Core-Shell Support for PEFC Electrocatalysts : A Case Study with Doped-SrTiO3

Abstract

Pt nanoparticles supported on carbon black (Pt/C) are widely used as polymer electrolyte fuel cell (PEFC) electrocatalysts. However, under the cathode high potential, carbon corrosion can occur leading to cathode electrocatalyst degradation. In addition, when air flows into the anode at the fuel starvation, carbon corrosion also occurs at the anode as well as at the cathode [1]. Therefore, it is desired to develop alternative electrocatalyst support materials. Metal oxide is one of the promising candidates. Especially, titanium oxide (TiO2) possesses excellent stability in PEFCs environment in both anode and cathode. Pt/TiO2 electrocatalyst has high durability, but the initial catalytic activity is much lower than that of Pt/C because the electrical conductivity of TiO2 is too low around room temperature [2],[3]. In this study, we use SrTi(Nb)O3 with higher electrical conductivity compared to TiO2. SrTi(Nb)O3 is a perovskite-type oxide with strontium ions on the A-site and titanium ions on the B-site. As strontium ions can easily dissolve in acid, the surface of SrTi(Nb)O3 will become titanium-rich by applying an acid treatment. In this way, we can prepare “Core-Shell support” consisting of TiO2-based “shell” with high stability in strongly-acidic PEFC environment and SrTi(Nb)O3 “core” with a high conductivity. The aim of this study is to develop such electrocatalyst with both high durability and activity using TiO2–based support. We made the acid treatment using HClO4 to SrTi(Nb)O3 prepared by sol-gel method to remove strontium ions from the surface of SrTi(Nb)O3. To impregnate Pt particles on this core-shell support (SrTi(Nb)O3 core - Ti(Nb)O2 shell), Pt(acac)2 method were applied. We then obtained Pt/SrTi(Nb)O3-Ti(Nb)O2. SrTi(Nb)O3 before and after the acid treatment was characterized by XPS to analyze the titanium-rich surface. We made half-cell tests to evaluate electrochemical activities of these electrocatalysts. Electrochemical surface area (ECSA) was evaluated by cyclic voltammetry (CV), and oxidation reduction reaction (ORR) activity was derived from kinetically controlled current (ik) in rotating disk electrode (RDE) measurements. Figure 1 shows the XPS spectra of SrTi(Nb)O3. It is confirmed that the surface of SrTi(Nb)O3 become Ti-rich as the Ti(2p) spectrum after the acid treatment was larger than that before the acid treatment. Figure 2 shows FE-SEM micrograph of the Pt/SrTi(Nb)O3 core - Ti(Nb)O2 shell electrocatalyst. This micrograph shows the highly dispersed impregnation of several-nanometer Pt particles on this support. We will report electrochemical activities in our presentation.