Synthesis, Structure and Electrochemical Properties of Ca Doped Lanthanum Manganite Powders As Supercapacitor Electrode Material
Authors: Binici Esra , Solak Nuri , Unal Ugur , Yilmaz Akkaya Ceren 
Published: May 2020

The rapidly increasing energy demand due to the depletion of fossil fuels and worsening environmental pollution have led to recent research interests on clean energy sources, energy conversion, and energy-storage systems. Therefore, supercapacitors also referred as electrochemical capacitors have gained a great deal of attention in industry as well as academia compared with the batteries and fuel cells because of the significant properties such as high power density produced by fast charge/discharge rate and long cycle life. Generally, conducting polymers, carbon-based materials and metal oxides have been extensively investigated to develop the most efficient electrode material for supercapacitors. Among the metal oxides, the perovskite oxides with ABO3 crystal structure has drawn lots of interest owing to exhibiting reversible faradaic surface reactions, their high intrinsic capacity, excellent thermal stability and also low cost among pseudocapacitive materials. On the other hand, LaMnO3 perovskite is a promising material in terms of accessibility, environmental friendliness, and also its capability to store charges through intercalation and reversible faradaic surface reactions in vacancies (both oxygen and cation).

Considering chemical states and type of defects are influenced by introduction of the dopant elements to main structure, in this study we have synthesized calcium doped LaMnO3 perovskites. The LaMnO3 (LMO) and La1-xCaxMnO3 (LCM-x) (x=0.1, 0.3, 0.5) nanocrystalline powders have prepared via the modified pechini method to obtain nanoparticles with high surface area, and the effect of calcium content on electrochemical properties studied with respect to pure LaMnO3. The crystalline structures of the nanopowders have characterized by X-ray diffraction (XRD, Bruker D8 Advance) and also elemental compositions of the samples have identified by X-ray fluorescence spectrometer (XRF, Bruker Tiger S8). The surface morphologies have determined with the field emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus). The Brunauer-Emmett-Teller method (BET, Micromeritics ASAP 2020) has been used for examination of the surface area. For surface electronic states of powders, X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha) have performed with Al K-alpha radiation source.

To evaluate the electrochemical properties of prepared samples, cyclic voltammetry (CV) and galvanostatic charge-discharge measurements (GCD) have carried out with three electrode system in 1M KOH electrolyte at the potential window of -0.1 to 1.4 V (vs RHE), and at different scan rates ranging from 2 to 100 mV. s-1. According to assessment of CV results, specific capacitance of LCM-0.3 has been found as 1114.3 F. g-1 nearly 5 times higher than that of pure LaMnO3 sample (238.1 F. g-1) at the scan rate of 2 mV. s-1. Compatible with the CV results, the LCM-0.3 sample have also performed the highest capacitance with 1499.2 F. g-1 at current density of 0.5 A. g-
1. The results have revealed that calcium doped lanthanum manganite based perovskites could be promising in the supercapacitor electrode materials area. Effect of chemical composition on structure, particle size, surface area and electrochemical performance will be discussed in detail.