Synthesis of a Pt/Carbon-Sphere Catalyst and Evaluation of Its Oxygen Reduction Reaction Activity in Acidic Environments

In this study, the use of carbon spheres of 500 nm diameter as a catalyst support in polymer electrolyte fuel cells was investigated. The carbon spheres were expected to promote diffusion and improve platinum utilization. The carbon spheres' exterior surface consisted of graphene oxide, which supported highly dispersed platinum nanoparticles. This graphene oxide wall was reduced during the platinum-supporting process. Scanning electron microscopy images and the crystallite size showed that platinum particles of diameter less than 10 nm were supported on the surface of the carbon sphere. The electrochemical surface area (ECSA) of the carbon sphere-supported platinum (Pt/CS) was lower than that of commercial Ketjen black-supported platinum (Pt/KB). However, the approximate platinum utilization rate, which was calculated from the crystallite size and Pt/CS ECSA, was over 20% higher than that of Pt/KB. The oxygen reduction reaction (ORR) activity of Pt/CS was approximately twice that of Pt/KB at 0.9 V, which is a kinetically controlled region. The difference between the ORR activities of Pt/CS and Pt/KB increased with the increasing effect of material diffusion. Electrochemical measurements indicated that the platinum utilization rate and ORR activity were enhanced by changing the catalyst support to carbon spheres. The improved platinum utilization rate and ORR activity were attributed to the material diffusion achieved by the catalyst layer constructed from carbon spheres being superior to that achieved by Pt/KB. The results of this study showed that the use of a uniform spherical material such as carbon spheres as the catalyst support improved the platinum utilization rate and ORR activity because platinum nanoparticles were highly dispersed on the catalyst surface.