Online Estimation of Membrane Water Content in Vehicular PEMFC by Complex Morlet Wavelet Transformations

The amount of water content in membrane electrode assembly (MEA) is an important factor affecting the efficiency and life of proton exchange membrane fuel cell (PEMFC), and there are several methods to measure it. However, it’s widely believed that the most feasible method to measure the water content in the MEA is an indirect way as described below: measure the electrochemical impedance spectroscopy (EIS), and take advantage of the positive correlation between the proton’s conductivity, which is reciprocal of specific resistance, and the water content in MEA. The traditional EIS measurement method has the shortcoming of high cost and slow speed, especially in low frequency bands, so this method is unsuitable for high-power vehicle fuel cell systems. In this paper, a measuring method which does not require a large number of experiments and only need to perform Morlet wavelet transform on the PEMFC voltage signal as well as current signal is proposed. The EIS of PEMFC is exactly the quotient obtained by dividing the voltage wavelet coefficient by the current wavelet coefficient. This measuring method is firstly verified in by simulation in a PEMFC equivalent circuit model established on the MATLAB/Simulink platform and then is tested in a commercial fuel cell simulation platform. The simulation results show that the EIS measured by wavelet transform is very close to the theoretical value and can effectively determine the level of water content in the proton exchange membrane in the current state. This positive result can be further applied to the dynamic control of water balance in automotive fuel cell systems.