Electrochemical performance of Ti3C2 supercapacitors in KOH electrolyte

Two-dimensional (2D) carbide Ti3C2 was synthesized by exfoliating Ti3AlC2 in HF solution and used for supercapacitive performance investigation in 3 M KOH electrolyte. The specific surface area (SSA) of as-synthesized Ti3C2 was 22.35 m2/g. Ti3C2-based supercapacitor electrodes exhibited good energy storage ability and had a volumetric capacitance 119.8 F/cm3 at the current density of 2.5 A/g. Moreover, the addition of carbon black into Ti3C2 powders greatly improved the performance of Ti3C2-based capacitors because carbon black restrained the preferred orientation of 2D Ti3C2, providing fast ion transport channels, and in turn, decreasing electrical resistance from 16.7 Ω to 3.5 Ω.


Introduction
Supercapacitors have fast charge/discharge rates, high power density, and good cyclability compared with batteries, and have attracted extensive research interest due to the increasing demand for portable and clean energy storage devices. In principle, supercapacitors store charges at the interface between electrode and electrolyte, thus large specific surface area (SSA) of electrode is favorable for high capacitance [1]. Therefore, many materials with large SSA were used as electrode, such as activated carbons [2,3], carbide-derived carbons [4,5], and carbon nanotubes [6,7]. Two-dimensional (2D) materials prepared by exfoliating precursors with layered structure [8,9] have high SSA, which are regarded as one of promising candidates for supercapacitor electrodes. The first and most investigated 2D material is graphene. Graphene or graphene-based electrodes for supercapacitors have been successfully prepared and their supercapcitor performance has been investigated extensively [10][11][12][13].
Recently, a new family of 2D materials was prepared by exfoliating ternary carbides or carbonitrides with the name of MAX phases [14,15]. The 2D materials were named as MXene to emphasize their graphene-like structure and the removing of A-site atoms from MAX structure. MAX phases, the precursors of MXenes, have the general formula of M n+1 AX n , where M is an early transition metal; A is an A-group element (mostly group IIIA or IVA); and X is either carbon or nitrogen [16,17].
MXenes have important application in many areas, such as hydrogen storage [18,19], lead adsorption [20], and catalysis [21]. Especially, as conductive and hydrophilic 2D materials [14], MXenes are promising electrode materials for electrochemical energy storage  [22][23][24][25]. Ti 3 C 2 is a typical MXene. Its performance as supercapacitor electrode was investigated. Additive-free paper-like Ti 3 C 2 can yield the volumetric capacitance of 350 F/cm 3 in KOH electrolyte [26], and Ti 3 C 2 with binder and carbon black can yield the capacitance of 415 F/cm 3 in H 2 SO 4 electrolyte [27]. In this paper, Ti 3 C 2 was fabricated and its supercapacitive performance with/without carbon black in KOH electrolyte was investigated.

Experimental methods
Ti 3 AlC 2 powders (98 wt% pure, 200 mesh) were made from the mixture of TiH 2 , Al, and TiC at 1450 ℃ for 2 h in Ar atmosphere [28]. 2D Ti 3 C 2 was produced by immersing Ti 3 AlC 2 in 49% HF (Aladdin Reagent, China) at 60 ℃ for 24 h followed by washing with deionized water for several times. Finally Ti 3 C 2 powders were centrifugally separated from the obtained suspension and dried in vacuum at 80 ℃.
X-ray diffraction (XRD) patterns of the as-fabricated powders were obtained with a diffractometer (Bruker AXS Co., Germany) using Cu Kα radiation. A field emission scanning electron microscope (FESEM; Hitachi, S4800, Japan) was used to characterize the microstructure of the as-prepared powders. Nitrogen sorption isotherm measurements were performed by an automatic gas adsorption analyzer (Autosorb-iQ-MP, Quantachrome, USA) at 77 K. The specific surface area (SSA) was calculated by Brunauer-Emett-Teller (BET) method.
Galvanostatic charge/discharge cycling and cyclic voltammetry (CV) tests were performed by a CSCT supercapacitor test system (Arbin, USA). Electrochemical impedance spectroscopy (EIS) was tested by a potentiostats-electrochemistry work station (Parstat 2273, Princeton Applied Research). Two types of working electrodes were prepared and labeled with Electrode-I and Electrode-II, respectively. Electrode-I was 95 wt% MXene and 5 wt% polytetrafluoroethylene (PTFE); Electrode-II was 85 wt% MXene, 10 wt% carbon black (CB, BP2000, Cabot Corporation, USA), and 5 wt% PTFE. The mixed slurries were pressed under a pressure of 10 MPa to completely adhere together as a disc with diameter of 13 mm and thickness of 0.15 mm. Finally, the electrodes were dried at 120 ℃ in vacuum for 4 h. Both Electrode-I and Electrode-II were assembled into symmetric supercapacitor devices and labeled as SC-1 and SC-2, respectively. The supercapacitive performance of SC-1 and SC-2 was characterized by three-electrode cells in 3 M KOH electrolyte.

Results and discussion
3. 1 Characterization of Ti 3 C 2 Figure 1(a) shows the XRD patterns of Ti 3 AlC 2 and Ti 3 C 2 MXene. During etching process, Ti 3 AlC 2 was exfoliated and Ti 3 C 2 MXene with low content of TiC  Figure 1(b) shows the FESEM image of the as-synthesized 2D Ti 3 C 2 . The original Ti 3 AlC 2 grains (shown in the inset of Fig. 1(b)) were fully exfoliated and 2D Ti 3 C 2 was formed with thickness of ~40 nm [15,21]. The N 2 sorption isotherm of the as-synthesized Ti 3 C 2 is shown in Fig. 1(c). The shape of hysteresis loop indicates slit-shaped mesopores. The SSA calculated from BET equation is 22.35 m 2 /g. Figure 2 shows the galvanostatic charge/discharge curves of the supercapacitors at different current density. The charge/discharge curves of both SC-1 and SC-2 are almost isosceles triangle, which indicate a good reversibility of SC-1 and SC-2. The capacitance of SC-2 is 71.2 F/g at the current density of 2.5 A/g, corresponding to volumetric specific capacitance of 119.8 F/cm 3 , which is higher than that of graphite oxide (110 F/cm 3 ) [29]. Figure 3 shows the capacitance decay with increasing discharge current density. The capacitance of SC-2 retains 60.4 F/g at the current density of 5 A/g. This indicates the electrodes of SC-2 possess the good conductivity and have good interfacial contact with electrolyte, which provide fast ion transport channels for KOH electrolyte. The capacitance of SC-2 is obviously larger than that of SC-1. Thus, it can be concluded that the addition of carbon black in Ti 3 C 2 MXene can significantly improve the performance of supercapacitors. Normally, because of the 2D structure, Ti 3 C 2 has obvious preferred orientation. Due to the pressure exerted during the electrode fabrication process, most Ti 3 C 2 sheets are lying parallel to the current electrode surface, namely, perpendicular to the direction of ion diffusion, which is unfavorable for the diffusion of electrolyte ions. However, if some carbon black particles are added and located between Ti 3 C 2 2D sheets, the preferred orientation is resisted, and thus more ion diffusion channels are generated by the random orientation of 2D Ti 3 C 2 , especially for those Ti 3 C 2 sheets parallel to the direction of ion diffusion. Therefore, the addition of carbon black can greatly increase ion conductivity. This explains the better performance of SC-2.

2 Electrochemical properties
The cyclic voltammograms (CV) at scan rate of 1 mV/s are presented in Fig. 4. Both CV curves are similar to rectangles and have good symmetry. Thus the supercapacitors are typical electrochemical capacitors.
The impedance spectra are shown in Fig. 5. From the inset, equivalent resistance of SC-1 and SC-2 devices can be calculated to be 16.7 Ω and 3.5 Ω, respectively. It is clear that Ti 3 C 2 with carbon black electrode yields smaller cell resistance and higher capacitance than pure Ti 3 C 2 electrode. A capacitance retention test was performed by   Fig. 6. There is almost no degradation in performance of SC-1 after 1000 cycles. Although the capacitance of SC-2 decays with increasing cycle numbers in the beginning cycles, the performance holds steady after 800 cycles, and remains 94.2% of the maximum capacitance. SC-2 still has a good cycling stability and can deliver the high volumetric capacity of 112.9 F/cm 3 after 1000 cycles.

Conclusions
Ti 3 C 2 with SSA of 22.35 m 2 /g was used to prepare supercapacitors. A volumetric capacitance of 119.8 F/cm 3 has been achieved at the current density of 2.5 A/g. It has a good cycling stability and can deliver a volumetric capacity of 112.9 F/cm 3 after 1000 cycles. Carbon black addition in Ti 3 C 2 can avoid the preferred orientation of Ti 3 C 2 , increasing ion diffusion channel and reducing the diffusion resistance of ion. Equivalent resistance was decreased from 16.7 Ω to 3.5 Ω. Capacitance retention test of Ti 3 C 2 -based supercapacitors.