The role of the micro-porous layer (MPL) in fuel cells: neutron imaging and advanced electrochemical analysis study.
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The role of the micro-porous layer (MPL) in fuel cells: neutron imaging and advanced electrochemical analysis study. Pierre Boillat1, Pierre Oberholzer1, Eberhard H. Lehmann2, Günther G. Scherer1 and Alexander Wokaun3 1 Electrochemistry Laboratory (ECL), Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland 2 Neutron Imaging and Activation group (NIAG), Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland 3 General Energy Department (ENE), Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland ABSTRACT The effect of the micro-porous layer (MPL) in polymer electrolyte fuel cells (PEFCs) was studied by a combination of in situ visualization of the liquid water distribution and advanced electrochemical analysis using helox and O2 pulses. Four cells with and without MPLs on the anode and cathode side were tested. Visualization studies showed that the significant changes in performance observed when using an MPL on the cathode side cannot be related to a reduction of the water content in the cathode side diffusion layer (GDL). The helox/O2 pulse analysis indicated that two different mechanisms are responsible for the performance loss without an MPL. INTRODUCTION The polymer electrolyte fuel cell (PEFC) is an electrochemical energy converter proposed as a replacement for internal combustion engines in automotive applications. Its operation is based on two electrochemical reactions – hydrogen oxidation and oxygen reduction – occurring at electrodes placed on both sides of a proton conducting membrane. The fine distribution of gaseous reactants is assured by a carbon fiber based porous layer called gas diffusion layer (GDL). Between the electrode and the gas diffusion layer having relatively large pores (typ. 10-50 μm), a layer having a fine porosity (typ. 0.01-0.1 μm pore size [1]) is usually inserted. This layer is called micro-porous layer (MPL) and several studies (e.g. [2, 3]) reported an improved performance when using it. However, the reason for such an improvement is not well understood yet. The proposed mechanisms are, (i) a reduction of the water saturation level inside the electrode or at its interface with the GDL [3, 4], (ii) an improved back transport of water to the anode – resulting in a reduction of water saturation in the cathode GDL [5] –, and (iii) an improved electrical contact [4]. We previously published a neutron imaging study of different operating fuel cells with and without MPLs on the anode and cathode side [6]. The cross water quantity and distribution was analyzed using high resolution neutron imaging with a newly designed multicell imaging setup. This paper presents further results on the same set of cells using higher current density (1 A/cm2) and includes an analysis of the sources of performance loss using the recently developed helox/O2 pulsing method [7]. While the previous publication was focused on the experimental setup and on the effect of the MPL on water transport, the additional results published here give a new insight into the loss mechanisms related to the absence of a
