Clean Technology 2009
S. Moghaddam, R. Masel, M. Shannon
University of Illinois at Urbana-Champaign, US
fuel cell, proton exchange membrane, nanoporous silicon, self-assembly
We report development of the first viable silicon-based proton exchange membrane (PEM). This was achieved through self-assembly of 3-mercaptopropyl-trimethoxysilane (MPTMS) molecules inside extremely high aspect ratio torturous silicon nanopores. This development can greatly benefit the micro fuel cell technology where a silicon-based MEMS/CMOS compatible PEM has long been sought. In recent years, creating a thermally and mechanically stable (i.e. non-swelling) PEM for micro fuel cells has garnered intense interest. Early efforts on fabricating silicon-based PEMs have focused on adding perfluorosulfonate ionomers such as Nafion in a silicon membrane with large pores . Unfortunately, the lack of adhesion between the perfluoro polymers and the silicon base structure, along with significant volume change of the polymers, results in failure of the membrane electrode assembly (MEA) over time. One approach to solve these problems is to covalently bond molecules with functional groups inside silicon nanopores (cf. Fig. 1). Efforts to do so have been largely unsuccessful due to difficulties in self-assembling molecules in torturous nanopores with extremely high aspect ratios. The common approach has been (e.g. in ) to soak a porous 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. But no evidence of functionalization deep within the pores has been presented; instead proton conductivity (or performance in a MEA) has been evaluated. In this study, a new technique for fabrication of relatively straight and uniform nanopores (~5 nm in diameter) in a silicon membrane (e.g. a 20 µm thick membrane) has been developed (cf. Fig. 2). The surface of the nanopores was then modified to facilitate silane-based self-assembly. A new process was developed to conduct self-assembly of MPTMS molecules on the surface of the pores. In this process, a dilute solution of MPTMS in benzene (1 mM) is used to avoid self-polymerization of the MPTMS molecules. Since one pore volume of the solution contained 3-4 orders of magnitude less molecules than necessary for complete coverage of all active sites within the pores, a setup was fabricated (cf. Fig. 3) that allows supply of solute-rich solution to the pores and extraction of the depleted solvent 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 (cf. Fig. 4). 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 developed MEA delivered (cf. Fig. 5) an order of magnitude higher power density than the state-of-the-art  (84 mW/cm2 versus17 mW/cm2).