Life Testing of LSM-YSZ Composite SOC Electrodes During Current-Switched Operation

Gareth. A. Hughes, Kyle J. Yakal-Kremski, Scott A. Barnett

Department of Materials Science and Engineering, Northwestern University 60208

Motivation Experimental Setup

Project Scope and Future Work

Microstructural Analysis

Electrochemical Evolution

Conclusions

This work was supported by the Global Climate and Energy Project under award 51922

Thanks to Dr. Dean Miller, Dr. Jon Hiller, and Dr. J. Scott Cronin

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LSM-YSZ Symmetric Cell Life Test

(1h Current Cycle - 0.5 A/cm2 at 800°C)

Integration of renewable energy sources with the grid requires systems to store excess energy. Solid oxide cells (SOCs) are a proposed new storage method

Aims of current work: • Develop life-testing methods with combined

microstructural and electrochemical characterization • Observe evolution of common air electrode material

(LSM-YSZ) in cyclic operation

[1] Mawdsley, J. R., et al. (2009). "Post-test evaluation of oxygen electrodes from solid oxide electrolysis stacks." International Journal of Hydrogen Energy 34(9): 4198-4207.

Long term durability is unclear: • Oxygen electrode delamination observed

during electrolysis operation • Effects of cycling not known

~ 10 μm LSM Current Collector

~ 20 μm LSM – YSZ Air-Electrode

~ 1 mm YSZ Electrolyte

~ 20 μm LSM – YSZ Air-Electrode

~ 10 μm LSM Current Collector

• Porous composite (La0.8Sr0.2)0.98MnO4-Y0.16Zr0.84O3-δ (LSM-YSZ) electrodes tested at 800 C.

• Symmetrical cells (same LSM-YSZ on both sides) used to isolate air electrode.

• Applied current direction switched every 30 minutes.

• Cells just reach steady state before current direction reversed.

• New test setup developed to avoid contamination by silver current collector grid due to long testing times (up to 1000 h).

• Multiple cells can be tested simultaneously in series using multiple leads.

• SOC operates similar to a flow battery • Electrolysis mode to charge • Fuel cell mode to discharge

System Configuration

Fuel Electrode

Oxygen Electrode

Electrolyte

Feedstock

Storage

Fuel

Storage

Oxygen

Storage

Renewable

Energy Source

Power

Out

e-

e-

e-

e-

O2-

Air

H2O CO2

CO H2 CH4

O2 or

out

• Max theoretical round-trip efficiency: = QFCVFC/QELVEL = VFC/VEL

• η > 80% possible for SOC at J= 0.5 Acm-2

However, this neglects the endothermic process during electrolysis mode.

• VEL > VTN needed to maintain stack temperature

VTN = thermoneutral voltage = H/zF

• Typical cell reaction (800°C): H2O H2 + (1/2)O2,

H= 248.3 kJ mole-1, VTN­ = 1.29 V Efficiency values are prohibitively low! • 66 % for H2O (VFC = 0.85V) • Other losses further decrease η

Utilize CH4-forming reactions to reduce VEL and increase η

FIB milling with serial imaging Zeiss Nvision 40 FIB/SEM Argonne National Laboratory, EMC

Elec

tro

lyte

LSM YSZ Ag

Furnace

Hot Zone

Pressure

Vessel

Quartz

Tube

Gas Flow and

Electrical

Systems

Cell Mount

Location

• Pressurized testing setup under development.

• Experimentally verify thermodynamic predictions.

• Combine with life testing to develop long-term energy storage solution

RΩ

RP

SOC electrochemical performance is measured by electrochemical impedance spectroscopy.

Results consist of two distinct parts:

• Polarization resistance (RP) – a measure of electrode processes such as charge transfer and gas diffusion.

• Ohmic resistance (RΩ) – a measure of electrolyte resistance.

102 Hz 103 Hz

104 Hz

105 Hz

Sets of symmetric cells tested at 1.5A/cm2 and 0.5A/cm2

• Higher current density displayed a significant increase in both RP and

RΩ.

• Largest change seen in RΩ, indicating potential electrolyte issues.

• Lower current density showed no significant change whatsoever.

Degradation rate depends on

current density

Degradation and Current Density

SEM analysis did not show any physical difference between tested and untested electrode structures.

• Delamination not observed.

• Resolution may not be sufficient to observe initial stages of delamination.

More detailed microstructural analysis will be completed using 3D FIB-SEM reconstruction method.

Tested at 1.5A/cm2 Untested Structure

Cathode Functional Layer Phase Segmentation

5 μm

• Current-switching testing method has been used to observe the performance of LSM-YSZ reversible electrodes.

• EIS used to track performance of LSM-YSZ over 1000 h of current cycling.

• Degradation occurred only at high current density while performance at low current remained constant.

• Microstructural changes were not observed via traditional SEM microscopy.

• Further analysis is underway using 3D FIB-SEM reconstruction to analyze wider-range and 3D-specific structure features.

• Results will help to develop a reversible SOC system for energy storage.

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LSM-YSZ Symmetric Cell Life Test

(1h Current Cycle - 1.5 A/cm2 at 800°C)

Rt

R

RP