Heonjoo Ha

Mechanically Stable Thermally Crosslinked Poly(acrylic acid)/Reduced Graphene Oxide Aerogels

Graphene oxide (GO) aerogels, high porosity (>99%) low density (∼3-10 mg cm(-3)) porous materials with GO pore walls, are particularly attractive due to their lightweight, high surface area, and potential use in environmental remediation, superhydrophobic and superoleophilic materials, energy storage, etc. However, pure GO aerogels are generally weak and delicate which complicates their handling and potentially limits their commercial implementation. The focus of this work was to synthesize highly elastic, mechanically stable aerogels that are robust and easy to handle without substantially sacrificing their high porosity or low density. To overcome this challenge, a small amount of readily available and thermally cross-linkable poly(acrylic acid) (PAA) was intermixed with GO to enhance the mechanical integrity of the aerogel without disrupting other desirable characteristic properties. This method is a simple straightforward procedure that does not include multistep or complicated chemical reactions, and it produces aerogels with mass densities of about 4-6 mg cm(-3) and >99.6% porosity that can reversibly support up to 10,000 times their weight with full recovery of their original volume. Finally, pressure sensing capabilities were demonstrated and their oil absorption capacities were measured to be around 120 g oil per g aerogel(-1) which highlights their potential use in practical applications.

An All-Stretchable-Component Sodium-Ion Full Battery

Stretchable energy-storage devices receive considerable attention due to their promising applications in future wearable technologies. However, they currently suffer from many problems, including low utility of active materials, limited multidirectional stretchability, and poor stability under stretched conditions. In addition, most proposed designs use one or more rigid components that fail to meet the stretchability requirement for the entire device. Here, an all-stretchable-component sodium-ion full battery based on graphene-modified poly(dimethylsiloxane) sponge electrodes and an elastic gel membrane is developed for the first time. The battery exhibits reasonable electrochemical performance and robust mechanical deformability; its electrochemical characteristics can be well-maintained under many different stretched conditions and after hundreds of stretching-release cycles. This novel design integrating all stretchable components provides a pathway toward the next generation of wearable energy devices in modern electronics.

Thermoplastic Elastomer‐Enabled Smart Electrolyte for Thermoresponsive Self‐Protection of Electrochemical Energy Storage Devices

Thermoresponsive smart electrolytes based on Pluronic solution are developed for active control and thermal self-protection of electrochemical energy-storage devices. Mechanistic studies reveal that the highly effective and reversible self-protection behavior is attributed to the sol-gel transition of the Pluronic solution upon temperature change. The transition temperature and the degree of performance suppression can be tuned over a wide range.

Linking Semiconductor Nanocrystals into Gel Networks through All‐Inorganic Bridges

For colloidal semiconductor nanocrystals (NCs), replacement of insulating organic capping ligands with chemically diverse inorganic clusters enables the development of functional solids in which adjacent NCs are strongly coupled. Yet controlled assembly methods are lacking to direct the arrangement of charged, inorganic cluster-capped NCs into open networks. Herein, we introduce coordination bonds between the clusters capping the NCs thus linking the NCs into highly open gel networks. As linking cations (Pt(2+)) are added to dilute (under 1 vol %) chalcogenidometallate-capped CdSe NC dispersions, the NCs first form clusters, then gels with viscoelastic properties. The phase behavior of the gels for variable [Pt(2+)] suggests they may represent nanoscale analogues of bridged particle gels, which have been observed to form in certain polymer colloidal suspensions.

Reduced-Graphene Oxide/Poly(acrylic acid) Aerogels as a Three-Dimensional Replacement for Metal-Foil Current Collectors in Lithium-Ion Batteries

We report the synthesis and properties of a low-density (∼5 mg/cm3) and highly porous (99.6% void space) three-dimensional reduced graphene oxide (rGO)/poly(acrylic acid) (PAA) nanocomposite aerogel as the scaffold for cathode materials in lithium-ion batteries (LIBs). The rGO-PAA is both simple and starts from readily available graphite and PAA, thereby providing a scalable fabrication procedure. The scaffold can support as much as a 75 mg/cm2 loading of LiFePO4 (LFP) in a ∼430 μm thick layer, and the porosity of the aerogel is tunable by compression; the flexible aerogel can be compressed 30-fold (i.e., to as little as 3.3% of its initial volume) while retaining its mechanical integrity. Replacement of the Al foil by the rGO-PAA current collector of the slurry-cast LFP (1.45 ± 0.2 g/cm3 tap density) provides for exemplary mass loadings of 9 mgLFP/cm2 at 70 μm thickness and 1.4 g/cm3 density or 16 mgLFP/cm2 at 100 μm thickness and ∼1.6 g/cm3 density. When compared to Al foil, the distribution of LFP throughout the three-dimensional rGO-PAA framework doubles the effective LFP solution-contacted area at 9 mg/cm2 loading and increases it 2.5-fold at 16 mg/cm2 loading. Overall, the rGO-PAA current collector increases the volumetric capacity by increasing the effective electrode area without compromising the electrode density, which was compromised in past research where the effective electrode area has been increased by reducing the particle size.

Polyhedral Oligomeric Silsesquioxane-Containing Thiol–ene Fibers with Tunable Thermal and Mechanical Properties

Polyhedral oligomeric silsesquioxanes (POSS) are versatile inorganic-organic hybrid building blocks that have potential applications as reinforcement nanofillers, thermal stabilizers, and catalyst supports for metal nanoparticles. However, fabrication of fibrous materials with high POSS content has been a challenge because of the aggregation and solubility limits of POSS units. In this paper, we describe a robust and environmentally friendly fabrication approach of inorganic-organic hybrid POSS fibers by integrating UV initiated thiol-ene polymerization and centrifugal fiber spinning. The use of monomeric liquids in this approach not only reduces the consumption of heat energy and solvent, but it also promotes homogeneous mixing of organic and inorganic components that allows integration of large amount of POSS (up to 80 wt %) into the polymer network. The POSS containing thiol-ene fibers exhibited enhanced thermomechanical properties compared to purely organic analogs as revealed by substantial increases in residual weight and a factor of 4 increase in modulus after thermal treatment at 1000 °C. This simple fabrication approach combined with the tunability in fiber properties afforded by tailoring monomer composition make POSS containing thiol-ene fibers attractive candidates for catalyst supports and filtration media, particularly in high-temperature and harsh environments.

Polymer/graphene oxide (GO) thermoset composites with GO as a crosslinker

Composites have historically been of intense interest due to their potential to replace glass, wood and metal at a fraction of the weight. Of the many choices of fillers, graphene oxide (GO) is generally attractive for its versatility and tunability in terms of surface functionality, size and shape. While the majority of GO has been incorporated into polymers and polymer precursors by physical mixing, this review focuses on research where GO has served as both a property enhancer and multifunctional crosslinker in thermosets. Methods for preparing GO (with and without additional functionalization) and incorporating it appropriately into thermosets are described. A review of characterization techniques, typically applied before and after GO is incorporated into thermosets and analytical methods for confirming important chemical reactions during crosslinking, is also provided. Finally, the resulting composite thermoset properties are surveyed throughout to connect preparation and characterization methods to their potential practical importance.