Layered Vanadium Phosphates as Electrodes for Electrochemical Capacitors: Part III. Inducing Fast Faradaic Reactions with Cu²⁺ Ions in the Interlayer Space

A novel synthesis approach for Cu 0.16 VOPO 4 ·2.5H 2 O material is reported, consisting of VOPO 4 layers incorporating water molecules and Cu 2+ ions within the interlayer space. The inclusion of Cu 2+ ions leads to significant changes in the previously reported electrochemical properties of VOPO 4 ·2H 2 O. Indeed, copper ion insertion leads to a reduced interlayer space, a higher surface area, and, consequently, to a higher specific charge. Moreover, the appearance of new fast faradaic reactions is depicted from the presence of new redox peaks in cyclic voltammograms. The capacity of this material is 93 Cg −1 in 3 M LiOH, 114 Cg −1 in 3 M NaOH, and 126 Cg −1 in 3 M KOH at a scan rate of 5 mVs −1 . It was determined that an intercalation process takes place across the entire operational range in all three electrolytes, even at scan rates as high as 500 mVs −1 . Additionally, electrochemical impedance spectroscopy (EIS) was used for a more comprehensive understanding of the electrochemical role of the interlayer Cu 2+ cations. EIS enables us to propose a new mechanism of electron transfer between VOPO 4 layers and Cu 2+ ion layers, which extends to neighboring layers, thus explaining the fast kinetic of the related faradaic reactions.