Chinese materials scientists used a one-pot molten-salt process to make the first hollow high-entropy carbide nanoparticles containing rare earths, and the best version split water efficiently without noble metals.
Splitting water into hydrogen and oxygen with electricity is one of the cleanest ways to make hydrogen fuel, especially when that electricity comes from wind or sunlight. But the reaction needs a catalyst, and the best catalysts for the hydrogen-producing half of the process are based on platinum and other scarce, expensive noble metals.
Researchers at Inner Mongolia University of Technology in China now report a new candidate made entirely from non-noble elements. Writing in the Journal of Advanced Ceramics, they describe hollow nanoparticles of so-called high-entropy carbides that include rare-earth elements, the first such materials to be made, and show that the best of them performs well as a hydrogen evolution catalyst.
“In this study, a family of hollow rare-earth-containing high-entropy carbide (RE-HEC) nanoparticles has been successfully synthesized for the first time,” said Wen Ma, a professor in the university’s School of Materials Science and Engineering and director of the Inner Mongolia Key Laboratory of New Materials and Surface Engineering.
Many metals, one crystal
High-entropy materials break a long-standing rule of metallurgy. Traditional alloys and compounds are built around one or two main elements with small additions of others. High-entropy materials instead combine five or more elements in roughly equal amounts, mixed randomly within a single crystal structure. The name refers to the high configurational entropy, a measure of disorder, that comes from mixing so many elements.
That disorder can produce unusual and useful properties. In catalysis, a surface containing many different metal atoms offers a wide variety of local environments, some of which may be especially good at binding and releasing the molecules involved in a reaction.
High-entropy carbides combine several metals with carbon. Until now, the nanocrystals made of these materials were all dense, solid particles, and most were based on transition metals from groups IVB to VIB of the periodic table, such as hafnium, niobium, tantalum, titanium and zirconium. Adding rare-earth elements has been difficult because their atoms are much larger and do not fit easily into the carbide crystal lattice. Making hollow particles has generally required templates and multi-step procedures.
One pot, molten salt
The Inner Mongolia team took a simpler route. They mixed metal oxides with carbon black and reduced them electrochemically in a bath of molten calcium chloride and sodium chloride at 800 °C. Running an electric current through the molten salt strips oxygen from the oxides and allows the metals to combine with carbon, all in a single step and without templates.
Earlier work using molten-salt electrochemistry had produced solid high-entropy carbide nanoparticles at higher temperatures. One approach yielded particles of 15 to 80 nanometres at 900 °C. Another made particles of 20 to 100 nanometres at 980 °C; those particles reached a hydrogen evolution overpotential of 350 millivolts at a standard current density, a measure of how much extra voltage is needed to drive the reaction.
The new study produced several compositions: a five-metal carbide of hafnium, niobium, tantalum, titanium and zirconium; versions that added a single rare earth, either gadolinium, cerium or yttrium; and a version containing two rare earths, cerium and yttrium together. All formed a single face-centred cubic crystal phase with the elements evenly distributed from the nanoscale up to the microscale.
The key to making hollow particles turned out to be the choice of carbon source. “The results show that the microstructure of HECs can be easily tailored from solid to hollow particles by selecting different carbon sources,” Ma said. “It was found that the electrocatalytic performance of HECs for HER is noticeably enhanced by the tailored hollow nanostructure compared to their solid counterparts.”
Two rare earths beat one
Hollow particles offer more exposed surface area than solid ones, since both the inner and outer surfaces can be accessible, giving reactants more places to interact with the catalyst.
The overpotential is a key figure of merit for such catalysts. Every millivolt of extra voltage needed to drive the reaction is energy lost as heat rather than stored in hydrogen, so lowering it directly improves the efficiency of an electrolyser. Platinum remains the benchmark because it needs very little overpotential, which is why finding cheaper materials that come close is a major goal.
The best performer was the hollow carbide containing both cerium and yttrium. Tested in an alkaline solution of potassium hydroxide, it required an overpotential of 167 millivolts to reach a current density of 10 milliamps per square centimetre, compared with the 350 millivolts reported for the earlier solid particles. It also showed a Tafel slope of 58.8 millivolts per decade, an indicator of how quickly the reaction speeds up as voltage increases, and a low charge-transfer resistance of 2.7 ohms.
“These findings collectively demonstrate that the dual RE incorporation into transition-metal-based HECs can not only construct more electroactive sites but also accelerate electrocatalytic kinetics, thereby yielding enhanced HER performance that far surpasses the single-RE counterparts,” Ma said. “This superior activity arises from the synergistic effects of two distinct RE species, Ce and Y.”
Toward cheaper hydrogen
Alkaline water electrolysis is a mature technology, widely used in industry, and catalysts that work well in alkaline conditions without precious metals are of particular interest for scaling up green hydrogen production. Reducing reliance on platinum-group metals could lower costs and ease supply concerns, since those metals are mined in only a few places in the world.
The researchers describe their synthesis as a general platform that could produce a wide range of hollow, rare-earth-containing carbides, offering a new design strategy for high-performance catalysts made from more abundant elements.
Questions remain. The release does not report particle sizes for the hollow materials, and the authors say more work is needed to understand how each individual rare earth affects the materials’ electronic structure and catalytic activity. Long-term stability under industrial operating conditions, a crucial test for any electrolysis catalyst, will also need to be established.
Still, the work shows that the molten-salt route can deliver complex, hollow nanostructures in a single step, and that adding the right pair of rare earths can significantly boost performance.
Notes
- “Hollow nanostructured rare-earth-containing high-entropy carbides: Facile electrochemical synthesis and effective electrocatalytic performance for hydrogen evolution.” Journal of Advanced Ceramics, 2026. DOI: 10.26599/JAC.2026.9221377.
- Tsinghua University Press. “First hollow rare-earth-containing high-entropy carbide nanoparticles: A facile electrochemical route yields efficient noble-metal-free hydrogen evolution catalyst.” EurekAlert!, 16 October 2026. https://www.eurekalert.org/news-releases/1148099.