Thin films for micro solid oxide fuel cells

doi:10.1016/j.jpowsour.2007.04.070

Journal of Power Sources

Volume 173, Issue 1, 8 November 2007, Pages 325-345

https://doi.org/10.1016/j.jpowsour.2007.04.070 Get rights and content

Abstract

Thin film deposition as applied to micro solid oxide fuel cell (μSOFC) fabrication is an emerging and highly active field of research that is attracting greater attention. This paper reviews thin film (thickness ≤1 μm) deposition techniques and components relevant to SOFCs including current research on nanocrystalline thin film electrolyte and thin-film-based model electrodes. Calculations showing the geometric limits of μSOFCs and first results towards fabrication of μSOFCs are also discussed.

Introduction

Solid oxide fuel cells (SOFCs) convert chemical energy with high efficiency directly into electricity and heat and can operate on a variety of fuels such as natural gas or hydrogen. As depicted in Fig. 1, the fuel supplying H₂ is fed into the anode compartment where it is oxidized, and the electrons released as a result are conducted to an external circuit [1]. The reaction products on the anode side of an SOFC are mainly water and CO₂. Air enters on the cathode side and oxygen is reduced here to O²⁻ by reaction with electrons from the external circuit. The O²⁻ ions can travel through the ion-conducting and gas-tight electrolyte, which separates the anode compartment from the cathode compartment. Once on the anode side the O²⁻ joins with hydrogen to form water. The driving force for an SOFC is the difference in oxygen partial pressure between the anode (low pO₂) and cathode (high pO₂). Open circuit voltage (OCV) is the voltage obtained at zero current that ranges from about 0.8–1.1 V and is a measure for the gas leakage or electronic leakage through the electrolyte. A low OCV reduces the SOFC power output.

The choice of materials for each component is given by the requirements resulting from the functions discussed above: The anode must be porous to allow gas access, should be a catalyst for fuel oxidation and must show electronic and ionic conductivity. Nickel satisfies the first two requirements, while yttria-stabilized zircona (YSZ) or cerium gadolinium oxide (CGO) fulfills the last. Therefore, two-phase cermets (ceramic-metal composites) that combine all three properties are used. The electrolyte should be dense and predominantly an ionic conductor like YSZ or CGO while the cathode should be porous to enhance gas access, catalytically active towards oxygen reduction, and a good ionic and electronic (mixed) conductor. For cathodes perovskites are commonly chosen, for example La_xSr_1−xMnO_3±δ (LSM), La_xSr_1−xCo_yFe_1−yO_3±δ (LSCF) or Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3±δ (BSCF) [2] or precious metals such as Pt [3].

Differing from this two-gas-chamber concept is the single-chamber SOFC [4], where the anode and cathode are exposed to the same gas atmosphere, a mixture of fuel and air in a safe ratio. Here the driving force is the locally different oxygen partial pressure at the electrodes, which is generated by the different selectivity of the anode and cathode towards fuel oxidation [5], [6], [7]. For a recent review see [8].

Typical operating temperatures for current thick-film-based two-chamber SOFCs are 800–1000 °C, placing heavy demands on the materials and complicating the sealing mechanism. Therefore, research trends towards lowering the operating temperature down to 500–600 °C. To compensate for the performance losses associated with a lower operating temperature, thin film components with lower ohmic resistance have been developed. Thin film components facilitate the fabrication of μSOFCs, leading to new applications for SOFCs, namely portable electronic devices such as laptops, personal digital assistants (PDAs) and scanners [9], [10].

The scope of this review primarily encompasses the emergent fields of thin film deposition for SOFC components, μSOFC fabrication with thin films, and the properties of the thin film materials themselves.

Section snippets

Thin film deposition techniques

The choice of an appropriate thin film deposition technique is strongly influenced by the material to be deposited and the desired film quality and available budget. This section gives an overview of the most relevant thin film deposition techniques for SOFC applications as well as some examples of SOFC materials where these techniques were used for deposition.

Cell components

In this section, the current work on thin film components (anode, electrolyte and cathode) for SOFCs is reviewed.

Cells

μSOFCs are currently an important issue in research [9], [10], because they offer more geographical independence, higher energy densities than batteries and the possibility of fast recharging by refueling. Furthermore, the use of thin films allows lower operating temperatures. Thin film deposition is combined with micro-machining techniques in order to realize fuel cells with micron dimensions. Typically such a μSOFC would look like that depicted in Fig. 10. The gas channels are micro-machined

Summary and conclusion

Many different techniques are available for thin film deposition of SOFC materials. Each method has its own advantages and drawbacks concerning microstructure and quality of the deposited films. Process complexity, choice of material, large area coating, equipment and process costs also have to be taken into account when it comes to industrial fabrication of thin films.

Sputtering is an easy process applicable to any material, with which large surfaces can be coated. For this reason it is

Acknowledgment

Financial support from Swiss Bundesamt für Energie under the project OneBat – Start and Kommission für Technik und Innovation OneBat Discovery Project and The financial support by the European Union, IP-project REAL-SOFC (SES6-CT-2003-502612), are gratefully acknowledged.

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Thin films for micro solid oxide fuel cells

Abstract

Introduction

Section snippets

Thin film deposition techniques

Cell components

Cells

Summary and conclusion

Acknowledgment

Solid State Ionics

Vacuum

Solid State Ionics

Vacuum

Thin Solid Films

Vacuum

Solid State Ionics

Nucl. Instrum. Methods Phys. Res. Sect. B-Beam Interact. Mater. Atoms

Solid State Ionics

Thin Solid Films

Appl. Surf. Sci.

Acta Mater.

J. Power Sources

J. Power Sources

Solid State Ionics

Prog. Mater. Sci.

Solid State Ionics

Mater. Sci. Eng., B

J. Phys. Chem. Solids

Appl. Surf. Sci.

Powder Technol.

Mater. Chem. Phys.

Prog. Cryst. Growth Charact. Mater.

Prog. Cryst. Growth Charact. Mater.

Mater. Sci. Eng., R

Thin Solid Films

Phys. Thin Films

J. Aerosol Sci.

Solid State Ionics

Mater. Chem. Phys.

Solid State Ionics

Thin Solid Films

Solid State Ionics

Solid State Ionics

Mater. Sci. Eng., A

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J. Eur. Ceram. Soc.

Solid State Ionics

Solid State Ionics

Thin Solid Films

Solid State Ionics

Acta Mater.

Solid State Ionics

Solid State Ionics

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High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

Nature

J. Electrochem. Soc.

Electrochem. Solid State Lett.

Chem. Lett.