Development and characterization of low Pt-loaded membrane electrode assemblies with focus on performance and durability

Abstract

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.