Citation: Liang, W.; Xu, R.; Nawwar, M.; Zhitomirsky, I. Multifunctional MXene–Fe 3 O 4 –Carbon Nanotube Composite Electrodes for High Active Mass Asymmetric Supercapacitors. Batteries 2023, 9, 327. https://doi.org/10.3390/ batteries9060327 Academic Editor: Marco Giorgetti Received: 7 April 2023 Revised: 6 June 2023 Accepted: 11 June 2023 Published: 16 June 2023 Copyright: © 2023 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/). batteries Article Multifunctional MXene–Fe 3 O 4 –Carbon Nanotube Composite Electrodes for High Active Mass Asymmetric Supercapacitors Wenyu Liang 1 , Rui Xu 1, *, Mohamed Nawwar 2 and Igor Zhitomirsky 3, * 1 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China; liangw@ustb.edu.cn 2 Technical Research Center (TRC), Cairo 12622, Egypt 3 Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S 1L7, Canada * Correspondence: ruixu@ustb.edu.cn (R.X.); zhitom@mcmaster.ca (I.Z.) Abstract: Ti 3 C 2 T x –Fe 3 O 4 –carbon nanotube composites were prepared for electrochemical energy storage in the negative electrodes of supercapacitors. The electrodes show a remarkably high areal capacitance of 6.59 F cm −2 in a neutral Na 2 SO 4 electrolyte, which was obtained by the development of advanced nanofabrication strategies and due to the synergistic effect of the individual components. Enhanced capacitance was achieved using the in-situ synthesis method for the Fe 3 O 4 nanoparticles. The superparamagnetic behavior of the Fe 3 O 4 nanoparticles facilitated the fabrication of electrodes with a reduced binder content. Good mixing of the components was achieved using a celestine blue co-dispersant, which adsorbed on the inorganic components and carbon nanotubes and facilitated their co-dispersion and mixing. The capacitive behavior was optimized by the variation of the electrode composition and mass loading in a range of 30–45 mg cm −2 . An asymmetric device was proposed and fabricated, which contained a Ti 3 C 2 T x –Fe 3 O 4 –carbon nanotube negative electrode and a polypyrrole–carbon nanotube positive electrode for operation in an Na 2 SO 4 electrolyte. The asymmetric supercapacitor device demonstrated high areal capacitance and excellent power-density characteristics in an enlarged voltage window of 1.6 V. This investigation opens a new avenue for the synthesis and design of MXene-based asymmetric supercapacitors for future energy storage devices. Keywords: MXene; iron oxide; superparamagnetism; polypyrrole; supercapacitor 1. Introduction Advanced energy storage technology is of significance today, due to the energy crises challenge worldwide [1–5]. MXenes have been extensively investigated as new energy storage materials [6–8] since their discovery in 2011 [9]. Due to their large surface area, good conductivity and rich surface chemistry, MXenes have demonstrated great potential for the development of high performance pseudocapacitors [10–13]. As the most widely studied material among the various types of MXenes, Ti 3 C 2 T x with the accordion-like particle shape contains alternation of Ti and C layers with a number of functional terminations, such as -O, -F and -OH [9]. Many efforts have been made to enhance the electrochemical performance of Ti 3 C 2 T x in composites with polymers [14–17], oxides [18–20], hydroxides [21,22] and carbon-based materials [23–25]. These studies showed that combining Ti 3 C 2 T x with other capacitive ma- terials is a promising strategy for the development of advanced supercapacitor electrodes. Fe-based materials, such as FeOOH, Fe 2 O 3 and Fe 3 O 4 , exhibit high capacitance in a relatively large negative potential range [26–30]. Therefore, they have great potential to be considered as candidates for composites with MXene with enhanced electrochemical performance. Ma et al. [31] employed Fe 2 O 3 nanoparticles intercalating flexible MXene hybrid paper and achieved an ultrahigh volumetric capacitance of 2607 F cm −3 as well as excellent cycling performance. In another research, a freestanding Ti 3 C 2 /FeOOH hybrid Batteries 2023, 9, 327. https://doi.org/10.3390/batteries9060327 https://www.mdpi.com/journal/batteries