Mats of free-standing manganese oxide (MnxOy ) nanowires have been fabricated as experimental electrode materials for rechargeable electro-chemical power cells and capacitors. Because they are free-standing, the wires in these mats are electrochemically accessible. The advantage of the mat-of-nanowires configuration, relative to other configurations of electrode materials, arises from the combination of narrowness and high areal number density of the wires. This combination offers both high surface areas for contact with electrolytes and short paths for diffusion of ions into and out of the electrodes, thereby making it possible to charge and discharge at rates higher than would otherwise be possible and, consequently, to achieve greater power densities.
The nanowires are fabricated in an electrolytic process in which there is no need for an electrode binder material. Moreover, there is no need to incorporate an electrically conductive additive into the electrode material; the only electrically conductive material that must be added is a thin substrate contact film at the anchored ends of the nanowires. Hence, the mass fraction of active electrode material is close to 100 percent, as compared with about 85 percent in conventional electrodes made from a slurry of active electrode material, binder, and conductive additive pressed onto a metal foil.
The locations and sizes of the nanowires are defined by holes in templates in the form of commercially available porous alumina membranes. In experiments to demonstrate the present process, alumina membranes of various pore sizes and degrees of porosity were used. First, a film of Au was sputtered onto one side of each membrane. The membranes were then attached, variously, to carbon tape or a gold substrate by use of silver or carbon paste. Once thus attached, the membranes were immersed in a plating solution comprising 0.01 M MnSO4 +0.03 M (NH4)2SO4. The pH of the solution was kept constant at 8 by addition of H2SO4 or NH4OH as needed. MnxOy nanowires were potentiostatically electrodeposited in the pores in the alu- mina templates. Depending on the anodic deposition potentials, MnxOy was deposited in various oxidation states [divalent (Mn3O4), trivalent (Mn2O3),or tetravalent (MnO2)]. The MnxOy wires were made free-standing (see figure) by dissolving the alumina templates,variously, in KOH or NaOH at a concentration of 20 volume percent.
This work was done by Nosang Myung, William West, Jay Whitacre, and Ratnakumar Bugga of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention.Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-30655, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
(NANO) Direct Electrolytic Deposition of Mats on MnxOy
(reference NPO30655) is currently available for download from the TSP library.
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Overview
The document discusses the direct electrolytic deposition of manganese oxide (MnOₓ) nanowires, highlighting their potential applications in high-power batteries and capacitors. Manganese oxides, particularly MnO₂, are recognized for their versatility in various electrochemical applications, including as electrodes in alkaline batteries, intercalation hosts for lithium batteries, and ultracapacitor electrodes.
The document outlines the environmentally friendly characteristics and cost-effectiveness of MnOₓ films compared to other transition metal oxides like NiO, CoO, and RuO₂. It details several preparation methods for MnOₓ thin films, including chemical vapor deposition, thermal decomposition, electron beam evaporation, sol-gel methods, and electrochemical deposition. The focus is on the direct electrochemical fabrication of freestanding MnOₓ nanowires using nano-templates created at the Jet Propulsion Laboratory (JPL).
The fabrication process involves coating nanoporous alumina membranes with sputter-deposited gold films, attaching them to substrates, and immersing them in a manganese sulfate plating bath. The solution's pH is maintained at 8, and various oxidation states of manganese oxides (Mn₃O₄, Mn₂O₃, and MnO₂) can be deposited depending on the anodic deposition potentials. The resulting nanowires are characterized as being in an amorphous phase, as indicated by X-ray diffraction patterns.
Figures in the document illustrate the anodizing process of alumina templates and the resulting electroplated MnOₓ nanowire arrays, showcasing their structural characteristics through scanning electron microscopy (SEM) images.
The document emphasizes that the power density of batteries is fundamentally influenced by the electrochemical kinetics at the electrode/electrolyte interface and the solid-state diffusion of lithium ions. Therefore, the morphology and structure of electrode materials are crucial for enhancing charge and discharge characteristics. The ability to engineer high surface area structures at the nanoscale is highlighted as a key factor in improving the performance of electrochemical devices.
Overall, this technical support package provides valuable insights into the innovative methods for fabricating manganese oxide nanowires and their implications for advancing energy storage technologies, particularly in the context of electric vehicles and portable electronic devices.
