Nano-porous Al/Au skeleton to support MnO2 with enhanced performance and electrodeposition adhesion for flexible supercapacitors

Abstract

A nano-porous Al/Au skeleton is constructed to effectively improve the utilization rate of the active MnO₂ and the overall adhesion between the current collector and MnO₂ in an electrodeposition system. The Al/Au current collector is prepared by first forming a nano-porous structure on the surface of Al foil through etching modification, and subsequently coating an ultra-thin Au layer onto the Al foil. The active MnO₂ is electrodeposited on the Al/Au current collector to fabricate a novel Al/Au/MnO₂ electrode. The nano-porous skeleton supports MnO₂ to grow autonomously inside-out. The ultra-thin Au layer acts as a transition layer to improve the overall conductivity of the current collector (0.35 Ω m^-1) and to improve the adhesion with MnO₂ as well. Owing to the highly porous structure, the electrochemical properties of the electrode are greatly improved, as evidenced by a remarkable specific capacitance of 222.13 mF cm^-2 at 0.2 mA cm^-2 and excellent rate capability of 63% capacitance retention at 6.0 mA cm^-2. Furthermore, the assembled solid-state symmetric supercapacitor exhibits a high energy density of 0.68 mW h cm^-3, excellent cyclic stability (86.3% capacitance retention after 2000 cycles), and prominent flexibility.

Fig. 1. The fabrication procedure of Al/Au/MnO₂ electrode.

Fig. 2. SEM images: (a) A3 foil; (b) internal and (c) surface of the as-prepared MnO₂. (d) EDS mapping. (e) XRD patterns of the A3/Au current collector and the AAM3 electrode. (f) Raman spectra of AAM3 electrode. (g) N₂ adsorption/desorption isotherms of the samples. (h) Pore size distribution of A3 foil and AAM3 electrode.

Fig. 3. XPS spectra of the AAM3 electrode: (a) survey spectrum, (b) Mn 2p, (c) O 1s, and (d) Au 4f.

Fig. 4. (a) The CV curves of AAM, AAM1 to AAM6 electrodes at the same scan rate of 25 mV s⁻¹. Electrochemical behaviors of the AAM3 electrode: (b) CV curves from 5 to 100 mV s⁻¹, (c) GCD curves from 0.2 to 6 mA cm⁻², (d) the relationship between the specific capacitance and the scan rate or current density, (e) EIS Nyquist plots, with the insets showing the amplified Nyquist plots at high to medium frequencies and the equivalent circuit, (f) the GCD cyclic stability of 2000 cycles at 3 mA cm⁻², with the inset showing the comparison of the first and the 2000^th cycle GCD.

Fig. 5. Electrochemical properties of the assembled solid-state supercapacitor under the voltage window of 0–0.8 V: (a) CV curves from 5 to 100 mV s⁻¹, (b) GCD curves from 0.1 to 1 mA cm⁻², (c) the relationship between the specific capacitance and the scan rate or current density, (d) Nyquist plots of the supercapacitor, (e) cyclic performance, (f) Ragone plots.

Fig. 6. (a) Schematic drawing of the supercapacitor and physical drawing of three supercapacitors connected in series. (b) CV curves of the symmetric supercapacitor under different bending states at 50 mV s⁻¹ within the voltage window of 0–0.8 V. (c) Images of three supercapacitors connected in series to charge and light up a red LED light under different bending states.