Low-Temperature SOFCs

Efforts to lower the operating temperature of SOFCs to values below 600°C have continued to the present. Two directions are followed in this work reducing the thickness of electrolytes having higher conductivities at lower temperatures that were mentioned in Section 8.7.1, and developing new composite electrolytes having even higher conductivites. 

Tsai et al. (1997) reported building soUd-oxide fuel cells with an electrolyte of yttrium-doped ceria that was 4 to 8 p,m thick. A thin layer (1 to 1.5 p,m) of a second electrolyte, of the YSZ type, was deposited between the electrolyte and the anode to eliminate the influence of electronic conductivity of the electrolyte. A cell having this bilayer electrolyte had an OCV of 98% of the theoretical EMF, which shows that there is no effect of the electronic component of conduction (without the second YSZ electrolyte, the OCV was only 50% of the theoretical value). At 600°C the electrolyte had a resistivity of 0.25 cm. At a current density of 400 mA/cm, the cell voltage was 0.4 V. The maximum energy density was 210 mW/cm. At 550°C, the resistivity of the electrolyte was 0.45 cm  

Doshi et al. (1999) used an electrolyte of ceria-doped gallate (CeGO) 30 p,m thick. In the cell, a two-phase cathode 250 p,m thick was used that consisted of a cermet of 45 to 55% silver and electrolyte material in the form of yttria-doped bismuth oxide (YDB). An Ni-CeGO cermet served as the anode. At a working temperature of 500°C and a ceU voltage of 0.6 V, the current density was 100 mA/cm at a cell voltage of 0.4 V, the current density was 3(K) mA/cm. It is interesting to note that in this cell, the contributions of the ohmic resistance of the electrolyte (0.7 cm ) and of cathode polarization (0.6 cm ) to the 

Another principle of building a cell with a thin electrolyte layer was used by Ito et al. (2005). An ultrathin layer of electrolyte (0.7 p,m) of the type of BaCeo.8Yo.2O3 was deposited onto a 40-p.m palladium membrane that is hydrogen-permeable. It served as the anode, hydrogen was supplied from the side opposite to the electrolyte. A cathode layer was deposited on top of the electrolyte layer. At a cell voltage of 0.6 V, current densities of about 1.5 A/cm were obtained at 400°C, and of 2.5 A/cm at 600°C. 

100 times higher than those of the pure ceria electrolyte and 10 times higher than those of YSZ-type electrolytes) were obtained at temperatnres between 400 and 600°C. 

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M.M. Seabaugh and B. Emley, Textured Composite Seals for Low Temperature SOFCs, SECA Core Technology Program, SOFC Seal Meeting, July 7, 2003. 

Yin Y, Li S, Xia C, and Meng G. Electrochemical performance of gel-cast NiO-SDC composite anodes in low-temperature SOFCs. Electrochim. Acta 2006 51 2594-2598. 

Ni-cermet type anodes have been improved by substituting the YSZ by ceria, gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC). Ceria seems to increase the catalytic activity of the cermet for hydrogen oxidation, while SDC and GDC improve the ionic conductivity of the anode. Ni-ceria cermets are considered the main candidate for low-temperature SOFC [74],... 

Certain oxides with a perovskite structure are generally applied to the cathode. For high temperature type SOFCs, doped-LaMn03 is used as the typical cathode. For low temperature SOFCs, LaSr(CoFe)03 or La(NiFe)03 are used as the cathode. Doped LaCo03 has a high electric conductivity and shows an excellent catalytic performance. However, the TEC of LaSrCo03 is larger than that of the electrolyte, and so Fe is substituted to reduce the TEC of the cathode. 

For high temperature SOFCs, a conventional commercial alloy cannot be used as the interconnector because they are easily oxidized under a high temperature oxidizing atmosphere. Therefore presently, special rare metals or doped-LaCr03 are used as the interconnector. For low temperature SOFCs, a commercial alloy can be used as the interconnector and thus, various alloys are considered as the interconnector. Austenite stainless steels have a high TEC in comparison to YSZ, and only ferritic stainless steels are considered as a candidate for the interconnector. 

Steele B C H etal, 2000b, Improving Gd doped ceria electrolytes for low temperature SOFC s. Material Research Society Symposium Proceedings. 

Maricle, D.L. et al.. Enhanced ceria a low-temperature SOFC electrolyte, Solid State Ionics, 52, 173-182 (1992). 

Perovskite related cathode materials have been shown to possess levels of ionic and electronic conductivity comparable to existing perovskite cathode materials. The development of these new cathodes is in the very early stages and significant research is required before these can compete with the established Lai.xSrxCoOs.g (LSC) and Lai.xSrxMnOg.g (LSM) based cathodes. However, one of the main advantages offered by these materials is the prospect of fast surface exchange and ionic diffusion at temperatures considerably lower than the established candidate materials thus enabling development of low temperature SOFC devices. 

FACTORS INFLUENCING THE LIFETIME OF SOFCs 21.7 LOW-TEMPERATURE SOFCs (LT-SOFC)... 

Yang L, Zuo C, Wang S, Cheng Z and Liu M (2008), A novel composite cathode for low-temperature SOFCs based on oxide proton conductors , Adv Mater, 20, 3280-3283. 

Miniature fuel cells integrable, portable / Low-temperature- SOFC working temp. < 500X, Ml ceramic setup y ... 

There can be seen one interesting feature in the low-temperature SOFC, that is, the cell performance of the anode support cells has been much improved compared with electrolyte self support cells. This is apparently due to the lowering of the ohmic resistance inside electrolyte. Furthermore, the electrode performance is also improved very much. This can be ascribed to better fabrication procedure. When... 

Barnett, S., E. Perry, and D. Kaufmann, Application of Ceria Layers to Increase Low Temperature SOFC Power Density , Proceedings of the Fuel Cells 97 Review Meeting. http //www.netl.doe.gov/publications/proceedings/97/97fc/FC6-6.pdf... 

Factors associated with (i)-(v) can be mitigated by searching for low-temperature electrolyte and cathode materials for low-temperature SOFCs since the rates of those steps involved in these factors are usually low at low temperatures. 

We have already discussed some materials that could be used as low-temperature SOFC electrolytes. Referring again to Figme 7.22, if we assume that the electrolyte should not contribute more than 0.15 Vcm to the total cell area specific resistance, then for a thickness (L) of 15tim, the associated specific ionic conductivity (a) of the electrolyte should exceed 10 Scm (since a = L/ASR = 0.0015/0.15). 

In addition, direct oxidation has also been investigated on novel ceramic anodes for SOFCs made of mixed ionic- and electronic-conducting materials. A good example is the work reported by Perry Mnrray et al. (1999) on ceria-doped zirconia anodes in low-temperature SOFCs. Many other materials are also being investigated and a recent review has been published by Irvine and Sauvet (2001). 

Fuel cells were invented more than 150 years ago, but their commercialization has been very slow. To date, they have been used primarily in space vehicles, but these systems are quite costly and not suitable for commercial applications. In the last decade, however, there has been a dramatic increase in research, and it is now clear that fuel cells will enter the commercial marketplace in the not too distant future. Currently, most attention is focused on two types of fuel cells, polymer-electrolyte membrane (PEM) fuel cells and solid-oxide electrolyte fuel cells (SOFCs) (Carrette et al., 2000 Minh 1993). PEM systems use a proton-conducting polymer as the electrolyte and operate at low temperatures SOFCs use an oxygen ion-conducting ceramic membrane as the electrolyte and operate at temperatures of 700 to 1,000°C. 

Doped ceria has been suggested as an alternative electrolyte for low temperature SOFCs [6, 31, 32]. Reviews on the electrical conductivity and conduction mechanism in ceria-based electrolytes have been presented by Mogensen et al. [33] and Steele [34], Ceria possesses the same fluorite structure as the stabilised zirconia. Mobile oxygen vacancies are introduced by substituting Ce " with trivalent rare earth ions as shown in Eq. (1). The conductivity of doped ceria systems depends on the kind of dopant and its concentration. A typical dopant concentration dependence of the electrical conductivity in the (Ce02)i -x(Sm203)x system as reported by Yahiro etal. [3 5] is shown in Figure 4.9. 

In Table 4.2, the conductivity data for doped ceria are summarised. Ce02-Gd203 and Ce02-Sm203 show an ionic conductivity as high as 5 x 10 S/cm at 500°C, corresponding to 0.2 Q cm ohmic loss for an electrolyte of 10 pm thickness. These compositions are attractive for low temperature SOFCs and have been extensively examined. 

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