High-performance lanthanum-ferrite-based cathode for SOFC
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 Ce1-xGdxO2 (CGO) [2], La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) [3] or ZrScO2 [1]. There are problems associated with Ce1-xGdxO2 and LSGM such as electronic conductivity in reducing atmosphere for Ce1-xGdxO2 [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 La1-xSrxMnO3 (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. La0.6Sr0.4Co0.2Fe0.8O3 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 Rp 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 Rp 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 (Ce0.9Gd0.1O3) 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 (La0.6Sr0.4)1-xCo0.2Fe0.8O3 (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 Rp 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, Rp 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, Rp 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)
Solid State Ionics
(1997)- et al.
Solid State Ionics
(1992) - et al.
J. Electrochem. Soc.
(1999) - et al.
Solid State Ionics
(1995) - et al.
Solid State Ionics
(1998) - et al.
Solid State Ionics
(1999) Solid State Ionics
(2002)J. Power Sources
(2002)- et al.
Nature
(2001)
Cited by (302)
Electrolyzer technologies for hydrogen economy
2023, Hydrogen Economy: Processes, Supply Chain, Life Cycle Analysis and Energy Transition for Sustainability