組裝壓力對質子交換膜燃料電池局部反應主導機制影響研究

Investigation of the Assembly Pressure Effects on the Local Dominant Mechanism of Cell Reaction in a PEMFC

本研究以多重物理耦合模式，應用數值方法解析質子交換膜電池中反應物質與電流傳輸行為，探討電池於不同組裝壓力及操作條件下的效應，包括內部各主要物種性質分佈情形以及電化學反應變化趨勢，歸納出在各運轉條件下，控制電池局部反應的主導因素。由數值計算結果發現在反應氣體供應充足，反應速率較慢之操作條件或電極區域，電池局部電流密度隨組裝壓力變化的趨勢與過電位變化趨勢一致，顯示該處局部反應主導機制為歐姆極化。當反應速率提高，或反應氣體供應不足的區域或操作條件下，電池局部電流密度隨組裝壓力變化趨勢與氧氣濃度變化趨勢一致，使該處反應主導機制轉為濃度極化。電池之整體性能輸出為局部區域兩種主要極化現象互相影響的整合結果，在初期壓縮階段，其隨組裝壓力增加而提高，當電池發生局部濃度極化主導情況時，此趨勢將逐漸反轉，最終造成整體性能隨組裝壓力增加而降低。

In this study, the transport processes in a fuel cell are resolved througha three dimensional multi-physics model by using the technique ofcomputational fluid dynamics considering the essential conservationprinciples. Special attention is focused on the influences of assembly pressure upon local physical property distributions such as oxygenconcentration and solid phase potential field, and also its effects on cellpolarization behaviors.The results show that at operating conditions or electrode regions withsufficient reactant supply and low reaction rate, the variation trend of localcurrent density according to assembly pressure variation coincides with that ofoverpotential, indicating that local cell reactions are dominated by ohmicpolarization. With the increase of reaction rate, a lack of reactant gasemerged at specific locations and operating conditions, and the variation trendof local current density according to assembly pressure variation is consistentwith that of oxygen concentration. Therefore, the local cell reaction isdominated by the concentration polarization. The average cell performanceis the combined result of the two major mechanisms in local regions. Duringthe early compression stage, it is elevated with the increase of compressionpressure until concentration polarization starts to control the local reaction andthe tendency is gradually reversed. Finally, increasing assembly pressuredecreases cell global performance.