Modeling of electrochemical energy storage and energy conversion devices
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With increasing interest in energy storage and conversion devices for automobile applications, the necessity to understand and predict life behavior of rechargeable batteries, PEM fuel cells and super capacitors is paramount. These electrochemical devices are most beneficial when used in hybrid configurations rather than as individual components because no single device can meet both range and power requirements to effectively replace internal combustion engines for automobile applications. A system model helps us to understand the interactions between components and enables us to determine the response of the system as a whole. However, system models that are available predict just the performance and neglect degradation. In the first part of the thesis, a framework is provided to account for the durability phenomena that are prevalent in fuel cells and batteries in a hybrid system. Toward this end, the methodology for development of surrogate models is provided, and Pt catalyst dissolution in PEMFCs is used as an example to demonstrate the approach. Surrogate models are more easily integrated into higher level system models than the detailed physics-based models. As an illustration, the effects of changes in control strategies and power management approaches in mitigating platinum instability in fuel cells are reported. A system model that includes a fuel cell stack, a storage battery, power-sharing algorithm, and dc/dc converter has been developed; and preliminary results have been presented. These results show that platinum stability can be improved with only a small impact on system efficiency. Thus, this research will elucidate the importance of degradation issues in system design and optimization as opposed to just initial performance metrics. In the second part of the thesis, modeling of silicon negative electrodes for lithium ion batteries is done at both particle level and cell level. The dependence of the open-circuit potential curve on the state of charge in lithium insertion electrodes is usually measured at equilibrium conditions. Firstly, for modeling of lithium-silicon electrodes at room temperature, the use of a pseudo-thermodynamic potential vs. composition curve based on metastable amorphous phase transitions with path dependence is proposed. Volume changes during lithium insertion/de-insertion in single silicon electrode particle under potentiodynamic control are modeled and compared with experimental data to provide justification for the same. This work stresses the need for experiments for accurate determination of transfer coefficients and the exchange current density before reasoning kinetic hysteresis for the potential gap in Li-Si system. The silicon electrode particle model enables one to analyze the influence of diffusion in the solid phase, particle size, and kinetic parameters without interference from other components in a practical porous electrode. Concentration profiles within the silicon electrode particle under galvanostatic control are investigated. Sluggish kinetics is established from cyclic voltammograms at different scan rates. Need for accurate determination of exchange current density for lithium insertion in silicon nanoparticles is discussed. This model and knowledge thereof can be used in cell-sandwich model for the design of practical lithium ion cells with composite silicon negative electrodes. Secondly, galvanostatic charge and discharge of a silicon composite electrode/separator/ lithium foil is modeled using porous electrode theory and concentrated solution theory. Porosity changes arising due to large volume changes in the silicon electrode with lithium insertion and de-insertion are included and analyzed. The concept of reservoir is introduced for lithium ion cells to accommodate the displaced electrolyte. Influence of initial porosity and thickness of the electrode on utilization at different rates is quantitatively discussed. Knowledge from these studies will guide design of better silicon negative electrodes to be used in dual lithium insertion cells for practical applications.