Anode-supported cells power densities

Figure 9. Anode supported cells- power density at 0.7 V and 750°C, using + 3%H20...

Virkar A, Wilson L (2003) Low-temperature, anode-supported high power density solid oxide fuel cells with nanostructured electrodes. Technical report. Department of Energy, USA... 

This result may be explained by the observation that tar deposits from hydrocarbons are only removed by oxygen, not H2O, at SOFC operating temperatures [62]. Figure 7 shows the performance of a typical anode-supported cell [19]. Because of the relatively low hydrogen content in the reformed fuel, open-circuit voltages are 1.0 V and concentration polarization is pronounced, but power densities are only slightly lower than when the cells were run with pure hydrogen fuel. 

The current state-of-the-art SOFC anode-supported cells based on doped zircona ceramic electrolytes, ceramic LSM cathodes, and Ni/YSZ cermet anodes are operated in the temperature range 700-800°C with a cell area specific resistance (ASR) of about 0.5 O/cm at 750°C. Using the more active ceramic lanthanum strontium cobalt ferrite (LSFC)-based cathodes, the ASR is decreased to about 0.25 Q/cm at this temperature, which is a more favorable value regarding overall stack power density and cost-effectiveness. 

Anode-Supported. Advances in manufacturing techniques have allowed the production of anode-supported cells (supporting anode of 0.5 to 1 mm thick) with thin electrolytes. Electrolyte thicknesses for such cells typically range from around 3 to 15 im (thermomechanically, the limit in thickness is about 20 to 30 im (the cathode remains around 50 am thick), given the difference in thermal expansion between the anode and the electrolyte). Such cells provide potential for very high power densities (up to 1.8 W/cm under laboratory conditions, and about 600 to 800 mW/cm under commercially-relevant conditions). 

Barnett, Perry, and Kaufmann (75) found that fuel cells using 8 im thick yttria-stabiUzed zirconia (YSZ) electrolytes provide low ohmic loss. Furthermore, adding thin porous yttria-doped ceria (YDC) layers on either side of the YSZ yielded much-reduced interfacial resistance at both LSM cathodes and Ni-YSZ anodes. The cells provided higher power densities than previously reported below 700 °C, e g., 300 and 480 mW/cm at 600 and 650 °C, respectively (measured in 97 percent H2 and 3 percent H2O and air), and also provided high power densities at higher temperatures, e g., 760 mW/cm at 750 °C. Other data (Figure 7-25) from the University of Utah (73) show power densities of 1.75 W/cm with H2/air and 2.9 W/cm with H2/O2 at 800 °C for an anode-supported cell. However, no data is presented with regard to electrodes or electrolyte thickness or composition. 

Currently, electrolyte-supported, cathode-supported, anode-supported, and metallic substrate-supported planar SOFCs are tmder development. In electrolyte-supported cells, the thickness of the electrolyte, typically YSZ, is 50-150 pm, making then-ohmic resistance high, and such cells are suitable only for operation at 1,000°C. In electrode-supported designs, the electrolyte thickness can be much lower, typically 5-20 pm, which decreases their ohmic resistance and makes them better suited for operation at lower temperatures. The anode (Ni/YSZ cermet) is selected as the supporting electrode, because it provides superior thermal and electrical conductivity, superior mechanical strength, and minimal chemical interaction with the electrolyte. Kim et al. [83] have reported power densities as high as 1.8 W/cm at 800°C for such anode-supported SOFCs. At Pacific Northwest National Laboratory [84, 85], similar anode-supported cells have been developed using 10 pm... 

All fuel cell experiments were carried out using the anode supported Ni SDC (SDC) / SDC / LSM (SDC) cells described above. Temperatures reported are those at the fuel cell reaction zone. Fig. 4 shows the voltage V and power density P versus the current density J for the anode-supported SOFC operated on the R = 0.6 fuel mixture at 200 mL.min. ... 

The suitability of lanthanum nickelate as an SOFC cathode has been examined by Virkar s group [138], They showed that LN performed poorly as a single-phase cathode in an anode-supported YSZ cell. However, with an SDC/LN composite interlayer the performance of the LN cathode increased substantially and the maximum power density of the cell with a YSZ thin electrolyte (-8 pm) was -2.2 Wear2 at 800°C, considerably higher than 0.3 to 0.4 Wcm-2 of similar cells with only LN or SDC interlayer. The results are significant as it shows that the composite MIEC cathodes perform much better than single-phase MIEC in the case of LN despite its mixed ionic and electronic conductivity. 

The PE MFC has a solid ionomer membrane as the electrolyte, and a platinum, carbon-supported Pt or Pt-based alloy as the electrocatalyst. Within the cell, the fuel is oxidized at the anode and the oxidant reduced at the cathode. As the solid proton-exchange membrane (PEM) functions as both the cell electrolyte and separator, and the cell operates at a relatively low temperature, issues such as sealing, assembly, and handling are less complex than with other fuel cells. The P EM FC has also a number of other advantages, such as a high power density, a rapid low-temperature start-up, and zero emission. With highly promising prospects in both civil and military applications, PEMFCs represent an ideal future altemative power source for electric vehicles and submarines [6]. 

Amendola et al. [38, 39] constructed Model 2 type cells with an air cathode and an anode made of highly dispersed Au/Pt particles supported on high-surface area carbon silk. An anion exchange membrane (AEM) was used as the electrolyte. The number of electrons utilized per molecule of BH4 oxidized (about 6.9 out of a possible 8) shows efficient utilization of the BH4 oxidation. Specific energy >180Whkg and power densities >20mWcm at room temperature and >60mWcm at 70 °C have been reported. 

A SEM micrograph of the cathode/electrolyte interface and preliminary results on the electrochemical activity of YSZ electrolyte-supported SOFCs containing Ni-YSZ anode and a LSCF-SDC composite cathode are shown in Fig. 14. As it can be seen in Fig. 14(a), the composite film not only has good adhesion to the electrolyte, but also possesses a porous microstructure which is required for the oxidant electrochemical reduction. It indicates that such a composite film can have a good performance as SOFC cathode. By the LSV technique, qualitative information about electrochemical activity of this SOFC was acquired. The power density curves (Fig. 14b) revealed that maximum power densities were 19, 26, 36 and 46 mW/cm2 at 800, 850, 900 and 950 °C. It is possible to compare these first results with literature data and safely state that the LSCF-SDC cathode composite is qualitatively better than other plain standard materials or cathode composites already reported. It should also be mentioned that the result obtained at 800 °C is similar to that reported by Mucdllo et al (Mucdllo et al., 2006) for a SOFC single cell with LSM-YSZ cathode, Ni-YSZ anode and 70 pm... 

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