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).

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).

The electrochemical properties of two commercial (Condias, Sumitomo) boron-doped diamond thin-film electrodes were compared with those of two types of boron-doped diamond thin film deposited in our laboratory (microcrystalline, nanocrystalline). Scanning electron microscopy and Raman spectroscopy were used to characterize the electrode morphology and microstructure, respectively. Cyclic voltammetry was used to study the electrochemical response, with five different redox systems serving as probes (Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/)(2+), IrCl(6)(2)(-)(/3)(-), 4-methylcatechol, Fe(3+/2+)). The response for the different systems was quite reproducibile from electrode type to type and from film to film for electrodes of the same type. For all five redox systems, the forward reaction peak current varied linearly with the scan rate(1/2) (nu), indicative of electrode reaction kinetics controlled by mass transport (semi-infinite linear diffusion) of the reactant. Apparent heterogeneous electron-transfer rate constants, k degrees (app), for all five redox systems were determined from deltaE(p)-nu experimental data, according to the method described by Nicholson (Nicholson, R. S. Anal. Chem. 1965, 37, 1351.). The rate constants were also verified through digital simulation (DigiSim 3.03) of the voltammetric i-E curves at different scan rates. Good fits between the experimental and simulated voltammograms were found for scan rates up to 50 V/s. k degrees (app) values of 0.05-0.5 cm/s were observed for Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/2+), and IrCl(6)(2)(-)(/3)(-) without any extensive electrode pretreatment (e.g., polishing). Lower k degrees (app) values of 10(-)(4)-10(-)(6) cm/s were found for 4-methylcatechol and Fe(3+/2+). The voltammetric responses for Fe(CN)(6)(3)(-)(/4)(-) and Ru(NH(3))(6)(3+/2+) were also examined at all four electrode types at two different solution pH (1.90, 7.35). Since the hydrogen-terminated diamond surfaces contain few, if any, ionizable carbon-oxygen functionalities (e.g., carboxylic acid, pK(a) approximately 4.5), the deltaE(p), i(p)(ox), and i(p)(red) values for the two systems were, for the most part, unaffected by the solution pH. This is in contrast to the typical behavior of oxygenated, sp(2) carbon electrodes, such as glassy carbon.

The self-limiting redox reaction of carbon nanofoam substrates with permanganate at room temperature in neutral-pH solutions produces conformal nanoscale MnO 2 deposits throughout the macroscopic thickness ͑ϳ0.17 mm͒ of the nanofoam structure. The nanoscale MnO 2 morphology ranges from ϳ10 nm layered ribbons and rods for a 4 h deposition to ϳ20 nm polycrystalline nanoparticles that form at long deposition times ͑20 h͒. The through-connected pore network of the carbon nanofoam is maintained at all deposition times ͑5 min to 20 h͒, although the average pore size shifts to smaller values and the cumulative pore volume decreases as the MnO 2 coatings grow and thicken within the nanofoam structure. The electrochemical capacitance of the resulting hybrid electrode structure is dominated by the pseudocapacitance of the MnO 2 and increases with MnO 2 loading ͑a function of the exposure time in permanganate͒, particularly at low charge-discharge rates and at ac frequencies Ͻ0.1 Hz. The significant enhancement in mass-, volume-, and footprint-normalized capacitance at high MnO 2 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 MnO 2 deposition.Manganese oxides are well-established cathode materials for both aqueous Zn/MnO 2 alkaline cells 1 and Li-ion batteries, 2 due primarily to the low cost and low toxicity of manganese precursors. More recently, applications of manganese oxides ͑here denoted as MnO 2 ͒ have been extended to electrochemical capacitors ͑ECs͒, 3-37 where MnO 2 stores electron and cation charge via redox processes that mimic the response of double-layer capacitance ͑i.e., faradaic pseudocapacitance͒. The electrochemical performance reported for MnO 2 in electrochemical capacitor configurations, typically using neutral-to mildly basic-pH aqueous electrolytes, is highly variable and depends on such factors as the specific crystal structure of the oxide ͑or lack thereof͒ and the morphology, microstructure, and macrostructure of composite electrodes containing the MnO 2 phase. 22 Recent studies have demonstrated that manganese oxides, when prepared as ultrathin ͑tens to hundreds of nanometers thick͒ deposits on planar current collectors, exhibit anomalously high specific capacitance ͑ϳ700 to 1380 F g −1 ͒ 38-40 compared to electrodes with micrometer thick MnO 2 deposits or to traditional composite configurations ͑ϳ150 to 250 F g −1 ͒. 4,5,13 The specific capacitance reported for ultrathin, nanoscopic MnO 2 rivals the performance of the state-of-the-art EC metal oxide, disordered hydrous ruthenium dioxide, 41 which typically provides specific capacitances Ͼ700 F g −1 , but has limited applicability due to the high cost of ruthenium precursors. Utilization of MnO 2 in an ultrathin, nanoscale configuration reduces resistance issues caused by the low electronic conductivity of MnO 2 ͑10 −5 -10 −6 S cm −1 ͒, minimizes the slow solid-state transport of charge-compensating cations through the ox...

Electrically conducting diamond powder was prepared by coating insulating diamond powder ͑8-12 m diam, ϳ2 m 2 /g͒ 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 ͑SEM͒ 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. 1331 cm −1 for deposition times up to 4 h, and was downshifted to as low as 1317 cm −1 for some particles after the 6-h growth. Electrical resistance measurements of the bulk powder ͑no binder͒ confirmed that a conductive diamond overlayer formed, as the conductivity increased from near zero ͑insulating, Ͻ10 −5 S/cm͒ for the uncoated powder to 1.5 S/cm after the 6-h growth. Ohmic behavior was seen in current-voltage curves recorded for the 4-h powder between ±10 V. Cyclic voltammetric i-E curves for Fe͑CN͒ 6 3−/4− and Ru͑NH 3 ͒ 6 3+/2+ were recorded to evaluate the electrochemical properties of the conductive powder when mixed with a polytetrafluoroethylene binder. At scan rates between 10 and 500 mV/s, ⌬E p 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 Ag/AgCl for 1 h ͑25°C͒ was performed to evaluate the morphological and microstructural stability of the conductive diamond in comparison with graphite and glassy carbon ͑GC͒ powders. The total charge passed during polarization was largest for the GC powder ͑0.88 C/cm 2 ͒ and smallest for conductive diamond powder ͑0.18 C/cm 2 ͒. 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.

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Carbon paper ͑CP͒ was modified with a layer of boron-doped nanocrystalline diamond ͑BND͒ 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 50 m with a thin layer ͑ca. 1 m͒ of electrically conducting diamond. The diamond layer was deposited by microwave plasma-assisted chemical vapor deposition using an argon-rich CH 4 /H 2 /Ar/B 2 H 6 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 ͑cathodic͒ at 1.25 mA/cm 2 ͑geom.͒ and evaluated in terms of ͑i͒ the particle size and distribution, ͑ii͒ the stability during anodic polarization, and ͑iii͒ the electrochemical activity for the reduction of dissolved oxygen. Pt particles with a diameter of 100-300 nm and a particle density of 10 8 -10 9 cm −2 were formed on all regions of the BND electrode. Importantly, the diamond-modified electrode exhibited superior morphological and microstructural stability during anodic polarization ͑1.4 V vs Ag/AgCl͒ 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 sp 2 carbon materials, particularly those used in fuel cells.

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