Insight into the catalytic activity of MXenes for hydrogen evolution reaction

For a long time, the hydroxyl (OH) radicals has attracted particular attention not only because it is the most important species in the photooxidation cycles in the atmosphere but also because it can oxidize volatile organic compounds (VOCs) to form secondary oxygenated gas species and aerosols, some of which can be toxic and carcinogenic. Since most people spend 80%–90% of their lifetime indoors, a better understanding of the formation, occurrence and reaction of OH radicals indoors is crucial to assess the potential impact on human health. Moreover, the secondary pollutants can be substantially reduced in indoor environments only with comprehensive knowledge of the oxidative chemistry and subsequent chains of chemical reactions that pollutants undergo. MXenes have exhibited great potential as cost-effective electrocatalysts for hydrogen evolution reaction (HER). However, insight into the origin of activity is still missing. Herein, on the basis of a systematical investigation of the HER performance of 20 MXenes (M2NO2 and M2CO2, M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W), a Fermi-abundance model is proposed to understand variation of the activity in different MXenes. It is found that the occupied p electronic states of surface O atoms play a decisive role in the HER activity of MXenes. More importantly, Ti2NO2 and Nb2NO2 are found to be promising HER electrocatalysts with the free energy for hydrogen adsorption close to zero. This work not only provides possible catalysts for HER, the developed Fermi-abundance model but also is applicable to other two-dimensional materials and may serve as a simple descriptor of the intrinsic HER activity.