Dual Electronic and Li ⁺ Ion Conducting Ytterbium–Hexaazatrinaphthalene Metal–Organic Framework and Its Carbon Nanotube Composites

Abstract

Despite spectacular recent advances in electronic metal-organic frameworks (MOFs) that exhibit either electronic or ionic conductivity, mixed ionic-electronic conducting (MIEC) MOFs with dual electron and ion transport capabilities are rare yet crucial for future energy storage and electrochemical applications. Here, we present a modular design strategy for MOF-based MIEC materials by combining a Li⁺-ion-conducting ytterbium-hexahydroxy-hexaazatrinaphthalene (Yb-HOHATN) framework based on an N-heteroaromatic ligand with electrically conductive multiwalled carbon nanotubes (CNTs), yielding Yb-HOHATN@CNT composites with impressive dual conduction properties. The Yb-HOHATN MOF features hexagonal channels decorated with Lewis basic N-sites that facilitate Li⁺ ion binding and transport, generating excellent ionic conductivity (7.3 × 10^-5 S/cm), while the ligand π-stacks support through-space charge transfer, leading to modest electrical conductivity (9.8 × 10^-8 S/cm). Solvothermal growth of Yb-HOHATN MOF on electrically conducting multiwalled carbon nanotubes (CNTs) yielded porous, crystalline Yb-HOHATN@CNT composites that displayed significantly improved electronic conductivity (5.4 × 10^-3 S/cm). The Li⁺-doped Yb-HOHATN@CNT composite simultaneously exhibited impressive electronic and ionic conductivities (1.1 × 10^-2 and 4.03 × 10^-6 S/cm, respectively), emerging as an effective MIEC. This work not only presents an intriguing dual ionic-electronic conducting MOF@CNT composite but also provides a generalizable blueprint for crystalline MIECs that seamlessly combine the Li⁺ ion-transport capabilities of heteroatomic ligand-based MOFs with the electronic conductivity of CNTs, thereby unveiling MOF@CNT composites as a promising platform for advanced energy storage and electronic technologies.