Correlation between spin state and activity for hydrogen evolution of PtN₂ monolayer

Spin plays a key role in physical and chemical reactions, such as oxygen evolution and hydrogen evolution reactions (OER/HER), but the spin-activity correlation has remained unclear. Based on a transition metal (TM)-doped PtN2 monolayer model with a well-defined spin center as an adsorption site, we here reveal that only an active spin state can enhance the strength of hydrogen adsorption, while an inert spin state offers very little influence. Specifically, the an unpaired electron along the out-of-plane direction such as in the dz(2) orbital, acting as an active spin state, will strongly hybridize with hydrogen, resulting in enhanced hydrogen binding energy because the dz(2) orbital is just enough to accommodate two electrons to form a bonding orbital. While the in-plane unpaired electron such as in the dx(2) similar to y(2) orbital plays a negligible role in an adsorbing hydrogen atom. This is verified by a series of single atom catalysts comprising of PtN2 monolayer by replacing a Pt atom with a TM (Fe, Co, Ni, Ru, Rh, Pd, Os, or Ir) atom or subsequent adsorbing a Cl atom. One of the most promising materials is Pd@PtN2-Cl that offers superior HER activity, even better than pure Pt. This work uncovers the nature of spin-activity correlation, thus paving the way for the design of high-performance catalysts through spin-engineering.