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

Proton Ceramic Steam Electrolysers Einar Vøllestad 1 , R. Strandbakke 1 , Dustin Beeaff 2 and T. Norby 1 1 University of Oslo, Department of Chemistry, 2 CoorsTek Membrane Sciences AS  Theoretical considerations on electrolysis operation  Development and performance of tubular Proton Ceramic Electrolysers (PCEs)

Literature data for Proton Ceramic Electrolysers (PCEs) Key question: What is the origin of the low faradaic efficiencies observed in many PCEs? η (%) Electrolyte Anode T emperature i ASR Ref (mA cm 2 ) ( Ω cm 2 ) 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- 700 2000 6-8 22 Gan, 2012 BCZYZ BCZY62 BSCF 600 1050 0.5 99 (?) Yoo, 2013 BCZY53 SSC-BCZY 700 400 1 - He, 2010 Degradation and decomposition in H 2 O

Operating Principles of Proton Ceramic Electrolysers (PCEs) e - U 2H 2 O O 2- 4H + 2H 2 O  O 2 + 4H + +4e - 4H + +4e -  2H 2 Z el,a R ion Z el,c O 2 R e- h + 0  e - + h + e - + h +  0 Electrolyte Anode Cathode

Potentials through a solid oxide electrolyser Electrolyte E F SOEC OCV E F SOFC H 2 O 2 x

Electronic conductivity distribution during PCE operation Electrolyte σ p SOEC σ p ∝ p O 2 1/4 ∝ exp( E F /4) σ p,OCV σ e H 2 O 2 x

The effect of partial electronic conductivity on faradaic efficiency 2.0 30 H 2 production (mL min 20 t e = 0 Voltage 1.5 10 t e = 0 -1 ) 1.0 0 0.0 0.5 1.0 1.5 2.0 Current

The effect of partial electronic conductivity on faradaic efficiency 2.0 30 H 2 production (mL min 20 t e = 0 t e = 0.25 Voltage 1.5 10 t e = 0 -1 ) t e = 0.25 1.0 0 0.0 0.5 1.0 1.5 2.0 Current

The effect of partial electronic conductivity on faradaic efficiency 2.0 30 H 2 production (mL min 20 t e = 0 t e = 0.25 t e = 0.5 Voltage 1.5 10 t e = 0 -1 ) t e = 0.25 t e = 0.5 1.0 0 0.0 0.5 1.0 1.5 2.0 Current

Electrode performance and steam content significantly influence faradaic efficiency Steam content dependence with fixed t H = 0.8 Anode dependence for with fixed t H = 0.8 90 p H2O = 0.95 Faraday efficiency (%) Faraday efficiency (%) 80 80 p H2O = 0.75 70 p H2O = 0.5 60 Anode performance 60 1.25 1.50 1.75 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Voltage (V) Voltage U N Z el,a R ion Z el,c R e-

Tubular half-cell production Wet milling of precursors Extrusion of BZCY72-NiO support Spray-coating BZCY72 electrolyte Dip-coating suspensions NiO based paste Solid State Reactive Sintering

Dense tubular half-cells achieved Dense electrolyte @ 1550 ° C – 24h 1610 ° C – 6h 40 microns

Steam electrode processing on reduced tubes Cap and seal segment using 1. glass ceramic from CoorsTek Deposit Ba 0.7 Gd 0.8 La 0.5 Co 2 O 6- δ as 2. steam electrode by paint brush Firing in dual atmosphere: 3.  1000°C  2% O 2 outside, 5% H 2 inside  E cell = 1.4 V during firing Cell 1 Gold paste applied as current 4. collector

Electrolysis with single phase BGLC electrode Current (A) 0.00 0.25 0.50 0.75 1.00 5 700°C 650°C n -1 ) o 4 600°C i t c H 2 production (NmL min u d 550°C o Faradaic efficiencies vs cell potential r p 700°C H 2 3 c i a 650°C d a 100 r a 600°C F 2 550°C 550°C 600°C Faradaic efficiency (%) 650°C 1 700°C 80 0 2.0 60 Potential (V) 1.5 700°C 650°C 600°C 550°C 40 Anode: 1.0 1.5 2.0 Cathode: p tot = 3 bar 1.0 Potential (V) p H 2 O = 1.5 bar p tot = 3 bar p O 2 = 80 mbar p O 2 = 30 mbar p H 2 = 0.3 bar 0 50 100 150 200 -2 ) Current density (mA cm

Electrolysis with single phase BGLC electrode Current (A) 0.00 0.25 0.50 0.75 1.00 5 700°C 650°C n -1 ) o 4 600°C i Impedance at 600°C for increasing t c H 2 production (NmL min u d 550°C o r p 700°C galvanostatic bias H 2 3 c i a 650°C d a r a 600°C F 2 550°C -1 1 0 0 2 ) // ( Ω cm 2.0 1 OCV Poor adhesion and delamination of the Z Potential (V) 50 100 2 1.5 electrode layer observed in post 300 700°C 650°C 600°C 550°C 3 characterization Anode: Cathode: p tot = 3 bar 1.0 p H 2 O = 1.5 bar p tot = 3 bar - Improved processing route needed 4 5 6 7 8 p O 2 = 80 mbar p O 2 = 30 mbar p H 2 = 0.3 bar / ( Ω cm 2 ) Z 0 50 100 150 200 -2 ) Current density (mA cm

Steam electrode processing on unreduced tubes BZCY72- Ba 0.5 Gd 0.8 La 0.7 Co 2 O 6- δ 1. applied as steam electrode  Fired in air at 1200°C for 5h  Infiltrated with nanocrystalline Ba 0.5 Gd 0.8 La 0.7 Co 2 O 6- δ  Thin Pt layer current collection Capped and sealed at 1000°C 2.  Semi-dual atmosphere to keep BGLC layer intact Cell 2 NiO reduction at 800°C in 10% 3. H 2 for 24h  Kept in electrolytic bias during reduction to avoid re-oxidation

Electrolysis with composite BZCY-BGLC electrode -2 ) Current Density (mA cm 0 100 200 20 Faradaic H 2 production -1 ) EIS at 300mA galvanostatic operation H 2 production (NmL min 600°C 15 Z real ( Ω ) 500°C 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10 4 700°C 400°C 5 600°C 2 0 -Z im 500°C 2.0 700°C 400°C 0 400°C 500°C Voltage (V) 600°C 700°C 1.5 -2 Anode: Cathode: p tot = 3 bar p H 2 O = 1.5 bar p tot = 3 bar 1.0 4 5 6 7 8 9 p O 2 = 30 mbar p O 2 = 30 mbar p H 2 = 0.5 bar Z real ( Ω cm 2 ) 0 1 2 3 Current (A)

Electrolysis with composite BZCY-BGLC electrode -2 ) Current Density (mA cm 0 100 200 20 Faradaic H 2 production -1 ) H 2 production (NmL min 600°C 15 Calculated ASR from IV curves 500°C 10 700°C 10 Calculated from d V / d I 400°C 5 500°C 8 2 ) 0 ASR ( Ω cm 6 2.0 600°C 400°C 500°C Voltage (V) 600°C 4 700°C 700°C 1.5 Anode: 2 Cathode: p tot = 3 bar p H 2 O = 1.5 bar p tot = 3 bar 1.0 0 1 2 3 p O 2 = 30 mbar p O 2 = 30 mbar p H 2 = 0.5 bar Current (A) 0 1 2 3 Current (A)

Improved faradaic efficiency primarily due to enhanced electrode kinetics 2.0 100 600C 4 -2 80 30 mA cm Faradaic efficiency (%) Voltage (V) 2 ) 60 1.5 // ( Ω cm 2 Cell 1 40 Z 0 20 Cell 1 1.0 Cell 2 Cell 2 -2 0 0 50 100 150 200 4 5 6 7 8 9 -2 ) / ( Ω cm Current density (mA cm 2 ) Z

Conclusions  Proton Ceramic Electrolysers may suffer from electronic leakage during operation 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.