Tuning The Nanostructure Of 2D Ti₃C₂ Mxene For Optimization Of The Electrochemical Performance

MXenes are a quickly growing class of two-dimensional?2D?materials comprised of transition metal carbides or nitrides.Titanium carbide?Ti₃C₂T_x,T refers to the surface groups of=O,-OH and-F,etc.?is the first reported and the best studied MXene with ultrahigh electronic conductivity among all MXenes?>10000 S/cm?,good packing density?up to 4g/cm³?,and high proton induced pseudocapacitance?up to 1500 F/cm³?.However,the restacking of MXene sheets when they are reassembled into free-standing films impedes ion transport,limiting the overall rate performance.Plenty of work have been made on solving the restacking problem,including intercalation method,vertical alignment and template sacrifice method.However,most of these methods alleviate the restacking at certain extent but at the expense of other performance.Herein,we developed a novel concept of controlled oxidation to conveniently and effectively solve the restacking problem of Ti₃C₂T_x.We systematically studied the effects of in situ anodic oxidation on the nanostructure and electrochemical performance of Ti₃C₂T_x.We successfully explained the electrochemical performance improvement by analyzing the morphology,surface chemistry and interlayer structure change during the oxidation process.By controlling the anodic potential or oxidation cycles,we successfully tuned the oxidation degree of Ti₃C₂T_x and thus best optimized electrochemical performance was obtained.The capacitance retention at 2000 mV/s?with respect to the lowest scan rate of 5 mV/s?increases gradually from 38%to 66%by tuning the degree of anodic oxidation.The high volumetric capacitance of Ti₃C₂T_x was also retained after oxidation because the increase in interlayer spacing is in atomic level?⁰.2 nm?.We further developed a scalable chemical oxidation technique by high concentration sulfuric acid.We reported for the first time that the Ti₃C₂T_x can be slowly oxidized in warm?⁶0??high concentration sulfuric acid and the resulted Ti₃C₂T_x slurry can be re-dispersed and filtrated into porous films with increased interlayer spacing.Moreover,electrochemically inactive TiO₂ side product can be dissolved into the warm high concentration sulfuric acid and then washed away,which further increased the final electrochemical performance of Ti₃C₂T_x.To best optimize the ion transport path way,we reduced the flake size of Ti₃C₂T_x by probe sonication before oxidation and further improved performance was obtained.An 1?m thick?0.4 mg/cm²?Ti₃C₂T_x film with best optimized nanostructure showed an excellent rate performance,and 208 F/g or 756 F/cm³ high capacitance was retained even when the scan rate increases to 10000 mV/s,which is 64%of the value at low scan rate of 5 mV/s.Moreover,the structure optimized Ti₃C₂T_x also showed high electrochemical performance at a really high mass loading of 12 mg/cm²?48?m?which is comparable to the unoptimized film with low mass loading of 1 mg/cm².This high mass loading offers the Ti₃C₂T_x a high areal capacitance of 3.2 F/cm² and meets the need for commercial applications.Therefore,the technique developed in this work offered Ti₃C₂T_x practical values for the first time.At last,we developed a facile laser irradiation method for fast oxidizing the Ti₃C₂T_x and studied the effect of laser power on the nanostructure and electrochemical performance of Ti₃C₂T_x.By optimizing the laser power and scanning line density,we successfully controlled the oxidation degree of Ti₃C₂T_x and obtained improved electrochemical performance.It only takes¹ s for laser oxidizing an 1 cm×1 cm square Ti₃C₂T_x film,showing great advantage over other oxidation methods.Moreover,the laser technique is contactless to the samples and avoids the use of other chemicals,which is convenient and effective.By systematically studied the effect of laser power on the nanostructure of Ti₃C₂T_x,we showed how the electrochemical performance is improved.