Electrosynthesis, functional, and structural characterization of a
water-oxidizing manganese oxide†
Ivelina Zaharieva,*
a
Petko Chernev,
a
Marcel Risch,‡
a
Katharina Klingan,
a
Mike Kohlhoff,
a
Anna Fischer
b
and Holger Dau*
a
Received 23rd January 2012, Accepted 7th March 2012
DOI: 10.1039/c2ee21191b
In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient
catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and
largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical
water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive
materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn
oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by
voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100
nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the
biological paragon, the water-splitting Mn
4
Ca complex of photosynthesis, with respect to mean Mn
oxidation state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without
calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in
water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive
Mn oxides and active ones (MnCat), extensive di-m-oxo bridging between Mn ions is observed.
However in the MnCat, the voltage-cycling protocol resulted in formation of Mn
III
sites and prevented
formation of well-ordered and unreactive Mn
IV
O
2
. Structure–function relations in Mn-based water-
oxidation catalysts and strategies to design catalytically active Mn-based materials are discussed.
Knowledge-guided performance optimization of the MnCat could pave the road for its technological
use.
a
Free University Berlin, Physics Department, Arnimallee 14, 14195 Berlin,
de; Fax: +49 30 838 56299; Tel: +49 30 838 53581
b
Technical University Berlin, Institute of Chemistry, Straße des 17. Juni,
10623 Berlin, Germany
† Electronic supplementary information (ESI) available: Further details
on the experimental techniques as well as on the protocols for
electrodeposition of the Mn oxides, CVs for all discussed Mn oxides,
electrochemical characterization of the MnCat, detection of O
2
,
stability tests, results from the attempt to activate the inactive Mn
oxides by a voltage-cycling protocol, elemental analysis, and
additional XAS data and simulations. See DOI: 10.1039/c2ee21191b
‡ Present address: Electrochemical Energy Laboratory, Massachusetts
Institute of Technology, 77 Massachusetts Ave, Cambridge, MA
02139, USA.
Broader context
Threatening global climate changes and unsecured supply of fossil fuels call for a global transition toward sustainable energy-
conversion systems. The storage of wind or solar energy by formation of energy-rich fuel molecules could play a central role in both
transient storage of the intermittently provided energy and replacement of fossil fuels in the transportation sector. Whether
hydrogen or a carbon-based fuel is the target, in any event the extraction of reducing equivalents and protons from water (that is,
water oxidation) is pivotal. In search of water-oxidation catalysts that ultimately could play a role at a global scale, we and others are
aiming at development of simple routes towards formation of Mn-based catalysts. Manganese excels by high availability and low
toxicity; and in oxygenic photosynthesis, nature has demonstrated that a Mn-based catalyst can oxidize water efficiently. When
aiming at an ‘artificial leaf’ with a Mn-based catalyst directly coupled to a solar-energy-converting material, the activity of the
catalyst (per area) needs to cope with the incoming photon flux. Moreover the device design typically requires the use of benign
synthesis and operation conditions, that is, temperatures close to room temperature and pH close to neutral. As an important first
step, we report a simple protocol for electrodeposition of a Mn-oxide catalyst that fulfils the above requirements.
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Introduction
Water oxidation is pivotal not only in powering life on earth by
means of photosynthesis but also in technological systems for
sustainable production of non-fossil fuels.
1
The vision of fuel
production powered by solar energy has driven the rapid devel-
opment of a research area today mostly described as ‘artificial
photosynthesis’ or ‘solar fuels’.
2–7
Whether production of
molecular hydrogen or a carbon-based fuel is targeted, in any
event the use of water as a source of reducing equivalents and
protons is essential. We are aiming at electrosynthesis (electro-
deposition) of a water-oxidation electrocatalyst that is based on
the element also used in biological photosynthesis, that is,
manganese.
In photosynthetic organisms, highly efficient water oxidation
is catalyzed by a metal-oxo Mn
4
Ca complex
8,9
embedded in
a specific protein environment. Inspired by nature’s catalyst for
water oxidation, functional Mn-based molecular mimics
10–12
and
Mn oxides
13–18
have been synthesized. Aside from fully molecular
systems for fuel production which assume the presence of an
electron acceptor, the water oxidation catalysts need to be con-
nected to an electrode, photoanodes or photoactive semi-
conductor particles.
6,17,19,20
Grafting of molecular Mn complexes
to surfaces may be possible.
21
Currently however, intricate
synthesis routes and comparatively rapid ligand degradation
disfavor their use as inexpensive and robust water oxidation
catalysts in technological systems. Manganese oxides directly
electrodeposited on electrodes represent an attractive option but
standard deposition routes result in oxides of comparatively low
catalytic activity (see Results section).
Recently, simple protocols to obtain electrodeposited catalysts
based on Co
22
or Ni
23
oxides have been developed. Their low cost
and operation under benign conditions (neutral pH range, room
temperature) are favorable for realizing the concept of person-
alized energy
24
where inexpensive and simple devices replace the
centralized large-scale energy conversion and distribution
systems. In comparison to Co and Ni as the catalytic metal
ion,
22,23
Mn is favored by:
(1) low price due to high abundance (Mn is the 10
th
most
abundant element in the Earth crust
25,26
);
(2) low toxicity while Co and Ni are potentially carcinogenic;
(3) similarity to the Mn complex in photosynthesis rendering
Mn-based electrocatalysts interesting functional models of the
biological catalyst.
In the 70’s and 80’s, it was shown in several investigations that
manganese oxides (e.g., formed by thermal decomposition) can
catalyze water oxidation, but mainly in alkaline solutions.
27–31
The active compounds in these first studies were described as
mixtures of a-Mn
III
2
O
3
and b-Mn
IV
O
2
(rutile structures)
27–29,31
or
as AMn
IV
O
3
(perovskite, A ¼La, Sr).
30,31
The current interest in
water oxidation catalysts based on earth-abundant metals has
stimulated researchers to explore in more detail the usage of Mn-
based electrocatalysts for water oxidation.
5,17,20,32,33
The proce-
dures for synthesis of the catalysts range from electrodeposition
routes at constant potential (originally developed for Mn oxides
designed for use in batteries)
20
through more intricate
electrodeposition routes involving rotating electrodes and
voltage-cycling protocols
17
to immobilization of pre-synthesized
molecular or colloidal Mn-based catalysts on the electrode
surface.
5,16,32
The resulting structures are mainly layered type Mn
oxides,
20,32
but also Mn
2
O
3
structures
16,17
and spinel (Mn
3
O
4
)
structures
6,16,19
with water oxidation activity have been reported.
The goal of the present study is to pave the road for usage of
electrodeposited Mn-based catalysts in water-splitting devices
similar to the ‘‘artificial leaf’’ presented by Nocera and
coworkers.
34
The catalysts need to support the current densities
required for directly light-driven water oxidation, that is, 1 to
15 mA cm
2
. The typical technological design of an artificial-leaf
device asks for catalyst deposition and operation under benign
conditions (neutral pH domain and room temperature).
To match the incoming flux of solar energy, current densities
below 1 mA cm
2
are clearly insufficient. Thus we use this current
density level to discriminate between low-activity and high-
activity catalysts. In the following, all catalysts with catalytic
currents below 1 mA (at pH 7 and 1.45 V vs. NHE) are consid-
ered to be low-activity catalysts or, at very low current levels, to
be an inactive material. (We note that the level for a high-activity
catalysts depends on the envisioned application. In alkaline
water-electrolysis at elevated temperatures, clearly higher current
densities can be reached often exceeding 100 mA cm
2
.
35
)
Anodic electrodeposition results in water-oxidizing Co- or Ni-
based materials
22,23
which have been identified as oxides with
extensive di-m-oxo bridging between high-valent metal ions.
36–38
Initially the Co-oxide catalyst was considered to be a cobalt-
phosphate catalyst
22
but also electrodeposition in various phos-
phate-free electrolytes has yielded active catalysts
39,40
of similar
atomic structure.
40
For electrodeposition of Mn oxides, the use
of phosphate buffer is hindered by the rapid precipitation of Mn
so that alternative electrolytes have to be employed.
Herein we describe the results of a screening of protocols for
electrodeposition of Mn oxides. We identify conditions for
electrosynthesis of Mn oxides that feature superior catalytic
activity at neutral pH. Manganese oxides that are highly active in
water oxidation are compared to low-activity oxides. Insight into
the Mn oxidation state and atomic structure of the amorphous
oxides is obtained by X-ray absorption spectroscopy. Ultimately,
we identify distinct structural features associated with the
observed catalytic activity and discuss the options for design of
novel protocols for electrodeposition of active materials.
Experimental
Electrodeposition of Mn oxides
The Mn oxides were electrodeposited from aqueous Mn
2+
solu-
tion (0.5 mM Mn
II
(CH
3
COO)
2
$4H
2
O in de-ionized water, in 0.1
M MgSO
4
or in 0.1 M Na(CH
3
COO)/CH
3
COOH buffer at pH 6)
on an inert working electrode (glass coated with indium tin oxide,
ITO, 8–12 Usq
1
, Sigma Aldrich). A standard three-electrode
system was used with a platinum mesh as a counter electrode and
a Hg/Hg
2
SO
4
electrode (saturated) as a reference electrode.
Herein, the indicated potentials are always referenced to the
normal hydrogen electrode (vs. NHE). For Mn oxide deposition,
iR compensation was not applied. In 0.1 M MgSO
4
electrolyte as
well as in the acetate buffer, the electrolyte resistance between
working and reference electrode was relatively low (50 U) so that
also the difference between the nominal and the iR-corrected
potential was small.
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For electrodeposition in low-salt electrolyte (de-ionized water
containing exclusively 0.5 mM Mn acetate), an electrolyte resis-
tance of 4.1 kUwas determined by impedance spectroscopy. At
this high level of the solution resistance, a stable iR-compensa-
tion is problematic. Consequently when using a voltage-cycling
protocol, the actual electrode potentials deviate significantly
from the nominal potentials of the working electrode. When
applying nominal potentials of 0.75 V and +2.15 V, the actual
iR-corrected potential at the working electrode would be
(depending on the actual currents) 0.5 to +0.25 and +1.2 to +
1.7 V, respectively. Further details and deposition protocols are
given in the ESI, S-1A and Fig. S1–S4†.
Electrochemical characterization
The catalytic activity of the electrodeposited Mn oxides was
tested in 0.1 M phosphate buffer (pH 7, adjusted with 0.1 M
KH
2
PO
4
and 0.1 M K
2
HPO
4
). For the test procedures the typical
electrolyte resistance (incl. the electrode) was about 40 U;iR
compensation at 80% was applied.
The Tafel plot was obtained as shown in Fig. S5a–S5d†. To
investigate the pH dependence, chronopotentiometric measure-
ments maintaining a preset current (3 mAcm
2
,10mAcm
2
, and
30 mAcm
2
) were performed.
For further details on electrochemical procedures, UV-vis
spectroscopy, scanning electron microscopy (SEM), elemental
analysis and detection of O
2
evolution see the ESI†.
X-ray absorption spectroscopy
XAS measurements were performed at the BESSY synchrotron
radiation source operated by the Helmholtz-Zentrum Berlin. The
measurements at the manganese K-edge were acquired at the
KMC-1 bending-magnet beamline at 20 K in a cryostat (Oxford-
Danfysik) with a liquid-helium flow system. The incident energy
was adjusted using a double-crystal monochromator (Si-111). A
13 element windowless Ge detector (Ultra-LEGe detectors,
Canberra) was used to measure the X-ray fluorescence emitted
from the sample (Mn K
a
line). Further details are given in the
ESI, S-1E†.
Results
Electrodeposition at constant and alternating potentials: from
inactive to active Mn oxides
First we investigated a set of protocols for anodic electrodepo-
sition of Mn oxides at constant potential. Similar protocols have
been used successfully for electrodeposition of Co- or Ni-based
materials which catalyze water oxidation.
22,23
For electrodeposition of Mn oxides at constant anode poten-
tial, 0.5 mM of Mn
2+
ions in three types of aqueous solution were
used: (1) de-ionized water, (2) electrolyte at high ionic strength
(0.1 M MgSO
4
), and (3) buffered electrolyte (0.1 M sodium
acetate buffer, pH 6). Different deposition potentials ranging
from 0.95 to 2.15 V were applied for 15 min. As a source of Mn
2+
ions, 0.5 mM Mn(II) acetate tetrahydrate was used but other
Mn(II) salts (sulfate, nitrate, chloride) result in formation of very
similar catalysts, as judged from the respective electrochemical
activity and atomic structure (ESI, Fig. S2 and S12e†).
The catalytic activity of each electrodeposited Mn oxide was
assessed in 0.1 M phosphate buffer at pH 7.0 (Fig. 1). (This
specific buffer system was chosen in order to facilitate direct
comparison to electrodeposited Co oxides.
22,41
) Essentially the
same CVs were detected when using 0.1 M Na(CH
3
COO)/0.1 M
CH
3
COOH solution (carefully adjusted to pH 7) instead of the
phosphate buffer. The electrolysis buffer did not contain any Mn
ions to avoid further deposition of Mn ions on the electrode
surface when applying positive potentials. Under these stan-
dardized buffer conditions, all Mn oxides obtained by electro-
deposition at constant potential exhibited either low catalytic
activity or were found to be virtually inactive (see ESI, Fig. S2†).
The highest catalytic current densities of around 200 mAcm
2
at
1.45 V were observed for manganese oxides deposited in acetate
buffer at potentials around 1.6 V. Aiming at current densities of
1mAcm
2
or higher, we consider the level of 200 mAcm
2
as
clearly too low for practical use.
Jaramillo and coworkers electrodeposited a Mn oxide by
consecutive application of two different potentials (5 min at 1 V
followed by deposition of 25 mC at 1.2 V) and recorded sizeable
catalytic currents at pH 13.
20
Using the protocol of Jaramillo and
coworkers for Mn-oxide electrodeposition in a deaerated solu-
tion, we find that, at neutral pH, the activity of this oxide is
comparable to the activity of the best Mn oxide obtained in the
experiments described above (Fig. 1) but still is low in compar-
ison to the Mn oxides described further below.
In conclusion, none of the herein used protocols for electro-
deposition of water-oxidizing Mn oxides at constant anode
potential results in electrocatalysts of satisfactory catalytic
activity. This contrasts starkly with analogous Co- and Ni-based
water-oxidation catalysts where active catalysts are obtained by
electrodeposition at constant potential in a variety of buffer
systems,
22,23
including acetate buffer and unbuffered solutions.
40
Fig. 1 Cyclic voltammograms (CVs) of electrodeposited Mn oxide films
recorded in 0.1 M phosphate buffer at pH 7 (the second CV cycle is
shown, sweep rate of 20 mV s
1
). Light blue, blue-green and black –
MnCat deposited according to protocols A, B and C; green and blue –
Mn oxide films deposited at constant potential of 1.35 V in de-ionized
water and in 0.1 M acetate buffer, respectively; red – Mn catalyst
deposited on FTO according to the procedure described in ref. 20;
orange – blank ITO as a control.
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Since electrodeposition at constant potential failed to produce
high-activity oxides, we investigated more complex protocols
involving periodic variation of the electrode potential. Three
different electrodeposition protocols yielded a Mn-based water
oxidation catalyst (MnCat) of superior catalytic activity (each
comprising 25 cycles):
(A) For electrodeposition in 0.1 M MgSO
4
solution, the anode
potential was changed stepwise between +1.4 V (for 29 s) and
+0.25 V (for 29 s).
(B) For electrodeposition in de-ionized water, the anode
potential was changed stepwise between +2.15 V (for 29 s) and
0.75 V (for 29 s).
(C) For electrodeposition in de-ionized water, the potential was
cycled by continuousvariation of the anode voltage between +2.15
V and 0.75 V (using triangular voltage variations as known from
cyclic voltammetry; a sweep rate of 100 mV s
1
; 58 s per cycle).
The three voltage protocols are schematically shown in the
ESI, Fig. S1†. The catalytic current is highest for protocol-C
(cyclic sweeping in de-ionized water; Fig. 1, black line), inter-
mediate for protocol-B and lowest for protocol-A. For all three
protocols, the catalytic activity is superior to the highest activities
obtained for deposition at constant potential. Notably, for
protocol-C the catalytic current densities are comparable to the
values reported for an electrodeposited Co oxide when tested in
the same buffer system (0.1 M phosphate buffer, pH 7.0).
The electrolyte resistance in the de-ionized water containing
exclusively 0.5 mM Mn acetate (protocols-B and C) was high
(4.1 kUcm
1
as determined by electrochemical impedance spec-
troscopy) because only the highly diluted Mn acetate served as an
electrolyte. Since no iR compensation was applied during elec-
trodeposition, the actual anode potentials in protocols-B and C
were clearly different from the nominal potentials. However the
maximum and the minimum value of the actual electrode
potential in B and C were comparable to the respective values
used in protocol-A. For further details, see ESI, Fig. S1–S4†. We
note that in the presence of 0.1 M MgSO
4
, protocol-C does not
result in formation of an active oxide. In protocol-A and B the
actual anode potentials did not differ, but in protocol-A MgSO
4
was present at a concentration of 0.1 M. The comparison of
protocol-A and B thus shows that high MgSO
4
concentrations
reduce the catalytic activity, as also confirmed by systematic
variation of the MgSO
4
concentration (data not shown). This
observation implies that the electrolyte salt influences the cata-
lytic activity and opens up the prospect of improvement of the
catalysts by electrolyte variation.
In the following, a highly active (protocol-C) and a virtually
inactive manganese oxide (deposited at constant potential) are
compared structurally and functionally. We focus on the MnCat
deposited according to protocol-C, not only because it is the
most active catalysts, but also because the absence of redox-inert
cations (e.g.,Mg
2+
,Ca
2+
,K
+
,Na
+
) simplifies the discussion of its
atomic structure. The high-activity MnCat will be compared to
a virtually inactive film deposited from an aqueous solution of
0.5 mM Mn acetate at a constant potential of +1.35 V. For both
MnCat and the chosen inactive oxide, similar amounts of Mn,
namely 4.4 mgcm
2
(80 nmol cm
2
) and 5.4 mgcm
2
(98 nmol
cm
2
), respectively, were deposited on the working electrode, as
determined by total reflection X-ray fluorescence (TXRF, see
ESI, S-11†).
Functional characterization
To track the oxidation state changes of the Mn ions, we used UV-
vis spectroscopy. Applying different oxidizing potentials to an
already formed film in Mn
2+
-free phosphate buffer, we did not
observe significant change of the spectral shape, but an overall
increase in extinction for an increase in potential (Fig. 2). Since at
higher potentials oxidation of Mn ions is expected, we assume
that the absorption increase reflects an increase in the average
Mn oxidation state. Interestingly, we do not detect any bleach-
ing. Thus we conclude that the extinction of the less oxidized
form of the Mn oxide is clearly lower than for the oxidized form.
Based on this assumption, we are using the extinction recorded at
a fixed wavelength (450 nm, indicated with an arrow in Fig. 2B)
to track qualitatively Mn oxidation-state changes (Fig. 3, insets).
Recently a similar approach has been described for tracking Mn
oxidation state changes in a birnessite-type Mn oxide grafted on
an electrode.
42
Fig. 3 facilitates the comparison of the current density (I¼dQ/
dt, per cm
2
) and the first derivative of the absorption (dA
450
/dt).
In the CV of the active MnCat between 0.45 V and 1.2 V, the
appropriately normalized derivative of the absorption signal
follows the current closely (Fig. 3A). Current and absorption
derivative imply a pseudocapacitive behavior which is explain-
able by the accumulation of redox equivalents and resembles the
behavior of Mn oxides designed for use in batteries.
43,44
Inte-
gration of the reductive (negative) current (from Fig. 3) indicates
that between 0.45 V and 1.2 V a sizeable fraction, that is, about
37% of the Mn ions change their oxidation state by one equiv-
alent. At 1.2 V however, the two signals (Iand dA
450
/dt) split
indicating the onset of the catalytic wave. This catalytic wave is
Fig. 2 (A) UV-vis absorption spectra of the MnCat and the inactive Mn
oxide in dried state. (B) Difference spectra of the MnCat obtained by
subtracting the spectrum recorded in phosphate buffer (pH 7) without
any potential applied (open circuit condition, OC) from the spectra
recorded in phosphate buffer at different potentials. In B, each potential
was applied for 3 min prior to recording the corresponding spectrum.
(The difference in working-electrode potential between the presented
spectra is 0.05 V.)
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lacking in the inactive Mn film (Fig. 3B), in which, surprisingly,
the accumulation of redox equivalents occurs even at an
enhanced level (oxidation state changes of 63% of the Mn ions).
In the inactive film, at least two redox transitions with midpoint
potentials of about 0.8 V and 1.15 V are discernible, possibly
assignable to Mn
II
–Mn
III
and Mn
III
–Mn
IV
transitions. In the
active MnCat separate redox transitions are not readily resolved,
likely explainable by extreme broadening of these redox transi-
tions by electronic interactions of Mn sites in the amorphous
material.
45,46
To verify that the current that is observed at higher potentials
results from catalytic water oxidation and O
2
formation, the O
2
-
evolution rate of the MnCat was monitored in parallel in Mn
2+
-
free electrolyte. The O
2
evolution was detected with a Clark-type
electrode at different (steady-state) potentials and during the CV
(see ESI, S-6†). For voltages exceeding 1.2 V, we detected O
2
formation at a level that corresponds to the respective steady-
state current. The rate of O
2
-formation predicted from the elec-
trical current (at 1.4 V, 2.56 mmol O
2
h
1
cm
2
) was only slightly
higher than the rate detected by the Clark electrode (2.4 mmol O
2
h
1
cm
2
), suggesting that the Faradaic efficiency is close to
100%. The turnover frequency (TOF) at 1.35 V (vs. NHE) and
room temperature was around 0.01 s
1
per deposited Mn ion and
evolved O
2
molecule. At the same overpotential (and similar
amounts of deposited metal ions), we determined a TOF of
0.017 s
1
for the Co-based electrocatalyst of ref. 22 and of 0.01 s
1
for the Ni-based electrocatalyst of ref. 23, both deposited at
constant potential.
We studied the pH dependence of the anode potential required
to reach a preset current level and determined a slope of about
60 mV per pH unit (Fig. 4A), as also reported for a Co-based
electrocatalyst.
41
This behavior is explainable by formation of an
intermediate state by Mn oxidation coupled to the release of one
proton. This oxidation and deprotonation step is completed
before onset of a relatively slow chemical step, the latter being
largely insensitive to both the anode voltage and the buffer pH.
47
The current–voltage relation (Tafel plot) of the MnCat at pH 7
in Mn-free phosphate buffer shows linearity of the log(i)vs.
potential relation (Fig. 4B). For molecular catalysts with
a reversible oxidation step prior to a turnover-limiting chemical
step, a slope of about 59 mV per decade (2.3 RT/F) is pre-
dicted. However, for the MnCat we determined a slope of about
80 mV per decade, suggesting that the interaction energy between
various oxidation sites in the amorphous catalyst is significant
and affects the potential dependence. The latter conjecture is in
line with the CVs of Fig. 3, which suggest extreme broadening of
the oxidation (or reduction) peaks explainable by strong inter-
actions between oxidation sites.
The Tafel plot of Fig. 4B suggests an overpotential of 565 mV
cm
2
at 0.5 mA cm
2
and of 590 mV at 1 mA cm
2
. The former
value was verified in the chronopotentiometric experiment of
Fig. S5d†. We note that these overpotentials are still high in
comparison to values for catalysts based on precious metals. For
example, for an electrodeposited Ir-based amorphous electro-
catalyst recently an overpotential of only 200 mV (at 0.5 mA
cm
2
) was reported.
48
For technological use, improvement of the
MnCat to reduce the overpotential will be of high importance.
Structural characterization
Both the active and the inactive films were visually uniform and
adhered well to the ITO substrate. To probe the morphology of
Fig. 3 Cyclic voltammograms (CVs) of electro-deposited Mn films,
which are active (A) or inactive (B) in water oxidation (sweep rate 20 mV
s
1
, second scan). The electrolyte was 0.1 M phosphate buffer (pH 7.0,
Mn
2+
-free buffer). Both the current (black line) and the absorption at
450 nm (dark red line in the inset) were recorded. The derivative of the
absorption reflects the current flow assignable to changes in the Mn
oxidation state (red lines in the main panels). Note the extended potential
range in comparison to Fig. 1 (lower limit of +0.45 V versus 0.9 V in
Fig. 1). At low electrode potentials, partial dissolution of the oxide film
may occur likely explaining the lower catalytic current in comparison to
CV data shown in Fig. 1.
Fig. 4 (A) pH dependence of the electrode potential required for three
anodic current densities (in 0.1 M phosphate buffer): 3 mAcm
2
– black,
10 mAcm
2
– red, and 30 mAcm
2
– blue. (B) Tafel plot of the Mn oxide
catalyst in 0.1 M phosphate buffer at pH 7. The slope of the solid line
corresponds to 76 mV per decade. The top scale provides the over-
potential (h) relative to the equilibrium potential at pH 7. We note that
also the data in panel-A suggests a (pH-independent) Tafel slope close to
80 mV per decade. For details see ESI, S-5†.
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the electrodeposited material at the nanometre scale, we
employed scanning electron microscopy (SEM) (Fig. 5). Both
Mn films do not cover completely the surface of the electrode as
the ITO layer is clearly visible between amorphous Mn oxide
aggregates. The morphologies of the MnCat and the inactive film
differ clearly. The active MnCat is composed of sand-rose like
structures whereas the inactive one resembles a fluffy network.
However, the surface area of the inactive oxide does not appear
to be smaller than the surface area of the active oxide, as judged
by visual inspection of the SEM image. This implies that the
difference in catalytic activity is not explainable by a smaller
surface area in the inactive oxide.
To elucidate the Mn oxidation state and the dominating
structural motifs in the amorphous Mn oxides, we employed X-
ray absorption spectroscopy (XAS).
49–51
The inset in Fig. 6 shows
the X-ray absorption near-edge structure (XANES) spectrum of
active and inactive Mn oxides, both frozen using the same
protocol (after application of 1.35 V for 2 min in Mn-free
phosphate buffer at pH 7; for details see ESI, S-1E†). Edge-rise
energies are indicative of the mean oxidation state of manga-
nese.
51
On the basis of a calibration employing simple Mn oxides,
the average oxidation state of manganese in the active Mn film
was estimated to be +3.8, while in the inactive film it was +4.0
(see ESI, Fig. S12a and S12b†). The shape of the XANES
spectrum of the inactive Mn
IV
oxide suggests the presence of
a regular layered Mn oxide of the birnessite type
52,53
whereas the
shoulders in the XANES of the active oxide point towards
a more heterogeneous ligand environment.
Fig. 6 shows the Fourier transforms (FTs) of the extended X-
ray absorption fine structure (EXAFS) spectra. Ligation of Mn
by 6 oxygen atoms at a distance of 1.89
A was determined by
EXAFS simulations (Table 1), confirming the prevalence of
Mn
IV
O
6
in both the active and the inactive oxides. (The lower
coordination number in the MnCat is explainable by the pres-
ence of Mn
III
–O distances that are elongated along the Jahn–
Teller axis so that they are not covered by the herein used
simulation approach.)
The second prominent FT peak corresponds to a Mn–Mn
distance of 2.86
A, typical for di-m-oxo bridging between Mn
IV
ions as found in the birnessite-type of layered Mn oxides.
52–54
In
a perfectly ordered layer formed from edge-sharing Mn octa-
hedra (that is, di-m-oxo bridging), for each Mn there are 6
neighboring Mn ions at 2.86
A(N
2.86
¼6), which is close to the
situation in the inactive Mn film where about 4.3 Mn–Mn vectors
of 2.86
A length are detected (per X-ray absorbing Mn ion). A
distance of 5.72
A(22.86
A) is expected in a well-ordered
layered Mn oxide and is indeed visible in the inactive film. For
the MnCat however, the number of 2.86
A Mn–Mn vectors is
clearly smaller, and instead a FT peak assignable to Mn–Mn
vectors of 3.45
A appears, pointing to the presence of mono-m-
oxo bridged Mn ions (corner-sharing octahedra).
52–54
(Addi-
tional EXAFS data and simulations are given in the ESI, S-12†.)
We conclude that the inactive oxide resembles a layered Mn
IV
O
2
with extensive di-m-oxo bridging and significant long-range
order. In clear contrast, the MnCat is a Mn
III/IV
oxide with
a comparable number of di-m-oxo and mono-m-oxo bridges. Any
long-range order is lacking in the MnCat.
The various protocols used for deposition at constant poten-
tial result in inactive oxides with XANES and EXAFS spectra
resembling closely the spectra of the inactive Mn oxide shown in
Fig. 6, suggesting largely identical oxidation-state level and
atomic structure (see ESI, Fig. S12f†). We also found that an
inactive oxide deposited at constant potential was not trans-
formed into an active MnCat by application of a voltage-cycling
Fig. 5 SEM image of the electrodeposited Mn oxides. The active
(MnCat, left) and inactive oxide (right), both do not form a continuous
layer so that the nanostructure of the ITO substrate is visible.
Fig. 6 X-ray absorption spectra of the MnCat (orange) and the inactive
oxide (green). The edge region of the spectrum (XANES) is shown in the
inset; the arrows mark shoulders in the MnCat spectrum. Each peak in
the Fourier-transformed EXAFS spectra relates to a specific structural
motif that is schematically depicted (O in red, Mn in purple). The spectra
obtained by EXAFS simulations are shown as thin black lines (fit
parameters in Table 1).
Table 1 Parameters obtained by simulation of the k
3
-weighted EXAFS
spectra. The simulated spectra correspond to the Fourier-transformed
EXAFS spectra shown in Fig. 6. The errors represent the 68% confidence
interval of the respective fit parameter (N, coordination number; R,
absorber-backscatter distance; s, Debye–Waller parameter)
NR[
A] s[
A]
Active Mn oxide (MnCat)
Mn–O 5.3 0.2 1.89 0.003 0.066 0.003
Mn–Mn, di-m-oxo 2.6 0.3 2.86 0.004 0.070 0.004
Mn–Mn, mono-m-oxo 2.5 0.3 3.45 0.01 0.070 0.004
Inactive Mn oxide
Mn–O 5.9 0.3 1.89 0.003 0.069 0.003
Mn–Mn, di-m-oxo 4.3 0.3 2.86 0.002 0.054 0.003
Mn–Mn, mono-m-oxo 1.2 0.2 3.49 0.01 0.054 0.003
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protocol (ESI, S-9†). For the active MnCat, variation of the
electrode material, annealing at 90 C, and prolonged applica-
tion of a constant potential of 1.35 V (in Mn-free phosphate
buffer for 2 h) did neither affect its oxidation state nor its atomic
structure (ESI, Fig. S12d†).
For technological applications, long-term stability of the
MnCat will be crucial. Currently the activity of the MnCat
decreases significantly within minutes when operated as a water
oxidation catalyst at +1.35 V (vs. NHE) in a Mn-free phosphate
buffer (ESI, S-8†) probably explainable by partial dissolution of
the oxide. The atomic structure, however, remains unchanged
(ESI, S-12†). Calcination at 90 C does not prevent this activity
loss whereas the stability of the catalytic current is greatly
improved if the Mn film is covered with Nafion (ESI, S-7 and S-
8†). Further optimization toward increased stability or self-
repair will be of high interest. Grafting of Mn nanoparticles on
a mesoporous scaffold
5,16
or coverage with nafion membranes
32
may represent interesting routes toward improved stability of the
herein reported MnCat.
Discussion
Electrodeposition of a water-oxidizing Mn oxide
Searching for an efficient protocol for formation of a Mn-based
water oxidation catalyst, we performed a screening of conditions
for electrodeposition of Mn oxides. At sufficiently positive
potential, Mn oxides are readily synthesized by anodic electro-
deposition at constant potential at very low ion concentrations
(only 0.5 mM of a Mn
2+
salt) as well as in highly concentrated
non-buffering (0.1 M MgSO
4
) and buffering (0.1 M Na acetate)
electrolytes. However the resulting oxides exhibit low catalytic
activity only (or they are virtually inactive).
We report an electrodeposited Mn oxide with sizable catalytic
activity at pH 7 in aqueous phosphate buffer, where the buffer
ions (HPO
4
) likely serve as an essential proton acceptor
(unpublished results, see also ref. 39). However, the active
material could not be prepared by electrodeposition at constant
anode potential. The active MnCat was deposited using
a protocol that involves recurrent oxidation-state changes of Mn
ions caused by a voltage-cycling protocol.
Previously, thin films of nanostructured Mn oxide were
deposited potentiostatically using a protocol that also involved
a voltage-cycling protocol.
17
These films were found to be active
for both oxygen reduction and water oxidation at pH 13, but
activity at pH 7 was not verified. Moreover the synthesis protocol
used in ref. 17 was not a mere electrodeposition and involved
calcination at 480 C, essentially excluding its use in the envi-
sioned artificial-leaf system.
Structure–function relation
In the knowledge-guided search for more efficient catalysts,
functional and structural characterization of the catalytically
competent material is of high importance. Herein we show by
EXAFS analyses that the formation of the high-activity MnCat
and also of the inactive Mn oxide film involves electro-oxidation
of dissolved Mn
2+
and formation of di-m-oxo bridges (most likely
from water). The presence of high amount of di-m-oxo bridged
Mn ions (a Mn–Mn distance of 2.86
A), a motif which also can
be viewed as edge-sharing of MnO
6
octahedra, typically results in
interconnected incomplete Mn
3
(m-O)
4
or possibly also complete
Mn
4
(m-O)
4
cubanes, which are basic structural motifs in both the
MnCat and the inactive Mn oxide. An additional feature
observed only in the catalytically active MnCat is the high
amount of mono-m-oxo connected Mn ions (a Mn–Mn distance
of about 3.4
A).
The MnCat reported herein shares two characteristic metal–
metal distances with the photosynthetic Mn
4
Ca complex, namely
a short Mn–Mn distance characteristic of di-m-oxo bridging
between Mn ions and a longer metal–metal distance around
3.4
A.
9,55
(For a recent discussion of the structure of the bio-
logical catalyst, see ref. 56.) The Mn–Mn vectors assigned to di-
m-oxo bridging in the photosynthetic Mn complex are shorter
than in the Mn oxides (around 2.72
A)
9,55,57
explainable by m
2
-O
bridging between only two Mn ions
58
as opposed to the preva-
lence of di-m
3
-O bridging (possibly also m
2
-OH) in the Mn oxides.
(We note that in the MnCat, m
2
-oxo bridging is not excluded
because a minor fraction of 2.7
A Mn–Mn vectors of, e.g., 10%
would be irresolvable in the EXAFS analysis.) In the Mn
4
Ca
complex of oxygenic photosynthesis, the 3.3–3.5
A vector
detected by EXAFS spectroscopy relates to a Mn–Ca distance in
a distorted Mn
3
Ca(m-O)
4
cube.
56,59,60
In the synthetic MnCat
however, neither Ca ions nor any other bivalent cation is present.
It is conceivable that, in the MnCat, Mn ions attached by mono-
m-oxo bridges replace Ca with respect to its structural role in the
photosynthetic Mn
4
Ca complex and in Mn
2
Ca-oxide particles.
61
Bridging oxides likely are mechanistically pivotal in biological
water oxidation.
8,62–64
Also in synthetic water-oxidizing oxides,
extensive di-m-oxo bridging between redox-active metal ions
emerges as a key motif.
1
Di-m-oxo bridging between first-row
transition metals has been found to be the dominating structural
motif for the Co-based electrocatalyst of Nocera and
coworkers,
36,37
for the Ni-based electrocatalyst,
38
for colloidal
Mn–Ca oxides,
61
and for the herein described MnCat. However,
extensive di-m-oxo bridging was observed also for the herein
synthesized inactive Mn oxides indicating that, aside from
extensive di-m-oxo bridging, further aspects of the electronic
structure are crucial for catalytic activity.
Both in the active MnCat and in b-Mn
IV
O
2
, a naturally
occurring mineral,
65
a combination between extensive di-m-oxo
and mono-m-oxo bridging is observed. However as opposed to
the MnCat, the catalytic activity of b-Mn
IV
O
2
is exceedingly
low.
29
Well-ordered, microcrystalline Mn oxides generally may
be largely inactive in water oxidation.
15,61
The prevalence of
coordinatively saturated m
3–5
-oxo bonds and the absence of
terminal coordination sites for water binding appear to preclude
water-oxidation activity in both well ordered colloidal (see ref.
61) and electrodeposited Mn oxides. In conclusion, we propose
that the relatively high extent of order in the Mn oxides deposited
at constant potential is the cause for their comparatively low
activity.
The anodic electrodeposition of Co and Ni oxides at constant
potential results in a formation of catalytically active oxides with
extensive di-m-oxo bridging and largely without mono-m-oxo
bridging between the redox-active metal ions.
36,38
Similar mono-
m-oxo-free Mn oxides (resembling birnessites) can be formed by
electrodeposition of Mn ions at constant potential (see Fig. 6 and
ref. 20), but they are found to exhibit low activity only (Fig. 1 and
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ESI, Fig. S2†). As opposed to Co
IV
and Ni
IV
dioxides, layered
Mn
IV
dioxides (e.g., birnessites
26
) are readily formed and repre-
sent an energetically stable modification.
26
Thus we propose that
there are two factors that render a Mn
IV
O
2
a low-activity catalyst
in the neutral pH domain. First, well-ordered Mn
IV
O
2
layers
result in the absence of readily (de)protonatable m
2
-O(H) groups
and the lack of terminal water coordination sites.
1,9
Second, the
energetic stability of reasonably well-ordered MnO
2
impedes
water-oxidation catalysis.
We conclude that the electrodeposition of high-activity Mn-
based catalyst asks for protocols that prevent formation of
a stable Mn
IV
O
2
modification. This can be realized, as demon-
strated in our work, by cycling the electrode potential during
electrodeposition. For electrodeposition with cycling potentials,
the reduction of the Mn
IV
O
2
oxide readily formed upon appli-
cation of high potentials likely facilitates formation of low-
order mixed-valent Mn
III/IV
oxide. A next step forward could be
development of new protocols for electrodeposition of an active
MnCat at constant potential, which may facilitate self-healing
of the catalyst in the presence of Mn
2+
ions, as reported for the
Co-based electrocatalyst.
66
Aside from voltage cycling, it may
be possible to develop further synthetic routes that prevent
formation of a stable Mn
IV
O
2
phase and enforce deposition of
a fraction of Mn ions in a lower oxidation state. Illumination
with visible or UV light is known to cause reduction of Mn
IV
oxides;
32,67,68
illumination during electrodeposition may
promote formation of an active MnCat. Moreover our
screening of electrolytes has been by no means exhaustive.
Future investigations may result in discovery of specific elec-
trodeposition solutions for which the electrolyte ions become
part of the Mn oxide and enforce formation of a low-order
mixed-valent oxide also for electrodeposition at constant
potential.
Conclusions
By choice of the voltage protocol, we could switch from elec-
trosynthesis of a Mn oxide largely inactive in water oxidation to
a high-activity oxide (MnCat). The herein presented MnCat is
superior to previously published Mn-based catalysts as it cata-
lyzes electrochemical water oxidation at neutral pH at rates that
approach the level needed for direct coupling to photoactive
materials. Even higher rates at lower overpotentials are desirable
and represent an important target for future research, as is
development and engineering of systems with long-term stability.
The voltage-cycling protocol resulted in an especially active
Mn oxide, the MnCat. In clear contrast to the Co- and Ni-based
catalysts, electrodeposition at constant potential resulted in an
inactive Mn oxide. Active and inactive oxides differ in their
nanostructure and at the atomic level. The latter difference
appears to be decisive. The disorder in the atomic structure of the
MnCat may facilitate m
2
-O(H) bridging and terminal ligation of
water, and thereby catalytic activity. We believe that the func-
tionally important disorder is jeopardized by the propensity of
manganese to form at highly oxidizing potentials relatively well-
ordered and stable (and thus inert) Mn
IV
dioxides. The voltage-
cycling protocol efficiently counteracts the formation of Mn
IV
O
2
.
As opposed to Mn, electrodeposition of Co or Ni at constant
potential results in formation of high-activity catalysts because
even at especially oxidizing potentials, stable Co
IV
O
2
and Ni
IV
O
2
are not readily formed.
The herein presented results may pave the road for a knowl-
edge-guided synthesis and optimization of water-oxidizing
(electro)catalysts based on manganese, being earth-abundant
and non-toxic and employed since billions of years for water
oxidation in oxygenic photosynthesis.
Acknowledgements
We thank Dr M. Haumann (FU Berlin) for stimulating discus-
sions. The XAS data were collected at the beamline KMC-1 of
the BESSY, a synchrotron radiation source operated by the
Helmholtz-Zentrum Berlin (HZB); we thank M. Mertin and Dr
F. Sch€
afers for excellent technical support at KMC-1. Financial
support by the Berlin Cluster of Excellence on Unifying
Concepts in Catalysis (UniCat) and the European Union (7
th
framework program, SOLAR-H2 consortium, #212508) is
gratefully acknowledged.
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