Methanol Crossover Rate Research Articles

Mathematical Modelling and Simulations of Active Direct Methanol Fuel Cell

A one dimensional isothermal model is proposed by modelling the kinetics of methanol transport at anode flow channel (AFC), membrane and cathode catalyst layer of direct methanol fuel cell (DMFC). Analytical model is proposed to predict methanol cross-over rate through the electrolyte membrane and cell performance. The model presented in this paper considered methanol diffusion and electrochemical oxidation at the anode and cathode channels. The analytical solution of the proposed model was simulated in a MATLAB environment to obtain the polarization curve and leakage current. The effect of methanol concentration on cell voltage and leakage current is studied. The methanol cross-over has the significant impact on cell performance. The presented model predicts higher leakage current with the increase of methanol feed concentration. The cell performance was predicted at 70°C and various methanol feed concentration. The proposed model was validated with the experimental polarization curve of active DMFC.

Water and heat management of vapor-fed direct methanol fuel cells with a non-isothermal triple-phase mass transfer model

Direct methanol fuel cells (DMFCs) operating with methanol vapor tends to operate under higher temperature, which contributes to improve cell performance and specific energy. However, the design of vapor-fed DMFCs with high performance and adaptability are still hindered by the complex water and heat management. To provide a comprehensive understanding of the critical multiphase heat and mass transfer, we develop a two-dimensional non-isothermal triple-phase mass transfer model for a passive vapor-fed DMFC. Effects of the electrochemical reactions and related thermal effect are fully investigated. Increasing the current density raises the operating temperature, thus enhancing the generation rate of methanol vapor and reducing concentration polarization under high current densities. With the enlargement of the open area ratio (from 0.1 to 0.3), an excessive methanol supply rate may cause deteriorated methanol crossover rates and excessively high temperatures, resulting in severe dehydration of polymer electrolytes and a degraded cathode overpotential (from −0.48V to −0.54V). For the DMFC with Nafion 112 membrane, the maximum current density and peak power density are 33.3% and 36.0% greater than that of Nafion 117 membrane. The simulation work will provide an exhaustive theoretical guidance for the efficient management of water and heat in DMFCs fed with neat methanol.

Numerical simulation of methanol crossover in flowing electrolyte-direct methanol fuel cell

A two-dimensional, comprehensive model for flowing electrode direct methanol fuel cell (FEDMFC) is developed to investigate cell performance and prevention of methanol crossover. Governing equations consists of the electron and proton transfer equations, mass and momentum conservation equation, energy transfer equations, species transfer of gas and liquid equations, dissolved water equations, as well as modified Tafel equations. The influence of operating parameters (such as flow rate and inlet temperature) and porous properties (such as porosity and permeability) of flowing electrode channel (FEC) on cell performance and methanol crossover are discussed in details. The results show that FEDMFC effectively reduces the methanol crossover rate with no significant reduction of power densities compared with direct methanol fuel cell (DMFC) under the same conditions. The Increase of the FEC flow rate can further reduce the methanol crossover rate. However, a higher flow rate will lead to a decrease in cell efficiency and stability. The FEC flow rate controlled at 1-3 sccm is more appropriate. Additionally, the lower inlet temperature of FEC is associated with lower methanol crossover rate and better cooling effect. The lower porosity is more conducive to reducing the effective diffusion coefficient of methanol, thus decreasing the methanol crossover rate.

Enhanced water management via the optimization of cathode microporous layer using 3D graphene frameworks for direct methanol fuel cell

Water management is one of the key roles in determining the output performance of a passive direct methanol fuel cell (DMFC). By decreasing the size of hydrophobic pores inside the cathode microporous layer (MPL), a high hydraulic pressure can be built up inside the cathode catalyst layer, which enhances the water recovery from cathode to anode and thus improves the cathode mass transfer. To achieve this, three-dimensional graphene frameworks are adopted to construct cathode MPL for the first time, whose intrinsic pore structure and characteristics contribute to an enhanced water management. Experimental results show that the performance and stability of the as-prepared DMFC are significantly improved. To illustrate the mechanism of the improvement, the in-situ measurements of polarization and methanol crossover are conducted, which reveals a greatly lowered water transport coefficient, along with a reduced in-situ methanol crossover rate. Since the role of polytetrafluoroethylene (PTFE) is weakened in the novel design of MPL, a further optimization via reducing PTFE content is realized, leading to a total increase of 43% of the output performance. The significant elevation of both the performance and stability of DMFC can expand its potential as a green energy source in practical applications.

Fuel and Water Management in Direct Methanol Fuel Cells

This study has designed and setup experiments to measure the performance of MEAs made from Johnson Matthey’s gas diffusion electrodes (anode: 3 mg/cm2 Pt and 1.5 mg/cm2 Ru; cathode: 1.25 mg/cm2 Pt). The polarization curves of MEAs with fuel and water management layers, which are porous materials with different wettability and mass transfer resistance, have been experimental measured with dilute methanol solutions (3M or less). In addition, the water crossover, and methanol crossover, as well fuel efficiency, energy efficiency have been estimated from changes of weight and concentration of methanol solutions before and after constant current discharge experiments. Results show that adding a fuel management layer between the methanol channel and the MEA can significantly reduce the water and methanol crossover rate and increase the fuel and energy efficiency. The net water flux across the electrolyte membrane could be less than the water flux caused by electro-osmotic drag flux. In other words, the water flux by diffusion and convection could be from cathode to anode when a fuel management layer is applied. However, the fuel management layer also reduces the water flux from the channel to the MEA and increases the ohmic resistance of the electrolyte membrane. A water management layer between the MEA and cathode channel is recommended to reduce the water loss through the cathode, reduce the water and methanol and water crossover rate, and increase the fuel efficiency. This water management layer does not deteriorate the water flooding since the operating current of methanol fuel cells (<0.5 A/cm2) are typically much lower than that of proton exchange membrane. The limiting current with or without the water management layer keeps almost the same. In addition, if a fuel management layer is applied to reduce the methanol crossover, a thinner electrolyte membrane is recommended because it requires less water to be hydrated and it has relatively low ohmic resistance.

Simulation of a hybrid vehicle powertrain having direct methanol fuel cell system through a semi-theoretical approach

Different operating scenarios can be used in a hybrid system based on a direct methanol fuel cell (DMFC) and a battery. In this paper, a DMFC system model is integrated into a model formed for a hybrid vehicular system that consists of a battery, a DMFC stack and its auxiliary equipments; and the model is simulated in Matlab/Simulink environment using a quasistatic approach. An algorithm for the energy management of the system is also developed considering the state of charge (SOC) of the battery. In the DMFC system model, the current and empirical data for the polarization curves as well as methanol crossover and water crossover rates are taken as the input parameters, whereas the stack voltage, the remaining methanol in the fuel tank, and the power demand of auxiliary equipments are taken as the output parameters. In this model, the methanol consumption, and the water and CO2 production are found applying mass balances for each component of the system. The results of the simulations help to give more insights into the operation of a DMFC based hybrid system.

Direct methanol fuel cell stack for auxiliary power units applications based on fumapem® F-1850 membrane

Direct methanol fuel cells (DMFCs) have been actively investigated from both fundamental and applied points of view due to their interesting perspectives for the application in the fields of auxiliary power supply and portable power sources. However, research efforts are still needed to solve the drawbacks presently affecting these devices, such as methanol cross-over constraints, high costs of materials, etc. The present paper addresses these issues by using a new FUMATECH proton exchange membrane (fumapem® F-1850), characterized by a nominal equivalent weight of 1800, as the electrolyte, and low electrode noble metal loadings compared to the state of the art. A membrane-electrode assembly (MEA) based on this membrane was investigated in a small area (5 cm2) direct methanol fuel cell at 60 °C and compared to a benchmark Nafion® 115 membrane in terms of performance and methanol cross-over. The F-1850 membrane showed a lower methanol cross-over rate than Nafion® 115, despite a lower thickness of the fumapem® membrane. This allowed to obtain a higher performance (74 vs. 64 mW cm−2) for the F-1850 membrane compared to Nafion® at ambient pressure. To study the scalability of the DMFC device and related components, the new membrane was integrated in large-area MEAs (100 cm2) and investigated in a 5-cell and 10-cell pressurized stack based on a bipolar design. The stack was tested at 75 °C, 5 M methanol solution at the anode and pressurized oxygen (2 atm. rel.) at the cathode, giving a normalized power density per cell of about 130 mW cm−2. The results obtained with the 10-cell stack showed a good agreement with the performance recorded with the 5-cell stack, confirming the scalability of the DMFC device.

Transport of highly concentrated fuel in direct methanol fuel cells

It is desirable to operate a direct methanol fuel cell (DMFC) with highly concentrated fuel to increase the specific energy of the fuel cell system; thus it could have a longer runtime. The high-concentration operation requires the large methanol concentration gradient across the transport path from fuel reservoir to the catalyst layer, including fuel reservoir, fuel reservoir/current collector interface, current collector, backing layer and micro-porous layer. In this work, we create the large methanol concentration gradient through lowering the porosity of the backing layer with nanosized carbon powder. The performance tests show that the use of the present backing layer enables the passive DMFC to operate with a high-concentration fuel up to 10.0M without increasing the methanol crossover rate.

Transient analysis of passive vapor-feed DMFC fed with neat methanol

A transient model is presented to investigate the transport phenomena for passive vapor-feed direct methanol fuel cell (DMFC). The pervaporation membrane and vapor transport layer are considered for the formation and transport of methanol vapor, respectively. We attempt to provide insight into the transient mass transport characteristics of DMFCs by testing different operation conditions, including current density, open area ratio of the vaporizer, and membrane thickness. The results show that the methanol crossover rate and water transport from the cathode to the anode are the key factors for improving the cell performance, and indicate that fuel efficiency, energy efficiency and energy density of the DMFCs are improved by increasing current density, decreasing open ratio of the vaporizer or increasing membrane thickness due to the reduced methanol crossover rate. The cathode micro-porous layer (MPL) is useful in enhancing water recovery flux and decreasing water losses.

Numerical modeling and simulations of active direct methanol fuel cell (DMFC) systems under various ambient temperatures and operating conditions

Direct methanol fuel cells (DMFCs) are potential candidates for portable backup power generation and auxiliary power units owing to their advantageous features, such as ease of fuel storage and delivery. Optimizing each component of a DMFC system is critical to improving the overall system performance and power density. This paper presents an active DMFC system model, in which a one-dimensional DMFC stack model is combined with major system components, including fuel and water tanks, liquid–gas separator, heat exchangers, pumps, and blowers. The model is implemented using a commercial flow-sheet simulator, ASPEN-HYSYS, and then applied to an active DMFC system to analyze the effects of the DMFC operating parameters and heat management. Special emphasis is placed on establishing active control strategies for the DMFC stack temperature, methanol crossover rate, and water recovery by optimizing the system components and operating conditions. Overall, this study helps identify innovative active DMFC system designs and configurations.

Study of physicochemical characterization of potassium-doped Nafion117 membrane and performance evaluation of air-breathing fuel cell in different alkali-methanol solutions

Direct methanol alkaline fuel cell (DMAFC) using a cation exchange membrane (CEM) was operated in passive condition and compared to passive direct methanol fuel cell (DMFC). A polymer electrolyte membrane for passive DMAFC was prepared by immersing Nafion117 membrane into KOH solution at temperature of 80 °C. The prepared membrane (potassium-doped Nafion117) exhibited higher mechanical properties and thermal stability than Nafion117. Ionic conductivity of K+ form Nafion117 membrane was measured to be lower than that for pristine Nafion117. Single cell in alkaline condition (passive DMAFC) exhibited better cell performance than that in the cell using Nafion117 in acidic condition (passive DMFC). The open circuit voltage (OCV) of passive DMAFC reached to about 1 V which was considerably higher than that for passive DMFC (0.45 V). The methanol crossover rate through membranes was also measured and the results implied that the potassium-doped Nafion117 membrane had lower permeability to methanol.

Studies on development of titania nanotube (TNT) based ePTFE-Nafion®-composite membrane for electrochemical methanol reformation

The expanded polytetrafluoroethylene (ePTFE) substrate based composite polymer electrolyte membrane was prepared using Nafion® ionomer and titania nanotube (TNT) as a filler in the matrix and demonstrated its use as electrolyte in a polymer electrolyte Membrane (PEM) based electrochemical methanol reformation (ECMR) for hydrogen generation. The composite membranes were characterised for their mechanical strength and thermal stability using dynamic mechanical analyser (DMA) and thermo–gravimetric analysis (TGA). Methanol cross-over studies were carried out for composite membrane and compared with commercial Nafion® 117 membrane. The PEM based ECMR cell with composite membrane shows comparable performance related to Nafion® 117 with lower methanol crossover rates.

High-performance direct methanol fuel cell with optimized membrane electrode assembly: A theoretical approach

Methanol/water crossover through proton-exchange membranes and low cell voltage resulting from sluggish catalytic reactions are major issues that must be overcome to commercialize direct methanol fuel cells (DMFCs). Three key performance indices (low methanol crossover rate (MCO), low water–transfer coefficient, and high cell voltage) can be improved by optimizing the design of the DMFC membrane electrode assembly for the cell operating conditions. Using computational techniques with a one-dimensional DMFC model, the present study proposes guidelines for constructing a high-performance DMFC with a focus on membrane electrode assembly (MEA). First, the membrane thickness is determined by balancing the MCO and ionic conductivity for both the fluorocarbon (FC) membrane and the hydrocarbon (HC) membrane. Second, the material composition of the cathode catalyst layer (cCL) is optimized by balancing charge transfer and mass transfer. Finally, the anode diffusion layer (aDL) is optimized according to the cell operating conditions. Compared to the referenced baseline cell, this optimization process is predicted to improve the cell voltage by 19% and decrease methanol crossover by 64%.

Investigation of cell orientation effect on transient operation of passive direct methanol fuel cells

A transient model for passive direct methanol fuel cell (DMFC) is developed to investigate the effect of cell orientation and operating condition. The results show that the passive DMFC with vertical orientation has better performance than the horizontal one, except the case of high current density, because a large amount of water produced in cathode is hard to be removed in vertical orientation, which is easier for horizontal orientation due to gravity. The passive DMFC with horizontal orientation is sensitive to methanol crossover, and moderate current density or voltage is necessary to ensure high energy efficiency. The anode micro-porous layer (MPL) plays an important role in reducing the rate of methanol crossover by providing flow resistance. The MPL in cathode has a significant effect on water transport by enhancing the water back-flow from cathode to anode, which prevents water removal. Therefore, the anode MPL and cathode MPL have different effects on horizontal orientation and vertical orientation. Additionally, the size of fuel tank can improve the energy density by providing more fuel, and the effect on fuel efficiency and energy efficiency is a bit obvious in vertical orientation than horizontal orientation.

An experimental study on the effect of membrane thickness and PTFE (polytetrafluoroethylene) loading on methanol crossover in direct methanol fuel cell

Methanol crossover from anode to cathode is a process which adversely affects the performance of a DMFC (direct methanol fuel cell). Increasing the electrolyte membrane thickness and addition of a MPL (microporous layer) using PTFE (polytetrafluoroethylene) loading are two techniques used to reduce methanol crossover by increasing the mass transfer resistance. This paper reports experiments carried out to study the effect of membrane thickness and PTFE loading in anode MPL on methanol crossover in a 25 cm2 DMFC. The rate of methanol crossover is indirectly measured by measuring the CO2 concentration at the cathode exit. The influence of PTFE content (0–20%) and membrane thickness (183 μm and 254 μm) on limiting current density, peak power density and cell efficiency are reported. The experimental results show that the methanol crossover current density is reduced by 24% using thicker membrane compared to MEA (membrane electrode assembly) with thin membrane. This leads to enhanced peak power density of 22 mW/cm2 with cell efficiency of 10%. About 20% of methanol crossover current density is reduced by 10% PTFE loading in anode MPL, which helps in improving peak power density from 13 to 24 mW/cm2 with cell efficiency of 8% compared to membrane as mass transfer resistance layer.

Investigation of NaOH concentration effect in injected fuel on the performance of passive direct methanol alkaline fuel cell with modified cation exchange membrane

A passive DMAFC (direct methanol alkaline fuel cell) using CEM (cation exchange membrane) is operated and compared to passive DMFC (direct methanol fuel cell) which is operated in acidic condition. Nafion117 membrane is applied as solid polymer electrolyte membrane for both the alkali and acidic conditions. Sodium conducting membrane is prepared by dipping Nafion117 membrane into NaOH solution. The Nafion117/NaOH membrane exhibits higher mechanical and thermal properties than pristine Nafion117. The methanol crossover rate through membrane is also measured using an electrochemical technique and the results reveal that the cell using sodium conducting membrane has lower permeability to methanol than that in the cell using proton exchange membrane. Passive DMAFC using a sodium conducting membrane exhibits a higher OCV (open circuit voltage) and superior cell performance than those in passive DMFC using a proton conducting membrane. The cell performance of DMAFC is investigated by varying the concentration of methanol and alkali solutions. Single cell performance suggests that passive DMAFC with Nafion117/NaOH membrane can produce a maximum power density which is remarkably higher than that in passive DMFC.

A Sensorless Methanol Concentration Controller Based on the Amplitude of Output Voltage for Direct Methanol Fuel Cells

There are steady interests in direct methanol fuel cells (DMFCs) as a clean energy technology for powering small and portable electronic devices because of several merits of DMFC systems such as high energy density, flexibility in power output from sub-watt to several hundred watts, and reliable operation, etc. Over the last decade, a number of worldwide efforts have been reported for improving the performance and durability of DMFCs. One of the key issues in DMFC systems is the methanol circulating unit that is required to reuse the exhaust methanol solution from DMFC stacks. Recirculation of the used methanol is accomplished by adjusting the concentration of the exhaust methanol solution to a pre-set level at a fuel mixing tank. In this fuel recirculation unit, electronic sensors are generally used to monitor and control the methanol concentration. However, the sensors are generally recognized to have many problems in terms of cost, size, durability and reliability. In order to solve these problems, there are resurgent research interests for the development of sensor-less control algorithms for DMFC systems. Several studies took advantages of methanol consumption equations or mathematical models to estimate the methanol concentration in the feed. Other studies presented the sensor-less controllers that used the fuel cell voltages and temperatures as the feedback parameters. The methods, however, suffer from the complexity in building database correlating methanol concentrations with other operating parameters such as output voltages, stack temperatures and methanol consumption rates and low degree of precision due to ever-changing performance of fuel cell electrodes. We have found that there is a close relationship between the methanol concentration and the amplitude of voltage fluctuation of DMFC. The output voltage of DMFC experiences fluctuations during operation even under a constant electric load because of uneven reaction rates at the electrodes that are caused by mainly formation of CO2 slugs at the anode and water flooding at the cathode. The CO2 slugs and water flooding can clog the pores of the electrodes and retard the access of reactants to the catalytic sites, leading to a fluctuation of output voltage. Water flooding at the cathode is estimated to be more influential than the CO2 slugs at the anode. In addition, the methanol concentration in the feed affects the methanol crossover rate from the anode to the cathode and thus influences water flooding at the cathode. Thereby the degree of voltage fluctuation is proportional to the methanol concentration. This work presents a novel sensor-less control strategy for adjusting the methanol concentration in re-circulating direct methanol fuel cell (DMFC) systems by utilizing the amplitude of voltage fluctuation of DMFC as a feedback parameter. The effects of various operating conditions on the fluctuation amplitude are analyzed, and a new sensor-less algorithm is proposed based on the relationship between the methanol concentration in the feed and the amplitude of voltage fluctuation. The feasibility of the newly proposed algorithm is evaluated and compared with a control method using a methanol sensor in a continuous run of a 200 W-class DMFC system. It is confirmed that the algorithm works very well and is able to control the methanol feed concentration within a small error bound. Moreover, this method enables steady operations of DMFC systems by minimizing the flooding phenomenon at the cathode. This sensor-less algorithm could be applied to any type of DMFC systems because it has an auto-tuning function.

One-dimensional steady state algebraic model on the passive direct methanol fuel cell with consideration of the intermediate liquid electrolyte

One-dimensional steady state performance equations of passive direct methanol fuel cell are formulated at the presence of intermediate liquid electrolyte. This model evaluates the effect of liquid electrolyte on the cell polarization curve in detail. It is shown that liquid electrolyte employed reduce the methanol crossover rate, increase the fuel utilization, and decrease the cathode voltage loss.The prediction shows that ohmic potential drop is significant at high polarization if low methanol concentration is fed. Cell is better performed without liquid electrolyte. On the other hand, the cathode voltage loss by the mixed potential is more influential at high methanol concentration. Thus, liquid electrolyte reduces the cathode over-potential, and cell performs better with the liquid film.

A micro-porous current collector enabling passive direct methanol fuel cells to operate with highly concentrated fuel

Operating direct methanol fuel cells (DMFCs) with concentrated fuel is desired to increase the energy density of the power pack. In this paper, we propose and fabricate a micro-porous anode current collector, which enables a passive DMFC to operate with methanol solutions as concentrated as 22.0M without sacrificing the performance generated with diluted methanol. It is found that the micro-porous current collector allows an accumulation of CO2 gas in the porous structure, forming a barrier to resist the transport of methanol to the catalyst layer. The reduced methanol concentration level in the anode catalyst layer tends to reduce the rate of methanol crossover and hence, improve the performance.

Effect of anode MPL on water and methanol transport in DMFC: Experimental and modeling analyses

The regulation of mass transport through anode diffusion layer is one of the major issue of direct methanol fuel cell. In fact it is critical to maintain an adequate methanol concentration in the anode electrode such that both the rate of methanol crossover and the mass transport loss can be minimized. In the present work the effect of anode micro-porous layer on system operation is investigated both experimentally and theoretically. The developed 2D two-phase isothermal model is validated with respect to three different typologies of measure at the same time, increasing results reliability. Model simulations highlight that anode micro-porous layer can cause an inversion of water diffusion flux through the membrane and enhances methanol gas diffusion mechanism, reducing methanol crossover. Finally the developed model is used as a tool to design an optimized anode diffusion layer.