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