Planar fibre reinforced carbon aerogels for application in PEM fuel cells

Abstract

Organic precursor aerogels have been prepared from resorcinol-formaldehyde (RF) with a very high molar ratio of resorcinol/catalyst of 1500, (RF/total) mass ratios between 30 and 25 wt. % (target density ≈ 300 kg m −3) and an F/ R-ratio between 1.5 and 3.0. These compositions show minimal shrinkage during ambient pressure drying. Subsequently, the precursors were pyrolysed at 1050°C to form carbon aerogels. The use of fibre additives from ceramic, carbon or polymeric fibres (novoloid, PAN, aramide) increase the stability of the carbon aerogel matrix drastically and allow to prepare thin planar sheets with thicknesses between 50 and 300 μm. By using untreated cellulose as the additive, a bimodal pore structure is generated. During pyrolysis the fibre–aerogel compounds were transformed into carbon aerogels exhibiting the morphology and planarity of the RF precursors. The carbon aerogels were investigated with respect to their structure, using scanning electron microscopy (SEM), small angle X-ray scattering (SAXS) and gas permeability measurements. The prepared aerogel sheets show a rather smooth surface and a somewhat coarse interior structure, which is advantageous for application in fuel cells. For the fibre reinforced gels, the structural changes due to a variation of the composition parameters is less distinct than for the non-reinforced gels. The structure of the reinforced gel depends strongly on the time, the sol was allowed to age prior to impregnation. In addition, it was found that the surface of certain fibre materials (e.g. cellulose) influence the gelation catalytically, while novoloid rather ‘slows’ the gelation down. The catalytic effect can be described by a semi-empirical model, involving a catalytic penetration depth.