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

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燃料电池中离聚物降解速度和反应性活性氧物种的关联实验：原位荧光光谱的应用

In situ fluorescence spectroscopycorrelates ionomer degradation to reactive oxygen species generation in anoperating fuel cell

Venkateshkumar Prabhakaran

Christopher G． Arges

Vijay Ramani

Abstract

The rate of generation of reactive oxygenspecies （ROS） within the polymer electrolyte membrane （PEM） of an operatingproton exchange member fuel cell （PEMFC） was monitored using in situ fluorescence spectroscopy． Amodified barrier layer was introduced between the PEM and the electrocatalystlayer to eliminate metal–dye interactions and fluorescence resonance energytransfer （FRET） effects during measurements． Standard fuel cell operatingparameters （temperature， relative humidity， and electrode potential） weresystematically varied to evaluate their influence on the rate of ROS generationduring PEMFC operation． Independently， the macroscopic rate of PEM degradationwas measured by monitoring the fluoride ion emission rate （FER） in the effluentstream at each operating condition． The ROS generation reaction rate constant（estimated from the in situ fluorescence experiments） correlated perfectly withthe measured FER across all conditions， demonstrating unequivocally for thefirst time that a direct correlation exists between in situ ROS generation andPEM macroscopic degradation． The activation energy for ROS generation withinthe PEM was estimated to be 12．5 kJ mol−1．

Fig． 1 Membrane electrode assembly setup  employed to investigate in situ ROS generation： （a） corresponds to the setup  resulting in metal–dye interactions； （b） shows the multi－layer membrane used  to obviate metal–dye interactions during in situ ROS estimation studies， and  the corresponding single layered membrane used in independent estimates of  fluoride emission rate

Fig． 2 （a） In situ ROS generation rates  at different relative humidities – 95％， 75％ and 50％ RH at 80 C and OCV  conditions， （b） total fluoride ion concentration at the exit stream in the  corresponding time－frame．

Fig． 3 （a） In situ ROS generation rates  at different temperatures – 100 C， 80 C，60 C and 40 C at 75％ RH and OCV  conditions （b） total fluoride emission flux at the exit stream （c） Arrhenius  plots for the in situ ROS generation rate and the macroscopic rate of PEM  degradation．

Fig． 4 （a） In situ ROS generation rates  at different cathode potentials – 0．8 V， 0．6 V and 0．4 V at 80 C and 75％ RH  （b） total fluoride ion concentration at the exit stream in the corresponding  time－frame．

Fig． 5 Correlation between in situ ROS  generation rate and the macroscopic rate of PEM degradation．

Conclusions

In situ fluorescence spectroscopy （using  6CFL as the fluorescent probe） and FER measurements were employed to study  the rates of ROS generation and the macroscopic rate of PEM during PEMFC operation．  To facilitate this study in catalyzed membranes， platinum–6CFL interactions  and FRET effects within the membrane electrode assembly were eliminated by using  a barrier layer comprising a thin Nafion s membrane modified with oxidized  R6G at both membrane／electrode inter－faces． The resultant experimental setup  was employed to study the in situ ROS generation rate under various operating  conditions by varying temperature， RH， and cathode overpotential．

Independently， fluoride ion emission  fluxes were computed under identical conditions using MEAs devoid of 6CFL and  oxidized R6G， but otherwise identical in all aspects． The results obtained  from these experiments clearly demonstrated， for the first time， the cause  and effect relationship between ROS generation within the PEM and the  macroscopic degradation of the PEM． The ROS generation rate was more  pronounced at lower RH （50％ RH ＞ 75％ RH ＞ 95％ RH）， higher cell temperature  （100 C ＞ 80 C ＞ 60 C ＞ 40 C）， and higher cathode potential （0．8 V  ＞ 0．6 V ＞ 0．4 V）． The ROS generation rate correlated perfectly with the  macroscopic rate of membrane degradation across all operating condition  variants， demonstrating the robustness of this experimental method． The activation  energy of the ROS generation process was estimated to be 12．5 kJ mol－1 ．  Calculations suggested that ROS generation occurred at least partly due to H2O2  decomposition， where the H2O2 was generated during the first step of the  oxygen reduction reaction． The  increase in ROS generation rate at higher temperatures was attributed to the  improved hydrogen peroxide generation and decomposition kinetics at the  cathode and to the increased fuel crossover rate due to softening of the Nafion  membrane． Future studies will investigate how fuel starvation， dry－out  conditions， and fuel cell start－up and shut－down cycles affect ROS generation  within the PEM， and， consequently， the macroscopic degradation of the PEM．  The ROS generation rate profile across the membrane thickness will also be evaluated，  as will the efficacy of free radical scavengers in mitigating ROS generation  and the optimal location of free radical scavengers within the PEM．