Thursday, June 26, 2008 - 3:10 PM
South American AB (Capital Hilton)

Polymer-Filled Composite Membranes

Xiangwu Zhang, North Carolina State University, Raleigh, NC

Proton exchange membrane fuel cells (PEMFCs) are promising candidates for “clean” power generation because they provide electricity without combustion and pollutants associated with burning fossil fuels. Perfluorosulfonic acid polymer membranes, such as Nafion®, are typically used as the electrolyte in low-temperature PEMFCs because of their excellent chemical, mechanical, and thermal stability, as well as their relatively high proton conductivity of ca. 0.08 S cm-1. However, the proton transport requires continuous water assistance, and as a result, Nafion® membranes have limited operating temperature (< 100 oC) because of the dehydration at high temperatures. Increase in membrane operating temperature can lead to high-temperature PEMFC technology, which solves or avoids many shortcomings associated with low-temperature PEMFC technology, including slow electrode kinetics, low CO tolerance, and difficult heat dissipation.

In this work, polymer-filled porous silica composite electrolyte membranes [porous silica-polystyrenesulfonic acid (porous silica-PSS)] were introduced by attaching polystyrenesulfonic acid (PSS) onto the inner surface of porous silica. The proton conductivity of porous silica-PSS increases with increase in surface proton density, and the highest conductivity (0.05 S cm-1 at 150 oC) is obtained at a surface proton density of 15.2 nm-2. It is also found that there is no dramatic conductivity drop for porous silica-PSS membranes at temperatures higher than 100 oC, indicating that a high operating temperature has been achieved.

Cell potentials of porous silica-PSS-based fuel cell and Nafion® 115-based fuel cell were measured. At 65 oC, Nafion®-based fuel cell has higher cell potential than porous silica-PSS-based fuel cell. However, when the temperature increases to 130 oC, the cell potential of Nafion®-based fuel cell drops dramatically because of the low operating temperature of Nafion® membrane. Porous silica-PSS-based fuel cell has even higher cell performance at 130 oC than at room temperature, which may be caused by improved electrode reaction kinetics at elevated temperature.