Thermal-assisted electrochemical synthesis of fluoride-free MXenes for efficient energy application

Abstract

Ultrathin two-dimensional (2D) materials have been intensively explored and exploited in a variety of sectors because of their kinetics-favored design and unique physicochemical features since the discovery of graphene. Specifically, 2D transition metal carbides and/or nitrides (MXene) have piqued the interest of researchers since their first exfoliation in 2011 owing to their exceptional electrical conductivity, stability, hydrophilicity, and porosity, leading to extensive investigations in electrocatalysis and energy storage applications. Over the last decade, direct use of hydrofluoric (HF) acid has been the most frequently applied approach for the synthesis of MXene. However, the acute poisonousness of HF acid restricts energy-related applications and mass-scale manufacture of MXenes. Therefore, it is highly motivating for researchers to develop a safe protocol to fabricate MXenes. The work aims to synthesize MXenes through a universal approach based on thermal-assisted electrochemical etching and explore their unique properties. The unique synthesis pathway for MXenes without the hazardous HF acid is shown, allowing the material to be produced within nine hours at 55°C, and its diverse applications are also discussed. As a proof-of-concept experiment to illustrate the high robustness and versatility of the synthesized HF-free MXene, a bifunctional zinc air/ion battery, and flexible zinc ion battery were constructed which exhibited a long cycle life of over 150 cycles with 100% retention. The 1D HF-free MXene with 98 nm width is functionalized by a mixture of O, OH, and/or Cl groups, resulting in a reduced lattice mismatch towards platinum with an electrochemical activity phase compared to HF-etched MXene, where Pt growth on the MXene sample is 1.15 nm in size. Such functionalization can also provide an effective linker to the transition metal ion, and the Pt/MXene heterostructure exhibits a low overpotential (33.3 mV) for generating hydrogen gas in an acidic solution without the competing behavior between fluorine to hydrogen. In conclusion, a general strategy for rapid and HF-free synthesis of 2D layered MXenes is proposed, which is especially appealing for efficient heavy metal absorption, multifunctional electrocatalytic, and energy storage applications. Following the successful fabrication of the HF-free precursor, the technique presented in this work was widely extended to other fields adopting 1D nanowires in 1D/3D architectures. Taking the advantage of epitaxial growth of MXenes, reactive metal substrate interaction can be tweaked on 2D MXene support, and our work demonstrated in this thesis paves the way for a novel, scalable, and safe approach for building a heterostructure catalyst without the use of explosive reduction gas.