Freestanding MXene-hydrogels prepared via critical density-controlled self-assembly: high-performance energy storage with ultrahigh capacitive vs. diffusion-limited contribution

Abstract

A solvated network of two-dimensional materials in the form of hydrogel offers a unique platform for the full utilization of surface-dominated properties on a macroscopic scale. However, the development of such hydrogels with ceramic sheets of MXenes is highly challenging, and hence, they are often critically dried to aerogel form to make them self-standing. Herein, we report the preparation of a freestanding thin MXene hydrogel via spontaneous, quasi-ordered assembly over a metal plate and concurrent partial surface de-functionalization. We established that self-standing hydrogels can only be assembled from MXene solutions above a critical dispersion concentration (C_ct) depending on the size of the sheets. On behalf of ordered porous structures with intrinsic hydrated ion-permeable channels, MXene hydrogels display very high surface capacitive contribution (>94%) in their pseudocapacitive energy storage performance. Consequently, the as-developed hydrogels display an excellent gravimetric capacitance of 391 F g⁻¹ and rate performance, and continue to display an outstanding performance >337 F g⁻¹ at a high enough mass loading up to ∼15 mg cm⁻². As a versatility of hydrogel structures, an electrolyte-protected mechanical compression method was employed to obtain a compact yet ion channel–decorated electrode. As a result, compact MXene hydrogels achieved an excellent volumetric capacitance of 1120 F cm⁻³ and a rate capability of 73% at 1000 mV s⁻¹.