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Title: Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes

Abstract

In this project we explored the possibility of mitigating silicon poisoning of Pr doped ceria (PCO) electrodes for oxygen reduction in SOFC. The approach was to dope PCO with various amount of lanthanum and study the new compositions in terms of their mixed conduction properties, their oxygen surface exchange kinetics, as well as their resistance to silicon poisoning. The effects of lanthanum doping on the transport properties and oxygen exchange kinetics of Lax(Ce0.9Pr0.1)1-xO2-δ (x=0, 0.1 and 0.2) have been investigated by electrical conductivity on bulk samples and optical relaxation on thin films respectively. Lanthanum substitution was observed to increase the magnitude of ionic conductivity to values comparable to that of single substituted LaxCe1-xO2-δ, with a maximum for a La content of x = 0.1, reaching σ = 9.5×10-3 S·cm-1 at 600 °C and a decrease in activation energy from ≈ 1 eV for Ce0.9Pr0.1O2-δ down to ≈ 0.75 eV for the lanthanum substituted compositions. At the same time, a significant electronic contribution to the total conductivity, estimated to be on the order of 10%, supports significant mixed ionic-electronic conductivity. Substitution with lanthanum increases the absolute values of the oxygen surface exchange coefficient from kchem = 6×10-7 cm·s-1 for Ce0.9Pr0.1O2-δ tomore » kchem = 1×10-6 cm·s-1 for La0.2(Ce0.9Pr0.1)0.8O2-δ at 550 °C. Moreover, one observes significant increases in activation energy from 0.7 eV for Ce0.9Pr0.1O2-δ to ≈ 1 eV for La substituted compositions, suggesting a change in the rate-limiting step. Possible sources for the change in the rate-limiting step are suggested. The composition with the highest La content of x = 0.2, although not exhibiting the highest ionic conductivity, shows the highest surface exchange rates, is suggested to result from enhanced segregation of catalytically active lanthanum to the surface. PCO and the La doped composition x = 0.2 were subjected to an artificial Si poisoning to compare their resistance to it in terms of surface exchange kinetics. It appears that the La doped composition, although still subject to degradation after poisoning, has a better resistance to Si poisoning than the initial composition. The effect of Lanthanum as a surface dopant on oxygen surface exchange kinetics has also been studied, and lead to an improvement of the kinetics by a factor 10. This study has been extended to other rare earth oxides as well as strontium, and they all improve the surface exchange rate substantially, although in different proportion that it will be interesting to further investigate in the future.« less

Authors:
;
Publication Date:
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org.:
USDOE National Energy Technology Laboratory
Contributing Org.:
Massachusetts Institute of Technology
OSTI Identifier:
1569264
Report Number(s):
DOE-MIT-26109-1
DOE Contract Number:  
FE0026109
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN

Citation Formats

Nicollet, Clement, and Tuller, Harry. Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes. United States: N. p., 2019. Web. doi:10.2172/1569264.
Nicollet, Clement, & Tuller, Harry. Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes. United States. doi:10.2172/1569264.
Nicollet, Clement, and Tuller, Harry. Fri . "Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes". United States. doi:10.2172/1569264. https://www.osti.gov/servlets/purl/1569264.
@article{osti_1569264,
title = {Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes},
author = {Nicollet, Clement and Tuller, Harry},
abstractNote = {In this project we explored the possibility of mitigating silicon poisoning of Pr doped ceria (PCO) electrodes for oxygen reduction in SOFC. The approach was to dope PCO with various amount of lanthanum and study the new compositions in terms of their mixed conduction properties, their oxygen surface exchange kinetics, as well as their resistance to silicon poisoning. The effects of lanthanum doping on the transport properties and oxygen exchange kinetics of Lax(Ce0.9Pr0.1)1-xO2-δ (x=0, 0.1 and 0.2) have been investigated by electrical conductivity on bulk samples and optical relaxation on thin films respectively. Lanthanum substitution was observed to increase the magnitude of ionic conductivity to values comparable to that of single substituted LaxCe1-xO2-δ, with a maximum for a La content of x = 0.1, reaching σ = 9.5×10-3 S·cm-1 at 600 °C and a decrease in activation energy from ≈ 1 eV for Ce0.9Pr0.1O2-δ down to ≈ 0.75 eV for the lanthanum substituted compositions. At the same time, a significant electronic contribution to the total conductivity, estimated to be on the order of 10%, supports significant mixed ionic-electronic conductivity. Substitution with lanthanum increases the absolute values of the oxygen surface exchange coefficient from kchem = 6×10-7 cm·s-1 for Ce0.9Pr0.1O2-δ to kchem = 1×10-6 cm·s-1 for La0.2(Ce0.9Pr0.1)0.8O2-δ at 550 °C. Moreover, one observes significant increases in activation energy from 0.7 eV for Ce0.9Pr0.1O2-δ to ≈ 1 eV for La substituted compositions, suggesting a change in the rate-limiting step. Possible sources for the change in the rate-limiting step are suggested. The composition with the highest La content of x = 0.2, although not exhibiting the highest ionic conductivity, shows the highest surface exchange rates, is suggested to result from enhanced segregation of catalytically active lanthanum to the surface. PCO and the La doped composition x = 0.2 were subjected to an artificial Si poisoning to compare their resistance to it in terms of surface exchange kinetics. It appears that the La doped composition, although still subject to degradation after poisoning, has a better resistance to Si poisoning than the initial composition. The effect of Lanthanum as a surface dopant on oxygen surface exchange kinetics has also been studied, and lead to an improvement of the kinetics by a factor 10. This study has been extended to other rare earth oxides as well as strontium, and they all improve the surface exchange rate substantially, although in different proportion that it will be interesting to further investigate in the future.},
doi = {10.2172/1569264},
journal = {},
number = ,
volume = ,
place = {United States},
year = {2019},
month = {10}
}