Perfluorosulfonic polymers, for instance, Naflon, are preferable candidates for membranes used in polymer electrolyte fuel cells owing to their high proton conductivity, excellent mechanical strength, and chemical stability for temperatures below 100°C. Nevertheless, the operating conditions need to be optimized to maintain 100% relative humidity at about 80 °C as well as low carbon-monoxide concentration in hydrogen gas. The functioning of polymer electrolyte fuel cells at higher temperatures raises the tolerance concentration of carbon monoxide poisoning for the platinum catalysts.
The increased operation temperature enhances the cell efficiency and simplifies the management system. For this reason, there is an urgent need for membrane electrolytes exhibiting high proton conductivities as well as intermediate temperatures. Inorganic-organic hybrid membranes possess the advantages of inorganic as well as organic phases. This is in the sense that functionality and flexibility of organics are blended with the thermal, mechanical, and chemical stability of inorganics. Therefore, the inorganic-organic hybrid membrane is preferable for implementation at intermediate temperatures.
Copolymerization of suitable monomers is a clear-cut approach for the formation of covalent bonds in the hybrid membranes. Researchers led by Professor Toshinobu Yogo at Nagoya University in Japan, demonstrated the one-pot synthesis of inorganic-organic hybrid membranes through copolymerization of N-vinylbenzotriazole, 1,5-divinyl-3-phenylpentamethyltrisiloxane, and 2-hydroxyethyl methacrylate acid phosphate. The research team did not require any hydrolysis-condensation for the preparation of the silicon-oxygen-silicon linkages reference to the fact that trisiloxane linkage was used for the inorganic backbone of the hybrid membrane. Their research work is published in Polymer.
The authors adopted the ac impedance method to measure proton conductivity of the hybrid membranes. They did this at various temperatures and relative humidity in a sealed vessel. They equilibrated the measurement cell at a desired relative humidity for one night at about 40 °C before measurement.
The Toshinobu Yogo and his team constructed inorganic-organic membranes from trisiloxane as well as aliphatic polymer chains bond with phosphonic acid groups, which were later copolymerized through one-pot method. The authors observed that the membranes were self-standing, possessed high thermal stability, high formability, and were homogeneous. The trisiloxane linkage in the hybrid membrane enhanced the thermal and oxidation stability of the membrane.
The hybrid membrane in the ratio 1:9:5 was found to have a high elastic modulus as compared to that of membranes with 2:8:5 and 3:7:5. The proton conductivity of the hybrid membranes was observed to rise with increasing temperature and relative humidity up to 130 °C. 1:9:5-ratio membrane was operated at 140 °C, 30% relative humidity for about 30 hours and indicated a peak power of about 7.8mWcm-2 at 10 hours. Reference to chemical design, one-pot synthesis of the hybrid membranes is presented as a versatile synthetic process for the polymer electrolyte fuel cells used at low relative humidity as well as intermediate temperatures.
Masaya Takemoto, Koichiro Hayashi, Shin-ichi Yamaura, Wei Zhang, Wataru Sakamoto, and Toshinobu Yogo. Synthesis of inorganic-organic hybrid membranes consisting of organotrisiloxane linkages and their fuel cell properties at intermediate temperatures. Polymer, volume 120 (2017), pages 264-271.Go To Polymer