Transmission Electron Microscopy Study on Platinum Nanoparticle Distribution in PEMFC Catalyst Layers under Different Ionomer/Carbon Ratios Fabricated by Direct Dry Deposition

Abstract

Proton exchange membrane fuel cells (PEMFC) are poised for market penetration in the automobile, portable and back-up power markets. Cost and durability issues are crucial to the performance of PEMFC, which primarily depends on the design of catalyst layers [1]. Current catalysts layers use platinum (Pt) supported on high-surface-area carbons (e.g., Ketjen Black® and Vulcan®) with perfluorosulfonic acid (PFSA) ionomer as a binding agent. It is widely believed that incorporation of ionomer can extend the reaction zone, increasing the proton conductivity and catalyst utilization [2]. The catalyst layers are predominately comprised of primary carbon particles, 20-50 nm in diameter. Stacking of primary carbon aggregates creates a unique 3D structure between the fractal structured carbon branches [3]. Depositing the carbon with Pt nanoparticles can block these micropores since the size of the Pt particles ranges between 2-4 nm. The PFSA ionomer is distributed on the surface of carbon agglomerates and covers intra-agglomerate pores (<20nm) at lower ionomer content, e.g., below 30 wt.% ionomer. Significant blocking of the pores is observed with higher ionomer content (>50% wt.) [4].Although researches have been focused on influence of ionomer/carbon ratio in catalyst layer microstructure, few have truly revealed the mechanism of how Pt nanoparticles are incorporated into the carbon matrix and the reason for certain ionomer/carbon ratio exhibit better electrochemical performance and higher catalyst utilization [4]. Transmission electron microscopy (TEM) is an advantageous tool for investigating the spatial distributions of components even for complicated functional materials, and can be used to analyze the composition of a local area at a nano-or atomicscale. For example, using TEM combined with electron energy loss spectroscopy (EELs) and energydispersive X-ray spectroscopy (EDS), researchers characterized Pt binary alloys for fuel cell catalysts and investigated the interaction with catalyst support materials. Oezaslan et al. discovered the existence of different morphology regimes in dealloyed Pt-Co and Pt-Cu particle ensembles, a core-shell structure of nanoparticles with 10-30nm in diameter [5]. The interaction of Pd 3 Pt 2 alloy nanoparticles with the carbon support was studied by Slanac et al. [6]. Aberration corrected STEM revealed a corrugated crystalline alloy core surrounded by a disordered alloy shell that created strong electronic binding of Pd to the 20-50 nm carbon particles, facilitate strong wetting on the carbon at the particle-support interface, which significantly enhanced the mass activity for oxygen reduction reaction (ORR) [6]. In addition, TEM also helps investigating the change of catalyst microstructure during electrochemical test. Akita et al. have studied Pt nanoparticle distribution before and after membrane electrode assembly (MEA) test and an aggregation process during MEA testing was found under high resolution TEM [7].Herein we apply a novel depositing method, the Reactive Spray Depositi...