Abstract
Electrochemical Impedance Spectroscopy (EIS) is a well-established technique for studying Polymer Exchange Membrane Fuel Cells (PEMFC) but data interpretation remains delicate, mostly because impedance models are either based on oversimplified equations or conversely, include too many correlated parameters. It is thus crucial to carefully choose the models to interpret impedance data, according to FC materials and operation conditions. Most of PEMFC impedance spectra are composed of two loops in Nyquist plot that can be perfectly represented by classical Randles Electrical Equivalent Circuit (EEC). However, several spectra show a straight line at high frequencies associated with proton conduction in the cathode catalyst layer. Assuming an interface electrode, the Randles EEC is poorly adapted to such spectra and one will rather use Transmission Line Models (TLM). However, since TLM do not usually consider mass transport, it is necessary to adapt the EEC, especially at the cathode. Such EEC can then be used as general FC models independently of the occurrence of the straight line at high frequencies, i.e. independently of the ratio between proton conduction and reaction kinetics limitations. These TLM EECs are then used to analyze the layer(s) at the origin of oxygen transport limitations: catalyst and/or the gas diffusion layer.
Highlights
Location of the main mass transfer resistance.—We have just shown that the modified Transmission Line Models (TLM) in Figure 6 is equivalent to the usual Randles Equivalent Circuits (EEC) when the ionic resistance through the catalyst layer is negligible, which is the case for instance with a surface electrode. This leads us to the conclusion that the Warburg impedance in the Randles circuit corresponds to an oxygen transport resistance that is physically located in the catalyst layer, which is inherently in contradiction with the hypothesis of a surface electrode, often put forward with Randles EEC
The main conclusion of this work is that TLM-like EEC modified to consider charge transfer resistance as well as oxygen transport resistance can be used instead of Randles EEC to model Polymer Electrolyte Membrane Fuel Cells (PEMFC) Cathode Catalyst Layer (CCL) impedance since they correspond to a general representation: the impedance of such TLM-like EEC tends toward that of a Randles EEC when the ion transport resistance through the catalyst layer becomes negligible compared to the Oxygen Reduction Reaction (ORR) kinetics parameters, i.e. the charge transfer resistance and the double layer capacitance
The occurrence of the straight line at high frequencies, depends on the ratio between the ionic resistance and the elements characterizing the reaction kinetics. This is the reason why the straight line is always observed with blocked electrodes, but appears only sometimes during fuel cell operation, i.e. with thick electrodes or electrodes with low ionomer content. These considerations lead us to the conclusion that the Warburg impedance in the Randles circuit corresponds to an oxygen transport resistance that is physically located in the cathode catalyst layer, which is in contradiction with the hypothesis of a surface electrode that governs the derivation of this EEC
Summary
In the second case (in operando), the cell is fed with H2 at the anode and air at the cathode This configuration allows analyzing reaction kinetics and mass transport losses which is usually done using a Randles EEC.[11] The straight line at high frequencies can sometimes be observed in operando for certain MEA,[12,13] and it is typically associated with the volumetric character of the catalyst layer tied up to ion transport through the porous electrode as a whole[14,15,16,17] or within the thin electrolyte film covering the reaction sites.[18] The 45° slope is typical of homogenous ion transport through the CL, whereas other values may result from non-homogeneous transport properties, which is more likely to occur with thick electrodes and/or at high current density.[13]. The impedance spectra measured with cell B consistently showed a straight line at high frequencies, while that line never appeared with cell A
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