Nickel-ceria cermet

Figure 3-15. Temperature dependence of condnctivity of rednced (a) and oxidized (b) nickel- ceria cermet...

A number of researchers have studied nickel/ceria cermet anodes for both zirconia and ceria-gadolinia electrolyte-based SOFCs [30,93-96], Nickel/ceria... 

Alloying the nickel of the anode to improve tolerance for fuel contaminants has been explored. Gold and copper alloying decreases the catalytic activity for carbon deposition, while dispersing the anode with a heavy transition metal catalyst like tungsten improves sulfur resistance. Furthermore, ceria cermets seem to have a higher sulfur tolerance than Ni-YSZ cermets [75],... 

The anodes consisting of a nickel catalyst and of cermet mixed with yttria-doped zirconia electrolyte that are used in conventional solid oxide fuel cells also lose their ability to work at lower temperatures because of a loss of conductivity by the ceramic. This suggests that, for the ceramic in the anode, a material having a higher conductivity at intermediate temperatures should be used. It was in fact shown that an anode made with a nickel/samaria-doped ceria cermet has a much lower polarization than the conventional variant. 

Nickel/Rare Earth Metal-Doped Ceria Cermet... 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Ceria has also been used as the ceramic part in nickel - or ruthenium - cermet anodes for hydrogen oxidation." Beneficial effects have been reported and interpreted as being probably due to the broadening of the three-phase boundary zone width. 

At temperatures below 1000°C the doped ceria has a considerable larger polarisation resistance, Rp, even in case of cermet with nickel. (The reaction rate is proportional to 1/Rp). At the lower temperatures the reaction may be accelerated by addition of fine Ni particle to the surface of the doped ceria. Impedance spectra at 850°C of Ni/Ceo jGdo iO, 95 cermet electrodes with and without small amounts of fine Ni particles (probably of nanometer size) are given in Fig. 15.13. The impedance is significantly affected. The dominant low frequency arc (fsummit = 1 Hz)... 

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

One of the problems with anode-supported cells is that any difference in thermal expansion between anode and electrolyte becomes more significant than in conventional high-temperature SOFCs. For this reason many developers use porous nickel cermet anodes with interfacial regions made of NiA SZ doped with ceria. Operating at temperatures below about 700°C means that metallic bipolar plates can be used, and the lower the temperature, the less exotic the steel needs to be. Ferritic stainless steels can be used below about 600°C, and these have the advantage that they have a low thermal expansion coefficient. Conventional doped LSM-YSZ cathodes can be used but there is much development in progress to improve cathode materials as the cathode overpotentials become more significant as the cell temperatures are lowered. A recent review of cathode materials has been published by Ralph (2001). 

Ceria has been used as the ceramic part (or as an addition) in nickel- or ruthenium-cermet anodes for hydrogen oxidation. " Beneficial effects have been reported and interpreted as most likely being due to the broadening of the three-phase boundary zone. However, one of the major drawbacks of using ceria in cells with YSZ-based electrolytes is its chemical reactivity with the YSZ electrolyte at high temperatures. Sintering of a doped ceria anode on a YSZ electrolyte at high temperatures (>1200°C) results in the formation of a reaction (diffusion) zone with limited oxide ion conductivity. ... 

One approach to overcoming the limitations of nickel anodes, which has met with some success, is to augment the oxidation activity of Ni/YSZ cermets through the addition of an oxide-based oxidation catalyst. For example, stable operation on dry methane has been reported at 650°C in an SOFC using an yttria-doped ceria interlayer between the YSZ electrolyte and the Ni/YSZ cermet anode [61]. Ceria is a well-known oxidation catalyst, and might be expected to increase the activity of the anode for the electrochemical oxidation of methane. This approach still requires, however, that the operating temperature be maintained below 700°C to suppress carbon deposition reactions that take place rai nickel. 

It is well known that the ionic conductivity of aliovalent-doped ceria solutions show a maximum at a certain dopant concentration and cation radius. Compared to divalent cation doping, however, trivalent dopants are observed to contribute higher conductivity values in ceria. Among trivalent rare earth metals, samarium and gadolinium are accepted as the most effective dopants. Therefore, we have selected a cermet made up of nickel and 20 at% samarium-doped ceria (SDC) as the anode material in our standard cells [12-14]. 

The main problem with the nickel-based anodes is their propensity to coke, that is to become coated with a carbon layer on reacting with hydrocarbon fuel. This carbon layer has two deleterious effects it can disrupt the anode by pushing the nickel particles apart and it can form a barrier at the nickel surface, preventing gas reactions. Typically, if a hydrocarbon such as methane is fed directly into an SOFC anode, then it may not remain functional after as little as 30 minutes as the coking proceeds. Additives to the Ni+YSZ cermet such as 5% ceria or 1% molybdena can inhibit this process [19]. Alternatively, metals other than nickel can be employed [20]. 

Figure 6.14 Stability of ceria-catalyzed copper cermet under different fuel conditions contrasts with rapid irreversible deactivation of nickel cermet [43].

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