Structural and electrochemical characteristics of clathrate hydrates, and their application to energy devices

Abstract

Ionic clathrate hydrates are icelike crystalline compounds wherein hydrophilic ions incorporated with water molecules build a clathrate framework while hydrophobic ions are embedded in the framework cavities. In the framework, various polyhedral cavities are stabilized by hydrophobic guest molecules with suitable shape and size, and counter ions, i.e. a halogen and hydroxide or proton, cause the framework to be distorted, creating numerous crystal structures. Importantly, ionized frameworks exhibit many peculiar features such as metal ion encagement, co-host inclusion, excellent thermal stability and proton conductivity. In particular, $OH^-$ ions induce the water framework to be proton-deficient and accelerate the reorientation and hydrogen bond breaking processes of water molecules, leading to fast conduction of $H^+/OH^-$ ions in the hydrogen-bonded framework. On the basis of the conducting behaviors, key variables to enhance the conductivity and thermal stability in several types of clathrate for practical application to solid ionic conductors have been investigated. Additionally, a new concept for fabrication of an amperometric $H_2$ sensor based on icelike clathrate hydrates is suggested. Fast responses and recovery to $H_2$ gas indicates that clathrate hydrates would be versatile materials for energy devices. First, the unique role of the co-guest is presented when it is additionally included in a ionic $(n-Pr)_4NOH$ hydrate. The present results provide strong evidence that tetrahydrofuran (THF) serves as a promoter for greatly enhancing the ionic conductivity in ionic clathrate hydrates. The ionic conductivity of the double $2THF\cdot(n-Pr)_4NOH\cdot 32H_2O$ hydrate $(\sigma = 1.06 \times 10^{-3} S \cdot cm^{-1})$ is two orders of magnitude higher than that of THF-free $(n-Pr)_4NOH-32H_2O (\sigma = 6.01 \times 10^{-6} S\cdot cm^{-1})$ at $-30\deg C$. This implies that the cubic structure II (CS-II or sII) host lattices formed by THF inclusion can prov...