Necessity to Avoid Titanium Oxide As Catalyst Support in PEM Fuel Cells
Authors: Zhang Jingxin , Coms Frank D. , Kumaraguru Swami 
Published: February 2020

Although fuel cell electric vehicles (FCEVs) has become a commercial reality, overall cost reduction as well as durability improvement continue to remain as the key challenges facing fuel cell developers. One of the area of durability concern in proton exchange membrane (PEM) fuel cells relates to the degradation of carbon supported Pt-based electro catalysts. [1] Carbon-supported (e.g., Vulcan or Ketjen black) Pt or Pt-alloy (e.g., PtCo) catalysts are used in PEM fuel cells to catalyse the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). In an automotive environment, the catalysts must survive highly dynamic operations. State-of-the-art high surface area carbon support suffer from severe corrosion at potentials above 1.1 V. [1] A few transient operations that contribute to the corrosion of carbon support including vehicle start-up/shutdown, cell reversal due to fuel starvation, as well as load cycling. The degradation of carbon support and the electrocatalyst can induce significant performance decay. Therefore, material that is inherently more corrosion resistant has been actively investigated as alternative catalyst support to carbon in PEM fuel cell applications.

In recent years, transition metal oxides are of interest as an alternate to carbon electro catalyst support due to their improved stability at higher potentials [2-3]. Furthermore, it is also believed that metal-oxide supports can possibly influence the intrinsic activity of the supported Pt (Pt-alloy) catalyst toward ORR due to strong metal-support interactions(SMSI). Titanium oxide has been demonstrated to be one of the suitable metal oxide, and was frequently reported as effective ORR catalyst support showing both significantly improved stability at high potential as well as improved ORR activity [3]. However, no long-term durability and possible negative effects of oxide supports on PEM fuel cell durability has been reported.

In this study, for the first time we report that the use of titanium oxide (TiO2) as catalyst support in the PEM fuel cell electrode can lead to significant degradation of the polymeric membrane, causing membrane thinning and increased gas crossover. Experiments were conducted using Pt supported on titanium oxide (TiO2) as either the anode HOR catalyst or the cathode ORR catalyst. The impact on membrane chemical degradation is evaluated at open circuit voltage (OCV) condition at 115 oC and 25% RH for 100 hours. The exhaust water from both the anode and cathode of the fuel cell during the OCV hold was collected and analyzed using ion chromatography (IC) to quantify the concentration of fluoride and determine the fluoride release rate (FRR) which is a measure of the rate of membrane chemical degradation. With measured FRR, a total fluorine inventory loss of the membrane is also calculated.

It was found that the total fluorine inventory loss is 26% and 15% for Pt/TiO2 used as anode and cathode catalyst, respectively, after 100hrs of OCV test. A comparable electrode with carbon supported Pt catalyst exhibit only about 0.3% total fluorine inventory loss at identical OCV conditions, which is more than 50X lower. Additional experiments were designed to confirm the impact of titanium oxide by adding titanium oxide only (without Pt) to either anode or cathode electrode while using Pt/C as anode or cathode catalyst. One reduced form of TiO2, i.e., Magneli phase (Ti4O7) which is shown to have improved electrical conductivity and has been investigated as Pt support is also tested in this experiment. The results show that by adding TiO2 or Ti4O7 to anode or cathode both increase the FRR compared titanium oxide free electrodes. The FRR release rate data with titanium oxide added in the cathode is shown in Fig. 1.

In this presentation, we will discuss the details of the experimental study on the impact of titanium oxide on polymeric membrane chemical degradation, including fluoride release rate, fluorine inventory loss, as well as the change in hydrogen crossover rate. Additional analysis on the location of chemical degradation (membrane close to anode vs. membrane close to the cathode) has also been conducted which may shed additional light on the fundamental mechanisms of TiOx induced membrane degradation. Finally, the necessity to find alternative metal oxide support rather than TiOx in PEM fuel cell application is highlighted

Figure 1