A thesis submitted to the Faculty of Engineering and the Built Environment,
University of the Witwatersrand, Johannesburg, in fulfillment of the
requirements for the degree of Doctor of Philosophy in Engineering.
Johannesburg, 2017 / Among of the components of the fuel cell, the polymer electrolyte membrane is
critical to the performance and life time of the cell. Over the years the mechanical
properties of the membrane, water management have tended to limit its wide
spread commercialization as an alternative source of the renewable energy for
portable power units. Fuel cell continues to attract extensive research interest as
potential source of renewable energy. This work focuses on the production of ionexchange
membrane (IEM) for hydrogen fuel cell, using cheap and locally
available starting materials. The polystyrene-butadiene rubber (SBR) of different
styrene and butadiene compositions, have been explored for functionality in fuel
cell application. The production process was conducted in three stages: the first
stage involved hydrogenation process followed by sulfonation process. The
second stage entailed the production of carbon nano-spheres for the blending in
the hydrogenated sulfonated polystyrene-butadiene rubber. The blending was also
done between hybrid nanoparticles and hydrogenated sulfonated polystyrenebutadiene
rubber. The third stage was the casting in thin film of blended solutions
employing the evaporative method and the use of casting tape machine technique.
The thin film was later on characterized and tested in a single fuel cell stack.
Controlled hydrogenation of SBR employing catalytic method was achieved with
maximum degree of hydrogenation in the range of:
90 – 92% for SBR with 23.5% styrene content and for SBR 25% styrene
content
76 – 80% for SBR with 40% styrene content and
82 – 92% for SBR with 52% styrene content.
The optimum conditions of this process were obtained using the Design of
Experiments.
SBR was also hydrogenated using a photocatalytic method and the percentage of
hydrogenation for all SBR compositions used was found in the range between 60
and 74%. The hydrogenation results using the catalyst were higher compared to
those obtained with the photocatalytic method. Therefore they were used to
develop the kinetic model for prediction of hydrogenation process. Langmuir –
Hinshelwood models were reviewed in this project as they explain these
heterogeneous catalytic processes. Data from the kinetic tests were fitted to
Langmuir – Hinshelwood models and reaction constants were found in the range
between 0.445 h-1 and 0.610 h-1 for the reaction temperature between 20 and
30°C.
The hydrogenated SBR of different compositions were effectively sulfonated with
chlorosulphonic acid employed as first sulfonating agent of concentrations 0.15,
0.175 and 0.25M for SBR 23.5 and 25% styrene content, for SBR 40% styrene
content and for SBR 52% styrene content, respectively. The degree of sulfonation
was found in the range between 56 and 72% depending on the rubber
composition. Trimethylsilyl chlorosulfonate used as the second sulfonating agent
was like wise attached to the same polymer back bone and the degree of
sulfonation was between 59 and 74% depending on the rubber’s styrene content.
Non-conductive carbon nanospheres (CNS) of uniform size of about 46 nm were
produced employing the non-catalytic chemical vapour deposition method at
1000°C. Acetylene and argon were respectively used as carbon source and carrier
gas, in a reactor of 16 mm in diameter. Successful blending of 4 wt%
nanoparticles and hydrogenated sulfonated styrene butadiene solution was
accomplished by magnetic stirring technique combined with ultrasonication at
60% amplitude. The blended solution was casted to produce a thin film membrane
of 156 μm thickness. Further the tensile strength test of the membranes has shown
an increase in Young’s Modulus by 72-120% for all the rubbers. This test was
done using TA.XTplus, Texture Analyser machine. The water uptake increment
was in the range of 20-27% and thermal stability in the range of 2-20% depending
on the rubber composition. Purchased electrodes from FuelCellsEtc (USA), were
pasted on both sides of the membranes by the means of hot press at 125oC for
about 5 minutes at a pressure of 40 kPa. The Membrane Electrode Assembly
(MEAs) fabricated were tested in the fuel cell stack. The highest power density of
approximately 85mW/cm2 was obtained for 52% styrene nanocomposite
membrane with 4% hybrid nanoparticles at the current density of 212.41mA/cm2
and the efficiency was between 41 and 43%. MEA fabricated with Nafion112
membrane was tested and yielded the open cell voltage of 0.79V, power density
of about 77.34mW/cm2 and efficiency of 45%. Results obtained disclose that the
MEA with nanocomposites based SBR 52% styrene composition yielded higher
power density and higher voltage than the one with Nafion 112 which is one of
the fuel cell membranes available on the market. The results obtained revealed
that the nanocomposite membranes with 4% hybrid nanoparticles (CNS + SiO2)
had higher voltage than the one with 4% CNS. These optimum conditions
obtained in this work may be adopted for a typical continuous production of the
membrane for hydrogen fuel cell. / MT2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/24237 |
| Date | January 2017 |
| Creators | Mufula, Alain Ilunga |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (xxxiii, 260 leaves), application/pdf, application/pdf, application/pdf |
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