Titanium dioxide (TiO2) nanomaterials have garnered extensive scientific interest since 1972 and have been widely used in many areas, such as sustainable energy generation and the removal of environmental pollutants. Although TiO2 possesses the desired performance in utilizing ultraviolet light, its overall solar activity is still very limited because of a wide bandgap (3.0–3.2 eV) that cannot make use of visible light or light of longer wavelength. This phenomenon is a deficiency for TiO2 with respect to its potential application in visible light photocatalysis and photoelectrochemical devices, as well as photovoltaics and sensors. The high overpotential, sluggish migration, and rapid recombination of photogenerated electron/hole pairs are crucial factors that restrict further application of TiO2. Recently, a broad range of research efforts has been devoted to enhancing the optical and electrical properties of TiO2, resulting in improved photocatalytic activity. This review mainly outlines state-of-the-art modification strategies in optimizing the photocatalytic performance of TiO2, including the introduction of intrinsic defects and foreign species into the TiO2 lattice, morphology and crystal facet control, and the development of unique mesocrystal structures. The band structures, electronic properties, and chemical features of the modified TiO2 nanomaterials are clarified in detail along with details regarding their photocatalytic performance and various applications.

Titanium Dioxide: From Engineering to Applications

Supercapacitors are increasingly used for energy conversion and storage systems in sustainable nanotechnologies. Graphite is a conventional electrode utilized in Li-ion-based batteries, yet its specific capacitance of 372 mA h g−1 is not adequate for supercapacitor applications. Interest in supercapacitors is due to their high-energy capacity, storage for a shorter period and longer lifetime. This review compares the following materials used to fabricate supercapacitors: spinel ferrites, e.g., MFe2O4, MMoO4 and MCo2O4 where M denotes a transition metal ion; perovskite oxides; transition metals sulfides; carbon materials; and conducting polymers. The application window of perovskite can be controlled by cations in sublattice sites. Cations increase the specific capacitance because cations possess large orbital valence electrons which grow the oxygen vacancies. Electrodes made of transition metal sulfides, e.g., ZnCo2S4, display a high specific capacitance of 1269 F g−1, which is four times higher than those of transition metals oxides, e.g., Zn–Co ferrite, of 296 F g−1. This is explained by the low charge-transfer resistance and the high ion diffusion rate of transition metals sulfides. Composites made of magnetic oxides or transition metal sulfides with conducting polymers or carbon materials have the highest capacitance activity and cyclic stability. This is attributed to oxygen and sulfur active sites which foster electrolyte penetration during cycling, and, in turn, create new active sites.

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Advanced materials and technologies for supercapacitors used in energy conversion and storage: a review

The last decade has witnessed an extensive uptake of clean and sustainable energy sources to meet the surging energy demand while mitigating the increasing levels of greenhouse gas emission and air pollution. Among various energy systems, electrochemical energy storage devices such as batteries and supercapacitors have attracted worldwide attention for use in electric-powered transport, portable electronics, and biomedical devices. Recently, new multifunctional supercapacitors, which combine energy storage capability with load-carrying and other functions, offer a new “two-birds-one-stone” strategy for next-generation energy storage systems to store energy beyond the traditional systems. Multifunctional supercapacitors show great promise in reducing the size and volume of devices, improving the charge storage capacity, and minimising the cost in materials and fabrication while bringing the benefits of additional functions to the systems. This review describes the recent advances in multifunctional supercapacitors in terms of materials design, device configuration, system integration, and applications. The unique features of multifunctional supercapacitors depend strongly on architectural designs and system integration, which allow elimination of certain components to reduce the size and weight, thus improving the overall system performance. The review focuses specifically on multifunctional supercapacitors with novel mechanical, surface/interfacial, thermal, electronic, photodetection and energy harvesting/conversion functions. In addition, challenges and opportunities for further developments in the emerging field of multifunctional supercapacitors are suggested and discussed.

Two-birds-one-stone: multifunctional supercapacitors beyond traditional energy storage

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics.

The design and development of self-charging supercapacitor power cells are rapidly gaining interest due to their ability to convert and store energy in an integrated device. Here, we have demonstrated the fabrication of a self-charging supercapacitor using siloxene sheets as electrodes and siloxene-based polymeric piezofiber separator immobilized with an ionogel electrolyte. The self-charging properties of the fabricated device subjected to various levels of compressive forces showed their ability to self-charge up to a maximum of 207 mV. The mechanism of self-charging process in the fabricated device is discussed via “piezoelectrochemical effect” with the aid of piezoelectrochemical spectroscopy measurements. These studies revealed the direct evidence of the piezoelectrochemical phenomenon involved in the energy conversion and storage process in the fabricated device. This study can provide insight towards understanding the energy conversion process in self-charging supercapacitors, which is of significance considering the state of the art of piezoelectric driven self-charging supercapacitors. Devices that are capable of energy harvesting and storage are attractive for meeting daily energy demands, however they are limited by efficiency. Here the authors fabricate a siloxene-based self-charging supercapacitor power cell and probe the piezoelectrochemical effect involved in the charging process.

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Probing the energy conversion process in piezoelectric-driven electrochemical self-charging supercapacitor power cell using piezoelectrochemical spectroscopy.

Vimal Kumar Mariappan

Papers

Probing the energy conversion process in piezoelectric-driven electrochemical self-charging supercapacitor power cell using piezoelectrochemical spectroscopy.

Nanostructured ternary metal chalcogenide-based binder-free electrodes for high energy density asymmetric supercapacitors

A High Efficacy Self-Charging MoSe2 Solid-State Supercapacitor Using Electrospun Nanofibrous Piezoelectric Separator with Ionogel Electrolyte

Two‐Dimensional Siloxene–Graphene Heterostructure‐Based High‐Performance Supercapacitor for Capturing Regenerative Braking Energy in Electric Vehicles

Copper tungsten sulfide anchored on Ni-foam as a high-performance binder free negative electrode for asymmetric supercapacitor