Hierarchically Structured Conductive Lanthanide Metal Organic Framework Nanorods for Ultrastable Flexible Magnesium Ion Capacitors

Abstract Aqueous multivalent metal ion capacitors are very attractive owing to their high capacitance, low cost, environmental benignity, and safety. Herein, hierarchically structured, conductive lanthanide metal‐organic frameworks (MOFs) assembled by nanorods for ultrastable flexible magnesium ion capacitors (MICs) is designed. Three MOFs, consisting of lanthanide metals (La, Ho, and Yb) and 2,3,6,7,10,11‐hexahydroxytriphenylene (HHTP), are structurally stabilized through the covalent bonds and electronically conductive due to π‐d conjugation. Among 3 MOFs, Ho‐HHTP electrodes in a full‐cell system achieved long‐term cyclic stability with a high capacitance retention of 65.0% after 12 000 cycles and a high Coulombic efficiency of 96.9%. Furthermore, flexible pouch cells are fabricated using Ho‐HHTP as anode, reduced graphene oxide as cathode, and poly(vinyl alcohol) (PVA)/Mg(ClO 4 ) 2 as gel electrolyte, demonstrating a low capacitance decay rate of 0.00444% per cycle during 10 000 cycles owing to the hierarchical structure and intrinsic electronic conductivity. As indicated by density functional theory and spectroscopic characterizations, Mg 2+ ions are faradaically stored in the catechol part of the ligand, and Mg 2+ inserted into the vacant Ho sites contributes to structural stability. Furthermore, operando 2D correlation Raman spectroscopy identified how Mg 2+ ions interact with the ligand of Ho‐HHTP and demonstrated the sequential change of peak change.