Hydrogen storage capacity of freeze cast microporous monolithic composites

Abstract

Low carbon hydrogen is a highly effective clean energy carrier due to its high gravimetric energy density (higher heating value of 142 MJ kg⁻¹) and, when it is oxidised to yield power and heat, the only product is water. However, the low volumetric energy density of hydrogen (<14 MJ L⁻¹ under any condition) requires heavy and complex storage tanks when stored as a high pressure gas (70 MPa) or a low pressure liquid (<0.16 MPa, 20 K). Highly adsorbent porous materials show potential to improve tank capacity by increasing the volumetric density, or decreasing the operating pressure, for a given amount of fuel, thereby making it beneficial for use in transport applications. Here, we demonstrate the use of freeze casting to manufacture highly adsorbent 3D structures that consist of a matrix of polymer of intrinsic microporosity 1 (PIM-1) filled with high surface area activated carbons (MSC-30 and MSC-30SS). We present the first reported hydrogen adsorption data for freeze cast monoliths and show that they generally follow a rule of mixtures in terms of hydrogen storage capacities of the matrix and filler, providing a route for the design of these materials. The addition of water into the freeze casting solution is also explored for the first time, which lead to an increased surface area and mass of hydrogen stored above that of PIM-1 powder. The experimental adsorpion data for the monoliths fit well to the Tóth isotherm, which allows their maximum storage capacity to be predicted. It is demonstrated that the monoliths formed are able to store more hydrogen than compression at 77 K for pressures below 0.4 MPa. The composites show potential for use in the ullage region of a liquid hydrogen tank, to reduce boil-off, increasing safety and reliability of storage tanks. Our work provides the first reported data for hydrogen storage capability of adsorptive composites, which show potential to be incorporated as three-dimensional inserts into liquid hydrogen storage tanks.