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Electrocatalytic oxygen reduction reaction (ORR) activities of the pyrochlore oxides Ln 2 Zr 2 O 7-d (LnZ) and Ln 2 Sn 2 O 7-d (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 LaMnO 3 (LaM). However, the E on value apparently sifted to a more positive potential probably due to LaMnO 3 , 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.

To explore new electrocatalysts for alkaline direct alcohol fuel cells (ADAFCs), oxygen reduction reaction (ORR) activity of the pyrochlores Ln 2 Ru 2 O 7-(LnR, Ln=Pr,Nd,Sm,Gd,Dy,Yb) and Ln 2 Ru 2-x Mn x O 7-(LnRMn x , Ln=Pr,Nd) were examined in 0.1 M KOH solution at 70 o 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 rotating ring-disk electrodes. In the LnR series, PrR with the highest ORR activity showed E on = ~ 0.85 V vs. reversible hydrogen electrode and a Eff 4 value above 90 %. Their E on and Eff 4 values revealed that LnR with a smaller atomic number had a higher ORR activity. This trend was in good agreement with that of electrical conductivities of the LnR. In addition, effects of Mn substitution for the Ru-site were confirmed in both PrRMn x and NdRMn x series. Their ORR activities increased with an increase in the amount of Mn incorporation. The NdRMn 0.25 exhibited the highest ORR activity (E on = 0.95 V and Eff 4 > 90%) in the formation range of the pyrochlore single phase. Moreover, the ORR selectivity was much higher than that of a conventional 20 mass% Pt/C catalyst: the E on value was 0.95 V even in 0.1 KOH containing 1 M methanol, ethanol, ethylene glycol and 2-propernol, whereas the E on the value was 0.7 V for the Pt/C catalyst.

The Mn-doped pyrochlores Pb2Ru1.5Mn0.5O7- (PRMns) samples were synthesized by a precipitation method, followed by heat-treatments at temperatures of 300-800oC. Effects of the heat-treatments on their electrocatalytic activities for oxygen reduction reaction (ORR) were examined by semi-steady state voltammetry with a rotating ring-disk electrode in 0.1 M KOH solution at 70oC. The PRMns showed the best ORR activity for the 500oC-heat-treated PRMn. The onset potential of the ORR current was over 1.0 V vs RHE, and the efficiency of 4-electron reduction was almost 100%. The maximum in the ORR activity for the 500°C-heat-treated PRMn resulted from a trade-off effect between their crystallinity and specific surface areas.

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