Acid-Functionalized Mesostructured Aluminosilica for Hydrophilic Proton Conduction Membranes

New proton exchange membrane (PEM) materials with high proton conductivities at elevated temperatures (ca. > 150 °C) offer the promise of improving PEM fuel cell operation. Higher operating temperatures would provide several benefits including: (i) improved resistance to carbon monoxide poisoning of the platinum (Pt) electrocatalyst at the anode, (ii) reduced need for feed-stream heat and humidity management, and (iii) faster reaction kinetics at both electrodes. Reforming of natural gas and other hydrocarbons provides a hydrogen feed stream that typically contains ∼ 10 % CO, with subsequent water-gas shift and preferential CO oxidation reactions able to reduce the concentration down to ∼ 10 ppm CO. However, carbon monoxide levels as low as 10 ppm can interfere with H2 adsorption on the dispersed Pt catalyst at the anode, detrimentally affecting the performance of PEM fuel cells operating at temperatures below 100 °C. At higher operating temperatures, it has been shown that H2 adsorption on the Pt electrocatalyst becomes competitive with CO adsorption, with the result that PEM fuel cells can stably function at higher CO levels, e.g., 100 ppm at temperatures ≥ 150 °C. Operating temperatures above 140 °C are therefore expected to improve significantly the performance of proton exchange membrane fuel cells that use hydrogen generated from hydrocarbon fuel sources. Currently, most PEM fuel cells use polymeric or polymerinorganic composite membrane materials that must be chemically stable, provide structural support, and have low permeability to diffusing reactant and product species, in addition to conducting protons. Widely used perfluorosulfonic-acid polymers, such as Nafion, provide and balance these properties under conditions in which they remain hydrated. For example, Nafion 117 exhibits typical conductivity values of ∼ 0.1 S cm under fully hydrated conditions at room temperature. However, as their extent of hydration decreases, such as occurs under conditions of low humidity or elevated temperatures, their proton conductivities also decrease. At temperatures above 150 °C, where CO poisoning of the Pt catalyst becomes less troublesome, water loss of perfluorosulfonic-acid-polymer PEMs is a major limitation, as their strongly hydration-dependent proton conductivities decrease to below 10 S cm. Consequently, there has been extensive interest in the development of alternative proton exchange membrane materials that retain high proton conductivities at elevated temperatures near 150 °C. The majority of new membrane materials are based on continuous organic polymer matrices with sulfonic-acid, aromatic, and/or acid-base functional groups or containing inorganic particle additives, such zeolites or silica. Each of these materials balances various membrane property attributes differently. For example, sulfonated aromatic polymers are characterized by good thermal and chemical properties, while possessing proton conductivities between 10–10 S cm at temperatures up to 140 °C. The higher conductivity values are obtained for aromatic polymer membranes possessing high degrees of sulfonation, although this tends to be accompanied by poorer mechanical properties at high temperatures, due to increased membrane swelling. Acid-base polymer membranes, such as H3PO4-doped polybenzimidazole (PBI), exhibit proton conductivity values up to 0.13 S cm at 160 °C. However, PBI-H3PO4 membranes similarly attain high proton conductivities at high acid-doping levels and high temperatures, conditions under which the membranes also exhibit swelling and poor mechanical properties. By comparison, inorganic-organic composite membranes, such as those containing hydrophilic zeolite, silica, or titania particles introduced into Nafion or other polymers tend to retain more water, with modest improvements in their proton conductivities. Likewise, improved water retention and proton conductivity have been obtained through the incorporation of inorganic particles into other proton exchange membrane materials. Alternatively, porous inorganic particles have been functionalized by proton-conducting species or filled with proton-conducting polymers. Extensive efforts to develop new organic-polymer-based PEMs are summarized in several comC O M M U N IC A TI O N