Doped MXene combinations as highly efficient bifunctional and multifunctional catalysts for water splitting and metal-air batteries

Designing highly efficient bifunctional and multifunctional catalysts for the hydrogen/oxygen evolution reaction (HER/OER) and oxygen reduction reaction (ORR) has attracted acute attention, toward the development of clean and renewable energy technologies. The rare introduction of non-noble TMs such as Fe/Ni/Cu at the surface of MXenes as single atoms (SAs) has shown modified mechanical and electrical properties. Using density functional theory (DFT) calculations along with machine learning (ML) based descriptors, we have systematically screened and investigated different MXene combinations based on stability of intermediates and adsorption free energies, exploring their ability to catalyze the reactions. The results reveal that most of the considered MXenes are thermodynamically stable with Ni/Cu/Fe doping, showing the best performance for the OER/ORR/HER, respectively. The surprisingly small overpotentials for these reactions exhibit desirable bifunctional catalysts for water splitting (eta(overall) = 0.48 V) and metal-air batteries (eta(overall) = 0.70 V), even superior to commercially used IrO2(111) and Pt(111) for the OER and HER/ORR, respectively. The high correlations between intermediates (scaling relations) and the varying interaction strength with the catalytic surface mostly govern the performance. The Bader analysis, d-band, and charge density difference show that doped metals not only act as active sites but also behave as an important charge transfer medium between the surface and adsorbate. The significantly low reaction barriers of Ni-Cr2ScC2O2 for all three reactions with robust reaction kinetics make it an efficient trifunctional catalyst.