Fluorine-Free Pt Nanocomposites for Three-Phase Interfaces in Fuel Cell Electrodes

Abstract

The first generation of proton exchange membrane fuel cells uses costly and unsafe perfluorinated sulfonic acid polymers (PFSAs) as membranes and as ionomers impregnating electrodes to achieve the three-phase boundaries. PFSAs imply paramount issues for large-scale manufacture, use, commercialization, and recycling. Alternative nonfluorinated polymers should allow obtaining not only membranes but also adequate ionomer suspensions in convenient solvents for preparing efficient catalytic layers, which has not yet been achieved. Here, we propose a universal solution consisting of the transposition of the three-phase boundary at the molecular level by grafting directly at the surface of carbon-supported Pt nanoparticles a nonfluorinated proton-conducting polymer combining the catalytic activity of the former and the transport properties of the latter. The length of the polystyrenesulfonate polymer chain (as a model polymer) and the number of polymer feet per platinum nanoparticles have been optimized in order to achieve the highest active surface area and activity possible. It was shown that low grafting density and high degree of polymerization gave the best configuration. The great potency of such nanocomposites as cathode catalysts for PEMFC was evidenced not only in a standard three-electrode cell but also under real working conditions in a single hydrogen/oxygen fuel cell, where higher activity and stability were obtained with a nanocomposite material in comparison to those with a classical Pt/C + Nafion electrode.