Recently, Profs. Jing-Lin Zuo and Mengning Ding in Nanjing University and Prof. Ya-Qian Lan inNanjing Normal University have made the important progress on the studies of conductive 2D metal–organic frameworks (MOFs). They exploited a redox-active and superprotonic conductive 2D MOF, and developed a new conducting mechanism of “coupled ionic/pseudo-capacitive conduction” from an ionic conductive MOF material. The related work entitled “High Electrical Conductivity in a 2D MOF with Intrinsic Superprotonic Conduction and Interfacial Pseudo-Capacitance” has been published in Matter (doi: 10.1016/j.matt.2019.12.018) on Jan. 22. The Co-first authors are Dr. Jian Su, Miss Wen He and Dr. Xiao-Min Li. Prof. Jing-Lin Zuo, Prof. Mengning Ding and Prof. Ya-Qian Lan are the corresponding authors. Dr. Lei Sun in the Northwestern University has involved in this work.
Porous MOFs combining both high protonic and electrical conductivity may bring unprecedented opportunities to clean energy technologies (such as in fuel cells). Guided by topology, they report two 2D MOFs (In-m-TTFTB and In-TTFOC in Figure 1) constructed via combination of [In(COO)4]− metal nodes and tetratopic tetrathiafulvalene (TTF)-based linkers, with ultrahigh ionic conductivity (6.66 × 10−4 and 1.30 × 10−2 S cm−1 at 303 K, 98% RH for In-m-TTFTB and In-TTFOC, respectively). The high proton conductivity ofIn-TTFOCresults from an efficient proton conduction pathway consisting of dimethylammonium cation, water molecules, and uncoordinated carboxylic acid groups in its structure. The low activation energy Ea indicates the Grotthuss mechanism. Additionally, a high electrical conductivity was simultaneously achieved with the pure protonic nature of the 2D MOF In-TTFOC. Further studies on the relationship between solid state direct current cyclic voltammetry and non-linear I‒V behavior, suggesting that the electrical conduction at the MOF-metal interface is enabled by the redox-switchable behavior of the TTF-based ligands. The result of electrical conduction indicating a full integration of ionic conduction into the overall electrical circuit. However, for pure ionically conductive materials (such as PEDOT), the lack of interfacial charge transfer pathway usually blocks the conduction pathway, and therefore only lead to an overall capacitive charging behavior.So, the authors propose that the unique redox-switchable ligands TTF can serve as the electrochemically active component, and the corresponding switches between its oxidation states provide a Faradaic current (as also proved by solid state CV) that is high enough to match up to the ionic current density in the MOF channel. Such interfacial Faradaic process can also be described as ligand-enabled pseudo-capacitance. This enables efficient charge transfer between the MOF and the metal electrode in an electrical device.
In summary, these results provide solid experimental evidence to exclude the possibility of the interfacial decomposition in the electrical measurement and confirms the Ionic-Capacitive conduction mechanism. This unique charge transport mechanism, protonic/pseudo-capacitance coupling, offers a new strategy for utilizing the ionic conductivity from MOFs to construct functional electronic devices. Thus, this project elaborates the possible principles for realizing efficient proton and electrical conduction in MOFs, extending their future applications to a variety of functional electronic, neuromorphic, and energy conversion/storage devices.
This work was supported by the State Key Laboratory of Coordination Chemistry, the Collaborative Innovation Center of Advanced Microstructures, the Key Laboratory of Mesoscopic Chemistry, the National Basic Research Program of China, the National Natural Science Foundation of China, and Natural Science Foundation of Jiangsu Province.
Figure 1. Structures of 2D MOFs In-m-TTFTB and In-TTFOC.
Figure 2. Conductive measurements and “Coupled Ionic/Pseudo-Capacitive Conduction” Mechanism for In-TTFOC.