Preparation of Pt/Carbon-Sphere As Cathode Catalyst of PEFC
Authors: Sakakibara Kan , Saida Takahiro 
Published: February 2020

Polymer electrolyte fuel cell (PEFC) is emerged as one of the candidates for automotive and residential power sources because has relatively small and lightweight and works under 100ºC. The electrocatalyst of PEFC consists of platinum nanoparticles, which size is several nanometers, and activated carbon as catalyst support. Catalyst support requires to two things at least; one is good electron conductivity, second one is high surface area in order to achieve dispersing of platinum particles. Activated carbon is cheap and meets two requirements needed as catalyst support. Therefore, activated carbon is widely used as catalyst support in PEFC. However, activated carbon is corroded under the start-up/shutdown conditions reaching the high potential region. Carbon corrosion leads to detachment and aggregation of platinum nanoparticles. These phenomena mean catalyst degradation, and are directly linked to decrease of cell performance. In addition, platinum utilization is not high in case of using activated carbon as catalyst support. This attributes that platinum nanoparticles is accommodated by complex pore structure of activated carbon. For realization of wide-spread commercialization of PEFC, inhibition of catalyst degradation and improvement of platinum utilization are necessary to build up the low cost system. Recently, graphitization carbon is focused as corrosive-resistant carbon support. Graphene, in particular, has high stability of thermodynamics and good electron conductivity. We succeeded already a preparation of carbon sphere composed from graphite oxide nanosheets (GOns) wall. GOns could reduce by the reductant, such as hydrazine and hydrogen, and is given electron conductivity by reduction treatments. Moreover, our carbon sphere is expected to decrease the substance diffusion resistance of oxygen and water, due to it has homogeneous size. In this study, platinum nanoparticles supported on carbon sphere, and its activity of oxygen reduction reaction and durability were evaluated.
Graphite oxide was synthesized by typical Hummers method. GOns colloid solution was obtained by shaking GO with 0.01 wt% TBAOH aqueous solution. Preparation method of carbon sphere was briefly described below. Polymer spheres were dispersed in cationic polymer aqueous solution and washed with ultrapure water. After that, their polymer spheres were mixed with GOns colloid solution. The above procedure was repeated five times. Platinum supported carbon sphere was prepared by a traditional impregnation method. Platinum hexahydrate ethanol solution was added to a carbon sphere dispersed with ethanol. And then, this mixed solution was dried at 60ºC. A dried powder containing platinum complex was reduced at 200ºC under the vacuum conditions. Its morphology and crystalline structure were observed SEM and XRD, raman spectroscopy. Electrochemical measurement was conducted by rotating disk electrode method. A working electrode was prepared by thin film method. Briefly, test catalyst was dispersed in mixed solution of ultrapure water and isopropyl alcohol. And then catalyst dispersion solution was dropped 10 µL on the mirror-polished glassy carbon electrode and dried at 60°C.
Carbon sphere as catalyst support was kept sphere shape after platinum deposition treatments. This platinum supported catalyst may form by stacked GOns wall and polymer sphere core. Because raman peaks attributed polymer sphere were not observed, but D- and G-band attributed GOns were observed. In diffraction peaks of XRD, clear peaks were observed at ~40° and ~45°. These diffraction peaks are attributed to Pt (111) and Pt (200) phases. On the other hand, diffraction peaks of polymer sphere were not observed. In addition, oxygen reduction activity and hydrogen adsorption/desorption current of platinum were also confirm by electrochemical measurement using rotating disk electrode method. Thus, we succeeded synthesis of platinum supported carbon sphere catalyst as new electrocatalyst. Its activity of oxygen reduction reaction and durability will be reported at the venue.