The effect of entropy-mediated water clustering transition on hydronium vehicle transport across a three-phase boundary: molecular dynamics simulation.

Understanding the hydronium vehicle transport mechanism within the three-phase boundary (TPB) is crucial for enhancing the efficiency of proton exchange membrane fuel cells. We employed constant-potential molecular dynamics (MD) simulations to investigate this process. Based on the particle spatial distribution, the TPB can be divided into an interfacial layer, a transport layer, and a bulk-like layer along the direction away from the electrode surface, each exhibiting distinct water structures. In the interfacial and transport layers, an intermediate water structure between linear and ring chains arises from hydronium polarization and a bridged hydrogen bond network, while the bulk-like layer shows ring tetrameric water clusters that grow with increasing potential. The transition potential is predicted using the thermodynamic entropy and energy distribution. Under potential regulation, entropy governs the nanostructure evolution by reorganizing water molecules into a more robust water network, which may in turn promote hydronium vehicle transport. The transition potential is verified from the energy distribution. These findings reveal a potential-regulated hydronium vehicle transport mechanism within the TPB, providing molecular-level insights into nanostructure evolution in cathode catalyst layers.