Oxygen reduction reaction on tantalum oxide-based catalysts prepared from TaC and TaN

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 ...

Section: Discussion About Active Site For Orr and Effect Of Solution mentioning

confidence: 99%

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confidence: 99%

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

Fujii

Satô

Takase

2014

“…We previously described the acidic stability and catalytic ORR activity of a number of materials, including tantalum-doped tungsten carbide, 15 tantalum oxynitride, [16][17][18] zirconium oxide, [19][20][21] titanium oxide, 22 zirconium oxynitride, 23,24 tantalum carbonitride, 25 and tantalum-oxide or zirconium-oxide-based compounds. [26][27][28] Recently, we revealed that zirconium oxide-based compounds prepared from oxy-zirconium phthalocyanine by heat treatment under low oxygen partial pressure showed high ORR activity. [28][29][30] These catalysts were composed of zirconium-oxide nano-particles supported by multi-walled carbon nanotubes (MWCNTs).…”

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confidence: 99%

Effect of Reheating Treatment on Oxygen-Reduction Activity and Stability of Zirconium Oxide-Based Electrocatalysts Prepared from Oxy-Zirconium Phthalocyanine for Polymer Electrolyte Fuel Cells

Okada

Ishihara

Matsumoto

et al. 2015

Zirconium oxide-based electrocatalysts with multi-walled carbon nanotubes as electroconductive support (ZrC x N y O z /MWCNT) were prepared by oxidation of oxy-zirconium phthalocyanine under low oxygen partial pressure and examined as new non-platinum cathodes for polymer electrolyte fuel cells. The effect of reheating treatment of ZrC x N y O z /MWCNT in the temperature range from 900 to 1200 • C for 1 h under nitrogen atmosphere on the activity and stability of the oxygen-reduction reaction (ORR) was investigated. The reheating treatment at 1000 • C enhanced the ORR activity because of the formation of oxygen vacancies, which might act as active sites for the ORR. The reheating treatment at temperatures above 1100 • C, however, led to the formation of carbonitrides which are unstable under cathode conditions, resulting in decrease in the ORR activity. The reheating treatment significantly increased the durability of ORR at both low (0.6 V vs. reversible hydrogen electrode (RHE)) and high (1.2 V) potential.The reheating treatment at higher temperatures promoted graphitization of the deposited carbon. Because the degradation in the high-potential region mainly originates from corrosion of the carbon materials, graphitization would be effective to improve the durability.

“…11, 13 Ota and co-workers also investigated the ORR activity of micrometric Ta and Zr oxides produced by heat-treatment of carbides, 11,14 nitrides, 14 or carbonitrides, 6,10,15,16 concluding that the formation of understoichiometric or defective oxides (i.e., of oxygen vacancies) would enhance their ORR activity, which was based on the observation that partial reduction of the formed oxides lead to improved ORR activity. 15,16 The same group also applied surface sensitive X-Ray absorption spectroscopy, which suggested a correlation between ORR activity and defect-induced strain of the Ta 2 O 5 oxide layer formed on top of a μm-sized TaCN core upon high-temperature oxidation.…”

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confidence: 99%

ZrO₂Based Oxygen Reduction Catalysts for PEMFCs: Towards a Better Understanding

Mittermeier

Madkikar

Wang

et al. 2016

The oxygen reduction reaction (ORR) activity of ZrO 2 based, carbon-supported nanoparticles is not conclusively reported in literature. This study examines the dependence of the ORR activity on the used precursors as well as on the heat-treatment atmosphere and temperature. We further determine the ORR activation energy and the ORR mechanism. Various precursors containing Zr and/or N were employed in the synthesis, and the ORR activity was measured by rotating (ring) disk electrode (R(R)DE) voltammetry in both acidic and alkaline electrolyte as well as by measurements in a single-cell polymer electrolyte membrane fuel cell (PEMFC) configuration. We show that even the most active ZrO 2 based ORR catalysts exhibit an activity gap of ca. two orders of magnitude compared to the DOE target of 300 A/cm 3 for PGM-free ORR catalysts, thus requiring further development. Our RRDE analysis suggests a primarily 2-electron ORR mechanism in the case of the tested catalysts in acid, which in turn provides a consistent temperature dependence between RDE and PEMFC experiments, allowing also for a mechanistic (re-) interpretation of experimental results in the literature.