Synthesis of a Highly Efficient Oxygen-Evolution Electrocatalyst by Incorporation of Iron into Nanoscale Cobalt Borides

Article abstract of DOI:10.1002/cssc.201801547

High-performance catalysts for the oxygen-evolution reaction in water electrolysis are usually based on expensive and rare elements. Herein, mixed-metal borides are shown to be competitive with established electrocatalysts like noble metal oxides and other transition-metal(oxide)-based catalysts. Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline solutions. (Co_0.7Fe_0.3)₂B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm^?2 in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO₂ and RuO₂. XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH)₂ are formed as active surface species, whereas the Fe remains in the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained by a one-step solution synthesis, calcined, and characterized by XRD, energy-dispersive X-ray spectroscopy, and SEM. Single crystals of (Co_1?xFe_x)₂B grown under chemical transport conditions were used for an unambiguous specification of the nanostructured particles by relating the cobalt/iron ratio to the lattice parameters.

Full text of DOI:10.1002/cssc.201801547

Accepted Article
Title: Highly Efficient Oxygen Evolution Electrocatalyst by Incorporation
of Iron into Nanoscale Cobalt Borides
Authors: Sebastian Klemenz, Jona Schuch, Stefan Hawel, Anne-Marie
Zieschang, Bernhard Kaiser, Wolfram Jaegermann, and
Barbara Albert
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ChemSusChem
Highly Efficient Oxygen Evolution Electrocatalyst by Incorporation of Iron
into Nanoscale Cobalt Borides
b,†
b
a
b
b
a
S. Klemenz,a, J. Schuch, S. Hawel, A.-M. Zieschang, B. Kaiser, W. Jaegermann, B. Albert
†
Abstract: High performance catalysts for the oxygen evolution reaction in water electrolysis are usually
based on expensive and rare elements. Here, mixed-metal borides are shown to be competitive with
established electrocatalysts like noble metal oxides and other transition metal(oxide)-based catalysts.
Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline
solutions. (Co0.7Fe0.3)2B shows an overpotential of η = 0.33 V (1.56 V vs. RHE) at 10 mA/cm2 in 1 M KOH
with a very low onset potential of ~1.5 V vs. RHE, comparable to the performance of IrO2 and RuO2. X-
ray photoelectron spectroscopy shows that the original catalyst is modified under reaction conditions
and indicates that CoOOH and Co(OH)2 are formed as active surface species, while the Fe remains in
the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained
by a one-step solution synthesis, calcined, and characterized by X-ray diffraction, energy-dispersive X-
ray spectroscopy and scanning electron microscopy. Single crystals of (Co1-xFex)2B grown under
chemical transport conditions were used for an unambiguous specification of the nanostructured
particles by relating the cobalt/iron ratio to the lattice parameters.
a. Eduard-Zintl-Institute of Inorganic and Physical Chemistry, Technische Universität Darmstadt,
Alarich-Weiss-Str. 12, 64287 Darmstadt, German, albert@ac.chemie.tu-darmstadt.de
b. Institute of Materials Science (Surface Science Division), Technische Universität Darmstadt,
Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany, kaiser@surface.tu-darmstadt.de
† These authors contributed equally to this project and should be considered co-first authors.
Electronic Supplementary Information (ESI) available: Data from energy-dispersive X-ray
spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron
microsopy
Introduction
Recently, Kaner et al. described how a boride’s properties can be adjusted for almost every
potential application by varying the interplay between metals and boron.[1] While it is well-
known that borides are interesting refractory materials for high-temperature uses,[2–4] their
activity as catalysts is far less explored. Earlier, we showed nanoscale Co2B and Ni7B3 to be very
interesting catalysts for the selective hydrogenation of natural products like citral,[5] and – in
an independent study – identified CoOOH as a highly active species for the oxygen evolution
reaction (OER),[6] (compare also ref.[7,8]). Electrochemical oxygen evolution with a Co and B
containing catalyst has been observed as early as 1980,[9,10] and recently such (semi)-
amorphous electrocatalysts impregnated or mixed with graphene or carbon nanotubes were
described to exhibit exceptionally high catalytic efficiencies for OER.[11–13] Furthermore,
electrocatalytic activities of other transition metal borides like MoB,[14] MoB2,[15] and Ni3B[16,17]
are currently under investigation. The benchmark catalysts for the OER are still noble metal
catalysts, namely IrO2 in acidic and RuO2 in alkaline media.[18,19] In order to replace those noble
metal catalysts more and more studies are focused on 3d transition metal oxides (e.g. Mn[20–
24], Fe[24–27], Co[6,24,26–28], Ni[24,26,27,29], Cu[30]) and mixtures of such compounds.[18,19,24,27,31–38] While
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the mechanisms of single metal catalysts are understood quiet well, the mechanism and the
synergy of bimetallic systems are less explored. Rossmeisl et al. showed that four consecutive
energy steps are needed to create oxygen out of water.[39–41] The energy required for each of
these steps is different, and it further varies depending on the catalyst chosen. Therefore it
should be advantageous to use a material mixture of e.g. two materials, at which the first
material is ideal for e.g. energy step one and two and the second material is ideal for e.g. energy
step three and four. Recent studies showed that one of these beneficial mixed-metal oxide
systems is a Ni-based catalyst mixed with Fe.[24,33–36,42] Small amounts of Fe incorporation
improved the OER performance of the Ni-based compound dramatically. Similar studies
showed that the incorporation of Fe into Co3O4 led to an improvement of the catalytic
performance.[43,44] More recently Burke et al. stated that the incorporation of Fe into CoOOH
enhances the intrinsic activity by a factor of 100.[7] The Fe ions were described to be the active
centres, with a CoOOH matrix stabilizing the catalyst in alkaline electrolytes. While many of
those presented non-noble metal catalysts described earlier in benchmark studies were
unstable under OER conditions,[18,19] we present here new nanoscale cobalt and iron containing
ternary borides (Co1-xFex)2B with optimized catalytic properties. In terms of electrical
conductivity,[45,46][45,46] these bimetallic borides are superior to transition metal oxides, which
usually possess semiconducting properties. The one-step, low-temperature synthesis of mixed
metal borides is reported, as well as the characterization of the crystalline ternary boride
particles. The principle of precipitation of borides in solvents using reducing agents has been
described as early as 1953,[47–50] and the method has recently been extended to produce new
nanoscale materials with specific structures and properties.[51–53] However, to our knowledge
this is the first report on the production of crystalline (compare to ref.[54] for amorphous alloys)
mixed-metal borides by bottom-up synthesis in solvents at low temperatures followed by
annealing at moderate temperatures. Contrary to conventional (high-temperature) routes for
boride synthesis, the low-temperature solution yields powders that are highly reactive. The
determination of the exact composition of the precipitates including their surfaces is very
challenging, but essential for further investigation of the role of the iron dopant in the catalyst.
Thus, to prove the formation of the nanoscale ternary compounds and to determine the metal-
to-metal ratio unequivocally, a series of single crystals was grown with selected compositions.
This allowed for an unambiguous identification of the catalysts. Surface species were identified
by X-ray photoelectron spectroscopy (XPS).
Experimental section
Synthesis of nanoscale (Co1-xFex)2B
Nanoscale ternary borides were synthesized following a modified protocol based on earlier
work.[48–50,53] The masses of the starting materials, cobalt(II) chloride hexahydrate (97 %, VWR
Chemicals) and ammonium iron(II) sulfate hexahydrate (99 %, AnalaR NORMAPUR), were
calculated for 1 g NaBH4 (> 97 %, Alfa Aeser) to obtain a molar ratio of tetrahydridoborate-to-
transition metal ion of four. The following steps were performed in an inert atmosphere.
Sodium tetrahydridoborate was dissolved in 100 ml of degassed water and slowly added
dropwise to a solution of the metal salts in 150 ml of degassed water. The reaction was stirred
at room temperature for one hour. The precipitate was separated and washed with 50 ml of
degassed water and 100 ml of dry acetone. After drying in vacuo the precipitate was transferred
into a Schlenk tube and annealed in vacuo for two hours at 500 °C.
Synthesis of single crystals of (Co1-xFex)2B
Single crystals of (Co1-xFex)2B were synthesized following a modified protocol we developed for
MnB4.[55,56] For the synthesis of 500 mg mixtures of cobalt (99.8 %, Merck), iron (99.97 %,
Chempur) and boron (98-99 %, HC Starck) were prepared according to the molar ratio desired
for the product phase. 100 mg of iodine (99.8 %, VWR Chemicals) was added to the mixture.
The reactants were sealed in a quartz ampoule in vacuo and heated to 1000 °C for three weeks.
Electrode assembly
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Powders of (Co1-xFex)2B (5 mg/mL) were added to a mixture of 49 % ethanol, 49 % ultra-pure
distilled water and 2 % nafion (Aldrich Chemistry). The ink was then blended using an ultrasonic
bath for at least 20 minutes. Glassy carbon (HTW GmbH) was used as a substrate material,
sanded with abrasive paper (400, 1000, 1500, 2000, 2500 grit size, Starcke) and polished with
Al2O3 paste (1 µm, 0.05 µm grit size, Buehler). Then the substrate was cleaned in an ultrasonic
bath with ethanol and ultrapure distilled water for ten minutes each. An amount of 15 µL of
the resulting ink was loaded onto the glassy carbon substrate with an Eppendorf pipette and
dried for at least 30 minutes under ambient conditions.
Pre-catalyst characterization
Nanoscale powders were annealed at 500 °C. Single crystals were ground to yield
microcrystalline powders, which were filled in glass capillaries (diameter: 0.5 mm). X-ray
diffraction (XRD) patterns were collected on a powder diffractometer (Stadi-P, Fa. Stoe,
Darmstadt) using Mo-Kα1 radiation. The program TOPAS (version 4.2, Fa. Bruker AXS) was used
for pattern analysis. Patterns were fitted using TCHZ-type pseudo-Voigt functions. Foe energy-
dispersive X-ray spectroscopy (EDS), the samples were measured on a carbon pad in a scanning
electron microscope (JSM 6400, Fa. JEOL Ltd.) at 20 kV with an EDS detector (Apollo-x, Fa. EDAX
Inc.). The SEM images were investigated in a XL30 FEG scanning electron microscope (Philips)
with an acceleration voltage of 10 kV.
Catalyst characterization
The catalytic performance tests with respect to OER were conducted using a GAMRY Interface
1000E potentiostat in a three-electrode setup with a Hg/HgO (1 M NaOH) reference electrode
in 1 M KOH (Carl Roth) solution. The catalysts were first activated using cyclic voltammetry (CV)
between 1 V and 1.5 V with a scan rate of 100 mV/s until no further shifts were observable in
the voltammograms. The activity was then obtained by a further CV measurement between
1 V and 1.8 V with a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS;
100 kHz - 1 Hz) was used to obtain the setup resistance for iR compensation. In this work all
electrochemical plots are calibrated to the Reversible Hydrogen Electrode (RHE).
XPS measurements were performed with monochromatic Al-Kα radiation (XR 50, SPECS Surface
Nano Analysis GmbH). A hemispherical electron analyzer (PHOIBOS 150, SPECS Surface Nano
Analysis GmbH) was used, calibrated with the core lines of copper, silver and gold. Peaks were
fitted using a Shirley background and a mixed Gaussian/Lorentzian peak shape. Prior to the
electrochemical measurements, samples of (Co1-xFex)2B were embedded in indium foil inside a
glove-box and then transferred to the UHV system in a nitrogen filled transport chamber. Only
a moderate pressure was applied to embed the powder into the indium foil in order to keep
the crystallites intact.
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Results and discussion
Crystal structure, morphology and composition
Amorphous precipitates of (Co1-xFex)2B with varying cobalt iron ratios were obtained by reacting
mixtures of iron and cobalt salts with sodium tetrahydridoborate in water; the powders were
then dried and annealed at 500 °C. Powder X-ray diffraction (Fig. S1 and S2) experiments
confirmed the formation of borides crystallizing with the tetragonal CuAl2-structure type, as
expected for Co2B or Fe2B.[57–59] The samples appeared to be almost mono-phasic at low iron
contents. At x ≥ 0.3, a second crystalline phase was observed and identified as α-Fe, which
became the main phase for x ≥ 0.7. For x = 0.6 and 1.0, traces of an additional phase,
presumably (Co/Fe)3B, were detected (Table S2 and S3). The successful incorporation of iron
into Co2B was accompanied by an increase in unit cell volume, as shown in Figure 1.
Figure 1: Lattice parameters a (diamonds) and c (circles) and unit cell volumes (squares) plotted against x in (Co1-xFex)2B for
nanoscale (black and white) and pulverized single crystals (gray). Dotted lines from literature.36
As an independent proof for the formation of ternary borides as nanoscale powders, single
crystals of mixed-metal borides were synthesized at high temperatures under chemical
transport conditions. Their iron-to-cobalt ratios and unit cell dimensions were determined and
used as reference values in Figure 1, in addition to selected values from literature.[59] The lattice
parameters of the powders and the single crystals coincide well at low iron contents.
Differences at higher Fe contents are attributed to the formation of the further side phases
mentioned above.
In addition, the chemical composition of the powders of (Co1-xFex)2B with x ≤ 0.3 were checked
with EDS. The iron content of the precipitates proved to be identical to the iron content of the
solution during synthesis (SI; Table S1). The SEM images in Figure 2 show the nano-structured
morphology of the pure Co2B phase (left) and of the most active ternary boride (Co0.7Fe0.3)2B
(right). While the morphology of Co2B reveals an agglomerate structure with short rods and/or
small spheres emerging out of it, the morphology of the (Co0.7Fe0.3)2B shows longer rods
forming a network-like structure. An additional transmission electron microscopy (TEM) image
underlining the sponge-like character of the sample is shown in the SI (Figure S3). The (Co1-
xFex)2B catalysts were further investigated by XPS. Here the XPS results of (Co0.7Fe0.3)2B are
discussed exemplarily for the other systems. The 2p photoemission spectrum of Co is depicted
in Figure 2(a). It shows signals at 778.4 eV (Co 2p3/2) and 793.5 eV (Co 2p1/2) at the same position
as Co0, as observed previously for binary cobalt borides.[60,61] A small high binding energy
shoulder of Co 2p3/2 (~781.3 eV) indicates partial oxidation.[62,63]
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The percentage of cobalt and iron was found to be 67.7 at% and 32.3 at% (see Fig. 3 and Fig.
S4), which is in good agreement with the assumed metal-to-metal ratio in (Co0.7Fe0.3)2B.
Furthermore, the metal-to-boron ratio was determined to be 1.95 and agreed very well with
the assumed composition. The B1s spectrum shows two peaks at 188.7 eV and at 194.0 eV
binding energy
Figure 2: Scanning electron microscopy (SEM) image of Co2B (left) and (Co0.7Fe0.3)2B (right).
(Figure 2c). The former can be attributed to the metal-boride, whereas the second is mainly
due to B2O3 and other boron-oxo-species.[64,65] In the O1s spectrum we observe a broad peak
from 535 eV down to 529 eV in binding energy (Figure 2d). Prominent features are observed at
533.5 eV, 532.5 eV and 530.5 eV. These can be assigned to boron and cobalt oxo-species.[6,64–
66]
Additional features at 534 eV and 535 eV binding energy are due to carbon-oxygen
compounds and adsorbed water, respectively.
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Figure 3: Exemplary XPS photoemission lines of the (Co0.7Fe0.3)2B catalyst; showing cobalt (a) before and (b) after
electrochemical treatment, while in (c) & (d) the photoemission lines of boron and oxygen before and after the
electrochemical test procedure are each shown together in one picture, respectively.
The XPS data on Fe (SI; Figure S2), B and O (Figure 2c & d) in general exhibit the signals that can
be expected for samples of (Co0.7Fe0.3)2B with minor contributions of oxides at the surface.
Electrochemical characterisation
The electrochemical behaviour of Co2B and (Co1-xFex)2B was investigated in 1 M KOH, using
glassy carbon as the substrate material. It is well-known from literature that cobalt features an
oxidation wave around 1.0 – 1.3 V, relating to Co(OH)2 + OH−
"
CoOOH + H2O + e−. Thus, the
electrocatalysts were first activated by cycling between 1 V and 1.5 V, before measuring the
catalytic activity. The cyclic voltammograms of (Co1-xFex)2B (x = 0, 0.3) after the activation
procedure are presented in Figure 4(a).
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Figure 4: Electrochemical measurements in 1 M KOH solution showing cyclic voltammograms of (a) pure Co2B (violet)
and (Co0.7Fe0.3)2B (olive) with a magnification of the redox-waves of cobalt, (b) cyclic voltammograms and (c) tafel
plots of all (Co1-xFex)2B with x ≤ 0.5 in comparison to pure Co2B and (d) a galvanostatic scan at 10 mA/cm² of
(Co0.7Fe0.3)2B. All CV curves were measured with a scan rate of 10 mV/s. The potential is plotted versus the
reversible hydrogen electrode (RHE) and is iR compensated.
The mixed cobalt-iron boride (Co0.7Fe0.3)2B proves to be one of the best catalysts free of noble
metals that has been described for the OER so far.[18,19] It exhibits a significant reduction of the
overpotential (η = 0.33 V; U = 1.56 V vs. RHE at 10 mA/cm²) as well as of the onset potential
(~1.5 V) compared to the binary dicobalt borides, which were investigated in our experiments,
too. The binary compounds showed an activity of 1.64 V (η = 0.41 V) at a current density of 10
mA/cm² and an onset potential around 1.57 V vs. RHE. This is in good agreement with literature
data, that already described cobalt boride to be exceptionally active (1.61 V, with η = 0.38 V at
a current density of 10 mA/cm²).[13,67] Phases of (Co1-xFex)2B with other values of x have also
been measured (Figure 4b – CV & Figure 4c – Tafel plots). Whereas for x = 0.1 and x = 0.2 similar
catalytic performances were obtained as for x = 0.3, the catalytic activities x > 0.3 were similar
to Co2B. This trend in performance can also be supported by tafel slope measurements for the
different phases shown in Figure 4c. (Co1-xFex)2B samples with 0.1 ≤ x ≤ 0.3 show tafel slopes of
around 40 mV/dec, while those with x = 0.4 or 0.5 have values around 50 mV/dec. Even these
values are considered to be an improvement compared to the tafel slope measured for binary
Co2B (63 mV/dec). These values are in good agreement with results from Xiao et al., who found
that Fe reduces the tafel slope of Co-based catalysts.[44] Furthermore, the onset potential is
affected by the incorporation of Fe. This effect is however only valid for a certain amount of
iron in the metal boride. While (Co1-xFex)2B catalysts with x = 0.1 and x = 0.2 show an onset
potential around 1.51 V vs. RHE, Co2B and (Co1-xFex)2B with x > 0.3 feature an onset potential
around 1.56 V vs. RHE. In addition to the activity, the stability of a catalyst is crucial for its
technological application. Therefore, the long-term performance of the most active phase
(Co0.7Fe0.3)2B, was investigated using a galvanostatic scan at a constant current density of 10
mA/cm² in 1 M KOH solution (Fig. 4d). The catalyst showed a strong improvement in
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performance with time during the first 2 h of the measurement and remained nearly constant
afterwards, showing no degradation of the catalyst over the complete 16 hours of
measurement time. We assume, that during the first two hours all sites in the network-like
structure are activated leading to the highest overall catalytic performance. The new ternary
cobalt-iron boride studied in this work shows an at least comparable activity and stability to
the standard OER catalysts IrO2 and RuO2.[18,19]
XPS surface analysis after electrochemical investigation
XPS analysis of the surface was additionally performed after the electrochemical
measurements, in order to observe changes introduced by the applied processes and the
composition of the catalytically active species at the surface. Figure 3b shows the Co 2p line of
the (Co0.7Fe0.3)2B catalyst after the electrochemical investigation. The original Co2B peak has
disappeared and is replaced by a strong feature at 779.9 eV and a clearly visible shoulder at
781.2 eV. The former being assigned to cobalt oxide, the latter to cobalt hydroxide. The peak
positions and shape of the photoemission lines corresponding to Co 2p might be associated
with Co3O4, consisting of cobalt in the two oxidation states Co+II (satellite at 785.7 eV) as well
as a Co+III (satellite at 789.8 eV).[28,62,68] However, this assignment can be excluded due to the
presence of a strong hydroxide signal in the photoemission line of O 1s (531 eV, Figure 3(d)).
But, these observations are fully consistent with the formation of a cobalt oxyhydroxide at the
surface, which was observed in cobalt containing catalysts before.[6,7,11,24,69,70] They are also in
agreement with the occurrence of the redox wave discussed above and shown in Figure 4a.
According to the literature, the O 1s photoemission line of CoOOH shows two signals (oxide:
529.3 eV and hydroxide: 530.9 eV) with similar intensities.[6,28,71] In our case, the hydroxide
species shows higher intensity. Therefore, we assume that the phase present at the surface is
not pure CoOOH, but rather a mixture of CoOOH (Co+III) and Co(OH)2 (Co+II). The O 1s spectrum
shows in general only very broad features. A high binding energy shoulder at 533 eV can be
due to a small amount of boron oxo-species remaining in the surface region, which is also
supported by the very weak B 1s signal, which is detected after the electrochemical
investigation at 191.8 eV (Fig. 3c).[64–66]
Figure 5: Schematic picture of the cross section of the rod-like structure and its compositional changes taking place during
the electrochemical testing procedure.
The here observed XPS-features agree well with XPS studies of the binary Co2B catalysts by
Masa et al.[11,13] They also showed the transformation from a non-active to an activated phase
after an electrochemical treatment. The XPS data measured for the Fe 3p region are presented
in the SI (Figure S4). An identification of the iron species is futile since the intensities of the Fe
2p/Fe 3p signals are too low for a meaningful interpretation. The role of a small amount of iron
to improve the catalytic activity in transition metal oxide compounds has been observed
previously.[7,24,29,34,36,42–44,69,72] From our XPS-data before and after the electrochemical
investigation, we propose that the structure of the (Co1-xFex)2B changes from a rod-like
structure with a thin oxide layer to one with a thick oxide layer (see Figure 5). Furthermore, this
surface oxide layer is formed mainly by the active catalyst phase CoOOH and Co(OH)2, whereas
the central area is preferentially composed of the Co/Fe-containing boride. Such a core-shell
structure has been previously suggested for such metal boride particles.[73] The metal boride in
the center of the particles provides an enhanced conductivity compared to catalysts that
consist of CoOOH/Co(OH)2 only.[45,46] This might enhance the catalytic activity, but further
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investigations will surely be necessary to achieve a better understanding of the synergy of
cobalt ions at the surface and iron atoms in the core of the catalyst that forms during the
electrochemical activation from the initial metal-boride system.
Summary and Conclusion
We have identified a new, highly efficient, and stable electrocatalyst for the OER in alkaline
media, which is comparable to noble-metal containing compounds like RuO2 and IrO2. The
mixed-metal boride is produced by a new bottom-up synthesis technique at low temperature,
followed by an annealing step at moderate temperature. By this route highly reactive metal
boride powders are formed. The powders were deposited by standard techniques on glassy
carbon substrates.
The OER active species is obtained from (Co1-xFex)2B by electrochemical activation (cycling) in 1
M KOH and its main surface species are identified as CoOOH and Co(OH)2. Its electrocatalytic
activity is higher than that of the active phase formed from the binary compound Co2B.
Obviously, the incorporation of iron into Co2B provides an electronic contribution that
improves the OER activity significantly. XPS analysis clearly indicates the formation of a complex
multinary oxide at the surface, which is obviously responsible for the favourable properties of
the catalyst. The surface composition resembles that found on mixed-metal
oxides/hydroxides,[27,34,36,74–77] The core of mixed-metal boride (M2B) contributes a better
electrical conductivity to the system.[45,46,73] The specific role of each individual component of
the catalyst is still unclear, but will be systematically investigated in the near future, using
further borides from bottom-up syntheses with varying compositions. It is highly desirable to
improve the understanding of catalytic mechanisms in order to foster the development of
specifically designed electrocatalysts that are stable and consist of abundant components.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support by the DFG in the framework of the Darmstadt Graduate School of Excellence
for Energy Science and Engineering (GSC 1070) as well as by the BMBF (Mangan 03EK3552) is
gratefully acknowledged. We would like to thank Dr. Stefan Lauterbach and Anna Müller for
help with the TEM investigations.
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