Metal Oxide-Based Compounds as Electrocatalysts for Oxygen Reduction Reaction
The cathode catalysts for polymer electrolyte fuel cells should have high stability as well as excellent catalytic activity for oxygen reduction reaction (ORR). Group 4 and 5 metal oxide-based compounds have been evaluated as a cathode from the viewpoint
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Metal Oxide-Based Compounds as Electrocatalysts for Oxygen Reduction Reaction Ken-ichiro Ota and Akimitsu Ishihara
Abstract The cathode catalysts for polymer electrolyte fuel cells should have high stability as well as excellent catalytic activity for oxygen reduction reaction (ORR). Group 4 and 5 metal oxide-based compounds have been evaluated as a cathode from the viewpoint of their high catalytic activity and high stability. Although group 4 and 5 metal oxides have high stability even in acidic and oxidative atmosphere, they are almost insulator and have poor ORR activity because they have large bandgaps. It is necessary to modify the surface of the oxides to improve the ORR activity. We have tried the surface modification methods of oxides into four methods: (1) formation of complex oxide layer containing active sites, (2) substitutional doping of nitrogen, (3) creation of oxygen defects without using carbon and nitrogen, and (4) partial oxidation of compounds which include carbon and nitrogen. These modifications were effective to improve the ORR activity of the oxides. The solubility of the oxide-based catalysts in 0.1 M H2SO4 at 30 C under atmospheric condition was mostly smaller than that of platinum black, indicating that the oxide-based catalysts had sufficient stability compared to the platinum. The onset potential of various oxide-based cathodes for the ORR in 0.1 M H2SO4 at 30 C achieved over 0.97 V vs. a reversible hydrogen electrode. This high onset potential suggests that the quality of the active sites of the oxide-based catalysts is mostly equivalent to that of platinum.
K.-i. Ota (*) • A. Ishihara Green Hydrogen Research Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan e-mail: [email protected]; [email protected] M. Shao (ed.), Electrocatalysis in Fuel Cells, Lecture Notes in Energy 9, DOI 10.1007/978-1-4471-4911-8_13, # Springer-Verlag London 2013
391
K.-i. Ota and A. Ishihara
a
140
Number of published papers
392
120 100
Total Organometallic complexes Chalcogenides
b Others 18% Chalcoge nides 12%
80 60
Organometallic complexes 69%
40 20 0 1960 1970 1980 1990 2000 2010 2020
Total 1056 (1964-2012.05)
Fig. 13.1 (a) Trend of number of the research paper deals with the non-platinum cathode catalysts from 1964. (b) Percentage of the paper about the non-precious metal oxygen reduction catalyst in an acid electrolyte
13.1
Introduction
Polymer electrolyte fuel cells (PEFCs) are expected as power sources for residential and transport applications due to their high theoretical efficiency of energy conversion, high power density, and low operating temperature. However, PEFCs have some serious problems to be solved before wide commercialization. In particular, a large overpotential of oxygen reduction reaction (ORR) must be reduced in order to obtain high energy efficiency. Although Pt is generally used as a cathode catalyst in the present PEFCs, its catalytic activity for the ORR is insufficient to obtain a high efficiency [1, 2]. Main issues
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