1. R&D of Low temperature SOFC at USN

Yttria-stabilized zirconia (YSZ) is commonly used as the electrolyte material for SOFCs operated at a temperature of ~1000^oC. Advantages of operation at such a high temperature include internal reforming of fuels such as natural gas, and high quality waste heat. However, these high temperature SOFCs suffer material constraints, high stress from differential thermal expansion of the cell system, high cost of manufacturing, and problems of long-term stability. Intermediate temperature SOFCs exploiting advantageous processing techniques are expected to overcome the problems related to this high operating temperature.

There are two major obstacles that have to be solved to operate SOFCs at intermediate temperatures, including the performance of electrolyte and electrodes. Lowering the operating temperature is possible with the use of alternative materials, appropriate cell design and manufacturing routes. As the operating temperature of an SOFC is reduced, the ohmic loss of the cell across the electrolyte can become a serious problem in an ordinary high temperature electrolyte material. This ohmic loss may be minimized through the use of higher ionic conductivity materials such as gadolinium-doped ceria or strontium-and-magnesium-doped LaGaO₃. Reducing the thickness of the electrolyte membranes will obviously decrease the electrolyte ohmic loss.

Another issue is the slow electrode reaction rates, which may result in polarization losses when the operating temperature is decreased. It is believed that the electrode reactions occur mainly at the triple points between oxygen ions/gas/electrons, so-called triple-phase-boundaries (TPBs). Therefore, both an extended reaction zone and a sufficient porous microstructure will enhance the electrode performance. The reaction zones for the Ni-YSZ cermet anode are intrinsically limited to contacting parts between the Ni and YSZ particles, even though these effective reaction zones can be enlarged two-dimensionally by increasing the anode thickness. Mixed ionic and electronic conductors (MIEC) offer a way to enlarge the TPBs over the entire particle surface, and are ideal electrode materials for both anode and cathode. In addition, nanomaterials, with dimensions down to the atomic scale (10-9 meters), represent a new generation of advanced materials with improved physical, chemical and mechanical properties. A feature of such nanomaterials is the high fraction of atoms that reside at grains and grain boundaries, largely enhancing the chemical activity. Nanostructured materials provide unprecedented opportunities for significantly improved materials performance.

US Nanocorp® (USN) is currently developing intermediate temperature SOFCs using a combination of new materials, novel manufacturing routes, and nanostructures. USN has already demonstrated that such novel low temperature SOFCs can be made successfully, based on an LSGM electrolyte and yttrium doped strontium titanate (SYT) anode using a plasma spray technique.

2. Technology Development
Sequential-Fab Plasma-Sprayed SOFC Components USN’s technology for the synthesis MIEC material and processing SOFC components is a radical departure from conventional SOFC materials and fabrication techniques. A schematic of USN’s SOFC technology is illustrated in Fig. 1, which includes powder synthesis, reconstitution of nanoparticles into sprayable agglomerates, plasma spray fabrication of LSGM and SYT onto the surface of a LSM substrate to form SOFC components, and a photograph of the plasma sprayed single SOFC components. Both planar and tubular SOFC components have been fabricated sequentially with desired porosity or density.

Fig. 1. Schematic representation of USN’s nanostructured anode materials synthesis and thermal spray technique for fabrication of SOFC components

USN’s nanoparticle synthesis route is a low cost wet chemical synthesis technique, which is easily being scaled up to large-scale production. Using this technique, MIEC SYT nanoparticles has been produced, and subsequently reconstituted into sprayable form capable of being fed into industrial thermal spray equipment. USN had developed unique plasma spray parameters that can produce both dense LSGM electrolytes and porous SYT anodes in a single sequential thermal spray operation. Since the SYT is a mixed ionic and electronic conductor anode high temperature ceramic material, no conductive metal such as nickel is needed to mix with the anode ceramics. The resultant cell components will be more durable and energy efficient when compared to conventional high temperature nickel-YSZ cermet anode SOFC systems.

Characterization of Plasma-Sprayed LSGM Electrolyte Layer
A dense electrolyte layer is critical for a successful SOFC operation. The LSGM material has two major favorable factors for processing dense electrolyte films by a plasma spray technique, including:

Beyond the benefit of cost savings, other technical advantages have also been identified with the plasma spray process. For example, in a conventional “co-firing” process in cell fabrication, the reaction has been verified at 1470^oC between electrode and electrolyte in the LSM/LSGM system, and the formation of a solid solution between two materials at 1270^oC was also reported. Obviously, the sintering-induced reaction/degradation can be eliminated in such an extremely rapid far-from-equilibrium plasma spray process.

XRD was carried out on plasma sprayed discs. A single LSGM phase was identified with severely broad peaks as shown in Figure 2 (a). No extra phases were observed after the LSGM powders were sprayed. A comparison of the impedance spectra for plasma sprayed and sintered LSGM at 800^oC as shown in Figure 2 (b). It reveals that the left side intersection points between plasma sprayed and sintered LSGM are almost the same, meaning that their conductivity is nearly identical. A value of 0.75 and 0.82 W are evident in Figure 2 (b) for ohmic resistances of sintered and plasma sprayed LSGM, respectively. However, the second intersection points are different for plasma sprayed and sintered LSGM cells. This is most likely due to the highly effective surface area of the plasma sprayed pellets because of the rough surface, leading to a larger interface area between LSGM pellet and Pt electrode.

Fig. 2. LSGM characterization: (a) XRD spectra for LSGM feedstock and as-sprayed disc, (b) Impedance spectrum,

The microstructure of the sprayed LSGM disc was examined using SEM as shown in Figure 3 (a). The main defect is isolated pores, probably attributed to the inclusion of some unmelted large-size particles of the original feedstock powder. The density of the plasma sprayed LSGM, measured by Archimedes principle, ranges from 93 - 96% of theoretical density. The information about porosity or gas-permeability resistance of the plasma sprayed LSGM could also be obtained by measuring open-circuit voltage (OCV) from a cell with the LSGM electrolyte. A cell was sequentially plasma sprayed with LSM as cathode, LSGM layer, and Ni-YSZ as anode of thickness 200, 300, 200µm, respectively. The OCVs were measured at different temperatures ranging from 500 – 700^oC. A value of 1.045 V was measured at 700^oC as shown in Figure 3 (b), which was slightly lower than that of the theoretical value. OCV data indicated that the plasma sprayed LSGM electrolyte has the desired gas tightness, and thus can isolate hydrogen and air effectively.

Fig. 3. LSGM characterization: (a) SEM image showing dense LSGM layer, (b) Open circuit voltage from the cell that consists of the LSGM layer as an electrolyte.

Advantages of USN’s LSGM electrolyte SOFC system: The proposed thermal spray technique to fabricate SOFCs demonstrated high performance at intermediate temperatures (600 – 800^oC) and low cost. Specifically,

3. Possible Applications
The potential applications of the proposed techniques include high efficiency SOFCs for distribution of electrical generation or utilities, transportation, portable power sources for space and defense applications, as well as gas purification and separation. This technique could be also used for the design and fabrication of membranes for gas separation, solid-state batteries, sensors, and electrochromic devices.

4. Commercialization Strategy
USN is currently seeking a corporate partner in the commercialization of this technology by implementing current lab results in the development of an actual device.

Conference Presentations

H. Zhang, X. Ma, J. Dai, S. Hui, J. Roth, T.D. Xiao, and D. Reisner, "Structure and Electrochemical Behavior of Plasma-Sprayed LSGM Electrolyte Films," 2002 MRS Fall Mtg., Boston, MA, Dec 2-6, 2002.

X. Ma, H. Zhang, J. Dai, J. Roth, J. Broadhead, D. Xiao, D. Reisner, and S. Hui, "Char. of Plasma Sprayed SOFCs for Intermediate Temps.," 2002 MRS Fall Mtg., Boston, MA, Dec 2-6, 2002.

S. Hui, X. Ma, H. Zhang, J. Dai, J. Roth, T.D. Xiao, and D.E. Reisner, "Plasma Sprayed LSGM Electrolyte for Intermediate Temp. SOFCs," 8th Intnl. Symp. on SOFCs, Paris, France, Apr 27-May 2, 2003.

X. Q. Ma, S. Hui, H. Zhang, J. Dai, J. Roth, T.D. Xiao, and D. Reisner, "Intermediate Temp. SOFC Based on Fully Integrated Plasma Sprayed Components," Intnl. Thermal Spray Conf., Orlando, FL, May 5-8, 2003.