Preparation and properties of coke powder activated carbon/α-Co(OH)2 composite electrode materials
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Coke powder activated carbon (CPAC) was prepared by dipping-calcined KOH activation method. Using CPAC as the raw material a series of composite electrode materials of CPAC/α-Co(OH)2 with different mass fractions of cobalt were synthesized by the Sol–gel method. The physical properties of the resulting samples were characterized by the field emission scanning electron microscopy and the X-ray diffraction. The results show that composite materials, CPAC/α-Co(OH)2, have a flower-like structure. The results of electrochemical performances show that the composite material has a good electrochemical capacity of 472.3 F g−1 with a cobalt doping amount of 30 wt %. By the cyclic voltammetry testing, we found that the anodic peak potential of the redox peaks in composite electrode materials shifted positively when the scan rate increased, while the cathodic peak potential shifted negatively, and that would cause a gradual increase of the peak potential difference of redox peaks. In contrast, the lower of the scan rate, the smaller of the peak potential difference and the better of the reversibility of composite material. The results of impedance testing show that CPAC/α-Co(OH)2 has a lower electrochemical impedance than that of CPAC.
KeywordsSpecific Capacitance Electrochemical Performance Cobalt Content Powder Activate Carbon Porous Carbon Material
Recently, as a kind of new energy store and conversion equipment, electrochemical supercapacitors (ECs) have generated great interests due to their large capacitance, long cycle life and quick charge/discharge performance, etc. [1, 2, 3, 4]. Based on the energy store mechanism, electrochemical supercapacitors can be separated into two different categories: electric double layer capacitors (EDLCs) and redox pseudocapacitors [5, 6, 7]. The electrode materials have an extremely high influence on the property of the ECs and an appropriate electrode material plays an important role in enhancing the energy density and the range of working potential windows [8, 9, 10]. So far, several kinds of porous carbon materials [11, 12, 13, 14, 15] have been widely applied in the electrochemical double-layer capacitors (EDLCs), owing to characteristics of abundant raw materials, low production cost, high special surface area, higher electrochemical stability, better electrical conductivity, and so on. But the specific capacitance of the porous carbon materials is lower, so it is necessary to improve the specific capacitance by depositing the metal oxides and metal hydroxides on their surface. Therefore, one can build the electrochemical supercapacitors, which have the advantages of both of electric double layer capacitors (EDLCs) and redox pseudocapacitors [16, 17]. Considering the low cost and excellent capacitance, the cobalt compounds have been widely used as electrode materials [18, 19]. And as we know, α-Co(OH)2 and β-Co(OH)2 are two different crystal structures of Co(OH)2 compounds, and the former has attracted much attention for its outstanding electrochemical performance [18, 20, 21].
Coke powder is generally known as the coke particles with an average diameter of less than 5 mm produced in the breaking process of coke, and treated as a waste because of its small particle size. Nowadays, the reuse of coke powder is mainly focused on the treatment of waster water after being activated by alkaline compounds . However, the coke powder has been rarely employed as electrode material. In this study, coke powder activated carbons (CPAC) were prepared by dipping-calcined activation method with the modifier of KOH using coke powder as the raw material , and the composite electrode materials, CPAC/α-Co(OH)2, were synthesized by the Sol–gel method. Moreover, the physics and electrochemical performances of the composite electrode materials were systematically investigated.
2.1 Preparation of CPAC/α-Co(OH)2 composite materials
CPAC were prepared according to our previous study . Dissolved certain quality of CoCl2·6H2O into 50 mL distilled water and stirred for 30 min at room temperature. Then CPAC were added with a little of alcohol, and made sure the mass ratio of cobalt respectively was 5, 10, 20, 30, 50 wt %. After subsequent stirring for 30 min, aqueous ammonia (10 wt %) was added into each of samples and the pH value was adjusted to between 9 and 10. And after stirring for 6 h and standing for 4 h, each of the precipitates was collected by filtration under reduced pressure. Finally, each of the samples was washed to pH value is 7 with distilled water and dried for 24 h at 80 °C. Cooling in the normal temperature, samples were fully grinded in the agate mortar, respectively.
2.2 Preparation of CPAC/α-Co(OH)2 electrode
CPAC/α-Co(OH)2 electrode was composed of composite materials, acetylene black and polytetrafluoroethylene (PTFE) emulsion with a mass ratio of 8:1:1 and drops of anhydrous ethanol were added to get paste sample. Then the paste sample was filled into a foam nickel with an apparent area of 10 × 10 mm2, dried at 80 °C for 2 h under vacuum and pressed to a sheet at the pressure of 10 MPa for 1–2 min to assure a good electronic contact and to form an effective quadrate coating, next drying at 80 °C for 4 h.
2.3 Characterization and electrochemical measurement
Morphology observation of the composite materials was performed on a FE-SEM (JSM-6701F, Japan) and a XRD (D/Max-2400, Rigaku, Japan) technique. The electrochemical measurement of electrodes were carried out using an electrochemical working station (CHI660B, Shanghai, China) in a three electrode cell at room temperature. In the normal three electrode system, CPAC/α-Co(OH)2 composite electrode was used as working electrode. Furthermore, a platinum gauze electrode and a saturated calomel electrode (SCE) were served as the counter electrode and the reference electrode, respectively, and KOH solution (2 mol L−1) was used as the electrolyte.
3 Results and discussion
3.1 FE-SEM analysis
3.2 XRD analysis
3.3 The electrochemical characterizations of CPAC/α-Co(OH)2
3.3.2 Cyclic voltammetry
It can be also seen from Fig. 5 that, the current response for the electrode is quite higher than others when the cobalt content is 30 wt %. This verdict is consistent with that of shown in Figs. 3 and 4. Besides, it is obvious that the peak potential absolute value of the reduction is equal to that of the oxidation, showing an approximate ideal symmetry. Undoubtedly, we conclude that the composite electrode materials exhibit a good reversibility characteristic.
3.3.3 AC impedance
A series of CPAC/α-Co(OH)2 composite electrode materials with different cobalt contents were prepared by the sol–gel method. The results obtained from electrochemical testing show that, with cobalt contents of 30 wt %, the optimal specific capacitance of the as-prepared composite electrode reaches up to 472.3 F g−1. The cyclic voltammetry curves reveal that active substances have a wider current window and an optimum electrochemical property at a scan rate of 5 mV s−1. The electrochemical impedance spectroscopy results indicate that CPAC/α-Co(OH)2 composite electrode materials exhibit an outstanding conductivity and a lower impendence compared with that of CPAC.
This work was financially supported by the Funds for Creative Research Groups of China (Grant NO.51121062) and Excellent Young Teachers in Lanzhou University of Technology Training Project (Grant NO.1005ZCX016).
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