Engineering Intrinsic Mixed Sodium Ionic–Electronic Conductivity in a Nickel Bis(diimine) Metal–Organic Framework

Abstract

Abstract Mixed ionic–electronic conductors (MIECs) are highly desirable for a wide range of electronic applications, however, developing single-phase MIECs with intrinsic transport properties under ambient conditions remains challenging. Herein, we report a Na+-ion and electron mixed-conducting metal–organic framework (MOF), Ni-HITP-SO3Na, by tethering anionic sulfonate side chains with Na+ counterions onto the 2,3,6,7,10,11-hexaiminotriphenylene (HITP) ligand of a d-π conjugated nickel bis(diimine)-based framework. Ni-HITP-SO3Na is highly crystalline with honeycomb networks and microporous with surface area of 412 m2 g–1. Remarkably, Ni-HITP-SO3Na exhibits dual transport without relying on conductive fillers, guest electrolytes, or external ion sources. Four-probe measurements reveal a high electronic conductivity of 0.7 ± 0.3 S cm–1, and DC polarization yields a sodium ionic conductivity of (3.7 ± 0.7) × 10–6 S cm–1 at room temperature in the solid state, which can be further improved to (1.5 ± 0.4) × 10–5 S cm–1 with the assistance of solvents. These results, to the best of our knowledge, demonstrate the first MOF that intrinsically combines electronic and sodium ionic conduction, delivering competitive mixed conductivity among reported MIECs, particularly under ambient temperature. The mixed conductivity originates from the intact d–π conjugation for electronic transport and the sulfonate-functionalized pore walls that afford low-barrier Na+ pathways via a relay transport mechanism involving transient bridging coordination between neighboring sulfonate groups. This work highlights embedding ion-transport functionality directly into conductive MOF backbones as an effective route to structurally well-defined, highly porous framework MIECs for electrochemical and iontronic devices.