High-performance organic pseudocapacitors via molecular contortion

Pseudocapacitors harness unique charge-storage mechanisms to enable high-capacity, rapidly cycling devices. Here we describe an organic system composed of perylene diimide and hexaazatrinaphthylene exhibiting a specific capacitance of 689 F g^-1 at a rate of 0.5 A g^-1, stability over 50,000 cycles, and unprecedented performance at rates as high as 75 A g^-1. We incorporate the material into two-electrode devices for a practical demonstration of its potential in next-generation energy-storage systems. We identify the source of this exceptionally high rate charge storage as surface-mediated pseudocapacitance, through a combination of spectroscopic, computational and electrochemical measurements. By underscoring the importance of molecular contortion and complementary electronic attributes in the selection of molecular components, these results provide a general strategy for the creation of organic high-performance energy-storage materials.

Temperature controlled diffusion of hydroxide ions in 1D channels of Ni-MOF-74 for its complete conformal hydrolysis to hierarchical Ni(OH)2 supercapacitor electrodes

Conformal hydrolysis of MOF precursors is a promising strategy to prepare hierarchical metal hydroxide electrode materials on a large scale with low cost and high efficiency. However, a complete transformation is challenging due to the normal "outside-in" conversion process. After studying the hydrolysis of Ni-MOF-74, which has regular 1D channels, we suggest that the transformation to Ni(OH)2 can occur simultaneously outside and within the precursor depending on the treatment temperature. Molecular dynamics simulations reveal that a higher temperature weakens the steric effects of OH- ions and facilitates the diffusion in the regular channels, and therefore, a complete transformation from Ni-MOF-74 to Ni(OH)2 is achieved. It is for the first time demonstrated that the 1D channels of MOFs are utilized for the complete conformal hydrolysis of Ni-MOF-74 to Ni(OH)2 electrode materials. Meanwhile, we also perform pioneering work illustrating that the complete conformal hydrolysis is the key to the improved supercapacitor performances of the MOF-derived Ni(OH)2 electrodes. The prepared Ni(OH)2 electrode under the optimized conditions has a specific capacity of 713.2 C g-1 at a current density of 1 A g-1, which is at least 28% larger than those of the Ni(OH)2 prepared at other temperatures. The detailed analyses based on CV and EIS of the obtained Ni(OH)2 electrodes indicate that the residual MOFs within electrodes due to incomplete hydrolysis significantly influence the diffusion length and diffusion efficiency of OH-, drastically lowering the supercapacitor performances.

Engineering PPy decorated MnCo2O4 urchins for quasi-solid-state hybrid capacitors

In this work, three dimensional PPy decorated MnCo₂O₄ urchins on Ni foam are fabricated via a hydrothermal strategy and an electro-polymerization process.

In Situ Growth of Layered Bimetallic ZnCo Hydroxide Nanosheets for High-Performance All-Solid-State Pseudocapacitor

Two-dimensional (2D) hydroxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Herein, we report ZnCo_1.5(OH)_4.5Cl_0.5·0.45H₂O nanosheets (thickness ∼30 nm) which crystallize in a layered structure and exhibit a high specific capacitance of 3946.5 F g^-1 at 3 A g^-1 for an electrochemical pseudocapacitor. ZnCo_1.5(OH)_4.5Cl_0.5·0.45H₂O was synthesized by a homogeneous precipitation method and spontaneously crystallized into 2D nanosheets in well-defined hexagonal morphology with crystal structure revealed by synchrotron X-ray powder diffraction data analysis. In situ growth of ZnCo_1.5(OH)_4.5Cl_0.5·0.45H₂O nanosheet arrays on conductive Ni foam substrate was successfully realized. Asymmetric supercapacitors based on ZnCo_1.5(OH)_4.5Cl_0.5·0.45H₂O nanosheets @Ni foam// PVA, KOH//reduced graphene oxide exhibits a high energy density of 114.8 Wh kg^-1 at an average power density of 643.8 W kg^-1, which surpasses most of the reported all-solid-state supercapacitors based on carbonaceous materials, transition metal oxides/hydroxides, and MXenes. Furthermore, a supercapacitor constructed from ZnCo_1.5(OH)_4.5Cl_0.5·0.45H₂O nanosheets@PET substrate shows excellent flexibility and mechanical stability. This study provides layered bimetallic hydroxide nanosheets as promising electroactive materials for flexible, solid-state energy storage devices, presenting the best reported performance to date.

Optimizing crystallinity and porosity of hierarchical Ni(OH)2 through conformal transformation of metal–organic framework template for supercapacitor applications

Conformal hydrolysis of MOFs (metal–organic frameworks) yields high-performance supercapacitor electrodes.