Self-Limiting Electrodeposition of Hierarchical MnO₂ and M(OH)₂/MnO₂ Nanofibril/Nanowires: Mechanism and Supercapacitor Properties

Abstract

Hierarchical nanostructures have generated great interest in the energy, materials, and chemical sciences due to the synergic properties of their composite architectures. Herein, a hierarchical MnO₂ nanofibril/nanowire array is successfully synthesized. The structure consists of a conformal layer of MnO₂ nanofibrils evenly distributed on the surface of the individual MnO₂ nanowires. The synthetic mechanism of this hierarchical structure is characterized by electrochemical measurements, Raman spectroscopy, EELS, and electron microscopy. This material was then investigated at slow scan rates for its charge storage mechanisms in different solvents. In aqueous electrolyte, the nanofibrils show a capacitance almost purely dedicated to double-layer and surface adsorption processes, while in an acetonitrile electrolyte, the nanofibrils' capacitance comes mainly from a cation insertion process. This material was also tested at high scan rates in aqueous solution for its practical supercapacitor capabilities. The material shows a large capacitance of 298 F/g at 50 mV/s and 174 F/g at 250 mV/s. It also maintains 85.2% of its capacitance after 1000 cycles. The material also displays easily controllable parameters such as nanowire length, nanowire diameter, and amount of nanofibril material which is shown here to affect the capacitance dramatically.

MnOx/carbon nanotube/reduced graphene oxide nanohybrids as high-performance supercapacitor electrodes

“…20 Raman spectroscopy was also used to analyze the MnO x structural features over large areas. 12 A Raman spectrum of the MnO x /CNT/ RGO is shown in Figure 3d, and the spectra of pristine graphite foil, CNT and RGO are shown in Supplementary Figure S9. The peak observed in Figure 3d at Raman shift of~640 cm − 1 can be attributed to the Mn-O stretching vibration in the basal plane of MnO 6 and/or the symmetric stretching vibration (Mn-O) of the MnO 6 group.…”

“…Specifically, the porous and flower-like structure could provide a large and accessible surface area to greatly enhance surface ion adsorption, improve the accessibility of cations and shorten the ion diffusion path. 2,17 The smaller and uniformly sized nanoparticles could also facilitate fast charge transfer on the surface or sub-surface of the active material; for example, Duay et al 12 found that the specific capacitance was much higher for small MnO 2 nanofibrils (5-10 nm) than for large MnO 2 nanowires (4.5 μm). In addition, although MnO x could be gradually oxidized to a higher valent state during the charge/discharge process, the structural features associated with the as-prepared MnO x , such as ionic (e.g., vacancies and misplaced ions) and electronic (electrons and holes) defects and mismatches at different phases, could still be preserved because of the slow and mild nature of the process.…”

“…According to the Pourbaix diagram, Mn 2 O 3 , Mn 3 O 4 and MnO 2 can all be formed from a Mn 2+ salt solution at room temperature. 12 Compared to single-phased MnO 2 , MnO x compounds (i.e., Mn atoms with multiple oxidation states and phases) usually contain both donor and acceptor sites in their microstructures as well as defects and mismatch induced by different phases, which can enable a higher charge storage capacity that is beneficial for supercapacitor applications. 13 However, despite a few examples of MnO x reported recently, 14 most previous studies simply denoted the synthesized Mn oxides as MnO 2 with few or no structural investigations.…”

Hybrids of NiCo 2 O 4 nanorods and nanobundles with graphene as promising electrode materials for supercapacitors

“…However, the searching for electrode materials with higher electrochemical performance are still seriously needed, because of the practical application of supercapacitors are hindered by their low specific capacitance, poor cycling stability and so on. Over the past few years, lots of transition metal oxides and metal hydroxide such as MnO 2 , NiO, Co 3 O 4 , Ni(OH) 2 , and NiAl-LDH [6][7][8][9][10][11][12][13][14], have been wildly studied, because of there lower price and lower toxic nature. However, the intrinsic low conductivity of these metal oxides limited the performance of the supercapacitors.…”

Hierarchically nanostructured transition metal oxides for supercapacitors

“…Recently, hierarchically nanostructured TMO materials have gradually become a research hotspot in SC field because of their unique structures [63][64][65][66][67]. These structures could offer large specific surface area, abundant accessible electroactive sites, and short ion diffusion pathway during the rapid charge-discharge process [63][64][65][66][67].…”