Analysis of a Fuel Cell System by a Step Response

A simple method for analysing a fuel cell system using the voltage response to a step change in load resistance is presented in this work. With the modified Randles model, where two resistors and two capacitors are implemented for the Warburg impedance, the response curve is fitted with three exponential curves. Six independent equations corresponding to six parameters of the model can be obtained from the fitted values, and they are solved under two specific assumptions for the initial state. The parameters are the charge double layer capacitance, the charge transfer resistance, a resistor-capacitor pair related to gas diffusion and another pair related to water diffusion. A down curve of the voltage for the step down change in load and an up curve for the step up change are fitted and parameters for both cases are evaluated separately. Analysis is performed by comparing the parameters at different current levels. The impedance is also simulated using the estimated parameters in order to show which parameter is the rate-determining factor of the fuel cell reaction with increasing current levels.