The prohibitive cost and scarcity of the noble-metal catalysts needed for catalysing the oxygen reduction reaction (ORR) in fuel cells and metal–air batteries limit the commercialization of these clean-energy technologies. Identifying a catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant transition-metal-oxide catalysts to replace platinum. Here, we demonstrate that the ORR activity for oxide catalysts primarily correlates to σ*-orbital (eg) occupation and the extent of B-site transition-metal–oxygen covalency, which serves as a secondary activity descriptor. Our findings reflect the critical influences of the σ* orbital and metal–oxygen covalency on the competition between O22–/OH– displacement and OH– regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.
Figures at a glance
Figure 1: ORR activity of perovskite transition-metal-oxide catalysts. 
a, ABO3 perovskite structure. b, Oxygen reduction activity of LaCu0.5Mn0.5O3 electrode in O2-saturated 0.1 M KOH at 10 mV s−1 scan rate and rotation rates of 100, 400, 900 and 1,600 r.p.m. The Koutecky–Levich analysis (inset) of the limiting currents (0.4 V) indicates a 4e− transfer reaction. c, Specific activities of LaCu0.5Mn0.5O3, LaMnO3, LaCoO3 and LaNiO3. The potential at 25 µA cm−2ox is used as a benchmark for comparison (shown as the intersection between the activity and the horizontal grey dashed line). d, Potentials at 25 µA cm−2ox of the perovskite oxides have an M-shaped relationship with d-electron number. Data symbols vary with type of B ions (Cr, red; Mn, orange; Fe, grey; Co, green; Ni, blue; mixed compounds, purple), where x = 0 and 0.5 for Cr, and 0, 0.25 and 0.5 for Fe. Error bars represent standard deviations of at least three measurements.
Figure 2: Role of eg electron on ORR activity of perovskite oxides. 
a, Potentials at 25 µA cm−2ox as a function of eg orbital in perovskite-based oxides. Data symbols vary with type of B ions (Cr, red; Mn, orange; Fe, grey; Co, green; Ni, blue; mixed compounds, purple), where x = 0 and 0.5 for Cr, and 0, 0.25 and 0.5 for Fe. Error bars represent standard deviations. b, The shape of the eg electron points directly towards the surface O atom and plays an important role during O22−/OH− exchange. O, B and H atoms are coloured red, blue and green, respectively.
Figure 3: Proposed ORR mechanism on perovskite oxide catalysts30. 
The ORR proceeds via four steps: 1, surface hydroxide displacement; 2, surface peroxide formation; 3, surface oxide formation; 4, surface hydroxide regeneration.
Figure 4: Role of B–O covalency on the ORR activity of perovskite oxides. 
