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

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 CO 2 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 Na 2 CO 3 ) into a cationic solution (0.15 M Co II and 0.05 M Fe III ) and varying the pH until the desired pH value is reached. Beginning at pH 2, pH increases induce precipitation of Fe III as ferrihydrite, which is the pristine reactional intermediate. From pH > 2, Co II sorption on ferrihydrite promotes a redox reaction between Fe III of ferrihydrite and the sorbed Co II . 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 Co III and free Co II generates pristine hydroxylated Fe II /Co III LDHs at pH 7. Above pH 7, free Co II hydrolysis proceeds, which is responsible for the local dissolution of pristine LDHs and their reprecipitation and then 3D organization into Co II 4 Fe II 2 Co III 2 LDHs. The progressive incorporation of Co II 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 Fe II is re-oxidized to Fe III , while Co III is re-reduced to Co II , returning to a Co II 6 Fe III 2 LDH. The nature of the interlamellar species (OH - , HCO 3 - , CO 3 2- and NO 3 - ) depends on their mobility and the speciation of anions in response to changing pH.