Spectroscopic Characterization of Mixed Fe–Ni Oxide Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Electrolytes

Mixed Fe–Ni oxide electrocatalysts for the oxygen evolution reaction in alkaline electrolytes were synthesized using three different approaches: evaporation induced self-assembly, hard templating, and dip-coating. For each synthesis method, a peak in oxygen evolution activity was observed near 10 mol % Fe content, where the mixed metal oxide was substantially more active than the parent metal oxide electrocatalysts. X-ray diffraction (XRD) analysis showed the formation of a mixed NiO/NiFe₂O₄ phase at low Fe concentrations, and formation of Fe₂O₃ at compositions above 25 mol % Fe. Raman vibrational spectroscopy confirmed the formation of NiFe₂O₄, and did not detect Fe₂O₃ in the electrocatalysts containing up to 20 mol % Fe. X-ray absorption near edge structure (XANES) showed the Fe in the mixed oxides to be predominantly in the +3 oxidation state. Extended X-ray absorption fine structure (EXAFS) showed changes in the Fe coordination shells under electrochemical oxygen evolution conditions. Temperature programmed reaction spectroscopy showed the mixed oxide surfaces also have superior oxidation activity for methanol oxidation, and that the reactivity of the mixed oxide surface is substantially different than that of the parent metal oxide surfaces. Overall, the NiFe₂O₄ phase is implicated in having a significant role in improving the oxygen evolution activity of the mixed metal oxide systems

In Situ Electrochemical X-ray Absorption Spectroscopy of Oxygen Reduction Electrocatalysis with High Oxygen Flux

An in situ electrochemical X-ray absorption spectroscopy (XAS) cell has been fabricated that enables high oxygen flux to the working electrode by utilizing a thin poly­(dimethylsiloxane) (PDMS) window. This cell design enables in situ XAS investigations of the oxygen reduction reaction (ORR) at high operating current densities greater than 1 mA in an oxygen-purged environment. When the cell was used to study the ORR for a Pt on carbon electrocatalyst, the data revealed a progressive evolution of the electronic structure of the metal clusters that is both potential-dependent and strongly current-dependent. The trends establish a direct correlation to d-state occupancies that directly tracks the character of the Pt–O bonding present

Synthesis and Characterization of [Ir(1,5-Cyclooctadiene)(μ-H)]₄: A Tetrametallic Ir₄H₄-Core, Coordinatively Unsaturated Cluster

Reported herein is the synthesis of the previously unknown [Ir­(1,5-COD)­(μ-H)]₄ (where 1,5-COD = 1,5-cyclooctadiene), from commercially available [Ir­(1,5-COD)­Cl]₂ and LiBEt₃H <i>in the presence of excess 1,5-COD</i> in 78% initial, and 55% recrystallized, yield plus its unequivocal characterization via single-crystal X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, electrospray/atmospheric pressure chemical ionization mass spectrometry (ESI-MS), and UV–vis, IR, and nuclear magnetic resonance (NMR) spectroscopies. The resultant product parallelsbut the successful synthesis is different from, vide infrathat of the known and valuable Rh congener precatalyst and synthon, [Rh­(1,5-COD)­(μ-H)]₄. Extensive characterization reveals that a black crystal of [Ir­(1,5-COD)­(μ-H)]₄ is composed of a distorted tetrahedral, <i>D</i>_2<i>d</i> symmetry Ir₄ core with two long [2.90728(17) and 2.91138(17) Å] and four short Ir–Ir [2.78680 (12)–2.78798(12) Å] bond distances. One 1,5-COD and two edge-bridging hydrides are bound to each Ir atom; the Ir–H–Ir span the shorter Ir–Ir bond distances. XAFS provides excellent agreement with the XRD-obtained Ir₄-core structure, results which provide both considerable confidence in the XAFS methodology and set the stage for future XAFS in applications employing this Ir₄H₄ and related tetranuclear clusters. The [Ir­(1,5-COD)­(μ-H)]₄ complex is of interest for at least five reasons, as detailed in the Conclusions section

Synthesis and Characterization of [Ir(1,5-Cyclooctadiene)(μ-H)]₄: A Tetrametallic Ir₄H₄-Core, Coordinatively Unsaturated Cluster

Reported herein is the synthesis of the previously unknown [Ir­(1,5-COD)­(μ-H)]₄ (where 1,5-COD = 1,5-cyclooctadiene), from commercially available [Ir­(1,5-COD)­Cl]₂ and LiBEt₃H <i>in the presence of excess 1,5-COD</i> in 78% initial, and 55% recrystallized, yield plus its unequivocal characterization via single-crystal X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, electrospray/atmospheric pressure chemical ionization mass spectrometry (ESI-MS), and UV–vis, IR, and nuclear magnetic resonance (NMR) spectroscopies. The resultant product parallelsbut the successful synthesis is different from, vide infrathat of the known and valuable Rh congener precatalyst and synthon, [Rh­(1,5-COD)­(μ-H)]₄. Extensive characterization reveals that a black crystal of [Ir­(1,5-COD)­(μ-H)]₄ is composed of a distorted tetrahedral, <i>D</i>_2<i>d</i> symmetry Ir₄ core with two long [2.90728(17) and 2.91138(17) Å] and four short Ir–Ir [2.78680 (12)–2.78798(12) Å] bond distances. One 1,5-COD and two edge-bridging hydrides are bound to each Ir atom; the Ir–H–Ir span the shorter Ir–Ir bond distances. XAFS provides excellent agreement with the XRD-obtained Ir₄-core structure, results which provide both considerable confidence in the XAFS methodology and set the stage for future XAFS in applications employing this Ir₄H₄ and related tetranuclear clusters. The [Ir­(1,5-COD)­(μ-H)]₄ complex is of interest for at least five reasons, as detailed in the Conclusions section