Reducing Polarization Using Novel Carbon Support in Polymer Electrolyte Fuel Cells

1. Introduction Fibrous carbon nanomaterials are expected to be appropriate Pt support materials. Their small diameter and long length contribute to the formation of large specific surface areas. Fibrous carbon materials have few inside internal pores, and their surface areas are due to the outside of the fibers. The fibrous structure is expected to facilitate both the smooth supply of fuel gases and the discharge of water. Carbon nanofilaments(CNFs) are fibrous carbon materials that have a cup-stack primary structure with graphene edges on the filament surface. The edges on the outer surfaces of CNFs are expected to act as support sites for highly dispersed Pt particles. Marimo-like carbon (MC) (1) comprises many CNFs that are interwoven to form a spherical secondary shape. We studied the relation between preparation conditions and properties of the formed Pt nanoparticles and revealed the effects of the microstructure of catalyst layers using MC 2. Experimental MC was synthesized by CVD using a fixed-bed flow-type reactor. An oxidized diamond-supported Ni catalyst was placed at the center of the reactor, and heated to the reaction temperature, after which a reactant gas was fed into the reactor in a stream of Ar at a flow rate of 30 mL min -1 . The reaction temperature was maintained at 823 K for 10 h, and the CH 4 reactant gas flow rate was 30 mL min -1 . An MC supported Pt catalyst(Pt/MC) was prepared using the nanocolloidal solution method (2) . Carbon materials were added to the solution before Pt particles were generated. Pt particles were directly deposited from the solution onto the surfaces of carbon materials. Pt/MC was used as the membrane electrode assembly (MEA) with different catalyst inks (I/C = 0.14) produced from the carbon supported Pt catalyst (Pt/C, I/C = 0.46). The surface area of the electrode was 5×5 cm 2 , and the amount of Pt in the electrode was 5 mg (0.2 mg cm -2 ). The I-V measurements were conducted at 353 K. Humidified hydrogen gas (200 ml min -1 , 100 % RH at 353 K) was supplied to the anode and humidified air (200 ml min -1 , 100 % RH at 353 K) was supplied to the cathode. The durability of the fuel cell was estimated using an accelerated degradation test (ADT). Electrodes were conditioned by potential cycling at a scan rate of 200 mV s -1 over a range of 50 -1200 mV vs. RHE. 3. Results and discussion Before ADT (0 cycle), the MEA using the Pt/C showed higher performance than the Pt/MC. The oxygen reduction reaction activities of Pt/MC were 41.9 A g -1 (mass activity) and 58.9 mA cm -2 (specific activity) and those of Pt/C were 83.9 A g -1 and 76.9 mA cm -2 . The Pt/C exhibited two times better performance than Pt/MC before ADT. However, after 500 cycles of ADT, Pt/MC exhibited the same performance as Pt/C and even better performance after 1000-2000 cycles. From this result, the cause of low performance using Pt/MC was perhaps determined to be the shortage breaking in the fuel cell during this operation. The mechanism of this behavior under cell operating conditions was not clear, but the good cell performance was confirmed shen using Pt/MC. Pt/MC exhibited high durability under ADT conditions. This result reveals that MC has superior oxidation-resistant properties under ADT conditions. MC comprises carbon nanofilaments, which have a graphene stacked structure. Only the edges of graphene sheets can be oxidized, the inside of graphene cannot be oxidized. Thus, the graphene structure is more stable against oxidation and decomposition compared with an amorphous structure. The unique structure of MC results in superior electrochemical properties. (1) K. Nakagawa, H. Oda, A. Yamashita, M. Okamoto, Y. Sato, H. Gamo, M. Nishitani-Gamo, K. Ogawa, T. Ando, J. Mater. Sci, 44 (2009) 221. (2) M. Eguchi, S. Yamamoto, M. Kikuchi, K. Uno, Y. Kobayashi, M. Nishitani-Gamo, T. Ando, Journal of Surface Finishing Society of Japan, 62 (2011) 179.