High-performance, high energy density symmetric supercapacitors based on δ-MnO₂ nanoflower electrodes incorporated...

The present work investigates liquid-based and liquid-free supercapacitors assembled using δ-MnO₂-nanoflower-based electrodes. An optimized electrode composition was prepared using acetylene black (AB), a polymer (PEO), a salt (LiClO₄), and δ-MnO₂ and used for device fabrication. The composite electrode was tested against a liquid electrolyte and a 'liquid-free' composite solid polymer electrolyte (CSPE) membrane. In a three electrode geometry, with 1 M solution of LiClO₄ as an electrolyte, the specific capacitance of the electrode was found to be ∼385 F g^-1, with a specific energy of ∼23 W h kg^-1 and specific power of ∼341 W kg^-1 (at 1 mA, 1 V). Dunn's method confirmed that the charge storage process was predominantly pseudocapacitive. When the device was assembled in a two-electrode Swagelok cell, a stable specific capacitance of ∼216 F g^-1 was observed with a specific energy of 30 W h kg^-1 and a specific power of 417 W kg^-1. The supercapacitors exhibited stable performance up to ∼7000 cycles with ∼90% capacitance retention and ∼97% coulombic efficiency. A combination of these cells could light two white light-emitting diodes (LEDs, 3 V) for at least ∼10 minutes. Further, all-solid-state supercapacitors (ASSCs) were fabricated using a Li⁺ ion (CSPE) membrane. The ASSCs exhibited a specific capacitance of ∼496 F g^-1 after ∼500 cycles, with a specific energy and power of ∼19 W h kg^-1 and ∼367 W kg^-1, respectively. The investigation reveals that the electrodes are versatile and show compatibility with liquid and solid electrolytes. The polymer in the electrode matrix plays an important role in enhancing device performance.