Characterization of fluoride (F^–) and carbon dioxide (CO₂) emissions in the exhaust from operating polymer electrolyte fuel cells (PEFCs) are crucial to determining the rates of membrane and ionomer degradation, as well as the rates of carbon corrosion in PGM-free catalysts. Most active PGM-free catalysts contain small amounts of transition metals, typically at less than 2 wt%. Transition metal ions, e.g., Fe²⁺, act as a Fenton reagent in the presence of H₂O₂ (Fe²⁺ + H₂O₂ → Fe³⁺ + OH^– + OH˙; Fe²⁺ + H₂O₂ → Fe³⁺ + H⁺ + OOH˙), producing harmful hydroperoxyl radicals (OOH˙) as a result. These radicals cause degradation of Nafion® membrane and ionomer, producing F^– as the main byproduct. By measuring the amount of emitted F^–, the extent of the membrane and ionomer degradation can be calculated. Furthermore, carbon corrosion is thought to be one of the leading causes of the decrease in the ORR activity of PGM-free catalysts due to the destruction of oxygen reduction active sites. Thus, detection of the amount of CO₂ generated during PGM-free catalyst corrosion can provide important information on the catalyst degradation mechanism. Parallel measurements of fluoride and CO₂ emissions at various fuel cell operating conditions, e.g., under varied voltage, relative humidity, and temperature, provide essential data that can be later used for the purpose of improving the durability of PGM-free catalysts.

In situ F^– and CO₂ detection systems are not commercially available. Therefore, LANL has developed a system to simultaneously measure both F^– and CO₂ emissions during fuel cell operation. The dehumidified gas is passed to a non-dispersive infrared (NDIR) detector to measure the amount of CO₂. Water is removed from the cathode exhaust by a combination of heat exchange and desiccation to avoid interference with the CO₂ measurements. The fluoride concentration is measured in the collected water using ion chromatography.

P. Zelenay, “Non-Precious Metal Fuel Cell Cathodes: Catalyst Development and Electrode Structure Design,” Hydrogen and Fuel Cells Program 2016 Annual Merit Review and Peer Evaluation Meeting, Washington, DC, June, 2016.

Hoon T. Chung et al., “Fuel Cell Performance of (Cyanamide + Polyaniline) – Iron-Carbon Cathode Catalyst,” 229^th ECS Meeting, San Diego, CA; May, 2016.