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Cite this: Chem. Commun., 2015,
51, 5005
Using nickel manganese oxide catalysts for
efficient water oxidation†
Prashanth W. Menezes,
a
Arindam Indra,
a
Ophir Levy,
a
Kamalakannan Kailasam,
a
Vitaly Gutkin,
b
Johannes Pfrommer
a
and Matthias Driess*
a
Nickel–manganese oxides with variable Ni : Mn ratios, synthesised
from heterobimetallic single-source precursors, turned out to be
efficient water oxidation catalysts. They were subjected to oxidant-
driven, photo- and electro-catalytic water oxidation showing
superior activity and remarkable stability. In addition, a structure–
activity relation could be established.
Splitting of water by an efficient catalyst is one of the major
aspects of renewable energy research at present.
1
Discovery of
such highly active catalysts with scalable, abundant, robust,
stable and low-cost materials is a promising solution for the
sustainable production of clean energy.
2
Over the years, first-
row transition metal oxides, particularly those with manganese
oxide based systems have been widely investigated for photo-
and electrochemical reactions,
3,4
not only due to their eco-
nomic and environmental benefits but also because of the fact
that nature enables solar to chemical energy conversion with a
Mn
4
CaO
5
cluster of photosystem II (PS II).
5
In this context, several manganese oxides have been extensively
explored for oxidant-driven, photo-catalytic and electro-catalytic
water oxidation especially with nanocrystalline and amorphous
manganese and calcium manganese oxides.
6–11
Recently, we inves-
tigated different routes for the synthesis of various manganese
oxides for efficient water oxidation.
12,13
On the other hand, nickel
based materials have drawn particular attention due to their
earth abundant nature as well as their lower water oxidation
potentials for efficient water oxidation catalysis.
14–18
Although,
we were successful in substitution of cobalt in manganese
oxides for enhanced redox oxygen catalysis,
19
theroleofnickelin
manganese oxide has been merely examined. This is indeed
because of the difficulties involved in the synthesis and the precise
control over the composition with a maximum of dispersion of the
nickel and manganese on the atomic level, and of the oxidation
states of the metals. A while ago, Fukuzumi et al. reported a NiMnO
3
phase toward water oxidation
20
but other Ni : Mn ratios (composi-
tions) of nickel manganese oxides have not been studied as yet.
Therefore, we opted for the single-source precursor (SSP) approach
to gain access to a new class of heterobimetallic nickel manganese
oxides versus nickel oxide as promising catalysts for efficient
oxidant-driven, photo- and electro-catalytic water oxidation.
First of all, nickel manganese and nickel oxalate SSPs were
prepared in micro-emulsions containing cetyltrimethylammonium
bromide (CTAB) as a surfactant, 1-hexanol as co-surfactant and
hexane as the lipophilic phase and mixed with aqueous solution
containing Ni
2+
,Mn
2+
and oxalate ions with tuneable ratios.
13
The
thus yielded oxalate SSPs were treated thermally in the presence of
dioxygen to form the respective oxide phases of various morpho-
logies. (see ESI,†synthesis). The latter method is a reliable ways to
access these low-cost materials in multigram-scale quantities.
All precursors were characterised extensively by state-of-the-art
techniques and the corresponding data are given in Fig. S1–S9
and Tables S1 and S2 (ESI†), respectively. The thermal degrada-
tion of the different heterobimetallic NiMn and homometallic Ni
precursors in the presence of dioxygen produced the pure oxides
phases Ni
6
MnO
8
(JCPDS 42-479), MnNi
2
O
4
( JCPDS 36-83),
NiMn
2
O
4
(JCPDS 1-1110) and NiO ( JCPDS 71-1179), respectively,
as confirmed by Powder X-Ray Diffraction (PXRD) analysis
(Fig. S10, ESI†). The chemical composition, quantification of
Ni : Mn ratio and their presence in the phase was obtained by
Inductively Coupled Plasma (ICP) Atomic Emission Spectro-
scopy (AES) and Energy Dispersive X-ray (EDX) analysis (Fig. S11
and Table S3, ESI†). Interestingly, just by tuning the nickel and
manganese ratio, various morphologies have been realised. As
shown in Fig. 1, the higher magnification Scanning Electron
Microscopy (SEM) images of Ni
6
MnO
8
showed a flower-type
morphology (B1mm) which consisted petals assembled from
a
Metalorganic Chemistry and Inorganic Materials, Department of Chemistry,
Technische Universita
¨t Berlin, Strasse des 17 Juni 135, Sekr. C2, D-10623 Berlin,
Germany. E-mail: matthias.driess@tu-berlin.de
b
The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology,
The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram,
Jerusalem, 91904, Israel
†Electronic supplementary information (ESI) available: Complete experimental
details and characterisation of precursors and oxides before and after oxidant-
driven, photo-catalytic and electro-catalytic water oxidation. See DOI: 10.1039/
c4cc09671a
Received 4th December 2014,
Accepted 13th February 2015
DOI: 10.1039/c4cc09671a
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nanoparticles and MnNi
2
O
4
displayed cubic type particles. NiMn
2
O
4
forms small nanochains whereas NiO exhibits bricks (B400 nm)
built of tiny nanoparticles (see also Fig. S12, ESI†). Further insights
on the morphology and particles size were gathered by Trans-
mission Electron Microscopy (TEM) and High Resolution (HR)
TEM images (Fig. S13, ESI†). In all cases the particles were well
crystalline with particle size of about 2–5 nm for Ni
6
MnO
8
,B50 nm
for MnNi
2
O
4
,B10 nm for NiMn
2
O
4
,andB5–10 nm for NiO,
respectively. The Fourier Transform Infrared (FTIR) spectrum of all
oxides is typical and characteristic for metal oxalates (Fig. S14,
ESI†). The highest Brunauer–Emmett–Teller (BET) surface area
was displayed for Ni
6
MnO
8
(51.9 m
2
g
1
) followed by NiMn
2
O
4
(39.6 m
2
g
1
), NiO (30.4 m
2
g
1
), and MnNi
2
O
4
(29.3 m
2
g
1
),
respectively (Fig. S15, ESI†).
The detailed bonding states of Ni, Mn and O were further
characterised by X-ray Photoelectron Spectroscopy (XPS). The XPS
core level spectra of Ni2p
3/2
and Ni2p
1/2
for Ni
6
MnO
8
,NiMn
2
O
4
andNiOexhibitedpeaksatbindingenergy(BE)ofB854.5 eV and
872.2 eV corresponding to Ni
2+
while the peaks of MnNi
2
O
4
shifted to the higher energy of 856 eV and 873.8 eV which can
be assigned as the mixture of Ni
2+/3+
(Fig. S16, ESI†).
21
The Mn2p
spectra of Ni
6
MnO
8
and MnNi
2
O
4
displayed two major peaks for
Mn2p
3/2
and Mn2p
1/2
at BE B643.5 eV and B655.0 eV that are
consistent Mn
4+
species whereas for NiMn
2
O
4
, the peak positions
were shifted to lower energy of 642.4 eV and 653.9 eV and are
characteristic for Mn
3+
(Fig. S17, ESI†).
22
The O1s spectrum for all
oxides exhibited a major O
2
peak assignable to bridging oxides
with two smaller ones that could be attributed to the surface
oxygen, physi- and chemisorbed water at or near the surface and
to the hydroxide species (Fig. S18, ESI†).
19
Oxidant-driven water oxidation experiments (see ESI†for details)
were conducted with all catalysts (Fig. S19, ESI†) in deoxygenated
aqueous solution of 0.5 M ceric ammonium nitrate (CAN) and the
rate of the oxygen evolution was calculated from the slope of the linear
fitting for the first 60 s. The Ni
6
MnO
8
was found to be extremely active
with a maximum rate of 1.41 mmol
O2
mol
M
1
s
1
considering
both nickel and manganese atomsareactive,andwasapproxi-
mately thrice higher than the MnNi
2
O
4
(0.52 mmol
O2
mol
M
1
s
1
).
However, for NiMn
2
O
4
, the rate was far lesser with the value of
0.19 mmol
O2
mol
M
1
s
1
and is comparable with the pure NiO
(0.15 mmol
O2
mol
M
1
s
1
). The surface area and the total number
of active sites present on the catalyst play a crucial role in water
oxidation. Therefore, the correlation of surface area normalised
plotsisshowninFig.S20(ESI†)andfollowsthesametrendas
that of total mass activity.
The photo-catalytic water oxidation was performed in a phosphate
buffersolutionofpH7inthepresenceof[Ru(bpy)
3
]
2+
(bpy = 2,2-
bipyridine) as a photosensitiser and S
2
O
82
as two electron acceptor
(Scheme S1, ESI†). In a similar trend to the oxidant-driven water
oxidation, the highest rate of oxygen evolution was exhibited by the
nickel-rich Ni
6
MnO
8
with a value of 1.00 mmol
O2
mol
M
1
s
1
that was
again 1.5 times higher than the other nickel-rich MnNi
2
O
4
phase
(0.69 mmol
O2
mol
M
1
s
1
) (Fig. 2). The rate of oxygen evolution for the
nickel-diluted NiMn
2
O
4
was 0.44 mmol
O2
mol
M
1
s
1
while NiO
showed only a limited activity (0.07 mmol
O2
mol
M
1
s
1
). To compare
the photo-catalytic activity, the commercial manganese oxides and
nickel oxide were measured as standards that again showed that
as-synthesised catalysts are highly active and of interest (Fig. S21,
ESI†). Surface normalisation discloses that the values for MnNi
2
O
4
are superior to Ni
6
MnO
8
4NiMn
2
O
4
cNiO due to their lower
surface area (Fig. S22, ESI†). Comparison of the mass and surface
normalised activity with other reported catalysts confirmed that the
diluted-manganese oxide based Ni systems produced higher oxygen
evolution than most of the known active nickel and manganese
based catalysts (Table S4, ESI†).
7–10,20
After the Clark electrode experiments, a set of experiments for
longer duration was also carried out separately (see ESI†)andthe
oxygen gas was collected in the head space of the reaction mixture
wasquantitativelyanalysedbyagaschromatograph(GC).Amaxi-
mum oxygen yield of 0.08 mL h
1
of O
2
was detected for Ni
6
MnO
8
and 0.07 mL h
1
for MnNi
2
O
4
(Table S5, ESI†). Moreover, it is not
enough to have catalysts that are extremely active but one of the
indispensible criteria is also to know the fate of the catalyst after the
photo-catalytic experiments, and therefore, PXRD and HRTEM inves-
tigation were conducted on high performance Ni
6
MnO
8
and
MnNi
2
O
4
catalysts. From PXRD and HRTEM images (Fig. S23, ESI†),
Fig. 1 SEM images of (a) Ni
6
MnO
8
, (b) MnNi
2
O
4
, (c) NiMn
2
O
4
(d) NiO.
Fig. 2 Dissolved oxygen concentration profiles measured by Clark elec-
trode containing NiMn- and Ni-oxide catalysts in S
2
O
82
–Ru(bpy)
32+
system using phosphate (pH 7) buffer (300 W Xe lamp with 395 nm
cut-off filter).
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it is clear that the crystallinity and the morphology of the nickel-rich
manganese oxides catalysts were preserved and stayed intact unveil-
ing the enhanced stability.
The electro-catalytic measurements (see ESI†) were per-
formed in alkaline 0.1 M KOH solution using Cyclic Voltam-
metry (CV) at a scan rate of 20 mV s
1
. The current for all
electrodes were initially increased during the first few cycles
and reached a steady value after 50 cycles (Fig. S24, ESI†), and
then stayed stable and were unchanged even after additional
cycling. As shown in Fig. 3, For Ni
6
MnO
8
, the anodic current
started growing at 1.54 (vs. the reversible hydrogen electrode,
RHE). The maximum current density of 5.85 mA cm
2
was
attained at 1.87 V. Similarly, for the MnNi
2
O
4
and NiMn
2
O
4
,
the current started increasing at 1.58 and 1.60 V, and the highest
current density was achieved at 2.83 mA cm
2
and 1.25 mA cm
2
at 1.87 V, respectively. Interestingly, for NiO, the current started
growing at 1.40 V itself and the CV’s featured a pair of anodic
and cathodic peaks centred B1.5 V vs. RHE corresponding to the
oxidation of NiO (NiO + OH
1e
-NiOOH), followed by a
current due to O
2
evolution.
23
Electrodes were also pre-
conditioned and forward and backward scans were performed,
with respect to the NiO/NiOOH redox reaction of NiO (Fig. S25,
ESI†).
16,24
It could also be seen that for the Ni
6
MnO
8
,MnNi
2
O
4
and NiMn
2
O
4
, a small redox peak exists, suggesting the partial
oxidation of NiO to NiOOH in the nickel manganese catalysts.
The estimated overpotential for water oxidation at 1 mA cm
2
for
NiO was 395 mV while Ni
6
MnO
8
,MnNi
2
O
4
NiMn
2
O
4
displayed
an overpotential of 480, 560 and 610 mV, respectively (Table S6,
ESI†). The overpotential obtained for NiO here is slightly higher
than the highly active ultra-small NiO nanoparticles but lesser
than other known NiO as well as Ni(OH)
2
nanoplates and
nanoparticles (see also Table S7, ESI†).
16–18,23,25,26
Also similar
trend was extended when normalised with the surface area
suggesting that more active sites are available on the surface of
NiO than the nickel manganese oxides (Fig. S26). Tafel slopes
were extracted in the potential range of 1.55 to 1.80 V and a Tafel
slope of 65 mV per decade was achieved for NiO whereas 88 mV
per decade for Ni
6
MnO
8
associated with a rate determining
chemical step preceded by a reversible electrochemical step at
equilibrium (Fig. S27, ESI†). Increase in the apparent Tafel slope
values were seen for MnNi
2
O
4
and NiMn
2
O
4
that could correspond
to a change in the reaction mechanism but would also be expected
if mass or ion transport limitations became significant.
27,28
How-
ever, from the above electrochemical behaviour, it can be inferred
that a higher content of Ni ions in the structure leads to lower Tafel
slopes and thus, beneficial electro-catalytical properties.
Furthermore, to test the stability of all catalysts, chrono-
amperometric experiments were carried out (Fig. S28, ESI†) and
the current values for NiO and NiMn
2
O
4
were maintained over
the period of 15 hours. In the case of Ni
6
MnO
8
, increased
current values were achieved demonstrating the exceptional
stability of catalysts on a long run. On the other hand, a slight
decrease in currents was observed for MnNi
2
O
4
.
After the long-term stability tests, the electrodes were further
characterised by TEM and CV. HRTEM images of NiO, Ni
6
MnO
8
and MnNi
2
O
4
revealed that an amorphous shell of NiOOH
appears on the surface of the catalysts, which has already been
well described for the Ni based catalysts (Fig. S29, ESI†).
16,23
After chronoamperometry, the NiO electrode was subjected to
CV attaining a lower overpotential (370 mV at 1 mA cm
1
) with
slightly lower current density, which unveils the impressive
nature of the catalyst with prolonged durability (Fig. S30, ESI†).
In addition to the alkaline media, the NiO catalyst was also
studied in neutral (pH 7) and slightly basic (pH 9) conditions
using phosphate and borate buffers and in KOH solution of
pH 11, but only resulting into lower activity (Fig. S31, ESI†). The
determined Tafel slope at pH 11 was lower than pH 13 elucidating
slower kinetics at lower pH (Fig. S32, ESI†).
Based on the higher activity of nickel-rich manganese
oxides for oxidant-driven and photo-catalytic water oxidation
(Ni
6
MnO
8
4MnNi
2
O
4
4NiMn
2
O
4
4NiO), and conversely,
nickel oxide (NiO 4Ni
6
MnO
8
4MnNi
2
O
4
4NiMn
2
O
4
) for
electrochemical OER, a structure activity relation can be
deduced. The crystal structure of Ni
6
MnO
8
is cubic (space
group Fm3m) and may be considered as rock-salt structure
where 6/8 of octahedral sites are occupied by Ni
2+
atoms and
1/8 by Mn
4+
atoms, and by vacancies.
29
The vacancies are ordered
in the alternative (111) planes (Fig. 4a). Under oxidant-driven and
photochemical conditions, not only the Ni
6
MnO
8
provides more
active sites due to the presenceofhighernumberofNi
2+
as active
centres that are supported and stabilised by Mn
4+
but also an
additional (extra) hole density drives this reaction efficiently.
However, both MnNi
2
O
4
and NiMn
2
O
4
crystallise in cubic (space
group Fd3m) system and belong to the spinel type (AB
2
O
4
)
structure (Fig. 4b).
30,31
The Mn
4+
ions occupy the tetrahedral sites
and the octahedral sites are preferred by Ni
2+/3+
for MnNi
2
O
4
whereas the tetrahedral sites are occupied by Ni
2+
and octahedral
by Mn
3+
for NiMn
2
O
4
. It has been already well described that the
octahedral sites in a spinel structure play a prominent role than
that of tetrahedral sites for water oxidation making Ni
2+/3+
higher
active than Mn
3+
and that itself explains the higher activity
of MnNi
2
O
4
in comparison to the NiMn
2
O
4
.
32–34
The NiO (cubic
Fm3m) adopts a rock-salt structure
35
similar to Ni
6
MnO
8
(Fig. 4c)
with octahedral Ni
2+
and O
2
and perhaps because of unavailable
supportofmanganese,displayslimitedactivity.
Fig. 3 Cyclic voltammograms (sweep rate 20 mV s
1
)ofnickelmanganese
and nickel oxide thin film catalysts in 0.1 M KOH (pH 13).
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The situation is partly reversed in the electrochemical water
oxidation. The NiO exhibits highest activity followed by nickel
manganese oxides. This is due to a large amount of amorphous
NiOOH, where Ni
3+
is hexa- coordinated (Fig. 4d),
36
is formed
on the surface of the electrodes (as shown by TEM and XPS)
under electrochemical conditions and the resulting amorphous
phase is known to be active for water oxidation by making the
system very efficient and has been already well established for
other nickel oxide and hydroxides.
16,23,25
Interestingly, for
Ni
6
MnO
8
, MnNi
2
O
4
, and NiMn
2
O
4
lesser amount of NiOOH is
generated depending on the amount of nickel present in the
catalysts. Therefore, it can be concluded that for oxidant-driven
and photo-catalytic water oxidation Ni
6
MnO
8
is efficient due to
the higher amount of Ni active sites stabilised by manganese
and higher structural-hole density whereas amorphous NiOOH
seems to be crucial for electro-catalytic water oxidation due to
its structural features.
In conclusion, we investigated for the first time, the oxidant-
driven, photochemical and electrochemical water oxidation
employing nickel manganese oxide-based catalysts (Ni
6
MnO
8
,
MnNi
2
O
4
,NiMn
2
O
4
) with various Ni : Mn ratios and morphologies,
starting from well-defined heterobimetallic nickel manganese
SSPs; their activities were compared with NiO. Nickel-rich manga-
nese oxides were found to be highly efficient with very high activity
for oxidant-driven and photo-catalytic water oxidation whereas NiO
exhibited higher performance and remarkable stability for electro-
catalytic water oxidation. Based on the crystallographic aspects, a
structure–activity relationship could be deduced from structural
features of the oxide systems. The latter relationship deduced here
can help to predesign new material to boost efficiency of water
oxidation.
Financial support by the BMBF (L2H project) and the DFG
(Cluster of Excellence UniCat) is gratefully acknowledged.
O. Levy would like to thank Einstein Foundation Berlin for
the financial support and Prof. David Avnir for the helpful
discussions.
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Fig. 4 Structural units of (a) Ni
6
MnO
8
, (b) MnNi
2
O
4
or NiMn
2
O
4
, (c) NiO
and (d) NiOOH (see text for details). Atom codes, blue: Mn
4+
or Ni
2+
;
green: Ni
2+
,Ni
3+/2+
or Mn
3+
; red: O
2
; gray: H
+
, yellow: vacancy.
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