Hierarchical manganese oxide/carbon nanocomposites for supercapacitor electrodes

Yiting Peng 0 1 Zheng Chen 0 Jing Wen 0 Qiangfeng Xiao 0 Ding Weng 0 Shiyu He 1 0 Department of Chemical and Biomolecular Engineering, University of California , Los Angeles, CA 90095, USA 1 Department of Materials Science and Engineering, Harbin Institute of Technology , Harbin, 150001, China MnO2/carbon nanocomposites with hierarchical pore structure and controllable MnO2 loading have been synthesized using a self-limiting growth method. This was achieved by the redox reactions of KMnO4 with sacrificed carbon substrates that contain hierarchical pores. The unique pore structure allows the synthesis of nanocomposites with tunable MnO2 loading up to 83 wt.%. The specific capacitance of the nanocomposites increased with the MnO2 loading; the conductivity measured by electrochemical impedance spectroscopy, on the other hand, decreased with increasing MnO2 loading. Optimization of the MnO2 loading resulted in nanocomposites with high specific capacitance and excellent rate capability. This work provides important fundamental understanding which will facilitate the design and fabrication of high-performance supercapacitor materials for a large variety of applications. - and Yunfeng Lu2 ( 1. Introduction Supercapacitors are emerging as a class of high-power energy-storage devices [14]; their broader uses, however, are still limited by their energy density [57]. Generally, a supercapacitor is based on the electrical double layers formed along carbon electrodes, which may provide capacitance of up to 300 F/g in an aqueous electrolyte [8, 9]. Oxides of transition metals, such as RuO2 [1012], MnO2 [1316], NiO [17, 18], Co3O4 [19], and V2O5 [20, 21], possess significantly higher capacitances; however, harvesting such capacitance has been limited by their low conductivity and redox kinetics. To address such intrinsic limitations, a common strategy is to integrate low-dimensional oxide materials with conductive components, such as carbon, which has led to the development of various nanocomposites with significantly improved energy density [2224]. Nevertheless, many essential questions about such composites, such as how the structure, composition and interfaces of the composites may affect the capacitive performance, remain open. We report herein the synthesis of MnO2/carbon nanocomposites with controlled structure and composition and the study of the role of these properties in determining capacitive performance. MnO2 is currently considered as one of the most promising redox components for supercapacitor applications owning to its high capacitance, low cost, and low toxicity. To date, various MnO2/carbon composites have been synthesized, such as composites with planar graphite [25], acetylene black [26, 27], ordered mesoporous carbon [28, 29], carbon nanotubes [3033], and carbon aerogels and nanofoams [34]. These have generally been synthesized by physical mixing of MnO2 with carbon [31], or electrochemically or chemically depositing MnO2 on carbon substrates [32, 33]. Among these synthesis methods, the chemical deposition of MnO2 through self-limiting redox reactions of KMnO4 and carbon is of particular interest [2529]. In such a synthesis, carbon substrates are exposed to KMnO4 solution at room temperature or an elevated temperature, and a spontaneous redox reaction described as 4MnO4 + 3C + 2H2O 4MnO2 + 3CO2 + 4OH (1) occurs producing MnO2 on the sacrificial carbon substrates [35]. The resulting MnO2 layer reduces the diffusion of the MnO4 ions, generating conformal MnO2 coatings on the conductive carbon substrates. Such a self-limiting growth confers several major advantages, such as intimate interfaces between the oxide and the carbon, nanosized oxide particles, and controllable oxide thickness. Nevertheless, the structure and composition of such MnO2/carbon composites are governed by transport of the KMnO4 in solution and its reaction with carbon, which is very rapid even at room temperature. Note that the diffusion of KMnO4 in porous carbon substrates, such as activated carbon and mesoporous carbons, is generally slow in comparison with the fast reaction kinetics [36, 37]. The rapid reactions result in the preferential formation of MnO2 layers located on the exterior of the carbon substrates, blocking inwards diffusion and reaction of KMnO4. This technical difficulty inevitably leads to a low MnO2 loading in the composites, which is detrimental for overall capacitance. Moreover, the formation of MnO2 is often associated with reduced pore accessibility and electron conductivity. In fact, it has been observed that increasing the oxide loading in such carbon composites by further promoting the redox reaction resulted in a reduction in the overall capacitance [38]. Therefore, the synthesis of MnO2/carbon composites with higher oxide loading and controlled pore structure for fast transport of ions, which retain the carbon framework needed for electron conductivity is essential to ensure a satisfactory capacitance performance. To address this challenge, carbon substrates with hierarchical pores (pores with multiple-length-scale diameters) rather than uniform pores were used in this work. The presence of large pores ensures the effective transport of KMnO4 to the substrate interior, while the mesopores provide high surface area for effective reaction. Such a structure allows the formation of composites with high oxide loadings that are homogenously and intimately coated on the sacrificial carbon scaffold. Moreover, the resulting nanocomposites still retain an interconnected porous structure, which facilitates effective electrolyte diffusion and charge transport and ensures higher specific capacitance and better rate capability. Resulting from this unique design, nanocomposites with MnO2 loadings of up to 80 wt.% have been achieved, which is a significant advance on the current state of the art. For the first time, electrochemical impedance spectroscopy has been used to study the synergic effects between the MnO2 mass loading and conductive carbon scaffold, providing insights leading to better capacitance performance. 2. Experimental 2.1 Synthesis of carbon substrate and MnO2/carbon The porous carbon substrates were synthesized by an assembly process using phenolic resol as the carbon precursor. Tri-block copolymer F127 (Mw = 12,600, PEO106PPO70PEO106, where PEO and PPO are poly(ethylene oxide) and poly(propylene oxide), respectively) was used as a soft template; silicate clusters formed by hydrolysis and condensation reactions of tetraethyl orthosilicate (TEOS) and colloidal silica particles (70100 nm in diameter) were used as the hard template. Briefly, F127 (1.6 g), 0.2 mol/L HCl (1.0 g), ethanol (8 g), TEOS (2.08 g), and colloidal silica (1.5 g) were mixed in a flask and vigorously stirred for 4 h. The mixture was transferred to a glass dish and after evaporation of the solvent for 12 h at room temperature, the mixture was put into an oven at 100 C for 24 h to eff (...truncated)