Univ of Michigan_Industry Day_Nikolla_r2.ppt
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Degradation of SOFC Electrodes Eranda Nikolla and Suljo Linic Department of Chemical Engineering University of Michigan, Ann Arbor, MI
Solid oxide fuel cells Convert the chemical energy of combustible fuels into electrical energy Anode Cathode Electrolyte SEM image of SOFC electrode-electrolyte assembly, Anode: Ni/YSZ, Electrolyte: YSZ , Cathode: LSCF O2 H2 H2O O2- V e-e-steam reforming
CH4 + H2O CO +H2
Cathode:
½ O2 + 2 e- O2- |Cathode Electrolyte:
O2- |Cathode O2- |Anode Anode:
H2 + O2- |Anode H2O + 2 e- H2 + ½ O2 H2O
TPB
SOFC assembly
| Anode: Ni/YSZ, Cu/ceria/YSZ |
| Electrolyte: YSZ |
| Cathode: strontium doped lanthanum manganate (LSM), mixed conducting materials such as La1-xSrxCo1-yFeyO3-z (LSCF), LaxSr1-xCoO3 (LSC), or even composites of Ag and LSCF (Ag/LSCF). |
Advantages of the SOFC technology
| Fuel flexibility |
| High efficiency |
| Stationary and auxiliary power |
| Portable power |
| Strategic advantages |
| Silent watch: low noise, low fuel consumption, low emission |
Critical problems
| Degradation in long-term performance |
| Degradation due to operating conditions |
| Temperature, thermal cycling, operating voltage and current |
| Degradation due to external factors |
| Chemical poisoning of anode and cathode |
| Carbon, sulfur, various environment-specific impurities |
| Poisoning of Cr-based interconnects |
Degradation of SOFC performance due to operating conditions
| Temperature |
| Higher T results in faster degradations |
| After 2000 hrs of operation at 850 oC and 300 mA/cm2, 100 % increase in cell over-voltage was observed (Riso National Laboratory in Denmark) |
| As T is lowered from 800 to 700 oC, the rate of degradation is lower by 50 %. |
| Mechanism not understood |
| Structural changes? |
| Operating voltage and current |
| Higher current (lower operating voltage) yield more rapid degradation |
| If I > 1 A/cm2 the cell deactivation is rapid |
| For ∆V < 0.7 V (T between 750 and 850 oC), the long-term stability is compromised |
| Mechanism not understood |
| Our hypotheses: anode oxidation, collapse of electrolyte micro-structure |
| Thermal cycling |
| Frequent cycling between high operating T and room T |
| One cycle between the room T and T = 950 oC doubles the resistance at cathode/electrolyte interface |
| Mechanism not understood |
| Hypotheses: Chemical transformations due to the T-induced reduction/oxidation reactions or structural failure due to the gradients in T |
Degradation of SOFC performance due to atmospheric conditions
| Cathode degradation |
| Cr interconnects |
| Oxidation of Cr under high T, oxygen- and H2O-rich conditions. |
| Environment-specific impurities present in air |
| Naval environments |
| humidity, SO2, salt-containing mist, saturated and unsaturated hydrocarbons, ammonia, halogen-containing gasses (Cl2 and HCl), various additives and solvents used in petrochemical industry, and many others. |
| Not well understood |
Degradation of SOFC performance due to atmospheric conditions
| Anode degradation |
| Carbon poisoning |
| During direct on-cell fuel utilization |
| Slippage through a reformer |
| Sulfur Poisoning |
| Environment-specific impurities |
| NaCl interact strongly with Ni |
| Solvents and additives in petrochemical industry |
Carbon poisoning of Ni
| Severe catalyst deactivation |
| Formation of graphitic layers, fullerenes, and nanotubes |
| Significant problem in SOFC |
| Leads to fuel cell fracture |
| Recycling/regeneration of the anode is not trivial |
Ni/YSZ fresh pellet Ni/YSZ pellet after rxn 50 nm Ni
C
50 nm
Our approach
| Utilize DFT calculations |
| Carbon chemistry over Ni model surfaces |
| Can we control the carbon chemistry? |
| Can we slightly perturb anode composition to improve SOFC performance? |
| Ni surface alloying |
| Experimental tests and characterization of the identified alloys |
| SOFC anodes |
Steam reforming over Ni(111)
| Methane steam reforming over Ni(111) |
| There exits a thermodynamic force to form CO and graphene |
Ni(111)
CO2 (g) Bengaard, H. S., et al, J. Catal, 2002
Proposed mechanism for the growth of carbon fibers on Ni Insitu TEM studies have demonstrated that the mechanism of carbon fiber growth involves C diffusion on the surface and nucleation on low-coordinated Ni sites
Possible strategies
| Promising materials |
| Low propensity for the formation of carbon deposits |
| Preferential C-oxidation versus C-C bond formation |
| Hinder the nucleation of carbon deposits |
| Alloys containing Ni |
| Low cost, excellent thermal properties and electrical conductivity |
DFT: C-O and C-C bond formation on Ni(111)
-2 Formation of C-C and C-O bonds are kinetically comparable on Ni(111)
Graphene C-C‡
C-O ‡ CO(g)
-1
C atom position
Energy (eV)
DFT: C-O and C-C formation on Sn/Ni(111) alloy
| C and O diffusion become kinetically important |
| The barrier for C diffusion is much higher than O diffusion |
O path C path Graphene
-2
-1
C atom position
C-O ‡
CO(g)
Carbon Nucleation
Sn/Ni surface alloy hinders the nucleation of carbon
Carbon chemistry on Ni and Sn/Ni
| Sn/Ni versus Ni |
| Higher rates of C-oxidation than C-C bond formation |
| Enhanced rates of the removal of carbon |
| Lower driving force for carbon nucleation |
| Partitioning between atomic carbon and sp2 carbon fragments |
Sn-Ni Surface Alloy
| DFT predictions |
| There is a thermodynamic driving force to form Sn-Ni surface alloy |
| Sn-Ni/YSZ synthesis and characterization |
| Evidence of the Sn-Ni surface alloy formation |
| STEM/EDS studies |
| XPS |
| EELS |
Nikolla. E, Holewinski. A, Schwank. J, Linic. S, JACS, 2006 Nikolla. E, Schwank. J, Linic. S, J. Catal, 2007 Nikolla. E, Schwank. J, Linic. S, Catal. Today, 2008
Solid oxide button cells
| Anode supported button SOFC |
| SOFC compartments |
| Anode (0.5-1mm) |
| Ni/YSZ (50/50 %wt) |
| Sn/Ni/YSZ |
| Electrolyte (~15m) |
| YSZ |
| Cathode (~30m) |
| La0.8Sr0.2MnO3 (LSM)/YSZ (50/50 %wt) |
Load
Anode Electrolyte Cathode
Air Fuel
Anode e-e-
Sn/Ni vs. Ni anode
Sn/Ni/YSZ|YSZ|LSM/YSZ SOFC Ni/YSZ|YSZ|LSM/YSZ SOFC
| Constant Voltage |
| 0.5V |
| Temperature |
| 740°C |
| Fuel Feed : |
| 5.3% isooctane / 1.8% Air / Nitrogen |
| The Sn/Ni anode cell is more stable than the Ni anode cell |
| The Sn/Ni open circuit voltage (OCV) was recovered after exposure to isooctane |
| The Ni cell fractured |
| No OCV recovery |
Current Density (A/cm2)
Sn/Ni anode at different conditions
I = 0.3 A/cm2 Temp = 740°C Feed = 5.3% isooctane / 1.8% Air / Nitrogen I = 0.2 A/cm2 Temp = 740°C I = 0.2 A/cm2 Temp = 740°C The Sn/Ni anode was stable under various operating conditions
Current Decrease Doubled isooctane flow-rate
SEM/EDS: Ni-SOFC
| Carbon is detected at the anode/electrolyte interface |
| The anode disintegrated |
| The electrolyte fractured |
Cathode Anode Electrolyte C = Blue Ni =Yellow C O Ni Zr
500m
SEM/EDS: Sn/Ni-SOFC
| No evidence of carbon on the Sn/Ni anode |
| The integrity of the fuel cell was maintained |
Anode
Conclusion
| Factors that result in SOFC degradation |
| Operating conditions (thermal cycling, voltage…) |
| Atmospheric conditions (sulfur, carbon, air impurities..) |
| Hybrid experimental/theoretical approaches can be utilized to identify robust SOFC electrode materials |
| Ni surface alloys anodes as carbon tolerant SOFC anodes |
Acknowledgements
| DOE-NETL |
| DOE-BES |
| NSF |
| ONR |
UNKNOWN-0.xls
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