Journal
ADVANCED ENERGY MATERIALS
Volume 5, Issue 10, Pages -Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/aenm.201402307
Keywords
aberration corrected electron microscopy; electrocatalysts; nanostructures; oxygen evolution reaction; X-ray absorption spectroscopy
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Funding
- Office of Science of the U.S. Department of Energy [DE-SC000499]
- Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [DE-AC02-05CH11231]
- DOE [DE-AC02-05CH11231]
- DOE Office of Biological and Environmental Research
- NIH [P41GM103393]
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Ce-rich mixed metal oxides comprise a recently discovered class of electrocatalysts for the oxygen evolution reaction (OER). In particular, at current densities below 10 mA cm(-2), Ni0.3Fe0.07Co0.2Ce0.43Ox exhibits superior activity compared to the corresponding transition metal oxides, despite the relative inactivity of ceria. To elucidate the enhanced activity and underlying catalytic mechanism, detailed structural characterization of this quinary oxide electrocatalyst is reported. Transmission electron microscopy imaging of cross-section films as-prepared and after electrochemical testing reveals a stable two-phase nanostructure composed of 3-5 nm diameter crystallites of fluorite CeO2 intimately mixed with 3-5 nm crystallites of transition metal oxides alloyed in the rock salt NiO structure. Dosing experiments demonstrate that an electron flux greater than approximate to 1000 e angstrom(-2) s(-1) causes the inherently crystalline material to become amorphous. A very low dose rate of 130 e angstrom(-2) s(-1) is employed for atomic resolution imaging using inline holography techniques to reveal a nanostructure in which the transition metal oxide nanocrystals form atomically sharp boundaries with the ceria nanocrystals, and these results are corroborated with extensive synchrotron X-ray absorption spectroscopy measurements. Ceria is a well-studied cocatalyst for other heterogeneous and electrochemical reactions, and our discovery introduces biphasic cocatalysis as a design concept for improved OER electrocatalysts.
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