Cerium and Sulfur Team Up to Forge Oxygen Vacancies for Faster Hydrogen from Water

Green hydrogen has a stubborn bottleneck, and it sits on the oxygen side of the reaction. Splitting water into hydrogen and oxygen is an electrochemical balancing act: while the hydrogen evolution reaction has been tamed by decades of catalyst research, the oxygen evolution reaction, or OER, remains a sluggish, energy-hungry four-electron process that drags down the overall efficiency of alkaline water electrolyzers. Now, a team of researchers led by Ruxin Lei of Chengdu University, working with colleagues at the Sichuan Institute of Product Quality Supervision and Inspection, Université de Montpellier, and Southwest Jiaotong University, reports a clever defect-engineering strategy that coaxes a cheap nickel-based catalyst into delivering some of the most impressive OER numbers yet recorded for a non-noble-metal system. Writing in the journal Ionics, the team describes how cerium and sulfur, acting together, seed a dense population of oxygen vacancies that transform the catalytic properties of ultrathin nickel-based nanosheets grown directly on nickel foam.

The material at the heart of the study, dubbed NiCeS/NF, is not a single crystalline compound but a deliberately messy multiphase framework. Using a molten-salt synthesis route, the researchers grew an array of ultrathin nanosheets in situ on a nickel foam substrate, producing a composite that contains nickel sulfide, nickel oxysulfide, and sulfate species all interwoven with one another. This phase diversity is not an accident of sloppy chemistry; it is the point. The boundaries where these different phases meet, known as heterointerfaces, create regions of structural strain and electronic mismatch that are rich in defects. Structural analyses confirmed that the resulting surface environment is defect-dense, providing exactly the kind of chemically active terrain where water oxidation reactions prefer to occur.

The real innovation, however, lies in the cooperative roles played by cerium and sulfur. Cerium, a rare-earth element famous in catalysis circles for its ability to shuttle between the +3 and +4 oxidation states, is a well-known promoter of oxygen vacancy formation in oxide materials. Sulfur, meanwhile, brings its own talents: it can modulate the electronic structure of nickel sites, and sulfur-treated metal catalysts have repeatedly shown enhanced OER activity in prior studies. When the two are combined, spectroscopic measurements revealed that they do more than add their individual effects. The synergistic coupling of Ce and S effectively redistributes the local electronic structure around the nickel centers, enriches the surface with oxygen-deficient species, and, crucially, accelerates the process of electrochemical surface reconstruction.

Surface reconstruction is a concept that has reshaped how electrochemists think about pre-catalysts. Many as-synthesized materials are not the true active phase at all; under the oxidizing potentials of the OER, they transform in situ into oxyhydroxide species, typically nickel oxyhydroxide, which is where the real catalysis happens. The rate and completeness of this transformation can make or break performance. In the NiCeS/NF system, the defect-rich, electronically modulated surface appears to fast-track this reconstruction, allowing the active oxyhydroxide phase to form more readily and more uniformly. In other words, the cerium and sulfur do not merely decorate the catalyst; they choreograph its evolution into its most active form under operating conditions.

The performance figures reported by the team are striking by any standard. In alkaline media, the NiCeS/NF electrode required an overpotential of just 187 millivolts to drive a current density of 10 milliamperes per square centimeter, a benchmark figure of merit for OER catalysts that reflects how much extra voltage beyond thermodynamic minimum must be supplied. The Tafel slope, which describes how rapidly current increases with applied voltage and offers a window into reaction kinetics, came in at a lean 44.1 millivolts per decade, indicating favorable reaction dynamics. These numbers place the material firmly in the company of the best non-precious-metal OER catalysts described in the recent literature.

Perhaps even more consequential for real-world electrolysis is the durability result. Industrial alkaline electrolyzers do not operate at the gentle 10 milliampere benchmark; they push currents of hundreds of milliamps to more than an ampere per square centimeter, and few laboratory catalysts survive such abuse for long. The NiCeS/NF electrode, however, sustained a punishing 1000 milliamperes per square centimeter for approximately 400 hours without catastrophic degradation. That combination of industrial-level current density and long-term stability is rare, and it addresses one of the most persistent criticisms of defect-engineered catalysts: that the very vacancies and interfaces that boost activity can also be points of structural weakness that dissolve or collapse under prolonged operation.

The findings also contribute to a growing body of evidence on the special role of rare-earth elements in reconstructive electrocatalysis. Recent studies have shown that cerium doping can induce lattice distortion in nickel-iron layered double hydroxides, and that cerium oxide can accelerate surface reconstruction of cobalt selenides into active oxyhydroxide interfaces. The Chengdu-led work extends this picture by pairing cerium with sulfur in a multiphase sulfide-oxysulfide-sulfate matrix, suggesting that the cooperative interplay of a redox-flexible rare-earth cation and a chalcogen that reshapes nickel’s electronic landscape may be a general design principle. If so, it opens a rational pathway for tuning not just what a catalyst is, but what it becomes under working conditions.

The broader context makes the advance timely. Alkaline water electrolysis is one of the leading candidate technologies for producing green hydrogen at scale using renewable electricity, and its cost competitiveness hinges on replacing or minimizing precious-metal catalysts such as iridium and ruthenium oxides, which are scarce and expensive. Nickel-based electrocatalysts, grown directly on conductive nickel foam substrates that double as current collectors, offer a self-supported electrode architecture that avoids the binders and additives that can add resistance and fail mechanically. By demonstrating that defect and interfacial engineering can push such earth-abundant systems to both high activity and industrial-grade durability, the study strengthens the case that hydrogen production need not depend on critical raw materials.

There are, of course, the usual caveats that separate laboratory triumphs from commercial deployment. The 400-hour stability test, while impressive, is still short compared with the multi-year lifetimes expected of commercial electrolyzer stacks, and the molten-salt synthesis route will need to prove itself scalable and economical at the electrode areas relevant to industry. The multiphase nature of the catalyst, so central to its performance, also complicates the task of understanding exactly which sites do the catalytic work, a question the authors address through spectroscopic evidence of electronic redistribution and oxygen-deficient species rather than a single atomic-scale structure. Continued operando studies, of the kind that have illuminated surface reconstruction in nickel sulfide and hydroxide systems, will be needed to pin down the mechanism in full detail.

Even so, the work offers a compelling demonstration that the chemistry of imperfection can be a source of perfection in electrocatalysis. By deliberately engineering oxygen vacancies through the joint action of cerium and sulfur, the researchers have shown that a catalyst’s defects can be designed, controlled, and exploited rather than merely tolerated. As the hydrogen economy matures, strategies like this one, which marry defect chemistry with interfacial engineering and self-supported electrode design, may prove essential to closing the gap between laboratory benchmarks and the gigawatt-scale electrolyzers that a decarbonized energy system will demand. The oxygen half of the water-splitting equation has long been the slow partner; with defect-rich, rare-earth-assisted nickel catalysts now performing at industrial current densities for hundreds of hours, that partnership is finally speeding up.

Subject of Research: Defect-engineered nickel-based electrocatalysts with cerium- and sulfur-induced oxygen vacancies for the oxygen evolution reaction in alkaline water electrolysis

Article Title: Ce and S co-induced oxygen vacancies for enhanced alkaline water electrolysis

Article References: Lei, R., Zhang, Y., Chen, L., Wu, Y., Chen, H., Chen, S., Zhu, Y., & Deng, J. (2026). Ce and S co-induced oxygen vacancies for enhanced alkaline water electrolysis. Ionics. https://doi.org/10.1007/s11581-026-07448-5

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07448-5

Keywords: oxygen evolution reaction, alkaline water electrolysis, oxygen vacancies, nickel-based electrocatalyst, cerium and sulfur co-doping, surface reconstruction, green hydrogen, electrocatalysis, nickel foam, molten-salt synthesis, rare-earth catalysis, Tafel slope