E. Kendrick

Developing apatites for solid oxide fuel cells: insight into structural, transport and doping properties

Materials displaying high oxide-ion conductivity have attracted considerable interest due to technological applications in solid oxide fuel cells (SOFCs), oxygen sensors and separation membranes. This has driven research into the identification of new classes of oxide-ion conductors, and in this review, work on the recently discovered apatite-type silicate/germanate oxide-ion conductors is presented. In contrast to the traditional perovskite- and fluorite-based oxide-ion conductors, in which conduction proceeds via oxygen vacancies, the research on these apatite systems suggests that the conductivity involves interstitial oxide-ions. In addition, the flexibility of the tetrahedral (Si/GeO4) framework also plays a crucial role in facilitating oxide-ion migration. Detailed doping studies have shown that the apatite structure is able to accommodate a large range of dopants (in terms of both size and charge state), and the influence of these dopants on the conductivity is discussed.

Atomic-scale mechanistic features of oxide ion conduction in apatite-type germanates

Atomistic modelling studies of the apatite-type oxide ion conductor La9.33(GeO4)6O2 show that a key role of the O4 channel oxygen atoms appears to be as a reservoir for the creation of interstitial oxide ion defects, while the migration of these defects proceeds via the GeO4 tetrahedra.

Neutron diffraction and atomistic simulation studies of Mg doped apatite-type oxide ion conductors

In this paper, detailed studies of the effect of Mg doping in the apatite-type oxide ion conductor La9.33Si6O26 are reported. Mg is confirmed as an ambisite dopant, capable of substituting for both La and Si, depending on the starting composition. A large enhancement in the conductivity is observed for Si site substitution, with a reduction for substitution on the La site. Neutron powder diffraction studies show that in agreement with cation size expectations, an enlargement of the unit cell is observed on Mg substitution for Si, with a corresponding increase in the size of the tetrahedral sites. For Mg substitution on the La site, a contraction of the unit cell is observed, and the neutron diffraction results indicate that there is preferential occupancy of Mg on the La2 (1/3, 2/3, approximately 0.5) site. Atomistic simulation studies show significant local structural changes affecting the oxide ion channels in both cases. Mg doping on the Si site leads to a local expansion of the channels, while doping on the La site results in a large displacement of the silicate O4 site, such that it encroaches the oxide ion channels. The observed differences in conductivities are discussed with respect to these observations.

Effect of oxygen content on the 29Si NMR, Raman spectra and oxide ion conductivity of the apatite series, La8+xSr2−x(SiO4)6O2+x/2

29Si NMR data have been recorded for the apatite series La8+xSr2-x(SiO4)6O2+x/2 (0 < or = x < or = 1.0). For x = 0, a single NMR peak is observed at a chemical shift of approximately -77 ppm, while as the La : Sr ratio and hence interstitial oxygen content is increased, a second peak at a chemical shift of approximately -80 ppm is observed, which has been attributed to silicate groups neighbouring interstitial oxide ions. An increase in the intensity of this second peak is observed with increasing x, consistent with an increase in interstitial oxide ion content, and the data are used to estimate the level of interstitial oxide ions, and hence Frenkel-type disorder in these materials. The increase in second 29Si NMR peak intensity/interstitial oxide ion content is also shown to correlate with an increase in conductivity. The effect of interstitial oxygen content can also be studied by means of Raman spectroscopy, with a new mode at 360 cm(-1) appearing for samples with x > 0 in the symmetric bending mode energy region of the SiO4 group. The intensity of this mode increases with increasing oxygen content, yielding results comparable to those from the NMR studies, showing the complementarities of the two techniques.

Protonic defects and water incorporation in Si and Ge-based apatite ionic conductors

Apatite-type oxide-ion conductors have attracted considerable interest as potential fuel cell electrolytes. Atomistic modelling techniques have been used to investigate oxygen interstitial sites, protonic defects and water incorporation in three silicate and three germanate-based apatite-systems, namely La8Ba2(SiO4)6O2, La9.33(SiO4)6O2, La9.67(SiO4)6O2.5, La8Ba2(GeO4)6O2, La9.33(GeO4)6O2, and La9.67(GeO4)6O2.5. The simulation models reproduce the complex experimental structures for all of these systems. The interstitial defect simulations have examined the lowest energy configuration and confirm this site to be near the Si/GeO4 tetrahedra. The water incorporation calculations identify the O–H protonic site to be along the O4 oxygen channel as seen in naturally occurring hydroxy-apatites. The results also show more favourable and exothermic water incorporation energies for the germanate-based apatites. This is consistent with recent experimental work, which shows that Ge-apatites take up water more readily than the silicate analogues.

Apatite germanates doped with tungsten: synthesis, structure, and conductivity

High oxygen content apatite germanates, La(10)Ge(6-x)W(x)O(27+x), have been prepared by doping on the Ge site with W. In addition to increasing the oxygen content, this doping strategy is shown to result in stabilisation of the hexagonal lattice, and yield high conductivities. Structural studies of La(10)Ge(5.5)W(0.5)O(27.5) show that the interstitial oxygen sites are associated to a different degree with the Ge/WO(4) tetrahedra, leading to five coordinate Ge/W and significant disorder for the oxygen sites associated with these units. Raman spectroscopy studies suggest that in the case of the WO(5) units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO(5) units, thus not contributing significantly to the conduction process.

Raman spectroscopy studies of apatite-type germanate oxide ion conductors: correlation with interstitial oxide ion location and conduction

A Raman spectroscopy study of the apatite series La8+xBa2−x(GeO4)6O2+x/2 is presented. The results show the presence of a new Raman band appearing at ∼645 cm−1, whose intensity increases with increasing interstitial oxide ion content. This new band is also observed in samples containing cation vacancies, consistent with previous suggestions that the presence of cation vacancies enhances Frenkel-type defect formation. The fact that the new band is in the stretching region of the spectra, rather than the bending region as observed for the silicate analogues, is consistent with the interstitial oxide ions being more closely associated with the Ge. This band is attributed to the presence of interstitial oxide ions leading to the formation of five coordinate Ge, in agreement with recent neutron diffraction and modelling studies. From the observation of a reduction in the intensity of this band with increasing temperature, it is suggested that the activation energy for conduction in these apatite germanates is a combination of the energy to “free” the interstitial oxide ions from the five coordinate Ge, and the energy for their subsequent migration. The former process is ascribed to the observed reduction in Raman intensity with an activation energy of 0.32 ± 0.06 eV. Thus the higher activation energy for the germanate apatites over the related silicates can be ascribed to the defect trapping associated with the closer association of the interstitial oxide ion with the tetrahedra in the former.

Neutron diffraction structural study of the apatite-type oxide ion conductor, La8Y2Ge6O27: location of the interstitial oxide ion site

Apatite-type rare earth silicates/germanates have attracted considerable interest recently due to their high oxide ion conductivities. Despite evidence in support of a conduction mechanism involving interstitial oxide ions, the exact location of the interstitial oxide ion sites continues to attract controversy. In this paper we report a neutron diffraction structural study for the high oxygen excess compound, La8Y2Ge6O27. The structural model indicates that the oxide ions are located between the GeO4 tetrahedra, leading to significant localised distortions. These results, coupled with recent modelling studies, hence, support the conclusion that oxide ion migration proceeds via these tetrahedra.

An investigation of the high temperature reaction between the apatite oxide ion conductor La9.33Si6O26 and NH3

There is growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs). However, the possible reaction between the electrolyte and ammonia, and its potential effect on performance, has received little attention. In this paper, we report an investigation of the high temperature (950 °C) reaction of the apatite-type oxide ion conductor, La9.33Si6O26, and ammonia. The results show that such treatment leads to nitridation of the sample, with evidence for Si loss leading to an increased La:Si ratio in the final product. From neutron diffraction studies, the composition of the final product was determined to be La9.7(1)Si6O22.6(2)N2.7(2), with structural and 29Si NMR data suggesting the presence of N both within the apatite anion channels, and bonded to Si. An interesting feature of the structural studies are the relatively low atomic displacement parameters compared to the comparable apatite oxide systems, La9.33 + xSi6O26 + 3x/2, which can be related to the lack of interstitial anions in the oxynitride. Further studies on samples heated in ammonia at lower temperatures (600, 800 °C) suggest lower N incorporation, particularly for the 600 °C treatment. Considering the correlation of ionic conductivity, and interstitial oxide ion content in apatite systems, the data suggests the potential use of apatite-type electrolytes in SOFCs utilising NH3 as the fuel should be limited to temperatures <800 °C.

Battery and solid oxide fuel cell materials

This article reviews the literature reported during 2011 on battery and solid oxide fuel cell materials. The review focuses in particular on anode, cathode and electrolyte materials for metal ion batteries such as lithium, sodium and magnesium, and on oxide and proton conducting electrolytes as well as anode and cathode materials used in solid oxide fuel cell applications.