Impact of the Metal–Organic Frameworks Polymorphism on the Electrocatalytic Properties of CeO2 toward Oxygen Evolution

Abstract

Hydrogen (H₂) is a viable alternative as a sustainable energy source, however, new highly efficient electrocatalysts for water splitting are still a research challenge. In this context, metal–organic frameworks (MOFs)-derived nanomaterials are prominent high-performance electrocatalysts for hydrogen production, especially in the oxygen evolution reaction (OER). Here, a new synthesis of two cerium oxide (CeO₂) electrocatalysts using Ce-succinates MOFs as templates is proposed. The cerium succinates polymorphs ([Ce₂(Succ)₃(H₂O)₂], Succ = succinate ligand) were obtained via hydrothermal reaction and room temperature crystallization, adopting monoclinic (C/2c) and triclinic (P1̅) crystalline structures, respectively, confirmed by X-ray diffraction (XRD). MOFs-Ce were also characterized by infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). CeO₂ electrocatalysts were obtained via MOFs-Ce calcination at 350 °C in air, and characterized by XRD with Rietveld refinement, HRTEM, SEM, FT-IR, and Raman spectroscopy, UV–vis spectroscopy, X-ray photoelectron spectroscopy. Electrocatalytic performances were investigated in KOH 1.0 M solution, and overpotentials were η = 326 mV (for CeO₂ (H) from monoclinic MOF-Ce) and η = 319 mV (for CeO₂ (RT) from the triclinic MOF-Ce) for a current density of 10 mAcm^–2. The Tafel slope values show the adsorption of intermediate oxygenated species as the rate-determining step. The high values of double-layer capacitance, the presence of oxygen vacancies, and low charge transfer resistance agree with the high performance in OER. Additionally, the materials were stable for up to 24 h, according to chronopotentiometry results.