Current distribution measurements with a free-breathing direct methanol fuel cell using PVDF-g-PSSA and Nafion 117 membranes

The methanol crossover and other mass transfer phenomena have been investigated in a free-breathing direct methanol fuel cell (DMFC). The current distribution profile along the MeOH flow channel was measured and information of local concentrations of the reacting species was obtained. The DMFC with a segmented cathode was found to be very useful for a detailed analysis of the interrelated parameters, which cause the local variations of the cell current. The connections between different operating parameters were clarified in detail for two different membranes. The measurements were done for both an experimental poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid) (PVDF-g-PSSA) membrane and the commercial Nafion^® 117 membrane, which have different methanol permeabilities. The MeOH concentration and the flow rate were varied in a wide range in order to determine their optimum values. The deviations from an even current density distribution were observed to increase as a function of MeOH concentration and decrease as a function of temperature. The power production of a free-breathing DMFC was observed to be proportional to the local oxygen concentration at the cathode side and inadequate air convection together with the MeOH crossover phenomenon was observed to decrease the cell performance locally.     

Water transport characteristics in a passive liquid-feed DMFC

A two-dimensional, two-phase, non-isothermal model was developed to investigate the water transport characteristics in a passive liquid-feed direct methanol fuel cell (DMFC). The liquid–gas two-phase mass transport in the porous anode and cathode was formulated based on multi-fluid model in porous media, and water and methanol crossover through the membrane were considered with the effect of diffusion, electro-osmotic drag, and convection. The model enabled numerical investigation of the effects of various operating parameters, such as current density, methanol concentration, and air humidity, as well as the effect of the cathode hydrophobic air filter layer, on the water transport and cell performance. The results showed that for the free-breathing cathode, gas species concentration and temperature showed evident differences between the cell and the ambient air. The use of a hydrophobic air filter layer at the cathode helped to achieve water recovery from the cathode to the anode, although the oxygen transport resistance was increased to some extent. It was further revealed that the water transport can be influenced by the ambient relative humidity.     

In situ measurements of water crossover through the membrane for direct methanol fuel cells

We show analytically that the water-crossover flux through the membrane used for direct methanol fuel cells (DMFCs) can be in situ determined by measuring the water flow rate at the exit of the cathode flow field. This measurement method enables investigating the effects of various design and geometric parameters as well as operating conditions, such as properties of cathode gas diffusion layer (GDL), membrane thickness, cell current density, cell temperature, methanol solution concentration, oxygen flow rate, etc., on water crossover through the membrane in situ in a DMFC. Water crossover through the membrane is generally due to electro-osmotic drag, diffusion and back convection. The experimental data showed that diffusion dominated the total water-crossover flux at low current densities due to the high water concentration difference across the membrane. With the increase in current density, the water flux by diffusion decreased, but the flux by back convection increased. The corresponding net water-transport coefficient was also found to decrease with current density. The experimental results also showed that the use of a hydrophobic cathode GDL with a hydrophobic MPL could substantially reduce water crossover through the membrane, and thereby significantly increasing the limiting current as the result of the improved oxygen transport. It was found that the cell operating temperature, oxygen flow rate and membrane thickness all had significant influences on water crossover, but the influence of methanol concentration was negligibly small.     

Analysis of an active tubular liquid-feed direct methanol fuel cell

A two-dimensional, two-phase, non-isothermal model was developed for an active, tubular, liquid-feed direct methanol fuel cell (DMFC). The liquid-gas, two-phase mass transport in the porous anode and cathode was formulated based on the multi-fluid approach in the porous media. The two-phase mass transport in the anode and cathode channels was modeled using the drift-flux and the homogeneous mist-flow models, respectively. Water and methanol crossovers through the membrane were considered due to the effects of diffusion, electro-osmotic drag, and convection. The model enabled a numerical investigation of the effects of various operating parameters, such as current density, methanol flow rate, and oxygen flow rate, on the mass and heat transport characteristics in the tubular DMFC. It was shown that by choosing a proper tube radius and distance between the adjacent cells, a tubular DMFC stack can achieve a much higher energy density compared to its planar counterpart. The results also showed that a large anode flow rate is needed in order to avoid severe blockage of liquid methanol to the anode electrode due to the gas accumulation in the channel. Besides, lowering the flow rate of either the methanol solution or air can lead to a temperature increase along the flow channel. The methanol and water crossovers are nearly independent of the methanol flow rate and the air flow rate.     

The mass transport based on convection effects in a passive DMFC under open-circuit conditions

The study investigates the open-circuit characteristics of a passive direct methanol fuel cell (DMFC) based on temperature-induced convection effects, including the reactants distributions at anode, the non-uniform temperature distribution and the methanol crossover. A two-dimensional, well-thought-out numerical model coupling with mass transfer and momentum transfer is exploited for DMFC to investigate its inner component and temperature distributions under open-circuit condition. In addition, a 4.0 cm² passive DMFC has been designed and manufactured by the laser-cutting technology for experimental verification. The average methanol crossover flux, methanol diffusion coefficient and crossover current are obtained, which coincide with the simulation data well. The temperature-induced convection simulation results show that the distorted temperature distribution becomes more obvious with higher methanol concentration. Furthermore, the polarization curve, cell temperature and open-circuit voltage (OCV) are measured by varying the methanol concentration to conduct more in-depth research on DMFC performance at open circuit state. The results indicate that the temperature is increased, whereas the OCV is decreased with the increase of methanol concentration, accompanied by the phenomenon of methanol crossover is aggravated. The paper provides the theory basis and the optimal operating parameters for safe start-up of DMFC.     

Effect of breathing-hole size on the electrochemical species in a free-breathing cathode of a DMFC

A three-dimensional numerical model is developed to study the electrochemical species characteristics in a free-breathing cathode of a direct methanol fuel cell (DMFC). A perforated current collector is attached to the porous cathode that breathes the fresh air through an array of orifices. The radius of the orifice is varied to examine its effect on the electrochemical performance. Gas flow in the porous cathode is governed by the Darcy equation with constant porosity and permeability. The multi-species diffusive transports in the porous cathode are described using the Stefan–Maxwell equation. Electrochemical reaction on the surfaces of the porous matrices is depicted via the Butler–Volmer equation. The charge transports in the porous matrices are dealt with by Ohm's law. The coupled equations are solved by a finite-element-based CFD technique. Detailed distributions of electrochemical species characteristics such as flow velocities, species mass fractions, species fluxes, and current densities are presented. The optimal breathing-hole radius is derived from the current drawn out of the porous cathode under a fixed overpotential.     

Heat and power management of a direct-methanol-fuel-cell (DMFC) system

In this paper, we describe the heat and the power management of a direct methanol fuel cell system. The system consists mainly of a direct methanol fuel cell stack, an anode feed loop with a heat exchanger and on the cathode side, a compressor/expander unit. The model calculations are carried out by analytical solutions for both mass and energy flows. The study is based on measurements on laboratory scale single cells to obtain data concerning mass and voltage efficiencies and temperature dependence of the cell power. In particular, we investigated the influence of water vaporization in the cathode on the heat management of a direct-methanol-fuel-cell (DMFC) system. Input parameters were the stack temperature, the cathode pressure and the air flow rate. It is shown that especially at operating temperatures above 90 °C, the combinations of pressure and air flow rate are limited because of heat losses due to vaporization of water in the cathode.     

Effect of cathode separator structure on performance characteristics of free-breathing PEMFCs

The performance of free-breathing polymer electrolyte membrane fuel cells (PEMFCs) was studied experimentally and the effect of the cathode separator structure on the cell performance was investigated. Two types of cathode separators were used for a cell with an 18 cm² active area: an open type with parallel rectangular open-slits and a channel type with straight vertical channels with open ends. The polarization curves, cell impedance, and contact pressure distribution of the separators were measured with each type of cathode structure. The result showed that it is difficult to realize a uniform contact pressure across the cell layers for the open type separator, and this results in higher contact resistance and poorer cell performance than the channel type separator. The channel type separator can maintain a low contact resistance, and the cell performance is strongly affected by the natural convection inside the channel. Optimization of the channel design of the channel type separator achieves good performance and this type of separator is superior for a free-breathing PEMFC. A computational three-dimensional analysis for the free-breathing channel type PEMFC with the different channel depths was performed, and it identified the influence of natural convection.     

Transient analysis for the cathode gas diffusion layer of PEM fuel cells

A one-dimensional, non-isothermal, two-phase transient model has been developed to study the transient behaviour of water transport in the cathode gas diffusion layer of PEM fuel cells. The effects of four parameters, namely the liquid water saturation at the interface of the gas diffusion layer and flow channels, the proportion of liquid water to all of the water at the interface of the cathode catalyst layer and the gas diffusion layer, the current density, and the contact or wetting angle, on the transient distribution of liquid water saturation in the cathode gas diffusion layer are investigated. Especially, the time needed for liquid water saturation to reach steady state and the liquid water saturation at the interface of the cathode catalyst layer and gas diffusion layer are plotted as functions of the above four parameters. The ranges of water vapour condensation and liquid water evaporation are identified across the thickness of the gas diffusion layer. In addition, the effects of the above four parameters on the steady state distributions of gas phase pressure, water vapour concentration, oxygen concentration and temperature are also presented. It is found that increasing any one of the first three parameters will increase the water saturation at the interface of the catalyst layer and gas diffusion layer, but decrease the time needed for the liquid water saturation to reach steady state. When the liquid water saturation at the interface of the gas diffusion layer and flow channels is high enough (≥0.1), the liquid water saturation at steady state is almost uniformly distributed across the thickness of the gas diffusion layer. It is also found that, under the given initial and boundary conditions in this paper, evaporation takes place within the gas diffusion layer close to the channel side and is the major process for water phase change at low current density (<2000 A m⁻²); condensation occurs close to the catalyst layer side within the gas diffusion layer and dominates the phase change at high current density (>5000 A m⁻²). For hydrophilic gas diffusion layers, both the time needed for liquid water saturation to reach steady state and the water saturation at the interface of the catalyst layer and gas diffusion layer will increase when the contact angle increases; but for hydrophobic gas diffusion layers, both of them decrease when the contact angle increases.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Neutron radiography and current distribution measurements for studying cathode flow field properties of direct methanol fuel cells

The influence of the cathode flow field properties on the water distribution and performance of direct methanol fuel cells (DMFCs) was studied. All measurements were performed with DMFC stack cells (A = 314.75 cm²). The local and temporal water distributions in the flow field channels during DMFC operation were visualized by means of through‐plane neutron radiography. Current and temperature distributions were measured simultaneously by the segmented cell technology. Additionally, the time‐dependent current distribution, cell performance and pressure drop were measured. Cathode flow field designs with channel and grid structures were compared. The cathode flow field channels were impregnated by either hydrophobizing or hydrophilizing agents or used as received. It turned out that hydrophobized and partially also untreated flow fields cause large water droplets in the cathode channels. The water droplets cause a blocking of the air flow and consequently a lower and more unstable (fluctuating) performance, less steady current and temperature distributions, and higher pressure drops between cathode inlet and outlet. Because of their two‐dimensional design, grid flow fields are less prone to water accumulations. The best results are achieved with a hydrophilized grid flow field that has a channel depth and width of 1.5 mm each (‘C‐GR15’). Copyright © 2013 John Wiley & Sons, Ltd.     

Two-dimensional two-phase thermal model for passive direct methanol fuel cells

A two-dimensional two-phase thermal model is presented for direct methanol fuel cells (DMFC), in which the fuel and oxidant are fed in a passive manner. The inherently coupled heat and mass transport, along with the electrochemical reactions occurring in the passive DMFC is modeled based on the unsaturated flow theory in porous media. The model is solved numerically using a home-written computer code to investigate the effects of various operating and geometric design parameters, including methanol concentration as well as the open ratio and channel and rib width of the current collectors, on cell performance. The numerical results show that the cell performance increases with increasing methanol concentration from 1.0 to 4.0 M, due primarily to the increased operating temperature resulting from the exothermic reaction between the permeated methanol and oxygen on the cathode and the increased mass transfer rate of methanol. It is also shown that the cell performance upgrades with increasing the open ratio and with decreasing the rib width as the result of the increased mass transfer rate on both the anode and cathode.     

Macroscopic analysis of characteristic water transport phenomena in polymer electrolyte fuel cells

Comprehensive analytical and numerical analyses were performed, focusing on anode water loss, cathode flooding, and water equilibrium for polymer electrolyte fuel cells. General features of water transport as a function of membrane thickness and current density were presented to illustrate the net effect of back-diffusion of water from the cathode to anode over a polymer electrolyte fuel cell domain. First, two-dimensional numerical simulation were performed, showing that the difference in molar concentration of water at the channel outlet is widened as the operating current density increases with a thin membrane (Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111), which was verified by Dong et al. [Distributed performance of polymer electrolyte fuel cells under low-humidity conditions. J Electrochem Soc 2005; 152: A2114–22]. Then, analytical solutions were compared with computational results in predicting those characteristics of water transport phenomena. It was theoretically estimated that the high pressure operation of fuel cells expedites water condensing and results in shorter anode water loss and cathode flooding locations. In this study, it was also found that a thin membrane (Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111) facilitates water transport in the through-membrane direction and therefore water concentration at the anode and cathode channel outlets reaches an equilibrium state particularly at low operating current densities. Moreover, the difference in the anode water concentration between Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111 and Nafion^®${\mathrm{Nafion}}^{\circledR }$ 115 membranes becomes intensified in the in-plane direction under the same water production condition, while the cathode water concentration profiles remains almost same.     

Study of sintered stainless steel fiber felt as gas diffusion backing in air-breathing DMFC

Adoption of a sintered stainless steel fiber felt was evaluated as gas diffusion backing in air-breathing direct methanol fuel cell (DMFC). By using a sintered stainless steel fiber felt as an anodic gas diffusion backing, the peak power density of an air-breathing DMFC is 24 mW cm⁻², which is better than that of common carbon paper. A 30-h-life test indicates that the degraded performance of the air-breathing DMFC is primarily due to the water flooding of the cathode. Twelve unit cells with each has 6 cm² of active area are connected in series to supply the power to a mobile phone assisted by a constant voltage diode. The maximum power density of 26 mW cm⁻² was achieved in the stack, which is higher than that in single cell. The results show that the sintered stainless steel felt is a promising solution to gas diffusion backing in the air-breathing DMFC, especially in the anodic side because of its high electronical conductivity and hydrophilicity.     

Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm⁻² power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.     

Evaluation of structural aspects and operation environments on the performance of passive micro direct methanol fuel cell

Passive micro direct methanol fuel cell (μDMFC) which operates based on fuel diffusion is preferred for portable applications for its structural simplicity. In this work, we have systematically investigated multiple variables including the hot-press conditions, current collector channel patterns, current collector open ratios, and their effects on the performance for passive μDMFC by experiments and simulations. Results indicate that vertical stripe pattern (VSP) is preferred for both anodes and cathodes due to the upward reaction products drift by natural convection. Open ratio of 45.6% and 35.8% are found to yield the best performance for anode and cathode, respectively. In addition, the external environmental conditions of vibration frequency, cell orientation, environmental temperature and atmospheric pressure are all discussed in detail in this work. The optimized fabrication, assembly and operation parameters shed light on the design considerations necessary for the wide adaptation of high-performance and durable passive μDMFC for portable applications.     

An approach for determining the liquid water distribution in a liquid-feed direct methanol fuel cell

In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL.     

Study of pulsed-current loading of direct methanol fuel cells using a new time-domain model based on bi-functional methanol oxidation kinetics

This work describes a non-linear time-domain model of a direct methanol fuel cell (DMFC) and uses that model to show that pulsed-current loading of a direct methanol fuel cell does not improve average efficiency. Unlike previous system level models, the one presented here is capable of predicting the step response of the fuel cell over its entire voltage range. This improved model is based on bi-functional methanol oxidation reaction kinetics and is derived from a lumped, four-step reaction mechanism. In total, six states are incorporated into the model: three states for intermediate surface adsorbates on the anode electrode, two states for the anode and cathode potentials, and one state for the liquid methanol concentration in the anode compartment. Model parameters were identified using experimental data from a real DMFC. The model was applied to study the steady-state and transient performance of a DMFC with the objective to understand the possibility of improving the efficiency of the DMFC by using periodic current pulses to drive adsorbed CO from the anode catalyst. Our results indicate that the pulsed-current method does indeed boost the average potential of the DMFC by 40 mV; but on the other hand, executing that strategy reduces the overall operating efficiency and does not yield any net benefit.     

Comparison of single-cell testing,short-stack testing and mathematical modeling methods for a direct methanol fuel cell

In this paper, a comparison between direct methanol fuel cell (DMFC) measurements performed on a single cell and a short-stack, and the results of a mathematical model for a DMFC, is presented. The testing of a short-stack, which consists of 5 cells with an active area of 315 cm², was performed at various current densities, permeation current densities, and cathode flow rates (CFR) in order to determine the voltage outputs of each cell. Methanol concentration and stack temperature results obtained from short-stack testing were then integrated into the single cell test and single cell mathematical model as the input parameters. For the mathematical modelling, transport equations originating from methanol, water, and oxygen were coupled with the electrochemical relations. Therefore, a comparison between these three methods is made in order to gain a deeper understanding of the effects of the operating parameters on DMFC performance. This study showed that the model could describe experimental results well when lower methanol concentrations (under 1.2 M) and temperature (under 60 °C) values are used as input parameters. The results also show very good agreement at lower methanol permeation rates and therefore lower temperatures. It is found that the voltage output for a given current density is higher for the theoretical model than that of the experimental studies; and the differences in the results can be up to 0.04 V for a cell.