Tesi sul tema "Anion exchange polymer membrane"

Cette thèse décrit la modélisation des performances AEMWE (chap 1) et ses dégradations (chap 2). Les modèles sont développés dans le code MePHYSTO développé au CEA dans la plateforme Matlab/Simulink. Le modèle de performance a été développé grâce aux caractérisations électrochimiques réalisées au CEA au cours du projet. Les phénomènes électrochimiques essentiels sont bien capturés, notamment l'effet de concentration en KOH et l'effet de couverture de bulles, et les courbes de polarisation sont correctement simulées.Concernant les dégradations, ces travaux s'appuient sur les résultats expérimentaux obtenus au CEA au cours du projet. Les résultats expérimentaux ont apporté plusieurs idées : les dégradations comportent à la fois des parties réversibles et irréversibles qui évoluent différemment. En effet, les dégradations réversibles augmentent avec le temps tandis que les parties irréversibles diminuent. Nous avons supposé que la partie réversible provenait de la présence des bulles dans l'anode qui la dénoie partiellement. Concernant la partie irréversible, plusieurs phénomènes interviennent. Nous avons quantifié les différentes contributions de ces dégradations grâce au modèle électrochimique que nous avons développé et aux courbes de polarisation fournies. Dans un premier temps, la dégradation du catalyseur est quantifiée via l'estimation du facteur de rugosité au début des courbes de polarisation. Dans un deuxième temps, l’évolution de la surtension d’échange d’ions entre l'électrolyte et le ionomère est quantifiée en ajustant le modèle à l’aide des courbes de polarisation. Ensuite, les dégradations associées au transport de masse sont analysées en détail. Nous avons supposé qu'elles sont induites par la perte de mouillabilité qui augmente la présence des bulles à l'anode et réduit ainsi les performances. Ceci est cohérent avec l’augmentation des dégradations réversibles que nous associons à la présence des bulles. L'évolution de l'angle de contact du PTL qui caractérise cette perte de mouillabilité est calculée selon une approche originale. Nous développons une méthode basée sur des simulations de l'écoulement dans la géométrie réelle du PTL à l'aide d'images tomographiques 3D et du code GeoDict. Les propriétés d'écoulement (perméabilité et pression capillaire) et l'angle de contact sont extraits de ces simulations et sont utilisés dans le code MePHYSTO pour calculer les performances à différents moments du vieillissement avec une bonne précision
This report describes the modelling AEMWE performances (chap 1) and degradations (chap 2). The models are developed in the MePHYSTO code developed at CEA in the Matlab/Simulink platform. The performance model has been developed thanks to the electrochemical characterization performed at CEA during the project. The essential electrochemical phenomena are captured including KOH concentration effect and bubble coverage effect and the IV curves are correctly simulated.Regarding the degradation, the work is based on the experimental results obtained at CEA during the project. The experimental results provided several ideas: the degradations include both reversible and irreversible parts that evolve differently. Indeed, the reversible degradations increases with time while irreversible parts decreases. We assumed the reversible part comes from the anode bubble coverage. Regarding the irreversible part, several phenomena are involved. We quantified the different contributions of these degradations thanks to the electrochemical model we developed, and the IV curves provided. First, the catalyst degradation is quantified via the estimation of the roughness factor at the beginning of the IV curves. Secondly, the ion-exchange over-potential evolution is quantified by fitting the model using the IV curves. Then, the degradations associated to the mass transport are analyzed in detail. We assumed that they are induced by the loss of wettability that increases the anode bubble coverage and thus, reduces the performances. This is coherent with the increase of the reversible degradations we associate to the bubble coverage. The evolution of the sinter contact angle that characterized this loss of wettability is calculated using an original approach. We develop a method based on simulations of the flow in the real geometry of the sinter using tomographic 3D picture and the GeoDict code. The flow properties (permeability and capillary pressure) and the contact angle are extracted from these simulations and are used in the MePHYSTO code to calculate the performances at different aged times with a good accuracy

Fundamental understanding of water transport and morphology is critical for improving ion conductivity in polymer electrolyte membranes (PEMs). Herein, we present comprehensive water transport measurements comparing anion-exchange membranes (AEMs) based on ammonium-functionalized poly(phenylene oxide) and cation-exchange membranes (CEMs) based on sulfonated poly(ether sulfone). We investigate the influence of counter ions, alkyl side chain, and degree of functionalization on water transport in AEMs and CEMs using pulsed-field-gradient (PFG) NMR diffusometry. Water diffusion in both AEMs and CEMs exhibit specific trends as a function of water uptake (wt%), indicating morphological similarities across common chemical structures. Furthermore, restricted diffusion reveals micron-scale heterogeneity of the hydrophilic network in both CEMs and AEMs. We propose a model wherein the hydrophilic network in these membranes has micron-scale distributions of local nm-scale dead ends, leading to changes in tortuosity as a function of water content, counterion type, and polymer structure. We furthermore parse tortuosity into two regimes, corresponding to nm-to-bulk and µm-to-bulk ranges, which reveal the importance of multi-scale morphological structures that influence bulk transport. This study provides new insights into polymer membrane morphology from nm to µm scales with the ultimate goal of controlling polymeric materials for enhanced fuel cells and other separations applications
MS

Ce travail porte sur la synthèse et la caractérisation de membranes polymères échangeuses d'anions, destinées à la protection de l'électrode à air dans une batterie lithium-air (en vue d'une application pour véhicule électrique). Ces matériaux à architecture de réseaux interpénétrés de polymères (RIP) associent un réseau polyélectrolyte cationique hydrocarboné, la poly(épichlorohydrine) (PECH), à un réseau de polymère neutre qui peut être soit hydrocarboné, soit fluoré. Tout d'abord, la synthèse du réseau polyélectrolyte et son assemblage sur l'électrode à air ont été optimisés. Une première série de RIP associant ce réseau PECH à un réseau de poly(méthacrylate d'hydroxyéthyle) a été synthétisée. Une seconde série de matériaux combinant ce même réseau PECH à un réseau de polymère fluoré a été développée. L'ensemble de ces matériaux a été caractérisé, et pour chaque série de RIP, la méthode de synthèse et la composition ont été optimisées. Les membranes RIP présentent des propriétés améliorées par rapport au réseau simple de PECH. L'électrode à air protégée par ces nouvelles membranes échangeuses d'anions présente une stabilité améliorée dans les conditions de fonctionnement de la batterie lithium-air. Plus précisément, une durée de vie de 1000 h est obtenue lorsque l'électrode à air a été modifiée avec un RIP fluoré, soit une augmentation d'un facteur 20 de la durée de vie de l'électrode non modifiée
This work focuses on the synthesis and characterization of polymer membranes to be used as anion exchange membranes for protection on an air electrode in a new lithium–air battery for electric vehicle. In these materials showing interpenetrating polymer networks (IPN) architecture, a hydrogenated cationic polyelectrolyte network, the poly(epichlorohydrin) (PECH), is associated with a neutral network, which can be either hydrogenated or fluorinated. First, the synthesis of the polyelectrolyte network and the membrane/electrode assembly were optimized. Second, a first IPN series associating the PECH network with a poly(hydroxyethyl methacrylate) network was synthesized. Third, the same PECH network was associated with a fluorinated polymer network. All the materials were characterized, and optimal synthesis methods as well as an optimal composition were determined for each association. The IPNs show improved properties compared with the single PECH network. The air electrode protected by these new anion exchange membranes shows improved stability in the working conditions of the lithium-air battery. Specifically, a lifetime of 1000 h was obtained when the electrode was modified with a fluorinated IPN, a 20-fold increase in the lifetime of the non-modified electrode

A novel free radical-based substitution reaction was developed for grafting aromatic/heteroaromatic compounds to perfluorosulfonic acid polymers (PFSAs). Two proton-exchange membranes perfluorobenzoic acid (PFBA) and perfluorobenzenesulfonic acid (PFBSA)—were synthesized for proton-exchange membrane fuel cells via the free radical-based reaction. The physical properties, in-plane ionic conductivities and fuel cell performance of two membranes were investigated. They exhibited different electrochemical and physical properties, possibly due to the formation of unique dimerized/trimerized structure of –CO2H groups in the PFBA membrane. A free radical-based thermolytic reaction under a high temperature (180 oC)/pressure (1000 psi) condition in the presence of TFA and hydrogen peroxide is first demonstrated. A novel perfluorotetrafluoroaniline (PFTFAn) polymer was synthesized from PFSA and 2,3,5,6-tetrafluoroaniline in one step via the thermolytic reaction. After doping H2SO4 in the PFTFAn polymer, a new conjugated acid membrane (H2SO4-doped PFTFAn) was obtained. The H2SO4-doped PFTFAn membrane displayed better chemical stability and mechanical properties than NafionTM due to the removal of –SO3H groups. The second part of this thesis deals with fluoropolymer-based anion-exchange membranes. A new class of coordinated metal/perfluoropolymer type composite membranes were synthesized and characterized for anion-exchange membrane fuel cells (AEMFCs). A membrane comprised of perfluoro(phenyl-2,2’:6’,2”-terpyridine) polymer, ZrO(ClO4)2 nanoclusters, and 2,2’:6’,2”-terpyridine displayed the highest conductivity of 23.1 mS/cm at 60 oC. The chemical stability test of composite membrane showed no conductivity loss after refluxing in 7 M KOH solution at 120 oC for 2,200 h. A H+ coordinated cage-shape molecule with a benzyl group (Bn-proton cage) was designed and synthesized as a base-stable anion-exchange group. By employing the free radical-based reaction, Bn-proton cage was grafted to a fluoropolymer to yield a stable anion-conductive membrane under alkaline conditions. The third part of this thesis is our design, synthesis and test of ionic liquids for reversible SO2 absorption. Novel di-cationic ionic liquids (DILs) were designed and synthesized for SO2 absorption. DILs were found to have better SO2 absorption capabilities than mono-cationic ionic liquids (MILs). A chloride-based DIL comprised of two N-methylimidazolium cations and a PEG9 (HO-(CH2CH2O)9-H) chain could reversibly uptake 3.710 mole SO2 per mole DIL under ambient conditions. The anion, temperature and water impact on SO2 absorption in DILs was investigated. Although replacing chloride with triflate or tosylate groups led to a reduced SO2 absorption for the DILs, a high selectivity against CO2 was observed in CO2 absorption test.

La recherche vise à proposer des membranes pour des dispositifs électrochimiques capables d'atteindre le bon compromis en terme de conduction ionique, de stabilité et de longue durée de vie pour une haute efficacité.Nous avons réalisé des membranes échangeuses des protons, d'anions ou amphotères à base de polymères aromatiques stables fonctionnalisés. Des groupes sulfonique on été introduit sur la squelette du PEEK, des groupes d'ammonium sur le PEEK et le PSU ou le deux au même temps pour échanger ensemble des protons et des anions.L'optimisation continue des paramètres de synthèse, le choix des différents polymères et/ou des groupes de fonctionnalisation et l'amélioration des procédures et des traitements des membranes coulée, a conduit à de bons résultats en termes de conductivité ionique, sélectivité et stabilité.L'étude des principaux paramètres des membranes démontre une stabilité thermique entre 140 et 200 ° C selon la membrane sélectionnée, un comportement mécanique caractérisé par une résistance à la traction et un module d'élasticité élevée et un relativement faible ductilité, influencé par le niveau d’ hydratation de la membrane ou l éventuelle présence de cross-link. En optimisant le degré de fonctionnalisation et les types de groupes de fonctionnalisation, nous avons obtenu une accordable absorption d'eau, une conductivité ionique élevé pour différent ions (jusqu'à ≃ 3 mS / cm pour le polymère conducteurs des anions) et une perméabilité aux ions vanadium très faible (applications dans RFB) jusqu'à ≃ 10-10 cm2/min, ce qui est bien au-dessous des données typiques de la littérature et un paramètre très important pour applications technologiques
The research aims to propose membranes for electrochemical devices alternative to the commercial ones able to reach the right compromise in term of good ionic conduction, stability and long life time for an high efficiency. We realized proton exchange, anion exchange and amphoteric membranes based on stable functionalized aromatic polymers (PEEK, PSU). We thus introduced sulfonic groups on a PEEK backbone to exchange protons or ammonium groups on PEEK and PSU to exchange anions. We also realized amphoteric membranes able to exchange at the same time both kinds of ions. The continuous optimization of synthesis parameters, the choice of different polymers and/or functionalization groups and the improvement of casting procedures and treatments of membranes, led to good results in terms of ionic conductivity, selectivity and stability.The study of the main parameters of the synthesized membranes demonstrates a thermal stability between 140 and 200°C depending on the selected membrane, a mechanical behavior characterized by a high elastic modulus and tensile strength and a relatively low ductility strongly influenced on the degree of hydration of the membrane as well as the eventual presence of cross-linking. Working on the degree of functionalization and the type of functionalizing groups, we obtained a tunable water uptake, an elevated ionic conductivity for different ions (up to ≃ 3 mS/cm for anionic conducting polymers) and a very low ion permeability (vanadium ions for RFB applications) down to ≃ 10-10 cm2/min, which is much below typical literature data for cation- and anion separation membranes and a challenge parameters for technological applications

Essai d'identification de l'origine de la "fuite h+" et de celle des différentes sources susceptibles d'alimenter le mouvement d'eau au sein des membranes échangeuses d'anions, selon que le matériau est fortement ou non "élusterisé", puis d'établissement de l'existence d'une corrélation étroite entre l'intensité de fuite protonique et celle de la perméabilité osmotique des membranes (ainsi que le laisse prévoir la théorie de Gierke dans le cas des membranes échangeuses de cations). Et enfin, évaluation du rôle du champ électrique, ainsi que celui de la composition de l'électrolyte sur les résultats.

2013/2014
Lo sviluppo sostenibile è una sfida prioritaria per la nostra società. La possibilità di costruire un futuro sostenibile, mantenendo al contempo alti standard nella qualità della vita e preservando risorse e ambiente, dipende dalla disponibilità di metodi per la produzione verde di energía e prodotti chimici. La produzione simultanea di prodotti chimici ed energía può essere ottenuta nelle celle a combustibile che impiegano combustibili liquidi (Direct Liquid Fuel Cells – DLFC), dispositivi in cui l’energia chimica contenuta nelle molecole di combustibile è convertita direttamente in energía elettrica. Le DLFC impiegano solitamente combustibili a base di piccole molecole organiche quali ad esempio alcoli ed acido formico. Questi combustibili sono di particolare interesse, dal momento che possono essere ottenuti a partire da biomassa, con un impatto minore sulle emissioni di gas serra rispetto ai combustibili fossili. Allo stato attuale le DLFC impiegano platino in quantità elevate. Questo per due ragioni: i) il platino è un buon catalizzatore sia per l’ossidazione di composti organici che per la riduzione dell’ossigeno e ii) il platino è stabile in ambiente acido. E’ importante sottolineare che le attuali DLFC impiegano membrane a scambio protonico come elettroliti e dunque richiedono ambienti fortemente acidi per avere un’adeguata conducibilità. Le DLFC impiegano carichi di platino maggiori di 1 mg cm-2, un fatto che ne limita molto la possibilità di diffusione commerciale. In questo lavoro, grazie alla disponibilità di membrane a scambio anionico ad elevata conducibilità (Tokuyama A-201), abbiamo sviluppato delle DLFC alcaline (Anion Exchange Membrane Direct Liquid Fuel Cells – AEM-DLFC). Ciò e’ stato fatto con l’obiettivo di eliminare il platino dai dispositivi. E’ infatti noto che il palladio è un catalizzatore molto attivo per l’ossidazione delle piccole molecole organiche in ambiente alcalino e che la reazione di riduzione dell’ossigeno puo’ essere catalizzata da composti di ferro e cobalto (es. ftalocianine). La tecnología qui riportatata si basa sull’impiego di anodi di palladio supportati da carbon black (Vulcan XC-72), membrane a scambio anionico e ftalocianine di ferro e cobalto subbortate da carbon black con maggiore area superficiale rispetto a quello impiegato all’anodo (Ketjen Black 600). Un fatto importante è che le ftalocianine di ferro e cobalto non sono attive per l’ossidazione di molecole organiche. Ciò è particolarmente rilevante per le fuel cells perché il cross-over del combustibile attraverso la membrana non produce significative cadute di potenziale e quindi dell’efficienza energetica. La parte sperimentale della tesi inizia con un capitolo in cui si decrivono le prestazioni di AEM-DLFC esenti da platino ed alimentate ad etanolo. Questa parte del lavoro è particolarmente rilevante dal momento che è la prima e completa caratterizzazione della performance energetica di questi dispositivi. In particolare si sono determinati i seguenti parametri: i) massima densità di potenza, ii) efficienza energetica e iii) l’energia prodotta per singolo batch di combustibile. Tutti questi parametri sono stati determinati in funzione della composizione del combustibile. Abbiamo scoperto che la composizione del combustibile che massimizza uno dei parametri sopra riportati generalmente ha effetti negativi sugli altri. E’ dunque necesario definire la composizione del combustibile in funzione della particolare applicazione cui il dispositivo è destinato. Abbiamo inoltre studiato l’effetto dell’aggiunta di un ossido promotore, la ceria, al catalizatore anódico, mostrando che le prestazioni migliorano significativamente. In alcuni casi l’efficienza energetica può essere migliorata anche di più del 100% grazie alla semplice aggiunta di dell’ossido promotore. Il capitolo successivo e’ dedicato alle celle a combustile che impiegano combustibili a base di formiato (Direct Formate Fuel Cells – DFFC). In questo caso si sono impiegati catalizzatori nanostrutturati di Pd supportato da Vulcan XC-72 e ftalocianine di ferro e cobalto, rispettivamente all’anodo ed al catodo, ottenendo un potenziale di circuito aperto superiore ad 1 V. Le celle alcaline al formiato hanno prodotto una densità massima di potenza superiore alle celle alcaline che impiegano metanolo ed etanolo, ed anche alle celle acide che impiegano acido formico. In particolare l’efficienza energetica delle celle al formiato è stata superiore di un fattore 4 a quella delle migliori celle alcaline ad etanolo. Questo e’ un punto cruciale per l’applicazione pratica della tecnología proposta. Infatti l’efficienza energetica e’ uno dei cardini per il raggiungimento della sostenibilità e, senza dubbio, il vincolo principale per i sistemi che devono produrre grandi quantita’ di energía, come la generazione stazionaria di energía elettrica. Anche nel caso delle celle al formiato, abbiamo osservato che la composizione del combustibile è essenziale nel definire la performance energetica. Abbiamo mostrato che la massima densità di potenza si ottiene con un combustibile che contiene formiato 2 M e KOH 2 M, mentre l’energia per singolo batch di combustibile, la migliore conversione del combustibile e l’efficienza energetica sono migliori per il formiato 4 M e KOH 4 M. Al fine di migliorare la capacità del palladio di catalizzare l’ossidazione elettrochimica di composti organici rinnovabili, abbiamo sviluppato un metodo elettrochimico originale per il trattamento delle superfici degli elettrodi. Il trattamento consiste nell’applicazione di un potenziale ad onda quadra (Square Wave Potential – SWP) che produce un aumento della rugosità superficiale e una modifica della distribuzione delle terminazioni cristalline della superficie, incrementando la densità degli atomi di Pd superficiali a basso numero di coordinazione (< 8). Il trattamento si è rivelato efficace nel migliorare la cinetica di ossidaizione dell’etanolo, dell’etilen glicole e del glicerolo. I trattementi sviluppati hanno prodotto incrementi dell’attività fino ad un fattore 5.6. L’analisi FTIR dei processi di ossidazione ha dimostrato che anche la distribuzione dei prodotti di ossidazione e’ affetta dal trattamento. In particolate abbiamo riscontrato un incremento nella capacità dei catalizzatori ottenuti per SWP di rompere il legame C-C. Il trattamento elettrochimico con potenziale ad onda quadra è stato sviluppato anche per le superfici di platino, con l’obbiettivo di fornire uno strumento per ridurne il contenuto nelle fuel cells quando non sia possibile eliminarlo completamente. Nel caso del platino si è riscontrato che il parámetro piu’ importante per l’efficienza del trattamento è il periodo dell’onda quadra. Le superfici più attive si sono ottenute con un periodo di trattamento di 120 minuti, mentra la stabilità massima si e’ avuta per campioni trattati con onde quadre con periodo di 360 minuti. Tramite esperimenti FTIR si è inoltre concluso che nel caso del platino il trattamento inibisce la rottura del legame C-C. Questo fatto è importante perchè limita la formazione di frammenti CO che sono le principali specie che avvelenano gli elettrocatalizzatori a base di platino. Il capitolo 7 è dedicato allo studio dei meccanismi di deattivazione dei catalizzatori di palladio per l’ossidazione elettrochimica in ambente alcalino di alcoli. L’argomento è rilevante poichè la deattivazione è una delle principali cause che limita la diffusione di questi dispositivi. Abbiamo dimostrato che la formazione di ossidi è la causa che determina maggiormente la degradazione della performance catalítica. Siamo giunti a questa conclusione combinando le informazioni proveniente da indagini elettrochimiche ed esperimenti che impiegano la radiazione di sincrotrone. L’analisi degli spettri XANES (Near Edge X-ray Absorption Spectroscopy) ha mostrato che il palladio è presente nella sua forma metallica nei catalizzatori freschi, mentre è completamente ossidato dopo l’impiego in fuel cells. Nello studio si conclude che per allungare la vita degli anodi a base di palladio è necesario che il catalizzatore anodico non sia esposto a potenziali superiori a 0.7 V. Ciò è possibile in pratica con una semplice elettronica di controllo da abbinare alla cella. Al fine di aumentare la cinetica di ossidazione abbiamo provveduto ad effettuare esperimenti di ossidazione dell’etanolo a temperatura intermedie (> 100 °C) in autoclave. Abbiamo osservato che l’incremento della temperatura aumenta in misura significativa la capacità dei catalizzatori di ossidare l’etanolo in ambiente alcalino. Questo fatto è stato ascritto prevalentemente al miglioramento della capacità di adsorbire specie idrossido alla superficie del palladio. Lo stesso miglioramento non è stato osservato per esperimenti condotti in ambiente acido. Si sono inoltre realizzati esperimenti di ossidazione dell’etanolo su superfici di carburo di tungsteno in matrice di cobalto. Si è provato che questo materiale non mostra un’attività significativa per l’ossidazione di etanolo in ambiente alcalino. In ogni caso si è osservato che il materiale è stabile in ambienti alcalini, in un range di temperatura compreso tra 100 e 200 °C. Questo fatto unitamente all’elevata conducibilità suggerisce che il carburo di tungsteno in matrice di cobalto possa essere impiegato come supporto per la fase attiva dei catalizzatori, quali appunto il palladio. Lo stesso materiale ha mostrato una debole attività nell’ossidazione dell’etanolo ad una temperatura di 50 °C in ambiente acido. La stabilità non era però suficiente per permettere la caratterizzatione delle proprietà catalitiche in soluzioni acide a temperatura superiori.
Amongst current societal challenges sustainability is certainly a priority. The possibility of building a sustainable future, while maintaining high standards in the quality of life and preserving environment and resources, strongly relies on the availability of methods for the green production of energy and chemicals. The production of chemicals together with the on-demand power generation can be achieved in Direct Liquid Fuel Cells (DLFCs), devices in which the chemical energy of a liquid fuel is converted into electrical energy. DLFCs usually employ Small Organic Molecules (SOMs), such as alcohols or formic acid, as fuels. These fuels can be obtained from biomass feedstock. Consequently their use generates a significantly lower atmospheric CO2 with respect to the use of fossil fuels, resulting in a potential mitigation of the “greenhouse effect”. At the present stage, DLFCs rely on the use of the rare and costly platinum. This is for two reasons: i) platinum is a good catalyst for both SOMs oxidation and Oxygen Reduction Reaction (ORR); ii) platinum is stable in acidic environment. It is worth mentioning that most of DLFCs employ proton exchange membranes as electrolytes and need strongly acidic conditions for achieving low resistivity. In these systems also the water management can be a problem, as it is attracted to the cathode side for polarization and water is frequently introduced in the feed stream to the fuel cell. At present acidic DLFCs operate with a platinum content largely exceeding 1 mg cm-2, a fact that severely hampers the diffusion of such devices. In this investigation, thanks to a low resistivity Anion Exchange Membranes (AEM), the Tokuyama A-201, we have developed efficient alkaline direct liquid fuel cells (AEM-DLFCs). This has been done with the purpose of eliminating platinum from the devices. Indeed it is known that palladium effectively catalyzes SOMs oxidation in alkali; besides, oxygen reduction reaction can also be effectively achieved by using iron and cobalt phtalocyanines (Pc). Consequently the membrane electrode assembly (MEA) of a AEM-DLFC can be assembled using: i) a palladium based anode, ii) a Tokuyama A-201 membrane and iii) a cathode containing FePc-CoPc/C as electrocatalyst obtained from the high temperature pyrolysis of FePc-CoPc. An important fact is that FePc-CoPc/C is not active at all for the oxidation of SOMs. This has the major implication that fuel crossover through the membrane does not result in significant potential (and so energy efficiency) drop in fuel cells. The experimental part of this thesis starts with a chapter devoted to the analysis of the energy performance of platinum-free AEM-DLFCs fueled with ethanol (Chapter 3). This work is the first exhaustive analysis of the energy performance of such devices. Particularly we have determined the major parameters that characterize the fuel cell operations: i) maximum power density, ii) energy efficiency and iii) energy delivered per single fuel batch. All these parameters have been determined as a function of the fuel composition. We have discovered that the fuel concentration that maximizes one of the parameters can be detrimental to the others with the fundamental consequence that fuel composition must be selected according to the selected application. The effect of adding a promoting oxide, CeO2, to the anode catalyst has also been investigated. In some cases efficiency can be improved up to the 100% by simply adding cerium oxide to the anode catalyst. We have also proved that DEFCs are suitable candidates for the µ-fuel cells technology as we have shown their ability to operate with no or little performance degradation for 3 months at low power density (< 1 mW cm-2). Chapter 4 is dedicated to the Direct Formate Fuel Cells (DFFCs). Nanostructured Pd/C and FePc-CoPc/C have been employed at the anode and cathode side respectively. A large open circuit voltage (≥1.0 V) was obtained. This has been attributed to the larger (as compared with DEFCs) Nernst potential of the DFFCs and the use of FePc-CoPc/C as cathode electrocatalyst to restrain the reduction of cell voltage by fuel crossover. Our DFFCs have shown maximum power density larger than state of the art AEM-DLFCs and also Direct Formic Acid Fuel Cells (DFAFCs). AEM-DFFCs are also very effective in exploiting the energy content of the fuel. Indeed we have shown that DFFCs energy efficiency is four times the energy efficiency of analogous DEFCs. This point is very important to exploit the technology as the energy efficiency is the key issue for achieving sustainability and the major constraints for systems devoted to massive energy production. Again we have found that fuel composition is essential for the performance. The best power density was obtained by the cell fuelled with 2 M formate plus 2 M KOH, while best delivered energy, fuel utilization and energy efficiency were delivered by cell equipped with 4 M formate plus 4 M KOH. To enhance the ability of palladium to catalyze SOMs oxidation in alkaline environment, we have developed an original electrochemical treatment (Chapter 5). The treatment consisted of the application of a Square-Wave Potential (SWP) to the electrode and resulted in surface roughening and change in the distribution of the crystal surface terminations. Particularly we have found that after SWP an increase of the density of low coordination (Coordination Number < 8) Pd surface atoms occurs. We have found significant activity enhancement (from 4 to 5.6 times as compared to untreated surface) for the oxidation of all the investigated alcohols. Furthermore, FTIR spectra have shown that the reaction products distribution was also affected. Particularly we determined an increased tendency of the SWP treated Pd surface to cleave the C-C bond as compared to the untreated ones. A tailored SWP treatment for enhancing the catalytic activity of platinum was also developed (Chapter 6). The essential reason behind the study is to provide a tool for reducing Pt content in fuel cells when it cannot be completely eliminated. For platinum, it has turned out that the period of the square wave is the most important parameter. The most active platinum surface for Ethanol Oxidation Reaction (EOR) in alkali has been produced with a square wave period of 120 min, while the maximum stability of the catalytic performance has been obtained with the sample produced with a period of 360 min. Via in situ FTIR we have also found that the treated samples limit C-C cleavage as compared to the untreated ones. This suggests that SWP on Pt could provide an effective strategy to minimize the formation of CO, a major poisoning agent for platinum based catalysts. Chapter 7 is devoted to the investigation of the degradation mechanism of palladium electrocatalysts in platinum-free AEM-DLFCs. This is among the main issues still preventing the full exploitation of palladium in DLFCs. We have demonstrated that palladium oxide formation is the major cause for the catalytic performance degradation. We came to this conclusion by combining the information derived from electrochemical measurements and synchrotron light experiments (X-ray Absorption Spectroscopy). X-ray Absorption Near Edge Structure (XANES) spectra of the Pd Kα edge before and after DEFC run have shown that Pd is present in its metallic form in the pristine catalyst, while it is almost completely oxidized after work in an ethanol fed fuel cell. This has enabled us to conclude that to extend the service life of palladium electro-catalysts in alkali, the anode potential has not to exceed 0.7 V. In practice this can be achieved with a simple electronic control of the device. Increasing the operating temperature of fuel cells is an alternative strategy to improve the performance of fuel cells fed with SOMs containing fuels. In chapter 8, palladium has been investigated as a catalyst for ethanol oxidation at intermediate temperatures (> 100 °C) in a pressurized vessel. We have found that the increase of the temperature dramatically enhances the ability of catalyzing EOR in alkali. This fact has been ascribed to the improved adsorption of the hydroxyl species on the palladium surface. The same enhancement has not been observed in acidic environment. A few experiments on the use of tungsten carbide in a cobalt matrix (WC-Co) have also been performed. We have proved that WC-Co does not catalyze significantly the ethanol oxidation reaction in alkaline media, while it does in acidic electrolyte at medium temperature (~50 °C). At larger temperature the stability in acidic environment is not enough to allow a reliable assessment of the catalytic performance. Larger stability has been achieved in alkali where tungsten carbide is a potential candidate for supporting other active phases such as noble metals.
XXVII Ciclo
1987

Cette thèse de doctorat étudie la synthèse, caractérisation structurale et activité pour la réaction de réduction de O2 (ORR) de catalyseurs Fe-N-C et de composites d’oxydes de manganèse supporté sur Fe-N-C, ainsi que leur utilisation en pile à combustible à membrane échangeuse d’anions (AEMFC). Tandis que les piles à membrane échangeuse de protons (PEMFC) requièrent aujourd’hui du platine dans ses catalyseurs pour atteindre des hautes performances, les piles AEMFC peuvent ouvrir la voie vers des piles sans métaux précieux. Si les catalyseurs Fe-N-C sont actuellement étudiés comme alternative au platine à la cathode des PEMFC, ils souffrent d’une faible activité et d’une durabilité limitée dans ce milieu. En revanche, on peut espérer que l’activité et la durabilité des catalyseurs Fe-N-C soient améliorées dans les AEMFC.Ce travail démontre la haute activité, stabilité et durabilité en milieu alcalin de catalyseurs Fe-N-C comprenant des sites FeNx à un atome de fer. Ils ont été préparés à partir de ZIF-8 et de sel de fer, pyrolysé sous Ar (Fe0.5-Ar) puis sous NH3 (Fe0.5-NH3). Leur activité a été mesurée en électrode à disque tournant (RDE) et en AEMFC, tandis que la stabilité a été mesurée en RDE et operando avec un spectromètre de masse (ICP-MS) en aval d’une cellule à flux (SFC), en électrolyte acide et alcalin. Le dispositif ICP-MS/SFC a été utilisé pour mesurer in operando la dissolution du fer. En électrolyte acide oxygéné, la vitesse de dissolution du fer du catalyseur le plus actif (Fe0.5-NH3) est 10 fois plus rapide que celle du catalyseur moins actif, Fe0.5-Ar. Ceci explique la faible stabilité des catalyseurs Fe-N-C pyrolysés sous NH3 en PEMFC. En revanche, en électrolyte alcalin, les vitesses de dissolution du fer sont faibles, même pour Fe0.5-NH3. Ces résultats vont de pair avec l’absence de changement d’activité en RDE après un test de dégradation accélérée. La nature des sites actifs a de plus été étudiée par spectroscopie d’absorption de rayons X en mode operando.Afin de réduire la quantité de peroxyde d’hydrogène sur Fe-N-C pendant l’ORR, plusieurs oxydes de manganèse ont été synthétisés et leur activité pour l’ORR et la réaction de réduction du peroxyde d’hydrogène (HPRR) évaluée. Il a été démontré par ICP-MS/SFC que même l’oxyde de manganèse le plus stable, Mn2O3, peut dissoudre une quantité importante de Mn pendant l’ORR en milieu alcalin. De plus, cette dissolution est due au peroxyde d’hydrogène produit pendant l’ORR. Des composites MnOx/Fe0.5-NH3 ont été étudiés pour les réactions ORR et HPRR. Tous ont montré une meilleure sélectivité pendant l’ORR que Fe0.5-NH3 seul, et l’effet le plus important fut avec Mn2O3.Avant d’étudier ces catalyseurs en AEMFC, une étude a été faite sur la compatibilité entre différents catalyseurs de l’ORR et/ou de l’oxydation de H2 (Pt/C, Fe0.5-NH3, PtRu/C, Pd-CeO2/C) et des ionomères échangeurs d’anion, en RDE dans 0.1 M KOH. Ceci a permis d’identifier certains problèmes entre les ionomères étudiés et les catalyseurs comprenant une faible quantité de métal (Fe0.5-NH3, Pd-CeO2/C).Les catalyseurs Fe0.5-NH3 et Mn2O3/Fe0.5-NH3 ont alors été étudiés en AEMFC avec un ionomère à base d’éthylène-tetrafluoroéthylène. Les deux catalyseurs atteignent une densité de courant de 80 mA cm-2 à 0.9 V, avec un chargement de 1.0-1.5 mg cm-2. Le pic de puissance sous H2/O2 est de 1 W cm-2 à 60°C, avec une AEM à base de polyéthylène basse densité, et de 1.4 W cm-2 à 65°C avec une AEM en polyéthylène haute densité. En comparaison, une densité de courant de 70 mA cm-2 à 0.9 V et un pic de puissance de 1.5 W cm-2 ont été obtenus avec 0.45 mgPt cm-2 à la cathode (40 wt% Pt/C) à 60°C, avec l’AEM en polyéthylène basse densité. Un test de durabilité de 100 h à 0.6 A cm-2 sous air a montré une bonne stabilité de Fe0.5-NH3.En conclusion, ce travail met en exergue l’application prometteuse des catalyseurs Fe-N-C à la cathode de piles AEMFC, afin de s’affranchir des catalyseurs à base de métaux précieux
This PhD thesis investigates the synthesis, structural characterization and oxygen reduction reaction (ORR) activity of Fe-N-C catalysts and composites of Fe-N-C and manganese oxides, and their application at the cathode of anion exchange membrane fuel cells (AEMFCs). Compared to proton exchange membrane fuel cells (PEMFCs), where platinum is today needed to reach high performance, AEMFCs hold the promise to reach high performance without precious metals in their catalysts. While Fe-N-C catalysts are currently investigated as an alternative to Pt/C for PEMFC cathodes, they suffer from lower activity and lower durability in the acidic medium of PEMFCs. In contrast, both the ORR activity and stability of Fe-N-C catalysts can be expected to be significantly improved in AEMFC.This PhD work demonstrates the high activity, stability and durability in alkaline medium of Fe-N-C catalysts with atomically-dispersed FeNx sites. They were prepared from a mix of ZIF-8 and iron salt, pyrolyzed in argon (Fe0.5-Ar) and then ammonia (Fe0.5-NH3). The activity was measured in a rotating disk electrode (RDE) and in AEMFC, while the stability was measured in RDE and in operando with mass spectroscopy (ICP-MS) coupled with a scanning flow cell, in both acid and alkaline media. The latter setup was used to measure Fe dissolution in operando. It was evidenced that, in oxygenated acid electrolyte, the iron leaching rate of the most active Fe-N-C catalyst (Fe0.5-NH3) is 10 times faster compared to the less active Fe0.5-Ar. This explains the reduced stability of ammonia-treated Fe-N-C catalysts in operating PEMFC. In contrast, in alkaline medium, very little demetallation was observed even for Fe0.5-NH3. This was correlated with almost unchanged activity after load cycling in RDE. The nature of the active sites was investigated with X-ray absorption spectroscopy, including in operando measurements.Then, to minimize the amount of peroxide species during ORR on Fe-N-C, different manganese oxides were synthesized and their activity for ORR and hydrogen peroxide reduction reaction (HPRR) were evaluated, while operando manganese dissolution was investigated with ICP-MS. It was found that even the most stable Mn-oxide, Mn2O3, leached a significant amount of Mn during ORR in alkaline medium. It was further demonstrated that the Mn leaching is associated with hydrogen peroxide produced during ORR. Composites of Fe0.5-NH3 and Mn-oxides were then investigated for ORR and HPRR. Improved selectivity during ORR was observed for all composites relative to Fe0.5-NH3 alone, but the effect was strongest for Mn2O3.Before investigating such catalysts in AEMFC, a study on the compatibility between different ORR and/or hydrogen oxidation reaction catalysts (Pt/C, Fe0.5-NH3, PtRu/C, Pd-CeO2/C) and anion exchange ionomers was performed in RDE in 0.1 M KOH. The study identified issues between the investigated ionomers and catalysts having low metal contents on the carbon support (Fe0.5-NH3, Pd-CeO2/C).The catalyst Fe0.5-NH3 and its composite with Mn2O3 were then investigated in AEMFC with an ethylene-tetrafluoroethylene ionomer. Both cathode catalysts reached a current density of ca 80 mA cm-2 at 0.9 V, with relatively low loading of 1.0-1.5 mg catalyst·cm-2. The peak power density with H2/O2 reached 1 W cm-2 at 60°C with a low density polyethylene AEM and 1.4 W cm-2 with high density polyethylene AEM at 65°C. By comparison, a current density of ca 70 mA cm-2 at 0.9 V and peak power density of 1.5 W cm-2 was reached with 0.45 mgPt cm-2 at the cathode (40 wt% Pt/C) with low density polyethylene AEM at 60°C. A durability test of 100 h at 0.6 A cm-2 in air showed good stability of the Fe0.5-NH3 catalyst.In conclusion, this work highlights the promising application of Fe-N-C catalysts at the cathode of AEMFCs for replacing precious metal catalysts

Kyoto University (京都大学)
0048
新制・課程博士
博士(工学)
甲第11583号
工博第2529号
新制||工||1344(附属図書館)
23226
UT51-2005-D332
京都大学大学院工学研究科物質ｴﾈﾙｷﾞｰ化学専攻
(主査)教授 小久見 善八, 教授 垣内 隆, 教授 田中 功
学位規則第4条第1項該当

Landfill leachates are often discharged to wastewater treatment plants (WWTPs) but their highly varied composition makes their treatment in WWTPs difficult. Landfill leachates contain bio-refractory organic matter which easily passes the biological treatment processes at WWTPs and increases the organic matter in the effluent. Leachates also interfere with the UV disinfection process at treatment plants. Another concern is the presence of large amounts of bio-refractory organic nitrogen in the leachates which makes it difficult for WWTPs to meet the tightening total nitrogen requirements. Studies were conducted to evaluate the applicability of anion exchange resins to remove organic matter, UV quenching substance and organic nitrogen from landfill leachates. Leachate samples based on varying age and treatment methods were utilized. The anion exchange resins were found to work effectively for all studied leachates. The resins were found to remove more bio-refractory UV absorbing substances as compared to total organic carbon (TOC), suggesting that anion exchange resins could be employed for removal of UV absorbing substances. Multiple regenerations of the resin showed slight loss in the capacity to remove UV and organic carbon. Fractionation of leachate samples showed effective removal of humic acid (HA) fraction which is responsible for most of UV quenching. The resin was also found to effectively remove the bio-refractory hydrophilic (Hpi) fraction which tends to persist even after HA fraction has bio-degraded. Membrane filtration (1000 Da and 3000 Da Molecular weight cut off) in conjunction with ion exchange resins achieved better removal of organic matter and UV254 absorbing substances. In addition, this also significantly improved the performance of resins. Significant removal of organic nitrogen was also observed using anion exchange though it was less than both UV and TOC. Around 80% removal of organic nitrogen associated with bio-refractory Hpi fraction was achieved using anion exchange suggesting ion exchange as a viable alternative for removing organic nitrogen.
Master of Science

The hydrogen, as energy vector, is considering one promising green, sustainable, low-cost alternative to hydrocarbon fuels. In the circular hydrogen economy, the fuel cell technologies play a crucial role of the energy conversion and, in particular, Anion Exchange Membrane Fuel Cell are retained to be very promising for the high-power delivery, the short waiting time before providing energy, the low working temperature. My PhD is focus on synthesis and characterization of anionic conducting polymer for fuel cell and electrolyzer applications. The first part of activities is focused on the study of new chemical modifications of polyfluorinated (Aquivion®), aliphatic polyketones, polystyrene polymer matrix to address the main drawbacks of the chemical and electrochemical stability and also the high cost. The synthesis methods involve the organic chemistry procedure for examples Pall-Knorr reaction, Baeyer-Villiger oxidation, methylation process. The physical-chemical characterization part is aimed to the better understand the properties of the functionalized polymer matrix. The polymer structure is investigated by spectroscopes technique for example FTIR and solid-state NMR while, the thermal properties and their stability are determined by TGA and DSC measurements. For the promising work of Aquivion® modification, I also performed accelerated ageing treatment for testing the chemical and electrochemical stability and I used them in for water Electrolyzer application. The functionalized polymers show interesting and promising properties for fuel cell and electrolyzer applications and, in particular, modified Aquivion® membranes show excellent stability in alkaline environmental and archive 130 mA cm-2 at 80°C. The results of Aquivion® modification are published on two international journals and the polyketones functionalization work is undergoing publication.
The hydrogen, as energy vector, is considering one promising green, sustainable, low-cost alternative to hydrocarbon fuels. In the circular hydrogen economy, the fuel cell technologies play a crucial role of the energy conversion and, in particular, Anion Exchange Membrane Fuel Cell are retained to be very promising for the high-power delivery, the short waiting time before providing energy, the low working temperature. My PhD is focus on synthesis and characterization of anionic conducting polymer for fuel cell and electrolyzer applications. The first part of activities is focused on the study of new chemical modifications of polyfluorinated (Aquivion®), aliphatic polyketones, polystyrene polymer matrix to address the main drawbacks of the chemical and electrochemical stability and also the high cost. The synthesis methods involve the organic chemistry procedure for examples Pall-Knorr reaction, Baeyer-Villiger oxidation, methylation process. The physical-chemical characterization part is aimed to the better understand the properties of the functionalized polymer matrix. The polymer structure is investigated by spectroscopes technique for example FTIR and solid-state NMR while, the thermal properties and their stability are determined by TGA and DSC measurements. For the promising work of Aquivion® modification, I also performed accelerated ageing treatment for testing the chemical and electrochemical stability and I used them in for water Electrolyzer application. The functionalized polymers show interesting and promising properties for fuel cell and electrolyzer applications and, in particular, modified Aquivion® membranes show excellent stability in alkaline environmental and archive 130 mA cm-2 at 80°C. The results of Aquivion® modification are published on two international journals and the polyketones functionalization work is undergoing publication.

Vätgas kan framställas från förnybara energikällor genom vattenelektrolys med anjonbytande membran (AEMWE). AEMWE har vissa fördelar jämfört med traditionell alkalisk vattenelektrolys och elektrolysmed protonledande membran. Till exempel finns det möjlighet att använda alkalisk elektrolyt (även rent vatten) och billiga platinagruppsmetallfria katalysatorer tillsammans med ett anjonbytesmembran. Den största utmaningen med tekniken är att uppnå utmärkt och stabil prestanda för membran och elektroder. AemionTM anjonbytande membran (AEMs) av olika tjocklek, vattenupptag och kapacitet undersöktes i ett AEMWE system med 5 cm2 elektrodarea. Elektrokemisk prestanda hos dessa kommersiella AEM studerades med hjälp av porösa nickel elektroder. Bland de undersökta membranen visade AF2-HWP8-75-X stabil prestanda med en högfrekvent resistans (HFR) på 90 mΩ•cm2 och kunde nå en strömtäthet på 0,8 A/cm2 vid 2,38 V med 1 M KOH vid 60 ˚C.  AEMWE med AF2-HWP8-75-X och olika elektrodkombinationer undersöktes under samma driftsförhållanden. En elektrodkombination med Raney-Ni och NiFeO som katod respektive anod visade bäst prestanda under utvärderingen och gav en strömtäthet på 1,06 och 3,08 A/cm2 vid 2,00 respektive 2,32 V. KOH-lösningens temperatur och koncentration sänktes till 45 ˚C respektive 0,1 M för att undersöka effekten av driftsparametrar på flödescellens prestanda. Flödescellen uppvisade god stabilitet under de nya driftsförhållandena, men dess prestanda minskade avsevärt. Den nådde en strömtäthet på 0,8 A/cm2 vid 2,25 V.
Hydrogen can be produced from renewable energy sources using a novel anion exchange membrane water electrolysis (AEMWE) system. AEMWE has some benefits over the currently used state-of-the-art alkaline and proton exchange membrane water electrolysis systems. For instance, there is a possibility of using alkaline electrolytes (even pure water) and low-cost platinum-group-metal free catalysts together with an ion exchange membrane. However, the main challenge is that the AEMWE system should show excellent and stable performance, depending on the stability of the membrane and the electrodes. AemionTM anion exchange membranes (AEMs) of different thickness and water uptake capacity were investigated using a 5 cm2 AEMWE system. The electrochemical behaviour of these commercial AEMs was studied using nickel (Ni) felt electrodes. Among the investigated AEMs, the AF2-HWP8-75-X showed stable performance with a high frequency resistance (HFR) of 90 mΩ•cm2 and was able to reach a current density of 0.8 A/cm2 at 2.38 V using 1 M KOH at 60 ˚C.  AEMWE systems based on AF2-HWP8-75-X and different electrode combinations were examined under the same operating conditions. An electrode combination with Raney-Ni and NiFeO as cathode and anode, respectively, showed the best performance during the degradation test and provided a current density of 1.06 and 3.08 A/cm2 at 2.00 and 2.32 V, respectively. The operating temperature and concentration of the KOH solution were reduced to 45 ˚C and 0.1 M, respectively, to study the effect of operating parameters on the flow cell performance. The flow cell showed good stability under the new operating conditions, but its performance was reduced significantly. It reached a current density of 0.8 A/cm2 at 2.25 V.

Anion exchange membrane fuel cells (AEMFCs) are an alternative to proton exchange membrane fuel cells (PEMFCs) with potential benefits that include low cost (i.e., platinum-free), facile electro-kinetics, low fuel crossover, and use of CO-resistant metal catalysts. Despite these advantages, AEMFCs have not been widely used because they require more highly conductive anion exchange membranes (AEMs) that do not exhibit impaired physical properties. Therefore, the issues that this research is dealing with are to maximize conductivity and to improve chemical stability. As model materials for these studies, I synthesize a series of multiblock copolymers with which polymer structures and morphologies can be easily controlled. Chapter 2 presents the synthesis and the chemical structure determination of the multiblock copolymers. With the objective of maximizing conductivity, an understanding of the impact of structural features such as organization, size, polarity and connectivity of ionic domains and channels within AEMs on ion/water transporting properties is necessary for the targeted and predictable design of an enhanced material. Chapters 3 to 5 describe three characterization techniques that reveal the role of these structural features in the transport process. Specifically, Chapter 3 demonstrates the possibility that the NMR relaxation times of water could be an indicator of the efficiency of ion channels. Low-temperature DSC measurements differentiate the state of water (i.e., bound water and free water) inside the membranes by measuring freezing temperature drop and enthalpy. Chapter 4 demonstrates that the number of water molecules in each state correlates with conductivity and suggests a major anion-conducting mechanism for the multiblock AEM systems. In Chapter 5, the measurement of the activation energy of diffusion characterizes ion transporting behavior that occurs on the sub-nanometer scale. For the characterization of the chemical stability of the AEMs under high pH conditions, I employ automated 1H NMR measurements as a function of time as well as diffusion-ordered NMR spectroscopy (DOSY) as shown in Chapter 6. Finally, I demonstrate that new multiblock copolymers are successfully utilized as an ionomer for a hybrid cell in Chapter 7. The properties of the polymer strongly influence overall cell performance. I believe that the combination of the techniques presented in this thesis will provide insight into the ion/water transporting mechanism in a polymer ion conductor and guidance for improving conductivity and the chemical stability of the AEMs.

It is desirable to increase the operation temperature of proton exchange membrane fuel cells above 100oC due to fast electrode kinetics, high tolerance to fuel impurities and simple thermal and water management. In this study
the objective is to develop a high temperature proton exchange membrane fuel cell. Phosphoric acid doped polybenzimidazole membrane was chosen as the electrolyte material. Polybenzimidazole was synthesized with different molecular weights (18700-118500) by changing the synthesis conditions such as reaction time (18-24h) and temperature (185-200oC). The formation of polybenzimidazole was confirmed by FTIR, H-NMR and elemental analysis. The synthesized polymers were used to prepare homogeneous membranes which have good mechanical strength and high thermal stability. Phosphoric acid doped membranes were used to prepare membrane electrode assemblies. Dry hydrogen and oxygen gases were fed to the anode and cathode sides of the cell respectively, at a flow rate of 0.1 slpm for fuel cell tests. It was achieved to operate the single cell up to 160oC. The observed maximum power output was increased considerably from 0.015 W/cm2 to 0.061 W/cm2 at 150oC when the binder of the catalyst was changed from polybenzimidazole to polybenzimidazole and polyvinylidene fluoride mixture. The power outputs of 0.032 W/cm2 and 0.063 W/cm2 were obtained when the fuel cell operating temperatures changed as 125oC and 160oC respectively. The single cell test presents 0.035 W/cm2 and 0.070 W/cm2 with membrane thicknesses of 100 µ
m and 70 µ
m respectively. So it can be concluded that thinner membranes give better performances at higher temperatures.

Several types of novel proton exchange membranes which could be used for both direct methanol fuel cells (DMFCs) and hydrogen/air fuel cells were investigated in this work. One of the main challenges for DMFC membranes is high methanol crossover. Nafion, the current perfluorosulfonic acid copolymer benchmark membrane for both DMFCs and hydrogen/air fuel cells, shows very high methanol crossover. Directly copolymerized disulfonated poly(arylene ether sulfone)s copolymers doped with zirconium phosphates and phenyl phosphonates were synthesized and showed a significant reduction in methanol permeability. These copolymer/inorganic nanocomposite hybrid membranes show lower water uptake and conductivity than Nafion and neat poly(arylene ether sulfone)s copolymers, but in some cases have similar or even slightly improved DMFC performance due to the lower methanol permeability. These membranes also show advantages for high temperature applications because of the reinforcing effect of the filler, which helps to maintain the modulus of the membrane, allowing the membrane to maintain proton conductivity even above the hydrated glass transition temperature (Tg) of the copolymer. Sulfonated zirconium phenyl phosphonate additives were also synthesized, and membranes incorporating these materials and disulfonated poly(arylene ether sulfone)s showed promising proton conductivity over a wide range of relative humidities. Single-Tg polymer blend membranes were studied, which incorporated disulfonated poly(arylene ether sulfone) with varied amounts of polybenzimidazole. The polybenzimidazole served to decrease the water uptake and methanol permeability of the membranes, resulting in promising DMFC and hydrogen/air fuel cell performance.
Ph. D.

Le celle a combustibile con membrane a scambio protonico (PEMFCs) sono alternative sorgenti di energia che offrono numerosi vantaggi come l’alta efficienza, l’alta densità di potenza e la bassa emissione d’inquinanti. Esse sono impiegate nelle macchine ad idrogeno e nei dispositivi elettronici quali computer e cellulari alimentati con metanolo. La diffusione su larga scala di queste tecnologie punta sullo sviluppo di membrane a scambio protonico di nuova generazione, il cui costo di produzione sia compatibile con un mercato di massa. Tali conduttori protonici devono esibire una buona conducibilità, stabilità chimica e termica. Nel presente lavoro, diverse strategie sono state impiegate per la preparazione di materiali a conduzione protonica a partire dai polimeri termoplastici aromatici: polietereterchetone (PEEK) e polifenilsolfone (PPSU). Di particolare rilevanza è la funzionalizzazione di tali polimeri mediante l’introduzione sulla catena aromatica di gruppi solfonici e gruppi contenenti silicio. Infatti, questo approccio sintetico permette di controllare la microstruttura del polimero, modulando il rapporto tra la fase idrofila e quella idrofoba, da cui dipendono fortemente le prestazioni dell’elettrolita. Diversi tipi di membrane sono state preparate impiegando: PEEK solfonato (SPEEK) e/o PPSU solfonato, variamente funzionalizzati con gruppi contenenti silicio, al fine di ottenere l’effetto sinergico derivante dalla combinazione di polimeri aventi diverse conducibilità protonica e caratteristiche meccaniche. I sistemi ibridi sono stati preparati mediante la reazione sol-gel che ha portato alla formazione dei legami covalenti Si-O-Si tra due derivati del PPSU diversamente funzionalizzati. Le membrane blend sono state invece preparate mescolando, durante il processo di casting, derivati del PEEK e/o PPSU. La caratterizzazione dei materiali ha riguardato l’analisi della struttura dei polimeri sintetizzati e delle proprietà chimico-fisiche ed elettrochimiche delle membrane. Risultati molto positivi sono stati ottenuti dai test eseguiti sulle membrane in un prototipo di cella a combustibile operante a metanolo diretto.
Proton exchange membrane fuel cells (PEMFCs) are promising power sources emerging among alternative energy conversion systems, because they can operate at relatively low temperature and offer numerous benefits, such as high efficiency, high power density and low polluting emissions. The present dissertation deals with the development of new proton conducting membranes having good conductivity, chemical and thermal stability, low methanol permeability and low cost. The main strategy used in this work was the preparation of sulfonated and silylated polyetheretherketone (PEEK) and polyphenylsulfone (PPSU) as membrane materials, because this synthetic approach represents a powerful tool to modulate the proton conductivity and hydrolytic stability of the electrolyte by the dosage of sulfonic acid groups and inorganic moieties covalently bound to the aromatic chains. Several types of proton exchange membranes were studied. Sulfonated and silylated PEEK and/or PPSU were used to prepare systems where two components resulted crosslinked by physical interactions or covalent bonds, obtaining the synergic effect of polymers having different conductivity and mechanical properties. • Sulfonated and silylated polyetheretherketone PhSi0.1S0.9PEEK (degree of sulfonation DS=0.9, and degree of silylation DSi=0.1) was synthesized via (i) sulfonation of PEEK, (ii) conversion of sulfonated polyetheretherketone (S0.9PEEK) into sulfonyl chlorinated derivative (PEEKSO2Cl), (iii) lithiation of PEEKSO2Cl and subsequent addition of PhSiCl3, followed by hydrolysis. The solubility of PEEKSO2Cl in organic solvent allows the silylation reaction to be carried out in homogeneous conditions. The structural characterization of the products by 1H and 13C NMR and ATR/FTIR spectroscopies highlighted the success of the synthetic pathway. The thermogravimetric analysis of PEEK derivatives indicated that the presence of the inorganic moieties stabilizes the aromatic matrix of the sulfonated polyetheretherketone. Blends of PhSi0.1S0.9PEEK and S0.5PEEK (DS=0.5) were prepared using different weight ratios of the two polymers. The membranes were characterized by water uptake measurements and electrochemical impedance spectroscopy (EIS). The results converge to indicate that the developed materials are promising electrolytes for PEMFC application. • Silylated and sulfonated polyphenylsulfone PhSi0.2S2PPSU (DS=2.0 and DSi=0.2) was synthesized via (i) lithiation of PPSU and subsequent addition of PhSiCl3, followed by hydrolysis, (ii) sulfonation by reaction with concentrated sulphuric acid. The chemical structure of polymers was investigated by 1H and 13C NMR, and ATR/FTIR, verifying the success of the developed synthetic route. Blends of PhSi0.2S2PPSU and S0.5PEEK were prepared, obtaining electrolytes with higher hydrolytic stability and increased proton conductivity with respect to those of pure S0.5PEEK membrane. Blend membranes showed also better performance in DMFC, where a reduced methanol permeability and adequately high power density values were observed, at temperature values as high as 100°C. All these features identify the prepared blend membranes as promising electrolytes for DMFC operating at intermediate temperatures. • Two silylated and sulfonated PPSU derivatives: Si0.2S2PPSU (DS=2.0 and DSi=0.2) and Si0.03S0.05PPSU (DS=0.05 and DSi=0.03) were synthesized following two different routes. In the first one, PPSU was silylated by reaction with SiCl4, then sulfonated by reaction with concentrated sulphuric acid, and Si0.2S2PPSU was obtained. In the second route, the use of the mild sulfonating agent ClSO3Si(CH3)3 allowed a careful control of the degree of sulfonation, and PPSU with a lower DS was obtained. Subsequent silylation by reaction with SiCl4 led to the final product Si0.03S0.05PPSU. An organic-inorganic hybrid polymer HSiSPPSU was synthesized by non-hydrolytic sol–gel reaction of Si0.2S2PPSU and Si0.03S0.05PPSU. The condensation between the silanol groups of the two polymers led to the formation of Si-O-Si bonds, as highlighted by analysis of ATR/FTIR spectra. The electrochemical characterization of HSiSPPSU membranes by EIS showed adequately high conductivity values to make the hybrid polymer a suitable candidate for application in PEMFCs operating at T > 100°C. The strategies followed in this work seems to be an effective way to overcome some drawbacks related to conventional polymer membranes currently used, demonstrating the relevant role played by synthesis in the preparation of electrolytes for PEMFCs.

Alkaline anion exchange membranes (AAEM) have been fabricated using polyethylene as the base polymer offering a low cost AAEM for electrolyser and fuel cell applications. This study focused on the synthesis and characterisation of AAEM with controlled degree of grafting (DOG) and ion-exchange capacity (IEC) with the following parameters investigated: low density polyethylene (LDPE) film thickness 30-130 μm, gamma radiation dose and monomer concentration. The corresponding IEC, water uptake (WU) and degree of swelling (DS) are reported. The performance of 74.6% DOG membrane in a hydrogen fuel cell showed a high open circuit voltage of 1.06 V, with a peak power density of 608 mW cm-2 at 50 °C under oxygen. The use of a membrane with a high DOG does not impact fuel cross-over significantly and provides improved fuel cell performance due to its high conductivity, water transport and resilience to dehydration. The AAEMs showed long term stability, at 80 °C, exhibiting a conductivity of ca. 0.11 S cm-1 over a period of 3300 h under nitrogen. The membrane showed a degradation rate of 5.7 and 24.3 mS kh-1 under nitrogen and oxygen, respectively. With the membrane lifetime defined as the duration of fuel cell operation until the conductivity of the membrane has reduced to a cut-off value of 0.02 S cm-1, the estimated lifetime of the membrane is 2 years under nitrogen and 5 months under oxygen operating at 80 °C. The fabricated anion exchange membranes were subjected to degradation tests in deionised water for electrolyser/fuel cell operation. After the degradation test, the decrease in ion exchange capacity (IEC) of the AEM, hence decrease in ionic conductivity, was influenced by the applied gamma radiation dose rate. The use of a high radiation dose rate produced membranes with improved stability in terms of % IEC loss due to shorter, more uniformly distributed vinylbenzyl chloride (VBC) grafts. For LDPE-based AEMs, increasing the applied radiation dose rate during grafting from 30 to 2000 Gy h-1 significantly reduced AEM % IEC loss from 38 to 11%, respectively. Analyses of both the aged functionalised membranes and their resulting degradation products confirmed the loss of not only the functional group, but also the VBC group, which has not been reported previously in the literature. Investigation of other amine functional groups revealed similar degradation via the removal of both VBC and head group. Oxidation reactions iii taking place at pH close to neutral are the main contributor to the IEC loss, in contrast to the widely reported E2 or SN2 attack on the head group in high alkalinity solutions. A parallel degradation mechanism is proposed to explain head group loss of AEMs, that involves peroxide radicals which are more dominant in low alkalinity solutions. The investigation of the degradation of a commercially available AEM (A201, Tokuyama Corp.) was performed and compared with the fabricated LDPE AEMs. Using similar membrane thickness, results revealed that the fabricated AEM exhibited superior stability to the commercial A201 membrane in terms of % IEC loss and ionic conductivity, both in fuel cell and electrolyser modes. It is believed that the faster degradation rate of the A201 membrane could possibly be due to the attack of OH- ions on both the head group and on the polymer backbone, the latter of which was not observed on the fabricated AEMs.

In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the
best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen.

A transient, one-dimensional, model of the membrane of a proton exchange membrane fuel cell is presented. The role of the membrane is to transport protons from the anode to cathode of the fuel cell while preventing the transport of other reactants. The membrane is modeled assuming mono-phase, multi-species flow. For water transport, the principle driving forces modeled are a convective force, an osmotic force (i.e. diffusion), and an electric force. The first of these results from a pressure gradient, the second from a concentration gradient, and the third from the migration of protons from anode to cathode and their effect (drag) on the dipole water molecules. Equations are developed for the conservation of protons and water, the conservation of thermal energy, and the variation of proton potential within the membrane. The model is solved using a fully implicit finite difference approach. Results showing the effects of current density, pressure gradients, water and heat fluxes, and fuel cell start-up on water concentration, temperature, and proton potential across the membrane are presented.
Master of Science

My research goals are to synthesize new zwitterionic perfluorosulfonimide (PFSI) monomer/polymers. They are expected to replace traditionally used perfluorosulfonic acid (PFSA) polymers as the electrolyte in PEM fuel cells. For the PFSI monomer preparation, we designed a nine-step synthesis route. Thus far, I have successfully completed the synthesis of 4- (2-bromotetrafluoroethoxy)-benzenesulfonyl amide, 4-acetoxybenzenesulfonic acid sodium salt, and 4-chlorosulfonyl phenyl acetate. The coupling reaction of 4-(2-bromotetrafluoroethoxy)- benzenesulfonyl amide with 4-chlorosulfonyl phenyl acetate, was troublesome due to slow reaction kinetics and byproducts. Additionally, I did a methodology study for the homopolymerziation of the perfluoro 3(oxapent-4-ene) sulfonyl fluoride monomer. We compared the weight average molecular weight (Mw) of different reaction conditions. The best Mw was achieved when the polymerization was carried out for five days at 100 °C and150 psi with 2 wt % initiator and 5 g of monomer. All the compounds were characterized by melting point, GC-MS, GPC, FT-IR, and 13C/1H/19F NMR.

Fuel cells have the potential to change the energy paradigm by allowing more efficient use of energy. In particular, Polymer Electrolyte Membrane Fuel Cells (PEMFC) are interesting because they are low temperature devices. However, there are still numerous challenges limiting their widespread use including operating temperature, types of permissible fuels and optimal use of expensive catalysts. The first two problems are related mainly to the ionomer electrolyte, which largely determines the operating temperature and fuel type. While new ionomer membranes have been proposed to address some of these issues, there is still a lack of fundamental knowledge to guide ionomer design for PEMFC. This work is a computational study of the effect of temperature and water content on sulfonated poly(ether ether ketone) and the effect of acidity on sulfonated polystyrene to better understand how ionomer material properties differ. In particular we found that increased water content preferentially solvates the sulfonate groups and improves water and hydronium transport. However, we found that increasing an ionomer’s acid strength causes similar effects to increasing the water content. Finally, we used Density Functional Theory (DFT) to study platinum nano-clusters as used in PEMFCs. We developed a model using the atom’s coordination number to quickly compute the energy of a cluster and therefore predict which platinum atoms are most loosely held. Our model correctly predicted the energy of various clusters compared to DFT. Also, we studied the interaction between the various moieties of the electrolyte including the catalyst particle and developed a force field. The coordination model can be used in a molecular dynamics simulation of the three phase region of a PEMFC to generate unbiased initial clusters. The force field developed can be used to describe the interaction between this generated cluster and the electrolyte.

Proton Exchange Membrane Fuel Cells (PEMFCs) are widely regarded as the next generation of portable power production devices, with uses ranging from powering automotive vehicles to laptops and smartphones. PEMFCs convert oxygen and hydrogen into water and usable electricity and have no moving parts, meaning that they can reach overall efficiencies of 60%. However current Polymer Electrolyte Membranes only work efficiently below 80 C and at high humidity. At this low temperature, CO poisoning of the Pt electrocatalysts means that only high-grade fuel (low CO concentration ≤ 2 ppm) and high catalyst loading are required. This means that the overall cost of a PEMFC is prohibitively expensive. To dramatically the reduce cost and increase the efficiency of a PEMFC, new membranes are required which work at 120 C, at which point CO poisoning is no longer a dominating issue. In this thesis, the synthesis of novel organic/inorganic hybrid polyurethane Polymer Electrolyte Membranes (PEMs) with covalently bound phosphonic acid moieties (PA) made from cheap source materials have been reported, which, for the first time, demonstrate high conductivities at high temperatures, for example a PEM made from triethoxysilylpropyl isocyanate, polyethylene glycol, 4,4’-methylene diphenyl diisocyanate and PA displayed a conductivity of 3  10-2 S cm-1 at 120 C and 100% RH. The membranes also display good mechanical, thermal and chemical stability making them ideal candidate PEMs for the use in PEMFCs. However further work needs to be done to reduce the thickness of the membranes from their current thickness of 200 m to just 20-30 m, which would dramatically increase their efficiency when used in a PEMFC, by reducing the Area Specific Resistance and increasing the output (usable) power.

The overall goal of this research is to synthesize two different monomers for proton exchange membrane (PEM) Fuel Cells. Such monomers are proposed to be polymerized to improve the efficiency and compatibility of electrodes and electrolytes in PEM fuel cells. The first target is to synthesize 4-diazonium-3-fluoro PFSI zwitterionic monomer. Three steps were carried out in the lab. First one was the ammonolysis of 3-fluoro-4-nitrobenzenesulfonyl chloride. Second reaction was the bromination of Nafion monomer. The next coupling reaction, between brominated Nafion monomer and the 3-fluoro-4-nitrobenzenesulfonamide, was failed. The obstacles involve the harsh reaction condition and troublesome purification procedure. The second target is to synthesize 5-nitro-1, 3-benzenedisulfonamide. According to the literature, this synthesis was also designed as three steps: 1)nitration of sodium 1, 3-benzenedisulfonate salt; 2)chlorination of sodium 5-nitro-1, 3-benzenedisulfonate salt; and 3)ammonolysis of 5- nitro-1, 3- benzenedisulfonyl chloride. This monomer is expected to be copolymerized for membrane electrolyte in PEM fuel cells.

Durability is one of the major issues for the successful commercialisation of polymer electrolyte membrane fuel cells (PEMFCs) and it mainly depends on the stability of the individual cell components. In order to minimise the durability issues, the development of new materials or modification to replace the existing fuel cell components is required. The typically used proton exchange membrane (PEM) is the perfluorosulfonated polymer such as Nafion® and electrocatalysts for PEMFC is high surface area carbon supported platinum electrocatalyst (Pt/C). A higher temperature of operation (>80 oC) of PEMFC would boost their performance by enhancing the electrochemical kinetics and also improve the carbon monoxide tolerance of platinum catalysts. A Nafion® type membrane is not suitable for higher temperature operation as its proton conductivity mainly depends on the hydration level. An approach to improve the proton conductivity of Nafion® based membranes is the incorporation of hydrophilic inorganic oxide materials into the Nafion® polymer matrix. A composite membrane based on graphite oxide (GO) has been developed and demonstrated as an alternative PEM for high temperature operation up to 120 oC. GO is an insulator and hydrophilic in nature. GO exhibits proton conductivity due to the presence of acidic functional groups like, carboxylic acid, hydroxyl groups and epoxy groups. Further functionalisation of GO with sulfonic acid (called SGO) improves the proton transport properties of GO which in turn improves the composite membrane proton conductivity. Free standing GO and SGO papers were fabricated and evaluated to understand their proton transport mechanism. The in-plane and through-plane proton conductivities of GO paper were 0.008 and 0.004 S.cm-1 at 30 oC and 25% RH respectively. The in-plane and through-plane proton conductivities of SGO paper were 0.04 and 0.012 S.cm-1 at 30 oC and 25% RH respectively. The fuel cell performance of a membrane electrode assembly made with SGO paper gave a maximum power density of 113 mW cm-2. GO/Nafion composite membranes were fabricated with different GO content. The composite membranes with an optimum of 4 wt% GO showed better mechanical strength (tensile strength of 8.17 MPa) and water uptake (37.2%) compared to recast Nafion. A GO (4 wt%) /Nafion composite membrane gave a high ion exchange capacity (IEC) value of 1.38 meq g-1. The proton conductivity of GO (4 wt%) /Nafion was 0.026 S.cm-1 at 120 oC. SGO/Nafion composite membrane showed improved proton ii conductivity (0.029 S.cm-1). The SGO/Nafion composite membrane gave peak power density of 240 mW cm-2, whereas GO/Nafion composite membrane gave a power density of 200 mW cm-2 at 120 oC and 25% RH. The stability and durability of GO and SGO/Nafion composite membranes was investigated under fuel cell operating conditions and compared with recast Nafion. A non fluorinated proton exchange membrane based sulfonated poly ether-ether ketone (SPEEK) was used to develop a composite membrane with SGO. SGO (4 wt%) /SPEEK composite membrane showed high IEC of 2.3 meq g-1 and proton conductivity of 0.055 S.cm-1 at 80 oC and 30% RH. SGO (4 wt%) /SPEEK composite membrane gave a power density of 378 mW cm-2 at 80 oC and 30% RH, which was higher than that of recast SPEEK (254 mW cm-2). Transition metal nitride based electrocatalyst support such as titanium nitride (TiN), has been used to replace carbon to support Pt and Pt-Co alloy for PEMFC cathode. Nafion® stabilised Pt nanoparticles supported on TiN (Pt/TiN) were prepared and evaluated as cathode electrocatalyst for PEMFC. Pt/TiN showed better electrocatalytic activity, stability and durability under fuel cell operating conditions compared to commercial Pt/C. Pt/TiN retained 66% of electrochemical active surface area (ECSA) after 1000 potential cycles (cycled between the potential range of +0.6 to +1.20 V vs. RHE) under fuel cell operating conditions. The ECSA of the Pt/C catalyst fell by 75%. Pt/TiN was also evaluated for its suitability in phosphoric acid based PEMFCs. Pt/TiN showed better durability than Pt/C under fuel cell operating conditions. Pt/TiN showed a two-fold increase in mass and specific activities than Pt/C as calculated from oxygen reduction reaction data at 0.9 V. An improved durability of Pt/TiN resulted from a Nafion® layer surrounding the Pt protecting from phosphate ion adsorption. Alloying of Pt with 3d transition metals changes the electronic structure of Pt (Pt becomes e- deficient) and enhances the electrocatalytic activity of PtM alloy compared to Pt. 3d transition metals such as Fe, Co and Ni are reported to be more active than other metals. Pt-Co alloy supported on TiN was prepared and evaluated. Pt-Co/TiN showed about +21 and +32 mV positive shifts in half-wave potential compare to Pt/TiN and conventional Pt/C respectively. After 5000 potential cycles, the ECSA of Pt-Co/TiN had decayed by about 55%, whereas Pt/TiN and Pt/C showed a greater loss in ECSA of 70%.

In this study two polychelatogens for borate and a polyelectrolyte for chromate retention (R) were designed for investigating the effect of pH and loading (g metal /g polymer) on the separation performances of the synthesized polymers using continuous polymer enhanced ultrafiltration. Increase in pH increased the retention of borate for all of the synthesized polymers. Decrease in the loading resulted in an enhancement in boron retention with PNSM and PNSL. When COP was utilized, retentions remained almost constant after a certain loading, probably due to possible adverse effects of high polymer concentrations on polymer conformation in aqueous solutions. Decrease in loading caused an increase in the retention of chromate until a loading of 0.01. After that a slight decrease was observed. Maximum Cr (VI) retention was obtained as 0.70 for a loading of 0.01 and a pH of 4. Effect of crowding on Cr(VI) retention was also investigated. It was observed that retention does not only depend on the loading but also on the concentrations of both Cr (VI) and PDAM. Effect of the presence of competing anions such as chloride and sulfate on the retention of chromate was investigated to see the effect of competing anion charge to the selectivity of the synthesized polyelectrolyte. Addition of both anions decreased the retention of Cr(VI) . Divalent sulfate decreased the retention more than monovalent chloride indicating that charge of the anion may be the predominant variable in the retention of chromate using PDAM. Finally, dynamic and static light scattering measurements were performed to investigate the conformational changes in the structure of the synthesized polymers at different pH values as well as in the presence of boron in the solution. In this study, it is shown that PEUF can be successfully applied to for boron and Cr (VI) retention with the synthesized polymers. Satisfactory retention values were obtained both for boron and Cr (VI). Even if the retention of Cr (VI) decreased with the addition of high amount of competing anions, significant Cr (VI) retentions could be obtained.

Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2011
High temperature polymer electrolyte membrane fuel cells (PEMFC) operating between 120-180 oC are currently of much research attention. The acid doped polybenzimidazole (PBI) membranes electrolyte are known for their tolerance to relatively high levels of carbon monoxide impurity in the feed. Most fuel cell modelling are theoretical in nature and are solved in commercial CFD platforms such as Fluent. The models require a lot of time to solve and are not simple enough to be used in complex systems such as CHP systems. This study therefore, focussed on developing a simple but yet accurate model of a high temperature PEMFC for a CHP system. A zero dimensional model for a single cell was developed and implemented in Engineering Equations Solver (EES) environment to express the cell voltage as a function of current density among others. Experimental results obtained from literature were used to validate and improve on the model. The validated models were employed for the simulation of the stack performance to investigate the effects of temperature, pressure, anode stoichiometry and the level of CO impurity in the synthesis gas, on the cell potential and overall performance. Good agreement was obtained from the simulation results and experimental data. The results showed that increasing temperature (up to 180oC) and acid doping level have positive effects on the cell performance. The results also show that the cell can operate with a reformate gas containing up to 2% CO without significant loss of cell voltage at elevated temperatures. The single cell model was extended to a 1 kWe high temperature PEMFC stack and micro-CHP system. The stacks model was validated with experimental data obtained from a test station. The model was used to investigate the performance of PEMFC and CHP system by using uncertainty propagation. The highest combined cogeneration system efficiency of 87.3% is obtained with the corresponding electrical and thermal efficiencies are 41.3% and 46 % respectively. The proposed fuel processing subsystem provides an adequate rate of CH4 conversion and acceptable CO-level, making it appropriate for integration with an HT PEMFC stack. In the steam methane reformer 97% of CH4 conversion is achieved and the water gas shift reactors achieve about 98% removal of CO.

Les piles à combustible à membrane échangeuse d'anions (AEMFC) ont récemment attiré l'attention en tant que piles à combustible alternatives à faible coût aux piles à combustible à membrane échangeuse de protons traditionnelles en raison de l'utilisation possible d'électrocatalyseurs non-nobles. Bien que l'AEMFC ressemble à la PEMFC, les problèmes de gestion de l'eau sont plus prégnants dans une AEMFC car l'ORR en milieu alcalin nécessite de l'eau, tandis qu'en même temps, de l'eau est produite en grande quantité du côté de l'anode. Pour mieux comprendre la gestion de l'eau dans ce type de pile à combustible, il faut d'abord développer et acquérir de l'expérience avec ce type de pile à combustible à l'échelle du laboratoire. Puisque les matériaux prêts à l'emploi n'existaient pas au commencement de la thèse, nous avons dû fabriquer nos propres assemblages électrode-membranes (AMEs) à partir des matériaux disponibles dans le commerce. Etant donné que la thématique de fabrication des AMEs est nouvelle pour les chercheurs du LEMTA, cette thèse est articulée en deux parties, une dédiée à la formulation, la préparation et l'optimisation des AMEs pour PEMFC ; et une autre dédiée au développement d'AEMFC. Les résultats ont indiqué que la composition et préparation de l'encre, ainsi que la manière de déposer l'encre modifient systématiquement la structure de l'électrode, de même que ses performances en piles à combustible. En outre, l'étude fournit des informations sur les procédures et les méthodes pour les tests en AEMFC. Ici, nous souhaiterions partager notre savoir-faire avec les nouveaux venus dans le domaine de la préparation des AMEs pour piles à combustibles à membranes échangeuses d'ions
Anion exchange membrane fuel cells (AEMFCs) have recently attracted significant attention as low-cost alternative fuel cells to traditional proton exchange membrane fuel cells as a result of the possible use of platinum-group metal-free electrocatalysts. Although AEMFC is a mimic of PEMFC but working in an alkaline medium, water management issues are more severe in AEMFC because ORR in alkaline media requires water, while at the same time water is produced at the anode side. To better understand water management in this type of fuel cell, it is necessary first to develop and gain experience with this kind of fuel cell on the laboratory scale. Since no ready-to-use materials are available at the beginning of the project, the necessity of fabricating homemade MEAs from commercially available materials becomes a reality that we must face. As MEA fabrication is a new topic to LEMTA's researchers, this is why this thesis was divided into two parts: one part dedicated to the formulation, preparation, and optimization of MEAs for PEMFC through physico-chemical and electrochemical characterizations; another part dedicated to the development of AEMFC. The results indicated that ink deposition, composition, and preparation systematically change the electrode structure and thus affect fuel cells performance. Furthermore, the study provides information on the AEMFC procedures and methods. Here, we would like to share our know-how with newcomers in the field of preparation of MEA in ion exchange membrane fuel cells

As membranas de troca aniônica são uma alternativa promissora para o desenvolvimento de eletrólitos mais eficientes para células a combustível alcalinas. Em geral, as membranas de troca aniônica são ionômeros capazes de conduzir íons hidroxila devido aos grupos quaternário de amônio e têm como característica elevado pH equivalente. Com o objetivo de desenvolver membranas aniônicas química e termicamente estáveis, com satisfatória condutividade iônica para aplicação em células a combustível alcalinas, as membranas aniônicas foram sintetizadas a partir de polímeros base de polietileno de baixa densidade (LDPE), polietileno de ultra alto peso molecular (PEUHMW), poli(etileno-co-tetrafluoroetileno) (PETFE) e poli(tetrafluoroetilleno-co-hexafluoroetileno) (PFEP) previamente irradiados nas fontes de radiação gama de 60Co ou com feixe de elétrons, para enxertia do monômero de estireno e funcionalizados com trimetilamina para incorporação dos grupos quaternário de amônio. As membranas resultantes foram caracterizadas por espectroscopia de ressonância paramagnética eletrônica (EPR), espectroscopia Raman, termogravimetria (TG), espectroscopia de impedância eletroquímica (EIS), além da determinação do grau de enxertia, capacidade de absorção de água por gravimetria e capacidade de troca iônica, por titulação. As membranas sintetizadas com os polímeros LDPE e UHMWPE pré-irradiados a 70 kGy com feixe de elétrons e armazenadas a baixa temperatura (-70 °C) por até 10 meses, mostraram resultados de condutividade iônica, quando na forma (OH-), de 29 mS.cm-1 e 14 mS.cm-1 a 65 °C, respectivamente. Os filmes de PFEP irradiados no processo simultâneo mostram níveis de enxertia insuficientes para a síntese de membranas aniônicas, necessitando maiores estudos para aperfeiçoar os processos de irradiação e enxertia. As membranas baseadas em PETFE, pré-irradiadas a 70 kGy com feixe de elétrons e armazenadas a baixa temperatura (-70 °C) por até 10 meses, mostraram maior condutividade iônica, quando na forma hidroxila (OH-), com valores de condutividade iônica entre 90 mS.cm-1 e 165 mS.cm-1 na faixa de temperatura entre 30 e 60 °C. Estes resultados mostraram que membranas de LDPE, UHMWPE e PETFE são eletrólitos promissores para a aplicação em células a combustível alcalinas.
Anion Exchange Membranes (AEMs) are a promising alternative to the development of more efficient electrolytes for alkaline fuel cells. In general, the AEMs are ionomeric membranes able to conduct hydroxide ions (OH-) due to the quatermary ammonium groups, which confer high pH equivalent to the AEM. In order to develop alkaline membranes with high chemical and thermal stability, besides satisfactory ionic conductivity for alkaline fuel cells, membranes based on low density polyethylene (LDPE), ultrahigh weight molecular weight polyethylene (UHWHPE), poly(ethylene-co-tetrafluoroethylene) (PETFE) and poly(hexafluoropropylene-co-tetrafluoroethylene) (PFEP) previously irradiated by using 60Co gamma and electron beam sources, have been synthesized by styrene-grafting, and functionalized with trimethylamine to introduced quaternary ammonium groups. The resulting membranes were characterized by electron paramagnetic resonance (EPR), Raman spectroscopy, thermogravimetry (TG) and electrochemical impedance spectroscopy (EIS). The determination of the grafting degree and water uptake were conducted by gravimetry and ion exchange capacity, by titration. The membranes synthesized with PELD and PEUHMW polymers pre-irradiated at 70 kGy and stored at low temperature (-70 °C), up to 10 months, showed ionic conductivity results, in hydroxide form (OH-), of 29 mS.cm-1 and 14 mS.cm-1 at 65 °C, respectively. The PFEP polymers irradiated by the simultaneous process showed insufficient grating levels for the membrane synthesis, requiring more studies to improve the irradiation and grafting process. The styrene-grafted PETFE membranes, pre-irradiated at 70 kGy and stored at low temperature (-70 °C), up to 10 months, showed ionic conductivity results, in hydroxide form (OH-), of 90 mS.cm-1 to 165 mS.cm-1, in the temperature range 30 to 60 °C. Such results have demonstrated that LDPE, UHMWPE and PETFE based AEMs are promising electrolytes for alkaline fuel cell application.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2007.
Committee Chair: Wong, C.P.; Committee Co-Chair: Liu, Meilin; Committee Member: Barefield, Kent; Committee Member: Collard, David; Committee Member: Fahrni, Christoph.

Lo sviluppo di elettroliti polimerici di nuova generazione è un requisito essenziale per la diffusione su grande scala delle celle a combustibile a membrana polimerica. Tali conduttori protonici devono esibire stabilità morfologica, idrolitica, meccanica ed adeguate proprietà di conducibilità (σ~ 0.01 Scm-1) a temperature superiori a 100 °C per bassi valori d’umidità relativa. Nel presente lavoro sono esplorate diverse strategie per la sintesi di polimeri conduttori ibridi organici-inorganici nanocompositi a partire da polimeri termoplastici aromatici. L'impiego di materiali ibridi permette di sfruttare l'effetto sinergico dovuto alla contemporanea presenza di una componente organica, nel caso specifico polimerica, e di una inorganica, nel caso specifico a base di silicio. Tale effetto sinergico si esplica nella possibilità di modulare e controllare la separazione tra la fase idrolifila ed idrofobica da cui fortemente dipendono le prestazioni dell'elettrolita polimerico. Membrane ibride di classe I a base di polietereterchetone solfonato (S-PEEK) sono così state sintetizzate insieme a numerosi esempi di membrane ibride di classe II a base di S-PEEK e polifenilsolfone solfonato (S-PPSU), contenenti come porzione inorganica atomi di silicio diversamente funzionalizzati. La caratterizzazione dei materiali ha riguardato l’analisi della struttura, le proprietà chimico fisiche ed il comportamento elettrochimico. Risultati molto positivi sono stati ottenuti principalmente con due dei sistemi investigati: una miscela polimerica a base di S-PEEK e S-PPSU sililato ed un polimero interconnesso tramite ponti -SO2- (SOPEEK) e sililato (SOSiPEEK).
The development of new generation polymer electrolytes is an essential prerequisite for grand scale commercialisation on of polymer electrolyte membrane fuel cells. These proton conductors must show good morphological, hydrolytic and mechanical stability and an appropriate conductivity (σ ~ 0.01 Scm-1) at a temperature above 100°C at low relative humidity. In this work, diverse strategies for synthesis of hybrid organic-inorganic proton conducting polymer nanocomposites were explored, based on aromatic thermoplastic polymers. The use of hybrid materials permits exploitation of the synergy between the simultaneously present organic polymeric component and an inorganic silicon-based part. These effects can be explained by the possibility to modulate and to control the separation between hydrophilic and hydrophobic parts, which strongly modify the properties of the electrolytic polymer. Hybrid materials of class I based on sulfonated poly-ether-ether-ketone (S-PEEK) were synthesized as well as several examples of hybrid materials of class II based on SPEEK and poly-phenyl-sulfone sulfonated (S-PPSU) and containing as inorganic part diverse functionalized silicon atoms. These materials were characterized from the point of view of structure, physical and chemical properties and electrochemical behaviour. Very positive results were obtained mainly for two investigated systems: a mixture of S-PEEK and S-PPSU silylated polymer and a cross-linked polymer, through -SO2- bridges (SOPEEK) and silylated (SOSiPEEK).

Per- and polyfluoroalkyl substances (PFASs) is a group of man-made, highly persistent chemicals. Due to the specific surface-active attributes of these molecules, applications are numerous and feed an economically important industry. During the last decade, PFASs have been detected globally in the environment, living organisms and tap water. The combination of toxic properties and high bioaccumulative potential, together with the discovery that conventional water treatment methods do not remove PFAS, renders further research on purification methods highly needed.  Three techniques of purifying water from PFASs were examined. Nanofiltration technology (NF) is a membrane filtration technique, which produces a purified product (the permeate) by generating an effluent of high contaminant concentration (the reject water). To decontaminate the reject water, adsorption by granular activated carbon (GAC) or anion exchange (AE) have been proposed. The efficiency of these three technologies was studied at Bäcklösa drinking water treatment plant (DWTP) in Uppsala. A nanofiltration pilot with two 270NF membranes (Dow Filmtech™), connected in series, was used. A high removal efficiency (>90%) was found for all PFASs. Furthermore, it was confirmed that the concentration in the permeate water was a function of the concentration in the incoming raw water; increased PFAS raw water concentration resulted in increased PFAS permeate concentration. Size-exclusion and electrostatic repulsion were deemed important mechanisms. For the comparison of GAC (Filtrasorb 400®) and AE (Purolite® A-600), a column experiment was set up. The perfluoroalkane (-alkyl) sulfonic acids (PFSAs) and perfluorooctanesulfonamide (FOSA) had similar removal efficiencies using both GAC and AE, and the efficiency increased with increasing chain length. AE was found to have a higher average removal efficiency of perfluoroalkyl carboxylic acid (PFCAs) (62-95%) than GAC (49-81%). In conclusion, longer chain length PFASs were removed more effectively than shorter-chained, and the PFSAs and FOSA showed higher removal efficiency compared to the PFCAs. Furthermore, linear isomers were removed more effectively than branched for GAC and AE. In contrast, the opposite was found for the NF membrane, where branched isomers were better retained.
Per- och polyfluorerade alkylsubstanser (PFAS) är en grupp syntetiska, ytterst persistenta kemikalier. På grund av deras ytaktiva egenskaper är de lämpliga för användning i många produkter och tillverkningsprocesser, och är således viktiga för en ekonomiskt betydande industri. Under det senaste årtiondet har PFAS påträffats i miljön, levande organismer och kranvatten världen över.  Kombinationen av toxiska egenskaper, en hög bioackumuleringspotential och upptäckten att konventionella reningsmetoder inte avlägsnar substanserna från vatten, gör att vidare forskning av reningsmetoder för PFAS är mycket angelägen. Tre reningsteknikers förmåga att rena vatten från PFAS undersöktes. Nanofiltrering (NF) är en membranfiltreringsteknik som utöver den renade produkten, permeatet, även framställer en biprodukt av hög föroreningsgrad, rententatet. För att rena rententatet har adsorption till granulärt aktivt kol (GAC) eller jonbytarmassa (AE) föreslagits. Teknikerna utvärderades på Bäcklösa Vattenverk i Uppsala.  Nanofiltreringen undersöktes i en pilotanläggning där två 270NF (Dow Filmtech™) membran var seriekopplade. En hög reningsgrad (>90%) konstaterades för alla typer av PFAS. Vidare visades PFAS-koncentrationen i permeatet vara en funktion av PFAS-koncentrationen i råvattnet; en ökad råvattenkoncentration gav en ökad permeatkoncentration. Storleksseparation och elektrostatisk repulsion befanns vara viktiga mekanismer som påverkade reningsgraden. För att undersöka de mekanismer som påverkar PFAS-adsorption jämfördes GAC (Filtrasorb 400®) och AE (Purolite® A-600) i ett kolonnexperiment. Reningsgraden för GAC och AE av perfluorerade sulfonsyror (PFSA) och perfluorooktan sulfonamider (FOSA) var lika hög och reningseffektiviteten ökade med ökande kolkedjelängd. AE återfanns ha en högre genomsnittlig reningsgrad av perfluorkarboxylsyror (PFCA) (62-95%) än GAC (49-81%). Sammanfattningsvis avlägsnades PFAS av längre kolkedjelängd mer effektivt än kortare kolkedjor, och PFAS med sulfonsyror och sulfonamider som funktionella grupper uppvisade en högre reningsgrad än karboxylsyrorna. Vidare renades linjära isomerer mer effektivt än grenade både genom GAC och AE. Däremot konstaterades det motsatta för NF-membranen, där grenade isomerer renades mer effektivt.
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Due to escalating energy costs and limited fossil fuel resources, much attention has been given to polymer electrolyte membrane (PEM) fuel cells. Gas diffusion layers (GDLs) play a vital role in a fuel cell such as (1) water removal, (2) cooling, (3) structural backing, (4) electrical conduction and (5) transporting gases towards the active catalyst sites where the reactions take place. The power density of a PEM fuel cell in part is dependent upon how uniform the gases are distributed to the active sites. To this end, research is being conducted to understand the mechanisms that influence gas distribution across the fuel cell. Emerging PEM fuel cell designs have shown that higher power density can be achieved; however this requires significant changes to existing components, particularly the GDL. For instance, some emerging concepts require higher through-plane gas permeability than in-plane gas permeability (i.e., anisotropic resistance) which is contrary to conventional GDLs (e.g., carbon paper and carbon cloth), to obtain a uniform gas distribution across the active sites. This is the foundation on which this thesis is centered. A numerical study is conducted in order to investigate the effect of the gas permeability profile on the expected current density in the catalyst layer. An experimental study is done to characterize the effects of the weave structure on gas permeability in woven GDLs. Numerical simulations are developed using Fluent version 6.3.26 and COMSOL Multiphysics version 3.5 to create an anisotropic resistance profile in the unconventional GDL, while maintaining similar performance to conventional GDL designs. The effects of (1) changing the permeability profile in the in-plane and through-plane direction, (2) changing the thickness of the unconventional GDL and (3) changing the gas stoichiometry on the current density and pressure drop through the unconventional GDL are investigated. It is found that the permeability profile and thickness of the unconventional GDL have a minimal effect on the average current density and current density distribution. As a tradeoff, an unconventional GDL with a lower permeability will exhibit a higher pressure drop. Once the fuel cell has a sufficient amount of oxygen to sustain reactions, the gas stoichiometry has a minimal effect on increases in performance. Woven GDL samples with varying tightness and weave patterns are made on a hand loom, and their in-plane and through-plane permeability are measured using in-house test equipment. The porosity of the samples is measured using mercury intrusion porosimetry. It is found that the in-plane permeability is higher than the through-plane permeability for all weave patterns tested, except for the twill weave with 8 tows/cm in the warp direction and 4 tows/cm in the weft direction, which exhibited a through-plane permeability which was 20% higher than the in-plane permeability. It is also concluded that the permeability of twill woven fabrics is higher than the permeability of plain woven fabrics, and that the percentage of macropores, ranging in size from 50-400 µm, is a driving force in determining the through-plane permeability of a woven GDL. From these studies, it was found that the graduated permeability profile in the unconventional GDL had a minimal effect on gas flow. However, a graduated permeability may have an impact on liquid water transport. In addition, it was found that graduating the catalyst loading, thereby employing a non-uniform catalyst loading has a greater effect on creating a uniform current density than graduating the permeability profile.

Par couplage diffusion-réaction, le transport d'un substrat non ionique à travers une membrane échangeuse d'ions, peut être facilité en choisissant un contre-ion apte à réagir réversiblement avec le substrat. Dans ce travail, nous avons réalisé le transport facilité à travers une membrane échangeuse d'anions. L'ion borate était utilisé comme transporteur et sa teneur dans la membrane était fixée par l'activité de l'acide borique. Nous avons mené simultanément la modélisation et l'expérimentation du transport. Par des études d'équilibre et de conductivité, nous avons déterminé les paramètres physicochimiques, tels que le coefficient de stabilité du complexe glucose-borate et le coefficient de diffusion des différentes espèces présentes. Nous avons, de plus, mis en évidence en présence de glucose, deux aspects originaux: la variation de l'accessibilité des sites membranaires et la variation de leurs interactions avec certains contre-ions. Dans la majorité des cas, nous avons choisi une activité d'acide borique relativement faible afin que les polyborates soient minoritaires devant les autres contre-ions. Dans une première série de mesures, nous avons réalisé le transport facilité du glucose en fonction de son activité, en maintenant constante celle de l'acide borique. Dans une seconde série, le transport a été réalisé en fonction de l'activité du transporteur. A partir des paramètres préalablement déterminés, nous avons élaboré un modèle théorique qui permet de comparer les valeurs théoriques et expérimentales des flux du glucose. Nous parvenons à un bon accord lorsque le transport a été réalisé en imposant pour l'acide borique une activité pour laquelle les paramètres du modèle ont été caractérisés