[Paper Review] One-pot green process to synthesize controllable surface terminations MXenes in molten salts

Miao Shen, Weiyan Jiang+4 more|arXiv (Cornell University)|Feb 20, 2021

MXene and MAX Phase Materials4 citations

TL;DR

This study presents a one-pot, green electrochemical etching method using molten salts (MS-E-etching) to synthesize fluorine-free, metallic-free Ti3C2Tx MXenes with controllable surface terminations. By spatially separating cathodic reduction and anodic etching, the method enables direct in-situ conversion of Cl- to O- and S- terminations, yielding MXenes with 225 F/g capacitance at 1 A/g, while enabling salt recycling for sustainability.

ABSTRACT

Surface terminations for 2D MXene have dramatic impacts on physicochemical properties. The commonly etching methods usually introduce -F surface termination or metallic into MXene. Here, we present a new molten salt assisted electrochemical etching (MS-E-etching) method to synthesize fluorine-free Ti3C2Tx without metallics. Due to performing electrons as reaction agent, the cathode reduction and anode etching can be spatially isolated, thus no metallic presents in Ti3C2Tx product. Moreover, the Tx surface terminations can be directly modified from -Cl to -O and/or -S in one pot process. The obtained -O terminated MXenes exhibited capacitance of 225 and 205 F/g at 1 and 10 A/g, confirming high reversibility of redox reactions. This one-pot process greatly shortens the modification procedures as well as enriches the surface functional terminations. More importantly, the recovered salt after synthesis can be recycled and reused, which brands it as a green sustainable method.

Motivation & Objective

• To develop a sustainable, one-pot method for synthesizing MXenes with controlled surface terminations.
• To eliminate metallic impurities and fluorine-based terminations common in conventional etching.
• To enable direct, in-situ modification of surface terminations from -Cl to -O and/or -S in a single reaction vessel.
• To enhance MXene electrochemical performance through tailored surface chemistry.
• To demonstrate recyclability of the molten salt medium for environmental and economic sustainability.

Proposed method

• Utilizes molten salt electrolytes (e.g., KCl–NaCl) as both solvent and ion-conducting medium for electrochemical etching.
• Employs spatial separation of anode (Ti3AlC2 etching) and cathode (reduction of oxygen or sulfur species) to prevent metallic contamination.
• Introduces O- and S-containing species (e.g., H2O, SO4²⁻) into the molten salt to enable in-situ surface termination modification.
• Performs electrochemical etching at elevated temperatures (e.g., ~700 °C) to facilitate ion diffusion and reaction kinetics.
• Controls reaction conditions (voltage, time, salt composition) to tune surface terminations from -Cl to -O and -S.
• Recovers and reuses the molten salt after synthesis, minimizing waste and enhancing green credentials.

Experimental results

Research questions

• RQ1 Can a one-pot, green process achieve fluorine-free, metallic-free MXene synthesis with controllable surface terminations?
• RQ2 How can surface terminations be directly modified from -Cl to -O and -S in a single reaction step?
• RQ3 What is the electrochemical performance of O-terminated MXenes synthesized via this method?
• RQ4 Can the molten salt medium be effectively recycled without loss of functionality?
• RQ5 To what extent does spatial separation of anodic etching and cathodic reduction prevent metallic impurities in the final MXene product?

Key findings

• The MS-E-etching method successfully produced fluorine-free, metallic-free Ti3C2Tx MXenes with tunable surface terminations.
• Surface terminations were directly converted from -Cl to -O and/or -S in a single pot, eliminating multi-step post-treatment.
• The -O-terminated MXenes exhibited a specific capacitance of 225 F/g at 1 A/g and 205 F/g at 10 A/g, indicating excellent rate capability.
• The molten salt electrolyte was recovered and reused, confirming the process’s sustainability and green credentials.
• Spatial separation of anodic etching and cathodic reduction effectively prevented metallic impurities in the final MXene product.
• The method significantly shortened synthesis procedures compared to conventional multi-step etching and surface modification routes.