Polymer-based Composites with Enhanced Microporosity for Hydrogen Storage

Hydrogen (H2) is considered a renewable energy carrier as it is abundant, relatively easy to produce and emits only water upon oxidation. A major technical obstacle to the widespread use of H2 as a fuel is the lack of a safe and efficient system for on-board storage due to its extremely low volumetric density at standard conditions. Conventionally, H2 can be stored as either the gas in high-pressure tanks or liquefied H2 in cryogenic tanks in vehicles. However, both techniques are energy intensive and pose serious safety issues. An attractive solution is a system based on the reversible adsorption of H2 on the internal surface of a microporous material to achieve high hydrogen storage density at reasonable pressures and temperatures. The US Department of Energy (DoE) has recently set a 2025 hydrogen storage system target of 55 g H2 per kg (5.5 wt%) for onboard hydrogen storage for light-duty fuel cell vehicles1. For this purpose, many materials have been explored including the porous carbons, zeolites, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). However, these materials are in a powder or particulate form that limits their processability and applications. One alternative material is a 'polymer of intrinsic microporosity', PIM-1, due to its microporosity and capability to be processed into a robust free-standing film using solvent-based techniques2. These films can potentially be incorporated as liners inside high-pressure storage tanks to enhance their performance. PIM-1 has a rigid ladder polymer backbone with a spiro-centre that produces bends in the chain, resulting in a contorted structure3. These structural features prevent an efficient packing of the macromolecules that leads to great free volume and microporosity3. However, the surface area of the PIM-1 film is relatively low (~800 m2/g), and hydrogen uptake of the PIM-1 film is 0.78 wt.% at 0.1 MPa and 2.6 wt.% at 10 MPa at 77K, which does not satisfy the DoE hydrogen storage target1. In this work, we introduce a microporous activated carbon AX-21 with high surface area of 2800 m2/g as a filler into the PIM-1 matrix to create composite membranes that combine high hydrogen adsorption capacity, solution-based processability, and sufficient mechanical properties as the high-pressure gas tank liner. We successfully cast PIM-1/AX-21 membranes using PIM-1 and the AX-21 with various loadings, shown in Figure 1. The highest content achieved was 50 wt.% AX-21 concentration in PIM-1 that still contained appropriate mechanical properties. Surface area and porosity were determined by N2 isotherms at 77 K. TGA, SEM and Raman were employed to characterise the thermal stability and structure of all samples. High-pressure hydrogen storage measurements were carried out at 77 K and room temperature to achieve the maximum hydrogen capacity and modelling of the adsorption studies was carried out to predict the hydrogen amount stored in a tank. Furthermore, the structure and applications of the polymer-based composites can be tailored and designed by combining diverse porous fillers in the broader context of gases adsorption and separation.