In response to the ever-increasing global energy demand and the need to move away from non-renewable and CO2-emitting fossil fuels as the primary energy production method, renewable energy sources have become more and more viable as energy production methods. However, given the unreliable and instantaneous nature of these energy sources, reliable, renewable energy storage methods are required. Hydrogen is an excellent candidate as a chemical energy store, as it is highly abundant, relatively easily produced as diatomic hydrogen (including from water electrolysis), and only produces water upon its complete combustion. Hydrogen also has the highest gravimetric energy density of any known chemical fuel, meaning that not very much of it is required relative to other chemical fuels. However, hydrogen gas is incredibly sparse, and therefore hydrogen has a very low volumetric energy density, making storage of the material a key challenge in the development of the so-called “hydrogen economy”. Most commonly, hydrogen is stored by compressing it to 70 MPa. However, this technique has a number of flaws, including the high expense of strong tanks (and in the case of light duty vehicles, lightweight materials are also required), and the inherent safety risks that high pressure, highly flammable gas poses.
One of the alternatives to compression is to store hydrogen by adsorption, which uses high surface area materials to densify hydrogen via the formation of weak physical bonds. This research line is well developed, and a number of different materials has been created that show good potential as hydrogen storage materials, such as activated carbons and metal organic frameworks. However, the vast majority of materials developed for this purpose are tailored only with the hydrogen uptake in mind, which can cause issues as the focus of development shifts from small scale tests to full tank scale. One adsorptive that shows a number of highly useful engineering properties on the large scale, such as good thermal resistance and solution processability, is the polymer of intrinsic microporosity PIM-1. This material can be processed into a number of morphologies without losing porosity, and shows good thermal and mechanical resistance. However, its adsorption capacity is rather limited, with the BET surface area generally reported in the 700 – 800 m2 g-1 range, and hydrogen uptake of 1.45 wt% at 77 K and 1 MPa.
This thesis presents two separate studies on attempting to improve the hydrogen uptake of PIM-1 without adversely affecting the material properties that make it attractive. The first of these was the creation of mixed-matrix-membrane style composite films solution cast from PIM-1 and the metal organic framework MIL-101. PIM-1 proved slightly difficult to synthesise consistently with high molecular weight, but MIL-101 is an easy hydrothermal synthesis. Film casting was successfully performed, producing flat, homogeneous films that maintained the MOF crystallinity. These materials were tested for their thermal properties – thermal decompositions proceeded according to the rule of mixtures of the two starting materials, whilst an increasing concentration of MOF was shown to decrease the specific heat capacity.
Both PIM-1 and MIL-101 were shown to adsorb nitrogen as previously reported. The composites showed increasing uptake with MIL-101 content, but at a lower rate than the rule of mixtures. This was a common theme for the N2 (77 K), CO2 (293 K) and low pressure H2 isotherms performed. High pressure isotherms up to 17 MPa were performed on PIM-1 for the first time, showing a maximum excess uptake of 1.8 wt% on the powder and 1.6 wt% on the film, both at 77 K. The composites showed improved uptake with increasing MIL-101, but the maximum uptakes did not meet the rule of mixtures. The uptakes at the highest pressure did, however. Multiple temperature isotherm sets were performed on the PIM-1 film and powder, as well as the 30 wt% composite. These data sets were hampered largely by machine faults, but contained sufficient valuable data to be able to proceed with parameter fitting. The sensitivity of the isotherms produced in this study to the value of skeletal density is also examined closely.
The second theme of improved H2 uptake in PIM-1 was to carbonise the material. TGA studies on PIM-1 showed good thermal stability in anoxic conditions, and TGA twinned with mass spectroscopy was able to confirm a previously proposed mechanism of thermal decomposition. Carbonised and activated PIM-1 film samples, and a carbonised powder, were produced using physical activation methods. The adsorption performance of the carbons was disappointing, as the uptakes of N2 and H2 (< 0.1 MPa) were reduced post-carbonisation, with little recovery in the activated film. CO2 uptakes were improved, however. High pressure H2 isotherms on both the carbonised and activated films showed unusual ‘stepping’ behaviour in the adsorption curve, but maximum uptakes for both (1.0 – 1.3 wt%) were less than that seen for PIM-1 alone.
Parameter fitting was performed on all of the high pressure H2 isotherms performed in this study, using a method previously proposed by the Mays group. The parameter fits all showed effective hydrogen densification in the adsorbate layer, although the repeatability of parameter values, and the smoothness of the parameters as a function of temperature were undermined by the low quality of some of the isotherms. Using the parameters acquired, it was possible to calculate the isosteric enthalpy of adsorption for PIM-1 powder (-9.5 kJ mol-1), film (- 8.0 kJ mol-1) and the 30 wt% composite (-9.3 kJ mol-1). The stored and deliverable hydrogen contained within tanks featuring the tested materials were estimated, although only the MIL-101 powder on its own competes with other hydrogen storage adsorbents currently reported.
|Date of Award||10 Jul 2018|
|Supervisor||Tim Mays (Supervisor) & Andrew Burrows (Supervisor)|