Hydrogen storage for aerospace applications

: (Alternative Format Thesis)

Abstract

The UK is required to achieve net zero greenhouse gas emissions by 2050 under the 2019 amendment to the 2008 Climate Change Act. In 2019, domestic and international flights were responsible for 8% of the UKs carbon emissions, making it one of the highest contributors from the transport sector. The aerospace industry is pushing towards zero emission aircraft but faces several challenges to achieve this goal. One option for zero emissions is hydrogen fuel. 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). This is not ideal for some applications. 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. These materials however, are usually produced in particulate form which can lead to handling and fouling issues, for example when inserted inside storage tanks. Polymers of intrinsic microporosity (PIMs) are one material that show potential for gas storage and separation applications, as they have good processability and the ability to retain porosity. However, their surface area is limited compared to other materials such as activated carbons and the surface area is linked to the hydrogen storage capacity. The challenge with activated carbons is that they are difficult to process into adsorbent inserts. Combining the PIM-1 as a matrix to hold the higher surface area carbon “fillers” would allow the hydrogen adsorption capability of the high surface area material to be retained whilst benefitting from the ease of forming of the polymer matrix to create usable adsorbent inserts.

This thesis demonstrates the use of freeze casting to manufacture highly adsorbent 3D structures, in monolithic and bead form, 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). Freeze casting is a well-established method for forming porous ceramics, normally from a suspension of water mixed with the ceramic and a small amount of dispersant. The suspension can either be cooled equally from all directions to form a homogeneous structure or directionally where it is cooled from one side and forms a lamellar structure. The solvent is then removed by sublimation in a freeze drier leaving the solid structure behind. In this project a solution of PIM-1 and chloroform is frozen.

Nitrogen adsorption isotherms at 77 K are presented and used to calculate the Brunauer Emmett Teller (BET) surface areas, as well as hydrogen adsorption data which shows that composite adsorbent inserts 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 explored for the first time, which led to an increased surface area and mass of hydrogen stored above that of PIM-1 powder, however, it did not have the same effect when carbon was added into the solution. The experimental adsorption data for the monoliths and beads fitted well to the Tóth isotherm, which allowed their maximum storage capacity to be predicted. The maximum hydrogen storage capacity demonstrated for the monoliths and beads formed was 9.58 wt.%, which shows potential to meet the United States (US) Department of Energy (DoE) requirements for light duty fuel cell vehicles (5.5 wt.%), although this requirement is for the whole system and not solely the material. High pressure isotherms (10 MPa) for PIM-1 and carbon powders are also presented to verify the predictions made by the Tóth isotherms. It is demonstrated that the monoliths formed are able to store more hydrogen than compression in a 1.4 l tank at 77 K for low pressures (100 kPa – 8000 kPa). The monoliths are therefore unlikely to be used in a high pressure compression tank (~70 MPa) but would show potential for use in the ullage region of a liquid hydrogen tank (~145 kPa). At this pressure the monoliths are shown to store up to 25 times more hydrogen than by compression. This would lead to reduced boil-off, increasing safety and reliability of storage tanks. This work provides the first reported data for hydrogen storage capability of adsorptive PIM-1 composites, which show potential
to be incorporated as three-dimensional inserts into hydrogen storage tanks.

Date of Award	26 Mar 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Bowen (Supervisor) & Tim Mays (Supervisor)