Improving synergism in Ni-prussian blue analog/CNT composite via coordination engineering for highly stable K+-ion capacitor†

Prussian blue analogs (PBAs), as redox-active metal–organic frameworks, offer great promise for hybrid supercapacitors but are hindered by low conductivity and limited cycling stability. In this work, we present a robust composite of nickel hexacyanoferrate (NiHCF) and carboxyl-functionalized multi-walled carbon nanotubes (CNTs), synthesized via a simple ultrasonication-driven coordination engineering method for K⁺-ion capacitor applications. The NiHCF/CNT composite, stabilised by coordination between the Ni²⁺/Fe³⁺ centers of NiHCF and the carboxylate groups on functionalized CNTs, achieves a high specific capacity of 223 C g⁻¹ at 1 A g⁻¹, significantly outperforming its pristine components. The composite exhibits exceptional electrochemical stability, with capacity increasing to ∼230% after 5000 cycles, attributed to the progressive activation of redox centers and improved electrolyte wettability. Density functional theory (DFT) calculations confirm enhanced electronic interactions and reduced bandgaps due to synergism between NiHCF and CNTs. The primary charge storage mechanism involves K⁺ ion (de)intercalation, as verified by ex situ P-XRD and EIS studies. A symmetric NiHCF/CNT//NiHCF/CNT supercapacitor device further demonstrates a high energy density of 18.07 Wh kg⁻¹ and a power density of 10 kW kg⁻¹, with 95.43% retention over 10 000 cycles. This study presents a rational design strategy focused on coordination bond formation between the metal centers of PBA and carboxyl groups on CNTs, which facilitates the effective compositization and enables enhanced charge storage capacity, exceptional cycling durability, and long-term performance in potassium-ion energy storage devices.