Structural evolution of Pt/CNF nanocomposites as a part of fuel cell's gas‐diffusion electrodes

Abstract

A search of new nanostructural materials (such as gas‐diffusion layers, electrodes, catalysts, membranes etc.) for a polybenzimidazole‐membrane fuel cell remains a crucial task nowadays. Reducing of noble metals content has become one of the most significant goals in development of fuel cells while a balanced union of electron and ion conductivity along with gas permeability and catalytic activity of electrodes serves as an essential condition of the hydrogen‐air fuel cell efficiency [1]. In this study carbon nanocomposites of nanofiber nonwoven mats, produced by electrospinning and decorated with Pt, after work inside membrane electrode assembly as gas‐diffusion cathode at 160‐180 °C were investigated by analytical TEM and STEM methods [2]. Initially a nanofiber surface is evenly covered with a thick layer of Pt nanoparticles of anisotropic elongated shape with a number of sub angstrom atomic steps on their surface ( Fig. 1a ). This type of defects plays an important part in platinum catalytic activity [2,3]. The investigations of such a nanocomposite after work as a gas‐diffusion cathode in a fuel cell revealed significant changes in the structure of the platinum layer. After several hours of work at a standard fuel cell working temperature (160 °C) the structure of metal nanoparticles changes slightly ( Fig. 1b ). Metal nanoparticles preserve their shape elongated in direction and specific distribution on fibers surface same as in initial nanocomposites. However, in this case platinum is covered with a thin amorphous layer. Some of the platinum nanocrystals demonstrate the signs of partial melting and a loss of initial acicular shape as well as the decrease of surface defects. After several days of work at standard and high fuel cell working temperature (160‐180 °C) the structure of catalyst changes considerably ( Fig. 1c ) and can be observed as melted conglomerates of unspecified shape and size of 40‐100 microns ( Fig. 2 ) with a cover of thin (5‐10 nm) amorphous layer.