Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni^2+/(3+δ)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH > NaOH ≈ KOH > RbOH > LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect.

Beamline P64 at PETRA III is dedicated to x-ray absorption spectroscopy experiments which require high flux, like quick extended x-ray absorption fine structure spectroscopy on the sub-second time scale, high-resolution resonant emission spectroscopy, and x-ray absorption fine-structure spectroscopy of highly diluted systems. The beamline is installed at the high-energy storage ring PETRA III. The source is a 2 m-long undulator. The beamline covers an energy-range from 4 keV to 44 keV, which was required by the user community. Mirrors can be used to reduce the intensity of higher harmonics, and to focus the beam. A conventional photo-diode and a 100-pixel high-purity Ge-detector are used in fluorescence extended x-ray absorption fine-structure spectroscopy for diluted samples.

Nanocrystalline NiO samples have been studied using the Ni K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy and recently developed modeling technique, combining classical molecular dynamics with ab initio multiple-scattering EXAFS calculations (MD-EXAFS). Conventional analysis of the EXAFS signals from the first two coordination shells of nickel revealed that (i) the second shell average distance R(Ni\char21{}Ni2) expands in nanocrystalline NiO compared to microcrystalline NiO, in agreement with overall unit cell volume expansion observed by x-ray diffraction; (ii) on the contrary, the first shell average distance R(Ni\char21{}O1) in nanocrystalline NiO shrinks compared to microcrystalline NiO; (iii) the thermal contribution into the mean-square relative displacement $\sigma${}2 is close in both microcrystalline and nanocrystalline NiO and can be described by the Debye model; (iv) the static disorder is additionally present in nanocrystalline NiO in both the first Ni\char21{}O1 and second Ni\char21{}Ni2 shells due to nanocrystal structure relaxation. Within the MD-EXAFS method, the force-field potential models have been developed for nanosized NiO using as a criterion the agreement between the experimental and theoretical EXAFS spectra. The best solutions have been obtained for the 3D cubic-shaped nanoparticle models with nonzero Ni vacancy concentration Cvac: Cvac$\approx${}0.4\char21{}1.2% for NiO nanoparticles having the cube size of L$\approx${}3.6\char21{}4.2 nm and Cvac$\approx${}1.6\char21{}2.0% for NiO thin film composed of cubic nanograins with a size of L$\approx${}1.3\char21{}2.1 nm. Thus our results show that the Ni vacancies in nanosized NiO play important role in its atomic structure relaxation along with the size reduction effect.