A Hybrid Electrode of Co3O4@PPy Core/Shell Nanosheet Arrays for High-Performance Supercapacitors
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Herein, combining solverthermal route and electrodeposition, we grew unique hybrid nanosheet arrays consisting of Co3O4 nanosheet as a core, PPy as a shell. Benefiting from the PPy as conducting polymer improving an electron transport rate as well as synergistic effects from such a core/shell structure, a hybrid electrode made of the Co3O4@PPy core/shell nanosheet arrays exhibits a large areal capacitance of 2.11 F cm−2 at the current density of 2 mA cm−2, a ~4-fold enhancement compared with the pristine Co3O4 electrode; furthermore, this hybrid electrode also displays good rate capability (~65 % retention of the initial capacitance from 2 to 20 mA cm−2) and superior cycling performance (~85.5 % capacitance retention after 5000 cycles). In addition, the equivalent series resistance value of the Co3O4@PPy hybrid electrode (0.238 Ω) is significantly lower than that of the pristine Co3O4 electrode (0.319 Ω). These results imply that the Co3O4@PPy hybrid composites have a potential for fabricating next-generation energy storage and conversion devices.
KeywordsCo3O4@PPy Core/shell nanosheet arrays Supercapacitors
With the rapid increasing demand in energy storage system for portable electronics and hybrid electric vehicles, supercapacitors have aroused widespread research interest owning to their high power density, fast charge–discharge rate and long lifespan [1, 2, 3]. As for a key component of the supercapacitors, electrode materials can be divided into three major types: carbon materials [4, 5], transition metal oxides [6, 7, 8] and conducting polymers (CPs) [9, 10]. Carbon materials store charges electrostatically through reversible ion adsorption at the electrode/electrolyte interface . In comparison, transition metal oxides and CPs exploit the fast and reversible Faradic redox process at the electrode surface, thus delivering a considerably high specific capacitance [12, 13]. Therefore, the electrode materials based on transition metal oxides and CPs are gradually becoming a research hotspot in the field of the supercapacitors [14, 15, 16].
Among various electrode materials, Co3O4 is one of the most extensively investigated pseudocapacitive materials because of its low cost, environmental friendliness and high theoretical capacitance (~3560 F g−1) . Importantly, it can provide multiple oxide states for reversible redox process . Despite these appealing features, the real specific capacitance obtained from various Co3O4 nanostructures [18, 19, 20] is still far below the theoretical value, which may be attributed to its intrinsic semiconducting characteristic . To overcome this problem, one effective method is fabricating addictive/binder-free electrode configuration to avoid the “dead surface” and tedious process in traditional slurry-coating electrode. Ni foam is widely used as the substrate to support metal oxides materials because of its good electrical conductivity and porous structure, which can enhance the electron transport and improve the active site of electrode materials. Simultaneously, another feasible method is designing three-dimensional (3D) hybrid electrode with large surface area and fast electron transport. Recently, integrating carbon materials, CPs, or noble metal nanoparticles onto electroactive materials has been demonstrated to be an effective synthesis route. Wang et al.  successfully prepared Co3O4@MWCNTs hybrid composites, which show superior electrochemical performance as positive electrode materials. As one of the most important CPs, polypyrrole (PPy) has been a promising pseudocapacitive electrode material because of its low cost, good electrical conductivity, relatively high capacitance, and outstanding mechanical flexibility . For instance, Liu et al.  fabricated a supercapacitor electrode composed of CoO@PPy hybrid nanowires, which delivers a remarkably large areal capacitance of 4.43 F cm−2 at 1 mA cm−2, excellent rate capability and cycling performance; Hong et al.  developed a Co3O4@Au-PPy core/shell nanowires electrode, which exhibits a high specific capacitance of 2062 F g−1 (6.39 F cm−2) at 5 mA cm−2, with ~68 % retention of the initial capacitance from 5 to 50 mA cm−2. However, Au as a noble metal is quite costly, and the in situ interfacial polymerization process is time-consuming. In contrast, electrodeposition technique has great advantages, such as convenient, low cost, controllable, and efficient. Thus, it is of great interest to develop a low cost and efficient route to fabricate 3D Co3O4@PPy hybrid electrode with enhanced electrical conductivity and excellent electrochemical performance for supercapacitor applications.
Based on above consideration, we designed a 3D core/shell nanostructure of uniform PPy thin layer on mesoporous Co3O4 nanosheet arrays as a hybrid electrode material through a solvothermal and electrodeposition process. A hybrid electrode made of as-grown Co3O4@PPy core/shell nanosheet arrays exhibits a large areal capacitance of 2.11 F cm−2 at the current density of 2 mA cm−2, which is superior to 0.54 F cm−2 of the pristine Co3O4 electrode. Meanwhile, this electrode also displays a good rate capability (1.37 F cm−2 at the current density of 20 mA cm−2). Most importantly, the Co3O4@PPy hybrid electrode demonstrates a superior cycling performance (~85.5 % capacitance retention after 5000 cycles). Furthermore, the equivalent series resistance (ESR) value of the Co3O4@PPy hybrid electrode (0.238 Ω) is significantly lower than that of the pristine Co3O4 electrode (0.319 Ω), indicting the enhanced electrical conductivtity.
2.1 Synthesis of Mesoporous Co3O4 Nanosheet Arrays
All reagents used in the work were of analytical grade. A hybrid electrode configuration was prepared by a facile two-step method, which can be easily scaled up. Typically, a piece of Ni foam (ca. 4 × 1 cm2) was carefully pretreated with 3 M HCl aqueous by ultrasonication for 30 min, and then cleaned with deionized water and absolute ethanol for several times. 2 mmol of Co(NO3)2·6H2O and 5 mmol of hexamethylenetetramine (HMT) were dissolved in 25 mL of deionized water and 25 mL of absolute ethanol under magnetic stirring for 30 min. Then, the resulting solution was transferred into a 60 mL Teflon-lined autoclave and a piece of cleaned Ni foam substrate was immersed into it. Subsequently, the autoclave was sealed and maintained in an electric oven at 90 °C for 8 h. After cooling down to room temperature naturally, the products were rinsed with deionized water and absolute ethanol for several times, and then dried at 60 °C for 2 h. Finally, the as-prepared samples were calcined at 300 °C in air for 2 h.
2.2 Synthesis of Co3O4@PPy Core/Shell Nanosheet Arrays
PPy thin layer was grown on the surface of mesoporous Co3O4 nanosheet arrays by electrodeposition. The procedure of eletrodeposition was accomplished in a three-electrode system by using the Ni foam-supported as-grown Co3O4 electrode materials as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl as the reference electrode. Electrolyte for electrodeposition of PPy was prepared by dissolving 0.4 mL of pyrrole (288 mM) and 0.1491 g of KCl (100 mM) into 20 mL of deionized water. Then, the Co3O4@PPy core/shell nanosheet arrays were synthesized at 0.8 V for a different duration of 2, 5, 8, and 10 min. Finally, as-prepared Co3O4@PPy hybrid electrode materials were rinsed with deionized water and absolute ethanol for several times, and then dried at 60 °C for 2 h.
2.3 Structure Characterization
As-synthesized products were characterized by D/Max-2550 PC X-ray diffractometer (XRD, Rigaku, Cu-Kα radiation), X-ray photoelectron spectroscopy (XPS, PHI5000VersaProbe), scanning electron microscopy (SEM, HITACHI, S-4800) and transmission electron microscopy (TEM, JEOL, JEM-2100F) equipped with an energy-dispersive X-ray spectrometer (EDX). The Co3O4@PPy samples were easily scraped off from the Ni foam substrate for the Fourier transform infrared (FTIR) test, and the FTIR spectrum was recorded on a Nicolet 6700 FTIR spectrometer (Bruker).
2.4 Electrochemical Characterization
Electrochemical measurements were performed on an Autolab electrochemical workstation (PGSTAT302N) using a three-electrode system and 1 M KOH as the electrolyte. A Pt foil and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The Ni foam-supported Co3O4@PPy and Co3O4 electrode materials (ca. 1 cm2 area) acted directly as the working electrode.
3 Results and Discussion
Comparison of performance metrics for the Co3O4@PPy electrode materials with several reported electrode materials in previous literatures
Co3O4@PPy hybrid composites
2.11 F cm−2 at 2 mA cm−2
Mesoporous Co3O4 nanosheets
0.54 F cm−2 at 2 mA cm−2
Co3O4@PPy@MnO2 core/shell/shell nanowires
1.13 F cm−2 at 1.2 mA cm−2
Co3O4@PPy@MnO2 ternary core/shell composites
0.55 F cm−2 at 0.5 A g−1
Co3O4@MnO2 core/shell nanowires
0.56 F cm−2 at 11.25 mA cm−2
Co3O4@NiO core/shell nanowires
1.35 F cm−2 at 6 mA cm−2
ZnO@MnO2@PPy ternary core/shell nanorods
1.793 F cm−2 at 2 A g−1
FEG/PPy hybrid composites
0.56 F cm−2 at 1 mA cm−2
In summary, a hybrid nanomaterial of Co3O4@PPy core/shell nanosheet arrays on Ni foam was prepared through a solvothermal and electrodeposition process. The Co3O4@PPy hybrid electrode exhibits a large areal capacitance of 2.11 F cm−2 at the current density of 2 mA cm−2, a ~4-fold enhancement compared with the pristine Co3O4 electrode. Furthermore, the Co3O4@PPy hybrid electrode also displays good rate capability (~65 % retention of the initial capacitance from 2 to 20 mA cm−2) and superior cycling performance (~85.5 % capacitance retention after 5000 cycles). In addition, the ESR value of the Co3O4@PPy hybrid electrode (0.238 Ω) is significantly lower than that of the pristine Co3O4 electrode (0.319 Ω). The outstanding electrochemical performance can enable the Co3O4@PPy hybrid composites to be a promising electrode material for next-generation energy storage and conversion devices.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21171035, 51472049 and 51302035), the Key Grant Project of Chinese Ministry of Education (Grant No. 313015), the PhD Programs Foundation of the Ministry of Education of China (Grant No. 20110075110008 and 20130075120001), the National 863 Program of China (Grant No. 2013AA031903), the Science and Technology Commission of Shanghai Municipality (Grant No. 13ZR1451200), the Fundamental Research Funds for the Central Universities, the Program Innovative Research Team in University (IRT1221), the Shanghai Leading Academic Discipline Project (Grant No. B603), and the Program of Introducing Talents of Discipline to Universities (No. 111-2-04).
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