59th Annual Report on Research 2014

Narrative Report

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 and conversion devices. In this study, we developed hierarchical vanadium oxide V₂O₅ nanowires/conductive polypyrrole (PPy) nanocomposites for LIB cathodes, to deliver excellent specific capacitances, better rate capability, lower inner resistance, and promising potentials to advance energy density of supercapacitors. To investigate the synergistic effect of nanocomposites on properties and the role of each component optimize the co-operation of distinct materials, a classic model was applied to stimulate and analyze the partitioned contributions from electric double-layer capacitance and pseudocapacitance of the composites with altering PPy amount. The optimal ratio of PPy to V₂O₅ was determined at 40% where the sum of polymer’s electric double-layer (EDL) capacitance and layered oxide’s subsurface Faradic capacitance could be maximized (Figure 1). Over loading of PPy weakened the adherence between electroactive materials and current collector and therefore restrained the total performances of electrodes. This work lays the foundation for outperformed hybrid electrode materials consisting of conductive polymer and transition metal oxide which applied in aqueous supercapacitors.

At the meantime, a large-scale production process composed of a novel Forcespinning^® technology followed by calcination (450 °C) was developed for a flexible/bendable energy storage material (Figure 2). 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, TiO₂ fibers with tunable polymorphs were prepared via the ForceSpinning^® process followed with a thermal treatment at various calcination temperatures (Figure 3). 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.

On the other hand, converting solar energy into other easily usable forms has attracted considerable interest in the last several decades. Recently, we have developed a robust and scalable fabrication process to form “caterpillar-like” branched ZnO nanostructures. As shown in Figure 4, our “caterpillar-like” branched ZnO nanostructures over perform other two types of ZnO nanostructures with its maximum current density as 0.524 mA cm^-2 at +1.2 V (vs. Ag/AgCl). This is 151% higher than that of the ZnO NW arrays (0.348 mA cm^-2) tested at the same conditions. The maximum PEC conversion efficiencies (η) for the “caterpillar-like” branched ZnO nanostructures is ~ 0.165% (at 0.89 V vs. RHE), 147% higher than the vertically-aligned NW array counterpart (0.112% at 0.85 vs. RHE).

Our group also conducted morphology-programmed hydrothermal synthesis to fabricate 3-D branched nanotree arrays with well-defined shape and size. Illuminated by the simulated solar light (Figure 5), our willow-like ZnO nanoforest stood out with its maximum current density of 0.919 mA/cm² at +1.2 V (vs. Ag/AgCl), which is more than 267% and 126% advancement in comparison with the ZnO NW arrays and the brush-like nanoforest. Our willow-like nanoforest distinguishes itself with PEC of 0.299% at 0.89 V (vs. RHE) from all previously reported homogeneous ZnO nanostructured photoanodes. With the morphology of ZnO nanoarchitectures transits from the NW arrays to nanoforests, the photoelectron lifetime was evidently prolonged, implying less charge trapping and more efficient charge separation.

During the No-Cost Extension Year of this program, one undergraduate student and two M.S. graduate students have worked on the program with interest in pursuing a doctoral degree. Now two of them have enrolled into the graduate school at Texas A&M University.

We have given thirteen presentations at six conferences. Also, five manuscripts have been peer-reviewed for publication, one is under review, and one is under preparation: 1. X. Sun, Q. Li, J. Jiang, and Y. Mao, “Morphology-tunable synthesis of ZnO nanoforest and its photoelectrochemical performance,” Nanoscale, 2014, 6(15), 8769-8780. DOI: 10.1039/C4NR01146E. 2. Q. Li, X. Sun, K. Lozano, and Y. Mao, “Facile and scalable synthesis of ‘caterpillar-like’ ZnO Nanostructures with enhanced photoelectrochemical water-splitting effect,” J. Phys. Chem. C 2014, 118, 13467-13475. 3. A. Altecor, K. Lozano, and Y. Mao, “Mixed-valent VO_x/polymer nanohybrid fibers for flexible energy storage materials,” Ceramic International 2014, 40, 5073-5077. 4. Y. Mao, “Branched nanostructures for photoelectrochemical water splitting,” Nanomaterials & Energy, 2014, 3(4), 103-128. 5. E. Garcia, Q. Li, X. Sun, K. Lozano, and Y. Mao, “TiO₂ fibers: tunable polymorphic phase transformation and electrochemical properties,” Journal of Nanoscience and Nanotechnology, 2014, in press. 6. X. Sun, J. Jiang, and Y. Mao, The optimal ratio of polypyrrole decoration on V₂O₅ nanofibers as electrode materials for energy storage, submitted (2014). 7. Q. Li, and Y. Mao, Nanoarchitectured Ni(OH)₂ nanotube arrays for high power and high energy asymmetric supercapacitors, under preparation (2014).

Figure 1. (A) Rate capability of V₂O₅@PPy composites arising from CD measurements at different current densities; (B) Rate capability of V₂O₅@PPy composites with different ratios of PPy/V₂O₅ at 0.2 A/g and 1.0 A/g, respectively.

Figure 2 Structural and electrochemical characterization of the nanohybrid fibers.

Figure 3 Characterization of the TiO₂ fibers.

Figure 4. Comparison of photoelectrochemical performance of three types of ZnO nanostructures.

Figure 5. Photoelectrochemical characterization of three nanostructured ZnO architectures.