Stability of TiO2-Based Materials under PEFC Operation Condition for Non-Platinum Cathodes
Authors: Kitamura Yuto , Matsuzawa Koichi , Nagai Takaaki , Ota Ken-ichiro , Ishihara Akimitsu 
Published: December 2020

1. Background
Pt/C catalysts used for cathodes of polymer electrolyte fuel cells (PEFCs) have problems such as high cost and insufficient durability against potential change [1]. Therefore, we have developed a non-platinum catalyst based on group 4 and 5 transition metal oxides because they are inexpensive and stable in acidic solution [2]. We successfully demonstrated that Nb-doped TiOx catalyst showed high onset potential for oxygen reduction reaction (ORR) and higher electrochemical durability than platinum [3]. However, the ORR current was much smaller than that of platinum catalyst. An increase in the surface area of oxides is effective to increase the ORR current. Nano-sizing of oxide particles is one of the solutions to increase the surface area of oxides. In general, the solubilities of nanoparticles are larger than those of large particles. Therefore, we need to evaluate the stability of nanoparticles in acidic solution. Some investigations regarding the stability of titanium oxide nanoparticle which is one candidate of group 4 oxide cathodes [4, 5, 6] have been already performed. However, the stability in PEFC operating condition has not been evaluated in detail. Therefore, in this study, the chemical stability was evaluated by measuring the dissolution behavior of titanium oxide nanoparticles in an acidic electrolyte simulating the operating environment of PEFC.
2. Experimental
A 600 mg of TiO2 nanopowder (US Research Nanomaterials, mean particle size: 6.0 nm, Anatase) was immersed in 1.0 M (pH = 0.25), or 0.32 M (pH = 0.66), or 0.10 M HClO4 (pH = 1.06) and stirred. The temperature was controlled by using a water or an oil bath in the temperature range from 30 to 90 ℃. At the desired time, the stirring was stopped, then a small amount of the solution (2 mL) was sampled and filtered through a 0.22 µm syringe filter. The sample solution was immediately diluted. The remaining solution was stirred again. This procedure was repeated. The concentration of titanium in the sample was measured by ICP-AES (Seiko Instruments).
3. Results and Discussion
Figure 1 shows the concentration of titanium of TiO2 nanopowder with diameter of 6.0 nm as a function of immersion time in 0.1 M HClO4 under air at 30, 50, 70, and 90 ℃. The concentration of titanium became constant after 800, 500, 300, and 150 h immersion at 30, 50, 70 and 90 ℃, respectively, meaning that dissolution reached equilibrium. The solubility of TiO2 nanopowder in 0.1 M HClO4 were 16, 6.2, 3.2, and 2.2 µmol dm-3 at 30, 50, 70, and 90 ℃, respectively. The solubility of TiO2 nanopowder decreases with increase in temperature and becomes chemically stable, which indicates that TiO2 nanoparticles are more suitable for PEFC operating condition at high temperature.
Figure 2 shows the van't Hoff plots of the solubility of the TiO2 nanopowder in HClO4 and the solubility of platinum black powder in 0.1 M HClO4 [7] is also plotted. The solubility of TiO2 decreases with increase in temperature, while the solubility of platinum black increases with increase in temperature in acidic electrolytes. From the slope of the approximate line of the van't Hoff plot of the solubility of TiO2 nanopowder, the apparent standard enthalpy change of dissolution of TiO2 nanoparticles, ΔHº , in 0.10 M HClO4 is estimated to be -29.1 kJ mol-1, and the dissolution reaction of TiO2 nanoparticles in an acidic electrolytes is exothermal. Furthermore, from the value of the standard Gibbs energy change of the dissolution reaction, ΔGº , calculated from the solubility of TiO2 nanopowder at 30 ℃, the apparent standard dissolution entropy change, ΔSº , in 0.10 M HClO4 was calculated to be -151 J mol-1 K-1.
Acknowledgments
This research is conducted with the support of the National Research and Development Corporation New Energy and Industrial Technology Development Organization (NEDO). In addition, the authors wish to thank the support of JSPS grants-in-aid for scientific research, Suzuki Foundation and Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation.
Reference
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Figure 1