Effect of pinholes in Nb4C3 MXene sheets on its electrochemical behavior in aqueous electrolytes

<p>Two-dimensional (2D) niobium carbide, Nb<sub>4</sub>C<sub>3</sub>T<sub>x</sub> (T<sub>x</sub>: O, OH, and F), a representative member of the 43 MXene structural motif, has shown promising electrochemical performance in acidic electrolytes. The capacitive performance of Nb<sub>4</sub>C<sub>3</sub>T<sub>x</sub> in neutral aqueous electrolytes has been reported as moderate, but little effort has been made to improve it. In this paper, we report a method to introduce nanopores (pinholes) in Nb<sub>4</sub>C<sub>3</sub>T<sub>x</sub> MXene flakes by adjusting the etching time. The pinholes generated during the etching process improve ion diffusion pathways, which are otherwise hindered by the restacking of the 2D flakes. The “holey Nb<sub>4</sub>C<sub>3</sub>T<sub>x</sub>” shows a 50 % improved rate capability at charge/discharge time scales of 1–2 s in 1 M Li<sub>2</sub>SO<sub>4</sub>, Na<sub>2</sub>SO<sub>4</sub>, and (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> electrolytes. Our strategy of controlling the permeability of Nb<sub>4</sub>C<sub>3</sub>T<sub>x</sub> sheets can potentially be applied to other MXenes, providing guidance for improving the capacitance and rate capability of 2D materials.</p>