Understanding the kinetics of polymer electrolyte fuel cells (PEFC) remains a key issue for the optimization of the catalyst layer structure with respect to fuel cell performance and ageing.

In this study, firstly the impact of hydrogen crossover from anode to cathode through the membrane on the measurable open circuit voltage (OCV) is analyzed extensively. For this purpose, all the other parasitic reactions like platinum oxide formation or carbon corrosion are neglected in a first approach and the influences of the operating conditions on the crossover flux by means of voltammetry measurements and also mass spectrometry in the cathode exhaust gas are examined. These experimental results yield an efficient parameterization of the crossover rate. Incorporated into a physical 1D through-plane cell simulation model, it allows for consistent predictions at and close to OCV conditions.

Subsequently, the activation overpotential is parameterized experimentally thanks to electrochemical impedance spectroscopy on a 25 cm² cell with a straight parallel channel flowfield under differential conditions. Therefore, spectra are recorded for a large range of conditions (Fig. 1a) and the distribution of relaxation times (DRT) is evaluated (Fig. 1b). As a result, an equivalent circuit is set up. Hence, it is possible to fit the experimental data and to extract the kinetic parameters for our model, as functions of electrical load, oxygen partial pressure and temperature. We finally use this extensive dataset for comparing different approaches for modelling the oxygen reduction reaction (ORR) on platinum, ranging from single-step Butler-Volmer kinetics to advanced microkinetic models that take into account the intermediates of the ORR and thereby also the potential dependent surface coverage of the electrode [1-2].

References:

1. Huang, J. Zhang, M. Eikerling, Phys. Chem. Chem. Phys., 2018, 20, 11776
2. Holewinski, S. Linic, J. Electrochem. Soc., 159 (11) H864-H870 (2012)