Water oxidation is pivotal in biological photosynthesis, where it is catalyzed by a protein-bound metal complex with a Mn4Ca-oxide core; related synthetic catalysts may become key components in non-fossil fuel technologies.
The emergence of disordered metal oxides as electrocatalysts for the oxygen evolution reaction and reports of amorphization of crystalline materials during electrocatalysis reveal a need for robust structural models for this class of materials. Here we apply a combination of low-temperature X-ray absorption spectroscopy and time-resolved in situ X-ray absorption spectroelectrochemistry to analyze the structure and electrochemical properties of a series of disordered iron-cobalt oxides. We identify a composition-dependent distribution of di-μ-oxo bridged cobalt–cobalt, di-μ-oxo bridged cobalt–iron and corner-sharing cobalt structural motifs in the composition series. Comparison of the structural model with (spectro)electrochemical data reveals relationships across the composition series that enable unprecedented assignment of voltammetric redox processes to specific structural motifs. We confirm that oxygen evolution occurs at two distinct reaction sites, di-μ-oxo bridged cobalt–cobalt and di-μ-oxo bridged iron–cobalt sites, and identify direct and indirect modes-of-action for iron ions in the mixed-metal compositions.
Understanding
the mechanism for electrochemical water oxidation
is important for the development of more efficient catalysts for artificial
photosynthesis. A basic step is the proton-coupled electron transfer,
which enables accumulation of oxidizing equivalents without buildup
of a charge. We find that substituting deuterium for hydrogen resulted
in an 87% decrease in the catalytic activity for water oxidation on
Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In situ
visible and quasi-in situ X-ray absorption spectroscopy reveal that
the hydrogen-to-deuterium isotopic substitution induces an equilibrium
isotope effect that shifts the oxidation potentials positively by
approximately 60 mV for the proton coupled CoII/III and
CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical
measurements indicate the absence of a kinetic isotope effect, implying
that the precatalytic proton-coupled electron transfer happens through
a stepwise mechanism in which electron transfer is rate-determining.
An observed correlation between Co oxidation states and catalytic
current for both isotopic conditions indicates that the applied potential
has no direct effect on the catalytic rate, which instead depends
exponentially on the average Co oxidation state. These combined results
provide evidence that neither proton nor electron transfer is involved
in the catalytic rate-determining step. We propose a mechanism with
an active species composed by two adjacent CoIV atoms and
a rate-determining step that involves oxygen–oxygen bond formation
and compare it with models proposed in the literature.
Water-oxidizing calcium-manganese oxides, which mimic the inorganic core of the biological catalyst, were synthesized and structurally characterized by X-ray absorption spectroscopy at the manganese and calcium K edges. The amorphous, birnesite-type oxides are obtained through a simple protocol that involves electrodeposition followed by active-site creation through annealing at moderate temperatures. Calcium ions are inessential, but tune the electrocatalytic properties. For increasing calcium/manganese molar ratios, both Tafel slopes and exchange current densities decrease gradually, resulting in optimal catalytic performance at calcium/manganese molar ratios of close to 10 %. Tracking UV/Vis absorption changes during electrochemical operation suggests that inactive oxides reach their highest, all-Mn(IV) oxidation state at comparably low electrode potentials. The ability to undergo redox transitions and the presence of a minor fraction of Mn(III) ions at catalytic potentials is identified as a prerequisite for catalytic activity.
Nickel–vanadium
layered double hydroxide has recently been
considered as a highly active, low-cost electrocatalyst and as a benchmark
non-noble metal-based electrocatalyst for water oxidation. The material
showed a current density of 27 mA/cm2 at an overpotential
of 350 mV, which is comparable to the best-performing nickel–iron-layered
double hydroxides for water oxidation in alkaline media. The enhanced
conductivity and facile electron transfer were suggested among important
factors for the high activity of nickel–vanadium layered double
hydroxide. In the present study, the stability of an Ni–V catalyst
was investigated by scanning electron microscopy (SEM), transmission
electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS),
X-ray diffraction (XRD), X-ray absorption near edge structure (XANES),
extended X-ray absorption fine structure (EXAFS), and electrochemical
characterization methods. These methods show that the initial Ni–V
catalyst during water oxidation in alkaline conditions is converted
from an initial α-Ni(OH)2 phase to a partially oxidized
α-Ni(OH)2/NiOOH phase and VO4
3– ions. We carefully evaluate the stability of the catalysts and analyze
the compositional changes during prolonged water-oxidation conditions
using inductively coupled plasma-optical emission spectroscopy (ICP-OES).
The
experiments using both Fe-free electrolyte and Fe-free nickel–vanadium
layered double hydroxide reveal that vanadium do not affect the water-oxidizing
activity of α-Ni(OH)2.
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