Insights into Redox Reactions and Ionic Transfers in Nickel/Iron Layered Double Hydroxide in Potassium Hydroxide

The pseudocapacitive properties and the mixed cation/anion-exchanger nature of layered double hydroxides (LDHs) were highlighted via in situ electrochemical and physicochemical analyses conducted in a KOH solution (1 mol/L) on a nano-Ni/Fe-LDH film. The reversible Ni­(II)/Ni­(III) oxidation/reduction reaction was monitored using an electrochemical quartz crystal microbalance (EQCM). The coupling of powerful electrochemical methods, EQCM and ac-electrogravimetry, and physicochemical techniques, EDX, XPS and in situ XRD, provided results clarifying the corresponding mass transfer mechanisms. Ac-electrogravimetry identified the nature, in terms of molar mass, kinetics, and concentration variation of all reversibly transferred species associated with the redox reactions involving Ni and water molecules. This methodology demonstrated a reversible anion intercalation/deintercalation into/from the interlayer spacing, in agreement with the redox reactions leading to modulation of the electric charge of the layers. This phenomenon was also identified by in situ XRD. In addition, the variation of the pH at the LDH/electrolyte interface via a catalyzed oxygen evolution reaction (OER) leads to cation electro-adsorption/desorption onto/from the external LDH surfaces.