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Ultra-Long Manganese Oxide Nanowires for Efficient Energy Storage


Manganese oxide, MnO2, excels as a hybrid electrical energy storage material. The man- ganese centers in MnO2 are capable of undergoing a reduction from 4+ to 3+ balanced by the intercalation of lithium ions to form LixMnO2 while its conductive surfaces simultaneously store energy as an electrical double layer capacitor. Additionally, MnO2 nanowire structure for batteries and super-capacitors can produce superior power relative to films of the same materials. These advantages make MnO2 promising candidate for electrical energy storage material. But the Achilles Heel of nanowires is their susceptibility to degradation and failure. Typically such MnO2 nanowires are attached to a current collector at just one end and electrical conductivity of the nanowire therefore plays a very important role. In this work, the electrical conductance of δ-MnO2 nanowires is directly measured in-situ in 1M LiClO4, acetonitrile as a function of the equilibrium Li content for nanowires with varying lateral dimensions. As the equilibrium electrode potential is decreased from 0.60 V to -0.80 V and lithium is intercalated, the electrical conductivity of MnO2 nanowires increases by up to one order of magnitude. The measured change in conductivity is also dependent on the width of nanowires. After doping at -0.80 V vs. MSE, the conductivity increases by 30% for an 870 nm wide nanowire array and 880 % for a 275 nm wide array. Additionally, the combination of poly (methylmethacrylate)(PMMA) to the propylene carbonate (PC) electrolyte is discovered to eliminate the degradation processes in MnO2 nanowire capacitors. The nanowires investigated here have a Au@ δ-MnO2, core@shell, architecture in which a central gold nanowire current collector is surrounded by an electrodeposited layer of δ- MnO2. In the liquid PC electrolyte, the identical δ-MnO2 capacitors fractures, delaminates, and separates from the gold nanowire current collector. The addition of PMMA to the PC electrolyte prevents these deleterious processes and extends cycle stability to 100,000 cycles and beyond. This is shown through reversible cycle stability up to 200,000 cycles with 96 % average coulombic efficiency for symmetrical δ-MnO2 nanowire capacitors operating across 1.2V– 1.8 V voltage windows in a PMMA gel electrolyte.

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