MXenes are two-dimensional metal carbides and nitrides first discovered in 2011, and they have since become one of the most versatile material platforms in nanotechnology. Made by chemically etching away layers from layered ceramic precursors known as MAX phases, they combine metallic electrical conductivity with mechanical flexibility and a chemically tunable surface. Their surfaces are decorated with terminations such as oxygen, fluorine, and hydroxyl groups, and these terminations, together with the choice of metal element, allow researchers to adjust the material’s work function, the energy required to lift an electron out of the solid. That tunability is precisely what makes MXenes attractive as electrode materials, because the quality of a transistor depends critically on how well the energy levels of the electrode and the semiconductor line up.

The contact problem is particularly acute for p-type transition-metal dichalcogenides, the class of 2D semiconductors that conducts current using holes rather than electrons. For n-type devices, low-work-function metals such as indium and bismuth can be deposited gently at low energy, preserving the delicate semiconductor surface. P-type devices, by contrast, demand high-work-function metals, and those metals, including platinum, palladium, and gold, all melt above 1000 degrees Celsius. Depositing them requires energetic vacuum processes that bombard the semiconductor surface, damaging the interface and undermining the very band alignment the metals were chosen to provide. The result has been a persistent bottleneck: the ideal electrode materials are the hardest ones to use without destroying the device.

The new study tackles this dilemma by replacing vacuum-deposited metals with solution-processed MXene films, which can be laminated onto the semiconductor under mild, low-cost conditions that minimize interface damage. The team benchmarked three synthetically mature MXenes: Ti3C2Tx, the most widely studied member of the family, along with Nb2CTx and Mo2CTx, chosen because their work function ranges differ from the titanium compound and thus expand the compositional space for contact engineering. As the semiconductor channel they used 2H-MoTe2, a representative p-type 2D material, grown as thin films by sputtering molybdenum onto silicon dioxide substrates followed by chemical vapor deposition tellurization.

Characterization confirmed that all four materials were synthesized with the intended structure and chemistry. Transmission electron microscopy showed the characteristic flake morphology of the MXenes with hexagonal diffraction patterns indicating high crystallinity, while atomic-resolution scanning transmission electron microscopy revealed the well-defined layered lattice of the 2H-MoTe2 films. Raman spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction verified phase purity and surface chemistry, with no significant oxidation detected in the MXene samples and no secondary phases in the semiconductor. These checks matter because even trace impurities or unwanted phases can dominate the behavior of an atomically thin device.

The decisive measurement came from ultraviolet photoelectron spectroscopy, which determined the work functions of Ti3C2Tx, Nb2CTx, and Mo2CTx to be 4.44, 4.72, and 4.23 electron volts respectively, while 2H-MoTe2 showed a work function of 4.40 electron volts with its valence band maximum sitting 0.33 electron volts below the Fermi level. The resulting band alignment diagram revealed that Nb2CTx provides the closest energetic match to the semiconductor’s valence band, meaning the lowest barrier for hole injection. Importantly, the work function of a MXene is not set by the metal element alone; surface terminations modify surface dipoles and charge redistribution, so the measured value reflects the combined contribution of composition and actual surface chemistry. In this case, Nb2CTx’s higher work function translated directly into better contacts.

Because MXenes are hydrophilic while the MoTe2 surface is intrinsically hydrophobic, the researchers could not simply coat one onto the other. Instead they used a polymer-assisted transfer strategy, laminating patterned gold-capped MXene source and drain electrodes onto the semiconductor film. A 20-nanometer gold capping layer improved lateral conductivity and protected the underlying interface, while a consistent spray-coating protocol ensured that differences between devices reflected the MXene composition rather than thickness variation. The choice of transfer polymer proved surprisingly consequential: devices made with cellulose acetate showed significantly degraded characteristics, likely due to unintentional doping, whereas polymethyl methacrylate yielded clean, reproducible interfaces, a sensitivity the authors note is common in TMD devices.

The electrical results were striking. Transistors with Nb2CTx contacts showed clear p-type behavior with hole mobilities reaching about 19.8 square centimeters per volt-second in individual devices and an on/off current ratio of roughly 5.23 times ten to the fourth. Across twenty identically patterned devices measured at random positions on the wafer, the average mobility was 17.4 plus or minus 1.2 square centimeters per volt-second with a narrow performance distribution, indicating excellent uniformity. Output curves were linear and symmetric, confirming low-resistance ohmic-like contacts. Compared head-to-head with platinum-contacted devices, the Nb2CTx devices performed significantly better, and they also surpassed previously reported CVD-grown MoTe2 transistors and nearly all previously reported MXene-based transistors in mobility. According to the authors, this is the first demonstration of wafer-scale MXene contact engineering in p-type systems and the first use of Nb2CTx as an effective contact material. The devices also proved robust, showing negligible degradation after three months in dry air.

Temperature-dependent measurements from 150 to 250 kelvin provided deeper physical insight. As temperature decreased, the on/off ratio rose to about ten to the fifth, because suppressing thermally assisted carrier emission reduces the off-state current. By fitting the current to the thermionic emission model appropriate for 2D semiconductors, the team extracted Schottky barrier heights from Richardson plots as a function of gate voltage. At large negative gate voltages the effective barrier dropped to only a few millielectron volts, signaling the onset of tunneling-assisted carrier injection and explaining the low contact resistance under strong turn-on bias. Together with cross-sectional electron microscopy showing a sharp, well-defined interface, these measurements confirm that the performance gains stem from genuinely clean, low-barrier contacts rather than artifacts.

The broader significance lies in the vast, largely unexplored compositional space of the MXene family. The authors suggest that other compositions such as V2CTx, Ta2CTx, and Ta4C3Tx, as well as the investigated MXenes with different surface terminations, could serve as promising p-type contact candidates if suitable work functions, stable terminations, and clean interfaces can be achieved. Future work with more precisely controlled surface chemistry should help establish a quantitative relationship linking composition, termination, work function, and contact quality. For now, the study offers a clear and practical recipe: pick the MXene whose measured work function best matches the semiconductor’s valence band, deposit it gently from solution, and laminate it without damage. If that approach generalizes, the bottleneck that has slowed p-type 2D electronics may finally be dissolving, opening the door to solution-processed, scalable 2D transistors and optoelectronic devices built without a single high-temperature deposition step.

Subject of Research: MXene electrode contacts for p-type two-dimensional semiconductor transistors

Article Title: MXenes Contacts for p‐type 2D Electronics

Article References: Guo, T., Chen, M., Wang, Y., Liu, C., Xu, X., Luo, L., Zhu, D., Chu, N., Zhang, X., Caraveo, A., Anthopoulos, T. D., Zhang, X., & Alshareef, H. N. (2026). MXenes Contacts for p ‐type 2D Electronics. Advanced Science, 13(56), Article e76591. https://doi.org/10.1002/advs.76591

Image Credits: AI Generated

DOI: 10.1002/advs.76591

Keywords: MXenes, Nb2CTx, 2D semiconductors, MoTe2, p-type transistors, work function, Schottky barrier, contact resistance, solution processing, field-effect transistors, transition-metal dichalcogenides, band alignment

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