Urea-Assisted Nickel-Manganese Phosphate Composite Microarchitectures with Ultralong Lifecycle for Flexible Asymmetric Solid-State Supercapacitors: A Binder-Free Approach

Abstract

The limited energy density and cyclability of supercapacitors are major roadblocks to their development as energy storage devices. To address these issues, a binder-free nickel–manganese (Ni–Mn) phosphate composite (NMP series) microarchitecture has been synthesized by the hydrothermal method on a nickel foam (NF) substrate using various urea dosages. Due to the influence of urea, microrod-/microplate-like morphologies of NMP series thin films evolved to micropetals. This study demonstrates a synergy between Ni and Mn metal ions and also the influence of different urea contents on the physicochemical properties of mesoporous NMP series thin films. Notably, the NMP-4 microarchitecture has a large surface area (7.5 m2 g–1), which provides more electroactive sites in electrochemical measurements. Accordingly, in the NMP series electrodes, the NMP-4 thin film demonstrated high electrochemical properties (the maximum specific capacity was found to be 901 C/g at a 5 mV/s scan rate) and retained 127% capacity over 6000 cycles, indicating good durability with a well-preserved microstructure throughout the cycling. Furthermore, a flexible asymmetric solid-state (FASS) supercapacitor was designed utilizing NMP-4 and reduced graphene oxide (rGO) as a cathode and an anode, respectively, in the poly(vinyl alcohol)-KOH (PVA-KOH) gel electrolyte with an extended operational voltage of +1.8 V. This FASS device provides a high specific capacity (192 C/g at 0.6 A/g current density), supreme energy density (48.2 Wh kg–1) at a power density of 575 W kg–1, and a desirable longevity of 108% over 5000 cycles. Moreover, the FASS device also demonstrated its practical applicability. The long-term stability suggests that the binder-free urea-assisted Ni–Mn phosphate composite is a good candidate for energy storage devices.