Papers by Author: Kenji Yoshihara

Abstract: 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.

Abstract: The activities of the pyrochlores Pb2Ru2-xMxO7-δ (M=Mn,Fe,Co,Ni)[PRM(0x)] toward electrochemical oxygen reduction (EOR) were examined in 0.1 M KOH and 0.05 M H2SO4 aqueous solutions. The onset voltage (Vo) and the efficiency (E4) of 4-electron reduction of oxygen for PRM(0x) were evaluated by semi-steady state voltammetry with rotating ring-disk electrodes. In PRM01, the order of the EOR activities was PRMn01 > PRCo01 > PRNi01 > PRFe01 in both solutions. This was consistent with the variation of the lattice parameters, i.e. PRM01 with a smaller lattice parameter showed higher EOR activity. In addition, PRMn05 showed higher EOR activity than PRMn01. The onset voltages Vo were 0.95 and 0.50 V vs. reversible hydrogen electrode in 0.1 M KOH and 0.05 M H2SO4, respectively, while E4 was almost 100 % in both solutions. These results indicate that the EOR activity of PRM(0x) depends on the incorporated metal ions on the B-sites and their contents.

Abstract: The activity of electrochemical oxygen reduction (EOR) for the pyrochlores Ln2Ru2-XO7-δ (Ln=Pr,Nd,Sm) [LnR] were examined in 0.1 M KOH aqueous solution at 70oC. The onset voltage (Vo) of the oxygen reduction current and the efficiency (E4) of 4-electron reduction of oxygen were evaluated by semi-steady state voltammetry with rotating ring-disk electrodes. PrR with the highest EOR activity showed Vo = ~ 0.85 V vs. reversible hydrogen electrode and E4 values above 80 %. Their Vo and E4 values show that LnR containing Ln with a smaller atomic number has a higher EOR activity, i.e. the order of the activity is PrR > NdR > SmR. This was in good agreement with that of the lattice parameters of LnR. These results indicate that the EOR activity of LnR depends on the kind and/or the size of the lanthanide metal ion on the A-site. PrR and NdR exhibited higher E4 values than known excellent Pb2Ru2O7-δ electrocatalyst containing toxic Pb.

Abstract: The O2-gas electrode (OGE) properties of the BaCe1-xGd0.2RuxO3-δ(BCGRx, x=0-0.45) oxides and the BaGd1-yRuyO3-δ(BGRy, y=0.5-1.0) oxides were examined with the cell of the type solid-state cell (OGE|BaCe0.8Gd0.2O3|OGE) below 300°C. Two single perovskite phases, BCGR0.15 in the BCGR series and BGR0.5 in the BGR series, exhibited OGE properties superior to those of known OGEs of Pt electrodes and La0.6Sr0.4Co0.98Ni0.02O3. Oxide-ion currents of the order of over one hundred μAcm-2 not yet reported were allowed to pass through the solid-state cell at low temperatures of ∼200°C.

Abstract: The O2-gas electrode (OGE) properties of the pyrochlores Pb2Ru2-XMXO7-δ(M=Mn,Fe,Co,Ni) [PRMx] were examined below 300°C with the cell of the type (PRMx | Ba0.975Ce0.8Gd0.2O3 (BCG) | PRMx). The PRMn electrodes showed good OGE properties comparable to those of Pb2Ru2O6.5 and superior to those of a Pt-paste electrode and the La0.6Sr0.4Co0.98Ni0.02O3 electrode, which are well-known representative OGEs. The results indicate that the PRMn electrodes can be used for OGEs in low-temperature applications including oxygen sensors.