The demand for renewable energy sources worldwide has gained tremendous research attention over the past decades. Technologies such as wind and solar have been widely researched and reported in the literature. However, economical use of these technologies has not been widespread due partly to cost and the inability for service during of-source periods. To make these technologies more competitive, research into energy storage systems has intensified over the last few decades. The idea is to devise an energy storage system that allows for storage of electricity during lean hours at a relatively cheaper value and delivery later. Energy storage and delivery technologies such as supercapacitors can store and deliver energy at a very fast rate, offering high current in a short duration. The past decade has witnessed a rapid growth in research and development in supercapacitor technology. Several electrochemical properties of the electrode material and electrolyte have been reported in the literature. Supercapacitor electrode materials such as carbon and carbon-based materials have received increasing attention because of their high specific surface area, good electrical conductivity and excellent stability in harsh environments etc. In recent years, there has been an increasing interest in biomass-derived activated carbons as an electrode material for supercapacitor applications. The development of an alternative supercapacitor electrode material from biowaste serves two main purposes: (1) It helps with waste disposal; converting waste to a useful product, and (2) it provides an economic argument for the substantiality of supercapacitor technology. This article reviews recent developments in carbon and carbon-based materials derived from biowaste for supercapacitor technology. A comparison between the various storage mechanisms and electrochemical performance of electrodes derived from biowaste is presented.

To contribute to solving global energy problems, a multifunctional CoFe₂O₄ spinel was synthesized and used as a catalyst for overall water splitting and as an electrode material for supercapacitors. The ultra-fast one-step electrodeposition of CoFe₂O₄ over conducting substrates provides an economic pathway to high-performance energy devices. Electrodeposited CoFe₂O₄ on Ni-foam showed a low overpotential of 270 mV and a Tafel slope of 31 mV/dec. The results indicated a higher conductivity for electrodeposited compared with dip-coated CoFe₂O₄ with enhanced device performance. Moreover, bending and chronoamperometry studies suggest excellent durability of the catalytic electrode for long-term use. The energy storage behavior of CoFe₂O₄ showed high specific capacitance of 768 F/g at a current density of 0.5 A/g and maintained about 80% retention after 10,000 cycles. These results demonstrate the competitiveness and multifunctional applicability of the CoFe₂O₄ spinel to be used for energy generation and storage devices.

Hydrogen is an efficient fuel which can be generated via water splitting, however hydrogen evolution occurs at high overpotential, and efficient hydrogen evolution catalysts are desired to replace state-of-the-art catalysts such as platinum. Here, we report an advanced electrocatalyst that has low overpotential, efficient charge transfers kinetics, low Tafel slope and durable. Carbon nanofibers (CNFs), obtained by carbonizing electrospun fibers, were decorated with MoS₂ using a facile hydrothermal method. The imaging of catalyst reveals a flower like morphology that allows for exposure of edge sulfur sites to maximize the HER process. HER activity of MoS₂ decorated over CNFs was compared with MoS₂ without CNFs and with commercial MoS₂. MoS₂ grown over CNFs and MoS₂-synthesized produced about 374 and 98 times higher current density at −0.30 V (vs. Reversible Hydrogen Electrode, RHE) compared with the MoS₂-commercial sample, respectively. MoS₂-commercial, MoS₂-synthesized and MoS₂ grown over CNFs showed a Tafel slope of 165, 79 and 60 mV/decade, capacitance of 0.99, 5.87 and 15.66 mF/cm², and turnover frequency of 0.013, 0.025 and 0.54 s⁻¹, respectively. The enhanced performance of MoS₂-CNFs is due to large electroactive surface area, more exposure of edge sulfur to the electrolyte, and easy charge transfer from MoS₂ to the electrode through conducting CNFs.

Interconnected hollow-structured carbon was successfully prepared from a readily available bio-waste precursor (orange peel) by pyrolysis and chemical activation (using KOH), and demonstrated its potential as a high-performing electrode material for energy storage. The surface area and pore size of carbon were controlled by varying the precursor carbon to KOH mass ratio. The specific surface area significantly increased with the increasing amount of KOH, reaching a specific surface area of 2521 m²/g for a 1:3 mass ratio of precursor carbon/KOH. However, a 1:1 mass ratio of precursor carbon/KOH displayed the optimum charge storage capacitance of 407 F/g, owing to the ideal combination of micro- and mesopores and a higher degree of graphitization. The capacitive performance varied with the electrolyte employed. The orange-peel-derived electrode in KOH electrolyte displayed the maximum capacitance and optimum rate capability. The orange-peel-derived electrode maintained above 100% capacitance retention during 5000 cyclic tests and identical charge storage over different bending status. The fabricated supercapacitor device delivered high energy density (100.4 µWh/cm²) and power density (6.87 mW/cm²), along with improved performance at elevated temperatures. Our study demonstrates that bio-waste can be easily converted into a high-performance and efficient energy storage device by employing a carefully architected electrode–electrolyte system.

The urea oxidation reaction (UOR) is a possible solution to solve the world’s energy crisis. Fuel cells have been used in the UOR to generate hydrogen with a lower potential compared to water splitting, decreasing the costs of energy production. Urea is abundantly present in agricultural waste and in industrial and human wastewater. Besides generating hydrogen, this reaction provides a pathway to eliminate urea, which is a hazard in the environment and to people’s health. In this study, nanosheets of CuCo₂O₄ grown on nickel foam were synthesized as an electrocatalyst for urea oxidation to generate hydrogen as a green fuel. The synthesized electrocatalyst was characterized using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The electroactivity of CuCo₂O₄ towards the oxidation of urea in alkaline solution was evaluated using electrochemical measurements. Nanosheets of CuCo₂O₄ grown on nickel foam required the potential of 1.36 V in 1 M KOH with 0.33 M urea to deliver a current density of 10 mA/cm². The CuCo₂O₄ electrode was electrochemically stable for over 15 h of continuous measurements. The high catalytic activities for the hydrogen evolution reaction make the CuCo₂O₄ electrode a bifunctional catalyst and a promising electroactive material for hydrogen production. The two-electrode electrolyzer demanded a potential of 1.45 V, which was 260 mV less than that for the urea-free counterpart. Our study suggests that the CuCo₂O₄ electrode can be a promising material as an efficient UOR catalyst for fuel cells to generate hydrogen at a low cost.

The rigid polyurethane foam (PU) is a versatile material, used especially for construction and household applications. The current situation demands a facile, cost-efficient, and greener approach for developing the polyurethanes from bio-derived materials. In this study, we present a novel bio-polyol synthesized using carvone, an extract from caraway, spearmint, or dill seeds via facile thiol-ene reaction. Our one-step reaction uses a UV irradiation to allow the room temperature conversion of the carvone to a high purity bio-polyol, as confirmed from the standard analytical characterizations. The hydroxyl number of 365 mg KOH/g close to its theoretical limit confirms the high conversion yield of the polyol for rigid PU synthesis. To overcome the flammability issues in PU, expandable graphite (EG) powder was used as an additive flame-retardant during the synthesis step. The resulting foams with EG maintained the uniform closed cell structure (>95%) with a high compression strength of 175 kPa. The addition of EG in PU results in the formation of a protective char layer during the flammability test and reduces the weight loss from 40.70% to 3.55% and burning time from 87 to 11 s. Our results confirm that the carvone-based polyol can be a novel alternative to the petroleum polyols for an industrial-scale application.