Abstract
In recent times, energy storage is pivotal since it minimizes the high supply and demand gap, increases dispatch ability and useful for a later time. However, the main challenges associated with the energy storage includes the loss of energy in “round trips” inefficiencies, additional costs and complexity as well as additional infrastructure and space requirements. The development of technologies that will permit energy storage for longer times, that are recyclable, green and sustainable for future generation are essential. Renewable energy has not been fully utilized globally due to it being intermittent, has high upfront costs, issues with logistical barriers, limited availability of infrastructure, sources having geographical limitations and they are not always 100 % carbon-free. Scientifically, the technology utilizing porous adsorbents for storing energy is favoured for the reason that the adsorbent is able to store large amounts of energy in a smaller volume, have high energy storage densities, offer reversible processes and recyclability of the adsorbent, less solvents used and easy methods of synthesis. Metal-organic frameworks (MOFs) have attained an unprecedent attention for numerous adsorption energy storage applications owing to their intriguing properties. MOFs have very high surface areas, large pore volumes, excellent chemical stabilities, high moisture stability, good thermal stability, tunable pore structures and unique surface morphologies.
In this work we presented five MOFs as adsorbents, namely, MIL-101(Cr), MIL-101(Fe), MIL-88B(Fe), UiO-66(Zr) and aluminium fumarate for adsorption thermal energy storage (ATES), carbon dioxide (CO2) and methane (CH4) adsorption assessment. The five MOF materials were synthesized by a viable hydro/solvothermal synthesis procedure. This synthesis method is the most extensively used and favoured procedure for MOF synthesis due to easily controlled process, simple, utilizes readily available solvents, and environmental benign synthesis procedure. These MOFs were derived from polyethylene terephthalate (PET) waste, except for aluminium fumarate. The 1,4-benzene dicarboxylic acid (1,4-H2BDC)-based MOFs have been explored as one of the most significant prototype MOFs for various adsorption energy storage applications. The challenges impeding wide commercialization of these MOFs is a lack of cheap, environmentally benign and scalable synthesis methods. In mitigating the negative environmental effects, the solution to develop a highly reproducible, facile and green routes to synthesize H2BDC-based PET waste as a direct precursor was considered. The physicochemical properties of the prepared materials were then determined by powder X-ray diffraction (PXRD), Brunauer, Emmett, Teller (BET), scanning electron microscopy (SEM) and thermogravimetric analyses (TGA). Dynamic vapour sorption analyzer (DVS) equipment was utilized to determine the water adsorption capacities at 25 and 55 °C, p/p0 = 1 bar. High pressure CH4 and CO2 gas adsorption measurements were accomplished using a high pressure volumetric analyzer (HPVA II) at 25 and 55 °C and 65 bar. PXRD measurements revealed crystalline microporous planes present in each MOFs identified by their respective signals at 2𝜃. BET measurements showed that the five the adsorbents presented varying BET surface areas, pore volumes and pore diameters. SEM micrographs of MOFs revealed defined and distinct shapes. TGA showed good thermal stabilities at different temperature when subjected to weight loss analysis. In thermal adsorption analysis, MIL-101(Cr) outperformed all other materials with water uptake of 1.53 gH2O/gads and relative pressure p/p0 = 0.99. MIL-101(Cr), UiO-66(Zr), MIL-101(Fe) and MIL-88B(Fe) presented good hydrothermal stabilities. The Dubinin-Astakhov (D-A) and Linear driving force (LDF) mathematical models showed that the model predict the experimental data with a mean relative deviation of 6 %. MIL-101(Cr) showed an increase in heat storage capacity and faster adsorption capacity, while aluminium fumarate showed lower heat storage capacity and slow adsorption kinetics. For CH4 adsorption measurements, MIL-101(Cr) outperformed all other materials present a maximum uptake capacity at both 25 and 55 °C with a maximum adsorption capacity of 220.52 cm3/g at 64.98 bar and 117.47 cm3/g at 64.76 bar, respectively. In CO2 adsorption, MIL-101(Fe) at temperature at 25 and 55 °C, surpassed all the other four adsorbents with a maximum capacity of 325.84 cm3/g at 64.98 bar and 282.25 cm3/g at 64.76 bar, respectively. CO2 presented higher adsorption capacities than CH4 adsorption measurements.
D-A fitting was followed to model both CH4 and CO2 adsorption on the MOFs at 25 and 55 °C. For CH4 adsorption, MIL-101(Cr) showed a slight inconsistency in adsorption capacities when fitted with D-A model. Nonetheless, the model offered better consistency at studied temperature and pressure giving the consistent adsorption capacities for the MIL-101(Fe), MIL-88B(Fe), UiO-66(Zr) and aluminium fumarate. The D-A model fitting for CO2 adsorption on MIL-101(Fe) showed inconsistencies at a subcritical temperature. LDF model for adsorption kinetics revealed that rate for CO2 adsorption was faster than CH4. The diffusion activation energy (Ea) of CH4 and CO2 decreased with the pressure, suggesting that the adsorption of CH4 and CO2 was easier at higher pressures. The prepared MOF materials were comparable to those reported in the literature. MIL-101(Cr) outperformed all other four MOF materials in ATES (heat storage), CH4 and CO2 storage applications in this study.