1. Electrochemical synthesis of ammonia from steam and nitrogen using an oxygen-ion conducting electrolyte
Jong Hoon Joo, Hyung Chul Yoon, Hana Jeoung, Ji Haeng Yu, Jong-Nam Kim, Young Min Woo, Jin Young Jang
Korea Institute of Energy Research (KIER), Daejeon, South Korea 1
Korea Institute of Energy Research

2. Overview
Hydrogen manufacturing by Solid Oxide Electrolysis Cells (SOECs)
Ammonia manufacturing by Solid Oxide Electrolysis Cells (SOECs)
 Electrochemical synthesis of ammonia from steam and nitrogen using an oxygen-ion conducting electrolyte
Korea Institute of Energy Research

3. Introduction SOFCs SOECs Fuel (H2) Steam rich + H2 O2 Air (O2) H2O
H2 rich
+ Steam,
Solid Oxide Fuel Cells (SOFCs)
Solid Oxide Electrolysis Cells (SOECs)
Anode RXN Cathode RXN Overall RXN
H2O + 2e- → H2 + O2- O2- → ½O2 +2e-
H2O → H2 + ½O2
Endothermic (ΔH < 0)
H2 + O2- → H2O + 2e-
½O2 + 2e- → O2- H2 + ½O2 → H2O
Exothermic (ΔH > 0)
Reaction heat
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4. Thermodynamic aspects
► Energy requirements for electrolysis
SOEC operating temp. ( oC)
Steam electrolysis ?
Why ???
∆G= ∆ H-T ∆ S
S. Herring (INL), 2005 Hydrogen, Fuel Cells & Infrastructure Technologies Program Review
▫ Overall thermal-to-hydrogen efficiency > 50%
▫ Electrical energy requirements for electrolysis
<
HTE: ~ 34 kWh/kg
Conventional: ~ 50 kWh/kg
Korea Institute of Energy Research

5. Oxygen ion conducting electrolyte
- Electrolyte Materials for SOFC/SOEC
900
800
700
600 (oC)
Electrical Conductivity (S/cm)
ScSZ (Scandia stabilized zirconia)
0.1
YSZ (Yttria stabilized zirconia)
0.01
0.8
0.9
1.0
1000/T (K-1)
1.1
1.2
[1] B.C.H. Steele, Nature 414 (2001) 345
Korea Institute of Energy Research

6. Button cell tests ▫ Button cell Button cell test unit
LSM
LSM-YSZ
YSZ
NiO-YSZ
▫ Button cell
active area: 0.5 ~ 1.0 cm2
cell thickness: 1 mm
sealing materials: Pyrex
Button cell test unit
Korea Institute of Energy Research

7. Button cell tests (SOEC)
1.5
YSZ (850oC) ScSZ (850oC) ScSZ (800oC) ScSZ (650 C) o
Cell Voltage / V
1.0
0.5
SOEC mode
SOFC mode
50% H O 2
0.0 -1
0 1
Current Density / A cm-2 2
Polarization resistance: SOEC mode > SOFC mode
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8. Current-voltage characteristics
► Button cell I-V tests
From Faraday’s law,
Hydrogen production rate is
�̇ =
𝐼�𝑚𝐻2 𝑛�
≅ 1 𝐶∙ 𝑠𝑠𝑠−1 × 𝑠𝑚 ∙ 𝑚𝑚� 3 −1 �̇
SOFC mode
2 × 𝐶∙ 𝑚𝑚�−1
SOEC mode
= 𝑠𝑚3 ∙ 𝑠𝑠𝑠−1
𝑚≅ × � 𝐼𝑑𝑑
▫ Hydrogen production rate : 8.3 cc/min∙cm2
▫ Over 30% steam content is required.
@ 1.3V ( ~ 100% current efficiency)
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9. Operation conditions ► Button cell operating conditions
▫ Impedance results:
Resistances decrease with temperature.
Rc - strong dependence on steam content
Rohm – no connection with steam content
▫ I-V results:
High steam content  high performance
No significant differences in H2 production rate with steam content at low temp
Korea Institute of Energy Research

10. Stack design Manifold glass sealing Stack structures
H2O (rich) + H2 (lean)
Stack structures
H2O (lean) + H2 (rich)
▫ Characteristics of KIER flat-tubular cell stack
All-ceramic stack (ceramic interconnector all-in-one)
High mechanical strength
Minimum sealing area and manifold
Minimum stack volume
Enhanced active area
Korea Institute of Energy Research

11. Processing Extrusion Machine work Dip-coating Spray-coating Sintering
Manufacturing step
Flat-tubular single cells
Stack module
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12. Stack development
Stack development
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13. Ammonia as an energy carrier
While the introduction of a hydrogen economy has its merits, the associated problems with on-board hydrogen storage are still a barrier to realization.
Ammonia and related chemicals can provide an alternative energy vector.
- Haber- Bosch process (250 bar, 450 oC)
N2 (g) + 3H2 (g)  2NH3 (g) Energy consumption: 36.GJ/ton NH3
- Solid-state electrochemical process (1 bar, oC)
3H2O(g) + N2 (g)  2NH3 + 3/2 O2 (g) 26 GJ/ton NH3
 Overall cost reduction: 1/2 of the current price of NH3 [2]
[2] J. Holbrook, Ammonia:The Promise of Green Fuel, Spring 2008
Korea Institute of Energy Research

14. Energy density
Fig.1. Volumetric versus gravimetric energy density of the most important energy carriers [3]
- Only ammonia and hydrides exhibit an energy density close to fossil fuels such as
coal and oil, much higher than compressed hydrogen.
[3] A. Zuttel et al., Philos. Trans. R Soc. A-Math Phys. Eng. Sci. (2010)
Korea Institute of Energy Research

15. Solid State Ammonia Synthesis
Solid State Ammonia Synthesis (SSAS) using H2 and N2 H2 e- H+
Proton conductor N2
NH3
Proton conductor electrolyte
Perovskite: SrCeO3, BaZrO3, CaZrO3, BaCeO3, SrZrO3 et al. Pyrochlore: La2Zr2O7, La2Ce2O7 et al.
Polymer: Nafion et al.
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16. Solid State Ammonia Synthesis using H2 and N2
Summary of the SSAS using H2 and N2
[4] A. Ibrahim et al., J. Solid State Electrochem. (2011)
Korea Institute of Energy Research

17. Solid State Ammonia Synthesis using H2O and N2
Using steam instead of hydrogen  cost saving (production and purification)
Oxygen ion conductor 2. Proton conductor
Air
H2O e- e-
O2- H+
3H2O +N2
3O2-  3/2O2 + 6e-
2NH3
N2 2NH3
3H2O  6H+ +3/2O2 + 6e-
- Drawbacks of proton conducting oxides: High sintering temp. (BaZrO3 ~ 1700 oC) Formation of secondary phases (phase instability) High grain boundary resistance
3H2O + N2 + 6e- 3O2- +2NH3
Korea Institute of Energy Research

18. e- O2- Experimental H2O + N2 NH3 +H2O +N2 +H2
Electrodes: Pt or (LSF)La0.6Sr0.4FeO3-(GDC)Ce0.9Gd0.1O2-δ
Electrolyte : O2- ion conductor (3YSZ, t: 90 ㎛)
O2-
H2O + N2
NH3 +H2O +N2 +H2
Overall cell reaction: 3H2O +N2  2 NH3 + 3/2O2
- N2 (50 cc/min) + 3% H2O
- Electrode area: 1cm2 -
Measuring temperature : oC
Electrochemical test
Current-voltage characteristic
Impedance spectroscopy
Korea Institute of Energy Research

19. Analysis of ammonia formation
Indophenol Blue Method
Phenol: 1ml
Sodium nitroprusside: 1ml
Alkaline citrate + Sodium hypochlorite: 2.5ml
Ammonia collection quantified by bubbling through solution.
Analyzed by spectrophotometer
Standard Curve 1.474
Range: mg/L
: ppm
1.000
Abs.
0.500
Error: ±0.013 mg/L
(95% confidence level)
0.000
-0.134
y = x
Correlation Coef f icient r2 = Multiple Correlation Coef f icient r2 =
1.000
1.500
Conc. (mg/l)
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20. Mixed conducting perovskite
Mixed ionic electronic conductor
Mixed conducting perovskites contain alkaline earth and rare earth cations on the A-site and a transition metal on the B-site.
For examples, La0.6Sr0.4CoO3-δ has a high ionic conductivity (≈
0.1 S/cm , δ ≈ 0.1 at 800 oC in 1 atm O2) caused by oxygen vacancy.
<Ideal cubic perovskite structure>
Korea Institute of Energy Research

21. Mixed conducting perovskite
Electrode Reactions
- Electronic conductor : Pt
- Mixed conductor : (La,Sr)FeO3-δ
Three-phase boundary (gas, electron,ion) area in electrodes is important for the oxygen ion transport.
Polarization resistance: Pt > Mixed conducting perovskite
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22. Impedance spectra at OCV
200
660 oC
Pt electrodes
150
12 Hz
-Z'' (Ω)
100 50
0.8 Hz
LSF-GDC electrodes
Z' (Ω)
Anode: air
Cathode: N2 (50 cc/min) + 3% H2O
Korea Institute of Energy Research

23. Current-voltage characteristics
1.0
660 oC
1.0
660 oC
0.8
0.8
Voltage (V)
Voltage (V)
0.6
0.6
0.4
0.4
0.2
Pt electrodes
0.2
LSF-GDC electrodes
0.0
0.0
0.0
0.2
1.2
1.4 5 10 15 20
Current (mA)
- LSF-GDC electrode  Higher current can be applied.
Korea Institute of Energy Research

24. Current-voltage characteristics
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
500 C o
500 C o
Voltage (V)
Voltage (V)
Pt electrode
LSF-GDC electrode
0.00
0.06 1
2 3 4 5
Current (mA)
- LSF- GDC electrode  80 times higher current than Pt at 500 oC
Korea Institute of Energy Research

25. Dependence of ammonia production rate on the applied current
660 oC
660 oC
2.0x10-10
2.0x10-10
Ammonia synthesis rate (mol/sec)
Ammonia synthesis rate (mol/sec)
Pt electrode
LSF-GDC electrode
1.5x10-10
1.5x10-10
1.0x10-10
1.0x10-10
5.0x10-11
5.0x10-11
Current (mA)
Ammonia production rate
- Pt-YSZ-Pt  1.2ⅹ mol/cm2∙sec at 660 oC 2
4 6
Current (mA) 8 10
- LSF-GDC/YSZ/LSF-GDC  1.7ⅹ mol/cm2∙sec at 660 oC
Pd-SCY-Ru  9.1ⅹ mol/cm2∙sec
Pt-Nafion-Ru  2.1ⅹ mol/cm2∙sec
[5] A. Skodra et al., Solid State Ionics (2009) [6] V. Kordali et al., Chem. Commun. (2000)
at 650 oC [5]
at 90 oC [6]
There are only two literature data (using H2O and N2)
Korea Institute of Energy Research

26. Dependence of ammonia production rate on the applied current
- Pt-YSZ-Pt  1.2ⅹ mol/cm2∙sec at 0.4 mA
Theoretical value (Faraday’s law ) : 1.4ⅹ mol/cm2∙sec at 0.4 mA
𝑚𝑚�𝑚𝑚𝑚𝑚�𝑣𝑚𝑣𝑣𝑚𝑚�
≈ 8.6 %
𝑡𝑡𝑚𝑡𝑚𝑚𝑡𝑡𝑡𝑚𝑣𝑣𝑚𝑣𝑚𝑚𝑚�
- LSF-GDC/YSZ/LSF-GDC  1.7ⅹ mol/cm2∙sec at 9 mA Theoretical value: 3.1ⅹ mol/cm2∙sec at 9 mA
𝑚𝑚�𝑚𝑚𝑚𝑚� 𝑣𝑚𝑣𝑚𝑚�
𝑡𝑡𝑚𝑡𝑚𝑚𝑡𝑡𝑡𝑚𝑣𝑣𝑚𝑣𝑚𝑚𝑚�
≈ 0.6 %
Conversion rate should be increased.
Korea Institute of Energy Research

27. Conclusions
Ammonia is synthesized from steam and nitrogen by using oxygen ion conducting electrolyte.
The maximum rate of ammonia production is 1.7ⅹ mol/cm2∙sec with perovskite electrode.
 about 2000 times larger than reported value (Pd-SCY-Ru)
 about 10 times larger than reported value (Pt-Nafion-Ru)
Further study is necessary to enhance the ammonia formation rate.
- Reaction mechanism (N2 dissociation et al.)
- Factors affecting the rate of ammonia formation (temperature, catalysis, conductivity)
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28. Thank you for your attention!!
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