X-ray Absorption Spectroscopy Studies of the Local Atomic and Electronic Structure of Iron Incorporated into Electrodeposited Hydrous Nickel Oxide Films

Abstract

We have utilized X-ray absorption fine structure (XAFS) spectroscopy to investigate the local atomic and electronic structure of iron incorporated into electrodeposited nickel hydroxide films. We found that cathodic codeposition from a solution containing Fe(II) and Ni(II) ions results in iron occupying Ni lattice sites in R-Ni(OH) 2 . The X-ray absorption near edge structure (XANES) shows that Fe is present as Fe(III) ions in the cathodically codeposited film. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that Fe is coordinated to oxygen at ∼2.00 Å and to Ni at ∼3.11 Å. This Fe-O bond length is smaller than the Fe(II)-O bond distance found in Fe(OH) 2 (∼2.10 Å) but is in good agreement with the average Fe-(III)-O bond distance found in FeOOH (R, γ). The Fe-Ni bond distance is in agreement with that of the Ni(II)-Ni(II) bond distance found in R-Ni(OH) 2 . Moreover, the radial structure function (RSF) around Fe shows a distinct peak at ∼5.8 Å, which is a fingerprint of the brucite (R-Ni(OH) 2 ) structure. On anodic oxidation of the codeposited film in KOH, we found that the Fe ions occupied Ni lattice sites in γ-NiOOH.The XANES shows that the Fe edge shifts to higher energy values, indicating an increase in the oxidation state of Fe on charging. Analysis of the EXAFS data shows that Fe is coordinated to oxygen at ∼1.94 Å and to Ni at ∼2.84 Å. The latter value is in good agreement with the Ni(IV)-Ni(IV) bond length found in γ-NiOOH. The RSF around Fe in the oxidized film shows a distinct peak at ∼5.4 Å, just as in the RSF of Ni in γ-NiOOH. The Fe-O bond distance of ∼1.94 Å is in good agreement with the Fe(IV)-O bond distance found in SrFeO 3 . Our results strongly suggest that the Fe ions in the oxidized film is nominally tetravalent but with the Fe-O bond possessing a high degree of covalency.

Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting

“…[ 46,47 ] However, shown in Figure 6 A, the most signifi cant increase in OER current density is obtained after the deposition of NiFe layer. For example, 600 mA cm −2 can be obtained at 1.55 V with NiFe/NiCo 2 O 4 /NF.…”

“…[ 17,47 ] However, the NiFe/NiCo 2 O 4 /NF composite electrode exhibits much higher OER activity than NiFe/NF electrode for bulk water electrolysis. This superior performance can be explained from the following aspects: First, the deposition of NiCo 2 O 4 nanofl akes layer increases the surface area dramatically, offering more catalytic active sites.…”

In SituX-ray Absorption Near-Edge Structure Study of Advanced NiFe(OH)xElectrocatalyst on Carbon Paper for Water Oxidation

“…50−52 Choy et al 53 reported the change of Fe(III) to Fe(IV) in SrFeO 3 and associated high degree of covalence in Fe−O bond, which was further demonstrated experimentally by Abbate et al 54 Apparently, the edge shift is much smaller than ionically bonded Fe due to the high covalence. Balasubramanian et al 50 conducted the Fe K-edge XANES study on α-Fe, Fe-doped α-Ni(OH) 2 , α-FeOOH, and Fe-doped γ-NiOOH. Gradual edge shift toward higher and higher photon energies is observed.…”

Electrochemical Deposition of Ni(OH)2 and Fe‐Doped Ni(OH)2 Tubes

“…[30][31][32] Four possible mechanisms of electrochemical deposition of Ni(OH) 2 from nickel nitrate solution have been proposed by MacArthur. [33] Furthermore, electrochemical deposition in Al 2 O 3 templates has recently been used to prepare inorganic micro-or nanotubes of Au, [34] Co, Fe, [35] Ni, [36] Cu, [37] Pt, [38] CdS, [39] and TiO 2 .…”

“…The Fedoped Ni(OH) 2 tubes contain only Fe III because two binding energies of 725.3 eV and 711.1 eV were detected, which are assigned to Fe 2p 1/2 and Fe 2p 3/2 in Fe 2 O 3 , respectively. [42] Balasubramanian et al [32] reported that cathodic codeposition of Fe II with Ni II led to the incorporation of Fe in the Ni lattice sites of α-Ni(OH) 2 2 tubes shows that the atomic ratio of Fe to Ni is 1:3.7. Since Corrigan's study shows that iron/nickel hydrous oxides with this atomic ratio are very useful in lowering the Tafel slope for catalyzing the oxygen evolution reaction, [29] we believe that the Fe III -doped α-Ni(OH) 2 ·0.75H 2 O tubes have potential uses as electrocatalysts for alkaline electrolysis to produce hydrogen.…”