The scale of the plastic problem provides the backdrop. Approximately 330 million tons of plastic are produced annually, and projections cited in the study indicate that plastic waste generation may reach 12 billion tons by 2050. PET alone accounts for roughly 30% of global plastic consumption, finding its way into beverage bottles, food packaging, containers, and textile fibers. Although mechanical recycling of PET is encouraged, effective recycling rates remain limited by collection difficulties, separation costs, and contamination. Because these polymers degrade slowly and can release toxic compounds such as phthalates and bisphenol-A, and because they fragment into microplastics already detected in water, soil, and air, researchers are increasingly turning to chemical upcycling strategies that convert plastic residues into high-value functional materials.

The Brazilian team chose one of the most ambitious of these strategies: turning PET waste into solid acid catalysts for esterification reactions related to biofuel production. The synthesis began with transparent two-liter bottles from a single soft drink brand, collected in Manaus, washed, dried, and cut into small pieces to ensure reproducibility. The plastic pieces were then subjected to chemical activation in aqueous medium with either zinc chloride (ZnCl2) or potassium carbonate (K2CO3) in a 1:1 mass ratio, heated at 80 °C for 3 hours under magnetic stirring, followed by heat treatment at 100 °C for 24 hours. The activated samples were carbonized in a tubular oven under a nitrogen atmosphere at temperatures ranging from 500 to 700 °C for the ZnCl2 route, while the K2CO3 route required 800 °C. After washing with hydrochloric acid solution and drying, the materials were functionalized by sulfonation in concentrated sulfuric acid, anchoring sulfonic acid (–SO3H) groups onto the carbon structure to confer catalytic activity.

The choice of activating agent proved decisive. ZnCl2 promotes pore formation through oxidative, templating, and gasification mechanisms: during pyrolysis, thermal dehydration of zinc oxide chloride hydrate generates gaseous ZnCl2, which reacts with carbon atoms via reduction reactions, eliminating parts of the carbon atoms and leaving behind channels that form the pore network. K2CO3, by contrast, develops pores primarily through dehydration reactions, the water-gas reaction, and the formation of gaseous potassium metal via reduction of the carbonate by carbon atoms above 760 °C. These distinct mechanisms produced markedly different textures. The K2CO3-activated sample CPK-8 achieved the highest specific surface area of 732 m2 g−1 and total pore volume of 0.61 cm3 g−1, with micropore and mesopore volumes of 0.28 and 0.32 cm3 g−1 respectively. The ZnCl2-activated sample carbonized at 600 °C, CPZn-6, reached 272 m2 g−1 with total, micropore, and mesopore volumes of 0.36, 0.14, and 0.22 cm3 g−1. Nitrogen adsorption isotherms classified the ZnCl2-derived materials as IUPAC type I(a), dominated by micropores centered around 0.9 nm, while CPK-8 showed type I(b) isotherms with H4 hysteresis characteristic of micro-mesoporous carbons with pores centered at 0.7 nm.

Counterintuitively, the material with the highest surface area was not the best catalyst. The K2CO3-activated sample achieved only 65% conversion of oleic acid to methyl oleate, and its performance decreased by 52% after the first reuse. The researchers attribute this to the fact that activation with K2CO3 favors more stable carbon surfaces with lower concentrations of oxygenated functional groups, which hinders effective incorporation of sulfonic acid groups during functionalization. The catalytic behavior of sulfonated carbons, the study concludes, depends more on the density and accessibility of acidic sites than on surface area alone.

The best overall performance came from CPZn-6*, the ZnCl2-activated sample carbonized at 600 °C and sulfonated at 80 °C for 2 hours. This catalyst achieved 88% conversion to methyl oleate and remained effective through the third reuse cycle, a stability advantage over comparable plastic-derived catalysts reported in the literature, which typically show pronounced conversion decreases after the second cycle. The sulfonic group density of CPZn-6*, measured by Boehm titration at 1.31 mmol g−1, is comparable or superior to catalysts derived from coconut shells (0.63–1.18 mmol g−1) and microwave-assisted syntheses (1.12 mmol g−1). Notably, CPZn-6* achieved its high conversion under relatively mild sulfonation conditions of 80 °C for 2 hours, whereas previous PET-derived catalysts required severe conditions of 150 °C for 15 hours and exhibited surface areas below 133 m2 g−1. Increasing the carbonization temperature from 500 to 700 °C raised conversion from 63% to 90% for CPZn-7*, but that catalyst, despite slightly higher acidity of 1.43 mmol g−1, lost stability after the second cycle. The 600 °C activation condition apparently promoted the most stable incorporation of –SO3H groups within the porous structure, contributing to improved resistance against leaching.

An extensive characterization campaign validated the structure and chemistry of the materials. Thermogravimetric analysis recorded three mass loss events for the activated carbons: water desorption between 30 and 150 °C, decomposition of the carbonaceous material between 150 and 350 °C, and thermal decomposition of the PET polymer into volatile species such as CO2 between 360 and 500 °C. For the sulfonated samples, mass losses between 150 and 360 °C attributed to strongly attached sulfonic groups were 14%, 10%, and 15% for CPZn-5*, CPZn-6*, and CPZn-7* respectively, consistent with the Boehm titration results. FT-IR spectroscopy confirmed covalently bonded sulfonic groups through a vibration band at 1,025 cm−1 associated with the O=S=O bond, while thermogravimetry coupled to mass spectrometry detected released water, SO2, and SO3 gases from decomposition of the sulfonic functionalities. Scanning electron microscopy revealed rough, irregular surfaces with cavities of various sizes in the activated carbon, which facilitate reagent diffusion and acid group anchoring; after sulfonation, the surface became smooth and compact. Energy-dispersive spectroscopy showed an increase in oxygen (16% by weight) and sulfur (5% by weight) after sulfonation, confirming effective anchoring of the acid sites.

X-ray photoelectron spectroscopy provided the most precise picture of how sulfur is bonded to the carbon matrix. The C1s spectrum of CPZn-6* showed a peak at 286.1 eV assigned to C–S bonds, strongly suggesting that sulfur was chemically incorporated into the carbonaceous structure rather than merely physically adsorbed. The high-resolution S2p spectrum revealed the characteristic doublet at 167.8 and 168.9 eV, binding energies characteristic of sulfur in the S6+ oxidation state associated with sulfonic acid groups. The O1s spectrum showed a component at approximately 534.0 eV attributed to O=S=O species, providing additional evidence of successful functionalization. According to the literature cited by the authors, the presence of C–S bonds in highly defective carbons derived from polymeric precursors contributes to improved thermal and chemical stability of sulfonated catalysts, explaining the resistance to leaching observed during reuse cycles.

The catalytic optimization revealed clear parameter sensitivities. Sulfonation temperature mattered greatly: 80 °C yielded the maximum 88% conversion, while 60 °C dropped the yield to approximately 70%, and temperatures above 100 °C also reduced performance, apparently because excessive temperatures partially degrade the carbonaceous structure and its anchoring sites. Sulfonation time, by contrast, was not the predominant parameter, with conversion stabilizing around 85% from 2 to 8 hours, suggesting rapid saturation of sulfonic groups on the PET-derived carbon matrix. In the esterification reaction itself, conducted in a high-pressure reactor with an oleic acid to methanol molar ratio of 1:12 and 5% catalyst loading, conversion increased with temperature up to 100 °C and with reaction time up to 1 hour, both reaching the 88% maximum. Higher temperatures decreased the yield, likely because accelerated methanol evaporation reduces reactant availability, and longer reaction times favored interaction of the water byproduct with the hydrophilic sulfonic groups, contributing to partial deactivation through competitive adsorption and site hydration.

The deactivation mechanism was confirmed spectroscopically. After four consecutive reaction cycles, the FT-IR band at 1,025 cm−1 attributed to sulfonic groups disappeared, and thermogravimetric profiles of the reused catalyst showed reduced mass loss associated with sulfonic group decomposition, corroborating the conversion drop below 50% in the third cycle due to leaching of the –SO3H groups. The catalytic activity of CPZn-6*, the authors conclude, is strongly associated with the preservation of surface acidic groups during reuse. Despite this limitation, the study demonstrates that PET waste can be converted into functional carbon-based catalysts with potential application in sustainable biofuel processes, offering what the researchers describe as a sustainable and promising alternative for reusing PET waste while mitigating the environmental problems associated with plastic accumulation in landfills, soils, and oceans.

Subject of Research: Porous carbons from pet waste: structure–property relationships and catalytic performance in esterification reactions

Article Title: Porous carbons from pet waste: structure–property relationships and catalytic performance in esterification reactions

Article References: Porous carbons from pet waste: structure–property relationships and catalytic performance in esterification reactions. (n.d.). https://doi.org/10.3389/frcrb.2026.1873847

Image Credits: AI Generated

DOI: 10.3389/frcrb.2026.1873847

Keywords: PET waste, porous carbon, heterogeneous catalysis, esterification, biodiesel, sulfonation, plastic upcycling, Frontiers in Carbon, Porous, carbons, waste, structure

The composite anode, assembled in a standard CR2032 3-volt half-cell, delivered a specific capacity of 3,412.44 milliampere-hours per gram at a rate of 0.56 C, with an energy density of 204.74 watt-hours per kilogram. The initial capacity of the half-cell, 3,103.42 milliampere-hours per gram, declined to 1,674.53 milliampere-hours per gram after 200 cycles at 2.57 C. Those numbers may not sound like a complete victory, but in the silicon anode field, where pristine silicon electrodes often collapse within a few dozen cycles, the retention of more than half the initial capacity over 200 cycles at a demanding rate is a meaningful achievement. The team attributes the resilience to a dual charge-storage mechanism that blends faradaic bulk reactions with pseudocapacitive surface processes.

The structural analysis reveals why the material behaves the way it does. X-ray diffraction showed sharp crystalline silicon peaks at 28.91, 47.76, 56.69, 69.69, and 76.97 degrees, corresponding to the (111), (220), (311), (400), and (331) planes, alongside a broad amorphous silicon dioxide hump near 14 degrees. Using the Scherrer equation, the team calculated an average crystallite size of 96.75 nanometers, closely matching the roughly 100-nanometer particle size supplied by the manufacturer. The amorphous silicon dioxide layer, far from being a defect, acts as a passivation shell that protects the crystalline core from further oxidation and helps absorb the mechanical strain of repeated lithium insertion and extraction.

Fourier-transform infrared spectroscopy confirmed the surface chemistry. Broad silanol bands between 3,300 and 3,500 inverse centimeters, together with Si-O-Si asymmetric stretching vibrations at 1,192.31, 1,084.17, and 885.56 inverse centimeters, revealed a hydrated, siloxane-rich surface. The team notes that these silanol groups enhance wettability, improving electrolyte penetration and lithium-ion diffusion during cycling. Notably, the absence of carbon-hydrogen stretching peaks in the 2,852 to 2,924 inverse centimeter region indicated high material purity with minimal organic contamination, while residual Si-H stretching modes at 1,868 and 2,258 inverse centimeters pointed to some surface hydride bonds.

Brunauer-Emmett-Teller and Barrett-Joyner-Halenda analysis painted a picture of a mesoporous architecture with a specific surface area of 61.89 square meters per gram and an average pore diameter of 38.50 nanometers. Field-emission scanning electron microscopy showed porous spherical clusters formed by aggregated particles ranging from 50 to 200 nanometers. Raman spectroscopy found a sharp first-order silicon-silicon vibration peak at 523 inverse centimeters, confirming a highly ordered crystalline lattice with minimal defects and low residual stress. Perhaps most strikingly, a water contact angle measurement showed the electrode surface to be superhydrophilic, with droplets spreading completely at a zero-degree angle, ensuring uniform electrolyte contact across the entire coating.

Cyclic voltammetry between 0.0 and 3.0 volts at scan rates from 20 to 200 millivolts per second revealed redox peaks near 1.5 and 1.0 volts corresponding to lithium insertion and deinsertion. The absence of a distinct intercalation peak between 1.0 and 0.5 volts suggested that lithium storage in this voltage window proceeds through surface-dominated faradaic processes rather than classical bulk intercalation. When the team measured stability at 200 millivolts per second across 700 cycles, the voltammetric profiles barely changed, indicating sustained redox activity and preserved electrochemical performance over long-term operation.

The charge-storage kinetics were dissected using a power-law relation between peak current and scan rate. The analysis showed that the anode stores the majority of its charge through diffusion-controlled bulk redox reactions, with a capacitive contribution of 37.79 percent at 200 millivolts per second. As the scan rate decreased, the diffusion-controlled contribution increased, driving higher specific capacity. The team suggests that the increased film thickness of the coated electrode allows deeper penetration of the LiPF6 electrolyte, promoting bulk redox activity throughout the active material rather than confining reactions to the surface.

Galvanostatic charge-discharge measurements across a wide range of C-rates, from 0.56 C all the way to 2,155.69 C, showed discharge plateaus associated with electrolyte decomposition and solid electrolyte interface formation at all current densities. The lithium-silicon alloy phases evolved systematically during cycling: Li15Si4 formed in the initial stage, transitioning to Li21Si5 as capacity declined, and later to Li13Si4. Despite the gradual capacity fade, structural integrity was maintained throughout the 200-cycle stability test, and the reversible recovery of capacitance and capacity when the rate was lowered back to 0.56 C demonstrated the material’s ability to withstand expansion-induced strain without catastrophic degradation.

Electrochemical impedance spectroscopy before and after cycling showed that the addition of activated charcoal and carbonized sucrose powder reduced the film resistance of the anode. These carbon additives stabilized the solid electrolyte interface, reduced continuous electrolyte decomposition, and helped form a relatively thin and conductive interphase layer. The team notes that the impedance spectra contained both semicircles and oblique lines, with the low-frequency linear region associated with lithium-ion diffusion into the electrode and the high-frequency semicircle reflecting charge-transfer resistance at the interface.

The study, published in Frontiers in Batteries and Electrochemistry, offers a pathway that sidesteps the expensive raw materials, complex synthesis routes, and harsh reaction conditions that have limited many previous silicon anode strategies. By combining commercially available porous silicon nanoparticles with two carbon sources that cost pennies, the researchers have assembled an anode that balances high conductivity, structural stability, and scalable fabrication. The work does not claim to have solved every problem facing silicon anodes, and the capacity fade over 200 cycles remains substantial. But the demonstration that a simple three-component composite can deliver over 3,400 milliampere-hours per gram with meaningful cycle life suggests that the road to practical high-energy silicon anodes may be paved with ingredients as ordinary as table sugar and charcoal.

Subject of Research: Dual mechanism enhanced capacitive storage and diffusion kinetics in hierarchically porous spherical cluster silicon nanoparticles-carbonized sucrose-activated charcoal anode enable high energy in lithium-ion CR2032 half-cell

Article Title: Dual mechanism enhanced capacitive storage and diffusion kinetics in hierarchically porous spherical cluster silicon nanoparticles-carbonized sucrose-activated charcoal anode enable high energy in lithium-ion CR2032 half-cell

Article References: Thombare, S., Humane, R., Kalubarme, R., Jadhav, H., Malavekar, D., Shaikh, S., & Lokhande, C. (2026). Dual mechanism enhanced capacitive storage and diffusion kinetics in hierarchically porous spherical cluster silicon nanoparticles-carbonized sucrose-activated charcoal anode enable high energy in lithium-ion CR2032 half-cell. Frontiers in Batteries and Electrochemistry, 5, Article 1913828. https://doi.org/10.3389/fbael.2026.1913828

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1913828

Keywords: Dual, mechanism, enhanced, capacitive, storage, diffusion, kinetics, hierarchically, porous, spherical, cluster, silicon