Advances in precursor system for silica-based aerogel production toward improved mechanical properties, customized morphology, and multifunctionality: A review.

Conventional silica-based aerogels are among the most promising materials considering their special properties, such as extremely low thermal conductivity (~15 mW/mK) and low-density (∼0.003-0.5 g.cm-3) as well as high surface area (500-1200 m2. g-1). However, they have relatively low mechanical properties and entail extensive and energy-consuming processing steps. Silica-based aerogels are mostly fragile and possess minimal mechanical properties as well as a long processing procedure which hinders their application range. The key point in improving the mechanical properties of such a material is to increase the connectivity in the aerogel backbone. Several methods of mechanical improvement of silica-based aerogels have been explored by researchers such as (i) use of flexible silica precursors in silica gel backbone, (ii) surface-crosslinking of silica particles with a polymer, (iii) prolonged aging step in different solutions, (iv) distribution of flexible nanofillers into the silica solution prior to gelation, and, most recently, (v) polymerizing the silica precursor prior to gelation. The polymerized silica precursor, as in the most recent approach, can be gelled either by binodal decomposition (nucleation and growth), resulting in a particulate structure, or by spinodal decomposition, resulting in a non-particulate structure. By optimizing the material composition and processing conditions of materials, the aerogel can be tailored with different functional capabilities. This review paper presents a literature survey of precursor modification toward increased connectivity in the backbone, and the synthesis of inorganic and hybrid systems containing siloxane in the backbone of the silica-based aerogels and its composite version with carbon nanofillers. This review also explains the novel properties and applications of these material systems in a wide area. The relationship among the materials-processing-structure-properties in these kinds of aerogels is the most important factor in the development of aerogel products with given morphologies (particulate, fiber-like, or non-particulate) and their resultant properties. This approach to advancing precursor systems leads to the next-generation, multifunctional silica-based aerogel materials.

Formation of Nanocomposite Solid Oxide Fuel Cell Cathodes by Preferential Clustering of Cations from a Single Polymeric Precursor.

Conventional composite cathodes used in solid oxide fuel cells (SOFCs) are fabricated by co-sintering of electrocatalyst and ionic conductor powders at 1100-1250 °C. The relatively high-temperature heat treatments required to ensure bonding among the powders and between the powders and electrolyte results in the formation of resistive phases and coarse microstructures corresponding to short triple-phase boundary (TPB) length and, consequently, low oxygen reduction activity. In the present work, to achieve long TPBs and avoid resistive phase formation, we propose to fabricate nanocomposite La0.8Sr0.2MnO3-Ce0.8Sm0.2O2 (LSM-SDC) and La0.8Ca0.2MnO3-Ce0.8Sm0.2O2 (LCM-SDC) thin film cathodes by a low-temperature method, which involves the use of a single polymeric precursor solution containing all the respective cations. Owing to the molecular level mixing and the liquid lack of any powder-based starting material, we envision that preferential clustering of cations forming nanoscale electrocatalyst and ionic conductor particles will take place upon heat treatment at relatively low temperatures of 600-800 °C. Here, we report for the first time in the literature, a correlation between the heat-treatment temperature-phase evolution-cluster formation-surface chemistry evolution and electrochemical activity of nanocomposite thin film cathodes fabricated from a single polymeric precursor. Our experiments reveal that highest electrochemical activity is achieved when the electrocatalyst phase is poorly crystallized, complete clustering of cations takes place, and A-site dopant segregation at the surface is minimal.

Mesopore-Rich Activated Carbons for Electrical Double-Layer Capacitors by Optimal Activation Condition.

In this study, activated polymer-based hard carbon using steam activation (APHS) with mesopore-rich pore structures were prepared for application as electrodes in electrical double-layer capacitors (EDLC). The surface morphologies and structural characteristics of APHS were observed using scanning electron microscopy and X-ray diffraction analysis, respectively. The textural properties were described using Brunauer-Emmett-Teller and Barrett-Joyner-Halenda equations with N2/77 K adsorption isotherms. APHS were prepared under various steam activation conditions to find optimal ones, which were then applied as electrode materials for the EDLC. The observed specific surface areas and total pore volumes of the APHS were in the range 1170-2410 m2/g and 0.48-1.22 cm3/g, respectively. It was observed that pore size distribution mainly depended on the activation time and temperature, and that the volume of pores with size of 1.5-2.5 nm was found to be a key factor determining the electrochemical capacity.

A Facile Approach towards Fluorescent Nanogels with AIE-Active Spacers.

A facile and efficient approach for design and synthesis of organic fluorescent nanogels has been developed by using a pre-synthesized polymeric precursor. This strategy is achieved by two key steps: (i) precise synthesis of core⁻shell star-shaped block copolymers with crosslinkable AIEgen-precursor (AIEgen: aggregation induced emission luminogen) as pending groups on the inner blocks; (ii) gelation of the inner blocks by coupling the AIEgen-precursor moieties to generate AIE-active spacers, and thus, fluorescent nanogel. By using this strategy, a series of star-shaped block copolymers with benzophenone groups pending on the inner blocks were synthesized by grafting from a hexafunctional initiator through atom transfer radical copolymerization (ATRP) of 4-benzoylphenyl methacrylate (BPMA) or 2-(4-benzoylphenoxy)ethyl methacrylate (BPOEMA) with methyl methacrylate (MMA) and tert-butyldimethylsilyl-protected 2-hydroxyethyl methacrylate (ProHEMA) followed by a sequential ATRP to grow PMMA or PProHEMA. The pendent benzophenone groups were coupled by McMurry reaction to generate tetraphenylethylene (TPE) groups which served as AIE-active spacers, affording a fluorescent nanogel. The nanogel showed strong emission not only at aggregated state but also in dilute solution due to the strongly restricted inter- and intramolecular movement of TPE moiety in the crosslinked polymeric network. The nanogel has been used as a fluorescent macromolecular additive to fabricate fluorescent film.