Synthesis and functionalization of advanced 2D transition metal carbides (MXENE): application to oxygen electroCATalysis. – MXENE-CAT

Development of new synthesis to control the MXene core and surface compositions. In this way, the MAX phases precursors of the MXenes, are prepared with adjusted compositions. The influence of the nature of the etching agent during the MXene synthesis is then studied.
Development of optimized free noble metals catalysts using functionalized MXenes as active supports of different oxides (Co, Ni, Fe) or sulfides (Mo). These oxides or sulfides are known to be efficient catalysts for the targeted reactions (OER, HER, ORR) at low overpotentials with high charge transfer kinetics.
Development of a complete characterization protocol based on the combination of long range (XRD) and short-range (STEM-EELS & NMR) structural orders, together with electronic structure characterizations (XPS & STEM-EELS) allowing to correlate the crystallography and electronic structure. The interpretation of the data obtained on these complex materials will be made quantitative thanks to a strong theoretical supports based on density functional theory simulations (RMN, XPS, EELS). This theoretical support is of utmost relevance in order to establish the chemistry/structure/properties relationship allowing to efficently guide the experimental works towards the most promissing MXene material for HER, OER and/or ORR. In addition, the interplay between surface functionalization and chemical composition of the MX core will be scrutinize by synthesizing (Ti,Mo)n+1CnTx solid solutions in order to evidence potential synergetic effects in terms of the electronic/catalytic properties that we aim at developing in the project.
Evaluation of the catalysts performances by classical electrochemical techniques combined with original in situ and operando electrochemical characterizations down to the single nano-objet level, in order to determine, inter alia, the involved reaction mechanisms and the morpho-structural restructurations during the reactions, allowing to guide towards the best catalyst formulation.

The MXENECAT project has led so far:

- To develop etching agents allowing to modulate the Ti3C2Tx surface chemistry and thus to adapt the protocol according to the targeted application.
- To better understand the experimental signals (XRD, XPS, Raman, EELS) of these complex materials that can serve as a guide for the MXene community in order to characterize them more precisely. The contribution of DFT simulations to interpret the experimental data should be emphasized.
- The development of stable and very active catalysts for OER and HER, two key reactions involved in applications related to energy conversion (electrolyser).
- The writing of 3 scientific articles in peer-reviewed journals, two others in submission and one popular article.

- Synthesis and characterization of Ti/Mo-based MXenes solid solutions (and the corresponding MAX phases) and use of the most promising as an electrocatalyst support for the various studied reactions (OER, ORR, HER).
- Optimization of sulfide/MXene compositions for the development of high-performance and stable electrocatalysts for the HER reaction.
- Development of new MXene-based composites containing oxides with Co, Fe and/or Ni elements in order to obtain highly efficient electrocatalysts for OER and ORR, on the basis of the already obtained results on the Co-LDH@Ti3C2Tx composite. Different approaches will be explored such as the polyol route, wet impregnation or electrodeposition.
- To better understand the surface properties of MXenes (one of the major aims of the project). Given that the possibilities of EELS characterizations are now relatively well established, we will now focus on the modeling of experimental data in XPS and NMR. This study combining experimental and theoretical approaches is completely original and should provide valuable data to the MXenes community to better understand these materials in the targeted applications. NMR dipole correlation experiments will also be performed to see the spatial proximity of the observed terminal groups.
- Work is planned to study these materials in TEM operando and in situ Raman and IR to study the surface restructuring phenomena during the reaction processes in order to better understand their mode of operation and thus guide us towards the optimized catalyst formulation.

Several communications in international and national conferences.
3 scientific articles in peer-reviewed journals and one popular article:

1. MXene Supported Cobalt Layered Double Hydroxyde Nanocrystals: Facile Synthesis Route for a Synergistic Oxygen Evolution Reaction Electrocatalyst
M. Benchakar et al. Advanced Materials Interfaces, 6, 1901328 (2019)
hal.archives-ouvertes.fr/hal-02324997/document
2. One MAX phase, different MXenes: A guideline to understand the crucial role of etching conditions on Ti3C2Tx surface chemistry
M. Benchakar et al. Applied Surface Science, 530, 147209 (2020)
hal.archives-ouvertes.fr/hal-02904636/document
3. On a two-dimentional MoS2/Mo2CTx hydrogen evolution catalyst obtained by the topotactic sulfurization of Mo2CTx MXene”
M. Benchakar et al. Journal of The Electrochemical Society, 167, 124507 (2020)
hal.archives-ouvertes.fr/hal-02937660/document
4. Le MXène : un mille-feuille aux multiples talents
L. Loupias et al. MicroScoop, N°80, juillet 2019. Tirage : 2500 exemplaires.
hal.archives-ouvertes.fr/hal-02325186/document

The MXENE-CAT project is focused on two main objectives (i) the synthesis, functionalization and advanced characterization of new 2D transition metal carbides, so-called MXenes, and (ii) their development, including composites, for applications as noble-metal free electrocatalysts for key reactions of the energy transition: oxygen evolution and reduction reactions (OER and ORR).
The MXenes is among the newest and largest family of 2D materials with already demonstrated applications in diverse fields (e.g. energy storage and conversion or electromagnetic interference shielding). They are synthesized by selectively etching the A elements from Mn+1AXn phases, a family of 70-plus different members of layered ternary carbonitrides, where M is a transition metal, A is a group 13 or 14 (i.e. group IIIA or IVA) element, X is C and/or N, and n = 1 to 3. Beyond the possibility to tune the MXenes’ composition and related properties by changing the MAX phase precursor, the etching process is also a key step since it leads to the surface functionalization of the MXene sheets with different T terminal groups (F, OH or O), which also deeply modify the MXenes properties. Although crucial for many applications, the MXenes’ functionalization is still in its infancy because of the limited number of etching processes and the need for characterization protocols allowing the accurate determination of the nature and location of the T-groups.
Our first objective is thus to develop new etching protocols, beyond the classical acidic media, coupled to post-treatments (thermal, chemical) to control and optimize the MXenes’ surface functionalization for the target applications. In addition, we will also develop a complete characterization protocol based on the combination of long range (XRD) and short-range (STEM-EELS & NMR) orders, together with electronic structure characterization (XPS & STEM-EELS). The interpretation of the data obtained on these complex materials will be made quantitative thanks to a strong theoretical supports based on density functional theory simulations (RMN, XPS, EELS). This theoretical support is of utmost relevance in order to establish the chemistry/structure/properties relationship allowing to efficently guide the experimental works towards the most promissing MXene material for a given application (OER and ORR in the present context). In addition, the interplay between surface functionalization and chemical composition of the MX core will be scrutinize by synthesizing (Ti,M’)n+1CnTx solid solutions with M’ = Nb or Mo (i.e. relevant elements for OER and ORR) in order to evidence potential synergetic effects in terms of the electronic/catalytic properties that we aim at developing in the project.
Based on the fundamental knowledge concerning the synthesis of new functional MXenes, our second objective is to optimize MXene and MXene-based composites (functionalized with Co3O4) in order to propose a performant and stable noble metal-free catalyst formulation capable of activating OER or ORR (and potentially both reactions) at low overpotentials with high charge transfer kinetics. The performances of these catalysts will be evaluated by classical electrochemical techniques combined with original in situ and operando electrochemical characterizations down to the single nano-objet level, in order to determine, inter alia, the involved reaction mechanisms and the morpho-structural restructurations during the reactions.
Beyond the target applications, the findings and the deeper understanding regarding the synthesis of well-controlled functionalized MXene-based systems and their advanced characterization should benefit the entire, and rapidly growing, MXene community and we expect this fundamental project to open the way to a whole new field of research. We could then hope to emerge as one of the national and international leading consortia on MXene synthesis and characterization.