The Formation Mechanism of Ordered Mesoporous Carbon with Network-Structure: A Novel Support Material for Pt-Based Catalysts in PEFC Cathodes

Abstract

INTRODUCTIONTo achieve carbon neutrality in the upcoming sustainable society, it is crucial to utilize hydrogen as a fuel in the transportation sector. Fuel cell vehicles are expected to play an important role in the future hydrogen society, but there are still challenges to be overcome before their widespread adoption. For example, Pt/C-based catalysts used in PEFC cathodes have an issue with poisoning of the Pt due to direct contact with the ionomer, especially during low-load operation. Recently, a new approach using accessible-pore carbons has been proposed, where Pt is placed at an appropriate position within the nanopores of a carbon support to suppress ionomer poisoning of Pt [1]. In order to realize this concept, it is essential to design a mesoporous carbon support that possesses nanopores with appropriate size and proper ordering. We have developed an ordered mesoporous carbon with a hierarchical network structure (Net-OMC) consisting of small OMC particles of several tens of nm in size, which are connected to form a network structure with well-developed primary pores. The Net-OMC exhibited highly ordered nanopores on the surface of the OMC primary particles, with a pore size ca. 5 nm and an interpore distance of ca. 8 nm. Pt nanoparticles were loaded selectively within the Net-OMC nanopores by the selective Pt-deposition technique developed by our research group. Pt/Net-OMC catalyst exhibited higher mass activity and durability for the ORR in comparison with those for a commercial Pt/CB catalyst [2]. In this study, the formation mechanism of Net-OMC and the relationship between the catalytic properties of Pt/Net-OMC and its pore structure was investigated in detail.EXPERIMENTALNet-OMC powder was synthesized with resol-nonionic surfactant micelles as a carbon source and structure-directing agent. Resol-F127 (nonionic surfactant) micelles were synthesized by mixing phenol, formaldehyde, water and F127 with sodium hydroxide. After hydrothermal treatment of the mixture, the resulting solid was filtered and washed thoroughly and dried under vacuum. The obtained powder was carbonized at 700 ºC followed by annealing at 1000 ºC. Pt was deposited on the Net-OMC powder by selective deposition techniques. Cyclic voltammetry and linear sweep voltammetry were carried out in N2-saturated and O2-saturated 0.1 M HClO4 solution at 25 ºC using the conventional rotating disk electrode (RDE) technique. Potential step cycling measurements simulating load cycling between 0.6 V and 1.0 V vs. RHE were carried out according to the FCCJ protocol. Coating of the catalyst on a glassy carbon disk substrate was carried out by an electrospray (ES) coating technique developed by our group [3]. N2 adsorption measurements at liquid nitrogen temperature were carried out. The morphology of the Pt/ns-OMC was observed by scanning transmission electron microscopy (STEM).RESULTS AND DISCUSSIONSTEM images for the ordered mesoporous carbon particles with network structure (Net-OMC) with varying concentration during hydrothermal synthesis are shown in Figure 1. In Fig. 1(a), the network structure formed by connection of OMC primary nanoparticles with developed macropores is clearly observed. The OMC nanoparticles were tightly connected via a thick necking structure. From the high magnification image (Fig. 1(a), inset), highly ordered nanopores were observed on the surface of the OMC particles. The OMC primary particle size and the connecting neck diameter changed with the resol micelle concentration during the hydrothermal treatment. When the concentration of the micelle solution was decreased, both the primary particle size and necking length decreased. At a concentration of 1.8 mmol l-1, the formation of monodisperse particles with roughly the same size from the resol-F127 micelle particles was observed, as estimated by dynamic light scattering analysis. Thus, it is believed that the Net-OMC structure is formed through the self-assembly of micelle particles during the hydrothermal synthesis. The controllable network structure of Net-OMC is expected to provide a favorable porous structure of the catalyst layer for the MEA measurement even during high-load operation.