La_0.7Sr_0.3Mn_1–xNi_xO_3−δElectrocatalysts for the Four-Electron Oxygen Reduction Reaction in Concentrated Alkaline Media

Electrocatalytic activity for the four-electron-transfer oxygen reduction reaction (ORR) was examined with perovskite-type La 0.7 Sr 0.3 Mn 1– x Ni x O 3—δ ( x = 0–0.5) compounds in concentrated KOH. Electrochemical measurements by means of a rotating ring–disk electrode technique revealed that 10% substitution of Mn by Ni atoms significantly improved ORR activity whereas further Ni substitution significantly decreased the catalytic performance. Moreover, La 0.7 Sr 0.3 Mn 0.9 Ni 0.1 O 3—δ could achieve the ideal reaction electron number (∼4) without the aid of carbon additives, whereas compounds with x > 0.1 achieved a number less than 4 in the absence of carbon cocatalysts, attributed to lowered reaction rates related to two-electron reduction from oxygen to peroxide. Combined with the results of X-ray photoelectron spectroscopy, the pronounced catalytic activity of La 0.7 Sr 0.3 Mn 0.9 Ni 0.1 O 3—δ could be correlated with the population of Mn 3+ states with (e g ) 1 configuration.