Oxygen Reduction Electrode Properties of Pyrochlores Ln₂Ru₂O_7-δ(Ln=Pr, Nd, Sm) in Aqueous Solutions

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.

Electrocatalytic Activity of the Pyrochlores Ln2M2O7−δ (Ln = Lanthanoids) for Oxygen Reduction Reaction

“…On the whole, the ORR current (i D ) and the efficiency (Eff 4 ) of 4-electron reduction of oxygen increased with decreasing atomic number of the Ln ion in both series. This trend is in good agreement with that for the LnR reported previously [7][8][9]. As for the effect of B-site ion, the LnZ series exhibited higher ORR activities than the LnS ones in the case of the same Ln ion; the Eff 4 values for the LaZ and LaS samples were more than 85 and 75%, respectively.…”

“…Figure 6 shows variations of the ORR activities as a function of the lattice parameter for the LnZ and LnS series along with that for the LnR series. In both LnZ and LnS series, on the whole the ORR activities (i D at 0.7 V and Eff 4 values) increase with an increase in the lattice parameter as with that of LnR series [7][8][9]. In the case of the LnR series, the electrical conductivity increased with an increase in the lattice parameter [9,10].…”

“…On the whole, they had similar morphologies and surface areas S BET around 4.5 m 2 g -1 . [7][8][9].…”

“…Sample electrodes were prepared by the following procedure [6][7][8][9]: 15.4 mg of the oxide sample milled for one hour at 500 rpm (Planetary ball mill P-6, Fritsch Japan) and 4.6 mg of carbon black (Vulcan XC-72R, Cabot) as an electric assisting additive were mixed with 2 mL of 2 mass% Nafion ethanol solution with an ultrasonic equipment for 30 min. The mixed paste (5 lL) was coated on the grassy carbon (GC) disk electrode of RRDE and dried at room temperature.…”

“…All the potentials shown in this paper were recalculated to RHE. The efficiency (Eff 4 ) of 4-electron reduction of oxygen was estimated by the following equation [6][7][8][9]:…”