Three‐Dimensional MXenes for Supercapacitors: A Review

Abstract

However, limited by their high contact resistance, insufficient charge storage, and low electrode density, the activated carbons only show an inadequate capacitance range of 60-200 F cm −3 . [7,8] In comparison, the pseudocapacitive materials, such as conducting polymers and metal oxides, exhibit higher capacitance based on their fast non-diffusion limited surface Faraday redox reactions, [9] whereas poor conductivity and unstable cycle performance restrict their wide-spectrum application. [10,11] Several efforts have been made to develop the intercalation pseudocapacitive electrode materials, which store charge via fast ion intercalation and thus combine the high energy density of batteries and the high power density of supercapacitors. [12,13] 2D transitional metal carbides/nitrides, also known as MXenes, have attracted much attention since firstly reported in 2011. [14] MXenes have a general formula of M n + 1 X n T x , where M is a transition metal, X is carbon or/and nitrogen, n can be an integer between 1 and 4, and T x stands for surface-terminating functional groups (-OH, -F, -O, etc.). [15,16] MXenes could be obtained by selectively etching the A atomic layer from their layered MAX phase precursors (M n + 1 AX n ) with different etchants. [14,17,18] Currently, more than 100 kinds of MXenes have been predicted, among which about 40 compositions have been experimentally synthesized and Ti 3 C 2 T x is the most studied member. Various compositions and ratios of M and X elements, together with different surface terminations, endow MXene materials with special and tunable structures and properties. As a result, they have rapidly aroused tremendous interest for various applications, including energy storage, [19][20][21][22][23] catalysis, [24] electromagnetic interference shielding, [25] water purification, [26] reinforcement for composites, [27] etc.More specifically, MXene materials are promising electrode materials for supercapacitors, as they have 2D lamellar structure, high electrochemically active surface, metallic-like electrical conductivity, tunable surface-terminating functional groups, good dispersibility in various solvents, and outstanding mechanical flexibility. [28][29][30] Due to their 2D structure and polar terminations, MXenes allow ions (e.g., H + , Li + , Na + ) to intercalate/de-intercalate fast between the layers. [20,31] In basic and neutral aqueous electrolytes, the intercalated ions are surrounded by water molecules, leading to an inner EDLC mechanism with common capacitances of 60-150 F g -1 , [32] while in H 2 SO 4 electrolyte, the intercalated H + protons can bond/ debond with the oxygen-containing terminations on the MXene Supercapacitors have the characteristics of high power density and long cycle life, but the low energy density limits their further development. The 2D transitional metal carbides/nitrides (MXenes) show great application prospects in the field of supercapacitors due to their superior volumetric capacitance, metallic-like conductivity, tunable surface termination...

Jujube Shell Based-Porous Carbon Composites Double-Doped by MnO2 and Ti3C2Tx: The Effect of Double Pseudocapacitive Doping on Electrochemical Properties

“…It was found that the low Ti 3 C 2 content could improve the specific capacitance of the composites, based on the CV curve integral area data. This might be because the appropriate Ti 3 C 2 content helped to cover the carbon surface evenly, while the excessive Ti 3 C 2 content led to the collapse of the multilayer structure, which is detrimental to ion storage [ 34 ]. Then the gradient sweep velocity of the JSPC&Ti 3 C 2 Tx-1 composite was measured ( Figure 4 b).…”

Hierarchical MXene/transition metal oxide heterostructures for rechargeable batteries, capacitors, and capacitive deionization

“…Nevertheless, MXene electrodes suffer from the following issues: (1) low capacity or capacitance; (2) MXene sheets can be easily restacked during the preparation of the electrodes; (3) the electronic conductivity of MXene sheets decreases with the oxidation of the MXene; and (4) termination -F groups may influence the electrolyte infiltration into the electrodes. [57][58][59][60][61][62][63][64][65][66][67][68] To solve the above problems and enhance the electrochemical performance of MXenes, a lot of effort has been made to, for example, construct few-layer MXene sheets, 69 nanostructured MXene, 70,71 porous MXene, [72][73][74][75][76][77] MXene/transition metal oxides (TMOs), [78][79][80][81] and MXene/transition metal chalcogenides (TMCs). [82][83][84][85] In particular, constructing hierarchical MXene/ TMO heterostructures can remarkably improve the electrochemical performance of MXene-based electrodes in batteries, capacitors, and CDI.…”

Zinc‐ion hybrid supercapacitors: Design strategies, challenges, and perspectives

“…Therefore, developing efficient energy storage systems has become significantly important. Numerous energy storage systems with superior electrochemical performance (long cycle stability and high energy density/power density) have emerged since the first commercialization of rechargeable lithium‐ion batteries in 1990, including sodium‐ion batteries, [ 1–3 ] lithium‐sulfur batteries, [ 4–6 ] aqueous batteries, [ 7,8 ] supercapacitors, [ 9,10 ] and so on. [ 11–16 ] Meanwhile, the batteries often have extremely high energy densities, but they have a short cycle life and safety risks due to severe redox reactions and dendritic complications.…”