Researchers at Ulm University have identified nickel dioxide (NiO₂) as the catalytically active surface of nickel electrodes in alkaline water electrolysis, overturning a decades-old assumption that the surface consisted of nickel oxyhydroxide (NiOOH). The findings, published in Nature Catalysis, resolve a long-standing debate about the material’s structure under reaction conditions and highlight implications for hydrogen production technologies.
Researchers Identify NiO₂ as Active Surface in Electrolysis
The study, led by Ph.D. student Justus Leist and Professor Timo Jacob at Ulm University’s Institute of Electrochemistry, utilized surface-enhanced Raman spectroscopy (SERS) and isotope labeling to determine the true surface composition. Previous assumptions held that nickel oxyhydroxide (NiOOH) was the active catalyst, but the team’s experiments revealed that the surface under reaction conditions is actually nickel dioxide (NiO₂).
This difference is fundamentally important from a chemical point of view,
Jacob explained. NiOOH contains hydrogen atoms, whereas NiO₂ does not. This means that the adsorption sites and reaction pathways on the surface, as previously assumed, must now be reassessed.
The research was conducted in situ, meaning the material was observed during the chemical reaction, ensuring accurate results.
Techniques Confirm NiO₂ as Active Material
The team employed SERS, a technique that enhances the interaction of a material with a laser by depositing a thin layer onto a rough gold surface. This allowed them to analyze the material’s structure during the oxygen evolution reaction (OER), a critical step in water electrolysis. To validate their findings, they used isotope labeling, comparing Raman spectra in normal water (H₂O) and deuterated water (D₂O). If the surface had been NiOOH, replacing hydrogen with deuterium would have altered the vibrational properties, but the results instead confirmed the presence of NiO₂.
Impact on Catalyst Design and Hydrogen Production
The discovery has significant implications for the design of more efficient catalysts in alkaline water electrolysis, a key technology for climate-neutral hydrogen production. Nickel is prized for its cost-effectiveness and durability, but understanding its active surface is critical for optimizing performance. The shift from NiOOH to NiO₂ as the active phase suggests that reaction mechanisms and adsorption sites must be re-evaluated.
PD Dr. Albert Engstfeld, who coordinated the study, highlighted the broader relevance: Although hydrogen is usually the focus of attention as a climate-neutral energy carrier, the greatest energy losses frequently occur at the oxygen electrode—that is, where the oxygen is generated.
The findings underscore the need for further research into the structural and electronic properties of NiO₂ to enhance catalytic efficiency.
Publication and Next Steps for Research
The study, titled Consequences of overtones in Raman spectra for assigning nickel anode surface structures as NiO₂ during alkaline electrolysis,
was published in Nature Catalysis on September 11, 2026. The research team called for additional studies to explore the oxidation state, magnetization, and geometry of NiO₂ under reaction conditions, as well as its interplay with other catalytic materials.