Nano-Scaled Mixed Conductors for High Performance SOFCs at ≤ 600°C
Mixed ionic-electronic conductors as LSCF (La0.58Sr0.4Co0.2Fe0.8O3-δ) and cermets like Ni/8YSZ (8 mol% Y2O3 stabilized ZrO2) are well-established cathode and anode materials for solid oxide fuel cells. At operating temperatures of 600 °C or below, electrode polarization losses as well as ohmic losses in the solid electrolyte rise steeply. Thereby, anode-supported cells (ASC), proving highest performance and excellent durability at temperatures > 700 °C, are unsuitable in their actual “standard-type” design.
In this contribution, the solid-gas electrochemistry at both electrode/electrolyte interfaces of a standard ASC was significantly improved by modifications using nanotechnology:
- A thin-film 8YSZ electrolyte of ~1.5 µm, made by a wet-chemical method, in combination with a dense Ce0.8Gd0.2O1.9 (CGO) thin-film buffer layer efficiently reduced the ohmic loss contribution by 80% .
- A nano-scaled La0.6Sr0.4CoO3-δ (LSC) thin-film cathode of 200 nm, deposited by metal organic deposition (MOD), cut the area specific resistance of the two-layer cathode of 30 µm down by two orders of magnitude, compared to µm-scaled cathode structures .
- A nano-scaled Ni/8YSZ anode layer of 10 to 100nm, grown in operando by a reverse current treatment (RCT) , lowered the area specific resistance of a standard µm scaled anode by up to 40% .
Anode-supported half cells were manufactured by Forschungszentrum Jülich. They consist of a regular anode support (thickness 0.5 mm, NiO/8YSZ), with a µm-scaled anode functional layer (AFL, thickness 7 µm, NiO/8YSZ) and an ultra-thin-film electrolyte (thickness 1.5 µm, 8YSZ). Onto this electrolyte, a dense CGO layer (thickness 850 nm) was deposited by physical vapor deposition (PVD) to prevent secondary phase formation. At IWE, the nano- scaled LSC thin-film cathode (thickness 200 nm) was deposited by spin-coating of a metal-organic solution on top. This required a modified fabrication route, including anode reduction at 800 °C prior to cathode deposition and two consecutive heat treatments (170 °C and 650 °C) of the LSC thin film. Subsequently, the cathode was completed by screen printing a µm-scaled LSC current collector on top. The nano-scaled anode/electrolyte interface was evolved from a reverse current treatment during cell operation at 600 °C (~1% H2O in H2).
Results and Discussion
The nanostructures and phase compositions of both electrode/electrolyte interfaces were studied by high-resolution and scanning electron microscopy methods, among others. MOD derived LSC thin-film cathodes typically consist of a heterointerface made of the perowskite La0.6Sr0.4CoO3-δ and the Ruddlesden-Popper type phase (La,Sr)2CoO4±δ , which, in combination with the nanoscaled microstructure, leads to an extremely enhanced oxygen surface exchange . The Ni/YSZ anode had undergone a complete rearrangement in the close vicinity to the electrolyte, resulting in an enormous increase of triple phase boundaries on the nanoscale . A reaction model explaining the solid-solid transformations during the reverse current treatment, which was set up in model experiments, is presented. Analysis by electrochemical impedance spectroscopy (EIS) and current/voltage (C/V) curves revealed an area specific resistance (ASR) of 359 mΩ∙cm² at 600 °C and 60% H2O in H2and a power density of 900 mW/cm² at 700 mV in dry hydrogen atmosphere. This performance is by 400% better than measured for a standard-type ASC with a LSCF cathode, which underlines the potential of introducing nanotechnology for mixed conducting electrodes in SOFC.
This study has created a better understanding, how custom-tailored nanostructures potentially support the performance of mixed-conducting electrodes for SOFC at ≤ 600 °C. Despite alternative approaches reported in literature [5, 6], quite simple nanotechnology strategies were applied, which are also applicable on the larger scale, so that we see great potential in this “nano” ASC design.
 F. Han, R. Mücke, T. Van Gestel, A. Leonide, N. H. Menzler, H. P. Buchkremer, D. Stöver, J. Power Sources 218 (2012) 157.
 J. Hayd, E. Ivers-Tiffée, J. Electrochem. Soc. 160 (2013) F1197.
 J. Szász, D. Klotz, H. Störmer, D. Gerthsen, E. Ivers-Tiffée, ECS Trans. 57 (2013) 1469.
 D. Klotz, B. Butz, A. Leonide, J. Hayd, D. Gerthsen, E. Ivers-Tiffée, J. Electrochem. Soc. 158 (2011) B587.
 A. Evans, A. Bieberle-Hütter, J. L. M. Rupp, L. J. Gauckler, J. Power Sources 194 (2009) 119.