Novel mixed nickel/cobalt hexacyanoferrate microcubes with synergistic effects for aqueous hybrid supercapacitors†

Combining different metals in coordination compounds is an efficient strategy to improve their various properties. Herein, mixed nickel–cobalt hexacyanoferrate (Ni_xCo_yHCF) microcubes of varying x : y molar ratios are synthesized via a co-precipitation route and comprehensively characterized to study their material and electrochemical properties. Ni_xCo_yHCF microcubes display the battery-type electrochemical energy storage mechanism in aqueous electrolytes. Among the samples, Ni₁Co₂HCF microcubes deliver the highest specific capacity of 134 mA h g⁻¹ (1068 F g⁻¹) at a specific current of 1 A g⁻¹. This significant enhancement in the capacity indicates the synergistic and cooperative effects between Ni and Co sites in Ni₁Co₂HCF microcubes. The asymmetric supercapacitor device assembled with Ni₁Co₂HCF microcubes delivers an excellent energy density of 74.4 μW h cm⁻² at a power density of 750 μW cm⁻² and retains 87.7% of its initial capacity after 2000 cycles at a current density of 5 mA cm⁻², indicating its robust structural integrity and electrochemical durability. This study highlights the promising potential of mixed-metal hexacyanoferrates as high-performance electrodes for aqueous supercapacitors.