**1. Introduction**

The increasingly prominent climate changes and limited availability of fossil fuel issues have stimulated a tremendous amount of research interest in highly efficient and clean energy storage and conversion devices [1–5]. The development of new classes of advanced two-dimensional (2D) layered materials, including graphene, MoS2, phosphorene, have promoted tremendous technological progress in those energy resources (e.g., supercapacitor, different-type metal ion batteries (MIBs)), which are attributed to their extraordinary properties [2,6–17]. The enhancement in the performance of these devices by incorporating layered materials indicated that the 2D materials commonly possess the following two unique characteristics:


**Citation:** Ji, C.; Cui, H.; Mi, H.; Yang, S. Applications of 2D MXenes for Electrochemical Energy Conversion and Storage. *Energies* **2021**, *14*, 8183. https://doi.org/10.3390/en14238183

Academic Editor: Lyes Bennamoun

Received: 12 September 2021 Accepted: 2 December 2021 Published: 6 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

As a novel family of transition metal carbides, carbonitrides, or nitrides, (e.g., Ti2CTx, Ti3CNTx, V2CTx), MXenes have attracted particular research enthusiasm since their discovery in 2011 [20]. The delaminated MXenes have elevated research on the novel 2D materials to a new era in the energy related fields due to their prominent and attractive properties, including adjustable composition, hydrophilic, conductivity, thermal conductivity, tunable band gap, and excellent mechanical strength [21–26]. However, MXenes themselves are easily to spontaneously stack and aggregate into multilayer structures due to strong van der Waals forces between the layers, which lead to the decrease in the interlayer space and sluggish kinetics for redox activities or intercalation/deintercalation behaviors [27]. Thus, the capabilities to enlarge the space between the interlayer and elaborately modulate pathways or active sites for MXenes draw lots of attention. As experienced by other 2D materials, several strategies have been proposed for successfully solving the challenges mentioned above, which includes the surface modification, heteroatom doping, or crumpling [28–33]. Recent investigation also demonstrated that the MXene are versatile for self-assembling into specific configuration with geometric flexibility [34]. In this sense, substantial efforts have also been made in methodologies of patterned MXene-based composite film or MXene-based conductive ink for fabricating more types of energy storage devices, which encompasses vacuum filtration, mask-assisted filtration, screen printing, extrusion printing technique, and directly writing, etc. In general, the materials and electrodes for various energy storage devices need to fulfill the requirements of stable physicochemical properties, superb energy storage performance, and high electron/ion conduction [35]. By now, available and optional strategies for enhancing the ion and electron transfer ability of MXene-based materials and the methodologies to design satisfactory patterns are still limited due to the primary development stage of MXenes [36–38]. It is highly needed to get intensive and systematical understanding about the progress of elaborately constructing advanced MXene-based electrode materials and versatile MXene-based flexible electrodes with favorable configurations. In this regard, the efforts on the synthetic method of MXene, the enhancement in electrochemical performance of MXene materials, and the research strategies and technologies to fabricate flexible electrodes for energy storage are briefly summarized. Meanwhile, the challenges and perspectives of MXene-based materials and technologies for the future energy storage applications are presented.
