Extreme Activation of Surface Proton Hopping Conduction Mechanism via Self-Assembly of Liquid-Crystalline Discotic...

The Grotthuss mechanism is a proton conduction mechanism where proton transports through the hydrogen-bonding network of water molecules. This mechanism enables high ionic conductivity, which is well utilized in nature and some proton-active artificial devices. Since this mechanism requires cleavage and reformation of hydrogen-bonding networks, its activation energy (Ea) is generally in the range of 10-15 kJ mol-1. Aiming to create a new mechanism beyond the Grotthuss mechanism, we focused on the surface proton hopping conduction (SPHC) mechanism, where a proton hops between neighboring sulfonate groups via bound water molecules. Generally, the SPHC mechanism requires a large Ea. Our idea is that constructing densely aligned sulfonate groups should lead to small Ea (≤10 kJ mol-1) and high proton transport efficiency. To realize high-density alignment of sulfonate groups with an average distance of ca. 5 Å, we employed the self-assembly of a liquid-crystalline (LC) discotic molecule TPES. TPES self-assembled into a hexagonal columnar LC structure in the presence of an appropriate amount of water. The TPES/H2O mixtures showed a maximum proton conductivity of 3.5 × 10-1 S cm-1 at 30 °C and a small Ea of 6.0 kJ mol-1 when the water content X = 53 wt %. We confirmed that extremely fast proton conduction and a small Ea were achieved through only bound water. The dynamics of this bound water were quantitatively evaluated by QENS measurement. These results led us to conclude that the high proton conductivity in the columnar LC materials is primarily based on an extremely activated SPHC mechanism.