Passive Air-breathing Direct Methanol Research Articles

Developing a passive air-breathing tubular direct methanol fuel cell fed with concentrated methanol

ABSTRACTThis study develops and investigates a fully passive air-breathing tubular direct methanol fuel cell (t-DMFC) with a steel-tube anode and a steel-mesh cathode. The effects of methanol concentration, cathode catalyst loading, mesh structure, and forced air convection are experimentally explored. Results indicate that the t-DMFC performs better at a relatively higher methanol concentration of 8 M. It is recommended to use a catalyst loading of 4 mg cm−2. Both the electrochemical impedance spectroscopy (EIS) and performance tests confirm that the 40-mesh setup is preferred at the cathode. The fuel cell yields a poor performance when the cathode works with forced air convection because the air-blowing operation reduces the cell temperature and this effect dominates the cell performance. The dynamic and constant-load behaviors are also inspected.

Lightweight current collector based on printed-circuit-board technology and its structural effects on the passive air-breathing direct methanol fuel cell

To realize lightweight design of the fuel cell system is a critical issue before it is put into practical use. The printed-circuit-board (PCB) technology can be potentially used for production of current collectors or flow distributors. This study develops prototypes of a single passive air-breathing direct methanol fuel cell (DMFC) and also an 8-cell mono-polar DMFC stack based on PCB current collectors. The effects of diverse structural and operational factors on the cell performance are explored. Results show that the methanol concentration of 6 M promotes a higher cell performance with a peak power density of 18.3 mW cm−2. The combination of current collectors using a relatively higher anode open ratio and inversely a lower cathode open ratio helps enhance the cell performance. Dynamic tests are also conducted to reveal transient behaviors and its dependence on the operating conditions. To validate the real working status of the DMFC stack, it is coupled with an LED lightening system. The performance of this hybrid system is also reported in this study.

Investigating the effect of operating parameters on the open circuit voltage of a passive DMFC

The variations of the open circuit voltages (OCVs) were studied in a passive air-breathing direct methanol fuel cell with an air-breathing cathode using Nafion 115 as the electrolyte membrane. The effects of some operating parameters such as cell temperature, cell orientation, and also methanol concentration on the OCV of fabricated fuel cell were investigated experimentally. The experimental results showed that the OCV values depend strongly on the cell orientation, cell temperature, and methanol concentration. The OCV values decrease with an increase in methanol concentration and cell temperature. Also, the OCV values in vertical orientation are lower than the OCV values in other orientations.

Manufacture, characterization and application of porous metal-fiber sintered felt used as mass-transfer controlling medium for direct methanol fuel cells

Fabrication, characterization and performance of a porous metal-fiber sintered felt (PMFSF) based on multi-tooth cutting and solid-phase sintering were studied. The PMFSF was used as the anodic methanol barrier in a passive air-breathing direct methanol fuel cell to mitigate the effects of methanol crossover. Compared with the commercial SUS316L felt made of bundle-drawn fibers, this self-made PMFSF has larger pore diameter, polarized pore distribution, irregular fiber shape, rougher surface, lower mass flow resistance and evident hydrophobicity. The results reveal that the use of a PMFSF significantly enhances the cell performance since it helps to maintain a balance between the reactant and product management while depressing methanol crossover. The PMFSF with a porosity of 70% yields the highest cell performance at a methanol concentration of 4 mol/L.

The effect of cell orientations and environmental conditions on the performance of a passive DMFC single cell

A single cell passive air-breathing liquid feed direct methanol fuel cell (DMFC) is designed and fabricated. Furthermore, the effects of cell orientation and environmental conditions such as temperature and relative humidity on the performance of such passive DMFC are tested experimentally. The obtained results indicate that both environmental temperature and relative humidity have significant effects on the performance of fabricated fuel cell. The experimental data contained within this work shows that under lower relative humidity and higher temperature, the passive air-breathing direct methanol fuel cell has higher power output and better performance. According to experimental results, flooding has a vital role on the cell performance in various relative humidity and temperatures. The results also show that cell orientation has a strong effect on the performance of passive DMFC. The best power output and performance were achieved under vertical orientation.

Corrosion Behavior of Porous Metal Fiber-Sintered Felt in Both Simulated and Practical Environments of a Direct Methanol Fuel Cell

This study deals with corrosion behaviors of the porous metal fiber-sintered felt (PMFSF), which is intended to be used as the anodic methanol barrier in a passive air-breathing direct methanol fue...

High-concentration operation of a passive air-breathing direct methanol fuel cell integrated with a porous methanol barrier

To realize high-concentration operation of a passive air-breathing direct methanol fuel cell (PAB-DMFC) is a critical issue that should be addressed before it is used for practical applications. This work reports the feasibility of using a porous metal fiber sintered felt (PMFSF) as the anodic methanol barrier to control methanol crossover (MCO) in order to feed the fuel cell with a higher concentration of methanol fuel. The effectiveness of this method is successfully validated. The optimization criterion of using this material is to maintain a balance between reactant and product management. Results demonstrate that a medium value of PMFSF thickness, i.e. 2mm yields a higher performance. The PMFSF porosity has two-fold effect on the cell performance, which interacts with the current collector setup including the opening pattern and ratio. The related mechanisms concerning such effects on mass transfer process are explained in detail. In addition, the dynamic characteristics of this PMFSF-based PAB-DMFC are also evaluated in this study.

Structural diversity and orientation dependence of a liquid-fed passive air-breathing direct methanol fuel cell

This paper investigates the interesting effects of structural diversity and operating orientation on the performance of a liquid-fed passive air-breathing direct methanol fuel cell (PAB-DMFC). The results indicate that a higher thickness of the GEFC®-10N membrane helps enhance the cell performance due to its ability in reducing methanol crossover (MCO). When the cell uses carbon cloth at the anode but carbon paper at the cathode as the diffusion media, it produces higher performances than other combinations. The work also confirms the merit of using a cathode diffusion layer since it improves water, methanol and heat management. As for the structural optimization of current collector, it is recommended to use the circular-hole-array pattern with a lower open ratio at the anode but the parallel-fence pattern with a higher open ratio at the cathode. It is further demonstrated that the vertical operation yields a higher cell performance at a lower methanol concentration while the horizontal operation performs better at a higher methanol concentration. Besides, the effects of opening pattern and working orientation on the CO2 evolvement behaviors are analyzed by using visualized methods. Detailed mechanisms related to the resultant phenomena are comprehensively provided in this work.

Effect of methanol concentration on the operation performances for passive air-breathing direct methanol fuel cell

The effect of methanol concentration on the performances for passive air-breathing direct methanol fuel cell (DMFC) was studied by single cell tests and methanol transfer theory. With the increase of methanol concentration, the Faradic efficiency and energy efficiency were decreased due to the methanol crossover effect. The maximum power density of 13.9 mW/cm2 and a 20-hour stable performance at a discharge current of 60 mA were obtained under the methanol concentration of 4 mol/L. The above results could be attributed to the synergy of methanol transport and methanol crossover.

The function of hydrophobic cathodic backing layers for high energy passive direct methanol fuel cell

Homemade wet-proofing carbon papers with back-flow effect were applied as backing layers in the cathode of passive air-breathing direct methanol fuel cell (DMFC) fed by pure methanol. With the increase of polytetrafluoroethylene (PTFE) content, the carbon papers exhibited different water transport resistance and generated different back-flow effects. Moreover, PTFE-treated carbon papers were observed by scanning electronic microscope (SEM) to investigate the function of the cross-linked microstructure. Maximum energy density (438 Wh L −1) of the improved pure methanol DMFC was obtained by using carbon paper with 40 wt.% PTFE content as the cathodic backing layer. This value was 6 times larger than that of the conventional DMFC fed by 2 M methanol solution.

Operational characteristics of a passive air-breathing direct methanol fuel cell under various structural conditions

The operational characteristics of a small-scale passive air-breathing direct methanol fuel cell (PAB-DMFC) are comprehensively investigated under both steady-state and dynamic conditions. As the most important operating parameter, methanol concentration has significant effects on the cell performance. For different methanol concentrations (e.g., 0.5 and 8 M), the structural adaptations are particularly discussed. The results show that the structural factors are closely related to the influence degree of methanol concentration. In this study, the characteristics of the open circuit voltage (OCV) under various structural and methanol-concentration conditions are presented. Besides, the effects of other operating conditions such as running time, forced air convection and refueling action on the cell performance are also evaluated. In addition, a series of dynamic operations of the PAB-DMFC are conducted under different load cycles. Accordingly, the transient phenomena such as voltage undershoot and overshoot are explored. A fundamental principle for evaluating the operational characteristics of a PAB-DMFC is to simultaneously take into account the mass transfer requirements such as reactant delivery, product removal, methanol/water crossover control and so on.

Dominance evaluation of structural factors in a passive air-breathing direct methanol fuel cell based on orthogonal array analysis

This study comprehensively investigates the dominance of various structural factors in a passive air-breathing DMFC by means of orthogonal array analysis (OAA). Two membrane types, two assembly patterns of the diffusion layer and two open ratios of the current collector are prepared. Three target variables are selected as the performance indexes including the maximum power density (MPD), limiting current density (LCD) and open circuit voltage (OCV). The range analysis (RA) method and effect curves (ECs) are used to characterize the OAA data. The RA results demonstrate that the current collector and diffusion layer combine to dominate the values of MPD and LCD in a wide range of methanol concentrations from 0.5 to 8 M. The dominant structural factors related to the value of OCV at different methanol concentrations are also explored. In addition, the effect curves show that a medium methanol concentration like 2 M generally promotes higher values of MPD and LCD, while a relatively lower methanol concentration like 0.5 M benefits a higher value of OCV than others in a general statistical sense.

Effects of structural aspects on the performance of a passive air-breathing direct methanol fuel cell

This study systematically investigates the effects of structural aspects on the performance of a passive air-breathing direct methanol fuel cell (DMFC). Three factors are selected in this study: (1) two different open ratios of the current collector; (2) two different assembly methods of the diffusion layer; and (3) three membrane types with different thicknesses. The interrelations and interactions among these factors have been taken into account. The results demonstrate that these structural factors combine to significantly affect the cell performance of DMFCs. The higher open ratio not only provides a larger area for mass transfer passage and facilitates removal of the products, but also promotes higher methanol crossover. The hot-pressed diffusion layer (DL) can mitigate methanol permeation while the non-bonded variant is able to enhance product removal. The increase of membrane thickness helps obtain a lower methanol crossover rate and higher methanol utilisation efficiency, but also depresses cell performance under certain conditions. In this research, the maximum power density of 10.7 mW cm −2 is obtained by selecting the current collector with a lower open ratio, the non-bonded DL, and the Nafion 117 membrane. The effect of methanol concentration on the performance of DMFCs is also explored.

Effect of wettability of anode microporous layer on performance and operation duration of passive air-breathing direct methanol fuel cells

The influence of the wettability of the anode microporous layer (MPL) on both cell performance and operation duration of air-breathing direct methanol fuel cells (DMFCs) was investigated. The experimental results demonstrated that a passive DMFC with a hydrophilic MPL (DMFC-L) in the anode gas diffusion layer (GDL) showed performance superior to that with a hydrophobic MPL (DMFC-B), when the performance evaluation was conducted by applying a holding time of 45 s at each current density. DMFC-B showed good performance at medium and high current densities when a holding time of 150 s was used. The observation of water accumulation on the cathode of DMFC-L indicated that the decreased performance resulted mainly from blockage of the oxygen supply path. The constant-current discharging test showed that DMFC-B exhibited a lower performance at the beginning of discharge. However, it showed a lower rate of water accumulation on the cathode and thereby relatively stable operation. Operating the passive DMFCs at high water evaporation rate confirmed the important role of the wettability of anode GDLs for cathode oxygen transport.

Analysis of the Transient Behavior of a Passive Air Breathing Direct Methanol Fuel Cell using 1-D Model Simulation

The transient behavior of a passive air breathing direct methanol fuel cell (DMFC) operated on vapor-feeding mode is studied in this paper. As a result of experiment, the output performance is sometimes unstable and there are some overshoots. A mathematical dynamic one-dimensional model for simulating transient response of the DMFC is presented. In this model a DMFC is decomposed into its subsystems using lumped model. A one-dimensional, non-isothermal, combined heat and mass transport model is developed that captures the coupling among water generation and transport, oxygen consumption, self-heating and natural convection. For validation, similar experiments were carried out. As a result of simulation and experiments, strong effects of feeding methanol concentration and water transport were found out. The model and experiment analysis provides several conclusions. The performance drop after peak point is caused by insufficiency of water at the anode. The excess water at the cathode makes performance recovery impossible.

Performance evaluation of passive direct methanol fuel cell with methanol vapour supplied through a flow channel

This work examines the effect of fuel delivery configuration on the performance of a passive air-breathing direct methanol fuel cell (DMFC). The performance of a single cell is evaluated while the methanol vapour is supplied through a flow channel from a methanol reservoir connected to the anode. The oxygen is supplied from the ambient air to the cathode via natural convection. The fuel cell employs parallel channel configurations or open chamber configurations for methanol vapour feeding. The opening ratio of the flow channel and the flow channel configuration is changed. The opening ratio is defined as that between the area of the inlet port and the area of the outlet port. The chamber configuration is preferred for optimum fuel feeding. The best performance of the fuel cell is obtained when the opening ratio is 0.8 in the chamber configuration. Under these conditions, the peak power is 10.2 mW cm −2 at room temperature and ambient pressure. Consequently, passive DMFCs using methanol vapour require sufficient methanol vapour feeding through the flow channel at the anode for best performance. The mediocre performance of a passive DMFC with a channel configuration is attributed to the low differential pressure and insufficient supply of methanol vapour.

06/01904 On the consequences of methanol crossover in passive air-breathing direct methanol fuel cells: Kho, B. K. et al. Journal of Power Sources, 2005, 142, (1–2), 50–55

06/01904 On the consequences of methanol crossover in passive air-breathing direct methanol fuel cells: Kho, B. K. et al. Journal of Power Sources, 2005, 142, (1–2), 50–55

On the consequences of methanol crossover in passive air-breathing direct methanol fuel cells

Passive direct methanol fuel cells (DMFC) are under development for use in portable power applications due to their enhanced energy density in comparison with active DMFCs. This study has been carried out to understand the unique properties of passive DMFCs, focusing on the internal temperature and the open circuit voltage (OCV), which change as a consequence of the methanol crossover phenomenon. The changes found in the passive DMFCs were very different from active ones because of different reactants supplying conditions. Methanol concentration in the built-in reservoir attached to the anode changed with time on stream and the OCV and the temperature changed correspondingly. Various experiments were conducted to show the unique properties of passive DMFCs at controlled conditions and configurations of the cell.