2Division
of Geological and
Planetary Sciences, California Institute of Technology, Pasadena, CA
91125,
USA
3Department of Chemistry, Occidental College, Los Angeles, CA 90041, USA
The
sustainable and economical conversion of solar energy into storable,
transportable fuels by solar-driven water splitting is a grand
challenge of 21st century
chemistry. The mechanisms by which heterogeneous materials perform the
anodic half-reaction, water oxidation, are not well understood. Here,
we describe in-situ spectroscopic measurements in nonaqueous media
designed to trap an exceptionally strong oxidant generated
electrochemically from an iron-containing nickel layered double
hydroxide ([NiFe]-LDH) material. Anodic polarization of this material
in acetonitrile produces prominent infrared absorption features (840
and 856 cm-1)
that are quenched by the addition of neutral or alkaline acetonitrile.
These vibrational spectroscopic signatures along with an extremely
narrow luminescence peak at 1633 nm indicate that the reactive
intermediate is a cis-dioxo-iron(VI) species. An absorption in the
Mössbauer spectrum of the material, which disappears upon exposure to
alkaline acetonitrile, is consistent with population of a high-valent
iron-oxo species. Importantly, chemical trapping experiments reveal
that addition of H2O to the polarized electrochemical cell produces hydrogen peroxide; and addition of HO–
generates oxygen. Re-polarization of the electrode restores the
iron(VI) spectroscopic signatures, confirming that the high-valent oxo
complex is active in the electrocatalytic water oxidation cycle. Tafel
slopes of [NiFe]-LDH in 1%-100% 1 M aqueous KOH (59.1 ± 0.7
mV/decade) confirm that the conditions employed are mechanistically
relevant to bulk water oxidation (S1). Our work demonstrates that
in-situ spectroscopy in nonaqueous media offers a powerful new approach
to the study of aqueous redox mechanisms.