Hydrazine monohydrate modifying Ti3C2Tx in alkaline electrolytes for high-performance capacitive properties

MXene has shown a bright future in the electrochemical field, benefiting from its excellent conductivity and surface hydrophilicity. However, the performance of MXene in the field of supercapacitors is seriously impaired by restacking and a large number of F-containing functional groups, especially in alkaline electrolytes. Herein, hydrazine monohydrate is used to intercalate Ti3C2Tx, and the surface chemistry of MXene is effectively optimized. In 2 M KOH, the specific capacitance of Ti3C2-HM after modification is 157 F/g, which is 35% higher than the original one.


Introduction
In recent years, MXene has been synthesized by etching the A atomics from the MAX phase [1].With the disappearance of the Al atomic layer, the remaining MX atomic layers are connected by weak intermolecular force, which can be easily broken, thus obtaining few or even single-layer MXene [2].Because of its unique structure, MXene has a bright application prospect in the field of electrochemical energy storage [3].There are many factors affecting the properties of MXene, such as its structure and surface chemistry.According to our previous research, the conductivity of Ti-based MXene can reach 4326S cm -1 [4], which makes the lamellar surface of Ti-based MXene provide electrons to active sites quickly.Besides, the specific surface area of MXene can be increased by expanding the interlayer spacing of MXene, thus providing more active sites.MXene shows good electrochemical performance in acidic electrolytes, but its capacitance value is low in alkaline electrolytes.Therefore, we mainly study the capacitive properties of Ti 3 C 2 T x in KOH electrolyte.
Herein, Ti 3 AlC 2 can be delaminated effectively by using an F-containing etchant, and the corresponding two-dimensional Ti 3 C 2 MXene has been obtained.After further intercalation with hydrazine monohydrate, the lattice parameter c of Ti 3 C 2 expanded to 26.4Å.In addition, hydrazine monohydrate can also optimize the surface functional groups of Ti 3 C 2 .The results of XPS showed that the oxygen-containing groups of Ti 3 C 2 -HM increased while the F content decreased.The specific capacitance of Ti 3 C 2 -HM reached 157 F/g, which is 35% higher than that of the original Ti 3 C 2 MXene.

Preparation of Ti 3 AlC 2 :
Ti, Al, and C powders are uniformly mixed in an atomic ratio of 3: 1.2: 2.8 and then cold pressed into discs.Subsequently, the cold-pressed discs were placed in a vacuum induction sintering furnace, heated to 1350℃, and kept for 1 h.After cooling with the furnace, Ti 3 AlC 2 can be obtained.

Preparation of Ti3C2Tx and Ti3C2Tx-HM:
The prepared 0.5 g Ti 3 AlC 2 was ground into powder and added to HCl/LiF etchant at room temperature for 24 h.After washing to neutrality, the multi-layered Ti 3 C 2 T x was obtained.Then, Ti 3 C 2 T x was dispersed in water and then divided into two parts after 30 min ultrasound.A part of the suspension was centrifuged and then directly vacuum-filtered to obtain Ti 3 C 2 T x 'paper'.At room temperature, 2μL hydrazine monohydrate was added to another part of the suspension.After 24 hours, The Ti 3 C 2 -HM 'paper' was obtained by centrifugation and filtration.The FE-SEM images of Ti 3 AlC 2 and its corresponding MXenes are shown in Figure 2. Ti 3 AlC 2 MAX is composed of tightly stacked plates with a diameter of 5~20 μm (Figure 2a).After etching, as shown in Figure 2b, random cracks appeared on the multilayer MXene block due to the disappearance of Al atomic layers.Ti-C layers are connected with each other by the van der Waals force.Compared with the stable chemical bond, this weak intermolecular force between layers is easily broken by an external force.Ultrasonic treatment of multilayer Ti 3 C 2 MXene can separate the layers, thus obtaining a single layer and a few layers of Ti 3 C 2 .As shown in Figure 2c, Ti 3 C 2 nano-layers can be stably suspended in an aqueous solution due to their same charge at the surface.Therefore, the free-standing Ti 3 C 2 MXene film can be obtained by vacuum filtration of this suspension.Figure 2d shows Ti 3 C 2 -HM MXene after hydrazine monohydrate treatment.It can be seen from the figure that hydrazine monohydrate has no obvious effect on the surface morphology of MXene, and Ti 3 C 2 -HM maintains a complete two-dimensional structure.Similarly, Ti 3 C 2 -HM films can be obtained by vacuum-assisted filtration.

Electrochemical performances of Ti 3 C 2 T x and Ti 3 C 2 -HM
The electrochemical performances of Ti 3 C 2 T x and Ti 3 C 2 -HM were carried out in a 3-electrode system, and the electrolyte was 2M KOH.The cyclic voltammetry (CV) curves are shown in Figures 3a and 3b.
In 2M KOH aqueous solutions, all the CV curves deviate from the rectangle.At 2 mV/s, a weak redox peak can be observed at -1 V, which indicates that the mechanism of both Ti 3 C 2 T x and Ti 3 C 2 -HM have pseudocapacitance contributions.With the increase in scanning rate, the redox peaks gradually blurred and disappeared.The GCD tests were carried out at 0.5, 1, 2, 5, and 10 A/g (Figures 3c and 3d).The excellent symmetry of GCD curves indicates that Ti 3 C 2 T x and Ti 3 C 2 -HM electrodes have great electrochemical reversibility.In addition, the specific capacitance of the electrode can be calculated from the GCD curves [5]: where ∆ is the discharging time, and ∆ is the potential window.The capacitance value of 116 F/g is obtained in Ti 3 C 2 T x at 0.5 A/g.Interestingly, Ti 3 C 2 -HM treated with hydrazine monohydrate has a longer charge-discharge time, which indicates that Ti 3 C 2 -HM has better performance.The calculation shows that the value of Ti 3 C 2 -HM is 157 F/g at 0.5 A/g.Particularly, in F 1s, it can be seen that the proportion of AlF x in Ti 3 C 2 T x MXene is as high as 52%, while that in Ti 3 C 2 -HM after hydrazine monohydrate treatment is only 19%.Due to the F-containing material being unstable in alkaline electrolytes [6], the electrochemical performance of the electrode can be seriously damaged.In addition, the increase of O is also very important in improving the capacitance of Ti 3 C 2 -HM.

Conclusions
In summary, the Ti 3 AlC 2 MAX phase was synthesized by pressure-less sintering, and the corresponding Ti 3 C 2 T x MXene was obtained by HCl/LiF etching.By exfoliation and delamination of hydrazine monohydrate, free-standing Ti 3 C 2 T x and Ti 3 C 2 -HM MXene 'paper' are obtained by vacuum infiltration.Ti 3 C 2 T x can not only be intercalated by hydrazine monohydrate molecules spontaneously to increase its lattice parameter c but also its surface chemistry can also be optimized.Compared with the original Ti 3 C 2 T x , the specific capacitance of modified Ti 3 C 2 -HM increased by 35%.
3 C 2 T x and Ti 3 C 2 -HM X-ray diffraction (XRD) of Ti 3 AlC 2 MAX powders and its corresponding MXenes, Ti 3 C 2 T x and Ti 3 C 2 T x -HM after etching and delamination by hydrazine monohydrate are shown in Figure1.The black vertical line at the bottom of the figure is the peak position corresponding to the standard card of Ti 3 AlC 2 (No 52-0875), which is perfectly matched with the powder diffraction XRD pattern of Ti 3 AlC 2 MAX phase obtained by pressure-less sintering (red diffraction spectrum in Figure1).Therefore, it can be seen that the Ti 3 AlC 2 MAX phase with high purity has been prepared.After an etching by HCl/LiF, all peaks of Ti 3 AlC 2 disappeared and were replaced by the (002) peak of Ti 3 C 2 T x MXene.The (002) peak broadens and shifts to a lower angle, which is caused by the etching of the Al atomic layer and the expansion of the c lattice parameter, from 18.6 Å in Ti 3 AlC 2 to 24.2 Å and 26.4 Å in MXenes.During hydrazine monohydrate treatment, N 2 H 4 and water molecules intercalate into MXene spontaneously, which further enlarges the interlayer spacing and lattice parameter.

Figure 3 .
Figure 3. CV and GCD curves of Ti 3 C 2 T x MXene (a, c) and Ti 3 C 2 -HM counterpart (b, d)

Figure 4 .Figure 4
Figure 4.The XPS spectra for Ti 3 C 2 T x MXene (a, b, and c) and Ti 3 C 2 -HM (d, e, and f) Figure 4 shows the XPS spectroscopy of Ti 3 C 2 T x and Ti 3 C 2 -HM.Ti 2p orbital splits into Ti 2p3/2 and Ti 2p1/2 due to the coupling of the orbital motion and spin motion of the electrons outside the nucleus of the Ti atom.The O 1s spectra show that Ti 3 C 2 -HM contains more O elements, including Ti-O and Ti-OH bonds.Particularly, in F 1s, it can be seen that the proportion of AlF x in Ti 3 C 2 T x MXene is as high as 52%, while that in Ti 3 C 2 -HM after hydrazine monohydrate treatment is only 19%.Due to the F-containing material being unstable in alkaline electrolytes[6], the electrochemical performance of the electrode can be seriously damaged.In addition, the increase of O is also very important in improving the capacitance of Ti 3 C 2 -HM.