Synthesis of Silica Aerogels via Supercritical CO₂ Drying for Efficient Hydrogen Storage

Developing efficient hydrogen storage technologies is vital for cultivating sustainable energy solutions. Hydrogen is a promising alternative to fossil fuels due to its ability to be produced from various domestic resources and its production of only water vapor as a byproduct in fuel cells. However, its low volumetric energy density and embrittlement necessitate adsorption-based storage at lower pressures and near-ambient temperatures. Aerogels, ultralight, three-dimensional, highly porous materials characterized by low density and a large specific surface area, have emerged as superior adsorbents for storage. This study investigates the synthesis of silica aerogels using supercritical CO₂ drying and evaluates their efficacy in hydrogen adsorption, addressing global clean energy demands.

A sol solution with a 1:7:4:0.01:0.01 molar ratio of tetraethyl orthosilicate (TEOS) to ethanol (EtOH) to water (H₂O) to hydrochloric acid (HCl) to ammonium hydroxide (NH₄OH) was prepared. To create a 240 mL solution, 75 mL of TEOS, 135 mL of EtOH, and 300 μL of 36.5-38.0% HCl diluted with 12.5 mL of deionized (DI) water were spun in a beaker on a hot plate at 400 rpm and 40°C for 30 minutes, followed by another 10 minutes after 400 μL of 28.0-30.0% NH₂OH diluted with 12.5 mL of DI water was added. After gelling in silicone molds for 4 hours and aging for three days in EtOH, DI water, and EtOH for 24 hours each, the sol-gels’ quality was tested using Fourier-transform infrared spectroscopy (FTIR), which confirmed silica bonds and ethanol.

Aerogels are made by replacing the liquid in a gel with air. The qualified alcogels were subjected to the supercritical CO₂ drying process, which removes the EtOH solvent and preserves the aerogel’s porous structure by eliminating capillary action that could collapse the gel. Drying at 40°C and 1800 psi for an hour produced aerogels with an expected translucent appearance and a thermogravimetric analysis (TGA) final weight percentage of 74.09%, which indicates the amount of residual liquid, either EtOH or air moisture, showed that most ethanol was removed. Scanning electron microscopy (SEM) was used to confirm porosity.

FTIR on the aerogel demonstrated a pronounced reduction in ethanol compared to the sol-gel, confirming the drying method’s success. Similarly, the relative peak intensities of silica and ethanol in Raman spectroscopy data for three batches of aerogels align with TGA and SEM observations. The Brunauer-Emmett-Teller (BET) method measured the average pore radius of our silica aerogel at 0.895 nm, which is within the optimal range for hydrogen storage at −196 °C (0.60 nm to 0.70 nm).¹

To test hydrogen storage capacity, a gravimetric method was used, leveraging a modified Sievert’s apparatus designed to address the low density of the aerogel. Gravimetric analysis was employed to quantify mass changes during hydrogen uptake and provide direct measurement of storage capacity. This apparatus consists of a known volume gas storage reservoir, and a sample reservoir with known volume and aerogel is placed inside. The sample reservoir is first vacuumed and then allowed to equalize with the pressurized storage reservoir, after which pressure drop is recorded and data analyzed to determine the gravimetric storage capacity (wt% H₂) and adsorption kinetics. This method enables the evaluation of aerogels for hydrogen storage. The preliminary tests showed a hydrogen weight capacity of 0.3% at room temperature and 1% at around 77K.

We gratefully acknowledge the Louis Morin Charitable Trust for their support of this work.

¹Suphakorn Anuchitsakol, et al. “Combined Experimental and Simulation Study on H2 Storage in Oxygen and Nitrogen Co-Doped Activated Carbon Derived from Biomass Waste: Superior Pore Size and Surface Chemistry Development.” RSC Advances, vol. 13, no. 51, 1 Jan. 2023, pp. 36009–36022, pubs.rsc.org/en/content/articlehtml/2023/ra/d3ra06720c, https://doi.org/10.1039/d3ra06720c. Accessed 7 Sept. 2024.