Superfast ice crystal-assisted synthesis of NiFe2O4 and ZnFe2O4 nanostructures for flexible high-energy density asymmetric supercapacitors

Highlights: • Superfast ice crystal-assisted synthesis of metal ferrite nanostructures. ii.The unique nanostructures show the presence of an abundance of voids with high surface areas. iii.The NiFe₂O₄ NP electrode shows a maximum specific capacity of 1403 C g⁻¹. iv.The ZnFe₂O₄ NR electrode shows a maximum specific capacity of 1005 C g⁻¹. • The asymmetric supercapacitor exhibits a high energy density of 99.55 Wh kg⁻¹. -- Abstract: An ultrafast (∼20 min), inexpensive and scalable ice crystal-assisted precipitation approach was developed to synthesize unique self-assemblies of nickel ferrite nanoparticles (NiFe₂O₄ NPs) and zinc ferrite nanorods (ZnFe₂O₄ NRs) that contain plenty of porous voids for supercapacitor applications. The void-rich NiFe₂O₄ NPs and ZnFe₂O₄ NRs provide the required electroactive sites, multiple redox couples, and fast ion transportation pathways for electrolyte ions. Due to the formation of these self-assembled porous networks of nanostructures and their high specific surface area, NiFe₂O₄ NP- and ZnFe₂O₄ NR-based electrodes demonstrate excellent charge storage properties. Particularly, the individual NiFe₂O₄ NP and ZnFe₂O₄ NR electrodes manifest high specific capacities of 1403 and 1005 C g⁻¹ at a current density of 1 A g⁻¹, respectively, and excellent durability with a high capacity retention (>97%) for up to 15,000 cycles. A flexible NiFe₂O₄ NP//ZnFe₂O₄ NR-based asymmetric supercapacitor (ASC) device was fabricated using NiFe₂O₄ NPs as the positive electrode, ZnFe₂O₄ NRs as the negative electrode, and a PVA-KOH electrolyte. Importantly, the flexible NiFe₂O₄ NP//ZnFe₂O₄ NR device exhibits a superhigh energy density of 99.55 Wh kg⁻¹ at a power density of 1.28 kW kg⁻¹. During the long-term stability tests, this flexible ASC device shows a capacity retention of 95% after 15,000 GCD cycles. Thus, the present work offers an alternative low-cost and rapid ice crystal-assisted precipitation approach for the development of self-assembled porous networks with void-rich structures to enhance the overall performance of energy storage devices.