The motivation begins with scale. Banana is the second most produced fruit in the world, with more than 300 varieties cultivated across Asia, Africa, Latin America and the Caribbean, and the production and consumption of bananas generate waste accounting for roughly 30 to 40 percent of the total fruit weight. Because peels are rich in carbon-containing organic compounds and cost essentially nothing, they are an attractive precursor for porous carbon materials. At the same time, paper-based microfluidic fuel cells have emerged as compact, low-cost devices that exploit the capillary action of paper to transport reactants without pumps, integrating channels, electrodes and reaction zones on a single disposable platform. Combining the two, the researchers reasoned, could yield portable energy systems that are cheap, green and easy to fabricate.

The synthesis route is straightforward enough to be reproduced in modest laboratories. Banana peels collected from a juice shop in Medellín, Colombia, were washed, dried at 80 degrees Celsius for 24 hours and pulverized. The powder was suspended in water, briefly microwave-heated, filtered and dried again. The resulting biomass was then mixed with an aqueous solution of zinc sulfate heptahydrate, the activating agent, at three different zinc salt to biomass weight ratios of 1:2, 1:1 and 2:1, producing materials labeled BCZ1, BCZ2 and BCZ3 respectively. After stirring, drying and pyrolysis under a nitrogen flow at 500 degrees Celsius, the solids were washed, dried and sieved to a particle size of 90 micrometers. The zinc sulfate plays a double role: it activates the carbon, developing porosity, and it supplies the zinc that ultimately forms the catalytic ZnO and ZnS phases dispersed through the carbon matrix.

Characterization revealed how strongly the mixing ratio shapes the final material. Fourier transform infrared spectroscopy confirmed the survival of lignocellulosic functional groups after carbonization, with bands assigned to O-H groups of cellulose and lignin, aromatic C=C stretching, and C-O and C-O-C vibrations. Thermogravimetric analysis mapped four stages of thermal decomposition, from moisture loss below 120 degrees Celsius through hemicellulose, cellulose and lignin degradation up to the formation of carbonaceous material and zinc species above 650 degrees Celsius. Single-point BET measurements gave specific surface areas of 232, 239 and 141 square meters per gram for BCZ1, BCZ2 and BCZ3 respectively, indicating that the two lower zinc ratios produced far more developed porous structures, while the highest zinc loading likely blocked pores and limited micropore development.

Microscopy and spectroscopy filled in the structural picture. Scanning electron microscopy showed irregular, porous morphologies for all three samples, studded with bright, faceted particles of zinc-based crystals anchored to the carbon surface, including hexagonal shapes associated with the wurtzite structure of ZnO. Energy-dispersive X-ray analysis confirmed the trend in composition: BCZ1 contained about 47 weight percent carbon and 35 percent zinc, BCZ2 was zinc-dominated at 54.5 percent, and BCZ3 held 45.8 percent zinc with a smoother but still porous surface. X-ray diffraction identified overlapping reflections from ZnO and ZnS superimposed on the broad background of disordered graphitic carbon, with crystallinity increasing as zinc content rose. Raman spectroscopy recorded the characteristic D and G bands near 1350 and 1580 per centimeter, and the D-to-G intensity ratios of 0.95 to 0.97 showed that all three materials share a similar, partially graphitized structure with a moderate density of defects, a balance the authors note is favorable for electrochemistry because graphitic domains conduct electrons while defect sites promote redox reactions.

To test the materials, the team built sandwich-style fuel cells on Whatman Fusion 5 filter paper, with carbon paper electrodes one by half a centimeter in size. The anodes were coated with catalytic inks of the banana-derived composites, while the cathode carried commercial platinum on carbon. Each fuel, roughly 35 microliters, was dropped at the tip of the paper strip and wicked into the electrodes by capillarity. Once the open-circuit voltage stabilized, linear sweep voltammetry traced the polarization and power curves. The cells were first run on ethanol dissolved in sulfuric acid at concentrations from 0.1 to 1.5 molar, then on tequila at 35 percent alcohol by volume, about 6 molar ethanol, and antioquian aguardiente at 29 percent, about 5 molar, with Brix measurements of 13 and 11 confirming that soluble sugars were far lower than the ethanol content.

The ethanol experiments exposed a clear concentration dependence and a clear winner. Power densities rose as ethanol concentration increased from 0.1 to 1.0 molar for BCZ1 and BCZ2, but beyond 1.0 molar every material lost performance, a decline the authors attribute to surface deactivation and the formation of a concentration boundary layer that hinders mass transport of ethanol to active sites and removal of products. BCZ3, with its higher zinc content, behaved differently, its power density falling already between 0.5 and 1.0 molar, consistent with stronger adsorption of reaction intermediates at elevated fuel concentrations. BCZ2 consistently delivered the best results, reaching 0.12744 milliwatts per square centimeter at 1.0 molar ethanol, an order of magnitude above BCZ1 and BCZ3 at the same concentration.

The explanation lies in a structural compromise. BCZ3 offers abundant zinc-derived active sites but its low surface area of 141 square meters per gram and reduced porosity limit reactant access. BCZ1 has an open, well-developed porous network but too few active sites to drive the alcohol oxidation reaction efficiently. BCZ2 sits at the sweet spot, combining a high surface area of 239 square meters per gram with a micro-mesoporous architecture that aids diffusion, plus enough well-dispersed zinc species to provide catalytic centers and conductivity. Because the Raman ratios were nearly identical across samples, the authors conclude that the degree of graphitization was not the deciding factor; rather, the synergy between porosity, accessible surface and active-species dispersion determined performance, pointing to an intermediate zinc content as the optimal design rule.

The real-sample demonstrations are the study’s most eye-catching result. With BCZ2 as the anode, the paper-based cells produced peak power densities of 0.22436 milliwatts per square centimeter on tequila and 0.21422 on antioquian aguardiente, the tequila edging ahead thanks to its higher alcohol content. Those figures are below the best laboratory ethanol cells reported in the literature, which have reached several milliwatts per square centimeter using dichromate oxidants and dual-electrolyte designs, but the earlier devices ran on synthetic ethanol solutions. Running on beverages straight from the bottle, the authors argue, is an important step toward deploying such cells as power supplies for disposable sensors under genuine field conditions, where a cheap, pump-free, single-use power source matters more than record output.

The broader significance is twofold. Scientifically, the work shows that zinc-assisted carbonization of biomass can tune the texture and composition of ZnO-ZnS/carbon composites, and that the coexistence of ZnO and ZnS domains uniformly dispersed in a conductive carbon framework is a viable, precious-metal-free strategy for alcohol oxidation anodes. Practically, it closes a loop between agricultural waste and portable energy: peels that would otherwise rot are converted, with a zinc salt, a microwave and a furnace, into electrodes that can squeeze electricity from leftover spirits. The authors suggest the approach offers a promising path toward greener portable energy systems, and if the banana peel anode keeps performing this well on real-world fuel, the humble fruit scrap may soon find a second life at the heart of disposable diagnostic devices.

Subject of Research: Banana peel-derived ZnO-ZnS/activated carbon anodes for alcohol-fueled paper-based microfluidic fuel cells

Article Title: Performance of anodes of ZnO-ZnS/activated carbon from banana peel in paper-based microfluidic fuel cells

Article References: Mediavilla, M., Dector, A., Olivares-Ramírez, J. M., Amaya-Cruz, D. M., & Villa, A. L. (2026). Performance of anodes of ZnO-ZnS/activated carbon from banana peel in paper-based microfluidic fuel cells. Discover Electrochemistry, 3(1), Article 46. https://doi.org/10.1007/s44373-026-00136-z

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00136-z

Keywords: banana peel, activated carbon, ZnO-ZnS composite, paper-based microfluidic fuel cell, anode, alcohol oxidation, ethanol, tequila, biomass waste valorization, electrocatalysis, portable power, sustainable materials