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
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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
Introduction
SOECs
SOFCs
Fuel (H 2 )
Steam rich
+ H2
O2
Air (O2 )
H2O
H 2 rich
+ Steam,
Solid Oxide Fuel Cells (SOFCs)
Solid Oxide Electrolysis Cells (SOECs)
Anode RXN
H2 + O2- → H2O + 2e-
H2O + 2e- → H2 + O2-
Cathode RXN
½O2 + 2e- → O2- H2 +
O2- → ½O2 +2e-
Overall RXN
½O2 → H2O
H2O → H2 + ½O2
Reaction heat
Exothermic (ΔH > 0)
Endothermic (ΔH < 0)
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Thermodynamic aspects
► Energy requirements for
electrolysis
SOEC operating temp. (600-1000oC)
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
• HTE:
~ 34 kWh/kg
Conventional: ~ 50 kWh/kg
electrolysis
<
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Oxygen ion conducting electrolyte
- Electrolyte Materials for SOFC/SOEC
Electrical Conductivity (S/cm)
900
600 (oC)
700
ScSZ (Scandia stabilized zirconia)
0.1
YSZ (Yttria stabilized zirconia)
0.01
0.8
[1] B.C.H. Steele, Nature 414 (2001) 345
800
0.9
1.0
1.1
1.2
1000/T (K-1)
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Button cell tests
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
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Button cell tests (SOEC)
Cell Voltage / V
1.5
YSZ
(850oC)
ScSZ
(850oC) o
ScSZ
(800oC)
ScSZ (650
C)
1.0
0.5
SOEC mode
SOFC mode
50% H2O
0.0
-1
0
1
2
Current Density / Acm-2
Polarization resistance: SOEC mode > SOFC mode
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7
Current-voltage characteristics
► Button cell I-V
tests
From Faraday’s law,
Hydrogen production rate is
𝐼� 𝐻
SOFC mode
SOEC mode
𝑚 2
�̇
𝑛
3
−1
=
�
−1 × 22400 𝑠𝑚 ∙ 𝑚𝑚�
≅
1
𝐶∙
𝑠𝑠𝑠
�̇
2 × 96485 𝐶∙ 𝑚𝑚
�−1
= 0.116 𝑠𝑚3 ∙ 𝑠𝑠𝑠−1
𝑚≅ 0.116 × � 𝐼𝑑𝑑
▫ Hydrogen production rate : 8.3 cc/min∙cm2 @ 1.3V ( ~ 100% current efficiency)
▫ Over 30% steam content is required.
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Operation conditions
► Button cell operating
conditions
▫ Impedance results:
▫ I-V results:
·Resistances decrease with temperature.
·High steam content  high performance
·Rc - strong dependence on steam content
·No significant differences in H2 production
·Rohm – no connection with steam content
rate with steam content at low temp
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Stack design
Manifold glass sealing
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 stack volume
· Minimum sealing area and manifold
· Enhanced active area
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Processing
Extrusion
Machine work
Manufacturing step
Dip-coating
Spray-coating
Flat-tubular single cells
Sintering
Stack module
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Stack development
 Stack development
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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, 300 - 700 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
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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)
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Solid State Ammonia Synthesis
 Solid State Ammonia Synthesis (SSAS) using H2 and N2
H2
e-
H+
N2
Proton conductor
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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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)
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Solid State Ammonia Synthesis using H2O and N2
 Using steam instead of hydrogen  cost saving (production and purification)
1. Oxygen ion conductor
2. Proton conductor
Air
H2O
e-
e-
H+
O2-
3H2O +N2
3O2-
 3/2O2 +
2NH3
6e-
3H2O + N2 + 6e- 3O2- +2NH3
N2
2NH3
3H2O  6H+ +3/2O2 +
6e- Drawbacks of proton
conducting oxides: High
sintering temp. (BaZrO3 ~
1700 oC) Formation of
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secondary phases (phase
Experimental
eElectrodes: 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 : 500-660 oC
 Electrochemical test
-
Current-voltage characteristic
-
Impedance spectroscopy
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Analysis of ammonia formation
 Indophenol Blue Method
1. Phenol: 1ml
2. Sodium nitroprusside: 1ml
3. Alkaline citrate + Sodium hypochlorite: 2.5ml
- Ammonia collection quantified by bubbling through solution.
- Analyzed by spectrophotometer
Standard Curve
1.474
Range: 0.01-1.5mg/L
1.000
Abs.
: 0.01-10 ppm
0.500
Error: ±0.013 mg/L
(95% confidence level)
0.000
0.000
-0.134
0.500
1.000
1.500
Conc. (mg/l)
y = 0.89441 x + 0.00000
Correlation Coef f icient r2 = 0.99929 Multiple
Correlation Coef f icient r2 = 0.99929
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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>
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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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Impedance spectra at OCV
200
660 oC
-Z'' (Ω)
150
12 Hz
Pt electrodes
100
50
0.8 Hz
LSF-GDC electrodes
0
0
-
50
100 150 200 250 300 350 400
Z' (Ω)
Anode: air
Cathode: N2 (50 cc/min) + 3% H2O
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Current-voltage characteristics
1.0
660 oC
0.8
Voltage (V)
Voltage (V)
0.8
0.6
0.4
0.2
0.0
0.0
660 oC
1.0
0.6
0.4
0.2
Pt electrodes
LSF-GDC electrodes
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
5
Current (mA)
10
15
20
Current
(mA)
- LSF-GDC electrode  Higher current can be applied.
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Current-voltage characteristics
1.4
1.2
1.4
500 oC
1.2
1.0
Voltage (V)
Voltage (V)
1.0
0.8
0.6
0.4
0.2
0.0
0.00
500 oC
0.8
0.6
0.4
0.2
Pt electrode
LSF-GDC electrode
0.0
0.02
0.04
Current (mA)
0.06
0
1
2
3
4
5
Current (mA)
- LSF- GDC electrode  80 times higher current than Pt at 500 oC
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Dependence of ammonia production rate on the applied current
660 oC
660 oC
-10
2.0x10
Pt electrode
1.5x10-10
1.0x10-10
5.0x10-11
0.0
0.0
Ammonia synthesis rate (mol/sec)
Ammonia synthesis rate (mol/sec)
-10
2.0x10
LSF-GDC electrode
1.5x10-10
1.0x10-10
5.0x10-11
0.0
0
0.
1
2
4
6
8
10
Current (mA)
0.
2
0.
3
- LSF-GDC/YSZ/LSF-GDC4  1.7ⅹ 10-10 mol/cm20.∙sec at 660 oC
-
0.
2∙sec at 650 oC [5]
Pd-SCY-Ru  9.1ⅹ 10-14 mol/cm
5
Pt-Nafion-Ru  2.1ⅹ 10-11 mol/cm2∙sec at 90 oC [6] There are
0. only two literature data (using H2O and N2)
6
[5] A. Skodra et al., Solid State Ionics (2009) [6] V.
Kordali et al., Chem. Commun. (2000)
Current (mA)
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Dependence of ammonia production rate on the applied current
 Ammonia production rate
- Pt-YSZ-Pt  1.2ⅹ 10-10 mol/cm2∙sec at 0.4 mA
Theoretical value (Faraday’s law ) : 1.4ⅹ 10-9 mol/cm2∙sec at 0.4 mA
𝑚𝑚�𝑚𝑚𝑚𝑚�𝑣𝑚
≈ 8.6 %
𝑣𝑚𝑚�
𝑡𝑡𝑚𝑡𝑚𝑚𝑡𝑡𝑡𝑚𝑣𝑣𝑚𝑣𝑚
𝑚�
- LSF-GDC/YSZ/LSF-GDC  1.7ⅹ 10-10 mol/cm2∙sec at 9 mA
Theoretical value: 3.1ⅹ 10-8 mol/cm2∙sec at 9 mA
𝑚𝑚�𝑚𝑚𝑚𝑚� 𝑣𝑚𝑣
𝑚𝑚�
𝑡𝑡𝑚𝑡𝑚𝑚𝑡𝑡𝑡𝑚𝑣𝑣𝑚𝑣𝑚
𝑚�
≈ 0.6 %
 Conversion rate should be increased.
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Conclusions
Ammonia is synthesized from steam and nitrogen by using oxygen ion
conducting electrolyte.
The maximum rate of ammonia production is 1.7ⅹ 10-10 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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Thank you for your attention!!
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