The trick behind the material lies in how it is made. Rather than precipitating nickel sulfide particles directly from solution, the researchers used a metal-organic framework, or MOF, as a sacrificial template. MOFs are crystalline lattices built from metal ions connected by organic linker molecules, and they are famous for being riddled with regularly spaced pores at the molecular scale. By converting this framework through a sulfurization strategy, the team preserved a memory of the original porous structure while transforming the chemistry from a hybrid organic-inorganic framework into a pure inorganic sulfide. The result is a sponge-like NiS2 architecture in which mesopores, pores in the range of roughly two to fifty nanometers, are abundant and interconnected, producing a high specific surface area that would be difficult to achieve by conventional synthesis routes.

That architecture matters because the performance of a battery or supercapacitor electrode is governed largely by how easily ions can reach the electroactive sites where charge storage happens. In a dense, poorly connected material, ions from the electrolyte must diffuse long distances through tortuous pathways, and much of the interior of the material is effectively wasted. In the sponge-like NiS2, the abundant mesopores act as highways for hydroxide ions and other electrolyte species, shortening diffusion lengths dramatically, while the large surface area exposes a vast number of electroactive sites for the surface redox reactions that store charge. The hierarchical porosity also helps accommodate the volume changes that accompany repeated charging and discharging, a key factor in the material’s observed cycling stability.

The electrochemical results are the heart of the study. When tested as a supercapacitor electrode in alkaline electrolyte, the NiS2 electrode delivered a specific capacitance of 1227.5 F g−1 at 0.5 A g−1, a value that reflects an exceptional density of accessible charge-storage sites. Specific capacitance measures how much electric charge a gram of electrode material can hold per volt of potential, and values above one thousand farads per gram are characteristic of high-performance pseudocapacitive materials, which store charge through fast, reversible surface reactions rather than purely electrostatic adsorption. Nickel sulfides are well suited to this role because nickel can shuttle between multiple oxidation states during charging and discharging, enabling rich redox chemistry at the electrode surface.

Perhaps the most commercially interesting finding is that the material is bifunctional. The same NiS2 electrode that excels in a supercapacitor configuration can also serve as the cathode in an aqueous nickel-zinc battery, where it delivers high specific capacity and stable long-term performance. Ni-Zn batteries are an attractive technology because they use water-based electrolytes, which are inherently safer and cheaper than the flammable organic solvents found in lithium-ion cells, and because zinc is abundant and easy to handle. The catch has historically been finding cathode materials that can withstand the demanding alkaline environment while storing enough charge to make the cells competitive. The demonstration that a single MOF-derived sulfide can serve both device types suggests a versatile platform rather than a one-off material.

The study situates itself within a broader surge of interest in aqueous energy storage. Water-based devices occupy a compelling middle ground between supercapacitors, which charge in seconds but store limited energy, and conventional batteries, which store more energy but charge more slowly and carry greater safety burdens. Recent literature highlighted in the paper’s reference list spans electrochemical activation tactics for aqueous devices, rechargeable zinc-based storage, molecularly crowding electrolytes for micro-supercapacitors, and flexible yarn-like Ni-Zn batteries for wearable electronics. Transition metal sulfides, including nickel sulfides such as Ni3S4 and NiS2, have emerged as leading candidates in this space because they combine high theoretical capacity, good electrical conductivity relative to oxides, and low cost.

The MOF-derived approach itself is part of a wider design philosophy in materials chemistry. By using a preassembled framework as a template, chemists can impose order on a material at the nanoscale that would be hard to achieve through direct synthesis. In this case, the sulfurization step converts the nickel-containing framework into NiS2 while retaining the sponge-like morphology, a process analogous to casting a sculpture in a mold. Earlier work by some of the same authors produced accordion-like iron-doped NiS2 and surface-amorphized nickel sulfides for aqueous storage, indicating that the group has been systematically exploring how morphology and surface chemistry tune the performance of nickel sulfide electrodes. The new sponge-like architecture extends that program by emphasizing interconnected mesoporosity as the central design feature.

There are, of course, caveats that temper any immediate industrial enthusiasm. The reported measurements were made on laboratory-scale electrodes in alkaline electrolyte, and translating a promising lab material into a commercial device requires solving problems that go beyond the cathode itself, including zinc anode durability, electrolyte optimization, and full-cell engineering. The paper’s own framing is careful: the authors describe the work as establishing NiS2 as a promising electrode material for aqueous energy storage devices and providing a feasible synthetic pathway for designing high-performance transition metal sulfide electrodes, rather than claiming a ready-made product. Still, the combination of high capacitance, cycling stability, and bifunctionality in a single low-cost material is exactly the kind of result that device engineers look for when deciding which candidates to pursue.

The broader significance of the work lies in its demonstration that structure, not just composition, is the lever that unlocks performance. Nickel disulfide has been recognized as a sustainable, low-cost electrode material before, but the sponge-like hierarchical architecture reported here shows how much headroom remains in a familiar compound when its porosity is engineered deliberately. As the demand for safe, inexpensive, grid-scale and wearable energy storage continues to grow, strategies that turn molecularly precise templates into functional inorganic electrodes are likely to play an increasing role. For now, a synthetic sponge made of nickel and sulfur, born from a crystalline framework and hardened into a charge-hungry electrode, offers a vivid example of how rational design at the nanoscale can translate directly into better electrochemical numbers.

Subject of Research: MOF-derived hierarchical porous NiS2 electrode materials for aqueous supercapacitors and Ni-Zn batteries

Article Title: MOF-derived sponge-like NiS2 with a hierarchical porous architecture for aqueous supercapacitors and Ni-Zn batteries

Article References: Gong, G., Liu, X., Ma, H., Lv, D., Luo, H., Liang, M., Zhou, Y., Wang, H., Zhao, Y., Lashari, N. U. R., & Miao, Z. (2026). MOF-derived sponge-like NiS2 with a hierarchical porous architecture for aqueous supercapacitors and Ni-Zn batteries. Ionics. https://doi.org/10.1007/s11581-026-07558-0

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07558-0

Keywords: NiS2, metal-organic framework, supercapacitor, Ni-Zn battery, aqueous energy storage, transition metal sulfide, mesoporous structure, electrode material, pseudocapacitance, sulfurization, energy storage, Ionics

A new study published in the journal Ionics by Zhenyun Zhao, Naihui Hou, and colleagues at Zhejiang Sci-Tech University, working with Yuwen Cheng of Anhui University of Technology, tackles that question head-on. The team’s central insight is that the counter anions in the electrolyte, long treated as passive spectators because they do not carry charge between electrodes, actually play a decisive role at the electrode-electrolyte interface. By combining electrochemical measurements with density functional theory simulations and spectroscopic evidence, the researchers show that ammonium ions and electrolyte anions form interfacial ion pairs that reshape the local electric field, alter charge-transfer barriers, and ultimately govern both capacitance and rate performance. The work provides one of the most systematic pictures to date of how anion identity controls interfacial ammonium-ion storage.

To isolate interfacial effects from the confounding variables of complex electrode architectures, the team needed a structurally well-defined model electrode. They chose TatDha-COF, a fully conjugated covalent organic framework synthesized through the condensation of 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline with 2,3-dihydroxyterephthalaldehyde in a sealed glass tube heated at 120 degrees Celsius for 72 hours, yielding red powders at 60% efficiency. The framework features a 3.2-nanometer pore size, a hollow microtubular morphology visible in electron microscopy, and a crystalline hexagonal lattice confirmed by powder X-ray diffraction peaks at 2.8 and 26.38 degrees. X-ray photoelectron spectroscopy verified the expected carbon, nitrogen, and oxygen chemistry, including imine C=N groups, C-N linkages, and hydroxyl C-OH functionalities. Critically, the one-dimensional channel architecture and highly exposed surface groups promote pseudocapacitive behavior, making the material an ideal platform for studying what happens at the interface rather than optimizing a commercial device.

The electrochemical comparison across three representative ammonium electrolytes produced a result that initially looks like a paradox. In 1 M ammonium acetate, TatDha-COF delivered the highest specific capacitance of the three salts tested, yet it suffered the poorest rate capability, with capacitance falling off more steeply as scan rates increased from 1 to 5 millivolts per second than in either 0.5 M ammonium sulfate or 1 M ammonium chloride. In ammonium sulfate, the opposite held true: excellent rate performance came at the expense of specific capacitance. Cyclic voltammetry revealed three pairs of redox peaks in the sulfate and chloride electrolytes but only two pairs in ammonium acetate across a stable window of -0.9 to 0.9 volts versus Ag/AgCl. A two-electrode coin cell in ammonium acetate retained 61% of its initial capacitance after 100 cycles at 1 ampere per gram while maintaining a Coulombic efficiency near 100%, indicating highly reversible charge-discharge processes despite gradual decay.

Kinetic analysis using the well-known b-value method, in which the slope of log peak current versus log scan rate distinguishes diffusion-controlled from adsorption-controlled behavior, added an important layer of nuance. Values approaching 1 indicate adsorption-controlled, capacitive processes, while values near 0.5 indicate diffusion control. For the dominant redox pair between -0.3 and 0 volts, the b values were 1.03 and 0.85 in ammonium sulfate, 0.82 and 0.70 in ammonium chloride, and 0.84 and 0.74 in ammonium acetate. This shows that the redox reaction was predominantly adsorption-controlled in all three electrolytes, but with a stronger adsorption contribution in ammonium sulfate. Taken together with the capacitance and rate data, the results suggest that ammonium acetate and ammonium chloride favor pronounced pseudocapacitive behavior, whereas ammonium sulfate leans toward electric-double-layer characteristics.

The theoretical heart of the paper lies in the density functional theory calculations, performed with the Vienna ab initio simulation package using the PBE functional, projected augmented wave method, and Grimme’s DFT-D3 dispersion correction. The team first identified the preferential adsorption site for bare NH4+ on the framework by comparing two distinct imine nitrogen environments, labeled N1 and N2. Adsorption at N1 proved more thermodynamically favorable because the ammonium ion can simultaneously form two hydrogen bonds, an N-H…N bond with an adjacent C=N group and an N-H…O bond with a neighboring hydroxyl, anchoring it firmly to the surface. All subsequent simulations therefore centered on the N1 site, providing a consistent atomic-scale stage on which to test how different anions modify the adsorption picture.

What emerged from the ion-pair calculations is the study’s most consequential finding. Although anions do not adsorb directly onto the negatively biased electrode, they can coordinate with the already-adsorbed ammonium ion, sitting on its distal side and forming stable interfacial complexes. The calculated adsorption energy for a [NH4+(SO4^2-)] pair was -4.09 electron volts, while a [NH4+(AC^-)3] complex with three acetate ions reached -3.05 electron volts, both substantially more favorable than bare NH4+ adsorption alone and both far lower than a single [NH4+(AC^-)] pair. This confirms that interfacial ion-pair formation is thermodynamically spontaneous. The stronger association of three acetate ions with one ammonium ion, compared with one sulfate paired with one ammonium, means the acetate complex is harder to rearrange and desorb during charging, which raises the interfacial charge-transfer barrier and slows ion diffusion, exactly the behavior observed in impedance measurements.

Experimental evidence backed the simulations. Ex situ X-ray photoelectron spectroscopy of the discharged electrode revealed a weak but discernible high-binding-energy component between 402 and 405 electron volts in the N 1s spectrum, consistent with chemically adsorbed ammonium, alongside a weakening of C=N peaks and strengthening of C-N and N…H-N features. More tellingly, the O 1s spectrum showed a new component at approximately 531.3 electron volts after discharge, assignable to the carboxylate group of acetate anions retained at the surface through strong association with ammonium. Electrochemical impedance spectroscopy at different voltage states completed the picture: the ammonium acetate electrolyte exhibited a notably larger charge-transfer resistance and a steeper Warburg slope than ammonium sulfate, indicating higher interfacial barriers and slower ion diffusion, in direct agreement with the computed ion-pair energetics.

The final piece of the puzzle came from Bader charge analysis, which quantifies how electrons redistribute across the interface. The total charge transferred to the [NH4+(AC^-)3] pair was 0.38 electrons, nearly eight times the 0.05 electrons transferred to [NH4+(SO4^2-)]. The reason lies in polarizability: acetate is far more polarizable than sulfate, and it strongly perturbs the ammonium ion’s symmetric tetrahedral electronic environment. The charge dispersion among the four hydrogen atoms of adsorbed ammonium grew from roughly 0.04 electrons in the sulfate complex to about 0.11 electrons in the acetate complex, a signature of strong anisotropic polarization. This distortion enhances the local positive charge density on the electrode-proximal side of the ammonium ion and increases the number of electrons transferred at the interface. Since more transferred electrons mean more stored charge, this mechanism neatly explains why ammonium acetate yields higher capacitance than ammonium sulfate.

The study thus resolves the capacitance-rate trade-off into a single coherent picture rooted in interfacial coordination chemistry. Acetate anions, by pairing tightly with adsorbed ammonium, boost the electron transfer number and pseudocapacitive charge storage but simultaneously immobilize the interface, degrading rate performance. Sulfate forms a less stabilizing, more mobile ion pair that sacrifices capacitance for speed. The work, which builds on earlier interface-focused studies of acetate electrolytes with MXene electrodes, establishes that electrolyte anions are not merely charge-balancing bystanders but active architects of the ammonium storage process. For a technology whose appeal rests on safety and cost, that realization could prove pivotal: rational electrolyte design, tuned at the level of the anion, now stands out as a powerful and largely unexplored lever for engineering the next generation of ammonium-ion supercapacitors.

Subject of Research: Role of electrolyte anions in interfacial ammonium-ion storage in supercapacitors

Article Title: Role of electrolyte anions in governing ammonium-ion storage performance

Article References: Zhao, Z., Hou, N., Ming, S., Su, Q., Cheng, Y., Wang, T., Chen, H., & Chen, W. (2026). Role of electrolyte anions in governing ammonium-ion storage performance. Ionics. https://doi.org/10.1007/s11581-026-07539-3

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07539-3

Keywords: ammonium-ion supercapacitors, electrolyte anions, electrode-electrolyte interface, covalent organic framework, interfacial ion pairs, pseudocapacitance, density functional theory, Bader charge analysis, electrochemical impedance spectroscopy, aqueous energy storage, ammonium acetate, charge transfer kinetics