Two Birds with One Stone: Self‐Supporting Anodes and Cathodes for Quasi‐Solid‐State Asymmetric Supercapacitors via Reactions of 2‐Thiobarbituric Acid with Fe and Co Foams

Abstract Advanced electrode materials with simple manufacturing processes and wide voltage windows are needed for the commercialization of high energy density supercapacitors. Herein, a facile method is presented for fabricating self‐supporting anodes and cathodes for quasi‐solid‐state asymmetric supercapacitors (QASCs) by hydrothermally reacting 2‐thiobarbituric acid (TBA) with Fe foam (IF) and Co foam (CF), yielding FeTBA 4 /FeOOH/IF and Co 9 S 8 /CF electrodes, respectively. Due to the perfect match between the two electrodes, the redox‐active TBA ligands in FeTBA 4 , the 2D ultrathin nanosheet structure of FeTBA 4 /FeOOH/IF, and multiple pairs of reversible redox reactions for suppressing water splitting, the configured Co 9 S 8 /CF//FeTBA 4 /FeOOH/IF QASC device delivers outstanding performance. The device possesses a wide operating voltage window of 1.6 V, leading to a high energy density of 82.64 Wh kg −1 at 486.38 W kg −1 and an equally impressive 35.36 Wh kg −1 at 4595.92 W kg −1 . Furthermore, a 98.5% capacitance retention is realized after 10000 charging–discharging cycles. Impressively, density functional theory (DFT) calculations reveal the unique pseudocapacitive reactions on the surface of Co 9 S 8 /CF and FeTBA 4 /FeOOH/IF electrodes. Importantly, this work guides the development of high‐energy‐density supercapacitors via the matching of electrodes and the use of redox‐active complex electrodes.