La0.1Ca0.9MnO3/Co3O4 for oxygen reduction and evolution reactions (ORER) in alkaline electrolyte

Rare-Earth/Manganese Oxide-Based Composites Materials for Electrochemical Oxygen Reduction Reaction

The main objective of this research was a systematic development of advanced micro/nanostructured materials based on the most used metal-oxides for ORR and metal-oxides with an extremely low-loading of Pt for comparison. Hybrid composites compared were: MnO2, La2O3, mixed lanthanum manganese oxides (LMO), and mixed lanthanum manganese oxides with reduced platinum load (LMO-Pt). The influence of the reduced amount of noble metal, as well as single oxide activity toward ORR, was analyzed. The complete electrochemical performance of the hybrid materials has been performed by means of CV, LSV, and EIS. It was shown that all synthesized catalytic materials were ORR-active with noticeable reduction currents in O2 saturated 0.1 M KOH. The ORR behavior indicated that the La2O3 electrode has a different mechanism than the other tested electrode materials (MnO2, LMO, and LMO-Pt). The EIS results have revealed that the ORR reaction is of a mixed character, being electrochemically and diffusion controlled. Even more, diffusion is of mixed character due to transport of O2 molecules and the chemical reaction of oxygen reduction. O2 diffusion was shown to be the dominant process for MnO2, LMO, and LMO-Pt electrolytic materials, while chemical reaction is the dominant process for La2O3 electrolytic materials.

Spray-Pyrolytic Tunable Structures of Mn Oxides-Based Composites for Electrocatalytic Activity Improvement in Oxygen Reduction

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.

Gram-Scale Synthesis of Bimetallic ZIFs and Their Thermal Conversion to Nanoporous Carbon Materials

The hybrid metal-organic frameworks (MOFs) with different Zn²⁺/Co²⁺ ratios are synthesized at room temperature with deionized water as the solvent. This use of deionized water can increase the yield of hybrid MOFs (up to 65–70%). After the pyrolysis, the obtained nanoporous carbons (NPCs) show a decrease in the surface area, in which the highest surface area is 655 m² g⁻¹. The as-prepared NPCs are subjected to activation with KOH in order to increase their surface area and convert cobalt nanoparticles (Co NPs) to Co oxides. These activated carbons are applied to electrical double-layer capacitors (EDLCs) and pseudocapacitors due to the presence of CoO and Co₃O₄ nanoparticles in the carbon framework, leading to significantly enhanced specific capacitance as compared to that of pristine NPCs. This synthetic method can be utilized in future research to enhance pseudocapacitance further while maintaining the maximum surface area of the carbon materials.

NiFeOx as a Bifunctional Electrocatalyst for Oxygen Reduction (OR) and Evolution (OE) Reaction in Alkaline Media

This article reports the two-step synthesis of NiFeOx nanomaterials and their characterization and bifunctional electrocatalytic activity measurements in alkaline electrolyte for metal-air batteries. The samples were mostly in layered double hydroxide at the initial temperature, but upon heat treatment, they were converted to NiFe2O4 phases. The electrochemical behaviour of the different samples was studied by linear sweep voltammetry and cyclic voltammetry on the glassy carbon electrode. The OER catalyst activity was observed for low mass loadings (0.125 mg cm−2), whereas high catalyst loading exhibited the best performance on the ORR side. The sample heat-treated at 250 °C delivered the highest bi-functional oxygen evolution and reduction reaction activity (OER/ORR) thanks to its thin-holey nanosheet-like structure with higher nickel oxidation state at 250 °C. This work further helps to develop low-cost electrocatalyst development for metal-air batteries.