Nickel is a promising, cost-effective and robust catalyst for alkaline water electrolysis—a key technology for the climate-neutral production of hydrogen. However, the actual structure of the active surface of such nickel electrodes under reaction conditions had not yet been conclusively determined. A research team at Ulm University has now refuted a decades-old assumption: The catalytically active surface does not consist of nickel oxyhydroxide (NiOOH), as was previously widely assumed, but is nickel dioxide (NiO₂). The findings have been published in the journal Nature Catalysis.