June 1998

Ž .Materials Letters 35 1998 383–391

Nickel incorporation into a hollow fibre microporous polymer:a preparation route for novel high surface area nickel structures

S. Sotiropoulos ), I.J. Brown, G. Akay, E. LesterChemical Engineering Department, Nottingham UniÕersity, UniÕersity Park, Nottingham NG7 2RD, UK

Received 5 September 1997; accepted 4 December 1997

Abstract

Ž .The incorporation of Ni metal into PolyHIPE a generic hollow fibre polymer produced by emulsion polymerisation wasattempted by both electroless deposition and electrodeposition and the resulting composites were characterised by a

Ž . Ž .Scanning Electron Microscope SEM and Energy Dispersive Analysis by X-ray EDAX analysis. The first methodproduced a conducting outer surface coating which did not penetrate significantly into the interior of the polymer.Electroplating succeeded in growing Ni through the polymer cells and pores. Subsequent burning out of the polymer matrixresulted in a granular interconnected Ni structure with a specific surface area of 49.5 m2 gy1, suggesting that Ni coatingsproduced by this method could be an alternative to Raney-nickel coated electrodes. q 1998 Elsevier Science B.V. All rightsreserved.

Keywords: High surface area Ni; Microporous polymers; Raney-nickel electrocatalysts

1. Introduction

The incorporation of metals intoronto microp-Ž .orous pore size 0.1 to 100 mm or nanoporous

˚Ž .several A to 100 nm matrices is relevant to a widerange of applications. These include catalytic mem-

w xbrane reactors 1 , high surface area electrocatalystsw xused in fuel cells and batteries 2–4 and modified

Želectrodes as electroanalytical devices and for de-signing the microenvironment in which electron

w x.transfer occurs 5 . Furthermore, when the microp-orous material is used in separationrfiltration pro-cesses, its partial metallisation offers exciting oppor-tunities for process improvement by electrochemical

) Corresponding author. Tel.: q44-115-9514182; fax: q44-115-4181.

w x w xscrubbing 6 or electrofiltration 7 of the species tobe separated. Finally, if the matrix pores are com-pletely filled with metal and the former is easy to

Ž .burn off as is the case for many polymeric matrices ,w xthen a reticulated metal structure can be obtained 8

and used as a high surface area electrode, filteringmedium or flame arrestor.

Ž .PolyHIPE Polymer PHP is a microporous mate-w xrial developed at Unilever 9 and Los Alamos Na-

w xtional Laboratories 10 . It is produced through theŽ .formation of a high internal phase emulsion HIPE

of water-in-oil, in which the volume of the aqueousdispersed phase is greater than about 75%, and the

Ž .subsequent polymerisation at 608C of the oil phaseŽwhich contains the monomer styrene and occasion-

. Žally other monomers, too and the cross-linker di-.vinylbenzene . This results in the production of a

00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0167-577X 97 00283-8

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391384

Žpolymer matrix with an extremely high voidage up.to 97% due to the evaporation of the water droplets

which were present in the precursor emulsion. Thestructure of PHP is therefore characterised by the

Žpresence of numerous cells of a 1–100 mm diame-. Žter interconnected by smaller pores of a 0.1 to 10

.mm diameter .The size of both the cells and thepores can be controlled by changing the mixing time,

Ž .the surfactant emulsion stabiliser concentration andthe monomer composition during the emulsification.The monomer composition can also be modified sothat the polymer is brittle or elastomeric. From anelectrochemical point of view, important PHP fea-tures include the possibility to render it hydrophilic

Žandror ionic conducting by choosing a modified.monomer or by sulfonation as well as its transfor-

mation into a carbonised material.Nickel foams have been successfully used as

three-dimensional porous electrodes both for thew x Želectrolytic recovery of Cu 11,12 wastewater treat-

. w xment and electroorganic synthesis 13 . Raney-nickelŽcoatings of typical specific areas ranging from 20 to

2 y1.100 m g have been used as electrocatalysts onnickel electrodes which are employed as hydrogen

w xanodes in alkaline fuel cells 14 , hydrogen cathodesw xin alkaline water electrolysis 15 and electrochemi-

w xcal hydrogenation of organics 16,17 . The variousmethods for the production of composite coatingsŽ .having both a nano- and a micro-porosity are men-

w xtioned in Ref. 18 and include cold rolling, plasmaspraying, annealing and cathodic codeposition of the

Ž .Raney-nickel precursor alloys NirAl or NirZn ona nickel support. Smooth Raney-nickel coatingsŽ .mainly exhibiting nanoporosity have also been pro-duced by sherardizing of nickel substrates i.e., by thereaction of Ni with Zn vapours at 4008C whichresults in the formation of a smooth NirZn precursor

w xcoating 19 .This paper reports some preliminary results of Ni

plating ontorthrough an elastomeric form of PHPusing two different methods namely, electroless plat-

w xing or electroplating 20 onto or through the poly-mer matrix. The possibility of producing high sur-face area Ni structures by subsequently burning offthe polymer is also investigated and both the precur-sor composite NirPolyHIPE and resulting reticu-lated Ni structures were characterised by a Scanning

Ž .Electron Microscope SEM and Energy Dispersive

Ž .Analysis by X-ray EDAX analysis, while the sur-face area of the latter was estimated by BET analy-sis.

2. Experimental

2.1. Preparation of PHP

The relative volumes of oil and aqueous phasesmixed to produce the HIPE determine the pore vol-ume of the resulting polymer after the polymerisa-tion and water evaporation has been completed. Inthis work, polymer of a 90% voidage was producedand, in a typical production, 500 ml of emulsion wasprepared by mixing 450 ml of oil phase with 50 mlof water phase. The composition of the oil phase for

Žthe preparation of elastomeric PolyHIPE was by. Ž .volume : 15% Styrene Aldrich, 99% , 62% 2-ethyl-

Ž . Žhexyl-acrylate Aldrich , 8% Divinylbenzene Al-. Ždrich, 80% and 15% Sorbitan Monooeleate Al-.drich, 95% . The aqueous phase was made of dis-

tilled water and contained 1% wrw Potassium Per-Ž .sulphate Aldrich, 99%q which acted as the poly-

merisation initiator. The presence of 2-ethyl-hexyl-acrylate in the oil phase renders PolyHIPE flexibleand hydrophilic, properties which are required for itssuccessful electroless plating. The emulsion was pro-duced in a stainless steel, water-tight mixing vesselof an approximate volume of 700 ml, equipped with

Ž .a Janke and Kunkel IKA Labortechnik Model RWŽ20DZM two-blade mixer. The correct volume 50

.ml of oil mixture was introduced into the mixingvessel and the stirrer blades were adjusted so thattheir bottom was in contact with the oil phase; this isessential to ensure that the subsequent addition ofwater under stirring results in a water-in-oil emul-sion. The 450 ml of water phase was placed in abeaker and was added to the mixing chamber with aWatson–Marlow peristaltic pump over a period of10 min. The mixer was turned on at 300 rpm whenthe first drops of water entered the chamber. Afterthe addition of water, the mixing continued for afurther 10 min. During formation, the emulsion vis-cosity increases and so the mixer power had to beadjusted to maintain a constant speed-mixing rate.The emulsion was then poured into high densitypolyethylene beakers and covered tightly to min-

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391 385

imise water evaporation during polymerisation whichwas performed for 6 h in an oven heated at 608C.After polymerisation had been completed, the coverwas removed and the samples remained for another24 h in the oven for residual water to be driven offthe pores. The Sorbitan Monoleate which was thesurfactant used as an emulsion stabiliser, was re-moved by washing several times with boiling iso-propyl alcohol in a socket setup.

From the bulk quantities of polymer thus pro-duced, disk-shaped samples of a typical thickness of0.5 cm and areas ranging from 1 to 3 cm2 were cutand used as plating substrates.

2.2. Ni electroless plating

Electroless plating is a chemical process in whichmetal ions from solution react with a reducing agentalso present in solution and are deposited onto cat-

w xalytic sites of the substrate to be plated 20 . Non-conducting polymer substrates first have to be ren-dered hydrophilic and also sensitised by the deposi-

Ž .tion of catalytic metal nuclei usually Pd beforesuccessful deposition of the metal of interest cantake place. Elastomeric PolyHIPE is adequately hy-drophilic due to the presence of 2-ethyl-hexyl-acrylate in its structure and was sensitised by 30 min

w x y1immersion in a solution containing 20 2 g lŽ . Ž . y1Tin II Chloride Aldrich, 98% , 0.2 g l Palla-Ž . Ž . y1dium II Chloride Aldrich, 99% and 10 ml l

Hydrochloric Acid at 20–258C. The chemistry ofsensitisation–nucleation of Pd catalytic sites is de-scribed by the following reaction:

Snq2 qPdq2 ™Snq4 qPd0 1Ž .

The sensitised samples were then washed for 5min in 250 ml of solution containing 50 ml of

ŽFluoroboric Acid solution Aldrich, 48% vrv water. Ž .solution and 15 g Oxalic Acid Aldrich, 99% , in

order to remove any colloidal tin chloride formed.All the above pretreatments have been performed inan ultrasonic bath to assist the mass transport ofreagents into the polymer pores. A standard Nickel

w xChloride–Hypophosphite bath 20 was used for thefinal Ni electroless plating step: 37 g ly1 Nickel

Ž . y1Chloride Hexahydrate Aldrich , 10 g l SodiumŽ . y1Acetate Trihydrate Aldrich, 98% , 15 g l Sodium

Ž . y1Succinate Aldrich, 99% , 20 g l Sodium Hy-Ž .pophosphite Hydrate Aldrich , pH controlled at 4.5

with acetic acid. The overall deposition reaction,occurring at 938C, is described by:

2H POy q2H OqNiqq™Ni0 qH2 2 2 2

q4Hqq2HPOyy 2Ž .3

The actual mechanism involves the reaction ofhypophoshite with water to produce protons and

Ž .hydride ions Reaction 3 below ; the latter reduce Niions with simultaneous evolution of gaseous hydro-

Ž .gen Reaction 4 below ; a side reaction betweenprotons and hydride ions also leads to hydrogen

Ž .evolution Reaction 5 :

H POy qH O™2HqqHyqHPOyy 3Ž .2 2 2 3

2HyqNiqq™Ni0 qH 4Ž .2

HqqHy™H 5Ž .2

The evolution of hydrogen gas poses a severelimitation in substantial plating of the interior of themicroporous sample since the gas trapped in thepores after the deposition has been initiated prohibitsfurther deposition by blanketing–drying out the inte-rior of the polymer. Thus, Ni is preferentially platedat the outer surface, resulting in a further exclusionof the reactant solution from the inner pores. Tocircumvent this problem, the plating was carried outin three steps. First, the sample was soaked for 30min in a nickel-containing solution with no reducing

Ž .agent and then rapidly transferred to a hot 938Chypophosphite solution for 5 min. In this way, apreliminary preferential deposition into the poreswas achieved without significant deposition on theouter surface. Second, the temperature in the finalplating bath was gradually raised from 608 to 938Cwithin 30 min so that both the metal ion and thereducing agent had time to diffuse into the poresbefore the reaction commenced. Third, and mostimportant, during the process the elastomeric samplewas periodically pressed and depressed with a glassrod so that the gas trapped in its interior was released

Žand transport to the pores was improved the same.effect could be achieved by the use of vacuum .

Plating was carried out until the bath had been

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391386

exhausted as indicated by hydrogen ceasing to beproduced.

2.3. Ni electrodeposition

The composite AurPolyHIPE electrodes ontowhich Ni was electroplated were prepared as fol-lows. One face of the polymer sample was covered

Žwith a thin layer of Gold paste or liquid Agar.Scientific and left overnight to dry. Silver-loaded

Ž .conducting epoxyresin RS Components was usedto make the electrical contact between the Au layerand commercial insulated copper wires. The Au-

Žcovered face of the sample together with the ex-.posed Ag-loaded resin was meticulously insulated

Žwith a thick layer of Lacomit varnish Agar Scien-.tific . This ensured that Ni deposition occurred from

the uncovered face side through the polymer poresand onto the AurPolyHIPE interface. A standard

w xNickel Sulphamate bath 20 served as the electro-plating solution at 608C: 600 g ly1 Nickel Sulpha-

Ž . Ž .mate Aldrich, 98% , 10 g Nickel Chloride Aldrich ,y1 Ž .40 g l Boric Acid Aldrich, 99.5q% and 10 mg

y1 Ž .l Sodium Dodecylsulphate Aldrich, 98% . A NiŽ .mesh Goodfellow, 99% shaped as a semi-cylinder

served as the anode of the electrolytic cell, facing theuncovered face of the AurPolyHIPE cathode. A

Ž .regulated 7A-35V DC GW GPR-3060 LaboratoryPower Supply was used to carry out the electroplat-ing at a constant current density of 30 mA cmy2 .

2.4. Burnout of the polymer matrix

The burnout of small samples of the NirPoly-HIPE composites was carried out under pure oxygenin a Stanton–Redcroft Thermogravimetric AnalyserŽ .TGA which also provided the weight loss due topolymer combustion. The temperature was raisedfrom 25 to 5008C within 1 h, stayed there for anotherhour and then decreased again to 258C within 1 h.

2.5. SEM, EDAX and BET analysis

SEM and EDAX experiments were carried outwith a Hitachi S-570 SEM. Surface area determina-tion of the polymer and metal microporous substratesŽ .based on BET analysis was performed in a Mi-

cromeritics ASAP 2000 equipment using Nitrogenadsorption.

3. Results and discussion

3.1. Ni electroless plating

The outer surface of a PolyHIPE sample treated ina Ni electroless deposition bath, as described inSection 2, is shown in the photograph of Fig. 1A. Itcan be seen that the sample is covered by a macro-scopically uniform Ni coating which was also foundto be electronically conducting. However, as shown

Ž .Fig. 1. A Photograph of the outer surface of a PHP sampleŽ .coated with Ni, by means of electroless deposition. B Pho-

tographs of cross-sections of PolyHIPE samples treated in a Nielectroless-plating bath.

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391 387

Ž . Ž . Ž .Fig. 2. A SEM micrograph of a blank untreated PolyHIPE sample. B SEM micrograph with superimposed EDAX map for Ni of theouter surface of a PolyHIPE sample treated in a Ni electroless-plating bath.

in the photographs of Fig. 1B which present cross-sections of similarly treated samples, this Ni coatingis not uniform throughout the sample; the Ni contentfades out as one moves from the outer surface intothe core of the sample.

A more detailed picture of the Ni electroless-platedpolymer at a microscopic level was obtained bymeans of SEM. Fig. 2A shows the SEM micrographof PHP before plating and an open structure can

Table 1Ž .Ni content as estimated by EDAX of a PolyHIPE sample treated

in a Ni electroless-plating bath, at various distances from the outersurface

Ž .Distance from the outer surface mm % wrw Ni

10 6650 28.7

100 123000 2.5

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391388

Ž . Ž .Fig. 3. Cross-section a and top view b of a PolyHIPErNickelcomposite electrode formed by Ni electrodeposition through Poly-HIPE

Žreadily be seen. It consists of large cells 20–40 mm.diameter, average diameter of 25 mm corresponding

to the water droplets of the initial water-in-oil emul-Žsion, interconnected by smaller pores 2–10 mm

.diameter, average diameter of 5 mm which corre-spond to the contact areas of the initially present

w xwater droplets 10 . Fig. 2B shows the SEM micro-graph and the superimposed EDAX map for Ni ofthe outer surface of a plated sample. It can be seenthat the outer surface is partially coated with Ni but

Table 2Ni content of the PolyHIPErNi composite along a cross-sectionperpendicular to the AurPolyHIPE interface

Ž .Distance from the AurPolyHIPE interface mm % wrw Ni

0 53.56100 47.73200 47.95300 45.77500 36.94

1000 35.97

still retains a significant degree of porosity. IsolatedNi nuclei can also be noted, extending through un-covered cells into the interior of the polymer. Amore quantitative picture of Ni penetration into thepolymer matrix is given in Table 1 which presents

Ž .the Ni content as given by EDAX as a function ofdistance from a sample’s outer surface. It is obviousthat the Ni content falls rapidly as one moves intothe core of the sample and at a distance of 3 mm isalready as low as 2.5% wrw. Therefore, it seemsthat, in spite of the steps we adopted to improve the

Ž .efficiency of electroless plating see Section 2 , theŽ . Ž .evolution of hydrogen by reactions 4 and 5 forms

a blanket over the polymer pores prohibiting themass transport of reagents to the interior of thesamples and thus limiting the plating close to the

Fig. 4. SEM micrograph of a cross-section of a PolyHIPErNickel composite electrode, 500 mm away from the AurPolyHIPE interface.

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391 389

Ž .Fig. 5. SEM micrographs of the microporous Ni structure produced after the PHP had been burnt off at 5008C. Magnifications: A =200,Ž . Ž .B =1000 and C =4500.

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391390

outer surface. Nevertheless, the production of a con-ducting Ni coating on the polymer surface mightprove useful in the preparation of a NirPolyHIPE

Ž .substrate instead of a AurPolyHIPE one onto whichNi could be electroplated as described in Section 3.2.

3.2. Ni electrodeposition

The photograph of Fig. 3 shows NirPolyHIPEcomposite structures produced by electrodepositionof Ni through the pores of PolyHIPE and onto theAurPolyHIPE electrode prepared as described in

Ž .Section 2. Image a is a cross-section of an electro-plated sample where Ni can be seen to have grownaround the middle of the sample and from the right-

Ž .hand side LacomiterAurPolyHIPE side towardsŽ .the bare PolyHIPE left-hand side. Image b is the

top view of another electroplated sample where pro-longed electroplating resulted in two Ni globuleshaving grown out of the polymer matrix. The unevengrowth of relatively thick metal deposits producedby electroplating through porous materials is a com-mon feature of this method for reticulated metalproduction and it has recently been modelled interms of uneven current distribution in these resistive

w xporous electrodes 8 .Fig. 4 shows the SEM micrograph of a composite

NirPolyHIPE structure produced by electroplating,500 mm away from the AurPolyHIPE electrodesurface. A closed, filled-in, structure can readily beseen containing 36.94% wrw Ni as given by EDAX.Table 2 presents the electroplated Ni content as afunction of the distance from the AurPolyHIPEinterface. It can be seen that, at least for a thicknessof up to 1 mm, the Ni content does not changedrastically, which is to be expected since electro-plated Ni is anticipated to occupy-grow through thesubstrate free volume and the latter is uniformthroughout the polymer matrix.

3.3. Burnout of the polymer matrix

The PolyHIPE component of a small part of aNirPolyHIPE composite material prepared by elec-troplating, was burnt off at 5008C as described in theSection 2. A 44.1% weight loss was found, which isin reasonable agreement with a 53–36% wrw Ni

Ž .content see Table 2 if one takes into account the

presence of the Au and Ag layers as well as that ofresidual salts from the electroplating bath. The SEMmicrographs of Fig. 5 show the resulting Ni structureat different magnifications. Fig. 5A clearly shows agranular structure with some honeycomb featureswhich exhibits less regularity than the matrix struc-ture shown in Fig. 2A. This is due to uneven currentdistribution in the pores during electroplating, as

w xdescribed in Ref. 8 . Fig. 5B takes a closer look atthe structure, highlighting its non-uniformity but alsothe matrix–metal deposit relationship observed lo-cally. That is, at the right-hand side of the micro-graph, one can clearly see Ni globules of a 10–20

Žmm diameter mean cell diameter of the PolyHIPE.matrix is 25 mm interconnected by smaller granules

Žof a 1–5 mm diameter mean pore diameter of the.PolyHIPE matrix is 5 mm . It seems, therefore, that

at least locally, the metal structure follows that of thevoid of the polymer matrix. Finally, the micrographof Fig. 5C shows that the Ni structure thus producedhas pores as small as 1 mm in diameter; these porescould correspond to the thickness of cell walls of the

Ž w x .PolyHIPE matrix see Ref. 10 and Fig. 2A .The BET analysis of the Ni structure described

above gave a value of 49.5 m2 gy1 for its specificsurface area. This surprisingly high surface areacould be due to uneven current distribution resultinglocally in high surface area deposits andror to highroughness of the deposited Ni. This value is compa-rable with that of Raney-nickel coatings used inalkaline fuel cells and in alkaline water electrolysisw x18 and implies an interesting potential applicationof the method described here for the production ofhigh surface area Ni.

4. Conclusion and future work

It was possible to electroplate Ni through the cellsand pores of PolyHIPE, a generic high surface areahollow fibre polymer matrix, and produce aNirPolyHIPE composite material. Subsequent burn-ing out of the polymer matrix resulted in a granularinterconnected Ni structure of a high specific surface

Ž 2 y1.area 49.5 m g , in the range of those of Raney-nickel coatings used as electrocatalysts.

Future work will focus on using this polymerŽmatrix method for the production of thin a few tens

( )S. Sotiropoulos et al.rMaterials Letters 35 1998 383–391 391

.of microns Ni coatings on Ni substrates. These aregoing to be characterised electrochemically both interms of their electrochemically active surface areaand their catalytic activity for hydrogen evolution.

Acknowledgements

The authors wish to thank EPSRC for a stu-dentship to I.J.B and the University of Nottinghamfor a New Lecturers’ Research Grant to S.S.

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