A polydopamine coating enabling the stable cycling of MnO2 cathode materials in aqueous zinc batteries.

MnO2 is a desired cathode candidate for aqueous zinc batteries. However, their cycling stability is seriously limited by active material dissolution, and pre-addition of Mn2+ salts in electrolytes is widely required to shift the dissolution equilibrium. Herein, we synthesize a polydopamine (PDA) coated MnO2 composite material (MnO2/PDA) to realize stable cycling in zinc cells without relying on pre-added Mn2+. The functional groups on PDA exhibit strong coordination ability with the Mn active material. It not only confines dissolved species within the cathode during discharge, but also enhances their deposition back to the cathode during charge to retrieve the active material. Thanks to this effect, the cathode achieves 81.1% capacity retention after 2000 cycles at 1 A g-1 in the 1 M ZnSO4 electrolyte, superior to 37.3% with the regular MnO2 cathode. This work presents an effective strategy to realize the stable cycling of manganese oxide cathode materials in aqueous zinc batteries.

Enhancing Two-Electron Reaction Contribution in MnO2 Cathode Material by Structural Engineering for Stable Cycling in Aqueous Zn Batteries.

MnO2 is a promising cathode for aqueous Zn batteries. However, the cycling stability is seriously hindered by active material dissolution, and the pre-addition of Mn2+ salts in electrolytes is widely required. Herein, we propose a structural engineering strategy for MnO2 to enhance the capacity contribution from the reversible two-electron transfer reaction of MnO2/Mn2+ and realize stable cycling in Mn2+-free electrolytes. By compositing with MoO3, MnO2 exhibits weakened Mn-O bonds, more oxygen vacancies, spontaneous generation of structural water, and thus a lowered energy barrier for Mn release during discharge. Meanwhile, the composite material presents stronger electrostatic attractions for dissolved Mn2+, which ensures highly reversible re-deposition during charge. As a result, the mass ratios between materials undergoing reversible two-electron and one-electron transfer reactions increase from 0.85 in MnO2 to 1.68 in the MnO2/MoO3 composite material. In the ZnSO4 electrolyte, the MnO2/MoO3 cathode achieves 92.6% capacity retention after 300 cycles at 0.1 A g-1 (>1900 h), superior to 62.7% for MnO2. MnO2/MoO3 also retains 80.1% capacity after 16 000 cycles at 1 A g-1 (>3200 h). This work presents an effective path to realize stable cycling of MnO2 in Zn batteries.

Polyoxometalates-Based Metal-Organic Frameworks Made by Electrodeposition and Carbonization Methods as Cathodes and Anodes for Asymmetric Supercapacitors.

Hybrid materials have obtained well-deserved attention for energy storage devices, because they show high capacitances and high energy densities induced by the synergistic effect between complementary components. Polyoxometalate-based metal-organic frameworks (POMOFs) possess the abundant redox-active sites and ordered structures of polyoxometalates (POMs) and metal-organic frameworks (MOFs), respectively. Here, an asymmetric supercapacitor (ASC) NENU-5/PPy/60//FeMo/C was fabricated in which both its electrodes are prepared from POMOF precursors. A typical POMOF material, NENU-5, was first connected with polypyrrole (PPy) through electrodeposition to form the cathode material NENU-5/PPy. Another representative POMOFs material, PMo12 @MIL-100, was carbonized to obtain the anode material FeMo/C. Cathode NENU-5/PPy exhibited an extraordinary capacitance of 508.62 F g-1 (areal capacitance: 2034.51 mF cm-2 ). In addition, anode FeMo/C shows excellent cyclic stability attributed to its unique structure. Finally, benefiting from the outstanding capacitances and structural merits of the anode and cathode, assembled asymmetric supercapacitor NENU-5/PPy/60//FeMo/C achieves an energy density of 1.12 mWh cm-3 at a power density output of 27.78 mW cm-3 , as well as a notable life of 10 000 cycles with an capacity retention of 80.62 %. Thus, the unique ASC is strongly competitive in high capacitance, long cycle life, and high energy-required energy storage devices.

Supercapacitor with high cycling stability through electrochemical deposition of metal-organic frameworks/polypyrrole positive electrode.

Electrochemical deposition is an environment-friendly method for functional film fabrication, benefiting from the facile tuning of structure and properties of the film. Metal-organic frameworks (MOFs) play a significant role in the field of energy sources because of their porous nature, large specific surface area and metal ion binding sites. However, the application of such materials is limited by their inherent insulator-like characteristics and poor electrical conductivity. In this study, we designed and fabricated a composite material comprising metal-organic framework/polypyrrole as the positive electrode of a supercapacitor via smear and electrodeposition methods. Electrochemical properties of the electrode materials were studied through galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) tests. The positive electrode has an excellent specific capacitance of 284.3 F g-1 (180.7 mF cm-2) with 1 mA cm-2 current density. Furthermore, this material as a positive electrode shows high cycling stability after 40 000 cycles (100.7% capacitance retention). This strategy can be extended to fabricate other MOFs and MOFs/conducting polymers composite electrodes for supercapacitors.