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Current Applied Physics 11 (2011) 790e793

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

The effect of bath conditions on the electroless nickel plating on the porouscarbon substrate

So-Young Cheon a, So-Yeon Park a, Young-Mok Rhymb, Doo-Hyun Kimb, Jae-Ho Lee a,*

aDept. of Materials Science and Engineering, Hongik University, 72-1 Sangsu-dong, Mapo-gu, Seoul 121-791, Republic of KoreabDiv. of Indus. Tech. Support, Korea Institute of Materials Science, 66 Sangnam-dong, Changwon, Gyeongnam 641-010, Republic of Korea

a r t i c l e i n f o

Article history:Received 7 September 2010Received in revised form20 November 2010Accepted 25 November 2010Available online 29 December 2010

Keywords:Electroless nickel platingPorous carbonPalladium chloride

* Corresponding author. Tel.: þ82 2 320 1483; fax:E-mail address: jhlee@hongik.ac.kr (J.-H. Lee).

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2010.11.076

a b s t r a c t

Electroless nickel plating is widely used technique in industries. In most cases, electroless nickel platingwas applied on the open surfaces and the rate of deposition was controlled with temperature and time.However, when the electroless plating is applied on the porous carbon, the rate of deposition is alsodependent on the activation process. In this research, electroless nickel plating on the porous carbon wasinvestigated. The porous carbon was selected as the substrate. The pore sizes of carbon substrates were16e20 mm and over 20 mm. Since hydrophobic surface prevented the penetration of solution into porouscarbon, the carbon surface changed from hydrophobic to hydrophilic after immersing the substrate in anammonia solution at 60 �C. The alkaline bath and acidic bath were used in electroless nickel plating. ThepHs were 9e11 in alkaline bath and 4e5 in acidic bath. The content of phosphorous in nickel deposit washigher in acidic bath than that in alkaline bath. As increasing pH in each bath conditions, the content ofphosphorous in nickel deposit was decreased. The rate of electroless plating in alkaline bath was fasterthan that in acidic bath. The minimum concentration of PdCl2 for the electroless nickel plating was10 ppm in acidic bath and 5 ppm in alkaline bath. The thickness of nickel was not significantly affected bythe concentration PdCl2.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Since fuel cells (FCs) have high theoretical thermal efficiencies,FC research and development has been aimed at commercial goalssince the discovery of the hydrogen FC concept by Grove in 1839[1]. A FC is an electrochemical cell that converts a source fuel intoan electrical current. Molten carbonate fuel cell (MCFC) is one of thehigh temperature fuel cells. One of themerits of MCFC is that a goodreaction rate is achieved by using a comparatively inexpensivecatalyst such as nickel. The nickel also forms the electrical basis ofthe electrode. It can use gases such asmethane and coal gas (H2 andCH4) directly, without an external reformer. MCFCs have short lifetime and low efficiency due to non uniform Ni3Al matrix of elec-trode. To solve this problem, electroless nickel plating on theporous carbonwas investigated. The porous electrode requires highsurface area for reaction and catalytic surface [2,3]. Instead ofelectrical power, reducing agents are used in electroless plating.And then uniform electroless plating is possible on the surface ofsubstrate with complicated shape. Electroless nickel plating is

þ82 2 333 0127.

All rights reserved.

widely used technique in industries. Electroless nickel plating is ofspecial interest due to its advantages such as uniform deposition,good corrosion and wear resistance, good electrical and thermalconductivity, and good solderability [4,5]. In this research, elec-troless nickel plating on porous carbon substrate was investigatedfor uniform deposition in acidic bath and alkaline bath.

2. Experimental procedures

Two different types of porous carbon substrates were used.Diameter of pore is 16e20 mm and over 20 mm. Fig. 1 shows thefractured surface of bare carbon. The ammonia solutionwas used topromote the wettability of the carbon substrate. The surfacesensitization was conducted by immersion of substrate in 10 g/LSnCl2 solution for 5 min followed by immersion in PdCl2 solutionfor the activation. After activation process, the samples werecleaned by distilled water. The compositions of electroless nickelplating baths were given in Table 1. The electroless nickel platingbath was composed of nickel salt, reducing agent and suitablecomplex agents. Nickel sulfate was used as nickel source. Thecomplex agents were used to buffer the change of pH. The hypo-phosphite was used as reducing agent to both acid and alkalinebath. The pHwas controlled 4e5 in acidic bath and 9e11 in alkaline

60

80

( n

m/m

in

)

a

Fig. 1. Bare carbon substrate (a) porosity �20 mm (b) porosity 16e20 mm.

0 20 40 60

20

40

60

80

Wettin

g an

lg

e

NH4OH pretreatment time (min.)

after 0hourafter 3hours

Fig. 2. Wetting angle by NH4OH pretreatment time (Immediately after the pretreat-ment and after 3 h of the pretreatment).

S.-Y. Cheon et al. / Current Applied Physics 11 (2011) 790e793 791

bath using NaOH and NH4OH. The temperaturewas fixed at 65 �C inacidic bath and at 60 �C in alkaline bath. Energy Dispersive X-raySpectrometer (EDS) and Field Emission Scanning Electron Micros-copy (FESEM) were used for analysis of nickel deposit.

4.0 4.2 4.4 4.6 4.8 5.020

40

dep

ositio

n rate

200

in

)

pHb

3. Results and discussion

The carbon substrate is hydrophobic and then it is required tochange the surface from hydrophobic to hydrophilic. The carbonsubstrates were immersed in ammonia solution at 60 �C to intro-duce NH� on carbon surface. The change of wetting angle withNH4OH pretreatment time was shown Fig. 2. The carbon substrateshowed strong hydrophobic surface before immersing ammoniasolution since the wetting angle is 85�, however the carbonsubstrate was changed to hydrophilic surface after immersing

Table 1Compositions of electroless nickel bath (a) acidic bath (b) alkaline bath.

Chemical Concentration (g/L)

(a)Nickel sulfate 21Sodium hypophosphite 24Lactic acid 28Propionic acid 2.2(b)Nickel sulfate 25Sodium citrate 50Sodium hypophosphite 25

ammonia solution for over 40min. Thewetting angle of carbonwasdecreased from 85� to 15� when substrate was immersed inammonia solution for 60 min. The decreased wetting angle grad-ually was increased since NH� on carbon surface disappeared [6].

In the hypophosphite-based electroless nickel acidic and alka-line bath, the overall reactions can be written as respectively [7]:

Ni2þ-complex þ 2H2PO2� þ 2H2O / Ni þ 2H2PO3

� þ 2Hþ

þ H2 þ complex (1)

Ni2þ-complex þ 2H2PO2� þ 4OH� / Ni þ 2HPO3

2�

þ 2H2O þ H2 þ complex (2)

9.0 9.5 10.0 10.5 11.050

100

150

dep

ositio

n rate ( n

m/m

pH

Fig. 3. Effects of pH on the deposition rate (a) acidic bath (b) alkaline bath.

Fig. 5. Cross section of NiP on porous carbon in the acidic bath (a) over 20 m

4.0 4.2 4.4 4.6 4.8 5.03

4

5

6

7

8

9

10P

( at.%

)

a

b

9.0 9.5 10.0 10.5 11.03

4

5

6

7

8

9

10

P( w

t.%

)

pH

pH

Fig. 4. Effects of pH on the P content (a) acidic bath (b) alkaline bath.

S.-Y. Cheon et al. / Current Applied Physics 11 (2011) 790e793792

The reaction rates were influenced by the pH in both reactions.The deposition rates of acidic bath and alkaline bath weremeasured and compared in Fig. 3. The results indicate that rate ofelectroless plating in alkaline bath was faster than that in acidicbath. As increasing pH in each bath conditions, the deposit ratewas increased. It can be predicted from the above equations. As pHincreased the reactions forced to the forward by Le Chatelier’sprinciple. Besides nickel deposition and hydrogen evolution,phosphorous was also deposited with nickel as secondary reac-tion [7].

H2PO2� þ H / H2O þ OH� þ P (3)

The contents of phosphorous in nickel deposit of acidic andalkaline bath were compared in Fig. 4. The content of phosphorousin nickel deposit was higher in acidic bath than that in alkalinebath. At pH 11, the content of phosphorous in nickel deposit wasbelow 4%. And at pH 5, phosphorous content in nickel deposit wasabout 5.5%. The pH dependency of phosphorous contents can bealso explained by the Le Chatelier’s principle in equation (3).

The nickel covered surfaces were shown in Fig. 5. The surface ofthe carbon substrate was completely covered, and the deposit ofnickel was uniform and showed strong adhesion through electro-less nickel plating. Since the rate of deposition was slow in acidicbath, the pore of carbon was maintained for longer time to giveuniform coating inside of pores. And the slow deposition rate wasadvantage to small size of pore.

When the electroless plating is applied on the porous carbonsubstrate, the thickness of nickel deposit is dependent on theactivation process prior to the electroless plating. The low Pdconcentration in activation process has advantage economically.The minimum concentration of Pd was investigated and the resultswere shown in Fig. 6. The minimum concentration of PdCl2 for theelectroless nickel plating was 10 ppm in acidic bath and 5 ppm in

m (b) 16e20 mm, and in the alkaline bath (c) over 20 mm (d) 16e20 mm.

Fig. 6. Effects of Pd concentration for electroless Ni plating in the acidic bath (a) 5 ppm (b) 10 ppm, and the alkaline bath (c) 0 ppm (d) 5 ppm.

S.-Y. Cheon et al. / Current Applied Physics 11 (2011) 790e793 793

alkaline bath. The thickness of nickel was not significantly affectedby the concentration PdCl2.

4. Conclusion

In this research, electroless nickel plating on the porous carbonwas investigated. The carbon surface changed from hydrophobicto hydrophilic as immersing the substrate in an ammonia solution.The nature of hydrophilic surface was maintained at least 3 h afterammonia solution treatment. The rate of electroless plating inalkaline bath is 4 times higher than that in acidic bath. The contentof phosphorous in nickel deposit was higher in acidic bath thanthat in alkaline bath. The minimum concentration of PdCl2 toactivate the substrate was 10 ppm in acidic bath and 5 ppm inalkaline bath.

Acknowledgment

This work was supported by Korea Materials & ComponentsIndustry Agency and also supported by 2010 Hongik Research Fund.

References

[1] L. Zhou, H. Lin, B. Yi, H. Zhang, Chem. Eng. J. 125 (2007) 187e192.[2] D. Lee, I. Lee, S. Chang, Electrochim. Acta 50 (2004) 755e759.[3] J. Larminie, A. Dicks, Fuel Cell Systems Explained, second ed., Willey, England,

2003, pp. 187e201.[4] M. Palaniappa, S.K. Seshadri, Mater. Sci. Eng. 460e461 (2007) 638e644.[5] J.H. Lee, I.G. Lee, T. Kang, N.S. Kim, S.Y. Oh, J. Microelectron. Packag. Soc., 26

(2003) 118e121.[6] K.R. Kull, M.L. Steen, E.R. Fisher, J. Memb. Sci. 246 (2005) 203e215.[7] H. Lee, Surface Engineering. Hyungseul Publishing, Korea, 1999, pp. 147e148.