A three-dimensional two-phase flow model for a liquid-fed direct methanol fuel cell

A three-dimensional, two-phase, multi-component model has been developed for a liquid-fed DMFC. The modeling domain consists of the membrane, two catalyst layers, two diffusion layers, and two channels. Both liquid and gas phases are considered in the entire anode, including the channel, the diffusion layer and the catalyst layer; while at the cathode, two phases are considered in the gas diffusion layer and the catalyst layer but only single gas phase is considered in the channels. For electrochemical kinetics, the Tafel equation incorporating the effects of two phases is used at both the cathode and anode sides. At the anode side the presence of gas phase reduces the active catalyst areas, while at the cathode side the presence of liquid water reduces the active catalyst areas. The mixed potential effects due to methanol crossover are also included in the model. The results from the two-phase flow mode fit the experimental results better than those from the single-phase model. The modeling results show that the single-phase models over-predict methanol crossover. The modeling results also show that the porosity of the anode diffusion layer plays an important role in the DMFC performance. With low diffusion layer porosity, the produced carbon dioxide cannot be removed effectively from the catalyst layer, thus reducing the active catalyst area as well as blocking methanol from reaching the reaction zone. A similar effect exits in the cathode for the liquid water.     

An ultrasound enhanced direct methanol fuel cell

A novel direct methanol fuel cell (DMFC) incorporating an ultrasonic transducer is introduced based on a recent provisional patent application [J. Ge, J. Han, H. Liu, Ultrasounically enhanced fuel cell system, U.S. Provisional Patent Application No. 60/815,268, June 21, 2006]. The ultrasound transducer is embedded in the methanol supply line and is used to enhance the performance of a DMFC. The technique of introducing ultrasound through methanol supply line significantly reduced the potential losses in ultrasound transmission to the reaction sites of the fuel cell. Series of experiments have been conducted to study the effect of the ultrasound on the performance of the DMFC. The experimental results showed that the high-frequency vibrations of the ultrasound through the methanol supply line enhance the cell performance significantly and consistently. The experimental results unequivocally demonstrated the feasibility of using ultrasound to enhance DMFC performance and the effectiveness of introducing ultrasound into a DMFC via methanol supply line to minimize the wave transmission losses.     

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.     

Development of a direct methanol fuel cell stack fed with pure methanol

Two passive fuel cell stacks with the same four MEAs in a series connection have been fabricated, tested, and compared. The dilute-stack was filled with 30 mL dilute methanol solutions (1–3 M), whereas the pure-stack was driven by 3 mL pure methanol. In the pure-stack, porous components were added on both sides of the MEAs to modify its mass transfer characteristics so that the stack could directly use pure methanol as fuel without having severe methanol crossover. The performance, fuel efficiency, energy efficiency, and electrochemical impedance spectroscopy (EIS) responses of the passive dilute-stack and pure-stack were measured at room temperature with different fuels. The pure-stack using pure methanol showed similar performance with the dilute-stack using 1 M methanol solution. The measured fuel efficiency and energy efficiency of the pure-stack were 53.6% and 13.3%, respectively, at 1.2 V. Since 100% methanol, instead of the less than 10% methanol solutions, was used as fuel, the energy density of the pure-stack per weight of fuel was more than 10 times higher than that of the dilute stack.     

High power direct methanol fuel cell for mobility and portable applications

Here, we report on a low cost and novel architecture Direct Methanol Fuel Cell (DMFC) for mobility and portable applications. DMFC is fast charged by a low cost liquid fuel, thus it is expected to be competitive with the hydrogen gas fuel cells. Our research efforts have culminated in the outstanding performance of DMFC with very high power density of 181 mW cm⁻² at 80 °C, under very low air pressure of 0.05atm. This exceptional DMFC performance was achieved by a modification of the hydrophobicity of the BPP (Bi-Polar Plate) flow field channels. Our study of the effects of the hydrophobicity of bipolar flow field plates give rise to fundamental understanding of the relationship between the two-phase flow, that occurs in the flow channels of the bipolar plates of DMFC cells. To the best of our knowledge, such performance was never achieved prior to this work.     

Mass transport analysis of a passive vapor-feed direct methanol fuel cell

A two-dimensional, two-phase, non-isothermal model using the multi-fluid approach was developed for a passive vapor-feed direct methanol fuel cell (DMFC). The vapor generation through a membrane vaporizer and the vapor transport through a hydrophobic vapor transport layer were both considered in the model. The evaporation/condensation of methanol and water in the diffusion layers and catalyst layers was formulated considering non-equilibrium condition between phases. With this model, the mass transport in the passive vapor-feed DMFC, as well as the effects of various operating parameters and cell configurations on the mass transport and cell performance, were numerically investigated. The results showed that the passive vapor-feed DMFC supplied with concentrated methanol solutions or neat methanol can yield a similar performance with the liquid-feed DMFC fed with much diluted methanol solutions, while also showing a higher system energy density. It was also shown that the mass transport and cell performance of the passive vapor-feed DMFC depend highly on both the open area ratio of the vaporizer and the methanol concentration in the tank.     

A semi-empirical model for efficiency evaluation of a direct methanol fuel cell

Energy density and power density are two of the most significant performance indices of a fuel cell system. Both the indices are closely related to the operating conditions. Energy density, which can be derived from fuel cell efficiency, is especially important to small and portable applications. Generally speaking, power density can be easily obtained by acquiring the voltage and current density of an operating fuel cell. However, for a direct methanol fuel cell (DMFC), it is much more difficult to evaluate its efficiency due to fuel crossover and the complex architecture of fuel circulation. The present paper proposes a semi-empirical model for the efficiency evaluation of a DMFC under various operating conditions. The power density and the efficiency of a DMFC are depicted by explicit functions of operating temperature, fuel concentration and current density. It provides a good prediction and a clear insight into the relationship between the aforementioned performance indices and operating variables. Therefore, information including power density, efficiency, as well as remaining run-time about the status of an operating DMFC can be in situ evaluated and predicted. The resulting model can also serve as an important basis for developing real-time control strategies of a DMFC system.     

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.     

Performance characteristics of a novel tubular-shaped passive direct methanol fuel cell

The present work consists of a tubular-shaped direct methanol fuel cell (DMFC) that is operated completely passively with methanol solution stored in a central fuel reservoir. The benefit of a tubular-shaped DMFC over a planar-shaped DMFC is the higher instantaneous volumetric power energy density (power/volume) associated with the larger active area provided by the tubular geometry. Membrane electrode assemblies (MEAs) with identical compositions were installed in both tubular and planar-shaped, passive DMFCs and tested with 1, 2, and 3 M methanol solutions at room temperature. The peak power density for the tubular DMFC was 19.0 mW cm⁻² and 24.5 mW cm⁻² while the peak power density for the planar DMFC was 20.0 mW cm⁻² and 23.0 mW cm⁻² with Nafion^® 212 and 115 MEAs, respectively. Even though the performance of the fuel cell improved with each increase in methanol concentration, the fuel and energy efficiencies decreased for both the tubular and planar geometries due to increased methanol crossover. The tubular DMFC experienced higher methanol crossover potentially due to a higher static fluid pressure in the anode fuel reservoir (AFR) caused by the vertical orientation of the tubular fuel reservoir. The performance of the tubular DMFC in this work represents an 870% improvement in power density from the previous best, passive, tubular DMFC found in the literature.     

Transient and steady-state analysis of catalyst poisoning and mixed potential formation in direct methanol fuel cells

The present dynamic model is developed to investigate the coupled reaction mechanisms in a DMFC and therein associated voltage losses in the catalyst layers. The model describes a complete five-layer membrane electrode assembly (MEA), with gas diffusion layers, catalyst layers and membrane. The analysis of the performance losses are mainly focused on the electrochemical processes. The model accounts for the crossover of both, methanol from anode to cathode and oxygen from cathode to anode. The reactant crossover results in parasitic internal currents that are finally responsible for high overpotentials in both electrodes, so-called mixed potentials. A simplified and general reaction mechanism for the methanol oxidation reaction (MOR) was selected, that accounts for the coverage of active sites by intermediate species occurring during the MOR. The simulation of the anode potential relaxation after current interruption shows an undershoot behavior like it was measured in the experiment [1]. The model gives an explanation of this phenomenon by the transients of reactant crossover in combination with the change of CO and OH coverages on Pt and Ru, respectively.     

A semi-empirical model considering the influence of operating parameters on performance for a direct methanol fuel cell

This research focuses on modeling the relationships between operating parameters and performance measures for a single stack direct methanol fuel cell (DMFC). Four operating parameters, including temperature, methanol concentration, and methanol and air flow rates, are considered in this work. Performance of the DMFC is described by the relationship between current density and voltage. The open circuit voltage and voltage drop in the closed circuit due to resistance, activation, and concentration polarization are influenced by the operating parameters. To consider both modeling accuracy and simplicity, a semi-empirical model is developed in this work by integrating theoretical and approximation models. Experiments were designed and conducted to collect the required data and to obtain the coefficients in the semi-empirical model. The error analysis indicates that our semi-empirical model is effective for predicating the DMFC's performance. The influence of the four operating parameters on the DMFC's performance is also analyzed based on this semi-empirical model. Possible applications of the semi-empirical model in the optimal control of fuel cell systems are also discussed.     

Sensor-less control of methanol concentration based on estimation of methanol consumption rates for direct methanol fuel cell systems

Adequate control over the concentration of methanol is critically needed in operating direct methanol fuel cell (DMFC) systems, because performance and energy efficiency of the systems are primarily dependent on the concentration of methanol feed. For this purpose, we have built a sensor-less control logic that can operate based on the estimation of the rates of methanol consumption in a DMFC. The rates of methanol consumption are measured in a cell and the resulting data are fed as an input to the control program to calculate the amount of methanol required to maintain the concentration of methanol at a set value under the given operating conditions of a cell. The sensor-less control has been applied to a DMFC system employed with a large-size single cell and the concentration of methanol is found to be controlled stably to target concentrations even though there are some deviations from the target values.     

A transient semi-empirical voltage model of a fuel cell stack

On the basis of a case study, we first analyze the voltage transient properties of a fuel cell stack. We then propose a semi-empirical model, which has been validated by three test runs in the paper. The results obtained in this paper show that the model-computed values correctly fit the experimental data and ensure that the suggested model is highly able to reflect the transient properties of the fuel cell stack voltage. The advantage of this model lies in a simple structure, represented by a small number of parameters. Therefore, it can easily be applied to the simulation of vehicle dynamics for design aims.     

Ultrasonic synthesis and evaluation of non-platinum catalysts for alkaline direct methanol fuel cells

Ultrasonic synthesis was investigated as a synthesis method of non-platinum catalysts for alkaline direct methanol fuel cells (alkaline DMFCs) such as 20% mass Pd/C, Au/C, and PdAu/C. Among four kinds of solvents, ethylene glycol was demonstrated to be the optimum solvent for the synthesis of those catalysts. When ethylene glycol was used, the synthesized metal nanoparticles were highly dispersed on carbon particles. The synthesized Pd/C and PdAu/C showed the high oxygen reduction reaction (ORR) activity in alkaline condition (0.5 M NaOH aqueous solution), which was comparable to conventional Pt/C. Moreover, they showed lower methanol oxidation reaction (MOR) activity. Membrane electrode assemblies (MEAs) containing the synthesized Pd/C cathode catalysts and alkaline ion exchange membranes were fabricated and evaluated by single cell tests. They showed high performance that was comparable to MEAs with Pt/C cathode. In addition, it was found that the synthesized Pd/C was relatively tolerant to methanol crossover.     

Systematic approach for modeling methanol mass transport on the anode side of direct methanol fuel cells

A systematic method for modeling direct methanol fuel cells, with a focus on the anode side of the system, is advanced for the purpose of quantifying the methanol crossover phenomenon and predicting the concentration of methanol in the anode catalyst layer of a direct methanol fuel cell. The model accounts for fundamental mass transfer phenomena at steady state, including convective transport in the anode flow channel, as well as diffusion and electro-osmotic drag transport across the polymer electrolyte membrane. Experimental measurements of methanol crossover current density are used to identify five modeling parameters according to a systematic parameter estimation methodology. A validation study shows that the model matches the experimental data well, and the usefulness of the model is illustrated through the analysis of effects such as the choice fuel flow rate in the anode flow channel and the presence of carbon-dioxide bubbles.     

Methanol and water crossover in a passive liquid-feed direct methanol fuel cell

Methanol crossover, water crossover, and fuel efficiency for a passive liquid-feed direct methanol fuel cell (DMFC) were all experimentally determined based on the mass balance of the cell discharged under different current loads. The effects of different operating conditions such as current density and methanol concentration, as well as the addition of a hydrophobic water management layer, on the methanol and water crossover were investigated. Different from the active DMFC, the cell temperature of the passive DMFC increased with the current density, and the changes of methanol and water crossover with current density were inherently coupled with the temperature rise. When feeding with 2–4 M methanol solution, with an increase in current density, both the methanol crossover and the water crossover increased, while the fuel efficiency first increased but then decreased slightly. The results also showed that a reduction of water crossover from the anode to the cathode was always accompanied with a reduction of methanol crossover. Not only did the water management layer result in lower water crossover or achieve neutral or reverse water transport, but it also lowered the methanol crossover and increased the fuel efficiency.     

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.     

Bifunctional activation of a direct methanol fuel cell

We report a novel method for performance recovery of direct methanol fuel cells. Lowering of air flow rate below a critical value turns the cell into bifunctional regime, when the oxygen-rich part of the cell generates current while the rest part works in electrolysis mode (electrolytic domain). Upon restoring the normal (super-critical) air flow rate, the galvanic performance of the electrolytic domain increases. This recovery effect is presumably attributed to Pt surface cleaning on the cathode with the simultaneous increase in catalyst utilization on the anode.     

Anionic-cationic bi-cell design for direct methanol fuel cell stack

A new fuel cell stack design is described using an anion exchange membrane (AEM) fuel cell and a proton exchange membrane (PEM) fuel cell in series with a single fuel tank servicing both anodes in a passive direct methanol fuel cell configuration. The anionic-cationic bi-cell stack has alkaline and acid fuel cells in series (twice the voltage), one fuel tank, and simplified water management. The series connection between the two cells involves shorting the cathode of the anionic cell to the anode of the acidic cell. It is shown that these two electrodes are at essentially the same potential which avoids an undesired potential difference and resulting loss in current between the two electrodes. Further, the complimentary direction of water transport in the two kinds of fuel cells simplifies water management at both the anodes and cathodes. The effect of ionomer content on the AEM electrode potential and the activity of methanol oxidation were investigated. The individual performance of AEM and PEM fuel cells were evaluated. The effect of ion-exchange capacity in the alkaline electrodes was studied. A fuel wicking material in the methanol fuel tank was used to provide orientation-independent operation. The open circuit potential of the bi-cell was 1.36 V with 2.0 M methanol fuel and air at room temperature.     

A dynamic model for high temperature polymer electrolyte membrane fuel cells

A dynamic one-dimensional isothermal phenomenological model was developed in order to describe the steady-state and transient behavior of high temperature polymer electrolyte membrane fuel cells (PEMFC). The model accounts for transient species mass transport at the bipolar plates and gas diffusion layers and the electric double layers charge/discharge. To record the impedance spectra, a small sinusoidal voltage perturbation was imposed to the simulator over a wide range of frequencies, and the resultant current density amplitude and phase were recorded.The steady-state behavior of the fuel cell, as well as the impedance spectra were obtained and compared to experimental data of two different fuel cells equipped with different MEAs based on phosphoric acid polybenzimidazole membrane. This approach is new and allows a deeper analysis of the controlling phenomena. The model fitted quite well the I-V curves for both systems, but fairly well the Nyquist plots. The differences observed in the Nyquist plots were attributed to proton resistance in the catalyst layer and the gas diffusion limitations to cross the phosphoric acid layer that coats the catalyst, phenomena not included in the proposed phenomenological model.