Cheap Molybdenum and Zinc Sulfide Electrode Gets a Boost Simply by Changing the Electrolyte

Supercapacitors promise to bridge the stubborn gap between the batteries that store a lot of energy slowly and the capacitors that deliver it in a flash. The catch has always been the electrode: the materials that can soak up and release enormous amounts of charge tend to be expensive, fragile, or short-lived. Now a team of researchers based at the University of Ha’il in Saudi Arabia, working with collaborators in Oman, China and elsewhere, reports a deceptively simple way to squeeze more performance out of one of the cheapest electrode chemistries available: change the concentration of the electrolyte bathing the material. Their study, published in the journal Ionics, shows that a molybdenum disulfide and zinc sulfide nanocomposite behaves remarkably differently in 6 molar potassium hydroxide than it does in 3 molar, and that this difference can be exploited to build a durable, noble-metal-free asymmetric supercapacitor.

The material at the heart of the work, labeled MZ-(1:1), is a nanocomposite of molybdenum disulfide (MoS₂) and zinc sulfide (ZnS) mixed in equal mass proportions. Both compounds belong to the family of transition-metal sulfides, a class of materials that has attracted intense attention in energy storage because their metal centers can shuttle between oxidation states during charging and discharging, storing charge through fast surface redox reactions rather than the sluggish diffusion processes that limit conventional batteries. Molybdenum disulfide, in particular, is a layered two-dimensional material whose sheets can host ions between them, while zinc sulfide brings its own electrochemical activity and structural stability to the partnership. The researchers synthesized the composite through a combination of hydrothermal processing, in which the precursors react in a sealed, pressurized water vessel at elevated temperature, and subsequent wet-processing steps that assemble the two sulfide phases together.

Before any electrochemistry took place, the team confirmed that both crystalline phases had indeed formed and coexisted in the final powder. X-ray diffraction, the workhorse technique for identifying crystal structures, revealed the characteristic diffraction signatures of both MoS₂ and ZnS, verifying that the synthesis had produced a genuine composite rather than a mixture of separate, unconnected phases. This structural verification matters because the entire premise of the composite approach rests on synergy: the two sulfides are supposed to complement each other electrochemically, with the redox activity of molybdenum ions in the +2 oxidation state and zinc ions in the +2 oxidation state contributing jointly to charge storage. If the phases had not integrated properly, the composite would simply behave like the average of its components rather than something better than either alone.

The electrochemical testing produced the study’s most striking finding. When the MZ-(1:1) electrode was cycled in a 6 molar potassium hydroxide electrolyte, it delivered a specific capacitance of 123 farads per gram at a current density of 1 ampere per gram. In the more dilute 3 molar electrolyte, the same electrode managed only 96 farads per gram under identical conditions. That difference of roughly 28 percent, achieved without changing anything about the electrode itself, underscores how much the electrolyte concentration shapes the charge storage behavior of sulfide-based pseudocapacitive materials. In alkaline electrolytes, hydroxide ions participate directly in the surface redox reactions that store charge, so a higher hydroxide concentration increases the availability of these charge-carrying species at the electrode interface, deepening the extent of the redox reactions and boosting the measured capacitance.

The authors attribute the enhanced performance to the synergistic redox activity of the molybdenum and zinc ions working in tandem. In such composites, the two metal centers can undergo overlapping but distinct faradaic reactions with hydroxide ions, effectively widening the window of potential over which the electrode stores charge and providing multiple parallel pathways for electron and ion transfer. The layered structure of MoS₂ offers abundant active sites and short diffusion distances for the electrolyte ions, while the ZnS phase contributes additional redox capacity and helps maintain the mechanical integrity of the electrode during repeated cycling. The systematic comparison between the two electrolyte concentrations elevates the work beyond a simple materials report: it offers a methodical explanation of how electrolyte engineering, a variable that is often treated as a fixed background condition, can be tuned deliberately to optimize sulfide electrodes.

Capacitance measured on a single electrode, however, is only half the story. Real devices need two electrodes working together, and asymmetric supercapacitors pair a battery-type or pseudocapacitive electrode with a capacitive counter-electrode to maximize the operating voltage window. The researchers assembled a full asymmetric device using activated carbon as the positive electrode and the MZ-(1:1) nanocomposite as the negative electrode. Activated carbon stores charge through the formation of an electrochemical double layer, a purely physical accumulation of ions at its vast internal surface, which complements the fast redox chemistry of the sulfide electrode. The pairing allowed the device to operate stably at a cell voltage of 1.1 volts, a respectable window for an aqueous alkaline system where water splitting normally constrains the upper voltage limit.

The device-level numbers tell a story of practical promise. In 6 molar potassium hydroxide, the MZ-(1:1)//activated carbon cell achieved an energy density of 9.3 watt-hours per kilogram at a power density of 2780 watts per kilogram. In the 3 molar electrolyte, the corresponding figures were 7.7 watt-hours per kilogram and 2680 watts per kilogram. While lithium-ion batteries store far more energy per kilogram, supercapacitors of this type can deliver their charge orders of magnitude faster and tolerate hundreds of thousands of cycles, making them attractive for applications that demand rapid bursts of power, such as regenerative braking, grid frequency regulation, and backup power for portable electronics. The fact that the denser electrolyte outperformed the dilute one at the device level, mirroring the half-cell results, reinforces the central claim that electrolyte concentration is a genuine optimization lever rather than an incidental detail.

Durability, the Achilles heel of many high-capacitance sulfide electrodes, proved surprisingly strong. After 6000 galvanostatic charge-discharge cycles at a current density of 2 amperes per gram, the device retained 94 percent of its initial capacitance in the 6 molar electrolyte and 85 percent in the 3 molar version. Cycle life of this order suggests that the composite structure withstands the mechanical stresses and dissolution processes that typically degrade metal sulfides in alkaline media. The zinc sulfide component may play a stabilizing role here, anchoring the structure and buffering the volume changes that accompany repeated redox reactions. For a device built entirely from abundant, inexpensive materials, this combination of rate capability and cycling endurance is a meaningful result.

The novelty of the study, as the authors frame it, lies less in the composite itself, which builds on a growing body of work on MoS₂-based and ZnS-based electrodes, and more in the formal, systematic treatment of how electrolyte concentration governs charge storage in this material system. Previous studies have explored MoS₂/ZnS composites and reported enhanced pseudocapacitive performance, but the explicit comparison of the same electrode in two electrolyte molarities, carried through from half-cell testing to full asymmetric device fabrication and long-term cycling, provides a template for how electrolyte engineering should be incorporated into electrode optimization workflows. It also demonstrates that the performance gains from a more concentrated electrolyte persist at the device level and do not come at the cost of stability.

The broader implications reach across the field of aqueous energy storage. Transition-metal sulfides are among the most heavily researched alternatives to noble-metal oxides for supercapacitor electrodes precisely because they are cheap, conductive, and electrochemically versatile, yet they have struggled to combine high specific capacitance with long-term cyclic stability in a single cost-effective system. By showing that a carefully formulated MoS₂/ZnS composite can deliver both, and that a simple change in electrolyte molarity unlocks an additional quarter of the available capacitance, the Ha’il-led team offers electrode designers a low-cost, easily implemented knob to turn. The MZ-(1:1) nanocomposite, they conclude, is a feasible and adaptable electrode material for advanced supercapacitor applications, and the electrolyte-concentration effect they documented may well prove transferable to the wider family of sulfide-based electrodes now under development worldwide.

Subject of Research: Electrolyte-concentration-dependent charge storage in MoS2/ZnS nanocomposite electrodes for asymmetric supercapacitors

Article Title: Electrolyte-dependent synergistic enhancement in MoS₂/ZnS nanocomposites electrode for asymmetric supercapacitors

Article References: Alshammari, M. F., Ali, N., Al-Hinaai, M. M., Hameed, R. A., Alshammari, F. K., Alanazi, K. D., Alhawsawi, M. A., Hegazy, A., Farghaly, O., Muhammad, D., & Shah, S. H. (2026). Electrolyte-dependent synergistic enhancement in MoS₂/ZnS nanocomposites electrode for asymmetric supercapacitors. Ionics. https://doi.org/10.1007/s11581-026-07580-2

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07580-2

Keywords: supercapacitors, MoS2, ZnS, nanocomposite, electrolyte concentration, pseudocapacitance, asymmetric supercapacitor, transition-metal sulfides, KOH electrolyte, energy storage, hydrothermal synthesis, electrode materials