Electrochemical Charge Storage Process of Ti₃C₂T_x in Neutral Aqueous Electrolyte

MXenes, a group of two-dimensional transition carbides/nitrides, have demonstrated remarkable performance as pseudocapacitive electrodes, owing to their unique transition metal oxide-like surface, large interlayer spacing and high electrical conductivity^[1]. The charge storage behavior of MXene electrodes are strongly dependent on the interaction between MXene surface chemistry and the electrolyte solution^[2]. In acidic electrolyte, the (de)intercalation of hydrated H⁺(from) into Ti₃C₂T<i>_x</i> interlayer leads to a reversible conversion between =O and -OH surface groups, which is accompanied by the Ti valence change^[3]. Consequently, Ti₃C₂T<i>_x</i> displays a pseudocapacitive behavior with a mirrored redox peaks emerged in the CV curve. In contrast, Ti₃C₂T<i>_x</i> electrodes typically exhibit EDL-like behavior in neutral aqueous electrolytes, with a rectangular CV and a sloping GCD curve within a narrow (ca. 1.2 V) potential window, resulting in a relatively lower capacitance (80 -120 F g^-1)^[4]. Recently, it has been found that the charge storage mechanism of MXene electrodes in different electrolytes can be elucidated through in-situ UV-Vis spectroscopy^[5].<br/>Engineering the MXene electrode and optimizing the electrolyte composition have the potential to further enhance the pseudocapacitive performance of MXene. An extended potential window of 1.6 V can be obtained by employing water-in-salt electrolyte (WIS). Using WIS electrolyte also induces a novel intercalation process that fully solvated Li⁺ ions intercalate into MXene layer at positive potential^[6]. Partially oxidizing Ti₃C₂T<i>_x</i> in the WIS electrolyte can further enhance the pseudocapacitance in the neutral aqueous electrolyte by activating a new surface redox reaction at &lt;−0.2 V. This is due to the higher valence of Ti (+2.86) in the partially oxidized Ti₃C₂T<i>_x</i>, which is more prone to be reduced than the Ti (+2.43) in the pristine MXene^[7].<br/>In our recent research, we try to improve the electrochemical performance of Ti₃C₂T<i>_x</i> by employing PEG-based Molecular crowding electrolyte (MCE) and designing MXene-based heterostructure. By introducing PEG as the molecular crowding agent into dilute LiTFSI electrolyte, a 25% increase in capacitance (100.8 F g^-1) is obtained in MCE. The capacitance increase is possibly due to additional intercalation process of fully-solvated Li⁺, accompanied by the co-insertion of the PEG molecules. Furthermore, We construct a Ti₃C₂@Naphthalenediimide (NDI) heterostructure, in which a n-type conjugated polyelectrolyte is incorporated with Ti₃C₂T<i>_x</i>MXene through a facial self-assemble process. Contributing from both the faradaic reaction from NDI and the pseudocapacitance from MXene, a high specific capacitance of 126.1 F g^-1is obtained in 1 M NH₄Cl electrolyte, which is 1.5 times higher than that of pristine Ti₃C₂T<i>_x</i>. Benefiting from the unique structure, Ti₃C₂@NDI also showed a superior rate performance of 102.8 F g^-1 at 10 A g^-1 (81% capacitance retention of 0.1 A g^-1) and a great cycling stability of 89% capacitance retention upon 10 000 cycles for NH₄⁺ storage.<br/>Reference:<br/>[1]Anasori et al., <i>Nature Reviews Materials</i>. <b>2017</b>, <i>2</i>, 16098.<br/>[2]Bergman et al., <i>Advanced Energy Materials.</i> <b>2023</b>, 202203154.<br/>[3]Shao et al., <i>ACS Energy Letters.</i> <b>2020</b>, <i>5</i>, 2873.<br/>[4]Lukatskaya et al., <i>Science.</i> <b>2013</b>, <i>341</i>, 1502.<br/>[5]Zhang et al., <i>Nature Energy.</i> <b>2023</b>, <i>8</i>, 567.<br/>[6]Wang et al., <i>ACS Nano.</i> <b>2021</b>, <i>15</i>, 15274.<br/>[7]Wang et al., <i>ACS Energy Letters.</i> <b>2022</b>, <i>7</i>, 30.