Electrochemical pressure impedance spectroscopy as a diagnostic method for hydrogen-air polymer electrolyte fuel cells

This work presents the implementation and analysis of electrochemical pressure impedance spectroscopy (EPIS) as an in situ diagnostic method for polymer electrolyte fuel cells (PEFCs). Inspired by electrochemical impedance spectroscopy (EIS), EPIS is an acoustic spectroscopic technique that analyses the response of the voltage signal to an applied pressure signal in the frequency domain. In EPIS, the cathode gas pressure is modulated as a sinusoidal wave and the voltage measured from the PEFC is monitored as the response signal. The EPIS measurements are sought as a means to probe cathode transport properties specifically as these processes are involving mass transport limitations during PEFC operation. The development of EPIS is sought to furnish tools to study, for example, water and oxygen transport in the MEA as a function of changes in its structure and its operating conditions. Flow channels are one of the core elements of PEFCs. To probe the contribution of the flow channel to an EPIS response, the flow channel is characterised by the response of the system as a function of correlations between pressure measured at the cathode inlet and outlet. The outlet pressure is excited as the input variable, and the response of the system is measured at the inlet, on the other side of the flow channels. An alternative way to characterise the flow channel is to apply a flow rate step excitation at the inlet of the gas flow stream and measure pressure at the flow channel inlet correspondingly. Both methods are discussed in this work. The experimental approach for EPIS requires an oscillating pressure signal in the form of a sinusoidal wave. In this work, oscillations are applied through the use of a: (i) mass flow controller (MFC) oscillation; (ii) back-pressure controller (BPC) oscillation; or (iii) acoustic speaker box (ASB) oscillation. The BPC method studies experimental parameters like the cathode stoichiometry ratio, cathode flow rate, oxygen partial pressure, and pressure oscillation amplitude. The MFC creates a pressure perturbation with a modulated flow and it probes the voltage change with respect to the perturbation in oxygen partial pressure and flow rate. The ASB method is proposed for future studies, and some theoretical basis for the ASB method is covered in the outlook section.