Electrocatalysts Prepared by Galvanic Replacement

Galvanic replacement is the spontaneous replacement of surface layers of a metal, M, by a more noble metal, M-noble, when the former is treated with a solution containing the latter in ionic form, according to the general replacement reaction: nM + mM(noble) (n+) -> nM(m+) + mM(noble). The reaction is driven by the difference in the equilibrium potential of the two metal/ metal ion redox couples and, to avoid parasitic cathodic processes such as oxygen reduction and (in some cases) hydrogen evolution too, both oxygen levels and the pH must be optimized. The resulting bimetallic material can in principle have a M-noble-rich shell and M-rich core (denoted as Mnoble(M)) leading to a possible decrease in noble metal loading and the modification of its properties by the underlying metal M. This paper reviews a number of bimetallic or ternary electrocatalytic materials prepared by galvanic replacement for fuel cell, electrolysis and electrosynthesis reactions. These include oxygen reduction, methanol, formic acid and ethanol oxidation, hydrogen evolution and oxidation, oxygen evolution, borohydride oxidation, and halide reduction. Methods for depositing the precursor metal M on the support material (electrodeposition, electroless deposition, photodeposition) as well as the various options for the support are also reviewed.