Inorganic Porous Framework Based Hybrid Materials for Effective Energy and Environment Applications

Abstract

In recent years, the global warming potential (GWP) triggered from the production of greenhouse gases (GHG) has been a significant concern for climate disasters and rising temperatures in the environment. An additional 30-40% of total GHG emissions (carbon dioxide, methane, ozone, nitrous oxide, hydrofluorocarbons, and chlorofluorocarbons) are caused by industrial and residential buildings in developed countries. The rise in the temperature of the earth's surface compelled research scientists and academicians to develop more efficient thermal insulation with higher energy efficiency for green buildings to save energy consumption. With increased space exploration missions in developed countries and heavy-duty construction industries, the demand for high-performance thermal and acoustic insulators tends to increase in efficiency in aerospace structures, spacesuits, industrial buildings, and space satellites. Conventional insulators (glass fiber, mineral wool, gypsum board, extruded polystyrene) have more significant potential in the market share due to their high performance per unit cost. Owing to the low thermal conductivity, the hydrophobic characteristics, non-decomposable nature, and toxicity of such synthetic materials minimize heat gain or loss in green energy buildings. A facile and cost-effective modified sol-gel synthesis is employed to synthesize flexible silica - cellulose hybrid aerogels (SCHA) using recycled cellulose fibers (RCF) of three-dimensional cellular skeletons, Kymene cross-linker, Tetraethylorthosilicate (TEOS) and methyltrimethoxy -silane as filler through simple freeze-drying. The effect of cellulose fiber concentrations, silica concentration, and ambient temperature on the thermal, acoustic, and oil adsorption characteristics was quantified comprehensively. The experiments considered a range of RCF weight fractions from 1 to 4 wt.% with a crosslinker and TEOS as silica precursors. The resultant SCHA aerogels were modified by a sialylation agent with surface hydroxyl groups to achieve superhydrophobic behavior and well impregnation of silica nanoparticles in the 3D cellulose structure. A variety of hierarchical nanoporous silica aerogels with diversified particle distributions were also synthesized from well-dispersed silica sols from traditional sol-gel method trailed by ambient pressure drying (APD), supercritical drying (SCD) and Freeze drying (FD) to check the shape and size of silica nanoparticles. Among the different particle sizes of aerogels, the silica aerogel with 0.4M TEOS/MTMS surface modification and supercritical drying attains 11.61 ± 2.96 nm constituent part with well slender size particle dissemination, elevated temperature confrontation, and lower thermal conductivity. In association with the outmoded two-step acid-base dilution of silica sols, the well-diffused form with surface alteration also delivers a minimum thermal conductivity of 0.01865 W/mK with greater thermal resistance. In addition, the drying shrinkage can be minimized by face modification by proper sialylation with TMCS. The concentrations of well- diffused silica sol were adjusted to achieve higher surface area, low density, large pore size and structure, and temperature resilience by controlling the reaction rate of sol-gel process. The resilient skeleton structure developed from the assembly of tiny particles can efficiently restrict the glutinous heat dissipation between aerogel networks without collapsing a porous network up to a higher temperature of 900°C. However, this network retains a similar pattern up to 1000°C with only 32% volume contraction after 2hr of heat treatments. At an elevated temperature of 1100-1200°C, the viscid heat flow between nanoparticles and the porous network cannot be meritoriously suppressed and starts to collapse after a shorter duration. However, the fabrication cost in FD was effective compared to SCD and provided a comparative tiny particle size with heat-resistant characteristics. Subsequently, the average thermal conductivity of hybrid aerogels was also estimated at a magnitude of 0.038-0.032 W/m K. An enhancement in the thermal conductivity is noted with an increase in wt.% of cellulose to the silica aerogel. At a temperature of about 40-50ºC, thermal degradability improved (as concluded from Thermogravimetry Analysis), with a minimum weight loss observed in hybrid aerogel over cellulose aerogel. A comparatively high sound absorption coefficient of 0.453- 0.628 at low frequency (1500 Hz) and 0.86-0.94 at high frequency (3600 Hz) was achieved with an average thickness of 8mm compared to cellulose aerogel. The compressive Young's modulus of hybrid aerogels was also augmented by 94.12% in comparison with pure silica aerogels due to the impregnation of cellulose network. The resultant SCHAs yield a stable superhydrophobic nature (water contact angle (WCA) of 163.4º±2.5, 160º±1.2, 168º±1.5) with the help of the sialylation process by a functional group modification for 1,2 and 4 wt.% of RCF in hybrid aerogel. Using nanofibers enables the aerogel to possess a highly remarkable undeviating pore size distribution, super elasticity, and compressibility characteristics. At the same time, the inclusion of silica nanoparticles improved its oleophilic performance. Compared with non-biodegradable, low adsorption commercial polypropylene foam, the HRCS aerogel provides an excellent oil adsorption capacity within a data range of 31.67-48.25 g.g-1 with 94% retention capacity and recyclable up to 10 number of cycles for various 1wt.% of RCF concentration. An optimized parameter of 1wt.% of cellulose concentration, 6 ml of Kymene, and 15 ml of TEOS achieves a higher oil adsorption capacity of 44.845 g/g. Moreover, the experimental values of 48.89 g/g of oil adsorption were observed with one wt.% of cellulose concentration, 9 ml of Kymene, and 14 ml of TEOS, respectively. An adsorption kinetics model and isotherm study were also done for suitable oil adsorption on hybrid aerogel. In a comparative analysis, the pseudo-second-order model is more authenticated for oil adsorption kinetics than the pseudo-first-order model. This auspicious feature of SCHA aerogel can be used as an alternative to hostile polymer-based oil absorbents due to its extraordinary oleophilic capacities. Subsequently, phase change composite materials with uniform 3D high thermal conductive shape stabilised aerogel are a potential solution for improving thermophysical properties and enhanced latent heat capacity applicable in thermal management. In this aspect, different weight percentages (25, 30, 35, and 40 wt.%) of graphene oxide (GO) and GO-Ferrous oxide (GO-Fe3O4) samples were synthesised by dispersing individual additives in a solvent followed by crosslinking and freeze drying for augmenting the heat transfer capacity and photothermal conversion of the solar thermal energy storage (TES). These samples were uniformly dispersed in a eutectic mixture (1:1) of paraffin wax (PW) and polyethylene glycol (PEG-6000), resulting in the development of a graphene oxide–Ferrous oxide aerogel (GOFA) composite. The controllable porous structure and hydrogen interactions with the external layer of GO and long- chain PCM benefitted in the good impregnation of PCM and compatibility impregnated into the composite. A higher mass fraction of GOFA-30-based PCM composite unveiled a comparatively higher phase change enthalpy of 119.64 J/g in melting phase and 119.85 J/g in the solidification phase with 98.73% energy conversion efficiency. In contrast to pure eutectic PCM (0.2593 W/mK), the thermal conductivity of GOFA composite is superior and provides a thermal conductivity of 1.7034-2.5156 W/mK for GOFA-25, GOFA-30, GOFA-35, and GOFA-40 PCM composite. The enhancement of thermal conductivity is ascribed to the increased heat conduction pathway facilitated by the conductive GO arrangement. The utilisation of spatially constrained eutectic phase change material (PCM) assembly resulted in the development of GOFA-PCM, which exhibited enhanced thermal conductivity, chemical and physical stability, and thermal consistency across 300 melting-solidification cycles. Benefitting from the thermal conduction path, the GOFA-based PCM also exhibits an excellent heat absorption capacity of 2320-4000 kJ in a shell and tube heat exchanger under a constant water flow rate of 1-2 LPM for quick response. The thermal responsiveness of GOFA-PCM composite samples is superior to that of eutectic PCM without any compromise in thermal performance. With a higher thermal storage capacity and higher heat transfer property, thermally reliable GOFA composites exhibited tremendous application potential in photothermal and thermal energy storage applications.