Letteratura scientifica selezionata sul tema "Anion exchange polymer membrane"

Anion exchange blend membranes (AEBMs) were prepared for use in Vanadium Redox Flow Batteries (VRFBs). These AEBMs consisted of 3 polymer components. Firstly, PBI-OO (nonfluorinated PBI) or F6-PBI (partially fluorinated PBI) were used as a matrix polymer. The second polymer, a bromomethylated PPO, was quaternized with 1,2,4,5-tetramethylimidazole (TMIm) which provided the anion exchange sites. Thirdly, a partially fluorinated polyether or a non-fluorinated poly (ether sulfone) was used as an ionical cross-linker. While the AEBMs were prepared with different combinations of the blend polymers, the same weight ratios of the three components were used. The AEBMs showed similar membrane properties such as ion exchange capacity, dimensional stability and thermal stability. For the VRFB application, comparable or better energy efficiencies were obtained when using the AEBMs compared to the commercial membranes included in this study, that is, Nafion (cation exchange membrane) and FAP 450 (anion exchange membrane). One of the blend membranes showed no capacity decay during a charge-discharge cycles test for 550 cycles run at 40 mA/cm2 indicating superior performance compared to the commercial membranes tested.

Solid anion exchange membrane (AEM) electrolytes are an essential commodity considering their importance as separators in alkaline polymer electrolyte fuel cells (APEFC). Mechanical and thermal stability are distinguished by polymer matrix characteristics, whereas anion exchange capacity, transport number, and conductivities are governed by the anionic group. The physico-chemical stability is regulated mostly by the polymer matrix and, to a lesser extent, the cationic head framework. The quaternary ammonium (QA), phosphonium, guanidinium, benzimidazolium, pyrrolidinium, and spirocyclic cation-based AEMs are widely studied in the literature. In addition, ion solvating blends, hybrids, and interpenetrating networks still hold prominence in terms of membrane stability. To realize and enhance the performance of an alkaline polymer electrolyte fuel cell (APEFC), it is also necessary to understand the transport processes for the hydroxyl (OH−) ion in anion exchange membranes. In the present review, the radiation grafting of the monomer and chemical modification to introduce cationic charges/moiety are emphasized. In follow-up, the recent advances in the synthesis of anion exchange membranes from poly(phenylene oxide) via chloromethylation and quaternization, and from aliphatic polymers such as poly(vinyl alcohol) and chitosan via direct quaternization are highlighted. Overall, this review concisely provides an in-depth analysis of recent advances in anion exchange membrane (AEM) and its viability in APEFC.

Perfluorinated polymers are widely used in polymer electrolyte membranes because of their excellent ion conductivity, which are attributed to the well-defined morphologies resulting from their extremely hydrophobic main-chains and flexible hydrophilic side-chains. Perfluorinated polymers containing quaternary ammonium groups were prepared from Nafion- and Aquivion-based sulfonyl fluoride precursors by the Menshutkin reaction to give anion exchange membranes. Perfluorinated polymers tend to exhibit poor solubility in organic solvents; however, clear polymer dispersions and transparent membranes were successfully prepared using N-methyl-2-pyrrolidone at high temperatures and pressures. Both perfluorinated polymer-based membranes exhibited distinct hydrophilic-hydrophobic phase-separated morphologies, resulting in high ion conductivity despite their low ion exchange capacities and limited water uptake properties. Moreover, it was found that the capacitive deionization performances and stabilities of the perfluorinated polymer membranes were superior to those of the commercial Fumatech membrane.

Cation-exchange, anion-exchange, and bipolar membranes play crucial roles in a variety of electrochemical processes and devices, including chloralkali cells, electrodialysis separations for water purification, proton-exchange membrane and hydroxide-exchange membrane (alkaline) fuel cells, redox flow batteries, and processes for direct air capture of CO2. The incorporation of polymeric nanofibers into such membranes provides an attractive and tunable method of creating materials with new nano-morphologies and highly desirable properties. The impregnation of an ionomer solution into a pre-formed nonwoven porous mat of electrospun polymer fibers is a well-known method of making reinforced proton-exchange membranes. The use of simultaneous dual-fiber electrospinning or the electrospinning of polymer blends can be used to intermix/incorporate/co-locate dissimilar and incompatible polymers on the nanoscale. Although less studied in the literature, these methods offer many interesting possibilities for new membrane structures with targeted and unique transport and mechanical properties. In this review talk, the use of dual fiber and blended polymer fiber electrospinning for membrane fabrication will be presented for the following: (1) Nanofiber reinforced cation (proton) exchange membranes, (2) Electrospun NafionTM/PVDF dual fiber and single-fiber membranes for H2/Br2 redox flow batteries, (3) Composite anion exchange membranes, and (4) Bipolar membranes with a 3D nanofiber junction. Materials and methods for membrane fabrication will be described and the properties of the membranes will be discussed.

Bipolar membranes (BPMs), typically laminated layers of anion-exchange and cation-exchange polymers, have the unique capability of splitting water at a potential near 0.83 V. Such membranes are used in electrodialysis membrane separation processes. They also have applications in water electrolyzers, CO2 electrolysis cells, and self-humidifying fuel cells. We report here on recent developments regarding BPMs with a high interfacial area, 3D nanofiber junction. Membranes were prepared by first creating a bipolar junction layer, by the simultaneous electrospinning of anion-exchange and cation-exchange polymers, with the addition of catalyst nanoparticles to facilitate water splitting. Solvent vapor exposure and hot-pressing closed all interfiber pores, to create a dense film of interpenetrating and interlocking nanofibers of positively and negatively charged polymers. In one fabrication method, dense films of solution cast anion and cation-exchange polymers were hot-pressed onto the opposing surfaces of the junction layer to create a tri-layer BPM. Membranes were also fabricated by electrospinning all three layers of the BPM: first spinning anion-exchange fibers, followed by dual fiber spinning with catalyst spraying, and then spinning only cation-exchange fibers. Solvent exposure and hot-pressing closed all interfiber voids. BPMs were made with a junction layer 12-15 mm thick, where the total membrane thickness was 50-80 mm. Membranes were prepared with both hydrocarbon and fluoropolymer ionomers, e.g., sulfonated poly(ether ether ketone) or perfluorosulfonic acid as the cation-exchange polymer and either quaternized poly(phenylene oxide), AEMIONä (an imidazolium-based polymer sold by Ionomr Innovations, Inc.), or PiperION (a poly(aryl piperidinium) from Versogen) as the anion-exchange polymer. A variety of different junction layer catalysts powders were examined, including Al(OH)3 and graphene oxide. Membranes were evaluated in an H-cell for the collection of steady-state current-voltage data and in a flow cell for long-term constant current water splitting (electrolysis) operation, typically with an aqueous Na2SO4 electrolyte. The 3D junction membranes performed exceptionally well: (i) the water splitting potential was low (near the expected value of 0.83 V), (ii) the transmembrane voltage drop was small at high currents (e.g., a voltage drop of 1.1 V up to 1.1 A/cm2), and (iii) the extended bipolar reaction zone for water splitting with interlocking fibers allowed for high current density operation with no evidence of membrane degradation. In this talk, the effect of membrane composition (the type/amount of anion and cation exchange polymers and choice of catalyst) and structure (the thickness of the layers) on short-term and long-term membrane performance will be discussed. Results will be presented for operating the bipolar membrane in water splitting and water generation modes.

Anion exchange membranes play a crucial part as the primary component of alkaline fuel cells, yet their optimization remains an ongoing endeavor. While research and development efforts have made strides in advancing anion exchange membranes, a pressing need exists to further refine their mechanical properties, ionic conductivity, and chemical stability, especially in comparison to proton exchange membranes. Block copolymers have emerged as promising candidates among the array of materials explored for enhancing anion exchange membranes due to their inherent advantages. These copolymers offer unparalleled flexibility in adjustment and boast superior mechanical properties, making them highly adaptable for modifying anion exchange membranes to meet desired specifications. In order to demonstrate the benefits of block copolymer, this paper primarily summarizes and examines the techniques for varying the material content, investigating composition to identify the block copolymer anion exchanging membrane with exceptional performance characteristics, and contrasting it with the random copolymer, polymer blend, and homopolymer exchange membrane. The results unequivocally demonstrate the efficacy of block copolymers in improving the material structure of exchange membranes by fine-tuning the polymer content. Notably, block copolymers outperform other copolymers in significantly enhancing the performance metrics of anion exchange membranes. In summary, studying block copolymers is a practical way to significantly enhance the performance and functionality of anionic exchange membranes, which will help the alkaline fuel cell industry move toward greater sustainability

Water electrolysis processes play a crucial role in transitioning to a climate-friendly society. They facilitate the integration of renewable energy, offer a clean and versatile energy carrier, decarbonize industries, improve energy storage and grid stability, and support the development of sustainable transportation solutions. As technology advances and economies of scale are realized, electrolysis is expected to play an increasingly significant role in the clean energy landscape, contributing to a more sustainable and resilient future. Various water-splitting electrolysis processes currently exist, including alkaline, solid oxide, proton exchange membrane (PEM), anion exchange (AEM), acidic-alkaline amphoteric, microbial, and photoelectrochemical methods [1]. In our research group, we are actively involved in membrane development for both PEM and AEM water electrolysis. In PEM electrolysis membrane development, our group explores several approaches, such as (a) aromatic main-chain block copolymers[2],(b) acid-base blend membranes using sulfonated and partially fluorinated aromatic polyether, polybenzimidazole, and a PSU-derived basic polymer[2], (c) poly(fluorene)-based sulfonated ionomers, (d) sulfonated and phosphonated poly(pentafluorostyrene) polymers with flexible side groups, and (d) nanophase-separated block copolymers based on phosphonated[3] or sulfonated pentafluorostyrene and octylstyrene. Additionally, we investigate (e) H+-conductive fiber-mat reinforced perfluorosulfonic acid (PFSA) polymers[4]. The development of anion exchange membranes in our research group includes (a) polystyrene-based side chain anion exchange polymers and their blends with polybenzimidazole[5], (b) polynorbornene-based optionally ionically and covalently crosslinked anion exchange polymers and membranes, (c) side chain anion exchange polymers and membranes prepared by polyhydroxyalkylation[6], and (d) anion exchange blend membranes made from polydiallyldimethylammonium salts and polybenzimidazole. This contribution highlights the application of two polymer types in PEM and AEM membrane water electrolysis, respectively: (A) PEM Water Electrolysis (PEMWE): Membranes from PEM types (a) and (b) demonstrated good performance. PEM (a) achieved 2.2 V@6 A/cm2, and PEM (b) reached 2.26 V@6 A/cm2 (compared to Nafion212: 2.26 V@6 A/cm2)[2]. These performances were accomplished with non-optimized membrane-electrode assemblies using Nafion as the electrode ionomer. Further performance improvements are expected with optimized electrodes containing the same ionomers as used in the membrane. (B) AEM Water Electrolysis (AEMWE): Blend membranes from AEM type (a) exhibited excellent alkali stability (no conductivity decrease after 1000 hrs of storage in 1M KOH@85°C) and good AEMWE performance (CuCo anode catalyst, 1M KOH, 70°C, 2 V@3 A/cm2)[5]. Type (c) AEMs were applied to a seawater electrolysis cell at 60°C, achieving a performance of 2 V@1 A/cm2 using completely noble metal-free catalysts in both the anode and cathode[6]. [1] M. F. Ahmad Kamaroddin, N. Sabli, T. A. Tuan Abdullah, S. I. Siajam, L. C. Abdullah, A. Abdul Jalil, A. Ahmad, Membranes 2021, 11. [2] J. Bender, B. Mayerhöfer, P. Trinke, B. Bensmann, R. Hanke-Rauschenbach, K. Krajinovic, S. Thiele, J. Kerres, Polymers 2021, 13. [3] S. Auffarth, M. Wagner, A. Krieger, B. Fritsch, L. Hager, A. Hutzler, T. Böhm, S. Thiele, J. Kerres, ACS Materials Lett. 2023, 5, 2039. [4] M. S. Mu'min, M. Komma, D. Abbas, M. Wagner, A. Krieger, S. Thiele, T. Böhm, J. Kerres, Journal of Membrane Science 2023, 685, 121915. [5] L. Hager, M. Hegelheimer, J. Stonawski, A. T. S. Freiberg, C. Jaramillo-Hernández, G. Abellán, A. Hutzler, T. Böhm, S. Thiele, J. Kerres, J. Mater. Chem. A 2023. [6] M. L. Frisch, T. N. Thanh, A. Arinchtein, L. Hager, J. Schmidt, S. Brückner, J. Kerres, P. Strasser, ACS Energy Lett. 2023, 8, 2387.

Crosslinked anion exchange membranes (AEMs) made from poly(vinyl alcohol) (PVA) as a backbone polymer and different approaches to functional group introduction were prepared by means of solution casting with thermal and chemical crosslinking. Membrane characterization was performed by SEM, FTIR, and thermogravimetric analyses. The performance of AEMs was evaluated by water uptake, swelling degree, ion exchange capacity, OH- conductivity, and single cell tests. A combination of quaternized ammonium poly(vinyl alcohol) (QPVA) and poly(diallyldimethylammonium chloride) (PDDMAC) showed the highest conductivity, water uptake, and swelling among other functional group sources. The AEM with a combined mass ratio of QPVA and PDDMAC of 1:0.5 (QPV/PDD0.5) has the highest hydroxide conductivity of 54.46 mS cm-1. The single fuel cell tests with QPV/PDD0.5 membrane yield the maximum power density and current density of 8.6 mW cm-2 and 47.6 mA cm-2 at 57 °C. This study demonstrates that PVA-based AEMs have the potential for alkaline direct ethanol fuel cells (ADEFCs) application.

In order to improve the performance of the anion exchange membrane (AEM) used in acid recovery from industrial wastewater, this study adopted a new strategy in which brominated poly (2,6-dimethyl-1,4-phenyleneoxide) (BPPO) and polyepichlorohydrin (PECH) were used as the polymer backbone of the prepared membrane. The new anion exchange membrane with a net structure was formed by quaternizing BPPO/PECH with N,N,N,N-tetramethyl-1,6-hexanediamine (TMHD). The application performance and physicochemical property of the membrane were adjusted by changing the content of PECH. The experimental study found that the prepared anion exchange membrane had good mechanical performance, thermostability, acid resistance and an appropriate water absorption and expansion ratio. The acid dialysis coefficient (UH+) of anion exchange membranes with different contents of PECH and BPPO was 0.0173–0.0262 m/h at 25 °C. The separation factors (S) of the anion exchange membranes were 24.6 to 27.0 at 25 °C. Compared with the commercial BPPO membrane (DF-120B), the prepared membrane had higher values of UH+ and S in this paper. In conclusion, this work indicated that the prepared BPPO/PECH anion exchange membrane had the potential for acid recovery using the DD method.

Anion exchange membranes fabricated through a one-step Menshutkin reaction with down-selected multifunctional alkyl halides and multifunctional tertiary amines within an ion-solvating matrix, poly(ethylene-co-vinyl alcohol), yielded alkaline-stable ammonium network polymers. Due to the vast simplicity in fabrication due to the quaternization/Menshutkin reaction between tertiary amine and alkyl bromides, which does not evolve any by-products that require purification, alkaline-stable membranes were fabricated in one step through facile mixing and curing of alkaline-stable ammonium network forming monomers. Prepared membranes showed controllable ion exchange capacity (IEC), conductivity, and mechanical strength by controlling of poly(ethylene-co-vinyl alcohol) amount which is an ion-solvating polymer. The selection of ammonium network chemical structure allowed for flawless retention of IEC and conductivity under conditions of 70°C, 1M KOH of over 300 h. Anion exchange membrane electrolysis membrane electrode assembly tests with optimized membranes showed a greater performance (1.78 A/cm2 at 2.0 V) and more enhanced water electrolyzer durability than that of commercial anion exchange 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

Abstract. Proton exchange membrane (PEM) electrolysis is one of the waters splitting techniques available for producing green hydrogen. As such, improvement of the membrane ion conductivity will result in improvement of hydrogen production. Ionic liquids have recently been reported to enhance ionic conductivity of PEM. Herein, a screening method to select suitable ionic liquids for the development of efficient proton exchange membrane. COnductor-like Screening MOdel for Realistic Solvents (COSMO-RS) was used to predict the ionic conductivity as well as the compatibility of the ions with the Nafion™ through the interpretation of σ-profile as well as interaction energy of the selected cations and anions. It was found that the anions namely of trifluoromethanesulfonate and nitrate with the cation of ammonium and imidazolium may be the best candidate for the ILs to be incorporated to Nafion™ for polymer electrolyte membrane (PEM) as the combination gives high ionic conductivity with considerable high interaction towards Nafion™. It is to be highlighted that the ionic liquids mainly interact with Nafion™ through the anion as implied by the high interaction energy of the anion towards Nafion™ compared to the cation.

The performance of three alkaline direct ethanol fuel cells (ADEFCs) is investigated. All three use identical anode and cathode electrodes, but one uses an anion exchange membrane (AEM) and the other two use non-permselective porous separators. Ethanol was chosen as the fuel because of its low toxicity, low carbon footage and market readiness. A direct comparison between ADEFCs with and without AEM is reported. The performance of each cell is studied under different operation conditions of temperature, reactants flow rate, ethanol and KOH concentrations. The results show that with low cost porous separator, the ADEFC can reach similar power output as those using expensive AEMs. With 1 M ethanol and 1 M KOH aqueous solution, the maximum power densities of 26.04 mW/cm2 and 24.0 mW/cm2 are achieved for the ADEFC employing AEM and non-woven fabric separator, respectively. This proves the feasibility of replacing AEM with non-permselective separators. The results suggest that improving the cathode structure in order to provide a better oxygen supply is a key factor to enhance the performance of an anion exchange membrane free ADEFC.