Effect of Sheet Morphology on the Scalability of Graphene-Based Ultracapacitors

Graphene is considered a promising ultracapacitor material toward high power and energy density because of its high conductivity and high surface area without pore tortuosity. However, the two-dimensional (2D) sheets tend to aggregate during the electrode fabrication process and align perpendicular to the flow direction of electrons and ions, which can reduce the available surface area and limit the electron and ion transport. This makes it hard to achieve scalable device performance as the loading level of the active material increases. Here, we report a strategy to solve these problems by transforming the 2D graphene sheet into a crumpled paper ball structure. Compared to flat or wrinkled sheets, the crumpled graphene balls can deliver much higher specific capacitance and better rate performance. More importantly, devices made with crumpled graphene balls are significantly less dependent on the electrode mass loading. Performance of graphene-based ultracapacitors can be further enhanced by using flat graphene sheets as the binder for the crumpled graphene balls, thus eliminating the need for less active binder materials

High-Yield Spreading of Water-Miscible Solvents on Water for Langmuir–Blodgett Assembly

Langmuir–Blodgett (LB) assembly is a classical molecular thin-film processing technique, in which the material is spread onto water surface from a volatile, water-immiscible solvent to create floating monolayers that can be later transferred to solid substrates. LB has also been applied to prepare colloidal thin films with an unparalleled level of microstructural control and thickness, which has enabled the discovery of many exciting collective properties of nanoparticles and the construction of bulk nanostructured materials. To maximize the benefits of LB assembly, the nanoparticles should be well dispersed in both the spreading solvent and on water. This is quite challenging since colloids usually need contrasting surface properties in order to be stable in the water-hating organic solvents and on water surface. In addition, many organic and polymeric nanostructures dissolve in those organic solvents and cannot be processed directly. Using water-liking spreading solvents can avoid this dilemma. However, spreading of water-miscible solvents on water surface is fundamentally challenging due to extensive mixing, which results in significant material loss. Here we report a conceptually simple strategy and a general technique that allows nearly exclusive spreading of such solvents on water surface using electrospray. Since the volume of these aerosolized droplets is reduced by many orders of magnitude, they are readily depleted during the initial spreading step before any significant mixing could occur. The new strategy drastically reduces the burden of material processing prior to assembly and broadens the scope of LB assembly to previously hard-to-process materials. It also avoids the use of toxic volatile organic spreading solvents, improves the reproducibility, and can be readily automated, making LB assembly a more robust tool for colloidal assembly and thin-film fabrication

Crumpled Graphene-Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes

Submicrometer-sized capsules made of Si nanoparticles wrapped by crumpled graphene shells were made by a rapid, one-step capillary-driven assembly route in aerosol droplets. Aqueous dispersion of micrometer-sized graphene oxide (GO) sheets and Si nanoparticles were nebulized to form aerosol droplets, which were passed through a preheated tube furnace. Evaporation-induced capillary force wrapped graphene (a.k.a., reduced GO) sheets around the Si particles, and heavily crumpled the shell. The folds and wrinkles in the crumpled graphene coating can accommodate the volume expansion of Si upon lithiation without fracture, and thus help to protect Si nanoparticles from excessive deposition of the insulating solid electrolyte interphase. Compared to the native Si particles, the composite capsules have greatly improved performance as Li ion battery anodes in terms of capacity, cycling stability, and Coulombic efficiency

Investigation on the Correlations between Droplet and Particle Size Distribution in Ultrasonic Spray Pyrolysis

In this study, the measurement of droplet size distribution by means of a laser diffraction technique in ultrasonic spray pyrolysis was investigated. Effects of precursor temperature, spray volume rate, carrier gas flow rate, and physicochemical properties of precursors were analyzed in detail. The results showed that the mean size of precursor droplets decreased with increasing precursor temperature, concentration, and addition of alcohol, while it increased with increasing spray volume rate and carrier gas flow rate. Bimodal droplet size distribution was observed probably due to droplet coagulation effect. A cyclone or an impactor was used to control the droplet size distribution. Uniform droplets were obtained using a cyclone that removed large droplets. For comparison, spray pyrolysis of salt aqueous solutions, such as ZrO(NO3)2 and Ni(NO3)2, were also conducted. Particle size distribution of the spray pyrolyzed powders was analyzed using scanning electron microscopy. The results showed that the particle size distribution of prepared solid particles was strongly dependent on, and could be controlled by, variation in the corresponding properties of the sprayed droplets

Synthesis of 3D Silver-Graphene-Titanium Dioxide Composite via Aerosol Spray Pyrolysis for Sensitive Glucose Biosensor

<div><p>A sensitive glucose biosensor was developed based on the adsorption of glucose oxidase by a three-dimensional silver-graphene-titanium dioxide (3D Ag-GR-TiO₂) composite electrode. Aerosol spray pyrolysis was employed to synthesize the 3D Ag-GR-TiO₂ composite using a colloidal mixture of a silver acetate precursor (C₂H₃AgO₂), graphene oxide, and TiO₂ nanoparticles. The effects of the operating temperature, gas flowrate, and TiO₂ concentration on the particle properties were investigated. The particle morphology of all 3D Ag-GR-TiO₂ composites was spherical in shape. The average sizes of composites could be controlled from 0.45 to 0.64 μm with the variation of process variables. Ag nanoparticles less than 10 nm in diameter were deposited on the surfaces of the TiO₂ nanoparticles and GR after a reduction process. The characteristics of the glucose biosensor fabricated with the as-prepared 3D Ag-GR-TiO₂ composite were assessed through cyclic voltammetry measurements. The biosensor exhibited a high current flow as well as clear redox peaks, resulting in a superior ability of the catalyst in terms of the electrochemical reactions. The highest sensitivity of glucose biosensor was obtained by 3D Ag-GR-TiO₂ composite, which was 12.2 μA/mM·cm², among 3D Ag-GR-TiO₂, 3D Ag-GR, and 3D GR-TiO₂ composites.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets

Unlike flat sheets, crumpled paper balls have both high free volume and high compressive strength, and can tightly pack without significantly reducing the area of accessible surface. Such properties would be highly desirable for sheet-like materials such as graphene, since they tend to aggregate in solution and restack in the solid state, making their properties highly dependent on the material processing history. Here we report the synthesis of crumpled graphene balls by capillary compression in rapidly evaporating aerosol droplets. The crumpled particles are stabilized by locally folded, π–π stacked ridges as a result of plastic deformation, and do not unfold or collapse during common processing steps. In addition, they are remarkably aggregation-resistant in either solution or solid state, and remain largely intact and redispersible after chemical treatments, wet processing, annealing, and even pelletizing at high pressure. For example, upon compression at 55 MPa, the regular flat graphene sheets turn into nondispersible chunks with drastically reduced surface area by 84%, while the crumpled graphene particles can still maintain 45% of their original surface area and remain readily dispersible in common solvents. Therefore, crumpled particles could help to standardize graphene-based materials by delivering more stable properties such as high surface area and solution processability regardless of material processing history. This should greatly benefit applications using bulk quantities of graphene, such as in energy storage or conversion devices. As a proof of concept, we demonstrate that microbial fuel electrodes modified by the crumpled particles indeed outperform those modified with their flat counterparts

Aerosol Synthesis of Cargo-Filled Graphene Nanosacks

Water microdroplets containing graphene oxide and a second solute are shown to spontaneously segregate into sack-cargo nanostructures upon drying. Analytical modeling and molecular dynamics suggest the sacks form when slow-diffusing graphene oxide preferentially accumulates and adsorbs at the receding air–water interface, followed by capillary collapse. Cargo-filled graphene nanosacks can be nanomanufactured by a simple, continuous, scalable process and are promising for many applications where nanoscale materials should be isolated from the environment or biological tissue