This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. During the electrochemical splitting of water, hydrogen is produced 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—that is, where the oxygen is generated," explains PD Dr. Albert Engstfeld, who coordinated this study together with professor Timo Jacob at the Institute of Electrochemistry at Ulm University. This oxygen evolution reaction therefore requires efficient catalysts: This is where nickel comes into play.
This transition metal is of particular interest for this application, as it exhibits high catalytic activity under alkaline conditions. Under reaction conditions, nickel forms oxidized surface structures, the nature of which determines how the oxygen evolution process unfolds in detail. How, then, can the molecular structure of the catalyst surface be analytically characterized?
"To obtain a valid result, it is crucial to observe the material while the chemical reaction is taking place—that is, in situ," emphasizes lead author Justus Leist, a Ph.D. student at the Institute of Electrochemistry. Surface-enhanced Raman spectroscopy (SERS) was used for this purpose. To perform this specialized analytical measurement technique, a thin layer of the material under investigation is deposited onto a rough gold surface.
This enhances the interaction of the material with an incident laser, and valuable insights into the material's structure can be gained from the reflected light. For decades, it had been assumed that the active surface of nickel catalysts during oxygen evolution consisted of nickel oxyhydroxide, chemically denoted as NiOOH. The Ulm researchers tested this assumption using isotope labeling.
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