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Electrochemical and thermochemical behavior of CeO2-δ

Citation

Chueh, William C. (2011) Electrochemical and thermochemical behavior of CeO2-δ. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:11042010-235339265

Abstract

The mixed-valent nature of nonstoichiometric ceria (CeO2- δ) gives rise to a wide range of intriguing properties, such as mixed ionic and electronic conduction and oxygen storage. Surface and transport behavior in rare-earth (samaria) doped and undoped ceria were investigated, with particular emphasis on applications in electrochemical and thermochemical energy conversion processes such as fuel cells and solar fuel production. The electrochemical responses of bulk- processed ceria with porous Pt and Au electrodes were analyzed using 1-D and 2-D transport models to decouple surface reactions, near-surface transport and bulk transport. Combined experimental and numerical results indicate that hydrogen electro-oxidation and hydrolysis near open-circuit conditions occur preferentially over the ceria | gas interface rather than over the ceria | gas | metal interface, with the rate-limiting step likely to be either surface reaction or transport through the surface oxygen vacancy depletion layer. In addition, epitaxial thin films of ceria were grown on zirconia substrates using pulsed-laser deposition to examine electrocatalysis over well-defined microstructures. Physical models were derived to analyze the electrochemical impedance response. By varying the film thickness, interfacial and chemical capacitance were decoupled, with the latter shown to be proportional to the small polaron densities. The geometry of microfabricated metal current collectors (metal = Pt, Ni) was also systematically varied to investigate the relative activity of the ceria | gas and the ceria | metal | gas interfaces. The data suggests that the electrochemical activity of the metal-ceria composite is only weakly dependent on the metal due to the relatively high activity of the ceria | gas interface. In addition to electrochemical experiments, thermochemical reduction-oxidation studies were performed on ceria. It was shown that thermally-reduced ceria, upon exposure to H2O and/or CO2, can be reoxidized to form H2, CO, and/or CH4. Analysis of gas evolution rates confirms that the kinetics of ceria oxidation by H2O and CO2 are dominated by surface reactions, rather than by ambipolar oxygen diffusion. Temperature-programmed oxidation experiments revealed that, even under thermodynamically favored conditions, carbonaceous species do not form on the surface of neat ceria, thereby giving a high CO selectivity when dissociating CO2. A scaled-up ceria-based solar reactor was designed and tested to demonstrate the feasibility of solar fuel production via thermochemical cycling.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Cerium oxide, impedance spectroscopy, fuel cells, solar fuel
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Haile, Sossina M.
Thesis Committee:
  • Johnson, William Lewis
  • Bhattacharya, Kaushik
  • Snyder, G. Jeffrey
  • Haile, Sossina M. (chair)
Defense Date:24 September 2010
Author Email:willchueh (AT) gmail.com
Funders:
Funding AgencyGrant Number
NSFUNSPECIFIED
Stanford Global Climate Energy ProgramUNSPECIFIED
eSolar, Inc.UNSPECIFIED
Josephine De Karman FellowshipUNSPECIFIED
U.S. Office of Naval ResearchUNSPECIFIED
Record Number:CaltechTHESIS:11042010-235339265
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:11042010-235339265
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6170
Collection:CaltechTHESIS
Deposited By: William Chueh
Deposited On:29 Mar 2011 16:30
Last Modified:26 Dec 2012 04:32

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