Proton Ceramic Steam Electrolysers

Transcription

1 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 proton ceramic electrolysis operation Development and performance of tubular Proton Ceramic Electrolysers (PCEs)

2 Literature data for Proton Ceramic Electrolysers (PCEs) Electrolyte Anode Temperature i (ma cm 2 ) ASR (Ωcm 2 ) η (%) SSY541 SSC ~8 Matsumoto, 212 BCZY53-Zn BSCF Li, 213 BZCY72 LSCF Babiniec, 215 BCZY53-Zn Key question: What is the origin of the low faradaic efficiencies observed in many PCEs? LSCM- BCZYZ Ref Gan, 212 BCZY62 BSCF (?) Yoo, 213 BCZY53 SSC-BCZY He, 21 Degradation and decomposition in H 2 O

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

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

5 Electronic conductivity distribution during PCE operation Electrolyte σ p po 2 1/4 exp(e F /4) σ p σ p,ocv SOEC σ e O 2 x H 2

6 The effect of partial electronic conductivity on faradaic efficiency Voltage = = 2 1 H2 production (ml min-1 ) Current

7 The effect of partial electronic conductivity on faradaic efficiency Voltage = = =.25 = H2 production (ml min-1 ) Current

8 The effect of partial electronic conductivity on faradaic efficiency Voltage = = =.25 =.25 =.5 = H2 production (ml min-1 ) Current

9 Faraday efficiency (%) Faraday efficiency (%) Electrode performance and steam content significantly influence faradaic efficiency Anode dependence for with fixed t H =.8 Steam content dependence with fixed t H =.8 9 p H2O = p H2O = Anode performance p H2O = Voltage (V) Voltage Z el,a R ion U N Z el,c R e-

10 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

11 Dense tubular half-cells achieved Dense 155 C 24h 161 C 6h 4 microns

12 log (R p ( cm 2 )) R p ( cm 2 ) Steam electrode: Ba 1-x Gd.8 La.2+x Co 2 O 6-δ T ( C) GBCF / BZCY [1] BSCF / BCY [3] Pr 2 NiO 4 / BCY [4] LSCF / BCY [4] BSCF / BCY [4] BGLC (x=) / BZCY [2] BCZF [5] : H. Ding et al., International Journal of Hydrogen Energy (21). 2: R. Strandbakke et al., Solid State Ionics (215). 3: Y. Lin et al., Journal of Power Sources (28). 4: J. Dailly et al., Electrochimica Acta (21). 5: M. Shang et al., RSC Advances, (213) 1/T (K -1 )

13 Steam electrode processing on reduced tubes 1. Cap and seal segment using glass ceramic from CoorsTek 2. Deposit Ba.7 Gd.8 La.5 Co 2 O 6-δ as steam electrode by paint brush 3. Firing in dual atmosphere: 1 C 2% O 2 outside, 5% H 2 inside E cell = 1.4 V during firing Cell 1 Area: 5 cm 2 4. Gold paste applied as current collector

14 Potential (V) Faradaic efficiency (%) H 2 production (NmL min -1 ) Electrolysis with single phase BGLC electrode Current (A) C 65 C 6 C 55 C Faradaic H 2 production 7 C 65 C 6 C 55 C 1 Faradaic efficiencies vs cell potential 8 55 C 6 C 65 C 7 C C 6 C 65 C 7 C 4 1. Cathode: p tot = 3 bar ph 2 =.3 bar Anode: p tot = 3 bar ph 2 O = 1.5 bar po 2 = 3 8 mbar Potential (V) Current density (ma cm -2 )

15 Potential (V) Z // ( cm 2 ) H 2 production (NmL min -1 ) Electrolysis with single phase BGLC electrode Current (A) C 65 C 6 C 55 C Faradaic H 2 production 7 C 65 C 6 C 55 C Impedance at 6 C for increasing galvanostatic bias Poor adhesion and delamination of the 2 55 C 7 C 6 C electrode layer observed in post 65 C Anode: characterization Cathode: p 1. tot = 3 bar p tot = 3 bar ph 2 O = 1.5 bar ph 2 =.3 bar po 2 = 3 8 mbar Improved processing route needed Z / ( cm 2 ) Current density (ma cm -2 ) 1 3 OCV 5 1 3

16 Steam electrode processing on unreduced tubes 1. BZCY72- Ba.5 Gd.8 La.7 Co 2 O 6-δ applied as steam electrode Fired in air at 12 C for 5h Infiltrated with nanocrystalline Ba.5 Gd.8 La.7 Co 2 O 6-δ Thin Pt layer current collection 2. Capped and sealed at 1 C Semi-dual atmosphere to keep BGLC layer intact 3. NiO reduction at 8 C in 1% H 2 for 24h Kept in electrolytic bias during reduction to avoid re-oxidation Cell 2 Area: 11 cm 2

17 Electrolysis with composite BZCY-BGLC electrode Voltage (V) H 2 production (NmL min -1 ) Current Density (ma cm -2 ) 1 2 Faradaic H 2 production 5 C 6 C 7 C 4 EIS at 3mA galvanostatic operation Z real ( ) C 2 6 C 2. 4 C 5 C 6 C -Z im 7 C 5 C 4 C C Anode: Cathode: p tot = 3 bar 1. p tot = 3 bar ph 2 O = 1.5 bar ph 2 =.5 bar po 2 = 3 mbar Current (A) Z real ( cm 2 )

18 Electrolysis with composite BZCY-BGLC electrode Voltage (V) H 2 production (NmL min -1 ) ASR ( cm 2 ) Current Density (ma cm -2 ) 1 2 Faradaic H 2 production 4 C 5 C 6 C 7 C 1 8 Calculated ASR from IV curves 5 C Calculated from dv / di 2. 4 C 6 6 C C 6 C 7 C 4 7 C Anode: Cathode: p tot = 3 bar 1. p tot = 3 bar ph 2 O = 1.5 bar ph 2 =.5 bar po 2 = 3 mbar Current (A) Current (A)

19 Voltage (V) Faradaic efficiency (%) Z // ( cm 2 ) Improved faradaic efficiency primarily due to enhanced electrode kinetics C 3 ma cm Cell 1 1. Cell 1 Cell 2 2 Cell Current density (ma cm -2 ) Z / ( cm 2 )

20 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 22 ma cm -2 at 6 C observed with > 8% faradaic efficiency Contact resistance may still contribute significantly to the ohmic resistance of the electrolyser

21 Acknowledgements The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/27-213) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n