The novelty of the work lies in its discipline. Rather than varying several synthesis parameters at once, the team isolated manganese precursor molarity as a single, independent lever—the variable that controls how supersaturated the solution becomes with Mn2+ ions at the moment of nucleation. Because the resulting powders contain no conductive scaffold or composite additives, the measured electrochemical behavior reflects the intrinsic properties of the β-MnO2 itself. That makes the study a unusually clean probe of a question that has long hovered over pseudocapacitor research: how exactly do structural defects govern whether charge is stored quickly at surfaces or slowly through bulk diffusion?

Electron microscopy revealed that the precursor concentration directly sculpted the material’s architecture. All three samples formed flower-like agglomerates of nanorods, but their mean diameter grew steadily with concentration, from 171 nanometers at 0.05 M to 268 nanometers at 0.20 M. Higher molarity means more Mn2+ supersaturation, faster nucleation and growth, and consequently larger, more developed structures. Morphology, however, turned out to be only half the story. Raman spectroscopy told a subtler tale about what was happening inside the crystal lattice.

The Raman ν2 vibrational mode of the material shifted from 647.1 to 638.5 inverse centimeters as the precursor concentration rose, while the peak broadened from 36.8 to 47.4 inverse centimeters. In the language of vibrational spectroscopy, that combination of shifting and broadening is a fingerprint of increasing oxygen-vacancy-related disorder—missing oxygen atoms in the crystal lattice that disturb the local bonding environment. Oxygen vacancies are not mere imperfections; in manganese dioxide they are known to enhance electronic conductivity and can open additional pathways for cation storage. The Indonesian team had, in effect, tuned the defect density of their electrodes simply by choosing how concentrated the starting solution should be.

To quantify how those defects translated into charge-storage behavior, the researchers turned to cyclic voltammetry and analyzed the current response using the well-established power-law framework, extracting the kinetic exponent known as the b-value at a fixed anodic potential. The b-value is a diagnostic dial: a value of 1.0 indicates ideal capacitive behavior, where current scales linearly with scan rate and charge is stored at surfaces, while 0.5 signals diffusion-controlled processes, where ions must slowly intercalate into the bulk. A b-value of 0.96 for the lowest-concentration sample (Mn-0.05) meant that electrode behaved almost like an ideal capacitor. The intermediate sample (Mn-0.10) dropped to 0.62, deep into diffusion-limited territory, and the highest concentration (Mn-0.20) recovered only partially to 0.74.

That non-monotonic pattern is the study’s most striking result. If morphology alone dictated kinetics, the trend should have been smooth. Instead, the intermediate composition was the most kinetically crippled of the three. The authors’ independent Dunn deconvolution analysis—which separates the current at each potential into surface-capacitive and diffusion-controlled contributions—reproduced the same ordering. At a scan rate of 10 millivolts per second, the capacitive fraction of stored charge was 80 percent for Mn-0.05, only 23 percent for Mn-0.10, and 44 percent for Mn-0.20. Those percentages are consistent with the values of 2b−1 predicted from the power-law exponents, a cross-check that lends confidence to the kinetic picture.

Electrochemical impedance spectroscopy filled in the mechanistic details. Fitting the spectra to an equivalent circuit comprising solution resistance, a charge-transfer resistance in parallel with a double-layer constant phase element, and a low-frequency constant phase element, the team achieved excellent fits with chi-squared values at or below 4.9 × 10−3. The charge-transfer resistance told a dramatic story: it plummeted from 3471 ohms for the intermediate sample and 174 ohms for the lowest concentration down to just 25 ohms for Mn-0.20. Meanwhile, the low-frequency exponent nL approached the Warburg limit of 0.53—the signature of semi-infinite diffusion—only for the highest-concentration material. In other words, the defect-rich, larger-structure Mn-0.20 moved charge across its interface with ease but then throttled it in slow, diffusion-limited storage.

The capacitance rankings added a final layer of nuance. Mn-0.20 delivered the highest specific capacitance, roughly 13 farads per gram, but its impedance was dominated by Warburg diffusion, meaning that capacity comes at the price of sluggish kinetics. Mn-0.05, by contrast, exhibited more ideal capacitive phase behavior, with phase angles near 70 degrees, but at a lower total capacity. Notably, capacitances derived independently from cyclic voltammetry, galvanostatic charge–discharge, and impedance spectroscopy ranked the three samples identically, and the galvanostatic and impedance values agreed to within 1.0 farad per gram—a level of internal consistency that strengthens the reliability of the conclusions.

For the supercapacitor field, the message is that precursor stoichiometry is not a dial for size alone; it is a dial for a coupled morphology–defect–kinetics relationship. Increasing manganese precursor concentration simultaneously enlarges the nanorod agglomerates and enriches the lattice with oxygen-vacancy disorder, and these two effects combine in a non-obvious way to redistribute charge between fast surface reactions and slow bulk diffusion. An electrode designer who wants rapid, high-power response might deliberately choose dilute precursor conditions to favor near-ideal capacitive behavior, while one chasing maximum capacity at moderate rates might accept the Warburg-limited regime of the concentrated recipe.

The work also speaks to a broader movement in electrochemistry toward defect engineering as a design principle. Oxygen vacancies in manganese oxides have been shown in numerous studies to boost conductivity and charge-transfer kinetics, but this study demonstrates that defect density can be controlled through something as mundane as the molarity of the starting salt—no dopants, no post-treatment, no conductive scaffolds required. Because the electrodes here are unsupported powders, the findings provide a baseline understanding of intrinsic material behavior that can later be layered onto composite and scaffold-based architectures. As demand grows for grid buffering, fast-charging electronics, and regenerative energy capture, understanding how a single synthesis variable redistributes the speed and location of charge storage brings the rational design of pseudocapacitive electrodes one concrete step closer.

Subject of Research: Effect of manganese precursor concentration on structural defects and charge-storage kinetics in hydrothermal beta-MnO2 supercapacitor electrodes

Article Title: Correlation between structural defects and charge storage kinetics in manganese oxide-based supercapacitor electrodes: the role of manganese precursor concentration

Article References: Nurfani, E., Mahardhika, L., Khamidy, N. I., Arundina, R. Y., & Marlina, R. (2026). Correlation between structural defects and charge storage kinetics in manganese oxide-based supercapacitor electrodes: the role of manganese precursor concentration. Ionics. https://doi.org/10.1007/s11581-026-07528-6

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07528-6

Keywords: supercapacitors, manganese dioxide, oxygen vacancies, hydrothermal synthesis, pseudocapacitance, charge-storage kinetics, Raman spectroscopy, electrochemical impedance spectroscopy, cyclic voltammetry, Dunn deconvolution, nanorods, defect engineering

The elegance of the design lies in pairing two materials with complementary chemistries. Manganese dioxide is a veteran of pseudocapacitor research: it is cheap, abundant, environmentally benign, and boasts a theoretical capacitance of roughly 1,370 farads per gram thanks to reversible Mn3+/Mn4+ redox reactions at or near its surface. But it has well-known Achilles heels. Its bulk is electrochemically nearly inaccessible, its electrical conductivity is poor, and it gradually dissolves and aggregates during repeated cycling. Prussian blue, the deep-blue iron hexacyanoferrate pigment first synthesized in the eighteenth century, brings exactly what manganese dioxide lacks. Its three-dimensional open framework of iron-nitrogen and iron-carbon coordination units, stitched together by cyanide bridges, creates interconnected channels through which electrolyte ions can migrate freely. More importantly, it hosts its own family of Fe2+/Fe3+ redox centers, adding a second, independent source of faradaic charge storage.

Fabrication followed a two-step sequence chosen for its industrial simplicity. First, strips of carbon cloth were activated in a hot mixture of nitric and sulfuric acid, which cleans the fibers and grafts oxygen-rich functional groups onto their surfaces, improving wettability and creating nucleation sites. The cloth was then sealed in an autoclave with acidified potassium permanganate solution and heated to 180 degrees Celsius for 24 hours, growing dense forests of manganese dioxide nanoneedles and nanorods directly on every fiber. In the second step, Prussian blue was electrodeposited onto the coated cloth by cyclic voltammetry in a solution of ferric chloride, potassium ferricyanide, hydrochloric acid, and supporting potassium chloride. The number of deposition cycles, varied from 15 to 100, became the tuning knob for how much Prussian blue loaded onto each electrode.

Microscopy revealed why the choice of 50 cycles proved decisive. With only 15 cycles, sparse Prussian blue particles dotted the carbon fibers, leaving most of the conductive network uncovered and the active material content too low. At 100 cycles, the opposite failure appeared: thick, cracked, agglomerated crusts of Prussian blue choked the porous architecture, blocking electrolyte diffusion and raising charge-transfer resistance. The PBMC-50 electrode, named for its 50 deposition cycles, hit the sweet spot, with a uniform, well-dispersed coating that preserved the interconnected fibrous framework of the cloth while maximizing the electroactive surface area. X-ray diffraction, performed on films grown on transparent conductive substrates to avoid interference from the carbon background, confirmed the coexistence of tetragonal alpha-MnO2 and face-centered cubic Prussian blue, with no impurity phases detected.

Electrochemical testing told a consistent story across every measurement technique. In a three-electrode configuration, the optimized PBMC-50 electrode delivered an areal capacitance of 636 millifarads per square centimeter at a modest current density of 0.7 milliamperes per square centimeter, and still retained 426 millifarads per square centimeter at 2 milliamperes per square centimeter. Cyclic voltammetry and galvanostatic charge-discharge measurements, which probe the electrode on different timescales, both independently identified PBMC-50 as the best performer, ruling out the possibility that the result was an artifact of a single method. Impedance spectroscopy added a mechanistic explanation: adding Prussian blue left the overall series resistance essentially unchanged, around 172.5 ohms, but sharply reduced interfacial charge-transfer resistance and produced a near-vertical low-frequency response with a phase angle approaching 80 degrees, a signature of efficient ion transport.

Kinetic analysis added a layer of nuance rarely reported for such electrodes. By separating the current response into surface-controlled capacitive and diffusion-controlled contributions, the researchers discovered markedly asymmetric behavior between the oxidation and reduction branches. The anodic b-value of 0.36 indicated a strongly diffusion-limited oxidation process, while the cathodic b-value of 0.66 pointed to a mixed regime. At the slowest scan rate of 2 millivolts per second, diffusion-controlled processes accounted for about 88 percent of the anodic charge and 77 percent of the cathodic charge, with surface-controlled contributions growing as the scan rate increased. This mixed, diffusion-influenced mechanism reflects the genuine bulk participation of the redox-active phases, a double-edged property that grants high capacity at moderate rates but limits the deepest active sites at very fast charging.

The practical payoff came when the team assembled complete devices. Using a polyvinyl alcohol-sodium sulfate gel electrolyte, they constructed six different symmetric and asymmetric all-solid-state supercapacitors and compared them head to head. The symmetric PBMC//PBMC configuration emerged as the clear winner, achieving an areal capacitance of 117 millifarads per square centimeter, more than four times that of the weakest configuration tested. After systematically mapping the voltage window with cyclic voltammetry and charge-discharge curves, the researchers settled on a wide operating range of 2.2 volts, extending from minus 1.4 to plus 0.8 volts. They deliberately avoided pushing to the highest voltage the device could technically tolerate, noting that beyond this range, polarization and incipient electrolyte oxidation begin to contaminate the response with parasitic, weakly reversible reactions.

The full-cell figures place the device among the competitive entries in the flexible supercapacitor field. The PBMC//PBMC device delivered a maximum areal energy density of 0.084 milliwatt-hours per square centimeter at a power density of 0.48 milliwatts per square centimeter, and held a capacitance of 31 millifarads per square centimeter even at a demanding 5 milliamperes per square centimeter. Endurance testing over 10,000 consecutive cycles at 7 milliamperes per square centimeter left 89.83 percent of the initial capacitance intact. Mechanical robustness proved equally impressive: capacitance retention measured at bending angles of 0, 90, and 180 degrees came in at 100, 99.14, and 98.67 percent respectively, meaning the device essentially did not notice being folded in half. Three devices wired in series successfully lit an array of ten red and yellow light-emitting diodes, and a single unit, light enough to rest on a plant leaf without bending it, demonstrated the ultralight character the design was after.

What distinguishes this work is not a single record-breaking number but the coherent integration of materials chemistry, electrode engineering, and device demonstration. By coupling manganese-based and iron-based redox systems on a conductive, mechanically resilient carbon cloth scaffold, the researchers eliminated the binders and insulating additives that typically degrade flexible electrode performance, while the Prussian blue framework simultaneously added redox capacity, improved ion pathways, and helped suppress the structural degradation that usually shortens manganese dioxide lifetimes. The hydrothermal-then-electrodeposition route uses inexpensive reagents, water-based processing, and standard laboratory equipment, making it a credible candidate for scale-up. As the market for wearables, electronic skin, medical sensors, and bendable displays continues to expand, electrode architectures of this kind, which balance energy density, durability, and manufacturability, may prove to be the quiet enabling technology behind the next generation of devices that flex with the human body.

Subject of Research: Binder-free Prussian blue-MnO2 heterostructure electrodes on carbon cloth for flexible solid-state supercapacitors

Article Title: Engineering Prussian blue-MnO 2 heterointerfaces on carbon cloth as binder-free electrodes for high-performance flexible solid-state supercapacitors

Article References: Moosavi, N., & Momeni, M. M. (2026). Engineering Prussian blue-MnO2 heterointerfaces on carbon cloth as binder-free electrodes for high-performance flexible solid-state supercapacitors. Results in Chemistry, 30, Article 103847. https://doi.org/10.1016/j.rechem.2026.103847

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103847

Keywords: flexible supercapacitors, Prussian blue, manganese dioxide, carbon cloth, binder-free electrodes, solid-state energy storage, pseudocapacitance, heterointerfaces, electrodeposition, wearable electronics, energy density, cycling stability