Isolation of pseudocapacitive surface processes at monolayer MXene flakes reveals delocalized charging mechanism

Pseudocapacitive charge storage in Ti3C2Tx MXenes in acid electrolytes is typically described as involving proton intercalation/deintercalation accompanied by redox switching of the Ti centres and protonation/deprotonation of oxygen functional groups. Here we conduct nanoscale electrochemical measurements in a unique experimental configuration, restricting the electrochemical contact area to a small subregion (0.3 mu m(2)) of a monolayer Ti3C2Tx flake. In this unique configuration, proton intercalation into interlayer spaces is not possible, and surface processes are isolated from the bulk processes, characteristic of macroscale electrodes. Analysis of the pseudocapacitive response of differently sized MXene flakes indicates that entire MXene flakes are charged through electrochemical contact of only a small basal plane subregion, corresponding to as little as 3% of the flake surface area. Our observation of pseudocapacitive charging outside the electrochemical contact area is suggestive of a fast transport of protons mechanism across the MXene surface. MXenes are two-dimensional nanomaterials that can be used as supercapacitors to store electrical energy. Here, the authors isolated chemical response from a single MXene flake and provide a new understanding of the surface chemistry of MXenes.

Automated summary

Through unique experimental configuration, researchers found that the pseudocapacitive charge storage in Ti3C2Tx MXenes in acid electrolytes involves proton intercalation/deintercalation, redox switching of the Ti centres, and protonation/deprotonation of oxygen functional groups. They also observed pseudocapacitive charging outside the electrochemical contact area, suggesting a fast transport of protons mechanism across the MXene surface. These findings provide new insights into the surface chemistry of MXenes.