Intellectual Merit. Polymer electrolyte fuel cells (PEFCs) are environmentally friendly electrochemical energy conversion devices with applications in the transportation, stationary power, portable electronics and military sectors. An important issue presently hindering PEFC commercialization is the oxidative degradation of polymer electrolyte membranes (PEMs, an important PEFC component) during fuel cell operation. The reactive oxygen species (ROS) responsible for PEM degradation include hydrogen peroxide (H2O2; indirect involvement) and free radicals derived from H2O2, including hydroxyl and hydroperoxyl radicals.
The proposed research will shed light on the issue of oxidative PEM degradation from a fundamental viewpoint. The rate of generation of ROS responsible for oxidative degradation of PEMs will be estimated in situ during fuel cell operation. The influence of operating parameters such as temperature, relative humidity, reactant gas concentration, PEM thickness, and electrode potential on the rate of generation of ROS will be determined through carefully controlled experiments. The macroscopic rate of PEM degradation will be independently estimated through fluoride emission rate (FER) measurements and will be correlated to the rate of generation of ROS under various operating conditions.
The free radical ROS directly responsible for PEM degradation are highly transient in nature with lifetimes of microseconds or less. Hence, estimating their generation rates during operation represents a considerable challenge. The proposed research will facilitate the design and implementation of a fluorescence spectroscopy based diagnostic technique to acquire ROS generation rates. A thin optical fiber will be embedded within the PEM, which in turn will contain a selective fluorescence agent. The modified PEMs will be used to prepare membrane electrode assemblies and will be tested at the desired operating conditions. In situ fluorescence spectroscopy will be used to probe the PEM environs during operation. Free radical ROS generated within the PEM will interact with the fluorescence agent, leading to a change in fluorescence signal. The rate of change of fluorescence will be used as a means of quantifying free radical ROS generation rates within the PEM.
Approaches for PEM degradation mitigation will also be studied as part of the proposed research. Non-stoichiometric rare earth metal oxide nanoparticles have the ability to capture free radicals by utilizing oxygen vacancies in their lattice; these vacancies can be regenerated in acidic media. The ability of cerium oxide nanoparticles (introduced into the PEM) to function as regenerative free radical scavengers will be investigated through ex situ and in situ fluorescence studies as a function of nanoparticle size, nanoparticle concentration, and pH.
Broader Impact. This study will enhance our understanding of fundamental aspects of the oxidative PEM degradation process and will provide new data on the rate of generation of ROS responsible for PEM degradation within an operating PEM. The fluorescence spectroscopy based diagnostic tool developed as part of this research can be used to study ROS generation rates and to establish degradation mechanisms in alternate PEM /electrocatalyst systems. The fundamental structure-property relationships obtained in this study using cerium oxide will guide the future design of superior regenerative free radical scavengers. In addition to research advancements, this proposal includes a comprehensive educational and outreach component that is motivated by the need to promote fuel cell technology and the concept of renewable energy economies. This need will be addressed at several levels by: 1) developing a course on fuel cell technology with a significant hands-on laboratory component and an emphasis on inductive learning; 2) introducing educational modules that relate experimentally observed fuel cell phenomena to traditional chemical engineering concepts into the freshman curriculum; 3) presenting lecture demonstrations in inner-city high schools in Chicago; and 4) promoting integration of research and education through graduate and undergraduate student mentorship.
