Direct electrochemical reduction of cellular hematite-based ceramics

Iron oxides are naturally abundant and versatile materials with broad range of applications including electrodes, catalysts, gas sensors, adsorbents, etc.. Many of the mentioned applications require suitable microstructures... [ view full abstract ]

Iron oxides are naturally abundant and versatile materials with broad range of applications including electrodes, catalysts, gas sensors, adsorbents, etc.. Many of the mentioned applications require suitable microstructures with high porosity and specific surface area, and permeability to fluids. In this scope, hematite-based ceramics, Fe_2-xAl_xO₃, were processed to obtain designed cellular microstructures in order to be further used as cathodes in electrochemical reactions. Three different nominal compositions of Fe_2-xAl_xO₃ (x = 0.10, 0.20 and 0.30) were processed by the emulsification of ceramic suspensions with liquid paraffin, using Fe₂O₃ and Al₂O₃ precursors. Reactive firing conditions between 1000 °C and 1400 °C were adjusted to design a cellular ceramic structure, possessing appropriate phase composition, percolation of porosity and pores interconnectivity. XRD/SEM/EDS studies were performed for the compositional and microstructural characterization of the cellular ceramics, while Archimedes method and impedance spectroscopy analysis were used for the investigation of porosity and percolation described by a constriction factor, respectively. Despite the segregation of a secondary phase on increasing the Al₂O₃ content, high porosities (37 – 66%) and suitable interconnectivity of cells were obtained. Cellular cathodes were used for the electrochemical tests in alkaline conditions (NaOH, 10 M) at 90 °C, with a platinum wire as anode and a reference electrode of Hg|HgO|NaOH (6 M). Combined XRD/SEM/EDS studies revealed the formation of iron (Fe⁰) crystals due to the reduction of iron, with Faradaic efficiency close to 100%. The impact of alumina content and contribution to the electroreduction mechanism was assessed. The results convincingly demonstrated that in-situ cathodic reduction of cellular iron oxide-based ceramics represents a promising approach for zero-valent iron (ZVI, Fe⁰) production for pollutant degradation and other applications.