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GO clad Co3O4 (Co3O4@GO) as ORR catalyst of anion exchange membrane fuel cell
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
GO clad Co3O4 (Co3O4@GO) as ORR catalyst of anion exchange membrane fuel cell Dong-Chao Wang, Nai-Bao Huang*, Yin Sun, Su Zhan, Jun-Jie Zhang Transportation Equipment and Ocean Engineering College, Dalian Maritime University, Dalian 116026, China
article info
abstract
Article history:
GO cladded Co3O4 (Co3O4@GO core/shell) was synthesized as ORR catalyst for anion ex-
Received 1 April 2017
change membrane fuel cell (AEMFC) by ultrasonic method. The obtained GO/Co3O4 was
Received in revised form
characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-
29 May 2017
SEM), high-resolution transmission electron microscopy (HRTEM), and Fourier transform
Accepted 31 May 2017
infrared spectroscopy (FT-IR). Physical characterizations confirmed the obtained catalyst
Available online 23 June 2017
was composed of GO lamina (2 nm) shell and 10 nm Co3O4 nanoparticle core. The optimal content of GO was 5%. At the moment, its onset potential and polarized current density
Keywords:
were identical with that of commercial 20% Pt/C catalyst. The limited current density for
GO
ORR reached 4.30 mA cm2, which was smaller than that of 20% Pt/C (4.62 mA cm2). The
Co3O4
prepared Co3O4@GO/C catalyst behaved excellent cycle stability after 1000th cycle. The
Core/shell catalyst
results indicated the prepared Co3O4@GO/C maybe a potential, efficient and low cost
Oxygen reduction reaction
catalyst for ORR in AEMFC.
Anion exchange membrane fuel cell
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
(AEMFC)
Introduction Because of the crisis from environmental pollution and fossil shortage, developing alternative energy has attracted great attention all over the word [1]. Due to having the advantages of zero-emission, high energy density, high efficiency, etc., fuel cells are considered to be the most attractive and competition energy resource. Compared with other fuel cells, although proton exchange membrane fuel cell (PEMFC) using proton exchange membrane (PEM) as electrolyte membrane has been used in cars, buses, and power station, etc., it lacks competition in large-scale application because of these disadvantages such as high cost, fuel crossover, and so on. If using anion-exchange membrane (AEM) replaces proton exchange membrane (PEM) in PEMFC, which is called anionexchange membrane fuel cell (AEMFC), some of the above
mentioned flaws will be overcome. Most importantly, nonprecious metal can be used as its catalyst. So, the study on AEMFC has been a promising technology [2e4]. For AEMFC, the cathodic oxygen reduction reaction (ORR) is very important in determining the performance of fuel cell. Upon to this point, platinum (Pt)-based materials, with high activity and low over potential, are known to be the most active electrocatalysts for ORR [5e7]. However, their large-scale applications are hindered because of high cost and scarcity of Pt resource [8,9]. Apart from these defects, Pt-based electrocatalysts are also subjected to deactivation from contaminants in fuel or oxidant [10]. Cobalt oxide, especially Co3O4, has been considered as a promising catalytic material due to its low cost, good electrical conductivity, and stability in alkaline media [11,12]. Co3O4 belongs to the normal spinel crystal structure based on a close-packed face centered cubic configuration of O2 ions, in
* Corresponding author. E-mail addresses: [email protected], [email protected] (N.-B. Huang). http://dx.doi.org/10.1016/j.ijhydene.2017.05.236 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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which Co2þ ions occupy the one-eighth of the tetrahedral A sites while Co3þ ions occupy one-half of the octahedral B sites [13]. The active sites of Co3þ ions on the Co3O4 play a decisive role for ORR. Although Co3O4 has a less ORR activity, when it was modified by carbon, its electrocatalytic activity was dramatically improved due to the synergy [14,15]. Among all of the carbon materials, graphene is the most promising matrix for accommodating numerous nanomaterials. The incorporation of nanomaterials into graphene produces high electronic transportation and provides a sufficient area for electrolyte contact, leading to structural stability [16e18]. Graphene, which consists of two-dimensional (2D) honeycomb sp2 carbon lattices with one-atomic thickness, has been paid attention widely for unique mechanical, electrical and thermal properties [19]. Great efforts have been made to utilize graphene based nano-materials as promising electrode materials for ORR [20e24]. For example, surprisingly high ORR activity had been observed when graphene doped by nitrogen. However, it has been reported that electrodes made of graphene may display decreased surface area due to formation of agglomerates or restacking of the graphene sheets [26]. This drawback can be overcome by the addition of transition metal oxides, such as Ni(OH)2, MnO2 or CoOx to graphene which leads to a considerable decrease of restacking of the graphene sheets, increasing the interlayer distance (pore size) between them and consequently the surface area exposed to the electrolyte [25e27]. When graphene mixed with metallic oxide, its electrocatalytic activity for ORR was also improved significantly [28]. However, to our knowledge, no studies have reported the activity of Co3O4@graphene oxide (GO) core/shell catalyst for ORR in alkaline media. In this study, carbon supported Co3O4@GO (Co3O4@GO/C) core/shell catalyst was prepared by ultrasound method. We investigated the ORR catalytic activity of Co3O4@GO/C in alkaline media. The obtained catalyst showed higher activity than Co3O4/C and better stability in alkaline solution.
Experiment Synthesis of Co3O4 Co3O4 was synthesized by the liquid phase precipitation method. Firstly, 6.8206 g Co(NO3)2$6H2O and 3.32 g NH4HCO3 were dissolved in ultrapure water (10 ml), respectively. Secondly, the obtained Co(NO3)2$6H2O solution was added into NH4HCO3 solution at room temperature and kept stirring for 3 h. Then, the mixture washed with ultrapure water and absolute ethanol, dried in an oven at 80 C for 12 h. Finally, they were kept in an oven at 120 C for 12 h. The obtained precursor was heated to 400 C with a heating rate of 5 C min1 and kept at 400 C for 3 h, and then cooled down to room temperature. All the chemical substrates used were analytical grade reagent and employed as received without further purification.
Preparation of Co3O4@GO A series of Co3O4@GO core/shell catalysts were prepared as follows. First, 0.002 g of GO was dispersed in 50 ml CH3OH by
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ultrasonication for 1 h, then 0.2 g of Co3O4 was added. The mixture was ultrasonicated for 30 min and stirred for 24 h. Second, the precipitate was filtrated, and washed with ultrapure water and absolute ethanol repeatedly. Finally, they were dried in an oven at 80 C for 5 h. That was Co3O4@GO core/shell composites. In this work, the Co3O4@GO core/shell catalysts with different mass ratios (marked as x-Co3O4@GO, x ¼ 0, 1%, 3%, 5%, 7%, 9%, the mass ratio of GO: Co3O4 equals 0:100, 1:100, 3:100, 5:100,7:100, 9:100) were obtained by the abovementioned method.
Fabricating Co3O4@GO/C catalyst Carbon supported x-Co3O4@GO(Co3O4@GO/C) was also prepared by ultrasonic method. To begin with, 0.1 g of carbon (XC72) was dispersed in 60 ml CH3OH by ultrasonication for 30 min. Next, 0.1 g of x-Co3O4@GO was added into the solution, ultrasonicated for 30 min and stirred for 12 h. Finally, the precipitate was filtrated, washed with ultrapure water and absolute ethanol, and then dried in an oven at 80 C for 5 h to obtain x-Co3O4@GO/C composites.
Characterization The X-ray diffraction (XRD) analysis was conducted on Rigaku DMAX Ultimaþ diffractometer with Cu Ka radiation (k ¼ 0.15406 nm) from 10 to 80 at a scanning rate of 8 $min1. Transmission electron microscope (TEM) images were performed on JEOL JEM-2100 instrument. Morphology was observed by Field emission scanning electron microscopy (FESEM, SUPRA 55 SAPPHIRE). Fourier transform infrared spectroscopy (FT-IR) spectrometer (PerkinElmer Frontier) was used to analyze the functional groups.
Electrochemical measurement All electrochemical tests were measured on VMP3 electrochemical workstation (EG&G) at room temperature. Working electrode was glassy carbon disk (GC, D ¼ 3 mm) having xCo3O4@GO/C. Platinum net and Ag/AgCl (saturated KCl solution) served as counter electrodes and reference electrodes, respectively. The preparation process of the working electrode was as follows: catalyst ink was formed by mixing 20 mg of xCo3O4@GO/C in 10 ml of CH3CH2OH. 100 ml catalyst ink and 10 ml Nafion were mixed, mixed sample were dropped on a polished GC by microsyringe and dried at room temperature. The electrochemical measurements were carried out in O2saturated 0.1 M KOH solution. The electron transfer number can also be calculated from the results of LSV. In addition, 20% Pt/C come from John Mathy.
Results and discussion Characterization of Co3O4@GO/C catalysts Fig. 1 presented the XRD patterns of Co3O4 nanoparticles, GO, XC-72 and x-Co3O4@GO/C core/shell catalysts. The assynthesized Co3O4 exhibited significant diffraction peaks at 18.9 , 31.4 , 37 , 38 , 44.9 , 59.5 and 65.3 , ascribed to the (1 1
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Fig. 1 e XRD patterns of series Co3O4@GO/C catalysts.
1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1) and (4 4 0) crystal phases (JCPDS no. 42-1467) [1] of face center cubic (fcc) structure, respectively, and were consistent with that of standard Co3O4. All diffraction peaks of the obtained Co3O4 were very incisive and strong, which displayed that the prepared Co3O4 had high crystallinity. There appeared a bread diffraction peaks at 10.27 in pristine GO, indexed to (001) plane of amorphous carbon [29]. For Co3O4@GO/C samples, the main characteristic peaks of Co3O4 can be clearly observed. In addition, all samples showed two weak peak located at 25 and 10.27 , respectively, assigned to the diffraction plane of graphite (0 0 2) and (0 0 1) carbon, respectively [30]. The results further demonstrated the existence of graphite and carbon phase. To obtained more detail information, FTIR spectra of Co3O4, GO and Co3O4@GO/C core/shell catalyst were characterized and shown in Fig. 2. For pure Co3O4, two peaks at 565 cm1 and 663 cm1 were shared to OB3 (B denotes the Co3þ in an octahedral hole) vibration and the ABO3 (A represents the Co2þ in a tetrahedral hole) vibration in the spinel lattice, respectively [31]. The stretching vibrations of skeletal C]C and labile oxygen functional groups in pure GO were found at 3470 cm1 (eOH), 1728 cm1 (C]O), 1634 cm1 (C]C), 1396 cm1 (CeO),
and 1096 cm1 (CeO), respectively. For 5%-Co3O4@GO/C sample, the characteristic bands of Co3O4 vibrations were found at 661 cm1 and 559 cm1. In addition, eOH stretching vibration at 3470 cm1 and CeO stretching vibrations of carboxyl at 1396 cm1 were slight shifted to 3474 cm1 and 1401 cm1, respectively. The above-mentioned band shifts associated with intensity decrement may be contributed by the interfacial interaction between GO and Co3O4. Fig. 3 demonstrated high-resolution field emission scanning electron microscope (FE-SEM) images of the prepared GO, Co3O4, XC72, 5%-Co3O4@GO and 5%-Co3O4@GO/C catalysts. The lump structures with wrinkles and folds surface are observed (Fig. 3a). For Co3O4 and XC-72, they were spherical nanoparticles with the size of about 5e10 nm and 40 nm, respectively (Fig. 3b and c). From Fig. 3e, we found that the small nanoparticles with size around 10 nm scattered on the large nanoscale particles with size around 50 nm. In 5%-Co3O4@GO/C catalyst, there existed elements of C, O, Co. The result demonstrated that 5%-Co3O4@GO/C are homogeneously loaded on XC72, which agreed with those of XRD and FT-IR. TEM was conducted to investigate the interaction of Co3O4 and GO. Fig. 4aed illustrated the TEM images of Co3O4, GO and 5%-Co3O4@GO/C. The typical TEM image (Fig. 4a) of GO sheets showed clear small folds and wrinkles [32]. From Fig. 4b, Co3O4 nanoparticles was about 10 nm, corresponded with that in SEM. Fig. 4c and d was the high-resolution TEM (HRTEM) image of Co3O4@GO. The distinct lattice fringes with an interplanar spacing of 0.46 nm were observed, which corresponded to the (1 1 1) plane of Co3O4 [1]. Scattered Co3O4 was clad by a layer of GO with a thickness of approximately 2 nm, which confirmed that the core/shell of Co3O4@GO had been obtained.
Electrochemical performance of Co3O4@GO/C core/shell catalysts
Fig. 2 e FT-IR spectra of GO, Co3O4, and 5% Co3O4@GO.
The electrocatalytic activity of Co3O4@GO/C was evaluated in 0.1 M KOH solution with O2 saturated. Fig. 5a showed the
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Fig. 3 e FE-SEM images of (a) GO; (b) Co3O4; (c) XC72; (d) 5% Co3O4@GO and (e) 5% Co3O4@GO/C; (f) EDS of Co3O4@GO/C.
cyclic voltammetry (CV) curves of all samples (between 0.2 and 1.2 V vs. SCE at the scan rate of 10 mV s1). The cathodic peaks current density for Co3O4@GO/C were higher than that for Co3O4/C. Meanwhile, with the content of GO increase, the corresponding peak current density increased gradually, and reached the maximum at 5%-GO, and then reduced. The more positive peak potential and the larger peak current were the easier and the higher. Compared with other samples, 5%-Co3O4@GO/C exhibits the highest activity. The results confirmed GO doped Co3O4
nanoparticles were able to dramatically improve electrocatalytic activity for ORR. Maybe, it was the synergistic catalytic effects between the GO and Co3O4 nanoparticles that contributed to facilitate ORR [33,34]. Fig. 5bed illustrated the effect of GO content in Co3O4@GO/ C catalysts on ORR, the linear sweep voltammetry of 5%Co3O4@GO/C at different rotation speeds ranging from 400 to 2400 rpm, and Koutechye-Levich plots in O2-saturated 0.1 mol l1 KOH solution. Consistent with CV results, RDE polarization curves (Fig. 5b) behaved the similar trend. That is,
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Fig. 4 e FE-TEM images of GO (a), Co3O4 (b) and FE-TEM images of Co3O4@GO (c,d).
Fig. 5 e Electrochemical performance of Co3O4/C, Co3O4@GO/C hybrid catalysts for ORR in O2-saturated 0.1 mol L¡1 KOH solution and Koutechye-Levich plot (a) CV curves (n ¼ 100 mV/s) (b) RDE polarization curves of hybrid catalysts with different GO content (n ¼ 10 mV/s) and the rotating speed of 1600 rpm (c) Linear sweep voltammetry of 5%- Co3O4@GO/C at different rotation speed (d) Koutechye-Levich plots of 5%-Co3O4@GO/C.
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5%-Co3O4@GO/C catalysts exhibited higher activity for ORR than that of Co3O4/C and other content of Co3O4@GO/C in terms of limiting diffusion current density and onset potential. For example, for 5%-Co3O4@GO/C, the current density reached 4.3 mA cm2 (@-0.8 V) and increased by about 48% (compared to Co3O4/C 2.92 mA cm2 @0.8 V). The limiting current density increased with rotate rate increasing due to intensify the mass transfer of electrolyte at higher speed [35,36]. According Kouteckye-Levich (K-L) equation, the relationship between current density and reciprocal of square root for rotate rate were regressed. So, the number of electron transfer (n) per oxygen molecule was calculated by Kouteckye-Levich (K-L) equation [37]:
1/jd ¼ 1/jk þ 1/(B u0.5)
(1)
B ¼ 0.62 nFC0 (D0)2/3 n1/6
(2)
where, jk represents the kinetic current density (mAcm2); u e represents the rotating rate of the electrode (rpm); n e represents the transferred electron number; F e represents the Faraday constant, 96486C; C0 e represents the oxygen bulk concentration, 1.2 103 mol cm3; D0 e represents the diffusion coefficient of oxygen, 1.9 105 cm2 s1; n e represents the kinetic viscosity (0.01 cm2 s1). From Fig. 5d, a straight fitting line was obtained, which mean a first order reaction of dissolved oxygen [38]. At the potential of 0.8 V, the n value of Co3O4@GO/C is calculated to be 3.65, suggesting that the Co3O4@GO/C electron followed four-electron transfer process. The above results proved GO cladding Co3O4 could enhance ORR activity. It was the following reason that contributed to the enhancement. Firstly, GO had good electrical conductivity, which was a great deal of benefit to electron transport. Secondly, after Co3O4 was encapsulated by GO lamina (2 nm), on one hand, the aggregation of Co3O4 nanoparticle could be
Fig. 7 e CV curves of Co3O4@GO core/shell measured at different cycles.
restricted. On the other hand, electron transfer across the interface of GO/Co3O4/C could be improved. Finally, the unique lamella core/shell structure, which enabled the fully exposure of electrolyte to active materials and provided short diffusion path for both electrons and ions, thus leading to faster kinetics, lower overpotential and higher electrocatalytic reactivity [39]. The polarization curves of 20% Pt/C (John Mathy) and 5%Co3O4@GO/C for ORR was showed in Fig. 6. The onset potentials of 5%-Co3O4@GO/C was similar to that of commercial 20% Pt/C. Moreover, the current density of prepared 5%Co3O4@GO/C was very close to that of commercial Pt/C. The diffusion-limited current density of 5%-Co3O4@GO/C at 0.8 V vs Ag/AgCl is 4.3 mA cm2 which is close to that of commercial 20% Pt/C (4.62 mA cm2). Long term stability test was evaluated to confirm the durability of the prepared catalyst. Fig. 7 showed that different scanning laps of cyclic voltammetric curves, Note that, the CV curve of 1000th coincided with that of 20th, which manifested the cathodic current density had no obvious change after 1000th cycle, and demonstrated excellent cycle stability. All the results indicated GO cladded Co3O4 would be a promising catalyst.
Conclusion
Fig. 6 e RDE polarization curves of Pt/C and 5% Co3O4@GO/C in O2-saturated 0.1 mol L¡1 KOH solution.
In summary, GO cladded Co3O4 (Co3O4@GO core/shell) catalyst for ORR in alkaline solution had been successfully fabricated by a facile method. The obtained Co3O4@GO core/shell was composed of GO lamina (2 nm) shell and 10 nm Co3O4 nanoparticle. Compared with Co3O4/C, the prepared Co3O4@GO/C ORR catalyst exhibited a more positive onset potential, higher cathodic current density (increased about 38%), and a higher electron transfer number in alkaline media. Moreover, the onset potential and the polarized current density of Co3O4@GO/C catalyst were identical with that of commercial 20% Pt/C catalyst except the former's limited current density
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(4.30 mA cm2) was smaller than the latter's (4.62 mA cm2). Co3O4@GO/C catalyst behaved excellent cycle stability after 1000th cycle. The results indicated the prepared Co3O4@GO/C maybe a potential, efficient and low cost catalyst for ORR in AEMFC.
[14]
[15]
Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 21676040, No. 21276036), the National Key Research and Development Program of China (No. 2016YFB0101206).
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GO clad Co3O4 (Co3O4@GO) as ORR catalyst of anion exchange membrane fuel cell Dong-Chao Wang, Nai-Bao Huang*, Yin Sun, Su Zhan, Jun-Jie Zhang Transportation Equipment and Ocean Engineering College, Dalian Maritime University, Dalian 116026, China
article info
abstract
Article history:
GO cladded Co3O4 (Co3O4@GO core/shell) was synthesized as ORR catalyst for anion ex-
Received 1 April 2017
change membrane fuel cell (AEMFC) by ultrasonic method. The obtained GO/Co3O4 was
Received in revised form
characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-
29 May 2017
SEM), high-resolution transmission electron microscopy (HRTEM), and Fourier transform
Accepted 31 May 2017
infrared spectroscopy (FT-IR). Physical characterizations confirmed the obtained catalyst
Available online 23 June 2017
was composed of GO lamina (2 nm) shell and 10 nm Co3O4 nanoparticle core. The optimal content of GO was 5%. At the moment, its onset potential and polarized current density
Keywords:
were identical with that of commercial 20% Pt/C catalyst. The limited current density for
GO
ORR reached 4.30 mA cm2, which was smaller than that of 20% Pt/C (4.62 mA cm2). The
Co3O4
prepared Co3O4@GO/C catalyst behaved excellent cycle stability after 1000th cycle. The
Core/shell catalyst
results indicated the prepared Co3O4@GO/C maybe a potential, efficient and low cost
Oxygen reduction reaction
catalyst for ORR in AEMFC.
Anion exchange membrane fuel cell
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
(AEMFC)
Introduction Because of the crisis from environmental pollution and fossil shortage, developing alternative energy has attracted great attention all over the word [1]. Due to having the advantages of zero-emission, high energy density, high efficiency, etc., fuel cells are considered to be the most attractive and competition energy resource. Compared with other fuel cells, although proton exchange membrane fuel cell (PEMFC) using proton exchange membrane (PEM) as electrolyte membrane has been used in cars, buses, and power station, etc., it lacks competition in large-scale application because of these disadvantages such as high cost, fuel crossover, and so on. If using anion-exchange membrane (AEM) replaces proton exchange membrane (PEM) in PEMFC, which is called anionexchange membrane fuel cell (AEMFC), some of the above
mentioned flaws will be overcome. Most importantly, nonprecious metal can be used as its catalyst. So, the study on AEMFC has been a promising technology [2e4]. For AEMFC, the cathodic oxygen reduction reaction (ORR) is very important in determining the performance of fuel cell. Upon to this point, platinum (Pt)-based materials, with high activity and low over potential, are known to be the most active electrocatalysts for ORR [5e7]. However, their large-scale applications are hindered because of high cost and scarcity of Pt resource [8,9]. Apart from these defects, Pt-based electrocatalysts are also subjected to deactivation from contaminants in fuel or oxidant [10]. Cobalt oxide, especially Co3O4, has been considered as a promising catalytic material due to its low cost, good electrical conductivity, and stability in alkaline media [11,12]. Co3O4 belongs to the normal spinel crystal structure based on a close-packed face centered cubic configuration of O2 ions, in
* Corresponding author. E-mail addresses: [email protected], [email protected] (N.-B. Huang). http://dx.doi.org/10.1016/j.ijhydene.2017.05.236 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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which Co2þ ions occupy the one-eighth of the tetrahedral A sites while Co3þ ions occupy one-half of the octahedral B sites [13]. The active sites of Co3þ ions on the Co3O4 play a decisive role for ORR. Although Co3O4 has a less ORR activity, when it was modified by carbon, its electrocatalytic activity was dramatically improved due to the synergy [14,15]. Among all of the carbon materials, graphene is the most promising matrix for accommodating numerous nanomaterials. The incorporation of nanomaterials into graphene produces high electronic transportation and provides a sufficient area for electrolyte contact, leading to structural stability [16e18]. Graphene, which consists of two-dimensional (2D) honeycomb sp2 carbon lattices with one-atomic thickness, has been paid attention widely for unique mechanical, electrical and thermal properties [19]. Great efforts have been made to utilize graphene based nano-materials as promising electrode materials for ORR [20e24]. For example, surprisingly high ORR activity had been observed when graphene doped by nitrogen. However, it has been reported that electrodes made of graphene may display decreased surface area due to formation of agglomerates or restacking of the graphene sheets [26]. This drawback can be overcome by the addition of transition metal oxides, such as Ni(OH)2, MnO2 or CoOx to graphene which leads to a considerable decrease of restacking of the graphene sheets, increasing the interlayer distance (pore size) between them and consequently the surface area exposed to the electrolyte [25e27]. When graphene mixed with metallic oxide, its electrocatalytic activity for ORR was also improved significantly [28]. However, to our knowledge, no studies have reported the activity of Co3O4@graphene oxide (GO) core/shell catalyst for ORR in alkaline media. In this study, carbon supported Co3O4@GO (Co3O4@GO/C) core/shell catalyst was prepared by ultrasound method. We investigated the ORR catalytic activity of Co3O4@GO/C in alkaline media. The obtained catalyst showed higher activity than Co3O4/C and better stability in alkaline solution.
Experiment Synthesis of Co3O4 Co3O4 was synthesized by the liquid phase precipitation method. Firstly, 6.8206 g Co(NO3)2$6H2O and 3.32 g NH4HCO3 were dissolved in ultrapure water (10 ml), respectively. Secondly, the obtained Co(NO3)2$6H2O solution was added into NH4HCO3 solution at room temperature and kept stirring for 3 h. Then, the mixture washed with ultrapure water and absolute ethanol, dried in an oven at 80 C for 12 h. Finally, they were kept in an oven at 120 C for 12 h. The obtained precursor was heated to 400 C with a heating rate of 5 C min1 and kept at 400 C for 3 h, and then cooled down to room temperature. All the chemical substrates used were analytical grade reagent and employed as received without further purification.
Preparation of Co3O4@GO A series of Co3O4@GO core/shell catalysts were prepared as follows. First, 0.002 g of GO was dispersed in 50 ml CH3OH by
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ultrasonication for 1 h, then 0.2 g of Co3O4 was added. The mixture was ultrasonicated for 30 min and stirred for 24 h. Second, the precipitate was filtrated, and washed with ultrapure water and absolute ethanol repeatedly. Finally, they were dried in an oven at 80 C for 5 h. That was Co3O4@GO core/shell composites. In this work, the Co3O4@GO core/shell catalysts with different mass ratios (marked as x-Co3O4@GO, x ¼ 0, 1%, 3%, 5%, 7%, 9%, the mass ratio of GO: Co3O4 equals 0:100, 1:100, 3:100, 5:100,7:100, 9:100) were obtained by the abovementioned method.
Fabricating Co3O4@GO/C catalyst Carbon supported x-Co3O4@GO(Co3O4@GO/C) was also prepared by ultrasonic method. To begin with, 0.1 g of carbon (XC72) was dispersed in 60 ml CH3OH by ultrasonication for 30 min. Next, 0.1 g of x-Co3O4@GO was added into the solution, ultrasonicated for 30 min and stirred for 12 h. Finally, the precipitate was filtrated, washed with ultrapure water and absolute ethanol, and then dried in an oven at 80 C for 5 h to obtain x-Co3O4@GO/C composites.
Characterization The X-ray diffraction (XRD) analysis was conducted on Rigaku DMAX Ultimaþ diffractometer with Cu Ka radiation (k ¼ 0.15406 nm) from 10 to 80 at a scanning rate of 8 $min1. Transmission electron microscope (TEM) images were performed on JEOL JEM-2100 instrument. Morphology was observed by Field emission scanning electron microscopy (FESEM, SUPRA 55 SAPPHIRE). Fourier transform infrared spectroscopy (FT-IR) spectrometer (PerkinElmer Frontier) was used to analyze the functional groups.
Electrochemical measurement All electrochemical tests were measured on VMP3 electrochemical workstation (EG&G) at room temperature. Working electrode was glassy carbon disk (GC, D ¼ 3 mm) having xCo3O4@GO/C. Platinum net and Ag/AgCl (saturated KCl solution) served as counter electrodes and reference electrodes, respectively. The preparation process of the working electrode was as follows: catalyst ink was formed by mixing 20 mg of xCo3O4@GO/C in 10 ml of CH3CH2OH. 100 ml catalyst ink and 10 ml Nafion were mixed, mixed sample were dropped on a polished GC by microsyringe and dried at room temperature. The electrochemical measurements were carried out in O2saturated 0.1 M KOH solution. The electron transfer number can also be calculated from the results of LSV. In addition, 20% Pt/C come from John Mathy.
Results and discussion Characterization of Co3O4@GO/C catalysts Fig. 1 presented the XRD patterns of Co3O4 nanoparticles, GO, XC-72 and x-Co3O4@GO/C core/shell catalysts. The assynthesized Co3O4 exhibited significant diffraction peaks at 18.9 , 31.4 , 37 , 38 , 44.9 , 59.5 and 65.3 , ascribed to the (1 1
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Fig. 1 e XRD patterns of series Co3O4@GO/C catalysts.
1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1) and (4 4 0) crystal phases (JCPDS no. 42-1467) [1] of face center cubic (fcc) structure, respectively, and were consistent with that of standard Co3O4. All diffraction peaks of the obtained Co3O4 were very incisive and strong, which displayed that the prepared Co3O4 had high crystallinity. There appeared a bread diffraction peaks at 10.27 in pristine GO, indexed to (001) plane of amorphous carbon [29]. For Co3O4@GO/C samples, the main characteristic peaks of Co3O4 can be clearly observed. In addition, all samples showed two weak peak located at 25 and 10.27 , respectively, assigned to the diffraction plane of graphite (0 0 2) and (0 0 1) carbon, respectively [30]. The results further demonstrated the existence of graphite and carbon phase. To obtained more detail information, FTIR spectra of Co3O4, GO and Co3O4@GO/C core/shell catalyst were characterized and shown in Fig. 2. For pure Co3O4, two peaks at 565 cm1 and 663 cm1 were shared to OB3 (B denotes the Co3þ in an octahedral hole) vibration and the ABO3 (A represents the Co2þ in a tetrahedral hole) vibration in the spinel lattice, respectively [31]. The stretching vibrations of skeletal C]C and labile oxygen functional groups in pure GO were found at 3470 cm1 (eOH), 1728 cm1 (C]O), 1634 cm1 (C]C), 1396 cm1 (CeO),
and 1096 cm1 (CeO), respectively. For 5%-Co3O4@GO/C sample, the characteristic bands of Co3O4 vibrations were found at 661 cm1 and 559 cm1. In addition, eOH stretching vibration at 3470 cm1 and CeO stretching vibrations of carboxyl at 1396 cm1 were slight shifted to 3474 cm1 and 1401 cm1, respectively. The above-mentioned band shifts associated with intensity decrement may be contributed by the interfacial interaction between GO and Co3O4. Fig. 3 demonstrated high-resolution field emission scanning electron microscope (FE-SEM) images of the prepared GO, Co3O4, XC72, 5%-Co3O4@GO and 5%-Co3O4@GO/C catalysts. The lump structures with wrinkles and folds surface are observed (Fig. 3a). For Co3O4 and XC-72, they were spherical nanoparticles with the size of about 5e10 nm and 40 nm, respectively (Fig. 3b and c). From Fig. 3e, we found that the small nanoparticles with size around 10 nm scattered on the large nanoscale particles with size around 50 nm. In 5%-Co3O4@GO/C catalyst, there existed elements of C, O, Co. The result demonstrated that 5%-Co3O4@GO/C are homogeneously loaded on XC72, which agreed with those of XRD and FT-IR. TEM was conducted to investigate the interaction of Co3O4 and GO. Fig. 4aed illustrated the TEM images of Co3O4, GO and 5%-Co3O4@GO/C. The typical TEM image (Fig. 4a) of GO sheets showed clear small folds and wrinkles [32]. From Fig. 4b, Co3O4 nanoparticles was about 10 nm, corresponded with that in SEM. Fig. 4c and d was the high-resolution TEM (HRTEM) image of Co3O4@GO. The distinct lattice fringes with an interplanar spacing of 0.46 nm were observed, which corresponded to the (1 1 1) plane of Co3O4 [1]. Scattered Co3O4 was clad by a layer of GO with a thickness of approximately 2 nm, which confirmed that the core/shell of Co3O4@GO had been obtained.
Electrochemical performance of Co3O4@GO/C core/shell catalysts
Fig. 2 e FT-IR spectra of GO, Co3O4, and 5% Co3O4@GO.
The electrocatalytic activity of Co3O4@GO/C was evaluated in 0.1 M KOH solution with O2 saturated. Fig. 5a showed the
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Fig. 3 e FE-SEM images of (a) GO; (b) Co3O4; (c) XC72; (d) 5% Co3O4@GO and (e) 5% Co3O4@GO/C; (f) EDS of Co3O4@GO/C.
cyclic voltammetry (CV) curves of all samples (between 0.2 and 1.2 V vs. SCE at the scan rate of 10 mV s1). The cathodic peaks current density for Co3O4@GO/C were higher than that for Co3O4/C. Meanwhile, with the content of GO increase, the corresponding peak current density increased gradually, and reached the maximum at 5%-GO, and then reduced. The more positive peak potential and the larger peak current were the easier and the higher. Compared with other samples, 5%-Co3O4@GO/C exhibits the highest activity. The results confirmed GO doped Co3O4
nanoparticles were able to dramatically improve electrocatalytic activity for ORR. Maybe, it was the synergistic catalytic effects between the GO and Co3O4 nanoparticles that contributed to facilitate ORR [33,34]. Fig. 5bed illustrated the effect of GO content in Co3O4@GO/ C catalysts on ORR, the linear sweep voltammetry of 5%Co3O4@GO/C at different rotation speeds ranging from 400 to 2400 rpm, and Koutechye-Levich plots in O2-saturated 0.1 mol l1 KOH solution. Consistent with CV results, RDE polarization curves (Fig. 5b) behaved the similar trend. That is,
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Fig. 4 e FE-TEM images of GO (a), Co3O4 (b) and FE-TEM images of Co3O4@GO (c,d).
Fig. 5 e Electrochemical performance of Co3O4/C, Co3O4@GO/C hybrid catalysts for ORR in O2-saturated 0.1 mol L¡1 KOH solution and Koutechye-Levich plot (a) CV curves (n ¼ 100 mV/s) (b) RDE polarization curves of hybrid catalysts with different GO content (n ¼ 10 mV/s) and the rotating speed of 1600 rpm (c) Linear sweep voltammetry of 5%- Co3O4@GO/C at different rotation speed (d) Koutechye-Levich plots of 5%-Co3O4@GO/C.
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5%-Co3O4@GO/C catalysts exhibited higher activity for ORR than that of Co3O4/C and other content of Co3O4@GO/C in terms of limiting diffusion current density and onset potential. For example, for 5%-Co3O4@GO/C, the current density reached 4.3 mA cm2 (@-0.8 V) and increased by about 48% (compared to Co3O4/C 2.92 mA cm2 @0.8 V). The limiting current density increased with rotate rate increasing due to intensify the mass transfer of electrolyte at higher speed [35,36]. According Kouteckye-Levich (K-L) equation, the relationship between current density and reciprocal of square root for rotate rate were regressed. So, the number of electron transfer (n) per oxygen molecule was calculated by Kouteckye-Levich (K-L) equation [37]:
1/jd ¼ 1/jk þ 1/(B u0.5)
(1)
B ¼ 0.62 nFC0 (D0)2/3 n1/6
(2)
where, jk represents the kinetic current density (mAcm2); u e represents the rotating rate of the electrode (rpm); n e represents the transferred electron number; F e represents the Faraday constant, 96486C; C0 e represents the oxygen bulk concentration, 1.2 103 mol cm3; D0 e represents the diffusion coefficient of oxygen, 1.9 105 cm2 s1; n e represents the kinetic viscosity (0.01 cm2 s1). From Fig. 5d, a straight fitting line was obtained, which mean a first order reaction of dissolved oxygen [38]. At the potential of 0.8 V, the n value of Co3O4@GO/C is calculated to be 3.65, suggesting that the Co3O4@GO/C electron followed four-electron transfer process. The above results proved GO cladding Co3O4 could enhance ORR activity. It was the following reason that contributed to the enhancement. Firstly, GO had good electrical conductivity, which was a great deal of benefit to electron transport. Secondly, after Co3O4 was encapsulated by GO lamina (2 nm), on one hand, the aggregation of Co3O4 nanoparticle could be
Fig. 7 e CV curves of Co3O4@GO core/shell measured at different cycles.
restricted. On the other hand, electron transfer across the interface of GO/Co3O4/C could be improved. Finally, the unique lamella core/shell structure, which enabled the fully exposure of electrolyte to active materials and provided short diffusion path for both electrons and ions, thus leading to faster kinetics, lower overpotential and higher electrocatalytic reactivity [39]. The polarization curves of 20% Pt/C (John Mathy) and 5%Co3O4@GO/C for ORR was showed in Fig. 6. The onset potentials of 5%-Co3O4@GO/C was similar to that of commercial 20% Pt/C. Moreover, the current density of prepared 5%Co3O4@GO/C was very close to that of commercial Pt/C. The diffusion-limited current density of 5%-Co3O4@GO/C at 0.8 V vs Ag/AgCl is 4.3 mA cm2 which is close to that of commercial 20% Pt/C (4.62 mA cm2). Long term stability test was evaluated to confirm the durability of the prepared catalyst. Fig. 7 showed that different scanning laps of cyclic voltammetric curves, Note that, the CV curve of 1000th coincided with that of 20th, which manifested the cathodic current density had no obvious change after 1000th cycle, and demonstrated excellent cycle stability. All the results indicated GO cladded Co3O4 would be a promising catalyst.
Conclusion
Fig. 6 e RDE polarization curves of Pt/C and 5% Co3O4@GO/C in O2-saturated 0.1 mol L¡1 KOH solution.
In summary, GO cladded Co3O4 (Co3O4@GO core/shell) catalyst for ORR in alkaline solution had been successfully fabricated by a facile method. The obtained Co3O4@GO core/shell was composed of GO lamina (2 nm) shell and 10 nm Co3O4 nanoparticle. Compared with Co3O4/C, the prepared Co3O4@GO/C ORR catalyst exhibited a more positive onset potential, higher cathodic current density (increased about 38%), and a higher electron transfer number in alkaline media. Moreover, the onset potential and the polarized current density of Co3O4@GO/C catalyst were identical with that of commercial 20% Pt/C catalyst except the former's limited current density
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(4.30 mA cm2) was smaller than the latter's (4.62 mA cm2). Co3O4@GO/C catalyst behaved excellent cycle stability after 1000th cycle. The results indicated the prepared Co3O4@GO/C maybe a potential, efficient and low cost catalyst for ORR in AEMFC.
[14]
[15]
Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 21676040, No. 21276036), the National Key Research and Development Program of China (No. 2016YFB0101206).
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