Fuel cells, and in particular solid-oxide fuel cells (SOFCs), enable high-efficiency conversion of chemical fuels into useful electrical energy and, as such, are expected to play a major role in a sustainable-energy future. A key step in the fuel-cell energy-conversion process is the electro-oxidation of the fuel at the anode. There has been increasing evidence in recent years that the presence of CeO2-based oxides (ceria) in the anodes of SOFCs with oxygen-ion-conducting electrolytes significantly lowers the activation overpotential for hydrogen oxidation. Most of these studies, however, employ porous, composite electrode structures with ill-defined geometry and uncontrolled interfacial properties. Accordingly, the means by which electrocatalysis is enhanced has remained unclear. Here we demonstrate unambiguously, through the use of ceria–metal structures with well-defined geometries and interfaces, that the near-equilibrium H2 oxidation reaction pathway is dominated by electrocatalysis at the oxide/gas interface with minimal contributions from the oxide/metal/gas triple-phase boundaries, even for structures with reaction-site densities approaching those of commercial SOFCs. This insight points towards ceria nanostructuring as a route to enhanced activity, rather than the traditional paradigm of metal-catalyst nanostructuring.
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This work was supported in part by the Stanford Global Climate & Energy Project and by the National Science Foundation under contract number DMR-0604004. Additional support was provided by the NSF through the Caltech Center for the Science and Engineering of Materials, a Materials Research Science and Engineering Center (DMR-052056). W.C.C. was also supported by an appointment to the Sandia National Laboratories Truman Fellowship in National Security Science and Engineering, sponsored by Sandia Corporation (a wholly owned subsidiary of Lockheed Martin Corporation) as Operator of Sandia National Laboratories under its US Department of Energy Contract No. DE-AC04-94AL85000. The authors also acknowledge C. M. Garland and D. A. Boyd of Caltech and M. W. Clift of Sandia for their assistance with analytical measurements, K. L. Gu of Caltech for sample fabrication, F. Ciucci of University of Heidelberg for numerical simulations and D. G. Goodwin and E. C. Brown of Caltech and F. El Gabaly and A. H. McDaniel of Sandia for valuable discussions. The Evans Analytical Group carried out focused ion beam imaging, and for that effort the authors are particularly grateful to H. Deng.
The authors declare no competing financial interests.
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Chueh, W., Hao, Y., Jung, W. et al. High electrochemical activity of the oxide phase in model ceria–Pt and ceria–Ni composite anodes. Nature Mater 11, 155–161 (2012). https://doi.org/10.1038/nmat3184
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