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A unique oxygen ligand environment facilitates water oxidation in hole-doped IrNiOx core–shell electrocatalysts

Nature Catalysis volume 1, pages 841–851 (2018)Cite this article

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Abstract

The electro-oxidation of water to oxygen is expected to play a major role in the development of future electrochemical energy conversion and storage technologies. However, the slow rate of the oxygen evolution reaction remains a key challenge that requires fundamental understanding to facilitate the design of more active and stable electrocatalysts. Here, we probe the local geometric ligand environment and electronic metal states of oxygen-coordinated iridium centres in nickel-leached IrNi@IrOx metal oxide core–shell nanoparticles under catalytic oxygen evolution conditions using operando X-ray absorption spectroscopy, resonant high-energy X-ray diffraction and differential atomic pair correlation analysis. Nickel leaching during catalyst activation generates lattice vacancies, which in turn produce uniquely shortened Ir–O metal ligand bonds and an unusually large number of d-band holes in the iridium oxide shell. Density functional theory calculations show that this increase in the formal iridium oxidation state drives the formation of holes on the oxygen ligands in direct proximity to lattice vacancies. We argue that their electrophilic character renders these oxygen ligands susceptible to nucleophilic acid–base-type O–O bond formation at reduced kinetic barriers, resulting in strongly enhanced reactivities.

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Fig. 1: Morphology and catalytic OER activity of pure IrOx and IrNiOx core–shell nanoparticles.
Fig. 2: Electronic structure of iridium centres in IrOx and IrNiOx core–shell nanoparticles.
Fig. 3: Local geometric structure of iridium centres in IrOx and nickel-leached, lattice defect-rich IrNiOx core–shell nanoparticles.
Fig. 4: Computed electronic structure of iridium oxides with various metal-vacancy densities.
Fig. 5: Long-range order, element-specific atomic pair correlations and a structure model.
Fig. 6: Iridium sites under OER in the nickel-leached IrOx shell of IrNi@IrOx core–shell nanoparticles.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank the Zentraleinrichtung für Elektronenmikroskopie of the Technische Universität Berlin for support with the TEM technique and R. Loukrakpam for recording the TEM micrographs. Financial support from the German Research Foundation through grant STR 596/3-1/-2 under Priority Program 1613 is gratefully acknowledged. We thank the Helmholtz-Zentrum Berlin for allocation of synchrotron radiation beamtime under the proposal 14201762-ST/R, I. Zizak for technical support at the μSpot beamline of BESSY, A. Bergmann (Fritz Haber Institute of the Max Planck Society) for contributing to data collection at BESSY and S. Shastri (APS, Argonne National Laboratory) for helping with the HE-XRD measurements. This work was supported in part by DOE-BES grant DE-SC0006877. The work also used resources of the Advanced Photon Source at the Argonne National Laboratory provided by the DOE Office of Science under contract number DE-AC02-06CH11357. We acknowledge the Höchstleistungsrechenzentrum Stuttgart for access to the supercomputer Hazel Hen. T.J. acknowledges the Alexander von Humboldt Foundation for financial support.

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Authors and Affiliations

  1. Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Berlin, Germany

    Hong Nhan Nong, Tobias Reier, Hyung-Suk Oh, Manuel Gliech & Peter Strasser

  2. Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, Germany

    Hong Nhan Nong & Detre Teschner

  3. Ernst-Ruska Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, Jülich, Germany

    Paul Paciok & Marc Heggen

  4. National Key Laboratory for Petrochemical and Refinery Technologies, Vietnam Institute of Industrial Chemistry, Hanoi, Vietnam

    Thu Ha Thi Vu

  5. Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Berlin, Germany

    Detre Teschner, Robert Schlögl & Travis Jones

  6. Department of Physics, Central Michigan University, Mount Pleasant, MI, USA

    Valeri Petkov

  7. Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju, Republic of Korea

    Peter Strasser

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Contributions

P.S. and H.N.N. conceived and designed the experiments. H.N.N. carried out the chemical synthesis and electrochemical experiments, and analysed the results. H.N.N., T.R. and H.-S.O. performed the operando XAS experiments. H.N.N. analysed the XAS data. V.P. carried out the resonant HE-XRD measurements and analysed the data. M.G. acquired the TEM images. T.J. carried out the DFT calculations. P.P. and M.H. performed the STEM-EDX measurements. H.N.N., P.S., T.J. and V.P. wrote the manuscript. All authors discussed the results, drew conclusions and commented on the manuscript.

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Correspondence to Travis Jones or Peter Strasser.

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Supplementary information

Supplementary Information

Supplementary Methods, Supplementary Figures 1–10, Supplementary Tables 1–4 and Supplementary References

Supplementary Video 1

3D model of an IrNiOx particle with a size of approximately 9 × 6.5 × 5 nm (Figure 5c). Large grey and green balls represent, respectively, 1,609 Ir and 548 Ni atoms forming the particle’s core. Small grey and red balls represent, respectively, 3,766 Ir and 9,761 O atoms forming the particle’s shell. Note that Ir atoms from the shell are sixfold coordinated (short grey bars) by oxygen atoms thus forming [IrO6] octahedra. The model is optimized in terms of energy by Molecular Dynamics and refined against the experimental atomic pair distribution data by reverse Monte Carlo as described in the Supplementary Methods

Supplementary Video 2

3D model of an IrNiOx particle with a size of approximately 9 × 6.5 × 5 nm (Figure 5c). The (Ir1609Ni548)-atom core of the particle is covered up by an (Ir3766O9761)-atom shell. Note that Ir atoms from the shell are sixfold coordinated by oxygen atoms (red balls) thus forming [IrO6] octahedra (in grey). The octahedra are linked together forming a continuous network riddled with Ir vacancies. The model is optimized in terms of energy by Molecular Dynamics and refined against the experimental atomic pair distribution data by reverse Monte Carlo as described in the Supplementary Methods

Supplementary Data

Cartesian coordinates of the 3D model of an IrNiOx core–shell particle

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Nong, H.N., Reier, T., Oh, HS. et al. A unique oxygen ligand environment facilitates water oxidation in hole-doped IrNiOx core–shell electrocatalysts. Nat Catal 1, 841–851 (2018). https://doi.org/10.1038/s41929-018-0153-y

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