Study on the Stability and Durability of Dual Metal-Doped TiO2 Catalyst Support for Pt

Abstract

One of the critical issues in a polymer electrolyte membrane fuel cell (PEMFC) is catalyst durability. Several degradation mechanisms for Pt catalysts have been identified, such as dissolution or redeposition via Ostwald ripening, and migration of Pt catalysts and corrosion of carbon support1. During typical startup and shutdown cycles or anode hydrogen starvation conditions, the carbon support may experience corrosion2 as described by the reaction below: C + H2O → CO2 + 4H+ + 4e- (E° = 0.207 VRHE at 25°C) This causes the structural damage to the carbon support and leads to a reduction of electrochemical surface area (ECSA) of the Pt catalyst, which results in reduced performance and lifetime. To resolve this issue, many studies are underway to investigate alternative non-carbon supports, which are more stable under corrosive environment. Recently, metal oxides have received strong attention as a possible replacement for carbon. Metal oxides also allow strong metal-support interaction that could promote catalytic activity. Titanium dioxide (TiO2) is the most commonly studied metal oxide as a non-carbon support for Pt3–5 considering its electrochemical stability and resistance to acid dissolution. However, its low electronic conductivity, which is far lower than carbon, limits its application. To address this problem, we synthesize TiO2 with metals and nonmetals to lower its band gap and improve electrical conductivity. While most published literatures claim improved stability with TiO2-based materials, the reported accelerated durability test (ADT) protocol is in the range of 0.6 to 1.0 V vs RHE, which accounts mostly for Pt stability. In our study, we develop ADT based on lower (0.6 to 1.0 V vs RHE) and higher (1.0 to 1.6 V vs RHE) potential ranges to study and quantify Pt and carbon support degradation mechanisms, respectively. Using rotating disk electrode, Pt and carbon support stability are studied in 0.1 M HClO4 electrolyte under nitrogen purging at room temperature. Electrocatalytic activities are obtained under O2-saturated electrolyte every 1000 cycles. Cyclic voltammetry results of 20% Pt/C show that the two ADT protocols exhibit distinct degradation features. No significant change of the double layer capacitance (Cdl) is observed from low potential ADT, while the Cdl is almost doubled for high potential ADT. Although the two ADT protocols show similar losses of ECSA measured from hydrogen adsorption method, the mass and specific activity yield a more severe degradation rate for higher potential ADT. The additional loss caused by the support degradation is then modeled using Tafel-based Butler-Volmer kinetic equation. The same experimental study is applied to TiO2-based catalyst supports doped with niobium (Nb) and vanadium (V). In the conference, we will present detail results from Pt/C compared with dual metal-doped TiO2 catalyst support.