Energy Storage Devices Based on Tungsten Oxides

WO₃ hosts a theoretical capacitance of 1112 F.g^-1 and is quite promising as an electrode material for energy storage devices, but it has disadvantages such as poor electrical conductivity and rate performance, and weak cycling stability. The main improvement methods can be divided into two parts: obtaining nanostructured single-phase tungsten oxides and obtaining multiphase structures consisting of tungsten oxide with other materials such as carbon materials, transition oxides, and organic materials.

Single-phase WO₃ nanostructures. Usually, nanostructured materials have a larger specific surface area by refining the size of the material, which allows them to be fully exposed to the electrolyte. The active materials inside are well exposed to ions and electrons so that redox reactions can be accelerated. Nanostructures based on WO₃, including quantum dots and nano particles, nanofibers, nanorods, nanotubes, nanochannels and nanowires, and nanoflakes and nanoplates, etc.

(Photo source: Han/Nanomaterials)

Cong et al. demonstrated that WO₃ quantum dots have better reversibility and superior rate performance according to a more symmetrical charge/discharge profile. In particular, the WO₃ nanosheets fabricated by Yin et al. can maintain almost 100% capacity retention after 10,000 cycles. Huang et al. obtained WO₃ samples with different morphologies by the hydrothermal method: nanorods, nanoplates, and microspheres assembled from numerous nanorods. Among them, the cactus-shaped WO₃ microspheres have a larger specific surface area and can obtain lower equivalent series resistance (Rs) and excellent cycling stability, showing the best capacitive performance.

In addition to the aforementioned nanostructures, there are other more complex and interesting morphologies assembled from smaller nano-units. For example, Shao et al. prepared a flying disk-like WO₃-nH₂O microstructure assembled by many nanorods. Due to this special micro/nanostructure, it has a high specific capacity of 391 F g^-1 at 0.5 A g^-1 and a good rate capacity of 298 F g^-1 at 10 A g^-1. After 2000 charge-discharge cycles, its capacitance retention is about 100%. Furthermore, by doping with Pd, Gupta et al. changed the morphology from nanosheet-assembled cabbage pure tungsten oxides to nanobrick-assembled cauliflower Pd-doped WO₃, achieving a larger surface area.

In addition to the aforementioned nanostructures, there are other more complex and interesting morphologies that are assembled from smaller nanocells. For example, Shao et al. prepared a flying disk-like WO₃-nH₂O microstructure assembled by many nanorods. Due to this special micro/nanostructure, it has a high specific capacity of 391 F g^-1 at 0.5 A g^-1 and a good rate capacity of 298 F g^-1 at 10 A g^-1 in energy storage devices. After 2000 charge/discharge cycles, its capacitance retention is about 100%. By doping with Pd, Gupta et al. changed the morphology from nanosheet-assembled cabbage pure WO₃ to nanobrick-assembled cauliflower Pd-doped WO₃, achieving a larger surface area.

Researchers He et al. fabricated a special hairball-like microsphere in which the core is assembled from a large number of nanorods and the shell is many other fluffy nanorods connecting each core, forming a porous three-dimensional structural network. Remarkably, when used as an electrode for SC, its initial capacitance remained at 93.4% even after 10,000 charge/discharge cycles. In addition, WO₃ three-dimensional nanorod arrays, cactus-like microsphere layered three-dimensional structures composed of numerous nanorods, and WO₃-H₂O flower-like layered structures composed of nanosheets were also reported, which showed greatly enhanced performance compared to most WO₃.

(Photo source: Han/Nanomaterials)

Reference: Han W, Shi Q, Hu R. Advances in electrochemical energy devices constructed with tungsten oxide-based nanomaterials[J]. Nanomaterials, 2021, 11(3): 692.

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