Features of the Degradation of the Proton-Conducting Polymer Nafion in Highly Porous Electrodes of PEM Fuel Cells

The stability of new membrane-electrode assemblies of a proton-exchange membrane fuel cell with highly porous electrodes and low Pt loading, based on the proton-conducting polymer Nafion, was characterized in conditions of electrochemical aging. A comprehensive study of the effect of the microstructure on the evolution of the electrochemical characteristics of the new assemblies was obtained by voltammetry, electrochemical impedance spectroscopy, X-ray powder diffraction, and scanning electron microscopy. Because high (>70%) porosity provides intensive mass transfer inside an electrode, structural-modifying additives-long carbon nanotubes-were introduced into the new electrodes. PEM fuel cells with electrodes of a conventional composition without carbon nanotubes were used for comparison. The aging of the samples was carried out according to the standard accelerated method in accordance with the DOE (Department of Energy) protocols. The results show two fundamental differences between the degradation of highly porous electrodes and traditional ones: 1. in highly porous electrodes, the size of Pt nanoparticles increases to a lesser extent due to recrystallization; 2. a more intense washout of Nafion and an increase in ionic resistance occur in highly porous electrodes. Mechanisms of the evolution of the characteristics of structurally modified electrodes under electrochemical aging are proposed.

Automated summary

This study characterized the stability of new membrane-electrode assemblies of a proton-exchange membrane fuel cell under conditions of electrochemical aging. The effect of microstructure on the evolution of the electrochemical characteristics was investigated using various techniques. The introduction of carbon nanotubes as structural-modifying additives in highly porous electrodes showed differences in degradation compared to traditional electrodes, including lesser increase in Pt nanoparticle size and a more intense washout of Nafion. Mechanisms of the evolution of structurally modified electrodes under electrochemical aging were proposed.