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Physical Chemistry Chemical Physics
This paper is published as part of a PCCP Themed Issue on:
Interfacial Systems Chemistry: Out of the Vacuum, Through the Liquid, Into the
Cell
Guest Editors: Professor Armin Gölzhäuser (Bielefeld) & Professor Christof Wöll (Karlsruhe)
Editorial
Interfacial systems chemistry: out of the vacuum—through the liquid—into the cell Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c004746p
Perspective
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Heterogeneous films of ordered CeO2/Ni concentric nanostructures for fuel cell applications Chunjuan Zhang, Jessica Grandner, Ran Liu, Sang Bok Lee and Bryan W. Eichhorn, Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/b918587a
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Heterogeneous films of ordered CeO2/Ni concentric nanostructures
for fuel cell applicationsw
Chunjuan Zhang,a Jessica Grandner,a Ran Liu,a Sang Bok Lee*ab and
Bryan W. Eichhorn*a
Received 7th September 2009, Accepted 27th October 2009
First published as an Advance Article on the web 1st December 2009
DOI: 10.1039/b918587a
Heterogeneous films of ordered CeO2/Ni concentric nanostructures have been fabricated through
template-assisted electrodeposition. The free-standing films of Ni metal (8 mm thickness) contain
ordered arrays of ceria tubes (200 nm OD, 100 nm ID). Ni/CeO2 coaxial nanotubes were also
obtained by tuning experimental conditions. The interfacial contact area within the 3-dimensional
oxide nanotube/nickel matrix is B100 times greater than 2-dimensional thin films of nickel and
ceria of the same area. The use of the film as an anode electrocatalyst/current collector is
demonstrated in a solid oxide fuel cell.
1. Introduction
Heterogeneous nanocomposite structures such as coaxial
nanowires and nanotubes are of growing interest due to their
high energy-conversion efficiencies, fast response times and
charge/discharge rates in solar cell,1–3 energy storage4–7 and
fuel cell8–11 applications. The superior performance of these
heterogeneous composite structures originates from the
synergistic combination of multiple functionalities of materials.
Such functionalities include very high surface areas, large
interfacial contact areas, high electrical conductivities, and
short charge diffusion lengths. For example, we recently
described a supercapacitor electrode with coaxial nanowires
of manganese oxide (MnO2)/poly(3,4-ethylenedioxythiophene)
(PEDOT) that showed much higher specific capacitance and
stronger mechanical strength compared to not only pure MnO2
or PEDOT bulk structures but also the pure nanostructures.12
Similarly, utilizing 3-dimensional nanostructured electrodes in
a solid oxide fuel cell (SOFC) can theoretically improve the cell
performance by dramatically increasing the interfacial contact
area between the catalyst, the current collector and the fuel,
(i.e. the triple phase boundary TPB). Prinz and coworkers have
pioneered the development of 2-dimensional SOFCs with
ultrathin electrolytes supported by porous nickel nanohole
arrays,13 as well as 3-dimensional micrometre-size SOFC
structures9 with up to a 75% increase of current densities
compared to flat 2-dimensional SOFCs. We describe here
new 3-D composite structures with highly ordered nanotube
arrays as free-standing films that were prepared by utilizing
template-assisted electrodeposition methods.
Porous anodized aluminium oxide (AAO) membranes
have been widely employed to prepare 1- and 3-dimensional
nano-architectures. Pioneered by Martin,14 Masuda and
Fukuda,15 and others, the investigation of AAO-based template
synthesis includes arrays of nanowires/nanotubes,12,16–35 as
well as template replicas of various materials such as metals
and metal oxides. Specifically, the two-step replication of
AAO15,36 advances itself as a generic method to make
well-ordered porous membranes of many possible materials,
resulting the same high pore density (B1010 pores cm�2) as the
original AAO membranes. With this method, replicated
porous films with various materials such as gold,15,37
nickel,13,37–39 titanium oxide,40 tungsten oxide36 and others41
have been fabricated. These porous films have been pursued
for use as electrodes/catalysts for photovoltaics, energy
storage and fuel cell applications to name a few. While
the porous metal oxide membranes are very fragile and
not freestanding, (i.e. they are always affixed to a substrate),
which limits their direct utilization in fuel cell and solar
cell applications, the porous metal films can be strong
and freestanding. However, filling the pores of the porous
metal films with catalysts requires a separate step, such
as sol–gel deposition, which can potentially accumulate
catalyst on the metal film surface and block the pores.
The use of electrochemical deposition to modify the
inner pore walls is also difficult because the electrodeposited
materials tend to grow faster on the exposed conductive
surface of the porous metal electrode rather than diffusing
into the pores.
We describe here a new process involving two sequential
steps of electrodeposition to make either highly ordered
nanocomposite structures of CeO2 nanotubes embedded in a
Ni film or ordered arrays of Ni/CeO2 coaxial nanotubes,
depending on the conditions. The nanocomposite films
generate relatively robust, free-standing structures containing
highly porous, catalytically-active CeO2 nanotubes.
2. Experimental
The ceria cathodic electrodeposition was performed
at �0.9 V vs. Ag/AgCl for 30 min. in a solution containing
aDepartment of Chemistry and Biochemistry, University of Maryland,College Park, MD 20742, USA. E-mail: eichhorn@umd.edu
bGraduate School of Nanoscience and Technology (WCU),Korea Advance Institute of Science and Technology (KAIST),Daejeon 305-701, Korea. E-mail: slee@umd.edu
w Electronic supplementary information (ESI) available: Additionalexperimental details. See DOI: 10.1039/b918587a
This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 4295–4300 | 4295
PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics
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100 g L�1 cerium nitrate hexahydrate and 10 mL L�1 30%
hydrogen peroxide in de-ionized water. The nickel was
electroplated at �1.0 V vs. Ag/AgCl for 40 min. in a solution
consisting of 0.65 M nickel sulfamate, 0.12 M nickel
chloride and 5.5 M boric acid. Whatman Anodisc AAO
templates were purchased from Fisher Scientific. All chemicals
were purchased from Sigma-Aldrich. Titanium etchant
TFT and gold etchant TFA were purchased from Transene
Company, Inc.
The electrodepositions were performed with a 2-channel
BiStat from BioLogic Science Instruments. The 700-nm-thick
titanium contact layer was sputtered by the AJA sputter
system (350 watts, 3 mTorr Ar plasma). The 100-nm-
thick gold contact layer was sputtered by Denton Desk
sputter system DESK III (50% Setpoint, 50 mTorr Ar
plasma, 10 min). Scanning electron micrographs (SEM)
were taken in a Hitachi SU70 SEM, and transmission
electron micrographs (TEM) were taken in a JEOL 2100F
Field Emission TEM. The energy dispersive X-ray
spectroscopic (EDS) analysis was taken by the INCAx-
sight EDS system from Oxford Instruments. Microtomed
samples were mounted in Spur epoxy resin and cut with
an American Optical ULTRACUT microtome at room
temperature.
The two-step electrodeposition method is shown in Fig. 1a.
First, a nanoporous AAO membrane (200 nm Whatman
anodisc) is coated with a titanium contact layer on one side,
which provides electrical contacts and functions as a blocking
layer. In the first step, ceria is electrodeposited as nanotubular
structures in the pores of AAO from a cerium nitrate/
hydrogen peroxide solution at �0.9 V vs. Ag/AgCl. The
membrane is then soaked in a 1.0 M sodium hydroxide
(NaOH) solution to dissolve the alumina template. The radial
etching of alumina around the ceria nanotube after deposition
in the AAO pore may be due to the somewhat porous nature
between grains (see Fig. S3 in the ESIw) of electrodepositedceria, thus etchant can diffuse through the pores of ceria
nanotube wall and dissolve the alumina. As a result, the ceria
nanotube arrays remain attached to the titanium contact layer
after the complete removal of the AAO. Subsequently, nickel
metal is electrodeposited from a nickel sulfamate/nickel
chloride/boric acid solution at �1.0 V versus Ag/AgCl. The
growth of nickel replicates the AAO structure and embeds the
CeO2 nanotubes (Fig. 1a). The titanium contact layer is
then removed using the commercial HF-based etchant
TFT (thin film Ti etchant). The resulting film has open-ended
CeO2 nanotubes on one side and partially-filled solid CeO2
on the contact layer side. Scanning electron micro-
graphs (SEMs) of the Ni/CeO2 films are shown in Fig. 1b
and 1c. The total thickness of the Ni/CeO2 films is B8 mm(Fig. 1c), which is defined by the thickness of the Ni
matrix. The CeO2 nanotubes are significantly longer than
the thickness of the Ni films (Fig. 2) but the excess
portions of the ceria nanotubes protruding from the Ni
surface are broken during the Ti layer etching process.
The CeO2 nanotubes sometimes have irregular shapes,
however, the tubes remain open and the irregularities
should not significantly affect the functionality or gas diffusion
properties.
3. Results and discussion
To investigate the mechanism of CeO2 nanotube/nanowire
formation, a brief study of ceria nanotube growth was
conducted. For this study, ceria nanotubes were grown in an
AAO template with a 100 nm Au contact layer and the same
cerium nitrate/hydrogen peroxide precursor solution. For
imaging purposes, gold was used as the AAO contact layer
in the ceria nanotube study instead of titanium since the
titanium etchant dissolves alumina quickly and complicates
analysis. In the presence of hydrogen peroxide, Ce3+ cations
are oxidized to cerium dioxide through a hydroxide-mediated
process described by the overall electrochemical half-cell
reaction:
Ce3+ + 2H2O2 + 3e� - CeO2 + 2H2O
After ceria was deposited, the gold contact layer was removed
using a commercial TFA (thin film Au etchant) gold etchant,
which exposed the bottom of the ceria nanotubes. As shown
in Fig. 2, the CeO2 nanotubes are 200–300 nm in diameter,
which are in the same dimensions as the AAO template pores.
For reference, the 200 nm Whatman anodisc has B200 nm
diameter pores on one side of the membrane and B400 nm
diameter pores on the other side and an average diameter
of 300 nm pores in the center. On the Au contact side of
the template, the electrodeposition of ceria in the AAO
pores initially gives solid CeO2 by filling the pores as
shown in Fig. 2b. As the deposition proceeds, ceria deposition
forms amorphous CeO2 nanotubes on the AAO walls that
becomes polycrystalline after annealing under nitrogen at
500 1C for 1 h. (Fig. 2c and f). Since CeO2 is not electrically
conductive at room temperature, more CeO2 is deposited
on the bottom of AAO channels closer to the Au electrode,
and the wall thickness of the CeO2 nanotubes decreases
gradually from bottom to top. The individual tubes are
more than 20 mm in length with smooth outer surfaces and
rough nanoparticle-packed inner surfaces. The ceria nano-
particles forming the nanotube structures are less than
50 nm in diameter and pack together to form 50 nm thick
walls (Fig. 2c). The very top portions of the ceria nanotubes
are thin with loosely packed with particles. These thin sections
are not structurally sound and break easily during the
TEM sample preparation process (sonication in ethanol). As
illustrated in Fig. 2d, the individual nanotubes (13 mm) are
shorter than the ceria nanotube bundles (23 mm, see the ESIw)that support longer nanotube structures due to added inter-
tubular interactions. While the ‘as-prepared’ tubes are
essentially amorphous, the annealed tubes are crystalline
(see X-Ray diffraction analysis, Fig. S1w). The high-resolutionTEM (Fig. 2f) and electron diffraction pattern (see ESIw) showthe 3.1 A lattice spacings associated with the h111i facet ofCeO2.
To obtain CeO2 nanotubes in solid Ni film matrices, it is
important to completely remove the AAO in a 21 min NaOH
soaking step. Longer soaking times displace the ceria
nanotubes and damage the Ti contact layers. With shorter
times, residual AAO template structures remain due to partial
AAO etching. In this scenario, Ni deposits in the space
between the alumina pore wall and ceria nanotube resulting
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in the formation of coaxial nanotubes of Ni/CeO2 embedded
in the residual AAO. Subsequent AAO dissolution gives
arrays of alumina-free coaxial Ni/CeO2 nanotubes as shown
in Fig. 3a. The thickness of the outer Ni tubes can be
controlled by adjusting the time of the AAO dissolution step.
Overall, this method allows control over the lateral dimensions
Fig. 1 (a) Schematic electrochemical template synthesis route of the highly ordered nanocomposite structure of cerium oxide nanotubes
embedded in a nickel matrix, and SEM (b) top-view and (c) side-view images of Ni–CeO2 nanocomposite films.
Fig. 2 SEM and TEM analyses of ceria nanotubes grown inside an AAO template after removal of the gold contact layer. The sample was
annealed under nitrogen at 500 1C for 1 h. (a) SEM image of the top surface (opposite the original gold contact layer side), showing ceria
nanotubes, B200 nm in diameter with B50 nm thick walls, that are still embedded in the AAO template; (b) SEM image of the bottom surface
(previously attached to the gold contact layer), showing solid ceria nanowires in the pores; (c) side view SEM image showing the inner granular
structure of the ceria nanotubes withB50 nm thick walls and less than 50 nm grain sizes; (d) and (e) TEM images of410 mm ceria nanotubes after
being released from the AAO template; (f) high resolution TEM image of a ceria nanotube showing the polycrystalline crystalline lattice with a
3.1 A h111i facet.
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of both coaxial components and is a potentially useful
approach for preparing multilayer coaxial structures.
Fig. 3a shows the SEM image of Ni/CeO2 coaxial nanotube
arrays after removal of the residual AAO support. The
microtomed cross-section (Fig. 3b) shows that the coaxial
structure persists down the length of the nanotube. The
local elemental composition and coaxial architecture are
easily distinguished using a 1.5 nm EDS probe in the STEM
mode of high resolution TEM. The EDS line-scans and phase
maps of the Ni/CeO2 coaxial nanotubes (Fig. 3c and d) show
high concentrations of nickel and cerium on the nanotube
edges and low concentrations in the center of nanotubes,
which are the signatures of a hollow tubular structure. As
expected, the nickel tubes grow concentrically on the exterior
of the ceria tubes. Due to the partial AAO removal after the
CeO2 deposition, the nickel nanotubes are B400 nm in
diameter whereas the ceria nanotubes retain the B300 nm
dimensions of AAO template. Occasional defects in the ceria
structures allow for Ni deposition inside the ceria nanotubes
that results in the formation of short Ni wires (see ESI,wFig. S3).
The high inner tube surface area coupled with the close
proximity of the ceria catalyst and Ni current collector gives
rise to large triple phase boundary (TPB) lengths for SOFC
applications. Moreover, the ceria nanotubes can perform as
both an electrocatalyst and a short-range current collector due
to its mixed ionic and electronic conductivity (MIEC). As a
proof of principle, we have fabricated an SOFC test cell
(Fig. 4) containing the CeO2 nanotube-embedded Ni film as
the anode attached to a thick 1.0 mm yttria-stabilized zirconia
(YSZ) electrolyte structural unit. A 50 mm thick lanthanum
strontium manganate (LSM)–YSZ porous film cathode was
deposited on the opposite side of the YSZ electrolyte (see ESIwfor details). Due to the high pore density (B1010 pores cm�2)
of the anode, the TPB area can be up to 30 times larger than
that of a two-dimensional anode structure. The total cell
current (I) from the SOFC comes both from the porous nickel
paste and the nickel–ceria nanocomposite film:
I = iNiANi + iNi–ceriaANi–ceria (1)
where iNi and iNi–ceria are the current densities from pure
nickel paste and pure nickel–ceria nanocomposite portions,
respectively, and ANi and ANi–ceria are the areas of nickel paste
and nickel-ceria films, respectively. ANi and ANi–ceria were
carefully measured before and after running the SOFC. Cell
currents (I) were obtained from the linear sweep voltammetry
(LSV) measurements. A separate control experiment with a
pure nickel paste anode under the exact same conditions
gives iNi. Therefore, the only unknown, iNi–ceria, could be
extracted from eqn (1). The preliminary LSV displayed in
Fig. 4 shows current densities two times higher than those of
two-dimensional cells containing Ni pastes or ceria films under
the same conditions. While this demonstration shows that
the free-standing Ni/CeO2 nanocomposites can be directly
incorporated into SOFC structures, the use of thick electro-
lytes and poor adhesion between the anode and electrolyte give
rise to only a small improvement in current density. However,
the nanostructured anode not only increases overall TPB
length, but also generates a pore structure with a tortuosity
of one. Fabrication of micro-SOFCs containing the Ni/CeO2
films with thin (o100 nm) electrolytes are under investigation
and will be described elsewhere.
Conclusions
In summary, dense and highly ordered nickel–ceria films
were prepared using AAO templating methods. By tuning
the AAO template dissolution process, the composite films
can be prepared as coaxial nickel–ceria nanotube bundles
or as a nickel matrix impregnated with an ordered array
of ceria nanotubes. Preliminary linear sweep voltammetry
studies showed that the free-standing films can be used in
Fig. 3 Electron micrographs of Ni–CeO2 nanotubes: (a) SEM image of Ni–CeO2 coaxial nanotube arrays after removal of the AAO template;
(b) TEM image of the microtomed cross-section Ni–CeO2 coaxial nanotubes; TEM images and corresponding EDS (c) line scan and (d) phase
maps of a Ni–CeO2 coaxial nanotube.
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high temperature SOFC applications. Further studies are
in progress.
Acknowledgements
This work is supported by the Office of Naval Research under
Contract #N000140510711. R. Liu and S. B. Lee are partially
supported by UMD-NSF-MRSEC under grant DMR
05-20471 and a KOSEF grant from the Korean government
(MEST, grant code: R31-2008-000-10071-0). The authors
thank Xin Zhang from Materials Science and Engineering
Department of University of Maryland for microtome cutting.
The authors acknowledge the NISP lab and FabLab of the
Nanocenter at University of Maryland.
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Fig. 4 SOFC performance data (voltage and power densities vs. current densities) of a membrane electrolyte assembly (top inset). The total cell
performance (Ni–CeO2 nanocomposite film + Ni paste anode) is shown in red. The contributions from the Ni paste are shown in green. The
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argon as the fuel and dry air as the oxidant.
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