Waste Palm Seed Extract Yields Powerful Supercapacitor Electrode Material

In a finding that could reshape how the world builds its next generation of supercapacitors, a team of Nigerian researchers has turned an underused agricultural by-product, the seed of the raffia palm Raphia hookeri, into the chemical engine of a remarkably efficient electrode material. By using a simple aqueous extract of the seeds to reduce and stabilize copper and nickel precursors, the group prepared copper oxide nanoparticles, nickel oxide nanoparticles and, crucially, a combined CuO–NiO nanocomposite whose electrochemical performance dwarfs that of its individual components. The work, published open access in Discover Electrochemistry, offers a rare combination of sustainability, low cost and competitive charge-storage figures in a field often dominated by elaborate, energy-intensive synthesis routes.

The motivation behind the study rests on a well-known problem in electrochemical energy storage. Supercapacitors are prized for their fast charge–discharge rates, long cycle life and high power delivery, but their performance is ultimately dictated by the electrode material. Transition metal oxides such as copper oxide and nickel oxide are attractive candidates because they are abundant, environmentally benign and rich in accessible oxidation states that support pseudocapacitive, Faradaic charge storage. Copper oxide brings high theoretical capacitance, while nickel oxide contributes strong chemical stability and a highly active Ni2+/Ni3+ redox couple. Yet both materials suffer from the same Achilles heel: modest electrical conductivity and structural degradation during repeated cycling, which erode practical performance over time.

The researchers, led by B. H. Akpeji of the Federal University of Petroleum Resources in Effurun, Delta State, attacked this limitation with a binary composite strategy. Rather than relying on either oxide alone, they combined CuO and NiO into a single heterostructured nanomaterial, betting that the interplay of the two phases would multiply electroactive sites, speed electron transport and buffer each oxide’s structural weaknesses. Their synthesis began with seeds collected in the Ukwani Local Government Area of Delta State and verified at the University of Benin herbarium under voucher number UBH-R673. A Soxhlet extraction with distilled water, run for roughly four hours, yielded a concentrated aqueous extract that was then subjected to systematic phytochemical screening.

That screening revealed a veritable chemical toolkit within the seed. Flavonoids, alkaloids, tannins, phenolic compounds, terpenoids, saponins, glycosides and reducing sugars were all present, each playing a distinct role in the nanomaterial formation that followed. Flavonoids, phenolics and alkaloids donate electrons that reduce Cu2+ and Ni2+ ions toward their oxide forms, while hydroxyl and carbonyl functional groups assist nucleation and stabilization. Tannins and saponins act as passivating agents that prevent the freshly formed nanoparticles from clumping together, a property that proved decisive for the later electrochemical results. In essence, the plant extract replaced the hazardous reducing agents and synthetic surfactants that conventional nanoparticle recipes demand.

Using the extract mixed with copper acetate and nickel acetate solutions in the presence of sodium hydroxide, the team observed characteristic color transitions, copper oxide turning from blue to dark black and nickel oxide to dark green, as the hydroxide intermediates dehydrated into the final oxides. The nanocomposite itself was assembled by combining equimolar CuO and NiO nanofluids and stirring them at 65 degrees Celsius for two hours before centrifugation and drying. A full characterization battery followed: UV–visible spectroscopy showed absorption peaks at 301 nanometers for CuO, 295 nanometers for NiO and 299 nanometers for the composite, with optical band gaps of 2.29, 3.21 and 2.40 electronvolts respectively. The composite’s band gap sits neatly between its parents, a hallmark of genuine electronic interaction and heterojunction formation between the two oxide phases.

Electron microscopy confirmed that the green route produced the fine, well-dispersed particles that high-performance electrodes require. The CuO nanoparticles averaged 21.0 nanometers and the NiO particles 14.9 nanometers, while the composite measured 16.2 nanometers with reduced agglomeration and enhanced dispersion. Powder X-ray diffraction reinforced the picture, resolving the monoclinic structure of CuO and the face-centered cubic phase of NiO and giving the composite a mean crystallite size of 19.18 plus or minus 5.40 nanometers, calculated from six indexed reflections using the Debye–Scherrer equation and verified statistically in SPSS. Energy-dispersive X-ray analysis confirmed the successful incorporation of copper at 47.30 weight percent, nickel at 32.10 percent and oxygen at 20.60 percent in the composite, while Fourier-transform infrared spectroscopy located the distinctive Cu–O and Ni–O lattice vibrations, with a clear band near 535 inverse centimeters signaling hybridization of the two oxide lattices.

The electrochemical payoff came in a three-electrode configuration using a glassy carbon working electrode, a platinum counter electrode and an Ag/AgCl reference in 2 molar potassium hydroxide. Cyclic voltammetry revealed reversible, Faradaic redox behavior for all three materials, but the composite’s integrated curve area was dramatically larger. Quantitatively, the CuO–NiO nanocomposite delivered a specific capacitance of 489.60 farads per gram and an energy density of 0.272 watt-hours per kilogram, against roughly 124 farads per gram and about 0.069 watt-hours per kilogram for each of the single oxides, a near fourfold leap in stored charge. Electrochemical impedance spectroscopy told the same story from another angle: the composite exhibited the lowest charge-transfer resistance of the set at 2.31 ohms, compared with 4.90 ohms for CuO and 2.34 ohms for NiO, alongside the highest double-layer capacitance at 126 microfarads, indicating faster electron-transfer kinetics, more accessible electroactive surface and superior interfacial charge storage.

The authors attribute this synergy to the heterojunction formed where the two oxides meet. Band alignment between the narrow-gap CuO and the wider-gap NiO redistributes charge at the interface, creating new electronic states that ease the movement of electrons, while the dual Cu2+/Cu3+ and Ni2+/Ni3+ redox couples multiply the sites available for reversible Faradaic reactions in the alkaline electrolyte. The phytochemical capping inherited from the seed extract adds further advantages, keeping particle sizes small, shortening ion diffusion pathways and preserving the porous, interconnected morphology that scanning electron microscopy revealed. Compared with previous CuO–NiO composites reported in the literature, some requiring controlled-atmosphere annealing or surfactants to reach similar capacitances, the raffia-mediated route achieved competitive figures with nothing more exotic than water, acetate salts, sodium hydroxide and plant chemistry.

Beyond the numbers, the study carries a broader sustainability argument. Raphia hookeri seeds are inedible and largely discarded, so converting them into functional nanomaterials adds value to agricultural waste without competing with food production, in line with circular-economy thinking and the growing field of biomass valorization. The work was funded by Nigeria’s Tertiary Education Trust Fund and conducted across the Federal University of Petroleum Resources and the University of Benin. The authors are careful to note that thermogravimetric analysis, which showed decomposition temperatures of 379, 394 and 355 degrees Celsius for CuO, NiO and the composite respectively, speaks to thermal behavior rather than long-term cycling stability, and that full-cell devices, rate capability testing and extended cycling remain the next milestones. Still, the demonstration that a humble palm seed can seed, quite literally, a fourfold capacitance improvement offers a compelling template for affordable, greener electrode manufacturing, and suggests that the future of energy storage may be growing in fields as much as it is being engineered in cleanrooms.

The choice of a hydrothermal route deserves particular attention when weighing the practical significance of this work. Hydrothermal processing, in which reactions proceed in a sealed aqueous medium under elevated temperature and pressure, is prized for producing crystalline oxides at relatively low temperatures without the need for post-synthesis calcination at extreme conditions. Coupling that method with a plant-derived extract means the reducing, nucleating and capping functions are all performed by biomolecules rather than synthetic reagents, which simplifies purification and reduces the environmental footprint of the entire workflow.

The electrochemical measurements also illustrate why pseudocapacitive metal oxides behave so differently from carbon-based double-layer electrodes. In a 2 molar potassium hydroxide electrolyte, hydroxide ions participate directly in the reversible redox reactions at the electrode surface, so the measured capacitance reflects genuine Faradaic charge transfer rather than simple electrostatic adsorption. The low solution resistance of 0.29 ohms recorded for the composite electrode indicates that the electrolyte and electrode interface offered minimal ohmic opposition, an important precondition for high-rate operation in practical devices.

The thermal analysis adds a complementary dimension to the characterization. Decomposition temperatures in the range of 355 to 394 degrees Celsius indicate that the organic residues inherited from the plant extract are largely removed or stabilized well below the operating temperatures of supercapacitors, which function near ambient conditions. This suggests the phytochemical capping does not introduce thermal liabilities during normal device use, even though it may influence long-term cycling behavior in ways that only extended testing can reveal.

From a materials-design perspective, the intermediate band gap of the composite relative to its parent oxides is a useful diagnostic. It signals electronic coupling across the heterojunction rather than a mere physical mixture, which is precisely the condition needed for the interfacial charge redistribution that underpins the observed synergy. Future work building on this platform could explore varying the CuO to NiO ratio, tuning annealing conditions, or pairing the composite with biomass-derived carbon substrates to push energy density further while retaining the low-cost, waste-valorizing character that makes the approach distinctive.

Subject of Research: Green synthesis of CuO–NiO nanocomposites from Raphia hookeri seed extract for supercapacitor energy storage

Article Title: Nanocomposite materials of CuO–NiO from Raphia hookeri seed for investigation of energy storage potentials

Article References: Akpeji, B. H., Iyasele, J. U., Elemike, E. E., Okhuarobo, L. O., & Akpeji, S. A. (2026). Nanocomposite materials of CuO–NiO from Raphia hookeri seed for investigation of energy storage potentials. Discover Electrochemistry, 3(1), Article 76. https://doi.org/10.1007/s44373-026-00163-w

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00163-w

Keywords: CuO–NiO nanocomposite, green synthesis, supercapacitor, Raphia hookeri seed, pseudocapacitance, transition metal oxides, phytochemicals, energy storage, electrochemical impedance spectroscopy, cyclic voltammetry, nanoparticles, biomass valorization