3D Porous MXene (Ti3C2Tx) Prepared by Alkaline-Induced Flocculation for Supercapacitor Electrodes

Abstract

2D layered MXene (Ti3C2Tx) with high conductivity and pseudo-capacitance properties presents great application potential with regard to electrode materials for supercapacitors. However, the self-restacking and agglomeration phenomenon between Ti3C2Tx layers retards ion transfer and limits electrochemical performance improvement. In this study, a 3D porous structure of Ti3C2Tx was obtained by adding alkali to a Ti3C2Tx colloid, which was followed by flocculation. Alkaline-induced flocculation is simple and effective, can be completed within minutes, and provides 3D porous networks. As 3D porous network structures present larger surface areas and more active sites, ions transfer accelerates, which is crucial with regard to the improvement of the superior capacitance and rate performance of electrodes. The sample processed with KOH (K-a-Ti3C2Tx) exhibited a high capacity of approximately 300.2 F g−1 at the current density of 1 A g−1. The capacitance of the samples treated with NaOH and LiOH is low. In addition, annealing is essential to further improve the capacitive performance of Ti3C2Tx. After annealing at 400 °C for 2 h in a vacuum tube furnace, the sample treated with KOH (K-A-Ti3C2Tx) exhibited an excellent specific capacitance of approximately 400.7 F g−1 at a current density of 1 A g−1, which is considerably higher than that of pristine Ti3C2Tx (228.2 F g−1). Furthermore, after 5000 charge–discharge cycles, the capacitance retention rate reached 89%. This result can be attributed to annealing, which can further remove unfavourable surface groups, such as –F or –Cl, and then improve conductivity capacitance and rate performance. This study can provide an effective approach to the preparation of high-performance supercapacitor electrode materials.

Recent Advances of Flexible MXene and its Composites for Supercapacitors

“…They displayed a specific capacitance of 300.2 F g À 1 at 1 A g À 1 and up to 400.7 F g À 1 after further annealing. [145] Wu et al found that strong proton acids can also trigger the three-dimensional assembly of Ti 3 C 2 T x nanosheets. As shown in Figure 9c, protons inhibit the ionization of À OH on the surface of Ti 3 C 2 T x and reduce the negative charge on the surface of Ti 3 C 2 T x nanosheets.…”

“…obtained a 3D porous structure of Ti 3 C 2 T x by adding KOH to Ti 3 C 2 T x colloids for simple and effective flocculation within a few minutes. They displayed a specific capacitance of 300.2 F g −1 at 1 A g −1 and up to 400.7 F g −1 after further annealing [145] . Wu et al.…”

Optimized preparation and performance evaluation of a bifunctional chitosan-modified flocculant

“…In addition, the large specic surface area and the loose porous fold structure of poly(CTS-g-AM-MAPTAC) also enhanced its ability to catch and sweep the occulates in the late occulation period, making the generated occulation settle down quickly and improve the occulation efficiency. [36][37][38] Flocculation and sterilization of E. coli suspension. As with the kaolin occulation removal experiment, this section also examined the effect of occulant dosage, pH and graing rate of poly(CTS-g-AM-MAPTAC) on the removal of E. coli in wastewater.…”

“…In addition, the large specific surface area and the loose porous fold structure of poly(CTS- g -AM-MAPTAC) also enhanced its ability to catch and sweep the flocculates in the late flocculation period, making the generated flocculation settle down quickly and improve the flocculation efficiency. 36–38…”

MXene: Nanoengineering and Application as Electrode Materials for Supercapacitors

“…2p 原子间轨道能级相差较大，轨道之间的杂化相对 较弱。因此，配体场分裂(10 Dq)相对于 d 轨道的 带宽较小，产生了金属能带结构，使得 MXene 具有 金属的性质 [21] 。 MXene 主要是通过形成 M-X 键调整 金属性质，同时导电性也受到它们的表面性质和形 态的极大影响 [22] 。MXene 的表面端接官能团-F 和-OH 基团的氧化态相似，只允许接收一个电子。而=O 基团的行为与之相反，在静止态占据了两个电子。 调整端接基团， 可以明显改变 MXene 的电子性质 [23] 。 MXene 的形态对电导率也有很大影响，单层和大尺 寸薄片比多层和小尺寸薄片具有更好的相互作用，； 通常会提高电导率 [24] 。与其他 2D 材料(如石墨烯或 金属硫化物/氢氧化物)相比， Ti3C2Tx 薄膜含有丰富的 官能团，有利于导电性 [25] 。 MXene 具有优异的导电性，确保了电子快速传 输，不仅可以实现高功率密度超级电容器，而且在 电极的制造过程中，甚至可以不用导电剂和集流体， 这有利于提高整个器件的能量密度 [26] 。 [27] 。在 可逆的电化学插层反应过程中，稳健的层间键合保 证了多层结构的稳定性。碳化物和氮化物 MXene 的 杨氏模量都随着层数 n 增加而降低。此外，氮化物 MXene 的层数 n 超过碳化物 [28] 。 机械性能会影响到电极的电化学性能，特别是 在未来的应用中，如在特定条件下，柔性电子器件 的电极会受到应力，发生弯曲和扭转，MXene 具有 优异的机械柔性，有助于保持良好的电化学性能， 便于其应用在柔性超级电容器和微型超级电容器中 [29] 。 2 MXene 的储能机理 [21] 。优化电解液或修饰表面端 基 T 对增强电容有一定的作用 [30][31][32] [21] 。 图 1 MXene 在( a)水系和(b)非水系 Li + 电解质中的结构变 化和电子结构变化示意图 [21] Fig nonaqueous Li + electrolytes [21] φ: inner potential; ηe: electron electrochemical potential 3 MXene 电极的纳米工程 [33] ，调节 MXene 的表面端基可有效改善其储能 性能 [34] 。改变合成环境(溶液类型、浓度、老化和 后处理)，可以控制功能表面端基，改变 MXene 的 表面润湿性和亲水性质。在温和的蚀刻条件(稀 HF， H2SO4/HCl)下合成的 MXene 具有较好的热稳定性 和较低的缺陷密度。通常，碱化和退火 [35] 可以进一 步去除不利的表面基团，如-F 或-Cl。一旦端基被移 除，大量表面和边缘的金属原子可用来快速地进行 离子扩散，从而提高电导率，提升赝电容和倍率性 能 [36][37] 。 Li 等 [38]…”