Cyano-Bridged Cu-Ni Coordination Polymer Nanoflakes and Their Thermal Conversion to Mixed Cu-Ni Oxides

Herein, we demonstrate the bottom-up synthesis of 2D cyano-bridged Cu-Ni coordination polymer (CP) nanoflakes through a controlled crystallization process and their conversion to Cu-Ni mixed oxides via a thermal treatment in air. The chelating effect of citrate anions effectively prevents the rapid coordination reaction between Cu²⁺ and K₂[Ni(CN)₄], resulting in the deceleration of the crystallization process of CPs. Specifically, with addition of trisodium citrate dehydrate, the number of nuclei formed at the early stage of the reaction is decreased. Less nuclei undergo a crystal growth by interacting with [Ni(CN)₄]^2-, leading to the formation of larger Cu-Ni CP nanoflakes. Following heat treatment in air, the -CN- groups present within the CP nanoflakes are removed and nanoporous Cu-Ni mixed oxide nanoflakes are generated. When tested as an electrode material for supercapacitors using a three-electrode system, the optimum Cu-Ni mixed oxide sample shows a maximum specific capacitance of 158 F g^-1 at a current density of 1 A g^-1. It is expected that the proposed method will be useful for the preparation of other types of 2D and 3D CPs as precursors for the creation of various nanoporous metal oxides.