Mass-transport properties of electrosprayed Pt/C catalyst layers for polymer-electrolyte fuel cells.

Abstract

Mass-transport properties of electrosprayed catalyst-layers based on Pt/C and ionomer (Nafion) are studied with hydrogen limiting-current technique, water-vapor-uptake, scanning transmission microscopy (STEM), single-cell testing, and impedance spectroscopy. The hydrogen limiting-current technique provides the transport resistance of the layers (RCLmt), which shows to be lower in electrosprayed layers compared with conventional layers, especially at very low platinum loadings (0.025 mgPt·cm−2) and low cell temperature, denoting superior mass-transport properties. Images of the distribution of Pt, F, and C elements reveal the ionomer preferentially interacting with the Pt nanoparticles. Water-vapor-uptake experiments show larger vapor absorption for electrosprayed than conventional catalyst layers. Such large water-vapor uptake capability is combined with superhydrophobicity, ie. very low interaction with water in liquid phase (wettability). Both apparently contradictory properties result from a particular configuration of the amphiphilic ionomer in the electrosprayed layers, and provide ideal conditions for high mass transport and ionic conductivity in a catalyst layer. Electrosprayed layers as cathode catalyst layers show peak response at a loading of 0.17 mgPt·cm−2 (18 μm layer thickness when using Pt/C 20 wt% catalyst) where they provide minimal mass-transport and polarization resistances.