High-performance lanthanum-ferrite-based cathode for SOFC

doi:10.1016/j.ssi.2004.09.007

Solid State Ionics

Volume 176, Issues 5–6, 14 February 2005, Pages 457-462

https://doi.org/10.1016/j.ssi.2004.09.007 Get rights and content

Abstract

(La_0.6Sr_0.4)_1-xCo_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₃ (LSCF/CGO) composite cathodes were investigated for SOFC application at intermediate temperature, i.e., 500–700 °C. The LSCF/CGO cathodes have been studied on three types of tape-casted electrolyte substrates including CGO electrolyte, Yttrium-stabilized Zirconia (YSZ) electrolyte coated with a thin layer of CGO, and YSZ electrolyte. Impedance spectra were measured to determine the polarization resistance (R_p) and series resistance (R_s) on cells in a symmetric configuration. R_p of 0.19 Ω cm² at 600 °C and 0.026 Ω cm² at 700 °C were obtained using LSCF/CGO cathode on CGO electrolyte. On the YSZ electrolyte with thin layer CGO coating, R_p of 0.6 Ω cm² at 600 °C and 0.12 Ω cm² at 700 °C were obtained. On the YSZ electrolyte directly, R_p of 1.0 Ω cm² at 600 °C and 0.13 Ω cm² at 700 °C were achieved. These results are roughly six times better than our typical LSM cathodes. Slightly higher R_s was observed in the samples with LSCF/CGO cathode on the YSZ electrolyte with CGO coating due to extra contribution from the thin CGO layer and the CGO/YSZ interface. For the samples with LSCF/CGO on YSZ, the R_s was the same as that of our best LSM samples, which indicates good adhesion between LSCF/CGO cathode and YSZ electrolyte. Aging experiment at 800 °C for the cathode of LSCF/CGO on YSZ electrolyte shows a degradation rate of 5×10⁻⁴ Ω cm²/h in R_p, while the R_s has no obvious change.

Introduction

Reduction of the operation temperature in an SOFC system has a vital importance in reducing cost of the system, which is an inevitable path for the commercialization of this technology. This is particularly imperative concerning small stacks for distributed combined heat and power system. Mass production of units requires cheap material components. Keeping the performance, i.e., the same power density, at lower temperature as it was with previous cell generations at higher temperature, call for reduced resistive losses both from electrolyte and electrodes. Reducing the electrolyte thickness is one way to decrease the resistance from electrolyte [1], another is to use other electrolytes with higher ionic conductivity than Yttrium-stabilized Zirconia (YSZ) such as Ce_1-xGd_xO₂ (CGO) [2], La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) [3] or ZrScO₂ [1]. There are problems associated with Ce_1-xGd_xO₂ and LSGM such as electronic conductivity in reducing atmosphere for Ce_1-xGd_xO₂ [2], and reaction with electrodes for LSGM [4]. Thus, zirconia-based electrolytes are still preferred by most SOFC developers. Our study of lanthanum strontium manganite (LSM)-YSZ composite cathodes on a YSZ-electrolyte shows that a significant resistive loss comes from the YSZ/cathode interface [5]. Improvements would be expected by further optimization of the interface at electrolyte/electrodes. The reasons mentioned above explain why the main effort in the SOFC field is currently still focused on YSZ electrolyte. Many studies show that the cathode polarization is the major contribution to the total loss in a cell. High percentage losses come from cathode when the operation temperature is in the range of 500–700 °C. There are good examples for La_1-xSr_xMnO₃ (LSM) composite cathode developments, which have lead to good electric performance of the anode supported cell operating at 750 °C [5], [6], [7], [8].

Further reduction of the SOFC operation temperature to 500 °C calls for better cathode materials than LSM. Studies of the (La,Sr)(Co,Fe)O₃ have been one of the most popular topics in the cathode research toward to intermediate temperature operation [9], [10], [11], [12], [13], [14]. (La,Sr)(Co,Fe)O₃ (LSCF) is a Mixed Electronic and Ionic Conductor (MIEC). It is assumed that the three-phase boundary (TPB) is extended from the encountering line of the gas/cathode/electrolyte to an enlarged surface of the cathode without ionic material presented. However, reports of composite cathodes show better properties than using the LSCF material alone [11], [14]. Fleig [15] has numerically calculated the width of the electrochemically active zone in a mixed conducting SOFC cathode and finds out that an increasing ionic conductivity of the mixed conductor broadens the electrochemical active region, but it is still confined to the vicinity of the TPB. This is a theoretical explanation of the experiment results mentioned here. LSCF offers higher conductivity when the Sr content increases. On the other hand, thermal expansion coefficient (TEC) is reduced to close to that of YSZ or CGO when the Fe content increases and at the same time the reactivity with YSZ becomes lesser. La_0.6Sr_0.4Co_0.2Fe_0.8O₃ is the most common chemical composition for compromise between conductivity, catalytic activity, TEC and reactivity with the electrolyte. Dusastre and Kilner [11] reported their optimization work on LSCF/CGO composite cathodes. The R_p of 0.6 Ω cm² at 590 °C was achieved on the CGO electrolyte. Recently, interesting results were reported by Murray et al. [14] in which the cathode R_p of 0.33 Ω cm² was achieved at 600 °C on a single crystal YSZ electrolyte. We have conducted systematic development of the LSM/YSZ composite cathodes, which has lead us to an improved power density of 0.8 W/cm² at 750 °C under 0.7 V for our anode supported cells [5]. This brought us to develop the LSCF cathode by the same method.

Section snippets

Experimental

Symmetric electrode cells were prepared to study the electrochemical properties of the LSCF/CGO composite cathodes on CGO (Ce_0.9Gd_0.1O₃) and YSZ (8 mol% Y₂O₃ in ZrO₂) electrolyte. Electrolyte substrates in 5×5 cm² were produced using tape casting. The thickness was approximately 200 μm. Cathodes were made using the composition either 70 wt.% LSCF+30 wt.% CGO (70/30) or 50 wt.% LSCF+50 wt.% CGO (50/50), respectively. Cathode material (La_0.6Sr_0.4)_1-xCo_0.2Fe_0.8O₃ (0≤x≤0.1) was made by dip

LSCF/CGO composite cathode on CGO electrolyte substrate

Fig. 1 shows the impedance spectra measured for a typical sample with 70 wt.% LSCF+30 wt.% CGO (70/30) composite cathode at different temperatures. At the low temperature of 496 °C, there is an arc at high frequency which is possibly originated from the grain boundaries of the CGO electrolyte shown in Fig. 1(a). At the high temperature the inductance tails were measured instead. All the impedance curves can be resolved to two semicircles using EQUIVCRT. A number of frequencies were pointed on

Conclusions

(1)
High performance LSCF/CGO composite cathodes have been produced in which the R_p of 0.19 Ω cm² at 600 °C and 0.026 Ω cm² at 700 °C were obtained on CGO electrolyte. Nano- and submicro-structured cathode is believed to be responsible for such excellent performance.
(2)
On the YSZ electrolyte with thin layer CGO coating, R_p of 0.6 Ω cm² at 600 °C and 0.12 Ω cm² at 700 °C were obtained that are roughly six times better than our LSM cathode.
(3)
On the YSZ electrolyte directly, R_p of 1.0 Ω cm² at 600 °C and

Acknowledgements

This work was financially supported by the Danish Energy Agency in the project DK-SOFC b, long-term SOFC R&D and our industry partner Haldor Topsoe A/S. Hanne Pedersen is acknowledged for assistance in the sample preparation.

References (18)

J.B. Goodenough
Solid State Ionics
(1997)
T. Kenjo et al.
Solid State Ionics
(1992)
J.-W. Kim et al.
J. Electrochem. Soc.
(1999)
L.-W. Tai et al.
Solid State Ionics
(1995)
J.M. Bae et al.
Solid State Ionics
(1998)
V. Dusastre et al.
Solid State Ionics
(1999)
S.P. Jiang
Solid State Ionics
(2002)
J. Fleig
J. Power Sources
(2002)
B.C.H. Steele et al.
Nature
(2001)

There are more references available in the full text version of this article.

Cited by (302)

Phase composition and proton uptake of acceptor-doped self-generated Ba(Ce,Fe)O<inf>3-δ</inf> – Ba(Fe,Ce)O<inf>3-δ</inf> composites
2024, Solid State Ionics
Self-generated Ba(Ce,Fe,In)O_3-δ composites were prepared by one-pot sol-gel synthesis. They consist of Ce-rich and Fe-rich phases, and are intended to supply the required protonic and electronic transport for air electrode materials in protonic ceramic fuel and electrolysis cells (PCFC, PCEC). Crystal structure, lattice parameters, and the relative phase amounts of the composites were obtained from X-ray diffraction. The local chemical composition and distribution of cations within the individual phases were characterized by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. Annealing experiments indicate that the miscibility gap of the BaCe_0.8-xFe_xIn_0.2O_3-δ system ranges from [Ce]/([Ce] + [Fe]) ratios of ~ 0.2 to ~ 0.9. The In³⁺ acceptor shows a tendency to accumulate in the Fe-rich phase, with the ratio In(Ce-rich phase)/In(Fe-rich phase) being in the range of 0.3–0.7. The proton uptake capacity of the materials, which was analyzed by thermogravimetry, increases with an increasing amount of In and decreasing amount of Fe in the precursor. Proton concentrations are in the range of 1–4 mol% at 400 °C. Further measurements on BaCe_0.4Fe_0.4Acc_0.2O_3-δ (Acc = Y, Yb, Gd, Sm, Sc) composites show that proton uptake is generally increased compared to the undoped system BaCe_0.5Fe_0.5O_3-δ. However, variations in the acceptor ion can tune the proton uptake only to a limited extent.
Preparation of gadolinium-doped ceria barrier layer for intermediate temperature solid oxide fuel cells by spin coating and evaluation of performance degradation by impedance analysis
2024, Ceramics International
Thin gadolinium-doped ceria (GDC) films were deposited via the cost-effective aqueous spin coating technique and with rapid one-step sintering at 1200 °C, to be used as barrier layer between a lanthanum strontium cobalt ferrite (LSCF) cathode and a scandia-ceria-stabilised zirconia (ScCeSZ) electrolyte within a solid oxide fuel cell (SOFC) configuration. A 5.5 μm-thick GDC film was deposited using a 50 wt% GDC slurry in 3 cycles. The single cell comprising this GDC film produced a peak power density of over 1.00 W•cm⁻² at 750 °C in hydrogen operation. Electrochemical impedance spectroscopy was carried out to investigate the SOFC performance evolution with potential changes in microstructure over time. An operational stability test was also conducted at a current density of 0.2 A•cm⁻² for over 1200 h. The degradation behaviour of each electrochemical process as a function of cell operating time was evaluated by the distribution of relaxation times method based on the obtained electrochemical impedance spectra.
Tailoring triple charge (O<inf>2</inf>−/H+/e−) conducting nature of Fe-based lanthanum doped samarium oxides for ceramic fuel cells (CFCs)
2023, Fuel
Lattice defects in rare earth (RE) doped single-phase semiconductor materials play a key role in structural modifications and physicochemical characteristics, resulting to enhance the ionic conductivity. In this work, we synthesized lanthanum doped samarium oxide with different mole ratios and applied it as a ceramics membrane fuel cell device to enable high peak power density (PPD) at low operational temperatures (520–400 °C). The samples are capable of triple charge H⁺/O²⁻/e⁻ conduction and enhance the catalytic function of the cell in terms of HOR and ORR kinetics by producing lattice defects. The fabricated Sm_1.5La_1.5Fe₅O₁₂ (2-SLFO) fuel cell device exhibits a high OCV of 1.13 V with a maximum PPD of 864.26 mW/cm² at 520 °C. Durability measurement verifies that 2-SLFO-based fuel cell device can be worked for 60 h under fuel cell conditions (H₂/air) at 520 °C. The first principle calculations using density functional theory (DFT) demonstrate that the higher oxygen vacancy concentration in the 2-SLFO sample may cause to the higher charge-transfer kinetics, subsequent in the enhancement of ions transportation of the fabricated fuel cell device.
Electrolyzer technologies for hydrogen economy
2023, Hydrogen Economy: Processes, Supply Chain, Life Cycle Analysis and Energy Transition for Sustainability
Increasing global greenhouse gas emissions and their adverse climate effects have highlighted the need to develop clean, net-zero-emission technologies. However, the intermittent nature of renewable energy technologies like wind, solar, and tidal power remains a significant challenge in reducing greenhouse gas emissions in the energy sector and achieving renewable energy targets. Water electrolyzers can play a pivotal role in decarbonization strategies achieving net-zero emissions by making green hydrogen production feasible. A brief overview of various commercialized and near-commercialization electrolyzer technologies is presented here. Their salient features cover their state-of-the-art component materials, advantages and disadvantages, future research priorities, and a list of some key commercial manufacturers. High efficiency, rapid electrolyzer response, low cost, and durability are some of the features that require further improvement for the broader acceptance of water electrolyzer technologies. A single electrolyzer technology is insufficient to address all of the challenges of combining renewable energy technologies with electrolyzers. The intended applications and geospecific factors will govern the choice of the water electrolyzer technology.
Evaluation of La<inf>0.6</inf>Sr<inf>0.4</inf>CoO<inf>3-δ</inf>-Ce<inf>0.85</inf>Sm<inf>0.075</inf>Nd<inf>0.075</inf>O<inf>2-δ</inf> composite cathodes for intermediate temperature solid oxide fuel cells
2022, Ceramics International
Aiming to lower the operating temperature of solid oxide fuel cells down to 500–700 °C, a Co-containing composite cathode La_0.6Sr_0.4CoO_3-δ-Ce_0.85Sm_0.075Nd_0.075O_2-δ (LSC-SNDC) is prepared by the solid-liquid method. The crystallinity, grain size and microstructure of LSC powders are analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The LSC-SNDC cathodes with different mass ratios (LSC: SNDC = 100:0, 90:10, 70:30, 60:40 and 50:50) are implemented into symmetrical cells and full cells using screen printing. The effect of cathode sintering temperatures (900, 950, 1000, 1050 °C) is investigated at the symmetrical cell level. The lowest cathode polarization resistance (R_p) is obtained with the 50:50 sample sintered at 950 °C, demonstrating a R_p (symmetrical cell) of 0.0248, 0.0319, 0.0597, 0.1107 and 0.2304 Ω cm² and a maximum power density (full cell) of 1963, 1535, 1013, 585, and 302 mW cm⁻² at 800, 750, 700, 650 and 600 °C, respectively. The LSC-SNDC (50:50) cathode exhibits a low thermal expansion coefficient of 12.3 × 10⁻⁶ K⁻¹ and good compatibility with other cell components, as confirmed from SEM observations. The current work demonstrates that LSC-SNDC is a promising cathode material for intermediate temperature solid oxide fuel cells.
Reversible electrodes based on B-site substituted Ba<inf>0.5</inf>Sr<inf>0.5</inf>Co<inf>0.8</inf>Fe<inf>0.2</inf>O<inf>3-δ</inf> for intermediate temperature solid-oxide cells
2022, Solid State Ionics
The Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ perovskite is a promising electrode material for solid-oxide electrochemical cells exhibiting excellent oxygen reduction reaction (ORR) activity and mixed ionic-electronic transport properties at intermediate temperatures but suffers from stability issues. In this work, the effect on stability and electrochemical performance of several B-site substituted cations (Sc, Zr, Y) are studied. The presence of these substituted cations improved stability by preventing (a) the formation of the detrimental hexagonal phase during long term air exposure experiments and (b) reducing the formation of carbonates in CO₂-containing atmospheres. Symmetrical cell testing revealed that the Sc-substituted material mixed with an ionic gadolinium-doped ceria (GDC) phase has the lowest polarization resistance among the materials and thus was chosen as the cathode for the full cell construction. The composite electrode achieved encouraging power density values ~877 mW/cm² at 700 °C. The material showed stable hydrogen generation during intermediate temperature electrolysis tests and its performance improved upon the introduction of CO₂ to carry out co-electrolysis. Furthermore, methane was obtained during co-electrolysis and Sabatier reaction at temperatures below 450 °C. The superior stability and performance of the Sc-substituted Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ make it an exciting candidate for application as a cathode in reduced temperature electrochemical cells.

View all citing articles on Scopus

View full text

Article preview

Solid State Ionics

Abstract

Introduction

Section snippets

Experimental

LSCF/CGO composite cathode on CGO electrolyte substrate

Conclusions

Acknowledgements

References (18)

Solid State Ionics

Solid State Ionics

J. Electrochem. Soc.

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

J. Power Sources

Nature

Cited by (302)

Phase composition and proton uptake of acceptor-doped self-generated Ba(Ce,Fe)O<inf>3-δ</inf> – Ba(Fe,Ce)O<inf>3-δ</inf> composites

Preparation of gadolinium-doped ceria barrier layer for intermediate temperature solid oxide fuel cells by spin coating and evaluation of performance degradation by impedance analysis

Tailoring triple charge (O<inf>2</inf><sup>−</sup>/H<sup>+</sup>/e<sup>−</sup>) conducting nature of Fe-based lanthanum doped samarium oxides for ceramic fuel cells (CFCs)

Electrolyzer technologies for hydrogen economy

Evaluation of La<inf>0.6</inf>Sr<inf>0.4</inf>CoO<inf>3-δ</inf>-Ce<inf>0.85</inf>Sm<inf>0.075</inf>Nd<inf>0.075</inf>O<inf>2-δ</inf> composite cathodes for intermediate temperature solid oxide fuel cells

Reversible electrodes based on B-site substituted Ba<inf>0.5</inf>Sr<inf>0.5</inf>Co<inf>0.8</inf>Fe<inf>0.2</inf>O<inf>3-δ</inf> for intermediate temperature solid-oxide cells