Methanol Crossover Rate Research Articles

Effect of Sorbed Methanol, Current, and Temperature on Multicomponent Transport in Nafion-Based Direct Methanol Fuel Cells

The CO2 in the cathode exhaust of a liquid feed direct methanol fuel cell (DMFC) has two sources: methanol diffuses through the membrane electrode assembly (MEA) to the cathode where it is catalytically oxidized to CO2; additionally, a portion of the CO2 produced at the anode diffuses through the MEA to the cathode. The potential-dependent CO2 exhaust from the cathode was monitored by online electrochemical mass spectrometry (ECMS) with air and with H2 at the cathode. The precise determination of the crossover rates of methanol and CO2, enabled by the subtractive normalization of the methanol/air to the methanol/H2 ECMS data, shows that methanol decreases the membrane viscosity and thus increases the diffusion coefficients of sorbed membrane components. The crossover of CO2 initially increases linearly with the Faradaic oxidation of methanol, reaches a temperature-dependent maximum, and then decreases. The membrane viscosity progressively increases as methanol is electrochemically depleted from the anode/electrolyte interface. The crossover maximum occurs when the current dependence of the diffusion coefficients and membrane CO2 solubility dominate over the Faradaic production of CO2. The plasticizing effect of methanol is corroborated by measurements of the rotational diffusion of TEMPONE (2,2,6,6-tetramethyl-4-piperidone N-oxide) spin probe by electron spin resonance spectroscopy. A linear inverse relationship between the methanol crossover rate and current density confirms the absence of methanol electro-osmotic drag at concentrations relevant to operating DMFCs. The purely diffusive transport of methanol is explained in terms of current proton solvation and methanol-water incomplete mixing theories.

Vapor Feed Direct Methanol Fuel Cell with Flowing Electrolyte

This paper reports on the design and characterization of a vapor feed direct methanol fuel cell with flowing acidic electrolyte. A vapor fuel feed has the advantage of reducing methanol crossover and facilitating the removal of CO2. We observed a maximum power density when using 10 M liquid methanol as the source for the vapor fuel feed. At this concentration, an optimal balance appears to exist between the desired increase in the rate of methanol oxidation and the undesired increase in the rate of methanol crossover, which both result from increasing the methanol concentration. Performance studies show that the system is kinetically limited by the oxidation of methanol.

Water and Fuel Crossover in a Direct Methanol Fuel Cell Stack

Water and fuel crossover in a direct methanol fuel cell (DMFC) stack under constant voltage discharge was quantitatively determined by a fuel circulation method. The water permeation across the electrolyte membrane can be split into three portions, including proton osmotic-drag, methanol migration-drag, and spontaneous water crossover. Among these three types of water crossover, spontaneous water crossover is the most significant. Water crossover rate increases significantly with increasing operating temperature. As dry air was used as oxidant in this experiment, there was a significant portion of water evaporation at the cathode outlet, which indicates that water recycle-ability from the cathode is directly dependent on the relative humidity. The rates of methanol crossover across the electrolyte membrane in a DMFC stack under constant voltage discharge are experimentally determined by accurately measuring the methanol concentrations within specific time periods. The rate of methanol crossover increases significantly with operating temperature, but only slightly with increasing discharge voltage.

Influence of cathode oxygen transport on the discharging time of passive DMFC

In this paper, the long-term discharge performances of passive DMFC at different currents with different cell orientations were investigated. Water produced in the cathode was observed from the photographs taken by a digital camera. The results revealed that the passive DMFC with anode facing upward showed the best long-term discharge performance at high current. A few independent water droplets accumulated in cathode when the anode faced upward. Instead, in the passive DMFC with vertical orientation, a large amount of produced water flowed down along the surface of current collector. The passive DMFC with vertical orientation showed relatively good performance at low current. It was concluded that the cathode produced less water in a certain period of time at smaller current. In addition, the rate of methanol crossover in the passive DMFC with anode facing upward was relatively high, which leaded to a more rapid decrease of the methanol concentration in anode. The passive DMFC with anode facing downward resulted in the worst performance because it was very difficult to remove CO 2 bubbles produced in the anode.

Exergy flow and energy utilization of direct methanol fuel cells based on a mathematic model

An exergetic analysis model for direct methanol fuel cell (DMFC) is established in the present paper. Expressions of electrical, thermal and total exergetic efficiencies have been deduced with consideration of methanol crossover and over potential in operation. Furthermore, energy utilization of a DMFC system is quantitatively calculated and changes of electrical efficiency and thermal efficiency at various current density, methanol concentration, operating temperature, and cathode pressure have been investigated. Some suggestions of optimal operating conditions of direct methanol fuel cell based on our findings are put forward. Results show that the thermal energy generated in a DMFC takes up a significant amount of exergy in total energy and should be sufficiently used to obtain high total efficiency in a DMFC, high methanol crossover rate is the predominant cause of energy loss when the fuel cell operates at low current density, and total exergetic efficiency of a DMFC reaches its peak value at relatively high current density.

Celtec-V: A Polybenzimidazole-Based Membrane for the Direct Methanol Fuel Cell

Celtec-V is a proton exchange membrane based on polybenzimidazole (PBI) comprising an interpenetrating network of polyvinylphosphonic acid designed for application in the direct methanol fuel cell. The properties and fuel cell performance of Celtec-V are investigated and compared against a Nafion 117 standard. It is shown that with the PBI-based membrane, fuel cell performance can be sustained to higher methanol feed concentration at around half the methanol crossover rate. Above methanol, Celtec-V outperforms Nafion 117. Furthermore, lower water permeation is observed, with Celtec-V having an electro-osmotic drag coefficient of around 1 compared to a value of 4–5 for Nafion 117. Room for improvement is identified in the ohmic resistance of the membrane and the cathode-membrane interface, where higher losses are observed at increasing current density.

A two-dimensional, two-phase mass transport model for liquid-feed DMFCs

A two-dimensional, isothermal two-phase mass transport model for a liquid-feed direct methanol fuel cell (DMFC) is presented in this paper. The two-phase mass transport in the anode and cathode porous regions is formulated based on the classical multiphase flow in porous media without invoking the assumption of constant gas pressure in the unsaturated porous medium flow theory. The two-phase flow behavior in the anode flow channel is modeled by utilizing the drift-flux model, while in the cathode flow channel the homogeneous mist-flow model is used. In addition, a micro-agglomerate model is developed for the cathode catalyst layer. The model also accounts for the effects of both methanol and water crossover through the membrane. The comprehensive model formed by integrating those in the different regions is solved numerically using a home-written computer code and validated against the experimental data in the literature. The model is then used to investigate the effects of various operating and structural parameters, such as methanol concentration, anode flow rate, porosities of both anode and cathode electrodes, the rate of methanol crossover, and the agglomerate size, on cell performance.

An algebraic model on the performance of a direct methanol fuel cell with consideration of methanol crossover

An algebraic one-dimensional model on the membrane-electrode-assembly (MEA) of direct methanol fuel cell (DMFC) is proposed. Non-linear regression procedure was imposed on the model to retrieve important parameters: solid polymer electrolyte conductivity κ m, exchange current density of methanol electro-oxidation at anode catalyst surface i oM,ref, and mass diffusivity of methanol in aqueous phase within the porous electrode D a that correspond to the experimentally measured polarization curves. Although numerical iteration is required for a complete solution, the explicit relationships of methanol concentration, methanol crossover rate, oxygen concentration and cell discharge current density do provide a clear picture of the mass transport and electrochemical kinetics within the various porous media in the MEA. It is shown the cathode mixed potential induced by the parallel reactions of oxygen reduction and oxidation of crossover methanol elucidates the potential drop of the cathode and the decrease of the cell open circuit voltage (OCV). Methanol transport in the membrane is described by the diffusion, electro-osmosis, and pressure induced convection. Detailed accounts of the effects of anode methanol and cathode oxygen feed concentrations on the cell discharge performance are given with correlation to the physical structure and chemical compositions of the catalyst layers (CLs).

Distribution profile of water and suppression of methanol crossover in sulfonated polyimide electrolyte membrane for direct methanol fuel cells

We have developed novel cross-linked sulfonated polyimide (c-SPI) membrane as an electrolyte for direct methanol fuel cells (DMFCs). When the DMFC using the c-SPI membrane (thickness = 155 μm), Pt–Ru dispersed on carbon black (Pt–Ru/CB) anode and Pt/CB cathode with a Nafion ® ionomer was operated at 80 °C and 0.1 A cm −2 with 1 M CH 3OH and oxygen (oxidant), the methanol crossover rate, j(CH 3OH), was suppressed to about 1/2 compared with that of the Nafion ® 117 membrane (thickness = 180 μm) with the same electrodes. It was found for both cells that the j(CH 3OH) was not so small as expected from the membrane thickness. In order to obtain a clue for the suppression of j(CH 3OH), the distribution profiles of water (containing CH 3OH) in thickness direction were investigated by measuring the specific resistances ( ρ) between Pt probes inserted into the electrolyte membrane. Values of ρ at the anode side were low irrespective of the discharge current density, because such a part of the membrane was humidified thoroughly by liquid water (1 M CH 3OH) allowing free penetration of CH 3OH into the swollen polymer. In contrast, the values of ρ at the cathode side were high at the low current density due to drying of the membrane contacting with oxidant gas (O 2 or air) in low humidity. We have succeeded to suppress the j(CH 3OH) further (about 1/2 at 0.2 A cm −2) by using bilayer c-SPI, having a low ion exchanging (low swelling) barrier layer at the anode side without increasing the ohmic resistance, compared with that of the single c-SPI.

Three-Dimensional Simulations of Liquid Feed Direct Methanol Fuel Cells

Recent research indicates that performance and design of a liquid feed direct methanol fuel cell (DMFC) is controlled not only by electrochemical kinetics and methanol crossover but also by water transport and by their complex interactions in the design regime for portable electronics applications. In this paper, a three-dimensional (3D), two-phase model is presented for DMFCs, in particular considering water transport and treating the catalyst layer explicitly as a component rather than an interface without thickness. Other features of the model are similar to an earlier version published in 2003. The DMFC model is based on the multiphase mixture formulation and encompasses all components in a DMFC using a single computational domain. A flow solver, Fluent, is employed to simultaneously solve flow, species, and charge-transport equations. Numerical simulations in 3D are carried out to explore mass transport phenomena occurring in DMFCs for portable applications as well as to reveal an interplay between the local current density and methanol crossover rate. Numerical results also indicate that the anode flow field design and methanol feeding concentration are two key parameters for optimal cell performance.

Real time measurements of methanol crossover in a DMFC

Methanol crossover is a serious problem in a direct methanol fuel cell (DMFC), which causes significant voltage loss and waste of fuel. Due to methanol crossover, most DMFCs must operate on a fuel with a very low methanol concentration; yet very low methanol concentration also causes a poor cell performance. Thus, it is very important to find the optimal operating conditions of methanol concentration and other operating parameters. In this research, methanol crossover rate in a DMFC is determined by measuring the carbon dioxide concentration at the cathode exit in real time. By measuring methanol crossover and cell performances at different inlet methanol concentrations and various operating conditions three types of characteristics are identified in the relationships between methanol crossover and cell current density. Further analysis of these relationships between methanol crossover and cell performances reveals the optimal methanol concentration and other operating parameters, at which the cell reaches optimal performance without incurring excessive methanol crossover. Furthermore, transient peaks of methanol crossover have been identified when the cell voltage suddenly changes. Analyses of these peaks show that they are caused by the hysteresis of methanol concentration at the interface between the anode catalyst layer and the membrane.

Experimental analysis of methanol cross-over in a direct methanol fuel cell

Methanol cross-over through the polymeric membrane is one of the main causes limiting direct methanol fuel cell performances. It causes fuel wasting and enhances cathode overpotential. A repeatable and reproducible measurement system, that assures the traceability of the measurement to international reference standards, is necessary to compare different fuel cell construction materials. In this work a method to evaluate methanol cross-over rate and operating condition influence is presented and qualified in term of measurement uncertainty. In the investigated range, the methanol cross-over rate results mainly due to diffusion through the membrane, in fact it is strongly affected by temperature. Moreover the cross-over influence on fuel utilization and fuel cell efficiency is investigated. The methanol cross-over rate appears linearly proportional to electrochemical fuel utilization and values, obtained by measurements at different anode flow rate but constant electrochemical fuel utilization, are roughly equal; methanol wasting, due to cross-over, is considerable and can still be higher than electrochemical utilization. The fuel recirculation effect on energy efficiency has been investigated and it was found that fuel recirculation gives more advantage at low temperature, but fuel cell energy efficiency results are in any event higher at high temperature.

Estimating the methanol crossover rate of PEM and the efficiency of DMFC via a current transient analysis

This work describes a relatively easy transient method for estimating the methanol crossover rate of proton exchange membrane (PEM) and the efficiency of direct methanol fuel cell (DMFC). The method uses a DMFC that includes an anode and cathode chamber; a PEM arranged between the anode and the cathode, and a small motor fan connected to the DMFC. An aqueous solution of methanol is fed into the anode chamber while the motor fan is operated in a loading state, allowing the methanol to crossover to the cathode. The methanol crossover rate of PEM and the efficiency of DMFC are obtained by the current transient analysis.

Diphenylsilicate-incorporated Nafion ® membranes for reduction of methanol crossover in direct methanol fuel cells

We synthesized organically modified silicate microparticles, known as diphenylsilicate (DPS), and showed that the synthesized DPS has a nano-layered microstructure. We utilized this material as a filler for fabricating Nafion ®|DPS composite membranes for mitigating the problem of methanol crossover in direct methanol fuel cells (DMFC). The SEM and XRD analyses indicated that the incorporated DPS microparticles were uniformly distributed in the native membrane. The DMFC performance tests demonstrated that the use of the Nafion ®|DPS composite membranes resulted in a lower rate of methanol crossover, higher open-circuit voltage (OCV) and better cell performance than did the pure Nafion ® membrane, especially with a higher methanol concentration such as 10 M.

DMFC: Galvanic or electrolytic cell?

We report measurements of local current distribution in a direct methanol fuel cell with single straight channels and segmented electrodes. The experiments directly confirm the existence of a bifunctional regime of DMFC operation: at low air flow rate the segments located close to the inlet of the oxygen channel generate current (galvanic domain), while the other segments consume current to produce hydrogen (electrolytic domain). The cell voltage in this regime in not a single-valued function of current in the external load. The electrolytic domain disappears at a critical air flow rate, which does not depend on current in the cell. We show that a model (A.A. Kulikovsky, Electrochem. Commun. 6 (2004) 1259) explains this critical behavior. Finally, we extend the method of measurement of methanol crossover rate suggested by Ye and Zhao (J. Electrochem. Soc. 152 (2005) A2238) to the case of arbitrary current in the load.

Carbon dioxide permeability of proton exchange membranes for fuel cells

The permeability of carbon dioxide and oxygen in Nafion® 1100 EW membrane has been studied as function of the relative humidity or water content using a mass spectrometer equipped with a membrane inlet. The rate of permeation of CO 2 was found to depend strongly on the water content of the membrane. It was similar to the oxygen permeation rate for the dry membrane but an order of magnitude higher for the fully humidified membrane. The data were used to correct the estimated methanol crossover rate and fuel efficiency in a commercial DMFC using Nafion® 117 as electrolyte by subtracting the contribution from the CO 2 permeation to the CO 2 flux from the cathode. The contribution varied between 4.5% and 12% of the flux depending on the mode of operation. In an operating DMFC the electrolyte/anode interface will be saturated with CO 2. For very efficient anodes (almost complete methanol conversion at the electrode/electrolyte interface) and the use of thin electrolyte membranes negligence of the CO 2 permeation contribution may thus lead to misleading conclusions regarding the magnitude of methanol crossover.

Effect of cell orientation on the performance of passive direct methanol fuel cells

A passive liquid feed direct methanol fuel cell (DMFC) with neither liquid pump nor a gas compressor was tested at different orientations. The experimental results showed that the vertical operation always yielded better performance than did the horizontal operation. It was further demonstrated that the improved performance in the vertical orientation was caused by the increased operating temperature as a result of a higher rate of methanol crossover, which resulted from the stronger natural convection in the vertical orientation. The constant current discharging tests showed that, although the vertical operation of the passive DMFC can yield better performance, the fuel utilization at this orientation is lower as a result of the increased rate of methanol crossover. It was also shown that the horizontal orientation with the anode facing upward rendered an effective removal of both CO 2 bubbles on the anode and liquid water on the cathode and thereby a relative stable operation. Finally, it was revealed that the horizontal orientation with the anode facing downward exhibited rather unstable and short discharging duration because of the difficulties in removing CO 2 bubbles from the anode and the liquid water from the cathode at this particular orientation.

Preparation and Characterization of Nafion/Poly(1-vinylimidazole) Composite Membrane for Direct Methanol Fuel Cell Application

The base monomer, 1-vinyl imidazole was impregnated in a Nafion 112 membrane and polymerized to poly(1-vinylimidazole) (PVI) by UV irradiation in order to reduce the methanol permeability of the Nafion membrane when used in a direct methanol fuel cell. As the PVI content in the composite membrane was increased, the equilibrium water uptake decreased and the size of the hydrated ion clusters was reduced, as confirmed by small angle X-ray scattering analysis. The electrochemical properties of the membrane, such as its proton conductivity, methanol permeability, and electro-osmotic drag, were also affected by the equilibrium water uptake and hydrated pore size. Even a small amount of the base polymer incorporated into the membrane resulted in a significant effect on its proton conductivity and methanol permeability. The methanol transport induced by the electro-osmotic drag was evaluated and the methanol permeability and limiting current density data obtained in this study. Although the absolute number of the electro-osmotic drag was not determined, the trend of change is discussed in relation to the bulk-like water in the composite membranes. The novel composite membrane exhibited improved cell performance compared with a plain Nafion membrane due to its reduced methanol crossover rate.

Novel, Ionic-Liquid-Based, Gel-Type Proton Membranes

We report a preliminary study on novel protonic membranes, obtained by the gelification of a selected polymer matrix in a highly stable ionic liquid solvent, i.e., 1,2-dimethyl-3--propylimidazolium bis(trifluoromethylsulfonyl)imide (DMPI-Im). The addition of an acid, chemically compatible with the ionic liquid thanks to the equality between the anionic components, gives rise to a reasonably high conductivity which, together with the good mechanical and thermal stability as well as an excellent control on the methanol crossover rate, make the membranes of interest for practical applications.

1D + 1D model of a DMFC: localized solutions and mixedpotential

Our 1D + 1D model of DMFC reveals a new effect. At infinitely small total current in the cell, near the channel inlet forms a “bridge”, a narrow region with finite local current density. The bridge short-circuits the electrodes, thus reducing cell open-circuit voltage. In our previous work the effect is described for the case of equal methanol λ a and oxygen λ c stoichiometries. In this Letter, we analyze the general case of arbitrary λ a and λ c. In the case of λ a > λ c current may occupy finite domain of the cell surface. Asymptotic solution for the case of λ a ≫ λ c shows, that the size of this domain is proportional to oxygen stoichiometry. In the opposite limit of λ a ≪ λ c local current exponentially decreases with the distance along the channel. Asymptotic solutions suggest that the bridge forms regardless of the relationship between λ a and λ c. In all cases local current density in the bridge increases with the rate of methanol crossover and decreases with the growth of the “rate-determining” stoichiometry. The expression for voltage loss at open-circuit is derived.