Water-Free Proton Conduction in Hexakis(p-Phosphonatophenyl)benzene Nanochannels

We elucidate the proton conduction mechanism in self-assembling stacks of phosphonic-acid-functionalized molecules (hexakis(p-phosphonatophenyl)benzene) at different temperatures (400–600 K) and at zero humidity conditions. We employ first-principles molecular dynamics simulations in combination with large-scale force-field simulations, forming a specific arrangement of the molecules in the columnar stacks. This arrangement leaves space for quasi-one-dimensional hydrogen bond nanowires along which protons are transported. We observe spontaneous autodissociation of the phosphonic acid groups, leading to proton displacements of up to 10 Å along the nanowires. Our simulations show that there is a fast (200 fs) and a slow (3–12 ps) component in the dynamics of the hydrogen bond network, corresponding to orientation fluctuations of the hydrogen bonds and persistent long-range proton transport, respectively. Our results support the hypothesis that significant proton conduction is possible in this compound at fully dehydrated conditions and at high temperatures. In such circumstances, the material may outperform the common Nafion polymer as membrane materials for proton exchange fuel cells.