In Situ Characterization of Novel Gas Diffusion Layers for Advanced Water Management in Fuel Cell
Authors: Forner-Cuenca Antoni , Manzi-Orezzoli Victoria , Biesdorf Johannes , Gubler Lorenz , Schmidt Thomas J. , Boillat Pierre 
Published: February 2020

In polymer electrolyte fuel cells (PEFCs), the porous gas diffusion layer (GDL) plays a multiple role in terms of transport. The solid phase consisting of carbon fibers is responsible for the electrical conduction and heat removal, and the void space provides pathways both for the supply of gaseous reactants and for the removal of liquid water. This multiple function results in a contradictory set of requirements for the material characteristics: a high thermal and electrical conductivity of the solid phase, a high permeability of the volume occupied by water and a high diffusivity of the space remaining for gas transport are simultaneously desired. This is particularly true for the cathode side GDL, in which water usually accumulates, and which requires a good diffusivity to avoid mass transport losses (1). The fibrous nature of state-of-the-art GDL materials readily provides a nearly optimal combination of high conductivity – in particular in the critical in-plane direction – and high porosity (typ. 70-80%). However, the distribution of the volume occupied by water and gas in the void space is not optimal: the water flows from the catalyst layer to the channels of the flow fields following the pathways having the lowest capillary pressure. Such pathways are tortuous and some of them are dead-ended, which results in an unnecessary filling of space which would otherwise be available for the diffusive gas transport.
Our approach consists in defining water transport pathways in the GDL by locally modifying the contact angle of the material. Starting from a carbon paper which is made hydrophobic by coating it with a fluoropolymer (as in usual GDLs), we create radicals in defined regions by exposing the sample to an electron beam through a mask. Subsequently, the contact angle of these activated regions is modified using a graft-copolymerization reaction with a hydrophilic monomer (2). In a recent publication (3), we demonstrated that the proposed method is suitable to graft a hydrophilic compound with a very good spatial definition, and that the water effectively preferentially fills the defined water pathways in an ex situ water injection experiment.
In the proposed contribution, we will present the latest results of in situ measurement with fuel cells using our novel material. To this purpose, an advanced set of characterization methods was applied: The water distribution in the operating cells was measured using neutron imaging (1, 4), while the impact of water on cell performance was characterized not only by monitoring the cell voltage, but also by applying our recently developed pulsed gas analysis (PGA) method (1) to evaluate the mass transport losses. These combined measurements demonstrated that the definition of hydrophilic pathways in the cathode gas diffusion layer effectively improve the performance (cf. Figure 1) by reducing the magnitude of the mass transport losses related to the diffusive transport of oxygen through the gas diffusion layer.
1. P. Boillat, P. Oberholzer, A. Kaestner, R. Siegrist, E. H. Lehmann, G. G. Scherer and A. Wokaun, Journal of the Electrochemical Society, 159, F210 (2012).
2. P. Boillat, L. Gubler, A. Forner, C. Padeste and F. Büchi, Patent Application EP14184065.2 (2014).
3. A. Forner-Cuenca, J. Biesdorf, L. Gubler, P. M. Kristiansen, T. J. Schmidt and P. Boillat, Advanced Materials, 27, 6317 (2015).
4. D. Kramer, J. B. Zhang, R. Shimoi, E. Lehmann, A. Wokaun, K. Shinohara and G. G. Scherer, Electrochim Acta, 50, 2603 (2005).

Figure 1