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

Article abstract of DOI:10.1039/c6ra16337h

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.

Full text of DOI:10.1039/c6ra16337h

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Spinel CoMn2O4 Nanoparticles Supported on Nitrogen and Phosphorus
Dual Doped Graphene Aerogel as Efficient Electrocatalysts for the
Oxygen Reduction Reaction
Wenhui Guo,a Xiuxiu Ma,a Xianlei Zhang,a Yaqing Zhang,a Dingling Yu,a Xingquan He,*a
aDepartment of Chemistry and Chemical Engineering, Changchun University of Science and
Technology, Changchun 130022, P. R. China.
∗ Corresponding author. Tel.+86ꢀ431ꢀ85583430
Eꢀmail address:hexingquan@hotmail.com (Xingquan He);

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Abstract
In this work, we present a novel hybrid composed of spinel CoMn2O4 nanoparticles
and N, P dualꢀdoped graphene aerogel (CoMn2O4/NPGA). The CoMn2O4/NPGA is
characterized by scanning electron microscopy (SEM), transmission electron
microscopy (TEM), Xꢀray diffraction (XRD), Thermogravimetric analysis (TGA),
Raman spectroscopy, and Xꢀray photoelectron spectroscopy (XPS), respectively. The
electrocatalytic activity of the CoMn2O4/NPGA composite towards the ORR was
assessed using linear sweep voltammetry method. Rotating disk electrode (RDE)
measurements expose that the asꢀobtained CoMn2O4/NPGA shows an 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 fourꢀelectron pathway. Simultaneously, the methanol tolerance and
operational stability of CoMn2O4/NPGA toward 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 Ptꢀfree catalyst in alkaline direct
methanol fuel cells.
Keywords: N, P dualꢀdoped graphene aerogel; CoMn2O4 nanoparticles; hydrothermal
reaction; oxygen reduction reaction
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 subjected to sluggish kinetics, and thus electrocatalysts for the ORR
are the key part in these fields.1ꢀ5 Pt or its alloys are the best known ORR catalysts.

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However, the sluggish ORR kinetics, crossover effects, poor stability in longꢀterm
operation, as well as the high price, have restricted their utilizations.
Until now, electrocatalysts based on nonprecious metal oxides have been vastly
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.6ꢀ16 Among these oxides, mixedꢀvalence
transitionꢀmetal oxides with a spinel structure (general formula AB2O4) have proven
to possess the 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 and carbon black, etc) to guarantee excellent conductivity. Among these
carbon materials, graphene, a unique sp2ꢀhybridized carbon, is the most hopeful
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.18ꢀ21 In particular, modification of graphene by the heteroatom
(e.g., N, S, B, or P) doped is possibly great to enhance catalytic activity with more
active reactive sites and higher electron transfer rate.22ꢀ25 Therefore,
heteroatomꢀdoped graphene supported spinel hybrid materials were synthesized and
27
displayed an excellent performance towards the ORR.26,
In addition to the
heteroatom doping, high electrical conductivity can also be obtained from
hierarchical structures. Hierarchical structures can provide adequate passageways to

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guarantee the electron transfer and diffusion of 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
threeꢀdimensional N, P dualꢀdoped 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 XCꢀ72) was purchased from Alfa Aesar. Hypophosphorous
acid was bought from Sinopharm Chemical Reagent Co., Ltd. Nafion (5%) was
purchased from SigmaꢀAldrich. All other reagents were of analytical grade and used
without further purification, including ethanol, NaNO3, KMnO4, H2O2, KOH,
MnCl2·4H2O, CoCl2·6H2O，urea and Dꢀglucose. Ultra pure water was obtained from
a MilliꢀQ water system (18.2 Mꢁ cm).
2.2. Synthesis of manganese dioxide noparticle (MnO2)
Manganese dioxide nanoparticles (MnO2) were synthesized according to the method
described in the literature.33 Simply, MnCl2·4H2O (0.003 mol) was dispersed in 100
ml ultra pure water under vigorous stirring. KMnO4 (0.002 mol) was added into 50ml

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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 ºC.
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 mlꢀ1, 10 ml), urea (1 g), and
Dꢀglucose (0.04 g) were decentralized by ultrasonic vibration for 25 min (Scheme 1A)
and then sealed in a Teflon lined stainlessꢀsteel autoclave. The autoclave was kept at
180 ºC for 12 h, and then cooled to room temperature spontaneously. (Scheme 1B)
After freezeꢀdrying 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, the product was put in 10 ml of 4 mol Lꢀ1 KOH aqueous solution
containing 2.5 ml hypophosphorous acid (the volume ratio of hypophosphorous acid
to water is 1:1) for one day before freezeꢀdrying. (Scheme 1C) For control
experimentation, nitrogenꢀdoped graphene aerogel (NGA), and nitrogen and
phosphorus coꢀdoped 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.
2.4
．Preparation of modified electrode

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Before modification, the working electrode was polished with 1.0, 0.3 and 0.05 ꢂm
aluminum oxide powder, respectively, then sonicated in distilled water, ethanol and
distilled water in turn. Afterwards, the cleaned glassy carbon (GC) electrode was
blowꢀdried with N2 at room temperature. The modification of the working electrode
was achieved using the dropꢀdry 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 mLꢀ1 was cast on the GC disk and left to dry.
The catalyst loading per area on the GC electrode was kept to be 280 ꢂg cmꢀ2. 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 under
scanning electron microscopy (SEM, JEOL JSMꢀ6701F electron microscope
operating at 5 kV), transmission electron microsopy (TEM, tecnai G220 SꢀTwin
transmission electron microscope operating at 200 kV) and Xꢀray diffractometer
(RIGAK, D/MAX2550 VB/PC, Japan). Thermogravimetric analysis (TGA) was
carried out with Pyris Diamond analyzer. The specimens were heated in a temperature
range from ambient temperature to 900 ºC in air with a heating rate of 10 ºC minꢀ1.
Raman spectra were operated on a TriVistaTM555CRS Raman spectrometer. The laser
frequency used was the 532 nm line. Simulations were performed by using Peakfit
software. Xꢀray photoelectron spectroscopy (XPS) measurements were performed
using an ESCLAB 250 spectrometer with a monochromatized Al Kα Xꢀray source

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(1486.6 eV photons) to determine surface chemical compositions and the bonding
states. Fitting was done using a nonlinear leastꢀsquares curveꢀfitting 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 reference. Linear sweep voltammetry (LSV) and longꢀterm
chronoamperometric measurements were performed at a GC rotating disk electrode
(d=5 mm). LSV measurements were carried out in N2 or O2ꢀsaturated solution at
rotation speeds varying from 200 to 2500 rpm and with a scan rate of 10 mV sꢀ1.
Chronoamperometric durability experiments were performed at a constant potential
of ꢀ0.4 V vs. SCE in an O2ꢀsaturated solution with a rotation speed of 1600 rpm.
RRDE (ddisk =5.61 mm) experiments were performed using a Pine Instrument
Company AFꢀMSRCE and a platinum ring, resulting in a collection efficiency of the
Pt ring electrode, N = 37%. These experiments were performed at 1600 rpm in N2 or

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O2ꢀsaturated KOH solution. The disk potential was swept at 10 mV sꢀ1. The Pt ring
electrode was polarized at 0.5 V vs. SCE for oxidizing 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 N2 or
O2ꢀsaturated 0.1 M KOH solution at room temperature.
3. Results and discussion
3.1. Characterization of the composite film
The morphologies and microstructures were enquired by the use of scan electron
microscopy (SEM) and transmission electron microscopy (TEM). SEM images
display that the NGA presented threeꢀdimentional folded and wrinkled network
architecture (Fig. 1a and b). For the asꢀsynthesized CoMn2O4/NPGA, CoMn2O4
nanoparticles attached to the crumpled NPGA were clearly observed without obvious
aggregation (Fig. 1c and d). Meanwhile, the EDS (Figure S1) suggested the presence
of C, Co, Mn, O, P, and N components in the CoMn2O4/NPGA hybrid.
The TEM image reveals that the NGA presented a crumpled and wrinkleꢀlike
structure (Fig. 2a), which was consistent with the SEM result. For the NPGA, EDS
indicated that the presence of C, N and P of the NPGA (Fig. S2). It indicated that N
and P were successfully incorporated into graphene oxide. The size of 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 of lots of open edge sites.36 The measured d
spacing of CoMn2O4 nanoparticles was around 0.247 nm in the HRTEM image of

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CoMn2O4/NPGA (Fig. 2c) corresponding to the (311) crystal plane of CoMn2O4.
Powder Xꢀray 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. 2 (d). All the characteristic peaks of the CoMn2O4/NPGA were readily
indexed to CoMn2O4 phase in accordance with the standard values (JCPDS Card no.
818ꢀ0408), which suggested the formation of CoMn2O4 crystalline phase.Besides, a
typical broad diffraction peak around 26º exposed 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 ºC at a heating rate of
10 ºC minꢀ1. TG curve showed that the total loading of CoMn2O4 was about 32.22%.
Xꢀray photoelectron spectroscopy (XPS) was employed to characterize 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,
respectively. As expected, a full survey of CoMn2O4/NPGA showed peaks
corresponding to the presence of P2p, Mn2p, Co2p, N1s, O1s, and C1s. The existence
of phosphorus could be introduced by hypophosphorous acid (the reducing agent).
The total phosphorus content was 1.22 at%. The highꢀresolution P2p spectrum
(Figure 3b) was deconvoluted into three peaks located at 134.6, 133.45 and 133.1 eV,
which should be attributed to CꢀOꢀPO3, PꢀN and PꢀC bonding, respectively.38, 39 The
CꢀOꢀPO3 bonding indicated that P was successfully bonded to carbon surface using
one bridging oxygen bonding.40 The formation of P–C confirmed that P atoms were

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incorporated into the carbon framework. In highꢀresolution Mn2p XPS spectrum
(Figure 3c), two peaks were observed at 641.5 and 652.91 eV, and other two peaks
located at 643.01and 654.4 eV of Mn2p3/2 and Mn2p1/2 spinꢀorbit doublets
deconvoluted peaks, which could be characterized to Mn2+ and Mn3+, respectively.41,
Above results indicated that the manganese in CoMn2O4/NPGA was Mn2+ and
42
Mn3+ (Mn2+/ Mn3+ ratio = 0.88, estimated from the corresponding peaks areas).
Similarly, the highꢀresolution Co2p spectrum of CoMn2O4/NPGA (Fig. 3d) showed
the peaks of Co2p3/2 and Co2p1/2. Co 2p3/2 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 shakeꢀup type
peaks of the Co2p3/2 edge.43 The Co2+/Co3+ ratio was estimated to be around 1.23
from their corresponding peak areas. The highꢀresolution N1s 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
N1s highꢀresolution XPS and P2p highꢀresolution XPS spectra of CoMn2O4/NPGA
were summarized in the Table S1.
Raman spectroscopy was used to characterize the structural information on the
carbonaceous materials and in particular disorder and defect structures. Results
obtained for NGA and CoMn2O4/NPGA in the spectral region of 900–1700 cmꢀ1 are
respectively presented in Fig. S4 (a) and (b). As a result of the fit, several parameters

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were extracted and presented in Table S2. Our samples all had five bands located at
ca. 1200 cmꢀ1, 1350 cmꢀ1 (D mode), 1500 cmꢀ1, 1590 cmꢀ1 (G mode) and 1620 cmꢀ1
(D′ mode) respectively. Lorentzian line shapes were used for D and G bands, whereas
Gaussian ones were used to simulate the D′ band as well as the bands centered at ca.
1200 cmꢀ1 and 1500 cmꢀ1.44 An upward shift of the Gꢀband of CoMn2O4/NPGA (1591
cmꢀ1) could be observed with respect to the position of the Gꢀband for the NGA (1585
cmꢀ1). This shift probably resulted from the high degree of disorder of
CoMn2O4/NPGA. It is in fact wellꢀknown 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 an evidence for the decrease of the ordering degree for
CoMn2O4/NPGA. ID/IG ratio allows evaluating the graphitization degree of a
carbonꢀbased material. The ID/IG is used as an indicator of the amount of defects in
the carbonꢀbased materials as well as to evaluate the inꢀplane crystallite size (La)
which is a measure of the interꢀdefects distance. One of the relations describing the
evolution of La with the integrated intensity ratio of G to D bands is the following one:
La (nm) = 2.4×10ꢀ10λ4laser × IG/ID, in which λlaser is the laser wavelength in nm.
From the La values calculated for CoMn2O4/NPGA and NGA, it was deduced that
CoMn2O4/NPGA had lower crystallite size than NGA.
To investigate the porosity of the CoMn2O4/NPGA and the NGA, the N2 sorption
measurement was performed. The nitrogen adsorption–desorption curves categorized

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as type IV isotherm, and a distinct hysteresis loop in the P/P° range of 0.5 to 1.0 (Fig.
4), which were 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 were 135.8 m2 gꢀ1 and 0.302 cm3 gꢀ1, 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 effective triple phase (solideꢀliquideꢀgas) region for
efficient mass transfer of O2 and electrolyte.47
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. Liner sweep voltammograms (LSVs) for the ORR on CoMn2O4/NPGA are
displayed in Fig. 5 (a). LSVs on Pt/C, NPGA and NGA modified electrode are also
given for comparison (Fig. 5 (b) and Fig. S5). It is seen from LSVs of the obtained
samples, 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 (c) shows the RDE measurements obtained on CoMn2O4/NPGA, MnO2/NGA,
NGA, NPGA and Pt/C with the rotation rate of 1600 rpm in an O2ꢀsaturated 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

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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 was less positive than that of Pt/C
(ꢀ0.028 V). The diffusion limiting current density on the CoMn2O4/NPGA was also
found to be obviously larger than that on 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 literatures (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.
The Tafel curves of the ORR on the 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. 5(d) .The CoMn2O4/NPGA and Pt/C catalysts exhibited the Tafel
slopes of 77 and 78 mV decade−1 at low overpotential, respectively, which implied the
similar catalytic mechanism of ORR on the CoMn2O4/NPGA and Pt/C.
It is well known that the electrochemical reduction reaction of oxygen can occur
via two primary possible pathways: including two electrons to produce H2O2 and
ꢀ
HO2 , and the other, a direct transfer of fourꢀelectron pathway to produce H2O and
OHꢀ in acidic and alkaline media, respectively. In order to understand the ORR
pathway on 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 ) on different catalysts can be

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determined by the following equations.49
200× IR / N
ID + IR / N
%HO2− =
(1)
4ID
ID + IR / N
n =
(2)
Where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the
Pt ring, which was determined to be 0.37.
Fig. 6 (a) presents the disk and ring current density for the CoMn2O4/NPGA and Pt/C
ꢀ
catalysts in an O2ꢀsaturated 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. 6 (b). The measured HO2 yield of the CoMn2O4/NPGA and
Pt/C was 14.68ꢀ17.78% and 1.59ꢀ3.54% in the applied potential range from −0.35 to
−0.6 V vs. SCE, respectively (based on Eq. (1)). The electron transfer number for the
CoMn2O4/NPGA and Pt/C was 3.64ꢀ3.70 and 3.93ꢀ3.97 (calculated from Eq. (2)) at
the given potentials from −0.35 to −0.6 V vs. SCE. The RRDE tests indicated that the
CoMn2O4/NPGA resulted in an almost fourꢀelectron transfer pathway to produce OHꢀ
for the ORR.
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 10000 s, while only 80.18% of its initial
current was retained for the commercial Pt/C catalyst under the same conditions.

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These results implied that the CoMn2O4/NPGA catalyst possessed better stability than
Pt/C.
Furthermore, for practical application in fuel cells, robust tolerance to methanol
oxidation was a critical criterion for cathode electrocatalysts. On the matter, the tests
on CoMn2O4/NPGA and Pt/C were performed in an O2ꢀsaturated 0.1 M KOH solution
with the addition of 3 M methanol in 300 seconds (Figure. 8). For the Pt/C, the
catalytic activity deviated seriously due to its undesirable activity for methanol
oxidation reaction (MOR) simultaneously occurring at the cathode. In contrast, when
measured under the same conditions, the curve was no notable change of
CoMn2O4/NPGA composite. These results indicated that the CoMn2O4/NPGA
catalyst had better methanolꢀtolerance compared with Pt/C.
4. Conclusion
In conclusion, a novel CoMn2O4/NPGA composite with high catalytic activity
towards the ORR was successfully fabricated from lowꢀcost raw materials under mild
conditions. The electrochemical measurements demonstrate that the CoMn2O4/NPGA
composite exhibited 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
occupies high activity, good methanol tolerance and excellent durability, which may
possess great promise as the ORR catalyst in alkaline DMFCs.
Acknowledgements
This work has been supported by the National Natural Science Foundation of China

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under Grant No. 21273024, and Natural Science Foundation of Jilin Province, China
under Grant No. 20160101298JC
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Figure captions
A
B
C
Scheme 1: Schematic illustration of the preparation of the CoMn2O4/NPGA. (A) Stable
suspension of GO, MnO2, Dꢀglucose and urea dispersed in a vial. (B) 3D graphene hydrogel
formed via a hydrothermal treatment. (C) The bulk aerogel of CoMn2O4/NPGA obtained after a
freezeꢀdrying process and its typical SEM image.

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

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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 JCPDS Card.

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Fig. 3. (a) XPS spectra of NGA, MnO2/NGA and CoMn2O4/NPGA composite; (b) highꢀresolution
P2p XPS spectra of CoMn2O4/NPGA; (c) highꢀresolution Mn2p XPS spectra of CoMn2O4/NPGA;
(d) highꢀresolution Co2p XPS spectra of CoMn2O4/NPGA; (e) highꢀresolution N1s XPS spectra
of CoMn2O4/NPGA.

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

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Fig. 5. LSV curves for ORR on (a) CoMn2O4/NPGA, (b) Pt/C at different rotation speeds from
+0.2 to ꢀ0.8 V in O2ꢀsaturated 0.1 M KOH with the scan rate of 10 mV sꢀ 1; (c) LSV curves of the
ORR on CoMn2O4/NPGA, MnO2/NGA, NPGA, NGA and Pt/C at 1600 rpm in O2ꢀsaturated 0.1
M KOH with the scan rate of 10 mV sꢀ 1; (d) Tafel plots for the ORR on the CoMn2O4/NPGA and
Pt/C in 0.1 M O2ꢀsaturated KOH solution at a rotation rate of 1600 rpm.

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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 sꢀ1with 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.

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Figure. 7. Current−time (i−t) chronoamperometric responses of CoMn2O4/NPGA and commercial
Pt/C catalysts at −0.4 V vs. SCE in O2ꢀsaturated 0.1 M KOH solution with a rotation rate of 1600
rpm.

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Figure. 8. Current−time (i−t) chronoamperometric for Pt/C and CoMn2O4/NPGA in O2ꢀsaturated
0.1 M KOH solution and addition of 3 M methanol in 300 seconds.

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Table
Table 1 Textural parameters of the NGA and CoMn2O4/NPGA
Sample
SBET a (m2 gꢀ1 )
Smes b (m2 gꢀ1 )
Smac c (m2 gꢀ1)
PV d (cm3 gꢀ1 )
NGA
276.0
135.8
190.9
85.1
38.3
0.87
0.30
CoMn2O4/NPGA
97.5
a
b
Specific surface area from multiple BET method; Micropore surface area from DFT method; c Macropore
surface area (Smac=SBET ꢀSmes ); d Total pore volume at P/P0=0.99.
Table 2 electrochemical results for ORR estimated from LSV
Electrocatalysts
Eonset
V vs. SCE
ꢀ0.094
E1/2
V vs. SCE
ꢀ0.20
JL (mA cmꢀ2)
ꢀ0.8 V vs. SCE
ꢀ5.39
CoMn2O4/NPGA
MnO2/NGA
NGA
ꢀ0.136
ꢀ0.24
ꢀ4.42
ꢀ0.147
ꢀ0.26
ꢀ4.47
NPGA
ꢀ0.142
ꢀ0.23
ꢀ4.71
Pt/C
ꢀ0.028
ꢀ0.17
ꢀ5.25
JL: limiting current density at ꢀ0.8 V vs. SCE