An Electrochemical Haber-Bosch Process*

V. Kyriakou, I. Garagounis, A. Vourros, E. Vasileiou, and M. Stoukides

Aristotle University of Thessaloniki (AUTH) and

Center for Research & Technology Hellas (CERTH)

Financial Support: CoorsTek Membrane Sciences

* JOULE, 4(1)pp. 142-158 (2020).

T. Eggeman, in “Kirk- Othmer Encyclopedia of Chemical Technology,” 4th edition, John Wiley (2008)

Industrial Ammonia Production

▪ Supports >50% of earth’s population

▪ High T, P → High capital cost

▪ Responsible for 1-2% energy consumption and CO2 emissions worldwide

➢ Hydrogen production (highly endothermic)

Methane steam reforming: CH4 + H2O ➔ CO + 3 H2

Water gas shift: CO + H2O ➔ CO2 + H2

➢ Preparation of synthesis gas (extreme purification)

➢ Pressurization (150-250 bar)

➢ Ammonia synthesis (exothermic): N2 + 3H2 ➔ 2 NH3

Overall Reaction: 3 CH4 + 6 Η2O + 4 Ν2 <==> 3 CO2 + 8 NH3

In industrial practice, the CO2/NH3 molar ratio is about 1.1 (instead of 0.4)

The main steps in the NH3 synthesis plant

Electrolyte: BaZr0.7Ce0.2Y0.1O2.9 (BZCY72), thickness: 30 μm

Counter Electrode: Ni-BZCY72, area: 20-25 cm-2

Working Electrode: VN-Fe, area : 8 cm-2

Tube length: 250 mm

* H. Iwahara, et al, Solid State Ionics 4, 359–363 (1981).

* * S. Robinson, et al, J. Membr. Sci. 446 (2013) 99–105; W.G. Coors, A. Manerbino, J. Membr. Sci. 376 (2011) 50–55.

Plants and bacteria produce NH3 at ambient conditions (nitrogenase, N2 , H+, e- )

Discovery* and development of high temperature H+ conductors

Development * * of ceramic membrane reactors with high H+ conductivity

✓ High H+ conductivity (10-6 moles H2 cm-2 s-1) at 450-700 oC

✓ The hydrogen source can be a hydrocarbon (e.g. steam reforming)

NH3

electrode

9.99 mm

H+

H+

H+

H+

CH4 , H2O

N2 + 6H+ +6e- 2 NH3

CO, CO2

N2

N2, H2, NH3

CH4 + H2O CO + 3H2

CO + H2O CO2 + H2

H2 2H+ + 2e-

➢ Methane conversion increases by H2 removal (operation at lower T)➢ Purification of hydrogen is eliminated

Combined SSAS and CH4-H2O reforming

Overall: 3 CH4 + 6 H2O + 4 N2➔ 8 NH3 + 3 CO2

2H+ +2e- H2

Anode side

0 30 60 90 120 150

40

50

60

70

80

90

100

550 oC

600 oC

650 oC

CH

4 c

on

ve

rsio

n,

%

Current, mA

0 30 60 90 120 15090

92

94

96

98

100

CO

2 s

ele

ctivity,

%

Current, mA

550 oC

600 oC

650 oC

Results of the combined process

0 30 60 90 120 1500

15

30

45

60

75

90

NH

3 r

ate

, m

mo

l. h-1. m

-2

Current, mA

550 oC

600 oC

650 oC

0 30 60 90 120 1500

3

6

9

12

15

18

NH

3 f

ara

daic

effic

iency

, %

Current, mA

550 oC

600 oC

650 oC

Cathode side

At the anode:

▪ Open-circuit: methane conversion > 60%

▪ Closed-circuit: methane conversion up to 85%

▪ Open-circuit: CO2 selectivity up to 90%.

▪ Closed-circuit: CO2 selectivity > 99%.

At the cathode:

▪ NH3 is formed at a rate of up to 1.95 x 10-9 mol.s-1.cm-2 with a

corresponding Faradaic Efficiency of 5.5%.

▪ FEs up to 14% at lower current densities

Summary of Experimental Results

Visualizing an electrochemical HB

SSAS Reactor: CH4 + Η2O ↔ CO2 + 6Η+ + 6e- (anode)

N2 + 6H+ + 6e- ↔ 2NH3 (cathode)

2H+ + 2e- ↔ H2 (cathode)

Protonic Ceramic Fuel Cell: H2 ↔ 2Η+ + 2e- (anode)

1/2O2 + 2H+ + 2e- ↔ H2O (cathode)

(The PCFC is assumed to operate at a 45% efficiency)

NH3, N2, H2

0 20 40 60 80 1000

2

4

6

8

10

12

EHB process

HB process

CO

2 e

mis

sio

ns,

kg

CO

2/k

g N

H3

Faradaic efficiency to NH3, %

PCMR operation window

FENH3 = 35%

0 20 40 60 80 1000

500

1000

1500

2000

PCMR operation window

Electrical

Thermal

Total

HB process

PCMR voltage = 0.3 V

Ener

gy, k

J.m

ol-1 N

H3

Faradaic efficiency to NH3, %

FENH3 = 35%

Electrochemical vs Industrial HB

➢ >35% FE is required to exhibit lower energy demands and CO2 emissions

than the industrial HB

The “Giddey” Requirements 1

➢ current density: 0.25-0.5 A cm-2

➢ current efficiency : >50% ➢ NH3 is production rates: 4.3 - 8.7 x 10-7 mol cm-2 s-1

The DOE (REFUEL) Requirements 2

➢ current density: 0.3 A cm-2

➢ Faradaic efficiency : 90% ➢ NH3 is production rates: 9.3 x10-7 mol cm-2 s-1

1 S..Giddey et al, Int'l J. Hydrogen Energy 38, pp. 14576-14594 (2013)2 I.McPherson, J. Zhang, JOULE, 4, pp. 12-14 (2020)]

What is the target

Increasing the Faradaic Efficiency to a 40-50% and the NH3

production rate to 5 x 10-7 mol cm-2 s-1 is a real challenge. It

will require intense collaboration among researchers in the

fields of electrochemistry, heterogeneous catalysis and

materials science.

But it is worth the effort…

Thank you for your attention!

Electrical energy demands

SSASREACTOR

CO2capture

CH4, H2O

N2,H2O

PCFC NH3 storage

Condenser2

Anode:CH4 +2H2Oà CO2 +8H+ +8e-

Condenser1

H2O

Condenser3

Cathode:N2 +8H+ +8e-à 2NH3 +H2

N2,H2

N2

H2O

N2 (H2)

N2 (H2)

Anode:2H2à 4H+ +4e-

Cathode:O2+4H+ +4e-

à 2H2O

Air(N2,O2)