Durability-Limiting Factor of Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) hold the promise of low-cost energy conversion. However, one of the most significant challenges in the development of AEMFCs has been their limited durability, ca. < 1,000 h, when compared to proton exchange membrane fuel cells (PEMFC). The most common explanations for loss in fuel cell performance (AEM degradation, (bi)carbonation, water management, and catalyst aggregation) do not fully explain the irreversible performance degradation that limits the durability of AEMFCs. In our study, we set out to identify the source or causes of that performance loss in AEMFCs. To mitigate known sources of degradation, we applied fully humidified conditions and periodically replenished the cell with a dilute NaOH solution. To resolve the current density dependence of cell degradation we conducted several extended duration experiments at different current densities and observed the lifetimes of those cells. Finally, we conducted a rotating disk electrode (RDE) and 1H NMR spectroscopic experiments to identify the durability-limiting factor. We found that when controlling for the known mechanism of performance loss significant and accelerating performance degradation was present. This pointed to a previously unreported means of performance loss. Lower current densities (higher operating potential) resulted in decreased cell lifetime. These tests led to the electrode rather than membrane degradation as the source for performance loss. Our studies with RDE and 1H NMR experiments identified the generation of phenolic compounds that accumulate at the cathode throughout fuel cell operation. Once produced, the phenolic groups remain tethered to the ionomer, and cannot be removed by replenishment of the cell with a caustic solution. While quick recovery of cell performance occurs after caustic treatment continued to fuel cell operation inevitably leads to more phenol group formation that consumes quaternary ammonium counter ions. Eventually, these compounds reach a critical concentration that results in cell failure. The proposed mechanism is consistent with the degradation behaviors during our long-term test. Finally, we will discuss the mitigation strategy of the observed AEMFC degradation.