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

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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

4296 | Phys. Chem. Chem. Phys., 2010, 12, 4295–4300 This journal is �c the Owner Societies 2010

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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.

4298 | Phys. Chem. Chem. Phys., 2010, 12, 4295–4300 This journal is �c the Owner Societies 2010

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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

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