Sol-Gel Pechini synthesis of SOFC nanostructured materials

Introduction In last years, new energy devices working with renewable sources were taken into account in order to give different choice to fossil fuel-based systems. Solid Oxide Fuel Cells (SOFCs) are electrochemical systems which produce energy by exploiting the ionic conduction of some materials; their efficiency can reach about 80% by co-generation systems. The fuel can be H2 or hydrocarbons that can give hydrogen by internal reforming. Depending on the type of electrolytic material, SOFCs can be protonic or anionic. Tipically SOFC materials are mixed oxides and their synthesis plays a decisive role in cell performances. For this reason, high purity and dense materials have to be obtained. Sol-Gel Pechini (SGP) method yielded powders with nano-crystalline structure and allow to reach high purity products[1][2][3]. However, this technique requires time-prolonged high temperature treatments in order to eliminate all the organic compounds; as a consequence, the polymeric complex methods are considered time-wasting and energy-expensive. The application of microwaves in synthetic chemistry has worldwide gained acceptance as a promising cost effec-tive method for heating and sintering a variety of materials, as it offers specific advantages in term of speed, energy efficiency, process simplicity, finer microstructures, and lower environmental hazards compared to conventional heat-ing methods[4][5][6]. For this reasons, a Microwave Assisted Sol-Gel Pechini (MWA-SGP) method was set-up in this work for the synthesis of nanostructured oxides for protonic or anionic electrolyte materials for SOFCs.. In this work, examples of SGP and MWA-SGP syntheses are showed for BaCeO3 and LaGaO3 based materials. Experimental With the aim of obtaining high purity and nanostructured BaCe0.65Zr0.20Y0.15O3-? (BCZY) and La0.80Sr0.20Ga0.83Mg0.17O3-? (LSGM), SGP synthesis were carried out. The starting materials for BCZY synthesis were Ba(NO3)2 (Sigma-Aldrich, 99+%), Ce(NH4)2(NO3)6 (Alfa Aesar, 99.5%), ZrO(NO3)2o2.35 H2O (Alfa Aesar, 99.9%), Y(NO3)3o6H2O (Alfa Aesar, 99.9%) as metal precursors and EDTA (ethylenediaminetetraacetic acid, Sigma-Aldrich, 99+%) and ethylene glycol (EG, Aldrich, 99+%) as complexing and polymerizing agents, respectively. The water content of zirconium salt was determined by thermogravimetric analysis. Ammonium hydroxide (Riedel de Haën, NH3 33%) was added to promote the dissolution of EDTA in deionized water (Millipore, Billerica MA, USA). A 10 mol% barium excess was introduced in all of the compositions in order to avoid a barium substoichiometry due to BaO evaporation. The weighed amount of barium nitrate was firstly dissolved in de-ionized water at 80°C. Hence, an aqueous solution of EDTA and ammonia (pH = 9-10) was added dropwise to the barium solution (solution A). In a separate beaker the stoichiometric amounts of cerium, zirconium and yttrium nitrate salts were dissolved in deionized water (solution B). The solution B was then added dropwise to the solution A to avoid irreversible precipitation (figure 1). For LSGM the starting materials were La2O3 (Alfa Aesar, 99.9%), SrCO3 (Alfa Aesar, 99+%), MgO (Alfa Aesar, 99+%), Ga2O3 (Alfa Aesar, 99.9%) as metal precursors and citric acid (Sigma-Aldrich, 99.5 %) and ethylene glycol (EG, Aldrich, 99+%) as complexing and polymerizing agents, respectively. The preparation of nitrate solution and the addi-tion of citric acid and ethylene glycol were carried out as reported by Zhai et al. [7]. The condensation reaction and then the gel formation were carried out, for both BCZY and LSGM, by heating the solution between 50°C and 110°C for 144 h in a hot plate. Upon evaporation of solvent, a viscous, white gel was ob-tained, that gradually became a brown porous solid. The dried gel was py