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

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.