Micromachined Dense Palladium Electrodes for Thin-film Solid Acid Fuel Cells

This thesis paves the way towards the microfabrication of a solid acid electrolyte based fuel cell (µSAFC), which has a membrane electrode assembly (MEA) consisting of a thin-film of water soluble electrolyte encapsulated between two dense palladium electrode membranes. This project work investigates microfabrication techniques to realize the dense palladium membranes and to understand their suitability as hydrogen diffusive electrodes (HDEs) for the µSAFC. A thin-film transfer technique has been shown as a successful method to make defect free micron and sub-micron thick palladium membranes. 1 µm thick dense palladium membranes supported on perforated silicon gas diffusive supports (GDS) has been fabricated as the fuel cell electrodes. The supporting GDS is a silicon microsieve having straight cylindrical pores, which show low pressure drop as well as good strength. A customized recipe based on deep reactive ion etching has been developed to anisotropically etch the micropores. At 423 K and 1.5 bar hydrogen pressure on the feed side, the palladium membranes show a hydrogen permeate flux within the range 6.3 x 10-6 to 8.8 x 10-6 mol H/cm2.s and a good selectivity (> 1500) for hydrogen with respect to helium. The α-β phase change (and embrittlement) happening in palladium in the presence of hydrogen at lower temperatures (< 473 K) is thoroughly studied in this work. According to our observation, the maximum allowed hydrogen pressure on the anode side for the µSAFC operating at a temperature of 423 K (i.e. 150 °C) is ~1.5 bar. Potassium dihydrogen phosphite - KH(PO3H) has been investigated as the electrolyte for the proposed µSAFC. A Ø 10 mm and 1 mm thick compressed disc of KH(PO3H) was found to have a ex-situ proton conductivity of 3.8 x 10-3 Ω-1cm-1 at 408 K just above the superprotonic transition temperature. The performance of KH(PO3H) was also tested in a conventional (porous carbon electrodes) MEA of Ø 12 mm. For an electrolyte thickness of 0.5 mm, the measured open circuit voltage was about 0.65 V. When a current of 0.5 mA/cm2 is drawn, the operational voltage drops to 0.3 V. Thin-films of KH(PO3H) were also fabricated down to 10 µm thickness using a dip-coating technique. Impregnation of inorganic particles (~14 nm size) was helpful in suppressing random crystallization of the solid acid while drying.