thermodynamic characterization of paired charge ......determine partial molar properties of double...
TRANSCRIPT
| | 24/10/2017 Marie Hoes 1
Thermodynamic characterization of paired charge-
compensating doped ceria for improved redox performance
of solar thermochemical H2O/CO2 splitting cycles
Marie Hoes, Christopher Muhich, Roger Jacot, Greta Patzke, Aldo Steinfeld
Solar Thermochemistry Workshop 2017, Jülich
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24/10/2017 Marie Hoes 2
Two-step solar thermochemical water splitting
Reduction:
Oxidation:
0 > ∆𝐻red − 𝑇red ∆𝑆red +1
2𝑆O2
𝑜
+ 𝑇red𝑅 ln𝑝O2
𝑝O2𝑜
0 > −∆ 𝐻red − ∆ 𝐻H2O − 𝑇ox −∆𝑆red + 𝑆H2
𝑜 − 𝑆H2O𝑜 + 𝑇ox𝑅 ln
𝑝H2
𝑝H2O
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24/10/2017 Marie Hoes 3
Modeling solar to H2 efficiency
Reduction
and
oxidation
reactions
Solar
collection
Steam
supply
Ehrhart, Muhich, Al-Shankiti, Weimer, Int. J. Hydrogen Energy. 2016. 41(44)
Oxygen
removal
ηSTH =𝑛 𝐻2
HHVH2
𝑄solar
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Solar Collection
α= 0.95
hconv= 0.0482 [kW/(m2*K)]
ηoptic = 0.9
C = 3000
DNI = 1 [kW/m2]
Oxygen removal and product separation
Purity H2 = 0.999
Tsep = 300 K
ηpump = 0.1
ηmech = 0.15
24/10/2017 Marie Hoes 4
Efficiency modeling - Conditions
ηSTH =𝑛 𝐻2
𝐻𝐻𝑉𝐻2
𝑄solar
Qsolar
𝑄Steam
𝑄oxygen removal
𝑄redox
𝑄Solar collection
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Solar to H2 efficiency
Tred = 1673 K
Pred = 10 Pa
εgg = 0.9
εss = 0
ηO2 removal = 0.1
[-]
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The effects of ΔHred on efficiency
ΔHred
1Bulfin et al., PCCP. 2016. 18 (23147-23154) 2Takacs et al., Acta Materialia. 2016. 103(15)
CeO21
Ce0.85Zr0.15O21
La0.6Sr0.4MnO32
La0.6Sr0.4Mn0.6Al0.4O32
425 kJ/mol
375 kJ/mol
262 kJ/mol
265 kJ/mol
Favors oxidation
Favors reduction
Tred = 1673 K
Pred = 10 Pa
εgg = 0.9
εss = 0
ηO2 removal = 0.1
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24/10/2017 Marie Hoes 7
Effects of tetravalent dopants on ceria reduction
Hypothesis:
Two charge compensating dopants act like tetravalent dopants
An
de
rsso
n, D
. e
t a
l. P
hys. R
ev. B
2007, 7
6, 1
74
11
9
Need:
CeO2 > ∆𝐻red > ZrxCe1−xO2
Problem:
No such tetravalent dopant
Area of interest
Si
Ge
Pb
Sn
Ti
Th Ce
Zr
Hf
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Effects of different dopants on ΔHred
High Solar to H2 efficiency
3+ & 5+
3+
4+ 5+ Reduction e
nth
alp
y
ScN
b
YN
b
LaN
b
Hf
Nb
Y
Zr
YV
Y
Nb
CeO
2
Muhich et al., to be submitted 2017
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Validating DFT calculations
Determine partial molar properties of double doped ceria
Calculate solar to H2 efficiency
24/10/2017 Marie Hoes 9
Goal of this work
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New:
Ce0.9Y0.05Nb0.05O2 YNb5
Ce0.9Sc0.05Nb0.05O2 ScNb5
Ce0.9La0.05Nb0.05O2 LaNb5
Ce0.75La0.125Nb0.125O2 LaNb12.5
Ce0.95La0.25Nb0.25O2 LaNb2.5
Ce0.9Nb0.1O2 Nb10
Ce0.9La0.1O2 La10
Ce0.9Y0.1O2 Y10
Reference:
Ce0.9Hf0.1O2 Hf10
CeO2
24/10/2017 Marie Hoes 10
Thermodynamic analysis - materials
Shortname
| | 24/10/2017 Marie Hoes 11
Nonstoichiometry – Hf10 & CeO2
𝛿 =𝛥𝑚
𝑚s⋅𝑀s𝑀o
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Nonstoichiometry – Comparison 5+ dopant
𝛿 =𝛥𝑚
𝑚s⋅𝑀s𝑀o
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Nonstoichiometry – Comparison 3+ dopants
𝛿 =𝛥𝑚
𝑚s⋅𝑀s𝑀o
| | 24/10/2017 Marie Hoes 14
Nonstoichiometry – Comparison double dopants 5%
𝛿 =𝛥𝑚
𝑚s⋅𝑀s𝑀o
| | 24/10/2017 Marie Hoes 15
Nonstoichiometry – Comparison dopant concentration
𝛿 =𝛥𝑚
𝑚s⋅𝑀s𝑀o
| | 24/10/2017 Marie Hoes 16
Defect model
2Ce′Ce + VO⋅⋅ = Ce′CeVO
⋅⋅Ce′Ce
2CeCex + OO
x = 2Ce′Ce + VO⋅⋅ + 1
2 O2
Van’t Hoff equation
Point defect:
Cluster defect:
YNb5
| | 24/10/2017 Marie Hoes 17
Defect model and experiment - comparison
YNb5 Solid: Kexp
Dashed: Kfit
| | 24/10/2017 Marie Hoes 18
Partial molar Enthalpy
ΔH
[k
J/m
ol]
| | 24/10/2017 Marie Hoes 19
Partial molar Enthalpy
ΔH
[k
J/m
ol]
| | 24/10/2017 Marie Hoes 20
Partial molar Enthalpy
ΔH
[k
J/m
ol]
| | 24/10/2017 Marie Hoes 21
Partial molar Enthalpy
ΔH
[k
J/m
ol]
| | 24/10/2017 Marie Hoes 22
Partial molar Enthalpy
ΔH
[k
J/m
ol]
| | 24/10/2017 Marie Hoes 23
Partial molar Entropy
ΔS
[J/(
K m
ol)
]
| | 24/10/2017 Marie Hoes 24
Partial molar Entropy
ΔS
[J/(
K m
ol)
]
| | 24/10/2017 Marie Hoes 25
Partial molar Entropy
ΔS
[J/(
K m
ol)
]
| | 24/10/2017 Marie Hoes 26
Solar to H2 efficiency
Tred = 1673 K
Pred = 10 Pa
εgg = 0.9
εss = 0
ηO2 removal = 0.1
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Charge-compensating double doped ceria:
Partial molar enthalpy between CeO2 and Hf10
Similar behavior to tetravalent doped ceria
Large property range
High modeled solar to H2 efficiencies
24/10/2017 Marie Hoes 27
Summary
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PREC Group
24/10/2017 Marie Hoes 28
Acknowledgements
CTI
| |
24/10/2017 Marie Hoes 29
Questions & Discussion
| | 24/10/2017 Michael Takacs 30
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Powders were produced with Pechini method
highly homogeneous and finely dispersed oxide metals are
obtained
Calcination at 1273 K
Uni-axially pressed into pellets (d=8 mm)
Sintering at 1773 K
24/10/2017 Marie Hoes 31
Synthesis
| | 24/10/2017 Marie Hoes 32
XRD
Before TG
After TG
YNb5
2 Theta [°]
Inte
nsi
ty
Inte
nsi
ty
CeO2
| | 24/10/2017 Marie Hoes 33
Sample characterization - ICP
Material
Element YNb5 ScNb5 LaNb5 LaNb2.5 LaNb12.5 Hf10 La10 Nb10 Y10
Ce 3.10 4.21 2.29 0.03 3.35 1.03 0.60 1.51 1.49
Hf 0.46
La 0.01 0.01 1.37 0.10
Nb 0.10 0.10 0.03 0.01 0.06 0.04
Sc 0.22
Y 0.09 0.22
Difference between nominal and actual value in wt-%
< 5%
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Sample characterization - SEM
Before TG After TG
YNb5
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Thermogravimetric (TG) experiments
Investigated range
Temperature: 1173 K – 1773 K
pO2 : 10-15 atm – 10-1 atm
La10, Y10, Nb10
Temperature: 1573 K – 1773 K
pO2 : 10-5 atm – 10-2 atm
| | 24/10/2017 Marie Hoes 36
Thermogravimetric (TG) experiments
La10, Y10, Nb10: 1573 K – 1723 K 10-5 atm – 10-2 atm
Rest: 1173 K – 1773 K 10-15 atm – 10-1 atm
| | 24/10/2017 Marie Hoes 37
Reference masses TG LaNb2.5
| |
Assumption:
Standard enthalpy of reaction is constant in considered temperature
range
24/10/2017 Marie Hoes 38
Van’t Hoff equation
2
ln( ) o
rHd K
dT R T
ln( )o
rHK
R T
| | 24/10/2017 Marie Hoes 39
Van’t Hoff plot
o o
l KH S
nRT R
12
22 O
2
MO pK
MO
2
o oO
o
const
1 H Sln
2
p
p RT R
with
1
| | 24/10/2017 Marie Hoes 40
Extracting partial molar properties
YNb5 Constant δ values needed
| | 24/10/2017 Marie Hoes 41
Defect model – δ range
YNb5
Delta range:
experimental data for
at least 3 temperatures
| | 24/10/2017 Marie Hoes 42
Defect model and experiment – ScNb5
| | 24/10/2017 Marie Hoes 43
Defect model and experiment – CeO2
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Influence of temperature range
Open symbols: Panlener
Closed symbols: Our data
CeO2
Panlener, R. J., et al., Phys. Chem.
Solids 1975, 36(11), 1213-1222.
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Van’t Hoff plot
YNb5
lnp𝑂2
p𝑜 = −2Δ𝐻𝑜
RT+ 2
Δ𝑆𝑜
R 𝛿=const
| | 24/10/2017 Marie Hoes 46
Van’t Hoff plot
YNb5
Case I Case II
lnp𝑂2
p𝑜 = −2Δ𝐻𝑜
RT+ 2
Δ𝑆𝑜
R 𝛿=const
| | 24/10/2017 Marie Hoes 47
Influence of extrapolation range
YNb5
ΔH
[k
J/m
ol]
| | 24/10/2017 Marie Hoes 48
Influence of temperature range
Panlener, R. J., et al., Phys. Chem.
Solids 1975, 36(11), 1213-1222.
Calculated partial molar properties are
highly sensitive to temperature range
Properties extracted from data in
different temperature range might not
be suitable
* Selected data usage
*
ΔH
[k
J/m
ol]
| |
CTI