In order to fulfil the future requirements of electrochemical energy storage, such as high energy density at high power demands, heterogeneous nanostructured materials are currently studied as promising electrode materials due to their synergic properties, which arise from integrating multi-nanocomponents, each tailored to address a different demand (e.g., high energy density, high conductivity, and excellent mechanical stability). In this article, we discuss these heterogeneous nanomaterials based on their structural complexity: zero-dimensional (0-D) (e.g. core-shell nanoparticles), one-dimensional (1-D) (e.g. coaxial nanowires), two-dimensional (2-D) (e.g. graphene based composites), three-dimensional (3-D) (e.g. mesoporous carbon based composites) and the even more complex hierarchical 3-D nanostructured networks. This review tends to focus more on ordered arrays of 1-D heterogeneous nanomaterials due to their unique merits. Examples of different types of structures are listed and their advantages and disadvantages are compared. Finally a future 3-D heterogeneous nanostructure is proposed, which may set a goal toward developing ideal nano-architectured electrodes for future electrochemical energy storage devices.

Hierarchical nanostructures have generated great interest in the energy, materials, and chemical sciences due to the synergic properties of their composite architectures. Herein, a hierarchical MnO₂ nanofibril/nanowire array is successfully synthesized. The structure consists of a conformal layer of MnO₂ nanofibrils evenly distributed on the surface of the individual MnO₂ nanowires. The synthetic mechanism of this hierarchical structure is characterized by electrochemical measurements, Raman spectroscopy, EELS, and electron microscopy. This material was then investigated at slow scan rates for its charge storage mechanisms in different solvents. In aqueous electrolyte, the nanofibrils show a capacitance almost purely dedicated to double-layer and surface adsorption processes, while in an acetonitrile electrolyte, the nanofibrils' capacitance comes mainly from a cation insertion process. This material was also tested at high scan rates in aqueous solution for its practical supercapacitor capabilities. The material shows a large capacitance of 298 F/g at 50 mV/s and 174 F/g at 250 mV/s. It also maintains 85.2% of its capacitance after 1000 cycles. The material also displays easily controllable parameters such as nanowire length, nanowire diameter, and amount of nanofibril material which is shown here to affect the capacitance dramatically.

MnO2 nanoparticle enriched poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires are fabricated by simply soaking the PEDOT nanowires in potassium permanganate (KMnO4) solution. The structures of these MnO2 nanoparticle enriched PEDOT nanowires are characterized by SEM and TEM, which show that the MnO2 nanoparticles have uniform sizes and are finely dispersed in the PEDOT matrix. The chemical constituents and bonding of these composite nanowires are characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, and infrared spectroscopy, which indicate that the formation and dispersion of these MnO2 nanoparticles into the nanoscale pores of the PEDOT nanowires are most likely triggered by the reduction of KMnO4 via the redox exchange of permanganate ions with the functional group on PEDOT. Varying the concentrations of KMnO4 and the reaction time controls the loading amount and size of the MnO2 nanoparticles. Cyclic voltammetry and galvanostatic charge-discharge are used to characterize the electrochemical properties of these MnO2 nanoparticle loaded PEDOT nanowires. Due to their extremely high exposed surface area with nanosizes, the pristine MnO2 nanoparticles in these MnO2 nanoparticle enriched PEDOT nanowires show very high specific capacitance (410 F/g) as the supercapacitor electrode materials as well as high Li+ storage capacity (300 mAh/g) as cathode materials of Li ion battery, which boost the energy storage capacity of PEDOT nanowires to 4 times without causing excessive volume expansion in the polymer. The highly conductive and porous PEDOT matrix facilitates fast charge/discharge of the MnO2 nanoparticles and prevents them from agglomerating. These synergic properties enable the MnO2 nanoparticle enriched PEDOT nanowires to be promising electrode materials for supercapacitors and lithium ion batteries.

Magnesium batteries have been considered to be one of the promising beyond lithium ion technologies due to magnesium's abundance, safety, and high volumetric capacity. However, very few materials show reversible performance as a cathode in magnesium ion systems. We present herein the best reported cycling performances of MnO2 as a magnesium battery cathode material. We show that the previously reported poor Mg(2+) insertion/deinsertion capacities in MnO2 can be greatly improved by synthesizing self-standing nanowires and introducing a small amount of water molecules into the electrolyte. Electrochemical and elemental analysis results revealed that the magnitude of Mg(2+) insertion into MnO2 highly depends on the ratio between water molecules and Mg(2+) ions present in the electrolyte and the highest Mg(2+) insertion capacity was observed at a ratio of 6H2O/Mg(2+) in the electrolyte. We demonstrate for the first time, that MnO2 nanowire electrode can be "activated" for Mg(2+) insertion/deinsertion by cycling in water containing electrolyte resulting in enhanced reversible Mg(2+) insertion/deinsertion even with the absence of water molecules. The MnO2 nanowire electrode cycled in dry Mg electrolyte after activation in water-containing electrolyte showed an initial capacity of 120 mA h g(-1) at a rate of 0.4 C and maintained 72% of its initial capacity after 100 cycles.