Performance Improvement Methodologies for Proton Exchange Membrane Fuel Cells
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Hydrogen based energy generation devices, especially fuel cells are expected to play a dominant role in stationary and portable power systems in the future. However, fuel cells have a few bottlenecks and performance degradation factors, which needs to be understood and addressed to make fuel cells a commercial success. The membrane electrode assembly (MEA) acts as the heart of the fuel cell system and the performance limitations occurring in MEAs need to be addressed for ensuring its durability. The MEA consists of catalyst layers (CLs), gas diffusion layers (GDLs), and proton exchange membrane (PEM). The focus of this thesis can be described as two objectives – 1) the understanding of performance limitation factors and 2) developing methodologies to mitigate these factors. Initially an attempt was made to understand the degradation of Pt catalysts using a first principles model accounting for the dissolution behavior. The results indicated that dissolution of Pt could not account for a major portion or the trend in catalyst activity loss. A semi-empirical model was developed to understand the dissolution behavior of Pt. To further understand the factors, which contribute to the degradation of Pt, experimental studies were carried out to understand the contribution of various degradation factors such as agglomeration due to Ostwald ripening, sintering of Pt and dissolution. In order to deconvolute the confounding mechanisms, image analysis was applied in addition to various physical and chemical characterization techniques. Pt sintering was simulated using mesoscale simulations to understand the effect of applied potential on Pt degradation. Our next goal was to mitigate the impact of flooding in fuel cells by removing the excess water produced in the system. Simulations were performed with testing the effect of flow rate change of reactant gases, as an agent to remove the excess water, by monitoring the voltage drop in the system with and without the use of a zero-dimensional first principles model. An attempt has also been made to eliminate the drying of PEM and flooding of GDL by controlling the humidity of the reactant gases fed to the system. A model was developed to find the humidification of gases under both isothermal and non-isothermal conditions. Experimental results were gathered and the deviation between the model and experimental data were analyzed. A conventional PID controller has been applied experimentally under both isothermal and non-isothermal conditions. In high temperature PEM fuel cells, which transfer protons using phosphoric acid in the PEM, acid leaching is a major issue. An attempt has been made to conceptualize the fundamental processes that leads to acid leaching in the membranes. Experiments were performed to understand the onset of acid leaching and a model was conceptualized for the same. The advantages of using modeling and simulation alongside experimental data to recognize and predict the various factors reducing the performance of fuel cells can be appreciated. A discussion of the various possible experiments and simulations for data collection and verification, focused towards performance improvement of fuel cell MEA has been provided.