Josef Mertens

Performance improvement of (La,Sr)MnO3 and (La,Sr) ×(Co, Fe)O3-type anode-supported SOFCs

Targets in the development of anode-supported or planar solid oxide fuel cells (SOFCs) are low operation temperatures, high durability, high reliability, high power density, and low production costs. During the past ten years steps have already been taken at Forschungszentrum Jiilich to lower the operating temperatures while maintaining the power output. This was achieved by optimizing processing and microstructural parameters of the electrodes. This paper presents the latest results concerning performance improvement through variations of the processing route and the microstructure of La 0.65 Sr 0.3 MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF)-type SOFCs. In the case of the LSM-type single cells, the following aspects relating to the electrochemical performance were investigated in more detail: (I) production of the anode substrate by tape casting versus warm pressing; (2) deposition of the anode functional layer (AFL) and electrolyte by screen printing versus vacuum slip casting; (3) use of noncalcined and non-ground YSZ for applying the cathode functional layer (CFL); and (4) sintering temperature of the CFL and cathode current collector layer (CCCL). In the case of LSCF-type cells, a systematic approach was initiated for optimizing the Ce 0.8 Gd 0.2 O 2-δ (CGO) diffusion barrier layer: (1) deposition techniques of the CGO layer and (2) sintering temperature of the screen-printed CGO layer. Results have shown that certain modifications of the processing route led to a slightly lower electrochemical performance, whereas others did not affect the performance at all. Regarding LSCF-type SOFCs, a slight improvement of the performance was achieved by optimizing the sintering temperature of the CGO layer.

Sintering Behavior of (La,Sr)MnO3 Type Cathodes for Planar Anode-Supported SOFCs

One of the main targets in the development of anode-supported solid oxide fuel cell (SOFCs) is to improve the electrochemical performance. This can be achieved by optimizing processing and microstructural parameters of the SOFCs. Variations of the thickness of the cathode functional layer and the cathode current collector layer, the grain size of the powders used for applying these layers, and the sintering temperature, can influence the electrochemical performance as such that lower operation temperatures become possible without detrimentally affecting the power output to a great extent. In this study the effect of variations of the sintering temperature of the cathode on (I) the microstructure, (2) the gas diffusivity and permeability in the cathode, and (3) electrochemical performance of FZJ-type anode-supported single cells, was investigated. The FZ-Julich cell design is based on anode-supported type cells, which are characterized by a relatively thick anode (thickness: 1.0-1.5 mm) consisting of a NiO/8YSZ cermet, a thin 8YSZ electrolyte, and a bi-layered cathode. The cathode distinguished two separated layers: first a cathode functional layer consisting of La 0.65 Sr 0.3 MnO 3 (LSM)/Y 2 O 3 -stabilized ZrO 2 (8YSZ) and a cathode current collector layer of pure La 0.65 Sr 0.3 MnO 3 (LSM). This study can be considered as a follow-up of that (Journal of Power Sources 141 (2005) 216-226) describing the improvement of the cell performance by a systematic variation of the microstructure. The experiments described in this paper and the corresponding results are part of a more extensive study to investigate in more detail the effect of the sintering temperature on the electrochemical performance of LSM-type SOFCs. Since research is still going on, conclusions, drawn in this contribution, are yet not definitive.

The Electrochemical Performance of Anode-Supported SOFCs with LSM-Type Cathodes Produced by Alternative Processing Routes

The electrochemical performance of La 0.65 Sr 0.3 MnO 3 -type (LSM) anode-supported single cells, produced by alternative production processes, has been investigated at intermediate temperatures. In particular, three different variations of the production route were investigated in more detail: (I) the use of nonground LSM powder for the cathode current collector layer, (2) the use of noncalcined and nonground YSZ powder for the cathode functional layer, and (3) the use of tape casting versus warm pressing as the production process for anode substrates. Results from electrochemical measurements performed between 700 and 900 °C with H 2 (3 vol % H 2 O) as fuel gas and air as the oxidant showed that performance increased with increasing grain size of the outer cathode current collector layer: the highest performance was achieved with nonground LSM powder. Furthermore, performance was not adversely influenced by the use of noncalcined and nonground YSZ for the cathode functional layer. Also the use of anode substrates with a thickness of about 0.7 mm and produced by tape casting, instead of those with a thickness of about 1.5 mm and applied by warm pressing, did not detrimentally affect the electrochemical performance of this type of SOFC.

Influence of microstructure and AlPO4 secondary-phase on the ionic conductivity of Li1.3Al0.3Ti1.7(PO4))3 solid-state electrolyte

A ceramic solid-state electrolyte of lithium aluminum titanium phosphate with the composition of Li1.3Al0.3Ti1.7(PO4)3 (LATP) was synthesized by a sol–gel method using a pre-dissolved Ti-source. The annealed LATP powders were subsequently processed in a binder-free dry forming method and sintered under air for the pellet preparation. Phase purity, density, microstructure as well as ionic conductivity of the specimen were characterized. The highest density (2.77g⋅cm−3) with an ionic conductivity of 1.88×10−4 S⋅cm−1 (at 30∘C) was reached at a sintering temperature of 1100∘C. Conductivity of LATP ceramic electrolyte is believed to be significantly affected by both, the AlPO4 secondary phase content and the ceramic electrolyte microstructure. It has been found that with increasing sintering temperature, the secondary-phase content of AlPO4 increased. For sintering temperatures above 1000∘C, the secondary phase has only a minor impact, and the ionic conductivity is predominantly determined by the microstructur...

Influence of microstructure and AlPO4 secondary-phase on the ionic conductivity of Li1.3

A ceramic solid-state electrolyte of lithium aluminum titanium phosphate with the composition of Li1.3Al0.3Ti1.7(PO4)3 (LATP) was synthesized by a sol–gel method using a pre-dissolved Ti-source. The annealed LATP powders were subsequently processed in a binder-free dry forming method and sintered under air for the pellet preparation. Phase purity, density, microstructure as well as ionic conductivity of the specimen were characterized. The highest density (2.77g⋅cm−3) with an ionic conductivity of 1.88×10−4 S⋅cm−1 (at 30∘C) was reached at a sintering temperature of 1100∘C. Conductivity of LATP ceramic electrolyte is believed to be significantly affected by both, the AlPO4 secondary phase content and the ceramic electrolyte microstructure. It has been found that with increasing sintering temperature, the secondary-phase content of AlPO4 increased. For sintering temperatures above 1000∘C, the secondary phase has only a minor impact, and the ionic conductivity is predominantly determined by the microstructur...

Influence of microstructure and AlPO 4 secondary-phase on the ionic conductivity of Li

A ceramic solid-state electrolyte of lithium aluminum titanium phosphate with the composition of Li1.3Al0.3Ti1.7(PO4)3 (LATP) was synthesized by a sol–gel method using a pre-dissolved Ti-source. The annealed LATP powders were subsequently processed in a binder-free dry forming method and sintered under air for the pellet preparation. Phase purity, density, microstructure as well as ionic conductivity of the specimen were characterized. The highest density (2.77g⋅cm−3) with an ionic conductivity of 1.88×10−4 S⋅cm−1 (at 30∘C) was reached at a sintering temperature of 1100∘C. Conductivity of LATP ceramic electrolyte is believed to be significantly affected by both, the AlPO4 secondary phase content and the ceramic electrolyte microstructure. It has been found that with increasing sintering temperature, the secondary-phase content of AlPO4 increased. For sintering temperatures above 1000∘C, the secondary phase has only a minor impact, and the ionic conductivity is predominantly determined by the microstructur...