Preparation of Sm<sub>0.5</sub>Sr<sub>0.5</sub>CoO<sub>3</sub>-La<sub>0.8</sub>Sr<sub>0.2</sub>Ga<sub>0.8</sub>Mg<sub>0.15</sub>Co<sub>0.05</sub>O<sub>3</sub> composite cathodes using wet-chemistry method and characterization of their properties