Graphene-Based Aerogels Possessing Superhydrophilic and Superhydrophobic Properties and Their Application for Electroreduction of Molecular Oxygen

A composite aerogel with superhydrophobic external surface has been synthesized from reduced graphene oxide and polytetrafluoroethylene taken in a weight ratio of 1 : 1. The porous structure of the aerogel has been studied by the standard contact porosimetry method (SCPM). The porosimetric curves measured with respect to octane and water intersect in the region of small pores, thereby leading to the fact that the specific surface area of the aerogel in water is much larger than that in octane, although octane is known to wet any material almost ideally. This phenomenon, which is referred to as "superhydrophilicity," is explained by the fact that, in the region of mesopores, a sample swells in water due to the hydration of surface –CO and –COH groups, which have been identified with the help of IR and Raman spectroscopies. Thus, the outside surface of the aerogel granules is superhydrophobic, while their interior is superhydrophilic in the region of small pores. As follows from the SCPM data, the total porosity and specific surface area of the aerogel are substantially larger than those of Vulcan XC-72 carbon black, which is a standard carrier for Pt catalysts used in fuel cells based on proton-exchange membranes. Oxygen electroreduction at the aerogel, containing Pt deposited in an amount of 28 µg/cm2, has been studied by the method of rotating disk electrode (RDE) in an aqueous 0.5 M H2SO4 solution, and the results obtained have been compared with the data on standard commercial Pt (20%)/Vulcan XC-72 catalyst. It has been shown that the limiting diffusion RDE currents for Pt supported on the hydrophobic–hydrophilic aerogel are markedly higher than those for the standard catalyst because of the easier access of oxygen to the reaction zone as compared with hydrophilic Vulcan XC-72 carbon black carrier.