Flowing Electrolyte-direct Methanol Fuel Cell Research Articles

Three dimensional modeling of a FE-DMFC short-stack

One of the primary challenges in fuel cell stack models is the lack of numerical methods and computational resources available to handle full-scale stack geometry to at least the same grid resolution and detailed physics as could be obtained in an equivalently detailed single cell model. To help resolve this challenge, a 3D modeling approach is proposed, and applied to a 3-cell Flowing Electrolyte-Direct Methanol Fuel Cell short-stack. In this approach, the flow fields, backing layers and membranes are solved numerically in a 3D manner, whereas the electrochemical performance is solved analytically. This approach allowed for the detailed physics to be incorporated into the model without the requirement of a high mesh density within the MEA, thus softening the computational load. The methanol concentration distributions at the anode catalyst layer of each cell for different methanol concentrations and sulfuric acid flow rates were found. In addition, the changes of crossover current density and cathodic activation polarization with current density were assessed. The model was used to shed light onto the mechanisms that lead to non-uniform flow behavior within the stack's cells; help identify methods to maintain a uniform flow and concentration distribution within the stack; and to provide methods to minimize methanol crossover to the cathode. The results showed that among the studied parameters, taking the methanol concentration as 2000 mol/m3 and sulfuric acid flow rate as 10 mL/min, cathodic activation polarization takes its lowest value.

Reduction of methanol crossover in a flowing electrolyte-direct methanol fuel cell

The flowing electrolyte-direct methanol fuel cell (FE-DMFC) is a type of fuel cell in which a flowing liquid electrolyte is used, in addition to two solid membranes, to reduce methanol crossover. In this study, FE-DMFCs having new materials and design were manufactured and studied. In this design, the flow field plates were made of stainless steel 2205 and had a pin type flow structure. PTFE treated carbon felts were used as the backing layers as well as the flowing electrolyte channel. Nafion® 115 or Nafion® 212 was used as the membranes. The polarization curves and methanol crossover current densities under different methanol concentrations and flow rates of sulfuric acid were measured using fully automated DMFC test stations. The performances of the FE-DMFCs were compared with those of the DMFCs having a single or double membrane. This study is, to the authors' knowledge, the first experimental study on measuring the methanol crossover in a FE-DMFC. The results of this study demonstrate that this technology enables a significant reduction of methanol permeation. At different cell current densities, Faradaic efficiencies up to 98% were achieved. It was shown that for a fixed flow rate of sulfuric acid solution (5 ml/min), at 0.1 A/cm2, the Nafion® 115 based FE-DMFC operating at 1 M yields the highest cell voltage (0.38 V). The maximum power density of the FE-DMFC (0.0561 W/cm2) was achieved when the cell operates with 3 M methanol concentration and 10 ml/min sulfuric acid solution at 0.3 A/cm2.

Effect of cathode flow field configuration on the performance of flowing electrolyte-direct methanol fuel cell

A major challenge in direct methanol fuel cells is the crossover of methanol, from the anode to the cathode. The flowing electrolyte (FE) concept is one potential method to reduce or eliminate the effects of this problem, and the corresponding fuel cell is known as a flowing electrolyte – direct methanol fuel cell (FE-DMFC). In this study, the effect that the cathode flow field has on the performance of the FE-DMFC was investigated in a COMSOL Multiphysics environment. Methanol concentration, oxygen concentration, pressure distribution and velocity distributions were simulated for a single serpentine, parallel serpentine, triple serpentine and a grid type flow field configuration. The results of this study demonstrate the importance of the velocity distribution on the oxygen transport and its impact on the fuel cell's performance. As such, of the tested flow fields, the grid flow field obtained the lowest performance due the formation of locally low concentration zones (or dead zones) causing an increase in the cathode's concentration polarization. The single serpentine flow field on the other hand provided the best performance due to the better reactant coverage over the CCL.

Two-dimensional multiphase non-isothermal modeling of a flowing electrolyte – Direct methanol fuel cell

A two-dimensional, non-isothermal, two-phase model is developed to investigate the non-isothermal and thermo-osmotic behavior within a flowing electrolyte direct methanol fuel cell (FE-DMFC). The physics of this fuel cell are examined and discussed for varied anode and cathode inlet temperatures, with particular emphasis being placed on the non-isothermal behavior of this fuel cell. The results show that the cathode's thermal entrance length is much larger than that of the anode's, and that the cathode's inlet temperature influences the overall cell temperature more significantly than that of the anode's. This was found to be primarily due to the cathode's high velocity and molecular diffusivity, which allowed for a high degree of convective cooling. This effect was found to cause the mid-plane temperature to be ∼10 °C lower than that of the set point temperature for the coldest air inlet temperature of 40 °C. Furthermore, it was also found that possible flooding conditions could occur within the cathode air channel, if either the anode's or cathode's inlet temperature is too low, relative to the fuel cell's set point temperature.

Experimental Analysis of a Small-Scale Flowing Electrolyte–Direct Methanol Fuel Cell Stack

A small-scale five-cell flowing electrolyte–direct methanol fuel cell (FE-DMFC) stack with U-type manifold configuration and parallel serpentine flow bed design was studied experimentally. The active area of a single cell was approximately 25 cm2. For every stack cell, diluted sulphuric acid was used as the flowing electrolyte (FE) which was circulated through a porous medium placed between two Nafion® 115 polymer electrolyte membranes. The stack performance was studied over a range of several operating conditions, such as temperature (50–80 °C), FE flow rate (0–17.5 ml/min), methanol concentration (0.5–4.0 M), and methanol solution flow rate (10–20 ml/min). In addition, the stack cell to cell voltage variations and the effects of the FE stream interruption on the output voltage were investigated at various operating loads. Experimental results showed that utilization of the FE effectively reduced methanol crossover and improved the stack power output. It was found that increasing the FE flow rate enhanced the stack capability to operate at higher inlet methanol concentrations without any degradation to the performance. The results also demonstrated that the stack power output can be directly controlled by regulating the FE stream especially at high operating currents.

A single domain approach to modeling the multiphase flow within a flowing electrolyte – direct methanol fuel cell

In this study, the well-known Multiphase Mixture Model (MMM) was improved with a new single domain approach which was used to model the flow behaviour and performance of a Flowing Electrolyte - Direct Methanol Fuel Cell (FE-DMFC). Emphasis was placed on the methanol and water crossover fluxes. Unlike the existing methods, the proposed method only requires the mixture variables, thereby decoupling the requirement for information about the gaseous state. This new approach was demonstrated by describing the physics of the fuel cell under base line operating conditions, as well as different cathode humidifications. The observed trends were found to be consistent with experimental and modeling results in DMFC literature. The model suggests that the flowing electrolyte channel (FEC) effectively washes unreacted methanol out of the system, yielding a negligible methanol crossover flux at the cathode catalyst layer. Furthermore, it was determined that the convection caused by the back-pressure within the FEC is an important factor to consider; since this effect is sufficiently strong to cause water to flow from the FEC to the anode. The model also suggests that decreasing the cathode humidification increases both the methanol and water crossover fluxes as well as the ohmic resistance within the membranes and FEC. This result suggests that some liquid water within the cathode could be beneficial for the fuel cell, to decrease these losses.

A comprehensive 1D model of a flowing electrolyte-direct methanol fuel cell with experimental validation

SUMMARY This paper presents the development of a new comprehensive single-phase model for a flowing electrolyte-direct methanol fuel cell (FE-DMFC) to determine the operating range of key input parameters for the optimum performance of the fuel cell. These parameters include the inlet concentration of the FE, as well as the flow rate and thickness of the flowing electrolyte channel (FEC). In addition, in-house FE-DMFC experimental results were used to validate the model for the first time. It was found that the experimental and modeling results were in a good agreement. The results of the parametric studies showed that an FE concentration within the range of 1.5 and 3.2 M would yield the best performance, while avoiding the corrosive nature of the liquid electrolyte. Furthermore, an FEC flow rate of 1 mL min−1, at an FEC channel thickness of 0.6 mm, was deemed to be sufficiently high to remove adequate amounts of methanol from the crossover stream. It was also determined that the thinnest possible FEC thickness should be used to provide high power output. Copyright © 2014 John Wiley & Sons, Ltd.

Nonisothermal Hydrodynamic Modeling of the Flowing Electrolyte Channel in a Flowing Electrolyte–Direct Methanol Fuel Cell

The performance of a direct methanol fuel cell (DMFC) can be significantly reduced by methanol crossover. One method to reduce methanol crossover is to utilize a flowing electrolyte channel. This is known as a flowing electrolyte–direct methanol fuel cell (FE–DMFC). In this study, recommendations for the improvement of the flowing electrolyte channel design and operating conditions are made using previous modeling studies on the fluid dynamics in the porous domain of the flowing electrolyte channel and on the performance of a 1D isothermal FE-DMFC incorporating multiphase flow, in addition to modeling of the nonisothermal effects on the fluid dynamics of the FE-DMFC flowing electrolyte channel. The results of this study indicate that temperature difference between flowing electrolyte inflow and the fuel cell have negligible hydrodynamic implications, except that higher fuel-cell temperatures reduce pressure drop. Reducing porosity and increasing permeability is recommended, with a porosity of around 0.4 and a porous-material microstructure typical dimension around 60–70 μm being potentially suitable values for achieving these goals.

Performance measurements of a single cell flowing electrolyte-direct methanol fuel cell (FE-DMFC)

The performance of a single cell flowing electrolyte-direct methanol fuel cell (FE-DMFC) was experimentally studied and its performance was compared to a regular DMFC. The active area of the fuel cell was approximately 25 cm2. Serpentine channels were used for both the methanol and air flows. Two combinations of Nafion® polymer electrolyte membranes (PEMs) were used in the MEAs. These were NR-212/N-117 (Type 1) and NR-212/NR-212 (Type 2). For this study, diluted sulphuric acid was used as the electrolyte, which flows through a channel made of a polyethylene porous material. The flowing electrolyte conditions (e.g., flow rate, channel thickness and sulphuric acid concentration), the methanol concentration, and the fuel cell temperatures were varied to study how these parameters affect the overall performance of the FE-DMFC. Type 1 MEA used in conjunction with 2 M methanol produced the highest current density. The power density decreased when the thickness of the flowing electrolyte channel was increased, and the performance of the fuel cell increased when the temperature of the fuel cell was increased, as expected. The results show that the performance of the present FE-DMFC was similar to a DMFC using the same active area and a single MEA (Nafion® N-117).

1D modeling of a flowing electrolyte-direct methanol fuel cell

In this study, the performance characteristics of a flowing electrolyte-direct methanol fuel cell (FE-DMFC) and a direct methanol fuel cell (DMFC) are evaluated by computer simulations; and results are compared to experimental data found in the literature. Simulations are carried out to assess the effects of the operating parameters on the output parameters; namely, methanol concentration distribution, cell voltage, power density, and electrical efficiency of the cell. The operating parameters studied include the electrolyte flow rate, flowing electrolyte channel thickness, and methanol concentration at the feed stream. In addition, the effect of the circulation of the flowing electrolyte channel outlet stream on the performance is discussed. The results show that the maximum power densities that could be achieved do not significantly differ between these two fuel cells; however the electrical efficiency could be increased by 57% when FE-DMFC is used instead of DMFC.

Modeling Methanol Crossover by Diffusion and Electro‐Osmosis in a Flowing Electrolyte Direct Methanol Fuel Cell

AbstractA CFD model is created to analyze methanol transport in a flowing electrolyte direct methanol fuel cell (FE‐DMFC) by solving the 3D advection‐diffusion equation, with consideration of electro‐osmosis. The average methanol flux at the anode and cathode surfaces is simulated and compared to equivalent direct methanol fuel cells. Methanol crossover is defined as methanol flux at the cathode surface, and the results reveal that methanol crossover can be drastically reduced by the flowing electrolyte. The performance of the FE‐DMFC at peak power current density is evaluated, and diffusion is shown to be the dominant contribution, although electro‐osmosis increases with current density. The power consumption of the electrolyte pump is shown to be negligible compared to the cell power output. This indicates that thin electrolyte channels with high flow rates could further improve the efficiency.

A parametric study of methanol crossover in a flowing electrolyte-direct methanol fuel cell

Direct methanol fuel cells (DMFCs) have significant potential to become a leading technology for energy conversion in a variety of applications. However, problems, such as methanol crossover reduce the efficiency and open circuit voltage of the cells. The novel design of flowing electrolyte-direct methanol fuel cells (FE-DMFCs) addresses this issue. Methanol molecules are effectively removed from the membrane electrode assembly (MEA) by the flowing electrolyte, and the unused fuel can be utilized externally. In this paper, a general 3D numerical computational fluid dynamics (CFD) model is established to simulate methanol crossover by convection–diffusion in the FE-DMFC. Illustrations of methanol concentration distribution and methanol molar flux densities are presented, and the performance is compared to conventional DMFCs. The results indicate that methanol crossover can be reduced significantly. A parameter study is performed where the influences of anode fuel feed concentration, electrolyte channel thickness and electrolyte volumetric flow rate on methanol crossover are evaluated. In addition, effects of various electrolyte channel orientations are determined. According to the simulations, counter flow is the superior choice of channel orientations to minimize crossover.

The deactivation of impregnated type cathodes due to metal vapors: J. Vaughn, K. Dudley and L. Lesensky, Vacuum, 11 (2), March 1961, 77–79. (Conference Issue)

A two-dimensional, non-isothermal, two-phase model is developed to investigate the non-isothermal and thermo-osmotic behavior within a flowing electrolyte direct methanol fuel cell (FE-DMFC). The physics of this fuel cell are examined and discussed for varied anode and cathode inlet temperatures, with particular emphasis being placed on the non-isothermal behavior of this fuel cell. The results show that the cathode's thermal entrance length is much larger than that of the anode's, and that the cathode's inlet temperature influences the overall cell temperature more significantly than that of the anode's. This was found to be primarily due to the cathode's high velocity and molecular diffusivity, which allowed for a high degree of convective cooling. This effect was found to cause the mid-plane temperature to be ∼10 °C lower than that of the set point temperature for the coldest air inlet temperature of 40 °C. Furthermore, it was also found that possible flooding conditions could occur within the cathode air channel, if either the anode's or cathode's inlet temperature is too low, relative to the fuel cell's set point temperature.

Electrode contamination and arc formation: H. de B. Knight, Brit. J. Appl. Phys., 12 (6), June 1961, 282–287

A two-dimensional, non-isothermal, two-phase model is developed to investigate the non-isothermal and thermo-osmotic behavior within a flowing electrolyte direct methanol fuel cell (FE-DMFC). The physics of this fuel cell are examined and discussed for varied anode and cathode inlet temperatures, with particular emphasis being placed on the non-isothermal behavior of this fuel cell. The results show that the cathode's thermal entrance length is much larger than that of the anode's, and that the cathode's inlet temperature influences the overall cell temperature more significantly than that of the anode's. This was found to be primarily due to the cathode's high velocity and molecular diffusivity, which allowed for a high degree of convective cooling. This effect was found to cause the mid-plane temperature to be ∼10 °C lower than that of the set point temperature for the coldest air inlet temperature of 40 °C. Furthermore, it was also found that possible flooding conditions could occur within the cathode air channel, if either the anode's or cathode's inlet temperature is too low, relative to the fuel cell's set point temperature.