Nano-Scaled Mixed Conductors for High Performance SOFCs at ≤ 600°C

Introduction

Mixed ionic-electronic conductors as LSCF (La_0.58Sr_0.4Co_0.2Fe_0.8O_3-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 Ce_0.8Gd_0.2O_1.9 (CGO) thin-film buffer layer efficiently reduced the ohmic loss contribution by 80% [1].
• A nano-scaled La_0.6Sr_0.4CoO_3-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 [2].
• A nano-scaled Ni/8YSZ anode layer of 10 to 100nm, grown in operando by a reverse current treatment (RCT) [3], lowered the area specific resistance of a standard µm scaled anode by up to 40% [4].

Experimental

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 La_0.6Sr_0.4CoO_3-δ and the Ruddlesden-Popper type phase (La,Sr)₂CoO_4±δ , which, in combination with the nanoscaled microstructure, leads to an extremely enhanced oxygen surface exchange [2]. 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 [3]. 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.

Conclusions

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.

 References

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[2]   J. Hayd, E. Ivers-Tiffée, J. Electrochem. Soc. 160 (2013) F1197.

[3]   J. Szász, D. Klotz, H. Störmer, D. Gerthsen, E. Ivers-Tiffée, ECS Trans. 57 (2013)  1469.

[4]   D. Klotz, B. Butz, A. Leonide, J. Hayd, D. Gerthsen, E. Ivers-Tiffée, J. Electrochem. Soc. 158 (2011) B587.

[5]   A. Evans, A. Bieberle-Hütter, J. L. M. Rupp, L. J. Gauckler, J. Power Sources 194 (2009) 119.

[6]   Y. Takagi, S. Adam, S. Ramanathan, J. Power Sources 217 (2012) 543.