Recent Advancements in Electrochemical Deposition of Metal-Based Electrode Materials for Electrochemical Supercapacitors

The demand for energy storage devices with high energy and power densities has increased tremendously in this rapidly growing world. Conventional capacitors, fuel cells, and lithium-ion batteries have been used as energy storage devices for the long term. However, supercapacitors are one of the most promising energy storage devices because of their high specific capacitance, high power density, and longer cycle life. Recent research has focused on synthesizing transition-metal oxides/hydroxides, carbon materials, and conducting polymers as supercapacitor electrode materials. The performance of supercapacitors can be improved by altering electrolytes, electrode materials, current collectors, experimental temperatures, and film thickness. Thousands of papers on supercapacitors have already been published, reflecting the significance and elucidating how much demanding such energy storage devices for this fast-growing generation. This review aims to illustrate the electrode materials loaded on various conductive substrates by electrochemical deposition employed for supercapacitors to provide broad knowledge on synthetic pathways, which will pave the way for future research. We also discussed the basic parameters involved in supercapacitor studies and the advantages of the electrochemical deposition techniques through literature analysis. Finally, future trends and directions on exploring metals/metal composites toward designing and constructing viable, high-class, and even newly featured flexible energy storage materials, electrodes, and systems are presented.

Magnetic Resonance Investigation of Vanadia and Vanadium−Molybdenum Gels Synthesized with Peroxovanadate Precursors

Vanadia gels and vanadium-molybdenum oxide gels were investigated using the magnetic resonance techniques, EPR spectroscopy and (51)V MAS NMR spectroscopy. The vanadium oxide gels were derived from the reaction of H(2)O(2) and V(2)O(5), and the vanadium-molybdenum oxide (VMoO) gels were derived from the reaction of peroxovanadates with an ammonium molybdate solution. EPR spectroscopy was utilized to determine quantitative information about the concentration of V(4+) paramagnetic species present in the samples and additional structural information about the V(4+) coordination environment. (51)V MAS NMR spectroscopy was used to elucidate the V(5+) electronic environment and how it changes as a function of molybdenum content. The observed line broadening of the (51)V NMR signal with increasing molybdenum content was correlated with an increase in the concentration of paramagnetic species as monitored by EPR spectroscopy. The evolution of various vanadium sites during thermal treatment was also investigated. This work provides further support for the hypothesis that the selectivity of VMoO catalysts in the oxidation of 1,3-butadiene to maleic anhydride is due to the presence of paramagnetic V(4+) sites.

17O MAS and 3QMAS NMR investigation of crystalline V(2)O(5) and layered V(2)O(5).nH(2)O gels

The environments for oxygen sites in crystalline V(2)O(5) and in layered vanadia gels produced via sol-gel synthesis have been investigated using (17)O MAS and 3QMAS NMR. For crystalline V(2)O(5), three structural oxygen sites were observed: V=O (vanadyl), V(2)O (doubly coordinated), and V(3)O (triply coordinated). Line-shape parameters for these sites were determined from numerical simulations of the MAS spectra. For the vanadia gels at various stages of dehydration, assignments have been proposed for numerous vanadyl, doubly coordinated, and triply coordinated oxygen sites. In addition, by correlating the (17)O MAS and 3QMAS NMR, (51)V MAS NMR, and thermogravimetric analysis data, the coordination of water sites has been established. On the basis of these results, the gel structure and its evolution at various stages of hydration have been detailed. Upon rehydration of the layered gel, we observed a preferred site for initial water readsorption. The oxygen atoms of these readsorbed water molecules readily exchanged into all types of oxygen sites even at room temperature.