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

Bipolar plates are the workhorses of a PEMFC stack. They separate individual cells, distribute hydrogen and air across the electrode surfaces, carry current from cell to cell, and help manage water and heat. Traditionally, plates were machined from graphite, which is conductive and corrosion-resistant but brittle, thick, and costly to manufacture at volume. Metallic plates, by contrast, can be stamped into thin, intricate flow fields in seconds, making them far lighter and cheaper for automotive applications. The catch is that metals corrode in the acidic, humid environment inside a fuel cell, and the passive oxide films that form on their surfaces are poor conductors. That oxide layer is precisely what drives up interfacial contact resistance, or ICR, the area-specific resistance measured between the plate and the adjacent porous transport layer under the compressive load of stack assembly.

Researchers have explored a zoo of protective coatings to solve this, from physical vapour deposited nitrides to amorphous carbon films. But vapour-phase techniques require expensive vacuum equipment and struggle to coat complex, porous geometries uniformly. Sevinc and Hazar took a different route: a combined aqueous electrodeposition and electrophoretic deposition (EPD) process, carried out entirely in water-based baths. Electrodeposition builds the metallic matrix of the coating, while EPD uses an electric field to drive charged graphene particles toward the substrate, embedding the carbon sheets into the growing film. The approach is low-cost, scalable, and, crucially, compatible with porous structures that vacuum processes cannot easily penetrate.

The team applied their Ni–Cr–Ta–graphene coatings to an unusually broad set of substrates: dense aluminum, dense nickel, and AISI 316L stainless steel, the three most commonly discussed metallic bipolar plate materials, plus porous aluminum and porous nickel structures. Porous flow fields are an emerging design concept in which the conventional machined channels are replaced by an open-pore metal foam, giving vastly more contact area for gas distribution and water management. Coating such structures without clogging the pores is a major challenge, and demonstrating that an EPD-based coating preserves pore accessibility is one of the study’s most practically significant results.

The researchers systematically varied the deposition bath composition, designated S1 through S5, and the deposition time, then examined the resulting surfaces with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). These techniques revealed how uniformly the coating covered each substrate and how the graphene and metallic elements were distributed. The morphology proved highly sensitive to processing conditions: too short a deposition time left the surface incompletely covered, while excessive deposition produced thicker, less uniform layers. An optimized deposition time of 120 seconds yielded comparatively uniform coatings across the series and delivered the best electrical contact behavior within the investigated set of formulations.

The headline numbers come from the ICR measurements, taken under compressive pressures relevant to real PEMFC stack assembly, up to 140 N/cm². Among all tested formulations, the coating designated S4 achieved the lowest interfacial contact resistance, approximately 78 mΩ cm² at 140 N/cm² on the aluminum substrate. To appreciate how substantial that improvement is, consider the benchmark the team used for comparison: an aluminum specimen pre-coated with nickel alone, representing a conventional baseline, exhibited roughly 269 mΩ cm² under the same conditions. The multi-component graphene-containing coating therefore cut the contact resistance to less than a third of the baseline value, a reduction of nearly 70 percent achieved purely through smarter surface chemistry and processing.

The authors are refreshingly candid about the remaining gap. The U.S. Department of Energy has set a target of 10 mΩ cm² for bipolar plate contact resistance, a figure that reflects what is needed for maximum stack efficiency in mass-market vehicles. Even the best S4 coating remains approximately 7.8 times higher than that target. The study’s results, in other words, represent a relative improvement within the investigated coating series rather than a solved problem. Further optimization of bath chemistry, particle loading, deposition parameters, and possibly post-treatment steps will be required before these coatings can meet the stringent demands of commercial fuel cell stacks. This honesty is valuable in a field where headline numbers sometimes obscure how far laboratory results still sit from deployment thresholds.

Why does adding graphene to a nickel–chromium–tantalum matrix help so much? The answer lies in the complementary roles of each constituent. Nickel provides the conductive metallic backbone and serves as the electroplating medium from the aqueous bath. Chromium contributes corrosion resistance, helping to shield the underlying metal from the acidic PEMFC environment and suppress the growth of insulating oxide films. Tantalum is prized in the bipolar plate literature for its exceptional passivity and stability in simulated fuel cell conditions, and prior studies have shown tantalum-modified stainless steel plates resisting corrosion effectively. Graphene, meanwhile, brings its legendary in-plane electrical conductivity and chemical inertness; sheets distributed through the coating create conductive pathways and can act as a barrier to corrosive species. Together, the four constituents aim to satisfy the two demands that usually trade off against each other: protecting the metal from corrosion while keeping the surface electrically accessible.

The combined electrodeposition–EPD strategy also matters from a manufacturing standpoint. Metal matrix composite coatings produced by co-depositing particles into an electroplated layer are an established and diversifying technology, but systematic studies on Ni–Cr–Ta–graphene systems, particularly on both dense and porous substrates, had remained limited. By showing that the same water-based process works across aluminum, nickel, and stainless steel, and that it coats porous foams while keeping their pores open, the Firat University team has expanded the design space available to fuel cell engineers. Aqueous processing avoids the capital cost and geometric limitations of PVD and CVD chambers, and it integrates naturally with existing electroplating infrastructure in the automotive supply chain.

The work, which draws on the first author’s doctoral dissertation on metal-based flow plates for PEM fuel cells in electric vehicles and was funded by Firat University’s Scientific Research Projects Coordination Unit, arrives at a moment when hydrogen technology is scaling up globally. Every reduction in ohmic loss inside the stack translates directly into more kilometers per kilogram of hydrogen, and bipolar plate coatings are among the most cost-effective levers for achieving that. Sevinc and Hazar’s results demonstrate that aqueous EPD-based Ni–Cr–Ta–graphene coatings provide an effective route to improving electrical contact performance while preserving the pore accessibility that next-generation porous flow fields demand. The DOE target remains the finish line, but this study maps out a credible, low-cost path toward it, one graphene-laden layer at a time.

Subject of Research: Ni–Cr–Ta–graphene composite coatings for reducing interfacial contact resistance of metallic bipolar plates in proton exchange membrane fuel cells

Article Title: Microstructural and contact resistance behavior of Ni–Cr–Ta–graphene coatings on metallic bipolar plate substrates

Article References: Sevinc, H., & Hazar, H. (2026). Microstructural and contact resistance behavior of Ni–Cr–Ta–graphene coatings on metallic bipolar plate substrates. Ionics. https://doi.org/10.1007/s11581-026-07541-9

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07541-9

Keywords: PEMFC, bipolar plate, graphene, electrophoretic deposition, contact resistance, coatings, nickel, chromium, tantalum, corrosion, fuel cells, porous metal