Nanoarchitectonics of Mesoporous CaFe₂O₄ Thin-Film Electrodes from Salt-Surfactant Lyotropic Liquid Crystalline Mesophases and Their OER Performance

Metal oxides of earth-abundant elements (such as Ca and Fe) are highly important for fabricating active electrodes for various electrochemical applications (such as electrocatalysis and photo-electrocatalysis). Here, we employed a molten-salt-assisted self-assembly process to fabricate CaFe2O4 thin-film electrodes on graphite rods. The roles of precursor type (nitrates and chlorides) and solvent (water and ethanol) have been addressed in the fabrication of the electrodes that are tested in oxygen evolution reaction (OER) in alkali media. The mesophases have an unusual orthorhombic structure that is likely transformed from a well-known 3D hexagonal phase by an elongation along the b-axis caused by the hydrolysis and condensation of the Fe(III) species in the lyotropic liquid crystalline media. Four sets of mesoporous electrodes with a high surface area are fabricated using nitrate and chloride precursors in aqueous media and nitrates in ethanol. The electrodes, fabricated from the chloride precursors, are not as porous as nitrates, but they display better performance in the OER. The electrodes, fabricated from ethanol solutions, outperform, are more robust, and display as low as 250, 342, and 642 mV overpotentials at 1, 10, and 100 mA/cm2 current densities with a Tafel slope of around 60 mV/dec. The electrode thickness has no role in the electrode performance and can be prepared as thin as tens of nanometers with good stability and OER performance.