(Invited) Experimental and Physically Based Modelling Analysis of Electrochemical Impedance in PEMFC to Interpret Performance Degradation Causes

Abstract

The required target for mass deployment of PEMFC in different applications is still not fully achieved, because of a severe performance decay, determined by the interaction of several degradation mechanisms [1]. The comprehension of such mechanisms and the influence of operating condition stressors is still not satisfactory to accurately predict fuel cell lifetime during real operation. Especially the components structural and functional alteration in time, observed by ex-situ techniques, can be hardly associated to a distinct contribution to performance loss. Electrochemical impedance spectroscopy is a promising in-situ technique to distinguish the contributions of different degradation phenomena, but nowadays data analysis is widely performed modelling the fuel cell through simplified electrical circuits that do not permit to achieve a solid physical interpretation of performance degradation causes. A physically based modelling approach, recently explored in the literature [2,3], can cover this gap, however it is not fully consolidated yet and the required effort to develop appropriate tools limits today its utilization to modelling experts. The present work aims at presenting the effectiveness of an approach that combines experimental and physically based modelling analysis of electrochemical impedance in investigating performance degradation occurring in different technologies [4-7]: HT-PEMFC, Low-Pt PEMFC, DMFC, PGM-free PEMFC. The general methodology is presented, highlighting the possible onset of different regimes, where distinct phenomena prevail. The continuum model based on macro-homogenous assumption, solves mass and ionic conservation coupled with Stefan-Maxwell diffusion and Ohm’s law; electrochemical reactions are described with Butler-Volmer kinetics. EIS is solved according to the approach reported in [5]. Subsequently some relevant cases are discussed in details, emphasizing the influence of heterogeneous degradation [5] and local mass transport [7]. One analysis reported in this work is the effect of heterogeneity of ageing on EIS, which determine an increase of the cathodic charge transfer resistance that is not ascribed to a loss of electrochemical active surface, but to the uneven distribution of the reaction rate. Another analysis reported in this work consists in the simulation of EIS with commercial CFD codes: the effect of 3D geometrical features was analyzed, e.g bends in the flow field, flow bypass between channels, interdigitated flow pattern. [1] Boroup, R., & al. (2007). Scientific aspects of Polymer Electrolyte Fuel Cell durability and degradation. Chemical Reviews, 107, 3904-3951. [2] Baricci, A., Zago, M., & Casalegno, A. (2014). A quasi 2D model of a High Temperature Polymer Fuel Cell for the interpretation of impedance spectra. Fuel Cells, 14, 926-937. [3] Kulikovsky, A.A., (2012). A physical model for catalyst layer impedance. Journal of Electroanalytical Chemistry, 669, 28-34 [4] Bresciani, F., Casalegno, a., Zago, M., & Marchesi, R. (2013). A parametric analysis on DMFC anode degradation. Fuel Cells, 14, 386–394. [5] Baricci, A., Zago, M., & Casalegno, A. (2016). Modelling analysis of heterogeneity of ageing in high temperature polymer electrolyte fuel cells: insight into the evolution of electrochemical impedance spectra. Electrochimica Acta, 222, 596–607. [6] Casalegno, A., Baricci, A., Bisello, A., Odgaard, M., Serov, A., & Atanassov, P. (2017). Insight into degradation mechanism in non-precious metal composite catalysts for PEMFC. 7th FDFC, Stuttgart. [7] Baricci, A., Mereu, R., Messaggi, M., Zago, M., Inzoli, F., & Casalegno, A. (2017). Modeling analysis of flow field geometrical features in Polymer Electrolyte Fuel Cells porous media. 7th FDFC, Stuttgart.