电流密度、温度、湿度分区电池研究燃料电池内部湿度分布，湿度对抗反极催化剂耐久性的影响

Study of relative humidity on durability ofthe reversal tolerant proton exchange membrane fuel cell anode using asegmented cell

Yajun Wang

Xiaojun Xie

Chang Zhou

Qi Feng

Yong Zhou

Xiao－Zi Yuan

Jiaoyan Xu

Jiantao Fan

Lin Zeng

Hui Li

Haijiang Wang

Abstract

Cell voltage reversal resulted fromhydrogen starvation at the anode is one of the factors that exacerbate theoverall degradation of proton exchange membrane fuel cells （PEMFCs）． Aneffective mitigation strategy is to involve oxygen evolution reaction （OER）catalysts into the anode to facilitate water electrolysis against carboncorrosion． As such， this paper aims to study the influence of relativehumidity （RH） on the performance and durability of reversal－tolerant－anodes（RTAs） during hydrogen starvation． An advanced segmented technique isemployed to examine the coupling reactions by simultaneously measuringcurrent density， RH and temperature in a fuel cell with a large active area．It is found that the inlet of RTAs undergoes degradation earlier than theoutlet and the membrane electrode assembly （MEA） with a RTA has an optimalhumidity during cell reversal． Results also show that the failure of theRTA MEA is due to a loss of electron conduction medium rather than thedeactivation of the OER catalyst． In addition， this work highlights theimportance of plate flow field design and the OER catalyst gradient design ofthe RTA MEA．

To guarantee a reliable measurement， a commercial－grade  integrated RH／T sensor （SENSIRION SHT31－DIS） is used during all the tests．

As shown in Fig． 3， a voltage source of  2．8 V is connected in series with the cell to prevent test station shutdown  when a negative voltage is detected due to cell voltage reversal．

The cathode is pumped with  fully－humidified compressed air and the anode is supplied with fully－humidified  nitrogen through the test station．

Fig． 1． An illustration of the PEMFC  operation under various conditions． Electrochemical reactions of the anode  and cathode under normal gas supply conditions are shown on the left side．  Electrochemical reactions of a MEA with and without RTA under hydrogen  starvation conditions are shown on the right side． Note that the dominant  reactions are highlighted．

Fig． 2． Schematic diagram of bipolar  plates and segments： （a） Current mapping cathode plate． （b） RH and temperature  measuring anode plate． （c） Schematic diagram of the segments and co－flow gas  supplies． S1 represents Segment 1； S2 represents Segment 2； and so on．

As shown in Fig． 2（c）， the anode， cathode  and coolant of the segmented fuel cell were fed in co－flow directions．

Table 1 The geometrical dimensions of  flow fields．

Fig． 3． Assembly diagram of the cell and  electrical connection of the testing system．

Fig． 4． （a） Comparison of polarization  curves between the segmented cell and the normal cell． （b） Voltage detected  by the test station during the cell reversal test．

Fig． 5． The current density （（a）–（c））， RH  （（d）–（f）） and temperature （（g）–（i）） change curves of different segments： （a），  （d） and （g） show the segments near the gas inlet； （b）， （e） and （h） show the  segments in the middle； and （c）， （f） and （i） show the segments near the gas  outlet．

Fig． 6． Electron micrographs of the  surface and cross－section topography of the RTA catalyst layer： （a） and （b）  are SEM images of RTA catalyst’s surface topography before and after the  voltage reversal test， respectively； （c） and （d） are SEM images of RTA  catalyst layer’s cross－section before and after the voltage reversal test，

respectively．

Fig． 7． The current density and  corresponding RH of each row segments on the anode side： （a） The segments in  row one， （b） The segments in row two， （c） The segments in row three， （d） The  segments in row four． Note that the cell was laid on its side during testing．

As can be seen from Fig． 5（d）–（f） and  Fig． 7（d）， the RHs of the bottom segments are higher than those of the upper  rows， and they fluctuate wildly during testing． This can be explained by the  orientation of the test cell， which is laid on its side during operation．  Under gravity effects， water on the anode and the cathode sides aggregates to  the bottom of the cell and causes a higher RH． Additionally， the RH  fluctuation of the bottom segments are a direct result of the continuous gas  purging． Obviously， the current density of the bottom segments declines  earlier than that of other segments due to the higher RH， indicating that the  bottom of the RTA degrade preferentially． Based on the above analysis， it  can be concluded that both appropriate reduction of anode RH and gradient  design of RTA can prolong the durability of the cell under hydrogen  starvation． Through gradient design， the distribution of IrO2 can be  optimized by containing more IrO2 at the higher RH region while keeping the  overall IrO2 content identical．

Fig． 8． The IrO2 content and durability  of MEAs： （a） and （b） are the schematic diagrams of the IrO2 content for RTA  MEA sample ＃1 and sample ＃2， respectively． （c）

and （d） are the degradation curves of a  conventional non－RTA MEA and RTA MEAs （sample ＃1 and sample ＃2） under fuel  starvation， respectively．

Conclusions

In this study， a novel segmented cell has  been proposed as an in situ diagnostic tool to gain a better understanding of  the internal reactant activity of a PEMFC． Using this technique， the  distributions of current， RH and temperature can simultaneously be measured  during the entire process of hydrogen starvation． This can be operated in a  single PEMFC with a large active area， as well as a stack．

The key finding from this research is  that the durability of the RTA has a strong correlation with the gas RH． An  RH over 55％ adversely affects the durability of RTA when hydrogen starvation  occurs． The segments near the gas inlet degrade preferentially due to  severe carbon corrosion resulted from the higher RH near the inlet． This  repeatable finding has not been previously disclosed and requires a modest  operating condition of RH to mitigate the degradation of the RTA MEA．

In addition， the results shed light on  the gradient design of the RTA MEA． Specifically， a higher amount of IrO2  near the gas inlet can prolong the reversal tolerance time while maintaining  a total overall content of IrO2． With respect to hydrogen starvation， this  work also highlights the importance of flow field design and fuel cell  stack orientation to mitigate the non－uniform distribution of RH and flooding．