(528f) The Investigation of Water Movement and Cell Performance of Solid State Alkaline Fuel Cells Based On the Pore Filling Membrane

Solid state alkaline fuel cells (SAFCs) have attracted tremendous attention because of several unique benefits including the usage of non-noble metal catalysts along with the restriction to carbonate precipitation in electrode when air is supplied to the cathode. Considering the cell reaction in SAFC, it is easily observed that in contrast to proton exchange membrane fuel cells (PEMFCs) water is not only generated at the anode but also reacted for the oxygen reduction reaction (ORR) at the cathode. Just this simple systematical difference leads to a significant challenge of SAFC system development because the water management methodology of PEMFC is no longer fit for SAFC system. It was confirmed that the existence and movement of water in membrane electrode assembly (MEA) reflect the significance of its optimization in the SAFC system. To focus on the challenge of water management on SAFC directly, water movement has been observed quantitatively under a real cell performance test.

For MEA fabrication, 3-methacryloylamino propyl trimethylammonium chloride (MAPTAC) pore-filling membrane (PFM) has been designed as a polymer electrolyte to control the water movement through membrane. Carbon-supported Pt catalyst powder was used as the catalyst for both anode and cathode. Poly (vinylbenzyl chloride) (p-VBC, Mn=50000) was used as the electrolyte polymer in electrodes. The humidified H₂ and O₂ were supplied to the anode and the cathode with flow rates of both 100 ml min^-1. The cell temperature was 50ºC. The RH in the cell entrance was controlled by humidifier and the RH in the cell exit was measured by a water collection test under several different current density levels for a certain time.

The water collection result showed that the whole water flux direction was from the anode to the cathode, while the direction of electro osmosis water is from the cathode to the anode because this water is dragged by OH^-. Therefore, the water diffusion from the anode to the cathode could be confirmed for the first time in this SAFC system. According to the water mass balance calculation, the RH in MEA could be calculated. The membrane resistance was calculated with different RHs. It is found that a low RH leads to a large membrane resistance. Based on this understanding, the cell performances were predicted with two extremely speculated situations with different RHs and water diffusion. The results predict that when RH is lower than 60 % the cell performance decreased significantly because of a large membrane ohmic loss and with low water diffusion. Oppositely, if the cell RH can be maintained larger than 95 % with good water diffusion, the performance could improve to a relative high level (250 mA cm^-2, 0.7 V). Based on this calculation, the suitable humidification condition has been found for this SAFC system. Under this optimized condition, a fuel cell test has been achieved and the performance is improved as model showing when the cell RH controlled above 95 % well.

Reference: [1] John R. Varcoe et al., Electrochem. Commun., 8 (2006) 839–843, [2] T. Yamaguchi et al., Journal of Membrane Science, 214 (2003) 283–292