Factors for Improvements of Catalytic Activity of Zirconium Oxide-Based Oxygen-Reduction Electrocatalysts

Abstract

Partially oxidized zirconium carbonitrides (Zr-CNO) were evaluated as a non-precious-metal cathode used for polymer electrolyte fuel cells. The zirconium-oxide-based catalysts were prepared from zirconium carbonitrides under controlled oxygen partial pressures from 10 −2 to 10 −18 atm at 1000 • C. The ORR activity of Zr-CNO showed a volcano-like tendency against the degree of oxidation (DOO). We found a significant lattice-parameter decrease for highly active Zr-CNOs, and this could be ascribed for the formation of oxygen-and zirconium-vacancies in monoclinic ZrO 2 . On the other hand, carbon originated from starting materials was deposited on the surface of Zr-CNOs during the partial oxidation process. We also found that the oxygen content on the surface of highly active Zr-CNOs was drastically decreased when the amount of carbon significantly decreased during partial oxidation processes. This result indicates that the deposited carbon acted as a strong reduction agency. From XPS and XRD analyzes, there is a linear relationship between the ORR current and amount of oxygen vacancy formed in monoclinic phase. Therefore, we believe that oxygen vacancy formed in monoclinic phase could act as active sites for the ORR.Polymer electrolyte fuel cells (PEFCs) are efficient and clean electrochemical power generation devices that produce electricity from H 2 and O 2 gases. Platinum-based oxygen reduction reaction (ORR) catalysts are used as the best catalyst for cathodes of PEFCs at present. Its high cost and poor availability, however, makes the development of non-noble metal catalysts highly desirable. While many studies have been done to develop non-platinum cathode catalysts for PEFCs, such as cobalt-or iron-based macrocyclic complexes (N 4 -chelate type such as phtalocyanines, porphyrins, and tetraazaanulens), 1-3 metal chalcogenides (Mo x Ru y Se z, Ru x X y (where X = S, Se, and Te)), 4-8 and carbon-based catalysts, 9-11 these compounds are, actually, unstable in acidic and oxidizing atmosphere, and less-stable than that of platinum based cathode catalysts. [11][12][13][14][15][16][17] In contrast, we have been focusing on the materials' stability to sever cathode conditions, viz., strong acidic and corrosive environments, and have developed the group 4 and 5 metal-oxidebased catalysts that have high ORR activities that are comparable to that of platinum, simultaneously exhibiting highly tolerant behaviors for the PEFC cathode condition. [18][19][20][21][22][23][24] The results of our studies on transition-metal-oxide-based ORR catalysts have suggested that the best ORR activity is obtained when transition-metal carbonitrides, such as ZrC x N y , TaC x N y , NbC x N y , are partially oxidized. Since the transition metal oxides themselves show no remarkable ORR activity, carbon and/or nitrogen that are included in the starting materials should play a role in emergence and enhancement of ORR activity of oxide-based ORR catalysts.

ZrO2Based Oxygen Reduction Catalysts for PEMFCs: Towards a Better Understanding

“…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. 10 In order to increase the very low catalyst dispersion (i.e., the electrochemically active surface area per gram of catalyst) obtained with the above synthesis approaches, methods to synthesize highly dispersed Zr oxide nanoparticles supported on carbon black were investigated.…”

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

“…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).…”

“…[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). We concluded that oxygen vacancies in the oxide particles acted as ORR active sites.…”

“…We concluded that oxygen vacancies in the oxide particles acted as ORR active sites. 28,30 Carbon derived from phthalocyanine was deposited during the heat treatment and partially covered the zirconium oxides. The deposited carbon formed local electron conduction paths on the surface of the zirconium oxides.…”

Stabilization and Activation of Zirconium Oxide Based Electrocatalysts as PEFC Cathode by Re-Heat Treatment

“…We focused on group 4 and 5 transition metal oxides, because they have high chemical stability under acidic atmosphere even in high potential region, and are much less expensive than platinum. In our previous study, zirconium oxide based compounds made from oxyzirconium phthalocyanines by heat treatment under low oxygen partial pressure showed high ORR activity (1)(2). Therefore, their durability and stabilization should be investigated.…”