Rheological behavior of ionomer dispersions and their incorporation in catalytic inks for use in PEMFC electrodes
Examensarbete för masterexamen
Materials chemistry (MPMCN), MSc
Proton exchange membrane fuel cells produce greenhouse gas emission-free electric ity which is needed in the ongoing climate crisis. At the center of the fuel cell is a proton exchange membrane sandwiched between two electrodes produced from a catalytic ink. The ionomer component in catalytic inks acts both as a binder and a proton conductor and is an integral part of the catalyst layer. It is therefore im portant to have a thorough understanding of its key characteristics to optimize inks for improved processing and electrode performance and durability. Various perfluo rinated sulfonic-acid (PFSA) ionomer dispersions with varying solvent matrices and ionomer amounts were mixed and their viscosity was tested before and after elevated temperature treatment as it is known to support the dispersion process of specific ionomers in solvent matrices. Only short-side chained and low equivalent weight ionomers showed significant change in viscosity. Higher alcohol concentrations as well as more sterically hindering alcohols in the solvent matrix lead to more thicken ing. Higher ionomer concentrations, higher temperatures, and longer heating times also lead to thicker dispersions up to a maximum viscosity where it is fully gelled. The influence of ionomer viscosity on ink mixing and dispersion was described as well. Heating catalytic inks with short-side chained PFSA ionomers thickens the ink but the resulting electrode decals are full of holes. Inks made by mixing pre thickened ionomer with catalyst powder are difficult to mix and disperse properly. Using a more sterically hindering solvent will give a very thick ink even without any heating. The rheological properties of catalytic inks can be altered by changing the parameters that affect the structure of short-side chained and low equivalent weight PFSA ionomers but more investigation is needed into the dispersing of the inks as well as how altering of these parameters affect the resulting fuel cell performance. The increased understanding of the ionomer component will help to optimize the ink development and electrode design at PowerCell in the future which in turn will lead to more efficient fuel cells making them a more viable alternative to fossil fuel-based energy production.
ionomer, fuel cells, perfluorinated sulfonic acid, solvent, PEM, catalytic ink, catalyst layer, rheology, viscosity