Development, Evaluation and Characterization of Hydrophilic Oxide/Polymer Composite Membranes for Polymer Electrolyte Fuel Cells Operating at Temperatures over 80oc

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The present transportation system based on the internal combustion engine is a major contributor to the issue of CO2 emission, and a paradigm shift is required toward to electrical motorization based on batteries and polymer electrolyte fuel cells (PEFCs). The motorization of heavy-duty vehicles (HDVs) by PEFCs is one of the essential targets. HDVs will require higher operating temperatures, at least 120oC, versus normal temperatures (60-80oC), for optimal heat management. A commercial perfluorosulfonic acid-based electrolyte (PFSA, e.g., NafionⓇ) shows better proton conductivity with chemical stability over a wide temperature range, although the conductivity decreases under low back pressure at 100~120 oC. In order to improve the proton conductivity in the higher temperature range, a composite membrane of NafionⓇ with a hydrophilic filler of Ta-doped TiO2 (Ta-TiO2) was synthesized. Ta-TiO2 nanoparticles with fused-aggregate network microstructure were synthesized by the flame oxide-synthesis method and were crushed by wet milling (Star Burst, Sugino Machine, Limited Co.). The fillers were mixed with NafionⓇsolution (20 wt.%, D2020 Chemours Co.) by use of a rotary ultrasonic dispersing processor (500 rpm, 60 min., PR-1, THINKY Co.), a planetary ball mill (500 rpm, 30 min., PULVERISETTE 6, FRITSCH Co.), and a defoaming processor (500 rpm、5 min., HM-400W, Kyoritsu Seiki Co.). The dispersion solution was coated in hot air by use of a die coating system (die coater, Taku-Dai Mini-50, Daimon Co., Ltd.), and were hot-pressed (140 ℃, 3 min., TCMD-2.5, TOHO KOGYO Co.) to form composite membranes (9 cm × 9 cm × 25 µmt). The cross-sectional backscattered electron image BSE) of the composite membrane was observed by scanning electron microscopy (SEM, acceleration voltage 2 kV, SU9000, Hitachi High-Tech Co.) and transmission electron microscopy (TEM, H-9500, Hitachi High-Tech Co.). The proton conductivity of the composite membrane was measured at 80 °C and 120 °C by the AC four-probe method. Water uptake of the composite membrane was measured by water vapor adsorption. The current-voltage (I-V) polarization curves of single cells using the prepared composite membrane (membrane thickness, 25 µm) were measured at 80 ℃, 100% RH and 120 ℃, 20% RH, back pressure 50 kPaG.The obtained composite membranes were soft and flexible. The BSE images indicated, based on their high contrast, that the Ta-TiO2 particles were uniformly dispersed in the composite membrane(Fig. 1(a)). The higher magnification of the cross-sectional TEM images also showed that the hydrophilic surface of Ta-TiO2 adhered to the Nafion® without any voids. The proton conductivity of the composite membrane at 80 °C and 80% RH increased with increasing Ta-TiO2 content and showed a maximum value of 0.13 S cm-1at 3 wt% and then decreased. The maximum proton conductivity was 1.3 times higher than that of Nafion®(Fig. 1(b)). The proton conductivity of the composite membrane (3 wt% of Ta-TiO2) at 120 °C, 20% RH reached 0.018 S cm-1, which was 1.8 times higher than that of a commercial Nafion® membrane, 0.01 S cm-1. The water uptake in each composite membrane was nearly independent of the concentration of the hydrophilic oxide at each humidity(Fig. 1(c)). Comparing with literature results, improvements of the proton conductivity by use of various types of additives have been reported, but the improvement of the proton conductivity by such a small percentage of hydrophilic oxide additive without affecting the water uptake in the membrane has not been reported, to our knowledge. We consider that the protonic hopping (proton mobility) might change as a result of adding the hydrophilic Ta-TiO2 in the membrane. The I-V performance of a single cell using a composite membrane preserved the same performance as that of a cell using an unmodified Nafion® membrane at 80 °C and 100% RH, and then improved at 120 °C and 20% RH(Fig. 1(d)).