INVESTIGATION OF OXIDATIVE DEGRADATION AND DEGRADATION MITIGATION IN POLYMER ELECTROLYTE FUEL CELLS USING IN-SITU FLUORESCENCE SPECTROSCOPY

Hydrogen/air polymer electrolyte fuel cells (PEFCs) possess high efficiency and modularity. However, significant technical advances are required to facilitate their commercialization in targeted applications. A key issue is component durability under an array of adverse operating conditions. The polymer electrolyte membrane (PEM) in a PEFC is one of the components whose long-term durability is of concern since it undergoes mechanical, thermal, and chemical degradation during fuel cell operation. The chemical (oxidative) degradation processes that take place in a PEM are attributed to reactive oxygen species (ROS) that are generated in-situ during PEFC operation. It is essential to quantify the rate of ROS generation within the PEM during PEFC operation prior to proposing an effective degradation mitigation strategy. This is a daunting challenge, given the high reactivity and very short lifetime of these species. The rate of generation of ROS within the PEM of an operating PEFC was accurately measured, for the very first time, using in-situ fluorescence spectroscopy. The influence of fuel cell operating parameters (temperature, relative humidity, and electrode potential/current density) on the rate of ROS generation was studied. The ROS generation reaction rate constant (estimated from the in-situ fluorescence experiments) correlated perfectly with the macroscopic rate of PEM degradation (estimated from the ex-situ fluoride emission rate) across all conditions, demonstrating unequivocally for the first time that a direct correlation existed between in-situ ROS generation and PEM macroscopic degradation. The utility of using regenerative free radical scavengers (FRS) such as CeO2 nanoparticles to mitigate ROS induced PEM degradation was also demonstrated using xxii xxii in-situ fluorescence spectroscopy. Though CeO2 was shown to scavenge the generated ROS, its scavenging efficacy declined with time and hence it was not truly a regenerative scavenger. The FRS efficacy was found to scale with the number of surface oxygen vacancies in its non-stoichiometric lattice. The regenerative FRS activity of CeO2 nanoparticles was improved by tuning its lattice via nitrogen doping (N-doping). It was demonstrated that N-doping increased both the number of Ce3+ active clusters in the lattice and the Ce-O bond distance; these structural attributes enhanced the regenerative ROS scavenging activity of CeO2. In addition, the influence of catalyst support on PEM degradation during PEFC operation was studied. A novel and highly corrosion-resistant non-carbon catalyst support (RuO2-SiO2; RSO) developed by our group was compared against a benchmark carbonbased catalyst support (Vulcan XC 72; C). It was found that the ROS generation rate, and hence the macroscopic PEM degradation rate, was lower when RSO was used as the electrocatalyst support in place of C. In conjunction with its remarkable corrosionresistance, this finding further illustrated the viability of RSO as an outstanding PEFC electrocatalyst support. Apart from PEM degradation, the applications of fluorescence spectroscopy in the context of other electrochemical devices was also discussed. Proofof- concept studies to study the Pt dissolution rate (in PEFC electrodes) and vanadium crossover rate (in vanadium redox flow batteries) were successfully undertaken; these areas, along with probing degradation processes secondary batteries, would be rich grounds for future study.PH.D in Chemical Engineering, May 201