La0.75Sr0.25Cr0.5Mn0.5O3 based anodes for Solid Oxide Fuel Cells

Recently, the study of environmentally friendly materials useful in conversion of energy has become an interesting topic in material science. Fuel cells are efficient devices that directly transform chemical energy of fuels into electrical energy, with low or no pollutants emission. In this research, the use of an alternative anode has been investigated. Ni-YSZ based state of the art materials, in fact, suffer of particle sintering and C-poisoning with C-containing gas mixtures. Moreover, the oxidation of Ni particles can contribute to destroy the device. La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) perovskite was first proposed by Tao et al. as an alternative anode.[1] Inspired by this work, LSCM is fully characterised and its electrochemical performance has been studied utilising both methane and hydrogen as fuel. The possibility of enhancing electrocatalytic proprieties has been tempted using impregnation and infiltration of 5wt% of nickel. LSCM is synthetized using Marcilly method, starting from metal precursors. [2] It is characterised by several techniques. XRD is used to understand the crystallographic structure and to evaluate the presence of all constituents inside the cell. TPR curves give information about the processes of reduction at different temperatures. It can be noted a single peak at 403°C related to the reduction of Mn4+ to Mn3+. The presence of this redox couple can be strategic to obtain a Mixed Ionic and Electronic Conductor (MIEC). The use of MIECs can enhance the performance and reduce the Triple Phase Boundary criticisms. The powder surface area is determined using BET technique. Data concerning structure and granulometry are obtained using SEM. XPS and EDX analysis permit the compositional study of the electrode and the evaluation of eventual segregation processes. To test electrochemical performance of LSCM, a symmetric cell is prepared based on electrolyte supported geometry, using YSZ as electrolyte. At first the ink optimization was carried out: different inks are developed and compared to have good adhesion between electrode and electrolyte. Optimized ink is deposited by tape casting technique. Further efforts have been invested in the electron collector. The electrochemical performance was tested utilising Electrochemical Impedance Spectroscopy. Both hydrogen and methane have been used as fuel and different working temperatures have been selected. To evaluate the activating effect of Ni, a composite anode Ni/LSCM made by impregnation has been studied. The slow amount of Ni chosen (5wt%) and the highly dispersed deposition obtained allows avoiding the disadvantages connected with its utilisation. Consistently, the catalytic activity and electronic conductivity of the anodic material are enhanced. [1] S. Tao, J.T.S. Irvine, Nat Mater.,2003, 2,320. [2] C. Marcilly, P. Courty, and B. Delmon, J. Am. Ceram. Soc., 1970, 53, 56