Anode Monitoring By Electrochemical Impedance Spectroscopy in Polymer Electrolyte Membrane Fuel Cells

Abstract

Although widely used to study the operation of Polymer Exchange Membrane Fuel Cells (PEMFC), Electrochemical Impedance Spectroscopy (EIS) remains a complex tool: experimental data are somewhat delicate to interpret because of the wide range of physical phenomena occurring in Membrane Electrode Assemblies (MEA) and in the flow field plates. Therefore, impedance models are often based on oversimplified equations or conversely, include too many correlated parameters. A typical example of the complexity of the phenomena governing fuel cell electrical behavior are the oscillations of oxygen concentration resulting from the measuring signal, that propagate along the gas channels and makes more difficult the interpretation of the low frequency loop [1, 2, 3]. In addition, most of the impedance models assume Fickian oxygen diffusion although, at least in fuel cells fed with air, Stefan-Maxwell equations should be used as in most stationary models [4, 5]. In this regard, oxygen diffusion and thus the low frequency impedance are strongly dependent on the overall convective flux and as a result, on water management [6].However, due to the characteristic times of oxygen diffusion, these limitations do not apply to the high frequency region of the impedance spectra. The high frequency impedance of a PEMFC remains mostly governed by the cathode [7] but at least in some cases, considering the anode in the Electrical Equivalent Circuit (EEC) is necessary to improve the quality of the fit of the model to the experimental data (Figure). Since the anode has an impact on fuel cell impedance, this means that it becomes possible to monitor its ageing while performing Accelerated Stress Tests (AST). Since most of FC lifetime issues presumably originate from the cathode [8, 9] anode degradation has been rarely studied in the literature. Nevertheless, Schwämmlein et al. showed recently that it may be significant in the case of repeated start-up and shut-down - because of potential cycling - and have a measurable impact on FC performance [10].In this work, we performed AST combining potential and humidity cycles to be as representative as possible of the stressful conditions encountered by the FC in real operating conditions. Commercially available MEA with an initial voltage of the order of 0.7 V at 0.5 A/cm² showed a drop in performance comprised between 400 and 900 µV/h, and a significant degradation of the anode operation was observed by EIS (Figure), confirmed by an increase of its local potentials, and linked to Pt ElectroChemical Surface Area (ECSA) losses. Post-mortem physico-chemical analysis of MEA will be carried out to bridge the decrease of performance with the degradation mechanisms occurring at the anode catalytic layer. Note that these tests were carried out using a 30 cm² segmented cell with 5 straight parallel gas channels [11] (30×1 cm² active area) and that its overall performances may be lower than with regular cells.