Single yttrium atom coordinated by nitrogen and oxygen with an asymmetric 4d orbit as efficient oxygen reduction electrocatalyst.

Developing transition metal-nitrogen-carbon (MNC) with the inert metal-atom center for the Fenton reaction is crucial to achieving precious metal-free electrocatalysis of oxygen reduction reaction (ORR). Herein, we report a new structure of YNC nanosheets for efficient ORR, which was synthesized by pyrolyzing the Y ion-containing self-polymerized compound of 2, 4, 6-triaminopyrimidine (TAP) as a new precursor. Results demonstrate that the precursor of TAP with high N content is capable of forming atomically dispersed specific YN4O moieties anchoring in N-rich carbon nanosheets, exhibiting excellent ORR performance with a higher half-wave potential of 0.88 V and 0.78 V in 0.1 M KOH and 0.5 M H2SO4 than FeNC synthesized by the same strategy. Meanwhile, the zinc-air battery and proton exchange membrane fuel cell tests also verify its feasibility for practical application with a maximum power output density of 151 mW cm-2 and 496 mW cm-2 respectively. Theoretical calculations further reveal that the axial O coordination in YN4 moiety causes an symmetry breaking of the d-orbital electrons of yttrium and weakens the spin polarization, which can shift the rate-limiting step from the *OH step to the *OOH step with a lower ORR overpotential than the classic FeNC. This work proves that YNC with single yttrium atom holds a great promise as a substitute for the conventional FeNC with active Fenton effect as a none-precious metal ORR electrocatalyst.