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Electrocatalysis in solid acid fuel cells

Citation

Louie, Mary W. (2011) Electrocatalysis in solid acid fuel cells. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05232011-144137426

Abstract

Solid state electrochemical reactions play a crucial role in many energy conversion devices, yet the pathways of many reactions remain unknown. The elusiveness of the reaction mechanisms is due, in part, to the complexity of electrochemical reactions; because electrochemical reactions require the interaction of many species (e.g., ions, electrons, and adsorbates) across multiple phases (e.g., electrolyte, catalyst, and gas phases), elucidation of the reaction pathway can quickly become complicated. In this work, we develop and utilize model catalyst | electrolyte systems, that is, structures of reduced complexity, to study electrode reactions in solid acid fuel cells which operate at intermediate temperatures of ~ 250 ºC. We employ AC impedance spectroscopy to explore the reaction pathway for hydrogen electro-oxidation over Pt thin films sputter-deposited atop the proton-conducting solid acid electrolyte CsH2PO4. We observed that hydrogen electro-oxidation occurs by diffusion of hydrogen through Pt, taking advantage of the entire Pt | CsH2PO4 interfacial area rather than being confined to the triple-phase sites. This insight opens up new avenues for developing high performance electrodes with low Pt loadings by eliminating the requirement that Pt-based electrodes be comprised of high triple-phase site densities long considered to be critical for Pt electrocatalysis. Indeed, even for flat, planar electrodes of very thin Pt films, we obtained a Pt utilization that is significantly higher than in typical composite electrodes.

We also demonstrate the efficacy of a new tool for probing the spatial heterogeneity of electrochemical reactions at the metal | electrolyte interface. We characterized oxygen electro-reduction kinetics at the nanoscale Pt | CsHSO4 interface at ~ 150 ºC using conducting atomic force microscopy in conjunction with cyclic voltammetry and AC impedance spectroscopy. Not only did we find the electrochemical activity for oxygen electro-reduction to vary dramatically across the electrolyte surface but the current-voltage data, when analyzed in the Butler-Volmer framework, exhibited a strong counter-correlation between two key kinetic parameters, the exchange coefficient and exchange current. Specifically, the exchange current spanned five orders of magnitude while the exchange coefficient ranged between 0.1 and 0.6. Such a correlation has not been observed before and points to the power of atomic force microscopy for electrochemical characterization at electrolyte | metal | gas boundaries in general.

As reduction in microstructural complexity is a key advantage in model electrode | electrolyte systems, we also sought to understand the bulk properties of solid acid compounds, specifically, the relationship between microstructure and the superprotonic phase transition, the latter of which lends solid acid compounds their high proton conductivities at intermediate temperatures. We found a correlation between phase transformation hysteresis and crystallographic compatibility of the high- and low-temperature phases of the Cs1–xRbxH2PO4 solid solution series. Therefore, it is to be expected that hysteresis, and therefore microcrack formation, can be minimized during phase transformation via the principle of crystallographic compatibility. This is confirmed in single crystals of CsHSO4, which was found to have higher crystallographic compatibility, lower hysteresis, and significantly fewer microcracks formed during phase transition compared to CsH2PO4. The apparent applicability of the theory of crystallographic compatibility implies a new tool for identifying solid acid compounds with suitable microstructures for fuel cell application and for model electrode | electrolyte systems.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:electrochemistry, AC impedance spectroscopy, conducting atomic force microscopy, platinum, martensite
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Mixed availability, specified at file level
Research Advisor(s):
  • Haile, Sossina M.
Thesis Committee:
  • Haile, Sossina M. (chair)
  • Giapis, Konstantinos P.
  • Bhattacharya, Kaushik
  • Goddard, William A., III
Defense Date:20 May 2011
Author Email:mlouie (AT) caltech.edu
Funders:
Funding AgencyGrant Number
National Science Foundation, Division of Materials ResearchDMR-0906543
NSF, Caltech Center for the Science and Engineering of MaterialsDMR-0520565
Department of Defense Army Research Office, Multidisciplinary University Research InitiativeARO W911NF-07-1-0410
Gordon and Betty Moore Foundation, Caltech Center for Sustainable Energy ResearchUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
International Centre for Diffraction Data, Ludo Frevel Crystallography ScholarshipUNSPECIFIED
Society of Women Engineers, Baker Hughes ScholarshipUNSPECIFIED
Record Number:CaltechTHESIS:05232011-144137426
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05232011-144137426
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6420
Collection:CaltechTHESIS
Deposited By: Mary Louie
Deposited On:31 May 2011 21:37
Last Modified:16 Apr 2013 23:05

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