Activating lattice oxygen of two-dimensional M _nX _n−1O₂ MXenes via zero-dimensional graphene quantum dots for water oxidation

The poor oxygen evolution reaction (OER) activity of two-dimensional (2D) transition metal carbides (MXenes) is a major obstacle to their application in highperformance water splitting and fuel cells due to the high energy barriers for the absorption of intermediates. Here, we demonstrate that the lattice oxygen of M_nX_n−1O₂ MXenes can be activated by 0D graphene quantum dots (GQDs), thereby activating the OER via the lattice-oxygen oxidation mechanism (LOM) instead of the conventional adsorbate evolving mechanism. The pH-dependent OER activity of M_nX_n−1O₂@GQDs and ¹⁸O isotope-labelling experiments with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) provide the direct evidence of LOM. Interestingly, the activated lattice oxygen amount can be controlled by the GQDs. The as-prepared 0D/2D Ti₃C₂O₂@GQDs heterostructure delivers a highly reduced overpotential of 390 mV (bare Ti₃C₂O₂: 530 mV) at a benchmark current density of 10 mA cm⁻². Through optimizing the thickness and the additional conductive substrate, the overpotential at 10 mA cm⁻² decreases to 250 mV, while the Tafel slope is reduced to 39 mV dec⁻¹; these values indicate the as-prepared heterostructure is superior to the state-of-the-art MXene-based OER catalysts. This work provides a new strategy to enhance the OER activity of M_nX_n−1O₂ and extends the application of LOM from perovskite to MXenes. [Figure not available: see fulltext.].