Ultrasmall Pt2Sr Alloy Nanoparticles As Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Hydrogen Evolution in Acidic Media

Electrocatalytic oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in acidic media are vital for the applications of renewable energy electrolyzers.However,the low mass activity of noble Pt urgently needs to be improved due to the strong binding energetics of oxygen species (*O) with Pt sites.Here we report fine PtxSr alloy (～2 nm) supported on N-doped carbon (NC) pyrolyzing from ZIF-8 as bifunctional electrocatalysts toward ORR and HER in acidic media.The representative Pt2Sr/NC exhi-bits an onset potential of 0.94 V vs.RHE and half-wave potential of 0.84 V toward ORR,and a low over-potential of 27 mV (10 mAcm-2) toward HER,respectively.Significantly,the mass activities of Pt2Sr/NC are 6.2 and 2.6 times higher than that of Pt/C toward ORR (at 0.9 V) and HER (at-30 mV),respectively,Simultaneously,Pt2Sr/NC possesses a retention rate of 90.97％ toward acidic ORR after 35000 s of contin-uous operation.Through density functional theory (DFT) calculations and X-ray photoelectron spec-troscopy analysis,the incorporation of Sr into Pt forming Pt2Sr alloy redistributes the electronic structures of Pt-Pt bonds,changing the rate-determining step for the ORR on Pt sites from the formation of *OH from *O to the generation of *OOH along with decreasing the energy barrier,which is also con-firmed by the downshift of d band center.Meanwhile,the downshift of d band center also leads to the optimization of the adsorption energy (H*) with Pt,significantly improving Pt2Sr/NC toward HER.