Electrocatalytic Oxygen Evolution by Cobalt-Containing Fe<sub>2</sub>O<sub>3</sub>-Based Nanoparticles and Nanocomposites
João Pedro Gonçalves de Souza Soares, Annaíres de Almeida Lourenço, Rafael A. Raimundo, Marco A. Morales, Rodolfo Bezerra da Silva, André Luiz Menezes de Oliveira, Daniel A. Macedo, Fausthon Fred da Silva
Abstract
High Resolution Image Download MS PowerPoint Slide The major challenge for hydrogen (H 2 ) production via water electrolysis in an alkaline medium is the anodic reaction called oxygen evolution reaction (OER), with four electron transfer steps, slow reaction kinetics, and high overpotentials. However, metal–organic frameworks (MOFs)-based metal-oxide nanoparticles with cobalt and iron have emerged in the literature as excellent OER electrocatalysts. As a result, this work focused on developing cobalt-containing iron-oxide nanoparticles obtained from iron(II)-etched ZIF-67 in methanol or ethanol. The experiments in ethanol resulted in a nanocomposite (Co:α-Fe 2 O 3 /γ-Fe 2 O 3 ), and the sample obtained in methanol presented a single phase (Co:α-Fe 2 O 3 ). The structure and chemical composition of the electrocatalysts were elucidated using X-ray powder diffraction (XRD) with Rietveld refinement, vibrational spectroscopy (infrared and Raman), Mössbauer spectroscopy, and magnetic measurements. Electron microscopy images (TEM and SEM) for both samples showed irregular-shaped agglomerates. The electrochemical measurements for OER in an alkaline medium (1.0 mol/L KOH) showed overpotentials of 297 mV and 295 mV for Co:α-Fe 2 O 3 /γ-Fe 2 O 3 and Co:α-Fe 2 O 3 electrocatalysts, respectively, for a current density of 10 mA cm –2 . The Tafel slope values for both catalysts suggest that the adsorption of intermediate species predominantly steps the electrocatalytic process. The materials were also stable for up to 15 h. After stability tests, both materials showed a reduction in overpotential, due to a surface transformation to a mixed-valence spinel structure observed by Raman spectroscopy, boosting the OER activity.