In the last years, supercapacitors (SCs) have been proposed as a promising alternative to cover the power density deficiency presented in batteries. Electrical double-layer SCs, pseudocapacitors, and hybrid supercapacitors (HSCs) have shown very attractive features such as high-power density, long cycle life, and tunable specific capacitance. The advances of these energy storage devices made by transition metal oxides (TMOs) and their production in pseudocapacitors and HSCs depend on chemical composition, crystalline structure, morphology, theoretical capacitance, and oxidation states. In this way, this critical review considers several metal oxides (RuO₂, MnO₂, V₂O₅, and Co₃O₄) and their different configurations with diverse carbon-based materials. Energy storage mechanisms and fundamental principles to understand the promising effect of metal oxides in SCs devices are thoroughly described. Special attention as regards to the energy storage mechanisms relative to the specific capacitance values is presented in the reviewed articles. This review envisages the TMO as a key component to obtain high specific capacitance SCs.

In this review we focus on the recent progress and current trends in biomass carbon-based composites containing mono-, bi- and trimetallic metal oxides and hydroxides for supercapacitor applications.

Activated carbon-based supercapacitor electrodes synthesized from biomass or waste-derived biomass have recently attracted considerable attention because of their low cost, natural abundance, and power delivery performance. In this work, purple-corncob-based active carbons are prepared by KOH activation and subsequently evaluated as a composite electrode for supercapacitors using either an acidic or an alkali solution as the electrolyte. The synthesis of the material involves mixing the purple corncob powder with different concentrations of KOH (in the range of 5% to 30%) and a thermal treatment at 700 °C under an inert atmosphere. Physicochemical characterizations were performed using scanning electron microscopy, Raman spectroscopy, N₂ physisorption analysis, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy, while the electrochemical characteristics were determined using cyclic voltammetry, a galvanostatic charge/discharge curve, and electrochemical impedance techniques measured in a three- and two-electrode system. Composite electrodes activated with 10% KOH had a specific surface area of 728 m² g^-1, and high capacitances of 195 F g^-1 at 0.5 A g^-1 in 1 mol L^-1 H₂SO₄ and 116 F g^-1 at 0.5 A g^-1 in 1 mol L^-1 KOH were obtained. It also presented a 76% capacitance retention after 50 000 cycles. These properties depend significantly on the microporous area and micropore volume characteristics of the activated carbon. Overall, our results indicate that purple corncob has an interesting prospect as a carbon precursor material for supercapacitor electrodes.

Aiming to mitigate disposal and water pollution problems, biochars were prepared from cocoa pod husk (CPH) waste for application in artificial wastewater simulating real conditions of textile effluents. Therefore, the morphological properties of biochars obtained via one/two step NaOH activation were compared and characterized by XRD, Raman spectroscopy, FESEM/EDS and the BET technique. According to these results, biochar obtained from the one-step method (AQ₁) was chemically characterized by Boehm titration as well as the point of zero charge and used for the removal of a mixture of brilliant green (BG), rhodamine B (RB) and methyl orange (MO) as artificial wastewater. To evaluate the remaining concentration of each dye after adsorption without previous separation, a derivative spectrophotometry method was applied because some overlap will occur. In order to overcome this problem, adequate wavelengths for each dye in the mixture were established: BG = 631, RB = 592 and MO = 377 nm. Additionally, optimum conditions were determined such as pH 6 and a dosage of 50 mg and the effect of the initial dye concentration was studied in the range of 5–20 mg L⁻¹. The adsorption rate of the ternary dye mixture was fitted to the pseudo second-order model and the affinity of AQ₁ towards each dye was in the following order: BG > RB > MO. Furthermore, the adsorption isotherm data were described by multi-component Langmuir and Freundlich models. Finally, the presence of sodium chloride was explored and the synergetic effect in the removal of BG and MO was demonstrated, whereas the adsorption of RB was not promoted under the influence of this salt widely used in the textile industry.