Effects of ice crystals on the proton transport in proton exchange membranes

The existence of ice crystals in the proton exchange membrane (PEM) under low-temperature conditions would affect the proton transport but remain elusive. In this work, molecular dynamics simulations were employed to systematically examine the microstructural evolution of PEMs with the presence of ice crystals and elaborate interactions between ice crystals and hydronium ion. The results show that the presence of ice significantly reduces the diffusion coefficient of hydronium ions, primarily due to the structural blocking effect of ice, which markedly disrupts the connectivity of water clusters and weakens the vehicle mechanism of proton. Additionally, ice would directly adsorb hydronium ions onto their surfaces, but also reduce the adsorption strength between sulfonate groups and hydronium ions, while these two effects are almost counterbalanced. Finally, a modified empirical model was proposed to describe how the proton conductivity reduces with the ice volume fraction.