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1 Carbon Science Vol. 6, No. 3 September 2005 pp Preparation of Bipolar Plate for Fuel Cell Using CNT/Graphite Nano-Composite Jongmin Choi, Taejin Kim, Minsoo Hyun, Donghyun Peck, Sangkyung Kim, Byungrok Lee, Jongsoo Park* and Doohwan Jung Korea Institute of Energy Research, Daejeon , Korea *Seung Lim Carbon Metal Co., Ltd Wonsi-dong, Danwon-Gu, Ansan, Gyeonggi-do, Korea Ì doohwan@kier.re.kr (Received April 25, 2005; Accepted September 16, 2005) Abstract Bipolar plates require some specific properties such as electrical conductivity, mechanical strength, chemical stability, and low permeability for the fuel cell application. This study investigated the effects of carbon nanotube (CNT) contents and process conditions of hot press molding on the electrical and physical properties using CNT 3~7 wt% added graphite nano-composites in the curing temperatures range of 140~200 C and pressure of 200~300 kg/cm 2. Bulk density, hardness and flexural strength increased with increasing CNT contents, curing pressure and temperature. With the 7 wt% CNT added noncomposite, the electrical resistance improved by 30% and the flexural strength increased by 25% as compared to that without CNT at the temperature of 160 C and pressure of 300 kg/cm 2. These properties were close to the DOE reference criteria as bulk resistance of 13 mωcm and tensile strength of 515 kg/cm 2. Keywords : DMFC, Bipolar plate, CNT, Fuel cell, Nanocomposite 1. l rv l v vr r l v r p rp r rp q p [1]. q r v ql rv p r m p q m, l p rp qrp p r np p rol p ep p p [2]. rv p p nafion q r vp pl kyp k p p, carbon paper, carbon plate, p p (bipolar plate) lv (Fig. 1). p p l rv l l p orv pp d p v r r l l p p q dp r, r vr~ p [3-6]. l p orv l l dˆp e qtp p p p q ql rv r~ dˆ rs np k 60% p p v l [7] rso pp n tn. l p p r, r r l pl l r plm. l ~p rso p p dˆ k 20% t[8]p r rp k, r r, r pnp r kr l p qrp v p [9]. l l ˆm l rvn l p p p r r r p o r r s rp o l lp, l s l CNT(carbon nanotube) r vr 3~7 wt% ~ l ~ m k l, r r r p m. 2. Fig. 1. Schematic view of the single cell assembly

182 J. M. Choi et al. / Carbon Science Vol. 6, No. 3 (2005) 181-187 Table 1. Specification of TIMREX KS 6 graphite Item Content Item Content Ash 0.1% max Crystallite height 60 nm Typical size (D50) 3.

2 182 J. M. Choi et al. / Carbon Science Vol. 6, No. 3 (2005) Table 1. Specification of TIMREX KS 6 graphite Item Content Item Content Ash 0.1% max Crystallite height 60 nm Typical size (D50) 3.4 μm Interlayer distance BET 20 m 2 /g nm Table 2. Specification of epoxy matrix Flexural strength Tg (with hardner) Melting point Absorptivity (85 o C, 85%RH, 100 hr) 162 MPa 113 o C 75~85 o C 0.16 < Table 3. Mixing ratio of bipolar plate composite Item Compositi on (wt%) Graphite Carbon nanotube Epoxy Hardner Sum 73~77 3~ e l n l(graphite)p Timcal p p 3.4 μm, r 20 m 2 /gp TIMREX KS6 n mp p Table 1. ~p r r p o l n multi-wall carbon nanotube(mwcnt)[10] v 20 nm, p 10~50 μm op ILJIN Nanotech p r p pn m. l l n p (matrix) l l e v rr r d F p n mp p Table 2m. l e v pr p rm vr triphenyl phoshine ~ m. ~p e rr l p qp pv v o l r vr ~ m. p p p s p Table 3 p, ~p r p o l lp 73~77 wt%, r r p o MWCNT ~7 wt%, l e 13.8 wt%, r 6.2 wt% pnl vr rrlr ~ m. Fig. 2. Experimental process for bipolar plate. 150 rpm, 24e m. d p s l 80 o C, 24e s, r vr v d p l e attrition milll 150 rpm, 12e k Ž e l p l. e e l m e p l r ~ 2 mm s rp o pr l m (dm 2 o C/min) 140~200 o C, k p 200~300 kgf/cm 2 l 20 ov l p p ~ e p r q m r rs p p ~ e rqp pk r pn, 5.0 cm(l)ë2.5 cm(w)ë0.2 cm(t) r l e, t 5 j r l, (Kyeongdo, 20ton), l (SATO SEIKI / Model D), r r (milliohm meter HP 4338A) ~rr (bulk resistivity) p r mp, r p m. o ~ r t p m t r r p e 1/100 mm, 1/1000 g v r l e (1) m. (g/cm 3 ) = M( g) (1) T( cm) W( cm) L( cm) x p p rs p p p rs rp Fig. 2m p, Table 3p l attrition millp pn l d e p m. p lˆml l e, r vr n l lˆml eˆ CNT ~ l Fig. 3. Test of three point flexural strength.

e p rqp rq t [11]l o ~p e p l e (2)l m. (kgf/cm 2 3 P( kgf) L( cm) ) = ------------------------------------------------ (2) 2 W( cm) t 2 ( cm) e p rr (interfacial contact resistance) rp Fig.

3 Preparation of Bipolar Plate for Fuel Cell Using CNT/Graphite Nano-Composite 183 ~rr (mωcm) = (4) 3. y» p T( cm) W( cm) R( mω) L( cm) Fig. 4. Schematic of the test assembly for interfacial contact resistance. rp k e (KYEONGDO/ ) n l 3rr š(three point bending)p r mp, e p Fig. 3. p l vr p m e p k 16:1 r. e p rqp rq t [11]l o ~p e p l e (2)l m. (kgf/cm 2 3 P( kgf) L( cm) ) = (2) 2 W( cm) t 2 ( cm) e p rr (interfacial contact resistance) rp Fig. 4l ˆ m p ee m. p p e p p r l p r p 4rr r ep Davies' [12]l p milliohm meter (HP 4338A) ky Žl r (1A) tl r~ rk (V) r m. r q p Ž p rl mp Ž e pl p Toray p p l r vv [13] n l Žl k p r rp v eˆ l p rr p e (3) m. Fig. 6p 300 kgf/cm 2 p k l m 140, 160, 180, 200 o C e p n, ~ CNTp s l l t p. l m p CNTp p m p ~p ˆ p. Fig. 7p Fig. 6 p s l rs ~ Fig. 6. Density of bipolar plate with different CNT content and with different molding temperature at curing condition of 300 kgf/cm 2. T( cm) W( cm) V( V) rr (mωcm 2 ) = (3) I( A) rs p p p ~r r p Fig. 5m p p r mp Micro OHM Meter(SOKEN / DAC-MR- IS) pn l r p r l e (4) m. Fig. 5. Schematic of the test assembly for bulk resistance. Fig. 7. Shore of bipolar plate with different CNT content and with different molding temperature at curing condition of 300 kgf/cm 2.

4 184 J. M. Choi et al. / Carbon Science Vol. 6, No. 3 (2005) p l ˆ p l m p m CNTp p p p l p v m. Fig. 6l lt v CNTp v l t v m m p v l vp v v p kv. Fig. 7l ˆ l p v CNTp p v, CNTp r nm p CNT μm p lpq pl matrix l p l v p. H.C. Kuan[14]p ~ v p r p k r kr, r r rp l m p p pp, l l p p l t ppp p pl. Fig. 8(a) k 200 kgf/cm l m 2 140, 160, C eˆ CNTp ~ p o 3~7 w% r rp e p n k CNT ~ l p lt p. m p n vp o p v l l pq, v CNT p p l v p q p lr pq p p p. CNTp p v v p pp, m 200 o C, k 200 kgf/cm l rq p p e p CNT 2 3 wt% ~ n 430 kgf/cm p lt 2 pp, 7 wt%p CNT ~ n 525 kgf/cm v m 2. p CNTp p v CNTp r p q p p l ppp p. v p vm o p p k e r eˆ p k r p [15]. Fig. 8(b) p CNTp m s l k p 300 kgf/cm v eˆ np l t 2 p. 200 kgf/cm p k l m p p p l t p 2 p p v p ˆ l. p k p v CNTm v p p v l s CNT ~ ~ rs p p. v k p, vp o p m l l pq pl v p q p lr pq v m r p p k p.» p p p ~ e carbon paperp pl r r p n p k (loading pressure)l r r m. Fig. 9 k 200 kgf/cm 2, CNT p 7 wt%p m 160, Ë n p k l rr ˆ pp, m 160 o Cp n q p Fig. 8. Flexural strength of composites according to CNT content and curing condition. (a) Curing pressure at 200 kgf/cm 2. (b) Curing pressure at 300 kgf/cm 2 Fig. 9. Interfacial contact resistivity of bipolar plate with 7 wt% CNT contents and with different molding temperature at curing condition of 200 kgf/cm 2

5 Preparation of Bipolar Plate for Fuel Cell Using CNT/Graphite Nano-Composite 185 Fig. 10. Interfacial contact resistivity of bipolar plate with 7 wt% CNT content and with different molding pressure at 160 o C. r p ˆ l. p m l rs ~ p k p p nl p rr p ˆ pp, 50 kgf/cm p l rr p tl 2 50 kgf/cm p l rr p 2 m m. p rs ~ l rvn p p n r r p ˆ o orv e 50 kgf/ cm p p dˆ ~ k p n p 2 p., l m p m p p rr p lt p p l vp l e v r rp rl p, ml l CNT pq p p pl pq l rl vp v Žp v l r p v p lv p. Fig. 10p CNT 7 wt%, m 160 o C s l, k kgf/cm p n p k l rr 2 p lt p. 300 kgf/cm l k e p kgf/cm e l r p q 2 ˆ. p p m l k p p lpqp p v vp Œ vp v p ~p r p p lv. Fig. 11p k 300 kgf/cm 2, k m 160Ël CNT p 0~7 w% v eˆ rs ~p p k l rr p l t p. l l t m p CNTp p v rr p p ˆ p, p CNTp p r r l p kv. Fig. 12 m l CNT ~rr p ˆ p. l ˆ m p CNT v l ~rr Fig. 9p rr o ˆ p. m 160 Cl o CNT v l q p p ˆ l. e l k US DOE p 10 mωcm[16]l v v pl Fig. 11. Interfacial contact resistivity of bipolar plate with different CNT content at curing conditions of 160 o C and 300 kgf/ cm 2. Fig. 12. Bulk resistivity of bipolar plate with different CNT content at conditions of curing 160 o C and 300 kgf/cm 2. r ~rr (13 mωcm)p lp pl. k m 140 o C er l e m 160~180 o C p, r p v po l s v p v p l n, ~p t p p p lv. m 180 o C p l ˆ r v v p v p n l pq l v Žp p lr r o v, r r l rl p v l qn p ˆ. e s p CNT ~ o l ~p r r rss p k 300 kgf/cm 2, m 160 o C s pl.

$186 J. M. Choi et al. / Carbon Science Vol. 6, No. 3 (2005) 181-187 Fig. 13. SEM image of the fractured surface bipolar plate. Fig. 13p CNT ~ ˆ ~p SEM p. Fig. 13(a) rrlr ~ v kp np, Fig.$

6 186 J. M. Choi et al. / Carbon Science Vol. 6, No. 3 (2005) Fig. 13. SEM image of the fractured surface bipolar plate. Fig. 13p CNT ~ ˆ ~p SEM p. Fig. 13(a) rrlr ~ v kp np, Fig. 13(b) rrlr ~ np. r rlr ~ v kp n p CNTp p p. r rlr ~ n Ž lpql CNT v ˆ l t p. k 20 nm p CNT l pqm v pl l p ˆ l t p. prp ~p r r v l rrp l p p m p. lpqm CNT v l q p rr r r l e pv p, rrlrp ~ p v p rp v l v l np t, r r n l rvn p p ~ rs np t p r l. ps l CNT ~ 25% ~r r 13 mωcm p p ~ rq pl. 4. l l ˆm l rvn l p p ~p r r r p o l, 3~7 wt% ~ m. k 200~300 kgf/cm 2, m 140~200 o C ol k l ~ p p p p p m. 1) CNT p v, m k p v v l. 2) r r r r k s p k 300 kgf/cm 2, m 160 o C m 3) rs ~p ~rr p 13 mωcm DOE tl r mp, CNT 7 wt% ~ e r ~ 25% l References [1] Steele, B. C. H.; Heinzel, A. Nature 2001, 345, 414. [2] Srinvasan, S. J. of The Electrochemical Society 1989, 41, 136. [3] Heinzel, A.; Mahlendorf, F.; Niemzig, O.; Kreuz, C. J. of Power Sources 2004, 131, [4] Stewart, Jr.; Robert, C. 1987, US Patent 4,670,300. [5] Uemura, T.; Murakami, S. 1988, US Patent 4,737,421. [6] Busick, D.; Wilson, M. 2001, US Patent 6,248,467. [7] Davies, D. P.; Adcock, P. L.; Turpin, M.; Rowen, S. J. J. of Applied Electrochem 2000, 30, 101.

7 Preparation of Bipolar Plate for Fuel Cell Using CNT/Graphite Nano-Composite 187 [8] Kirchain, I. B-O. R.; Roth, R. J. of Power Sources 2002, 109, 71. [9] Scholta, J.; Rohland, B.; Trapp, V.; Focken, U. J. Power Sources 1999, 84, 231. [10] Tanaka, K.; Yamabe, T.; Fukui, K. The Science and Technology of Carbon Natubes, Vol. 1, ed. ELSEVIER SCI- ENCE Ltd, Oxford, 1997, 2. [11] nlr k e, l KS L 1601, l t, [12] Davies, D. P.; Adcock, P. L.; Turpin, M.; Rowen, S. J. J. of Applied Electrochem 2000, 30, 102. [13] Hentall, P. L.; Lakeman, J. B.; Mepsted, G. O.; Moore, J. M. J. Power Sources 1999, 80, 235. [14] Kuan, H.-C. et al. J. of Power Source 2001, 134, 14. [15] Zafar Iqbal. et al. 2003, US Patent 6,572,997. [16] David, H. et al. Hydrogen, Fuel Cells, and Infrastructure Technologies, FY 2003 Progress Report, 2003, IV-197. [17] David, H. et al. Hydrogen, Fuel Cells, and Infrastructure Technologies, FY 2003 Progress Report, 2003, IV- 200.

국706.fm

국706.fm Carbon Science Vol. 7, No. 4 December 2006 pp. 271-276 Effect of Heating Rate and Pressure on Pore Growth of Porous Carbon Materials Kwang Youn Cho, Kyong Ja Kim and Doh Hyung Riu Division of Nano Materials