Wetting and evaporation on a carbon cloth type gas diffusion layer for passive direct alcohol fuel cells

Gas diffusion layers (GDL) have a crucial importance in passive air breathing direct alcohol fuel cells as they play a pivotal role in governing fuel distribution on the anode side and in preventing water flooding on the cathode side. We report here a rapid, cost-effective, ex-situ method to study the wettability of GDLs by process fluids. A commercial Teflon® treated carbon cloth featuring a single microporous layer was used, because carbon cloths are anticipated to outperform carbon paper type GDLs at high humidity and high current density. The GDL structure was characterized by SEM, 3D microCT reconstruction, and surface profilometry. Wettability by aqueous alcohol mixtures was investigated by contact angle measurement and infrared thermography. Ethanol containing fuel offered better spreading characteristics than methanol, especially on the microporous side of the GDL. The surface behaviour of water was studied by recording the evaporation profile of a sessile water droplet using time dependent contact angle measurement and simultaneous weight loss measurement and thermography. The applicability of the Teflon-containing carbon cloth as a GDL was verified by its hydrophobic behaviour and its ability to reject water. We found evidence that the first stage of water evaporation occurs in constant contact angle mode, then a wetting mode transition takes place at approx. 0.65 relative evaporation time and the evaporation proceeds in constant contact radius mode.