Cao Guan

A New Type of Porous Graphite Foams and Their Integrated Composites with Oxide/Polymer Core/Shell Nanowires for Supercapacitors: Structural Design, Fabrication, and Full Supercapacitor Demonstrations

We attempt to meet the general design requirements for high-performance supercapacitor electrodes by combining the strategies of lightweight substrate, porous nanostructure design, and conductivity modification. We fabricate a new type of 3D porous and thin graphite foams (GF) and use as the light and conductive substrates for the growth of metal oxide core/shell nanowire arrays to form integrated electrodes. The nanowire core is Co3O4, and the shell is a composite of conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT) and metal oxide (MnO2). To show the advantage of this integrated electrode design (viz., GF + Co3O4/PEDOT-MnO2 core/shell nanowire arrays), three other different less-integrated electrodes are also prepared for comparison. Full supercapacitor devices based on the GF + Co3O4/PEDOT-MnO2 as positive electrodes exhibit the best performance compared to other three counterparts due to an optimal design of structure and a synergistic effect.

Synthesis of Free‐Standing Metal Sulfide Nanoarrays via Anion Exchange Reaction and Their Electrochemical Energy Storage Application

Metal sulfides are an emerging class of high-performance electrode materials for solar cells and electrochemical energy storage devices. Here, a facile and powerful method based on anion exchange reactions is reported to achieve metal sulfide nanoarrays through a topotactical transformation from their metal oxide and hydroxide preforms. Demonstrations are made to CoS and NiS nanowires, nanowalls, and core-branch nanotrees on carbon cloth and nickel foam substrates. The sulfide nanoarrays exhibit superior redox reactivity for electrochemical energy storage. The self-supported CoS nanowire arrays are tested as the pseudo-capacitor cathode, which demonstrate enhanced high-rate specific capacities and better cycle life as compared to the powder counterparts. The outstanding electrochemical properties of the sulfide nanoarrays are a consequence of the preservation of the nanoarray architecture and rigid connection with the current collector after the anion exchange reactions.

Solution synthesis of metal oxides for electrochemical energy storage applications

This article provides an overview of solution-based methods for the controllable synthesis of metal oxides and their applications for electrochemical energy storage. Typical solution synthesis strategies are summarized and the detailed chemical reactions are elaborated for several common nanostructured transition metal oxides and their composites. The merits and demerits of these synthesis methods and some important considerations are discussed in association with their electrochemical performance. We also propose the basic guideline for designing advanced nanostructure electrode materials, and the future research trend in the development of high power and energy density electrochemical energy storage devices.

Nanoporous walls on macroporous foam : rational design of electrodes to push areal pseudocapacitance

Supercapacitors, also known as electrochemical capacitors, are considered the most promising energy storage devices owing to their high power densities and long lifespan. [ 3–5 ] The fast charge and discharge capability make supercapacitors favorable for applications in hybrid vehicles, portable electronics, and backup energy systems. [ 6–10 ] Carbonaceous materials, including carbon nanotubes and graphene, are being widely studied as alternatives to conventional graphites. [ 2 , 11–14 ] However, carbon-based materials usually show low energy density as they store charges electrostatically at their surfaces, so they have intrinsically low specifi c areal capacitance ( C a ) in the range of 10 − 40 μ F cm − 2 . Transition metal oxides/hydroxides store charges with surface faradaic (redox) reactions, which enable higher energy density compared to carbon. Metal oxides/hydroxides such as MnO 2 , NiO, Ni(OH) 2 , CoO x and their compounds have recently come into focus in the design of high-energy-density charge-storage materials. [ 15–27 ] Despite these efforts, practical energy storage applications still require higher specifi c capacitance. One way out is to design nanometer-scale electrode materials with very large surface areas and structural stability. In this context, porous nanostructures are of great interest because they can reduce ionic and electronic diffusion distance and provide large electrode/electrolyte contact area. For example, porous Ni and Au electrodes as current collectors have recently been reported, which signifi cantly improve the specifi c capacitance when covered with the pseudoactive material MnO 2 . [ 28 , 29 ] Also, nanoporous graphene electrodes with ∼ 4 nm pores drastically enhance the specifi c capacitance up to 166 F g − 1 . [ 30 ] For metal oxides,

A general strategy toward graphene@metal oxide core–shell nanostructures for high-performance lithium storage

We demonstrate a simple, efficient, yet versatile method for the realization of core–shell assembly of graphene around various metal oxide (MO) nanostructures, including nanowires (NWs) and nanoparticles (NPs). The process is driven by (i) the ring-opening reaction between the epoxy groups and amine groups in graphene oxide (GO) platelets and amine-modified MO nanostructures, respectively, and (ii) electrostatic interaction between these two components. Nearly every single NW or NP is observed to be wrapped by graphene. To the best of our knowledge, this is the first report that substrate-supported MO NWs are fully coated with a graphene shell. As an example of the functional properties of these compound materials, the graphene@α-Fe2O3 core–shell NPs are investigated as the lithium-ion battery (LIB) electrode, which show a high reversible capacity, improved cycling stability, and excellent rate capability with respect to the pristine α-Fe2O3. The superior performance of the composite electrode is presumably attributed to the effectiveness of the graphene shell in preventing the aggregation, buffering the volume change, maintaining the integrity of NPs, as well as improving the conductivity of the electrode.

Iron Oxide-Decorated Carbon for Supercapacitor Anodes with Ultrahigh Energy Density and Outstanding Cycling Stability

Supercapacitor with ultrahigh energy density (e.g., comparable with those of rechargeable batteries) and long cycling ability (>50000 cycles) is attractive for the next-generation energy storage devices. The energy density of carbonaceous material electrodes can be effectively improved by combining with certain metal oxides/hydroxides, but many at the expenses of power density and long-time cycling stability. To achieve an optimized overall electrochemical performance, rationally designed electrode structures with proper control in metal oxide/carbon are highly desirable. Here we have successfully realized an ultrahigh-energy and long-life supercapacitor anode by developing a hierarchical graphite foam-carbon nanotube framework and coating the surface with a thin layer of iron oxide (GF-CNT@Fe2O3). The full cell of anode based on this structure gives rise to a high energy of ∼74.7 Wh/kg at a power of ∼1400 W/kg, and ∼95.4% of the capacitance can be retained after 50000 cycles of charge-discharge. These performance features are superior among those reported for metal oxide based supercapacitors, making it a promising candidate for the next generation of high-performance electrochemical energy storage.

Highly Stable and Reversible Lithium Storage in SnO2 Nanowires Surface Coated with a Uniform Hollow Shell by Atomic Layer Deposition

SnO2 nanowires directly grown on flexible substrates can be a good electrode for a lithium ion battery. However, Sn-based (metal Sn or SnO2) anode materials always suffer from poor stability due to a large volume expansion during cycling. In this work, we utilize atomic layer deposition (ALD) to surface engineer SnO2 nanowires, resulting in a new type of hollowed SnO2-in-TiO2 wire-in-tube nanostructure. This structure has radically improved rate capability and cycling stability compared to both bare SnO2 nanowires and solid SnO2@TiO2 core-shell nanowire electrodes. Typically a relatively stable capacity of 393.3 mAh/g has been achieved after 1000 charge-discharge cycles at a current density of 400 mA/g, and 241.2 mAh/g at 3200 mA/g. It is believed that the uniform hollow TiO2 shell provides stable surface protection and the appropriate-sized gap effectively accommodates the expansion of the interior SnO2 nanowire. This ALD-enabled method should be general to many other battery anode and cathode materials, providing a new and highly reproducible and controllable technique for improving battery performance.

Hollow core–shell nanostructure supercapacitor electrodes: gap matters

Hollow core–shell nanorods with a nanogap are designed and constructed with the assistance of atomic layer deposition (ALD) for energy storage applications. As a demonstration, CoO nanorods and NiO nanowalls are enclosed by a TiO2 nanotube shell, forming the “wire in tube” and “wall in box” structures, respectively. A thin sacrificial layer of Al2O3 is deposited by ALD and removed eventually, forming a nanogap between the CoO core (or the NiO nanowall) and the TiO2 shell. When they are tested as supercapacitor electrodes, an evident difference between the solid core–shell nanostructure and hollow ones can be found; for example, the hollow structure shows ∼2 to 4 times the capacitance compared to the solid wires. The electrochemical properties are also superior compared to the bare nanorods without the nanotube shell. The enhancement is ascribed to the conformal hollow design which provides enlarged specific surface areas and a shorter ion transport path. It is prospected that such a positive nanogap effect may also exist in other electrochemical cell electrodes such as lithium ion batteries and fuel cells.

A High Energy and Power Li‐Ion Capacitor Based on a TiO2 Nanobelt Array Anode and a Graphene Hydrogel Cathode

A novel hybrid Li-ion capacitor (LIC) with high energy and power densities is constructed by combining an electrochemical double layer capacitor type cathode (graphene hydrogels) with a Li-ion battery type anode (TiO(2) nanobelt arrays). The high power source is provided by the graphene hydrogel cathode, which has a 3D porous network structure and high electrical conductivity, and the counter anode is made of free-standing TiO(2) nanobelt arrays (NBA) grown directly on Ti foil without any ancillary materials. Such a subtle designed hybrid Li-ion capacitor allows rapid electron and ion transport in the non-aqueous electrolyte. Within a voltage range of 0.0-3.8 V, a high energy of 82 Wh kg(-1) is achieved at a power density of 570 W kg(-1). Even at an 8.4 s charge/discharge rate, an energy density as high as 21 Wh kg(-1) can be retained. These results demonstrate that the TiO(2) NBA//graphene hydrogel LIC exhibits higher energy density than supercapacitors and better power density than Li-ion batteries, which makes it a promising electrochemical power source.

A Flexible Quasi‐Solid‐State Nickel–Zinc Battery with High Energy and Power Densities Based on 3D Electrode Design

A flexible quasi-solid-state Ni-Zn battery is developed by using tiny ZnO nanoparticles and porous ultrathin NiO nanoflakes conformally deposited on hierar chical carbon-cloth-carbon-fiber (CC-CF) as the anode (CC-CF@ZnO) and cathode (CC-CF@NiO), respectively. The device is able to deliver high performance (absence of Zn dendrite), superior to previous reports on aqueous Ni-Zn batteries and other flexible electrochemical energy-storage devices.