Analysis of electrode performance in molten carbonate electrolysis cell (MCEC)

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1 Analysis of electrode performance in molten carbonate electrolysis cell (MCEC) Lan Hu, Göran Lindbergh, Carina Lagergren KTH Royal Institute of Technology Stockholm, Sweden

2 Outline Background Experimental set-up The performance in MCEC Conclusions Acknowledgements

3 Background What is MCEC? q Fuel Cell (MCFC) Chemcial energy Electricity H 2 /CO O 2 CO 3 O 2 H 2 O Anode Electrolyte Cathode Anode: H 2 +CO 3 H 2 O+ +2 CO+CO Cathode: ½O CO 3 Overall: H 2 + ½O 2 +,Cat H 2 O+,An CO+ ½O 2 +,Cat 2,An

4 Background What is MCEC? q Fuel Cell (MCFC) Chemical energy Electricity q Electrolysis Cell (MCEC) Electricity Chemical energy H 2 /CO H 2 O Anode CO 3 O 2 Electrolyte Cathode O 2 H 2 /CO O 2 H 2 O + H 2 / CO Cathode CO 3 Electrolyte O 2 Anode Anode: H 2 +CO 3 H 2 O+ +2 CO+CO Cathode: ½O CO 3 Overall: H 2 + ½O 2 +,Cat H 2 O+,An CO+ ½O 2 +,Cat 2,An Anode: CO 3 ½O Cathode: H 2 O+ +2 H 2 +CO CO+CO 3 Overall: H 2 O+,Cat H 2 + ½O 2 +,An 2,Cat CO+ ½O 2 +,An

cell efficiency (3) Decreasing costs q Fuel gas

5 Why to operate MCEC? Renewable electricity Reversible cell MCFC+MCEC Gas storage Biogas plant! q Cell (1) The maturity level of cell (2) Improving cell efficiency (3) Decreasing costs q Fuel gas production (high-temperature electrolysis) Electricity for grid Filling station (1) High efficiency (2) Lower voltage used

6 Issues to tackle q Is it feasible to run the molten carbonate electrolysis cell? q How is the performance of the cell and individual electrodes in the electrolysis cell?

Experimental set-up 3 cm2 lab-scale cell Hydrogen electrode Porous Ni-Cr alloy Oxygen electrode Porous Ni, oxidized and lithiated in-situ Electrolyte 62/38% Li2/K2CO3 in

7 Experimental set-up 3 cm2 lab-scale cell Hydrogen electrode Porous Ni-Cr alloy Oxygen electrode Porous Ni, oxidized and lithiated in-situ Electrolyte 62/38% Li2/K2CO3 in LiAlO2 matrix Standard gases for the cell Fuel gases 64/16/20% H2/CO2/H2O Oxidant gases 15/30/55% O2/CO2/N2 Ref. gases 33/67%O2/CO2 /Au-wire The standard operating temperature: 650 C

8 The performance in MCEC The cell The Ni hydrogen electrode The NiO oxygen electrode

9 The performance of the cell 1,5 1,4 1,3 without ir - correction with ir - correction 0,4 0,3 OC V MC FC (OC V- 0.2V) MC E C (OC V+0.2V) Cell voltage E / V 1,2 1,1 1,0 0,9 MC E C MC F C - Im (Z ) / Ω cm 2 0,2 0,1 0,0 0,8-0,1 0,7 0,6-0,6-0,5-0,4-0,3-0,2-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 Current density i / A cm - 2-0,2 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 Re (Z) / Ω cm 2 R tot (Ω cm 2 ) R p (Ω cm 2 ) R Ω (Ω cm 2 ) MCFC mode MCEC mode (1) It is feasible to operate the electrolysis cell! (2) The cell shows better performance in MCEC mode.

10 The stability of the cell in MCEC mode 1,4 1,3 MC E C mode: A/cm 2 1,2 Cell voltage E / V 1,1 1,0 0,9 0,8 50h 50h 246h 50h 15h MC F C mode: 0.16 A/cm 2 * 149h 396h 63h 0, Time t / h The long-term test shows the cell performs quite stable in both MCEC and MCFC modes.

11 The performance of the Ni hydrogen electrode Operating temperature varying from 600 to 675 C using standard gases for the cell. 0,15 0,10 0,15 0,10 OC V Fuel cell mode (OC V+0.05V) E lectrolysis cell mode (OC V- 0.05V) 650 C Overpotential η / V 0,05 0,00-0,05-0,10 Electrolysis cell mode Fuel cell mode 600 C 625 C 650 C 675 C - Im (Z ) / Ω cm 2 0,05 0,00-0,05-0,15-0,6-0,5-0,4-0,3-0,2-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 Current density i / A cm - 2-0,10 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 Re (Z)/ Ω cm 2 The Ni hydrogen electrode performs slightly better for hydrogen oxidation than for water electrolysis.

12 The performance of the Ni hydrogen electrode The fuel gas 80/20% H 2 / with humidity varying from 10% to 30%. Cell temperature was 650 C. 0,15 0,15 Electrolysis cell mode 0,10 0,10 Overpotential η / V 0,05 0,00-0,05 Electrolysis cell mode Fuel cell mode 10% H 2 O - Im (Z ) / Ω cm 2 0,05 0,00 increas ed H 2 O - 0,10 20% H 2 O 30% H 2 O - 0,05-0,15-0,6-0,5-0,4-0,3-0,2-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 Current density i / A cm - 2-0,10 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 Re (Z)/ Ω cm 2 Increasing the water content from 10% to 30%, the polarization (mass-transfer) of the Ni hydrogen electrode decreased in electrolysis cell mode.

13 The performance of the Ni hydrogen electrode The fuel gas consists of different amounts of ranging from 20% to 40%, 20% H 2 O and H 2 balance. Cell temperature was 650 C. 0,15 The current densities (A cm -2 ) at overpotential of 0.05V Overpotential η / V 0,10 0,05 0,00-0,05-0,10 Electrolysis cell mode Fuel cell mode 20% C O 2 30% C O 2 40% C O 2-0,15-0,6-0,5-0,4-0,3-0,2-0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 Current density i / A cm % 30% 40% MCFC 0.3 MCEC may play several roles in electrolysis cell mode: (1) Electrolysis of to generate CO (2) The impact of on electrolyte production (3) Water gas shift reaction

14 The performance of the NiO oxygen electrode Operating temperatures varying from 600 to 675 C, using standard gases for the cell. Overpotential η / V 0,2 600 C 625 C 650 C 0,1 675 C 0,0-0,1-0,2 Electrolysis cell mode Fuel cell mode - 0,6-0,5-0,4-0,3-0,2-0,1 0,0 0,1 0,2 0,3 0,4 0,5 Current density i / A cm Im (Z ) / Ω cm 2 0,3 0,2 0,1 OC V Fuel cell mode (OC V- 0.15V) E lectrolysis cell mode (OC V+0.15V) 650 C 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 Re (Z)/ Ω cm 2 Both the charge-transfer and masstransfer polarization of the NiO electrode are much lower in electrolysis cell mode. The NiO oxygen electrode shows much better performance in electrolysis cell mode than in fuel cell mode.

15 Conclusions q It is feasible to operate the cell in MCEC and MCFC modes by using the same electrode materials. Moreover, the cell shows better performance in electrolysis cell mode. q The cell can operate stable in MCEC and MCFC modes during long-term test. q The polarization of the Ni hydrogen electrode for water electrolysis is higher than that for hydrogen oxidation at different temperature and water content. q content has an important impact on the Ni hydrogen electrode in electrolysis cell. q The polarization of the NiO oxygen electrode is lower in electrolysis cell mode. Both the charge-transfer and masstransfer polarizations are lower in electrolysis cell mode.

16 Acknowledgements q The financial supports from China Scholarship Council (CSC) for the PhD student and from StandUp for Energy, are appreciated. q The cell components were provided by Ansaldo Fuel Cells in Italy.