Charge transfer and storage in nanostructures

Efficient storage and conversion of electrical charge in materials, to a voltage and current, provides the basis for batteries and capacitors. Given the widespread usage of portable electronics there is a continual need to further enhance the energy and power density of such devices, which could be accomplished through the use of nanostructured materials. The large surface area to volume ratio and the possibilities of new materials physics and chemistry provide the rationale for their use and is discussed. The former aspect considers the relevance to the area-dependent capacitance as well as the parasitic elements that reduce the charge and energy delivery from the theoretical maximum values. Specific instances of electrode materials, as well as the electrode-electrolyte interface and electrolyte properties, with respect to their capability and prospects are examined. Alternate internal and external surface dependent Faradaic reactions and concomitant pseudocapacitance based mechanisms, seem to have the ability to bridge the large energy densities of batteries to the power density of the capacitors perhaps helping in realizing a truly useful hybrid device. While much of the report relates to presently used devices such as Li-ion batteries and activated carbon based electrochemical capacitors, the relevant principles are shown to be valid for other types of charge conversion agents such as photoelectrochemical and dye-sensitized solar cells. The review also considers perspectives on alternate materials and architectures.