PT/mesoporous carbon nanocatalysts for polymer electrolyte fuel cell

Mesoporous materials are attractive because of their highly controllable ordered pore structure, and they are possibly used as catalytic nanoreactors if catalysts are successfully deposited within the pores. Among those mesoporous materials, mesoporous carbon (MC) materials were chosen owing to their high electric conductivity, and Pt was deposited inside the pores in order to apply for electrode materials. MC materials are commonly synthesized using mesoporous silica template, but the resulting pores are 2-3 nm sized and probably not large enough to deposit catalyst particles in the pores. Here, MC with relatively large pores was synthesized through self-organization of a surfactant and carbon precursors followed by carbonization. The obtained MC had 7-8 nm sized pores, and Pt nanoparticles were deposited inside the pores of MC by addition of Pt cations followed by reduction to metal Pt. The size and dispersion of Pt nanoparticles depended on the starting Pt precursors, but well dispersion of 1 nm or smaller Pt nanoparticles inside the pores was obtained without sintering. The properties as cathode of polymer electrolyte fuel cell were examined. To do so, Nafion® should also be introduced to mespores. Developing of an ideal triple phase boundary by introducing catalyst nanoparticles and ionomers into the pores and further building up the nanoreactor array were expected. When oxygen reduction reactivity with Pt deposited MC (Pt/MC) thin film rotating disk electrode was examined, the addition of Nafion® lead to increase in oxygen reduction reaction (ORR) current comparing to that without Nafion®. Therefore, a triple phase boundary was successfully developed inside the pores. Oxygen reduction reactivity depended on which Pt precursors were in use or how easily Nafion® could access to the pores. When Nafion® was dispersed in more hydrophobic solvent, Nafion® was more easily accessible to the pores, and as a result, higher ORR current was obtained. Furthermore, ORR current at Pt/MC thin film rotating disk electrode appeared to be not under control of oxygen diffusion from the electrolyte solution in the region where Pt/CB thin film rotating disk electrode showed the diffusion limited current. Then, oxygen transfer and storage processes within the pores were investigated, and the process was a characteristic function of Pt/MC. Mesopores can produce a channel connecting catalysts and external environment and favorably transferring oxygen, and oxygen is diffused through Nafion® inside the pores.