Figure S1. (a) Blocks of Ti3AlC2 (top) and Al-Ti3AlC2 (bottom), (b) mass loss during the washing of Al-Ti3AlC2 with HCl, (c) image of the purple filtrate from the acid washing process, (d) Al-Ti3AlC2 particles after acid washing using HCl, (e) higher magnification of (d).

Textile-based electronics enable the next generation of wearable devices, which have the potential to transform the architecture of consumer electronics. Highly conductive yarns that can be manufactured using industrial-scale processing and be washed like everyday yarns are needed to fulfill the promise and rapid growth of the smart textile industry. By coating cellulose yarns with Ti 3 C 2 T x MXene, we produce highly conductive and electroactive yarns, which can be knitted into textiles using industrial knitting machine. We show that yarns with MXene loading of ~77 wt.% (~2.2 mg cm -1 ) have conductivity of up to 440 S cm -1 . After washing for 45 cycles at temperatures ranging from 30 °C to 80 °C, MXene-coated cotton yarns exhibit minimal increase in resistance while maintaining constant MXene loading. The MXene-coated cotton yarn electrode offers a specific capacitance of 759.5 mF cm -1 at 2 mV s -1 . A fully knitted textile-based capacitive pressure sensor is also prepared which offers high sensitivity (gauge factor of ~6.02), wide sensing range of up to ~20 % compression, and excellent cycling stability (2,000 cycles at ~14 % compression strain). This work provides new and practical insights towards the development of platform technology that can integrate MXene in cellulose-based yarns for textile-based devices.

Scaling the production of synthetic 2D materials to industrial quantities has faced significant challenges due to synthesis bottlenecks whereby few have been produced in large volumes. These challenges typically stem from bottom‐up approaches limiting the production to the substrate size or precursor availability for chemical synthesis and/or exfoliation. In contrast, MXenes, a large class of 2D transition metal carbides and/or nitrides, are produced via a top‐down synthesis approach. The selective wet etching process does not have similar synthesis constraints as some other 2D materials. The reaction occurs in the whole volume; therefore, the process can be readily scaled with reactor volume. Herein, the synthesis of 2D titanium carbide MXene (Ti3C2Tx) is studied in two batch sizes, 1 and 50 g, to determine if large‐volume synthesis affects the resultant structure or composition of MXene flakes. Characterization of the morphology and properties of the produced MXene using scanning electron microscopy, X‐ray diffraction, dynamic light scattering, Raman spectroscopy, X‐ray photoelectron spectroscopy, UV–visible spectroscopy, and conductivity measurements show that the materials produced in both batch sizes are essentially identical. This illustrates that MXenes experience no change in structure or properties when scaling synthesis, making them viable for further scale‐up and commercialization.

MXene for supercapacitor application which shows outstanding proton-induced pseudocapacitance in acidic aqueous electrolytes. [14,15] High electronic conductivity (up to 15 000 S cm −1), high packing density (up to 4 g cm −3), along with high pseudocapacitance endow Ti 3 C 2 T x ultrahigh volumetric capacitance (≈1500 F cm −3), which gives Ti 3 C 2 T x incomparable advantages over other electrode materials for supercapacitors. [16] However, Ti 3 C 2 T x electrodes suffer from long ion transport pathways due to the stacking nature of 2D materials, leading to ultra-low rate performance in a thick electrode. When used as a power supply for electronic devices where high areal energy densities at high rates are required, MXene electrodes need to be thick enough to ensure high charge storage capability. In this context, the ion transport issue becomes more critical because the low rate performance will deteriorate with the increase of film thickness. [17] Numerous efforts have been made to alleviate the restacking issue of Ti 3 C 2 T x film electrodes. Typical strategies include interlayer insertion of graphene, carbon nanotube or other nanomaterials, pillared structure design, template sacrifice method, vertical alignment, and etching holes. [17-27] However, by most of the reported approaches, the rate performances are increased at the expense of volumetric capacitance because of the introduction of inactive materials, excess spacing, or active materials with lower volumetric capacitance. For example, ≈15% decrease

MXenes, two‐dimensional (2D) transition metal carbides and/or nitrides, possess surface termination groups such as hydroxyl, oxygen, and fluorine, which are available for surface functionalization. Their surface chemistry is critical in many applications. This article reports amine functionalization of Ti3C2Tx MXene surface with [3‐(2‐aminoethylamino)‐propyl]trimethoxysilane (AEAPTMS). Characterization techniques such as X‐ray photoelectron spectroscopy verify the success of the surface functionalization and confirm that the silane coupling agent bonds to Ti3C2Tx surface both physically and chemically. The functionalization changes the MXene surface charge from −35 to +25 mV at neutral pH, which allows for in situ preparation of self‐assembled films. Further, surface charge measurements of the functionalized MXene at different pH values show that the functionalized MXene has an isoelectric point at a pH around 10.7, and the highest reported positive surface charge of +62 mV at a pH of 2.58. Furthermore, the existence of a mixture of different orientations of AEAPTMS and the simultaneous presence of protonated and free amine groups on the surface of Ti3C2Tx are demonstrated. The availability of free amine groups on the surface potentially permits the fabrication of crosslinked electrically conductive MXene/epoxy composites, dye adsorbents, high‐performance membranes, and drug carriers. Surface modifications of this type are applicable to many other MXenes.