Studies on the Electrocatalytic Activity, Electrical Conductivity, and Oxygen Transport Properties of La0.5Sr0.5Co0.8Fe0.2-xBxO3- [B = Cu, Ni, & Al; x = 0 – 0.2] Perovskite Oxides

Abstract

A series of oxygen-deficient Co-rich La0.5Sr0.5Co0.8Fe0.2-xBxO3-δ (B = Cu, Ni, & Al: x = 0 - 0.2) perovskite oxides were synthesized and systematically characterized to study the electrocatalytic activity, electrical conductivity, and oxygen transport properties. The characterization focused on crystal structure, oxygen non-stoichiometry, surface morphology, oxygen evolution reaction (OER), and electrical transport behaviour. XRD study revealed that all studied powder oxides crystallized in a single phasic cubic structure. B-site cation substitution enhanced oxygen non-stoichiometry (δ) in the studied oxides. The change of surface oxygen vacancies and oxidation state of the B-site with substitution favours hand in hand with the weighty enrichment of OER activity, electrical conductivity, and oxygen transport properties The electrochemical study of La0.5Sr0.5Co0.8Fe0.2-xCuxO3-δ (x = 0 - 0.2) perovskite oxides using KOH electrolyte indicated a decrease in OER overpotential and the Tafel slope with increasing Cu substitution level which is associated with an increase in the mass and specific activity. The study confirms the attainment of the lowest OER overpotential and the Tafel slope of 500 mV and 60.5 mV dec-1 for La0.5Sr0.5Co0.8Cu0.2O3-δ. The enhancement in electrocatalytic activity is correlated to the increase in oxygen non-stoichiometry of the oxides with increasing substitution levels. The study predicts La0.5Sr0.5Co0.8Cu0.2O3-δ may have potential as an OER electrocatalyst for water-splitting applications. The high-temperature electrical transport behavior study on the dense sample indicated an enhancement of electrical conductivity, oxygen diffusion coefficient (Dchem), and surface charge transfer coefficient (Kchem) with increasing Cu-substitution levels. Enhancement in electrical transport characteristics is correlated to the defects in the sample and grain size. Among the investigated oxides, the highest conductivity value was observed for the La0.5Sr0.5Co0.8Cu0.2O3-δ oxide, which is near about 1250 S cm-1. The La0.5Sr0.5Co0.8Cu0.2O3-δ oxide was found to show superior Dchem (1.1×10-4 cm2 s-1) and Kchem (8.5×10-5 cm s-1) values at 900 ℃ compared to the other compositions. Further, an inclusive study focused on La0.5Sr0.5Co0.8Fe0.2-xNixO3-δ (x = 0 - 0.2) perovskite oxides investigate their electrocatalytic activity, electrical conductivity, Dchem, and Kchem. The electrocatalytic activity of the catalysts in an alkaline solution for the OER was evaluated. The results showed that increasing the Ni-substitution level reduced the OER overpotential and the Tafel slope. The observed behaviour is associated with an increase in mass and specific activity. Significantly, the La0.5Sr0.5Co0.8Ni0.2O3-δ oxide displayed a lower overpotential and the Tafel slope of 508 mV and 54.9 mV dec-1 among the studied oxides. This result was attributed to the increased oxygen non-stoichiometry, which generated more active sites for the electrocatalyst. Furthermore, a study of the electrical transport behaviour of the studied oxides revealed an increase in electrical conductivity, Dchem, and Kchem with increasing Ni-substitution levels. The enrichment in electrical transport properties can be ascribed to defects within the oxide and grain size variation. The La0.5Sr0.5Co0.8Ni0.2O3-δ oxide exhibited the highest conductivity value of 1595 S cm-1 among the studied oxides. The La0.5Sr0.5Co0.8Ni0.2O3-δ oxide outperformed the other oxides in terms of Dchem (1.50 × 10-4 cm2 s-1) and Kchem (2.34 × 10-5 cm s-1) values at 900 °C. Similarly, the electrocatalytic activity, electrical conductivity, Dchem, and Kchem of La0.5Sr0.5Co0.8Fe0.2 xAlxO3-δ (x = 0 - 0.2) perovskite oxides were studied thoroughly. The electrocatalytic activity of La0.5Sr0.5Co0.8Fe0.2-xAlxO3-δ perovskite oxides with KOH electrolyte reduced OER verpotential and the Tafel slope. Moreover, the Al substituted level increased, which enhanced the mass and specific activity. A lower overpotential and the Tafel slope of 490 mV and 50.6 mV dec-1 are displayed by the La0.5Sr0.5Co0.8Fe0.1Al0.1O3-δ electrocatalyst than all other investigated oxides. The improved activity is likely ascribed to the combination of oxygen non-stoichiometry and larger surface area, which increases the active sites of the electrocatalyst for OER. The electrical transport properties of the prepared oxides were studied, and it was found that electrical conductivity and oxygen transport parameters enhanced with increasing Al-substitution levels. The electrical conductivity and oxygen transport parameters, specifically Dchem and Kchem depend strongly on the oxygen non-stoichiometry and change in grain size in the samples. The La0.5Sr0.5Co0.8Fe0.1Al0.1O3-δ oxide exhibited the lowest conductivity value of 690 S cm-1. On the other hand, La0.5Sr0.5Co0.8Fe0.1Al0.1O3-δ oxide showed greater Dchem (5.5×10-5 cm2 s-1) and Kchem (2.32×10-5 cm s-1) value among the studied oxides at 900 °C.