Anion Exchange Membrane Water Electrolysis over Superparamagnetic Ferrites

Abstract

Oxygen evolution reaction (OER) is usually the bottleneck in water electrolysis due to its sluggish kinetics, there is a need of more efficient and stable catalysts, nowadays resulting in increased costs in the production of green hydrogen. Ideally, these improved catalysts should also be critical-raw-material-free. We have synthesized nanosized spinel ferrites CoFe2O4, NiFe2O4, ZnFe2O4, and a high-entropy spinel ferrite Zn0.2Mn0.2Ni0.2Co0.2Fe2.2O4 by simple coprecipitation reaction in an automated reactor on gram scale. The powder X-ray diffraction and transmission electron microscopy studies revealed crystallite sizes of 2035 nm. Insight into the oxidation states and cation distribution in the mixed spinel systems was gained through X-ray photoelectron and Mössbauer spectroscopies. Studies on the magnetic properties at room temperature revealed largely superparamagnetic response of the prepared materials, indicating that quantum spin exchange interactions catalyse oxygen electrochemistry. The activity of all spinel ferrites was tested for OER in half-cell laboratory measurements as well as full-cell anion exchange membrane electrolyzer (AEMEL), where Zn0.2Mn0.2Ni0.2Co0.2Fe2.2O4 shows the lower overpotential of 432 mV at the current density of10 mA/cm2. All the synthesized ferrites demonstrated good stability up to 20 h, with NiFe2O4 being the most active in high current density experiments up to 2 A/cm2. Computational calculations shed light on the superior catalytic activities of NiFe2O4 and Zn0.2Mn0.2Ni0.2Co0.2Fe2.2O4, these are the two strongly correlated oxides that exhibit the highest magnetization and the smallest band gaps, corroborating the recent principles determining the activity of magnetic oxides for electron transfer reactions.