Samarium Sulfide Boosts One Rare-Earth Electrode but Fails Another

In the crowded race to find better electrode materials for supercapacitors and aqueous energy-storage devices, rare-earth sulfides have quietly built an impressive résumé. Compounds built from lanthanum, cerium, dysprosium, samarium, and their neighbors have all shown pseudocapacitive behavior in water-based electrolytes, offering a cheaper and safer alternative to the corrosive or flammable chemistries that dominate much of the battery world. Yet a fundamental question has lingered beneath the steady stream of single-material reports: when researchers improve one of these sulfides with a second rare-earth additive, is the improvement a general property of the additive, or does it depend intimately on which host crystal it lands in? A new study from a team at Nanyang Normal University and collaborating institutions in China, published in the journal Ionics, delivers a strikingly clear answer: the host decides everything.

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