Deciphering the influence of fluorine on the electrochemical performance of MAX and derived MXene by selective electrophilic fluorination

The emerging class of MAX phases and corresponding MXenes offers a unique advantage of tunable surface functionalities, such as -O, -OH, and -F, which endow them with excellent electrochemical activity. This study focuses on the fluorination of the MAX phases and corresponding MXenes using an electrophilic fluorinating agent, Selectfluor (SF). The fluorination process was carried out to selectively fluorinate the MAX phase while minimizing etching effects, showcasing the distinct role of the electrophilic fluorine precursor over the conventional nucleophilic fluorinating agents. Intriguingly, the fluorinated MAX phase, with a fluorine content of 3.21 at%, demonstrated significantly enhanced electrochemical performance, exhibiting a two-fold increase in the specific capacitance compared to pristine MAX. Moreover, selective fluorination is further extended to MXene derivatives prepared through the conventional route. Using SF, we facilitated electrolyte-ion transport through the functionalized surface, resulting in an enhancement of ∼1400% in energy storage capacity after the fluorination of MXene. The observed improvement in the electrochemical performance can be attributed to the formation of electrochemically active Ti-F and C-F moieties at the surface as opposed to surface hydroxyls and oxidized MXene. The high electronegativity of fluorine atoms contributes to fast ion diffusion at the electrode surface, enhancing wettability and leading to superior electrochemical performance. As a result, our work introduces a novel and simple solution-based methodology for selectively fluorinating MAX phases and their MXene derivatives, unlocking their potential for enhanced electrochemical applications. This methodology can be extended to various MAX phases and their derivatives, offering precise control over the surface moieties in electrochemical systems where fine-tuning is essential for optimal performance. Overall, this study significantly contributes to advancing the understanding and utilization of fluorinated MAX and MXene materials in electrochemical energy storage and beyond.