Engineering the Interlayer Spacing by Pre‐Intercalation for High Performance Supercapacitor MXene Electrodes in Room Temperature Ionic Liquid

Abstract

MXenes exhibit excellent capacitance at high scan rates in sulfuric acid aqueous electrolytes, but the narrow potential window of aqueous electrolytes limits the energy density. Organic electrolytes and room‐temperature ionic liquids (RTILs) can provide higher potential windows, leading to higher energy density. The large cation size of RTIL hinders its intercalation in‐between the layers of MXene limiting the specific capacitance in comparison to aqueous electrolytes. In this work, different chain lengths alkylammonium (AA) cations are intercalated into Ti3C2Tx, producing variation of MXene interlayer spacings (d‐spacing). AA‐cation‐intercalated Ti3C2Tx (AA‐Ti3C2), exhibits higher specific capacitances, and cycling stabilities than pristine Ti3C2Tx in 1 m 1‐ethly‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide (EMIMTFSI) in acetonitrile and neat EMIMTFSI RTIL electrolytes. Pre‐intercalated MXene with an interlayer spacing of ≈2.2 nm, can deliver a large specific capacitance of 257 F g−1 (1428 mF cm−2 and 492 F cm−3) in neat EMIMTFSI electrolyte leading to high energy density. Quasi elastic neutron scattering and electrochemical impedance spectroscopy are used to study the dynamics of confined RTIL in pre‐intercalated MXene. Molecular dynamics simulations suggest significant differences in the structures of RTIL ions and AA cations inside the Ti3C2Tx interlayer, providing insights into the differences in the observed electrochemical behavior.

Manipulating the Interlayer Spacing of 3D MXenes with Improved Stability and Zinc‐Ion Storage Capability

“…[ 18–21 ] Thus, it is highly desired to engineer the interlayer spacing of MXenes to improve their electrolyte‐accessible surface area and enhance the charge‐transport properties to achieve their full charge‐storage capability. [ 22 ] Furthermore, synthetic MXene materials are rarely used for multivalent ion energy storage, [ 23–27 ] such as zinc‐ion hybrid supercapacitor (ZHSC). [ 28–31 ]…”

“…[18][19][20][21] Thus, it is highly desired to engineer the interlayer spacing of MXenes to improve their electrolyte-accessible surface area and enhance the charge-transport properties to achieve their full chargestorage capability. [22] Furthermore, synthetic MXene materials…”

EDL structure of ionic liquid-MXene-based supercapacitor and hydrogen bond role on the interface: a molecular dynamics simulation investigation

“…1,10 Novel two-dimensional transition metal carbides and carbonitride MXenes have become hopeful electrode candidates with high capacitance and superior conductivity (up to 24 000 S cm −1 ) 11 compared to most common pseudocapacitive materials. 12–16 The Ti 3 C 2 T X has a remarkable volume capacitance ≈1500 F cm −3 in aqueous acidic and ionic liquid (IL) electrolytes. 17,18 In the preparation of MXenes, the transition metal layer surface will be functionalized spontaneously, generally introducing –OH, O, and –F functional groups, which are available for a rapid reversible redox reaction to store energy.…”

“…12–16 The Ti 3 C 2 T X has a remarkable volume capacitance ≈1500 F cm −3 in aqueous acidic and ionic liquid (IL) electrolytes. 17,18 In the preparation of MXenes, the transition metal layer surface will be functionalized spontaneously, generally introducing –OH, O, and –F functional groups, which are available for a rapid reversible redox reaction to store energy. 19,20 In the acid electrolyte, the O groups are the active site for the fast Faraday reduction reaction, while the –F groups hinder the transport of ions on the nanoflakes and limit their energy storage.…”

“…5,24–27 In particular, due to the wider electrochemical window, ILs are expected to achieve higher energy storage. 1,15,28–32 Compared to the common ILs, the functional ILs will remarkably change their physico-chemical properties such as their large electrochemical windows when introducing functional groups on cations, such as hydroxyl groups, which makes them strong contenders in electrochemistry. 29,33–37 Yeon et al 38 confirmed that the hydroxyl functionalized ILs exhibit wide electrochemical windows (5.4–6.4 V).…”