Porous cerium-zeolite bifunctional ORR/OER electrocatalysts in alkaline media

Zeolite ZSM-5 and zeolite β were modified by aqueous ion exchange with cerium and then calcined (cal) to obtain Ce-ZSM-5, Ce-ZSM-5 cal, Ce-β, and Ce-β cal electrocatalysts. X-ray powder diffraction analysis, Fourier Ttransform infrared spectroscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray photoelectron spectroscopy, fluorescence spectroscopy, and Brunauer-Emmett-Teller method revealed changes in the structure and porosity of zeolites upon calcination. Voltammetry, chronoamperometry, and electrochemical impedance spectroscopy were used for testing four zeolites for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline media. OER starts the earliest at Ce-β cal with onset overpotential 50, 70, and 110 mV lower than Ce-ZSM-5 cal, Ce-ZSM-5, and Ce-β. Ce-β cal further showed the lowest OER Tafel slope (114 mV dec⁻¹). Consequently, the highest OER current density was recorded in the case of Ce-β cal, followed by Ce-β, Ce-ZSM-5 cal, and Ce-ZSM-5. Regarding ORR, Ce-ZSM-5 cal showed the lowest Tafel slope (70 mV dec⁻¹) with the highest current densities that remained constant during the chronoamperometry test with a negligible decrease of 4%. It could be concluded that calcined forms exhibit better performance for OER and OER than their parent, non-calcined forms due to more active sites available for OER/ORR and decreased charge-transfer resistance.