Effects of Oxygen Vacancies and Reaction Conditions on Oxygen Reduction Reaction on Pyrochlore-Type Lead-Ruthenium Oxide

Abstract

A series of pyrochlore-type lead-ruthenium oxide (Pb 2 Ru 2 O 7-δ ) containing various amounts of oxygen vacancies was prepared by varying the heat-treatment temperature, and the oxygen reduction reaction (ORR) activity as well as the electrochemical redox property in acid and alkaline solution was evaluated using rotating disk electrode. The specific ORR activity was increased with increasing the amount of oxygen vacancies, and linearly correlated to the charge amount of electrochemical redox of Pb 2 Ru 2 O 7-δ both in acid and alkaline solution. We concluded that the surface oxygen vacancy participates in the electrochemical redox of Pb 2 Ru 2 O 7-δ , and at the same time, behaves as the active site for ORR. The ORR activity in alkaline solution was much higher than that in acid solution, possibly because of the difference of the facility of electrochemical desorption of adsorbed oxygen species in the ORR process. Additionally, the effect of humidity on the ORR activity was investigated by single cell test. Proton exchange membrane fuel cells (PEMFCs) are promising alternative power sources for home co-generation system and transportation applications due to advantages such as low emission and high efficiency. There are, however, still many problems remaining for widespread commercialization of PEMFCs. In the current PEMFCs, platinum based catalyst is used as the cathode electrocatalyst for oxygen reduction reaction (ORR). From the view of cost and availability, alternative electrocatalyst to platinum is strongly required. While many types of non-platinum electrocatalyst have been reported, such as metal-oxides, metal macrocyclic complex, metal chalcogenides, and nitrogen-containing carbon, none of them has been applied to practice due to poor performance or durability. 1-12We have been focusing on lead-ruthenium oxide (Pb 2 Ru 2 O 7-δ ) as a candidate of non-platinum electrocatalyst for ORR. [13][14][15][16] Pb 2 Ru 2 O 7-δ has a pyrochlore-type structure with some oxygen vacancies, and shows high electronic conductivity. [17][18][19][20][21][22][23][24][25][26] Horowitz et al. 20 have firstly reported that Pb 2 Ru 2 O 7-δ which is prepared by co-precipitation method in aqueous solution and post heat-treatment at 300-750• C shows high ORR activity in alkaline solution. Afterwards, several researches aimed at application of Pb 2 Ru 2 O 7-δ to electrochemical devices such as fuel cell, metal-air battery and oxygen sensor have been conducted. 20,21,[27][28][29][30][31][32][33][34] For example, Goodenough et al. 31,35 have reported that Pb 2 Ru 2 O 7-δ shows ORR activity and high stability not only in alkaline solution but also in acid solution and proton exchange membrane such as Nafion. Also, we have found that partial substitution of other cations such as Bi, In, or Sb with Pb site in Pb 2 Ru 2 O 7-δ have a positive effect on the ORR activity.13 Nevertheless, the ORR activity of Pb 2 Ru 2 O 7-δ is needed to be drastically enhanced for practical use as the cathode electrocatalyst of PEMFC. For this purpose, it ...

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Unveiling the Catalytic Origin of Nanocrystalline Yttrium Ruthenate Pyrochlore as a Bifunctional Electrocatalyst for Zn–Air Batteries

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