Progress in Energy and Combustion Science

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1 Progress in Energy and Combustion Science 35 (2009) Contents lists available at ScienceDirect Progress in Energy and Combustion Science journal homepage: Mass transport phenomena in direct methanol fuel cells T.S. Zhao *, C. Xu, R. Chen, W.W. Yang Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China article info abstract Article history: Received 24 October 2008 Accepted 9 January 2009 Available online 20 February 2009 Keywords: Fuel cell Direct methanol fuel cell Mass transport Two-phase flow Water management Water flooding Cell performance Clean and highly efficient energy production has long been sought to solve energy and environmental problems. Fuel cells, which convert the chemical energies stored in fuel directly into electrical energy, are expected to be a key enabling technology for this century. This article is concerned with one of the most advanced fuel cells direct methanol fuel cells (DMFCs). We present a comprehensive review of the state-of-the-art studies of mass transport of different species, including the reactants (methanol, oxygen and water) and the products (water and carbon dioxide) in DMFCs. Rather than elaborating on the details of the previous numerical modeling and simulation, the article emphasizes: i) the critical mass-transport issues that need to be addressed so that the performance and operating stability of DMFCs can be upgraded, ii) the basic mechanisms that control the mass-transport behaviors of reactants and products in this type of fuel cell, and iii) the previous experimental and numerical findings regarding the correlation between the mass transport of each species and cell performance. Ó 2009 Elsevier Ltd. All rights reserved. Contents 1. Introduction General description of mass transport in DMFCs Critical mass transport issues Through the membrane On the anode On the cathode Mass transport of methanol on the anode Two-phase flow behavior in the anode flow field Mass transport of methanol from the flow field to the catalyst layer Effect of the operating conditions Effect of the flow field design Effect of the anode diffusion layer Mass transport of methanol through the membrane (methanol crossover) Effect of the operating conditions Effect of the membrane Effect of the anode diffusion layer Water transport through the membrane (water crossover) Effect of the operating conditions Effect of the MEA design Mass transport of water in the cathode (water removal) Effect of the operating conditions Effect of the cathode diffusion layer Effect of the cathode flow field Water recovery from the cathode to the anode Internal recovery * Corresponding author. Tel.: þ ; fax: þ address: metzhao@ust.hk (T.S. Zhao) /$ see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi: /j.pecs

2 276 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) External recovery Mass transport of oxygen at the cathode Concluding remarks Acknowledgements References Introduction A direct methanol fuel cell (DMFC) is an electrochemical energyconversion device that converts chemical energy of liquid methanol into electrical energy directly. Because of its unique advantages, such as higher energy densities, facile liquid fuel storage, and simpler system structures, the DMFC has been identified as one of the most promising power sources for portable and mobile applications [1 4]. Although promising, the DMFC technology is facing some challenging technical issues that need to be resolved before the widespread commercialization of this type of fuel cell. One of the technical issues is related to the mass transport of different species in the DMFC. Mass transport not only affects the performance and operating stability of the DMFC, but also influences the volumetric energy density of the DMFC system [7 14]. Over the past decade, the mass transport of methanol, gas CO 2, oxygen and water has been extensively studied [2 14]. The purpose of this article is to present a comprehensive review of the advances in the study of mass transport of different species in DMFCs and to discuss about the future research directions. The remainder of this paper is organized as follows: Section 2 gives a general description of the mass-transport processes of reactants and products in DMFCs; Section 3 discusses about the critical issues of mass transport of each species; Sections 4 and 5 review the mass transport of methanol and gas CO 2 at the anode and methanol crossover through membranes, respectively; Sections 6 8 deal with the mass transport of water, including water crossover, water removal, and water recycle; and Section 9 briefly reviews the mass transport of oxygen at the cathode. Finally, a summary is given in Section General description of mass transport in DMFCs Fig. 1 illustrates a typical single DMFC that consists of a membrane electrode assembly (MEA) sandwiched by anode and cathode bipolar plates with the machined flow fields. The MEA is a multi-layered structure that is composed of an anode diffusion layer (DL), an anode catalyst layer (CL), a polymer electrolyte membrane (PEM), a cathode diffusion layer (DL), and a cathode catalyst layer (CL). The function of the membrane is to conduct protons from the anode to the cathode, and to provide an electronic insulator between the anode and the cathode. Typically, perfluorinated sulfonic acid ion exchange membranes developed by DuPont and trademarked as Nafion Ò are used in DMFCs. The function of each DL is to provide a support to the corresponding CL and to distribute reactants over the catalyst layer, and to conduct electricity to the current collector. The DLs at both the anode and the cathode usually consist of two layers, a backing layer that is made of carbon cloth or carbon paper, and a microporous layer (MPL) that is composed of hydrophobic polymer and carbon powder. Different from those DLs, both CLs are made of catalysts mixed with ionomer to provide triple-phase boundaries for the methanol oxidation and oxygen reduction reactions, to facilitate the simultaneous transport of protons, electrons and reactants/ products. On the anode, methanol solution is supplied through the anode flow field to the anode CL, where part of methanol is oxidized to generate electrons, protons, and CO 2, while the remainder is directly transported to the cathode through the membrane; the permeation of methanol from the anode to the cathode through the membrane is termed as methanol crossover, which creates a mixed potential and decreases the cathode potential. On the cathode, oxygen/air is supplied through the cathode flow field and transfers through the cathode DL to the cathode CL, where major part of oxygen reacts with the protons that are conducted through the membrane from the anode and the electrons that come from the external circuit to form water, while the remaining part of oxygen electrochemically reacts with the permeated methanol. The gas CO 2 generated on the anode and the liquid water generated on the cathode are then vented out of the cell. The electrochemical reaction on the anode is CH 3 OH þ H 2 O/CO 2 þ 6H þ þ 6e (1) while the electrochemical reaction on the cathode is 6H þ þ 6e þ 3=2O 2 /3H 2 O (2) It follows that the overall reaction in the DMFC is CH 3 OH þ 3=2O 2 /CO 2 þ 2H 2 O (3) Clearly, the mass transport processes on both the anode and the cathode involve multi-component two-phase flows both in the e - e - Load CH 3 OH+H 2 O+CO 2 CL (5~25µm) Air/O 2 +H 2 O+CO 2 H 2 O CO 2 CH 3 OH e - e - e - e - PEM (20~200µm) H + H 2 O CH 3 OH H 2 O DL (100~300µm) CH 3 OH+H 2 O Air/O 2 Fig. 1. Schematic of a liquid-feed direct methanol fuel cell (DMFC). O 2

3 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) porous media and in the flow channels, and are inherently coupled with electrochemical reactions. During the fuel cell operation, the electrochemical consumption of reactants (i.e., methanol, oxygen, and water) leads to a reduction in each reactant concentration in the CLs which in turn results in a noticeable voltage loss, known as the mass-transport polarization or concentration loss. The reactant concentration in each CL decreases with the discharging current; when the concentration approaches zero as a result of the crisis in transporting reactant from the flow field to the CL, the corresponding current is termed as the limiting current, at which the cell voltage drops to zero. In general, the limiting current of a fuel cell can be caused by either the methanol or oxygen transport limitation, whichever comes first. From this point of view, seemingly, it would be sufficient to achieve better DMFC performance if the mass transport of both methanol and oxygen could be enhanced. However, as discussed in the next section, the mass-transport issues are much more complicated than just the enhancement of mass transport. 3. Critical mass transport issues Since the fuel cell is a multi-layered plane structure, the requirements for mass transport of species involved in the electrochemical reactions in the in-plane and through-plane directions are different. In the following, the critical mass-transport issues both in the in-plane and through-plane directions of the membrane, anode, and cathode are discussed Through the membrane The problem of methanol crossover associated with the use of Nafion membranes in DMFCs is detrimental to the cell performance, as methanol crossover can pose two technical problems. First, methanol permeated from the anode can be oxidized with the help of the cathode catalyst to form the parasitic current, resulting in a mixed potential on the cathode, which lowers the cell voltage. Second, methanol crossover leads to a waste of fuel, lowering the fuel efficiency. Hence, the rate of methanol crossover has to be reduced to alleviate its impacts on the DMFC performance. Although tremendous efforts have been made to modify existing Nafion membranes and develop new ones with a lower rate of methanol crossover, to date, no membranes that not only meet a higher ion conductivity and lower rate of methanol crossover but also meet other chemical and physical requirements have been seen. For this reason, at present dilute methanol solutions ( M) are usually fed to a DMFC so that the rate of methanol crossover can be lowered. Although feeding dilute methanol solution is effective to improve the cell performance at low currents, it becomes ineffective with increasing current. The high electrooxidation rate of methanol at high currents may result in methanol starving in the anode CL, increasing the concentration loss. Hence, for present Nafion membranes, a critical mass-transport issue is how to control the mass transport rate of methanol through a membrane such that both the rate of methanol crossover and the concentration loss can be reduced in the entire current range. In addition to methanol crossover, water crossover through the membrane from the anode to the cathode is another through-plane mass transport issue. As indicated by Reaction (1), water is a reactant in the anodic process at a molecular ratio of 1:1 (water:- methanol). Practically, since a dilute methanol solution is needed, the water:methanol molecular ratio on the anode substantially exceeds the stoichiometric 1:1. Unfortunately, since Nafion membranes are permeable to water, a significant fraction of liquid water at the anode can transport, along with methanol, through the membrane and arrives at the cathode. Water crossover through membranes can cause two challenging problems for the DMFC technology. First, it results in a water loss from the anode. Thus, a make-up water system is needed, complicating the overall fuel cell system design. More importantly, water crossover exaggerates the cathode water flooding problem, which is discussed in Section 7. Hence, how to reduce the rate of water crossover through membranes is another critical mass-transport issue On the anode As mentioned earlier, methanol crossover not only leads to a decrease in the cell voltage, but also wastes the fuel. Hence, the rate of methanol crossover must be minimized. As methanol crossover depends greatly on diffusion across the membrane, reducing the methanol concentration in the anode CL can significantly reduce the rate of methanol crossover. To reduce the methanol concentration in the anode CL for a given anode design, the feed methanol concentration must be sufficiently low. On the other hand, however, when the feed methanol concentration is too low, the low rate of methanol transfer can cause the methanol concentration in the anode CL to be too low, resulting in a large mass-transport loss and thus lower cell voltage. Hence, it is critical to maintain an adequate methanol concentration in the anode CL such that both the rate of methanol crossover and the masstransport loss can be minimized. However, it is challenging to achieve this, as the methanol concentration in the anode CL is intrinsically related to the local concentrations of water and CO 2.A change in one of the three mass transport processes of, methanol, water and CO 2, will cause a change in the other two local concentrations in the anode CL. Hence, how to maintain an adequate methanol concentration in the anode CL by controlling the through-plane mass transport processes of methanol, CO 2, and water through the anode flow field and DL is a critical issue. It should be pointed out that an adequate methanol concentration in the anode CL can be achieved by feeding a high or low methanol concentration to the flow field, depending on the mass transfer resistance through the anode DL. If the mass transfer resistance from the flow channel to the anode CL is sufficiently large, the fuel cell can be operated with high methanol concentration operation. On the other hand, if no particular measures are taken to increase the mass transfer resistance through the design of the anode DL, the fuel cell can only be operated with low methanol concentration operation. Although most of the previous work regarding the mass transport of methanol was focused on low methanol concentration operation (typically M), operating the fuel cell with high methanol concentration is the future direction, as this increases the volumetric energy density of the DMFC system. With respect to the in-plane mass transport of methanol on the anode, the uniformity of the methanol distribution over the electrode is a critical mass-transport issue on the DMFC anode. This issue can be more clearly elaborated by looking at Fig. 2. As shown in Fig. 2a, since part of the electrode area is covered by channel ribs (around 50%), the electrode is divided into the channel and underrib regions. Due to the difference in the mass-transport length, the methanol concentration in each channel region will be higher than that in each under-rib region, resulting in a concentration difference between the channel region and the rib region. Additionally, as shown in Fig. 2b, due to the consumption of methanol, the methanol concentration in the anode CL decreases along channel length. These two effects can result in a non-uniform methanol concentration distribution in the anode CL over the entire electrode, lowering the cell voltage. Hence, how to uniformly distribute the methanol is a critical in-plane mass transport issue. In summary, the critical issue on the anode mass transport is how to more evenly distribute methanol over the entire electrode,

4 278 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) a b High concentration region High concentration region Low concentration region Uneven distribution of fuel concentration resulting from channel-rib coverage High concentration region Low concentration region by adjusting the feed methanol concentration in the flow field. However, it should be noted that the through-plane mass transport of oxygen cannot be varied by changing feed oxygen concentration (either pure oxygen or air). Hence, the critical issue of the throughplane mass transport on the DMFC cathode is how to alleviate the water flooding problem and enhance oxygen transport. Similar to the methanol transport in the in-plane direction, due to the oxygen reduction reaction, the oxygen concentration (water concentration) decreases (increases) with channel length. As a result, the maximum concentration polarization of oxygen (or water flooding) occurs in the downstream region of the flow field. Hence, in the in-plane direction, it is critically important to enhance the in-plane mass transport of oxygen so that the oxygen concentration in the downstream region can be maximized. Another critical in-plane mass-transport issue on the cathode is how to enhance under-rib convection to avoid the water accumulation under the ribs and minimize the oxygen concentration difference between the channel and under-rib regions. In summary, the critical mass-transport issues on the cathode are as follows: 1) In the through-plane direction, it is critically important to enhance the mass transport of water and oxygen. 2) In the in-plane direction, the mass transport of water and oxygen also needs to be enhanced so that oxygen can be more uniformly distributed over the electrode. 4. Mass transport of methanol on the anode 4.1. Two-phase flow behavior in the anode flow field and in the meantime to maintain an adequate methanol concentration in the anode CL, such that the rate of methanol crossover and the concentration loss everywhere can be minimized and thus the cell voltage can be maximized On the cathode Uneven distribution of fuel concentration along channel length Fig. 2. Schematic illustration of the uneven fuel distribution in the in-plane direction of a DMFC. On the cathode, oxygen is transported from the flow channel through the cathode DL to the cathode CL, while the product, water, needs to be transported back to the flow channel and vented out of the cell. If the rate of water removal from the cathode is too low, the liquid water will accumulate on the cathode, forming the so-called water flooding problem, which increases substantially the mass transfer resistance of oxygen and lowers the cell voltage. In particular, the water flooding problem becomes more serious with increasing current, which may eventually lead to the so-called limiting current as a result of the mass transport limitation of oxygen. In the preceding section, it is mentioned that with respect to the through-plane mass transport of methanol on the anode, an adequate methanol concentration in the anode DL can be achieved The anode flow field of the DMFC is to supply methanol solution through the anode DL to the anode CL and transport out gas CO 2 from the cell. Hence, the flow in the flow field is in the form of gas liquid two phases. Since the two-phase flow behavior in the flow field affects not only the mass transport of methanol to the anode CL, but also the removal of gas CO 2 from the cell, it is directly related to cell performance [2,14 45]. To gain better understanding of the mass transport of methanol and gas CO 2 on the DMFC anode, twophase flow behaviors in different anode flow fields have been extensively studied [2,10,14,31 35,39 54]. The commonly used flow fields are sketched in Fig. 3, and the comparison between the flow fields has been summarized by Qian et al. [22]. Most of the previous studies have been focused on the parallel flow field [27,29,32 34,40,45,49]. Argyropoulos and Scott et al. [27,33,34,49] studied the gas CO 2 bubble flow characteristics in a DMFC with two different parallel flow channels. It was observed that the amount of gas in the channel rapidly increased with current density, and accordingly the flow pattern in the flow channel changed from bubbly to slug and annular flow regimes. Moreover, it was found that channel blocking by gas slugs often occurred in part of the flow channels of the parallel flow field. The channels blocked by gas slugs may restrict the through/in-plane transport of methanol to the anode CL, thereby increasing the concentration polarization. However, the channel blockage could be significantly reduced by increasing anode liquid flow rate. Lu et al. [32,40] further explored the mechanism of the gas CO 2 bubble dynamics in a parallel anode flow field. They found that gas bubbles were held on the carbon paper by surface tension until they grew into larger slugs before detachment, indicating that the effect of surface tension in bubble dynamics in the anode channel is important. Once the bubbles grew to a sufficient size, they detached and swept along the backing surface of the channel, which may clear all small bubbles preexisting on the downward backing surface, and make new bubbles grow renewedly from the smallest size to the full detachment

5 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) Fig. 3. Different flow fields for DMFCs: (a) serpentine; (b) parallel; (c) spot; and (d) interdigitated. diameter. As a result, the gas CO 2 removal became regularly intermittent. Similar phenomena have also been reported by Liao et al. [45]. They recorded the emergence, growth, coalescence, and removal of CO 2 gas bubbles in a parallel anode flow field and performed a series of parametric studies. In addition to the parallel flow field, the two-phase flow behavior in the serpentine anode flow field has also been visually studied [2,29,47]. Similar periodical repetition of a process of bubble formation, detachment and coalescence to gas slug was observed in the serpentine flow field. The effects of cell orientation, methanol solution flow rate, and operation temperature were also investigated. Furthermore, the different two-phase flow patterns in parallel and serpentine anode flow fields were compared, as illustrated in Fig. 4. It was found that under the same conditions more gas CO 2 bubbles were presented in the parallel flow field as the sweeping rate of the gas bubbles in the parallel flow field was lower than that in the serpentine flow field. Interestingly, the channelclogging phenomenon was never encountered under all testing conditions in the single serpentine flow field, indicating that the serpentine flow field has superior gas CO 2 removal ability than does the parallel flow field. Recently, Wong et al. [44] visually studied an in-house fabricated micro DMFC with various-sized micro single serpentine channels down to mm and demonstrated that the CO 2 bubble behavior depended on the channel width. They found that bubble evolution and removal in smaller flow channels occurred in a periodic manner because of transient capillary blocking. Shrinking channel width resulted in not only longer gas slugs but also a longer residence time of gas slug blocking in the flow channel, indicating that gas CO 2 management is especially critical for compact portable DMFC systems with micro-scale flow channels. Since the gas evolution in the flow channel starts from the surface of the permeable wall, i.e., the backing layer (BL), the twophase flow pattern may also be influenced by the properties of the BL. For this reason, many researchers investigated the effect of the BL structure on the two-phase behavior in the anode flow field. As mentioned above, BLs are commonly made of carbon paper and carbon cloth with the hydrophobic treatment, which show different structures illustrated in Fig. 5a and b. Argyropoulos et al. [33,34] and Lu et al. [32] compared the gas CO 2 bubble behaviors on the surfaces of these two commonly used BLs. It was observed that the CO 2 bubbles were produced more uniformly and with a smaller size from the carbon cloth, whereas large gas slugs were formed on the surface of the carbon paper and blocked the channel. The difference in bubble behavior between carbon paper and carbon cloth can be attributed to the different structure and surface wettability. Yoshizawa et al. [56] found that the carbon paper had a uniform pore size distribution with a high peak at 50 mm, while the carbon cloth had a broad pore size distribution from 5 to 100 mm, as shown in Fig. 5c. This difference in the pore size distribution gave rise to the fact that the CO 2 bubbles emerged more uniformly from carbon cloth than that from carbon paper. Therefore, the BL made of carbon cloth exhibited better through/inplane methanol transport. In addition, the effect of the PTFE treatment of the BL on the gas CO 2 bubble dynamics has also been investigated. Zhang et al. [42] showed that uniform CO 2 gas bubbles

6 280 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) (S-1) 50 ma cm -2 (S-2) 100 ma cm -2 (S-3) 200 ma cm -2 (P-1) 50 ma cm -2 (P-2) 100 ma cm -2 (P-3) 200 ma cm -2 Fig. 4. CO 2 gas bubble behavior in a single serpentine flow field and a parallel flow field with 1.0 M methanol fed at 2.0 ml min 1 and at 60 C [35]. with smaller size formed on hydrophilic anode BL, whereas nonuniform bubbles with larger size emerged over the hydrophobic anode BL. In addition to the visualization studies on the active DMFC with the methanol solution supplied with liquid pumps, passive DMFCs without liquid pumps were also visually investigated. In passive DMFCs, in addition to the parallel flow field, the current collector with plurality of perforated holes often serves as the passages of fuel and gas CO 2 [31,48,50]. Yang et al. [31] compared the gas CO 2 removal behavior between a parallel and a perforated anode flow field and found that the perforated flow field was poor to remove the gas CO 2 since gas bubbles were prone to block some holes. Subsequently, the through-plane transport resistance of methanol was increased, lowering the cell performance. However, the parallel flow field allowed easy removal of gas CO 2 along the channels, which led to a better performance for the cell with a parallel flow field. Recently, Litterst et al. [29] presented a new flow-field layout that enabled fully passive CO 2 gas bubble removal in micro DMFCs. They used tapered channel design to create an intrinsic transport mechanism that passively removes gas bubbles from the electrode by capillary forces, leading to better gas CO 2 removal. In summary, previous visualization studies of gas CO 2 bubble behavior in the DMFC anode field revealed that gas bubbles first form and grow on the surface of the anode BL and then depart into the channel region. The anode flow field design affects the twophase patterns, thereby affecting the mass transport of methanol and cell performance. In addition, the properties of anode BL, including pore structure and wettability, can also influence the gas bubble formation, departure and the two-phase flow in the channel Mass transport of methanol from the flow field to the catalyst layer As the mass transport of methanol from the anode flow field through the anode DL to the anode CL involves the counter liquidgas two-phase flow in the layered porous structures with different length scales, it is difficult to solve the problem theoretically. Nevertheless, the mass transport process can be characterized with an overall effective mass transfer coefficient, such as [3] k overall ¼ 1 1=k s þ 1=k dl (4) where k s is the hydrodynamic mass transfer coefficient between channel flow and the BL surface, and k dl is the effective mass transfer coefficient in the anode DL. It has been understood that gas bubbles in the flow channel affect the hydrodynamic mass transfer at the channel/anode DL interface in two opposite directions. On one hand, the presence of gas bubbles accelerates the velocity of the methanol solution, increasing the hydrodynamic mass transfer coefficient k s [20,44]. On the other hand, gas bubbles may cover some areas of the anode DL surface, reducing the effective transport area for the mass transport of methanol from the channel to the anode DL [44,46]. Therefore, the hydrodynamic mass transfer coefficient k s through the channel/anode DL interface can be expressed as: k s ¼ dð1 bþh (5) where d is the open ratio of the flow field, h is the mass transfer coefficient at the channel/anode DL interface, and b is the ratio of the gas coverage fraction on the channel/anode DL interface. Although it is difficult to quantify both b and h at this stage, Eq. (5) indicates that both b and h affect the mass transport of methanol. The mass transfer coefficient h can simply be obtained from the heat and mass transfer analogy for a fully developed single-phase laminar duct flow [46,57]. The mass transport of methanol in the anode DL is much more complicated. First, the gas CO 2 emerged in the anode DL may occupy some pores, blocking the through-plane transport of methanol. Second, the coverage by the ribs may require the inplane transport of methanol in the anode DL to distribute the

7 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) Pore capacity (mm 3 /mm 2 ) a b c 0.1 Carbon paper Carbon cloth Pore diameter (mm) Fig. 5. SEM images of carbon paper (a) and carbon cloth (b), and the comparison of porosity distribution of carbon paper and carbon cloth (c) [56]. reactant over the whole electrode. The rib coverage results in a longer mass transport length in the anode DL under the rib region, causing a non-uniform distribution of methanol over the electrode [58]. Third, methanol solution flowing in a serpentine or interdigitated flow field may cause a pressure difference between two adjacent channels, leading to channel-to-channel convection under ribs (referred to as under-rib convection hereafter), which may also contribute to the in-plane methanol transport in the anode DL. These effects can be implicitly incorporated into the effective mass transfer coefficient in the anode DL k dl, which depends on the thickness, permeability and porosity of the anode DL, methanol diffusivity, width of the collector ribs and channels, CO 2 gas void faction, and the channel-to-channel pressure difference. The above discussion indicates that the mass transport of methanol from the flow field to the anode CL in principle depends on the hydrodynamic mass transfer coefficient between the flow channel and the BL surface k s and the effective mass transfer coefficient in the anode DL k dl. Although it is rather difficult to separately determine k s and k dl, the overall mass transfer coefficient of the mass transport from the flow channel to the anode CL for given DMFC component hardware can be experimentally determined by measuring the limiting current density as follows [3]: k overall ¼ i lim 6F C in i lima 12FQ where C in represents the concentration of the feed methanol concentration, and Q is the flow rate of methanol solution Effect of the operating conditions The DMFC operating conditions, including the flow rate of methanol solution, discharging current, temperature, and other parameters, can affect the mass transport process of methanol from the channel to the anode CL and the corresponding overall masstransport coefficient [3,4,60 63]. The discharging current may be an important parameter that affects the mass transport of methanol, as it is related to the rate of CO 2 generation [3,16,37]. On one hand, an increase in current will lead to an increase in the gas void fraction in the flow channel. At a fixed flow rate of methanol solution, increasing the gas void fraction in the flow channel increases the liquid-phase velocity. The increased liquid velocity can lead to a higher hydrodynamic mass transfer coefficient h at the channel/anode DL interface, enhancing the through-plane methanol transport. On the other hand, however, the increased gas void fraction in the flow channel will also reduce the effective transport area for methanol transport from the flow channel to the anode DL surface, increasing the throughplane methanol transport resistance. Moreover, the increased gas void fraction in the anode DL as a result of the increased current density may reduce the methanol transport path such that the through-plane mass transfer resistance of methanol in the anode DL is increased. Therefore, there exists a competition between these effects when increasing the current density. Xu et al. [3] experimentally investigated the effect of discharging current, on the mass transport of methanol in a DMFC with both the serpentine and the parallel anode flow field. The results are shown in Fig. 6.Itis clear that the overall mass transfer coefficient was nearly independent of the current density for both of the flow fields, indicating that the increased hydrodynamic mass transfer coefficient h at the channel/anode DL interface is compensated by the lowered mass transfer of methanol in the anode DL. Therefore, it can be concluded that the discharging current has little effects on the overall methanol mass transfer coefficient in the DMFC. The flow rate of methanol solution can also influence the mass transport of methanol [3,14,19]. Increasing the flow rate can enhance the in-plane mass transport of methanol and reduce the difference of methanol concentration between the inlet and the outlet, thereby leading to a more uniform distribution of methanol over the electrode. In the meantime, the hydrodynamic mass transfer coefficient h at the channel/anode DL interface and the (6)

8 282 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) a b Overall methanol transport coefficient, k overall, m s -1 Overall methanol transport coefficient, k overall, m s x x x x Methanol flow rate 0.5x Current density, ma cm x x x x x ml min ml min -1 Methanol flow rate x Current density, ma cm -2 Fig. 6. Overall methanol transfer coefficient vs. cell current density for different flow rates (temperature: 75 C): (a) serpentine flow field, and (b) parallel flow field [3]. removal rate of immobile gas slug covering the anode DL can also increase with increasing the flow rate [20,35]. As a result, the through-plane mass transport of methanol is also enhanced with increasing the flow rate of methanol solution. Xu et al. [3] have demonstrated that increasing the flow rate of methanol solution led to an increase in the overall mass-transport coefficient, as shown in Fig. 7. In addition, Scott et al. [37] also investigated the variation in the overall mass transfer coefficient with temperature. They found that the coefficient could increase from 3.6e 6 to 7.2e 6ms 1 as the temperature increased from 70 to 95 C. This approximate doubling in the overall mass transport coefficient with the increase of temperature was mainly attributed to the exponential increase in methanol diffusion coefficient Effect of the flow field design One of the functions of the anode flow field is to distribute methanol to the electrode. Hence, the flow field design plays a significant role on both the through-plane and in-plane mass transport of methanol. Over the past decade, the effects of the anode flow field design on the mass transport of methanol and distribution of methanol over the anode CL have been extensively investigated, including the flow field pattern, size of the channels and ribs, open ratio, and other design parameters. Different flow field patterns, such as parallel, serpentine, plot, zigzag, or combined versions, have been used in the DMFC anode [3,10,29,39,44,52 55,61 66]. Different flow field designs can affect Overall methanol transport coefficient, k overall, m s x x x x x10-5 Fitted curve for the serpentine flow field Fitted curve for the parallel flow field Serpentine flow field Parallel flow field Methanol flow rate, ml min -1 Fig. 7. Overall mass transfer coefficient vs. methanol flow rate for different current densities (temperature: 75 C) [3]. the mass transport of methanol in both the through-plane and inplane directions. First, the gas bubble behaviors in different flow fields are different, which can influence the mass transport of methanol from the channel to the surface of the anode DL, i.e., the through-plane methanol transport. Second, different flow fields result in different liquid pressure distribution through the flow channel, which can influence the in-plane mass transport in the anode DL under the ribs, the so-called the under-rib convection. The under-rib convection can enhance the in-plane methanol transport to the anode CL underneath the ribs, and thus the uniformity of methanol concentration over the anode CL can be improved. The under-rib convection in the serpentine flow field is usually stronger than that in the parallel flow field, hence the use of the serpentine flow field results in better cell performance. Fig. 7 shows the overall methanol transport coefficients with both the serpentine and the parallel anode flow fields [3]. It was found that the overall methanol transfer coefficient for the serpentine flow field was much larger than that for the parallel flow field. More importantly, it was found that the increment of the measured k overall as a result of the increased methanol flow rate for the serpentine flow field was much larger than that of the parallel flow field. The reason is that the under-rib convection increases with the methanol flow rate as a result of the increased pressure drop between the flow channels. The investigation of the in-plane methanol transport enhancement by the under-rib convection can also be found elsewhere [51,62,65]. For example, by modifying the conventional serpentine flow field, Xu and Zhao [51] developed a new flow field with enhanced under-rib convection to achieve the better methanol transport and more uniform distribution. More recently, Hyun et al. [62] numerically investigated the distribution of the methanol concentration with different flow fields, including parallel, serpentine and zigzag patterns. It was found that the zigzag flow field induced more uniform under-rib convection than did the other types of flow fields, and thus led to more uniform distribution of methanol over the whole electrode. Both the channel size and the rib size can also affect the through- and in-plane mass transport of methanol. Varying the channel size, such as channel width or channel depth, may influence the gas bubble behavior and change the liquid velocity in the channel, thereby the cell performance. Yang and Zhao [35] investigated the effect of channel width of a serpentine flow field on cell performance. It was shown that decreasing the channel width from 3 to 1 mm enhanced both through- and in-plane mass transport of methanol as a result of the faster gas bubble removal and enhanced under-rib convection, thereby leading to better cell performance.

9 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) Wong et al. [52] investigated the effect of channel size of a microscale serpentine flow field, with the channel depth varying from 1000 mmto410mm. It was found that reducing the channel depth of the micro-scale channel can lead to two opposite impacts on the mass transport. On one hand, the reduction in channel size can lead to a higher liquid flow rate, which will yield a higher mass transfer coefficient at the channel/anode DL interface and under-rib convection, and thus improving the cell performance. On the other hand, however, the reduction in channel size can also cause serious gas slug blockages, which can hinder the mass transport of methanol, thus degrading cell performance. The trade-off between these two effects resulted in an optimal channel depth for the same channel width and the same open ratio when the same methanol flow rate was supplied. Similarly, Amphlett et al. [64] reported that for the parallel flow field, an intermediate channel depth showed the best cell performance. The open ratio is another key parameter of the flow field design. Increasing the open ratio can enhance the mass transport of methanol and improve the uniformity of methanol distribution, which has been proved by Yang and Zhao [35]. However, a too high open ratio may also lead to a sharp increase in the internal cell resistance since the flow field also acts as the current collector in DMFCs, degrading the cell performance. Therefore, the open ratio of the anode flow field should be as high as possible to distribute the methanol uniformly without dramatically increasing the internal cell resistance. In this regard, a noticeable try has been reported by Arisetty et al. [39,54], who used metal foam with very high open ratio as the anode flow field of a DMFC Effect of the anode diffusion layer As one of the key components of a DMFC, the anode DL typically consists of a BL and a coated thin MPL. Because of its unique importance, the effect of the anode DL on methanol transport has been studied extensively [4,19,22,25,28,36 38,42,67 72]. Let us discuss about the mass transport in the anode BL first. The BL, thicker and having larger pores (ranging from 20 to 80 mm) than the MPL, is not only to facilitate the through-plane mass transport of methanol from the flow channel to the electrode surface, but also to enhance the in-plane mass transport and make the reactant more uniformly distribute over the entire electrode. The mass transport of methanol in the anode BL is affected by material properties and design parameters, such as wettability, pore size and distribution, porosity, thickness, and others. The details are discussed below. Since the mass transport of methanol is accompanied with the liquid-gas two-phase flow in the anode BL, the effect of wettability is significant. The BL wettability can be changed by PTFE treatment. The effect of the PTFE content in the anode BL on mass transport and cell performance has been investigated by several researchers [4,36,42,68,71]. It was observed that an increase in the PTFE content led to an increase in the through-plane mass transfer resistance of methanol, as the increased hydrophobic pores as a result of the increased PTFE content resisted the liquid transport and led to a decrease in the liquid saturation in the BL. In addition to the change in wettability, it should be mentioned that the PTFE treatment may also change the pore morphology of the BL. Xu et al. [4] showed that during the PTFE treatment of carbon papers, PTFE was prone to form thin films, which covered open pores and thus increased the mass transport resistance. Therefore, from the viewpoint of enhancing methanol transport to reduce the concentration polarization, the BL without PTFE treatment is preferred in the anode of a DMFC. To enhance the mass transport of methanol, some researchers even made the BL more hydrophilic by adding some hydrophilic agent, such as Nafion [42,68]. It was shown that adding an appropriate amount of Nafion (e.g., 10 wt%) could enhance the methanol transport due to the more hydrophilic property, but too much Nafion also resulted in a lower porosity and blocked the transport of methanol and CO 2 in the anode [42,68].To achieve more hydrophilic anode BLs, the pores of the carbon paper/ cloth can also be partially filled with certain hydrophilic metal oxide compounds, such as aluminum oxide (Al 2 O 3 ), tin oxide (SnO 2 ), titanium oxide (TiO 2 ), and ruthenium oxide (RuO 2 ) et al. [67,70]. The effect of the thickness of the anode BL has also been investigated [4,36]. Xu et al. [4] have shown that the in-plane methanol transport depended highly on the thickness of the anode BL. When the BL was too thin, methanol was difficult to penetrate to the regions under ribs, which caused the local methanol concentration to be low, increasing the concentration polarization. It was found that a thicker BL could lead to a more uniform methanol distribution over the electrode, thereby improving cell performance. On the other hand, however, it was found that the throughplane mass transfer resistance increased with the increase of the BL thickness. It should be note that the increased mass transfer resistance as a result of using a thicker BL is not a problem, as there is plenty of room to increase the feed methanol concentration at the inlet of the flow field. In addition, the effect of the porosity of the BL has also been numerically investigated by Yang and Zhao [19], which showed that an increase in the porosity led to a significant decrease in the mass transport resistance. Changing the properties of the BL can also be achieved by using different porous materials, such as carbon paper, carbon cloth, metal wire cloth, metal foam, stainless steel fiber felt, and so on [32 34,36,54,72]. The different porous structure and wettability between these different porous materials may lead to different mass transport behaviors of gas CO 2 and methanol. For example, metal wire cloth [36] and stainless steel fiber felt [72] were also found to show superior methanol transport and cell performance due to their more hydrophilic property and good porous structures. In addition to the effect of the BL on the mass transport of methanol, although thinner, the MPL plays a more important role in the mass transport process. The material properties and design parameters of the MPL, such as carbon type, wettability, porous structures, and thickness affect the mass transport of methanol, mainly in the through-plane direction [25,68,70]. Lu et al. [25] found that a more hydrophobic MPL could significantly increase the through-plane mass transport resistance. As such, a higher methanol concentration can be fed to the DMFC. Shao et al. [68] investigated the effect of carbon type on the mass transport using carbon powders with different carbon diameters and pore structures. It was shown that smaller carbon powders with higher surface area and more hydrophilicity, such as Black Pearl 2000, could enhance the through-plane mass transport of methanol. They also found that the hydrophilic MPL with Nafion as binding agent yielded a smaller through-plane mass transport resistance than did the hydrophobic MPL using PTFE as the binding agent, and a thicker MPL led to a larger mass transport resistance. In summary, the mass transport of methanol at the anode is influenced by the material properties and design parameters of the anode DL and flow field as well as operating conditions. It should be emphasized whether the mass transport of methanol needs to be enhanced or suppressed depends on the feed methanol concentration. The key is to achieve a higher rate of mass transport everywhere over the surface of the MEA under the condition of a lower rate of methanol crossover, as discussed next. 5. Mass transport of methanol through the membrane (methanol crossover) This section addresses the mass transport of methanol through the membrane. In general, methanol crossover through the

10 284 T.S. Zhao et al. / Progress in Energy and Combustion Science 35 (2009) membrane is due to three transport mechanisms [73]: electroosmotic drag by proton transport, diffusion by methanol concentration gradient, and convection by the hydraulic pressure gradient between the anode and the cathode. The total flux of methanol crossover can thus be expressed as [73]: c ac=m c cc=m J mc ¼ D m þ c ac=mk m DP i þ X l m l m m ac=m n d (7) F where D m is the effective diffusivity of methanol in the membrane; l m is thickness of the membrane; c ac/m and c cc/m represent methanol concentration at the interface between the anode CL and the membrane and between the membrane and the cathode CL, respectively; K m is a constant related to the effective hydraulic permeability; m is the viscosity of liquid methanol solution; DP is the difference in the liquid pressure across the membrane; n d is the electro-osmotic drag coefficient of water; and X ac/m is the methanol molar fraction at the interface between the anode CL and the membrane. Typically, methanol crossover depends greatly on diffusion across the membrane. As a result, the rate of methanol crossover can be reduced by decreasing the diffusivity, increasing the membrane thickness or reducing the methanol concentration at the anode CL. The effects of membrane compositions, MEA design, and operating conditions on methanol crossover have been extensively studied Effect of the operating conditions It has been revealed that the flux of methanol crossover highly depends on the operating conditions of DMFCs, including anode methanol concentration, current density, temperature, cathode pressure, anode flow rate, and so on [71,73 77]. Fig. 8 illustrates the effect of the feed methanol concentration and discharging current density on methanol crossover. The rate of methanol crossover increases with increasing the concentration of the feed methanol solution, due primarily to the increased diffusion flux from the anode to cathode. Unlike the effect of methanol concentration, the effect of the discharging current density on methanol crossover is more complex. The current density changes the rate of methanol crossover mainly by the electro-osmosis. However, it should be noted that the rate of methanol crossover caused by the electroosmotic drag also depends on the methanol concentration at the MeOH permeation equiv. current (ma cm -2 ) MeOH conc. (M) Cell current density (A cm -2 ) Fig. 8. The rate of methanol crossover through a DMFC with a Nafion 117 membrane at 90 C. The rate is given as a current equivalent to complete oxidation to CO 2 of all methanol arriving at the cathode [77] anode CL/membrane interface, as indicated by Eq. (7). At the relatively low methanol concentration, the contribution of the electro-osmosis to the total flux of methanol crossover is small. Therefore, the rate of methanol crossover mainly depends on diffusion at low methanol concentration operating conditions. An increase in the current density results in a lower methanol concentration in the anode CL as a result of the higher methanol consumption rate. The lowered methanol concentration at the interface reduces the diffusion of methanol from the anode to the cathode, thus decreasing the rate of methanol crossover. However, this is not the case at relatively high methanol concentration operating conditions. In this case, the methanol concentration at the interface between the anode CL and the membrane is always high. Increasing the current density may significantly increase the electro-osmotic drag flux. The increase in the electro-osmotic drag flux overweighs the decrease in the diffusion flux. As a result, the rate of methanol crossover increases with current density, as shown in Fig. 8. In addition, previous experimental studies [71,74] revealed that the operating cathode gas pressure and operating cell temperature also showed a significant effect on methanol crossover. Pressurizing the cathode gas could reduce methanol crossover due to the enhanced back convection from the cathode to the anode, while increasing the temperature led to an increase in methanol crossover due to the increased methanol diffusivity. Moreover, the methanol flow rate can also affect methanol crossover. Increasing the methanol flow rate will enhance both the through-plane and in-plane methanol transport, thus increasing the average methanol concentration in the anode CL. This will obviously increase the flux of methanol crossover, which has been reported by Han and Liu [73] Effect of the membrane At present, Nafion membranes are commonly used for DMFCs. Methanol crossover through this type of membrane is usually significant [73,78,79]. The common strategy to mitigate methanol crossover is to use a thicker Nafion membrane. Eq. (7) clearly indicates that increasing the thickness of membrane can significantly decrease the methanol diffusion flux. However, a thicker membrane leads to an increase in the internal cell resistance, degrading cell performance. The trade-off between these two effects turns out that a Nafion membrane with an intermediate thickness, such as Nafion 115 or 117, exhibits the best performance. Other active approaches to reduce methanol crossover is to develop alternative materials that are less permeable to methanol, i.e., with low methanol diffusivity and electro-osmotic drag coefficient. Extensive efforts have been devoted to the modification of fluorinated and non-fluorinated membrane (PBI (polybenzimidazole), speek (sulfonated polyetheretherketone), AMPS (asymmetric based acrylic), DPS (diphenylsilicate), etc.) through the addition of inorganic components, or by depositing a super thin barrier layer made of the methanol-impermeable proton conducting (MIPC) materials such as Pd [74,80 85]. However, the new membrane materials that have lower methanol permeability usually exhibit lower proton conductivity than the original Nafion membrane. Although this problem could be minimized by employing a thinner membrane, but thinner membranes could potentially result in a poor mechanical strength and long-term stability problems [84]. Therefore, the development of an ideal lowcost membrane with a low methanol permeability and higher proton conductivity and durability are still underway. More detailed information about the-state-of-the-art status of the membranes for DMFCs can be found elsewhere [83].