On the role of the microporous layer in PEMFC operation

Transcription

1 The final publicatin is available at Elsevier via This manuscript versin is made available under the CC-BY-NC-ND 4.0 license On the rle f the micrprus layer in PEMFC peratin Jeff T. Gstick, Maris A. Iannidis*, Michael W. Fler and Mark D. Pritzker Department f Chemical Engineering, University f Waterl, Waterl, ON N2L 5N5 *Crrespnding Authr: miannid@uaterl.ca

2 Abstract The cnditin f liquid ater breakthrugh at the cathde f plymer electrlyte fuel cells (PEMFC) is studied experimentally and data n crrespnding ater saturatin and capillary pressure are prvided fr gas diffusin layers (GDL) ith and ithut a micrprus layer (MPL). The data demnstrate that the GDL saturatin at ater breakthrugh is drastically reduced frm ca. 25% t ca. 5% in the presence f MPL. This bservatin is cnsistent ith cnsideratins f invasin perclatin in finite-size lattices and suggests an explanatin fr the rle f MPL in imprving PEMFC perfrmance at high current densities. Keyrds: plymer electrlyte membrane fuel cell, gas diffusin layer, micrprus layer, ater management, capillary pressure, perclatin

3 1. Intrductin The accumulatin f excessive prduct ater in plymer electrlyte membrane fuel cells (PEMFC) is a factr limiting perfrmance at high current densities. Liquid ater generated by the cathde reactin perclates thrugh the prus gas diffusin layer (GDL) tards the gas channel. Ntably, the capillary number and viscsity rati prevailing during displacement f the gaseus phase by ater are such that a highly ramified, finger-like pattern characteristic f invasin perclatin is established [1,2]. Cnsequently, ater saturatin in the GDL and resistance t gaseus reactant transprt bth increase until ater first breaks thrugh at the GDL-channel interface. The liquid saturatin and capillary pressure assciated ith ater breakthrugh in GDL materials are clearly imprtant fr understanding resistance t gaseus reactant transprt due t ater accumulatin. Effrts t imprve PEMFC ater management have fcused n (1) impregnating the GDL pres ith a hydrphbic plymer and (2) adding a micr-prus layer (MPL) n ne face f the GDL. A recent systematic experimental study f these treatments by Ramasamy et al. [3] has clearly demnstrated imprved mass transfer hen these treatments are applied, bth in terms f increased limiting current as ell as reduced mass transfer resistance measured via AC impedance. The MPL treatment has been shn t be particularly beneficial althugh the actual functin f this layer is still under debate. The MPL presumably creates better electrical and thermal cntact beteen the catalyst layer and the GDL by prviding a smther, mre cntinuus interface, but the benefits f the MPL are mst nticeable at higher current cnditins indicating that it smeh imprves mass transfer. Since the MPL actually adds a diffusive resistance t mass transfer, it is generally thught that the MPL smeh alters the liquid ater distributin in the cell t a mre favrable arrangement fr

4 gas phase transprt [4,5]. The MPL questin has been recently addressed ith full-cell mdels [6,7], hich predict that the MPL acts as a capillary barrier t ater entering the cathde GDL and frces ater t permeate frm the cathde t the ande. Experimental attempts t cnfirm this mechanism have been incnclusive r cntradictry. Using a transparent fl field t mnitr ater in the ande, Spernjak et al. [8] fund that ater appears in the ande channels nly hen an MPL is used n the cathde side, hich they ffer as prf fr the functin f MPL as a capillary barrier. Similar experiments by Ge and Wang [9], hever, have attributed the appearance f ater in the ande t cndensatin since ater drplets nly appeared n channel alls and ere never bserved emerging frm the GDL. Atiyeh et al. [10] cnducted quantitative ater balance experiments and fund n discernible increase in ater cllected frm the ande hen an MPL is present n the cathde fr any given set f perating cnditins. In this rk ex-situ measurements are presented f the ater saturatin and assciated capillary pressure precisely at the pint f ater breakthrugh in GDL samples ith and ithut MPL. It is demnstrated that the ater saturatin at breakthrugh is dminated by finite-size effects ing t the very small thickness f the GDL. Namely, hen the entire inlet face f the GDL is expsed t ater (i.e. n MPL), a significant fractin f the pre vlume is invaded by nn-perclating clusters f pres filled ith ater and S at breakthrugh in the sample tested. When access f ater t the inlet face f the GDL is restricted t a very small area, the number f nn-perclating clusters f ater-filled pres present at breakthrugh [2] is drastically reduced and S In the presence f MPL, S at breakthrugh, suggesting that the MPL restricts the number f pints f entry f ater int the GDL.

5 2. Experimental The simultaneus measurement f capillary pressure and ater saturatin at breakthrugh as achieved ith a simple mdificatin f a technique detailed elsehere [11]. In rder t determine the pint n the capillary pressure curve here breakthrugh ccurred, a small circular piece (1 cm diameter) f dry hydrphilic membrane as inserted abve the GDL sample and bel the hydrphbic capillary barrier. The test prceeded as nrmal, ith each capillary pressure change resulting in a saturatin change up t the pint f ater breakthrugh. The breakthrugh pint can be easily detected since the saturatin changes drastically and rapidly due t icking f ater int the dry hydrphilic membrane placed abve the sample. Figure 1(left) shs the ater uptake respnse btained frm this test. The pint f ater breakthrugh is clearly visible as a sudden change in GDL ater uptake, alling the saturatin and capillary pressure just befre breakthrugh can be easily determined. Figure 1(right) shs that the capillary pressure curve up t the pint f breakthrugh btained ith the mdified methd is essentially the same as the capillary pressure curve btained fr the same material ith the unmdified methd [11]. Slight differences in the t curves are due t the fact that they ere btained frm different samples. T independently verify the breakthrugh pints fund using the mdified capillary pressure device, a secnd test as perfrmed similar t that f Benziger et al. [12]. A piece f GDL as affixed t the end f a tube and the head f ater abve the sample as increased in 5 cm increments, aiting 5 minutes at each pint, until breakthrugh as bserved. This simple test des nt prvide saturatin at breakthrugh, but cnfirms that the mdified capillary pressure device bserves the crrect breakthrugh pressure.

6 Tests ere perfrmed n SGL10BA and SGL10BB gas diffusin layers. These materials had the same GDL substrate and hydrphbic plymer cntent (5 t%). The SGL10BB sample had an m thick MPL applied n ne side [13]. In tests ith the SGL10BB sample, the MPL as placed facing the ater inlet as ell as aay frm it. These tests are referred t as SGL10BB-I and SGL10BB-O, respectively. Tests ere als cnducted n an SGL10BA sample here the inlet face f the sample is cvered ith an impermeable PTFE mask having a 600- m hle punctured in the center fr ater fl (dented SGL10BA-P). Tests n SGL10BA ith the full face expsed t liquid ater are dented as SGL10BA-F.

7 3. Results and Discussin The main findings f this study are summarized in Figure 2. Regarding the plain GDL substrate material (SGL10BA), it is evident that the Pc S relatinship fr ater injectin is significantly affected by finite-size effects. Since GDL materials are very much thinner (ca pres acrss) than they are ide [14], a significant fractin f the sample pre vlume is invaded by nn-perclating clusters befre breakthrugh if the entire inlet face f the sample is expsed t ater (SGL10BA-F). In this case, ater breakthrugh ccurs at P 2500 Pa and a high saturatin ( S ). By cmparisn P 3800 Pa and c S 0.05 hen nly a very small fractin f the GDL inlet face is expsed t ater (SGL 10BA-P). This situatin is illustrated in the left and middle panels f Figure 3. c The Pc S relatinship fr ater injectin int SGL10BB is different depending n hether the MPL is fund n the inlet face (SGL10BB-I) r the utlet face (SGL10BB-O) f the sample. In the frmer case, the data are displaced tards higher capillary pressures, as expected since ater invasin int the GDL is cntrlled by the smaller pre sizes f the intervening MPL. In the latter case, the Pc S relatinship in the l capillary pressure range is essentially the same as in test SGL10BA-F, hich is expected since ater is first invading the substrate material in bth cases. In either case (SGL10BB-I r SGL10BB-O), the maximum ater saturatin achieved is abut 75%, demnstrating that the MPL is nt significantly filled ith ater at the maximum capillary pressure established. When the MPL is n the utlet face, ater breakthrugh in SGL10BB ccurs at P 6700 Pa and S Such high ater saturatin is easily understd since the GDL is filled substantially by ater c at the capillary pressure necessary fr ater t invade the smaller MPL pres. Water

8 breakthrugh in sample SGL10BB hen the MPL is n the inlet face ccurs at a similarly high capillary pressure ( P 6000 Pa), but in this case S c The fact that very similar breakthrugh saturatin is bserved in tests SGL 10BA-P and SGL10BB-I supprts the cnclusin that the MPL limits ater access t the GDL face. Water breaks thrugh the MPL at a fe islated lcatins at the MPL-GDL interface, in the same ay ater drplets emerge frm the GDL at the GDL-gas channel interface [2,15,16], regardless f hether r nt the MPL is cracked. Subsequent ater perclatin thrugh the GDL starts frm these lcatins, as ppsed t the entire face f the GDL, as depicted in Figure 3(right). This renders mst pres n the GDL face inaccessible t ater, dramatically reducing GDL saturatin at breakthrugh by reducing the number f dead end clusters (a finite-size effect). In this manner, reduced gas diffusivity is cnfined t a smaller part f the transprt dmain (the MPL), resulting in imprved perfrmance at high current densities. The breakthrugh pressures bserved in tests SGL10BA-P and SGL10BB-I shuld nt be expected t be the same due t the mre hydrphbic nature and smaller pres f the MPL cmpared t the hle punctured in the PTFE mask. Furthermre, breakthrugh pressures f the rder f 6000 Pa, bserved in tests SGL10BB-Iand SGL10BB-O are ler than expected frm the pre sizes f the MPL (<1 m). Rather, a breakthrugh pressure in this range indicates that cracks and defects in the MPL are the main pathays fr ater invasin. Interestingly, fl thrugh the MPL via cracks uld be beneficial since it eliminates the frmatin f dead-end clusters in the MPL that are characteristic f perclatin prcesses and cntribute significantly t ater saturatin at breakthrugh hen finite-size effects are imprtant. In fact, it might be advantageus t design the MPL ith strategically placed hles.

9 4. Cnclusins An explanatin fr the efficacy f the MPL in fuel cell peratin is prpsed based n finitesize effects related t invasin perclatin in thin GDLs. Water perclatin thrugh the MPL results in limited access f ater t the GDL inlet face. Cnsequently, far feer dead-end ater clusters cme int being at breakthrugh and therefre much ler ater saturatin is reached. Experimental evidence is prvided by a mdified versin f a recently reprted capillary pressure measurement technique [11] that alls bth the pressure and saturatin at breakthrugh t be simultaneusly identified. The saturatin at breakthrugh drps frm 27% t less than 3% hen an MPL intervenes beteen the GDL and the ater surce. A similar result is btained hen an therise impermeable mask ith a small puncture is placed beteen the ater and the GDL. These findings suggest that the effectiveness f the MPL might be further imprved by designing them ith large hles fr ater passage int the GDL, hile leaving the micr-prsity dry fr gas transprt.

10 5. References [1] R.Lenrmand, C.Zarcne, A.Sarr, Jurnal f Fluid Mechanics 135 (1983) 337. [2] P.K.Sinha, C.Y.Wang, Electrchim Acta 52 (2007) [3] R.P.Ramasamy, E.C.Kumbur, M.M.Mench, W.Liu, D.Mre, M.Murthy, Int J Hydrgen Energy 33 (2008) [4] U.Pasagullari, C.Y.Wang, J Electrchem Sc 151 (2004) A399. [5] J.H.Nam, M.Kaviany, Int J Heat Mass Trans 46 (2003) [6] A.Z.Weber, J.Neman, J Electrchem Sc 152 (2005) A677. [7] U.Pasagullari, C.Y.Wang, K.S.Chen, J Electrchem Sc 152 (2005) A1574. [8] D.Spernjak, A.K.Prasad, S.G.Advani, J Per Surces 170 (2007) 334. [9] S.Ge, C.Y.Wang, J Electrchem Sc 154 (2007) B998. [10] H.K.Atiyeh, K.Karan, B.Peppley, A.Phenix, E.Hallip, J.Pharah, J Per Surces 170 (2007) 111. [11] J.T.Gstick, M.A.Iannidis, M.W.Fler, M.D.Pritzker, Electrchem Cmm 10 (2008) [12] J.Benziger, J.Nehlsen, D.Blackell, T.Brennan, J.Itescu, J Membrane Sci 261 (2005) 98. [13] J.T.Gstick, M.W.Fler, M.A.Iannidis, M.D.Pritzker, Y.M.Vlfkvich, A.Sakars, J Per Surces 156 (2006) 375. [14] J.T.Gstick, M.A.Iannidis, M.W.Fler, M.D.Pritzker, J Per Surces 173 (2007) 277. [15] S.Litster, D.Sintn, N.Djilali, J Per Surces 154 (2006) 95. [16] X.G.Yang, F.Y.Zhang, A.L.Lubay, C.Y.Wang, Electrchem and Slid-State Let 7 (2004) A408.

11 6. Figure Captins Figure 1: Experimental results frm ater breakthrugh test. Left: Pressure and saturatin time traces fr SGL10BA shing the breakthrugh pint. Right: Cmparisn f capillary pressure curves btained using riginal and mdified methd up t the breakthrugh pint.

12 Figure 2: Capillary pressure curves fr SGL10BA and SGL10BB ith breakthrugh pints verlaid as filled markers ith crrespnding shapes. F: Full face expsed t ater, P: Punctured mask beteen ater and GDL, I: MPL facing ater inlet, O: MPL facing aay frm ater inlet.

13 Figure 3: Prpsed cnfiguratin f ater in the GDL ith different injectin cnditins. Left: Full face injectin leads t several dead-end clusters and a single breakthrugh cluster. Middle: Pint surce injectin leads t a single breakthrugh cluster. Right: Perclatin thrugh the MPL creates cnditins similar t pint-like injectin int the GDL. Grey shs ater and dashed line marks regins that uld be filled if they ere accessible.