research article development of pd alloy hydrogen ...research article development of pd alloy...

Research ArticleDevelopment of Pd Alloy Hydrogen SeparationMembranes with Dense/Porous Hybrid Structure forHigh Hydrogen Perm-Selectivity

Jae-Yun Han,1 Chang-Hyun Kim,2 Sang-Ho Kim,3 and Dong-Won Kim1

1 Department of Advanced Materials Engineering, Kyonggi University, Gyeonggi-do 443-760, Republic of Korea2Department of SDM (Semiconductor Display Mechatronics), Kyonggi University, Gyeonggi-do 443-760, Republic of Korea3 School of Energy, Materials & Chemical Engineering, Korea University of Technology and Education,Chungcheongnam-do 330-760, Republic of Korea

Correspondence should be addressed to Dong-Won Kim; [email protected]

Received 14 July 2014; Revised 16 September 2014; Accepted 17 September 2014; Published 13 October 2014

Academic Editor: Zoe Barber

Copyright © 2014 Jae-Yun Han et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

For the commercial applications of hydrogen separation membranes, both high hydrogen selectivity and permeability (i.e., perm-selectivity) are required. However, it has been difficult to fabricate thin, dense Pd alloy composite membranes on porous metalsupport that have a pore-free surface and an open structure at the interface between the Pd alloy films and the metal support inorder to obtain the required properties simultaneously. In this study, we fabricated Pd alloy hydrogen separation membranes withdense/porous hybrid structure for high hydrogen perm-selectivity. The hydrogen selectivity of this membrane increased owing tothe dense and pore-free microstructure of the membrane surface.The hydrogen permeation flux also was remarkably improved bythe formation of an open microstructure with numerous open voids at the interface and by an effective reduction in the membranethickness as a result of the porous structure formed within the Pd alloy films.

1. Introduction

Pd-based membranes have attracted a great deal of attentionfor their use in hydrogen separation and purification owingto their high theoretical permeability, infinite selectivity,and chemical compatibility with hydrocarbon containinggas streams [1, 2]. Pd-based membranes are prepared in acomposite form consisting of a dense Pd alloy film, whichprovides the high hydrogen perm-selectivity and a porousmetal/ceramic support with appropriate mechanical strengthto support the top layer [3–5]. Porousmetal supports [4, 6–9]are promising candidates because of their better mechanicalstrength, resistance against cracking, ease of module fabrica-tion and sealing, and thermal expansion coefficients similarto that of Pd and its alloys. However, typical metal supportslike porous stainless steel support (PSS) and porous nickelsupport (PNS) have irregular, large surface pores, and veryrough surfaces [7, 10–13]. These lead to the inhomogeneoussputter deposition of Pd on the porous metal support and

numerous pores at the surface of the membrane remain evenafter a heat treatment to achieve alloying and crystallizationof Pd.

Many researchers have developed various surface pre-treatments such as alumina sol-gel coating [7, 10, 13, 14] andmicropolishing [4, 12, 13, 15] in order to remove the surfacepores of porous metal supports. A sol-gel coated aluminalayer is frequently introduced on PSS surface in order tomoderate the average pore size, smoothen the surface, andprevent intermetallic diffusion of the metal support elements[7, 10, 14, 16]. The micropolishing pretreatment is veryeffective in leveling off the rough PNS surface and it almostcompletely plugs the surface pores.

Other researchers have focused on decreasing the thick-ness of the densemembrane in order to improve the hydrogenselectivity and permeability. In our previous studies [12,13, 19], we reported on the fabrication of Pd alloy mem-branes with a dense and pore-free surface. These membranes

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2014, Article ID 438216, 10 pageshttp://dx.doi.org/10.1155/2014/438216

2 Advances in Materials Science and Engineering

exhibited infinite hydrogen selectivity (H2/N2) at 723K and

a transmembrane pressure difference of 689 kPa. However,they did not have sufficient hydrogen permeability for varioushydrogen separation applications because of the membranewith large thickness and closed structure at the interfacebetween the Pd alloy films and the modified metal support.Therefore, it has been difficult to fabricate thin and dense Pdalloy films that are pore-free at the membrane surface andhave an open structure near the interface in order to obtainhigh hydrogen perm-selectivity.

In this study, we developed Pd alloy membrane withdense/porous hybrid structure for obtaining both highhydrogen selectivity and permeability simultaneously. Thismembrane had high hydrogen selectivity owing to its denseand pore-free membrane surface, combined with improvedhydrogen permeation flux not only because of its openmicrostructure with numerous voids near the interface butalso because of the effectively reduced membrane thicknessvia the porous structure formed within the Pd alloy films.In this paper, we observed the morphology of the fabricatedmembranes and studied the formation mechanism of thishybrid structure.The effect of the dense/porous hybrid struc-ture on the hydrogen perm-selectivity was also investigated.

2. Experimental

A PNS with good chemical affinity to Pd was prepared bycompression of Ni powders and subsequent sintering. Nipowders were purchased from Sigma Aldrich Co. (with anaverage particle size of 5 𝜇m) and fromNano Technology Co.(with an average particle size of 100 nm). In order to enhancethe mechanical and thermal durability of the Ni support, thesupport surface was modified by mixing Ni powders withdifferent average grain size of 5 𝜇m and 100 nm in the ratio of4 : 1 (wt%), respectively.Then, themixture wasmilled for 24 husing a zirconia ball. The mixed-powders were compressedwithout binder in a cylindrical metal mold having a diameterof 1 inch. The compressed powders were heat treated at973K for 2 h under H

2atmosphere. Additionally, a porous

Al2O3support was fabricated in a similar manner in order

to identify the properties of the interface structures independence of the support material.

The sintered PNS was micropolished using an autopol-isher (GLP S-20/25, purchased from GLP Korea) in order tofill the surface pores and, thereby, to smoothen the surface,which was carried out with #400 to #2000 grit SiC paperand then with a 3 𝜇m/1 𝜇m wet, fine diamond slurry. Thesupport was thenwashed using isopropyl alcohol and acetonein an ultrasonic cleaner. Plasma surface modification of theNi support was performed in a mixture of 10% H

2/Ar at a

working pressure of 1.333 × 10−2 kPa and radio frequency(RF) power of 100W for 5min in order to remove surfaceimpurities induced by air contamination and to activate themicropolished surface.

Using advanced continuous dc magnetron sputtering,Pd (99.95% target purity) and Cu (99.99% target purity)were in situ deposited on the modified Ni support tothicknesses in the range of 4∼10 𝜇m and 0.4∼1 𝜇m, respec-tively, under conditions of 298∼673K substrate temperature,

1.333 × 10−2 ∼1.333 × 10−4 kPa working pressure, 40∼160Wdc power, and 20 sccm Ar flow. Furthermore, Cu metal wasadopted for alloying and surface densification as well as forits low cost andhigh resistance against poisoning by hydrogensulfide and sulfurous constituents [4, 12].

The Cu-reflow heat treatment was then applied to fillthe voids at the surface of the membrane and to facilitatealloying and crystallization of the separate films. Alloying,crystallization, and densification of the Pd/Cu films wereperformed at 923K for 2 h in a H

2/Ar mixture at a working

pressure of 1.333 × 10−2 kPa. In our previous studies [4, 12,13, 19], the obtained Pd-Cu alloy membranes had a dense,pore-free surface after the Cu-reflow process under the sameconditions.

Two types of Pd alloy filmswith 4𝜇mand 10𝜇mthicknesswere prepared by sputter deposition andCu-reflowprocesses.The Pd alloy films with a thickness of 4 𝜇m were used tocompare microstructures between closed interface, partiallyopen interface, and open interface. We studied the heattreatment-induced interfacial reaction between Pd films andNi particles. These samples were only applied for analyzingthe cross-sectional structures. The Pd alloy films with athickness of 10 𝜇m were prepared for permeation test. Afterpermeation test, the Pd alloy films with a thickness of 10 𝜇mwere used to identify the relation between themicrostructureof films and their hydrogen perm-selectivity.

The hydrogen permeation flux and selectivity were mea-sured using an apparatus that consisted of a membrane cell,furnace, temperature controller, pressure gauge/controller,and gas chromatograph, as described in the previous litera-ture [12, 20].Themeasuring temperatures were in the range of623∼723K, and the transmembrane pressure difference was50∼200 kPa under the flow of mixed gas (H

2: N2= 1 : 1 v/v).

The morphology of the Pd alloy films was investigatedby field emission scanning electron microscopy (FE-SEM),transmission electron microscopy (TEM), and atomic forcemicroscopy (AFM). The structures and compositions weredetermined byX-ray diffraction (XRD) and energy dispersivespectroscopy (EDS), respectively.

3. Results and Discussion

For commercial applications of Pd alloy hydrogen separationmembranes, both high hydrogen selectivity and permeabilityare required [3, 13, 21]. Therefore, much effort has beenfocused on achieving high hydrogen permeability whilemaintaining the high hydrogen selectivity. However, sincethese two properties are incompatible with each other, it hasbeen difficult to enhance the two properties simultaneously.To further increase the hydrogen permeability, the densePd alloy layer must be thin, such that the hydrogen flux islinearly related to the reciprocal of the membrane thickness.In addition, it is necessary to form an open structure inthe vicinity of the interface between the Pd alloy films andthe modified metal support in order to reduce the hydrogenpermeation resistance caused by the blocked interface.

Figure 1 shows FE-SEM cross-sectional photographs ofthe Pd alloy films formed on a porous nickel metal andalumina ceramic supports using the process described in

Advances in Materials Science and Engineering 3

the previous section. As can be seen in Figure 1, the Pd-Cualloy membranes had a dense, pore-free surface on top ofthe Al

2O3and modified Ni supports, essentially achieved by

the Cu-reflow process that was applied to eliminate pores atthe surface of the membrane and to facilitate alloying andcrystallization of the separate Pd and Cu films [4, 12, 13, 19].The interfaces between the Pd alloy films and the modifiedsupports show entirely different microstructures dependingon the support material. In case the Al

2O3ceramic was used

as support, the Al2O3ceramic did not satisfy the Hume-

Rothery rules for a homogeneous solid solution with Pdmetal because of the poor chemical affinity between Al

2O3

and Pd. This poor affinity resulted in a separate and closedinterface between Al

2O3and the Pd film deposited directly

on it, which led to low hydrogen permeation caused by theinterface resistance against hydrogen flow [13], as illustratedin Figure 1(a). On the other hand, themetals Pd andNi, whichhave the same crystal structure, similar atomic radius, samevalence, and similar electronegativity, completely satisfied theHume-Rothery rules for a homogeneous solid-solution alloy[12, 22, 23]. This resulted in the observed open structurecomposed of a large number of open voids near the interfacebetween the Pd film and the Ni support, as shown inFigure 1(b), which facilitated hydrogen gas flow via the opensites. Thus, it is expected that the hydrogen permeability canbe significantly enhanced by the formation of this structure.

Figure 2 illustrates schematically the process and themechanism for the formation of a three-dimensional (3D),widely open structure, which was fabricated via 3 stepsincluding the micropolishing of PNS, continuous in situPd/Cu sputtering, and Cu-reflow heat treatment. Figure 2(a)shows the entire micropolishing process that was applied toobtain the pore-free and flat surface of the Ni support. Roughgrinding was performed to remove larger pores at the surfaceusing SiC paper and fine grinding to eliminate smaller poresand to maintain the smooth surface using a polishing clothandwet diamond slurry. As shown in Figure 2(a), the sinteredNi support was composed of larger pores with a range of0.1∼5 𝜇m and had a very rough surface. Such a surface stateoffers high hydrogen flux to the Pd alloy membranes, but itleads to inhomogeneous deposition of Pd on the Ni supportduring sputtering. Thus, the surface pores and scratches onthe PNSwere completely removed and the surfacewas leveledoff by the micropolishing treatment, as can be seen from thesurface morphology of the modified support in Figure 2(a).The modified Ni support with its pore-free and flat surfaceserved as a high-quality substrate for the uniform, fine Pdnucleation by magnetron sputtering. Furthermore, most ofthe surface pores that had been filled by fine Ni debris duringthe micropolishing processes were regenerated after the Cu-reflow heat treatment. This may be attributed to the mutualdiffusion of fine Pd deposits and small Ni particles near theinterface and the resintering of the fine Ni particles in themodified Ni support by the thermal energy and activatedreaction induced by the Cu-reflow heat treatment.

Continuous in situ Pd and Cu deposition using anadvancedmagnetron sputtering system was also newly intro-duced in order to enhance the properties of the Pd alloyhydrogen membranes. In case the sputtering was carried

out at a high substrate temperature of 673K, a low workingpressure of 1.333 × 10−4 kPa, and a high dc power of 160W,we obtained uniform and dense Pd/Cu films, as shown in theFE-SEM and TEM images of Figure 2(b).The high mobilitiesand reactivities of Pd andCu atoms activated by the high tem-perature, low working pressure, and high dc power facilitatedthe initial homogeneous nucleation of Pd nanoparticles, andeventually promoted the growth of uniformly fine-grained,dense films during successive sputtering depositions.

Finally, the sputtered Pd/Cu films were subjected to theCu-reflow heat treatment at 923K for 2 h in a mixture of 10%H2/Ar in order to achieve complete alloying/crystallization

and to ensure a dense/porous hybrid structure. Figure 2(c)shows the corresponding FE-SEM image, XRD pattern,and cross-sectional EDS composition depth profile of themembrane after the final heat treatment. Verified by FE-SEM and XRD, this membrane had a uniform, dense surfacemicrostructure without micropores and formed a stable,single-phase Pd alloy. From the cross-sectional EDS com-position depth profile, it was observed that Pd and Ni fromthe Pd alloy coating layer and the modified Ni support,respectively, mutually diffused at the interface during theCu-reflow heat treatment process. These main componentsexactly satisfied the requirements to form a homogeneoussolid solution, as suggested by the Hume-Rothery rules[12, 22, 23]; Pd and Ni metals with high chemical affinityare completely soluble in the solid state over the entirecompositional range at 923K, as illustrated by the Pd-Niphase diagram given in Figure 2(d) [24]. The FE-SEM andTEM images shown in Figure 2 clearly exhibit 3D, widelyopen structure composed of a large number of voids nearthe interface between the Pd alloy films and the modified Nisupport.

Considering the results of the FE-SEM, TEM, XRD,and EDS analyses, it should be concluded that uniform,fine Pd deposits and small Ni particles having good mutualchemical affinity are totally integrated at their interfaceby interdiffusion. Moreover, resintering of embedded Niparticles is generated in the modified Ni support by self-diffusion and the reaction of activated Ni particles duringthe Cu-reflow heat treatment. Thus, the results confirmedthat the 3D, widely open structure of the Pd alloy membranewas formed by the combined effect of improved interfacialreaction (by using nanosized fine particles) and improvedthermal/chemical reaction (resulting from activation by theCu-reflowprocess). Furthermore, bothmaterial (Figure 2(d))and process requirements (Figures 2(a), 2(b), and 2(c)) forthe formation of the open structure within the Pd alloyfilms were satisfied simultaneously, schematically describedin Figure 2. It is likely that very significant changes in thehydrogen perm-selectivity can be attributed to variations ofthe microstructures of the Pd alloy membranes.

Thin films prepared by sputtering show a wide rangeof microstructures and their relevant properties, both ofwhich are highly dependent on the sputtering conditions.Figure 3 shows surface and cross-sectional FE-SEM imagesof as-deposited Pd/Cu and Pd-Cu alloy films after the Cu-reflowheat treatment in dependence of the sputter deposition


Pdcoating

layer

PorousNi

support

Porous

support

Chemical affinity between

Pd film and support materials

(b)

Open structure

(a)

Closed structure

Al2O3

“dissatisf

action” “satisfaction”

Figure 1: Chemical affinity between Pd film and supportmaterials: (a) closed structure formed at interface between Pd film and porous Al2O3

support; (b) open structure formed at interface between Pd film and porous nickel support.

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(b) Advanced sputtering process for uniformly dense Pd/Cu film microstructure

Cu layer

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Surface Cross-section

Rotation Rotationpolishing clothSiC paper

(a) Micro-polishing processfor pore-free and flat surface

3D widely open structure

Micro-pores filling

(a), (b), (c): process requirement(d): material requirement

TEM image

(d) Chemical affinity between Pd film and Ni support

atomic percent palladium

Weight percent palladiumNi Pd

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800600400200

0

(c) Heat treatment process for alloyingand the formation of hybrid-structure

Pd-Cu-Nialloy

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Pd

Cu

Ni

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×103

2𝜃 (𝜃)Depth (𝜇m)

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1455∘C 1555

∘C

354.3∘C

L

60.1%, 1237∘C(Ni, Pd)

Magnetic trans.

0 10 20 30 40 50 60 70 80 90 100

Supp

ort

Coa

ting

laye

r

Tem

pera

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(∘C)

Figure 2: Process and mechanism for the formation of 3D widely open structure: (a) micropolishing process for pore-free and flat surfaceof the support (SEM images); (b) advanced sputtering process for uniformly fine films (SEM/TEM images); (c) heat treatment process foralloying, crystallization, and the formation of a dense/porous hybrid structure (SEM/XRD/EDS profile); (d) Pd-Ni alloy phase diagram.


Pd-Cu alloy films after Cu-reflow processSputter as-dep. Pd/Cu films

Surface Cross-section Surface Cross-section

Cu-reflowheat-

treatment

Large pores

Pore-free surface Open structure

Partially open structure

Closed structure

Micro-pores

PNS

Pd-Cu alloy films

PNS

PNS

PNS

Pd/Cu films

PNS

films

(a) Coarse columnar structure

(c) Fine columnar structure

(e) Dense structure

(b) Coarse columnar structure

(d) Fine columnar structure

(f) Dense structure

PNS

filmsPd/Cu

Pd/Cu

Pd-Cu alloy films

Pd-Cu alloy films

(923K-2h)

Figure 3: Surface and cross-sectional FE-SEM images of sputter as-deposited Pd/Cu and Pd-Cu alloy films after the Cu-reflow process forvarious sputtering variables: (a), (c), (e) sputter as-deposited films; (b), (d), (f) Pd-Cu alloy films after heat treatment.

conditions. As can be seen from the film microstructuresshown in Figure 3, considerable structural differences inthe as-deposited Pd/Cu and heat-treated Pd-Cu alloy filmswith a thickness of 4𝜇m were observed, depending onthe sputtering variables including the substrate temperature,working pressure, and dc power. As shown in Figure 3(a),when the sputtering was carried out at room temperature, aworking pressure of 1.333 × 10−2 kPa, and dc power of 40W,the resulting Pd/Cu coating layer had a coarse columnarstructure and a very rough surface. Moreover, as evidentfrom the morphology of the films shown in Figures 3(a) and3(b), the inhomogeneous deposition of the films during thesputtering process resulted in abnormal grain growth, whichled to large pores at the surface of the films and a closedstructure near the interface after theCu-reflowheat treatmentat 923K for 2 h in a H

2/Ar mixture. This ultimately caused

the deterioration of the hydrogen perm-selectivity and long-term stability of the Pd alloy membranes. It can also be seenthat the Pd/Cu films deposited by sputtering at the substratetemperature of 473K, working pressure of 1.333 × 10−3 kPa,and DC power of 80W had a fine columnar structure withsmooth surface compared to the coarse columnar structure.After the Cu-reflow heat treatment, the Pd-Cu alloy filmshad a few micropores at the surface and a partially openstructure near the interface, as illustrated in Figure 3(d). Incontrast, as shown in Figure 3(e), the Pd/Cu films depositedby sputtering at a high substrate temperature of 673K, anextremely low working pressure of 1.333 × 10−4 kPa, andhigh dc power of 160W had a uniformly fine grained,

dense structure with pore-free surface. The reason for theformation of this desired film morphology is the same asthat described above for Figure 2(b), namely, homogeneousnucleation of Pd nanoparticles. As shown in Figure 3(f),the Pd alloy hydrogen membranes having a dense, pore-free microstructure at the surface and a completely openstructure near the interface between the Pd alloy films and themodified Ni support were obtained after the Cu-reflow heattreatment process, which is expected to increase the hydrogenpermeability without reducing the ideal H

2/N2selectivity.

Therefore, the microcrystalline morphology of the Pd/Cufilm formed by the sputtering method plays an importantrole in improving the characteristics of the Pd alloy hydrogenseparation membranes.

In order to further study the heat treatment-inducedinterfacial reaction betweenPdfilms andNi particles, we haveinvestigated the change of the cross-sectional compositionprofile of the Pd alloy films and the Ni support for variousinterface structures that are shown in Figure 3. Figure 4shows the cross-sectional composition depth profiles ofthe Pd-Cu alloy films with 4𝜇m thickness after the Cu-reflow heat treatment at 923K for 2 h for different interfacestructures between the Pd alloy film and the modified Nisupport. The average compositions of the alloy films andthe metal support including closed, partially open, and openinterface structures were determined by EDS scan analyses.The scan path is marked by arrows in Figure 4. From theEDS results, it was observed that Pd, Cu, and Ni metalswith high chemical affinity weremutually diffused during the


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)

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Sputter as-dep. interface

Pd-Cu alloy films microstructures

after the Cu-reflow process

Cross-sectional compositionprofiles of the Pd-Cu alloy films

(a)

(b)

(c)

(d)

(e)

(f)

EDS detection range

PdNi

Cu

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eigh

t per

cent

(%)

0 1 2 3 4 5 6 7 8 9 10Depth (𝜇m)

0 1 2 3 4 5 6 7 8 9 10Depth (𝜇m)

Figure 4:The cross-sectional FE-SEM images and composition profiles of the heat-treated Pd-Cu alloy films with 4 𝜇m thickness for variousinterface structures depending on the sputter as-deposited microstructures: (a), (d) closed interface; (b), (e) partially open interface; (c), (f)open interface; arrow lines in (a), (b), and (c) are EDS detection range.

Cu-reflow heat treatment. The largest amount of mutuallydiffused Pd and Ni atoms was found for Pd alloy filmshaving a closed interface structure, as shown in Figures 4(a)and 4(d). That is, the composition of Ni at the membranesurface was estimated to be 23 wt% for the films with aclosed interface structure (Figure 4(d)) and 8 wt% for anopen interface structure (Figure 4(f)), indicating significantNi interdiffusion into the Pd alloy films in the former case.

The reason for this experimental result can be attributedto the coarse columnar microstructures composed of largevoids near columnar valleys, as mentioned above in case ofFigure 3, which facilitated rapid mutual diffusion of Pd andNi atoms during the Cu-reflow heat treatment. Furthermore,the interfacial reactivity of Pd deposits and Ni particles wasrather high for the specimens with open interface structure(allowing rapid changes in the Pd and Ni composition at the


interface, as shown in Figure 4(f)) compared to the closedand partially open interface structures (corresponding torelatively gradual changes in the composition at the interface,as shown in Figures 4(d) and 4(e)). This could be explainedby the significantly increased thermal reaction of uniformlydistributed fine Pd and Ni particles (nanosize effect for theopen interface specimen).This increased interfacial reactivityin case the open structure near the interface is in good agree-ment with the results discussed above for Figure 2.Therefore,the open structure composed of porous microstructures withnumerous open voids near the interface was formed by theadvanced sputtering method and subsequent Cu-reflow heattreatment process on modified Ni support.

The main focus of the present study is to assess theeffect of the open structure near the interface on thehydrogen permeability.Therefore, hydrogen permeation testswere carried out and the hydrogen permeation flux of afabricated membrane with dense/porous hybrid structurewas compared to that of a conventional membrane having aclosed interface structure. To identify the characteristics ofthe hydrogenmembranes, the surface state and film thicknessof the two membranes were kept the same (dense, pore-free morphology, and 10 𝜇m in thickness, resp.). Thus, theinfluence of surface structure andmembrane thickness on thehydrogen permeation flux for Pd alloy membranes was keptconstant during the permeation tests. The membrane withlarge thickness of 10𝜇m was chosen to prevent degradationof the hydrogen permeability by intermetallic diffusion ofNi from the support into the membrane surface during thehigh-temperature of the Cu-reflow process. Consequently,the surface composition of the two membranes annealed at923K resulted in 90wt% of Pd and 10wt% of Cu. The Nicomposition at the membrane surface was not detected byEDS analysis, and any variation of the hydrogen permeabilityas a function of the Ni content was ruled out given theconstant surface composition.

Figure 5 represents the cross-sectional FE-SEM imagesof the developed and the conventional Pd alloy membranesmanufactured under the above conditions.The pictures wereobtained using these membranes after permeation test. Thepresent membrane with dense/porous hybrid structure wasobtained by the advanced process, as discussed in greaterdetail in Figure 2, whereas the conventional membrane hav-ing both purely dense structure and closed interface wasfabricated by fine columnar Pd/Cu deposition and roughlypolished surface treatment, as previously stated [4]. Suchmicrostructural problem associated with the conventionalmethod was the difficulty in forming open interface betweenthe Pd alloy films and the metal support, which led to theadverse effect of the hydrogen permeability.

Figure 6 shows the hydrogen perm-selectivity of thepresent Pd-Cu alloy membranes having a dense/poroushybrid structure (open interface) as opposed to a purelydense structure (closed interface) formed by conventionalmembrane process. The hydrogen permeation flux and theideal selectivity (H

2/N2) were measured using our previously

described measurement system [12, 20] at a transmem-brane pressure difference of 50–200 kPa. The temperaturewas increased from room temperature to 723K during the

measurements. Figure 6 represents the dependency of thehydrogen permeation flux and selectivity on the pressuredifference at various temperatures.The hydrogen permeationflux depends on the pressure difference with the pressureexponent of 0.5 indicating Sievert’s law flux dependence.The ideal H

2/N2selectivity of the two membranes was

infinite within our measuring accuracy (the level of N2was

below the detection limit of the measurement system duringthe permeation tests), regardless of the structure near theinterface between the Pd alloy membranes and the modifiedNi support. This is primarily because of the dense and pore-free surface morphology as a result of the Cu-reflow heat-treatment. The hydrogen permeation flux increased withincreasing temperature and pressure difference and reached2.4 × 10−1molm−2 s−1 for the fabricated membrane withdense/porous hybrid structure (open interface) and 1.1 ×10−1molm−2 s−1 for the conventional membrane with purelydense interface structure (closed interface) at 723K and200 kPa. Thus, the fabricated membrane with dense/poroushybrid structure had 2.2 times higher hydrogen permeationflux than the conventional one having a purely dense interfacestructure. This may be attributed to the open microstructurewith numerous voids at the interface (the porous structurefacilitates hydrogen flux without permeation resistance at theinterface) and the effective thickness reduction of the densestructure from 10 𝜇m to less than 5𝜇m as a result of theformation of the porous structure within the Pd alloy films(the hydrogen flux is inversely proportional to the thicknessof the dense membrane) in comparison to the conventionalmembranes having a purely dense structure, as illustrated inFigure 5.

Ni content also has effect on hydrogen permeation. Ourprevious study shows that H

2permeability increases with

decrease of Ni content [13]. As shown in Figure 4, thedense/porous hybrid structure decreases Ni content as well asdense layer so that the hydrogen permeation flux increased.

Table 1 shows the comparison of hydrogen permeabilitybetween the present membrane and other literatures [17,18]. We achieved 2.2 times higher hydrogen permeationflux by dense/porous hybrid structure. However, the presentmembrane has relatively low hydrogen permeability com-pared with Pd membranes deposited on porous ceramicsand ceramic modified porous stainless steels because nickeldecreased hydrogen solubility. Despite low hydrogen perme-ability, there are some advantages in Pd-Cu alloy membranedeposited on porous nickel support.

(1) While Pd membranes easily suffer from hydrogenembrittlement due to the 𝛼-𝛽 phase transition attemperatures below the critical points of the Pd-Hsystem [25], no 𝛼-𝛽 phase transition occurred in Pd-Cu alloy membrane [26]. Therefore, it was possible todesign and build up a simple hydrogen purifier [27].

(2) Very good adhesion between membrane layer andsupport provided very easy diversemodulationmeth-ods like knife-edge sealing [20] and diffusion bonding[28].


Dense structure

(improvement of hydrogen selectivity)

Dense structure- Closed interface

(decrease of hydrogen permeability)

Closed interface

- Pore-free surface and dense structure

(a)

Porous structure - Open interface and effective membrane thickness reduction

(improvement of hydrogen permeability)

Dense structure

Open interface

(improvement of hydrogen selectivity)- Pore-free surface and dense structure

(b)

Figure 5: The cross-sectional FE-SEM images of (a) purely dense structured and (b) dense/porous hybrid structured Pd-Cu alloy films.

0 50 100 150 200 2500.00

0.05

0.10

0.15

0.20

0.25

0.30

H2

perm

eatio

n flu

x (m

ols−1

m−2)

H2/

N2

sele

ctiv

ity

H2/N2 selectivity723K

673K623K

∞

Pup0.5

− Pdown0.5 (Pa0.5)

(a)

0 50 100 150 200 2500.00

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0.10

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perm

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ols−1

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N2

sele

ctiv

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H2/N2 selectivity723K

673K623K

∞

Pup0.5

− Pdown0.5 (Pa0.5)

(b)

Figure 6: Hydrogen permeation flux and selectivity of membranes with (a) the purely dense structure and (b) dense/porous hybrid structureas a function of the pressure difference at various temperatures.

Table 1: Comparison of hydrogen permeability between the present membrane and other types of literature.

SupportMethod

Membrane Temp.(K)

Permeability(molmm−2 s−1 Pa−0.5)

Selectivity(H2/N2)

Ref.Material Material Thick.(𝜇m)

Al2O3 ELP∗ Pd 3.5 773 1.9𝐸 − 08 4300 [17]PSS ELP Pd 7.5 873 1.9𝐸 − 08 685 [18]PNS Sputtering PdCu 12 773 3.5𝐸 − 09 Infinity [13]

PNS Sputtering PdCu 10 723 4.6𝐸 − 09 InfinityThis work

(purely densestructure)

PNS Sputtering PdCu 10 723 9.9𝐸 − 09 InfinityThis work

(dense/poroushybrid structure)

∗ELP: electroless plating.


In our previous study [4], a long-term annealing testwas continued for 1500 h at a temperature of 823K and thecomposition of elements in themembrane remained constantduring the long-term annealing test.

It should be noted that the hydrogen permeation fluxand the ideal H

2/N2selectivity of the Pd alloy membranes

were simultaneously enhanced by introducing the uniquedense/porous hybrid structure. In this study, we success-fully fabricated Pd alloy hydrogen separation membraneswith dense/porous hybrid structure on modified Ni supportusing advanced sputtering deposition and Cu-reflow heattreatment for high hydrogen perm-selectivity. Therefore, itis expected that this type of membrane will be applicable toindustrial processes for hydrogen purification and a varietyof further applications, including hydrogen separation.

4. Conclusions

We successfully developed Pd alloy hydrogen separationmembrane with a dense/porous hybrid structure for highhydrogen perm-selectivity. The hydrogen selectivity of thismembrane was increased by the dense and pore-freemicrostructure at the surface. The hydrogen permeation fluxalso was significantly improved not only because of the openmicrostructure with numerous open voids near the interfacebut also because of the effective thickness reduction of thedense membrane as a result of the formation of the porousstructure within the Pd alloy films. This indicates that therequirements for simultaneous high hydrogen permeabilityand high hydrogen selectivity essential for commercial appli-cations are satisfied by our modified fabrication process.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgement

This research was supported by the Basic Science ResearchProgram through theNational Research Foundation of Korea(NRF) funded by the Ministry of Education and ScienceTechnology (NRF-2012R1A1A2007010).

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