Yttria stabilised zirconia

The activation energy represents the ease of ion hopping, as already indicated above and shown in Fig. 2.5. It is related directly to the crystal structure and in particular, to the openness of the conduction pathways. Most ionic solids have densely packed crystal structures with narrow bottlenecks and without obvious well-defined conduction pathways. Consequently, the activation energies for ion hopping are large, usually 1 eV ( 96 kJ mole ) or greater and conductivity values are low. In solid electrolytes, by contrast, open conduction pathways exist and activation energies may be much lower, as low as 0.03 eV in Agl, 0.15 eV in /S-alumina and 0.90 eV in yttria-stabilised zirconia. 

CeOj and ThOj have the cubic fluorite structure. Fig. 2.15, and can be doped with large amounts of, for example, Ca, La or Gd to give extensive ranges of cubic solid solutions. ZrOj is cubic only above 2400°C, however, and requires 8% of dopant to stabilise the cubic form to room temperature (as in YSZ, yttria-stabilised zirconia). 

Both liquid and solid electrolytes can be used, ranging from molten halides, such as a eutectic mixture of LiCl and KCl, to very sophisticated solid-state electrolytes such as calcia or yttria stabilised zirconia, CSZ, YSZ, which are conductors of oxygen ions. 

The hydrogen oxidation within a fuel cell occurs partly at the anode and the cathode. Different models were supposed for the detailed reaction mechanisms of the hydrogen at Ni-YSZ (yttria stabilised zirconia) cermet anodes. The major differences of the models were found with regard to the location where the chemical and electrochemical reactions occur at the TPB (three-phase boundary of the gaseous phase, the electrode and the electrolyte). However, it is assumed that the hydrogen is adsorbed at the anode, ionised and the electrons are used within an external electrical circuit to convert the electrical potential between the anode and the cathode into work. Oxygen is adsorbed at the cathode and ionised by the electrons of the load. The electrolyte leads the oxide ion from the cathode to the anode. The hydrogen ions (protons) and the oxide ion form a molecule of water. The anodic reaction is... 

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... 

Hendriks, M.G.H.M., ten Elshof, J.E., Bouwmeester, H.J.M., Verweij, H. The defect structure of the double layer in yttria-stabilised zirconia. Solid State Ionics 2002,154, 467-72. 

G.Z. Cao, H.W. Brinkman, J. Meijerink, K.J. de Vries and A.J. Burggraaf, Pore narrowing and formation of ultra thin yttria-stabilised zirconia layers in ceramic membranes by chemical vapor deposition. /. Am. Cer. Soc., 76 (1993) 2201-2208. 

On the other hand, the largest disadvantage is that the protonic resistance of the Sr-Ce-Yb oxide was comparatively larger than that of a polymer-electrolyte-membrane fuel cell (PEM-FC) and was comparable with 0 ion conductivity of an yttria-stabilised zirconia (YSZ). Consequently, as seen in Figs. 4 and 5, the current density through the Sr-Ce-Yb oxide fuel cell was order of niA/cni and was much smaller than that of PEM-FC. This is because a thin ceramic is very difficult to manufacture. The protonic conductivity of the Sr-Ce-Yb oxide itself was around one-tenth smaller than that of PEM. Moreover, the conductivity was order of 10 " S/cm when a CH4 and HjO mixture was supplied directly to the cell without external reformer. The overall conductivity became around 10 -fold less than that of PEM, because the rate-controlling step was in the steam-reforming reaction. 

Haering, C., Roosen A., and Schichl, H., Degradation of the electrical conductivity in stabilised zirconia systems Part I Yttria-stabilised zirconia. Solid State Ionics 176 (2005) 253-259. 

X-ray Absorption Near-Edge Spectroscopy Yttria Stabilised Zirconia... 

These restrictions notwithstanding, yttria-stabilised zirconia polycrystal (Y-TZP) femoral heads presently make up about 25% of the total annual number of hip joint implants in Europe and 8% in the United States. Between 1985 and 2001 more than 400000 Y-TZP femoral heads were implanted worldwide. However, safety concerns continue to plague zirconia femoral heads. The U.S. Food and Drug Administration (FDA, 2013) issued a safety alert in August 2013 to orthopaedic surgeons that the French manufacturer St Gobain Desmarquest had recalled an unimplanted inventory of zirconia femoral heads that were found to fracture at a higher rate than expected in some patients 13-27 months after implantation. 

YSZ (Yttria-Stabilised Zirconia) Cubic Fluorite Structure... 

The principal types of cell in use have an electrol5de that conducts CF ions, so that the reaction products are generated at the anode (Figure 5.9a). Here the electrol5de is usually yttria-stabilised zirconia (YSZ) containing x/2 oxygen... 

The usual anode is a cermet (composite material made of a ceramic and a metal). Porous Ni-YSZ (yttria-stabilised zirconia) is the state-of-the-art electrode, presenting electronic and ionic conductivities in order to increase the number of reaction sites, called triple phase boundaries. It corresponds to the area where 0 , e and H2 are all present for the time required for the oxidation reaction to occur. No single phase has been found to completely fit all the requirements for an anode thermal and chemical compatibilities with the electrolyte, mixed ionic and electronic conductivity, high electro-catalytic activity and stability in reductive atmosphere. 

The prevalent material for substrates and anodes in anode-supported solid oxide fuel cells (SOFCs) are porous composites of oxygen-ion conducting ceramics such as yttria-stabilised zirconia (YSZ) and Nickel. Cells based on such substrates and anodes have been found to show very good performance. ... 

Figure 7.19 Permeance of hydrogen, nitrogen and carbon dioxide versus temperature for a platinum loaded membrane made of yttria stabilised zirconia [530].

Kusakabe et al. prepared yttria-stabilised zirconia membranes on a-alumina tubes and impregnated them with platinum and rhodium for the preferential oxidation of carbon monoxide [530]. The carbon monoxide concentration could be decreased from 10 000 ppm in the feed to 30 ppm in the permeate, at 150 °C reaction temperature by the platinum containing membrane. However, the ceramic membrane also showed permeance for carbon dioxide and methane. The latter was oxidised to a certain extent over the platinum catalyst of the membrane. The rhodium catalyst showed inferior performance and even some activity towards methanation at a 250 °C reaction temperature in the absence of oxygen in the feed. 

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