Iron-doped nickel oxyhydroxides, Nix(Fe1-x)OyHz, are among the most promising oxygen evolution reaction (OER) electrocatalysts in alkaline environments. Although iron (Fe) significantly enhances the catalytic activity, there is still no clear consensus on whether Fe directly participates in the reaction or merely acts as a promoter. To elucidate the Fe’s role, we performed operando X-ray spectroscopy studies supported by DFT on Nix(Fe1-x)OyHz electrocatalysts. We probed the reversible changes in the structure and electronic character of Nix(Fe1-x)OyHz as the electrode potential is cycled between the resting (here at 1.10 VRHE) and operational states (1.66 VRHE). DFT calculations and XAS simulations on a library of Fe structures in various NiOyHz environments are in favor of a distorted local octahedral Fe(III)O3(OH)3 configuration at the resting state with the NiOyHz scaffold going from α-Ni(OH)2 to γ-NiOOH as the potential is increased. Under catalytic conditions, EXAFS and HERFD spectra reveal changes in p-d mixing (covalency) relative to the resting state between O/OH ligands and Fe leading to a shift from octahedral to square pyramidal coordination at the Fe site. XES measurements and theoretical simulations further support that the Fe equilibrium structure remains in a formal Fe(III) state under both resting and operational conditions. These spectral changes are attributed to potential dependent structural rearrangements around Fe. The results suggest that ligand dissociation leads to the C4v symmetry as the most stable intermediate of the Fe during OER. This implies that Fe has a weakly coordinated or easily dissociable ligand that could serve to coordinate the O-O bond formation and, tentatively, play an active role in the Nix(Fe1-x)OyHz electrocatalyst.

The zinc/copper hexacyanoferrate (Zn/CuHCF) cell has gained attention as an aqueous rechargeable zinc-ion battery (ZIB) owing to its open framework, excellent rate capability, and high safety. However, both the Zn anode and the CuHCF cathode show unavoidable signs of aging during cycling, though the underlying mechanisms have remained somewhat ambiguous. Here, we present an in-depth study of the CuHCF cathode by employing various X-ray spectroscopic techniques. This allows us to distinguish between structure-related aging effects and charge compensation processes associated with electroactive metal centers upon Zn²⁺ ion insertion/deinsertion. By combining high-angle annular dark-field-scanning electron transmission microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy, and elemental analysis, we reconstruct the picture of both the bulk and the surface. First, we identify a set of previously debated X-ray diffraction peaks appearing at early stages of cycling (below 200 cycles) in CuHCF. Our data suggest that these peaks are unrelated to hypothetical Zn_xCu_1–xHCF phases or to oxidic phases, but are caused by partial intercalation of ZnSO₄ into graphitic carbon. We further conclude that Cu is the unstable species during aging, whose dissolution is significant at the surface of the CuHCF particles. This triggers Zn²⁺ ions to enter newly formed Cu vacancies, in addition to native Fe vacancies already present in the bulk, which causes a reduction of nearby metal sites. This is distinct from the charge compensation process where both the Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺ redox couples participate throughout the bulk. By tracking the K-edge fluorescence using operando XAS coupled with cyclic voltammetry, we successfully link the aging effect to the activation of the Fe³⁺/Fe²⁺ redox couple as a consequence of Cu dissolution. This explains the progressive increase in the voltage of the charge/discharge plateaus upon repeated cycling. We also find that SO₄^2– anions reversibly insert into CuHCF during charge. Our work clarifies several intriguing structural and redox-mediated aging mechanisms in the CuHCF cathode and pinpoints parameters that correlate with the performance, which will hold importance for the development of future Prussian blue analogue-type cathodes for aqueous rechargeable ZIBs.

Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8–1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. These reactions are proposed to proceed via the Langmuir–Hinshelwood mechanism. In contrast, for the potential range of 1.1–1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface.

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 Ni2+/(3+delta)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH>NaOH approximate to 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. It is commonly accepted that electrolyte alkali metal cations modify the catalytic activity for oxygen evolution reaction. Here the authors challenge this assumption, showing that the activity is actually affected by a change in the electrolyte pH rather than a specific alkali cation.

We present an unusual, yet facile, strategy towards formation of physically mixed Ni-Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)(2) and Fe(OOH) catalysts self-assemble into atomically intermixed Ni-Fe catalysts with unexpectedly high activity.