. Fig. 1 shows the TG curve of the NiFe₂O₄/MWCNT powder measured in an air flow from 30 to 800 °C. It can be clearly observed that there is a weight decrease of about 1.4 wt% from 30 to 405 °C, which is ascribed to the desorption of physically absorbed water as well as the decomposition of the hydroxide compounds in the precursor. The weight loss of about 5.4 wt% from 405 to 625 °C is ascribed to the combustion loss of the MWCNTs in the NiFe₂O₄/MWCNT nanohybrid.

Fig. 1 TG curve of the as-prepared NiFe₂O₄/MWCNT in an air flow from 30 to 800 °C.

The XRD profiles of NiFe₂O₄ and NiFe₂O₄/MWCNT are shown in Fig. 2. The characteristic peaks of the NiFe₂O₄/MWCNT nanohybrid and pure NiFe₂O₄ can be well indexed as cubic spinel phase (PDF#54-0964) with no collateral peaks. Furthermore, the intense and sharp peaks indicate that both the pure NiFe₂O₄/MWCNT nanohybrid and pure NiFe₂O₄ are well crystallized. No additional peaks ascribed to the MWCNTs were observed due to a very low content of the MWCNTs in the nanohybrid.

Fig. 2 XRD patterns of NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid.

The morphology and microstructure of the as-synthesized NiFe₂O₄/MWCNT nanohybrid were investigated using transmission electron microscopy (TEM). By using the one-pot hydrothermal treatment, the NiFe₂O₄ nanoparticles grew successfully and were homogeneously dispersed on the MWCNTs, as is evidenced by the low-magnification TEM image (Fig. 3a). The large length to diameter ratio of the MWCNTs results in the evident aggregation of the final hybrid. This unique network structure can create more active sites and allows the easy transport of O₂ and the electrolyte in the NiFe₂O₄/MWCNT nanohybrid during the electrocatalysis process. The high-magnification TEM image (Fig. 3b) further reveals that these NiFe₂O₄ nanoparticles dotted on the outer walls of the MWCNTs have typical sizes of about 10–20 nm. Meanwhile, high-resolution TEM (HR-TEM) can provide more information about the crystal structure of the NiFe₂O₄ nanoparticles and the contacting interface of the NiFe₂O₄ nanoparticles and MWCNTs (Fig. 3c and d). The measured d spacing of 0.18 nm is labelled as the lattice spacing of the (400) plane of NiFe₂O₄ and the lattice spacing about 0.33 nm is consistent with the interlayer distance of graphite (Fig. 3d). It is clear that the NiFe₂O₄ nanocrystals are closely cross-linked with the MWCNTs, resulting in more disordered crystal planes for NiFe₂O₄ and the MWCNTs at the interfaces. This disordered structure could lead to the formation of more defects, which could be beneficial for adsorbing and activating oxygen molecules and thus improves the electrocatalytic activity for the ORR and OER.

Fig. 3 (a) and (b) TEM images of the NiFe₂O₄/MWCNT nanohybrid with different magnifications, (c) high-resolution TEM image of the NiFe₂O₄/MWCNT nanohybrid, and (d) enlarged part of the selected area in panel (c).

The surface chemical composition and cation oxidation states of NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid were characterized by X-ray photoelectron spectroscopy (XPS). As expected, the XPS survey spectra in Fig. 4a exhibits Ni 2p, Fe 2p, O 1s and C 1s peaks for both pristine NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid. The de-convolution of the Ni 2p peak of NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid shows four peaks (Fig. 4b). In the pristine NiFe₂O₄ sample, the peaks at a binding energy of 855.9 and 873.6 eV correspond to Ni 2p_3/2 and Ni 2p_1/2, respectively.¹⁸ The satellite peaks at around 862.5 and 880.7 are two shake-up type peaks of Ni at the high binding energy side of the Ni 2p_3/2 and Ni 2p_1/2 edge. The Ni 2p_3/2 and Ni 2p_1/2 main peaks and satellite peaks demonstrate the presence of Ni²⁺cations in NiFe₂O₄. Nevertheless, after the hybridization of the NiFe₂O₄ nanoparticles with MWCNTs via in situ hydrothermal growth, there is an up-shift of ca. 3.5 eV for Ni 2p_3/2 and 1.2 eV for Ni 2p_1/2 at the binding energy of the peaks for the NiFe₂O₄/MWCNT nanohybrid compared with those for pristine NiFe₂O₄. This blue-shift of the binding energies for the Ni 2p peaks reveals the strong coupling between the NiFe₂O₄ nanoparticles and the MWCNTs in the nanohybrid possibly via Ni–O–C bonds during the hydrothermal treatment. The Fe 2p spectra are shifted into two peaks (Fig. 4c). The peaks at a binding energy of around 711.5 and 725.1 eV are assigned to Fe 2p_3/2 and Fe 2p_1/2, respectively, suggesting the presence of Fe³⁺cations in the as-synthesized samples.¹⁹ Meanwhile, no shift in the binding energy of the Fe 2p XPS peaks is exhibited after the hybridization of NiFe₂O₄ and the MWCNTs when comparing the Fe 2p spectra of NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid. As shown in Fig. 4d, the O 1s spectrum of NiFe₂O₄ shows a peak at 530.4 eV,²⁰ corresponding to the lattice oxygen in the Ni/Fe–O framework while a positive shift of about 0.4 eV in the binding energy for the O 1s peak is observed for the NiFe₂O₄/MWCNT nanohybrid. It also indicates the strong coupling via Ni–O–C bonds between NiFe₂O₄ and the MWCNTs in the NiFe₂O₄/MWCNT nanohybrid.

Fig. 4 (a) Full XPS spectra and (b–d) high-resolution Ni 2p (b), Fe 2p (c), and O 1s XPS spectra of NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid.

Raman spectroscopy also provides more information about the structural properties of the as-prepared NiFe₂O₄/MWCNT nanohybrid. As displayed in Fig. 5, the characteristic D and G bands of carbon materials locate at around 1349 and 1582 cm⁻¹, respectively, for pure MWCNTs. The D band is a double-resonance Raman mode, which can be understood as a measurement of structural disorder coming from amorphous carbon and any defects.²¹ The G band originates from the tangential in-plane stretching vibrations of the carbon–carbon bonds within the graphitic sheets.²² The second order 2D band at 2693 cm⁻¹ is activated independently of defects through a two-phonon double resonance process, which is responsible for its dispersive nature and sensitivity to the structural ordering of the tube walls and the graphitic electronic structure.²³ For the NiFe₂O₄/MWCNT nanohybrid, all of the D, G and 2D bands show a red-shift, compared with pure MWCNTs. It could indicate the existence of the coupling between NiFe₂O₄ and the MWCNTs, in good agreement with the results of HR-TEM and XPS. Furthermore, the ratio of the intensities of the D and G bands, R = I_D/I_G, can be used to evaluate the disorder density of carbon materials.²⁴ The I_D/I_G ratios for the MWCNTs and NiFe₂O₄/MWCNT are calculated to be 0.16 and 0.26, respectively. The I_D/I_G ratio increases after the MWCNTs were hybridized with NiFe₂O₄, which reveals that the disorder density and defect of the graphitic carbon sheets increase accordingly.

Fig. 5 Raman spectroscopy of the MWCNTs and NiFe₂O₄/MWCNT.

The ORR activity of the NiFe₂O₄/MWCNT nanohybrid as measured with CV and RDE in a conventional three-electrode system is shown in Fig. 6. In order to understand the electrocatalytic performance of NiFe₂O₄/MWCNT during the ORR process, the ORR activities of pure NiFe₂O₄ and the MWCNTs are also included for comparison.

Fig. 6 (a) CV curves of the NiFe₂O₄/MWCNT nanohybrid in N₂ and O₂-saturated 0.1 M KOH solution, (b) LSVs of the MWCNTs, NiFe₂O₄, and the NiFe₂O₄/MWCNT nanohybrid in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm, (c) Tafel plots of the MWCNTs, NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid calculated from the corresponding LSV curves measured in O₂-saturated 0.1 M KOH at a rotating speed of 1600 rpm, (d) LSVs for the ORR by the NiFe₂O₄/MWCNT nanohybrid at different rotation rates, (e) Kouteky–Levich plots of the NiFe₂O₄/MWCNT nanohybrid based on the LSVs in O₂-saturated 0.1 M KOH solution at different rotation speeds, and (f) the transferred electron number n of the MWCNTs, NiFe₂O₄ and NiFe₂O₄/MWCNT based on the LSVs in the potential range of −0.53–−0.62 V (vs. Ag/AgCl).

Fig. 6a demonstrates the CV curves of the NiFe₂O₄/MWCNT nanohybrid in N₂ and O₂-saturated 0.1 M KOH solution. The NiFe₂O₄/MWCNT nanohybrid shows no redox peak in the potential range from 0 to −0.9 V (vs. Ag/AgCl) in N₂-saturated 0.1 M KOH solution. However, a reduction peak corresponding to the ORR at −0.35 V (vs. Ag/AgCl) can be observed in O₂-saturated solution, indicating the occurrence of the ORR on the surface of the NiFe₂O₄/MWCNT nanohybrid. Fig. 6b exhibits the LSVs of the MWCNTs, NiFe₂O₄ and the NiFe₂O₄/MWCNT nanohybrid in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. The NiFe₂O₄/MWCNT nanohybrid shows a higher diffusion-limiting current density and more positive onset potential than pure NiFe₂O₄ nanoparticles and MWCNTs. Due to the high electric conductivity of MWCNTs, the hybridization of the NiFe₂O₄ nanoparticles with MWCNTs can endow the resulting nanohybrid with high electric conductivity, which is favourable for increasing the ORR activity. The ORR activity of the NiFe₂O₄/MWCNT nanohybrid is enhanced compared to those of pure MWCNTs and NiFe₂O₄, also suggesting the synergistic effect of the two components in the nanohybrid is possibly ascribed to the strong coupling between NiFe₂O₄ and the MWCNTs. The corresponding Tafel plots for the samples in the low overpotential region are shown in Fig. 6c. The NiFe₂O₄/MWCNT nanohybrid shows a smaller Tafel slope (93 mV per dec) than pure NiFe₂O₄ (115 mV per dec) and the MWCNTs (101 mV per dec), which can indicate the enhanced ORR kinetics after the hybridization of the NiFe₂O₄ nanoparticles with the MWCNTs. The LSVs for the ORR of the NiFe₂O₄/MWCNT nanohybrid at different rotation rates are shown in Fig. 6d. As can be seen, the diffusion limiting currents are improved with the increasing rotation rates. Fig. 6e demonstrates the corresponding Koutecky–Levich plots obtained from the inverse current density (j⁻¹) as a function of the inverse of the square root of the rotation rate (ω^−1/2) for NiFe₂O₄/MWCNT at −0.53, −0.56, −0.59 and −0.62 V (vs. Ag/AgCl), respectively. The plots are almost parallel and linear, indicating the first-order dependence of the kinetics of the ORR of the NiFe₂O₄/MWCNT surface. Fig. 6f shows the transferred electron number n of the MWCNTs, NiFe₂O₄ and NiFe₂O₄/MWCNT based on the LSVs (Fig. 6d and S1†) in the potential range of −0.53–−0.62 V (vs. Ag/AgCl). The transferred electron number n of the MWCNTs and NiFe₂O₄ is in the range of 2.34–3.12 and 2.20–2.62, respectively. For NiFe₂O₄/MWCNT, the transferred electron number n is in the range of 3.95–4.00. These results indicate that the integration of NiFe₂O₄ and the MWCNTs could not only significantly improve the electrocatalytic activity of NiFe₂O₄/MWCNT but also obtain the close 4e⁻ ORR catalytic pathway to obtain the maximum energy capacity.

In order to further confirm the catalytic activities and reaction pathways of the NiFe₂O₄/MWCNT nanohybrid for the ORR, the LSVs of NiFe₂O₄/MWCNT and Pt /C (Fig. 7a) were further obtained by the rotating ring-disk electrode (RRDE) in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. The ring potential was held at 0.2 V (vs. Ag/AgCl). The formation of peroxide species (HO₂⁻) and the electron transferred number n based on the corresponding RRDE data during the ORR procedure are shown in Fig. 7b. The measured HO₂⁻ yield of the NiFe₂O₄/MWCNT nanohybrid and commercial Pt/C is 15.7–19.7% and 7.17–7.36%, respectively, over the potential range from −0.53 to −0.62 V (vs. Ag/AgCl). The calculated electron transferred numbers n for the NiFe₂O₄/MWCNT nanohybrid and Pt/C are 3.61–3.68 and 3.84–3.85, respectively. It suggests that an intrinsic four-electron pathway for the ORR by the NiFe₂O₄/MWCNT nanohybrid is the dominant mechanism. This result is consistent with that calculated from the slopes of the K–L plots.

Fig. 7 (a) LSVs on the RRDE for the NiFe₂O₄/MWCNT nanohybrid and commercial Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm (the ring potential: 0.2 V (vs. Ag/AgCl)), (b) the calculated electron transfer number n and peroxide percentage at various potentials based on the corresponding RRDE data in (a).

The electrocatalytic activity of the NiFe₂O₄/MWCNT nanohybrid as well as the control catalysts for the OER was also measured. The anodic LSV curves for the OER by various electrocatalysts collected in N₂-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm are shown in Fig. 8a. Note that the onset potential of the NiFe₂O₄/MWCNT nanohybrid (0.42 V vs. Ag/AgCl) is smaller than those of pure NiFe₂O₄ (0.56 V), the MWCNTs (0.49 V) and commercial Pt/C (0.54 V). Furthermore, the current density of NiFe₂O₄/MWCNT reaches 39.5 mA cm⁻² at 1.0 V, which is much larger than those of NiFe₂O₄ (11.7 mA cm⁻²), the MWCNTs (23.7 mA cm⁻²) and commercial Pt/C (27.8 mA cm⁻²). The OER kinetics was investigated via the Tafel plots in the low over-potential region (Fig. 8b). At low over-potentials, the Tafel slope is 93, 148, 112 and 105 mV per dec for the NiFe₂O₄/MWCNT nanohybrid, NiFe₂O₄, the MWCNTs and commercial Pt/C, respectively. It implies a much faster OER activity of the NiFe₂O₄/MWCNT nanohybrid by the Tafel slopes than those for pristine NiFe₂O₄, the MWCNTs, and commercial Pt/C. These results clearly indicate that the NiFe₂O₄/MWCNT nanohybrid possesses a much higher OER electrocatalytic activity than NiFe₂O₄ and the MWCNTs, which is mainly ascribed to the unique network structures of the hybrids and the strong coupling and synergistic effect between NiFe₂O₄ nanoparticles and the MWCNT matrix. The present NiFe₂O₄/MWCNT nanohybrid (0.42 V and 24.5 mA cm⁻²) significantly outperforms the previously reported catalysts (e.g. Co₃O₄/graphene (0.54 V and 7.3 mA cm⁻²),²⁵ NiCo₂O₄/graphene (0.55 V and 19.8 mA cm⁻²),²⁶ CoFe₂O₄/graphene (0.54 V and 13.6 mA cm⁻²),²⁷ and CoFe₂O₄/biocarbon (0.48 V and 17.7 mA cm⁻²))^19a in terms of the onset potentials and limiting current density at 0.8 V (Table S1†), implying the very high excellence of MWCNTs as a matrix to cross-link transition metal oxide nanoparticles as highly efficient electrocatalysts.

Fig. 8 (a) Anodic LSVs of the MWCNTs, NiFe₂O₄, the NiFe₂O₄/MWCNT nanohybrid and commercial Pt/C in N₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm, and (b) the corresponding Tafel plots of the MWCNTs, NiFe₂O₄, the NiFe₂O₄/MWCNT nanohybrid and commercial Pt/C.

Another main challenge for metal–air battery development is the durability of the catalysts. The stability of NiFe₂O₄/MWCNT and commercial Pt/C for the ORR and OER was examined by the chronoamperometric method in O₂-saturated and N₂-statured 0.1 M KOH solution, respectively, at a rotation rate of 1600 rpm. As shown in Fig. 9a, the ORR current density of NiFe₂O₄/MWCNT decreases by about 13.1% at a constant potential of −0.6 V (vs. Ag/AgCl) over 20 000 s of continuous operation, while the ORR current density of commercial Pt/C decreases by 29.9% after 20 000 s. For the OER durability (Fig. 9b), the current density of the NiFe₂O₄/MWCNT nanohybrid decreases by only 15.9% while commercial Pt/C decreases by about 34.5% at a constant potential of 0.8 V after 20 000 s. These results indicate that the NiFe₂O₄/MWCNT nanohybrid has long-time stability for both the ORR and OER. The strong coupling between the NiFe₂O₄ nanoparticles and the MWCNTs can prevent the detachment and aggregation of the NiFe₂O₄ nanoparticles during the catalytic process, which enhances the cycling stability of the electrode.

Fig. 9 Current–time (i–t) chronoamperometric responses for the ORR (a) and the OER (b) on NiFe₂O₄/MWCNT and commercial Pt/C in O₂-saturated 0.1 M KOH solution at −0.6 V (vs. Ag/AgCl) for the ORR and in N₂-saturated 0.1 M KOH solution at 0.8 V (vs. Ag/AgCl) for the OER at a rotating speed of 1600 rpm.

Conclusions

In summary, NiFe₂O₄ nanoparticles were successfully cross-linked with the outer walls of multiwalled carbon nanotubes (MWCNTs) via a simple and scalable hydrothermal procedure. The particle size of the NiFe₂O₄ nanoparticles coupled with the MWCNTs was about 10–20 nm. The final NiFe₂O₄/MWCNT composite demonstrates a unique network structure, affording more active sites and allowing the easy transport of O₂ and electrolyte during the catalytic process. High-resolution TEM, X-ray photoelectron spectroscopy and Raman analysis confirmed the existence of the strong coupling between the NiFe₂O₄ nanoparticles and the MWCNTs. The results of the electrochemical measurements indicate that NiFe₂O₄/MWCNT possess promising activity for both the ORR and OER, in terms of the onset potential and current density. The hybridization of NiFe₂O₄ nanoparticles with MWCNTs also results in a close four-electron reaction pathway for the ORR, implying that the maximum energy efficiency could be obtained. Moreover, the NiFe₂O₄/MWCNT nanohybrid demonstrates superior durability investigated by the current–time chronoamperometric test for both the ORR and OER when compared to commercial Pt/C. The excellent catalytic activities and stabilities of the NiFe₂O₄/MWCNT nanohybrid are mainly owing to the strong coupling and synergistic effect between the NiFe₂O₄ nanoparticles and the MWCNT matrix, as well as the network structure.

Acknowledgements

The authors thank the financial support from the Shanghai Institute of Ceramics, the Key Project for Young Researcher of State Key Laboratory of High Performance Ceramics and Superfine Microstructure, the One Hundred Talent Plan of Chinese Academy of Sciences, the National Natural Science Foundation of China (No. 21307145), the Youth Science and Technology Talents “Sail” Program of Shanghai Municipal Science and the Technology Commission (No. 15YF1413800), and the Research Grant (No.14DZ2261200) from the Shanghai Government.

Notes and references

1. (a) L. Hu, H. Zhong, X. Zheng, Y. Huang, P. Zhang and Q. Chen, Sci. Rep., 2012, 2, 986; (b) B. Guan, D. Guo, L. Hu, G. Zhang, T. Fu, W. Ren, J. Li and Q. Li, J. Mater. Chem. A, 2014, 2, 16116; (c) J. Kim, X. Yin, K. C. Tsao, S. Fang and H. Yang, J. Am. Chem. Soc., 2014, 136, 14646.
2. (a) Y. C. Lu, D. G. Kwabi, K. P. C. Yao, J. R. Harding, J. Zhou, L. Zuin and Y. Shao-Horn, Energy Environ. Sci., 2011, 4, 2999; (b) Y. C. Lu, Z. C. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., 2010, 132, 12170; (c) H. Xia, Y. Wan, W. Assenmacher, W. Mader, G. Yuan and L. Lu, NPG Asia Mater., 2014, 6, e126.
3. (a) L. Ma, S. Sui and Y. Zhai, J. Power Sources, 2008, 177, 470; (b) D. Yu, Y. Xue and L. Dai, J. Phys. Chem. Lett., 2012, 3, 2863; (c) R. M. P. Li, Y. Zhou, Y. Chen, Z. Zhou, G. Liu, Q. Liu, G. Peng and J. Wang, RSC Adv., 2015, 5, 44476.
4. (a) C. T. Hung, N. Yu, C. T. Chen, P. H. Wu, X. Han, Y. S. Kao, T. C. Liu, Y. Chu, F. Deng, A. Zheng and S.-B. Liu, J. Mater. Chem. A, 2014, 2, 20030; (b) J. Y. Choi, R. S. Hsu and Z. W. Chen, J. Phys. Chem. C, 2010, 114; (c) L. Wang, F. Yin and C. Yao, Int. J. Hydrogen Energy, 2014, 39, 15913.
5. (a) X. Han, T. Zhang, J. Du, F. Cheng and J. Chen, Chem. Sci., 2013, 4, 368; (b) M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei and H. Dai, J. Am. Chem. Soc., 2013, 135, 219; (c) J. Wang, R. Ma, Y. Zhou and Q. Liu, J. Mater. Chem. A, 2015, 3, 12836; (d) J. A. Koza, Z. He, A. S. Miller and J. A. Switzer, Chem. Mater., 2012, 24, 3567.
6. B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133, 5587.
7. S. Ci, J. Zou, G. Zeng, Q. Peng, S. Luo and Z. Wen, RSC Adv., 2012, 2, 5185.
8. L. Feng, Y. Yan, Y. Chen and L. Wang, Energy Environ. Sci., 2011, 4, 1892.
9. G. Zhang, B. Y. Xia, X. Wang and X. W. Lou, Adv. Mater., 2013, 26, 2408.
10. Z. Pu, Q. Liu, C. Tang, A. M. Asiri, A. H. Qusti, A. O. Al-Youbi and X. Sun, J. Power Sources, 2014, 257, 170.
11. D. U. Lee, B. J. Kim and Z. Chen, J. Mater. Chem. A, 2013, 1, 4754.
12. M. Prabu, P. Ramakrishnan and S. Shanmugam, Electrochem. Commun., 2014, 41, 59.
13. (a) W. Xiong, F. Du, F. Liu, A. Perez, M. Supp, T. S. Ramakrishnan, L. Dai and L. Jiang, J. Am. Chem. Soc., 2010, 132, 15839; (b) Y. Tang, B. L. Allen, D. R. Kauffman and A. Star, J. Am. Chem. Soc., 2009, 131, 13200; (c) W. Zhang, A. U. Shaikh, E. Y. Tsui and T. M. Swager, Chem. Mater., 2009, 21, 3234; (d) Y. Liang, H. Wang, P. Diao, W. Chang, G. Hong, Y. Li, M. Gong, L. Xie, J. Zhou, J. Wang, T. Z. Regier, F. Wei and H. Dai, J. Am. Chem. Soc., 2012, 134, 15849.
14. R. Hajihashemi, A. M. Rashidi, M. Alaie, R. Mohammadzadeh and N. Izadi, Mater. Sci. Eng., C, 2014, 44, 417.
15. A. A. Ensafi, B. Arashpour, B. Rezaei and A. R. Allafchian, Mater. Sci. Eng., C, 2014, 39, 78.
16. K. Stojak Repa, D. Israel, J. Alonso, M. H. Phan, E. M. Palmero, M. Vazquez and H. Srikanth, J. Appl. Phys., 2015, 117, 17C723.
17. J. Zhang, H. Zou, Q. Qing, Y. Yang, Q. Li, Z. Liu, X. Guo and Z. Du, J. Phys. Chem. B, 2003, 107, 3712.
18. (a) C. Jin, F. Lu, X. Cao, Z. Yang and R. Yang, J. Mater. Chem. A, 2013, 1, 12170; (b) M. Fu, Q. Jiao and Y. Zhao, J. Mater. Chem. A, 2013, 1, 5577.
19. (a) S. Liu, W. Bian, Z. Yang, J. Tian, C. Jin, M. Shen, Z. Zhou and R. Yang, J. Mater. Chem. A, 2014, 2, 18012; (b) W. Bian, Z. Yang, P. Strasser and R. Yang, J. Power Sources, 2014, 250, 196.
20. L. Liu, L. Sun, J. Liu, X. Xiao, Z. Hu, X. Cao, B. Wang and X. Liu, Int. J. Hydrogen Energy, 2014, 39, 11258.
21. J. By A, R. Saito, G. Dresselhaus and M. S. Dresselhaus, Philos. Trans. R. Soc. London, Ser. A, 2004, 362, 2311.
22. M. S. Dresselhaus, G. Dresselhaus, R. Saito and A. Jorio, Phys. Rep., 2005, 409, 47.
23. (a) M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. G. Cancado, A. Jorio and R. Saito, Phys. Chem. Chem. Phys., 2007, 9, 1276; (b) P. Delhaes, M. Couzi, M. Trinquecoste, J. Dentzer, H. Hamidou and C. Vix-Guterl, Carbon, 2006, 44, 3005.
24. S. Osswald, M. Havel and Y. Gogotsi, J. Raman Spectrosc., 2007, 38, 728.
25. C. Sun, F. Li, C. Ma, Y. Wang, Y. Ren, W. Yang, Z. Ma, J. Li, Y. Chen, Y. Kim and L. Chen, J. Mater. Chem. A, 2014, 2, 7188.
26. D. U. Lee, B. J. Kim and Z. Chen, J. Mater. Chem. A, 2013, 1, 4754.
27. W. Bian, Z. Yang, P. Strasser and R. Yang, J. Power Sources, 2014, 250, 196.

---

Footnote

† Electronic supplementary information (ESI) available: Additional figures and table mentioned in the text. See DOI: 10.1039/c5ra14713a

---