In this work, using
electrochemical active Fe as an ion-exchange
element (attack side) and the Na
x
MnFe(CN)
6
slurry with a high solid content (MnHCF) as a template (defensive
side), a series of binary hexacyanoferrates are prepared via a simple
Mn/Fe ion-exchange process, in which Na
x
FeFe(CN)
6
(FeHCF) and solid solution Na
x
(FeMn)Fe(CN)
6
are concentrated on the shell and
the core, respectively. The proportions of the two structures are
mainly controlled by the competition between the ion-exchange rate
in the bulk material and dissolution-reprecipitation rate. Slowing
down the attacking rate, such as the use of a chelating agent complexed
with the attacker Fe, is advantageous to form a thermodynamically
metastable state with homogeneous distribution of elements since the
diffusion of Fe
2+
in the solid MnHCF is relatively fast.
The shell FeHCF could be adjusted by the dissolution-reprecipitation
rate, which is driven by the solubility difference. Adding the chelating
agent in the defensive side will promote the dissolution of MnHCF
and reprecipitation of FeHCF on the surface. Meanwhile, with the increase
of Fe sources, the thickness of the shell FeHCF increases, and correspondingly
the content of solid solution decreased due to FeHCF is more stable
than solid solutions in thermodynamics. Finally, such a design principle
in this case study could also be generalized to other ion-exchange
processes. Considering the difference of two components in solubility,
the larger difference can make the core/shell structure more clear
due to the enhancement of dissolution–reprecipitation route.