Phase control of ultrafine FeSe nanocrystals in a N-doped carbon matrix for highly efficient and stable oxygen reduction reaction

Transition metal chalcogenides have been known as cost-effective and energy-efficient electrocatalysts for the oxygen reduction reaction (ORR). Crystal phase control is vital for tailoring their ORR performances. Herein, hexagonal (h-FeSe) and tetragonal FeSe (t-FeSe) ultrafine nanocrystals are jointly encapsulated in a N-doped carbon matrix without agglomeration. Their phase evolution at different pyrolysis temperatures is explicitly elucidated. The resultant material that contains the highest amount of h-FeSe nanocrystals exhibits remarkable performances with a positive onset potential of 0.97 V, large limiting current density of 5.4 mA cm⁻²and low H₂O₂yield of 6.6%. The material also delivers outstanding catalytic stability and methanol crossover tolerance. Theoretical studies confirm that h-FeSe outperforms t-FeSe in O₂adsorption and O-O bond dissociation of *OOH intermediates on active Fe-sites. Thus, h-FeSe is more efficient than t-FeSe towards alkaline ORR. We believe it will provide great inspiration for designing other ORR-efficient transition metal-based electrocatalysts by controlling crystal phases.