Direct Growth of Birnessite-Type MnO2 on Activated Carbon Cloth for a Flexible Supercapacitor

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1. Introduction Carbon cloth (CC), which is composed of numerous uniform carbon fibers, has attracted significant attention in recent years as an electrode for use in flexible supercapacitors, owing to the low cost of this material, along with its unique 3D structure, high surface area, chemical stability, electrical conductivity and flexibility. However, because the active part of CC consists solely of an exfoliated carbon shell, as-purchased CC exhibits lower gravimetric capacitance (1~2 F g-1) than other carbonaceous materials, such as graphene and carbon nanotubes (100~200 F g-1). Thus, CC is effective when used as a scaffold to support highly capacitive materials. In the present study, commercial CC pieces were first treated using facile wet processes and the resulting material served as the substrate for the direct growth of a birnessite-type MnO2 film.1 During this process, the mass loading of the MnO2 was optimized in terms of the areal capacitance (capacitance normalized by the unit area of the electrode) and the electrochemical utilization. An asymmetric supercapacitor was subsequently built by assembling the optimized MnO2/CC together with activated carbon (AC)-coated CC as the positive and negative electrodes, respectively. We report the fabrication of a MnO2-modified CC electrode with optimized loading and the construction of a device with high performance and good stability. 2. Experimental MnO2 was deposited electrochemically in 2 mM MnSO4 aqueous solution containing 50 mM KCl, in which the treated and untreated CC pieces were used as the working electrodes. The CC piece with the optimized mass loading of MnO2 was used as the positive electrode together with an activated carbon-coated CC (AC/CC) as the negative electrode to build a two-electrode asymmetric cell. First, this cell was assembled in an open beaker, such that the potentials of both electrodes could be monitored with respect to a Ag/AgCl reference electrode while the voltage between the two electrodes was controlled within the range of 0 to 2.0 V. After a stable response was observed, the equilibrated electrodes were detached and re-assembled in an acrylic holder with a cellulose filter paper (pre-soaked in 0.5 M Na2SO4 electrolyte) sandwiched between the MnO2- and AC-modified CC pieces to construct a solid-state supercapacitor (The cell volume, 0.07 cm3). 3. Results and discussion A simple wet process was developed to enhance the EDLC performance of commercial CC, including oxidation followed by reduction with NaBH4. This treatment was found to enhance the areal capacitance from 4 to 78 mF cm-2. The mass loading of birnessite-type MnO2 film could be varied by controlling the delivered charge density during electrodeposition. Based on the magnitude of areal capacitance (C A) and the electrochemical utilization, the optimal MnO2 loading appeared to be approximately 4 mg cm-2. The MnO2-modified CC positive electrode with the optimized mass loading was combined with an AC-modified CC negative electrode to build an asymmetric supercapacitor operating at a cell voltage of 2.0 V, in which the potential windows of 0.73 and 1.27 V were assigned to the MnO2 and AC electrodes, respectively. The volumetric capacitance (C vol) values of each experimental cell were collected as a function of scan rate. The C vol at 2 mV s-1 increased in the order symmetric AC/CC < asymmetric MnO2/CC//AC/CC < symmetric MnO2/CC, which can largely be attributed to the capacitance of the single electrode. With increasing scan rate, the symmetric MnO2/CC cell exhibited a significantly decreased C vol due to slow kinetics. In contrast, the asymmetric cell showed a gentle decrease, equivalent to better rate-capability, indicating a contribution from the EDLC due to the AC negative electrode. The volumetric capacitance of the MnO2//AC asymmetric supercapacitor at scan rates above 20 mV s-1 was much higher than those of the MnO2 and AC symmetric supercapacitors. The asymmetric MnO2/CC//AC/CC cell exhibited high volumetric energy density (0.978 mW cm-3) and power density (0.158 W cm-3) with excellent cycling stability (94.4% after 10,000 cycles). This exceptional performance per unit volume can be attributed to the following reasons: an increase in the mass loading of MnO2 resulting from the pre-treatment of the commercial CC,an improvement in the rate capability, due to the specific structure of the electrodeposited MnO2 (birnessite) and the optimization of the loading mass based on considering the electrochemical utilization of the MnO2 on the CC, andavoidance of side reactions at the positive and negative electrodes in the asymmetric cell. Reference 1) M. Nakayama et al, “A Direct Electrochemical Route to Construct a Polymer/Manganese Oxide Layered Structure”, Inorg. Chem. , 43, 8215-8217 (2004).