Hierarchical MnO2/activated carbon cloth electrode prepared by synchronized electrochemical activation and oxidation for flexible asymmetric supercapacitors

Jie Zhang,Jiangbo Sun,Tofik Ahmed Shifa,Di Wang,Xiaofeng Wu,Yanbin Cui

doi:10.1016/j.cej.2019.04.202

Abstract

Despite appealing supercapacitive properties, the flexible asymmetric supercapacitor devices (FASCs) are still suffering from low mass loading and limited operating voltage, leading to unsatisfactory energy densities. Herein, we introduced a high-voltage anodic electro-deposition process (AED) which is a novel avenue enabling the synthesis of hierarchical MnO2 on activated carbon cloth (H-MnO2/ACC). Interestingly, the concomitant activation of carbon cloth substrate is found to be beneficial to improve the conductivity and hydrophilic nature of our novel electrode. A FASC based on H-MnO2/ACC-300 and reduced carbon cloth (RCC) was assembled using a mixed ionic liquid gel (ionogel) electrolyte. Benefiting from highly conductive paths derived from intimately attached fiber-MnO2 interfaces, hierarchically interpenetrated lamella MnO2 porosity and concentric MnO2 interlayer voids, as well as the merits of ionic liquid, the resultant FASC delivers an output voltage as high as 4 V and an impressive volumetric energy density of 3.82 mWh/cm3. Furthermore, the optimized H-MnO2/ACC-300 electrode with high mass loading can even retain 94.2% of initial capacitance upon 5000 cycles in 1 M Na2SO4. The unique H-MnO2/ACC can aid in the rational design towards flexible electronic devices with high mass loading and this synthetic strategy opens up enormous possibilities for the fabrication of electrodeposited materials.

Highlights

Flexible electronic devices are indispensable parts in modern life, applied widely in mobile electrical products, smart textile and artificial electronic skin, etc. [1,2,3,4,5,6,7] In this regard, electrochemical supercapacitors (SCs) have drawn considerable attention and emerged as a promising power source of flexible and lightweight electronic devices owing to their high-power density and long lifetimes [1,3,5]
The unique hierarchical MnO2 (H-MnO2)/ACC can aid in the rational design towards flexible electronic devices with high mass loading and this synthetic strategy opens up enormous possibilities for the fabrication of electrodeposited materials
Flexible supercapacitors were assembled with as-prepared H-MnO2/ ACC-300 as positive electrode and reduced carbon cloth (RCC) as the negative one, respectively

Summary

Introduction

Flexible electronic devices are indispensable parts in modern life, applied widely in mobile electrical products, smart textile and artificial electronic skin, etc. [1,2,3,4,5,6,7] In this regard, electrochemical supercapacitors (SCs) have drawn considerable attention and emerged as a promising power source of flexible and lightweight electronic devices owing to their high-power density and long lifetimes [1,3,5]. With regard to reported active materials anchored on the surface of CC substrate, nanoscale layered manganese dioxide (MnO2) among transition-metal oxides (TMOs) with rich valence states, stands out as one of the most eye-catching choices for high-performance SC electrodes [1,17,24,25,26,27,28]. Based on Ostwald ripening, Song et al developed a two-step route to prepare a high mass loading MnO2 electrode with remarkable capacitance of 618 mF cm−2 at 200 mV s−1 [18]. All these methods involve multi-step processes which are costly, complicated and timeconsuming. The H-MnO2/ACC electrodes were systematically investigated in three-electrode cell, delivering superior electrochemical performance

Experimental section

Assembly of fiber-shaped asymmetric supercapacitor

Materials characterization

Results and discussions