Avaliação de parâmetros sintéticos na preparação de eletrocatalisadores para OER utilizando a ZIF-67 como molde de síntese

Abstract

The growing demand for clean and sustainable energy sources drives the development of technologies that enable green hydrogen production. Hydrogen generation via water electrolysis is an environmentally friendly route; however, it requires high energy input, mainly due to the Oxygen Evolution Reaction (OER) semi-reaction. Therefore, the use of electrocatalysts is essential to make this reaction feasible. A variety of catalyst compositions, morphologies, and structures have already been tested, with metal oxides standing out as particularly promising. The use of Zeolitic Imidazolate Frameworks (ZIFs) for the synthesis of oxides has proven to be an effective route, in which modifications to the synthetic pathway result in distinct materials. In this context, this thesis reports the investigation of synthetic parameters (metals, precursor salts, and calcination temperature) in the production of materials using ZIF-67 as a synthesis template and the application of ZIF-67-derived materials as electrocatalysts for the OER. ZIF-67 was modified via impregnation with Fe, Ni, Mn, and Zn, using different counter-ions from the metal salts (sulfate, chloride, and nitrate), followed by calcination at 260, 350, and 500 °C in an oxidizing atmosphere to identify the most effective synthetic route. The materials were characterized using structural, morphological, thermal, and spectroscopic techniques, and evaluated for their electrocatalytic performance. The results revealed the presence of characteristic sulfate bands, both after impregnation and post-calcination (in the case of modification with sulfate salts), as well as bands attributed to ZIF-67 and the spinel structure of Co₃O₄ in all synthesized materials. Impregnation with sulfate salts proved to be the most beneficial for enhancing OER activity, while the nitrate-derived materials exhibited inferior performance. Additionally, lower calcination temperatures resulted in more favorable overpotentials. Electrochemical analyses indicated that the ZIF-67/Fe-S(350) catalyst exhibited the best electrocatalytic performance, with an overpotential of 237 mV to achieve a current density of 10 mA cm⁻², on eletrolyte KOH 1 mol/L (η = 289 mV for 100 mA cm⁻²), maintaining η = 269 mV after 500 cyclic voltammetry (CV) cycles.