燃料电池中的三维流场设计对氧和水传质的促进作用

Flow field design with 3D geometry for proton exchange membrane fuel cells

Xiaohui Yan
Cheng Guan
Yao Zhang
Kaicheng Jiang
Guanghua Wei
Xiaojing Cheng
Shuiyun Shen
Junliang Zhang

Abstract

It has been well recognized that the power density of fuel cells is limited by two key issues known as water flooding and oxygen starvation． Since flow field plays a critical role on the mass transport in fuel cells， a flow field design enabling improved water management and enhanced oxygen transport is highly desired to address these problems． In this work， two types of flow fields with three－dimensional channel geometry are proposed and developed． One flow field is designed to own waved channels to induce local oxygen convection flux from flow channel／diffusion layer interface to catalyst layer in order to enhance the oxygen supply． The other one owns the waved channels with gradient channel depth that results in increasing flow velocity at both in－plane and through－plane directions from upstream region to downstream region， accommodating the uneven distribution of oxygen concentration． The experimental results clearly demonstrate that the 3D channel geometry is capable of improving cell performance especially at high current densities， which can be attributed to the enhanced oxygen transport and water removal as illumined by a numerical simulation．

Fig． 1． Schematic of flow fields with different 3D geometry．

Table 2 Design parameters of different flow fields．

Fig． 2． Photographs of the fabricated flow fields： （a） conventional 2D flow field； （b） 3D flow field with waved channel； （c） gradient 3D flow field with waved channel and gradually decreased channel depth．

Fig． 3． Cell performance with various flow fields tested at 80 °C， 100％ RH and 50 kPa of back pressure （H2 ： Air＝2．0 ： 2．0）．

这个性能差异有些小，在效率比较高的区域没有表现出显著差异。

如果标记各电密的HFR和iR－free会让数据更丰富

Fig． 4． EIS results with different flow fields tested at the current density of （a） 1200 mA cm−2； （b） 1600 mA cm−2．

Fig． 5． Comparison between the experimental and numerical polarization curves with （a） 2D flow field； （b） 3D flow field； （c） gradient 3D flow field．

Fig． 6． Flow velocity at through－plane direction with （a） 2D flow field； （b） 3D flow field； （c） gradient 3D flow field．

在向下缩颈的位置有TP方向流速的增加。

Fig． 7． Mass fraction of O2 with （a） 2D flow field， （b） 3D flow field， （c） gradient 3D flow field； and mass fraction of H2O with （d） 2D flow field， （e） 3D flow field， （f）gradient 3D flow field at the GDL／CL interface．

在气体扩散层和催化层界面上，无论是氧的质量分数还是水的质量分数，性能更优的流场质量分数在整个流场区的分布更加均匀，有利于电流密度的均匀化。

Fig． 8． Flow velocity at in－plane direction within the channels of （a） 2D flow field， （b） 3D flow field， （c） gradient 3D flow field； （d－f） depict the respective zoomed－in channel at downstream region．

d－f没有标坐标系，但是可以理解到在向下缩颈的位置有IP方向流速的增加。

Table 3Pressure drop and net power of fuel cells （active area – 25 cm2） with different flow fields at a current density of 2．0 A cm−2

这个表格的数据应该是仿真的结果，但是2．0 A cm−2已经处于功率下降区，实际应用过程中难以应用。在峰值功率点计算是否是这个趋势更有价值一些。

如果配合分区电流测试或者微区水分布测试可以更清楚地证明缩颈对燃料电池上是否有价值。

第二个设计和第三个设计表明如何确定缩颈的尺寸给仿真也带来一个更严峻的课题，变截面的流道在哪里开始变换，带来的优势是什么，带来的缺点是什么。

在这篇文章的设计中第一个设计流道的尺寸比较大，在母流道的尺寸比较小时缩颈的优势有多大，缩颈设计对压力和湿度的敏感性是怎么样的都是采用这种设计时需要考虑的问题。

Conclusion

In this work， two types of flow fields with 3D channel geometry are
designed and fabricated to improve fuel cell performance． Both experimental and numerical investigations were performed to illuminate
the effects of flow channel geometry． Main conclusions are listed as
follow：

（1） The waved channel geometry is able to induce convection flux at
through－plane direction， which greatly enhances oxygen supply
from flow channel to catalyst layer， resulting in a higher local
concentration and thus smaller concentration polarization．
（2） When oxygen flows through a channel wave， the z－velocity， i．e．，
flow velocity at through－plane direction， changes from positive to
negative， indicating the existence of local vortex． Such vortex is capable of removing the water accumulated in the GDL via inertial
effect that in turn improves the water management．

在电池中引入湍流也表明在固定位置流速和状态的不确定性
（3） The gradient 3D flow field can create increasing flow velocity at
both in－plane and through－plane directions along the channel
length， which is able to overcome the severe water flooding and
oxygen starvation at downstream region， and thus enabling a more
uniform current density distribution．

（4） The 3D channel geometry enlarges the pressure drop as well as
pump power due to the higher flow resistance． Nevertheless， the net
power output with 3D and gradient 3D flow fields improves by
87．4％ and 114％ relative to conventional 2D flow field at a high
current density of 2．0 A cm−2， owing to the improved oxygen
transport and water removal．