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A novel electrode architecture for passive direct methanol fuel cells.pdf

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A novel electrode architecture for passive direct methanol fuel cells R. Chen, T.S. Zhao * Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China Received 13 October 2006; received in revised form 4 November 2006; accepted 6 November 2006 Available online 5 December 2006 Abstract The supply of cathode reactants in a passive direct methanol fuel cell (DMFC) relies
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  A novel electrode architecture for passive direct methanol fuel cells R. Chen, T.S. Zhao  * Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China Received 13 October 2006; received in revised form 4 November 2006; accepted 6 November 2006Available online 5 December 2006 Abstract The supply of cathode reactants in a passive direct methanol fuel cell (DMFC) relies on naturally breathing oxygen from ambient air.The successful operation of this type of passive fuel cell requires the overall mass transfer resistance of oxygen through the layered fuelcell structure to be minimized such that the voltage loss due to the oxygen concentration polarization can be reduced. In this work, wepropose a new membrane electrode assembly (MEA), in which the conventional cathode gas diffusion layer (GDL) is eliminated whileutilizing a porous metal structure for transporting oxygen and collecting current. We show theoretically that the new MEA enables ahigher mass transfer rate of oxygen and thus better performance. The measured polarization and constant-current discharging behaviorshowed that the passive DMFC with the new MEA yielded better and much more stable performance than did the cell having the con-ventional MEA. The EIS spectrum analysis further demonstrated that the improved performance with the new MEA was attributed tothe enhanced transport of oxygen as a result of the reduced mass transfer resistance in the fuel cell system.   2006 Elsevier B.V. All rights reserved. Keywords:  Fuel cell; Passive DMFC; Metal foam; Mass transfer resistance; Cell performance; Oxygen transport 1. Introduction The direct methanol fuel cell (DMFC) has been recog-nized as the most promising power source for portable elec-tronic devices because this type of fuel cell offers theadvantages of higher energy density as a result of the useof liquid methanol, as well as simpler and more compactstructure. Over the past decade, extensive effort [1–8] hasbeen made to the study of the active DMFC with the fuelfed by a liquid pump and oxidant supplied by a gas com-pressor. Nevertheless, these auxiliary devices not onlymake the fuel cell system complex but also decrease theachievable energy density and power density due to theparasitic power losses. It may not be practical to operatethe DMFC under such conditions for powering portableelectronic devices. Therefore, as a potential portable powersource, it is essential to eliminate some auxiliary devices inorder to decrease the volume and weight and to increasethe energy efficiency of the DMFC. For this reason,various DMFC systems that operate under the passive con-ditions, i.e., air breathing and passive methanol solutionsupply, have been proposed and studied [9–20]. This typeof passive DMFC not only offers the advantage of simpleand compact systems but also makes it possible to elimi-nate the parasitic power losses for powering ancillarydevices required in the active DMFC. Because of theseadvantages, the passive DMFC has received much moreattention in the area of small fuel cells. Chu and Jiang[10] investigated the effect of operating conditions on theperformance and energy efficiency of a small passiveDMFC. Liu et al. [11] studied sintered stainless steel fiberfelt as the gas diffusion layer in an air-breathing DMFC.The effect of methanol concentration was also studied inthis work. Bae et al. [12] investigated the effect of methanolconcentration, catalyst loading, fuel and oxidant supplymodes on the performance of a passive DMFC. Shimizuet al. [13] reported their activities regarding the researchand development of DMFCs that operated passively atroom temperature. Recently, Liu and Zhao et al. [18,19]found that the main reason for a higher methanol concen-tration leading to the improvement of the performance is 1388-2481/$ - see front matter    2006 Elsevier B.V. All rights reserved.doi:10.1016/j.elecom.2006.11.004 * Corresponding author. Tel.: +852 2358 8647; fax: +852 2358 1543. E-mail address:  metzhao@ust.hk (T.S. Zhao). www.elsevier.com/locate/elecom Electrochemistry Communications 9 (2007) 718–724  because of the increased methanol permeation rate, whichincreases the operating temperature and thus improvesthe electrochemical kinetics of both methanol oxidationand oxygen reduction reactions.Previous studies showed that the passive DMFC usuallyhas to be operated at relatively higher methanol concentra-tion, as diffusion is the main mechanism of methanol trans-port from a built-in fuel reservoir to the anode catalystlayer. Moreover, the higher methanol concentration canincrease the energy density of the fuel cell system, whichis desired in passive DMFCs. However, at present, theincrease in methanol concentration is limited by the prob-lem of methanol crossover. Therefore, there is plenty of room up to pure methanol, meaning that methanol trans-port on the anode is actually not a problem. In contrast,the transport of oxygen on the cathode of the passiveDMFC is a challenging problem, because the supply of oxygen in this type of fuel cell relies on naturally breathingoxygen from ambient air. As a result, the passive DMFCoften operates under oxygen-starving and water-floodingconditions. With the constraint without any external meansof air movement, it is critical to design and optimize thecathode architecture of the passive DMFC to ensure ahigher oxygen transfer and water removal rate.Typically, the cathode of the conventional MEA, asdepicted in Fig. 1a, is composed of a cathode catalyst layerand a gas diffusion layer (GDL) made of carbon paper orcarbon cloth with a coated micro porous layer (MPL).Oxygen is transported to the catalyst layer via the cathodecurrent collector and the GDL from ambient air, in whichoxygen reacts with the migrated protons and the incomingelectrons or directly reacts with the permeated methanol toproduce water and heat. The generated water is then trans-ported backward to ambient. As a result, the mass trans-port in the GDL causes the main mass transfer resistanceon the cathode. It is essential to reduce the mass transferresistance in the GDL so as to enhance the oxygen trans-port and water removal on the air-breathing cathode andthus improve the cell performance. Our previous work[20] showed that the porous current collector for the pas-sive DMFC can dramatically enhance the oxygen transportand water removal rate compared with the conventionalperforated-plate current collector. Hence, in this work,we propose a new membrane electrode assembly (MEA),in which the conventional cathode GDL is eliminated whileutilizing a porous metal structure for transporting oxygenand collecting current. Both theoretical and experimentalresults show that this type of passive DMFC can not onlyprovide a higher oxygen transfer rate but also a more effec-tive water removal. As a consequence, the cell with the newMEA yielded higher performance and more stable opera-tion than did the cell with the conventional MEA. 2. Theoretical Consider the transport process of oxygen in the conven-tional MEA, as shown in Fig. 1a, in which oxygen istransported to the catalyst layer through the current collec-tor and the GDL from ambient air. The oxygen flux  N  O 2  tothe catalyst layer can be expressed as:  N  O 2  ¼ C  1 O 2  C  CLO 2 1 h þ  l ccc  D eff  ; cccO2 þ  l gdl  D eff  ; gdlO2 ð 1 Þ where  C  1 O 2 and  C  CLO 2 represent the oxygen concentration inthe ambient air and catalyst layer;  D eff  ; cccO 2 ,  D eff  ; gdlO 2 and  h  de-note the effective diffusivity of oxygen in the current collec-tor and GDL, the mass transfer coefficient at the currentcollector surface, respectively. In the cathode catalyst layer,oxygen is not only electrochemically reduced but also di-rectly reacts with the permeated methanol. As a result,the oxygen flux can be related to the current by Faraday’slaw:  N  O 2  ¼ i þ i p 4  F    ð 2 Þ where  i   is the current density,  i  p  is the parasitic currentdensity corresponding to the flux of methanol crossover O 2 H 2 O h    M  e   t   h  a  n  o   l  s  o   l  u   t   i  o  n  r  e  s  e  r  v  o   i  r CH 3 OH+H 2 OCO 2 1234567 l ccc l gdl O 2 H 2 O h    M  e   t   h  a  n  o   l  s  o   l  u   t   i  o  n  r  e  s  e  r  v  o   i  r CH 3 OH+H 2 OCO 2 1234571-Anodecurrentcollector2-Anode gas diffusion layer3-Anode catalyst layer5-Cathode catalyst layer6-Cathode gas diffusion layer7-Cathode current collector4-Proton exchange membrane ab Fig. 1. Schematic of the passive DMFC with (a) conventional MEA and(b) new MEA. R. Chen, T.S. Zhao / Electrochemistry Communications 9 (2007) 718–724  719  N  crossover , i.e.,  i  p  = 6 FN  crossover . Tafel equation with consid-ering the effect of methanol crossover is employed to de-scribe the cathodic electrochemical kinetics as: i þ i p  ¼ i ref O 2 C  CLO 2 C  ref   exp  a  F   RT   g c    ð 3 Þ where  i ref O 2 is the exchange current density of oxygen,  C  ref  isthe reference concentration of oxygen and  g c  is the cathodeoverpotential. Combining Eqs. (1)–(3), we can obtain: g c  ¼  RT  a  F    ln i þ i p   C  ref O 2 i ref  C  1 O 2  i þ i p 4  F   1 h þ  l ccc  D eff  ; cccO2 þ  l cdl  D eff  ; gdlO2   0BB@1CCA ð 4 Þ Eq. (4) indicates that the cathode overpotential increaseswith the mass transfer resistance in each layer, including  1 h at the current collector surface,  l gdl  D eff  ; gdlO2 in the GDL, and l ccc  D eff  ; cccO2 in the current collector. Apparently, eliminating theGDL will result in a lower overall mass transfer resistance,thus lowering the cathode overpotential. In line with thisidea, we propose a new design of MEA, as shown inFig. 1b, in which the conventional GDL is eliminated whileutilizing a porous metal structure for transporting oxygenand collecting current. As will be demonstrated experimen-tally in subsequent sections, this new architecture allowsfor a higher mass transfer rate of oxygen and thus betterperformance. 3. Experimental 3.1. Membrane electrode assembly The membrane electrode assembly (MEA) having anactive area of 2.0 cm  ·  2.0 cm was fabricated in-houseemploying a Nafion 115 membrane and two electrodes.The employed Nafion 115 membrane with a thickness of 125  l m was pretreated in this work. The pretreatment pro-cedures included boiling the membrane in 5 vol.% H 2 O 2 ,washing in DI water, boiling in 0.5 M H 2 SO 4  and washingin DI water for 1 h in turn. The pretreated membranes werekept in the DI water prior to the fabrication of MEAs. Asingle-side ELAT electrode from ETEK was used in theanode, where carbon cloth (E-TEK, Type A) was used asthe backing support layer with 30 wt% PTFE wet-proofingtreatment. The catalyst loading on the anode side was4.0 mg/cm 2 with 80 wt% PtRu (1:1 a/o) on optimized car-bon. Furthermore, 0.8 mg/cm 2 dry Nafion  ionomer wascoated onto the surface of the electrode. On the cathode,the catalyst layer was fabricated in-house by the decalmethod [21]. The well-mixed catalyst ink was sprayed ontothe Teflon blank to form a catalyst layer. The catalyst layerwas then transferred onto the membrane by hot pressingthe catalyst coated Teflon blank and the anode electrodeon the each side of the membrane at 135   C and 4 MPafor 3 min. The cathode catalyst loading was about2.3 mg/cm 2 using 60 wt% Pt on Vulcan XC-72 with15 wt% Nafion as the bonding agent. Carbon paperTGPH-090 with 20 wt% PTFE, on which a MPL wascoated, was put on the cathode as the GDL. The decalmethod described above can ensure that the passiveDMFCs with the new and conventional MEAs be investi-gated for the same cathode catalyst layer, membrane andthe anode. 3.2. Single cell fixture The MEA mentioned above was sandwiched between ananode and a cathode current collector. The entire cell setupwas then held together between an anode and a cathode fix-ture, both of which were made of transparent organic glass.A 5.0-mL methanol solution reservoir was built in theanode fixture. Methanol was transferred into the catalystlayer from the built-in reservoir, while oxygen, from thesurrounding air, was transferred into the cathode catalystlayer through the opening of the cathode fixture. The celltemperature was measured by a thermocouple (Type T),which was installed on the outer surface of the anode gasdiffusion layer. The anode current collector was made of a perforated 316L stainless steel plate of 1.5 mm in thick-ness. A plurality of 2.6-mm circular holes was drilled inthe anode current collector, serving as the passages of fuel,which resulted in an open ratio of 47.8%. A 200-nm plati-num layer was sputtered onto the surface of the anode cur-rent collector to reduce the contact resistance with theelectrode. Porous current collector for the cells with boththe new and conventional MEAs was fabricated and tested,as shown in Fig. 2. The porous current collector was made Fig. 2. The current collector made of metal foam.720  R. Chen, T.S. Zhao / Electrochemistry Communications 9 (2007) 718–724  of a Ni–Cr alloy metal foam plate of 1.0 mm in thickness.The Ni–Cr alloy metal foam supplied by the RECEMAT  International offers over 95% porosity and the estimatedaverage pore diameter of 0.4 mm. A 200-nm gold layerwas sputtered onto the surface of the porous current collec-tors to reduce the contact resistance. 3.3. Electrochemical instrumentation and test conditions An Arbin BT2000 electrical load interfaced to a com-puter was employed to control the condition of dischargingand record the voltage–current curves. The temperature of the cell was measured by the Arbin BT2000 built-infunction.All the experiments of the passive DMFCs were per-formed at room temperatures ranging from 18.3 to18.6   C and the relative humidity of 68–73%. Prior to theperformance test, the MEA was installed in an active cellfixture and activated at 70   C about 24 h. During the acti-vation period, 1.0 M methanol was fed at 1.0 mL min  1 ,while oxygen was supplied under atmospheric pressure ata flow rate of 50 mL min  1 . 4. Results and discussion Prior to the polarization and constant-current discharg-ing tests, we measured the internal cell resistances of thepassive DMFCs with the new and conventional MEAs at2.0 M and 4.0 M methanol solution. The results are pre-sented in Table 1. It is seen from this table that the internalcell resistance with the new MEA is higher than that withthe conventional MEA. The increased internal cell resis-tance was attributed to the poor direct contact betweenthe cathode catalyst layer and the porous current collector,due primarily to the gap in pore size between the catalystlayer and the porous metal foam. It is also found thatthe measured internal cell resistance at 4.0 M is lower thanthat at 2.0 M as a result of the higher operating tempera-ture enhancing the proton transport in the Nafionmembrane.The cell performance of the passive DMFC with 4.0 Mmethanol concentration is shown in Fig. 3. It can be foundfrom Fig. 3 that the new MEA exhibited slightly highervoltages at zero and low current densities than did the con-ventional MEA; with increasing current density, the incre-ment of the cell performance became significantly larger.This is because the demand of oxygen on the cathode islarger due to the higher rate of methanol crossover at thehigher methanol concentration, which consumes the addi-tional oxygen. The cell with the conventional MEA maynot provide the sufficient oxygen on the cathode under thissituation. However, this is not the case for the cell with thenew MEA. The increased oxygen transfer rate as a result of the lowered overall mass transfer resistance improves theelectrochemical kinetics of oxygen reduction and thusyields a slightly higher OCV and higher voltages at low cur-rent densities. As the current density increases, the demandof oxygen on the cathode increases. Although the cell withthe new MEA has a higher internal cell resistance,the improved electrochemical kinetics as a result of theincreased oxygen transfer rate not only compensatedthe decreased voltage due to higher internal cell resistancebut also yielded higher voltages than did the cell withthe conventional MEA. We also made IR corrections tothe measured voltages, which are also shown in Fig. 3. Itis seen that the cell with the new MEA yielded much betterperformance than did the cell with the conventional MEAas a result of the increased oxygen transfer rate. Addition-ally, the cell operating temperature was measured and com-pared in Fig. 4. It is seen that the cell operatingtemperature is almost the same for the both MEAs. Thisfact further confirms that the improved performance ismainly caused by the enhanced oxygen transfer rate withthe new MEA. In summary, the lowered overall masstransfer resistance of the cell with the new MEA is themajor reason that yielded the better performance at highmethanol concentration.To investigate the operation stability of the passiveDMFC with the new MEA, we also performed the long-term operation tests. It should be noted that for the passiveDMFC the methanol consumption due to the electrochem-ical reaction and methanol crossover will cause a decreasein methanol concentration in the fuel reservoir. Therefore,the discharging behavior for the both MEAs with the Table 1The measured internal cell resistances and temperaturesMethanolconcentration (M)Internal cell resistance(mohm cm 2 )Cell temperature(  C)NewMEAConventionalMEA2.0 1472 1136 26.54.0 1464 1112 33.6 00.20.40.60 30 60 90 1200102030 New MEAConventional MEANew MEA with IR correctionConventional MEA with IR correction Current density (mA/cm 2 )    C  e   l   l  v  o   l   t  a  g  e   (   V   )   P  o  w  e  r   d  e  n  s   i   t  y   (  m   W   /  c  m    2    ) Fig. 3. Comparison in cell performance between the new and conven-tional MEAs. Anode: 4.0 M methanol; room temperature: 18.4   C;relative humidity: 69%. R. Chen, T.S. Zhao / Electrochemistry Communications 9 (2007) 718–724  721
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