A Comprehensive Physical Impedance Model of Polymer Electrolyte Fuel Cell Cathodes in Oxygen-free Atmosphere

Abstract

Electrochemical impedance spectroscopy (EIS) is an indispensable tool for non-destructive operando characterization of Polymer Electrolyte Fuel Cells (PEFCs). However, in order to interpret the PEFC’s impedance response and understand the phenomena revealed by EIS, numerous semi-empirical or purely empirical models are used. In this work, a relatively simple model for PEFC cathode catalyst layers in absence of oxygen has been developed, where all the equivalent circuit parameters have an entire physical meaning. It is based on: (i) experimental quantification of the catalyst layer pore radii, (ii) application of De Levie’s analytical formula to calculate the response of a single pore, (iii) approximating the ionomer distribution within every pore, (iv) accounting for the specific adsorption of sulfonate groups and (v) accounting for a small H2 crossover through ~15 μm ionomer membranes. The derived model has effectively only 6 independent fitting parameters and each of them has clear physical meaning. It was used to investigate the cathode catalyst layer and the double layer capacitance at the interface between the ionomer/membrane and Pt-electrocatalyst. The model has demonstrated excellent results in fitting and interpretation of the impedance data under different relative humidities. A simple script enabling fitting of impedance data is provided as supporting information.

Interpreting Ionic Conductivity for Polymer Electrolyte Fuel Cell Catalyst Layers with Electrochemical Impedance Spectroscopy and Transmission Line Modeling

“…where, R 0 is the electrolyte resistance for a unit length, Z 0 is the interfacial impedance for unit length, and l is the length of each pore. 18 In contrast to Obermaier et al, 19 which considered the pore size distributions, and a specific TLM for each pore, we simplify the entire catalyst layer to be represented by the TLM of a single cylindrical pore with representative parameters. Hence the impedance of the catalyst layer can be represented as:…”

“…Figure 1c shows the equivalent circuit adopted from Obermaier et al 19 The model neglects the impedance response of the anode catalyst layer, as it is shorted by the fast hydrogen oxidation reaction. Also because of the absence of the reactive gas there are no Faradaic resistances in the cathode catalyst layer.…”

“…18 Obermaier et al developed a one-dimensional TLM with additional elements describing the physics of species adsorption and side reactions making the model more comprehensive but at the same time requiring catalyst layer morphology as a model input. 19 Previously, we used a hydrogen pump (HP) technique to measure effective ionic conductivity of the pseudo-catalyst layers (PCLs) without Pt over a relative humidity (RH) range of 50%-120%. 20 For the HP set up, multiple PCLs were placed between two membranes.…”

Investigation of Hydrogen Oxidation and Evolution Reactions at Porous Pt/C Electrodes in Nafion-Based Membrane Electrode Assemblies Using Impedance Spectroscopy and Distribution of Relaxation Times Analysis

“…This proton-pump design has been beneficial for investigating the kinetics associated with HOR and HER operation at Pt/C in PEM-based MEAs under standard and pressurized conditions, allowing accurate determinations of the exchange current and rate-limiting steps associated with the HOR/HER at Pt/C surfaces. − Analyses of the mass transport resistance under hydrogen limiting currents in the proton-pump design have allowed a breakdown of the different contributions to the mass transport resistance in this current regime . Further, by placing one of the electrodes under inert conditions and the other under hydrogen flow, the ionic conduction pathway and corresponding ionic conductivity and active catalyst area of the catalyst layers can be determined using physical models. , While different resistances associated with HOR and HER operation have been determined using the proton-pump design independently, there has been little empirical work investigating each of the contributions to the cell impedance during operation.…”

“…27 Further, by placing one of the electrodes under inert conditions and the other under hydrogen flow, the ionic conduction pathway and corresponding ionic conductivity and active catalyst area of the catalyst layers can be determined using physical models. 28,29 While different resistances associated with HOR and HER operation have been determined using the proton-pump design independently, there has been little empirical work investigating each of the contributions to the cell impedance during operation.…”