Immobilized hindered amine radical scavenger for durability enhancement of perfluorosulfonic acid membrane in PEMFCs

Membrane durability is one of the crucial considerations that restrict the development of proton exchange membrane fuel cells (PEMFCs). The widely used and inexpensive stabilizer hindered amine 4-amino-2,2,6,6-tetramethylpiperidine (TEMP) was selected as the free radical scavenger and tethered onto the skeleton of carboxylfunctionalized metal-organic frameworks for addressing its migration problem. The resulted inorganic-organic hybrid dopant (UIO-66-TEMP) was then incorporated into the Nafion matrix to alleviate the attacks of free radicals on the membrane in the fuel cell working circumstances. Besides the improved mechanical strength and oxidative stability, and restricted water swelling and hydrogen crossover, the resulted hybrid membranes also possess excellent proton conductivity up to 176 mS cm-1, endowing them with high power density up to 612.78 mW cm-2, 1.5 times higher than recast Nafion. After accelerated durability testing for 120 h, the OCV decay rate of the hybrid membranes decreased to 0.74 mV h-1, only 40% of recast Nafion (1.89 mV h-1). Meanwhile, the optimized hybrid membrane still maintained a high peak power density of 463.62 mW cm-2 and a low hydrogen crossover density of 4.44 mA cm-2, which were superior to recast Nafion after OCV durability tests. Hindered amines can be "regenerated" by the "Denisov cycle" to ensure that they can be used sustainably in the fuel cell. The results demonstrate that the novel UIO-66-TEMP decorated with hindered amine radical scavengers are promising nanofillers for mitigating the membrane degradation and improving the proton conductivity, thereby significantly enhancing the performance and durability of PEMFCs.