Challenges in the Application of Manganese Oxide Powders as OER Electrocatalysts: Synthesis, Characterization, Activity and Stability of Nine Different MnxOy Compounds

Article abstract of DOI:10.1002/zaac.202000180

Manganese oxides are seen as potential electrocatalysts for the alkaline oxygen evolution reaction (OER). To find the most suitable OER catalyst among the large number of known manganese oxide compounds, several comparative studies of selected Mn_xO_y materials in water oxidation catalysis were reported in recent years with, in some cases, conflicting results. In this study, nine different manganese oxide powders differing in structure and/or composition were synthesized, characterized and compared regarding their OER activity and stability using a consistent set of experimental parameters. It turned out that the activity generally depends strongly on the manganese oxide compound. α-MnO₂ manganese oxides of the hollandite-type were found to be more active than those with a lower oxidation state or other crystal structures. The most active catalyst cryptomelane, α-(K)MnO₂, reached a current density of 10 mA/cm² at 1.77±0.02 V in LSV measurements. At a potential of 1.8 V, the current density was approximately 15 mA cm^?2. In contrast, the samples with the lowest activity exhibited values less than 1 mA cm^?2 at the same potential. The stability experiments revealed a fast decrease in activity of all samples within the first minutes of measurement and an almost complete activity loss after 60 min. Conductivity differences are discussed as a likely reason for the observed differences in performance.

Full text of DOI:10.1002/zaac.202000180

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.202000180
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Challenges in the Application of Manganese Oxide
Powders as OER Electrocatalysts: Synthesis,
Characterization, Activity and Stability of Nine Different
Mn O Compounds
x
y
[a]
[a]
[a, b]
Justus Heese-Gärtlein, Anna Rabe, and Malte Behrens*
th
Dedicated to Professor Christoph Janiak on Occasion of His 60 Birthday
Manganese oxides are seen as potential electrocatalysts for the
alkaline oxygen evolution reaction (OER). To find the most
suitable OER catalyst among the large number of known
manganese oxide compounds, several comparative studies of
type were found to be more active than those with a lower
oxidation state or other crystal structures. The most active
catalyst cryptomelane, α-(K)MnO , reached a current density of
2
2
10 mA/cm at 1.77�0.02 V in LSV measurements. At a potential
À 2
selected Mn Oy materials in water oxidation catalysis were
of 1.8 V, the current density was approximately 15 mAcm . In
x
reported in recent years with, in some cases, conflicting results.
In this study, nine different manganese oxide powders differing
in structure and/or composition were synthesized, characterized
and compared regarding their OER activity and stability using a
consistent set of experimental parameters. It turned out that
the activity generally depends strongly on the manganese
contrast, the samples with the lowest activity exhibited values
less than 1 mAcm at the same potential. The stability experi-
ments revealed a fast decrease in activity of all samples within
the first minutes of measurement and an almost complete
activity loss after 60 min. Conductivity differences are discussed
as a likely reason for the observed differences in performance.
À 2
oxide compound. α-MnO manganese oxides of the hollandite-
2
Introduction
have raised considerable interest as promising candidates for
OER catalysts.
[5]
A key technology to produce clean hydrogen is water
electrolysis with its thermodynamic potential difference of
Based on numerous theoretical and experimental studies on
powders and deposited thin films, a detailed but not yet fully
consistent picture of factors that contribute to the intrinsic
activity of manganese oxides has been elaborated. In this
introduction, only a snapshot of the literature can be high-
lighted for the sake of brevity and the reader is referred, for
[1]
1
.23 V. The critical step that leads to high overpotentials is the
[2]
anodic oxygen evolution reaction (OER) and it is a major
challenge for catalyst research to find and develop materials
that can lower this overpotential. Ideally, these electrocatalysts
should be based on elements which are inexpensive, abundant,
and non-toxic. Considering these points, and based upon the
[3]
[
5a]
example, to the recent review article of Melder et al.
for
further information. The OER activity of manganese oxides has
been related to different properties such as the oxidation state
[4]
inspiration from nature’s photosystem II, manganese oxides
[
6]
[7]
[8]
of manganese, the crystallographic phase, lattice strain,
[9]
[10]
coordinatively unsaturated sites, oxygen vacancies, structur-
al motifs like μ-oxo bridges or defective pseudo-cubanes,
but also non-chemical properties like specific surface area or
electric conductivity. In addition, many studies have shown
[11]
[12]
[
a] Dr. J. Heese-Gärtlein, A. Rabe, Prof. Dr. M. Behrens
University of Duisburg-Essen, Faculty of Chemistry and Center for
Nanointegration Duisburg-Essen (CENIDE)
Universitätsstr. 7, 45141 Essen, Germany
E-mail: malte.behrens@uni-due.de
[
13]
[14]
that the surface properties of manganese oxides change
[5f,g,11a,15]
significantly under operation conditions
and that ex-situ
[
b] Prof. Dr. M. Behrens
Ertl Center for Electrochemistry and Catalysis
Gwangju Institute of Science (GIST)
investigations rather describe a pre- or post-catalytic state that
may differ from the working state of the catalyst.
1
5
23 Cheomdan-gwagiro (Oryang-dong), Buk-gu, Gwangju
00-712, South Korea
Several manganese oxides with various properties are
[16]
known, such as the compounds Mn O , Mn O , and MnO and
3
4
2
3
2
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/zaac.202000180
their polymorphs. Comparative studies of these and other
manganese oxide compounds have been published.
[7b–d,14,17]
©
2021 The Authors. Zeitschrift für anorganische und allgemeine
With regard to the relative performance trend of the different
manganese oxides, there is not always agreement between the
different studies, which in part may also be due to variations in
the experimental conditions. It was also recently shown that the
relative trend in OER may depend on the driving force for water
Chemie published by Wiley-VCH GmbH. This is an open access
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oxidation, chemical or electrochemical, for different manganese
accommodate a wide variety and a large amount of ions and
[7d]
[16k,30]
oxides and even for differently prepared variants of the same
water molecules.
Similar to the above-mentioned tunneled
[14]
compound α-(K)MnO2. The work presented here provides a
comparative study of a large set of nine different manganese
oxides that were all synthesized as powders and tested under
the same experimental conditions with the aim to determine
the range of the electrocatalytic properties of different
manganese oxide powders and to seek for correlations with
materials properties such as oxidation state and exposed
surface area. Furthermore, we aim to identify the most
promising manganese oxide based OER catalysts for a more
manganese oxides, birnessite shows potential as battery
[
31]
[32]
material, cation absorbent, or as catalyst in chemical and
[33]
electrochemical water oxidation.
In addition to the MnO phases, also oxides with a lower
2
average oxidation state of manganese were investigated in this
[
34]
study, for example α-Mn O with its mineral name bixbyite.
2
3
The structure of bixbyite can be described as superstructure of
the fluorite-type with ordered anion vacancies and distorted
[35]
MnO octahedra. Among others, bixbyite is known to be an
6
[14]
detailed study.
active catalyst in CO oxidation and N O decomposition and can
2
[
36]
We have chosen the powder approach because it allows to
unambiguously identify and characterize the different com-
pounds using routine powder techniques such as powder-XRD
also be used as an adsorbent in waste water treatment. The
[37]
mineral hausmannite
(Mn O ) crystallizes in a tetragonal
3 4
deformed spinel-type structure. The deformation originates
[18]
III
[38]
or N -physisorption.
A
literature-reported
protocol was
from the Jahn-Teller distortion of the Mn O octahedra.
2
6
chosen to deposit the powder suspensions on glassy carbon
electrodes for their electrochemical characterization. In terms of
OER conditions, alkaline electrolytes have been chosen as this is
the typical medium for first-row transition metal oxide catalysts.
All experimental conditions have been kept constant for all
nine samples ensuring the best possible comparability.
Hausmannite can be used as oxidation catalyst and as sensing
material. A synthetic manganese(II,IV) oxide without a known
[39]
[
16l,40]
related naturally occurring mineral is Mn O .
The mono-
5
8
IV
clinic Mn O consists of layered edge-linked distorted Mn O
octahedra interrupted by Mn vacancies. Between the octahe-
dra, a Mn cationic layer was proposed. The manganese(II)
atoms are coordinated by oxygen atoms and build distorted
Mn O trigonal prisms. Mn O is rarely noticed in literature
5
8
6
IV
2
+
In the following, the compounds, which have been inves-
tigated in this work, will be introduced shortly. Manganese(IV)
oxides exist in various polymorphs all based on MnO6
octahedra, which differ in connectivity pattern that may give
rise to open framework structures. MnO2 is an idealized
composition since the average oxidation state of manganese is
II
[40]
6
5
8
but found attention as electrocatalyst for the OER at neutral
[41]
pH.
In this study, the before-mentioned nine Mn O compounds
x
y
were synthesized according to literature-reported routes and
characterized by powder XRD, thermogravimetry, nitrogen
physisorption, and SEM. Their catalytic performances were
measured by using the same experimental set up and
conditions.
[16a,17,19]
commonly slightly below +IV caused by a mixed valence.
Thus, many of these compounds accommodate certain addi-
tional cations next to water molecules in the framework’s void
space. The tetragonal hollandite structure (α–MnO ) consists of
2
edge-sharing MnO octahedra which form 2×2 tunnels with a
6
[
20]
spacing of the wall atoms of 4.8 Å. The tunnels commonly
+
incorporate cations like K (mineral named cryptomelane), Results
+
2+
[21]
NH4 or Ba (the actual mineral hollandite). Cryptomelane
[
22]
was widely studied since a long time,
containing hollandite (NH4 -hollandite) attracted less attention.
The amount of cations can vary and decrease even to zero
resulting in pure α-MnO without a large stabilizing foreign
cation. Hollandite-type manganese oxides are investigated as
electrode components for rechargeable lithium batteries, and
catalysts for the degradation or selective oxidation of
chemicals.
Orthorhombic ramsdellite
while ammonium
Characterization
+
The success of the syntheses was verified by PXRD measure-
ments. The fingerprint comparison of the XRD patterns with
literature data indicates phase pure and highly crystalline
samples (Figure 1). The structural data were successfully refined
by the Rietveld method. The graphical results are shown as
supporting information in Figure S1. The obtained lattice
parameters, the domain sizes in terms of integral-breadth based
2
[23]
[23]
[42]
[
16h,24]
[25]
(R-MnO ) consists of 2×1
2
channels which are formed by double chains of edge-sharing
volume-weighted column heights (L -IB) and selected addi-
Vol
MnO octahedra connected by common corners. As in α-MnO ,
tional structural information are shown in Table 1. The refined
6
2
the channels are orientated along the crystallographic <001>
lattice parameters of the different manganese oxide samples
[16j,26]
[16j,23,38b,40,43]
direction.
A further tunneled MnO phase is pyrolusite (β-
are similar to values described in literature
and
2
MnO ), an isotype to tetragonal rutile TiO . The MnO octahedra
confirm the targeted lattice identity of the synthesized phases.
Within the hollandite-group, the lattice parameter a varies in
dependence of the incorporated tunnel cation. Counter-
2
2
2
are edge-sharing along the <001> direction and crosslinked
[27]
by corners building 1×1 channels. Ramsdellite and pyrolusite
[28]
were studied as electrode material for lithium batteries.
Beside the tunneled phases, manganese oxides can also
intuitively, the α-MnO , which is free of any foreign cation
2
reveals the largest parameter and thus the largest unit cell
whereas the cryptomelane containing potassium shows the
smallest unit cell. This may be best explained by the additional
[29]
crystallize in layered structures, for example birnessite
(δ-
MnO ). Edge-sharing MnO octahedra form layers with a
2
6
interlayer distance of about 7 Å. This inter-layer space is able to
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[
23]
Figure 1. XRD patterns of different manganese oxides (offset for clarity) and the corresponding reference pattern (black): α-MnO , β-
2
[43c]
[16j]
[45]
[46]
[38b]
[40]
MnO2, R-MnO2, δ-MnO2, α-Mn O , Mn O ,
Mn O .
2
3
3
4
5 8
electrostatic bonding in the presence of extra-cations that may
lead to lattice contraction.
Scanning Electron Microscopy images (Figure 2) were
recorded for all samples. Uniform microstructures were ob-
served for each sample consistent with their phase-purity
moisture and/or the tunnels or inter-layers, the manganese
oxides exhibit two prominent phase transitions during thermal
treatment in air: The transformation into bixbyite starting at
about 500°C (except bixbyite itself) and the thermal reduction
into hausmannite starting at approximately 900°C. In case of
determined by XRD. Except for the KÀ birnessite, all MnO -
hausmannite as the starting material, a thermal oxidation into
bixbyite due to the oxygen partial pressure in air is observed
followed by re-reduction into hausmannite due to high temper-
ature. The results of the TGA are in line with the phase-purity of
the starting materials observed by XRD and confirm thermal
stability of the catalyst in air up to approximately 500°C.
2
polymorphs show a needle-shaped morphology. KÀ birnessite,
bixbyite and Mn O develop sponge-like aggregates, whereas
5
8
hausmannite crystallizes in a rather isotropic, faceted shape. All
samples exhibit a low to moderate BET surface area between 1
2
À 1
and 42 m g . The elemental Mn:O ratio is in most cases close
to the ideal one. Deviating compositions can be caused by
varying water content and flexible oxidation states of man-
ganese. Especially the tunneled (hollandite-group) and layered
In summary, the characterization data suggests that the
nine phase-pure manganese oxides hausmannite (Mn O ),
3
4
bixbyite (α-Mn O ), Mn O , ramsdellite (R-MnO ), pyrolusite (β-
2
3
5
8
2
(
birnessite) manganese oxides show this variability. Interestingly
MnO ), KÀ birnessite (δ-(K)MnO ), cryptomelane (α-(K)MnO ),
2
2
2
+
the same molar amount of NH4 was incorporated in the
NH À hollandite (α-(NH )MnO ), and pure α-MnO with uniform
4
4
2
2
+
NH À hollandite as K in the cryptomelane (KÀ hollandite). The
microstructures have been successfully synthesized. In addition
to the nominal average oxidation states of manganese ranging
from +2.67 to +4, also the particle morphologies and the
4
foreign cation/manganese atomic ratio of approximately 10% is
[44]
at the lower end of what was reported before,
while
2
À 1
KÀ birnessite can accommodate higher amounts of potassium
specific surface areas differ between 1 and 42 m g .
between its layers.
All samples were characterized by TGA (Figure 3) and the
results are discussed here briefly based on an ex situ-XRD
analysis, which can be found together with a deeper discussion
in the supporting information (Figure S2). In addition to the
weight loss at low temperature starting at approximately
Electrocatalysis
An OER activity screening of the synthesized samples was
performed in 1 M KOH after loading the catalysts powders on
glassy carbon electrodes as described in the experimental
1
00°C, which is assigned to the loss of water from adsorbed
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Table 1. Experimental composition, specific surface area, lattice parameters and crystalline domain size of different manganese oxide
samples obtained by Rietveld refinement. The crystal system of the hollandite structure type (lines 1–3) is tetragonal, space group (S.G) I
/m (No. 87) with 8 formula units in the unit cell (Z). The crystal structure of birnessite is orthorhombic (S.G.: C m c m, No. 63, Z=4).
Pyrolusite crystallizes in the rutile structure type (tetragonal, S.G.: P 4 /m n m, No. 136, Z=2), and ramsdellite in the VO -type
4
2
2
(
orthorhombic, S.G.: P b n m, No. 62, Z=4. Mn5O8 crystallizes in the monoclinic Cd Mn O type (S.G.: C 1 2/m 1, No. 12, Z=2). The bixbyite
2 3 8
structure type is cubic (S.G.: I a À 3, No. 206, Z=16), and hausmannite adopts a distorted spinel structure of the CdMn O type (tetragonal,
2
4
S.G.: I 4 /a m d, No. 141, Z=4).
1
[
a]
[b]
[c]
Sample
Composition
MnO2.3
S.A.
Lattice parameters
Domain size
/nm
2
(Formula)
/m /g
/Å, °
α-MnO2
27
a=b= 9.8715(4)
c= 2.8541(2)
α=β=γ= 90
a=b= 9.8062(19)
c= 2.8526(5)
α=β=γ= 90
a=b= 9.8498(14)
c= 2.8524(4)
α=β=γ= 90
a= 4.685(4)
20.49(19)
Cryptomelane
α-(K)MnO2
K MnO
2.1
9
48(9)
0.1
NH À hollandite
(NH ) MnO
1.9
33
42
23.4(3)
14.5(2)
4
4 0.1
α-(NH )MnO
4
2
KÀ birnessite
δ-(K)MnO2
K MnO
2.2
0.2
b= 2.839(2)
c= 14.247(11)
α=β=γ= 90
a=b= 4.404(4)
c= 2.872(2)
α=β=γ= 90
a= 4.462(9)
Pyrolusite
β-MnO2
MnO1.8
MnO2.0
15
35
25(2)
Ramdellite
7.25(8)
R-MnO2
b= 9.480(18)
c= 2.861(5)
α=β=γ= 90
a= 10.156(17)
b= 5.780(10)
c= 4.875(8)
Mn O8
Mn O
2
7.25(8)
5
5
7.8
α=γ= 90
β= 109.609(15)
a=b=c= 9.407(3)
α=β=γ= 90
a=b= 5.7628(11)
c= 9.4728(18)
α=β=γ= 90
bixbyite
Mn O
13
1
49.2(9)
23.4(3)
2
2.9
Mn O3
2
hausmannite
Mn O
3.0 4.0
Mn O4
3
[a] The total elemental composition was determined by atomic absorption spectroscopy (AAS) and the oxygen content was calculated
from the difference to 100% neglecting structural water. [b] Specific surface area as determined by nitrogen physisorption using the BET
method. [c] Volume-weighted mean column height (LVol-IB)
section. As seen in the rotating disk electrode voltammograms
Figure 4 A and B), the activities of the different manganese
pyrolusite and ramsdellite exhibit a clear increase in their slope,
but at 1.8 V they do not reach values exceeding 2 mA/cm . The
2
(
oxides were all clearly larger than that of bare glassy carbon,
but were found to differ substantially for the different
manganese oxides. Comparison with literature data of other
transition metal oxides and a comparative measurement of a
mixed nickel cobalt oxide benchmark powder in the same setup
group of hollandite-type manganese oxides show the best
performance with a clear order α-MnO <NH À hollandite<
2
4
cryptomelane. The onset potential of the five most active
manganese oxides seems rather similar with the major differ-
ence in the slope of the curves after the onset.
(
99% p.a., Sigma-Aldrich Chemie GmbH), which reached 10 mA/
The stability of the OER was determined by holding the
potential at 1.8 V for a duration of about 1 hour and the results
is shown on a relative scale in Figure 4C. The current densities
scale with, but are not identical to the current densities
determined in the LSV experiments shown in Figure 5. A
comparison and the development of the measured absolute
current density with time is shown as supporting information in
Figure S3. For the most active samples, the deviation was
largest and the initial values at steady potential were found to
be about 40% of the transient experiment. All manganese oxide
2
cm at a potential of 1.63�0.02 V, show that the electrocatalytic
[47]
activities of manganese oxides are relatively low. The highest
2
activity was found for cryptomelane, which reaches 10 mA/cm
at 1.77�0.02 V, followed by the other members of the α-MnO2
series. Zooming-in towards the onset of the catalytic current in
Figure 4B shows that the three catalysts hausmannite, birnessite
and bixbyite hardly show a clear onset of current in the
investigated potential range up to 1.8 V. This group of catalyst
can be described as nearly inactive. The curves of Mn O ,
5
8
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Figure 2. Scanning electron micrographs, BET surface areas and chemical formulas of different manganese oxide samples.
Discussion
The tunneled α-MnO2 samples turned out to be the most
promising manganese oxide based OER catalysts in this screen-
ing study. Results of more detailed investigations of the most
[
14]
promising Cryptomelane sample were recently published and
other α-MnO samples will be the topic of forthcoming papers.
2
Here, the discussion is focused on the attempt to correlate the
OER activity with the nominal oxidation state, exposed surface
area as measured using the BET technique and double layer
capacitance C among the set of nine samples.
dl
This attempt is presented in Figure 5. The activity is shown
as current density reached at an applied potential of 1.8 V vs.
RHE with an error margin of 15.4% (Figure S5). Looking at the
oxidation states, no clear correlation can be seen. The group of
inactive catalysts covers almost the full range of oxidation
states. At best, one may infer that average oxidation states
below approximately +3.5 do not lead to high performant
catalysts. However, the presence of an oxidation state higher
than +3.5 alone clearly is not sufficient to make a manganese
oxide a good electrocatalyst since the group of those six
catalysts with this high oxidation state alone cover almost the
Figure 3. Thermogravimetric analysis of the different manganese
oxides.
samples exhibit a fast decrease in activity from the very
beginning of the measurement. After 10 min the activity loss
amounts to at least 50% and after 60 min to 100% of the
respective starting value. Interestingly the stability trend is
similar to the one of the activities. Thus, the current density loss
within the hollandite-group is slower than for the low active
catalysts like bixbyite, Mn O , KÀ birnessite or hausmannite. In
2
full range in activity from about 0.5 to more than 15 mA/cm .
Considering the literature-reported dynamic change in surface
[5f,g]
oxidation state
if a manganese oxide is subjected to an
5
8
summary, however, it must be stated that the manganese
oxides are clearly unstable under the reaction conditions
applied here.
oxidizing potential, it may be debated if such a correlation is
anyway reasonable to expect. In the operating state, a
potential- and pH-dependent self-adjustment of the oxidation
state at the surface can be expected that may only be indirectly
related to the initial (bulk) oxidation state of the pre-catalyst.
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Figure 5. Current density at 1.8 V shown together with the BET
surface area, double layer capacitance C and average oxidation
dl
state of manganese. The oxidation states of cryptomelane, α-MnO2,
ramsdellite, Pyrolusite, KÀ birnessite, bixbyite and hausmannite are
[
17]
based on literature data. The oxidation state of NH À hollandite is
4
assumed to be the same as cryptomelane, the one of Mn O was
5
8
calculated from AAS measurement. The error of the current density
was estimated to be 15.4% based on five-fold measurements of a
[14]
cryptomelane catalyst (Figure S5). The typical error bar of the BET
surface area is smaller than the symbol size in the Figure. Error bars
for C were determined from multiple fits of the high frequency
dl
region of the Nyquist plots.
Also, the exposed surface area determined by nitrogen
physisorption does not show a clear trend with the catalytic
performance. In fact, the two samples with the lowest specific
surface areas are the best and worst catalysts. Again, the group
of the three inactive samples cover the full range of exposed
2
À 1
surface areas between 1 and 42 m g showing that also this
parameter does not correlate properly with the electrocatalytic
performance.
Cdl was extracted from electrochemical impedance spectro-
scopy data. Again, no clear trend with current densities can be
observed. Replicate measurements and changes to the starting
Figure 4. OER activities of the different manganese oxides, A and B:
parameters of the fits led to strong deviations for C , resulting
dl
rotating disk electrode voltammograms recorded in 1 M KOH,
in large error bars, making a reasonable interpretation of the
observed data unsubstantial. Although the Cdl results for the
most active catalysts seem to follow the BET values, no further
interpretation was possible due to the large uncertainty of the
measurements. An exemplary fit for cryptomelane is shown in
Figure S6. For Pyrolusite no reasonable fit was possible.
À 1
5
mVs scan rate and 1600 rpm rotation speed, C: loss of current
density during long-time measurement at 1.8 V.
[15b]
According to the Pourbaix diagram,
even the formation of
[
14]
Mn(VII) and corrosion in form of permanganate is thermody-
namically predicted at pH>13 and a potential of 1.8 V, which
however was visually not observed in this study. A likely
explanation is the deviation of the (lower) local pH in the kinetic
measurement at the working catalyst surface.
These results fit to our recently reported observations
that the electrochemical OER performance of differently synthe-
sized cryptomelane samples scale with the conductivity and the
double layer capacitance (electrochemically active surface area)
rather than with the BET-surface area. In that study, the
electrical conductivity was proposed to play the major role for
the OER activity. A crucial role of conductivity can also be
supposed for this comparative study of different manganese
oxide compounds. It is suggested by the different slope at
similar onset in the LSVs for the most active samples (Figure 4
The activity trend within the group of samples containing
manganese with an oxidation state > +3.5 was also reported
by Meng et al. who compared the OER performance in
[48]
0
.1 molar KOH of cryptomelane, pyrolusite and birnessite. The
authors identified cryptomelane as most active catalyst fol-
lowed by pyrolusite and birnessite in agreement with the
present work.
[14]
B). In our previous study on cryptomelane, the conductivity
was proposed to depend on the particle morphology, in
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particular the needle aspect ratio and their absolute lengths. Conclusions
For the group of hollandite-type catalysts in this study, a similar
reactivity trend scaling with the needle length can be observed
Nine manganese oxide phases differing in structure and
(
see SEM micrographs in Figure 2, left hand column: α-MnO <
composition were synthesized and characterized by Rietveld
2
NH À hollandite<cryptomelane).
refinement of X-ray diffraction data, N physisorption experi-
4
2
A role of conductivity being the most crucial parameter for
the OER over manganese oxides may also explain the apparent
deviation between the reported high performance of birnessite
and our rather poor OER results using this material. Frey et al.
investigated the catalytic activity of various manganese oxides
ments, scanning electron microscopy, thermogravimetric analy-
sis and atomic absorption spectroscopy. A screening of all
Mn O samples regarding their electrocatalytic OER perform-
x
y
ance showed strong differences in activity. Cryptomelane was
the best catalysts and reaches a current density of over
4
+
À 2
in the chemical water oxidation catalysis using Ce as oxidizing
15 mAcm at a potential of 1.8 V. In contrast to hausmannite,
[17]
agent. The authors reported a higher surface-area normalized
bixbyite and KÀ birnessite which were not even able to surpass
À 2
activity of birnessite than, for example, of Ramsdellite or α-
a value of 1 mAcm . No linear or clear dependence of the
MnO . They proposed that this high activity is related to
average manganese oxidation states, double layer capacitance
or the BET surface area on the OER activity was observed. All
manganese oxides were unstable and completely lost their
whole performance within 1 hour. Conductivity differences are
discussed as a likely reason for the different relative perform-
ance orders of manganese oxide phases in water oxidation
observed in the literature for chemical and here for electro-
chemical OER.
2
accessible internal volumes in the interlayer space of birnessite
and its ability of incorporate water and foreign cations
compared to oxides with dense structure like bixbyite or
hausmannite. A high activity was confirmed here for cryptome-
lane, but birnessite was similarly inactive as hausmannite in our
electrochemical OER study. This difference might be explained
with the different driving forces in electrochemical and
chemical water oxidation. The latter is free from constraints by
low conductivity as the reductant is in the same solution as the
4
+
oxidant. The reduction reaction (of Ce ) happens at the same Experimental Section
surface and does not require charge carrier transport through
the catalyst layer. Thus, the intrinsic surface activity as
determined by chemical water oxidation may indeed be similar
for cryptomelane and birnessite, but the lower conductivity of
the latter may prevent its application as a (bulk) electrocatalyst.
Severe stability issues were observed for all manganese
oxide electrocatalysts. Further studies on cryptomelane have
shown that the bulk crystal structure was relatively stable and
only subjected to slight changes. Smooth activity loss curves
with no indications for mechanical instability were observed at
similar current densities that were found to be much more
stable on, for example, a cobalt-based electrode prepared
according to the same protocols (see Figure S7) suggest an
electrochemical reason intrinsic to the manganese oxide
powder catalysts being responsible for the observed instability.
However, this hypothesis requires further research, e.g. with
surface sensitive methods.
Synthesis
[43a]
α-MnO : Adapted from Ohzuku et al.,
α-MnO was synthesized
2
2
by thermal treatment of bixbyite (α-Mn O ) powder with a sulfuric
acid solution in a reflux apparatus. Therefore, 0.3 g α-Mn O and
113 ml 4 molar H SO solution (H SO : >95%, Thermo Fisher
2
3
2
3
2 4 2 4
Scientific GmbH, demineralized water: conductivity <5 μS) were
stirred (500 rpm) in a 250 ml round-bottom flask for 24 h at a
temperature of 80°C (yield=85–91%). The bixbyite powder was
[14]
synthesized by calcination of Manganese(III) acetate (97% p.a.,
Sigma-Aldrich) in a muffle furnace for 24 h at 800°C (heating rate:
À 1
n+
+
32 Kmin ). No cations other than Mn and H were involved in
the synthesis giving rise to a hollandite-type material without
+
+
foreign cations, such as K or NH4 called “α-MnO ” in this text.
2
Cryptomelane: To synthesize cryptomelane analogous to the
literature procedure,
[
43b]
2 ml HCl (37%, VWR Chemicals) was added
to 160 ml of a 0.1 molar KMnO4 (>99% p.a., Carl Roth GmbH)
solution. After stirring (500 rpm) for 30 min at room temperature
the solution was hydrothermally treated for 8 h at 150°C in a
The complete loss of activity within one hour of operation
in our test, which represents typical conditions for non-noble
catalysts in alkaline media, seems prohibitive for an application
in electrolyzers. Nevertheless, two encouraging aspects are i)
the fact that activity and stability show a similar trend within
the series of catalysts meaning that more active samples were
also found to be more stable, though on a low level, and ii) that
the powder approach used here might over-emphasize the
conductivity and stability issues compared to differently
prepared electrodes being composed of electrodeposited
Teflon-lined steel autoclave (inner volume: 250 ml). (In our previous
[14]
comparative study of different cryptomelane samples,
sample was labeled “HT-cryptomelane”.)
this
NH À hollandite: Based on the same route like pure α-MnO ,
4
2
NH À hollandite was synthesized via the procedure of Ohzuku
4
[43a]
et al..
and (NH ) SO (>99% p.a., Sigma-Aldrich Chemie GmbH) were
4 2 4
Therefore, bixbyite (made by calcination of Mn(CH COO) )
3 3
added in the molar ratio 1:1 to the sulfuric acid solution (4 mol/l)
and treated for 24 h at 80°C.
Pyrolusite: For the synthesis of Pyrolusite, a method reported by
[
51]
[
8,49]
Gao et al. was used. 7.5 mmol (NH ) S O (>98% p.a., Alfa Aesar/
Thermo Fisher Scientific GmbH) and 7.5 mmol MnSO ·4H O (>98%
p.a., Sigma-Aldrich Chemie GmbH) were dissolved in 160 ml H O.
films
carbon
or nanoparticles on a conductive support such as
that were found to be promising.
4 2
2
8
[5b,50]
4 2
2
The solution was hydrothermally treated for 12 h at 140°C in a
Teflon-lined autoclave (inner volume: 250 ml).
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Ramsdellite: Ramsdellite was synthesized by the same route like
electrode acted Ag/AgCl/3 M KCl and platinum foil. 5.0 μl of a
catalyst ink the ink was drop-coated onto the polished glassy
carbon electrode and dried in air at room temperature for about
[16i,51]
Pyrolusite, however, a lower temperature of 85°C was applied.
[52]
KÀ birnessite: Based on the route published by Ching et al., 50 ml
2
h. The modified electrodes were subjected to continuous
of a 0.38 molar KMnO solution was mixed with 20 ml of a 1.4 molar
4
potential cycling until reproducible voltammograms were obtained
before catalytic measurements. The details and sequence of the ink
preparation and measurements are summarized in Scheme 1.
Stability tests were done in separate measurements with freshly
prepared and conditioned electrodes. The stability results shown in
Figures 4c and S3 are not iR corrected, but subsequent iR correction
Glucose (99% p.a., Alfa Aesar/Thermo Fisher Scientific GmbH)
solution. The precipitate was kept in solution for 4 h and afterwards
isolated, washed and dried in air at 80°C for 12 h. The obtained
À 1
precursor was calcined at 425°C for 2 h (heating rate: 17 Kmin ) in
a muffle furnace in air.
for the NH À Hollandite catalyst showed only marginal changes in
Bixbyite: Bixbyite was synthesized by solvothermal treatment of 2 g
4
the current densities that could not explain the deviations shown in
Figure S3b.
pyrolusite in a Teflon-lined autoclave (inner volume: 250 ml) in
[
53]
1
60 ml Ethanol for 24 h at 130°C.
All reported current densities were calculated using the geometric
surface area of the electrode. Based on 5-fold measurement of a
Hausmannite: The calcination of pyrolusite in a muffle furnace for
À 1
4
h at 1000°C (heating rate: 32 Kmin ) led to hausmannite.
2
selected samples, the error of the potential at 10 mA/cm was
Mn O : A brown precursor was obtained by mixing 100 ml of a
5
8
determined to be �0.02 V and the error of the current density at
1.8 V was found to be 15.4%. These errors were assumed to be
representative for all catalysts. The measured potentials were
0
1
.4 molar Mn(NO3) (>98% p.a., Carl Roth GmbH) solution with
00 ml of a 1 molar NaOH (>98.5% p.a., VWR Chemicals) solution.
2
The precipitate was washed, dried in air at 80°C for 24 h and finally
[18,54]
converted to the reversible hydrogen electrode (RHE) scale
À 1
[40]
calcined at 400°C for 3 h (β=16 Kmin ) in a muffle furnace.
assuming pH 14 with the following equation:
All obtained powders were isolated and washed with demineralized
water until the conductivity of the filtrates fell below 20 μScm .
À 1
Finally, the samples were dried in static air at 80°C for 24 h.
Characterization
Powder XRD patterns in the 2θ range from 5 to 90° were recorded
with a Bruker D8 Advance diffractometer in Bragg-Brentano
geometry by using a position sensitive LYNXEYE detector (Ni-
filtered Cu-K radiation). A step size of 0.01° and a counting time of
α
0.3 s were applied. Samples were finally dispersed with ethanol on
a glass disk inserted in a round PMMA holder. The latter was
subjected to gentle rotation during scanning. To verify the structure
and calculate the lattice parameters a Rietveld refinement was
[
42]
performed using the TOPAS software (Bruker). High-resolution
SEM images were taken with a JEOL JSM7500F equipped with a
cold-field emission gun. An acceleration voltage of 5.0 kV and an
emission current of 10 μA were applied. All experiments were
performed under high vacuum. Samples were fixed on the metallic
holder by using a conductive carbon paste. For determining the
BET surface N2 physisorption experiments were performed at
À 196°C with a NOVA 3200e (Quantachrome GmbH & Co. KG) after
degassing the samples at 80°C for 2 h in vacuum. BET surface areas
were calculated from p/p data between 0.05 and 0.3. Thermogravi-
0
metric analysis (TG) was carried out with a Netzsch STA 449 F3
Jupiter thermoanalyzer. Powdered sample (~50 mg) was placed in
a corundum crucible and heated from room temperature to
À 1
1
000°C. A heating rate of 5°Cmin and a mixture of 21% O in
2
À 1
Argon at a flowrate of 100 mLmin were applied. The metal
content of the as-prepared samples was determined by atomic
absorption spectroscopy (Thermo Electron Corporation, M-Series)
after dissolving the powders in hydrochloric acid. The oxygen
contents reported in Table 1 were calculated from the difference to
100% neglecting structural water.
Electrocatalysis
The electrochemical measurements were carried out in a cell with
three-electrode setup and a 1 M KOH solution as the electrolyte. An
Autolab potentiostat/galvanostat (PGSTAT12, Eco Chemie, Utrecht,
The Netherlands) coupled to a Metrohm RDE rotator (1600 rpm)
was used. The glassy carbon tip of the working electrode had a
Scheme 1. Flow scheme of the measurement and electrode prepa-
ration protocol utilized in the framework of the MANGAN project.
For this study, the current density at 1.8 V obtained in the last step
was evaluation as performance parameter. The raw data obtained
during conditioning, EIS and CV is shown for a selected sample as
supporting information in Figure S4.
2
geometric area of 0.126 cm . As reference electrode and counter
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Journal of Inorganic and General Chemistry
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Chem. 2013, 125, 13447–13451; d) A. Iyer, J. Del-Pilar, C. K.
RT
zF
0
ERHE ¼ EAg=AgCl=3MKCl þ E
þ
ꢀ pH:
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Ag=AgCl=3MKCl
The uncompensated resistance was determined from the Bode plot
of the EIS measurements and a 100% iR-drop compensation was
performed using
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ERHE;corrected ¼ ERHE À ðRs*JÞ:
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The double layer capacitance was determined by an electro-
chemical circuit fit using the NOVA 2.0.1 software. The Randles
circuit was used as a cell model for the high frequency region of
the Nyquist plot according to McCrory et al.. Only the part of the
Nyquist plot behaving according to electron transfer being rate-
limiting was considered for the fits, despite an additional linear
behavior of the plot hinting towards secondary diffusion limita-
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The authors acknowledge the Financial support of the BMBF
funded project MANGAN (FKZ: 03SF0511B) and the fruitful
collaboration and discussions with the participating working
groups. A.R. and M.B. are grateful for funding by the Deutsche
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thank Fatih Özcan for providing the presented SEM images.
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Manuscript received: April 23, 2021
Revised manuscript received: April 29, 2021
Accepted manuscript online: May 2, 2021
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These are not the final page numbers!

ARTICLE
Dr. J. Heese-Gärtlein, A. Rabe,
Prof. Dr. M. Behrens*
1
– 11
Challenges in the Application of
Manganese Oxide Powders as
OER Electrocatalysts: Synthesis,
Characterization, Activity and
Stability of Nine Different Mn O
x
y
Compounds