Beyond static descriptors: the need for operando insights into MXene catalysts for the hydrogen evolution reaction

MXenes have emerged as promising electrocatalysts for the hydrogen evolution reaction (HER) owing to their tunable electronic structure, diverse surface terminations, and high electrical conductivity. Over the past decade, numerous theoretical studies have proposed activity descriptors to rationalize HER performance on MXene surfaces, including hydrogen adsorption free energy, surface termination composition, electronic structure of transition-metal centers, and defect formation. While these descriptors provide useful guidelines for understanding catalytic trends, they are typically derived from static surface models or ex situ characterization, which may not accurately represent the dynamic catalyst-electrolyte interface under operating electrochemical conditions. During HER, interactions with the electrolyte, applied potential, and reaction intermediates can modify surface terminations, alter metal electronic structure, reorganize interfacial water networks, and generate new defect sites. Such processes can significantly influence catalytic activity and challenge the validity of descriptors defined for idealized surfaces. In this review, we discuss the current understanding of activity descriptors for MXene-based HER electrocatalysts and highlight the importance of probing the active interface under reaction conditions. Particular emphasis is placed on the role of operando spectroscopic techniques, including Raman spectroscopy, infrared spectroscopy, X-ray absorption spectroscopy, and photoelectron spectroscopy, in directly monitoring the evolution of surface chemistry and electronic structure during catalysis. By bridging theoretical predictions with experimental observations, operando investigations provide critical insight into the dynamic nature of MXene catalytic interfaces and offer pathways for validating descriptor-activity relationships. These advances are expected to guide the rational design of next-generation MXene electrocatalysts for efficient hydrogen production.