Nickel's Active Surface Isn't What Chemists Assumed for Decades, Ulm Team Finds
Watching nickel electrodes while they split water, researchers found the working layer is nickel dioxide, not nickel oxyhydroxide, forcing a rethink of hydrogen catalyst chemistry.
A decades-old assumption about one of the cheapest catalysts for making hydrogen has been overturned. A research team at Ulm University in Germany reports in Nature Catalysis that the active surface of nickel electrodes during alkaline water splitting is nickel dioxide, NiO₂, and not nickel oxyhydroxide, NiOOH, as chemists have widely believed.
The question matters because nickel is a promising, low-cost, durable catalyst for alkaline water electrolysis, a key route to climate-neutral hydrogen. In electrolysis, hydrogen forms at the cathode and oxygen at the anode. "Although hydrogen is usually the focus of attention as a climate-neutral energy carrier, the greatest energy losses frequently occur at the oxygen electrode," said Albert Engstfeld, who coordinated the study with Professor Timo Jacob at Ulm's Institute of Electrochemistry. That oxygen evolution reaction needs an efficient catalyst, and under alkaline conditions nickel is highly active, forming oxidized surface layers whose structure shapes how the reaction runs.
Pinning down that structure is difficult because the surface exists only while the reaction is under way. "To obtain a valid result, it is crucial to observe the material while the chemical reaction is taking place—that is, in situ," said lead author Justus Leist, a Ph.D. student at the institute. The team used surface-enhanced Raman spectroscopy, which puts a thin layer of the sample on a rough gold surface to amplify its interaction with a laser, so the reflected light reveals the material's structure.
To test the NiOOH assumption, the researchers ran the measurements in ordinary water and in heavy water, where hydrogen is replaced by its isotope deuterium. Swapping the isotope normally shifts the vibrations of hydroxyl groups. "If the nickel surface were indeed composed of NiOOH, the replacement of the hydrogen isotope would have to result in characteristic changes in the Raman spectra," Leist said. It did not.
The team then ran electrochemical measurements using cyclic voltammetry and correlated them with density functional theory calculations. The observed spectra could be convincingly reconstructed, and the experimental and theoretical results lined up. Their conclusion is that under reaction conditions the surface corresponds to nickel dioxide.
The difference is more than a labeling issue. "NiOOH contains hydrogen atoms, whereas NiO₂ does not," Jacob said. "This means that the adsorption sites and reaction pathways on the surface, as previously assumed, must now be reassessed." Models of how oxygen forms on nickel, and ideas for improving it, have been built on the old picture.
The finding does not by itself deliver a better electrolyzer. But catalyst design depends on knowing what the active surface really is, and for oxygen evolution on nickel, the answer in this study differs from the textbook one.
