Ulm team identifies nickel dioxide as the active surface of nickel electrolysis anodes

The central experiment is easy to describe. The Ulm group recorded Raman spectra of a working nickel surface in ordinary water and again in heavy water, where deuterium takes the place of hydrogen ^[5]. O-H and O-D groups vibrate differently, so a band that comes from hydrogen on the surface should move when the water is swapped ^[5]. "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," said Justus Leist, the PhD student who is the paper's lead author ^[6][12]. The spectra showed no such change ^[7].

A band that stays put rules out an O-H vibration, but it does not identify the surface by itself. The identification came from cyclic voltammetry, cross-checked against density functional theory calculations ^[8]. With that combination, according to the phys.org account, the team could reconstruct the measured spectra ^[8]. Their conclusion is that under reaction conditions the surface is nickel dioxide ^[1]. NiOOH has a hydrogen atom in its formula; NiO2 has none ^[15].

Leist said it is "crucial to observe the material while the chemical reaction is taking place," and the measurements were made that way ^[4]. Surface-enhanced Raman spectroscopy needs help to see a thin surface layer, so the nickel was deposited as a thin film on rough gold to strengthen its interaction with the laser ^[3]. The test electrode is a model system ^[3]. The account does not say whether the same surface forms on the thicker nickel electrodes in commercial electrolysers, and the study as described is about structure, with no new catalyst or efficiency result attached.

The second finding may travel further than the first. Bands in Raman spectra that have often been assigned to short-lived reaction intermediates are overtones, a physical effect rarely seen in this class of materials, according to the Ulm team ^[10]. "Based on simplified theoretical models, such bands are usually physically forbidden and only become accessible through extended models," Leist said ^[11]. Any mechanism inferred from one of those bands was inferred from a misassigned signal ^[10].

Hydrogen is collected at the cathode, but oxygen comes off the anode ^[14]. "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 Timo Jacob at Ulm's Institute of Electrochemistry ^[2][12]. The oxidized structures nickel forms there decide how oxygen evolution proceeds in detail ^[13]. I think the first people to act on this will be the groups that model that reaction. Their calculations start from an assumed surface, and for decades the assumed surface contained hydrogen ^[1][15]. Jacob said that "the adsorption sites and reaction pathways on the surface, as previously assumed, must now be reassessed" ^[9].