Fuel cells are clean, compact and modular energy generation devices that have the potential to revolutionalize the production of electricity and thermal energy. Depending on the type of electrolyte used, there are various kinds of fuel cells, one of which, the proton exchange membrane fuel cell (PEMFC) is attracting enormous attention due to its great potential in transportation, residential and portable applications. The “heart” of the PEMFC is "membrane electrode assembly" (MEA) which is made of a thin proton exchange membrane coated on both sides with a catalyst layer and carbon fiber paper. The MEA enables the essential reaction to happen between hydrogen and oxygen that produces electricity. While many breakthroughs have been made on PEMFC over the past few years, both technical and economic barriers for PEMFC commercialization still exist. The two major technical barriers specifically associated with the PEMFC are: (i) high fuel permeability and low proton conductivity of proton exchange membrane at high temperature (>100 oC); the state-of-the-art electrolyte material for PEMFC, perfluorosulfonic acid (PFSA) such as Nafion, requires humidification to conduct protons. Membranes made with PFSA require close to 100% relative humidity to achieve proton conductivity high enough for fuel cell applications (~0.1 S/cm), dictating humidification of the fuel and oxidant streams. This requirement and the thermal stability of PFSA limit the operating temperature of the fuel cell to <100ºC (typically 60–80ºC), while adding to the complexity, size, weight, and cost of the fuel cell system. Proton exchange membrane with reduced need for external humidification and the ability to operate at temperatures above 100°C would simplify the fuel cell systems and advance their development for automotive and stationary applications. (ii) insufficient durability and catalytic activity of carbon supported Pt catalysts. The Pt nanoparticles are prone to aggregation under the operating conditions in the long-run, which results in Pt surface area loss and hence performance loss over a period of time. In addition the catalyst support i.e. carbon black is susceptible to corrosion, especially in transportation applications under the actual driving cycle conditions. As the carbon is corroded away, Pt nanoparticles will be lost from the carbon support and aggregate into larger particles.
The objective of my Ph. D research is to develop novel proton exchange membrane and catalyst for the MEA of PEMFC, which is divided into two main research areas:
I. To develop Nafion/zeolite nanocomposites proton exchange membrane with low fuel permeability and high proton conductivity for operation up to 120°C or higher with low humidity.
Zeolite materials happen to be primarily composed of silica, and depending on the zeolite structure type, it can have very high surface area due to its microporosity. We expect the Nafion/zeolite nanocomposites proton exchange membrane should have such properties: (a) Higher working temperature: the porous structure of zeolite should retain water molecules well above its boiling point; (b) Higher proton conductivity: sulfonated zeolite nanocrystals are much better proton conductivity than other inorganic materials such as silica; (c) Lower fuel permeability: the nanochannels of zeolite nanocrystals can allow protons to pass through while blocking the passage of fuels such as methanol and hydrogen. First, zeolite nanocrystals has been synthesize by our novel microemulsion and microwave method, then Nafion/zeolite nanocomposite membranes are fabricated by casting method and in-situ method successfully and they can be used to improve performance of methanol feed PEMFC. Third, Nafion/Zeolite nanocomposite membranes will be demonstrated to work well in PEMFC at higher temperature (120-140 oC).
II. To develop novel nanotube-based electrocatalysts with high durability and catalytic activity.
(i) Develop and demonstrate functionalized CNTs and conductive polymer nanotubes as excellent candidates for catalyst support for PEMFC. Increasing the utilization of Pt in electrode structure and improving durability by developing novel nanotube-based catalyst support such as: carbon nanotubes (CNTs) and conductive polymer nanotubes. CNTs have been used to replace traditional carbon powders as the catalyst support in PEMFCs because of their unique structural, mechanical, and electrical properties. It was found that CNTs can improve mass transport and Pt utilization due to their elongated morphology, and they are more corrosion resistant than carbon black. Single walled, double walled and multi-walled CNTs supported Pt (2-3 nm) and Pt-alloy electrocatlysts (Pt/SWNT, Pt/DWNT and Pt/MWNT) have been prepared successfully. Double walled CNT has shown the best catalytic activity and durability among them. The optimization of MEA by them is still on the way. At the same time, conductive polymer nanowires and nanotubes have also been developed and used as Pt and Pt-alloy electrocatalysts catalyst, which have shown improved catalytic activity for the PEMFC.
(ii) Develop and demonstrate Platinum nanotubes (PtNTs) and Pt-alloy-NTs as high durable and catalytic activity catalysts for PEMFC. PtNTs and Pt-alloy-NTs have been synthesized successfully and directly used as a supportless catalyst with the goal of greatly reducing the use of Pt and improving the catalytic activity and durability. PtNTs have unique tube morphology with dimensions at multiple length scales that combine a number of remarkable properties unavailable from any other single catalyst system, which will enhance the catalytic activity and durability.
All these benefits of using the Nafion/zeolite nanocomposite membrane, the novel CNTs supported Pt catalyst and PtNTs and Pt-alloy-NTs can eventually lead to the high proton conductivity, lower fuel permeability, enhanced catalytic activity, increased Pt utilization, and higher durability of MEA, thus effectively improving the performance and durability and reducing the cost of MEA which are still the major obstacles for commercialization of the PEMFC technology.
References
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