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Identifying and understanding charge storage mechanisms is important for advancing energy storage. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradaic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti3C2T x MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.

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The diverse family of two-dimensional (2D) carbides, nitrides, and carbonitrides of early transition metals called “MXenes” has been quickly growing, exhibiting excellent optical, electronic, and mechanical properties. Recently, many attractive characteristics of MXenes have been demonstrated, including plasmonic behavior, optical nonlinearities, and highly efficient photothermal conversion, enabling their successful utilization in a variety of optical and optoelectronic devices. In this article, we provide an overview of selected optical phenomena and related device applications of MXenes, such as plasmonics, photothermal effects, ultrafast optics, and nonlinear optics. We also provide a perspective on future research directions for realizing the full potential of MXenes for photonic applications. With the versatility of available compositions and their exceptional optical properties, MXenes have the potential to facilitate development of many novel tunable nanophotonic devices.

MXenes constitute a rapidly growing family of 2D materials that are promising for optoelectronic applications because of numerous attractive properties, including high electrical conductivity. However, the most widely used titanium carbide (Ti3C2Tx) MXene transparent conductive electrode exhibits insufficient environmental stability and work function (WF), which impede practical applications Ti3C2Tx electrodes in solution‐processed optoelectronics. Herein, Ti3C2Tx MXene film with a compact structure and a perfluorosulfonic acid (PFSA) barrier layer is presented as a promising electrode for organic light‐emitting diodes (OLEDs). The electrode shows excellent environmental stability, high WF of 5.84 eV, and low sheet resistance RS of 97.4 Ω sq−1. The compact Ti3C2Tx structure after thermal annealing resists intercalation of moisture and environmental contaminants. In addition, the PFSA surface modification passivates interflake defects and modulates the WF. Thus, changes in the WF and RS are negligible even after 22 days of exposure to ambient air. The Ti3C2Tx MXene is applied for large‐area, 10 × 10 passive matrix flexible OLEDs on substrates measuring 6 × 6 cm. This work provides a simple but efficient strategy to overcome both the limited environmental stability and low WF of MXene electrodes for solution‐processable optoelectronics.