Crystallization of Fe3+ in an alkaline aqueous pseudocapacitor system

Abstract

For the first time, we have studied the crystallization behavior of FeCl₃ in an aqueous alkaline pseudocapacitor system, where FeCl₃ can be effectively transformed into Fe₂O₃·H₂O colloids by an in situ crystallization process in KOH electrolyte under an electric field. In alkaline aqueous solution, the Fe³⁺ cations firstly react with OH⁻ and crystallize into goethite Fe₂O₃·H₂O colloids. During the electrochemical tests, the crystallization of Fe³⁺ was interrupted by an external electric field due to a Faradaic redox reaction, resulting in fine and highly electroactive Fe₂O₃·H₂O colloids. For the currently designed FeCl₃ pseudocapacitor system, a very high capacitance of 977 F g⁻¹ at the current density of 3 A g⁻¹ and a high energy density of 23.6 Wh kg⁻¹ at the power density of 3400 W kg⁻¹ can be obtained. Rate performance tests show that 86% of the capacitance can be maintained after the galvanostatic current density increases from 3 to 15 A g⁻¹. Our current pseudocapacitor system can provide a versatile method for the construction of high-performance inorganic pseudocapacitors by in situ crystallization via chemical/electrochemical reactions in room temperature aqueous solution.