The story begins with one of the most abundant and cheapest starting materials imaginable. Fresh Bauhinia variegata flowers were gathered from the campus of P P Savani University in Surat, washed thoroughly with tap water, double-distilled water and ethanol, and dried first in the sun and then overnight at about 100 degrees Celsius. The dried petals were ground into a fine powder and mixed with zinc chloride in a one-to-two weight ratio, then left for a full month to allow deep impregnation. Zinc chloride activation is a well-established chemical route to porous carbon: during subsequent high-temperature treatment it promotes dehydration, aromatization and the development of an extensive pore network throughout the carbon matrix.

Carbonization itself took place in a tubular furnace under a continuous flow of nitrogen gas. The temperature was ramped from room temperature to approximately 900 degrees Celsius at a controlled 5 degrees Celsius per minute and held there for thirty minutes, driving off oxygen-containing surface groups and locking in the porous architecture. After natural cooling, the blackened product was crushed, washed with concentrated hydrochloric acid to strip out residual activating agent and inorganic impurities, and rinsed repeatedly with hot deionized water until the filtrate reached a neutral pH. X-ray photoelectron spectroscopy confirmed the thoroughness of this purification: no zinc signal appears in the spectra, because at 900 degrees Celsius zinc species largely volatilize thanks to the element’s high vapor pressure, and any survivors were dissolved away during the acid wash.

The innovation that elevates this work beyond routine biomass carbonization is the manganese doping step. Rather than relying on simple soaking, the team dispersed the activated carbon in ethanol and added manganese(III) oxide corresponding to a 2 weight percent manganese loading, then subjected the suspension to 70 minutes of high-intensity probe sonication at 750 watts and 20 kilohertz in pulsed mode. Acoustic cavitation, the rapid formation and collapse of microscopic bubbles in the liquid, generates intense local temperature and pressure spikes that drive precursor particles into the carbon pores and anchor them to the surface with unusual uniformity. Ethanol was replenished periodically during treatment to compensate for evaporation, and the dried powder was subsequently annealed at 450 degrees Celsius under nitrogen for three hours to strengthen the metal-carbon interactions and stabilize the manganese species.

The spectroscopic fingerprints of the resulting Mn-doped activated carbon, abbreviated Mn-AC, tell a convincing story. High-resolution XPS of the C 1s region shows peak broadening relative to the pristine material, evidence of increased surface functionalization after manganese loading. The O 1s spectrum deconvolutes into components assigned to Mn-O-C bonding near 530.0 electron-volts, surface hydroxyl groups near 531.6 electron-volts, and highly oxygenated species near 533.5 electron-volts, reflecting the strong metal-carbon interactions forged during sonication. The Mn 2p region displays the characteristic 2p3/2 and 2p1/2 doublet, confirming coexisting Mn3+ and Mn4+ oxidation states on the carbon surface. Scanning electron microscopy revealed the complementary morphological shift: smooth, compact, plate-like structures in the pristine carbon give way to a roughened, granular, nodular texture after doping, with energy-dispersive X-ray mapping confirming that carbon, oxygen and manganese are distributed evenly across the framework without agglomeration.

With the material characterized, the team fabricated two symmetric electrochemical double-layer capacitor cells using a gel polymer electrolyte composed of poly(vinylidene fluoride-co-hexafluoropropylene) and the ionic liquid 1-ethyl-3-methylimidazolium tricyanomethanide. Electrode slurries containing 90 weight percent active material and 10 weight percent binder were drop-cast onto graphite sheets to a thickness of 50 to 80 micrometers, with a mass loading of roughly 1 milligram per square centimeter. Electrochemical impedance spectroscopy measured from 100 kilohertz down to 10 millihertz delivered the first clear verdict: the Mn-AC cell achieved a specific capacitance of 20 farads per gram at low frequency, compared with 15 farads per gram for the undoped carbon. The authors attribute the gain to increased density of polar surface functionalities and adsorption sites, improved electrolyte wetting and ion accessibility within the pores, and additional fast surface redox contributions from the manganese species.

Cyclic voltammetry reinforced the picture. After identifying an optimized window of minus one to plus one volt, the researchers swept scan rates from 5 to 100 millivolts per second. The Mn-AC cell produced CV profiles that were noticeably more rectangular than those of the pristine device, a hallmark of superior double-layer formation, and the calculated specific capacitances at 5 millivolts per second were 13.20 and 8.59 farads per gram respectively. Galvanostatic charge-discharge testing added the durability data: the pristine carbon delivered about 2 farads per gram at 1 milliampere with an energy density of 0.38 watt-hours per kilogram and a power density of 1160 watts per kilogram, while Mn-AC reached approximately 9.80 farads per gram, an energy density of 1.42 watt-hours per kilogram and a power density of 1020 watts per kilogram. Crucially, both cells remained roughly stable over 10,000 charge-discharge cycles.

The second act of the study is arguably the more striking. Counter electrodes are a critical and often expensive component of dye-sensitized solar cells, which conventionally rely on platinum. Here the researchers coated layers of AC and Mn-AC onto fluorine-doped tin oxide glass, paired them with titanium dioxide working electrodes sensitized with N719 dye, and sandwiched a previously reported PEO-KI/I2-EMImSCN polymer electrolyte between them. Under one-sun illumination from a solar simulator, the Mn-AC device nearly doubled the power conversion efficiency of the pristine-carbon cell, driven primarily by a substantial increase in short-circuit current density while open-circuit voltage and fill factor stayed comparable. The improvement points to enhanced electrocatalytic activity toward the iodide-triiodide redox reaction and faster charge transfer at the electrode-electrolyte interface, both direct consequences of manganese incorporation.

The broader context makes the result notable. Recent literature has seen explosive interest in biomass-derived carbons, from machine-learning-guided designs predicting surface areas near 2822 square meters per gram and capacitances around 322 farads per gram, to nitrogen- and phosphorus-doped porous carbons achieving energy densities approaching 49 watt-hours per kilogram. Flower-derived carbons in particular have proven versatile, serving as dye adsorbents, carbon dioxide capture media and aptasensor substrates. What this study adds is a demonstration that a modest, sonochemically delivered transition-metal dose can simultaneously upgrade a carbon’s capacitive behavior and its electrocatalytic performance, without sacrificing the porous framework or requiring scarce, costly noble metals. The authors caution that excessive manganese loading could block pores and hinder ion transport, an effect not specifically investigated here, and that no systematic optimization of manganese content was performed.

The ionic liquid electrolyte deserves its own mention. Unlike volatile, flammable organic solvents, ionic liquids are essentially non-volatile salts that remain liquid below 100 degrees Celsius, offering wide operating voltage windows and a highly ionized environment. The team’s earlier work identified EMImTCM as particularly well suited to both supercapacitor and dye-sensitized solar cell platforms, and pairing it with a PVdF-HFP gel matrix here produced a mechanically robust, safer electrolyte suited to flexible device concepts. Looking forward, the authors suggest that polymer-based electrolytes, tuned through further doping and composite formation, could push device performance well beyond what aqueous-electrolyte systems reported in most prior literature achieve. For now, the takeaway is simple and compelling: a tree that dots the streets and gardens of South Asia sheds flowers that, with a zinc chloride bath, a blast of nitrogen and seventy minutes of ultrasound, become a genuine dual-purpose energy material.

Subject of Research: Sonochemically manganese-doped activated carbon derived from Bauhinia variegata flowers for supercapacitor and dye-sensitized solar cell electrodes

Article Title: Sonochemically manganese doped activated carbon from Bauhinia variegata for dual energy application

Article References: Punetha, V. D., Dhapola, P. S., Singh, P. K., Matiyani, M., Singh, R., Kumar, S., & Pathak, V. (2026). Sonochemically manganese doped activated carbon from Bauhinia variegata for dual energy application. Discover Electrochemistry, 3(1), Article 71. https://doi.org/10.1007/s44373-026-00159-6

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00159-6

Keywords: activated carbon, Bauhinia variegata, manganese doping, sonochemical synthesis, supercapacitor, dye-sensitized solar cell, biomass-derived carbon, ionic liquid electrolyte, energy storage, electrocatalysis, XPS, gel polymer electrolyte