Activating lattice oxygen of two-dimensional MnXn−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 MnXn−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 MnXn−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 MnXn−1O2 and extends the application of LOM from perovskite to MXenes.