Degradation mechanisms and advanced characterization and testing (II)

Degradation Mechanisms in Solid Oxide Cells and Systems Workshop Proceedings 275

Element diffusion in SOFCs: multi-technique characterization approach M. Morales1, A. Slodczyk1, A. Pesce2, A. Tarancón1, M. Torrell1, B. Ballesteros3, J.M. Bassat4, J. P. Ouweltjes5, D. Montinaro2 and A. Morata1*

1IREC, Catalonia Institute for Energy Research, Jardins de les Dones de Negre 1, 2º, Sant Adrià del Besós, Barcelona, 08930, Spain. 2SOLIDPower SpA, Viale Trento 117, Mezzolombardo, 38017, Italy. 3Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain 4CNRS, ICMCB, 87 avenue du Dr. A. Schweitzer, F-33608 Pessac, France 5HTceramix SA, Avenue des Sports 26, CH-1400 Yverdon-les-Bains, Switzerland

*Corresponding author e-mail address: amorata@irec.cat

The state-of-the-art materials for SOFCs are yttria-stabilized zirconia as electrolyte and lanthanum strontium cobalt ferrite as cathode. However, the formation of insulating phases between them requires the use of diffusion barriers, typically made of gadolinia doped ceria. The study of the stability of this layer during the fabrication and in operando is currently one of the major goals of the SOFC industry. In this work, the cation inter-diffusion at the cathode/barrier layer/electrolyte region is analysed for an anode-supported cell industrially fabricated by conventional techniques, assembled in a short-stack and tested under real operation conditions for 3000 h. A comprehensive study of this cell, and an equivalent non-operated one, is performed in order to understand the inter-diffusion mechanisms with possible effects on the final performance. A multi-scale characterization of the CGO barrier layer and its interfaces has been carried out, combining different complementary analysis techniques at micro- and nanoscale levels. The morphologic study carried out by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) is completed with the observation of element distribution maps obtained by Energy Dispersive X-Ray Spectroscopy (EDX), Electron Probe Micro Analysis with Wavelength Dispersive X-

Ray (EPMA-WDX), Secondary Ion Mass Spectroscopy (SIMS) and Electron Energy Loss Spectroscopy (EELS). Finally, micro-Raman Spectroscopy has provided complementary information about the presence of specific phases at a local level. The analyses evidence that the cation diffusion is occurring during the fabrication process. Despite the significant diffusion of Ce, Gd, Zr, Y and Sr cations, the formation of typically reported CGO-YSZ solid solution is not observed while the presence of isolated grains of SrZrO3 is proved. All in all, this study presents new insights into the stability of the typically employed diffusion barriers for solid oxide cells that will guide future strategies to improve their performance and durability.

Acknowledgements This work was carried out in the frame of

Endurance Project, funded by European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207. The research was supported by Generalitat de Catalunya-AGAUR (M2E exp. 2014 SGR 1638), and European Regional Development Funds (“FEDER Programa Competitivitat de Catalunya 2007-2013”).

WORKSHOP PROCEEDINGS

DEGRADATION MECHANISMS IN SOLID OXIDE CELLS AND SYSTEMS

FEBRUARY 17, 2017BARCELONA, SPAIN

Element diffusion in SOFCs: multi-technique characterization approachM. Morales, A. Slodczyk, A. Pesce, A. Tarancón, M. Torrell, B. Ballesteros, J.M. Bassat, J. P. Ouweltjes, D. Montinaro and A. Morata

Workshop Barcelona. 17th February 2017

M. Morales, A. Slodczyk, A. Pesce, V. Miguel-Pérez, A. Tarancón, M. Torrell,

B. Ballesteros, J.M. Bassat, J. P. Ouweltjes, D. Montinaro, A. Morata

Element diffusion in SOFCs: multi-technique characterization approach

Outline

State-of-the-art of diffusion barrier layer between cathode and electrolyte.

Case of study: porous CGO barrier layer (Screen-Printing).

Electrochemical aging tests of cells.

Characterization of cation diffusion (Sr, Ce, Zr,…) at the barrier layer

region, and their correlation with cell manufacturing and aging.

Degradation mechanisms of the barrier layer.

Conclusions.

High cell performance/durability cell

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

Requirements for a high performance/durability:

• Low ohmic loss by YSZ electrolyte.

• Low polarisation by Anode Functional Layer + Substrate.

• Low ohmic and polarisation loss by LSCF cathode.

• Low ohmic loss by CGO Barrier Layer between cathode-electrolyte.

M. Morales et al. Materials Issues for Solid Oxide Fuel Cells Design.

Handbook of Clean Energy Systems. Wiley, Vol. 2 (18) (2015) 1165-1182.

SOFC:

Stability of reaction area at

electrodes and electrolyte-

electrode interface

Decreasing operation

temperature

Low cost of raw materials and manufacturing and effective

heat insulation

Low chemical changes and

interfaces

Low energy in fast start-up

and operation

Cost-effective seals and

metal interconnects

Low cost High reliability

High performance

Requirements for an ideal

SOFC

LSCF highly reactive with YSZ electrolyte: Barrier layer required between YSZ and LSCF

Motivation - CGO Barrier Layer

Anode: Ni/YSZ

Electrolyte: YSZ

Cathode: LSCF

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

T sintering

(Testing)

T sintering

(Testing)

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

Anode: Ni/YSZ

Electrolyte: YSZ

STRONTIUM ZIRCONATES

Cathode: LSCF

OK?

• Porosity and thickness of barrier layer.

• Cation diffusion (Sr,Zr): resistive

phases formation (SrZrO3).

• CGO-YSZ interdiffusion (loss of dopant)

High resistance layer:

> 1000 times more resistive

than YSZ

CGO Barrier Layer for different deposition methods: RS contributions

Knibbe et al. J. Am. Ceram. Soc. 93 (2010) 2877-2883

Ni/YSZ

YSZ

CGO

LSC-CGO

RTotal = RP + RS

Ni/YSZ

LSC-CGO

YSZ

SrZrO3/CGO

YSZ/CGO

Cross-sectional view schematic

CGO

resistive phases formation

porosity and thickness

interdiffusion

Sintering temperature of CGO bulk material

Low T 600 800 1000 1100 1200 1300 1400 High T

Porous

Low ionic

conductivity

Dense

High ionic

conductivity

In a screen-printed CGO barrier layer: high sintering temperatures are required

for densification (↓ Rohm), but the cation inter-diffusion represents a limitation:

↓ Porosity

↑ Microstructural/Chemical stability

↑ Inter-diffusion: Ce, Zr, Gd…

(electrolyte - barrier layer)

↑ Formation of resistive phases

(SrZrO3) during cathode sintering

↑ Loss of dopant (Gd) in barrier layer

optimization of sintering temperature of B.L. decreasing the thickness of B.L. decreasing the sintering temperature of cathode

Fabrication strategies for improving a screen-printed B.L. are limited to:

Hi. Mitsuyasu et al. Solid State Ionics 113-115 (1998) 279-284.

A. Martínez-Amesti et al. J. Power Sources 192 (2008) 151-157.

S. Uhlenbruck et al. Solid Sate Ionics, 180 (2009) 418-423.

R. Knibbe et al. J. Am. Ceram. Soc., 93(9) (2010) 2877-2883.

M. Izuki et al. J. Power Sources, 196 (2011) 7232-7236.

F. Wang et al. Solid State Ionics 262 (2014) 454-459.

M. Kubicek et al. Phys. Chem. Chem. Phys. 16 (2014) 2715-2726.

F. Wang at al. J. Power Sources, 258 (2014) 281-289.

D. The et al. J. Power Sources 275 (2015) 901-911.

G. Nurk et al. J. Electrochem. Soc. 163 (2016) F88-F96.

CGO Barrier Layer fabricated by conventional methods

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

Cells fabricated and tested

under real conditions in a

short-stack at long-term (3000h)

Analysis of cation diffusion at

LSCF/CGO/YSZ region of cells using

different analysis techniques: XRD,

Raman Spectroscopy, SEM-EDX,

EPMA-WDX, TEM, STEM-EDX-EELS…

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF ENhanced DURability materials for

Advanced stacks of New solid oxide

fuel CElls.

FP7/2007-2013 and FCH-JU-2013-1

Case of study 1: degradation analysis of screen-printed CGO B.L.

M. Morales, V. Miguel-Pérez, A. Tarancón, A. Slodczyk, M. Torrell, B. Ballesteros, J.P. Ouweltjes, J.M. Bassat, D. Montinaro, A. Morata. J. Power Sources 344 (2017) 141–151. (ENDURANCE project)

Objectives: analysis of LSCF/CGO/YSZ region, for fresh and aged cells, using multi-technique

characterization approach, and their correlation with cell manufacturing and aging.

SEM-EDX at LSCF/CGO/YSZ region

SEM micrographs

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

SEM-EDX

Fre

sh

ce

ll A

ged

ce

ll

EPMA-WDX elemental analysis at LSCF/CGO/YSZ region

a) b)

Fresh cell

Aged cell

Electrolyte

Barrier layer

Electrolyte

Barrier layer

Elemental amount (%)

I I I

0 50 100

BSE images of the cross section and EPMA-WDX elemental distribution maps

and profiles of Ce, Sr, Zr and La at LSCF/CGO/YSZ interface

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

Ce and Zr interdiffusion

Presence of Sr at barrier layer During manufacturing process

No CeO2-ZrO2 solid solution was formed

2D Raman mapping at YSZ/CGO/LSCF region

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

10 µm

Po

siti

on

(µ

m) 6.5

4.5 2.5 0 -2 -3.5

Interface

with cathode

↓Gd

CGO migration from the barrier layer

Interface with

electrolyte

Interface

with cathode

Interface with

electrolyte

Loss of Gd (approaching to the electrolyte)

TEM-EDX at pores of CGO barrier layer

Anode: Ni/YSZ

Electrolyte: YSZ

Barrier layer: CGO

Cathode: LSCF

TEM-EDX analysis of CGO surfaces at the barrier layer

The formation of SrZrO3 takes place mainly in the

regions close to the pores of CGO barrier layer.

0 50 100 150 200 250 3000

50

200

300

400

500

600

Co

un

ts

Position (nm)

Ce

Zr

Sr

200 nm

The grain boundaries (and also pores)

of the CGO barrier layer are preferential

paths for the Zr-Sr diffusion.

TEM-EDX at grain boundaries of CGO barrier layer

Conclusions

Post-test analysis of fresh and aged cells with porous barrier layer allowed to determine the

mechanisms of cation diffusion at the CGO barrier layer region:

• In both cells, the cation diffusion at CGO barrier layer region was mainly caused during the

fabrication (sintering process).

• Raman analysis indicated that no CeO2-ZrO2 solid solution was formed at the CGO/YSZ interface,

and confirmed the loss of dopant (Gd) and Ce migration from the B.L. to deep inside electrolyte.

• Porous and grain boundaries of CGO barrier layer are preferential paths for the Zr-Sr diffusion.

• Accumulation of Sr and Zr forming SrZrO3 is detected at free surfaces close to CGO/YSZ interface.