Hollow rare-earth high-entropy carbide nanoparticles made by molten-salt electroreduction boost hydrogen evolution

Hydrogen made by splitting water with electricity sits at the heart of most credible roadmaps for a low-carbon energy system, yet the technology’s economics remain hostage to a single component: the electrocatalyst that drives the hydrogen evolution reaction. Platinum-group metals are still the benchmark for that reaction, and their scarcity and cost continue to block large-scale deployment. Transition metal carbides have emerged as some of the most promising substitutes, because their d-band electronic structure resembles that of platinum, they conduct electricity well, and they operate across a broad pH range. The catch has been that simple, single-metal carbides bind hydrogen intermediates too strongly, which caps their intrinsic catalytic activity no matter how well they conduct.

Combining five or more metals into a single-phase solid solution offers a way around that limitation. The high-entropy effect, already demonstrated for high-entropy alloys and oxides, can deliver catalytic behavior far better than any individual component. High-entropy carbides, however, are still in their infancy. Multiple metallic elements are chemically immiscible within a carbide lattice, and producing nanosized carbides with controlled size and morphology is difficult. The synthesis routes developed so far, including electrical-discharge-induced bulk-to-nanoparticle transformation, ultra-fast Joule heating, nanoconfined impulse synthesis on carbon nanotube films, pyrolysis of metal-ion-impregnated zeolitic-imidazolate-framework-derived carbon, and solid-phase combustion synthesis, have all expanded the compositional space. Every material reported to date, though, has been a dense, solid particle.

That constraint represents a missed opportunity. Hollow nanostructures offer high surface-to-volume ratios and abundant interior voids, which translate into more exposed active sites, lower mass-transport resistance, and shorter charge-transport pathways. Conventional hollow synthesis, however, relies on templates, which means multi-step preparation and often harsh template-removal conditions. In parallel, rare-earth elements are known to tune the electronic structure of transition-metal alloys and oxides and to boost catalytic activity, but rare-earth-containing high-entropy carbide catalysts remain scarce, largely because the large atomic-size mismatch between rare earths and transition metals hinders incorporation of rare-earth atoms into the carbide lattice. Fabricating hollow structures and introducing rare earths into high-entropy carbides are thus two formidable challenges that conventional synthesis has struggled to meet simultaneously.

Molten-salt electrolysis offers a mild, low-temperature alternative that converts inexpensive oxide precursors directly into nanomaterials in a single step. Sure and Schwandt first produced single-phase (NbTaTiZrHf)C nanoparticles of 15 to 80 nanometers by electro-deoxidation of a metal oxide and graphite mixture in molten calcium chloride at 900 degrees Celsius. Yang and colleagues later coupled anodic carbon dioxide capture with in situ cathodic carburization in the MSCC-ET process, preparing (NbTaTiZrHf)C of 20 to 100 nanometers in a lithium chloride-calcium chloride-potassium chloride melt at 980 degrees Celsius with carbon dioxide as the sole carbon source, and reported an alkaline hydrogen evolution overpotential of 350 millivolts at 10 milliamperes per square centimeter with a Tafel slope of 61 millivolts per decade. Both studies, however, were confined to five-component d-block carbides with solid particulate morphologies.

Now a team led by Wen Ma at the Inner Mongolia Key Laboratory of New Materials and Surface Engineering, Inner Mongolia University of Technology, has broken both barriers at once. The researchers achieved the first synthesis of hollow nanostructured high-entropy carbides with compositions (HfNbTaTiZr)C, (HfNbTaTiZrRE)C where the rare earth is gadolinium, cerium, or yttrium, and (HfNbTaTiZrCeY)C, through a direct electroreduction process in molten calcium chloride-sodium chloride at 800 degrees Celsius. All of the as-synthesized carbides exhibit a single face-centered cubic phase and compositional homogeneity from the nanoscale to the microscale. Critically, the team found that the microstructure can be tailored from solid to hollow particles simply by selecting a different carbon source, dispensing with templates altogether.

When evaluated as hydrogen evolution catalysts in 1.0 molar potassium hydroxide, the engineered hollow architecture combined with rare-earth incorporation substantially enhanced performance, and the dual-rare-earth compositions outperformed their single-rare-earth analogues. The best performer, (HfNbTaTiZrCeY)C, reached an overpotential of just 167 millivolts at 10 milliamperes per square centimeter, with a Tafel slope of 58.8 millivolts per decade and a charge-transfer resistance of only 2.7 ohms. Those figures compare favorably with the 350-millivolt overpotential and 61-millivolt-per-decade Tafel slope reported for the earlier solid five-component carbide, and they were achieved without a trace of noble metal.

The work was published in the Journal of Advanced Ceramics on September 10, 2026. In the paper, the authors describe how a family of hollow rare-earth-containing high-entropy carbide nanoparticles was successfully synthesized for the first time, exhibiting a single face-centered cubic structure obtained by a one-pot electroreduction process from oxides and carbon black in molten chlorides at 800 degrees Celsius. The results show that the microstructure of these carbides can be easily tailored from solid to hollow particles by selecting different carbon sources, and that the electrocatalytic performance for hydrogen evolution is noticeably enhanced by the tailored hollow nanostructure compared with solid counterparts.

The dual rare-earth incorporation appears to do more than add sites. According to the researchers, adding two distinct rare-earth species, cerium and yttrium, into the transition-metal-based carbides can not only construct more electroactive sites but also accelerate electrocatalytic kinetics, yielding enhanced hydrogen evolution performance that far surpasses the single-rare-earth counterparts. The superior activity arises from the synergistic effects of the two rare-earth elements working together within the carbide lattice, a combination that the large atomic-size mismatch had long made difficult to realize.

The study establishes a general and facile platform for synthesizing a wide family of hollow rare-earth-containing high-entropy carbide nanoparticles and suggests a new design strategy for high-performance noble-metal-free catalysts. The authors note that further work is needed to clarify how individual rare-earth elements govern the electronic structure and activity, a question that will determine how far this design principle can be pushed. Other contributors include Hongxia Liu, Jinqiao Zhang, Shiyu Qin, and Yu Bai from the School of Materials Science and Engineering at Inner Mongolia University of Technology in Hohhot, China. The work was supported by the Carbon Peak and Carbon Neutrality Research Program of Higher Education Institutions in Inner Mongolia Autonomous Region, the Natural Science Foundation of Inner Mongolia Autonomous Region, the National Natural Science Foundation of China, and Science and Technology Projects of Inner Mongolia Autonomous Region.

For the hydrogen economy, the implications are straightforward. A catalyst that combines abundant hafnium, niobium, tantalum, titanium, and zirconium with modest amounts of cerium and yttrium, synthesized in a single electrochemical step at 800 degrees Celsius, delivered performance approaching that of far more complex engineered systems while avoiding both noble metals and template chemistry. If the molten-salt electroreduction route proves scalable, the combination of hollow architecture and dual rare-earth tuning could become a standard design principle for the next generation of earth-abundant water-splitting catalysts, bringing low-cost green hydrogen a meaningful step closer to reality.

Subject of Research: First hollow rare-earth-containing high-entropy carbide nanoparticles synthesized by molten-salt electroreduction as noble-metal-free hydrogen evolution catalysts

Article Title: First hollow rare-earth-containing high-entropy carbide nanoparticles: A facile electrochemical route yields efficient noble-metal-free hydrogen evolution catalyst

Article References: First hollow rare-earth-containing high-entropy carbide nanoparticles: A facile electrochemical route yields efficient noble-metal-free hydrogen evolution catalyst. Original publication (Bibliographic details unavailable.)

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Keywords: high-entropy carbides, hydrogen evolution reaction, rare-earth elements, molten-salt electrolysis, electrocatalysis, hollow nanoparticles, noble-metal-free catalysts, water splitting, transition metal carbides, green hydrogen, cerium, yttrium