Fabrication And Characterization Of Composite Membranes For Polymer Electrolyte Fuel Cells

As the key material of proton exchange membrane fuel cell(PEMFC), the properties of the membrane are critical to the performance and durability of PEMFC. However, the high cost is needed to manufacture perfluorosulfonic acid(PFSA), which has restricted their applications for PEMFCs; another problem is the complexity of the water management system, especially under the situations of high temperature and low humidity. These problems have promoted an effort to develop membranes with the same desirable chemical and mechanical properties, but more cheaply to be produced. As composite membrane has much inexpensive polytetrafluoroethylene(PTFE) and has some merits such as high mechanical strength and better dimensional stability, more and more researchers pay their attentions to the PFSA/PTFE composite membranes. In this paper, we put forward a novel technics to prepare composite membrane based on the kinetic process of PFSA's filling into the pores of PTFE. Additionally, the mechanical strength and durable of the membrane were enhanced by raising the manufacture temperature. Based on the technics above, we also prepared the self-humidifying and water containing membrane by sol-gel and self-assembly methods. At the same time, some conclusions had been summarized as following:(1) Compared with the prevalent impregnation technics, the impregnation in vacuum is helpful for the filling of the PFSA solutions. It is indicated from the SEM photograph that the filling degree of the composite membrane we prepared is superior to Gore-select membrane. We also found that it was invalid to raise the solution temperature to improve the filling degree of the composite membrane. The modifying of alkali metals to PFSA can enhance the glass transition temperature of PFSA effectively. Preparing the membrane at high temperature after ion modifying can promote the combination of PFSA and PTFE. The performance of H⁺-Nafion/PTFE composite membrane without ion modifying descended abruptly after 144 hours, while the Na⁺-Nafion/PTFE composite membrane worked steadily after 336 hours. (2) The conducticity of the proton exchange membrane depends on the water-retaining property of the membrane deeply. In order to promote the water-response property of the composite membrane, we prepared water-containing composite membranes by sol-gel technics. In this research, the SiO₂ nanoparticles can distribute in the membrane uniformly. The water loss of the Nafion/SiO₂/PTFE membrane is 1.48Wt.%, which is 43% of Nafion/PTFE membrane at 80℃; at 100℃, the water loss of Nafion/SiO₂/PTFE membrane is 59.2% of Nafion/PTFE membrane; at the high temperature zone, such as 120℃, the Nafion/SiO2/PTFE membrane also exhibits a excellent water-retaining property. The cell performance of the Nafion/SiO₂/PTFE membrane is superior to that of Nafion111 and Nafion/PTFE membrane under the situations that: fuel cell temperature is 60℃, humidifying temperature is 50℃.(3) As the react gas penetration of the thin composite membrane, the performance of composite membrane is usually low than that of Nafion membrane. Pt nanoparticles in the membrane can catalyze the react gases to form water, which humidifys the membrane and also avoid the mixed potential effect. We obtained a self-humidifying composite membrane by self-assembled technics. It is indicated that the Pt particles assembled by static force is about 2nm and. Pt loading increased with the assembly time(the Pt loading is 9.57μg/cm2 after 36h); As the Pt loading is ultra low, the embedded Pt particles have little influence to the membrane conductivity(before assembling the conductivity is 0.113 Scm^-2, after assembling it is 0.107 Scm^-2). For the composite membrane we prepared, the optimum Pt loading is 9.57μg/cm².