A locally resolved investigation on direct methanol fuel cell uneven components fading: Local cathode catalyst layer tuning for homogeneous operation and reduced degradation rate

C Rabissi,M Zago,P Gazdzicki,L Guétaz,S Escribano,L Grahl-Madsen,A Casalegno

doi:10.1016/j.jpowsour.2018.09.094

Abstract

Durability issues of direct methanol fuel cell still hinder technology widespread commercialization; uneven aging of MEA components, generally harsher in air outlet region, is known to exasperate overall performance degradation. In a previous work, the authors selected a stable cathode electrode, demonstrated to fade homogenously: uneven water-related limitations, such as dehydration and flooding, were revealed to locally worsen performance at cathode inlet and outlet regions, leading to current redistribution. Aiming to reduce degradation rate, in this work homogeneous current distribution during operation is pursued by tuning MEA properties to meet local operating conditions. A properly improved 1D+1D physical model is used to support the development of a gradient MEA, featuring 1.6 mg cm−2 and 0.8 mg cm−2 of catalyst and ionomer respectively at inlet/outlet and center regions of cathode electrode. Tests based on custom macro-segmented cell demonstrated 55% more homogeneous current distribution, controllable during operation by means of cathode air stoichiometry. 500 h degradation test revealed 70% decreased degradation rate from uniform MEA (11 μV h−1) with a homogenous fading of performance. An 18% lower Pt nanoparticle growth at cathode outlet and limited ionomer degradation at cathode inlet were identified by ex-situ analyses (TEM and XPS), indicating locally mitigated fading mechanisms.

Highlights

Thanks to the employment of a high energy density liquid fuel, permitting convenient and quick recharging [1,2,3], direct methanol fuel cell (DMFC) is a promising technology for stationary and portable power production from very small to medium scale as well as for light and industrial vehicular sectors
Despite segmented cell-based works are increasing in number [21,22,23,24,25,26,27,28], few works about DMFC degradation correlate local performance analysis during ageing with local degradation measured by ex-situ analysis, i.e. transmission electron microscopy (TEM) and X-Ray photoelectron spectroscopy (XPS), limiting results reliability and generality
4.1.1 Model assisted design of local optimized cathode catalyst layer (CCL) The improved 1D+1D DMFC model is firstly validated with respect to the experimental data reported in [29], referring to a DMFC adopting a novel CCL material with uniform catalyst loading distribution (UNF-MEA); values of the most relevant model parameters are reported in the supplementary materials

Summary

Introduction

Thanks to the employment of a high energy density liquid fuel, permitting convenient and quick recharging [1,2,3], direct methanol fuel cell (DMFC) is a promising technology for stationary and portable power production from very small to medium scale (e.g. portable electronics to off-grid applications) as well as for light and industrial vehicular sectors. Such applications require at least several thousand hours of lifetime without any relevant degradation of performance; durability issues of this technology [4] still constitute a critical point which limit technology widespread commercialization. Modelling analyses dealing with local performance investigation found in the literature [30,31,32] generally lack of rigorous and experimental validation, reducing the reliability of model results or limiting model prediction capabilities

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Results

Conclusion