FeWO4 As Electrode Material for High Volumetric Capacitance Supercapacitors

The volumetric capacitance of supercapacitors is one of the limiting factors of today’s stationary applications. Further research on new materials with improved storage properties is necessary to meet the requirements concerning high-performance energy storage devices while considering a reasonable manufacturing cost and a low environmental impact. In order to meet those requirements, our interest has focused on high-density oxides operating in aqueous electrolytes.

Metal tungstates (M²⁺WO₄) represent an important group of inorganic high-density oxides with excellent functional and electronic properties. Iron tungstate (FeWO₄), crystallizes in the wolframite-type structure, characterized by distorted [MO₆] and [WO₆] octahedra. It is well known for its magnetic, photocatalytic and photoluminescence properties and has recently been studied in lithium-ion batteries but its electrochemical properties in supercapacitors have not yet been reported. FeWO₄ has been synthesized via various low-temperature methods (polyol-mediated, hydrothermal synthesis…) in order to obtain high specific surface area nanoparticles which have been characterized and electrochemically tested as supercapacitor electrode materials in aqueous electrolytes. Concomitantly, zinc tungstate was synthesized by the same method. Specific surface area ranging from 10 to 80 m²/g have been achieved.

The different oxides have been tested in neutral aqueous based electrolytes (LiNO₃ 5M). ZnWO₄ does not show any sign of electrochemical activity whatever the potential window. FeWO₄ exhibits a typical pseudocapacitive behavior i.e. CVs without any redox peaks but typical rectangular shape. The material exhibits very promising properties that seem to be due to Fe²⁺/Fe³⁺ surface redox reactions and are closely related to its specific surface area. The synthesis conditions, materials characterizations and electrochemical performance will be detailed in the presentation. For FeWO₄ obtained by polyol-mediated synthesis, a high volumetric capacitance of 210 F.cm^-3 was measured at 20 mV.s^-1 with less than 5% fade over 10 000 cycles. Furthermore, unlike most previously investigated iron based electrodes, a unique pseudocapacitive behavior emphasizes the role of the crystallographic structure.