Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Summary

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Abstract

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ^1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion². Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation^8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition^{8, 10, 13, 14} ("coking") and sulfur poisoning^{11, 15} and the manner in which surface modifications stave off this degradation¹⁶. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated^17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H₂S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

Protocol

1. Fabrication of a YSZ-embedded Mesh Anode Cell

1. Weigh out two batches of 0.2 g of YSZ powder.
2. Compress one batch YSZ powder in a cylindrical stainless steel mold (13 mm in diameter) with a uniaxial dry press at a pressure of 50 MPa for 30 sec.
3. Cut a <1-cm piece of Ni mesh and place it onto the surface of YSZ disc inside the mold.
4. Add the other 0.2 g of YSZ powder on top of the Ni-mesh inside the mold and flatten the surface of the powder using a ram.
5. Uniaxially press the Ni mesh sandwiched between packs of YSZ powder at a pressure of 300 MPa for 30 sec.
6. Extract the pressed Ni/YSZ pellet from the mold.
....

Results

Sulfur Poisoning Analysis

Shown in Figure 4 are typical I-V and I-P curves of a cell with a Ni mesh electrode under H₂ and 20 ppm H₂S condition. Clearly, the introduction of even just a few ppm of H₂S can poison the Ni-YSZ anode and cause considerable performance degradation.

In order to more intensively understand the poisoning behavior of the Ni-YSZ anode, AC impedance spectroscopy of the cell was performed under open.......

Discussion

Sulfur Poisoning Analysis

The impedance spectra shown in Figure 5 suggest that sulfur poisoning is a surface or interfacial phenomenon rather than one that affects the bulk of the material. Specifically, the quick poisoning of the Ni mesh electrode (Figure 6) might result from the direct exposure of Ni electrode to the fuel gas and subsequent sulfur adsorption; gas diffusion would not limit the rate of this process as much as in the case of a thick porous Ni/YSZ .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Nickel mesh	Alfa Aesar	CAS: 7440-02-0
Ni Foil	Alfa Aesar	CAS: 7440-02-0
YSZ powder	TOSOH	Lot No:S800888B
Ag paste	Heraeus	C8710
Barium oxide	Sigma-Aldrich	1304-28-5
Silver wire	Alfa Aesar	7440-22-4
Acetone	VWR	67-64-1
Ethanol	Alfa Aesar	64-17-5
UHP H2	Airgas		99.999% purity
100 ppm H2S/H2	Airgas		Certified custom mix
n-type Si AFM tip	MikroMasch	NSC16	10 nm tip radius
Au coated AFM tip	MikroMasch	CSC11/Au/Cr	20-30 nm tip radius
Raman Spectrometer	Renishaw	RM1000
Ar Ion laser	ModuLaser	StellarPro 150
He-Ne laser	Thorlabs	HPL170
Atomic Force Microscope	Veeco	Nanoscope IIIA
Moving Raman Stage	Prior Scientific	H101RNSW
Optical Microscope	Leica	DMLM
Scanning Electron Microscope	LEO	1550
Tube Furnace	Applied Test Systems	2110
Polisher	Allied High Tech Products	MetPrep
6 μm Grinding media	Allied High Tech Products	50-50040M
3 μm Polishing media	Allied High Tech Products	90-30020
1 μm Polishing media	Allied High Tech Products	90-30015
0.1 μm Polishing media	Allied High Tech Products	90-32000
Raman chamber	Harrick Scientific	HTRC