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Chapter 6. Molybdate Based Ceramic Electrode Materials 179 (Na/K)xWO3 and the perovskite SrMoO3 have applications in electrochromics (the color can be changed by slight modifications to the material that change the valence, e.g. NaxWO3 [25, 26]), very high conductivity oxides (conductivities reaching 105 S/cm at room temperature [27-32], see also section 6.3.2), and catalysis (oxidation of hydrocarbons [33-35]). Ni-Mo and Ni-W alloy electrodes with fine micro- or nano-structures have been reported as highly electrocatalytic negative-electrodes for alkaline electrolysis cells, improving upon the performance of Ni electrodes [36-39]. In the perovskite SrMoO3, the Mo valence is 4+. One may consider the ionic radius and the Goldschmidt tolerance factor, Table 6-1 and equation (6.2). (6.2) If the dimensionless value of the tolerance factor t is close to 1, the ions are the ideal size for fitting into their cavities in the perovskite structure and the structure is expected to adopt the ideal cubic symmetry. Note that the tolerance factor is very close to 1 for SrMoO3 (Mo4+), as well as for SrTiO3 (Ti4+). This will be revisited in section 6.3.1. In double perovskites, Mo is also on the B site, but Mo6+ or Mo5+ is paired with a 2+ or 3+ valence ion respectively. The differences in charge and ionic radii result in 6+/2+ or 5+/3+ ordering and a superlattice structure (see Table 6-1 for the relevant ionic radii). Of the 2+ ions, Mg is the most stable against reduction, according to thermodynamics [40]. The mechanism for high electrochemical performance proposed by Huang et al for the double perovskite anode Sr2MgMoO6 is mixed conductivity. When the material is reduced, oxygen is lost from the crystal structure, resulting in the formation of oxygen vacancies and electrons, see equation (6.1). A fraction of the Mo6+ ions accept the electrons and become Mo5+. Both the 6+ and 2+ ions in the double perovskite must have coordination flexibility to allow release of oxygen from the crystal lattice [22]. The electromagnetic properties of double perovskites vary significantly depending on the partner 2+/3+ ion [41-43]. For double perovskites, a ―fitness factor‖ has also been proposed to more exactly discriminate crystal systems than the Goldschmidt tolerance factor [44]. Table 6-1. Ionic radii (Å) for VI-fold coordination [45]. ls = low spin; hs = high spin. Valence +4 0.65 0.66 +5 0.61 0.62 +6 0.59 0.60 Ti 0.605 Nb 0.68 0.64 Valence Mg +2 0.72 +3 Mn 0.67(ls) 0.83(hs) 0.58(ls) 0.645(hs) Ni 0.69 0.56(ls) 0.60(hs) Fe 0.61(ls) 0.78(hs) 0.55(ls) 0.645(hs) Co 0.65(ls) 0.745(hs) 0.545(ls) 0.61(hs) Mo WPDF Image | Electrolysis of CO2 and H2O
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