高速时间分辨中子CT高衬度探测、定量分析燃料电池活性全区域分层（膜电极和阴阳极流道）液态水量和分布

High－speed 4D neutron computed tomographyfor quantifying water dynamics in polymer electrolyte fuel cells

Ralf F． Ziesche

Jennifer Hack

Lara Rasha

Maximilian Maier

Chun Tan

Thomas M． M． Heenan

Henning Markötter

Nikolay Kardjilov

Ingo Manke

Winfried Kockelmann

Dan J． L． Brett

Paul R． Shearing

Abstract

In recent years， low－temperature polymerelectrolyte fuel cells have become an increasingly important pillar in azero－carbon strategy for curbing climate change， with their potential to powermultiscale stationary and mobile applications． The performance improvement is aparticular focus of research and engineering roadmaps， with water managementbeing one of the major areas of interest for development． Appropriatecharacterisation tools for mapping the evolution， motion and removal ofwater are of high importance to tackle shortcomings． This articledemonstrates the development of a 4D high－speed neutron imaging technique，which enables a quantitative analysis of the local water evolution． 4Dvisualisation allows the time－resolved studies of droplet formation in the flowfields and water quantification in various cell parts． Performance parametersfor water management are identified that offer a method of cell classification，which will， in turn， support computer modelling and the engineering ofnext－generation flow field designs．

XCT技术研究水传递过程的缺陷：

1．there are limitations of imaging water transport  with X－rays due to the poor contrast between water and carbon fibres of the  GDL， 低衬度

2．a limited FoV that is a trade－off between  high spatial and temporal resolution区域有限

3．radiation induced material damage by  high intensity synchrotron X－ray beams， often leading to a drop in cell  performance． 辐射破坏

the non－destructive nature of neutrons  allows （owing to the lack of radiation damage） long－term measurements in steady－state  operation and the study of cell operation under

extreme conditions such as high or low  temperatures， and with water changing to vapour or ice

Fig1：Single  serpentine flow field design， fuel cell and imaging processing． a The flow  field design incorporated in the cell endplates for both anode and cathode  with hydrogen and air in－ and outlets， respectively． b Cell assembly on the  rotation stage using nylon screws and nuts． c Tomograms were collected in 40s during each of ±370° forward and  backward rotations in the neutron beam with an L／D collimation of ca． 70． d  Data processing， including flat fielding projections， generation of  time－dependent sinograms （below） with water volume build－up （increasingly  dark grey values from 0 to 12）， 3D reconstruction， followed by water  quantification in the anode and cathode flow fields， and in the MEA．

goniometer：测角器

L／D beam divergence：光束发散度，标准名称是中子束有效准直比

collimation：准直比

GB∕T 31362－2015 无损检测 热中子照相检测 中子束L／D比的测定

sinogram：正弦图（应该指的是恒速运动投影出来的图类似正弦）

tomogram：X线断层照片

Fig2：Operating parameters and segmented water evolution in the single  serpentine anode and cathode flow fields． a Voltage curves for current holds  from 100mAcm−2 to 700mAcm−2； b Current curves for potential hold values 0．7， 0．5 and 0．3V． c Water volume  build up during cell operation at 400mAcm−2 in the anode and cathode channels． There  is less water formation in the anode channel than in the cathode， with a  higher concentration in the lower cell section． The cathode displays a  more homogeneous water distribution with a slightly increasing water  volume gradient towards the bottom of the cell． The MEA exhibits a  steady accumulation of water in the first 500s before  stabilising．

这里区分了两个流道和膜电极。

文中后面没有对沿着流道不同长度位置液态水的分布仿真和实际测定结果进行对比，有些遗憾。

Fig．  3 Water volumes in the cathode and anode flow fields during constant current  and potential hold． The water volumes increase with increasing current  density and decreasing potential for the cathode （a， b） and anode （c， d） side  with a greater water volume accumulating at the cathode． After a  short time， the water amount appears to reach a maximum dependent on the  current and potential height before starting to expel water． Graphs e and  f show the water volume changes as a function of the current and potential，  respectively， for anode and cathode flow fields． The slopes of the linear  regressions represent the current／voltage and time－dependent water volume  changes．

0．7A／cm2时恒流条件下流道中的液态水量的测定值直到450s才保持稳定，而电压值在100s左右就变化不明显了。我们常规说的稳态是用电压的稳定状态，要么就是操作条件的稳定状态。

而低电流密度下稳定时长更长。

Fig． 4 MEA water evolution， calculation  of the current and potential－dependent water formation and time to reach the  water volume equilibrium

a and b The volume increase of the water  generated inside the MEA for constant current density and potential，  respectively． Higher currents／ lower potentials cause a faster water uptake，  which saturates for higher operational cell power． c and d Determine the  maximal water volumes in the MEA． To use the same saturation function the  potential values in d are reversed with a maximal open circuit potential of  1．0V． e and f Evaluates the time needed to reach the water  equilibrium for given current and potential densities of 95 ％ and 99 ％ of  equilibrium， with corresponding fit functions for e． A fit for f is not  possible due to the large uncertainties and small number of measurements．

0．7A／cm2时恒流条件下膜电极中的液态水量的测定值稳定和流道中的液态水量时间相当。

Fig． 5 Theoretical and observed water  evolution inside the single serpentine fuel cell a and b The theoretical and  experimental water volumes for different current densities and potentials as  dashed and solid lines， respectively， normalised to an active area of 1cm2． c and d Compare theoretical and experimentally determined  time－dependent water increases over the current and potential．

The experimental water volumes increase  by about three quarters， as approximately one－quarter of the H2O is expelled  from the cell in the gas stream．