Proton Ceramic Steam Electrolysers Einar Vllestad 1 , R. Strandbakke - - PowerPoint PPT Presentation

Proton Ceramic Steam Electrolysers

Einar Vøllestad1, R. Strandbakke1, Dustin Beeaff2 and

• T. Norby1

1University of Oslo, Department of Chemistry, 2CoorsTek Membrane Sciences AS

• Theoretical considerations on electrolysis operation
• Development and performance of tubular Proton

Ceramic Electrolysers (PCEs)

Literature data for Proton Ceramic Electrolysers (PCEs)

Electrolyte Anode T emperature i (mA cm2) ASR (Ωcm2) η (%) Ref SSY541 SSC 600 100 4 ~80 Matsumoto, 2012 BCZY53-Zn BSCF 800 55 20 50 Li, 2013 BZCY72 LSCF 700 100 6 50 Babiniec, 2015 BCZY53-Zn LSCM- BCZYZ 700 2000 6-8 22 Gan, 2012 BCZY62 BSCF 600 1050 0.5 99 (?) Yoo, 2013 BCZY53 SSC-BCZY 700 400 1

• He, 2010

Key question: What is the origin of the low faradaic efficiencies observed in many PCEs?

Degradation and decomposition in H2O

Operating Principles of Proton Ceramic Electrolysers (PCEs)

Anode Cathode Electrolyte

2H2O O2 4H+

U e- 2H2O  O2 + 4H+ +4e- 0  e- + h+

h+

e- + h+  0 Rion Zel,a Zel,c Re-

4H+ +4e-  2H2

O2-

Potentials through a solid oxide electrolyser

OCV SOEC

EF EF Electrolyte

O2 H2 x

SOFC

Electronic conductivity distribution during PCE operation

SOEC σp ∝ pO2

1/4 ∝ exp(EF/4)

σp

O2 H2 x

Electrolyte σe σp,OCV

The effect of partial electronic conductivity

• n faradaic efficiency

0.0 0.5 1.0 1.5 2.0 1.0 1.5 2.0

Voltage Current

10 20 30

H2 production (mL min

• 1)

te = 0 te = 0

The effect of partial electronic conductivity

• n faradaic efficiency

0.0 0.5 1.0 1.5 2.0 1.0 1.5 2.0

Voltage Current

10 20 30 te = 0.25

H2 production (mL min

• 1)

te = 0 te = 0 te = 0.25

The effect of partial electronic conductivity

• n faradaic efficiency

0.0 0.5 1.0 1.5 2.0 1.0 1.5 2.0

Voltage Current

10 20 30 te = 0.5 te = 0.25

H2 production (mL min

• 1)

te = 0 te = 0 te = 0.25 te = 0.5

Electrode performance and steam content significantly influence faradaic efficiency

1.25 1.50 1.75 60 80

Faraday efficiency (%) Voltage (V)

1.1 1.2 1.3 1.4 1.5 1.6 1.7 60 70 80 90

pH2O = 0.5 pH2O = 0.75

Faraday efficiency (%) Voltage

pH2O = 0.95

Anode performance UN Rion Zel,a Zel,c Re-

Steam content dependence with fixed tH = 0.8 Anode dependence for with fixed tH = 0.8

Tubular half-cell production

Dip-coating suspensions NiO based paste

Wet milling of precursors Solid State Reactive Sintering Extrusion of BZCY72-NiO support Spray-coating BZCY72 electrolyte

Dense tubular half-cells achieved

Dense electrolyte @ 1550°C – 24h 1610°C – 6h

40 microns

1.

Cap and seal segment using glass ceramic from CoorsTek

2.

Deposit Ba0.7Gd0.8La0.5Co2O6-δ as steam electrode by paint brush

3.

Firing in dual atmosphere:

• 1000°C
• 2% O2 outside, 5% H2 inside
• Ecell = 1.4 V during firing

4.

Gold paste applied as current collector

Steam electrode processing on reduced tubes

Cell 1

Electrolysis with single phase BGLC electrode

1 2 3 4 5 550°C 600°C 650°C 700°C

Current (A)

700°C 650°C 600°C 550°C

H2 production (NmL min

• 1)

F a r a d a i c H

2

p r

• d

u c t i

• n

0.00 0.25 0.50 0.75 1.00 50 100 150 200 1.0 1.5 2.0

700°C 600°C 550°C 650°C

Potential (V) Current density (mA cm

• 2)

Anode: pO2 = 30 mbar pH2 = 0.3 bar ptot = 3 bar ptot = 3 bar pO2 = 80 mbar pH2O = 1.5 bar Cathode:

1.0 1.5 2.0 40 60 80 100 550°C 600°C 650°C 700°C

Faradaic efficiency (%) Potential (V)

Faradaic efficiencies vs cell potential

Electrolysis with single phase BGLC electrode

1 2 3 4 5 550°C 600°C 650°C 700°C

Current (A)

700°C 650°C 600°C 550°C

H2 production (NmL min

• 1)

F a r a d a i c H

2

p r

• d

u c t i

• n

0.00 0.25 0.50 0.75 1.00 50 100 150 200 1.0 1.5 2.0

700°C 600°C 550°C 650°C

Potential (V) Current density (mA cm

• 2)

Anode: pO2 = 30 mbar pH2 = 0.3 bar ptot = 3 bar ptot = 3 bar pO2 = 80 mbar pH2O = 1.5 bar Cathode:

Impedance at 600°C for increasing galvanostatic bias

4 5 6 7 8 3 2 1

• 1

OCV 50 100 300

Z

// (Ωcm 2)

Z

/ (Ωcm 2)

Poor adhesion and delamination of the electrode layer observed in post characterization

• Improved processing route needed

1.

BZCY72- Ba0.5Gd0.8La0.7Co2O6-δ applied as steam electrode

• Fired in air at 1200°C for 5h
• Infiltrated with nanocrystalline

Ba0.5Gd0.8La0.7Co2O6-δ

• Thin Pt layer current collection

2.

Capped and sealed at 1000°C

• Semi-dual atmosphere to keep BGLC

layer intact

3.

NiO reduction at 800°C in 10% H2 for 24h

• Kept in electrolytic bias during

reduction to avoid re-oxidation

Steam electrode processing on unreduced tubes

Cell 2

Electrolysis with composite BZCY-BGLC electrode

5 10 15 20 400°C 500°C 600°C

H2 production (NmL min

• 1)

Faradaic H2 production 700°C 100 200

Current Density (mA cm

• 2)

1 2 3 1.0 1.5 2.0 Anode: pO2 = 30 mbar pH2 = 0.5 bar ptot = 3 bar ptot = 3 bar pO2 = 30 mbar 400°C 500°C 600°C

Voltage (V) Current (A)

700°C pH2O = 1.5 bar Cathode:

4 5 6 7 8 9

• 2

2 4 Zreal (Ωcm

2)

400°C 500°C 600°C

• Zim

700°C 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Zreal (Ω)

EIS at 300mA galvanostatic operation

Electrolysis with composite BZCY-BGLC electrode

5 10 15 20 400°C 500°C 600°C

H2 production (NmL min

• 1)

Faradaic H2 production 700°C 100 200

Current Density (mA cm

• 2)

1 2 3 1.0 1.5 2.0 Anode: pO2 = 30 mbar pH2 = 0.5 bar ptot = 3 bar ptot = 3 bar pO2 = 30 mbar 400°C 500°C 600°C

Voltage (V) Current (A)

700°C pH2O = 1.5 bar Cathode:

1 2 3 2 4 6 8 10 500°C

ASR (Ωcm

2)

Current (A)

600°C 700°C Calculated from dV / dI

Calculated ASR from IV curves

Improved faradaic efficiency primarily due to enhanced electrode kinetics

4 5 6 7 8 9

• 2

2 4 Cell 1

Z

// (Ωcm 2)

Z

/ (Ωcm 2)

Cell 2 600C 30 mA cm

• 2

50 100 150 200 1.0 1.5 2.0

Cell 1 Cell 2 Voltage (V) Current density (mA cm

• 2)

20 40 60 80 100

Faradaic efficiency (%)

Conclusions

 Proton Ceramic Electrolysers may suffer from electronic leakage during

• peration due to relatively high p-type conductivity in oxidizing conditions



Operation at high overpotentials will induce higher electronic conductivity within the electrolyte material



Improved electrode performance and higher steam pressures may reduce electronic leakage

 Tubular PCEs were made based on BZCY-NiO tubular supports, spray

coated BZCY72 electrolytes and BGLC steam electrodes



Enhanced faradaic efficiencies observed with improved anode performance



Current densities of 220 mA cm-2 at 600°C observed with > 80% faradaic efficiency



Contact resistance may still contribute significantly to the ohmic resistance of the electrolyser

Acknowledgements

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 621244.