Block‐Copolymer‐Assisted One‐Pot Synthesis of Ordered Mesoporous WO_3−x/Carbon Nanocomposites as High‐Rate‐Performance Electrodes for Pseudocapacitors

Abstract

An ordered mesoporous tungsten‐oxide/carbon (denoted as m‐WO3−x‐C‐s) nanocomposite is synthesized using a simple one‐pot method using polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) as a structure‐directing agent. The hydrophilic PEO block interacts with the carbon and tungsten precursors (resol polymer and WCl6), and the PS block is converted to pores after heating at 700 °C under a nitrogen flow. The m‐WO3−x‐C‐s nanocomposite has a high Brunauer–Emmett–Teller (BET) surface area and hexagonally ordered pores. Because of its mesoporous structure and high intrinsic density of tungsten oxide, this material exhibits a high average volumetric capacitance and gravimetric capacitance as a pseudocapacitor electrode. In comparison with reduced mesoporous tungsten oxide (denoted as m‐WO3−x‐h), which is synthesized by a tedious hard template approach and further reduction in a H2/N2 atmosphere, m‐WO3−x‐C‐s shows a high capacitance and enhanced rate performance, as confirmed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. The good performance of m‐WO3−x‐C‐s is attributed to the high surface area arising from the mesoporous structure, the large interconnected mesopores, and the low internal resistance from the well‐dispersed reduced tungsten oxide and amorphous carbon composite structure. Here, the amorphous carbon acts as an electrical pathway for effective pseudocapacitor behavior of WO3‐x.

Mesoporous materials for energy conversion and storage devices

“…Pseudocapacitors usually suffer from low power density and short cycle life because of the poor electrical conductivity and stability of the active materials, such as conducting polymers and metal oxides 169,170 . In this regard, mesostructured porous carbons are viable alternative hosts because they can increase the electrical conductivity and mechanical stability, as well as provide a confined space for the growth of active species at the nanoscale [171][172][173][174] . In general, an enhanced rate capacitance and cycling stability can be achieved using mesostructured porous carbons; however, both the volumetric and gravimetric capacitances are greatly decreased when the mass of the inactive conductive material is considered.…”

Hydrothermal preparation and supercapacitive performance of flower-like WO3·H2O/reduced graphene oxide composite

“…The capacitive behaviors of the nanostructured tungsten oxides have been also investigated as supercapacitor electrodes [23][24][25][26][27][28][29]. However, the low electrical conductivity and poor rate performance of tungsten oxide pseudocapacitors have been their major demerits.…”

“…Especially, the engineered WO 3 hierarchical nanostructures are expected to exhibit desirable electrochemical property for their shortened diffusion lengths for ions and large exposed surface for access of electrolyte. Another practical approach toward improving performance is to combine WO 3 with other conductive materials such as conductive polymers, carbon aerogel and graphene nanosheets to fabricate composites [25,26,28,30]. Amongst, graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice, has been confirmed to be an ideal candidate to fabricate composites due to its fascinating properties such as superior electrical conductivity, large surface area and excellent mechanical flexibility [31].…”

“…However, the low electrical conductivity and poor rate performance of tungsten oxide pseudocapacitors have been their major demerits. In order to combat these drawbacks, optimizing morphology and structure is confirmed as a feasible strategy [23,25]. Especially, the engineered WO 3 hierarchical nanostructures are expected to exhibit desirable electrochemical property for their shortened diffusion lengths for ions and large exposed surface for access of electrolyte.…”

Highly densified carbon electrode materials towards practical supercapacitor devices

“…So the best strategy is to wrap MnO2 with porous graphene facilitating both electrical conductivity and charge transfer. Jo et al [69] synthesized an ordered mesoporous tungsten-oxide/carbon nanocomposite using a template method and obtained a specific volumetric capacitance of 340 F cm −3 due to the combined effect of the interconnected mesoporous structure of the carbon and the high intrinsic density and psudocapacitance of tungsten oxide. In another work, Xiao et al [70] obtained a freestanding MoO3−x/CNT composite film, exhibiting a specific gravimetric capacitance of 337 F g −1 along with a specific volumetric capacitance of 291 F cm −3 and an excellent rate capability.…”