58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Growing demands for high-power applications such as portable electronics, electric vehicles, and other power supply devices have triggered substantial efforts on high-energy and high power-density energy storage devices. Among those energy storage devices, electrochemical supercapacitors can deliver high power, but they suffer from low energy density, while lithium ion batteries (LIBs) are limited in practical applications. Therefore, it is in impending need to develop versatile electrode materials which can be utilized in both of supercapacitors and LIBs.

In this study, we developed hierarchical vanadium oxide V₂O₅ nanowires/conductive polypyrrole (PPy) nanocomposites for LIB cathodes. It shows that active cathode V₂O₅ nanowires provide good electrochemical performance, and conductive polypyrrole further enhance the electrical conductivity. Moreover, the unique hierarchical architecture provides several advantages. At the meantime, in order to effectively utilize all the desired functions of each component, we are focusing on studying the synthesis-structure-function relationship of the proposed ternary nanocomposites. A clearer understanding has achieved on the effects of conductive polymers on electrical conductivity and interpenetrating structure with hierarchical porous channels on electrolyte transport efficiency.

More specifically, nanocomposites with active cathode V₂O₅ nanowires/conductive polypyrrole were prepared. Firstly, V₂O₅ nanowires were synthesized by a hydrothermal process. In the next step, we synthesized conductive PPy to glue V₂O₅ nanowires together by a redox process in aqueous solution. We have chemically and structurally characterized the nanocomposites using different techniques. XRD, SEM and STEM provide information about the phase, size and shape of the components of the nanocomposites (Figure 1). We have also measured the electrochemical properties of the nanocomposites. Electrochemical results demonstrate that our prepared V2O5/PPy nanocomposite exhibits excellent cycling and rate behavior, and is a promising candidate as an anode material for supercapacitors (Figure 2). This information is correlated with the chemical and structural information.

On the other hand, the miniaturization of supercapacitors and LIBs has not kept pace with advances of microelectromechanical systems, biomedical and autonomous devices, whose desired on-board power storage happens in exceptionally small geometric scales. In our study, we have demonstrated that hree-dimensional (3D) ZnO/MnO₂ core/shell branched nanowire arrays exhibit five times higher areal capacitance, better rate performance and smaller inner resistance than their nanowire array counterparts (Figure 3). These novel 3D architectures offer promising designs for powering microelectronics and other autonomous devices with exceptionally small geometric scales.

At the meantime, a large-scale production process composed of a novel ForceSpinning^® technology followed by relatively low-temperature calcination (450 ℃) was developed for a flexible/bendable energy storage material of heterogeneous vanadium oxide/polyvinylpyrrolidone derivative nanofibers with bark-like topography (Figure 4). It overcomes the low fabrication-efficiency and cost-performance shortcomings of previous techniques. The mechanical flexibility from moderately cross-linked polymeric backbones and the mixed valence-induced high electronic conductivity (4.48×10⁴ S m^-1) from vanadium oxide concurrently endow these nanohybrid fibers as high performance flexible electrode materials for LIBs.

Moreover, a series of 1D nanoparticle-assembled TiO₂ fibers with tunable polymorphs were prepared via the novel and large scale ForceSpinning^® process of titanium tetraisopropoxide (TTIP)/polyvinylpyrrolidone (PVP) precursor fibers followed with a thermal treatment at various calcination temperatures (Figure 5). The influence of polymorphic phase of the TiO₂ fibers on the electrochemical performance was investigated. The polymorphic amorphous/anatase/rutile TiO₂ fibers prepared at 450 ℃ achieved a highest capacitance of 21.2 F g^-1 (6.61 mF cm^-2) at a current density of 200 mA g^-1. This work helps bridge the gap between nanoscience and manufacturing. It also makes polymorphism control of functional materials a potential strategy for further improving supercapacitive output of metal oxides.

During the Year Two of this program, two undergraduate students (Edna Garcia and Aleksey Altecor as volunteers) and one M.S. graduate student (Xing Sun) have worked on the program with interest in pursuing a doctoral degree in the field of physical science and materials engineering. I have trained them on synthesis, surface functionalization, and structural and property characterization of functional nanomaterials. Now Aleksey Altecor has enrolled into the graduate school at Rice University.

The students and the PI have attended three conferences and given five presentations: 1. Y. Mao, 37^th International Conference on Advanced Ceramics and Composites, Daytona Beach, FL, January 27-Feburary, 1 2013 (invited); 2. E. Garcia, Y. Mao, 142^nd Minerals, Metals & Materials Society (TMS) Annual Meeting, San Antonio, TX (March 3-7, 2013); 3. X. Sun, Y. Mao, 142^nd Minerals, Metals & Materials Society (TMS) Annual Meeting, San Antonio, TX (March 3-7, 2013); 4. Q. Li, A. Altecor, K. Lozano, Y. Mao, 142^nd Minerals, Metals & Materials Society (TMS) Annual Meeting, San Antonio, TX (March 3-7, 2013); 5. Y. Mao, Xing Sun, Nanotech Conference & Expo 2013, National Harbor, MD (May 12-17, 2013).

Figure 1. (A) SEM image of as prepared V₂O₅ nanoribbons; (B, C and D) SEM, STEM, and TEM image of as-prepared V₂O₅/PPy, respectively; (E) XRD pattern of V₂O₅ nanoribbons; and (F) Raman spectra of V₂O₅nanoribbons.

Figure 2. (A) Capacitance vs ratio of PPy/V₂O₅ (wt%). (B) Specific capacitance vs current density for the composites with different PPy/V₂O₅ (wt%) ratio. (C) Charge-dicharge curves of V₂O₅ coated with 5%(wt) PPy at different current densities. (D) Nyquist plots.

Figure 3 (a) CV curves of the 3D ZnO/MnO₂ core/shell nanoforest electrode; (b) CV curves of different working electrodes; (c) charge/discharge (CD) curves of different working electrodes at current density of 0.02 mA/cm²; (d) specific discharging capacitances at different current densities of electrodes.

Figure 4 (A-C) TEM of the nanohybrid fibers. (B & C) High-resolution TEM of the nanorods on the nanohybrid fibers. The inset shows the SAED pattern. (D) CV curves of the nanohybrid fibers before and after rolling at 50 mV s^-1. (E) Cycling performance of the nanohybrid fibers up to 30 cycles at 100 mA g^-1.

Figure 5 (A) XRD patterns of the TiO₂ fibers obtained after calcining the TTIP/PVP microfibers at different temperatures. Electrochemical properties of the TiO₂ fibers. CV curves of (B) the TF400-650 samples at a scan rate of 100 mV s^-1 and (C) the TF450 at various scan rates. Galvanostaic charge/discharge measurements of (D) the TF400-650 at a current density of 200 mA g^-1and (E) the TF450 at various current densities.