Chairperson, Graduate Committee: Steven R. ShawSeger, Eric Matthew2013-06-252013-06-252010https://scholarworks.montana.edu/handle/1/2238This thesis demonstrates in-situ characterization of a 5kW solid oxide fuel cell (SOFC) stack and a 165W proton exchange membrane fuel cell (PEMFC) stack at the electrical terminals, using impedance spectroscopy and time-domain modeling. The SOFC experiments are performed using excitation from the power electronic ripple current and exogenous excitation generated from several different sources including a hybrid system which uses a secondary power source for the generation of the small-signal currents. The PEMFC experiments are performed using exogenous excitation from a boost converter. In contrast to typical off-line analysis using specialized instrumentation, the measurements are made as the stacks deliver power to their respective loads. The power electronic switching waveform is used as a source of excitation. This technique could be implemented on-line for continuous condition assessment of the stack. The results show typical data from the stack, comparison of model predictions and measured data, and whole-stack impedance spectroscopy results.enFuel cellsProton exchange membrane fuel cellsSolid oxide fuel cellsImpedance spectroscopyThesisCopyright 2010 by Eric Matthew Seger