Combustion synthesis and properties of nanosize oxides : studies on solid oxide fuel cell materials

Abstract

Salient Features The low?temperature initiated, self?propagating, and gas?producing combustion process, developed and used for the preparation of technologically important oxide materials, has now been successfully extended for the synthesis of nanosized catalytic, magnetic, and solid oxide fuel cell (SOFC) materials. The salient features of the present investigations are:

Nanosized Catalytic Oxides Nanosized catalytic oxides such as titania and ceria–zirconia solid solutions have been prepared using a solution combustion process, and their catalytic activity has been investigated. Nanosized titania has been synthesized by the thermal decomposition/combustion of:

TiO(N?H?COO)?·2H?O, TiO(C?O?)·2H?O, redox mixtures containing TiO(NO?)? and CH?/ODH/TFTA in stoichiometric amounts.

Titania powders were also prepared by sol–gel and hydrothermal processes. Combustion?derived titania exhibits:

high surface area (78–100 m²/g), nanosized particles confirmed by XRD line broadening (8–15 nm), TEM (10–25 nm), surface area-derived particle size (18–25 nm).

Combustion?derived anatase titania shows better photocatalytic activity toward the degradation of methylene blue in aqueous dispersions than commercial (Merck) titania. The sol–gel derived titania, despite having a surface area of 100 m²/g, exhibits poor photocatalytic activity, likely due to its highly agglomerated nature (15.9 ?m). Loading platinum on combustion?derived titania did not significantly alter its catalytic activity.

Ceria–Zirconia (Ce???Zr?O?) Solid Solutions Nanosized ceria–zirconia solid solutions were synthesized using a single?step solution combustion process involving cerous nitrate, zirconyl nitrate, and carbohydrazide.

TEM and XRD line broadening confirm primary particle sizes of 6–11 nm. Surface area varies between 38–120 m²/g, decreasing with increasing zirconia content.

Temperature?programmed reduction (TPR) studies indicate:

Ce?.?Zr?.?O? releases oxygen in two steps:

320 °C (surface oxygen), 590 °C (bulk oxygen).

Sintering Behaviour of Nanosized Titania Combustion?derived titania could be sintered to 99% theoretical density at 1100 °C for 2 hours, while commercial titania reached only 88% theoretical density at 1400 °C. Sintered titania exhibits:

high hardness (10.5 GPa), fracture toughness (3.3 MPa·m½).

Nanosized Magnetic Oxides Nanosized barium and strontium hexaferrites were prepared by solution combustion using metal nitrates and fuels such as ODH, MDH, and MH. Due to their nanosize, superparamagnetic behaviour was observed, as evident from the Mössbauer spectrum of as?formed BaFe??O??. Magnetic properties:

BaFe??O?? (50 nm, 62 m²/g):

Ms = 84 emu/g, Hc = 4697 Oe.

SrFe??O?? (35 nm, 73 m²/g):

Ms = 90.9 emu/g, Hc = 6187 Oe.

Coercivity decreases with increasing particle size. The squareness ratio (Mr/Ms) is close to 0.5, the theoretical value for single?domain particles with uniaxial anisotropy.

Iron(III) Vanadate (FeVO?) FeVO? was prepared via solution combustion using ferric nitrate, NH?VO?, and ODH. Properties:

Weakly crystalline, surface area 30 m²/g. Particle size 20–60 nm. Room?temperature magnetic moment ?eff = 5.19 ?B, slightly lower than the spin?only value (5.92 ?B) for Fe³?. Obeys Curie–Weiss law between 25–300 K. Weiss constant (?) = –30 K indicates antiferromagnetic ordering. Activation energy from DC electrical conductivity: 0.60 eV.

Solid Oxide Fuel Cell (SOFC) Components The following SOFC components were synthesized via solution combustion using metal nitrates and ODH/TFTA:

Cathodes: La?.??Ba?.??MnO?, La?.??Sr?.??MnO?, Pr?.??Sr?.??MnO?, Nd?.??Sr?.??MnO? Anode: Ni/YSZ (30 vol% Ni) Electrolyte: 8 mol% YSZ Interconnect: La?.?Ca?.?Cr?.?Co?.?O?

All materials are:

nanosized, high surface area (9–40 m²/g), sinter?active, phase?pure. Properties:

ComponentConductivity at 1173 K (S/cm)Thermal Expansion Coefficient at 1173 K (×10?? K?¹)La?.??Sr?.??MnO?2.0212.63La?.??Ba?.??MnO?1.5610.91Pr?.??Sr?.??MnO?1.1410.20Nd?.??Sr?.??MnO?1.4010.7030 vol% Ni/YSZ4011.608 mol% YSZ0.1110.70La?.?Ca?.?Cr?.?Co?.?O?2311.65 These combustion?derived SOFC materials satisfy the stringent requirements of SOFC applications.

Future Scope The future of nanomaterials depends on our ability to tailor material properties by designing structures at the nanometer scale and developing cost?effective, environmentally friendly production methods at industrial scale. Solution?based chemical methods are gaining global importance due to:

versatility, simplicity, scalability, low cost.

Among these, the solution combustion method has proven especially effective for producing fine oxide powders. Future research directions include:

fabrication of complete SOFC cells and stacks, long?duration testing (up to 1000 hours), overcoming current challenges in interconnect sintering, particularly with lanthanum chromite, developing interconnect materials that sinter at lower temperatures and exhibit improved conductivity and thermal expansion compatibility.