Porous Metal Oxide Nanotubes Modified Proton Exchange Membranes for Fuel Cells Operating at Elevated Temperature and Low Humidity

Polymer electrolyte membrane fuel cells (PEMFC) have been widely accepted as one of the most promising power sources for automobile application due to high energy conversion efficiency and environmental compatibility. However, the current PEMFC technology, utilizes perfluorosulfonic acid (PFSA) polymer membranes, e.g., Nafion, as electrolyte, is limited owing to expensive components and poor performance at elevated temperature (100 – 200 °C). PEMFC operated at elevated temperature results in enhancing the performance due to improving electrode kinetic and reduction of adsorption poisoned species such as CO [1]. The deterioration of PEMFC performance at elevated temperature is mainly attributed to the loss of proton conductivity of the Nafion membrane which results in drastic increase in the ohmic over potential [2]. Proton conductivity of Nafion membrane is strongly influenced by the water content and the conductivity is maximum when the membrane is fully hydrated. Operating PEFC at elevated temperature leads to membrane dehydration which dramatically reduces the proton conductivity in order of magnitude. Thus, it is highly desirable to enhance proton conductivity Nafion membrane at elevated temperature in order to achieve high PEFC performance.       
       Porous metal oxides nanotubes (TiO₂, ZrO₂, and CeO₂) with diameter in a range of 30 – 180 nm were synthesized by calcining electrospun metal ions embedded polyacrylonitrile nanofiber, which was prepared using a conventional single spinneret electrospinning technique, at 600 ^oC under an air atmosphere. Compared to a commercial membrane (Nafion^®212) operated at 80, 100, and 110 ^oC, the Nafion-TiO₂ membrane delivered improved power density at 0.6 V.  The performance of composite membranes was evaluated by making MEA and subsequently the composite membranes were investigated the H₂/O₂ fuel cell at 100 ^oC under 100 %RH and at 80 ^oC under 18 %RH.  Fig. 1B illustrates the polarization and power density plots obtained at 100 ^oC under 100 %RH. It was found that the power density at 0.6 V obtained from Nafion-TiO₂ composite membrane operated at 100 ^oC was 730 mW/cm², which is 1.6 times higher than commercial membrane (NRE-212) and recast Nafion membrane. On the other hand, the Nafion-TiO₂ composite membrane delivered maximum power density of 640 mW cm^-2, which is 3 times higher than that of the performance obtained from NRE-212 and recast Nafion membranes (Fig. 1C). The higher PEMFC performance at 100 ^oC and low RH was due to the hygroscopic property of TiO₂ which can retain water at 100 ^oC and due to the tubular morphology which facilitates the proton transport channel through membrane.                                                          
     We also fabricated inorganic proton cluster modified graphene oxide Nafion composite membrane. The deposition of phosphotungstic acid (PW) on the modified graphene oxide sheet with positively charged amine was carried out through an electrostatic interaction between PW and the modified graphene sheets to make heterogeneous system. The modified graphene oxide (mGO) was synthesized through a condensation reaction. A mixture of mGO and PW aqueous dispersion was stirred to fabricate PW-mGO. The interaction between inorganic cluster and the modified graphene oxide was confirmed by UV-visible, IR and cyclic voltammetry studies and these results revealed that that an alteration of the electronic structure of deposited cluster as a result of strong hybridization between the graphene oxide and PW [3]. The NRE-212/PW-mGO composite membrane exhibited much higher proton conductivity at relatively low humidity than the Nafion-212 (NRE-212), and the composite membrane showed 4-fold higher maximum power density PEMFC operated under 18 % RH than that of pristine Nafion-212 membrane. The synthesis of composite perfluorosulfonic acid membrane, conductivity and fuel cell performance at elevated temperature under low relative humidity will be discussed in this presentation.

References

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