In light of the world‟s growing demand for energy that is environmentally friendly and sustainable, energy sources such as hydrogen have been considered potential contenders. Hydrogen, which can be used for energy storage, can be produced efficiently by the membrane based Hybrid Sulfur (HyS) thermo-chemical process consisting of a decomposition and an electrolysis step. During the HyS electrolysis step, SO2 and H2O are converted to H2 and H2SO4, which implies that the proton exchange membranes (PEMs) to be used for this process should have a high proton conductivity, limited SO2 cross-over and good H2SO4 stability. In order to find alternatives to the costly and high-temperature unstable Nafion®, the aim of this study was to evaluate the H2SO4 stability of various novel membranes. To structure the study, the novel PEM materials were grouped according to the PBI-type base component within the blend membranes, resulting in three groups comprising non-PBI based membranes, PBIOO based membranes and F6-PBI based membranes. Nafion®212 was included as reference PEM. By repeating the H2SO4 treatment with three different Nafion®212 samples, the obtained Nafion® data was also used to determine the experimental and analytical error margins for the study. The stability of all membranes was determined by submerging the membrane samples in 80 wt% H2SO4 at 80 °C for 120 hours. To determine the influence of the acid on the membranes, all samples were characterised before and after the H2SO4 treatment and compared in terms of their acid stability. Physical characterisation of the PEMs included the evaluation of weight and thickness changes, while IEC, SEM-EDX, FTIR and TGA were used to elucidate possible chemical changes due to the H2SO4 treatment. According to the Nafion®212 data, which had been obtained in triplicate for each of the analytical techniques, the experimental error of both the analytical and H2SO4 treatment remained below 10 %, except for the SEM-EDX sulfur-content where significantly larger errors were observed. In spite of the high error margins of the SEM-EDX data (S-content), its results, combined with the results from the other analytical techniques, resulted in a better understanding (both physical and chemical) of the effect the H2SO4 had on the membrane. This further facilitated the evaluation and comparison of the various blended PEM materials in terms of their H2SO4 stability, and the subsequent relation obtained between the observed stability and the chemical constitution and cross-linking of the membranes. After the 80 wt% H2SO4 treatment, significant weight losses were reported for the non-PBI based and PBIOO based membrane groups in comparison with the minimal changes noted for the F6-PBI based group and Nafion®212. Furthermore, significant thickness changes were reported for most of the PBIOO based membranes. The small weight and thickness changes observed for the F6-PBI confirmed the improved stability of this group of membranes in an H2SO4 environment, most likely due to the protective role of the partially fluorinated basic polymer and the known strength of the C-F bonds present. The results showed a clear correlation between the H2SO4 stability and the specific polymers present in the PEM blends investigated. Specific effects found included sulfonation, salt formation, hydrolysis and the accompanied dissolution of membrane fragments. Significant physical changes, for example ascribed to sulfonation of the concerned polymers, were supported by increased IEC measurements and peak intensities of the FTIR spectra, corresponding to the additional –SO3H groups present, while a variation in TGA signals served to further support the altered membrane composition and structure due to the H2SO4 treatment. In the case of dissolution, the corresponding chemical changes (analytical techniques) were supported by the decreased peak intensities of FTIR spectra, IEC measurements and TGA signals associated with degradation of the polymer backbone. It was shown that the stability of the blended membranes depended on the composition (blend components) of the membrane and the effective cross-linking (interaction) between the blend components. For all three groups examined, it became apparent that blend components sFS and sPSU were, for example, more stable than sPEEK and that ionical cross-linking seemed more effective than covalent cross-linking of blend components. When considering all membranes tested, the non-PBI based blend membranes consisting of (s)PSU and PFS copolymers in the presence of fluorinated cross-linkers and the PBIOO-sPSU blended membranes including most of the F6-PBI based membranes showed sufficient stability to be recommended for SO2 electrolysis. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

In light of the world‟s growing demand for energy that is environmentally friendly and sustainable, energy sources such as hydrogen have been considered potential contenders. Hydrogen, which can be used for energy storage, can be produced efficiently by the membrane based Hybrid Sulfur (HyS) thermo-chemical process consisting of a decomposition and an electrolysis step. During the HyS electrolysis step, SO2 and H2O are converted to H2 and H2SO4, which implies that the proton exchange membranes (PEMs) to be used for this process should have a high proton conductivity, limited SO2 cross-over and good H2SO4 stability. In order to find alternatives to the costly and high-temperature unstable Nafion®, the aim of this study was to evaluate the H2SO4 stability of various novel membranes. To structure the study, the novel PEM materials were grouped according to the PBI-type base component within the blend membranes, resulting in three groups comprising non-PBI based membranes, PBIOO based membranes and F6-PBI based membranes. Nafion®212 was included as reference PEM. By repeating the H2SO4 treatment with three different Nafion®212 samples, the obtained Nafion® data was also used to determine the experimental and analytical error margins for the study. The stability of all membranes was determined by submerging the membrane samples in 80 wt% H2SO4 at 80 °C for 120 hours. To determine the influence of the acid on the membranes, all samples were characterised before and after the H2SO4 treatment and compared in terms of their acid stability. Physical characterisation of the PEMs included the evaluation of weight and thickness changes, while IEC, SEM-EDX, FTIR and TGA were used to elucidate possible chemical changes due to the H2SO4 treatment. According to the Nafion®212 data, which had been obtained in triplicate for each of the analytical techniques, the experimental error of both the analytical and H2SO4 treatment remained below 10 %, except for the SEM-EDX sulfur-content where significantly larger errors were observed. In spite of the high error margins of the SEM-EDX data (S-content), its results, combined with the results from the other analytical techniques, resulted in a better understanding (both physical and chemical) of the effect the H2SO4 had on the membrane. This further facilitated the evaluation and comparison of the various blended PEM materials in terms of their H2SO4 stability, and the subsequent relation obtained between the observed stability and the chemical constitution and cross-linking of the membranes. After the 80 wt% H2SO4 treatment, significant weight losses were reported for the non-PBI based and PBIOO based membrane groups in comparison with the minimal changes noted for the F6-PBI based group and Nafion®212. Furthermore, significant thickness changes were reported for most of the PBIOO based membranes. The small weight and thickness changes observed for the F6-PBI confirmed the improved stability of this group of membranes in an H2SO4 environment, most likely due to the protective role of the partially fluorinated basic polymer and the known strength of the C-F bonds present. The results showed a clear correlation between the H2SO4 stability and the specific polymers present in the PEM blends investigated. Specific effects found included sulfonation, salt formation, hydrolysis and the accompanied dissolution of membrane fragments. Significant physical changes, for example ascribed to sulfonation of the concerned polymers, were supported by increased IEC measurements and peak intensities of the FTIR spectra, corresponding to the additional –SO3H groups present, while a variation in TGA signals served to further support the altered membrane composition and structure due to the H2SO4 treatment. In the case of dissolution, the corresponding chemical changes (analytical techniques) were supported by the decreased peak intensities of FTIR spectra, IEC measurements and TGA signals associated with degradation of the polymer backbone. It was shown that the stability of the blended membranes depended on the composition (blend components) of the membrane and the effective cross-linking (interaction) between the blend components. For all three groups examined, it became apparent that blend components sFS and sPSU were, for example, more stable than sPEEK and that ionical cross-linking seemed more effective than covalent cross-linking of blend components. When considering all membranes tested, the non-PBI based blend membranes consisting of (s)PSU and PFS copolymers in the presence of fluorinated cross-linkers and the PBIOO-sPSU blended membranes including most of the F6-PBI based membranes showed sufficient stability to be recommended for SO2 electrolysis. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

Proton-Exchange Membrane (PEM) fuel cells are extremely attractive for replacing internal combustion engines in the next generation of automobiles. However, two major technical challenges remain to be resolved before PEM fuel cells become commercially successful. The first issue is that CO, produced in trace amounts in fuel reformer, severely limits the performance of the conventional platinum-based PEM fuel cell. A possible solution to the CO poisoning is higher temperature operation, as the CO adsorption and oxidation overpotential decrease considerably with increasing temperature. However, the process temperature is limited in atmospheric fuel cells because water is critical for high conductivity in the standard PEM. An increase in operating pressure allows higher temperature operation, although at the expense of parasitic power for the compressor. Further the conventional PEM, Nafion? is limited to 120°C due to it's low glass transition temperature. Thus, the design of higher temperature PEMs with stable performance under low relative humidity (RH) conditions is considered based on a proton transport model for the PEM and a fuel cell model that have been developed. These predictive models capture the significant aspects of the experimental results with a minimum number of fitted parameters and provides insight into the design of higher temperature PEMs operating at low RH. The design of an efficacious high temperature, low RH, PEM was based on enhancing the acidity and water sorption properties of a conventional PEM by impregnating it with a solid superacid. A systematic investigation of the composite Nafion?inorganic PEMs comprising experiments involving water uptake, ion-exchange capacity (IEC), conductivity and fuel cell polarization is presented in the work. The most promising composite is the nano-structured ZrO2/Nafion?PEM which exhibits an increase in the IEC, a 40% increase in water sorbed and a resulting 24% conductivity enhancement vs. unmodified Nafion?112 at 120°C and at RH < 40%.

Nitrogen-rich porous organic frameworks show great promise for use as acid-doped proton conducting membranes, due to their high porosity, excellent chemical and thermal stability, ease of synthesis, and high nitrogen content. Aided by very high surface area and pore volume, the material has the ability to adsorb high amounts of H3PO4 into its network, which creates a proton rich environment, capable of facile proton conduction. The morphology and chemical environment, doping behavior, and proton conduction of these materials were investigated. With such high acid-doping, ex-situ studies revealed that under anhydrous conditions, PA@BILP-16 (AC) produced a proton conductivity value of 5.8 x 10-2 S cm-1 at 60 °C and PA@ALP-6 showed a slightly higher value of 5.91 x 10-2 S cm-1 at 60 °C. With such promising results, in-situ experiments with various analogues are scheduled to be conducted in the near future.

No description available.

A designed series of directly copolymerized homo- and disulfonated copolymers containing controlled degrees of pendant sulfonic acid groups have been synthesized via nucleophilic step polymerization. Novel sulfonated poly (arylene ether sulfone) copolymers using 4,4'-bisphenol A, 4,4'-biphenol, hexafluorinated (6F) bisphenol AF, and hydroquinone, respectively, with dichlorodiphenyl sulfone (DCDPS) and 3,3'-disodiumsulfonyl-4,4'-dichlorodiphenylsulfone (SDCDPS) were investigated. Molar ratios of DCDPS and SDCDPS were systematically varied to produce copolymers of controlled compositions, which contained up to 70 mol% of disulfonic acid moiety. The goal is to identify thermally, hydrolytically, and oxidatively stable high molecular weight, film-forming, ductile ion conducting copolymers, which had properties desirable for proton exchange membranes (PEM) in fuel cells. Commercially available bisphenols were selected to produce cost effective alternative PEMs. Partially aliphatic bisphenol A and hexafluorinated (6F) bisphenol AF produced amorphous copolymers with different thermal oxidative and surface properties. Biphenol and hydroquinone was utilized to produce wholly aromatic copolymers. The sulfonated copolymers were prepared in the sodium-salt form and converted to the acid moiety via two different methodologies and subsequently investigated as proton exchange membranes for fuel cells. Hydrophilicity increased with the level of disulfonation, as expected. Moreover, water sorption increased with increasing mole percent incorporation of SDCDPS. The copolymers' water uptake was a function of both bisphenol structure and degree of disulfonation. Furthermore, the acidification procedures were shown to influence the Tg values, water uptake, and conductivity of the copolymers. Atomic force microscopy (AFM) in the tapping mode confirmed that the morphology of the copolymers could be designed to display nanophase separation in the hydrophobic and hydrophilic (sulfonated) regions. Morphology with either co-continuous hydrophobic or hydrophilic domains could be attained for all the sulfonated copolymers. The degree of disulfonation required for continuity of the hydrophilic phase varied with biphenol structure. Proton conductivity values for the sulfonated copolymers, under fully hydrated conditions, were a function of bisphenol and degree of sulfonation. However, at equivalent ion exchange capacities the proton conductivities were comparable. A careful balance of copolymer composition and acidification method was necessary to afford a morphology that produced ductile films, which were also sufficiently proton conductive. The copolymers of optimum design produced values of 0.1 S/cm or higher, which were comparable to the commercial polyperfluorosulfonic acid material Nafion™ control. / Ph. D.

Mechanical fatigue testing has shown that MEAs (membrane electrode assemblies) fail at lower stresses than PEMs (proton exchange membranes) at comparable times under load. The failure of MEAs at lower stresses is influenced by the presence of mud cracks in the catalyst layers acting as stress concentrators. Fatigue testing of MEAs has shown that smaller-scale cracking occurs in the membrane within these mud cracks, leading to leaking during mechanical fatigue testing and the failure of the membrane. In addition, this testing of MEAs has further established that the cyclic pressurization pattern, which affects the viscoelastic behavior of the membranes, has a significant effect on the relative lifetime of the MEA. To investigate this behavior, pressure-loaded blister tests were performed at 90 °C to determine the biaxial fatigue strength of Gore-Primea® Series 57 MEAs. In these volume-controlled tests, the leak rate was measured as a function of fatigue cycles. Failure was defined as occurring when the leak rate exceeded a specified threshold. Post-mortem characterization FESEM (field emission scanning electron microscopy) was conducted to provide visual documentation of leaking failure sites. To elucidate the viscoelastic behavior of the MEA based on these results, testing was conducted using a DMA to determine the stress relaxation behavior of the membrane. This data was then used in a FEA program (ABAQUS) to determine its effect on the mechanical behavior of the MEAs. A linear damage accumulation model used the ABAQUS results to predict lifetimes of the membrane in the MEAs. The models showed that under volume-controlled loading, the stress decays with time and the stress dropped towards the edges of the blisters. The lifetimes of the MEAs varied depending on the cycling pattern applied. This is important for understanding failure mechanisms of MEAs under fatigue loading, and will help the fuel cell industry in designing membranes that better withstand imposed hygrothermal stresses experienced during typical operating conditions. / Master of Science

"The need for an efficient, non-polluting power source for vehicles in urban environments has resulted in increased attention to the option of fuel cell powered vehicles of high efficiency and low emissions. Of various fuel cell systems considered, the proton exchange membrane (PEM) fuel cell technology seems to be the most suitable one for the terrestrial transportation applications. This is thanks to its low temperature of operation (hence, fast cold start), and a combination of high power density and high energy conversion efficiency. Besides automobile and stationary applications (distributed power for homes, office buildings, and as back-up for critical applications such as hospitals and credit card centers), future consumer electronics also demands compact long-lasting sources of power, and fuel cell is a promising candidate in these applications. The goal of a cost effective and high performance fuel cell has resulted in very active multidisciplinary research. Although significant progress has been made on PEM fuel cells over the last twenty years, further progress in fuel cell research is still needed before the commercially viable fuel cell utilization in transportation, potable and stationary applications. A chief goal among others is the design of PEM fuel cells that can operate with impure hydrogen containing traces of CO, which has been the objective of this research. Standard Pt and PtRu anode catalyst has been studied systematically under practical fuel cell conditions, in an attempt to understand the mechanism and kinetics of H2/CO electrooxidation on these noble metal catalysts. In the study of Pt as anode catalyst, it was found that the fuel cell performance was strongly affected by the anode flow rate and cathode oxygen pressure. A CO electrooxidation kinetic model was developed taking into account the CO inventory in the anode, which can successfully simulate the experimental results. It was found that there is finite CO electrooxidation even on Pt anode with H2/CO as anode feed. Thus, anode overpotential and outlet CO concentration is a function of anode inlet flow rate at a constant current density. The on-line monitoring of CO concentration in PEM fuel cell anode exit has proved that the ~{!0~}ligand mechanism~{!1~} and ~{!0~}bifunctional mechanism~{!1~} coexist as the CO tolerance mechanisms for PtRu anode catalyst. For PtRu anode catalyst, sustained potential oscillations were observed when the fuel cell was operated at constant current density with H2/CO as anode feed. Temperature was found to be the key bifurcation parameter besides current density and the anode flow rate for the onset of potential oscillations. The anode kinetic model was extended further to unsteady state which can reasonably reproduce and adequately explain the oscillatory phenomenon. The potential oscillations are due to the coupling of anode electrooxidation of H2 and CO on PtRu alloy surface, on which OHad can be formed more facile, preferably on top of Ru atoms at lower overpotentials. One parameter bifurcation and local linear stability analysis have shown that the bifurcation experienced during the variation of fuel cell temperature is a Hopf bifurcation, which leads to stable potential oscillations when the fuel cell is set at constant current density. It was further found that a PEM fuel cell operated in an autonomous oscillatory state produces higher time-averaged cell voltage and power density as compared to the stable steady-state operation, which may be useful for developing an operational strategy for improved management of power output in PEM fuel cells with the presence of CO in anode feed. Finally, an Electrochemical Preferential Oxidation (ECPrOx) process is proposed to replace the conventional PrOx for cleaning CO from reformate gas, which can selectively oxidized CO electrochemically while generating supplemental electrical power without wasting hydrogen."

The need for sustainable energy use has motivated the exploration of renewable alternative fuels and fuel conversion technology on a global scale. Fuel cells, which convert chemical energy directly into electrical energy with high efficiency and low emissions, provide a promising strategy for achieving energy sustainability. The current progress in fuel cell commercialization is mainly in portable and stationary applications, but fuel cell technology for transportation applications, which make up a substantial portion of the global energy market, have seen little commercial success. Proton exchange membrane fuel cells (PEMFCs) have high potential for addressing the future energy needs of the transportation energy sector. However, one of the prevailing limitations of the PEMFC is the availability of high-performance, cost-effective electrolyte materials. These materials may be realized in the near future by developing multi-functional polymer blends targeted at specific performance capabilities. Due to the near-infinite possibilities of polymer combinations and processing techniques high-throughput polymer characterization techniques are necessary to effectively and systematically screen for optimal materials and relevant structure-property relationships. In this work, a high-throughput mass transport assay (HT-MTA) has been developed to characterize water flux and permeability at multiple sample locations in parallel. The functionality of HT-MTA was evaluated using standard Nafion® films and a model semi-interpenetrated polymer network with commercial polyvinylidine fluoride as the host matrix for a proprietary polyelectrolyte supplied by Arkema, Inc. To further demonstrate the utility of HT-MTA, the instrument was incorporated into the lab's current high-throughput characterization toolset and used to investigate the mechanisms and effects of rapid free radical degradation of Nafion® membranes based on various concentrations of hydrogen peroxide and iron(II) sulfate in solution. The results have been used suggest the effects of these regent components on preferential degradation pathways and will prove to be useful in later simulating the membrane performance during in-situ fuel cell lifetime which is both time-intensive and costly. The high-throughput toolset was also used to develop a novel optimized blend consisting of polyetherimide (PEI), a low-cost high performance resin, and sulfonated PEI (S-PEI) made using a relatively mild post sulfonation reaction with trimethylsilyl chlorosulfonate. The effects of blend composition and thermal annealing on film performance were evaluated and the polymer system was shown to have optimal performance properties that should prove to be useful in other high-performance applications where mechanical strength is critical. In general, this work shows promising results for efficiently developing advanced polymer materials using high-throughput screening techniques.

Hydrophilic/hydrophobic block copolymers as proton exchange membranes (PEMs) has become an emerging area of research in recent years. Three series of hydrophilic/hydrophobic, fluorinated/sulfonated multiblock copolymers were synthesized and characterized in this thesis. These copolymers were obtained through moderate temperature (~100°C) coupling reactions, which minimize the ether-ether interchanges between hydrophobic and hydrophilic telechelic oligomers via a nucleophilic aromatic substitution mechanism. The hydrophilic blocks were based on the nucleophilic step polymerization of 3,3′-disulfonated, 4,4′-dichlorodiphenyl sulfone with an excess 4,4′-biphenol to afford phenoxide endgroups. The hydrophobic (fluorinated) blocks were largely based on decafluoro biphenyl (excess) and various bisphenols. The copolymers were obtained in high molecular weights and were solvent cast into tough membranes, which had nanophase separated hydrophilic and hydrophobic regions. The performance and structure-property relationships of these materials were studied and compared to random copolymer systems. NMR results supported that the multiblock sequence had been achieved. They displayed superior proton conductivity, due to the ionic proton conducting channels formed through the self-assembly of the sulfonated blocks. The nano-phase separated morphologies of the copolymer membranes were studied and confirmed by atomic force microscopy. Through control of a variety of parameters, including ion exchange capacity and sequence lengths, performances as high, or even higher than those of the state-of-the-art PEM, Nafion, were achieved. / Ph. D.