Rational Design of Highly Stable and Active MXene-Based Bifunctional ORR/OER Double-Atom Catalysts.

Designing highly active and bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts has attracted great interest toward metal-air batteries. Herein, an efficient solution to the search for MXene-based bifunctional catalysts is proposed by introducing non-noble metals such as Fe/Co/Ni at the surfaces. These results indicate that the ultrahigh activities in Ni1/Ni2- and Fe1/Ni2-modified MXene-based double-atom catalysts (DACs) for bifunctional ORR/OER are better than those of well-known unifunctional catalysts with low overpotentials, such as Pt(111) for the ORR and IrO2 (110) for the OER. Strain can profoundly regulate the catalytic activities of MXene-based DACs, providing a novel pathway for tunable catalytic behavior in flexible MXenes. An electrochemical model, based on density functional theory and theoretical polarization curves, is proposed to reveal the underlying mechanisms, in agreement with experimental results. Electronic structure analyses indicate that the excellent catalytic activities in the MXene-based DACs are attributed to the electron-capturing capability and synergistic interactions between Fe/Co/Ni adsorbents and MXene substrate. These findings not only reveal promising candidates for MXene-based bifunctional ORR/OER catalysts but also provide new theoretical insights into rationally designing noble-metal-free bifunctional DACs.