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PdeCu alloy membrane deposited on CeO2

modified porous nickel support for hydrogenseparation

Shin-Kun Ryi a,*, Hyo-Sun Ahn b, Jong-Soo Park a, Dong-Won Kim c

aEnergy Materials Center, Korea Institute of Energy Research (KIER), 102 Gajeong-ro, Yuseong-gu,

Daejeon 305-343, South KoreabUmicore Korea Limited, 71 3Gongdan 2Ro, Seobukgu, Cheonan 331-200, South KoreacDepartment of Advanced Materials Engineering, Kyonggi University, Suwon, South Korea

a r t i c l e i n f o

Article history:

Received 29 July 2013

Received in revised form

9 September 2013

Accepted 8 November 2013

Available online 12 December 2013

Keywords:

Hydrogen

Separation

Pd-based

Diffusion barrier

Ceria

* Corresponding author. Tel.: þ82 42 860 315E-mail address: h2membrane@kier.re.kr

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.11.0

a b s t r a c t

In this study, the hydrogen permeation behavior of a Pd93eCu7 alloy membrane deposited

on ceria-modified porous nickel support (PNS) was evaluated. PNS, which has an average

pore size of 600 nm, was modified by alumina sol. Alumina sol was prepared using pre-

cursors that had a mean particle size of 300 nm. Alumina-modified PNS was further treated

with ceria sol modification to produce a smoother surface morphology and narrow surface

pores. A 7 mm thick Pd93eCu7 alloy membrane was made on an alumina-modified PNS and

a ceria-finished membrane was fabricated by magnetron sputtering followed by Cu-reflow

at 700 �C for 2 h. SEM analysis showed that the membrane deposited on a ceria-finished

PNS contained more clear grain boundaries than the membrane deposited on the

alumina-modified PNS. The membrane was mounted in a stainless steel permeation cell

with a gold-plated stainless steel O-ring. Permeation tests were then conducted using pure

hydrogen and helium at temperatures ranging from 673 to 773 K and feed side pressures

ranging from 100 to 400 kPa. These tests showed that the membrane had a hydrogen

permeation flux of 2.8 � 10�1 mol m�2 s�1 with H2/He selectivity of >50,000 at a temper-

ature of 773 K and pressure difference of 400 kPa.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Dense palladium and its alloy membranes have attracted

great attention as an energy-efficient and cost-effective

method for hydrogen separation because of their high

hydrogen permeability, infinite hydrogen selectivity, and

chemical compatibility with other gas mixtures. While

commercially available palladium based self-supported

5; fax: þ82 42 860 3134.(S.-K. Ryi).2013, Hydrogen Energy P31

membranes are often fabricated by the cold rolling method,

utilization of composite structures that consist of a thin

membrane film and porous substrate can reducematerial cost

as well as improve the hydrogen permeation rate. The mate-

rials that have been used for supports include ceramics [1],

glass [2], stainless steel [3], Inconel [4], Hestalloy [5,6], and

nickel [7]. Since porous metal supports have been used to

improve the mechanical strength of the support, they can be

more readily sealed into a commercial unit. However, atomic

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 6 9 8e4 7 0 3 4699

interdiffusion of metals between the thin Pd/Pd alloy layer

and the metal components occur during high temperature

processing [3]. Diffusion barriers have been used to prevent

direct contact between the Pd/Pd alloy layer and the metallic

support. The properties of the diffusion barrier in the Pd or Pd

alloy coating are highly important to the performance of the

membrane. The diffusion barrier should be highly porous so

as to increase the membrane surface area and maximize

hydrogen permeation flux. The diffusion barrier should also

be inert not only with metals but also with the gas mixture. In

addition, adhesion between the diffusion barrier and the

metals (palladium layer and support) should be high enough

for to ensure mechanical stability of the membranes. Com-

mon materials that have been used for diffusion barriers

included Al2O3 [3,5,6], ZrO2 [8], yttria stabilized zirconia (YSZ)

[8,9], TiO2 [8], WO3 [10], Cr2O3 [11], CeO2 [12] and TiN [13].

Among these ceramic materials, ceria has a high melting

point (2750 K) and its linear thermal expansion coefficient is

the closest to palladium and nickel as shown in Table 1.

However, there is conflicting information on the use of a ceria

coating as a diffusion barrier because of the difficulty in

obtaining stable ceria sol.

In this study, a porous nickel support (PNS) was modified

with ceria sol prepared using commercially available ceria

powder as a precursor material. The dense PdeCu alloy com-

posite membrane was then prepared on ceria-modified PNS

using the sputtering and Cu-reflow method. Hydrogen perme-

ation tests throughtheas-preparedPdeCumembranewerealso

conducted and the membrane was analyzed by SEM/EDX.

2. Experimental

2.1. Membrane preparation

The PdeCu alloy composite membrane was prepared by

sputtering Pd and Cu on the modified PNS, which was fol-

lowed by Cu-reflow at 700 �C under vacuum (w10�3 bar). PNSs,

whichwere developed by KIER, were used as the substrates for

the PdeCu alloy composite membranes [7]. The PNS used in

this study was made of nickel powder and provided by INCO

Inc. The purity of the nickel was 99.9% with an average par-

ticle size of 3 mm. The nickel powder was pressed and then

coated with an aluminum nitrate solution using the incipient

wetness impregnation method to obtain 0.1 wt.% Al after

Table 1 e Linear thermal expansion coefficient of variousmaterials.

Materials Linear thermal expansion coefficient[10�5 K�1]

Pd 1.17

Cu 1.70

Ag 1.95

Ni 1.30

PSS 1.73

CeO2 1.10

ZrO2 1.00

Al2O3 0.65

TiO2 0.92

drying and calcination at 500 �C for 10 h. 20 g of the alumina-

modified nickel powder was compressed without a binder in a

cylindrical metal mold that had a diameter of 51 mm and

treated at 900 �C for 2 h to obtain a diameter of 50 mm. This

alumina sol coatingmethod can produce PNS that is thermally

stable. In addition, by using this approach, interdiffusion be-

tween the PdeCu alloy layer and nickel substrate at high

temperature (>450 �C) is prevented. As shown in Fig. 1, the

PNS contained micron-sized pores on its surface. Pore distri-

bution analysis using a mercury porosity meter showed that

the average pore size of the PNS was w600 nm. Before ceria-

modification, the pore size of the surface was reduced by the

alumina sol as shown in a previous study [14]. To ensure that

the alumina particles filled the entrance pores of the PNS, a

vacuum was applied to the other side of the PNS and excess

alumina particles on the PNS were carefully removed. The

ceria sol was prepared using precursors with a mean particle

size of<25 nm. 0.05 g of the boehmite powder (Sasol) and 0.1 g

of the ceria powder (Aldrich) were added to 20mL of deionized

water and then dispersed using an ultrasonic agitation for

10 min to prepare the ceria oxide sol. The alumina modified

PNS was coated with the ceria sol using a dip coating method.

After drying at ambient temperature, the ceria modified PNS

was calcined at 700 �C for 2 h under hydrogen. The modifica-

tion process is shown in Fig. 1.

A PdeCu alloy membrane was prepared by sputtering Pd

and Cu on the PNS, which was followed by the Cu-reflow

method. A detailed description of the fabrication methods

used has been described in previous papers [7,15]. The thick-

ness of the PdeCu alloy filmwas controlled to be around 7 mm.

2.2. Permeation measurements

Permeation tests were conducted using pure hydrogen and ni-

trogen at temperatures ranging from 673 to 773 K and feed side

pressures ranging from 100 to 400 kPa. The permeate side was

maintained at ambient pressure. The permeation apparatus

consisted of a membrane module, furnace, temperature

Fig. 1 e Schematic of the modification of porous nickel

support (PNS).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 6 9 8e4 7 0 34700

controller, pressure gauge/controller and mass flow controller.

Gases were introduced using a mass flow controller (MCF,

Brooks5850Eseries) andthe feedsidepressurewasregulatedby

a pressure controller (Alicat PC series). The temperature was

increased to 623 K at a rate of 3 K/min, while heliumwas being

introduced on both the feed and permeate sides of the mem-

brane in order to prevent the membrane film from being

damaged by the phase change [16,17]. Once the temperature

reached 623 K, hydrogen, instead of He, was introduced on the

feed side. The hydrogen permeation flux and helium leakage

was measured by a digital soap-bubble flow meter (Alltech,

Model 4074) whose detection limit is 1 � 10�2 ml min�1.

3. Results and discussion

3.1. Ceramic modification effects

The largestpore sizeon the surface is very importantbecause it

can determine the minimum thickness required to achieve a

pinhole-free Pd-based layer. According to Uemiya’s study, the

thickness of a palladium layer strongly depended on the

quality of the support, such as its pore size distribution and the

amount of defects on the surface [18]. The suggested relation-

ship between the thickness of the Pd layer and pore sizewere a

thickness of 13 mm vs. a pore size of 0.3 mm, 4.5 mm vs. 0.2 mm,

2.2 mm vs. 0.1 mm and 0.8 mm vs. 5 nm. Similarly, Mardilovich

et al. reported that the minimum thickness of palladium

required to achieve a dense layer by electroless plating was

approximately 3 times the diameter of the largest pores in the

Fig. 2 e SEM images of fresh PNS (a), alumina-m

support [19]. The fresh PNS was shown to contain some large

pores (Fig. 2(a)) and its largest pore size was greater than 5 mm.

Based on these results, it can be inferred that a membrane

thickness larger than 15e200 mmwould be required to produce

a pinhole-free Pd-based layer on the fresh PNS. Ryi et al. [5,6]

and Li et al. [3] modified porous Hastelloy and stainless steel

substrates using two different sized aluminum oxide sols

(2.5 mm, 0.3 mm). They could not form a continuous aluminum

oxide layer on the surface of the fresh disc when using the

0.3 mm sized aluminum oxide sol because of the large sized

pores of the disc. However, they were able to achieve pinhole-

free substrates after applying an aluminumoxide coatingwith

the 2.5 mm aluminum oxide sol. In a previous study, we filled

the large pores of PNS using aluminum oxide sol that had a

precursor particle size of 0.3 mm. Because of small pore size

compared with commercially available porous Hastelloy and

stainless steel substrates, our PNS could be successfully

modified with 0.3 mm aluminum oxide sol using a vacuum

assisting method. The alumina-modified PNS is depicted in

Fig. 2(b) and (d), i.e. 10 times magnification in Fig. 2(b). The

surfaceporesand roughness of the coateddiscweredrastically

reduced after applying the aluminum oxide filling.

The alumina-modified PNS was further modified with

<25 nm cerium oxide sol in order to further reduce the pore

size and surface roughness. Fig. 2(c) shows a SEM image of the

ceria-finished PNS. Compared with the alumina-modified

PNS, the ceria-modified PNS contained a smaller pore size

and surface roughness. A cross-sectional SEM image of ceria-

finished PNS shows that the thickness of ceria was w500 nm

(see the sub-figure in Fig. 2(c)). Fig. 3 shows the hydrogen

odified PNS (b, d), and ceria-finished PNS (c).

Fig. 3 e Hydrogen permeation flux of PNS after each

ceramic coating: test was carried at room temperature.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 6 9 8e4 7 0 3 4701

permeation flux of PNS after each coating. After the first

modification step with larger particles, the hydrogen perme-

ation flux of the PNS decreased to w78% of the permeation

flux of the fresh PNS and the hydrogen permeation flux of the

ceria-finished PNS was almost the same as the alumina-

modified PNS because the ceria layer was very thin

(w500 nm). Therefore, the disc remained very permeable after

all pre-treatment stepswith a hydrogen permeance as large as

1.42 � 10�5 mol m�2 s�1 Pa�1. The performance of the ceria-

finished PNS was also tested with nitrogen. The ideal selec-

tivity of hydrogen to nitrogen was determined to be w3.4,

which was very close to the 3.74 ideal value via Knudsen

diffusion, suggesting that no large pores or cracks were pre-

sent in the coating layer. This result also suggests that the

ceria-finished PNS is suitable for preparing thin Pd-based

composite membrane for hydrogen separation.

3.2. Membrane characterization

Images of the surface of the two different membranes after

Cu-reflow are compared in Fig. 4. Themembrane deposited on

Fig. 4 e Surface SEM images of the PdeCu alloy membrane dep

PNS (b).

ceria-finished PNS contained more clear grain boundaries.

SEM analysis results indicated that the PdeCu alloy layer was

well deposited on the surface of the alumina-modified PNS.

No obvious cracks or pinholes were observed on the surface of

the membrane.

3.3. Hydrogen permeation behavior

Before hydrogen permeation behavior tests, the membranes

deposited on alumina-modified PNS and ceria-finished one

were stabilized at 823 K and pressure difference of 100 kPa for

�15 h Fig. 5 compares the hydrogen permeation behavior be-

tween the two membranes. The hydrogen permeation flux of

the membrane on alumina-modified PNS increased as a func-

tion time andwas stabilized after 10 h,while themembrane on

ceria-finished PNS remained constant over the period. Hemra

et al. reported that annealing of palladiummembrane at 973 K

for 8 h in nitrogen atmosphere increased hydrogen perme-

ability and themembrane annealed at higher temperature had

lower surface roughness [20]. After permeation tests, the

morphologies of the membranes were analyzed by SEM and

shown in the sub-figures in Fig. 5. After stabilization, the

morphology of the membrane on ceria-finished PNS was not

changed, while the grain in the membrane on alumina-

modified PNS became clearer than the fresh membrane. The

stabilization tests indicate that the Cu-reflow time of 2 h at

973 K was enough for stabilization of the membrane on ceria-

finished PNS, while the membrane on alumina-modified PNS

needed more annealing time for stabilization.

The hydrogen permeation behavior tests were conducted

with pure hydrogen at temperatures ranging from 678 to 773 K

and at pressure differences across the membrane of

100e400 kPa. Fig. 6 shows the dependency of the hydrogenflux

on the feed side pressure for the prepared composite mem-

brane at various temperatures. As expected, the hydrogen flux

increasedwith increasing temperature and feed side pressure.

The hydrogen flux increased to 2.8 � 10�1 mol m�2 s�1at a

temperature of 773 K and pressure difference of 400 kPa. The

heliumleakageof themembranemeasuredat a temperatureof

773 K and pressure difference of 400 kPa and was below the

detection limit of the digital soap-bubble flow meter

(1� 10�2mlmin�1¼ 4.48� 10�6molm�2 s�1 when considering

osited on alumina-modified PNS (a) and ceria-finished

Fig. 5 e Comparison of permeation behaviors between Al2O3 modified and CeO2 modified membranes at 550 �C: pressuredifference was 100 kPa; sub-figures are surface SEM images after permeation tests.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 6 9 8e4 7 0 34702

effective membrane area). This indicated the selectivity of H2/

He was >50,000. As shown in Fig. 6, a pressure exponent of 0.5

provided good fits, indicating that the diffusion of hydrogen

atoms through the bulk PdeCu alloy layer is rate-limiting, as in

the well-known Sieverts’ law.

Hydrogen transport through a dense Pd-based alloy

membrane is an activated process and the relationship be-

tween the hydrogen permeability and temperature can be

described by Arrhenius law:

Ql¼ Q0

lexp

��Ea

RT

�(1)

where Q is the hydrogen permeability (mol m m�2 s�1 pa�n), l

is the membrane thickness (m), Ea is the activation energy

(kJ mol�1), R is the universal gas constant (8.314 J mol�1 K�1)

and T is the temperature in Kelvin. The activation energies for

Pd-based membranes have been reported to range from 5.4 to

38 kJ mol�1 [21]. The activation energy is known to be affected

by differences in the microstructure or impurities on the

surface, in the bulk, or at the grain boundaries [22]. The

Fig. 6 e Hydrogen permeation flux as a function of the

difference between the pressure exponent of 0.5 at various

temperatures.

activation energy of the membranes evaluated from the

Arrhenius plot of the overall hydrogen permeability against

the reciprocal temperature in Fig. 7 was 23.1 kJ mol�1 at a

pressure difference of 100 kPa, which is a little bit larger than

the membrane deposited on the alumina-modified PNS

(19.0 kJ mol�1) tested at higher pressure difference (2000 kPa)

[14] and similar to the values reported for other Pd-based

membranes at lower pressures [5,23,24].

4. Conclusions

Analysis of the membrane and hydrogen permeation tests at

high pressure differences for the PdeCu composite mem-

branes deposited on the ceria-finished porous nickel support

with sputtering followed by Cu-reflow revealed that:

� A defect-free PdeCu alloy membrane could be obtained by

sputtering followed by Cu-reflow on the ceria-finished

porous nickel support.

Fig. 7 e Arrhenius plot of overall hydrogen permeability as

a function of reciprocal temperature.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 6 9 8e4 7 0 3 4703

� The hydrogen flux increased up to 2.8 � 10�1 mol m�2 s�1

with H2/He selectivity of >50,000 at a temperature of 773 K

and pressure difference of 400 kPa.

� The membrane had an n-value of 0.5, indicating that

hydrogen permeation is controlled by diffusion in the bulk

PdeCu alloy layer.

� The activation energy of the membranes evaluated from

the Arrhenius plot of the overall hydrogen permeability

against the reciprocal temperature was 23.1 kJ mol�1.

Acknowledgment

This work was conducted under the framework of Research

and Development Program of the Korea Institute of Energy

Research (KIER) (B3-2461-6).

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