Review: Transition-metal-doped MXenes as emerging catalysts for the hydrogen evolution reaction

Transition-metal (TM)-doped MXenes stand out as highly promising earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), paving the way for cost-effective and sustainable green hydrogen generation. This review surveys the latest developments in TM-doped MXene systems for HER, emphasizing how diverse fabrication routes, including in situ incorporation during MAX-phase synthesis and post-etching modifications via atomic layer deposition (ALD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical deposition, shape their electronic properties and catalytic efficiency. Insights from d-band center theory, operando spectroscopic techniques, and deliberate active-site tuning are integrated to illustrate pathways toward optimal hydrogen binding energetics (ΔG_H* ≈ 0 eV). Benchmark comparisons reveal that carefully engineered TM-doped MXenes frequently deliver overpotentials, Tafel slopes, mass activities, and other metrics that approach or outperform conventional noble-metal catalysts. The review also critically evaluates ongoing limitations, such as susceptibility to oxidation, dopant instability, and nanosheet restacking, while exploring architectural innovations and protective modifications as effective countermeasures. Looking ahead, the discussion highlights transformative opportunities in machine-learning-guided discovery, exploitation of quantum phenomena, and nature-inspired fabrication routes to bridge the gap toward practical, large-scale implementation.