Anion exchange membrane water electrolysis over superparamagnetic ferrites†

The oxygen evolution reaction (OER) is usually the bottleneck in water electrolysis due to its sluggish kinetics, resulting in increased costs in the production of green hydrogen. Therefore, there is a need for more efficient, stable, and ideally, critical-raw-material-free catalysts. To this end, we have synthesized nanosized spinel ferrites CoFe₂O₄, NiFe₂O₄, and ZnFe₂O₄, and a high-entropy spinel ferrite Zn_0.2Mn_0.2Ni_0.2Co_0.2Fe_2.2O₄ through a simple coprecipitation reaction in an automated reactor on a 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 spectroscopy studies. The activity of all spinel ferrites was tested for the OER through half-cell laboratory measurements and full-cell anion exchange membrane electrolysis (AEMEL), where Zn_0.2Mn_0.2Ni_0.2Co_0.2Fe_2.2O₄ showed the lowest overpotential of 432 mV at a current density of 10 mA cm⁻². 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 cm⁻². In addition, studies on the magnetic properties at room temperature revealed a largely superparamagnetic response of the prepared materials, indicating that quantum spin-exchange interactions facilitate oxygen electrochemistry. Computational calculations shed light on the superior catalytic activities of NiFe₂O₄ and Zn_0.2Mn_0.2Ni_0.2Co_0.2Fe_2.2O₄, 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 in electron transfer reactions.