At present, there is an increasing interest in the development of new highly efficient and low cost catalytic systems in the field of global energy issues.
Layered double hydroxides (LDHs) are lamellar compounds whose structure consists of positively charged brucite-like layers with interlayer anions balancing the positive charge. LDHs containing transition metals have been widely investigated since they are very promising materials for a large number of possible applications due to their versatility, tunable properties, wide range of compositions and low cost.
Recently, these materials are attracting much interest in the area of electrochemistry for applications such as batteries, supercapacitors, sensors, fuel cells and as efficient electrocatalysts for oxygen evolution reaction (OER). For all these applications a fundamental property is that the active material must be well adherent to the conductive support, thus guarantying the formation of a mechanically stable coating.
Our group has proposed and optimized an electrochemical approach, based on the electrochemical generation of hydroxides by cathodic reduction of nitrate ions, in order to obtain LDH films on any kind of conductive supports of any shape and dimension, including porous substrates and transparent or flexible electrodes. The features of the deposited LDH (amount, thickness and composition) and the adherence of the film to the electrode surface depend on the applied potential, the composition of the electrolytic solution and on the surface morphology of the electrode itself (1).
This contribution is aimed to investigate the performance of LDH films containing Cobalt or Nickel as bivalent cation and Iron and Alluminium as the trivalent one for two important energy applications, i.e. as supercapacitors and as electrocatalysts for OER. In particular our results show that LDHs containing Co as the bivalent metal can be considered good candidate for supercapacitors development, especially when the trivalent metal is Fe: actually for Co/Fe LDH the redox process significantly involves also the inner surface of the LDH, giving a peculiar electrochemical behaviour, where the capacitance has both Faradaic and charge separation origin. This guarantees a wider voltage window than the one displayed by the Al based LDH, making that material even more suitable for supercapacitor development.
As to OER electrocatalysts are concerned the best performances in terms of onset potential, current density at a fixed potential and TOF were obtained for iron-based LDHs both containing Ni or Co as the bivalent metal, our results proving that the presence of iron is crucial to significantly enhance the OER performances of LDHs (2).
(1) E. Scavetta, A. Mignani, D. Prandstraller, D. Tonelli, Chem.Mater. 19, 2007, 4523–4529.
(2) Y. Vlamidis, E. Scavetta, M. Gazzano, D. Tonelli, Electrochimica Acta, 188, 2016, 653-660