Comprehension of the Route for the Synthesis of Co/Fe LDHs via the Method of Coprecipitation with Varying pH

Abstract

Co/Fe-based layered double hydroxides (LDHs) are among the most promising materials for electrochemical applications, particularly in the development of energy storage devices, such as electrochemical capacitors. They have also been demonstrated to function as energy conversion catalysts in photoelectrochemical applications for CO2 conversion into valuable chemicals. Understanding the formation mechanisms of such compounds is therefore of prime interest for further controlling the chemical composition, structure, morphology, and/or reactivity of synthesized materials. In this study, a combination of X-ray diffraction, vibrational and absorption spectroscopies, as well as physical and chemical analyses were used to provide deep insight into the coprecipitation formation mechanisms of Co/Fe-based LDHs under high supersaturation conditions. This procedure consists of adding an alkaline aqueous solution (2.80 M NaOH and 0.78 M Na2CO3) into a cationic solution (0.15 M CoII and 0.05 M FeIII) and varying the pH until the desired pH value is reached. Beginning at pH 2, pH increases induce precipitation of FeIII as ferrihydrite, which is the pristine reactional intermediate. From pH > 2, CoII sorption on ferrihydrite promotes a redox reaction between FeIII of ferrihydrite and the sorbed CoII. The crystallinity of the poorly crystalized ferrihydrite progressively decreases with increasing pH. The combination of such a phenomenon with the hydrolysis of both the sorbed CoIII and free CoII generates pristine hydroxylated FeII/CoIII LDHs at pH 7. Above pH 7, free CoII hydrolysis proceeds, which is responsible for the local dissolution of pristine LDHs and their reprecipitation and then 3D organization into CoII4FeII2CoIII2 LDHs. The progressive incorporation of CoII into the LDH structure is accountable for two phenomena: decreased coulombic attraction between the positive surface-charge sites and the interlayer anions and, concomitantly, the relative redox potential evolution of the redox species, such as when FeII is re-oxidized to FeIII, while CoIII is re-reduced to CoII, returning to a CoII6FeIII2 LDH. The nature of the interlamellar species (OH−, HCO3−, CO32− and NO3−) depends on their mobility and the speciation of anions in response to changing pH.