Supercapacitors represent a major technology to store energy for many applications including electronics, automobiles, military, and space. Despite their high power density, the energy density in supercapacitors is presently inferior to that of the state-of-the-art Li-ion batteries owing to the limited electrochemical performance exhibited by the conventional electrode materials. The advent of two-dimensional (2D) nanomaterials has spurred enormous research interest as supercapacitor electrode materials due to their fascinating electrochemical and mechanical properties. This Review discusses cutting-edge research on some of the key 2D supercapacitor electrode materials including transition metal dichalcogenides, transition metal oxides and hydroxides, MXenes, and phosphorene. Various synthetic approaches, novel electrode designs, and microstructure tuning of these 2D materials for achieving high energy and power densities are discussed.

Recent Advances in Two-Dimensional Nanomaterials for Supercapacitor Electrode Applications

Emergent Pseudocapacitance of 2D Nanomaterials

Cation substitution is a promising strategy for modulating the structural properties and optimizing the electrochemical performance of spinel cobalt oxide (Co3O4); however, the underlying mechanism of this action induced by different cation substitutions has not yet been clearly addressed. Herein, a systematic investigation is performed to elucidate the effect of cation substitution on the pseudocapacitive performance of spinel cobaltite (MCo2O4; M = Mn, Ni, Cu, and Co) mesoporous nanowires grown on nickel foam (NF). Theoretical and experimental analyses reveal that the substitution of Co by transition metals (i.e., Mn, Ni, and Cu) in the lattice of Co3O4 can simultaneously improve charge transfer and ion diffusion, thereby exhibiting enhanced electrochemical properties. Herein, as a representative example, MnCo2O4 achieves a high specific capacitance of 2146 F g−1 at a current density of 1 A g−1, while 92.1% of its initial capacitance is retained after 5000 cycles. An asymmetric supercapacitor with MnCo2O4 as the positive material and activated carbon (AC) as the negative material delivers a high energy density of 56.1 W h kg−1 at a power density of 800 W kg−1, and a favorable energy density of 29.3 W h kg−1 at a power density as high as 8000 W kg−1.

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Effect of cation substitution on the pseudocapacitive performance of spinel cobaltite MCo2O4 (M = Mn, Ni, Cu, and Co)

Two-dimensional Spinel Structured Co-based Materials for High Performance Supercapacitors: A Critical Review

Two-dimensional Co3O4 thin sheets assembled by 3D interconnected nanoflake array framework structures with enhanced supercapacitor performance derived from coordination complexes

Olivaceous two-dimensional (2D) multi-layer graphene-like Co3O4 thin sheets (CQU-Chen-Co-O-1) with vertically aligned nanosheets as basic building units were first prepared on a large scale by direct hydrothermal decomposition of a mixed aqueous solution of cobalt(II) nitrate and acetic acid without the assistance of any template or surfactant. The resulting products exhibited excellent pseudocapacitive performance with a high specific capacitance of 1752 and 1862 F g−1 at the current density of 5 mV s−1 and 1 A g−1, respectively, as well as good rate capability (63.64% capacitance surplus) and high cycling performance (99.5% surplus after 2000 cycles).

Engineering 2D multi-layer graphene-like Co3O4 thin sheets with vertically aligned nanosheets as basic building units for advanced pseudocapacitor materials

Weifan Chen

Papers

Two-dimensional Co3O4 thin sheets assembled by 3D interconnected nanoflake array framework structures with enhanced supercapacitor performance derived from coordination complexes

Engineering 2D multi-layer graphene-like Co3O4 thin sheets with vertically aligned nanosheets as basic building units for advanced pseudocapacitor materials