Hydrothermal synthesis of iron oxides in the presence of catecholate capping agent for supercapacitor electrodes

• Caffeic acid (CA) was used as a capping agent for hydrothermal synthesis of Fe 2 O 3. • Fe 2 O 3 nanoparticles showed high capacitance for supercapacitor applications. • Increasing CA concentration resulted in increased capacitance and conductivity. • γ-Fe 2 O 3 or α- Fe 2 O 3 phases were synthesized from Fe 3+ or Fe 2+ precursors. • The obtained γ-Fe 2 O 3 nanoparticles combine high magnetization and high capacitance. Caffeic acid (CA) is used as a capping agent for hydrothermal synthesis of iron oxides from FeCl 2 and FeCl 3 solutions. Iron oxide nanoparticles are utilized for the fabrication of supercapacitor anodes with mass loading of 40 mgcm −2 for operation in 0.5 M Na 2 SO 4 electrolyte. Nanoparticles of γ-Fe 2 O 3 , prepared from FeCl 2 solutions show ferrimagnetic properties and highest electrochemical capacitance of 4.49 Fcm −2 . The synthesis from FeCl 3 solutions results in the formation of antiferromagnetic α-Fe 2 O 3 phase at low CA concentrations, its transformation to the metastable γ-Fe 2 O 3 with increasing CA concentration and highest electrochemical capacitance of 4.35 Fcm −2 . The increase in CA concentration in solutions results in increasing capacitance and decreasing resistance. The obtained γ-Fe 2 O 3 nanoparticles exhibit valuable magnetic and electrical charge storage properties that are significantly superior to those of other magnetoelectric materials.