Pyrazine-Embedded 2D Conjugated Metal-Organic Framework with Quasi-Honeycomb Lattice for High-Capacitance Lithium-Ion Storage.

As a unique class of framework electronic materials, 2D conjugated metal-organic frameworks (2D c-MOFs) exhibit intrinsic porosity, superior electrical conductivity, and abundant active sites. These properties endow them with great potential in electrochemical lithium-ion storage. However, the development of 2D c-MOF-based capacitors has encountered a bottleneck in enhancing Li-ion storage capacitance, and the design of high-capacitance MOF electrode materials has remained a challenge. Herein, we synthesize a Cu-OHDDQP (octahydroxy-dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino[2,3-i]phenazine) 2D c-MOF with a quasi-honeycomb lattice by employing a nonplanar D-symmetric conjugated ligand embedding redox-active pyrazine moieties. The quasi-honeycomb lattice features a dual-porous tessellation of C-symmetric and C-symmetric pores. Notably, when utilized as active material for electrochemical lithium storage, Cu-OHDDQP achieves a record-high gravimetric specific capacitance among reported 2D c-MOFs of 452 F g in aqueous lithium electrolyte, along with a decent cycling stability of 90% after 1000 cycles. Such high capacitance is attributed to both the quasi-honeycomb lattice leading to higher surface area and the redox-active pyrazine moieties offering extra lithium-adsorption sites and associated pseudocapacitance. This work demonstrates that rational ligand design enables high-capacitance MOF electrodes materials, highlighting the potential of conductive MOFs for electrochemical energy technologies.