A R T I C L E S
Surendranath et al.
oxidation is thought to precede a turnover-limiting electrochemi-
cal step involving the removal of one proton and one electron
have recently described the self-assembly of a highly active
cobalt-based oxygen evolving catalyst that forms as a thin film
on inert electrodes when aqueous solutions of Co salts are
1
7-20
2+
to form a surface oxide species (Scheme 1).
The shift in
mechanism between the pH extremes has been attributed to the
kinetic facility of oxidizing hydroxide ion relative to water.
As such, studies of the OER on platinum oxides establish the
importance of evaluating electrode kinetics across a wide pH
range.
electrolyzed in the presence of phosphate (Co-Pi) or borate
2
1
39,40
(Co-Bi);
more recently, we have used a similar strategy to
4
1
discover a Ni-Bi catalyst. These catalysts are of interest
because they: (1) form in situ under mild conditions on a variety
of conductive substrates;
3
9-41
(2) exhibit high activity in pH
3
9,40
In contrast to precious metal oxides, first-row transition metal
spinels and perovskites have been studied mechanistically under
7-9 water at room temperature;
(3) are functional in salt
water; (4) are comprised of inexpensive, earth-abundant
4
0
2
2-31
39,40
only alkaline conditions
because oxide dissolution ac-
materials;
(5) self-heal by reversing catalyst corrosion at
2
1
42
companies O evolution under acidic conditions. Even under
2
open circuit upon application of a potential; (6) can be
interfaced with light absorbing and charge separating materials
to effect photoelectrochemical water splitting;
alkaline conditions, a mechanistic consensus has been elusive
because the identity of the substrate, method of preparation,
and thickness of the oxide layer strongly impact the current-
voltage characteristics and the observed reaction order with
respect to hydroxide. The variability in kinetic data may be
ascribed, in large part, to a barrier to electron transfer imposed
by the oxide film. These barriers may be of an ohmic or
nonohmic nature and serve to obscure the kinetics of the
4
3-45
and (7) are
functional models of the oxygen-evolving complex of Photo-
4
6
system II. The simple operation of the catalyst from conven-
tional water sources under benign conditions is an important
step toward providing distributed solar energy storage at low-
4
7
cost.
The further development of new catalysts for water splitting
will benefit from an understanding of the OER process of Co-
Pi. EXAFS studies support a structural model wherein Co-Pi is
2
0,30,32-36
interfacial reaction chemistry.
While some of these precious and nonprecious metal oxides
37
48
have been optimized for use in commercial electrolyzers, these
technologies remain expensive for nonconcentrated solar energy
composed of cobaltate clusters of molecular dimensions. In
4
8
49
addition, XANES and EPR studies are consistent with a
proportion of cobalt centers attaining an oxidation state of IV
during water oxidation catalysis. Against this backdrop, we now
report the electrochemical kinetics and 18O isotope studies of
the OER catalyzed by Co-Pi in neutral water. The ability to
perform electrokinetic studies of an amorphous thin film
consisting of cobaltate clusters circumvents the difficulties
imposed by electron transport barriers in interpreting the
electrokinetics of the aforementioned metal oxide catalysts.
Controlled low potential deposition of ultrathin catalyst films
3
8
storage applications. As such, we have turned our attention
to developing inexpensive, highly manufacturable, water-
splitting catalysts for nonconcentrated solar energy storage. We
(
16) Appleby, A. J. In Modern Aspects of Electrochemistry; Bockris,
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