The research, led by Liang Li and corresponding author Yuqi Chen, set out to test whether samarium sulfide modification, which had shown promise in earlier work on other rare-earth sulfide electrodes, would produce comparable benefits when applied to two closely related hosts: neodymium sulfide, Nd2S3, and praseodymium sulfide, Pr2S3. On paper, the experiment looks almost like a control study. Neodymium and praseodymium sit side by side on the periodic table, their trivalent ions differ only modestly in radius, and both form sesquisulfides with similar structural motifs. If samarium modification were a universal performance enhancer, both composite electrodes should have improved in step. Instead, the team found that the same treatment that supercharged the neodymium-based material actually degraded the praseodymium-based one, a result with real consequences for how composite electrodes are designed.

The first surprise came before any electrochemistry was attempted. When the researchers synthesized the nominal compounds NdSmS3 and PrSmS3, powder X-ray diffraction coupled with Rietveld refinement revealed that the products were not simple mixtures or straightforward solid solutions. Instead, the diffraction patterns showed substantial structural reconstruction, and the data could best be described using an average structural model derived from the P-3m1 space group. In other words, introducing samarium into these hosts did not merely decorate the surface or substitute gently into existing sites; it rebuilt the crystal architecture into something new. Spectroscopic follow-up using Raman spectroscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy confirmed the picture at the local scale, revealing pronounced changes in the lanthanide–sulfur bonding environments compared with the parent Nd2S3 and Pr2S3 compounds.

Surface area and porosity, measured through nitrogen adsorption–desorption experiments following established IUPAC protocols, provided the link between this atomic-scale restructuring and the materials’ behavior as electrodes. In pseudocapacitive and battery-type electrodes, the interplay between accessible surface, pore structure, and charge-transfer kinetics governs how much charge can be stored and how quickly it can be delivered. The structural reconstruction induced by samarium modification altered all of these parameters, but crucially it did so in different directions for the two host systems, setting the stage for the electrochemical divergence that followed.

When the electrodes were tested in aqueous sodium sulfate electrolyte, the neodymium-based pair delivered the kind of result materials chemists dream about. The nominal NdSmS3 composition showed improved high-rate behavior, meaning it could still store and deliver substantial charge at fast charging and discharging rates where many electrodes falter. More impressive still was its durability: after 1200 charge–discharge cycles, the modified electrode retained 98.9 percent of the discharge capacity it measured at cycle 100. The unmodified Nd2S3 electrode, by comparison, retained 93.5 percent over the same window. In a field where capacity fade of several percent per hundred cycles is common, holding on to nearly 99 percent of capacity across more than a thousand cycles marks the samarium-modified neodymium sulfide as a genuinely robust candidate for long-lived aqueous devices.

The praseodymium-based pair told the opposite story. The nominal PrSmS3 composition exhibited lower capacity and lower Coulombic efficiency than the pristine Pr2S3 from which it was derived. Coulombic efficiency, the ratio of charge extracted during discharge to charge invested during each cycle, is a sensitive indicator of parasitic side reactions and irreversible processes at the electrode surface; its decline signals that the samarium-induced restructuring created pathways for stored charge to leak away rather than enhancing reversible storage. Two nominally identical modifications, differing only in whether neodymium or praseodymium occupied the host lattice, produced outcomes on opposite sides of the performance ledger.

Impedance spectroscopy added a crucial nuance that lies at the heart of the study’s message. Fitting of the electrochemical impedance data showed that the charge-transfer resistance, a measure of how easily electrons and ions exchange at the electrode–electrolyte interface, dropped substantially in both composites. For the neodymium pair, the fitted value fell from 93.71 ohms for pristine Nd2S3 to 30.26 ohms for the samarium-modified composition. For the praseodymium pair, it fell from 45.77 ohms to 27.65 ohms. On the conventional wisdom that lower interfacial resistance means a better electrode, both modifications should have succeeded. Only one did. The authors draw the pointed conclusion that reduced interfacial resistance alone does not ensure enhanced reversible storage, a caution that resonates well beyond rare-earth sulfides and speaks to a common pitfall in electrode research, where impedance improvements are frequently cited as proof of overall superiority.

To understand why the two hosts diverged, the team turned to kinetic analysis of the cyclic voltammetry data using Dunn analysis and b-value determination, complementary methods that separate charge stored through fast, surface-dominated capacitive processes from slower diffusion-limited faradaic reactions. The analysis revealed distinctly different charge-storage kinetics in the two host systems, meaning that the samarium modification shifted the balance between surface-controlled and diffusion-controlled storage in opposite ways depending on the host cation. An electrode can have a fast interface and still store most of its charge through poorly reversible pathways, and the kinetic fingerprints showed that this is precisely what happened in the praseodymium system.

First-principles calculations completed the mechanistic picture. Using density functional theory relaxation, the researchers examined oxygen-free structural configurations of the nominal NdSmS3 and PrSmS3 compositions derived from the P-3m1 model suggested by the diffraction data. The calculations showed that both configurations retain physically reasonable rare-earth–sulfur coordination environments, lending computational support to the experimentally observed reconstruction rather than pointing to some pathological structural failure in the praseodymium case. The divergence in electrochemical performance therefore cannot be blamed on an obviously broken crystal; it emerges from subtler differences in how the reconstructed lattices accommodate and release charge, differences that the kinetic analysis captures but that resist simple chemical intuition.

The broader lesson of the study is one that composite-electrode research has been slow to internalize. Additive-based modification strategies are often reported on a single host and then generalized, with the implicit assumption that a dopant or secondary phase that helps one compound will help its chemical cousins. This work demonstrates that the electrochemical effect of Sm2S3 modification depends strongly on the Ln2S3 host, and it does so with a clean, internally consistent body of evidence spanning diffraction, spectroscopy, porosimetry, electrochemistry, and computation. For engineers scouting materials for aqueous sodium-ion supercapacitors and related devices, the practical takeaway is that screening must be done host by host, and that metrics like charge-transfer resistance must be read alongside capacity retention and Coulombic efficiency rather than in isolation. For the field at large, the neodymium-based composite now stands as a promising, durable aqueous electrode, while the praseodymium result serves as a vivid reminder that in electrode chemistry, context is not a detail; it is the whole story.

Subject of Research: Host-dependent electrochemical behavior of samarium sulfide-modified rare-earth sulfide composite electrodes in aqueous sodium sulfate electrolyte

Article Title: Host-dependent electrochemical response of Sm2S3-modified Ln2S3 (Ln = Nd, Pr) composite electrodes in aqueous Na2SO4

Article References: Li, L., Zhang, L., Chen, Y., Liu, Z., Yan, H., Song, Y., Jia, J., & Wang, L. (2026). Host-dependent electrochemical response of Sm2S3-modified Ln2S3 (Ln = Nd, Pr) composite electrodes in aqueous Na2SO4. Ionics. https://doi.org/10.1007/s11581-026-07504-0

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07504-0

Keywords: rare-earth sulfides, samarium sulfide, neodymium sulfide, praseodymium sulfide, composite electrodes, supercapacitors, aqueous electrolyte, sodium sulfate, charge-transfer resistance, cycling stability, X-ray diffraction, first-principles calculations

The chemistry behind the process is elegantly economical. Rather than relying on high-temperature furnaces fed with purified metal and boron precursors, or on wet-chemical routes that generate solvent waste, the team used an oxide-based electrolyte containing sodium tetraborate, Na2B4O7, and cobalt hydroxide. When heated above its melting point, this salt bath becomes an ionic liquid capable of conducting charge, and the applied current drives the co-deposition of cobalt and boron onto the steel cathode. The approach belongs to the family of molten salt electrochemical processes that have attracted growing attention as greener alternatives for producing metals and compounds directly from their oxides, sidestepping energy-intensive reductions and corrosive reagents. Because both feedstocks are inexpensive and widely available, the authors position the method as fast, economical and environmentally friendlier than conventional boride synthesis.

What makes the study particularly interesting is the level of control the researchers achieved. When they ran the electrolysis at a current density of 200 milliamperes per square centimeter, X-ray diffraction revealed that the deposited powders consisted of metallic cobalt together with a single boride phase, Co3B. Cranking the current up to 600 milliamperes per square centimeter changed everything: the higher overpotential and faster deposition kinetics pushed the reaction toward boron-rich chemistry, yielding a multiphase mixture of CoB, Co2B and Co3B alongside residual metallic cobalt. In other words, the boron content of the final composite could be tuned simply by adjusting how hard the cell was driven, without reformulating the electrolyte or changing the temperature.

Characterization went well beyond phase identification. Scanning electron microscopy and transmission electron microscopy allowed the team to trace the growth mechanism of the composite structure and examine its morphology in detail, while Brunauer-Emmett-Teller surface area measurements confirmed that the powders possessed an accessible surface area suitable for electrode-electrolyte interaction, a prerequisite for any material hoping to store charge efficiently. This combination of structural and textural analysis matters because supercapacitor performance is exquisitely sensitive to how much of the electrode surface the electrolyte can actually reach; a material with impressive intrinsic chemistry but a closed, inaccessible microstructure will squander most of its potential.

The electrochemical payoff was substantial. Fabricated into electrodes and tested by cyclic voltammetry and galvanostatic charge-discharge, the boride-rich composite produced at 600 milliamperes per square centimeter delivered an areal capacitance of 4180 millifarads per square centimeter, along with an energy density of 78 milliwatt-hours per square centimeter and a power density of 2.8 watts per square centimeter. Its counterpart grown at the lower current density, dominated by Co3B and metallic cobalt, reached 2900 millifarads per square centimeter, 48 milliwatt-hours per square centimeter and 2.3 watts per square centimeter. The comparison makes a clear case that the multiphase, boron-rich composition is not an incidental byproduct of harsher conditions but the active ingredient in the enhanced performance.

Why would a mixture of several cobalt boride phases outperform a simpler composite? Transition metal borides have emerged in recent years as serious contenders for pseudocapacitive electrodes, the family of materials that store charge through fast, reversible surface redox reactions rather than purely electrostatic ion adsorption. Cobalt borides in particular have been incorporated into composites with clays, MXenes, graphene and activated carbon in a string of recent studies, and the Istanbul group itself has previously reported nickel boride electrodes made by similar molten salt routes. Multiple coexisting phases can offer complementary redox activity, improved electronic pathways and abundant interphase interfaces where charge transfer is facilitated. The multiphase Co/CoxB powders appear to exploit exactly such synergy, with the residual metallic cobalt likely contributing conductivity while the varied boride phases supply redox-active sites.

To understand how the electrodes actually store charge, the researchers applied the Trasatti approach, a classical analysis that separates surface-controlled capacitive contributions from diffusion-controlled ones by examining how voltammetric currents scale with scan rate. The verdict: both electrodes operate through a mixed capacitive-diffusive mechanism. That hybrid behavior is generally desirable in pseudocapacitive materials, since surface-dominated storage supports high-rate operation while diffusion-controlled contributions deepen the total charge available at slower rates. It also suggests that the composites function as genuine battery-capacitor hybrid materials, combining some of the energy density of battery-type cobalt chemistry with the rate capability of capacitive storage.

The team also interrogated the electrodes after cycling, using X-ray photoelectron spectroscopy to see what the electrochemical punishment had done to the surface. The spectra revealed persistent Co-B bonding along with Co(OH)2 species on the electrode surface, a signature of the surface transformation that cobalt-based pseudocapacitive materials typically undergo in alkaline electrolytes. Documenting this surface chemistry is more than academic housekeeping; it connects the observed capacitance to specific surface species and provides a baseline for assessing how the active layer evolves over a device’s lifetime, a central concern for any electrode material hoping to leave the laboratory.

The broader significance of the work lies in how it reframes electrode manufacturing. Molten salt electrolysis has historically been the domain of extractive metallurgy, most famously in processes that reduce metal oxides directly to metals in fused salt baths. Applying it to co-deposit two elements into a functional composite, and tuning the product’s phase composition through current density alone, points toward a scalable and low-waste synthesis platform for energy materials. If boride electrodes for supercapacitors, and perhaps for electrocatalysis and hydrogen evolution, can be grown in one step from cheap oxide salts, the arithmetic of manufacturing high-performance storage devices starts to look considerably friendlier than routes that demand multiple chemical synthesis stages, sacrificial solvents and costly precursors.

Challenges remain before molten-salt-grown cobalt borides can power anything in your pocket. The 900-degree-Celsius processing temperature demands robust equipment, and the reported metrics are areal values measured on laboratory electrodes rather than the gravimetric figures and long-cycle-life data that device engineers ultimately need. Still, the study delivers a compelling demonstration that sustainability and performance need not be traded against each other. A one-hour electrochemical bath in molten borax, steered by nothing more than the current dial, produced electrodes storing roughly 44 percent more charge per unit area than their lower-current counterparts, with energy and power densities to match. As the search intensifies for electrode materials that can bridge the gap between batteries and capacitors, this multiphase cobalt boride chemistry, grown from humble ingredients in a single pot of molten salt, is a result worth watching closely.

Subject of Research: Sustainable molten salt electrolysis synthesis of multiphase Co/CoxB composite electrodes for supercapacitor applications

Article Title: Multiphase Co/CoxB composite electrodes produced via sustainable molten salt electrolysis: structural and electrochemical performance evaluation

Article References: Arslan-Kaba, M., & Kartal Sireli, G. (2026). Multiphase Co/CoxB composite electrodes produced via sustainable molten salt electrolysis: structural and electrochemical performance evaluation. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13806-5

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13806-5

Keywords: supercapacitors, cobalt boride, molten salt electrolysis, energy storage, pseudocapacitance, composite electrodes, electrochemistry, Co3B, energy density, Trasatti analysis, X-ray photoelectron spectroscopy, sustainable materials