车载大面积燃料电池电堆局部燃料饥饿衰减造成的MPL黏附催化层，裂缝、铂环铂片、阴极催化层减薄和电堆各位置衰减不均一现象

Local Fuel Starvation Degradation of an AutomotivePEMFC Full Size Stack

M． Obermaier

M． Rauber

A． Bauer

T． Lochner

F． Du

C． Scheu

Abstract

The achievement of durability targets is animportant challenge for the commercialization of fuel cell electric vehicles（FCEV）． In order to meet the requirements， knowledge about the most severedegradation mechanisms of fuel cell stacks under automotive conditions iscrucial． In the present work， degradation analysis of an automotive full sizestack is performed． Herein， we focus on defectsat the cathode catalyst layer and their interrelation including inhomogeneousadhesion of the microporous layer on the catalyst layer， crack formation，cathode catalyst layer thinning and wrinkling of the catalyst coated membrane．In addition， we report linear andcircular Pt depositions on top of the cathode catalyst layer， which have tothe best of our knowledge not been described in literature yet． For the latter，a degradation mechanism based on liquidwater formation， local fuel starvation and current density distribution at theinterface between microporous layer and cathode catalyst layer is postulated．Finally， a fast indication for stack degradation is suggested by correlatingdifferent degradation phenomena． This improved stack analysis approach allowedus to detect local differences in degradation on both cell and stack level．

An automotive full size stack of the EU Inspire  project，within the predevelopment stage was aged by applying conditions of different  automotive driving points at a fuel cell test station for approximately 100 h．

The stack as illustrated in Figure 1A，  consists of more than 350 cells with an approximate active area of 300 cm 2  each．

运行时间并不长。

Fig．1 （A） Schematic of an automotive full  size stack indicating the location of cells under investigation． （B） Location  matrix of the MEA placed on top of the CCL． The CCL shows increased microporous layer adhesion （dark region from  H2 to M6） at the anode outlet．

极板和膜电极的方向是垂直于屏幕的，堆垛是上下的。从这个描述上看这样更合理一些。

看这个图容易把歧管看成装配的端板。

Fig．2 Assembly of different degradation  phenomena observed on the CCM： （A） Optical microscopy image of CCL （top－view）  showing MPL adhesion （black） on CCL （gray）． （B） SEM image of CCL （top－view）  showing MPL adhesion （black）， crack  formation and circular and linear Pt deposition （white）．（C） 3D－optical  microscopy image showing buckling and linear wrinkling on the ACL． The inset  shows corresponding holes and cracks in the MPL． （D） SEM image of CCM cross－section  showing CCL layer thinning and formation of a Pt band close to the CCL． （E）  SEM image of a BOL CCL （top－view）． （F） SEM image of BOL CCM cross－section．

Fig．3 Different types of macroscopic Pt  depositions on top of the CCL imaged by SEM． （A） Ring－like Pt deposition． （B）  Disc－like Pt deposition． （C） Linear Pt deposition．

Fig．4 （A） Correlation of surface area  ratio of adhered MPL， CCL layer thickness （blue） and crack surface area ratio  （orange） of different segments of 6 cells within stack region 1， 6 and 8． （B）  Spatially resolved correlation of surface area ratio of adhered MPL （solid）  and CCL thickness （dashed） along segment 4 （see Fig． 1（B）） of a single cell  of region 1 （purple） and region 6 （yellow） of the stack．

Fig．5 （A） Distribution of MPL adhesion on  an exemplary cell of the stack， showing increasing degradation from anode  inlet to anode outlet． Ai／Ci and Ao／Co indicate positions of anode／cathode  inlet and outlet， respectively． （B） Horizontal development of degradation  from anode inlet to outlet region （as depicted by arrow in A） for different  cells of the stack． Region 4 and 5 show similar results as region 6 and are  excluded from the graph for reasons of clarity． （C） MPL free area of the CCL  as a measure for cell degradation at different stack positions． Increased  degradation （lower MPL free area） in the middle of the stack slightly shifted  to the end of the stack （regions 4–6） as well as at both ends of the stack （regions  1 and 8） were observed． Cells with increased degradation showed lower  performance in terms of in situ measured cell voltage．

Fig．6 Schematic of postulated formation  mechanism of Pt deposition on top of the CCL． （A） Dissolution of Pt in water  within a void （here： buckle） at elevated cathode potential due to local fuel  starvation． （B） Pt deposition close to contact area of MPL／CCL （high current  density for Pt－ion

reduction） at reduced cathode potential．

Conclusions

Different degradation phenomena were  found on cells of an automotive full size stack operated under different  automotive driving points． By correlating the observed degradation phenomena  of crack formation， CCL thinning and  Pt deposition， which are all related to local fuel starvation， to the degradation  of MPL adhesion， a fast indicator for degradation inhomogeneities in  particular for fuel starvation degradation within single cells and  between different cells of the stack were presented． Buckling （wrinkling of  the CCM） and linear wrinkling of the CL was observed continuously throughout  the cell and stack， the other degradation phenomena were found to increase  towards the anode outlet and exhibited different extent at different  locations of the stack．

An increased degradation in terms of MPL  adhesion in the second half of the stack as well as a slightly increased  degradation at both ends of the stack were detected． A potential rise at the  CCL due to fuel starvation was assumed to be the main cause of degradation．  Gas flow inhomogeneities and self－reinforcing effects due to the mutual  dependence of water accumulation and fuel starvation were thought to be  responsible for the different extents of degradation along the stack．

Finally a degradation mechanism for the  formation of Pt deposition was proposed． In brief， voids between MPL and  catalyst layer being filled with liquid water are suggested to be the  starting point of the Pt deposition． A potential rise according to the  reverse current mechanism is suggested to lead to Pt dissolution in water filled  voids．

After resupply of H2 and corresponding  decrease of potential， dissolved Pt deposits preferentially at the edge of water  filled voids． This might be both due to increased cathode potential in the  center of the void due to high in－plane electron resistance of the degraded  CCM， and an increased Pt ion concentration at the edges of the water filled  voids．