Combined electrochemical and DFT investigations of iron selenide: a mechanically bendable solid-state symmetric supercapacitor

Enhancing energy storing capability with the aid of unique nanostructured morphologies is beneficial for the development of high performance supercapacitors. Developing earth abundant and low-cost transition metal selenides (TMSs) with enhanced charge transfer capabilities and good stability is still a challenge. Herein, state of the art for iron selenide with a nanoflake surface architecture, synthesized with the aid of a simple, industry-scalable and ionic layer controlled chemical approach, namely the successive ionic layer adsorption and reaction (SILAR) method, is presented. The iron selenide electrode yields a capacitance of 671.7 F g−1 at 2 mV s−1 scan rate and 434.6 F g−1 at 2 mA cm−2 current density through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) studies, respectively, with 91.9% cyclic retention at 4000 cycles. The developed bendable solid-state supercapacitor reveals a remarkable power density of 5.1 kW kg−1 with outstanding deformation tolerance, including its use in a practical demo to run a small fan, demonstrating its capability for advanced energy storage applications. A complementary first-principles density functional theory (DFT) approach is used in combination with the experimental supercapacitive performance to achieve an understanding of the electronic structure.