Abstract
Hybrid nanomaterials based on manganese, cobalt, and lanthanum oxides of different morphology and phase compositions were prepared using a facile single-step ultrasonic spray pyrolysis (USP) process and tested as electrocatalysts for oxygen reduction reaction (ORR). The structural and morphological characterizations were completed by XRD and SEM-EDS. Electrochemical performance was characterized by cyclic voltammetry and linear sweep voltammetry in a rotating disk electrode assembly. All synthesized materials were found electrocatalytically active for ORR in alkaline media. Two different manganese oxide states were incorporated into a Co3O4 matrix, δ-MnO2 at 500 and 600 °C and manganese (II,III) oxide-Mn3O4 at 800 °C. The difference in crystalline structure revealed flower-like nanosheets for birnessite-MnO2 and well-defined spherical nanoparticles for material based on Mn3O4. Electrochemical responses indicate that the ORR mechanism follows a preceding step of MnO2 reduction to MnOOH. The calculated number of electrons exchanged for the hybrid materials demonstrate a four-electron oxygen reduction pathway and high electrocatalytic activity towards ORR. The comparison of molar catalytic activities points out the importance of the composition and that the synergy of Co and Mn is superior to Co3O4/La2O3 and pristine Mn oxide. The results reveal that synthesized hybrid materials are promising electrocatalysts for ORR.
Highlights
Economic development and extensive use of fossil fuels has led to fast depletion of energy resources
The hybrid nanomaterials based on rare earth/transition metal oxides were synthesized with facile and cost-effective ultrasonic spray pyrolysis (USP) procedure, bearing in mind the methodology as follows
The other group of materials was prepared by the same synthesis procedure, but without manganese component—it was only based on cobalt and lanthanum oxide, in order to investigate the influence of USP-synthesized Mn oxide within hybrid oxide electrocatalysts
Summary
Economic development and extensive use of fossil fuels has led to fast depletion of energy resources. The development of clean energy storage and conversion devices, such as metal–air batteries, supercapacitors, fuel cells, and other renewable energy technologies, is in the main focus of numerous researchers and laboratories worldwide [1,2]. The efficiency of these energy devices mainly depends on the electrochemical oxygen reduction reaction (ORR) that occurs at the cathode side, as limiting reaction [3]. Poor rate capability, sluggish kinetics, and serious voltage gap of oxygen electrode reactions limit the performance of energy devices that rely on ORR [4,5,6]. To overcome the above-mentioned issues, lowering the amount of noble metals and exploring new catalytic materials for ORR have triggered extensive research interests
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