A critical component of the PEMFC is the polymeric ionomer membrane which enables proton conduction and provides a crucial barrier to both reactant gas crossover and electrical shorting. Many fuel cell systems employ thin perfluorosulfonic acid (PFSA) based membranes due to their superior mechanical and chemical stability properties. The high chemical stability of PFSA membranes is due primarily to the highly inert Teflon®-like backbone of the polymer chain. Despite the high inherent chemical stability of PFSA ionomers, the thin membranes produced from them are not immune to the chemical degradation processes that occur within the highly oxidizing environment of an operating fuel cell. Over hundreds or thousands of hours, chemical degradation processes can physically weaken a membrane to the point where it no longer provides an effective gas crossover barrier. PEM chemical degradation is the result of a complex interplay between mechanical and chemical degradation processes, both of which are strong functions of operating conditions. In particular, hot and dry operating conditions induce high rates of chemical degradation whereas RH cycling between wet and dry states causes mechanical degradation.
The primary product of a PEMFC is water, generated via the four-electron oxygen reduction reaction. However, smaller amounts of highly oxidizing species such as hydroxyl radical (•OH), and hydrogen peroxide (H2O2) are generated either chemically or electrochemically. These, and other, oxidizing species can react with non-fluorocarbon end groups or with acid side chain functional groups to degrade and weaken the polymeric membrane until causing physical breach. In the absence of effective mitigation, PEMs will survive no more than a few hundred hours operating under typical automotive duty cycles involving large humidity and potential variations. Therefore, effective chemical degradation mitigations strategies are required for PEMFCs to become a practical transportation solution. This presentation addresses the details of PFSA chemical degradations and mitigation strategies from a chemically rigorous, mechanistic perspective. There are two principal strategies to decrease the rates and impact of chemical degradation. The first involves decreasing the concentration of reactive oxidant species, or “oxidative stress”, formed in the fuel cell. The second strategy involves deactivation of the oxidants before membrane damage occurs. This approach is typically affected by fast, efficient and reversible redox chemistry. The redox approach can be accompanied by a cell performance penalty. The most effective mitigation systems employ both strategies. Minimization of oxidative stress through cell design lessens the demand on the deactivation chemistry, thereby minimaxing the impact of cell performance.
An operating PFSA-based PEMFC creates a highly aggressive chemical environment that is dominated by the oxidizing power •OH. In the PEMFC, more than 99% of •OH is deactivated via hydrogen atom abstraction reactions with H2 and H2O2, creating •H and •OOH respectively. The small fraction of •OH that escapes deactivation is responsible for the high rates chemical damage to PFSA membranes. The •OH-induced, PFSA damaging chemistry is primarily initiated by hydrogen atom abstraction from perfluorocarboxylic acid ionomer end groups. In addition, •OH acts as a chemical ratchet by inflicting low frequency, but highly consequential chain scission reactions which increase the concentration of carboxylic acid end groups thereby further increasing the rate of chemical degradation. Fortunately, PFSA lifetimes are greatly extended by the use of •OH redox quenchers such as Ce3+ and Mn2+ that efficiently reduce the radical to H2O. The successful application of these redox quenchers is due to their high activity toward •OH reduction and the rapid and benign reduction of the oxidized metal to the reduced form via reaction with ubiquitous H2O2. The greater than 300-fold activity of advantage of Ce3+ toward •OH quenching compared to PFSA hydrogen atom abstraction reactions enables large membrane lifetime extensions without significantly sacrificing performance. Because high membrane loadings (> 5% proton exchange) of Ce3+ lead to fuel cell performance losses as protons are displaced, there is a practical limit to the amount of scavenger employed. MEA lifetimes approaching 10,000 hours and beyond can be enabled with non-penalizing Ce3+ loadings by employing robust, low oxidative stress MEA designs. Furthermore, precautions must be taken to control local operating conditions and contamination levels that minimize the creation of localized regions of high oxidative stress that may exceed the antioxidant quenching capacity. Finally, the remarkable efficiency of the Ce3+ mitigation system can be viewed in terms of relative rates, determined by the rates of •OH creation and quenching. The key to achieving desired application durability depends on controlling the degree of oxidative stress at beginning of life and the additional stress created throughout FCS lifetime.