The Power of Plasma

Innovative plasma spray techniques are providing increased efficiency and cost savings in the sequential formation of multi-layered SOFC structures

Solid oxide fuel cells (SOFCs) offer a highly efficient, pollution-free means to generate electricity through electrochemical and chemical reactions between hydrogen/hydrocarbon fuels and oxygen. The modular and distributive nature of SOFCs makes them an attractive component for the clean, safe, flexible power supplies envisioned for the future. However, challenges to SOFC commercialization remain, including selecting the proper materials, designing a successful manufacturing approach, developing the right cell and stack architecture, achieving acceptable performance and reliability levels, and reducing particularly high manufacturing and maintenance costs.

A promising solution is to develop intermediate temperature SOFCs (IT-SOFCs) that operate in the 650-850°C range. But maintaining a high power level at lower operating temperatures requires high-performance cells that comprise porous electrodes and a dense electrolyte with the appropriate microstructures for high electrochemical performance.

Plasma spray has been widely used in a variety of industries to form coatings from metal, ceramic and cermet materials injected into an extremely hot plasma plume (>20,000 K) (see Figure 1). The main advantage of the plasma spray process compared to other coating techniques is its versatility in achieving a wide range of coatings with different functions and microstructures. Plasma spray is therefore an ideal process for sequential fabrication of the multiple layers in a SOFC unit.

Innovative plasma spray techniques have been developed to produce thin-film IT-SOFCs and stacks either from solid powder or solution precursor feedstock. With plasma spray, the sequential and consecutive fabrication of the cathode, electrolyte, anode and interconnection layers can be realized in an integrated procedure with the aid of an automated robot manipulating system. SOFC producers that fabricate their cells using plasma spray can avoid the time consuming "co-fire" procedure required in conventional cast-sinter processes, and can also reduce the risk of detrimental phase formations that occur as a result of solid-state reactions between two conjunct layers at high sintering temperatures (1400°C). In particular, plasma spray techniques pioneered for the fabrication of nanostructured electrodes for high-performance primary and rechargeable lithium batteries* are showing a great deal of promise when applied to SOFCs and stacks.

*Developed by US Nanocorp, Inc., Farmington, CT.

Air Plasma Spray

Plasma spray can be performed in air or in a controlled environment with regulated pressure. The plasma plume can be a reducing, oxidizing or neutral flame, depending on the working plasma gases and the nature of the surrounding atmosphere. It is the property of the plasma itself that is believed to significantly influence the structure of the deposited coating, which, in turn, determines the catalytic activity or ionic conductivity of the resulting material. The plasma spray parameters must therefore be optimized to process the fuel cell materials into desirable lattice structures and favorable microstructures.

Figure 2 shows a schematic for producing a triple-layered planar SOFC unit using air plasma spray. Powder feedstock is injected into the plasma torch and then rapidly heated, melted and accelerated to a high velocity. The fused materials are rapidly cooled and solidified to form a layer as they impact the substrate. The substrate temperature is kept relatively low (typically less than 200°C) to avoid causing a solid-state reaction, interdiffusion or severe thermal stress.

In the cathode-supported SOFC design adopted by US Nanocorp, a cathode layer is first plasma sprayed onto a dissolvable substrate. The electrolyte and anode layers are then applied in sequences, with the plasma spray conditions optimized in terms of the lattice structure, porosity and thickness. Generally, the anode and cathode layers need to be highly porous for better fuel/oxygen permeability and catalytic activity, and the electrolyte layer needs to be fully dense and thin. The plasma-sprayed anode layer (YSZ+Ni or strontium titanate) and cathode layer (lanthanum strontium manganite) have a typical porosity of 25-35% and a thickness of 300 µm and 150 µm, respectively.

A challenge, however, is how to fabricate the thin (<20 µm) electrolyte layer with the full density (>97%) that is required to provide satisfactory gas tightness and internal resistance loss. A study was performed using a perovskite-type ceramic compound (strontium and magnesium-doped LaGaO₃ [LSGM]) as the electrolyte material. LSGM has a higher oxygen ionic conductivity at intermediate temperature ranges and a lower melting point than traditional electrolyte materials, such as yttria-stabilized zirconia. This material characteristic allows the feedstock to be fully melted under optimal plasma spray conditions, and also tolerates a higher thickness (up to 50 µm) without significant polarization loss from electrolyte resistance. X-ray analyses have indicated that the as-sprayed LSGM layer is amorphous, which will degrade its conductivity.

A post heat treatment was conducted at the IT-SOFC's operating temperature range of 450-850°C. Crystallization of the LSGM deposit started in the 500°C range, and full crystallization was identified above 700°C (see Figure 3). This means that the transition from amorphous to crystalline LSGM can be completed during the first heat-up of the cell/stack units.

Electrochemical impedance spectrum (EIS) analyses have verified that the conductivity of heat-treated LSGM layers is equivalent to that of sintered LSGM pellets. Figure 4 shows a sectioned tubular SOFC unit composed of a LSM tube support, a plasma-sprayed electrolyte layer, and an anode layer.

Solution Plasma Spray

Catalytic activity of the anode layer is crucial for SOFC performance; as a result, a porous microstructure with a large surface area for penetrating fuel is required. In a typical Ni+YSZ anode layer, the fuel oxidation reaction occurs at three phase boundaries (TPBs)-ionic, electronic and gas. Increasing the number of active sites at the TPBs will increase the catalytic activity of the anode, and therefore the electrochemical performance of the cell. With conventional plasma spray technology, the majority of formed pores have a micrometer size and a pancake-like lamellar shape. Thus, limited numbers of pore-related TPBs exist in a plasma-sprayed anode layer.

A new plasma spray technique called solution plasma spray (SPS^TM) has been used to successfully produce porous anode layers. Figure 5 illustrates the SPS process for applying a layer. Unlike the powder feedstock used with conventional plasma spray techniques, SPS uses an aqueous solution feedstock. A liquid delivery system pumps the precursors from the reservoirs to an atomizer that generates a fine-droplet fog. The fog stream is fed into the plasma, where the liquid droplets experience a series of physical changes and chemical reactions in multiple sequential steps that most likely involve evaporation, droplet breakdown, pyrolysis reaction and melting. The SPS-formed layer features a nanostructured, ultrafine splat; three-dimensional pores; and a nanometer and micrometer pore size distribution. In an anode layer of lanthanum oxide-doped ceria (LDC)+NiO fabricated using the SPS technique, fine particles (less than 2 µm) were agglomerated to form a highly porous matrix with a large surface area (see Figure 5).

Another concern with the anode layer is the formation of a detrimental phase at the interface between the LSGM electrolyte and LDC (or YSZ)+NiO anode. A suitable separation layer (2 µm thick or less) of LDC can act as a diffusion barrier to eliminate the solid-state reaction between LSGM and Ni at the interface. However, it is impossible to apply that thin layer with a conventional plasma spraying process due to the use of large-sized feedstock (20-100 µm). Since the SPS process is capable of generating ultrafine droplets, the resultant ultrafine particles and splat are considered ideal for producing the buffer layer. When the SPS method is used, the LDC thin layer is applied between the LSGM electrolyte and the LDC+Ni anode.

Cell Evaluation and Performance

Single cells comprising LSM cathodes, LSGM electrolytes and Ni-YSZ anodes were fabricated by air plasma spraying. The electrical performance of the cells was evaluated by measuring the open circuit voltage (OCV) and voltage-current density dependency in the 500 to 800°C temperature range. As shown in Figure 6, the OCV was 0.95 V at 600oC, and 1.045 V at 700°C-identical to the OCVs reported for similar cell systems fabricated through other techniques. The OCV data indicate that the plasma sprayed electrolyte layer has enough gas tightness to separate hydrogen and air effectively. The power density was measured as 80-150 mW/cm² at the tested temperatures. Current work is devoted to improving the electrode performance for increasing cell power density.

Successful IT-SOFCs

Both the air plasma spray process and solution plasma spray process have demonstrated high application efficiency, material versatility, tailored microstructure, process readiness and scale-up capability in comparison to other existing processes for the manufacture of SOFCs. These plasma spray techniques have produced porous electrode layers (LSM, Ni+YSZ and Ni+LDC) and a dense electrolyte layer (YSZ, LSGM). The results of cell evaluation have verified that both processes are highly promising for the sequential fabrication of multi-layers in thin-film IT-SOFCs in planar or tubular form.

For more information about plasma-sprayed SOFC technologies, contact US Nanocorp, Inc., 74 Batterson Park Rd., Farmington, CT 06032; (888) 626-6888; fax (860) 678-7569; e-mail info@usnanocorp.com ; or visit http://www.usnanocorp.com .