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., MFe 2 O 4 , MMoO 4 and MCo 2 O 4 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., ZnCo 2 S 4 , 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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Adsorption to date is the most effective and utilized technology globally to remove several pollutants in wastewater. In this approach, many adsorbents have been synthesized, tested and used for the elimination and separation of the contaminants such as radionuclides, heavy metals, dyes and pharmaceutical compounds both at lab and industrial scale. However, there are many challenges to adsorption processes such as reducing the high cost, through means of separation of suspending adsorbents to be used again, as well as the ease to synthesize. Two methods that have shown promising results and gained significant interest is that of magnetic nanomaterials and biosorbents due to their effective, safe, eco-friendly, low cost and low-energy intensive material properties. Magnetic nanomaterials act as efficient adsorbents due to their ease of removal of contaminants from wastewater using an applied magnetic field but also their advantageous surface charge and redox activity characteristics. On the other hand, biosorbents have a synergistic effect with their efficient adsorption capacity to remove contaminants, high abundance and participation in waste minimization, helping alleviate ecological and environmental problems. This review highlights, discusses and reports on the state-of-the-art of these two promising routes to adsorption and provides indications as to what are the optimum materials for utilization and insight into their efficiency, reusability and practicality for the removal of pollutants from wastewater streams. Some of the main material focuses are zero-valent iron, iron oxides, spinel ferrites, natural and waste-based biosorbents.