Search for: All records

Abstract M₅X₄MXenes, a subclass of 2D transition metal carbides, have attracted attention as the thickest 2D material synthesized. Early studies show their promising electrocatalytic activity but overlooked how metal composition and interlayer spacing affect hydrogen evolution reaction (HER). To address this gap, three M₅X₄MXenes, Mo₄VC₄, (TiTa)₅C₄, and (TiNb)₅C₄, are systematically studied and their interlayer spacing and composition modulated through ion exchange with tetramethyl ammonium (TMA⁺vs. Li⁺), providing new insights into their HER activity. These findings reveal that TMA⁺‐intercalated Mo₄VC₄exhibits superior HER activity, achieving areal and gravimetric overpotentials of 172 and 90 mV, respectively, due to its composition (presence of Mo) and expanded interlayer spacing that enhances proton accessibility. The Li⁺exchange increases the overpotential to 212 and 131 mV at 10 mA areal and gravimetric current density, respectively, as reduced interlayer spacing restricts access to active Mo sites. In contrast, (TiNb)₅C₄and (TiTa)₅C₄display higher overpotentials, making them more suitable for supercapacitor or aqueous battery applications due to the wider electrochemical window. This study provides critical insights into the interplay between metal composition and interlayer engineering in M₅X₄MXenes, establishing TMA‐Mo₄VC₄as a promising candidate for sustainable hydrogen production. 

Abstract MAX phases, ternary transition metal carbides and nitrides, represent one of the largest families of layered materials. They also serve as precursors to MXenes, two‐dimensional (2D) carbides and nitrides. The possibility of oxygen substitution in the carbon sublattice, forming oxycarbide MAX phases and MXenes, was recently reported using secondary ion mass spectrometry. However, while the effect of oxygen substitution on the properties of MXenes was investigated, little is known about its effect on the properties of MAX phases. Here, we explore the influence of process parameters (e.g., particle size, synthesis temperature, annealing time, etc.) and oxygen presence in the lattice on the oxidation resistance of Ti₃AlC₂MAX phase powders. We show that X‐ray diffraction measurements can identify oxygen substitution and assist in selecting MAX precursors to synthesize stable and highly conductive MXenes. Eliminating the substitutional oxygen from the MAX phase lattice increases the onset of oxidation by 400°C, from approximately 490 to 890°C. Finally, we discuss the impact of oxygen substitution in the MAX phases on the synthesis of MXenes and their resulting properties.