Towards Gas Distribution Layers

Abstract

Today, gas diffusion layers (GDLs) are stochastic, carbon based porous materials consisting of carbon fibres (~ 10 µm Ø) in which diffusion is the dominating transport mode for the gas phase. The stochastic nature of these materials requires bulk descriptors for (anisotropic) pore/throat size distributions, bulk electronic/thermal conductivity, tortuosity, effective diffusion coefficients asf.1 , 2 Here, we introduce a novel two-layered structure that not only offers unique design possibilities but also exceeds in performance tests conventional GDL materials under various operating conditions.The investigated GDL material consists of a gold-coated PET fabric (Sefar). Its two-layered design resembles a combination of GDL and microporous layer (MPL). The simpler, regular structure (Fig. 1) increases the effective diffusivity – both in-plane and through-plane - as compared to Toray 60 carbon paper and shows no notable local variation. Furthermore, only one layer of throats exists for product water to emerge, thus lowering the required break through pressure and leading to faster water removal. The structure and transport properties can be finely adjusted to the fuel cell geometry or the desired operating conditions.The material and structural properties were investigated ex situ as well as for fuel cell operation. The electrochemical performance was compared to commercial Toray 60 carbon paper (with and without wet proofing/with and without MPL) in a wide parameter range (50-80°C TCell, 60-100 % RHCath, 1.5-2 bar).Especially at high cathode gas humidity and colder operating temperatures, the new material outperforms the conventional GDL material and reaches higher current densities (up to 200 mA/cm2 or 100 mV gain, cf. Fig. 2). It seems that this structure still distributes the feed gas well enough across the catalyst layer and removes product water straight to the channel. The detailed water transport modes were investigated by operando 4D-X-ray tomographic microscopy. The results show a dry structure over a wide range of current densities, and the crucial alignment between fibres and ribs: only in closed off pores below a rib, water accumulated.These new, woven GDL materials show the possibilities of deterministic, designed structures to controllably tune single transport properties and improve water management in PEFC GDLs. Furthermore, transport properties as e.g. the electrical conductivity are adjustable even locally. The simpler structure might also narrow the gap between modelling and validation due to the regular structure and easy accessible material properties.