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1 Carbon Letters Vol. 8, No. 4 December 2007 pp The Characterization of the Resin Bonded Graphite Composite Bipolar Plate using Isotropic Graphite Powder for PEM Fuel Cell Kwang Youn Cho 1,, Doh Hyung Riu 1, Seung Hun Hui 1, Hong Suk Kim 2, Yoon Jung Chung 3 and Yun Soo Lim 3 1 Division of Nano Materials Application, KICET, Kumchen-gu, Seoul , Korea 2 LG Micron Ltd, LG Material & Parts R&D Center, 1271, Sangrog-gu, Ansan-city, Gyeonggi-do, , Korea 3 Department of New Materials Engineering, Myongji Univ. Cheoin-gu, Yongin-city, Kyunggido , Korea (Received October 10, 2007; Accepted December 5, 2007) Abstract In this study, graphite composites were fabricated by warm press molding method to realize commercialization of PEM fuel cells. Graphite composites have been considered as alternative economic materials for bipolar plate of PEM fuel cells. Graphite powder that enables to provide electrical conductivity was selected as the main substance. The graphite powder was mixed with phenolic resin and the mixture was pressed using a warm press method. First of all, the graphite powder was pulverized with a ball mill for the dense packing of composite. As the ball milling time increases, the average size of particles decreases and the size distribution becomes narrow. This allows for improvement of the uniformity of graphite composite. However, the surface electrical resistivity of graphite composite increases as the ball milling time increases. It is due to that graphite particles with amorphous phase are generated on the surface due to the friction and collision of particles during pulverizing. We found that the contact electrical resistivity of graphite particles increases as the particle size decreases. The contact electrical resistivity of graphite powders was reduced due to high molding pressure by warm press molding. This leads to improvement of the mechanical properties of graphite composite. Hydrogen gas impermeability was measured with the graphite composite, showing a possibility of the application for bipolar plate in fuel cell. And, I-V curves of the graphite composite bipolar plate exhibit a similar performance to the graphite bipolar plate. Key words : Isotropic graphite powder, Phenolic resin, Fuel cell, Bipolar plate, Electrical conductivity rp l p o p ~l vop l rrp rp lr p. rp ~l vop l rv tp l pr tl ˆ n l p m. qr v l rv ˆp l rvl q m, r p, e e p wp el l p p p p. v q r v l rvp n q kw } p dˆ p l np pl l rvp n l r p p l p. q l rv dˆp p Žp r~ dˆ p 52% r q p tp v. qr v l rv dˆp p o Žp p p e n. q pn Žp lp rq, e p l p l pl rq np p n p. lp ~ r p Ž q p e. Ž q pn o n p p nl n, n r m p r r, p m p Œ p, rp kr p p. p k n p seˆ q q d p d p ot Ž l q rs v l q Ž p Š p. d p d p n, r, qp r r p o n v, el k l p l r r r rp p [1, 2]. v l q Žp vr p l p r r, rp kr,, r k l l rvn Žp r p ˆ, el p r k p l k [3]. v l q Ž l Besmann p ˆ o q Žp rs l lp ~ p Ž q p p p Showa Denko l r r p n ps l

2 The Characterization of the Resin Bonded Graphite Composite Bipolar Plate using Isotropic Graphite Powder for PEM Fuel Cell 327 Table 2. Properties of phenolic resin(resol Type) Items Unit Property Remark Hardening Time min Non Volatile Contents % Specific Gravity o C Dilution % 2000 Me-OH Fig. 1. Thermogravimetric curve of phenolic resin as function of temperature in air, measured with a heating rate of 10 o C/min. p n l l rv Žp r r,, r p p m [4]. L. L. Shaw CNT n l ql rvn Žp rs l p eˆ k p l m [5, 6]. l l rp r p l p pn l rp kr Phenolic resinp q n l v l q rs m. Ball millingp pq rl v l qp r r r p m. Ball milling r p lpqp m ˆl m p l v l ql lpql p r r m., k rp r l PEM fuel cell bipolar platen v l q rs m. v l q bipolar plate e r q e p o o v l l k (Warm press molding)p e p rs l Œ, r r, I-V ee l sl n TOYO TANSO l p rs lq bipolar platem l PEM fuel celln bipolar plate p rn p m. x w w l l e r r n p TOYO TANSO p l G348p r vr n m. p rp p s r n p sˆp v KC 4703 n m. l G348 v KC 4703p p Table 1, 2l ˆ l. l p p l r, r r o 72e Ball millingp ee m. vm l p p np v ˆ m 10 : 90p l n m. l vp el l p s j Œl srp vm l p t 40 : 60, 30 : 70, 20 : 80, 10 : 90 m. p qp v l pq pl p Hot-plate o l 50 ~ 100 rpmp 12e p m. rp ˆmp r o 80 o C l e 12e p e ˆmp mr r m. Fig. 1 p vp l t p 90 o C k m p l p t m 350 o C p t pl. 90 ~ 200 o C v pp v p qp v 3 orp p l r p m p. 200 ~ 350 o C v s p r v p 350 o C p l l p pl [7]. p vp l t p ˆmp p v p v k 80 o Cp s l 6e q k m n m r q p p r m. rs p v 12 mm o l 10, 50, 100, 150, 200 MPap k l m. p m 180 o C l 10 ov l p lv m. rs e p mr pp 180 ~ 220 o C ol 12e ov m. vp lr l rrp q l o l t p m n dod Mettler p TGA/ Table 1. Properties of isotropic graphite Grades Specific Gravity Specific Resistance (µω m) Flexural Strength (MPa) Shore Hardness - Thermal Conductivity (W/mK) G

3 328 Kwang Youn Cho et al. / Carbon Letters Vol. 8, No. 4 (2007) qp ˆ l p ˆ r vrp p pn l OLYMPUS OLYMPUS BX51 Optical microscope n l m. v l q p ~ Œ p PMI Capillary Flow Porometer (Model CFP-1500AELT) r m r dk p kl 5.5 atm v rp k p v eˆ r m. I-V p LG Micron PEM Fuel cell test stationp n l rv e p ee m. e s p em d s Fig. 2. Schematic diagram of specific electrical resistivity testing system for the isotropic graphite powder. SDTA851 Thermal Analysis Systemp. l n e p kp k 20 mgpl, tl 25 o C 700 o C v 10 o C/minp dm m. s p l rs l p p MELVERN MASTERSIZER pn l r m s t rq (SEM, TAPCON SM300)p m. l ˆ d Jobin-Yvon LabRam HR (High Resolution) n l D-peak (disorder peak) 1350 cm 1 l p G-peak (graphite peak) 1590 cm 1 m. l p l p v r o XRD p e (1) p 002 l m. G( l )=[3.44-d (002) ]/[ ] (1) l l d (002) = λ/(2sinθ), d : interlayer spacing, θ : Bragg s angle, λ : Wavelength of Cu Ka X-ray( Å) l p pq rr o Fig. 2m p milliohmic multimeter n l l p k k tp k p r p r l r p r m. s rq qp r r r r rp Changmin Tech Sheet Resist Tester (SR 3000) pn l 4 probe method r m. s rs qp KS L 1591 p e (Model 4202, Instron JAPAN) n l 500 kg load cell, cross head speed 0.5 mm/minp s l r m. v l qp m pp KS L 3114l p l r w q rn(at201, METTLER, Switzerland)p n l r m. s rs v l Fig. 3. Particle diameters of isotropic graphite powder as a function of ball milling time.

4 The Characterization of the Resin Bonded Graphite Composite Bipolar Plate using Isotropic Graphite Powder for PEM Fuel Cell 329 Fig. 4. Fig. 5. SEM image of isotropic graphite powder after ball milling. Polarizing microscope images of resin bonded graphite composite as a function of ball milling time. 하에서 anode(h )를 흐름속도 209 ml/min로 주입했고 cathode (Air)는 흐름속도 630 ml/min로 주입하면서 실시하였다 결과 및 고찰 경을 이용하여 관찰하였으며 결과는 Fig. 5와 같다. 수지결합 흑연복합재 시험편은 Ball milling을 통해 분쇄된 등방성흑연분말과 분쇄 전 등방성흑연분말을 수지와 흑연분말 을 2 : 8로 각각 배합하여 제조하여 관찰하였다. 분쇄 전 흑연 3.1. 흑연분말의 분쇄와 전기적 특성 전도성 충진제인 등방성흑연분말 입자간 최밀 충진을 이루 어 수지결합 흑연복합재의 전기적, 기계적 특성을 향상시키고 자 Ball milling을 통해 입자의 크기, 형상을 작고 균일하게 입 도를 조절하였다. Fig. 3은 Ball milling 시간에 따른 등방성흑 연분말의 입도변화를 나타내었다. Ball milling시간 24시간 후 분말의 평균입경은 22.9 µm이고 72시간의 평균입경은 µm로 작아졌다. 그리고 흑연분말의 입경분포 또한 Ball milling 시간이 길어짐에 따라 분포범위가 좁아졌다. Fig. 4은 72시간 Ball milling 후의 흑연분말의 전자현미경 사진이다. 입자들이 전반적으로 작고 균일하게 분포되어 있다. 그리고 입자들은 분 쇄가 진행이 되면서 입경이 작아졌지만 판상의 흑연결정 특성 상 작은 크기의 판상형상으로 분쇄되었다. 전도성 충진제인 흑 연의 편광성을 이용하여 입자간 분포 및 응집상태를 편광현미 Surface electrical resistivity of resin bonded graphite composite as a function of particle size(µm). Fig. 6.

5 330 Kwang Youn Cho et al. / Carbon Letters Vol. 8, No. 4 (2007) p n qp n rp p pv p m, Ball milling 72e l p n qp n rp p r p q p pv p l. Ball milling l r r p o qp r r p r m. Ball milling v p pq q p r pq p rp l r r p p m. Fig. 6p q r r r p v kp l p n qp r r p 28.2 mω/cm 2 pl 24e, 72e l p n qp r r p 33.95, mω/cm 2 p ˆ l m Ball milling e p l pq q p p p l p n v l Fig. 7. Raman curves of isotropic graphite powder as a function of ball milling time. Fig. 8. XRD curves of isotropic graphite powder as a function of ball milling time.

6 The Characterization of the Resin Bonded Graphite Composite Bipolar Plate using Isotropic Graphite Powder for PEM Fuel Cell 331 qp r r p kv p p p. Fig. 7 l p l p ˆ p o r Raman p. v p 1350 cm 1 p disorder peakp ƒv 1590 cm p 1 graphite peakp qkr D/G ƒv ppp p. p v media pq l l p lpq p pl r ˆ r p p Ž p. Fig. 8p Ball milling l p lpq p r ˆ q XRD r p. d(002) p l l Ball milling e lp 74.7 ~ 77.6 p ˆ l l p pl v kk. p v r p r ƒv Raman m p p XRD p l l s X p q p m Œ pl I x =I o e l p l (µ/ρ)x XRD ˆ Cu(ka)p l v rn l X p lpq Œ p 1 µm p pl l p l p pq l d(002) p v, r lp lpq l vt r p. p Ball millinge mediam pq p l p l p p p Ž. r r p t r ˆl rp e p lv l p l r pq qkr pq rr p kv, v l qp r r p kv p Ž. Fig. 9 lpq r r ˆ r r l m p r q r rl~ l v q r ˆl l p prk p r l r r p r m. l p r l p v r r p v m. Fig. 10. Specific electrical resistivity of graphite powder as a function of load(kg f ). p v l qp r r p pqp l m p pq p r ˆl p r r v, r r rp. rp pq l r l p rr p p p r kv pl l r r p kv. l p kv r kr n l l l r r p kv p Ž. v, r r p rr p pl lpqp r l l kv p Ž l p lpqp q kv r v r~p rr p k r r p kv p Ž. l r e rk p e r r Fig. 10l ˆ l. rk p v r r p k r. p rk p kv pq rr p e Fig. 9. Specific electric resistance of graphite powder as a function of volume. Fig. 11. Surface electrical resistivity of resin bonded graphite composite as a function of mixture proportion.

7 332 Kwang Youn Cho et al. / Carbon Letters Vol. 8, No. 4 (2007) Fig. 12. Density and bending Strength of resin bonded graphite composite as a function of mixture proportion. Fig. 13. Surface electrical resistivity of resin bonded graphite composite as a function of forming pressure. rp v l qp r r p p Ž. k p kvl r p p m v p pq r ˆ skv rq p p o r r r p krv pq q~p r l p r l pl m p v p Ž. w w»» p Fig. 11p l vp l r r p ˆ l. lpq p v r r p m. p k l m p lp v pp v r r p v lpq q~p rr p pr p v kk. l v pl m ˆ Fig. 12 l lp v p v v mv l / v p ƒv qp v rp qkv k r m. Figs p kl r r,, ˆ l. k p v lpq p r v r r p kr., v m. v p n 200 MPap k l p pl p 180 o Cl pp v p p p k l p v l q l p p l l svp p p Ž. Fig. 15 s rq v l q p e p d Œ p r p. dk 5atm v 0.5 atm/minp dk p de ˆ r m. l Œ d p 95 cc/minp Fig. 14. Density and Bending Strength of resin bonded graphite composite as a function of forming pressure. Fig. 15. Permeability of resin bonded graphite composite as a function of gas pressure.

8 The Characterization of the Resin Bonded Graphite Composite Bipolar Plate using Isotropic Graphite Powder for PEM Fuel Cell p v l q l rv Žp I-V p e p. e p sl n TOYO TANSO l G458qv rs l Žp I-V p r m. GDLp 3M p anode/cathode type-i 3M 2950ol Pt loading kp 0.4 mg/cm 2 p 3M p 7-layer teflon gasketp n mp. 100% R.H, anode(h 2 ) 209 ml/min, cathode(air) 630 ml/min o p t eml ee m. p MEAp activationp o 24 e ee e p v m. rp Žp r r p p mv op e s l OCV Current density l sp l Ž r p p p p. Fig. 16. Electrical conductivity of resin bonded graphite composites as a function of mixture proportion. Fig. 17. I-V curves of resin bonded graphite composite bipolar plate for PEM fuel cell. l dkp v l 5atml 51 cc/min kr. p rp dk p kv Œ pp kv p p v l qp ˆ p k dkp v p l v l qp k r dœ p m kv p Ž. Fig. 16p 4point probe method v l qp r r r p. lp p v r r l v: l wt% pp 1:9l r r p ˆ lp p 33 MPa p l n r r k p ll. v : l wt% p 2:8l 151 S/cmp ˆ PEM fuel celll r r r ˆ r 57 MPap p p ˆ rp pp p m. 17p vrv ep PEM fuel cell test stationp l o v pn l l k p net-shapep rs Ball millinge p lvl l pq q kv o skr. pq p qp p Ž p ppp p m. r l n v l qp n p pv p mp l p n v l q p n rp p r pq p pv p l. v l p n v l qp r r p m kr. p v p media lpq l l p lpq p p pl r p l opp }p pp l p pq qkr lpq r kr r r r p kr v l qp r r p kv p Ž. p o l rk l r r l p rk p l pq rr r p e v l qp r r p lk. v lpq q~p rr p p r p v k p p. l p v lpq p rp k r r r p v lpq q~p rr r p p r p v kk. l p v p v mv r p p p v p rp qkv p m. k p v lpq p r ˆ k v r r p kr. k l l, v m. v k p e p p p p m. v l qp d Œe, p dk l Œ d p 95 cc/minpl dkp v l 5atml 51 cc/min kr. p v l qp ˆ p k dkp v p l dœ pp m kv p Ž. v : l

9 334 Kwang Youn Cho et al. / Carbon Letters Vol. 8, No. 4 (2007) wt% p 2:8l 151 S/cmp ˆ r 57 MPa p p p ˆ l rvn bipolar plate q rp pp p m. o v p v l q l rvn bipolar platep I-V p e, sp TOYO TANSO G458 qvp l bipolar platem OCV Current density l r p p p p. 3&'&3&/$&4 [1] Wind, R. S.; Kaiser, W.; Bohm, G. J. Power Sources 2002, 105, 256. [2] Kumar, A. ; Reddy, R. G. J. Power Sources 2004, 129, 62. [3] Lee, H.-S.; Ahn, S.-H.; Jun, E.-S.; Ahn. B.-K. Transaction of KSAE 2006, 14, 39. [4] Besmann, T. M.; Klett, J. W.; Henry, J. J.; Edgar, L. C. J. The Electrochemical Society 2006, 30, 39. [5] Wu, M.; Shaw, L. L. International Journal of Hydrogen Energy 2005, 30, 373 [6] Joo, W.-K.; Song, J.-G.; Choi, H.-S. Transaction of KSAE06-S , 1804 [7] Cho, D.-H.; Ahn, Y.-S.; Lee, S.-C.; Yoon, K.-H. Korean Journal of Materials Research 1997, 7, 838