Morphology-Dependent Enhancement of the Pseudocapacitance of Template-Guided Tunable Polyaniline Nanostructures

Polyaniline is one of the most investigated conducting polymers as supercapacitor material for energy storage applications. The preparation of nanostructured polyaniline with well-controlled morphology is crucial to obtaining good supercapacitor performance. We present here a facile chemical process to produce polyaniline nanostructures with three different morphologies (i.e., nanofibers, nanospheres, and nanotubes) by utilizing the corresponding tunable morphology of MnO₂ reactive templates. A growth mechanism is proposed to explain the evolution of polyaniline morphology based on the reactive templates. The morphology-induced improvement in the electrochemical performance of polyaniline pseudocapacitors is as large as 51% due to the much enhanced surface area and the porous nature of the template-guided polyaniline nanostructures. In addition, and for the first time, a redox-active electrolyte is applied to the polyaniline pseudocapacitors to achieve significant enhancement of pseudocapacitance. Compared to the conventional electrolyte, the enhancement of pseudocapacitance in the redox-active electrolyte is 49%–78%, depending on the specific polyaniline morphology, reaching the highest reported capacitance of 896 F/g for polyaniline full cells so far

Substrate Dependent Self-Organization of Mesoporous Cobalt Oxide Nanowires with Remarkable Pseudocapacitance

A scheme of current collector dependent self-organization of mesoporous cobalt oxide nanowires has been used to create unique supercapacitor electrodes, with each nanowire making direct contact with the current collector. The fabricated electrodes offer the desired properties of macroporosity to allow facile electrolyte flow, thereby reducing device resistance and nanoporosity with large surface area to allow faster reaction kinetics. Co₃O₄ nanowires grown on carbon fiber paper collectors self-organize into a brush-like morphology with the nanowires completely surrounding the carbon microfiber cores. In comparison, Co₃O₄ nanowires grown on planar graphitized carbon paper collectors self-organize into a flower-like morphology. In three electrode configuration, brush-like and flower-like morphologies exhibited specific capacitance values of 1525 and 1199 F/g, respectively, at a constant current density of 1 A/g. In two electrode configuration, the brush-like nanowire morphology resulted in a superior supercapacitor performance with high specific capacitances of 911 F/g at 0.25 A/g and 784 F/g at 40 A/g. In comparison, the flower-like morphology exhibited lower specific capacitance values of 620 F/g at 0.25 A/g and 423 F/g at 40 A/g. The Co₃O₄ nanowires with brush-like morphology exhibited high values of specific power (71 kW/kg) and specific energy (81 Wh/kg). Maximum energy and power densities calculated for Co₃O₄ nanowires with flower-like morphology were 55 Wh/kg and 37 kW/kg respectively. Both electrode designs exhibited excellent cycling stability by retaining ∼91–94% of their maximum capacitance after 5000 cycles of continuous charge–discharge

Enhanced Rate Performance of Mesoporous Co₃O₄ Nanosheet Supercapacitor Electrodes by Hydrous RuO₂ Nanoparticle Decoration

Mesoporous cobalt oxide (Co₃O₄) nanosheet electrode arrays are directly grown over flexible carbon paper substrates using an economical and scalable two-step process for supercapacitor applications. The interconnected nanosheet arrays form a three-dimensional network with exceptional supercapacitor performance in standard two electrode configuration. Dramatic improvement in the rate capacity of the Co₃O₄ nanosheets is achieved by electrodeposition of nanocrystalline, hydrous RuO₂ nanoparticles dispersed on the Co₃O₄ nanosheets. An optimum RuO₂ electrodeposition time is found to result in the best supercapacitor performance, where the controlled morphology of the electrode provides a balance between good conductivity and efficient electrolyte access to the RuO₂ nanoparticles. An excellent specific capacitance of 905 F/g at 1 A/g is obtained, and a nearly constant rate performance of 78% is achieved at current density ranging from 1 to 40 A/g. The sample could retain more than 96% of its maximum capacitance even after 5000 continuous charge-discharge cycles at a constant high current density of 10 A/g. Thicker RuO₂ coating, while maintaining good conductivity, results in agglomeration, decreasing electrolyte access to active material and hence the capacitive performance

Effect of Postetch Annealing Gas Composition on the Structural and Electrochemical Properties of Ti₂CT_<i>x</i> MXene Electrodes for Supercapacitor Applications

Two-dimensional Ti₂CT_<i>x</i> MXene nanosheets were prepared by the selective etching of Al layer from Ti₂AlC MAX phase using HF treatment. The MXene sheets retained the hexagonal symmetry of the parent Ti₂AlC MAX phase. Effect of the postetch annealing ambient (Ar, N₂, N₂/H₂, and air) on the structure and electrochemical properties of the MXene nanosheets was investigated in detail. After annealing in air, the MXene sheets exhibited variations in structure, morphology, and electrochemical properties as compared to HF treated MAX phase. In contrast, samples annealed in Ar, N₂, and N₂/H₂ ambient retained their original morphology. However, a significant improvement in the supercapacitor performance is observed upon heat treatment in Ar, N₂, and N₂/H₂ ambients. When used in symmetric two-electrode configuration, the MXene sample annealed in N₂/H₂ atmosphere exhibited the best capacitive performance with specific capacitance value (51 F/g at 1A/g) and high rate performance (86%). This improvement in the electrochemical performance of annealed samples is attributed to highest carbon content, and lowest fluorine content on the surface of the sample upon annealing, while retaining the original two-dimensional layered morphology and providing maximum access of aqueous electrolyte to the electrodes

Surface Passivation of MoO₃ Nanorods by Atomic Layer Deposition toward High Rate Durable Li Ion Battery Anodes

We demonstrate an effective strategy to overcome the degradation of MoO₃ nanorod anodes in lithium (Li) ion batteries at high-rate cycling. This is achieved by conformal nanoscale surface passivation of the MoO₃ nanorods by HfO₂ using atomic layer deposition (ALD). At high current density such as 1500 mA/g, the specific capacity of HfO₂-coated MoO₃ electrodes is 68% higher than that of bare MoO₃ electrodes after 50 charge/discharge cycles. After 50 charge/discharge cycles, HfO₂-coated MoO₃ electrodes exhibited specific capacity of 657 mAh/g; on the other hand, bare MoO₃ showed only 460 mAh/g. Furthermore, we observed that HfO₂-coated MoO₃ electrodes tend to stabilize faster than bare MoO₃ electrodes because nanoscale HfO₂ layer prevents structural degradation of MoO₃ nanorods. Additionally, the growth temperature of MoO₃ nanorods and the effect of HfO₂ layer thickness was studied and found to be important parameters for optimum battery performance. The growth temperature defines the microstructural features and HfO₂ layer thickness defines the diffusion coefficient of Li-ions through the passivation layer to the active material. Furthermore, ex situ high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction were carried out to explain the capacity retention mechanism after HfO₂ coating

Thermoelectric Properties of Strontium Titanate Superlattices Incorporating Niobium Oxide Nanolayers

A novel superlattice structure based on epitaxial nanoscale layers of NbO_<i>x</i> and Nb-doped SrTiO₃ is fabricated using a layer-by-layer approach on lattice matched LAO substrates. The absolute Seebeck coefficient and electrical conductivity of the [(NbO_<i>x</i>)_<i>a</i>/(Nb-doped SrTiO₃)_<i>b</i>]₂₀ superlattices (SLs) were found to increase with decreasing layer thickness ratio (<i>a</i>/<i>b</i> ratio), reaching, at high temperatures, a power factor that is comparable to epitaxial Nb-doped SrTiO₃ (STNO) films (∼0.7 W m^–1 K^–1). High temperature studies reveal that the SLs behave as n-type semiconductors and undergo an irreversible change at a varying crossover temperature that depends on the <i>a</i>/<i>b</i> ratio. By use of high resolution X-ray photoelectron spectroscopy and X-ray diffraction, the irreversible changes are identified to be due to a phase transformation from cubic NbO to orthorhombic Nb₂O₅, which limits the highest temperature of stable operation of the superlattice to 950 K

Influence of Stacking Morphology and Edge Nitrogen Doping on the Dielectric Performance of Graphene–Polymer Nanocomposites

We demonstrate that functional groups obtained by varying the preparation route of reduced graphene oxide (rGO) highly influence filler morphology and the overall dielectric performance of rGO-relaxor ferroelectric polymer nanocomposite. Specifically, we show that nitrogen-doping by hydrazine along the edges of reduced graphene oxide embedded in poly­(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) results in a dielectric permittivity above 10 000 while maintaining a dielectric loss below 2. This is one of the best-reported dielectric constant/dielectric loss performance values. In contrast, rGO produced by the hydrothermal reduction route shows a much lower enhancement, reaching a maximum dielectric permittivity of 900. Furthermore, functional derivatives present in rGO are found to strongly affect the quality of dispersion and the resultant percolation threshold at low loading levels. However, high leakage currents and lowered breakdown voltages offset the advantages of increased capacitance in these ultrahigh-k systems, resulting in no significant improvement in stored energy density