Octahedral spinel electrocatalysts for alkaline fuel cells

Significance We present a systematic study of a family of 15 types of octahedral spinel electrocatalysts for the oxygen reduction reaction (ORR) in alkaline fuel cells. Three specific candidates, MnCo 2 O 4 /C, CoMn 2 O 4 /C, and CoFe 2 O 4 /C, exhibited very promising ORR activity. In particular, CoMn 2 O 4 /C could rival the ORR activity and selectivity of Pt/C. Such performance was ascribed to a partial charge transfer between Co and Mn. The most active, MnCo 2 O 4 /C, exhibited a unique Co–Mn core–shell structure with heterogeneous valence states, which were probed at the single-nanoparticle level using scanning transmission and high-energy-resolution electron-loss near-edge structure. We believe that our approach, of combining advanced electron microscopy and spectroscopy, will enable the design of highly active nanocatalysts with unprecedented atomic-scale precision.