(609a) Composite Inorganic-Organic Proton Exchange Membrane for Hydrogen Fuel Cells

We report, for the first time, the development of a viable silicon-based proton exchange membrane (PEM). To do so, we developed a set of key processes for fabricating nanopores uniformly within a silicon membrane and to self-assemble dense, contiguous layer of functional molecules on the nanopores' wall. A membrane electrode assembly made using the Si-nanopore PEM delivered 102 mW/cm2 using dry hydrogen and air at room temperature, which is 5 times more than the state-of-the-art reported. This development can greatly benefit the micro-fuel cell technology where a silicon-based MEMS/CMOS compatible PEM has long been sought.

Early efforts to fabricate silicon-based PEMs have focused on adding perfluorosulfonate ionomers such as Nafion?µ in a silicon membrane with microscale holes. Unfortunately, poor adhesion between the perfluoro polymers and the silicon base structure, along with significant volumetric change of the polymers during operation, often results in failure of the membrane electrode assembly (MEA). In addition, the filling process of the holes is difficult to mass produce for commercialization of micro-fuel cells. One approach to solve these problems is to fabricate a nanoporous silicon membrane and covalently bond molecules with functional groups on the pores' walls in a dense, contiguous layer. Efforts to do so have been largely unsuccessful due to difficulties of: 1) robust wafer-scale processing of nanoporous silicon membranes; and 2) self-assembly of molecules in torturous nanopores with extremely high aspect ratios. The common approach in fabricating porous silicon membranes (through an electrochemical etching process) does not result in uniform open pores over the entire membrane and has high sensitivity to thickness variations across the standard silicon wafers. For functionalizing the nanopores, the common approach has been to soak the membrane in the solution containing the self-assembly molecules. Diffusion is relied upon to deliver molecules through torturous nanopores with aspect ratios in the 1000s, which inherently provides uneven coverage. Moreover, no evidence of functionalization deep within the pores has been presented.

Here we present a technique for: 1) fabricating nanoporous silicon membranes with uniform open pores independent of the thickness variations across the membrane and wafer; and 2) uniform functionalization of the pores' wall along the entire membrane thickness. A 20 ?Ým thick silicon membrane was fabricated through KOH etching process. The membrane was made porous through a self-terminating process, in which etching automatically stops when each pore is etched through the entire membrane thickness making the process insensitive to the thickness non-uniformities. Figure 2 shows the cross section of the membrane. The surface of the pores was hydroxilated and the self-assembly process was conducted. In order to avoid self-polymerization of the 3-mercaptopropyl-trimethoxysilane (MPTMS) molecules, a dilute solution of MPTMS in benzene (1 mM) was used. One pore volume of the solution contained 3-4 orders of magnitude less molecules than necessary for complete coverage of all hydroxylated sites within the pores. To supply sufficient molecules to all sites, an apparatus was fabricated that allowed continuous supply of solute-rich solution to the pores and extraction of the depleted solvent through diffusion from the bottom of the pores.

Full penetration of the functional group inside the membrane was verified using Time of Flight-Secondary Ion Mass Spectroscopy (ToF-SIMS) with depth profiling. The thiol end of the MPTMS molecule was then oxidized in nitric acid. A MEA was built by applying Pt-based catalyst and sputter coating of Cr/Au electrodes on both sides of the membrane. The MEA delivered 5 times higher power density (102 mW/cm2 versus 20 mW/cm2) than previously reported.