A facile hard-templating synthesis of mesoporous spinel CoFe₂O₄ nanostructures as promising electrocatalysts for the H₂O₂ reduction reaction

Abstract

Mesoporous spinel cobalt ferrite (CoFe₂O₄) nanostructures were synthesized via a facile Al₂O₃-assisted hard-templating (HT) strategy. Their physicochemical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectra (SEM-EDS), X-ray photoelectron spectra (XPS) and nitrogen sorption measurements. Their electrocatalytic performances towards H₂O₂ reduction reaction (HRR) were investigated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) tests. The obtained CoFe₂O₄ materials exhibit a superior mesoporous nanostructure with a particle size of around 20 nm, a specific surface area (SSA) of 140.6 m² g⁻¹ and a mesopore volume of 0.2410 cm³ g⁻¹, which favor their desirable electrocatalytic activity. A current density of 123 mA cm⁻² at −0.39 V (vs. Hg/HgO) in 3 M NaOH and 0.5 M H₂O₂ electrolytes was delivered for HRR. Moreover, the CoFe₂O₄ electrode exhibits a good stability for the catalytic reaction, showing the promising applications for H₂O₂-based alkaline fuel cells (AFCs).

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1. Introduction

The ever-growing global energy crisis and climate warming issues have triggered the worldwide interest in developing sustainable and clean energy devices.^1,2 Fuel cells (FCs), as a promising alternative for efficient and environmentally harmless power generation, have been actively investigated in recent years.^3–5 FCs are generally divided into two categories according to whether the electrolyte is acidic or alkaline. Alkaline fuel cells (AFCs) have usually aroused more concerns than the acidic ones due to the potential use of lower-cost non-Pt catalysts to achieve faster kinetics of the oxygen reduction reaction (ORR).⁶ The use of non-Pt catalysts can substantially reduce the cost of the fuel cell system. Moreover, the less-corrosive nature of an alkaline environment ensures a potential greater longevity. Recently, several new types of low-temperature liquid-based AFCs by employing hydrogen peroxide (H₂O₂) as the oxidant have been also illuminated, such as direct borohydride fuel cells (DBFCs)^7–10 and metal semi-fuel cells (MSFCs),^11–15 which were developed as underwater or space power sources working in air-free environments. Compared with the conventional oxygen (O₂) gas oxidant, the applied H₂O₂ liquid oxidant exhibits many advantages of easier feeding and assembly, faster intrinsic reduction kinetics and carbon-free merit.^10,13–15

One of the key requirements for H₂O₂-based AFCs is to explore much cheaper cathode catalysts for H₂O₂ reduction reaction (HRR) to replace the traditional noble metals catalysts, such as platinum (Pt), palladium (Pd) and their alloys.^{10,11,13,16,17} Thus, the search for alternative low-cost transition metal oxides catalysts, for example, Co₃O₄,^12,13,18,19 MnO₂,²⁰ Fe₂O₃,²¹etc., have brought much attention in recent years in the fuel cell community. Among of them, Co₃O₄ has attracted extensive attention due to its excellent electrocatalytic activity. However, the use of toxic and expensive cobalt is still a matter of concern. The partial substitution of Co in Co₃O₄ with other low cost, abundant and non-toxic elements (e.g. Ni, Mn, Fe, etc.) is regarded as an effective strategy to alleviate these problems. To date, Ni and Mn substituted candidates (i.e. NiCo₂O₄ (ref. 22–25) and Mn_1.5Co_1.5O₄ (ref. 26)) have been reported as the cathode catalysts for the HRR, whereas Fe substituted ones have been rarely investigated.

It has been reported that the cobalt ferrospinels, Co_xFe_3−xO₄ (0 < x < 1), are technologically important materials with regards to their structural, electronic, magnetic and catalytic properties.²⁷ The material has a common inverse spinel face-centered-cubic (fcc) structure packing with Co²⁺ ions occupied mostly at the 16d octahedral (Oct) sites and Fe³⁺ ions distributed evenly in the 16d Oct-sites and 8a tetrahedral (Tet) sites.²⁸ Such a structure has shown good electrical conductivities due to the electron hopping between different valence states of metals in Oct-sites^29,30 and should also provide desirable electrochemical activity due to their necessary surface redox active centers.³¹ Of which, the spinel cobalt ferrite (CoFe₂O₄) materials have attracted particular interest and have been extensively used in various fields of magnetism,^32,33 chemical sensors,^34,35 supercapacitors,^36,37 Li-ion batteries (LIBs),^38,39 and electrocatalysis.^40,41 Still, the CoFe₂O₄ materials have been rarely reported as a cathode catalyst for HRR, so far.

It is well accepted that the performance of a catalyst is mainly governed by the electrochemical activity and kinetic feature of the active materials and thus it is crucial to enhance the ion/electron transport rate in the electrode and at the electrode/electrolyte interface.⁴² It is noteworthy that one-dimensional (1D) porous nanostructures are one of the best systems due to their short transport pathways for electrons and ions.⁴³ More importantly, mesoporous nanostructures are capable of making the penetration of electrolytes into the whole electrode matrix facilely and further reduce the diffusion resistance, and therefore overcome the primary kinetic limits of electrochemical processes.⁴⁴ Therefore, rational design and fabrication of mesoporous nanostructures is imperative for the high-performance electrocatalysts for HRR.

Hard-templating (HT) method is a well-known strategy to fabricate functional mesoporous nanostructures by employing a variety of hard templates including silica particles, carbon spheres, polystyrene spheres, etc.^45–50 Herein, we opened up a self-constructed Al₂O₃ hard template derived from co-precipitation of Al(NO₃)₃ precursor solution to fabricate the mesoporous CoFe₂O₄ nanostructures, which show a desirable electrocatalytic activity and stability towards HRR, and thus a promising application for H₂O₂-based AFCs.

2. Experimental

2.1 Materials synthesis

All chemicals used in this study were of analytical grade and were used without further purification. A typical procedure is as follows. Firstly, 4.85 g Fe(NO₃)₃·9H₂O, 1.75 g Co(NO₃)₂·6H₂O and 2.25 g Al(NO₃)₃·9H₂O were dissolved in 120 mL deionized water and followed by the addition of NH₃·H₂O until a pH level of 10 was reached. After the stirring at 50 °C for 4 h, the obtained precipitate was collected by centrifugation and dried overnight at 70 °C, afterward annealed in a muffle stove at 450 °C under air for 2 h. Subsequently, the product was etched with 2 M KOH solution at 50 °C for 24 h to remove Al₂O₃ template, afterward filtrated by centrifugation and washed with deionized water until a neutral pH level, and finally dried overnight at 70 °C to obtain the ultimate CoFe₂O₄ materials.

2.2 Characterizations

X-ray diffraction (XRD) patterns were recorded on a Rigaku Dmax 2500 diffractometer equipped with graphite monochromatized Cu Kα radiation source (λ = 1.5406 Å) at a scanning speed of 4^° min⁻¹ in the 2θ range of 10–80^°. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectra (EDS) were observed using a Philips XL 30 and JEOL JSM-6700F microscope. X-ray photoelectron spectra (XPS) were measured by using an ESCALAB-MKII spectrometer (UK) with Al Kα radiation (1486.6 eV), and the raw spectra were curve-fitted by non-linear least squares fittings with a Gauss–Lorentz ratio (80 : 20) through the XPSPEAK41 software. Nitrogen sorption measurements were conducted at 77 K using a Micromeritics ASAP 2020 Analyzer, prior to analysis, the sample was degassed under vacuum at 120 ^°C for 24 h, the specific surface area (SSA) was calculated using the multipoint Brunauer–Emmett–Teller (BET) method, the pore size distribution (PSD) and pore volume data were calculated from the desorption branch based on the Barrett–Joyner–Halenda (BJH) equation.

2.3 Electrode fabrication and electrochemical measurements

The CoFe₂O₄ electrodes were fabricated by simply pressing the mixture of 50 wt% active materials (as-synthesized CoFe₂O₄) and 50 wt% conductive binder (teflonized acetylene black, TAB, the mass ratio of AB and PTFF is 2 : 1) onto the stainless steel mesh current collector. The mass loading of active materials was about 5 mg cm⁻².

The electrocatalytic performances of CoFe₂O₄ electrodes were examined by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) tests in a conventional three-electrode electrochemical setup containing a CoFe₂O₄ working electrode (WE), a Pt plate (3 cm × 4 cm) counter electrode (CE), and a Hg/HgO (1 M KOH, aqueous) reference electrode (RE). EIS experiments were taken under open circuit voltage (OCV) with an alternating current (AC) amplitude of 5 mV over frequency range of 100 KHz–10 mHz. The electrolytes were 3 M NaOH with different H₂O₂ concentrations (0, 0.3, 0.5, 0.7 and 0.9 M). All the tests were carried out at room temperature (about 25 °C).

3. Results and discussion

3.1 Physicochemical properties of the CoFe₂O₄ materials

Fig. 1a shows the XRD patterns of as-synthesized CoFe₂O₄ materials. The resultant diffraction peaks agree with the standard patterns for spinel CoFe₂O₄ phase (JCPDS no. 22-1086), and no peaks of impurities were detected, indicating a temperature of 450 °C was sufficient to drive to the formation of CoFe₂O₄ and the complete removal of template. The broad diffraction lines illustrate the nanostructural property of CoFe₂O₄ materials. The crystallite particle size of CoFe₂O₄ is calculated to be 15 nm by the Debye–Scherrer formula based on the strongest peak of (311) reflection. The lattice parameter (a₀) of the cubic lattice CoFe₂O₄, determined from the crystal plane (311), is calculated to be 0.8345 nm, which is close to the standard value of 0.8395 nm for spinel CoFe₂O₄ (JCPDS no. 22-1086), indicating the pure CoFe₂O₄ phase was yielded.

Fig. 1 XRD patterns (a), SEM images (b and c) and EDX spectra (d) of as-synthesized CoFe₂O₄ materials.

The SEM images of the CoFe₂O₄ materials are shown in Fig. 1b and c. The observed particles exhibit the basically uniform grain-like morphology with a diameter of around 20 nm. Moreover, some larger secondary agglomerate particles (100–200 nm in size) are also observed because of the agglomeration of primary nanoparticles during the calcinations.⁵¹ In addition, the materials display many nanosized space intervals among particles because of the successful removal of the template, and this porous texture may largely boost the ion/electron transportation processes at and within the electrode/electrolyte interface, which is very crucial for the facilitation of electrochemical kinetics and catalytic activity of the catalysts. The EDX spectra, as depicted in Fig. 1d, show typical O, Fe and Co elements with an O/Fe/Co atomic ratio close to the stoichiometric ratio of 4/2/1 in the spinel CoFe₂O₄, further confirming that the obtained material is a pure spinel CoFe₂O₄ phase without any impurities. Herein, the signals of Si element originated from the silicon substrate as support during the measurement, and Au was used in order to enhance the surface conductivity of the materials for the analysis and is also visible in the spectra.

Nitrogen sorption measurements were further conducted to quantitatively investigate the surface area and porosity properties of the CoFe₂O₄ materials. Typical nitrogen adsorption–desorption isotherms, pore volume and PSD plots are shown in Fig. 2a and b respectively. The nitrogen adsorption–desorption isotherms exhibit type IV isotherms with a distinct H3 hysteresis loop in a wide P/P₀ range of 0.5–1.0, a typical mesoporous characteristics. The SSA, total pore volume, mesopore volume and average pore diameter are 140.6 m² g⁻¹, 0.2413 cm³ g⁻¹, 0.2410 cm³ g⁻¹ and 6.53 nm respectively. Moreover, the pore size mainly locates at 5.6 nm, with the majority of the pores falling in the optimal size ranges (2–10 nm) for the diffusion of electroactive species in electrode materials.⁵² Here, the as-synthesized CoFe₂O₄ materials exhibit superior mesoporous nanostructures to the reported mesoporous NiCo₂O₄ nanostructures for methanol electrooxidation (130.5 m² g⁻¹, 0.17 cm³ g⁻¹)⁵³ and H₂O₂ electroreduction (89.94 m² g⁻¹, 0.195 cm³ g⁻¹).²⁵ Note that the large SSA and mesopore volume can greatly improve the electrode/electrolyte interfacial area and ion/electron transportation pathways, which can therefore boost the electroactive sites and electrochemical kinetics for electrocatalytic reactions. Accordingly, the superior surface structures of the CoFe₂O₄ materials are of huge benefit to HRR.

Fig. 2 Nitrogen adsorption–desorption isotherms (a), pore volume and PSD (b) of as-synthesized CoFe₂O₄ materials.

XPS were conducted to evaluate the surface chemical bonding states and compositions of the CoFe₂O₄ materials, and the spectra are shown in Fig. 3. Three typical signals of O_1s, Fe_2p and Co_2p core levels were detected from the survey scan plots (Fig. 3a). Of which, the O_1s spectra, as presented in Fig. 3b, show four oxygen contributions, denoted as O₁ (529.0 eV), O₂ (531.0 eV), O₃ (532.0 eV), and O₄ (533.0 eV), which associate with the typical of metal–oxygen bond,⁵⁴ the oxygen in hydroxyl groups,⁵⁵ the high number of defect sites with low oxygen coordination in the material with small particle size,⁵⁶ and the multiplicity of physi-/chemisorbed water at and within the surface,⁵⁴ respectively. The Fe_2p spectra, as depicted in Fig. 3c, are composed of two spin–orbit doublets characteristic of Fe 2P_3/2 (710.1 eV, Fe³⁺ in Oct-site; 712.4 eV, Fe³⁺ in Tet-site) and Fe 2P_1/2 (723.8 eV, Fe³⁺ in Oct-site; 726.1 eV, Fe³⁺ in Tet-site) and two shakeup satellites (identified as “Sat.”).^32,57 The Co_2p spectra, as shown in Fig. 3d, consist of two spin–orbit doublets characteristics of Co 2P_3/2 (780.0 eV, Co²⁺ in Oct-site; 781.7 eV, Co²⁺ in Tet-site; 783.3 eV, Co³⁺ in Oct-site) and Co 2P_1/2 (795.2 eV, Co²⁺ in Oct-site; 796.2 eV, Co²⁺ in Tet-site; 798.5 eV, Co³⁺ in Oct-site) and two shakeup satellites.^32,57 These results demonstrate that the surface of the CoFe₂O₄ materials has a composition containing Fe³⁺, Co²⁺ and Co³⁺ species. Therefore, the formula of the spinel CoFe₂O₄ can be described as follows: [Co²⁺_yFe³⁺_1−x]_Tet[Co³⁺_zCo²⁺_1−y−zFe³⁺_1+x]_OctO₄ (0 < x, y, z < 1). It is anticipated that the transition of solid state redox couple of Co³⁺/Co²⁺ in spinel CoFe₂O₄ structure may be responsible for the notable electrocatalytic performance towards HRR.¹⁹

Fig. 3 XPS patterns of as-synthesized CoFe₂O₄ materials. Full scan (a), O_1s core levels (b), Fe_2p core levels (c) and Co_2p core levels (d).

3.2 Electrocatalytic behavior of the CoFe₂O₄ electrode

The electrocatalytic performances of the CoFe₂O₄ electrode towards HRR were in turn examined by CV, CA and EIS tests. Fig. 4a shows the CV plots of CoFe₂O₄ electrode in 3 M NaOH containing different H₂O₂ concentrations (0, 0.3, 0.5, 0.7 and 0.9 M) at a scan rate of 10 mV s⁻¹. As can be seen the figure, the onset potential for H₂O₂ reduction is around −0.15 V (vs. Hg/HgO) and a sharp increase in cathodic current is clearly observed compared with the one in blank electrolytes, indicating the high electrocatalytic activity of the CoFe₂O₄ electrode towards HRR. Moreover, the peak or limit current increased approximately linearly with the increase of H₂O₂ concentration (Fig. S1, ESI†), indicating the diffusion-controlled feature of the reduction process.¹⁴ The limit current densities are found to be 85, 123, 134 and 176 mA cm⁻² when the H₂O₂ concentrations are in turn 0.3, 0.5, 0.7 and 0.9 M, and the performances are even comparable to those of reported Co₃O₄ electrodes,^14,19 suggesting that the high electrocatalytic activity of the CoFe₂O₄ electrode can be achieved even though its much lower Co active species, which is largely owing to the superior mesoporous nanostructures of the CoFe₂O₄ active materials that largely boosted the electroactive sites and electrochemical kinetics for the catalytic reaction. Note that the current became some fluctuated with the increase of H₂O₂ concentration, which is due to the perturbation of oxygen yielding from the unavoidable H₂O₂ chemical decomposition in strong basic electrolytes.¹⁴

Fig. 4 CV plots in 3 M NaOH with different H₂O₂ concentrations at 10 mV s⁻¹ (a), CA curves in 3 M NaOH with 0.7 M H₂O₂ at various (−0.2, −0.3, −0.4 V) polarization potentials (b), Nyquist plots in 3 M NaOH without (c) and with (d) different concentrations of H₂O₂ of as-fabricated CoFe₂O₄ electrode.

In view of the facts that both the carbon black (AB) and CoFe₂O₄ electrodes can also catalyze the reduction of oxygen yielding from H₂O₂ decomposition in the strong alkaline medium, and therefore it is necessary to clarify their specific contributions to the HRR process. The CV plots of the TAB and CoFe₂O₄ electrodes in blank and O₂ saturated 3 M NaOH solutions at a scan rate of 10 mV s⁻¹ were all recorded (Fig. S2, ESI†). For TAB electrode, a reduction peak centering at about −0.34 V was observed and the peak current was about 1.3 mA cm⁻², which can be related with the mixed processes of oxygen reduction to hydroxyl ions through a four-electron pathway and to hydrogen peroxide via a two-electron route.^58,59 For the CoFe₂O₄ electrode, a reduction peak (2.2 mA cm⁻²) was detected at around −0.2 V, which associates with the major process of oxygen reduction to the hydroxyl ions through the four-electron mechanism.⁶⁰ Since the current responses of the above two processes are very small, and therefore, the HRR on the CoFe₂O₄ electrode mainly goes through the direct two-electron pathways,¹⁴ although it still involves the minor contributions of these oxygen reduction processes.

CA tests were performed to investigate the stability of the CoFe₂O₄ electrode for HRR. Fig. 4b shows the current density–time (i–t) curves of the same electrode at various polarization potentials in 3 M NaOH with 0.7 M H₂O₂ for consecutive 600 s. At −0.2 and −0.3 V, the as-chosen mixed kinetic-diffusion control potentials regions, the current reached to steady-state after several seconds and exhibited almost no decay afterward, indicating the CoFe₂O₄ electrode being stable for H₂O₂ reduction. At −0.4 V, the selected diffusion-control potential point, the current density lightly fluctuated and decreased gradually with the elapsing of time, largely resulting from the consumption of H₂O₂ near the electrode surface.¹⁴ It is considered that H₂O₂ electroreduction does not produce any poisoning species that cause catalyst poisoning, and the catalytic stability mainly depends on the stability of the catalyst itself in the testing solution.²⁶ Herein, the CA tests reveal the fact that the CoFe₂O₄ electrode possesses a good stability for HRR.

EIS measurements were also conducted to deeply examine the catalytic behavior of the CoFe₂O₄ electrode for HRR. Fig. 4c and d illustrate the Nyquist plots of the electrode in 3 M NaOH with different H₂O₂ concentrations (0, 0.1, 0.3, 0.5, 0.7 and 0.9 M). In the absence of H₂O₂, the Nyquist plots (Fig. 4c) display a depressed semicircle at high frequency region and a declined line at low frequency region which are just divided by the knee frequency (f_knee) (also see Fig. S3, ESI†). In the presence of H₂O₂, the Nyquist plots (Fig. 4d) consist of two depressed semicircles. The semicircle at the high frequency region resembles the one without H₂O₂ and is nearly independent of the H₂O₂ concentrations, associating with the electrochemical transition of solid state redox couple of Co³⁺/Co²⁺ in the spinel CoFe₂O₄ electrode, which is most likely responsible for the catalytic activity of HRR.¹⁹ The semicircle at low frequency region can be related with the H₂O₂ electroreduction process since it appears only in the presence of H₂O₂. Moreover, the diameter of this semicircle becomes smaller when the H₂O₂ concentration becomes larger, which is because that the reduction rate depends obviously upon the H₂O₂ concentration, and the higher the concentration, the faster the reaction (Fig. 4a) and the smaller the diameter of the semicircle (Fig. 4d). Here, the EIS performances of the CoFe₂O₄ electrode are similar to those of reported NiCo₂O₄ electrodes under different H₂O₂ concentrations^24,25 and Co₃O₄ electrode under different polarization potentials,¹⁹ since the increase of H₂O₂ concentration and polarization potential are both effective ways to increase the kinetic rate of HRR.

Based on the above analysis, the catalytic mechanism for HRR on the CoFe₂O₄ electrode, which should bear an analogy to those of NiCo₂O₄ (ref. 24 and 25) and Mn_1.5Co_1.5O₄ (ref. 26) electrodes, can be illustrated as follows:

M³⁺ – ˙OHHO˙ – M³⁺ → 2M³⁺ + 2OH⁻ (3)

2M³⁺ + 2e⁻ → 2M²⁺ (4)

where M represents Co species. The overall reaction processes can be interpreted as follows. Firstly, one mole of H₂O₂ is absorbed by two moles of M²⁺ atoms. Next, the electrons transfer from M²⁺ to the O atoms, resulting in the weakness of O–O bond. And then, the elongation of O–O bond and the redistribution of electron occur, and an OH⁻ species is released accompanied by the change of M²⁺ to M³⁺. Finally, M³⁺ is reduced to M²⁺, which will ultimately lead to the formation of the second OH⁻. Of which, the steps one and four are considered as the rate determining steps (RDE).

In order to obtain quantitative information from EIS spectra, the experimental impedance spectra were simulated according to the proposed equivalent circuits models, which are given in the insets of Fig. 4c and d. From these figures, we can see that a good agreement between the experimental (hollow symbols) and simulated (solid lines) data was obtained. The proposed model circuits consist of six elements in all: the inductance (L), the equivalent series resistance or the ohmic resistance (R_e), the constant phase element (CPE) (Q), the charge-transfer resistance (R_ct), the diffusion impedance (T), and the limit capacitance (C_l).⁶¹ Here, the tangent hyperbolic impedance (T) was employed instead of Warburg impedance (W) because the phase angle of diffusion section is higher than 45°. The CPE (Q) is used in place of double-layer capacitance (C_dl) at the electrode/electrolyte boundary, indicating the deviation from the ideal behavior of a perfect capacitor. The impedance of CPE is defined as the following equation:⁶²

Z_CPE = [(Y₀jw)ⁿ]⁻¹ (5)

where Y₀ is the frequency-independent constant relating to the surface electroactive properties, w is the radial frequency, the exponent n arises from the slope of log Z vs. log f and has values −1≤ n ≤ 1. If n = 0, the CPE behaves as a pure resistor; n = 1, CPE behaves as a pure capacitor, n = −1, CPE behaves as an inductor; while n = 0.5 corresponds to Warburg impedance, which is related with the domain of mass transport control arising from the diffusion of ions to and from the electrode/electrolytes interface.

The fitting values of the circuit elements are given in Table S1 (ESI†). As can be seen in the table, the value of R_ct1 decreased largely from 5.64 Ω cm² to 0.80–0.70 Ω cm² after the addition of different concentrations of H₂O₂, indicating the even faster electrochemical kinetics (i.e. the quicker transition of Co³⁺/Co²⁺ redox couple) of the CoFe₂O₄ electrode in the presence of H₂O₂, which might be also contributable to the notable catalytic activity for HRR. Here, such kind of case is different from the reported NiCo₂O₄ electrodes,^24,25 which presented a lightly larger charge transfer resistance for the transitions of redox couples of Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ after addition of H₂O₂. Note that, in the presence H₂O₂, the R_ct1 values (0.80–0.70 Ω cm²) are almost independent of the H₂O₂ concentrations, indicating that the high catalytic activity can be maintained for the CoFe₂O₄ electrode even under higher H₂O₂ concentration, whereas the R_ct2 values for HRR process reduced from 19.9 to 5.13 Ω cm² when the H₂O₂ concentration increased from 0.3 to 0.9 M, largely owing to the faster reduction rates (i.e. larger exchange current density) under higher H₂O₂ concentrations (Fig. S4, ESI†). In all, the above EIS analysis reveals the fact that the CoFe₂O₄ electrode is capable of delivering a high catalytic activity for HRR.

4. Conclusions

Mesoporous spinel CoFe₂O₄ nanostructures were synthesized via a facile Al₂O₃-assisted hard-templating route. The CoFe₂O₄ materials exhibit a superior mesoporous nanostructure with a particle size of around 20 nm, a specific surface area of 140.6 m² g⁻¹ and a mesopore volume of 0.2410 cm³ g⁻¹, which are of huge benefit to boost their electrocatalytic activity. The CoFe₂O₄ electrode exports a current density of 123 mA cm⁻² at −0.39 V (vs. Hg/HgO) in 3 M NaOH and 0.5 M H₂O₂ electrolytes for HRR and exhibits also a good stability for the reaction, showing the promising applications for H₂O₂-based AFCs in the near future.

Acknowledgements

This project was financially supported by Scientific Research Foundation for the Returned Overseas Chinese Scholars and State Education Ministry (SRF for ROCS and SEM) and Hundred Talents Program of Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Relationship between limit current density and H₂O₂ concentration; CV plots of TAB and CoFe₂O₄ electrodes in blank and O₂ saturated 3 M NaOH solutions; Bode plots of CoFe₂O₄ electrode in 3 M NaOH solutions; relationship between exchange current density and H₂O₂ concentration. The fitted values of impedimetric parameters of the CoFe₂O₄ electrode for HRR. See DOI: 10.1039/c3ra45560b

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