平板式微型無薄膜燃料電池性能分析

Abstract

摘 要 本論文使用商業套裝軟體CFDRC考慮質傳、流場、電化學效應，針對平板式無薄膜燃料電池之傳輸現象、電池性能、氧氣消耗率作模擬分析。此燃料電池分別以甲酸、氧氣溶於硫酸溶液為燃料、氧化劑，而兩溶液分別由燃料電池兩入口流入流道，在流道內相鄰成層流，取代了薄膜。而論文中以有限體積法(Finite Volume Methode)求解統御方程式，考慮改變實驗可行之參數：濃度、流量、流道外型等，探討無膜燃料電池之傳輸現象、電池性能、氧氣消耗率，找尋出能改善電池性能之有效參數，藉由本研究將有助於平板式無薄膜燃料電池之設計。研究結果顯示，陰極側傳輸現象主導平板式無薄膜燃料電池之性能，藉由高流量、高氧氣濃度或者較厚的陰極觸媒層可以有顯著改善，但高流量和陰極觸媒層厚度之影響效果有限，意味兩參數皆存在一最佳值，其原因主要還是陰極側低傳輸現象所致。此外，流道外型亦為影響電池性能之重要參數，改變流道長度、寬度即是改變反應面積，燃料電池之總電流、總功率也隨之增加；流道厚度對於燃料電池之內電阻、氧氣消耗率有顯著的影響，流道厚度薄者，內電阻越小電池性能越好，但易發生燃料穿透，所以需要高流量防止燃料穿透，然而同截面流速下流道薄者其氧化劑供給量倍減，但電池性能因內電阻變小而增加，於是氧氣消耗率隨之倍增。

ABSTRACT The transport of oxygen, the cell performance and oxidant utilization in the planar membraneless microchannel fuel cell (PM2FC) are considenred by numerical simulation in this study. The physical model including the mass transport, the flow and the electrochemistry is simulated by commercial software CFDRC. The fuel is formic acid dissolved in dilute sulfuric acid solutions. Then the oxidant is oxygen dissolved in dilute sulfuric acid solutions. Both fuel and oxidant streams enter and flow in parellel through the microchannel. In the microchannel the occurrence of laminar flow separates both streams and eliminates the need of a membrane. This study solves governing equations through finite volume methode. It considers the effects of the flow, the concentration, and the geometric size of the system to examine the transport of oxygen, the cell performance and oxidant utilization of the PM2FC and to find singnificant parameters helpful to the cell performance. The results are helpful to the design of PM2FC. The results show that the cell performance of PM2FC is mainly restricted by the transport of oxygen in the cathode, which can be improved significantly by using higher flow rate or oxygen concentration, or a thicker catalyst layer. However, the effects of the flow rate and thickness of catalyst layer are limited, which is limited by the low transport of the cathode electrode. It signifies that two parameters must exist optimal conditions. Besides, the geometry of the microchannel is an important parameter to improve the cell performance. To change the length or width of the microchannel is equal to change the reaction area, which makes the current and power of the fuel cell increase. The thickness of the microchannel influences obviously the ohmic losses and oxidant utilization. The thiner mirochannel has less ohmic losses and better cell performances but tends to have the fuel crossover, which we need high flow rate to prevent. However at the same velocity the thiner microchannel offers less flux of the oxidant stream, the cell performance increases because of more ohmic losses. Then oxidant utiliztion increases.