Electrochemical quartz crystal microbalance study of mass transport in thin film of a redox active Ni–Al–Cl layered double hydroxide

Films of redox active Ni–Al–Cl layered double hydroxides (LDHs) lost mass on oxidation and gained mass on reduction. The size of the mass changes depended on the composition and on the pH of the electrolyte solutions. In pH 8 potassium borate or phosphate buffers, the decreases in mass on oxidation were small. An apparent mass per mole of electrons transferred (MPE) of only 4–6 g/mol was obtained. This suggests that compensation for the positive charge generated by oxidation of the Ni(II) sites occurred by loss of protons from the LDH lattice accompanied by the loss of some water from the LDH interlayer spaces. The mass changes were two to three times larger when sodium was present in the buffers. For example, the MPE in a pH 8 potassium borate buffer containing 4 mM sodium increased to 19 g/mol. This was attributed to a contribution from the expulsion of sodium ions adsorbed in the LDH films to the charge neutralization mechanism. The MPE increased when the pH of the electrolyte increased. At pH 10 the mass loss was also no longer reversible. This was attributed to the decomposition of the Ni–Al LDH when it was subjected to potential scanning in the more basic solutions. In non-buffered potassium acetate or sodium acetate solutions, the MPEs were found to decrease with decrease in the scan speed, from 20 to 22 g/mol at 50 mV/s to as little as 6 g/mol at 5 mV/s in potassium acetate.