Development of a Composite Electrolyte Membrane with Chain-like Microstructure CeO2-Based Oxide for Achieving High Durability and Performance in Polymer Electrolyte Fuel Cells
Authors: Shudo Kazuki , Ohno Ryuji , Uchida Makoto , Kakinuma Katsuyoshi 
Published: December 2024

Polymer electrolyte fuel cells (PEFCs) are expected to be applied in heavy-duty vehicles (HDVs), in which the operating temperature approaches around 120 ℃. The higher operation temperature, in comparison with that of passenger cars, leads to a significant decrease of the proton conductivity of perfluorosulfonic acid-based electrolyte membranes (PFSA, such as Nafion) under typical atmospheric conditions. Metal oxide surfaces, on which adsorbed water is stable even at temperatures exceeding 100 ℃, have the potential to maintain the water uptake at elevated temperatures, if the oxide is mixed with the PFSA membrane. In our Center, the improvement of proton conductivity over a wide temperature range has been confirmed in composite membranes prepared from Ta-TiO2 and Nafion.1) In the present study, Nafion / CeO2-based oxide composite membranes were fabricated to investigate their proton conductivity and microstructure, as well as to examine the durability of the electrolyte membrane.
CeO2-based nanoparticles (CeO2, Nb-CeO2, Zr-CeO2) with chain-like microstructure were prepared by the flame spray pyrolysis method. Each type of nanoparticle was mixed with Nafion dispersion (20 wt.%, DE2020 CS, Chemours) at various ratios, coated and dried by use of a die coater apparatus, and then hot-pressed (140 ℃, 3 min.) to form a membrane. The microstructure was observed by scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), and transmission microscopy (TEM). The proton conductivity was measured by the four-probe alternating current method, and water uptake was measured by a water vapor adsorption method. Additionally, the open circuit voltage of a single cell with the fabricated composite membranes (OCV test) was characterized to confirm the durability of the membrane.
The proton conductivity of a composite membrane with 3 wt.% Nb-CeO2 was 1.3 times higher than that of commercial Nafion (NRE211, Chemours) at 80 ℃ and 80% RH, and 1.6 times higher at 80 ℃ and 20% RH. The water content of the composite membranes was independent of the Nb-CeO2 content, and the intensity of a SAXS peak for water clusters (ca. 3 nm) was comparable to that of Nafion under all relative humidity conditions. OCV durability tests using composite membranes with 3 wt.% Zr-CeO2 maintained high OCV values up to 1500 hours of operation, even though unmodified Nafion membranes were unable to maintain high OCV values for more than 250 hours. These results indicate that even small amounts of CeO2-based oxide can improve the proton conductivity and the chemical stability.

Acknowledgement

This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO). The results of the OCV test were evaluated by the Fuel Cell Cutting-Edge Research Center Technology Research Association (FC-Cubic).

Reference

(1) Ohno, K. Shudo, T. Tano, and K. Kakinuma, ACS Appl. Energy Mater.,
6, 10098-10104 (2023).