Spinel CoMn₂O₄ nanoparticles supported on a nitrogen and phosphorus dual doped graphene aerogel as efficient electrocatalysts for the oxygen reduction reaction

Abstract

In this work, we present a novel hybrid composed of spinel CoMn₂O₄ nanoparticles and a N, P dual-doped graphene aerogel (CoMn₂O₄/NPGA). The CoMn₂O₄/NPGA is characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activity of the CoMn₂O₄/NPGA composite towards the ORR was assessed using a linear sweep voltammetry method. Rotating disk electrode (RDE) measurements show that the as-obtained CoMn₂O₄/NPGA shows excellent ORR activity in an alkaline medium comparable to the benchmark Pt/C catalyst. Electrochemical measurements reveal that the ORR on CoMn₂O₄/NPGA proceeds through an almost four-electron pathway. Simultaneously, the methanol tolerance and operational stability of CoMn₂O₄/NPGA toward the ORR are prominently higher than those of commercial Pt/C. All these conspicuous properties suggest that our proposed CoMn₂O₄/NPGA may be used as a prospective Pt-free catalyst in alkaline direct methanol fuel cells.

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1. Introduction

The oxygen reduction reaction (ORR) is vital in electrochemical energy storage and conversion technologies including fuel cells and metal–air batteries. Nevertheless, the ORR process is subject to sluggish kinetics, and thus electrocatalysts for the ORR play a key part in these fields.^1–5 Pt and its alloys are the best known ORR catalysts. However, the sluggish ORR kinetics, crossover effects and poor stability in long-term operation, as well as the high price, have restricted their utilization.

Until now, electrocatalysts based on nonprecious metal oxides have been widely investigated. The first row transition metal (Mn, Fe, Co, Ni, Cu, etc.) oxides have especially attracted tremendous attention because of their outstanding ORR activity, prominent stability and low cost.^6–16 Among these oxides, mixed-valence transition-metal oxides with a spinel structure (general formula AB₂O₄) have been proven to possess reinforced electrochemical properties from single metal oxides for the ORR.^9,17 However, they have accomplished little success in the recent state of investigation and development because of their poor electrical conductivity.

Hence spinel oxides are usually supported on conducting carbon (carbon nanotubes, graphene, carbon black, etc.) to guarantee excellent conductivity. Among these carbon materials, graphene, a unique sp²-hybridized carbon, is the most promising carbon support. Large specific surface area, high electronic conductivity, high chemical stability and light weight make graphene a perfect terrace for anchoring or growing nanomaterials.^18–21 In particular, modification of graphene by heteroatom (e.g., N, S, B, or P) doping is an attractive possibility to enhance catalytic activity with more active reactive sites and higher electron transfer rate.^22–25 Therefore, heteroatom-doped graphene supported spinel hybrid materials were synthesized and displayed excellent performance towards the ORR.^26,27 In addition to the heteroatom doping, high electrical conductivity can also be obtained from hierarchical structures. Hierarchical structures can provide adequate passageways to guarantee the electron transfer and diffusion of the electrolyte.^28–32 We speculate that the combination of spinel oxides with heteroatom doped hierarchical structures will lead to high electrocatalytic performance of the obtained hybrids.

Herein, we present a novel hybrid composed of CoMn₂O₄ spinel nanoparticles and a three-dimensional N, P dual-doped graphene aerogel (CoMn₂O₄/NPGA). The obtained CoMn₂O₄/NPGA composite material is an efficient ORR catalyst with high catalytic activity in an alkaline medium comparable to the benchmark Pt/C catalyst, and displays better methanol tolerance and durability than Pt/C. The superior performance makes it a likely catalyst for the ORR in alkaline fuel cells.

2. Experimental section

2.1. Materials and reagents

Graphite powder was purchased from Sinopharm Chemical Reagent Co., Ltd. Pt/C (20 wt% Pt on Vulcan XC-72) was purchased from Alfa Aesar. Hypophosphorous acid was bought from Sinopharm Chemical Reagent Co., Ltd. Nafion (5%) was purchased from Sigma-Aldrich. All other reagents were of analytical grade and used without further purification, including ethanol, NaNO₃, KMnO₄, H₂O₂, KOH, MnCl₂·4H₂O, CoCl₂·6H₂O, urea and D-glucose. Ultrapure water was obtained from a Milli-Q water system (18.2 MΩ cm).

2.2. Synthesis of manganese dioxide nanoparticles (MnO₂)

Manganese dioxide nanoparticles (MnO₂) were synthesized according to a method described in the literature.³³ Simply, MnCl₂·4H₂O (0.003 mol) was dispersed in 100 ml ultrapure water under vigorous stirring. KMnO₄ (0.002 mol) was added into 50 ml KOH aqueous solution (pH = 12), and then the solution was transferred into the above MnCl₂·4H₂O solution with stirring at room temperature. The obtained suspension was continuously stirred at room temperature for 3 h. Finally, the formed brown precipitate was filtered, washed with distilled water and absolute ethanol, and dried overnight at 60 °C.

2.3. Synthesis of CoMn₂O₄/nitrogen phosphorus dual doped graphene aerogel (CoMn₂O₄/NPGA)

The graphite oxide (GO) was gained from the natural graphite by a modified Hummers’ method.^34,35 MnO₂ (0.01 g), GO (2 mg ml⁻¹, 10 ml), urea (1 g), and D-glucose (0.04 g) were decentralized by ultrasonic vibration for 25 min (Scheme 1A) and then sealed in a Teflon lined stainless-steel autoclave. The autoclave was kept at 180 °C for 12 h, and then cooled to room temperature spontaneously (Scheme 1B). After freeze-drying for 10 h, the obtained MnO₂/nitrogen doped graphene aerogel (MnO₂/NGA) was dipped into CoCl₂·6H₂O solution (0.2 g, 5 ml) for three days. Following this, the product was put in 10 ml of 4 mol l⁻¹ KOH aqueous solution containing 2.5 ml hypophosphorous acid (the volume ratio of hypophosphorous acid to water is 1 : 1) for one day before freeze-drying (Scheme 1C). For control experiments, nitrogen-doped graphene aerogel (NGA), and nitrogen and phosphorus co-doped graphene aerogel (NPGA) were also fabricated through the same procedure as CoMn₂O₄/NPGA except for adding MnO₂, cobalt salt and hypophosphorous acid, and MnO₂ and cobalt salt, respectively.

Scheme 1 Schematic illustration of the preparation of the CoMn₂O₄/NPGA. (A) Stable suspension of GO, MnO₂, D-glucose and urea dispersed in a vial. (B) 3D graphene hydrogel formed via hydrothermal treatment. (C) The bulk aerogel of CoMn₂O₄/NPGA obtained after a freeze-drying process and its typical SEM image.

2.4. Preparation of the modified electrode

Before modification, the working electrode was polished with 1.0, 0.3 and 0.05 μm aluminum oxide powder, then sonicated in distilled water, ethanol and distilled water in turn. Afterwards, the cleaned glassy carbon (GC) electrode was blow-dried with N₂ at room temperature. The modification of the working electrode was achieved using the drop-dry method. 1.0 mg CoMn₂O₄/NPGA and 1.0 ml ethanol were mixed ultrasonically to obtain a uniform ink. A certain amount of the catalyst ink at a concentration of 1 mg ml⁻¹ was cast on the GC disk and left to dry. The catalyst loading per area on the GC electrode was kept as 280 μg cm⁻². The same amount of the NGA, NPGA, MnO₂/NGA or Pt/C (20 wt%) catalyst was also loaded onto a GC electrode for comparison.

2.5. Characterization

The morphologies and structures of the fabricated samples were acquired using scanning electron microscopy (SEM, a JEOL JSM-6701F electron microscope operating at 5 kV), transmission electron microscopy (TEM, a tecnai G220 S-Twin transmission electron microscope operating at 200 kV) and an X-ray diffractometer (RIGAK, D/MAX2550 VB/PC, Japan). Thermogravimetric analysis (TGA) was carried out with a Pyris Diamond analyzer. The specimens were heated in a temperature range from ambient temperature to 900 °C in air with a heating rate of 10 °C min⁻¹. Raman spectra were obtained using a TriVista™555CRS Raman spectrometer. The laser frequency used was the 532 nm line. Simulations were performed using Peakfit software. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCLAB 250 spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV photons) to determine surface chemical compositions and bonding states. Fitting was done using a nonlinear least-squares curve-fitting program (XPSPEAK41 software). XPS deconvolution conditions showed that the background type was Shirley and the FWHM value, as well as the percentage Lorentzian–Gaussian value of these peaks split by the same peak should be close. The Brunauer–Emmett–Teller (BET) surface areas and pore volumes were evaluated using nitrogen adsorption–desorption isotherms measured on an ASAP2020 volumetric adsorption analyzer at 77 K.

2.6. Electrochemical measurements

Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) experiments were performed on a CHI 660E electrochemical workstation (CH Instruments, USA) in a conventional three electrode cell using the coated GC electrode as the working electrode, a platinum wire as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference. Linear sweep voltammetry (LSV) and long-term chronoamperometric measurements were performed using a GC rotating disk electrode (d = 5 mm). LSV measurements were carried out in N₂ or O₂-saturated solution at rotation speeds varying from 200 to 2500 rpm and with a scan rate of 10 mV s⁻¹. Chronoamperometric durability experiments were performed at a constant potential of −0.4 V vs. SCE in an O₂-saturated solution with a rotation speed of 1600 rpm. RRDE (d_disk = 5.61 mm) experiments were performed using a Pine Instrument Company AF-MSRCE and a platinum ring, resulting in a collection efficiency of the Pt ring electrode of N = 37%. These experiments were performed at 1600 rpm in a N₂ or O₂-saturated KOH solution. The disk potential was swept at 10 mV s⁻¹. The Pt ring electrode was polarized at 0.5 V vs. SCE to oxidize the hydrogen peroxide ion during oxygen reduction at the modified GC disk electrode. During the test, the trachea was put on the solution surface. All the experiments were carried out in a N₂ or O₂-saturated 0.1 M KOH solution at room temperature.

3. Results and discussion

3.1. Characterization of the composite film

The morphologies and microstructures were investigated by the use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images display the fact that the NGA presented a three-dimensional folded and wrinkled network architecture (Fig. 1a and b). For the as-synthesized CoMn₂O₄/NPGA, CoMn₂O₄ nanoparticles attached to the crumpled NPGA were clearly observed without obvious aggregation (Fig. 1c and d). Meanwhile, the EDS (Fig. S1†) suggested the presence of C, Co, Mn, O, P, and N components in the CoMn₂O₄/NPGA hybrid.

Fig. 1 SEM images of: (a and b) the NGA and (c and d) the CoMn₂O₄/NPGA composite at different magnifications.

The TEM image reveals that the NGA presented a crumpled and wrinkle-like structure (Fig. 2a), which was consistent with the SEM result. For the NPGA, EDS indicated the presence of C, N and P in the NPGA (Fig. S2†). This indicated that N and P were successfully incorporated into graphene oxide. The size of the CoMn₂O₄ nanoparticles supported on NPGA was in the range of 5–25 nm (Fig. 2b). The surface folding and wrinkling have been demonstrated to be advantageous for sensing and electrocatalytic applications because there are lots of open edge sites.³⁶ The measured d spacing of CoMn₂O₄ nanoparticles was around 0.247 nm in the HRTEM image of CoMn₂O₄/NPGA (Fig. 2c) corresponding to the (311) crystal plane of CoMn₂O₄.

Fig. 2 TEM images of (a) NGA and (b) CoMn₂O₄/NPGA; (c) HRTEM images of the CoMn₂O₄/NPGA; (d) XRD patterns of NGA, NPGA, CoMn₂O₄/NPGA and the JCPDS card.

Powder X-ray diffraction (XRD) was used to expose the crystalline nature of the obtained samples. The XRD profiles of the NGA, NPGA and CoMn₂O₄/NPGA are shown in Fig. 2d. All the characteristic peaks of the CoMn₂O₄/NPGA were readily indexed to the CoMn₂O₄ phase in accordance with the standard values (JCPDS card no. 818-0408), which suggested the formation of CoMn₂O₄ crystalline phase. A typical broad diffraction peak around 26° exposed the fact that the interlayer spacing of NGA was smaller than that of GO (8.32 Å, 10.6°) but larger than that of graphite (3.36 Å, 26.5°), demonstrating the efficient reduction of graphene oxide.³⁷

Fig. S3† shows a TGA curve of the CoMn₂O₄/NPGA up to 900 °C at a heating rate of 10 °C min⁻¹. The TG curve showed that the total loading of CoMn₂O₄ was about 32.22%.

X-ray photoelectron spectroscopy (XPS) was employed to characterize the electronic configurations of the atoms and the surface elemental compositions in the hybrid. Fig. 3a presents the XPS survey spectra of NGA, MnO₂/NGA and CoMn₂O₄/NPGA. As expected, a full survey of CoMn₂O₄/NPGA showed peaks corresponding to the presence of P 2p, Mn 2p, Co 2p, N 1s, O 1s, and C 1s. The existence of phosphorus could be introduced by hypophosphorous acid (the reducing agent). The total phosphorus content was 1.22 at%. The high-resolution P 2p spectrum (Fig. 3b) was deconvoluted into three peaks located at 134.6, 133.45 and 133.1 eV, which should be attributed to C–O–PO₃, P–N and P–C bonding, respectively.^38,39 The C–O–PO₃ bonding indicated that P was successfully bonded to the carbon surface using one bridging oxygen bond.⁴⁰ The formation of P–C confirmed that P atoms were incorporated into the carbon framework. In the high-resolution Mn 2p XPS spectrum (Fig. 3c), two peaks were observed at 641.5 and 652.91 eV, and another two peaks located at 643.01 and 654.4 eV of Mn 2p_3/2 and Mn 2p_1/2 spin–orbit doublets deconvoluted peaks, which could be characterized as Mn²⁺ and Mn³⁺, respectively.^41,42 The above results indicated that the manganese in CoMn₂O₄/NPGA was Mn²⁺ and Mn³⁺ (Mn²⁺/Mn³⁺ ratio = 0.88, estimated from the corresponding peaks areas). Similarly, the high-resolution Co 2p spectrum of CoMn₂O₄/NPGA (Fig. 3d) showed the peaks of Co 2p_3/2 and Co 2p_1/2. The Co 2p_3/2 spectrum exhibited components associated with Co³⁺ and Co²⁺ cations which were respectively centered at 780.9 and 782.2 eV. The satellite peaks located at 784.2 (Co²⁺) and 788 eV (Co³⁺) were two shake-up type peaks of the Co 2p_3/2 edge.⁴³ The Co²⁺/Co³⁺ ratio was estimated to be around 1.23 from their corresponding peak areas. The high-resolution N 1s XPS spectra of CoMn₂O₄/NPGA are revealed in Fig. 3e. The total nitrogen content was as high as 2.35 at%. The N 1s peaks of CoMn₂O₄/NPGA clearly indicated that the peaks of nitrogen functionalities appeared at 400.10 eV (pyrrolic N) and 398.79 eV (pyridinic N). Relative surface concentrations of nitrogen and phosphorus species obtained by N 1s high-resolution XPS and P 2p high-resolution XPS spectra of CoMn₂O₄/NPGA are summarized in Table S1.†

Fig. 3 (a) XPS spectra of NGA, MnO₂/NGA and the CoMn₂O₄/NPGA composite; (b) high-resolution P 2p XPS spectra of CoMn₂O₄/NPGA; (c) high-resolution Mn 2p XPS spectra of CoMn₂O₄/NPGA; (d) high-resolution Co 2p XPS spectra of CoMn₂O₄/NPGA; (e) high-resolution N 1s XPS spectra of CoMn₂O₄/NPGA.

Raman spectroscopy was used to characterize the structural information of the carbonaceous materials and in particular disorder and defect structures. Results obtained for NGA and CoMn₂O₄/NPGA in the spectral region of 900–1700 cm⁻¹ are respectively presented in Fig. S4(a) and (b).† As a result of the fit, several parameters were extracted and are presented in Table S2.† Our samples all had five bands located at ca. 1200 cm⁻¹, 1350 cm⁻¹ (D mode), 1500 cm⁻¹, 1590 cm⁻¹ (G mode) and 1620 cm⁻¹ (D′ mode). Lorentzian line shapes were used for the D and G bands, whereas Gaussian ones were used to simulate the D′ band as well as the bands centered at ca. 1200 cm⁻¹ and 1500 cm⁻¹.⁴⁴ An upward shift of the G-band of CoMn₂O₄/NPGA (1591 cm⁻¹) could be observed with respect to the position of the G-band for the NGA (1585 cm⁻¹). This shift probably resulted from the high degree of disorder of CoMn₂O₄/NPGA. It is in fact well-known that the G band of disordered solids is shifted to higher Raman wave numbers than the G band of ordered ones.⁴⁵ The highly disordered character of CoMn₂O₄/NPGA was moreover in fair agreement with the observed D and G line widths of these two samples (Table S2†). The increase in both line widths was evidence for the decrease of the ordering degree for CoMn₂O₄/NPGA. The I_D/I_G ratio allows the evaluation of the graphitization degree of a carbon-based material. The I_D/I_G ratio is used as an indicator of the amount of defects in the carbon-based materials as well as to evaluate the in-plane crystallite size (L_a) which is a measure of the inter-defect distance. One of the relations describing the evolution of L_a with the integrated intensity ratio of G to D bands is the following one: L_a (nm) = 2.4 × 10⁻¹⁰λ_laser⁴ × I_G/I_D, in which λ_laser is the laser wavelength in nm.

From the L_a values calculated for CoMn₂O₄/NPGA and NGA, it was deduced that CoMn₂O₄/NPGA had a lower crystallite size than NGA.

To investigate the porosity of the CoMn₂O₄/NPGA and the NGA, N₂ sorption measurements were performed. The nitrogen adsorption–desorption curves are categorized as type IV isotherms, and there is a distinct hysteresis loop in the P/P⁰ range of 0.5 to 1.0 (Fig. 4), both of which are strong indications of capillary condensation and multilayer adsorption.⁴⁶ The BET surface areas and pore volumes are summarized in Table 1. It is seen from Table 1 that the BET specific surface area and total pore volume for the CoMn₂O₄/NPGA are 135.8 m² g⁻¹ and 0.302 cm³ g⁻¹, respectively, which evidently decreased compared to the NGA due to the incorporation of the CoMn₂O₄ into NPGA. Both CoMn₂O₄/NPGA and NGA exhibited the existence of mesopores and macropores which provided an effective triple phase (solid–liquid–gas) region for efficient mass transfer of O₂ and electrolyte.⁴⁷

Fig. 4 Nitrogen adsorption–desorption isotherms and pore size distribution (inset) for CoMn₂O₄/NPGA (a) and NGA (b).

Table 1 Textural parameters of the NGA and CoMn₂O₄/NPGA

Sample	SBET^a (m^2 g^−1)	Smes^b (m^2 g^−1)	Smac^c (m^2 g^−1)	PV^d (cm^3 g^−1)
a Specific surface area from the multiple BET method.b Micropore surface area from the DFT method.c Macropore surface area (Smac = SBET − Smes).d Total pore volume at P/P^0 = 0.99.
NGA	276.0	190.9	85.1	0.87
CoMn2O4/NPGA	135.8	97.5	38.3	0.30

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3.2. Electrochemical performance for ORR

In order to probe the ORR performance of CoMn₂O₄/NPGA, both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements were carried out. Linear sweep voltammograms (LSVs) for the ORR on CoMn₂O₄/NPGA are displayed in Fig. 5a. LSVs on Pt/C, NPGA and the NGA modified electrode are also given for comparison (Fig. 5b and S5†). It is seen from the LSVs of the obtained samples, that with increase of the rotation speed, the current density was enhanced obviously due to the shortened diffusion layer.⁴⁸ Apparently, the ORR onset potentials were kept almost immobile under different rotation speeds.

Fig. 5 LSV curves for the ORR on (a) CoMn₂O₄/NPGA, and (b) Pt/C at different rotation speeds from +0.2 to −0.8 V in O₂-saturated 0.1 M KOH with a scan rate of 10 mV s⁻¹; (c) LSV curves of the ORR on CoMn₂O₄/NPGA, MnO₂/NGA, NPGA, NGA and Pt/C at 1600 rpm in O₂-saturated 0.1 M KOH with a scan rate of 10 mV s⁻¹; (d) Tafel plots for the ORR on CoMn₂O₄/NPGA and Pt/C in 0.1 M O₂-saturated KOH solution at a rotation rate of 1600 rpm.

Fig. 5c shows the RDE measurements obtained on CoMn₂O₄/NPGA, MnO₂/NGA, NGA, NPGA and Pt/C with a rotation rate of 1600 rpm in an O₂-saturated 0.1 M KOH solution, and the corresponding kinetics parameters for the ORR are summarized in Table 2. As seen from Table 2, the ORR onset potential of CoMn₂O₄/NPGA commenced at about −0.094 V vs. SCE, which obviously shifted positively in comparison with those of MnO₂/NGA (−0.136 V vs. SCE), NGA (−0.147 V vs. SCE) and NPGA (−0.142 V vs. SCE), while it was less positive than that of Pt/C (−0.028 V). The diffusion limiting current density of CoMn₂O₄/NPGA was also found to be obviously larger than that of MnO₂/NGA, NGA and NPGA, and was close to that of Pt/C. Our CoMn₂O₄/NPGA catalyst was also compared with those spinel materials formerly reported in the literature (Table S3†). It is noteworthy that the performance of our catalyst, including the onset potential and limiting current density, was close to or even better than those of previously reported spinel materials.

Table 2 Electrochemical results for the ORR estimated from LSV^a

Electrocatalysts	Eonset, V vs. SCE	E1/2, V vs. SCE	JL (mA cm^−2), −0.8 V vs. SCE
a JL: limiting current density at −0.8 V vs. SCE.
CoMn2O4/NPGA	−0.094	−0.20	−5.39
MnO2/NGA	−0.136	−0.24	−4.42
NGA	−0.147	−0.26	−4.47
NPGA	−0.142	−0.23	−4.71
Pt/C	−0.028	−0.17	−5.25

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The Tafel curves of the ORR on CoMn₂O₄/NPGA and Pt/C can be derived from the polarization curve of linear sweep voltammetry with a rotation speed of 1600 rpm, as shown in Fig. 5d. The CoMn₂O₄/NPGA and Pt/C catalysts exhibited Tafel slopes of 77 and 78 mV per decade, respectively, at low overpotential, which implied a similar catalytic mechanism of the ORR on CoMn₂O₄/NPGA and Pt/C.

It is well known that the electrochemical reduction reaction of oxygen can occur via two primary possible pathways: one with two electrons producing H₂O₂ and HO₂⁻, and the other, a direct transfer in a four-electron pathway to produce H₂O and OH⁻ in acidic and alkaline media, respectively. In order to understand the ORR pathway of the CoMn₂O₄/NPGA electrode, RRDE experiments were performed. During the ORR process, the produced hydrogen peroxide species at the disk electrode can be detected by the ring electrode. The electron transfer number (n) and percentage of hydrogen peroxide ion (% HO₂⁻) with different catalysts can be determined by the following equations:⁴⁹

where I_D is the disk current, I_R is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be 0.37.

Fig. 6a presents the disk and ring current density for the CoMn₂O₄/NPGA and Pt/C catalysts in an O₂-saturated 0.1 M KOH solution. The yield of peroxide species (HO₂⁻) and the electron transfer number (n) calculated based on the corresponding RRDE data are shown in Fig. 6b. The measured HO₂⁻ yield of CoMn₂O₄/NPGA and Pt/C was 14.68–17.78% and 1.59–3.54% in the applied potential range from −0.35 to −0.6 V vs. SCE, respectively (based on eqn (1)). The electron transfer number for CoMn₂O₄/NPGA and Pt/C was 3.64–3.70 and 3.93–3.97 (calculated from eqn (2)) at the given potentials from −0.35 to −0.6 V vs. SCE. The RRDE tests indicated that CoMn₂O₄/NPGA resulted in an almost four-electron transfer pathway to produce OH⁻ for the ORR.

Fig. 6 (a) RRDE linear sweep voltammograms of the ORR on CoMn₂O₄/NPGA and Pt/C in O₂-saturated 0.1 M KOH at a scan rate of 10 mV s⁻¹ with a rotation speed of 1600 rpm. The ring electrode is polarized at 0.1 V vs. SCE; (b) peroxide percentage and electron transfer number (inset) of CoMn₂O₄/NPGA and Pt/C at fixed potentials of −0.35, −0.4, −0.45, −0.5, −0.55 and −0.6 V vs. SCE.

It is essential to estimate the stability of a new kind of electrocatalytic material using chronoamperometric durability measurements. The experiments were performed at a constant potential of −0.4 V vs. SCE in 0.1 M KOH solution saturated with O₂. As depicted in Fig. 7, the CoMn₂O₄/NPGA catalyst exhibited a much slower decay with a higher current retention (92.27%) after 10 000 s, while only 80.18% of its initial current was retained for the commercial Pt/C catalyst under the same conditions. These results implied that the CoMn₂O₄/NPGA catalyst possessed better stability than Pt/C.

Fig. 7 Current–time (i–t) chronoamperometric responses of CoMn₂O₄/NPGA and commercial Pt/C catalysts at −0.4 V vs. SCE in O₂-saturated 0.1 M KOH solution with a rotation rate of 1600 rpm.

Furthermore, for practical application in fuel cells, robust tolerance to methanol oxidation is a critical criterion for cathode electrocatalysts. To this end, tests on CoMn₂O₄/NPGA and Pt/C were performed in an O₂-saturated 0.1 M KOH solution with the addition of 3 M methanol at 300 seconds (Fig. 8). For Pt/C, the catalytic activity deviated seriously due to its undesirable activity for the methanol oxidation reaction (MOR) simultaneously occurring at the cathode. In contrast, when measured under the same conditions, there was no notable change in the curve of the CoMn₂O₄/NPGA composite. These results indicated that the CoMn₂O₄/NPGA catalyst had better methanol-tolerance compared with Pt/C.

Fig. 8 Current–time (i–t) chronoamperometric responses for Pt/C and CoMn₂O₄/NPGA in O₂-saturated 0.1 M KOH solution with addition of 3 M methanol at 300 seconds.

4. Conclusion

In conclusion, a novel CoMn₂O₄/NPGA composite with high catalytic activity towards the ORR was successfully fabricated from low-cost raw materials under mild conditions. The electrochemical measurements demonstrate that the CoMn₂O₄/NPGA composite exhibited a more positive onset potential and amazingly high current density towards the ORR compared to NGA, which should be ascribed to P doping and the synergistic effect between CoMn₂O₄ and NPGA. The CoMn₂O₄/NPGA composite exhibits high activity, good methanol tolerance and excellent durability, and shows great promise as an ORR catalyst in alkaline DMFCs.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China under Grant No. 21273024, and the Natural Science Foundation of Jilin Province, China under Grant No. 20160101298JC.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16337h

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