The material belongs to a family called transition metal phosphides, compounds in which phosphorus atoms bond to metal atoms and create electronic structures well suited to splitting water. Nickel phosphide in particular has been studied for years as a hydrogen evolution catalyst, and variants doped with additional metals have steadily improved performance. The West Anhui team, led by Lei Yang and Xueqin Yuan, pushed this strategy to its logical extreme. Instead of adding one or two dopants to a nickel phosphide host, they combined iron, cobalt, nickel, copper and manganese in near-equal proportions with phosphorus, producing a compound with the stoichiometry (FeCoNiCuMn)12P5. In such high-entropy materials, no single metal acts as the host; all five principal elements share a common crystal lattice in a single-phase solid solution.

High-entropy materials first attracted attention in 2004, when researchers demonstrated that alloys containing multiple principal elements could form surprisingly stable single-phase structures rather than segregating into a messy mixture of intermetallic compounds. The high configurational entropy of mixing stabilizes the disordered solid solution, and the resulting lattice, distorted by atoms of different sizes, exhibits properties that its individual constituents do not. In recent years the concept has migrated from structural alloys into catalysis, with high-entropy oxides, sulfides and phosphides all showing promise for electrochemical water splitting. What has often been missing, however, is clean experimental proof that the entropy itself, rather than the mere presence of extra elements, is responsible for the gains.

The Chinese team addressed this ambiguity with a deliberately staged set of comparison samples. Alongside the five-metal (FeCoNiCuMn)12P5, they synthesized (FeCoNiCu)12P5, (FeCoNi)12P5 and simple Ni12P5, forming a gradient in which each successive composition adds one more metal and therefore more configurational entropy while keeping the underlying phosphide structure. This gradient control experiment allowed the researchers to isolate the entropy effect: if catalytic performance improves monotonically as elements are added, the improvement can be attributed with confidence to the growing configurational disorder rather than to any single beneficial additive.

Synthesis proceeded in two straightforward steps. First, a hydrothermal reaction grew precursor structures directly on nickel foam, a porous, conductive metal substrate commonly used as an electrode support. The nickel foam serves double duty, providing both a three-dimensional scaffold for the catalyst and an integrated current collector, which eliminates the need for polymer binders that can add electrical resistance. Second, a phosphidation treatment converted the precursors into phosphides. The resulting (FeCoNiCuMn)12P5 adopted a striking hollow ring-like morphology, microrings whose hollow centers and thin walls expose generous surface area to the electrolyte and shorten the distances over which ions and electrons must travel during operation.

Comprehensive characterization confirmed that the product was what the researchers intended. X-ray diffraction showed a single crystalline phase rather than a composite of separate metal phosphides, and electron microscopy at both scanning and transmission scales, including high-angle annular dark-field imaging with energy-dispersive spectroscopy, verified that iron, cobalt, nickel, copper, manganese and phosphorus were distributed homogeneously throughout the material. That homogeneity matters: a true solid solution places all five metals in intimate atomic contact, creating a dense landscape of chemically distinct surface sites, whereas phase-separated mixtures would bury much of the compositional variety at internal interfaces where it could do little catalytic work.

The electrochemical results followed the entropy gradient exactly as the design predicted. Tested for the hydrogen evolution reaction, the five-element high-entropy microrings required an overpotential of just 102 millivolts to drive a current density of 10 milliamperes per square centimeter, and they displayed a Tafel slope of 61 millivolts per decade of current. The Tafel slope is a diagnostic of reaction kinetics: smaller values mean the reaction accelerates rapidly as voltage increases, pointing to efficient charge transfer at the catalyst surface. An overpotential of 102 millivolts at the benchmark current density places this noble-metal-free catalyst in competitive territory for alkaline water electrolysis, particularly for a material requiring no platinum-group elements.

Just as importantly, the lower-entropy counterparts performed measurably worse, in strict sequence. The four-metal (FeCoNiCu)12P5 came next, followed by the three-metal (FeCoNi)12P5, with single-metal Ni12P5 trailing the field. This monotonic trend across the compositional series is the paper’s central evidentiary claim. Because morphology, synthesis route and crystal structure were held essentially constant across the series, the systematic improvement with each added element constitutes what the authors describe as compelling experimental evidence validating the critical role of configurational entropy in boosting hydrogen evolution electrocatalysis. The team also reports that the high-entropy microrings demonstrated outstanding long-term durability, an essential qualification for any catalyst hoping to survive the corrosive, high-current conditions of industrial electrolysis.

Why does entropy help? The authors attribute the enhancement to a synergistic high-entropy effect that modulates the electronic structure and increases the density of accessible active sites. Each of the five metals contributes a different electronegativity and a different set of d-orbital energies, so the electronic landscape at the catalyst surface becomes a richly varied terrain rather than a uniform plain. Some sites will bind hydrogen atoms with near-ideal strength, close to the thermodynamic optimum for the reaction, while neighboring sites with different electronic character can assist in water dissociation or charge transfer. Lattice distortion from the mismatched atomic sizes may further strain the surface and tune bond strengths. Meanwhile, the sheer number of distinct site geometries raises the probability that a hydrogen or water molecule landing anywhere on the surface finds a favorable configuration, effectively raising the density of usable active sites beyond what a single-composition phosphide can offer.

The hollow microring geometry compounds these electronic advantages with structural ones. Hollow architectures are prized in electrocatalysis because their thin walls place nearly all active material within diffusion reach of the electrolyte, and their interiors can buffer the volume changes and gas evolution that degrade denser structures over time. Combined with the direct growth on conductive nickel foam, the microrings form an electrode in which every catalytic site is electrically wired and electrolyte-accessible, minimizing wasted material and resistance losses.

The broader significance of the study lies less in any single performance number than in its methodological clarity. Claims that high-entropy effects enhance catalysis have proliferated across the water-splitting literature, but many rest on comparisons between materials that differ in more than just entropy, leaving room for confounding explanations. By constructing a controlled entropy gradient within one crystal structure family and one synthesis platform, the West Anhui team has produced a template for how such claims should be tested. The work also provides a viable synthetic route toward high-entropy phosphide microrings, a morphology class that had not previously been reached with this five-metal composition.

There remain familiar caveats. Overpotentials and Tafel slopes measured on small laboratory electrodes do not translate directly to the current densities of industrial electrolyzers, and long-term stability under those harsher conditions would need separate demonstration. The authors note that no datasets were generated or analysed beyond those in the study, and the funding came from the National Natural Science Foundation of China, the Key Program of West Anhui University and an undergraduate innovation training program, an indication that the work also served as a training ground for early-career researchers. Still, the combination of abundant elements, a simple two-step synthesis, a durable electrode architecture and clean evidence for the entropy effect gives the result real practical and scientific weight.

As the hydrogen economy moves from promise to infrastructure, catalysts that pair platinum-class kinetics with iron-and-nickel-class economics will be decisive. This study suggests that the periodic table’s middle row, scrambled at high entropy and shaped into hollow rings, may be one of the places to find them.

Subject of Research: High-entropy transition metal phosphide microrings for enhanced hydrogen evolution reaction electrocatalysis

Article Title: Rational Synthesis of High-Entropy (FeCoNiCuMn)12P5 Microrings and Their Enhanced Hydrogen Evolution Reaction Performance

Article References: Yang, L., Wan, Y., Zhao, Z., Zhao, Z., Tian, X., Yin, J., Li, Y., Wang, J., Xu, L., & Yuan, X. (2026). Rational Synthesis of High-Entropy (FeCoNiCuMn)12P5 Microrings and Their Enhanced Hydrogen Evolution Reaction Performance. Catalysis Letters, 156(11), Article 301. https://doi.org/10.1007/s10562-026-05549-4

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05549-4

Keywords: high-entropy phosphide, hydrogen evolution reaction, electrocatalysis, transition metal phosphide, water splitting, nickel foam, microring morphology, configurational entropy, green hydrogen, Tafel slope, solid solution, catalysis

The appeal of metal sulfide anodes lies in their chemistry. Compounds containing two electrochemically active metals can undergo multiple redox reactions, and the presence of sulfur helps accommodate some of the volumetric strain that develops when metals alloy with lithium or sodium. Bismuth sulfide, for example, offers a theoretical capacity of about 625 mAh per gram through the formation of Li3Bi or Na3Bi phases. Crucially, bismuth sulfide and antimony sulfide form a complete solid solution across the entire compositional range, meaning researchers can tune the bismuth-to-antimony ratio freely to optimize performance. Previous studies of related compositions, such as Bi0.94Sb1.06S3 nanorod clusters, had already shown improved cycling stability compared with single-metal sulfide anodes, but the fabrication method itself remained a limiting factor in how uniformly the active material adhered to the current collector.

The Indian team’s solution was to eliminate the polymer binder altogether. Conventional electrodes are made by mixing active material with conductive additives and a glue-like binder, then coating the slurry onto metal foil. Binders add dead weight, can impede electron transport, and may fail mechanically over time. Instead, the researchers used electrophoretic deposition, a technique in which charged particles suspended in a liquid migrate toward and deposit onto an oppositely charged electrode under an electric field. By applying a constant DC voltage of around 300 volts in an isopropanol-based bath containing nickel nitrate, poly(acrylic acid), and carbon black, they fabricated complete electrodes on copper foil in roughly three minutes. The method is inherently scalable and produces a film whose adhesion, porosity, and uniformity can be controlled through deposition parameters rather than through binder chemistry.

Synthesis of the active material itself relied on a one-step hydrothermal process. Graphene oxide, prepared by a modified Hummer’s method from graphite powder, was dispersed in water and sonicated to achieve partial exfoliation. Bismuth nitrate and antimony trichloride precursors were dissolved separately and combined, then thioacetamide was added as the sulfur source. The negatively charged graphene oxide sheets electrostatically attracted the positively ionized Bi3+ and Sb3+ species, promoting a uniform distribution of metal ions across the carbon surface. Under hydrothermal conditions of 200 degrees Celsius for 24 hours in a Teflon-lined autoclave, the amorphous bismuth-antimony-sulfur complex nucleated and grew into crystalline nanorods anchored on the reduced graphene oxide sheets. Prolonged treatment favored unidirectional growth along the (011) crystallographic plane, producing the one-dimensional nanorod clusters that give the material its distinctive morphology.

Characterization confirmed the composite’s structure in detail. X-ray diffraction revealed an orthorhombic Bi1.09Sb0.91S3 phase matching the standard reference pattern, with crystallite sizes of approximately 22 nanometers in the composite compared with about 30 nanometers for the pristine sulfide. Raman spectroscopy showed the characteristic D-band and G-band of reduced graphene oxide, and thermogravimetric analysis quantified the composition at roughly 94 weight percent sulfide and 6 weight percent graphene. Electron microscopy showed nanorod clusters with average diameters near 35 to 40 nanometers and lengths around 0.35 micrometers, well anchored to the graphene sheets without phase segregation. Nitrogen adsorption measurements revealed that adding graphene increased the surface area from about 11 square meters per gram to roughly 48 square meters per gram, providing more electrochemically active reaction sites and shorter ion diffusion pathways.

Electrochemical testing in lithium-ion coin cells highlighted the graphene’s contribution. The composite anode delivered an initial discharge capacity of about 1269 mAh per gram, with the substantial first-cycle irreversibility attributed to electrolyte decomposition and solid electrolyte interphase formation, a familiar feature of nanostructured metal sulfide anodes. From the second cycle onward the electrode stabilized, and over 100 cycles at 0.5 amperes per gram it maintained average capacities near 506 mAh per gram, substantially outperforming the pristine sulfide electrode, which managed about 331 mAh per gram. Rate capability testing told a similar story: at 4 amperes per gram the composite still delivered about 128 mAh per gram, more than double the 58 mAh per gram of the bare material, and when the current was lowered back to 0.5 amperes per gram the composite recovered roughly 353 mAh per gram, evidence of excellent structural reversibility.

Sodium-ion performance followed the same pattern. Cyclic voltammetry identified the expected sequence of reactions: conversion of the sulfide into metallic bismuth, antimony, and sodium sulfide, followed by alloying of the metals into Na3Bi and Na3Sb, with the reverse reactions on charge. Over 33 cycles at 0.1 amperes per gram, the composite averaged about 413 mAh per gram with a Coulombic efficiency near 95 percent, compared with roughly 257 mAh per gram for the pristine electrode. Impedance spectroscopy revealed the underlying kinetics: the composite’s charge transfer resistance after cycling measured just 12.5 ohms in lithium cells and 21.7 ohms in sodium cells, dramatically lower than the pristine material’s values, and calculated diffusion coefficients confirmed faster ion transport through the graphene-supported structure. The team noted that sodium-ion gains were more modest than lithium-ion gains because the larger sodium ion is intrinsically slower and more constrained by the crystal lattice.

Perhaps the most compelling evidence came from postmortem analysis using synchrotron X-ray absorption spectroscopy and electron microscopy. Ex situ XANES measurements at the Bi L3-edge and Sb K-edge showed the reduction of trivalent bismuth and antimony to their metallic states during discharge, with irreversible edge shifts consistent with the formation of pseudo-crystalline Li3Bi, Na3Bi, Li3Sb, and Na3Sb phases. Cross-sectional electron microscopy quantified the mechanical toll of cycling: the pristine electrode swelled from about 10.9 micrometers to 29 to 34 micrometers, nearly a threefold increase, while the graphene composite expanded only to about 20.4 micrometers in lithium cells and 18.2 micrometers in sodium cells. The pristine electrode’s nanorods fragmented and agglomerated into dense aggregates riddled with voids, whereas the composite largely retained its nanorod morphology, with the graphene sheets acting as a mechanical buffer that absorbed volume fluctuations and prevented active material from detaching from the copper collector.

The study demonstrates that combining a tunable bimetallic sulfide solid solution with a conductive, flexible graphene scaffold and a fast, binder-free deposition process can produce an anode that works effectively in both lithium and sodium chemistries. Because sodium-ion batteries rely on abundant, inexpensive materials and are viewed as a complement to lithium technology for grid-scale storage, an anode strategy that serves both systems is particularly valuable. The electrophoretic deposition step, completed in minutes at ambient conditions, avoids the energy-intensive drying and calendering complexities of slurry coating while producing electrodes with a practical mass loading of about 1.3 milligrams per square centimeter. As the authors conclude, the Bi1.09Sb0.91S3/reduced graphene oxide composite represents a promising dual-functional anode candidate, and the scalable fabrication route suggests a plausible path from laboratory coin cells toward practical electrode manufacturing.

Subject of Research: Binder-free bismuth-antimony sulfide nanorod anodes on reduced graphene oxide for lithium-ion and sodium-ion battery storage

Article Title: Binder-free Bi₁.₀₉Sb₀.₉₁S₃ nanorod clusters anchored on reduced graphene oxide via electrophoretic deposition for enhanced lithium and sodium-ion storage

Article References: Dashairya, L., Das, D., & Saha, P. (2026). Binder-free Bi₁.₀₉Sb₀.₉₁S₃ nanorod clusters anchored on reduced graphene oxide via electrophoretic deposition for enhanced lithium and sodium-ion storage. Discover Electrochemistry, 3(1), Article 28. https://doi.org/10.1007/s44373-026-00116-3

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00116-3

Keywords: lithium-ion batteries, sodium-ion batteries, bismuth sulfide, antimony sulfide, reduced graphene oxide, electrophoretic deposition, binder-free electrode, anode materials, nanorods, solid solution, XANES, energy storage