An Easy and Ecological Method of Obtaining Hydrated and Non-Crystalline WO(3−x) for Application in Supercapacitors

In this work, we report the synthesis of hydrated and non-crystalline WO(3) flakes (WO(3−x)) via an environmentally friendly and facile water-based strategy. This method is described, in the literature, as exfoliation, however, based on the results obtained, we cannot say unequivocally that we have obtained an exfoliated material. Nevertheless, the proposed modification procedure clearly affects the morphology of WO(3) and leads to loss of crystallinity of the material. TEM techniques confirmed that the process leads to the formation of WO(3) flakes of a few nanometers in thickness. X-ray diffractograms affirmed the poor crystallinity of the flakes, while spectroscopic methods showed that the materials after exfoliation were abundant with the surface groups. The thin film of hydrated and non-crystalline WO(3) exhibits a seven times higher specific capacitance (C(s)) in an aqueous electrolyte than bulk WO(3) and shows an outstanding long-term cycling stability with a capacitance retention of 92% after 1000 chronopotentiometric cycles in the three-electrode system. In the two-electrode system, hydrated WO(3−x) shows a C(s) of 122 F g(−1) at a current density of 0.5 A g(−1). The developed supercapacitor shows an energy density of 60 Whkg(−1) and power density of 803 Wkg(−1) with a decrease of 16% in C(sp) after 10,000 cycles. On the other hand, WO(3−x) is characterized by inferior properties as an anode material in lithium-ion batteries compared to bulk WO(3). Lithium ions intercalate into a WO(3) crystal framework and occupy trigonal cavity sites during the electrochemical polarization. If there is no regular layer structure, as in the case of the hydrated and non-crystalline WO(3), the insertion of lithium ions between WO(3) layers is not possible. Thus, in the case of a non-aqueous electrolyte, the specific capacity of the hydrated and non-crystalline WO(3) electrode material is much lower in comparison with the specific capacity of the bulk WO(3)-based anode material.