This article discusses the growing importance of semiconductor nanowires in energy applications and the feasibility of using them in flexible energy storage devices, specifically in flexible supercapacitors (SCs).
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Importance of Semiconductor Nanowires in Energy Applications
Semiconductor nanowires are primarily nanosystems with tunable 2-200 nm cross-sectional dimensions and lengths ranging from a few hundred nanometers to several micrometers. These nanowires can confine photon and electron travel to a single dimension.
Semiconductor nanowires can be used in different applications, such as energy storage, electronics, photonics, and biomedical. In energy applications, semiconductor nanowires have attracted significant attention due to their tunable transport properties, unique surface chemistry, and large surface area.
Specifically, 1D semiconductor nanowire building blocks can be effective as harvesting units or channels for solar, mechanical, chemical, and thermal energy applications and as storage media for electrochemical energy.
1D semiconductor nanowires can control photon, phonon, and electron transport, which makes them suitable for solid-state energy harvesting, storage, and conversion applications. For instance, indium phosphide (InP) nanowires used in single nanowire-based photovoltaic cells display a maximum power conversion efficiency of 17.8%, while silicon germanium (SiGe) nanowires used in nanogenerators demonstrate a maximum power output of 7.1 µW/cm2.
Similarly, the core-shell hierarchical cobalt sulfide@molybdenum disulfide nanowire array used in SCs displays 95.7 W h kg-1 energy density at 711.2 W kg-1 power density, while three-dimensional (3D) freestanding hierarchical spinel copper cobaltate nanowires supported on a nickel foam used in lithium-air batteries demonstrate an exceptional specific capacity of 13654 mAh g-1 at 0.1 mA cm-2.
Flexible Energy Storage Devices Based on 1D Semiconductor Nanowires
In recent years, low-dimension inorganic semiconductor materials, such as 1D semiconductor nanowires, have gained prominence for application in flexible electronics, including flexible energy storage devices such as flexible SCs, owing to their superior electrical properties and exceptional mechanical flexibility.
SCs have gained considerable attention as energy storage devices in cameras, cell phones, laptops, and hybrid electric vehicles as they can provide higher power than batteries and store more energy compared to conventional capacitors.
However, the energy density of SCs must be increased substantially without affecting their cycle life and power density to meet the rising demands for next-generation flexible and portable devices.
Energy density in an SC can be increased by maximizing the cell voltage and device capacitance. Several nanostructured electrodes have been designed and fabricated to increase the device's capacitance.
Among them, 1D semiconductor nanowires and nanowire heterostructures have demonstrated significant potential as SC electrodes as they provide a large interfacial area between the electrolyte and electrode for charge transport and a shorter diffusion path for deintercalation/intercalation of active species.
High-performance SC electrodes have been developed using titanium dioxide (TiO2) and titanium nitride (TiN) semiconductor nanowires and TiO2/manganese dioxide (MnO2) core-shell nanowire heterostructures.
These 1D semiconductor nanowires and nanowire heterostructures demonstrated significantly improved stability and performance in applications for SCs due to suppressed irreversible oxidation reactions and improved electrical conductivities.
Moreover, the 1D nanowires and core-shell nanowires were directly grown on flexible conductive substrates, which provided good strain accommodation and enabled the fabrication of flexible solid-state SC devices without requiring a binder.
For instance, hydrogenated TiO2 (H-TiO2) based core-shell nanowires and stabilized TiN nanowire arrays were successfully used in flexible asymmetric SCs and flexible SCs, respectively, to improve the SC performance.
Although TiN displays significant potential as an SC electrode material due to its superior electrical conductivity of 4000-55500 S/cm and mechanical stability, several studies have shown that TiN electrode suffers from substantial capacitance loss in an alkaline electrolyte solution.
Solid-state polymer electrolytes with a limited amount of water can be used to enhance the TiN electrode stability. This electrolyte can effectively suppress the TiN electrochemical oxidation reaction and mechanically stabilize the TiN nanowires by retaining their contacts and structures during cycling.
Thus, the TiN electrode in solid-state polymer electrolyte displayed a significantly improved stability with substantially high capacitance retention of 82% after 15000 cycles compared to the TiN electrode in one M potassium hydroxide electrolyte solution.
Additionally, the solid-state TiN SC device realized a high volumetric energy density of 0.05 mWh/cm3. The effective stabilization of nitride materials such as TiN nanowires can therefore facilitate the development of high-performance and flexible SCs.
Similarly, the energy density of flexible asymmetric SCs was improved significantly using an H-TiO2 nanowire as a core/conducting scaffold to support electrochemically active carbon and MnO2 shells.
The H-TiO2 nanostructures acted as promising scaffolds to support the MnO2 as the hydrogenation of pristine TiO2 significantly increased its carrier density. The synthesized solid-state asymmetric SC device demonstrated excellent mechanical stability as a flexible energy storage device, and exceptional performance by retaining 91.2% of the initial capacitance after 5000 cycles.
Additionally, the maximum energy densities of these H-TiO2@MnO2//H-TiO2@carbon devices with 1.8 V cell voltage in both solid-state and aqueous electrolytes were considerably higher compared to the values obtained for only MnO2-based asymmetric SCs.
Developing complex nanostructured electrodes based on semiconductor nanowires can advance the design and fabrication of high-performance and flexible asymmetric SCs.
To summarize, 1D semiconductor nanowires can play a crucial role in the future in the fabrication of next-generation flexible and energy-efficient high-performance energy storage devices.
References and Further Reading
Chen, K., Pan, J., Yin, W., Ma, C., Wang, L. (2023). Flexible electronics based on one-dimensional inorganic semiconductor nanowires and two-dimensional transition metal dichalcogenides. Chinese Chemical Letters, p. 108226. https://doi.org/10.1016/j.cclet.2023.108226
Lu, X., Zhai, T., Yu, M., Li, Y., Tong, Y. (2013). Semiconductor Nanowires and Nanowire Heterostructures for Supercapacitors. Advanced Optoelectronics for Energy and Environment. doi.org/10.1364/AOEE.2013.ASu3B.7
Nehra, M., Dilbaghi, N., Marrazza, G., Kaushik, A., Abolhassani, R., Mishra, Y. K., Kim, K. H., Kumar, S. (2020). 1D semiconductor nanowires for energy conversion, harvesting and storage applications. Nano Energy, 76, p. 104991. https://doi.org/10.1016/j.nanoen.2020.104991
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