1 Korea Institute of Energy Research Electrochemical synthesis of ammonia from steam and nitrogen...

1Korea Institute of Energy Research

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

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

Solid Oxide Fuel Cells (SOFCs) Solid Oxide Electrolysis Cells (SOECs)

Anode RXN

Cathode RXN

Overall RXN

H2 + O2- → H2O + 2e-

½O2 + 2e- → O2- H2 +

½O2 → H2O

Exothermic (ΔH > 0)

H2O + 2e- → H2 + O2-

O2- → ½O2 +2e-

H2O → H2 + ½O2

Endothermic (ΔH < 0)Reaction heat

SOFCs SOECs

Air (O2)

Fuel (H2)

H2O H2 rich

+ Steam,

Steam rich

+ H2

O2


•HTE: ~ 34 kWh/kg Conventional: ~ 50 kWh/kg<

Thermodynamic aspects

Energy requirements for electrolysis

SOEC operating temp. (600-1000oC)

Steam electrolysis ?


• S. Herring (INL), 2005 Hydrogen, Fuel Cells & Infrastructure Technologies Program Review

Overall thermal-to-hydrogen efficiency > 50%

Electrical energy requirements for electrolysis

Why ??? ∆G= ∆ H-T ∆ S

[1] B.C.H. Steele, Nature 414 (2001) 345

- Electrolyte Materials for SOFC/SOEC

0.8 0.9 1.0

1000/T (K-

1)

1.1 1.2

0.01

0.1

600 (oC)


700

800

900

Oxygen ion conducting electrolyte

YSZ (Yttria stabilized zirconia)

ScSZ (Scandia stabilized zirconia)

Button cell test unit

Button cell

-active area: 0.5 ~ 1.0 cm2

-cell thickness: 1 mm

-sealing materials: Pyrex

LSM


LSM-YSZ

YSZ

NiO-YSZ

Button cell tests

-1 20 1

Current Density / A cm-2

0.0

0.5

1.0

1.5

YSZ (850oC) ScSZ (850oC) ScSZ (800oC) ScSZ (650 C)

o

50% H O

2

Polarization resistance: SOEC mode > SOFC mode

Button cell tests (SOEC)

SOEC mode SOFC mode

Korea Institute of Energy Research7

Hydrogen production rate : 8.3 cc/min∙cm2

Over 30% steam content is required.

@ 1.3V ( ~ 100% current efficiency)

Button cell I-V tests

Current-voltage characteristics

From Faraday’s law,

Hydrogen production rate is

*= 𝐼𝑚𝐻2 𝑛

≅ 1 𝐶∙ 𝑠𝑠𝑠−1 × 22400 𝑠𝑚 ∙ 𝑚𝑚

3 −12 × 96485 𝐶∙ 𝑚𝑚−1= 0.116 𝑠𝑚3 ∙ 𝑠𝑠𝑠−1

𝑚≅ 0.116 × 𝐼𝑑𝑑

SOEC mode

SOFC mode


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

Button cell operating conditions

Operation conditions


Manifold glass sealingH2O (rich) + H2 (lean)

Stack structuresH2O (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

Stack design


Extrusion Dip-coating Spray-coating Sintering

Manufacturing step Flat-tubular single cells Stack module

Machine work

Processing


Stack development


Stack development

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

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 ascoal and oil, much higher than compressed hydrogen.

Korea Institute of Energy Research[3] A. Zuttel et al., Philos. Trans. R Soc. A-Math Phys. Eng. Sci. (2010)

Solid State Ammonia Synthesis

N2 NH3

Solid State Ammonia Synthesis (SSAS) using H2 and N2

H2

e-

H+Proton conductor

Proton conductor electrolyte

Perovskite: SrCeO3, BaZrO3, CaZrO3, BaCeO3, SrZrO3 et al.

Pyrochlore: La2Zr2O7, La2Ce2O7 et al.

Polymer: Nafion et al.


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)


O2-

e-

3H2O +N2

3O2- 3/2O2 + 6e-

2NH3

e-

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

H+

Using steam instead of hydrogen cost saving (production and purification)

1. Oxygen ion conductor 2. Proton conductor


Air H2O

3H2O + N2 + 6e- 3O2- +2NH3

Solid State Ammonia Synthesis using H2O and N2

- N2 (50 cc/min) + 3% H2O

- Electrode area: 1cm2

- Measuring temperature : 500-660 oC

Electrodes: Pt or (LSF)La0.6Sr0.4FeO3-(GDC)Ce0.9Gd0.1O2-δ

Electrolyte : O2- ion conductor (3YSZ, t: 90 )

H2O + N2

O2-

e-

Overall cell reaction: 3H2O +N2 2 NH3 + 3/2O2

Experimental

NH3 +H2O +N2 +H2


Electrochemical test

- Current-voltage characteristic

- Impedance spectroscopy


Standard Curve 1.474

Conc. (mg/l)

0.000

-0.1340.000 0.500

y = 0.89441 x + 0.00000Correlation Coef f icient r2 = 0.99929 Multiple Correlation Coef f icient r2 = 0.99929

1.000 1.500

1.000

0.500

Range: 0.01-1.5mg/L

: 0.01-10 ppm

Error: ±0.013 mg/L

(95% confidence level)

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

Analysis of ammonia formation

Mixed conducting perovskite

<Ideal cubic perovskite structure>


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.

Electrode Reactions

- Electronic conductor : Pt - Mixed conductor : (La,Sr)FeO3-δ


Mixed conducting perovskite

-Three-phase boundary (gas, electron,ion) area in electrodes is important for the oxygen ion transport.

-Polarization resistance: Pt > Mixed conducting perovskite

Impedance spectra at OCV

0 50 100 150 200250Z' (Ω)

300 350400

0

50

100

150

200

0.8 Hz


660 oC

Pt electrodes

12 Hz

LSF-GDC electrodes

- Anode: air- Cathode: N2 (50 cc/min) + 3% H2O

0.0

0.2

0.4

0.6

0.8

1.0

660 oC

0.0

0.2

0.4

0.6

0.8

1.0


660 oC


Pt electrodes LSF-GDC electrodes

- LSF-GDC electrode Higher current can be applied.

0.0 0.2 0.4 0.6 0.81.0

1.2 1.4 0 5 10 15 20

Current (mA) Current (mA)

500 C

o1.4

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


Pt electrode LSF-GDC electrode

- LSF- GDC electrode 80 times higher current than Pt at 500 oC


0.00 0.020.04

0.06 0 1 2

3

4 5

Current (mA) Current (mA)

There are only two literature data (using H2O and N2)


0.0

0.00.0

0.1

0.2

0.3

0.4

0.5

0.6

Current (mA)

Ammonia production rate

-

Pt-YSZ-Pt 1.2ⅹ 10-10 mol/cm2∙sec at 660 oC

5.0x10-

11

1.0x10-

10

1.5x10-

10

2.0x10-

10

0 2 4

6

Current (mA)

8 10

5.0x10-

11

1.0x10-

10

1.5x10-

10

2.0x10-

10

Dependence of ammonia production rate on the applied current

Pt electrode LSF-GDC electrode

- LSF-GDC/YSZ/LSF-GDC 1.7ⅹ 10-10 mol/cm2∙sec at 660 oC

660 oC 660 oC

- Pd-SCY-Ru 9.1ⅹ 10-14

mol/cm2∙sec- Pt-Nafion-Ru 2.1ⅹ 10-11 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]

Dependence of ammonia production rate on the applied current

- 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

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 𝑣 𝑚𝑣𝑚𝑚𝑡𝑡𝑚𝑡𝑚𝑚𝑡𝑡𝑡𝑚𝑣𝑣𝑚𝑣𝑚𝑚

≈ 8.6 %


𝑚𝑚 𝑚𝑚𝑚𝑚 𝑣𝑚𝑣𝑚𝑚𝑡𝑡𝑚𝑡𝑚𝑚𝑡𝑡𝑡𝑚𝑣𝑣𝑚𝑣𝑚𝑚

≈ 0.6 %

Conversion rate should be increased.

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)

Thank you for your attention!!


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1 Korea Institute of Energy Research Electrochemical synthesis of ammonia from steam and nitrogen...

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