The rapid advancement in the miniaturization, integration, and intelligence of electronic devices has escalated the demand for customizable micro‐supercapacitors (MSCs) with high energy density. However, efficient microfabrication of safe and high‐energy MXene MSCs for integrating microelectronics remains a significant challenge due to the low voltage window in aqueous electrolytes (typically ≤0.6 V) and limited areal mass loading of MXene microelectrodes. Here, we tackle these challenges by developing a high‐concentration (18 mol kg⁻¹) “water‐in‐LiBr” (WiB) gel electrolyte for MXene symmetric MSCs (M‐SMSCs), demonstrating a record high voltage window of 1.8 V. Subsequently, additive‐free aqueous MXene ink with excellent rheological behavior is developed for three‐dimensional (3D) printing customizable all‐MXene microelectrodes on various substrates. Leveraging the synergy of a high‐voltage WiB gel electrolyte and 3D‐printed microelectrodes, quasi‐solid‐state M‐SMSCs operating stably at 1.8 V are constructed, and achieve an ultrahigh areal energy density of 1772 μWh cm⁻² and excellent low‐temperature tolerance, with a long‐term operation at −40°C. Finally, by extending the 3D printing protocol, M‐SMSCs are integrated with humidity sensors on a single planar substrate, demonstrating their reliability in miniaturized integrated microsystems.

MXene, a new intercalation pseudocapacitive electrode material, possesses a high theoretical capacitance for supercapacitor application. However, limited accessible interlayer space and active sites are major challenges to achieve this high capacitance in practical application. In order to stimulate the electrochemical activity of MXene to a greater extent, herein, a method of hydrothermal treatment in NaOH solution with reducing reagent-citric acid is first proposed. After this treatment, the gravimetric capacitance of MXene exhibits a significant enhancement, about 250% of the original value, reaching 543 F g⁻¹ at 2 mV s⁻¹. This improved electrochemical performance is attributed to the tailoring of an interlayer structure and surface chemistry state. An expanded and homogenized interlayer space is created, which provides enough space for electrolyte ions storage. The –F terminations are replaced with O-containing groups, which enhances the hydrophilicity, facilitating the electrolyte’s accessibility to MXene’s surface, and makes MXene show stronger adsorption for electrolyte ion-H⁺, providing sufficient electrochemical active sites. The change in terminations further leads to the increase in Ti valence, which becomes more prone to reduction. This work establishes full knowledge of the rational MXene design for electrochemical energy storage applications.

Exploring anode materials with an excellent electrochemical performance is of great significance for supercapacitor applications. In this work, a N-doped-carbon-nanofiber (NCNF)-supported Fe₃C/Fe₂O₃ nanoparticle (NCFCO) composite was synthesized via the facile carbonizing and subsequent annealing of electrospinning nanofibers containing an Fe source. In the hybrid structure, the porous carbon nanofibers used as a substrate could provide fast electron and ion transport for the Faradic reactions of Fe₃C/Fe₂O₃ during charge–discharge cycling. The as-obtained NCFCO yields a high specific capacitance of 590.1 F g⁻¹ at 2 A g⁻¹, superior to that of NCNF-supported Fe₃C nanoparticles (NCFC, 261.7 F g⁻¹), and NCNFs/Fe₂O₃ (NCFO, 398.3 F g⁻¹). The asymmetric supercapacitor, which was assembled using the NCFCO anode and activated carbon cathode, delivered a large energy density of 14.2 Wh kg⁻¹ at 800 W kg⁻¹. Additionally, it demonstrated an impressive capacitance retention of 96.7%, even after 10,000 cycles. The superior electrochemical performance can be ascribed to the synergistic contributions of NCNF and Fe₃C/Fe₂O₃.