Fast Energy Storage in Two-Dimensional MoO₂ Enabled by Uniform Oriented Tunnels

Abstract

While pseudocapacitive electrodes have potential to store more energy than electrical double-layer capacitive electrodes, their rate capability is often limited by the sluggish kinetics of the Faradaic reactions or poor electronic and ionic conductivity. Unlike most transition-metal oxides, MoO 2 is a very promising material for fast energy storage, attributed to its unusually high electronic and ionic conductivity; the one-dimensional tunnel is ideally suited for fast ionic transport. Here we report our findings in preparation and characterization of ultrathin MoO 2 sheets with oriented tunnels as a pseudocapacitive electrode for fast charge storage/release. A composite electrode consisting of MoO 2 and 5 wt % GO demonstrates a capacity of 1097 C g −1 at 2 mV s −1 and 390 C g −1 at 1000 mV s −1 while maintaining ∼80% of the initial capacity after 10,000 cycles at 50 mV s −1 , due to minimal change in structural features of the MoO 2 during charge/ discharge, except a small volume change (∼14%), as revealed from operando Raman spectroscopy, X-ray analyses, and density functional theory calculations. Further, the volume change during cycling is highly reversible, implying high structural stability and long cycling life.

Laser‐Induced Graphene/MoO2 Core‐Shell Electrodes on Carbon Cloth for Integrated, High‐Voltage, and In‐Planar Microsupercapacitors

“…The presence of a superficial MoO 3 is not surprising and it seems to be unavoidable for the MoO 2 nanoparticles due to the gradual oxidation by the exposure to air. [ 39 ]…”

“…[38] To confirm the existence of MoO 2 nanoparticles and LIG, and further distinguish their carbonization degree. [39] The Raman spectra of pristine CC, LIG, and MoO 2 /LIG-CC are given in Figure 3d. A total of three main peaks, the D (≈1350 cm −1 ), G (≈1580 cm −1 ), and 2D (≈2690 cm −1 ) peaks, can be clearly seen.…”

“…[16,38] Moreover, the low-intensity peaks of 128, 266, 362, 491, 566, 742, and 816 cm −1 at low wavenumbers verify the MoO 2 nanoparticles' presence. [26,39] These characteristic peaks are relatively lower in the inside region of the CC, possibly because the Mo ions ink were not soaked enough by the inner layer.…”

“…The presence of a superficial MoO 3 is not surprising and it seems to be unavoidable for the MoO 2 nanoparticles due to the gradual oxidation by the exposure to air. [39] Figure 3g shows the TEM image of the MoO 2 /LIG scratched from the CC substrate; MoO 2 nanoparticles with nonuniform sizes were supported by the LIG fragment, revealing a wide range of size distribution with most nanoparticles below 100 nm. This multi-scale structure can offer great electrochemical properties since its high specific surface area is conducive to charge accumulation and chemical redox reactions.…”

Comparative Studies on Two-Dimensional (2D) Rectangular and Hexagonal Molybdenum Dioxide Nanosheets with Different Thickness

“…Jacob et al described the deformation in MoO 2 at high temperature and acknowledged a phase transition occurring at specific temperature with transformation of distorted rutile structure ( P 2 1 / c ) into a hexagonal rutile structure ( P 4 2 / mnm ) [ 27 ]. In addition, the electronic structure and properties of molybdenum oxide materials vary with thickness [ 16 ], and MoO 2 nanostructures have been widely used in electro-chemical supercapacitors [ 28 ], catalysis [ 18 ], sensing [ 29 ], energy storage [ 30 ], electrochromic displays [ 31 ], and energy conversion regimes [ 32 ] because of their superior charge transport properties [ 24 ]. Furthermore, various methods have been used for synthesis of diverse morphologies MoO 2 for achievement of exceptional properties.…”