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Synthesis of novel MXenes materials

Grant number: 20/15632-6
Support type:Scholarships in Brazil - Post-Doctorate
Effective date (Start): March 01, 2021
Effective date (End): February 28, 2023
Field of knowledge:Physical Sciences and Mathematics - Chemistry - Physical-Chemistry
Principal Investigator:Sydney Ferreira Santos
Grantee:Rafael Kenji Nishihora
Home Institution: Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas (CECS). Universidade Federal do ABC (UFABC). Ministério da Educação (Brasil). Santo André , SP, Brazil
Associated research grant:19/22183-6 - Electrocatalysis VI: fundamental and applied aspects of emerging and classical problems in electrochemical energy conversion, AP.TEM

Abstract

The recent discovery of a 2D-layered compounds family, namely "MXenes", has attracted great attention of the scientific community, motivated by their peculiar structural and electronic characteristics, allowing their use on several potential applications. MXenes is the denomination of group of transition metal carbides, nitrides or carbonitrides obtained by chemical delamination of 3D ternary (or quaternary) compounds known as MAX phases. The precursor compounds (MAX phases) have defined stoichiometry as M(n+1)AXn, with n = 1, 2, or 3, in which "M" is a d-block transition metal, "A" is an element of the group 13 or 14 (e.g., Si, Al, Ge or Sn) and "X" is carbon, nitrogen, or both. These phases have hexagonal structure (space group P63/mmc), where the layers "M" and "A" are intercalated. The "X" atoms are located into the interstitial position of the octahedra formed by "M" elements. MXenes have as general formula M(n + 1)XnTx (n = 1-3), where M represents a transition metal (such as Sc, Ti, Zr, Nb and others), X is carbon and/ or nitrogen and Tx represents the hydroxyl, oxygen or fluorine terminations derived from the synthesis procedure. Since the discovery of Ti3C2Tx 2D compound in 2011, nearly thirty compounds of this class have already been synthesized and many more were predicted theoretically. Some MXenes have been investigated for energy conversion and storage (electrodes for supercapacitors, solar cells, Li and Na ion batteries, and fuel cells) demonstrating high potentiality for these applications but many others MXenes synthesized were not investigated concerning their electrocatalytic properties while others were theoretically predicted but not synthesized so far. The first challenge to develop novel MXenes is to produce appropriate MAX phase precursors which are synthesized by reactive sintering. This solid-state route demands appropriate setting of several processing variables which must be carefully adjusted since commonly there is a competition between the possible phases formed during the material's processing, demanding a good knowledge of thermodynamics and kinetics of the solid-state phase transformations that take place during the sintering process to achieve high purity MAX phases. To obtain the MXenes, the MAX phase precursor must be chemically etched and exfoliated in other to remove the "A" element and produce lamellas of the binary carbide. There are several protocols reported in the literature to produce MXenes and a constant search to improve these protocols aiming to decrease the use of hazardous chemicals such as HF. The chemical compositions and microstructures of MXenes have been engineered aiming to enhance their properties. An interesting approach to tune the properties of MXenes, still underexplored, is the "alloying" of the MXene compound with a second transition metal. In the case of Ti-based MXenes, Ti can be mixed with different transition metals such as Zr, Nb, and V aiming to produce MXene solid solutions. This approach was used to produce (Ti0.5V0.5)3C2, (Nb0.8,Ti0.2)4C3Tx and (Nb0.8,Zr0.2)4C3Tx MXenes. This approach can be used to design the compound chemical composition and also its microstructure (synthesized ordered or disordered solid solutions, for instance). In addition, the controlling of the functional groups (O, F, or OH), originated from the synthesis route or surface treatments, on the surface of the MXenes is another approach to tune the electrocatalytic properties of MXenes. In this project, novel MXenes with designed chemical compositions, microstructures, and surface chemistry with be synthesized, characterized and tested as electrocatalysts for HER and ORR.