Fuxin Wang

Three dimensional architectures: design, assembly and application in electrochemical capacitors

Currently, supercapacitors (SCs) are considered to be one of the most promising energy storage devices, mainly due to their unique properties such as high output power, long cycling stability, and fast charge/discharge capability. Nevertheless, the low energy density of SCs still limits their promotion in practical applications. Given this, designing three dimensional (3D) architectures for SC electrodes is perceived as an efficient strategy because well-constructed 3D structures could enable reduced “dead surface”, good electron transport kinetics, hierarchical porous channels and short ionic diffusion distances. This review aims to describe the current progress of different synthetic processes with respect to the preparation of 3D SC electrodes and focuses on both template-assisted strategies and non-template strategies. We summarize recently proposed methods, novel structures, and electrochemical performances for these 3D electrodes. The advantages and disadvantages accompanying them are also analyzed. Finally, we discuss the challenges and prospects of the fabrication of 3D SC electrodes.

Carbon Quantum Dot Surface-Engineered VO2 Interwoven Nanowires: A Flexible Cathode Material for Lithium and Sodium Ion Batteries

The use of electrode materials in their powdery form requires binders and conductive additives for the fabrication of the cells, which leads to unsatisfactory energy storage performance. Recently, a new strategy to design flexible, binder-, and additive-free three-dimensional electrodes with nanoscale surface engineering has been exploited in boosting the storage performance of electrode materials. In this paper, we design a new type of free-standing carbon quantum dot coated VO2 interwoven nanowires through a simple fabrication process and demonstrate its potential to be used as cathode material for lithium and sodium ion batteries. The versatile carbon quantum dots that are vastly flexible for surface engineering serve the function of protecting the nanowire surface and play an important role in the diffusion of electrons. Also, the three-dimensional carbon cloth coated with VO2 interwoven nanowires assisted in the diffusion of ions through the inner and the outer surface. With this unique architecture, the carbon quantum dot nanosurface engineered VO2 electrode exhibited capacities of 420 and 328 mAh g(-1) at current density rate of 0.3 C for lithium and sodium storage, respectively. This work serves as a milestone for the potential replacement of lithium ion batteries and next generation postbatteries.

MnO2 nanomaterials for flexible supercapacitors: performance enhancement via intrinsic and extrinsic modification

Increasing power and energy demands for next-generation portable and flexible electronics have raised critical requirements (flexibility, stretch-ability, environmental friendliness, lightweight, etc.) for the energy storage devices. Flexible supercapacitors (SCs), as one of the most promising next-generation energy storage devices, have stimulated intensive interest owing to their outstanding features including small size, low weight, ease of handling, excellent reliability, and high power density. Manganese oxide (MnO2), has attracted much interest in the development of flexible SCs with high electrochemical performance. Yet, the poor electronic and ionic transport in MnO2 electrodes still limits its promotion in practical applications. This review aims to describe the recent progress in the application of MnO2 materials in the development of flexible SCs and summarizes the intrinsic modification of MnO2via crystallinity, crystal structure, and oxygen vacancy introduction and the extrinsic modification of MnO2via non-three-dimensional (3D) and 3D flexible conductive scaffolds for high performance flexible SCs. Moreover, we also discuss briefly on the current challenges, future directions, and opportunities for the development of high-performance MnO2 based flexible SCs.

Photohole Induced Corrosion of Titanium Dioxide: Mechanism and Solutions

Titanium dioxide (TiO2) has been extensively investigated as photoanode for water oxidation, as it is believed to be one of the most stable photoanode materials. Yet, we surprisingly found that TiO2 photoanodes (rutile nanowire, anatase nanotube, and P25 nanoparticle film) suffered from substantial photocurrent decay in neutral (Na2SO4) as well as basic (KOH) electrolyte solution. Photoelectrochemical measurements togehter with electron microscopy studies performed on rutile TiO2 nanowire photoanode show that the photocurrent decay is due to photohole induced corrosion, which competes with water oxidation reaction. Further studies reveal that photocurrent decay profile in neutral and basic solutions are fundamentally different. Notably, the structural reconstruction of nanowire surface occurs simultaneously with the corrosion of TiO2 in KOH solution resulting in the formation of an amorphous layer of titanium hydroxide, which slows down the photocorrosion. Based on this discovery, we demonstrate that the photoelectrochemical stability of TiO2 photoanode can be significantly improved by intentionally coating an amorphous layer of titanium hydroxide on the nanowire surface. The pretreated TiO2 photaonode exhibits an excellent photocurrent retention rate of 97% after testing in KOH solution for 72 h, while in comparison the untreated sample lost 10-20% of photocurrent in 12 h under the same measurement conditions. This work provides new insights in understanding of the photoelectrochemical stability of bare TiO2 photoanodes.

In Situ Hydrothermally Grown TiO2@C Core-Shell Nanowire Coating for Highly Sensitive Solid Phase Microextraction of Polycyclic Aromatic Hydrocarbons

Nanostructured materials have great potential for solid phase microextraction (SPME) on account of their tiny size, distinct architectures and superior physical and chemical properties. Herein, a core-shell TiO2@C fiber for SPME was successfully fabricated by the simple hydrothermal reaction of a titanium wire and subsequent amorphous carbon coating. The readily hydrothermal procedure afforded in situ synthesis of TiO2 nanowires on a titanium wire and provided a desirable substrate for further coating of amorphous carbon. Benefiting from the much larger surface area of subsequent TiO2 and good adsorption property of the amorphous carbon coating, the core-shell TiO2@C fiber was utilized for the SPME device for the first time and proved to have better performance in extraction of polycyclic aromatic hydrocarbons. In comparison to the polydimethylsiloxane (PDMS) and PDMS/divinylbenzene (DVB) fiber for commercial use, the TiO2@C fiber obtained gas chromatography responses 3-8 times higher than those obtained by the commercial 100 μm PDMS and 1-9 times higher than those obtained by the 65 μm PDMS/DVB fiber. Under the optimized extraction conditions, the low detection limits were obtained in the range of 0.4-7.1 ng L-1 with wider linearity in the range of 10-2000 ng L-1. Moreover, the fiber was successfully used for the determination of polycyclic aromatic hydrocarbons in Pearl River water, which demonstrated the applicability of the core-shell TiO2@C fiber.