We demonstrate that, when distributed as nanoscale coatings on the walls of carbon nanofoam substrates, manganese oxides exhibit voltammetric signatures in LiOH-containing alkaline electrolytes that are characteristic of either electrochemical capacitors or batteries, depending on the potential range investigated. Pseudocapacitance is observed for positive potential ranges, and ex-situ X-ray absorption spectroscopy confirms that the native layered birnessite MnOx structure is retained as the Mn oxidation state is toggled between 3.72 and 3.43. When the cycling range is extended to more negative potential limits, well-defined reduction and oxidation features are observed, with an associated reversible change in the Mn oxidation state of 0.71 after 25 cycles. For these deep-discharge conditions, high charge-storage capacities are facilitated by the reversible interconversion of birnessite and γ-MnOOH forms of the nanoscale MnOx coating. Solid-state 7Li NMR is used to investigate the role of Li+ from the alkaline electrolyte in enhancing the cycling stability of the MnOx−carbon nanofoam.
The self-limiting reaction of aqueous permanganate with carbon nanofoams produces conformal, nanoscopic deposits of birnessite ribbons and amorphous MnO2 throughout the ultraporous carbon structure. The MnO2 coating contributes additional capacitance to the carbon nanofoam while maintaining the favorable high-rate electrochemical performance inherent to the ultraporous carbon structure of the nanofoam. Such a three-dimensional design exploits the benefits of a nanoscopic MnO2-carbon interface to produce an exceptionally high area-normalized capacitance (1.5 F cm-2), as well as high volumetric capacitance (90 F cm-3).
Carbon paper (CP) was modified with a layer of boron-doped nanocrystalline diamond (BND) in order to improve the material's chemical resistance and microstructural stability during exposure to aggressive electrochemical environments. In the procedure, carbon fibers in the CP were coated up to a depth of about with a thin layer (ca. ) of electrically conducting diamond. The diamond layer was deposited by microwave plasma-assisted chemical vapor deposition using an argon-rich source gas mixture. X-ray diffraction and Raman spectroscopy confirmed the presence of a crystalline diamond overlayer. The electrodeposition of Pt electrocatalyst particles was used to probe the electrochemical activity of the diamond-coated electrode. The metal phase was formed using pulsed galvanostatic deposition (cathodic) at (geom.) and evaluated in terms of (i) the particle size and distribution, (ii) the stability during anodic polarization, and (iii) the electrochemical activity for the reduction of dissolved oxygen. Pt particles with a diameter of and a particle density of were formed on all regions of the BND electrode. Importantly, the diamond-modified electrode exhibited superior morphological and microstructural stability during anodic polarization ( vs ) as compared to CP, both in the presence and absence of Pt. The results demonstrate that surface modification with electrically conducting diamond is a means to improve the dimensional stability of carbon materials, particularly those used in fuel cells.
The self-limiting redox reaction of carbon nanofoam substrates with permanganate at room temperature in neutral-pH solutions produces conformal nanoscale deposits throughout the macroscopic thickness of the nanofoam structure. The nanoscale morphology ranges from layered ribbons and rods for a 4 h deposition to polycrystalline nanoparticles that form at long deposition times (20 h). The through-connected pore network of the carbon nanofoam is maintained at all deposition times (5 min to 20 h), although the average pore size shifts to smaller values and the cumulative pore volume decreases as the coatings grow and thicken within the nanofoam structure. The electrochemical capacitance of the resulting hybrid electrode structure is dominated by the pseudocapacitance of the and increases with loading (a function of the exposure time in permanganate), particularly at low charge–discharge rates and at ac frequencies . The significant enhancement in mass-, volume-, and footprint-normalized capacitance at high mass loadings is accompanied by a modest increase in the Warburg resistance that develops as the pore size and void volume of the nanofoam substrate are reduced by internal deposition.
Electrically conducting diamond powder was prepared by coating insulating diamond powder ( diam, ) with a thin boron-doped layer using microwave plasma-assisted chemical vapor deposition. Deposition times from 1 to 6 h were evaluated. Scanning electron microscopy (SEM) revealed that the diamond powder particles become more faceted and more secondary growths form with increasing deposition time. Fusion of neighboring particles was also observed with increasing growth time. The first-order diamond phonon line appeared in the Raman spectrum at ca. for deposition times up to 4 h, and was downshifted to as low as for some particles after the 6-h growth. Electrical resistance measurements of the bulk powder (no binder) confirmed that a conductive diamond overlayer formed, as the conductivity increased from near zero (insulating, ) for the uncoated powder to after the 6-h growth. Ohmic behavior was seen in current-voltage curves recorded for the 4-h powder between . Cyclic voltammetric i-E curves for and were recorded to evaluate the electrochemical properties of the conductive powder when mixed with a polytetrafluoroethylene binder. At scan rates between 10 and , for both redox systems was high, ranging from 140 to 350 mV, consistent with significant ohmic resistance within the powder/binder electrode. Our results at this point suggest that the resistance is mainly due to poor particle-particle connectivity. Anodic polarization at 1.6 V vs for 1 h (25°C) was performed to evaluate the morphological and microstructural stability of the conductive diamond in comparison with graphite and glassy carbon (GC) powders. The total charge passed during polarization was largest for the GC powder and smallest for conductive diamond powder . SEM images taken of conductive diamond powder after polarization showed no evidence of microstructural degradation, while significant morphological and microstructural changes were seen for the GC powder.
Coenzyme B 12 is an organometallic compound that catalyzes biological rearrangement reactions. Homolytic cleavage of the unusual cobalt-carbon bond in the coenzyme initiates the free-radical reaction. In an attempt to understand the factors that might be important in the mechanism, we synthesized a series of model complexes [LCo{(DO)(DOH)bn}R] + with a folded equatorial ligand and determined the crystal structures. Semi-empirical calculations with these and other model compounds provide evidence for a transelectronic influence; when the Co-N bond is shortened, the Co-C bond lengthens. These results are compared to density functional calculations carried out by others.
The design and fabrication of three-dimensional multifunctional architectures from the appropriate nanoscale building blocks, including the strategic use of void space and deliberate disorder as design components, permits a re-examination of devices that produce or store energy as discussed in this critical review. The appropriate electronic, ionic, and electrochemical requirements for such devices may now be assembled into nanoarchitectures on the bench-top through the synthesis of low density, ultraporous nanoarchitectures that meld high surface area for heterogeneous reactions with a continuous, porous network for rapid molecular flux. Such nanoarchitectures amplify the nature of electrified interfaces and challenge the standard ways in which electrochemically active materials are both understood and used for energy storage. An architectural viewpoint provides a powerful metaphor to guide chemists and materials scientists in the design of energy-storing nanoarchitectures that depart from the hegemony of periodicity and order with the promise--and demonstration--of even higher performance (265 references).
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