Oxidatively Doped Tetrathiafulvalene-Based Metal-Organic Frameworks for High Specific Energy of Supercapatteries

Poor electrical conductivity and instability of metal-organic frameworks (MOFs) have limited their energy storage and conversion efficiency. In this work, we report the application of oxidatively doped tetrathiafulvalene (TTF)-based MOFs for high-performance electrodes in supercapatteries. Two isostructural MOFs, formulated as [M(py-TTF-py)(BPDC)]center dot 2H2O (M = NiII (1), ZnII (2); py-TTF-py = 2,6-bis(4 '-pyridyl)TTF; H2BPDC = biphenyl-4,4 '-dicarboxylic acid), are crystallographically characterized. The structural analyses show that the two MOFs possess a three-dimensional 8-fold interpenetrating diamond-like topology, which is the first example for TTF-based dual-ligand MOFs. Upon iodine treatment, MOFs 1 and 2 are converted into oxidatively doped 1-ox and 2-ox with high crystallinity. The electrical conductivity of 1-ox and 2-ox is significantly increased by six similar to seven orders of magnitude. Benefiting from the unique structure and the pronounced development of electrical conductivity, the specific capacities reach 833.2 and 828.3 C g-1 at a specific current of 1 A g-1 for 1-ox and 2-ox, respectively. When used as a battery-type positrode to assemble a supercapattery, the AC parallel to 1-ox and AC parallel to 2-ox (AC = activated carbon) present an energy density of 90.3 and 83.0 Wh kg-1 at a power density of 1.18 kW kg-1 and great cycling stability with 82% of original capacity and 92% columbic efficiency retention after 10,000 cycles. Ex situ characterization illustrates the ligand-dominated mechanism in the charge/discharge processes. The excellent electrochemical performances of 1-ox and 2-ox are rarely reported for supercapatteries, illustrating that the construction of unique highly dense and robust structures of MOFs followed by postsynthetic oxidative doping is an effective approach to fabricate MOF-based electrode materials.

Automated summary

In this work, oxidatively doped tetrathiafulvalene (TTF)-based metal-organic frameworks (MOFs) were used as high-performance electrodes in supercapatteries. These MOFs exhibited significantly increased electrical conductivity and high specific capacities. When used as positrodes in a supercapattery, they showed great energy density and cycling stability. The results indicate that constructing unique dense and robust structures of MOFs followed by postsynthetic oxidative doping is an effective approach for fabricating MOF-based electrode materials.