(147d) Sodium Silicate Based Sol-Gel Structures as Proton Exchange Membranes for Microfluidic Fuel Cells

In this work, we report a new approach to fabricating ultra-thin proton exchange membranes (PEM) that can be readily integrated to a microfluidic fuel cell. The proposed approach takes advantage of the larger capillary forces within sub-micrometer scale channels to retain nanoliter sized volumes of a precursor material between two compartments carrying the fuel and oxidant. This material is then provided a suitable physical/chemical treatment to transform it into a PEM with thicknesses of the order of 10µm. In this design, the mechanical stability of the membrane is not compromised however, as this structure is embedded between two thick glass substrates. The proposed approach to fabricating integrated PEM in microfluidic fuels cells has been demonstrated in this work by creating a sodium silicate based sol-gel structure within an array of sub-micrometer scale channels (nanochannels) that bridge two micrometer sized conduits (microchannels). The device is then operated by filling up one of the microchannels with a fuel e.g., 1M formic acid, and the other with an oxidant e.g., 0.15M KMnO₄ in 0.5M H₂SO₄. Upon bringing in contact two platinum electrodes to these solutions, a voltage is generated. In this circuitry, the porous silicate structure acts as the PEM that preferentially allows protons to pass through its pores. With our current design and the choice of fuel/ oxidant, we have been able to generate an open circuit potential of 1.3V and a maximum current of about 40µA. Interestingly, the power output of our device is an order of magnitude greater than that reported for other microfluidic fuel cells with integrated PEMs and the same choice of fuel/oxidant. Moreover, a detailed characterization of our current device shows that its power output is limited by the Ohmic resistance of the microchannels rather than that of the PEM. In this situation, a further enhancement in the maximum current deliverable by our device is possible through optimization of the microchannel design.