Electrocatalytic Performance of M₅X₄ MXenes for Hydrogen Evolution Reaction

M₅X₄ MXenes, 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 M₅X₄ MXenes, 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 Mo₄VC₄ exhibits 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)₅C₄ and (TiTa)₅C₄ display 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 M₅X₄ MXenes, establishing TMA-Mo₄VC₄ as a promising candidate for sustainable hydrogen production.