Abstract
Understanding water management is a crucial aspect in the development of improved polymer electrolyte fuel cells (PEFCs). Separating the performance degradation due to dehydration, water flooding and reactant starvation in PEFCs is a major challenge. In this study, acoustic emission (AE) analysis, a non-invasive and non-destructive diagnostic tool, is utilised to probe water formation and removal inside an operating fuel cell. In the acoustic emission as a function of polarisation (AEfP) method, AE activity from the PEFC is measured in terms of cumulative absolute AE energy (CAEE) hits during operation at discrete points on the polarisation curve. AEfP can identify the presence of liquid water in flow channels and correlate its formation and removal with the level of cell polarisation, and consequent internal temperature. Correlation between acoustic activity and water generation, supply and removal is achieved by varying current (polarisation), cathode air feed relative humidity (RH) and cell temperature, respectively. Features such as initial membrane hydration, liquid water formation, ‘flushing’ and the transition from ‘wet-channel’ to ‘dry-channel’ operation are identified using AE analysis, thereby providing a powerful and easy to implement diagnostic for PEFCs.
Highlights
Acoustic methods are emerging as powerful probes of the internal workings of electrochemical power systems and have the advantage of relatively low cost and non-invasive operation
The acoustic activity from the polymer electrolyte fuel cells (PEFCs) is computed based on three parameters: amplitude, which is the peak acoustic emission (AE) signal excursion obtained in a hit; energy, which is the area under an AE hit generated; and counts, which is the number of excursions exceeding the AE threshold
Considering points on the polarisation curves of equal current density as having the same rate of water generated, relative humidity (RH) as a measure of the water introduced to the cell from reactant flow, and temperature as a driving force for water to be removed from the cell, it is possible to analyse the AE results based on different cell hydration conditions
Summary
Acoustic methods are emerging as powerful probes of the internal workings of electrochemical power systems and have the advantage of relatively low cost and non-invasive operation. Legros et al [4,5] were the first to utilise AE analysis as a water diagnostic tool for PEFC Their initial reports indicated AE activity resulting from dehydration of the Nafion membrane and attributed it to structural changes occurring in the membrane during drying. AE from a PEFC was measured under different conditions, including without membrane electrode assembly (MEA), with MEA at open circuit voltage (OCV) and with MEA under load, respectively. These identified that hydrodynamics in the gas flow channels, water uptake and release by the MEA and electrochemical reactions can all contribute to the AE signal [5]. The frequency range for analysis was 100 kHz to 0.1 Hz, with 10 points per decade, and an AC modulation amplitude of 5% of the DC (direct current) input signal
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