Magnéli Phase Ti₉O₁₇- Catalyst Support Materials for ORR

Abstract

The durability of catalysts in polymer electrolyte membrane fuel cells (PEMFCs) is an important issue that must be addressed before their commercialization since catalyst durability directly reflects the life and cost of fuel cell power-generation systems [1]. Although many factors could play a role in the mechanism of catalyst degradation, the major contributors are believed to be i) dissolution of Pt and re-deposition (Ostwald ripening), ii) coalescence via crystal migration, and iii) detachment of Pt particles from the carbon support [2]. Among these, Pt detachment is strongly affected by the corrosion of the carbon support materials. Conventional carbon blacks, such as Vulcan XC72, are superior catalyst-support materials because of their cost and physicochemical properties. However, carbon is not stable based on thermodynamic considerations under the cathode conditions in a PEMFC: C + 2H2O → CO2 + 4H+ + 4e− E0 = 0.207 V vs. RHE Although this reaction proceeds slowly at potentials <0.8 V in a PEMFC [3], corrosion reactions are significantly accelerated under high potential conditions. Therefore, alternative catalyst-support materials that are highly oxidation-resistant are necessary. In order to improve the stability of the catalyst support, considerable work has been done over the past few decades on non-carbon support materials [4]. In this presentation, we have focused on oxygen-deficient, sub-stoichiometric titanium oxide (TinO2n−1, known as Magnéli phase) as a potential oxidation-resistant cathode catalyst support in PEFCs. This leads to improving specific activity for the oxygen reduction reaction (ORR) due to the formation of Pt–Ti alloy particles on the TiOx support, as well as the improve dispersion of Pt due to the deposition of smaller catalyst particles. The Magnéli phase Ti9O17 is synthesized and characterized (Fig. 1) by XRD. The Pt/Ti9O17 will be synthesized, and the ORR activity and stability will be investigated. A comparative study will be performed to those of a conventional Pt/C catalyst. References: Chem. Rev., 107, 3904 (2007) Top. Catal., 46,285(2007) ECS Trans., 1 (8), 3 (2006) J. Mater. Chem., 19, 46 (2009) Figure 1

Magnéli TiO2 as a High Durability Support for the Proton Exchange Membrane (PEM) Fuel Cell Catalysts

“…The stability of the PEMFC catalyst support is usually evaluated through the AST start-stop cycles developed by Nissan. The AST cycle involved a square wave from 0.6 V to 0.95 V with a 3 s dwell time at each potential [28]. The stability of catalysts (Pt/C and Pt/Ti 9 O 17 ) is evaluated for 12,000 cycles according to the protocol scheme shown in Figure 1.…”