Enhanced Performance of Ti₃C₂T_x (MXene) Electrodes in Concentrated ZnCl₂ Solutions: A Combined Electrochemical and EQCM-D Study

The need for improved batteries and supercapacitors, which are not based on lithium compounds, promotes significant research efforts to find suitable alternative systems based on various mono and multivalent cations capable of delivering high energy and power density with good long-term stability. The progress in aqueous Zn-ion batteries and supercapacitors obtained over the past years lead to the development of new structures and compounds that enable revisable hosting of Zn-ions while keeping good structural integrity. Yet, as aqueous electrolytes involve also the generation and co-insertion of protons it is necessary to carefully define what is the charge storage mechanism in these Zn insertion compounds. In this work, the use of Ti₃C₂T_x as an anode for the Zn-ion system was evaluated for the first time in different ZnCl₂ concentrations. Remarkable changes in the charge storage mechanism and the performance of the Ti₃C₂T_x electrodes were observed by moving from dilute Zn electrolytes (1 M) to higher concentrations. The high acidity of the concentrated ZnCl₂ solutions results in an additional charge storage mechanism that arises from redox interactions between the MXene and the released protons and hence enhanced capacity values (60 mAh/g with good rate capability and capacity retention of 89.4% after 9000 cycles). In-situ EQCM-D measurements were employed to shed light on the charge storage mechanism of the Ti₃C₂T_x and the exact identity of the charge carrier inserted into the electrodes. The demonstrated EQCM-D based analysis provides precise quantification of the Zn-ion to proton ratio in the examined system and can be further applied for other aqueous Zn-ion electrodes and multivalent systems as well.