Electrocatalytic Activity of the Pyrochlores Ln₂M₂O_7−δ (Ln = Lanthanoids) for Oxygen Reduction Reaction

Abstract

Electrocatalytic oxygen reduction reaction (ORR) activities of the pyrochlore oxides Ln₂Zr₂O_7−δ (LnZ) and Ln₂Sn₂O_7−δ (LnS) (Ln = La, Pr, Nd, Sm) were examined in 0.1 M KOH solution at 70 °C. The onset potential (E _on) of the oxygen reduction current and the efficiency (Eff ₄) of 4-electron reduction of oxygen were evaluated by semi-steady state voltammetry with a rotating ring-disk electrode. In both LnZ and LnS series, the E _on values were ~0.85 V versus reversible hydrogen electrode. A relation was found between the E _on values and the lattice parameters; i.e. on the whole, the ORR activity became high with an increase in the lattice parameters. When the Ln ion was the same, the LnZ series exhibited higher ORR activities than the LnS series. The pyrochlore LaZ with the highest ORR activity showed a Eff ₄ value higher than 85%. Moreover, Mn-incorporation to LaZ led to a mixed-oxide (1–xLaZ−xLaM) of LaZ and the perovskite LaMnO₃ (LaM). However, the E _on value apparently sifted to a more positive potential probably due to LaMnO₃, and the magnitude of the cathodic ORR current increased with an increase in the mixing content up to x = 0.3. The mixed-oxide 0.7LaZ–0.3LaM exhibited the highest ORR activity (E _on = ~0.90 V and Eff ₄ > 95%), which was comparable to that of a conventional 20 mass% Pt/C catalyst.