Efficient transport of active species in triple-phase boundary through "Paddle-Effect" of ionomer for alkaline fuel cells

The activity of the catalyst layer is crucial for the performance of alkaline anion exchange membrane fuel cells (AEMFCs). However, the microscopic transport mechanism of active species (gas, H2O, OH-) in the triple-phase boundary (TPB), which is composed of gas, liquid, and solid catalyst particles, has not yet been elucidated so far. Herein, quaternized polyphenylene oxide (PPO) ionomers with different hydratability are synthesized and characterized by the electrochemical performance analysis and molecular dynamics simulations. The results indicate that the mass transport of active species in the TPB depends on the free flow of water and the "locked" water layer hinders the displacement and transport of active species on the catalyst surface. Furthermore, claw shaped hydrophobic substituents of the ionomers are able to agitate the locked water transport layer on the catalyst surface in a manner that resembled the motion of a "paddle" and thus enhances active species transport in the TPB. The free-water-dominated active species transport mechanism and the "paddle effect" of the claw shaped ionomer substituent lay the foundation for the construction of highly active catalyst layer.