molecules
Review
Iron Is the Active Site in Nickel/Iron Water
Oxidation Electrocatalysts
Bryan M. Hunter
ID
, Jay R. Winkler * and Harry B. Gray *
Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA; bhunter@caltech.edu
*
Correspondence: winklerj@caltech.edu (J.R.W.); hbgray@caltech.edu (H.B.G.); Tel.: +1-626-395-2834 (J.R.W.);
+1-626-395-6500 (H.B.G.)
Academic Editors: Björn Åkermark, Eric V. Johnston and Markus D. Kärkäs
Received: 28 February 2018; Accepted: 12 April 2018; Published: 14 April 2018
Abstract:
Efficient catalysis of the oxygen-evolution half-reaction (OER) is a pivotal requirement for
the development of practical solar-driven water splitting devices. Heterogeneous OER electrocatalysts
containing first-row transition metal oxides and hydroxides have attracted considerable recent interest,
owing in part to the high abundance and low cost of starting materials. Among the best performing
OER electrocatalysts are mixed Fe/Ni layered double hydroxides (LDH). A review of the available
experimental data leads to the conclusion that iron is the active site for [NiFe]-LDH-catalyzed alkaline
water oxidation.
Keywords:
oxygen evolution reaction; layered double hydroxide; electrocatalyst
1. Background
The urgency to develop new technologies that harness energy and natural feedstocks in a
sustainable fashion has never been more apparent. With global power consumption growing at
an exponential rate, only one resource is truly capable of powering the planet: the sun. Sunlight is
reliable, clean, and free.
The long-standing goal of converting radiant solar energy into chemical fuels [
1
–
3
] has received
renewed attention in the past decade [
4
–
6
]. Solar-driven water splitting to H
2
and O
2
remains the
preeminent objective. Although considerable progress has been made in the development of scalable
devices, there is a clear need for robust earth-abundant materials that can catalyze the oxygen evolution
half-reaction (OER) in both acidic and alkaline media (Scheme 1) [5,7].
Scheme 1.
Oxygen Evolution Half-Reactions (OER).
Heterogeneous water oxidation electrocatalysts containing first-row transition metal oxides and
hydroxides have been extensively explored, owing in part to the high abundance and low cost of
starting materials [
7
]. FeOOH is the most active single first-row transition metal water oxidation
catalyst [
8
], and iron impurities on gold also have shown excellent performance [
9
]. Among the
best OER catalysts, however, are mixed Fe/Ni layered double hydroxides (LDH). The M(OH)
2
LDH
hexagonal crystal lattice is a brucite structure consisting of layers of edge-shared M(OH)
6
octahedra [
10
].
The interlayer spacing is defined by the crystallographic
c
-axis length. In anhydrous materials,
c
is
about 4.5 Å; the layers expand when H
2
O intercalates leading to
c
~7–8 Å. Partial incorporation of
M
3+
ions in place of M
2+
is accompanied by intercalation of anions between the layers to balance the
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extra positive charge, producing large interlayer spacings (
c
~7–8 Å). The most active [NiFe]-LDH
OER catalysts have a Ni:Fe ratio of ~3:1. Before examining the role of Fe in these materials, it is helpful
first to review the electrochemical and electrocatalytic properties of the parent LDH: Ni(OH)
2
.
2. Nickel Hydroxide Electrodes
The important role of Ni-hydroxides in battery and electrolyzer technology has stimulated a vast
amount of research into the structural, chemical, and electrochemical properties of this material [
10
,
11
].
Most discussions of Ni(OH)
2
electrochemistry begin with a square scheme known as the Bode cycle
(Figure 1) [
12
]. This cycle identifies two Ni(II)-hydroxides (
α
-Ni(OH)
2
,
β
-Ni(OH)
2
) and two oxidized
forms defined as
γ
-Ni(O)(OH),
β
-Ni(O)(OH). The formal potentials for the two couples are as follows:
E
◦
(
γ
-Ni(O)(OH)/(
α
-Ni(OH)
2
) = 0.37 V vs. Hg/HgO;
E
◦
(
β
-Ni(O)(OH)/(
β
-Ni(OH)
2
) = 0.50 V [
13
].
The primary structural distinction between hexagonal crystals of
α
-Ni(OH)
2
and
β
-Ni(OH)
2
is the
interlayer spacing. The layers in
α
-Ni(OH)
2
·
x
H
2
O (0.4
≤
x
≤
0.7) are intercalated with water molecules,
leading to
c
~8 Å, whereas
c
~4.6 Å in anhydrous
β
-Ni(OH)
2
[10].
Figure 1.
Bode cycle indicating electrochemical phase changes for Ni(OH)
2
.
Although the Ni(O)(OH) materials formally contain Ni(III), a large amount of early research
suggested that the average oxidation state in
γ
-Ni(O)(OH) was greater than +3. On the basis
of electrochemical and iodometric measurements, Corrigan concluded that oxidation of Ni(OH)
2
produces a material with an average oxidation state of +3.67 [
14
–
16
]. Notably, these studies did
not take into account the iron impurities that were later found to contaminate most Ni(OH)
2
samples (
vide infra
) [
14
,
17
,
18
]. On the basis of the calculated oxidation state change, the couple
in Scheme 2 was proposed [
16
,
19
]. The redox process in Scheme 2 corresponds to a structural
rearrangement of
α
-Ni(OH)
2
to
γ
-Ni(O)(OH) [
10
]. This interpretation was further supported by
X-ray spectroscopic measurements that indicated average oxidation states of +3 in
β
-Ni(O)(OH) and
≥
+3.5 in
γ
-Ni(O)(OH) [20,21].
Scheme 2.
Proposed 1.67 e
−
per Nickel Atom Couple [19].
Not all of the data are consistent with this simple interpretation. In particular, the charge
and discharge cycles are not symmetric, indicating that
γ
-Ni(O)(OH) represents a major structural
rearrangement in a portion of the material. Under galvanostatic control, ca. 1 electron per nickel center
is transferred to the electrode at the first plateau (0.4 V vs. Hg/HgO), then the potential increases to
0.6 V vs. Hg/HgO [
16
]. This behavior, often called “overcharging,” indicates that the single “pre-wave”
in LDH materials is direct oxidation to Ni(III).
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Furthermore, under cathodic cycling of the electrode, only 1.12–1.32 electrons per Ni atom can
be delivered. Following cycling, the electrode remains black, never reverting to the almost colorless
Ni(OH)
2
[
16
]. Although Corrigan attributes the asymmetric cycling to the insulating behavior of
Ni(OH)
2
preventing some NiOOH from being reduced, there is nevertheless irreversible restructuring
of the material. This restructuring, while of importance to applications involving battery materials,
apparently does not play a role in OER catalysis (the electrode does not visibly darken during in situ
anodization in nonaqueous media) [22].
3. Iron Incorporation into Ni(OH)
2
Catalysis of water oxidation by nickel hydroxide has been studied since the 1960s [
11
,
12
].
However, is nickel the active site? Not likely. Iron, a potentially better catalytic metal, is present
in most Ni(OH)
2
samples. What is more, the intentional incorporation of iron into Ni-LDH structures
dramatically enhances OER activity. This behavior has been attributed to enhanced Ni catalysis due to
the Lewis acidity of Fe(III) [
23
]. However, Fe(III) is more than a Lewis acid; these redox-active ions in
the Ni-LDH lattice cause a charge imbalance in the M(OH)
6
layers that is compensated by interlayer
anions [
24
]. For [NiFe]-LDH in strongly alkaline solution, that anion is almost always CO
3
2
−
[
25
].
Boettcher suggested that layered structures are critically important for highly efficient water oxidation
catalysis [
26
]. However, exfoliation of iron-containing nickel LDHs into individual “sheets” led to
improved performance [27].
Corrigan first recognized the influence of adventitious (as well as intentional) iron incorporation
into nickel oxide electrodes [
14
]. In one 1987 study, effects were apparent even at 0.01% iron loading.
In films of high iron content (10%), major changes in Tafel slopes were observed; notably, slopes were
lowered from ca. 70 mV/decade (pure nickel) to 25 mV/decade. Those who swear by Tafel slopes
would conclude that incorporation of iron fundamentally changed the nature of the catalytic reaction.
Perhaps of greater relevance is that a > 200-mV decrease in overpotential accompanied the Tafel slope
change. Given these findings, it is shocking that most subsequent work largely ignored the role of
incidental iron incorporation.
More than twenty-five years after Corrigan’s study, Boettcher and coworkers systematically
characterized nickel(II) hydroxide films with added iron [
18
]. They found that incorporation of iron
increased film conductivity by a factor of 30, but that this finding alone was insufficient to explain the
improved activity. Furthermore, they observed that iron migrates into LDH materials with alarming
ease. Upon redox cycling in the highest-purity KOH electrolyte available, these investigators found a
95:5 Ni:Fe ratio after just 12 min. Interestingly, iron incorporation in CoOOH materials shows similar
effects [28].
In a subsequent study, Boettcher’s group reported that the Ni(III)/Ni(II) “pre-wave” did not
correlate with water oxidation activity, further calling into question the role of nickel in iron-containing
catalysts. Importantly, the magnified effect of Fe in spin-cast films is unique; other metals, notably La,
Mn, Ce, and Ti, do not provide long-term improvement [29].
In other work, Boettcher was able to incorporate iron directly from solution at edge and defect sites,
showing that these specific sites are directly related to OER performance, while bulk iron substitution
only influences the nickel charging reaction [
17
]. The observation that iron affects nickel charging
but not the overpotential for water oxidation is strong evidence that nickel-only materials cannot
oxidize water at potentials that are active for [NiFe]-LDH catalysts. Indeed, the OER catalytic wave in
highly Fe-free Ni(OH)
2
electrodes occurs at potentials at least 200 mV more positive than for electrodes
containing even a trace of Fe. Moreover, the catalytic wave shifts further anodically upon Fe-free
Ni(OH)
2
electrode cycling, behavior unlike that of electrodes in the presence of trace Fe [
18
]. The role
of iron is not simply to modulate the oxidizing capability of nickel. Iron is the one and only active site.
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4. Nickel Is Not Oxidized to Ni(IV)
It is our view that the early literature on nickel oxide electrodes in aqueous alkaline solution
is largely irrelevant to the discussion of key steps in [NiFe]-LDH catalyzed OER. Incorporation of
Fe(III) into the Ni(OH)
2
lattice requires the intercalation of anions between M(OH)
6
layers, preventing
formation of the
β
-Ni(OH)
2
(brucite) structure and interfering with the Bode cycle. Oxidation of Ni
in this material might be expected to produce
γ
-Ni(O)(OH), which, in spite of its chemical formula,
is thought to be primarily Ni(IV). Boettcher’s work demonstrates that as the proportion of Fe(III) in
the lattice increases, the NI(II) oxidation wave shifts anodically: in a 75:25 Ni:Fe material, the Ni(II)
oxidation wave cannot be observed prior to the onset of catalytic OER current [
18
,
30
]. Boettcher
rationalized this shift on the basis of the increased positive charge in the M(OH)
6
lattice. Moreover,
his work with Fe-free Ni(OH)
2
indicated that its overpotential for OER is extremely large, far greater
than that of a material containing Fe. The poorer performance of the nickel-only material also revealed
previously unseen features, including a new oxidation at ~0.6 V versus Hg/HgO, which is past the
onset of water oxidation in iron-containing samples. The similarity of these potentials and those found
galvanostatically is worth noting. It is clear that the presence of iron makes the oxidation of nickel
more difficult, precluding the formation of Ni(IV).
It is likely that nickel electrochemistry in [NiFe]-LDH is similar to that in [NiAl]-LDH [
31
].
Interestingly, Qiu and Villemure found that charging [NiAl]-LDH in 0.2 M KOH at 0.6 V vs. SCE
(~0.74 vs. Hg/HgO) produces Ni(III). This material can be reduced readily back to Ni(II). Charging
at 0.8 V versus SCE, these investigators found that nickel was in the +3.6 oxidation state, and that
the reaction was reversible. Higher potentials produced some Ni(IV), but reduction in this case only
gave Ni(+2.6) [
32
]. Although nickel reaches a high oxidation state in these materials, the potentials are
much higher than OER catalyzed by [NiFe]-LDH nanosheets. OER in the nickel/aluminum materials
was observed at 580 mV vs. Hg/HgO in 7 M KOH [33].
5. The Role of High-Oxidation-State Iron in OER
The potentials for oxidation of the metal centers in [NiFe]-LDH are moderated by accompanying
deprotonation of H
2
O and OH
−
ligands; at high states of electrode charge, these deprotonation
reactions will produce terminal oxo ligands. Oxo ligands are far more likely to be found on
high-oxidation-state iron [Fe(IV), Fe(V), Fe(VI)] than high-oxidation-state nickel [Ni(IV)] at potentials
relevant to water oxidation in alkaline media. The stabilities of metal–oxo complexes decrease
across the periodic table, owing to the addition of
d
electrons to
π
* orbitals (there is an “oxo wall”
between Fe-Ru-Os and Co-Rh-Ir in the periodic table) [
34
]. Relevant to this discussion, there are no
well-characterized Ni(IV) oxos, as expected (nickel is on the wrong side of the wall).
Pourbaix (potential-pH) diagrams for nickel and iron illustrate that at pH 14 the
Fe(VI)/Fe(III) couple is virtually identical with the
γ
-Ni(O)(OH)/Ni(OH)
2
couple [
35
,
36
]. Moreover,
tetraoxoferrate(VI) does not to react with Ni(III), suggesting that the Ni(III)/Ni(II) couple is above the
Fe(VI)/Fe(V) couple [
37
]. For these reasons, iron in LDH materials is oxidized to high (>IV) oxidation
states before nickel is oxidized to Ni(IV).
We recently reported in situ infrared, luminescence, and Mössbauer spectra consistent with the
presence of cis-dioxo Fe(VI) in [NiFe]-LDH (the ground state of cis-dioxo Fe(VI) is a spin-triplet; the
signature spin-flip excited-singlet to triplet emission is shown in Figure 2) [
22
]. These spectroscopic
features were present in a minority (ca. 3%) of iron sites, as would be expected for localization at
corner sites in LDH nanosheets. Kinetics modeling revealed that a measurable population of reactive
intermediates could be produced by limiting access to substrate (H
2
O and HO
−
) [
22
]. Taken together,
the evidence strongly indicates that Fe(VI) plays a key role in water oxidation. We were not the first to
come to this conclusion, as Lyons and Brandon previously suggested that a high-oxidation-state iron
species might be the active site [38].
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Figure 2.
In red: near-IR luminescence spectrum (applied potential, 2.3 V vs. Pt) of [NiFe]-LDH. In
blue: reference spectrum of the material taken at the open circuit potential (O.C.).
In-operando
XAS studies of these materials indicate that, upon polarization of a 75:25 Ni:Fe
material, the Ni K-edge and pre-edge shift to higher energies, consistent with an increase in the
Ni oxidation state [
39
]. The Fe-edge, however, does not exhibit a potential-induced shift. EXAFS
analysis of polarized electrodes indicates that both Ni–O and Fe–O distances decrease to ~1.9 Å (from
d
(Ni–O) = 2.06 and
d
(Fe–O) = 2.01 Å in the unpolarized electrode). The Ni–O bond shortening is
consistent with the increase in Ni oxidation state, but the Fe bond contraction is difficult to rationalize
if the Fe oxidation state remains unchanged. A possible explanation for this behavior is that the
lattice contraction that accompanies Ni oxidation imposes a stronger ligand field on the Fe(III) centers,
thereby inducing crossover from a high-spin electronic structure to a low-spin state [Fe–O distances
of ~1.9 Å are consistent with low-spin Fe(III)] [
40
]. Synergistic coupling of Ni(II,III) with Fe(III) spin
states could account for the anodic shift in
E
◦
(
γ
-Ni(O)(OH)/(
α
-Ni(OH)
2
) as the concentration of Fe
increases [18,30].
Why, then, is there no observable shift in the K-edge for iron? The kinetics of the process being
observed also is of critical importance. The failure to observe high-oxidation-state Fe
in situ
may simply
be a consequence of the high OER activity of the material: rapid reaction with HO
−
could deplete the
population of Fe(VI) as rapidly as it is formed, leading to steady-state concentrations too small to be
detected by techniques such as XAS [
41
]. For this reason, we performed our studies at low substrate
concentrations, which sufficiently slowed the relative rate of consumption of intermediates.
Taken together, the experimental facts—the variation in water oxidation activity upon the
incorporation of iron, the relative difficulty in oxidizing Ni(III) to more reactive species, and the
direct evidence of high-oxidation-state iron in [NiFe]-LDH materials—lead to the firm conclusion that
iron is the active site for [NiFe]-LDH-catalyzed alkaline water oxidation.
Acknowledgments:
This work was supported by the NSF CCI Solar Fuels Program (CHE-1305124) and the
Arnold and Mabel Beckman Foundation. B.M.H. acknowledges a Fellowship from the Resnick Sustainability
Institute at Caltech.
Conflicts of Interest:
The authors declare no conflict of interest.
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