缓解干湿循环的机械因素造成燃料电池电极质膜失效的五种方法(其五)燃料电池电解质膜在混合模式加速测试下的原位诊断和分区降解分析

电化学能源科学与技术 2022-03-29

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燃料电池电解质膜在混合模式加速测试下的原位诊断和分区降解分析
In-situ diagnostics and degradation mapping of a mixed-mode accelerated stress test for proton exchange membranes

Yeh-HungLai

Gerald W.Fly

Abstract

With increasing availability of more durable membrane materials for proton exchange membrane fuel cells, there is a need for a more stressful test that combines chemical and mechanical stressors to enable accelerated screening of promising membrane candidates. Equally important is the need for in-situ diagnostic methods with sufficient spatial resolution that can provide insights into how membranes degrade to facilitate the development of durable fuel cell systems. In this article, we report an accelerated membrane stress test and a degradation diagnostic method that satisfy both needs. By applying high-amplitude cycles of electrical load to a fuel cell fed with low-RH reactant gases, a wide range of mechanical and chemical stressful conditions can be created within the cell which leads to rapid degradation of a mechanically robust Ion Power™ N111-IP membrane. Using an in-situ shorting/crossover diagnostic method on a segmented fuel cell fixture that provides 100 local current measurements, we are able to monitor the progression and map the degradation modes of shorting, thinning, and crossover leak over the entire membrane. Results from this test method have been validated by conventional metrics of fluoride release rates, physical crossover leak rates, pinhole mapping, and cross-sectional measurements.

Fig. 1. Exploded fuel cell assembly, including the segmented circuit board for current distribution measurement

分区电流测试设备

Fig. 2. Photograph of the segmented graphite anode flowfield plate, which contains a 10X10 array of segments that are separated by electrically insulating and gas impermeable adhesive. The gas in/out ports are located at the lower left and upper right corners.

极板

Fig. 3. A 36-min snapshot of the cell current density and high frequency resistance (HFR) profiles during the durability test in the counter-flow configuration at a cell temperature of 80 C and 50% RH at the inlets.

需要注意,加电流HFR迅速下降,而减电流HFR慢吞吞上升。干多湿少是常态。

对流是严格的对流,不是我们常规考虑重力的对流。根据HFR判断干湿状态。

It should be noted that although the cell temperature are well maintained in the flowfield plates, due to the thermal resistance from the membrane to the flowfield through the electrodes and GDL and interfaces between those layers, one would expect that the non-uniform current distribution at the wet condition would result in some temperature non-uniformity while the uniform current distribution at the dry condition should lead to uniform membrane temperature distribution.

电流密度的变化幅度在两种情况下不一样。

分区电流测试同时进行温度测试并进行对照很有意义。

Fig. 5. The average shorting and hydrogen crossover currents of the cell in the counterflow test. The shorting/crossover current measured at Diagnostic Steps 1, 2, and 3 are denoted as “SHORTING+DIFFUSIVE”, “SHORTING+DIFFUSIVE+CONVECTIVE”, and“SHORTING”, respectively. Notice the currents at Steps 1 and 2 diverge at t=7300 min.

短路电流、扩散渗氢电流(氢氮压力相等)、对流渗氢电流(氢压力大于氮压力)的区分。

7300min时有较强的对流产生,表明有针孔产生。

Fig. 6. The (a) thinning, (b) leak rate, and (c) shorting maps at t ¼ 6600 min of the counter-flow test.

Fig. 7. The (a) thinning, (b) leak rate, and (c) shorting maps at t=8000 min of the counter-flow test.

分区电流密度比较大的地方对应对流渗氢电流比较大。

氟离子释放一直持续,当膜平均减薄了20%时有局部针孔出现,而且针孔造成的对流渗氢电流增加很快。

Fig. 9. The comparison of the bubble pinhole map and the crossover leak map from the counter-flow test. The locations of leaks from the bubble pinhole test are represented by the solid circle symbols.

对流渗氢电流、针孔、气泡的位置对应关系。

实验使用的是Ion Power™ N111-IP membrane (25 mm)

Fig. 10. The comparison of the remaining membrane thickness along the 5th column of Fig. 7(a) using the crossover and microscopic cross-sectional measurements. Multiple cross-sectional thickness measurements were conducted on samples taken from each segment. The square symbol represents the mean thickness. The upper and lower
tick marks represent the minimum and maximum thicknesses, respectively.

横轴对应的是

thinning转化为thickness和SEM截面法的对应关系。

相对于总厚度25微米,局部厚度减薄11微米(6600min)不会产生严重的对流。局部厚度减薄14微米(8000min)严重的对流,这时局部厚度减薄和对流就不存在严格的对应关系了。

Fig. 12. The current distribution maps under the cell current densities of (a) 0.08 A cm2 and (b) 0.8 A cm2 in the co-flow test. The arrows in the figure represent the reactant gas flow directions at the gas inlet/outlet ports.

同流条件,电流密度的分布优于该极板设计条件下的逆流条件。

11900min才会出现严重的对流。

Fig. 14. The thinning maps at (a) t=8000 min, (b) t=11,200 min, and (c)
t=11,900 min from the co-flow test.

Fig. 15. The leak rate maps at (a) t=11,200 min, (b) t=11,900 min, and (c)t=16,600 min in the co-flow test.

Fig. 16. The shorting maps at (a) t=16,600 min and (b) t=19,100 min of the co-flow test.

和逆流不同,同流会出现短路现象,但出现16600min时仍比逆流8000 min时的状态好。

Fig. 17. The averaged membrane volume loss and leak rate over time in the co-flow test. Both volume loss and leak rate are determined from the H2 crossover measurement and integrated over the 100 segments

局部针孔的出现延迟。

Fig. 18. The correlation between the leak rate and thinning at various diagnoses from the co-flow test. Each symbol represents the leak rate and thinning from a current distribution segment.

相对于总厚度25微米,局部厚度减薄13微米不会产生严重的对流。局部厚度减薄16微米对流比较严重,但这时局部厚度减薄和对流仍存在对应关系。

Conclusion

We have developed a mixed-mode accelerated stress test by
applying current cycling between 0.08 and 0.8 A cm2. The test
condition had induced both voltage cycling as well as hydration
cycling.
The test was shown to accelerate the failure of a mechanically
robust membrane, Ion Power™ N111-IP. To quantify the degree
of degradation and to differentiate degradation modes, we
utilized an in-situ shorting/crossover diagnostic method to periodically
determine the shorting resistance, thinning, and crossover
leak rate of the membrane as the test progressed. Results from the
in-situ diagnostics agreed well with conventional diagnostic metrics
such as fluoride release rates and physical crossover leak rates.
Applying this diagnostic procedure in a specialized segmented fuel cell fixture that provided 100 local current density measurements
via a 10 by 10 array of flowfield segments over 50 cm2 of active area,
we were able to monitor the degradation progression and map the
three basic types of degradation within the membrane. The
degradation maps of thinning and crossover leaks were validated
by membrane thickness measurements using the cross-sectional
microscopy and the pinhole bubble test. From the viewpoint of
achieving the greatest reduction in test times, the counter-flow
configuration is preferred. However, the greatly simplified current
and humidity distribution and a more predictable failure location
within the cell, combined with still significant test time reduction,
may make the co-flow configuration an attractive option for many.

这个结论仅限于某种设计。逆流或者同流下设计更改,比如交叉流可以带来结论的不同。分区电流原位诊断方法和混合模式加速测试是这篇文章值得学习的地方,找出设计的薄弱点,缓解干湿循环的机械因素造成燃料电池电极质膜失效,而不是逆流或者同流这一简单的结论。

Although the use of segmented fuel cell fixture has greatly
increased the ability of understanding degradation behavior and is
highly recommended if available, it is believed that the accelerated
testing and diagnostic methodology demonstrated in this article
without the use of current distribution tool should be sufficient to
provide a viable test method to enable the durability testing of
robust membranes beyond the existing tests recommended by U.S.
DOE.

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