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.
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