| | 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

| |

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

| |

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

| |

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

| | 24/10/2017 Marie Hoes 5

Solar to H2 efficiency

Tred = 1673 K

Pred = 10 Pa

εgg = 0.9

εss = 0

ηO2 removal = 0.1

[-]

| | 24/10/2017 Marie Hoes 6

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

| | 24/10/2017 Marie Hoes 8

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

| |

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

| |

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

| | 24/10/2017 Marie Hoes 12

Nonstoichiometry – Comparison 5+ dopant

𝛿 =𝛥𝑚

𝑚s⋅𝑀s𝑀o

| | 24/10/2017 Marie Hoes 13

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

| |

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

| |

PREC Group

24/10/2017 Marie Hoes 28

Acknowledgements

CTI

| |

24/10/2017 Marie Hoes 29

Questions & Discussion

| | 24/10/2017 Michael Takacs 30

| |

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%

| | 24/10/2017 Marie Hoes 34

Sample characterization - SEM

Before TG After TG

YNb5

| | 24/10/2017 Marie Hoes 35

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

| | 24/10/2017 Marie Hoes 44

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.

| | 24/10/2017 Marie Hoes 45

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