Effect of stainless steel meshes on the performance of passive micro direct methanol fuel cells

Effect of stainless steel meshes on the performance of passive micro direct methanol fuel cells

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Effect of stainless steel meshes on the performance of passive micro direct methanol fuel cells ~ o a,**, J.P. Pereira b, A.M.F.R. Pinto a,* D.S. Falca a

CEFT, Departamento de Engenharia Quı´mica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, S/n, 4200-465 Porto, Portugal b TU Delft, Department of Biotechnology, Office 1.841, Julianalaan 67, 2628 BC Delft, The Netherlands

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abstract

Article history:

An important issue related with Micro-DMFC is the two-phase flow that can occur on both

Received 3 December 2015

sides of the cell. An efficient product removal is crucial to obtain higher performances,

Received in revised form

especially in passive Micro-DMFC. Passive devices do not need moving parts to feed

4 May 2016

oxidant and fuel to the cell, they use only natural convection/diffusion to fuel and oxidant

Accepted 5 May 2016

supply without any additional power consumption, therefore the product removal is less

Available online xxx

efficient than in active ones. In the expectation of improving products removal from the cell, stainless steel meshes can be used between the membrane electrode assembly (MEA)

Keywords:

and the current collectors. Several meshes with different characteristics are available in

Passive micro-direct methanol fuel

the market. In this work, different meshes are tested in a passive fuel cell with 2.25 cm2 of

cell

active area at the anode and/or cathode sides looking for higher performances. Visuali-

Stainless steel mesh

zation studies at the anode side using a digital camera are also performed and constitute

Two-phase flow

an important tool to observe carbon dioxide bubbles' profiles. The significantly higher

Visualization studies

performances obtained when using the meshes were explained by the combined effect of an enhanced current collection and a methanol crossover reduction leading to better distributed CO2 bubbles pattern. The best power output obtained was 29.3 mW/cm2 using stainless steel meshes at both the anode and cathode sides, which is almost the double when comparing to the one obtained with the cell operating without mesh. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Micro Direct Methanol Fuel Cells (Micro-DMFC) have attracted much attention since they use a liquid fuel, easy to handle and store, and their miniaturization promises higher efficiency and performance for power generating devices. The potential of micro direct methanol fuel cells is generally recognized as a possible technology to the replacement of conventional batteries [1e3]. This technology provides high power density with an

attractive cost-to-power ratio. However, for Micro-DMFCs to become commercially feasible, the control of the multiphase flows at the micro scale is a crucial issue. The CO2 bubbles formed at the anode side can disturb or even block the flow. On the cathode side, the water produced is injected into the channels developing a two-phase flow that hinders the water management [4e6]. Special attention has been devoted in the last years to avoid channels obstruction caused by carbon dioxide and/or water accumulation. A promising option reported by several authors seems to be the use of stainless steel meshes

* Corresponding author. ** Corresponding author. ~ o), [email protected] (A.M.F.R. Pinto). E-mail addresses: [email protected] (D.S. Falca http://dx.doi.org/10.1016/j.ijhydene.2016.05.059 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. ~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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placed between the MEA and the flow channels. The purpose of these meshes is to help the carbon dioxide/water removal and to increase the cell performance. Scott et al. [7] first examined the feasibility of using stainless steel mesh (SSM) materials as flow beds for active DMFCs, and verified promising behaviors in terms of gas removal and electrical cell performance. Furthermore, it was clear from the results that the cell performance was influenced by the type and geometry of the mesh used. The authors verified that small open ratios, combined with large mesh strand width and thickness, could significantly improve the cell performance. Shao et al. [8] developed a novel anode structure based on Ti mesh for DMFCs, thinner and more hydrophilic than the conventional porous carbon-based anode. Although the Ti mesh anode showed an enhanced performance compared to that of the conventional anode in a DMFC operating with low methanol concentrations, it showed a lower performance when higher concentrations of methanol were employed. The authors [8] related this fact with its opening structure, which facilitated methanol crossover to the cathode. Chetty et al. [9] prepared platinum based binary and ternary catalysts onto a titanium mesh by using thermal decomposition, and investigated the performance of the mesh-based fuel cell. When operating with 1 M methanol at 80  C, the authors attained maximum power densities of 38 mW/cm2 and 22 mW/ cm2 using 1 bar of oxygen and air, respectively. Zhu et al. [10] produced a passive 8-cell DMFC twin-stack for long-term operation with titanium nitride (TiN) plated mesh between the current collectors and the MEA. This was used to minimize the internal resistance of the stack. The peak power density of the stack was 7 mW/cm2 and 10.2 mW/cm2 at 3 M methanol without and with mesh, respectively. Zhang et al. [11,12] designed and investigated a high-performance metallic Micro-DMFC. The research group [11,12] added two sheets of SSM coated with TiN between the MEA and the current collectors of an active MicroDMFC, and obtained high performances. Furthermore, the authors concluded that lower open ratios generated better cell performances, since a lower open ratio represents a larger contact area between mesh and GDL resulting in greater current-collecting ability. On the other hand, the smaller the opening area the better the cell performance, due to the fact that smaller openings produce smaller CO2 bubbles and smoother two-phase flow patterns in the channels. A recent work, performed by Shivrastava et al. [13], suggests stainless steel wire meshes to replace current collectors. The authors found a more uniform fuel distribution at catalyst layer and a higher cell temperature with the use of these meshes as current collectors. Better performances were found using the mesh made of relatively thick wires. This study identify the stainless steel wire mesh as promising material to replace the perforated plate current collectors in passive DMFC's. These studies have revealed that employing stainless steel meshes can significantly enhance the DMFC performance. In general, stainless steel meshes may have several functions such as inhibiting methanol crossover, reducing the internal resistance, improving electrical conductivity, drawing CO2 bubbles from the anode GDL and breaking them into smaller sizes via the mesh structure, and compressing GDLs more homogeneously to prevent deformation caused by the current collectors [7,11,12]. The impact of the mesh geometry on cell performance can be analyzed by operating cells with different

open ratios and opening areas and by comparing the results with data reported in available literature. The analysis of two-phase flow phenomena within MicroDMFCs allows a deeper understanding of the coherence of fluid motion, channel blockage, and cell performance. The most elemental method of experimental investigation is flow visualization, an effective method to investigate qualitatively the dynamic behavior of CO2 gas bubbles and water droplets within fuel cells. Several authors reported the use of digital camera systems for in situ visualization of the two-phase flow through transparent fuel cell fixtures, capturing motionless pictures that allow to relate two-phase effects and cell polarization [14e16]. Different approaches were also attempted by other authors for flow visualization in micro-channels such as laser induced fluorescence techniques [17,18] and, most recently, micro-PIV [19e21]. However, these techniques still encounter serious seeding problems, especially regarding micro-gas flows. In this work, visualization studies are performed using a digital camera system at the anode side to investigate the CO2 bubbles formation and profile during the experiments with a passive micro methanol fuel cell. Several meshes are tested at the anode and/ or cathode micro fuel cell sides looking for higher performances.

Experiment The experimental fuel cell is constituted by: - Two acrylic end plates: the anode one is 1 cm thick with a 1.8 cm3 methanol reservoir and the cathode one is opened and 4 mm thick; - Two stainless steel current collectors: the anode has holes on the active area with a diameter of 1 mm, and the space between holes is 0.5 mm; the cathode has holes on the active area with a diameter of 1 mm and the space between holes is also 1 mm; - Two stainless steel meshes: the characteristics of the meshes used are presented in Table 1. - Two silicon gaskets; - One MEA: the MEA used is based on Nafion 117, with a catalytic load of 4 mg/cm2 PtB at cathode side and 4 mg/cm2 Pt/ Ru at anode side. The GDL layer used on both sides is carbon cloth PTFE treated with MPL with a thickness of 410 mm. A picture of the passive fuel cell is presented in Fig. 1. In the experiments, the Micro-DMFC with an active area of 2.25 cm2 was operated at atmospheric pressure and room temperature by feeding aqueous methanol solution into the anode reservoir. In preliminary experiences, to obtain the optimum methanol concentration, different methanol

Table 1 e Stainless steel meshes characteristics used in this work. Denomination Open ratio (%) Mesh Mesh Mesh Mesh

1 2 3 4

31.0 38.7 44.3 51.9

Opening area (mm2)

Strand width (mm)

0.032 0.069 0.247 0.837

0.140 0.160 0.250 0.355

~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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The open-circuit voltage is much lower than the thermodynamic equilibrium cell voltage (1.21 V) mainly due to the occurrence of methanol crossover. This effect is well known and considered as a major drawback in DMFCs technology. It reduces the fuel utilization and also depresses the cathode potential. Furthermore, the permeated methanol on the cathode reacts electrochemically with oxygen at Pt-based oxygen reduction reaction sites. The effect of methanol concentration on fuel cell performance could lead to two different consequences: an increase on methanol concentration leads to a higher coverage of the electrocatalyst sites by methanolic species, but also increases the concentration gradient between the anode and cathode side with a resulting increase of methanol crossover through the membrane. Therefore, the influence of methanol concentration on cell performance is a result of both the positive and negative effects referred before. The results from Fig. 2 show that the cell voltage and power density increase with an increase of the methanol feed concentration, until a concentration of 4 M. Until this value of concentration, it seems that the methanol oxidation enhancement due to higher concentrations is more relevant than the negative effect on methanol crossover. Using a 5 M methanol solution leads to performance losses, because higher methanol concentrations result in increased methanol crossover. At the cathode side, methanol reacts with the oxygen to form a mixed potential. Therefore, higher methanol concentrations lead to higher mixed potentials, inducing a lower cell performance. Visualization tests are crucial to obtain qualitative information about two-phase flow issues. Taking into account this premise the cell was photographed at the anode side for each value of current imposed, after 3 min. In Fig. 3 the pictures are presented for the current values indicated, for the best performance obtained using 4 M methanol concentration. Fig. 3 shows a similar bubble profile for all the imposed current density values with a bubble appearing at the superior left side of the cell. During the experiments, it was slowly released through the small opening in cell fixture and substituted by a similar one appearing at the same place. The main difference found in the two-phase flow pattern for

Fig. 1 e Picture of the passive Micro-DMFC used in the experiments.

concentrations in the range of 1e5 M were used operating the cell without SSM. In the following experiences the effect of the different SSM is studied at the anode and cathode sides. Visualization studies are also performed using a Canon EOS 30D camera. The pictures were taken at the anode side regarding the open-circuit current (initial time ¼ 0 s), and then for each current value imposed after regular time intervals of 3 min.

Results and discussion The performance of the passive Micro-DMFC was determined by a set of experiments, in order to obtain the voltage and power density polarization curves. The influence of methanol concentration and the effect of stainless steel meshes placed at anode and/or cathode side was carefully investigated. In Fig. 2 the polarization and power density curves for five methanol concentrations (1e5 M) tested are presented for the cell operating without mesh. These preliminary experiences were performed in order to determine the methanol concentration that conducts to higher cell performances. 0.6

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5M - V

1M - P

2M - P

3M - P

4M - P

5M - P

Fig. 2 e Voltageepower polarization curves for different methanol concentrations. Operating conditions: temperature 25  C, pressure 1 atm. ~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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increasing values of current density was the higher rate of bubble formation and release which was expected due to the increased electrochemical reaction rates. The use of stainless steel meshes at the anode and/or cathode side placed between the current collectors and the MEA seems to be advantageous to improve fuel cell performances. Taking this idea into consideration, four SSMs with different characteristics were independently tested at the anode and cathode sides. Fig. 4 represents the voltageepower polarization curves for the four different stainless steel meshes used exclusively at the anode side. For all the cases tested, the added meshes conduct to better performances when compared to the cell operation without mesh. The best performances were

obtained with mesh 4, the mesh with larger open ratio, opening area and strand width. This goes against the statements of Scott et al. [7] and Zhang et al. [11,12], who verified that smaller open ratios and opening areas lead to higher cell performances. This can be due to the fact that these authors tested SSMs on active fuel cells, while in this study the SSMs were tested on a passive fuel cell. In the passive system, small opening areas may cause a higher resistance within the cell, decreasing its efficiency; therefore the results obtained in this study are in accordance with this observation. Visualization of the bubbles profile formed when using mesh 4 at the anode side showed no significant difference when compared to the ones obtained when the cell was operated without any mesh (please see Fig. 3). Moreover, for

Fig. 3 e Anode cell side pictures at different current values for the cell operating without any mesh. ~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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Without Mesh - V

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Mesh 2 - P

Mesh 3 - P

Mesh 4 - P

Fig. 4 e Voltageepower polarization curves for different stainless steel meshes used at the anode side. Operating conditions: temperature 25  C, pressure 1 atm, methanol concentration 4 M.

the four different meshes tested, the bubble profiles follow the same trend. The better performances obtained when using the meshes for the methanol concentration tested, can be explained by the combined effect of an enhanced current collection and a methanol crossover reduction. On one hand, the metallic mesh acts as an additional metallic surface increasing the collection of current and on the other hand, the mesh placed at the anode side increases the resistance to the methanol crossover due to an increased higher anode thickness. Less methanol crosses the membrane to the cathode side, resulting in a higher methanol concentration in anode side available for anodic reaction, conducting to better cell performances. Furthermore, the importance of the reduction of methanol crossover is put in evidence by the significantly higher values of the OCV obtained when using the different meshes (Fig. 4). The positive effect of introducing the mesh structure between the anode current collector and the anode catalytic layer is

also confirmed by the significantly higher limiting current density achieved when using meshes 1 to 4. A similar analysis to the one performed for the anode side was implemented for the cathode side. Four different meshes were placed at the cathode side and the cell performance was evaluated. The voltageepower polarization curves obtained are presented in Fig. 5. Fig. 5 shows that the cell performance increases when using the four meshes tested, compared to the cell tested without any mesh. Moreover, the best result was obtained for the mesh 3. Visualization studies were also performed and are presented in Fig. 6 for the best performance, using mesh 3 operating the cell at the values of current density indicated. The CO2 bubble's profile is quite different from the previous presented pictures. The bubbles, in a much higher number, are now dispersed. The introduction of the SSM at the cathode side, between the MEA and the current

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Fig. 5 e Voltageepower polarization curves for different stainless steel meshes used at cathode side. Operating conditions: temperature 25  C, pressure 1 atm, methanol concentration 4 M. ~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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Fig. 6 e Anode cell side pictures at different current values for the cell operating with mesh 3 at the cathode side.

collector has a very positive impact on the cell performance due to a combination of different effects. The SSM surface acts as an additional distributor of current, facilitating the ORR reaction (starving for electrons) and consequently preventing the poisoning of the Pt sites by methanol crossing the MEA. This enhancement in distribution of current has impact both in the decrease of the cathode overpotential and also contributes to a decrease of the crossover due to a lower methanol concentration gradient. More methanol will react at the right place, the anode catalytic layer, increasing the bubbles number and improving the gas distribution over the active area. Additionally, the authors also believe that the SSM placed at the

cathode side can eventually help on the removal of water droplets, as suggested by Yi et al. [22]. Given the better performances obtained when using SSMs, combinations of meshes used simultaneously at the anode and cathode sides were also explored. The polarization curves obtained are presented in Fig. 7. Taking into account that better results are obtained when using mesh 4 at the anode side and mesh 3 at the cathode side, the authors attempted to test this combination simultaneously. However, due to the imposed compression and leaking problems, since these meshes are thicker, the experiment could not be accomplished. Moreover, the combination of mesh 3 both at the anode side and the cathode side could not be investigated due

~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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Without Mesh - P

Mesh 2 a, Mesh 3 c - P

Mesh 2 a, Mesh 2 c - P

Mesh 1 a, Mesh 2 c - P

Fig. 7 e Voltageepower polarization curves for different stainless steel meshes used at both sides: a e anode and c e cathode. Operating conditions: temperature 25  C, pressure 1 atm, methanol concentration 4 M.

to the same reason. The tests were thus performed considering a combination of mesh 2 simultaneously at the anode and the cathode sides, mesh 2 at the anode and mesh 3 at the cathode, and mesh 1 at the anode and mesh 2 at the cathode. As can be seen in Fig. 7, better results are obtained when mesh 2 is used on both anode and cathode sides. Visualization studies were also performed for all the experiments and the pictures corresponding to the best performance are presented in Fig. 8. The CO2 bubbles profile seems to be a mix between the ones presented for the cases using SSM at the anode and cathode sides separately. The generation of a larger bubble can be seen at the superior left side regarding lower current values, but considering larger current values, it is also visible the occurrence of small bubbles on a great extent of the active area. For higher current values, both half electrode reaction rates increase due to a better coverage of the electrocatalyst sites by methanolic species at the anode and to an enhanced current distribution at the cathode which in turn positively affects the methanol crossover as previously explained. Higher current values lead therefore to an increase on the CO2 bubbles production rate with a higher number of bubbles occupying a larger portion of the active area. Table 2 summarizes the maximum power outputs obtained for the best results considering each set of experiments performed. The best cell performances were obtained using the mesh 2 on both sides. Comparing the impact of the use of stainless steel meshes on the anode and cathode side independently, it seems to be more advantageous the SSM use on the anode side, however the differences in the cell performance regarding these cases are not as significant as when compared to the cell performance without SSM. Comparing the power density obtained in this work operating without mesh and the ones reported by Torres et al. [23] (13 mW/cm2) and by Ahmad et al. [24] (14 mW/cm2) in similar operating conditions, the results are quite satisfactory. Zhang et al. [25] obtained a power output of 27.1 mW/cm2 using a self-breathing micro DMFC (methanol is supplied in active way) with an improved cathode current collector with titanium. Even compared with this semi-passive micro DMFC, the

results presented in this work using stainless steel meshes at both sides are bigger. The great results using meshes at both cathode and anode sides are superior to the power densities obtained with larger passive methanol fuel cells. Chen et al. [26] obtained a maximum power density of 20 mW/cm2 with a passive fuel cell with 4 cm2 of active area while Tang et al. [27] reached 10.7 mW/cm2 operating with a passive fuel cell with 9 cm2 active area.

Conclusions In this work, the effect of the SSM use placed between the GDL and the current collectors at the anode and/or cathode side of a passive Micro-DMFC on the cell performance was studied. Preliminary tests indicated that the best methanol concentration was 4 M for the cell operating without mesh. The subsequent tests were done using this methanol concentration value. Several meshes with different characteristics were tested separately at the anode and cathode sides. All the experiments done using stainless steel meshes generated better performances. Meshes with larger open ratios, opening areas and strand width seem to produce better results; however they also lead to higher compression and leaking problems at cell clamping. SSM combinations used simultaneously at the anode and cathode sides were also studied. The best performance obtained concerning power density was 29.3 mW/cm2 using mesh 2 at both the anode and cathode sides, which is almost the double when comparing to the one obtained with the cell operating without mesh. Visualization tests were also performed for all the experiments for the anode side and different CO2 bubbles profiles were found. The pattern leading to higher performances consists on the formation of a large bubble at the superior left side of the cell, along with several small bubbles distributed through the active area. The results presented with a passive Micro-DMFC operating with stainless steel meshes demonstrated that the SSM use is a simple and effective way to significantly increase the cell performance.

~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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Fig. 8 e Anode cell side pictures at different current values for the cell operating with mesh 2 at the anode and cathode sides.

Comparison of the obtained best power densities, with values from literature, even for larger passive methanol fuel

Table 2 e Maximum power output of the more relevant data obtained in this work. Mesh used Without mesh Mesh 4 at anode side Mesh 3 at cathode side Mesh 2 at anode side and mesh 2 at cathode side

Power output (mW/cm2) 16.1 25.6 23.8 29.3

cells, suggests that size reduction does not compromise cell performance.

Acknowledgments ~ o acknowledges the post-doctoral fellowship (SFRH/ D.S. Falca ~ o para BDP/76063/2011) supported by the Portuguese “Fundac¸a ^ncia e Tecnologia” (FCT), POPH/QREN and European Soa Cie cial Fund (ESF). The project PTDC/EQU-FTT/112475/2009 supported by FCT is also acknowledge. POCI (FEDER) also supported this work via CEFT.

~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059

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~ o DS, et al., Effect of stainless steel meshes on the performance of passive micro direct methanol Please cite this article in press as: Falca fuel cells, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.059