Unveiling the Energy Storage Mechanism of MXenes under the Acidic Condition through Transitions of Surface Functionalizations

The high capacitive performance of MXenes in acidic electrolytes have made them potential electrode materials for supercapacitors. In this study, we conducted a structural analysis of MXene surface functionalizations by identifying the surface group distribution pattern and revealed the energy storage process of MXene surface chemistry by combining a complete Pourbaix stability diagram and density functional theory (DFT) calculations. The Pourbaix diagram indicated that pH controls the surface termination; an acidic pH generates favorable initial surfaces of MXenes with a specific distribution of functional groups (ten hydroxyls with respect to eighteen total locations for the selected surface unit). Using this, we report the charging and uncharging process of MXenes with transitions of surface oxygenic groups (-OH and -O) via hydrogen ion adsorptions and desorptions on the MXene surface. Our results demonstrated that transitions of surface functionalizations contribute to comparable pseudocapacitance and electrostatic capacitance for MXenes. These findings provide insights into understanding the MXene energy storage mechanism by controlling surface functionalizations through the experimental reaction environment and synthesis condition.