Electrocatalytic Water Oxidation by a Trinuclear Copper(II) Complex

ACS Catalysis, vol. 11, pp. 7223–7240

Abstract

We report a trinuclear copper(II) complex, [(DAM)Cu 3 (μ 3 -O)][Cl] 4 ( 1, DAM = dodecaaza macrotetracycle), as a homogeneous electrocatalyst for water oxidation to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5. Electrocatalytic water oxidation at pH 7 occurs at an overpotential of 550 mV with a turnover frequency of ∼19 s –1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs NHE confirm the evolution of dioxygen with Faradaic efficiencies of 81% and 45%, respectively. Rinse tests conducted after CPE studies provide evidence for the homogeneous nature of the catalysis. The linear dependence of the current density on the catalyst concentration indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H 2 O versus D 2 O) point to involvement of a proton in or preceding the rate-determining step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2 show no evidence of H 2 O 2 formation and support selectivity to form dioxygen. Freeze-quench electron paramagnetic resonance studies during electrolysis provide evidence for the formation of a molecular copper intermediate. Experimental and computational studies support a key role of the phosphate as an acceptor base. Moreover, density functional theory calculations highlight the importance of second-sphere interactions and the role of the nitrogen-based ligands to facilitate proton transfer processes.

Authors 15

1. Ana M. Geer

   University of Virginia

   Affiliation as printed

   Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

2. Charles B. Musgrave

   California Institute of Technology

   Affiliation as printed

   Materials and Process Simulation Center, Department of Chemistry, California Institute of Technology, Pasadena, California 91125, United States

3. Christopher Webber

   University of Virginia

   Affiliation as printed

   Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

4. Robert J. Nielsen

   California Institute of Technology

   Affiliation as printed

   Materials and Process Simulation Center, Department of Chemistry, California Institute of Technology, Pasadena, California 91125, United States

5. Bradley A. McKeown

   University of Virginia

   Affiliation as printed

   Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

6. Chang Liu

   University of Virginia

   Affiliation as printed

   Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

7. P. Philipp M. Schleker

   Forschungszentrum Jülich · Max Planck Institute for Chemical Energy Conversion

   Affiliation as printed

   Department of Heterogeneous Reactions, Max Planck Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany

   Institute of Energy and Climate Research - Fundamental Electrochemistry (IEK-9), Forschungszentrum Jülich, 52425 Jülich, Germany

8. Peter Jakes

   Forschungszentrum Jülich

   Affiliation as printed

   Institute of Energy and Climate Research - Fundamental Electrochemistry (IEK-9), Forschungszentrum Jülich, 52425 Jülich, Germany

9. Xiaofan Jia

   University of Virginia

   Affiliation as printed

   Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

10. Diane A. Dickie

    University of Virginia

    Affiliation as printed

    Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

11. Josef Granwehr Aachen Institute of Technical and Macromolecular Chemistry

    Forschungszentrum Jülich · RWTH Aachen University

    Affiliation as printed

    Institute of Energy and Climate Research - Fundamental Electrochemistry (IEK-9), Forschungszentrum Jülich, 52425 Jülich, Germany

    Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, 52074 Aachen, Germany

12. Sen Zhang

    University of Virginia

    Affiliation as printed

    Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

13. Charles W. Machan corresponding

    University of Virginia

    Affiliation as printed

    Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States

14. William A. Goddard corresponding

    California Institute of Technology

    Affiliation as printed

    Materials and Process Simulation Center, Department of Chemistry, California Institute of Technology, Pasadena, California 91125, United States

15. T. Brent Gunnoe corresponding

    University of Virginia

    Affiliation as printed

    Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States