Abstract
Layered double hydroxides (LDHs) are anionic clays which have found applications in a wide range of fields, including electrochemistry. In such a case, to display good performances they should possess electrical conductivity which can be ensured by the presence of metals able to give reversible redox reactions in a proper potential window. The metal centers can act as redox mediators to catalyze reactions for which the required overpotential is too high, and this is a key aspect for the development of processes and devices where the control of charge transfer reactions plays an important role. In order to act as redox mediator, a material can be present in solution or supported on a conductive support. The most commonly used methods to synthesize LDHs, referring both to bulk synthesis and in situ growth methods, which allow for the direct modification of conductive supports, are here summarized. In addition, the most widely used techniques to characterize the LDHs structure and morphology are also reported, since their electrochemical performance is strictly related to these features. Finally, some electrocatalytic applications of LDHs, when synthesized as nanomaterials, are discussed considering those related to sensing, oxygen evolution reaction, and other energy issues.
Highlights
Layered double hydroxides (LDHs) are lamellar compounds having molecular formula [M(II)1−x M(III)x (OH)2 ]x+ (An− x/n )·mH2 O, where x ranges from 0.22 to 0.33, M is a metal and An− is a n− valent anion
The aim of this paper is to summarize the most commonly used methods to synthesize and characterize LDHs and to present some of the most outstanding electrocatalytic applications of LDHs, when synthesized as nanomaterials, considering in particular those related to sensing, Oxygen Evolution Reaction (OER), and in general to the energy issues
The presence of a cation that can undergo reversible redox processes is mandatory in such applications because it acts as a redox mediator and provides sufficient electrical conductivity to the material by promoting electron hopping
Summary
Layered double hydroxides (LDHs) are lamellar compounds having molecular formula [M(II)1−x M(III)x (OH)2 ]x+ (An− x/n )·mH2 O, where x ranges from 0.22 to 0.33, M is a metal and An− is a n− valent anion. LDHs, and that is possible when transition metals (such as Co and Ni) are present in the brucitic layers In such a case, an inner reaction involving the redox active metal centers of the clay occurs within a proper potential window. The most commonly used is the chemical route, which involves bulk synthesis, and other synthetic approaches are available, among which the electrochemical deposition This strategy ensures the obtainment of LDH films on any kind of conductive supports of any shape and dimension, including porous substrates and transparent or flexible electrodes. As stated above, when redox-active metals are present in the brucitic layers, i.e., a reversible electrochemical process involving these cationic sites can occur within an appropriate potential range, LDHs materials can act as redox mediators to electrocatalyze the oxidation of many compounds. The aim of this paper is to summarize the most commonly used methods to synthesize and characterize LDHs and to present some of the most outstanding electrocatalytic applications of LDHs, when synthesized as nanomaterials, considering in particular those related to sensing, OER, and in general to the energy issues
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