Corrosion-Resistant PEFC Cathode Catalysts Based on Sub-Stoichiometric Titanium Oxide Supports

Corrosion of carbonaceous catalyst support materials such as carbon black has been known for conventional Pt/C catalysts, which is recognized as one of the cause of catalyst degradation [1]. To improve the durability of cathode catalyst, especially at high potentials during startstop periods, a more oxidation-resistant catalyst support at high potentials (>1.0 V) is desired for automotive and residential applications. Oxygen-deficient, sub-stoichiometric titanium oxide (TiOx), which is known as Magnéli phase, is not only a corrosion-resistant but also electrically conductive material, so the TiOx is attractive as durable catalyst supports. Since the specific surface area of the supports is very important for improving the Pt-mass activity of the catalysts, we have developed a new preparation route using pulsed UV laser irradiation to TiO2 fine particle dispersed in liquid to obtain submicron-nano particles of Magnéli phase oxides [2]. By using these TiOx supports, we reported formation of Pt-Ti alloy catalysts on the TiOx (Pt-Ti/TiOx), and found that the catalysts show good ORR activity and superior oxidation-resistant property under high potential conditions by RDE studies in 0.1M HClO4 solution [3]. This work intends to examine the electrochemical activity and stability of the Pt-Ti/TiOx catalysts under PEFC operating conditions and to investigate influences of the support materials. TiOx support was prepared by reducing fine TiO2 particles using Nd:YAG laser operated at λ = 355 nm. After UV laser irradiation to TiO2 particles (BET: 20m/g, 70 nm diameter), spherical TiOx particles with a diameter around 50~300 nm were obtained. Pt-Ti alloy nanoparticles were deposited on the obtained TiOx by impregnation-reduction method. Figure 1 shows a typical TEM image of the prepared 10wt.%Pt-Ti/TiOx catalyst particles. Crystal structure and composition of the deposited Pt-Ti particles were examined by XRD and analytical TEM observations, indicating the formation of ordered Pt-Ti alloy with average composition of Pt75Ti25 (Cu3Au structure). The electrochemical measurements of Pt-Ti/TiOx electrode bonded to Nafion membrane were carried out by using a single-cell operated at 80C and ambient pressure. Figure 2 shows cell performance curve of 20wt.%PtTi/TiOx cathode under fully humidified H2/O2 along with that of a commercial 40wt.%Pt/XC72 cathode. Whereas electrochemically active area (ECA) of Pt-Ti/TiOx was much smaller than that of Pt/XC72 (ca.40% of Pt/XC72), these electrodes showed similar performance curves. As reported previously [3,4], Pt3Ti alloy shows ca. 2-fold higher ORR activity than pure Pt, so that this activity enhancement of Pt-Ti/TiOx compensates the smaller ECA to give similar apparent performance with Pt/XC72. Figure 3 shows typical results of catalyst support durability tests (accelerated stress tests) for 40wt.%Pt/XC72 and 20wt.%Pt/TiOx cathode by potential cycling (potential sweep between 1.0/1.5V, 500mV/s, 80C, anode/cathode = humidified H2/N2). For Pt/XC72 the redox peaks of Hupd and Pt oxide became smaller with increasing in cycle number, which is mainly due to carbon corrosion at high potentials. As for Pt-Ti/TiOx, these redox peaks are fairly stable, even after 5000 cycles. These differences of ECA stability were also reflected in the cell performance after potential cycling tests. Details of synthesis, stability, and durability of Pt-Ti/TiOx catalyst will be discussed at the meeting.