Microstructure And Electrochemical Properties Of La₂NiO_4+δ Porous Electrode

La2NiO4+δ-based layer-structured compounds have attracted increasing attention as a novel candidate material for the cathode of intermediate temperature solid oxide fuel cells (SOFCs) because of their electronic-ionic mixed conducting characteristics. Superfine La2NiO4+δpowder has been derived from a ployaminocarboxylate complex precursor with diethylenetriaminepentaacetic acid (H5DTPA) as ligand. Electrochemical cells with a three-electrode configuration of La2Ni04+δ/Ce0.8Sm0.2O1.9/Pt have been fabricated by the screen-printing technique using the superfine La2NiO4+δpowder as the starting powder of the slurry. The electrochemical properties of porous La2NiO4+δelectrodes on dense substrates of the electrolyte have been investigated based on the electrochemical cells with respect to preparation conditions and the resulting microstructures.Electrochemical impedance spectroscopy, Chronopotentiometry and Tafel plot analysis were employed to investigate the electrochemical properties of La2Ni04+δelectrodes. The electrochemical parameters, such as polarization resistance, cathodic overpotential and exchange current density, were analyzed with respect to the composition of the slurry and sintering temperature of the electrodes. It was found that the electrode polarization mainly originated from the charge transfer process at the electrode/electrolyte interface and the oxygen exchange process on the surface of the porous electrodes, with the latter process dominating the overall electrode reaction.The electrocatalytic activity of the porous La2NiO4+δelectrodes has been evaluated in relation to their microstructural features, including the grain size, connection between the grains, the porosity of the electrode and the adhesion of the electrode to the electrolyte. A close dependence of the electrocatalytic activity on the microstructure was confirmed. It was found that a tight adhesion between the electrode and the electrolyte favored the interfacial charge transfer process, while fine grain size together with reasonable connectivity between the grains benefited the surface oxygen exchange process. From a microstructural viewpoint, the grain size and connectivity between the grains acted as the key contributing factors to the electrocatalytic activity of the porous electrodes. Adopting the superfine powder to fabricate the electrode was suggested to be favorable to modifying the microstructure and improving the electrocatalytic activity.Employing the powder clacined at 900℃as the starting powder, adopting the slurry with a solid content of 70% for screen printing and sintering the electrode at 950℃was determined to be the optimum preparation conditions based on comparative research. At 800℃, the electrode prepared under the conditions exhibited satisfactory electrochemical properties compared with literal results, showing a polarization resistance of 0.38Q·cm2, an overpotential of 103mV at a current density of 200mA·cm-2 and an exchange current density of 102mA·cm-2.