Exploring the Synergistic Ni-Fe-W Interplay in Double Perovskites to Understand the Operando Electronic Transformations Driving High Oxygen Evolution Reaction Activity and Stability
Authors: Hales Natasha , Schmidt Thomas J. , Fabbri Emiliana 
Published: December 2024

Novel flame-spray synthesized perovskite oxides of the composition BaSrNixFeyWzO6 exhibit excellent oxygen evolution reaction (OER) activity under alkaline conditions. In particular, BaSrNi0.9Fe0.1WO6, with a catalytic turn over frequency (TOF) six-times that of NiO, displays a competitive Tafel slope of 41 mV/dec in 0.1 M KOH electrolyte. The double perovskite structure, with its tunable AA’BB’O6 formula, provides a unique opportunity to enhance the intrinsic activity of Ni sites by co-doping with Fe and W. Comparing the transformations of BaSrNi0.9Fe0.1WO6 (BSNFW) and BaSrNiWO6 (BSNW) using ex-situ and operando X-ray absorption spectroscopy (XAS) allows a deep exploration of the relationship between the electronic state of the B-site metals, structural changes induced by doping, and the mechanism of oxygen evolution.

Ex-situ soft XAS at the Ni/Fe L2,3
-edge and O K-edge reveals that the electron-withdrawing W6+ stabilizes the Ni2+ state in the linear W-O-Ni chains in pristine BSN(F)W; however, cyclic voltammetry (CV) under OER conditions induces a surface transformation of Ni2+→Ni3+. In BSNW, Ni3+ formation facilitates the hybridization of Ni(eg) and O(2p) orbitals, activating the lattice oxygen for OER participation via the lattice oxygen mechanism (LOM).1 Henceforth, when the redox active Ni(3d) band donates electrons to the external circuit during OER catalysis, subsequent holes can be refilled by electrons from the O(2p) band, generating oxygen vacancies. Conversely, the presence of Fe3+ dramatically changes the post-OER O K-edge spectral features of BSNFW; pi-donation from O(2p) to Fe(t2g) orbitals induces a transformation of high to low-spin Fe3+ and prevents OER-induced Ni-O hybridization. Hence, it can be inferred that BSNFW favors the traditional adsorbate evolution mechanism (AEM) of OER catalysis.2
Ex-situ hard XAS at the W L3
-edge of BSNW shows a decrease in the post-OER white-line intensity, associated with surface W dissolution; this reflects the structural instability associated with lattice oxygen activation, a common problem also for the benchmark alkaline perovskite Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF).3 However, incorporation of Fe into to BSNW, leading to the BSNFW composition, results in high stability and durability under OER conditions, as revealed by W L3
-edge analysis, inductively coupled plasma optical emission spectroscopy (ICP-OES) and operando X-ray diffraction (XRD).

Operando XAS at the Ni K-edge during cyclic voltammetry provides a unique insight into the influence of Fe on the electronic transformations of Ni during the OER (Figure 1). Ni2+ sites in BSNFW experience a strong irreversible oxidation to Ni3+ during the first CV cycle, followed by stable and reversible transformations between Ni2+ and Ni3+ (Figure 1a). Comparatively, BSNW requires a longer activation period and higher oxidative potentials before stable, reversible changes in the Ni absorption edge are observed (Figure 1b); this suggests that Ni-Fe charge transfer via pi-bonding orbitals increases the reversibility of the Ni electronic changes. Moreover, the incorporation of structural Fe induces a strong cathodic shift in the Ni redox peak potential of BSNFW; the lower overpotential for the Ni2+→Ni3+ transformation leads to a greater increase in the Ni absorption edge at high potentials. Indeed, a closer examination of the final CV cycle reveals that the oxidative transformation of Ni in BSNFW appears to be coupled with the Ni redox process, whereas for BSNW it occurs after the Ni redox (Figure 1c, d). Such operando characterizations provide a valuable insight into the synergistic Ni-Fe relationship regularly exploited in OER catalysis, though often poorly understood in complex multi-elemental systems. The highly competitive OER activity of BaSrNi0.9Fe0.1WO6 establishes this underexplored class of Ni-based double perovskites as a promising avenue for further, targeted exploration.
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Figure 1