Wenhui Guo,
Xiuxiu Ma,
Xianlei Zhang,
Yaqing Zhang,
Dingling Yu and
Xingquan He*
Department of Chemistry and Chemical Engineering, Changchun University of Science and Technology, Changchun 130022, P. R. China. E-mail: hexingquan@hotmail.com; Tel: +86-431-85583430
First published on 29th September 2016
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.
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.
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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.
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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 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 2p3/2 and Mn 2p1/2 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 2p3/2 and Co 2p1/2. The 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 shake-up type peaks of the Co 2p3/2 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.†
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 ID/IG ratio allows the evaluation of the graphitization degree of a carbon-based material. The ID/IG 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 (La) which is a measure of the inter-defect 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 × 10−10λlaser4 × 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 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
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Fig. 4 Nitrogen adsorption–desorption isotherms and pore size distribution (inset) for CoMn2O4/NPGA (a) and NGA (b). |
Sample | SBETa (m2 g−1) | Smesb (m2 g−1) | Smacc (m2 g−1) | PVd (cm3 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/P0 = 0.99. | ||||
NGA | 276.0 | 190.9 | 85.1 | 0.87 |
CoMn2O4/NPGA | 135.8 | 97.5 | 38.3 | 0.30 |
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.
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 |
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
![]() | (1) |
![]() | (2) |
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.
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. These results implied that the CoMn2O4/NPGA catalyst possessed better stability than Pt/C.
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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.
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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. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16337h |
This journal is © The Royal Society of Chemistry 2016 |