Developing electrocatalysts (precious and non-precious) for PEM fuel cells applying metal organic frameworks

Les piles à combustible ont un grand potentiel pour une utilisation en tant que dispositifs alternatifs de conversion d'énergie pour de nombreuses applications. Les piles à combustible PEM sont considérées comme des remplaçants potentiels du moteur à combustion interne des véhicules automobiles, en raison de leurs émissions réduites et d'une meilleure efficacité. Un catalyseur à base de Pt est nécessaire pour faciliter à la fois la réaction d'oxydation de l'hydrogène (HOR) et la réaction de réduction de l'oxygène (ORR) qui se produisent à l'anode et à la cathode d'une PEMFC, respectivement. La vitesse d'ORR est intrinsèquement très lente et est considérée comme le facteur limitant de la performance des PEMFCs. Afin de produire de l'énergie à un rythme acceptable pour les applications du monde réel, une quantité importante de catalyseur au Pt est nécessaire. Celui-ci est habituellement sous la forme de nanoparticules de platine uniformément réparties sur un matériau de support en carbone poreux (Pt/C). Le Pt est un métal noble extrêmement coûteux avec une abondance naturelle très limitée. Ainsi, la commercialisation à grande échelle de PEMFC nécessite des progrès importants dans le développement de catalyseurs à la fois pour réduire la quantité de platine et renforcer la durabilité du catalyseur. Dans ce travail de recherche, nous avons utilisé des réseaux de coordination hybrides métal composé organique (MOF) comme seul précurseur pour préparer des électrocatalyseurs pour PEMFC. En raison de leur cristallinité, de leur porosité et de leur structure hybride, ces matériaux ont un potentiel pour être appliqués comme précurseur d'électrocatalyseurs de PEMFC. La structure tridimensionnelle bien définie de ces matériaux peut produire une forte densité de sites métalliques actifs distribués uniformément à travers leur structure et disposés régulièrement. Ainsi, ils peuvent améliorer l'utilisation du catalyseur. Les groupes de liaison organiques du précurseur à base de MOF sont convertis en carbone lors de l'activation thermique, tout en maintenant le réseau poreux, ce qui conduit à des catalyseurs ayant une grande surface spécifique et des sites actifs uniformément distribués sans la nécessité d'un autre support de carbone. Des précurseurs MOF contenant du Pt et du Fe ont été synthétisés et utilisés comme le précurseur exclusif pour développer à la fois des électrocatalyseurs à base de Pt et de métaux non précieux (Fe) pour PEMFC. L'expérience construite avec des électrocatalyseurs à base de Pt a été le premier essai de mise en œuvre de métaux précieux à base de MOFs pour développer des électrocatalyseurs PEMFC. L'électrocatalyseur à base de Pt dérivé de ce précurseur contenant du Pt MOF a démontré une performance catalytique comparable à celle disponible dans le commerce Pt/C en particulier pour les HOR du côté de l'anode. Pour préparer un électrocatalyseur non-précieux, un MOF contenant du Fe appartenant à une nouvelle classe de matériaux MOF, autre que les ZIFs, a été synthétisé et utilisé comme le précurseur unique d'électrocatalyseurs. Ce fut le premier rapport sur l'utilisation d'un précurseur MOF non-ZIF pour le développement d'électrocatalyseurs ORR. Cet électrocatalyseur à base de Fe a révélé une activité prometteuse en ORR et les performances de pile à combustible PEM lorsqu'il est appliqué à la couche de catalyseur cathodique de la MEA correspondante. En outre, l'effet de la composition de l'encre de catalyseur préparée à partir du dérivé MOF électrocatalyseur à base de Pt, en termes de teneur en ionomère Nafion, a été étudiée sur la performance globale du PEMFC via un modèle CFD macroscopique. La tendance prédite à partir des calculs de modélisation a ensuite été examinée expérimentalement à la recherche de la teneur optimale en ionomère Nafion. De plus, les électrocatalyseurs produits par la transformation thermique des MOFs à base de Pt sur noir de carbone, ont été étudiés par spectroscopie d'impédance. Les précurseurs (MOF-253) et leurs produits de thermolyse ont été pris en compte dans cette étude. Il a été observé que les matériaux soumis à la thermolyse à différentes températures passaient par différents états de conductibilité, depuis des isolants jusqu'à des matériaux de conductance voisine de celle des métaux. Ces données présentaient une augmentation de conductance avec la température et des valeurs élevées à température ambiante.Fuel cells have great potential for use as alternative energy conversion devices for a wide variety of applications. Proton exchange membrane fuel cells (PEMFCs) are considered to be potential replacements for internal combustion engines in automobiles, owing to their reduced emissions and better efficiency. A platinum (Pt)-based catalyst is required to facilitate both hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) which occur at the anode and cathode of PEMFCs, respectively. The ORR kinetic is inherently very sluggish and is considered the limiting factor facing the performance of PEMFCs. In order to generate power at an acceptable rate for real world applications, a significant amount of Pt catalyst is required. This is traditionally in the form of Pt nanoparticles evenly distributed on a porous carbon support material (Pt/C). Pt is an extremely expensive noble metal with very limited natural abundance. Thus, large-scale commercialization of PEMFCs requires significant advances in catalyst development in order both to reduce the amount of Pt metal and to enhance catalyst durability. In this research work, we employed Metal-Organic Frameworks (MOFs) as a sole precursor for preparing PEMFC electrocatalysts. Owing to their crystalline, porous, hybrid structure, these materials have potential to be applied as PEMFCs electrocatalyst precursor. The clearly-defined three-dimensional structure of these materials can produce a high density of metal active sites evenly distributed through their regularly arranged structure. They can therefore enhance catalyst utilization. The organic linkers of the MOF-based precursor would be converted to carbon during thermal activation while maintaining the porous framework, leading to catalysts with high surface area and uniformly distributed active sites without the need for a carbon support. Pt and Fe containing MOF precursors were synthesized and used as the sole precursor to develop both Pt and non-precious (Fe)-based electrocatalysts for PEMFCs. A Pt-based electrocatalyst was the first reported on implementation of precious metal containing MOFs for developing PEMFC electrocatalyst. The Pt-based electrocatalyst derived from this Pt-containing MOF precursor demonstrated catalytic performance comparable to the commercially available Pt/C especially for HOR at the anode side. To prepare a non-precious electrocatalyst, Fe containing MOF belonging to a different class of MOF materials other than ZIFs was synthesized and used as the sole electrocatalyst precursor. This was the first report on using non-ZIF MOF precursor for ORR electrocatalyst development. This Fe-based electrocatalyst revealed promising ORR activity and PEM fuel cell performance when applied at the cathodic catalytic layer of the corresponding membrane electrode assembly (MEA). In addition, the effect of catalyst ink composition prepared from the MOF derived Pt-based electrocatalyst, in terms of Nafion ionomer content, on the overall performance of PEMFC was investigated via a macroscopic CFD model. The trend predicted from the model calculations was then surveyed experimentally in search for the optimum Nafion ionomer content. Furthermore, the products of thermal transformation of Pt-based MOF into carbon-black based electrocatalyst were studied using a.c. impedance spectroscopy. Along with the electrocatalyst precursor, thermolysis products of parent MOF-253 (Al-containing) were considered in these studies. The materials subjected to thermolysis at increasing temperatures were found to pass through different conduction states starting from insulator and ending up with a particular metal-like conductance with positive temperature dependence and high ambient conductivity

Study of Water Transport Phenomena on Cathode of PEMFCs using Monte Carlo Simulation

This dissertation deals with the development of a three-dimensional computational model of water transport phenomena in the cathode catalyst layer (CCL) of PEMFCs. The catalyst layer in the numerical simulation was developed using the optimized sphere packing algorithm. The optimization technique named the adaptive random search technique (ARSET) was employed in this packing algorithm. The ARSET algorithm will generate the initial location of spheres and allow them to move in the random direction with the variable moving distance, randomly selected from the sampling range, based on the Lennard-jones potential of the current and new configuration. The solid fraction values obtained from this developed algorithm are in the range of 0.631 to 0.6384 while the actual processing time can significantly be reduced by 8% to 36% based on the number of spheres. The initial random number sampling range was investigated and the appropriate sampling range value is equal to 0.5. This numerically developed cathode catalyst layer has been used to simulate the diffusion processes of protons, in the form of hydronium, and oxygen molecules through the cathode catalyst layer. The movements of hydroniums and oxygen molecules are controlled by the random vectors and all of these moves has to obey the Lennard-Jones potential energy constrain. Chemical reaction between these two species will happen when they share the same neighborhood and result in the creation of water molecules. Like hydroniums and oxygen molecules, these newly-formed water molecules also diffuse through the cathode catalyst layer. It is important to investigate and study the distribution of hydronium oxygen molecule and water molecules during the diffusion process in order to understand the lifetime of the cathode catalyst layer. The effect of fuel flow rate on the water distribution has also been studied by varying the hydronium and oxygen molecule input. Based on the results of these simulations, the hydronium: oxygen input ratio of 3:2 has been found to be the best choice for this study. To study the effect of metal impurity and gas contamination on the cathode catalyst layer, the cathode catalyst layer structure is modified by adding the metal impurities and the gas contamination is introduced with the oxygen input. In this study, gas contamination has very little effect on the electrochemical reaction inside the cathode catalyst layer because this simulation is transient in nature and the percentage of the gas contamination is small, in the range of 0.0005% to 0.0015% for CO and 0.028% to 0.04% for CO2. Metal impurities seem to have more effect on the performance of PEMFC because they not only change the structure of the developed cathode catalyst layer but also affect the movement of fuel and water product. Aluminum has the worst effect on the cathode catalyst layer structure because it yields the lowest amount of newly form water and the largest amount of trapped water product compared to iron of the same impurity percentage. For the iron impurity, it shows some positive effect on the life time of the cathode catalyst layer. At the 0.75 wt% of iron impurity, the amount of newly formed water is 6.59% lower than the pure carbon catalyst layer case but the amount of trapped water product is 11.64% lower than the pure catalyst layer. The lifetime of the impure cathode catalyst layer is longer than the pure one because the amount of water that is still trapped inside the pure cathode catalyst layer is higher than that of the impure one. Even though the impure cathode catalyst layer has a longer lifetime, it sacrifices the electrical power output because the electrochemical reaction occurrence inside the impure catalyst layer is lower

The effect of flow field design on the degradation mechanisms and long term stability of HT-PEM fuel cell

Philosophiae Doctor - PhDFuel cells are long term solution for global energy needs. In current fuel cell technologies, Proton Exchange Membrane (PEM) fuel cells are known for quick start-up and ease of operation compared to other types of fuel cells. Operating PEM fuel cells at high temperature show promising applications for stationary combined heat and power application (CHP). The high operating temperature up to 160°C allows waste heat to be recovered for co-generation or tri-generation purposes. The commercially available PEM fuel cells operating at 160⁰C can tolerate up to 3% CO without significant loss of performance, making HT-PEM fuel cell viable choice when reformate is used. In reality these advantages convert to very little balance-of-plant compared to Nafion® based fuel cells operating at 60°C. However, there are some problems that prevent high temperature fuel cells from large scale commercialization. The cathode is said to have sluggish reaction kinetics and high cell potentials and operating temperature during fuel cell start-up may cause severe degradation. The formation of liquid water during the shut-down can cause the phosphoric acid to leach from the cell during operation. Efforts are being made to reduce the cost and increase the durability of fuel cell components (such as catalyst and membrane) at high temperatures. Apart from degradation issues, the problems are related to cost and performance. The performance of the PEM fuel cells depends on a lot of factors such as fuel cell design and assembly, operating conditions and the flow field design used on the cathode and anode plates. The flow field geometry is one important factor influencing the performance of fuel cells. The flow fields have significant effect on pressure and flow distribution inside the fuel cell. A homogeneous distribution of the reactant gases over the active catalyst surface leads to improved electrochemical reactions and thus enhances the performance of the fuel cell. So, the design of flow fields is one of the important issues for performance improvement of PEM fuel cell in terms of power density and efficiency. There are different types of flow fields available for PEM fuel cells such as serpentine, pin, interdigitated and straight flow fields but the most obvious choice is multiple serpentine. The same can be used for high temperature PEM fuel cell (HT-PEMFC) application with ease because of absence of liquid water during the high temperature operation and no need for complex water management

Electrospraying of polymer solutions for the generation of micro-particles, nano-structures, and granular films

S'ha realitzat un estudi sobre els mecanismes de formació de micropartícules polimèriques i les seves pel•lícules granulars, a partir de l'assecat de microgotes de electrosprays. L'estudi se centra en diferents solucions de tres polímers insolubles en aigua: polimetil(metacrilat), poliestirè, i etil cel•lulosa. L'assecat d'aquests electrosprays dóna lloc a diverses morfologies de partícula, que han estat determinades mitjançant microscòpia d'escombrat electrònic, i han estat caracteritzades en funció del solvent, concentració del polímer, el seu pes molecular, i la humitat relativa ambient. Les morfologies obtingudes inclouen una varietat d'estructures de partícula globulars i filamentoses, que, a humitat relativa elevada, poden desenvolupar porositat. Aquestes característiques morfològiques han estat explicades mitjançant models qualitatius que involucren fenòmens fluid dinàmics i sobre separació de fases, presents en sistemes relacionats amb els estudiats. Un dels fenòmens fluid dinàmics involucrats clau són les inestabilitats coulòmbiques de gotes elèctricament carregades. A més, la interacció de no solvent de l'aigua en la precipitació del polímer pot donar lloc a textures poroses sobre la superfície de les partícules. Les diferents formes de textura han estat explicades en referència als fenòmens de breath figure formation (BFF), i a la inversió de fases induïda per vapor (vapor induced phase separation, o VIPS). També hem estudiat el creixement de les pel•lícules granulars formades a partir de les partícules polimèriques. Demostrem que la càrrega elèctrica transportada per les partícules cap a la pel•lícula influeix fortament en la dinàmica de creixement d’aquesta. Un millor coneixement dels mecanismes estudiats en aquesta tesi hauria de permetre dissenyar nous processos de manufactura de partícules i recobriments basats en electrospray. Se ha realizado un estudio sobre los mecanismos de formación de micropartículas poliméricas y sus películas granulares, a partir del secado de microgotas de electropras. El estudio se centra en diferentes soluciones de tres polímeros insolubles en agua: polimetil(metacrilato), poliestireno, y etil celulosa. El secado de estos electrosprays da lugar a diversas morfologías de partícula, que han sido determinadas mediante microscopía de barrido electrónico, y han sido carSe ha realizado un estudio sobre los mecanismos de formación de micropartículas poliméricas y sus películas granulares, a partir del secado de microgotas de electropras. El estudio se centra en diferentes soluciones de tres polímeros insolubles en agua: polimetil(metacrilato), poliestireno, y etil celulosa. El secado de estos electrosprays da lugar a diversas morfologías de partícula, que han sido determinadas mediante microscopía de barrido electrónico, y han sido caracterizadas en función del solvente, concentración del polímero, su peso molecular, y la humedad relativa ambiente. Las morfologías obtenidas incluyen una variedad de estructuras de partícula globulares y filamentosas, que, a humedad relativa elevada, pueden desarrollar porosidad. Estas características morfológicas han sido explicadas mediante modelos cualitativos que involucran fenómenos fluido dinámicos y sobre separación de fases, presentes en sistemas relacionados con los estudiados. Uno de los fenómenos fluido dinámicos involucrados clave son las inestabilidades coulómbicas de gotas eléctricamente cargadas. Además, la interacción de no solvente del agua en la precipitación del polímero puede dar lugar a texturas porosas sobre la superficie de las partículas. Los diferentes tipos de texturas han sido explicadas en referencia a los fenómenos de breath figure formation (BFF), y a inversión de fases inducida por vapor (vapor induced phase separation, o VIPS). También hemos estudiado el crecimiento de las películas granulares formadas a partir de las partículas poliméricas. Demostramos que la carga eléctrica transportada por las partículas hacia la película influye fuertemente en la dinámica de crecimiento de ésta. Un mejor conocimiento de los mecanismos estudiados en esta tesis debería permitir diseñar nuevos procesos de manufactura de partículas y recubrimientos basados en electrospray.A study has been made of the mechanisms underlying the formation of polymeric microparticles and of their granular films, by drying of electrospray microdroplets. The study is focused on different solutions of three water-insoluble polymers: polymethyl(methacrylate), polystyrene, and ethyl cellulose. The drying of such electrosprays result in diverse particle morphologies, which have been determined by scanning electron microscopy, and have been characterized as a function of the solvent, polymer concentration, polymer molecular weight, and ambient relative humidity. The morphologies obtained include a variety of globular and filamented particle structures, which, at elevated relative humidity, can develop porosity. These morphological features have been explained using qualitative models involving fluid dynamic and phase separation phenomena which are known to occur in closely related systems. One of the key fluid dynamic phenomena involved is the coulombic instability of electrically charged droplets. In addition, the non-solvent interaction of water on the precipitation of the polymer can lead to porous textures on the particles surfaces. The different kinds of textures have been explained by reference to breath-figure formation (BFF) and vapor induced phase separation (VIPS) phenomena. We have also studied the growth of the granular films of such polymer particles. We show that the electrical charge transported by the particles to the film have a strong influence on the film growth dynamics. The better understanding of the mechanisms studied in this thesis, should help design new manufacturing processes of particles and coatings based on electrospray

Multiphase Mass Transfer and Capillary Properties of Gas Diffusion Layers for Polymer Electrolyte Membrane Fuel Cells

A detailed understanding of mass transport and water behavior in gas diffusion layers (GDLs) for polymer electrolyte membrane fuel cells (PEMFCs) is vital to improving performance. Liquid water fills the porous GDL and electrode components, hindering mass transfer, limiting attainable power and decreasing efficiency. The behavior of liquid water in GDLs is poorly understood, and the specific nature of mass transfer of multiphase flow in GDLs are not known. There is no clear direct correlation between easily measurable ex-situ GDL material properties and mass transfer characteristics. This thesis addresses this knowledge gap through a combination of test procedure development, experimentation and numerical pore scale modeling. Experimental techniques have been developed to measure permeability and capillary properties of water and air in the GDL matrix. Pore network modeling is used to estimate transport properties as a function of GDL water saturation since these are extremely difficult to determine experimentally. A method and apparatus for measuring the relationship between air-water capillary pressure and water saturation in PEMFC gas diffusion layers is described. The developed procedure of Gas Controlled Porosimetry is more effective for understanding the behaviour of water in GDL material then traditional methods such as the method of standard porosimetry and mercury intrusion porosimetry. Capillary pressure data for water injection and withdrawal from typical GDL materials are obtained, which demonstrated permanent hysteresis between water intrusion and water withdrawal. Capillary pressure, defined as the difference between the water and gas pressures at equilibrium, is positive during water injection and negative during water withdrawal. The results contribute to the understanding of liquid water behavior in GDL materials which is necessary for the development of effective PEMFC water management strategies and the design of future GDL materials. The absolute gas permeability of GDL materials was measured. Measurements were made in three perpendicular directions to investigate anisotropic properties of various materials. Most materials were found to be significantly anisotropic, with higher in-plane permeability than through-plane permeability. In-plane permeability was also measured as the GDL was compressed to different thicknesses. Typically, compression of a sample to half its initial thickness resulted in a decrease in permeability by an order of magnitude. The relationship between measured permeability and compressed porosity was compared to various models available in the literature, one of which allows the estimation of anisotropic tortuosity. The results of this work will be useful for 3D modeling studies where knowledge of permeability and effective diffusivity tensors is required. A pore network model of mass transport in GDL materials is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions of the pores. With the use of experimental data obtained the geometric parameters of the pore network model were calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials. The model was subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, transport properties of GDLs that are very difficult to measure were estimated, including the relative permeability of water and gas, and the effective gas diffusivity as functions of water saturation. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. The pore network model was also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but water flooding becomes a significant source of concentration polarization as the GDL becomes increasingly saturated with water. This work has significantly contributed to the understanding of mass transfer in gas diffusion layers in PEMFC through experimental investigation and pore scale modeling

Development and Characterization of Low Pt-Loaded Membrane Electrode Assemblies with Focus on Performance and Durability

For many applications of polymer electrolyte membrane fuel cell (PEMFC), the loading attributed to platinum as catalyst is still too high for this technology to penetrate into the mass market. However, this high loading of platinum is still necessary to achieve the performance and service life targets. Therefore, reducing the loading of precious group metals is a major challenge to low temperature PEM fuel cell community. The performance of the membrane electrode assembly (MEA) with low Pt loading depends on the optimization of numerous parameters like catalyst activity, proton conductivity of ionomer, ionomer to catalyst ratio, diffusion media, operating conditions, and last but not the least the microstructure of the electrode, which is determined by the coating method. An efficient electrode with low platinum loading and durable performance requires a thin but porous catalyst layer, in which the catalyst particles and ionomer are homogenously distributed with a large surface area. The fundamental goal of this dissertation is to understand the relationships between structural properties and performance, and to derive strategies for a goal oriented development. In the first part of the study, PEMFC electrodes were fabricated with the same Pt loading by means of diverse coating techniques. Current-voltage curves, electrochemical analysis, and physical characterizations are evaluated to interpret the influence of microstructure caused by the coating methods on performance and durability. In order to obtain different catalytic layer structures, the electrodes were produced using six different coating techniques with the same Pt loading. The selected coating techniques are wet spraying, screen printing, inkjet printing, dry spraying, doctor-blade and drop casting. Similar drying conditions were maintained after all the wet coating processes. The physical and electrochemical characterizations of the individual catalyst layers (CL) were investigated under identical operating conditions. The results show that wet spraying and screen printing showed the highest performance due to the low proton resistance. The lowest efficiencies were observed in doctor-blade and drop-cast techniques, which are associated with particularly low protonic conductivity. Microstructure investigation by focus-ion-beam scanning electron microscope analysis were used to determine transport properties such as porosity, permeability, diffusivity and inverse tortuosity by image analysis in GeoDict. A comparison of peak power density and effective transport parameters shows that an increase in permeability, diffusivity and porosity correlates strongly with increasing power. A dimensionless classification of the transport properties of the MEA with a point system and their summation can describe the observed performance very well. Consequently, the measured and analyzed transport parameters seem to be sufficient for predicting the performance of a membrane electrode assembly (MEA). This can help to optimize coating techniques and thus increase MEA performance together with service life. Furthermore, the dry coating technology developed at DLR was improved in order to produce MEAs nearly 50 % more efficient than before. Additionally, the effect of ionomer with diverse side chain length as well as the significance of membrane thickness is also studied for long and short term application upon load cycling test. This research further provides a deep insight into the importance of ionomer and its microstructure both in the electrode and the membrane in PEM fuel cell, which influences the performance and also the long term stability. After 600 hours of load cycle operation with the cells, roughly 120 mV of drastic degradation was observed owing to the higher gas crossover through thinner membrane, while the performance can be increased approximately 16 % due to the shorter side chain of ionomer. Another important result of this work is the investigation of the influence of the drying process of MEA production on the electrode microstructure, i.e. the open porosity, the ionomer distribution and the size of the reactive interface. An unconventional drying method known as freeze drying, shows three-fold improvement in the porosity and promising ionomer distribution in CL. Consequently, this can reduce the transport limitations and improve the peak power density about 34 % compared to the conventional drying technique. Furthermore, a transient 2D physical continuum model was applied and simulations were performed to numerically investigate the influence of different drying methods on PEM fuel cell performance. Both experimental and simulation data emphasize the fact that the sublimation of the catalyst layer improves the architecture by optimizing porosity, permeability and tortuosity. These above-mentioned properties of the microstructure of the catalytic layer significantly improve water management and diffusion properties, which has an impact on performance and reduced mass transport limitation. This work is able to identify important process engineering relationships between the microstructure of CL and its performance. In addition, promising manufacturing processes, drying methods and operating conditions were found, which should allow a targeted improvement of CL performance in the next step