Preparation of Catalyst for a Polymer Electrolyte Fuel Cell Using Different Loading Methods

Abstract

1. IntroductionPlatinum (Pt) exhibits high catalytic activity in polymer electrolyte fuel cells (PEFCs). However, reducing the amount of Pt is necessary because it is rare and expensive. For a Pt catalyst support, we have applied Marimo-like carbon (MC) which is a novel spherical carbon material comprising entangled carbon nanofilaments (CNFs). Since the CNFs in MC have a cup-stack primary structure of the graphene sheets, the sheet edge can effectively function as a site for the Pt catalyst. Although the supported Pt catalysts have been typically prepared via a wet process, a better loading method is being explored. In this study, the catalyst is prepared via a dry process, where Pt nanoparticles (NPs) generated in a gas phase are directly deposited onto the support surface. The catalytic activity is improved due to increasing the specific surface area which facilitates highly dispersive Pt loading with a small particle size.2. ExperimentalThe catalyst supports used were carbon black and MC. MC was synthesized by chemical vapor deposition1,2), using an oxidized diamond-supported Ni catalyst as the catalyst core. Methane was used as the reaction gas. The Pt loading by the dry process was performed using magnetron sputtering system, nanojima®3); Pt atoms or ions generated by sputtering a Pt disk target were grown into Pt NPs in a gas phase by introducing a cooled buffer-gas. Alternatively, Pt NPs were loaded using a nano-colloidal solution method in the wet process; Pt precursor was added to the carbon suspension with a sodium hydroxide solution, where the reducing agent for the platinum precursor yielded metallic Pt NPs on the carbon surfaces. The obtained suspension was centrifuged, washed and dried to prepare the Pt/C catalyst. The amounts of Pt loading were 6-8 wt%. The characteristics of the catalyst was evaluated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The catalyst performance was evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) with a rotating ring disk electrode (RRDE).3. Results and discussionThe morphologies of the loaded Pt catalysts were observed using TEM. The Pt NPs supported on the MC prepared by the dry process were ultra-fine NPs with the average size of 2.25 nm in diameter. In the dry process, highly dispersed and small Pt NPs were prepared because the Pt NPs synthesized in the beam could be supported directly on the carbon surfaces. In the wet process, on the other hand, the average diameter of the Pt NPs on the MC was 2.68 nm. The larger size in the wet process is caused by grain growth in the reduction reaction, because reduced Pt on the carbon surfaces acts as a catalyst. These results suggest that the dry process was more viable than the wet process for the preparation of a supported Pt catalyst with a small size, increasing the specific effective Pt surface area.The electrochemical measurements were performed for the prepared catalysts. For each sample, the electrochemical surface area (ECSA) value was evaluated from the CV curve based on the hydrogen adsorption peak. When MC was used as the catalyst support, the ECSA value was higher than that with a catalyst support of carbon black. Since carbon black is porous, hindered diffusion of the substances (i.e. reaction gases and/or product water) results in much less catalytic activity of Pt supported in a nanopore. Therefore, carbon black causes a decrease in the ECSA. In contrast, MC is a fibrous carbon, and the Pt NPs supported on the CNFs surfaces can be effectively used. The results have revealed that the fibrous structures in the MC support exhibit a higher ECSA value than that for the porous support of carbon black.1) K. Nakagawa et al., J. Mater. Sci. 44, 221-226 (2009). 2) T. Ando et al., US Patent No. 7608331 (2004.5.24). 3) A. Nakajima et al., US Patent No. 10283333 (2019.5.7). Figure 1