Three-Dimensional Hierarchical Core/shell Electrodes Using Highly Conformal TiO2 and Co3O4 Thin Films for High-Performance Supercapattery Devices

Redox-Active Two-Dimensional Tetrathiafulvalene-Copper Metal-Organic Framework with Boosted Electrochemical Performances for Supercapatteries

Metal-organic frameworks (MOFs) have attracted noticeable attention as promising candidates for electrochemical energy storage. However, the lack of electrical conductivity and the weak stability of most MOFs result in poor electrochemical performances. Here, a tetrathiafulvalene (TTF)-based complex, formulated as [(CuCN)₂(TTF(py)₄)] (1) (TTF-(py)₄ = tetra(4-pyridyl)-TTF), is assembled by in situ generation of coordinated CN^- from a nontoxic source. Single-crystal X-ray diffraction analysis reveals that compound 1 possesses a two-dimensional layered planar structure, which is further stacked in parallel to form a three-dimensional supramolecular framework. The planar coordination environment of 1 is the first example of a TTF-based MOF. Attributed to the unique structure and redox TTF ligand, the electrical conductivity of 1 is significantly increased by 5 orders of magnitude upon iodine treatment. The iodine-treated 1 (1-ox) electrode displays typical battery-type behavior through electrochemical characterizations. The supercapattery based on the 1-ox positrode and AC negatrode presents a high specific capacity of 266.5 C g^-1 at a specific current of 1 A g^-1 with a remarkable specific energy of 62.9 Wh kg^-1 at a specific power of 1.1 kW kg^-1. The excellent electrochemical performance of 1-ox is one of the best among those reported supercapatteries, demonstrating a new strategy for developing MOF-based electrode materials.

Boosting Pseudocapacitive Behavior of Supercapattery Electrodes by Incorporating a Schottky Junction for Ultrahigh Energy Density

Pseudo-capacitive negative electrodes remain a major bottleneck in the development of supercapacitor devices with high energy density because the electric double-layer capacitance of the negative electrodes does not match the pseudocapacitance of the corresponding positive electrodes. In the present study, a strategically improved Ni-Co-Mo sulfide is demonstrated to be a promising candidate for high energy density supercapattery devices due to its sustained pseudocapacitive charge storage mechanism. The pseudocapacitive behavior is enhanced when operating under a high current through the addition of a classical Schottky junction next to the electrode-electrolyte interface using atomic layer deposition. The Schottky junction accelerates and decelerates the diffusion of OH‒/K+ ions during the charging and discharging processes, respectively, to improve the pseudocapacitive behavior. The resulting pseudocapacitive negative electrodes exhibits a specific capacity of 2,114 C g-1 at 2 A g-1 matches almost that of the positive electrode's 2,795 C g-1 at 3 A g-1. As a result, with the equivalent contribution from the positive and negative electrodes, an energy density of 236.1 Wh kg-1 is achieved at a power density of 921.9 W kg-1 with a total active mass of 15 mg cm-2. This strategy demonstrates the possibility of producing supercapacitors that adapt well to the supercapattery zone of a Ragone plot and that are equal to batteries in terms of energy density, thus, offering a route for further advances in electrochemical energy storage and conversion processes.

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)]·2H₂O (M = Ni^II (1), Zn^II (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∼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∥1-ox and AC∥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.

New nickel-rich ternary carbonate hydroxide two-dimensional porous sheets for high-performance aqueous asymmetric supercapattery

Transition metal carbonate hydroxides (M-CHs) are promising candidates for electrode materials in supercapattery, due to their low-cost preparation and high-energy features. However, they also suffer from ionic kinetics bottlenecks without efficient morphological design. Tailoring the chemical compositions and nanostructures of electrode materials to realize high performance is significant for meeting the current demand for electrical energy storage devices. Therefore, we present a simple hydrothermal method for constructing better electrochemically active M-CHs with ternary metal components and hierarchical nanostructures that are assembled by interwoven nanosheets. Benefiting from higher contents of Ni species and superior two-dimensional/three-dimensional (2D/3D) pore structures, the fabricated cobalt-nickel-zinc carbonate hydroxides with Co/Ni/Zn molar ratios of 2:3:1 (CoNiZn-231) delivered the best specific capacity of 1130.8 C g^-1 at 1 A g^-1, decent rate performance (67.2% in 1-10 A g^-1), and excellent cycling performance (92.6% over 10,000 cycles) in comparison with the majority of mono/bimetallic materials. Then, an alkaline hybrid (CoNiZn-231//activated carbon (AC)) device is developed, which shows a high energy density of 31.62 Wh kg^-1 at a power density of 646 W kg^-1 and an excellent capacity retention of 99.27% after 10,000 cycles. Herein, the rational design of trimetallic compositions and hierarchical structures of carbonate hydroxides is described, which provides good choices for the synthesis of high-performance electrode materials in electrochemical energy storage applications.