Binder-free pinecone-like NiSe/MnSe nanostructure arrays via electrochemically controlled one-step synthesis towards high-performance semi-solid-state supercapacitor

Bimetallic selenides with nanostructures have attracted widespread interest to improve the electrochemical properties of supercapacitors due to their intrinsic properties such as high energy storage capacity, stability, and reliability. Herein, we report the novel synthesis of binder-free pinecone-like nanostructure arrays of Ni₂MnSe₄ on nickel foam via a facile electrodeposition technique. The electrode materials with various stoichiometric ratios of Ni:Mn as Ni_xMn_3-xSe₄ (x = 1, 2, and 2.5) are synthesized to study the effect on their surface morphology and electrochemical properties. Ni₂MnSe₄ is optimized with pinecone-like structure arrays formed by intraconnected rough nanosheets (two-dimensional structure) with interconnected microstructures (pinecone structure), which further offers continuous channels for rapid electron/ion transfer and accelerates the diffusion kinetics of the counter ions, thus leading to improved capacitive property of the electrode material. The as-prepared Ni₂MnSe₄ electrode exhibits the areal capacitance value of 2160 mF cm⁻² at 4 mA cm⁻² with superior cycling stability (92 % of capacitance retention over 10,000 cycles). Additionally, the Ni₂MnSe₄ is utilized as a positive electrode for a semi-solid-state hybrid supercapacitor device (SHSC). The assembled SHSC delivers the energy and power density values of 0.15 mWh cm⁻² and 15.51 mW cm⁻², respectively with excellent prolonged cycling stability.