The study of the macroscopic and atomic scale properties of zeolites has lead to various other uses for zeolites besides their catalytic properties. These uses are an ongoing focus in the Davis group as well as the use of mesoporous materials for similar functions.

Zeolitic Thin Films

As part of the Davis group’s focus on advanced materials, we have been developing novel applications for zeolites, such as zeolitic thin films, which can be used as separation/catalytic membranes, chemical sensors, adsorption coatings, and low dielectric constant (low-k) materials for computer chips, among a host of other applications. The development of pure-silica zeolitic thin films is of particular interest to the semiconductor industry, whose attempts to decrease the size of features in microprocessors have been limited by a resulting increase of cross-talk noise and energy dissipation, due to the use of nonporous silica (k = 4.0) as the dielectric film insulating the wiring between transistors in an integrated circuit. In order to reduce these problems, an ultra-low-k material (k < 2.0) needs to be developed. Typically, the dielectric constant of a material is decreased by introducing porosity (air has a k = 1.0), but porous silicas such as sol-gel and mesoporous silicas tend to have low mechanical strength and high hydrophilicity (water’s k = 80 – 90) that make them poor candidates for use in integrated circuits. Pure-silica zeolites, however, avoid these problems because a) they have dense crystalline, porous structures, which yield high mechanical strength and high heat conductivity, b) they are hydrophobic, which should reduce water adsorption, and c) they have a narrow pore size distribution of very small pores, which should minimize electric breakdown and power dissipation. Therefore, we are developing methods to synthesize pure-silica zeolitic thin films of various topologies by in-situ crystallization techniques that are suitable for extension into the semiconductor development process.

Proton Conductivity in Zeolite-like Materials

Improving the materials used as membranes in fuel cells has been a major focus as a way to improve their efficiency. Yan and coworkers proposed that zeolites added to polymer electrolytes may be effective in reducing methanol crossover in direct methanol fuel cells or in increasing the operating temperature in hydrogen proton exchange membrane fuel cells. Such additives should be thermally stable, hydrophilic, have low methanol permeability, and be good proton conductors. In their previous work, Yan and coworkers reported the proton conductivity of the aluminosilicate zeolite beta and zeolite beta functionalized with organosulfonic acid groups.[1] They found that the organofunctionalized zeolites exhibited excellent proton conductivities, ranging from 1.2x10 -3 to 1.2x10 -2 S/cm, which are only one to two orders of magnitude below those exhibited by Nafion under water-saturated, room temperature conditions.

In our recent work, we have focused on the proton conductivities for a series of zincosilicates. These zincosilicates are microporous, crystalline materials with zeolite-like frameworks. More importantly, they have a high ion density resulting from the need for two counter cations or protons per zinc in the framework as opposed to the one-to-one ratio of counter cation or proton to aluminum in traditional zeolites. Due to these properties, we anticipate that zincosilicate materials, or even a sulfonic acid fuctionalized zincosilicate, may exhibit comparable or higher proton conductivities than their zeolite counterparts.

[1] Holmberg, B.A., et al., Micropor. and Mesopor. Mater., 2005. 80(1-3): p. 347-356.

 

Proton conductivity in organically modified Mesoporous Silica

A key part of a DMFC (direct methanol fuel cell) is a polymer electrolyte membrane which is sandwiched between an anode and a cathode. The function of the PEM is to allow facile proton conduction and to keep the fuels at the anode and cathode separated.

The model membrane currently used is a perfluorosulfonic acid polymer (Nafion, Dupont) which exhibits excellent proton conductivity via spherical hydrophilic conductive channels. The major drawback of this polymer is its high methanol crossover rate which leads to and erosion of the fuel cell efficiency.

Extensive research has been devoted to the development of new proton conducting membranes to replace Nafion. Some of these materials include nonperfluorinated ionomer membranes, heteroatom substituted and organically functionalized zeolites and acid functionalized mesoporous materials. With the exception of heteroatom substituted zeolites, almost all have a sulfonic acid as the ionic group and give conductivities ranging from 10 -2-10 -7 S/cm. We believe that the wide range of conductivities observed is mainly due to the differences in their channel structure (microscopic to mesoscopic size with different geometries) which leads to different mechanisms of proton conduction.

Our interest is thus focused on understanding the relationship between the membrane geometry and the mechanism of proton transport in an attempt to match Nafion’s conductivity while decreasing its methanol crossover rate. Towards this goal, we are making attempts to utilize the structural uniformity of organically functionalized mesoporous materials to study the effect of the water channel size on proton conductivity and methanol permeability.

By fixing the geometry of the system (cylindrical pores arranged in a hexagonal fashion) and keeping the type and number of acid sites constant, we should be able to isolate the effect of the channel size by simply varying the pore size of our materials. Analysis of the proton conduction of these materials by NMR, impedance spectroscopy and other techniques should allow us to back out the mechanism of proton conduction which could vary from Grothuss (structural diffusion) to vehicular or a mixture of both and most importantly attribute this to a specific geometry and pore size. Methanol permeability tests can be conducted by constructing simple diffusion cells. These studies will provide us with the mechanistic understanding that will be required for the construction of an efficient proton conductive membrane for a DMFC.