氧化铈硫酸溶液中溶解现象和作为自由基淬灭添加剂燃料电池耐高电位循环效果分析：晶粒尺寸、载量、湿度对结果的影响

Effect of CeOx Crystallite Size on theChemical Stability of CeOx Nanoparticles

Dustin Banham

Siyu Ye

Tommy Cheng

Shanna Knights

S． Michael Stewart

Mahlon Wilson

Fernando Garzon

Abstract

CeOx is an excellent free radical scavengerto improve polymer electrolyte membrane durability． However， this metal oxidewill dissolve during accelerated stress testing （AST）， with the resultingcations transporting to the cathode catalyst layer （CCL） leading to performancereduction／degradation of the PEMFC． Controlling the rate of CeOxdissolution is therefore of great importance， as it may be possible to maintainsufficient Ce cations for free radical scavenging while minimizing the impactof these cations on the CCL． Here the effect of CeOx crystallite size on CeOxdissolution was investigated． Three CeOx additives were prepared having crystallitesizes of 6， 13， or 25 nm． An ex－situ method was used to evaluate thechemical stability of these three CeOx samples， as well as one commerciallyavailable CeOx． It was determined that surface area， rather than crystallitesize， is the best predictor of chemical stability． In－situ membraneelectrode assembly AST cycling was then performed， demonstrating that when lowloadings of CeOx （0．006 mg／cm2） are used， the ex－situ method correctly predictstrends in end of life （EOL） performance． Finally， it is shown that increasingthe anode RH during AST cycling leads to significantly higher EOL performancelosses．

合成过程Ceria（III）acetate＋ammonium hydroxide生成沉淀再加热焙烧。

Figure 1． XRD spectra of the four CeOx  samples．

Table I． Physical properties of the four  CeOx samples．

Figure 2． N2 sorption isotherms for the  four CeOx samples

Figure 3． Gas sorption and XRD estimated  particle sizes for the four CeOx samples．

Figure 4． TEM images for （a） LANL－200◦C and （b） LANL－800◦C．

Figure 5． （a–e） UV－vis spectra for the  five solutions of Ce3＋／Ce4＋． （f） Theoretical UV－vis spectrum for a linear  combination of the Ce3＋／Ce4＋ stock solutions in a 3：1 Ce3＋：Ce4＋ ratio．

这个图里f Theoretical ［Ce3＋］ （mM） ＝ 0．75  ［Ce4＋］ （mM） ＝ 0．025也不是3：1呀，不知道是不是应该是［Ce4＋］ （mM） ＝ 30：1。

To study the chemical stability of the  CeOx samples， the samples were dispersed in 1MH2SO4 and heated at 50◦C over a period of three days with periodic sampling for  UV－visible spectroscopy

这个图和文献可用于燃料电池的掺杂型二氧化铈：酸中溶解度低、过氧分解速度快、自由基选择性适中中是同一个标定图。

the Ce4＋ stock solution was 10 x more  dilute vs． the Ce3＋ solution due to the much stronger absorption of Ce4＋ vs．  Ce3＋

Figure 6． Concentration of （a） Ce3＋ and  （b） Ce4＋ during the three days of heating as measured by UV－vis．

和商用材料相比，LANL 800 °C释放更多的三价铈，更少的四价铈。

Table II． Total mass dissolved after 3  days of heating at 50◦C for the four CeOx  samples

Figure 7． （a） Surface area （m2／g） and （b）  crystallite size （nm） of the four CeOx samples vs． the total mass dissolved  over three days．

Figure 8． （a） Impact of conditioning time  on BOL performance and （b） impact of crystallite size and loading on BOL  performance （after 24 h of conditioning）． Temperature： 75◦C， Pressure： 136 kPa， Inlet RH： 100％， Fuel／oxidant： H2／Air．

这里：The CeOx additives were spray coated  onto the anode catalyst layer prior to assembling the MEAs．

Figure 9． （a） BOL and EOL （following 4700  cathode potential cycles from 0．6–1．3 V） performance for MEA－LANL－800◦C， MEA－Commercial－CeOx （both with a CeOx loading of 0．025  mg／cm2）， and a baseline MEA． （b） Performance loss at 0．45 A／cm2 for MEAs  containing LANL－200， 600， or 800◦C CeOx samples （CeOx  loadings of 0．006 mg／cm2）， following 4700 cathode potential cycles from  0．6–1．3 V． Temperature： 75◦C， Pressure： 136 kPa，  Inlet RH： 100％， Fuel／oxidant： H2／Air．

Figure 10． BOL and EOL performance for  MEA－LANL－800◦C and a baseline MEA，

showing the impact of anode RH （50 or  100％） on EOL performance following AST cycling． Diagnostic conditions：  Temperature： 75◦C， Pressure： 136 kPa， Inlet RH： 100％ （anode  and cathode）， Fuel／oxidant： H2／Air． All CeOx additive loadings were 0．025  mg／cm2．

Conclusions

The impact of CeOx crystallite size， and  anode relative humidity during accelerated stress testing （AST）， on membrane  electrode assembly （MEA） end of life （EOL） performance was examined． CeOx samples  were prepared using three different calcination temperatures （200， 600， and  800◦C）， leading to crystallite sizes of 6  （LANL－200◦C）， 13 （LANL－600◦C）， or  25 （LANL－800◦C） nm． A method for monitoring ex－situ  chemical stability was developed， and used to characterize the three LANL  CeOx samples， as well as one commercial CeOx．

The chemical stability was found to  increase in the following order： LANL－200◦C ＜  Commercial ＜ LANL－600◦C ＜ LANL－800◦C． Following the chemical stability tests， each of the four CeOx  samples were evaluated in－situ． MEA beginning of life （BOL） performance demonstrated  no dependence on conditioning time， crystallite size， or loading of the CeOx  additives suggesting that any initial dissolution of CeOx has a negligible  impact on performance． It was then demonstrated that when relatively high  loadings of CeOx are used （0．025 mg／cm2） in the MEA， trends in chemical  stability that are observed using the ex－situ test do not correlate with  in－situ AST testing， likely due to saturation of the cathode catalyst layer  （CCL） by Ce cations for even the most stable CeOx additive． However， when  lower CeOx loadings （0．006 mg／cm2） are used， the ex－situ AST correlates perfectly  with the in－situ AST results． Finally， anode RH （during AST cycling） was  found to have a major impact on EOL performance， with higher anode RHs  resulting in significantly more severe EOL losses， likely due to increased  CeOx dissolution．

不理解为什么没有添加失效分析的结果，湿度影响的原因是什么。