Acoustic Emission Analysis of Polymer Electrolyte Membrane Fuel Cells

Abstract

Diagnosis of water dynamics in a polymer electrolyte membrane fuel cell (PEMFC) is an effective way to evaluate the performance of a PEMFC. Several methods, like neutron imaging [1], x-ray scanning [2] and optimal imaging [3] are employed for direct water evaluation in PEMFCs. However, these methods have certain limitations in terms of their cost, complexity, radiation exposure and availability to use. Recently, we developed a non-destructive and portable tool, called acoustic emission as a function of polarisation (AEfP) that was utilised to diagnose water management in PEMFCs by probing the hydration levels generated during cell polarisation [4]. Acoustic emissions (AE) are elastic waves that are generated as a result of mechanical perturbation caused by water impacting the flow-fields in a PEMFC [4,5]. In this study, a detailed analysis of the acoustic signal parameters, like amplitude and peak counts, is presented. The polarisation and corresponding acoustic activity analysis were performed on a PEMFC at 40%, 70% and 100% reactant RH, and at 45 ⁰C and 60 ⁰C cell start-up temperature conditions. An increase in the set cell temperature by 15 ⁰C resulted in a drop in limiting current density by 52% at 40% RH, 50% at 70% RH and 35% at 100% RH, as a result of cell dehydration during the temperature rise, while operating the fuel cell. Cell dehydration at higher operating temperatures was also investigated by electrochemical impedance spectroscopy (EIS) measurements. The amplitude established the intensity of the generated AE, while peak counts established the proportion of AE generated beyond a threshold, respectively, and these are influenced by current density, reactant humidity and cell temperature. Furthermore, the physical impact created from release of water into flow-fields leads to acoustic emissions, with average amplitude between 29 – 35 dB, as seen in Fig. 1, irrespective of the cell’s operating conditions. Peak counts represent the sizeable AE events developed beyond a threshold, which are dependent on the level of water generation inside the cell; these were greater in number at lower cell temperatures and vice versa. Overall, the parametric analysis of acoustic activity for the PEMFC was instrumental in establishing the relationship between a cell’s operating conditions and its associated performance, with optimal and dehydrated cell conditions resulting in stable and reduced numbers of generated acoustic events, respectively, which were related to the flow of water and its physical state in the flow-fields.