Portable DMFC system with methanol sensor-less control

This work develops a prototype 20 W portable DMFC by system integration of stack, condenser, methanol sensor-less control and start-up characteristics. The effects of these key components and control schemes on the performance are also discussed. To expedite the use of portable DMFC in electronic applications, the system utilizes a novel methanol sensor-less control method, providing improved fuel efficiency, durability, miniaturization and cost reduction. The operating characteristics of the DMFC stack are applied to control the fuel ejection time and period, enabling the system to continue operating even when the MEAs of the stack are deteriorated. The portable system is also designed with several features including water balance and quick start-up (in 5 min). Notably, the proposed system using methanol sensor-less control with injection of pure methanol can power the DVD player and notebook PC. The system specific energy and energy density following three days of operation are 362 Wh kg⁻¹ and 335 Wh L⁻¹, respectively, which are better than those of lithium batteries (∼150 Wh kg⁻¹ and ∼250 Wh L⁻). This good energy storage feature demonstrates that the portable DMFC is likely to be valuable in computer, communication and consumer electronic (3C) markets.     

A robust methanol concentration sensing technique in direct methanol fuel cells and stacks using cell dynamics

The electrochemical behaviour of direct methanol fuel cells (DMFCs) is sensitive to methanol concentration; thus, to avoid external sensors, it is a promising candidate to monitor the concentration of methanol in the fuel circulation loop, which is central to the efficient operation of direct methanol fuel cell systems. We address this issue and report on an extremely robust electrochemical methanol sensing technique that is not sensitive to temperature, cell degradation and membrane electrode assembly (MEA) type. We develop a temperature independent empirical correlation of the dynamic response of cell voltage to step changes in current with methanol concentration. This equation is successfully validated under various operating scenarios at both the single cell and stack levels. Our sensing method achieves an impressive accuracy of ±0.1 M and this is expected to increase the reliability of methanol sensing and simplify the control logic of DMFC systems.     

Measurement and estimation of species distribution in a direct methanol fuel cell

Determination of methanol concentration in a direct methanol fuel cell is crucial for design improvement and performance enhancement. Methanol and water concentrations in a direct methanol fuel cell are experimentally and numerically investigated. In the experimental program, a single cell direct methanol fuel cell is developed and an experimental setup is devised to measure methanol and water concentrations and performance of the cell depending on operating conditions. In theoretical program a mathematical model which includes fluid flow, species distribution, electric field and electrochemistry is adapted and numerically solved. The results showed that the performance of a Direct Methanol Fuel Cell (DMFC) is mainly influenced by operating temperature. A large drop in methanol concentration methanol is measured at the inlet section of cell. The mathematical model is found to satisfactorily capture main physics involved in a DMFC.     

Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system

Despite serious methanol crossover issues in Direct Methanol Fuel Cells (DMFCs), the use of high-concentration methanol fuel is highly demanded to improve the energy density of passive fuel DMFC systems for portable applications. In this paper, the effects of a hydrophobic anode micro-porous layer (MPL) and cathode air humidification are experimentally studied as a function of the methanol-feed concentration. It is found in polarization tests that the anode MPL dramatically influences cell performance, positively under high-concentration methanol-feed but negatively under low-concentration methanol-feed, which indicates that methanol transport in the anode is considerably altered by the presence of the anode MPL. In addition, the experimental data show that cathode air humidification has a beneficial effect on cell performance due to the enhanced backflow of water from the cathode to the anode and the subsequent dilution of the methanol concentration in the anode catalyst layer. Using an advanced membrane electrode assembly (MEA) with the anode MPL and cathode air humidification, we report that the maximum power density of 78 mW/cm² is achieved at a methanol-feed concentration of 8 M and cell operating temperature of 60 °C. This paper illustrates that the anode MPL and cathode air humidification are key factors to successfully operate a DMFC with high-concentration methanol fuel.     

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.     

Study on operational aspects of a passive direct methanol fuel cell incorporating an anodic methanol barrier

This paper presents an investigation concerning the effects of operating conditions on the performance of a passive direct methanol fuel cell (DMFC). A self-developed porous metal fiber sintered plate (PMFSP) is used as the methanol barrier between the fuel reservoir and current collector at the anode in order to alleviate the effect of methanol crossover. The effectiveness of using this method is validated. A series of operating conditions such as operating orientation, methanol concentration, ambient temperature, forced air convection and dynamic load are evaluated. Results show that the use of a PMFSP promotes a higher cell performance during vertical operation than horizontal orientation. The effect of methanol concentration depends on the PMFSP porosity. A relatively lower porosity is favorable for high-concentration operation. The cell performance gets improved when increasing the ambient temperature and adopting forced air supply at the cathode. Compared with the traditional structure, the use of a PMFSP makes the fuel cell insensitive to the change of blowing intensity. In addition, the dynamic characteristics of the PMFSP-based passive DMFC are also reported.     

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.     

A flow field enabling operating direct methanol fuel cells with highly concentrated methanol

In this work, an anode flow field that allows a direct methanol fuel cell (DMFC) to operate with highly concentrated methanol is developed and tested. The basic idea of this flow field design is to vaporize methanol solution in the flow field by utilizing the heat generated from the fuel cell so that the methanol concentration in the anode catalyst layer can be controlled to an appropriate level. The flow field is composed of two parallel flow channel plates, separated with a gap. The upper plate with a grooved serpentine flow channel is to vaporize a highly concentrated methanol solution to ensure the fuel to be completely vaporized before it enters the gap, while the lower plate, perforated to form a serpentine flow channel and located between the gap and the membrane electrode assembly (MEA), is to uniformly distribute the fuel onto the anode surface of the MEA. The test results show that this unique flow field design enables the DMFC operating with 16.0-M methanol to yield a power output similar to that with the conventional flow field design with 2.0-M methanol, significantly increasing the specific energy of the DMFC system. Finally, the effects of methanol solution flow rates and operating temperature on cell performance are investigated.     

Operational characteristics of the direct methanol fuel cell stack on fuel and energy efficiency with performance and stability

This paper is presented to investigate operational characteristics of a direct methanol fuel cell (DMFC) stack with regard to fuel and energy efficiency, including its performance and stability under various operating conditions. Fuel efficiency of the DMFC stack is strongly dependent on fuel concentration, working temperature, current density, and anode channel configuration in the bipolar plates and noticeably increases due to the reduced methanol crossover through the membrane, as the current density increases and the methanol concentration, anode channel depth, and temperature decreases. It is, however, revealed that the energy efficiency of the DMFC stack is not always improved with increased fuel efficiency, since the reduced methanol crossover does not always indicate an increase in the power of the DMFC stack. Further, a lower methanol concentration and temperature sacrifice the power and operational stability of the stack with the large difference of cell voltages, even though the stack shows more than 90% of fuel efficiency in this operating condition. The energy efficiency is therefore a more important characteristic to find optimal operating conditions in the DMFC stack than fuel efficiency based on the methanol utilization and crossover, since it considers both fuel efficiency and cell electrical power. These efforts may contribute to commercialization of the highly efficient DMFC system, through reduction of the loss of energy and fuel.     

Comparison of numerical simulation results with experimental current density and methanol-crossover data for direct methanol fuel cells

The simulation results of a one-dimensional (1D) direct methanol fuel cell (DMFC) model are compared with the current density and methanol-crossover data that are experimentally measured under several different cell designs and operating conditions. No fitting parameters are employed for the comparison and model input parameters obtained from the literature are consistently used for all the cases of comparison. The numerical predictions agree well with the experimental data and the 1D DMFC model successfully captures key experimental trends that are observed in the cell current density and methanol-crossover data. This clearly illustrates that the present DMFC model can be applicable for optimizing DMFC component designs and operating conditions. In addition, the model simulations further indicate that the reduction of the methanol concentration in the anode catalyst layer is critical to simultaneously suppress both the electro-osmotic drag (EOD) and the diffusion aspects of methanol crossover.     

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.     

A study on the overall efficiency of direct methanol fuel cell by methanol crossover current

This paper was presented to determine the methanol crossover and efficiency of a direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol concentration, methanol flow rate, cathode flow rate, and cathode backpressure. The methanol crossover measurements were performed by measuring crossover current density at an open circuit using humidified nitrogen instead of air at the cathode and applied voltage with a power supply. The membrane electrode assembly (MEA) with an active area of 5 cm² was composed of a Nafion 117 membrane, a Pt–Ru (4 mg/cm²) anode catalyst, and a Pt (4 mg/cm²) cathode catalyst. It was shown that methanol crossover increased by increasing cell temperature, methanol concentration, methanol flow rate, cathode flow rate and decreasing cathode backpressure. Also, it was revealed that the efficiency of the DMFC was closely related with methanol crossover, and significantly improved as the cell temperature and cathode backpressure increased and methanol concentration decreased.     

Heat and mass transfer effects in a direct methanol fuel cell: A 1D model

Models are a fundamental tool for the design process of fuel cells and fuel cell systems. In this work, a steady-state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC, is presented. The model output is the temperature profile through the cell and the water balance and methanol crossover between the anode and the cathode. The model predicts the correct trends for the influence of current density and methanol feed concentration on both methanol and water crossover. The model estimates the net water transfer coefficient through the membrane, α, a very important parameter to describe water management in the DMFC. Suitable operating ranges can be set up for different MEA structures maintaining the crossover of methanol and water within acceptable levels. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations.     

Modelling and analysis of a direct methanol fuel cell with under-rib mass transport and two-phase flow at the anode

For the past decade, extensive mathematical modelling has been conducted on the design and optimization of liquid-feed direct methanol fuel cells (DMFCs). Detailed modelling of DMFC operations reveals that a two-phase flow phenomenon at the anode and under-rib convection due to the pressure difference between the adjacent channels both contribute significantly to mass-transfer in a DMFC and its output performance. In practice, comprehensive simulations based on the finite volume technique for two-phase flow require a high level of numerical complexity in computation. This study presents a complexity-reduced mathematical model that is developed to cover both phenomena for a realistic, but fast, in computation for the prediction and analysis of a DMFC prototype design. The simulation results are validated against experimental data with good agreement. Analysis of the DMFC mass-transfer is made to investigate methanol distribution at anode and its crossover through the proton-exchange membrane. From a comparison of the influence of two-phase flow and under-rib mass-transfer on DMFC performance, the significance of gas-phase methanol transport is established. Simulation results suggest that both the optimization of the flow-field structure and the fuel cell operating parameters (flow rate, methanol concentration and operating temperature) are important factors for competitive DMFC performance output.     

Effect of water concentration in the anode catalyst layer on the performance of direct methanol fuel cells operating with neat methanol

This paper reports on a chromatography-based method for determining the water concentration in the anode catalyst layer (CL) of a direct methanol fuel cell (DMFC). By this method, the effect of the water concentration in the anode CL on the product distribution of the methanol oxidation reaction (MOR), the anode potential, and the cell internal resistance is experimentally investigated in a DMFC operating with neat methanol. Interestingly, it is found that the main product of the anode MOR is still carbon dioxide even when the water concentration in the anode CL is extremely low. The experimental data also show that an increase in the water concentration in the anode CL decreases the internal resistance, the production of by-products (methyl formate and methylal), and the anode potential. As the mole ratio of water to methanol increases beyond a critical value, however, both the internal resistance and the anode potential tend to be stabilized at the points under diluted methanol operating conditions.     

A direct methanol fuel cell system with passive fuel delivery based on liquid surface tension

The existing direct methanol fuel cell (DMFC) systems are fed with a fixed concentration of fuel, which are either a diluted methanol solution or an active fuel delivery driven by an attached active pump. Both approaches limit the power conversion density or degrade the overall efficiency of the DMFC system significantly. Such disadvantages become more severe in small-scale DMFCs, which require a high conversion efficiency and a small physical space suitable for portable electronics. In this paper, passive fuel delivery based on a surface tension driving mechanism was designed and integrated in a laboratory-made prototype to achieve consumption depending on fuel concentration and power-free fuel delivery. Unidirectional methanol-to-water smooth flow is achieved through the capillaries of a Teflon PTFE (polytetrafluoroethylene) membrane based on the difference in liquid surface tension. The prototype was demonstrated to exhibit a better polarization performance and to last for an extended operating time compared to conventional DMFCs. Its high efficiency and load regulation performance were also demonstrated in contrast to an active DMFC supplied with a constant concentration fuel. The fuel delivery driven by the liquid surface tension effect demonstrated here is believed to be more applicable for future small-scale DMFCs for portable electronics.     

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.     

Design and simulation of a liquid electrolyte passive direct methanol fuel cell with low methanol crossover

This study investigates an aqueous solution of sulfuric acid that serves as the liquid electrolyte (LE) in a passive direct methanol fuel cell (DMFC). The addition of an LE can reduce methanol crossover and increase the fuel utilization significantly. To improve the performance of an LE-DMFC, a mathematical model is developed to optimize the thicknesses of both the LE layer and the Nafion membrane. The maximum power density of the LE-DMFC is improved by approximately 30% compared with a conventional DMFC (C-DMFC) when each is fed by methanol solutions of the same concentration. Due to the low methanol crossover of the LE-DMFC, a highly concentrated methanol solution can be directly fed into the LE-DMFC. The discharge time and volume energy density of the LE-DMFC are two times longer and three times greater than those of the C-DMFC, respectively. In addition, fuel utilization increases by approximately 100%.     

Effect of operating conditions on the performance of a direct methanol fuel cell with PtRuMo/CNTs as anode catalyst

PtRu/CNTs and PtRuMo/CNTs catalysts have been synthesized by microwave-assisted polyol process and used as the anode catalysts for a direct methanol fuel cell (DMFC). The catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The effect of different anode catalysts, membrane electrode assembly (MEA) activation, methanol concentration, methanol flow rate, oxygen flow rate and cell temperature on the DMFC performance has been investigated. The results show that the PtRu or PtRuMo particles with face-centered cubic structure are uniformly distributed on CNTs, and the addition of Mo to PtRu/CNTs makes the binding energies of each Pt species shift to lower values. PtRuMo/CNTs is a promising anode catalyst for DMFCs, and the appropriate operating conditions of the DMFC with PtRuMo/CNTs as the anode catalyst are MEA activation for 10 h, 2.0–2.5 M methanol at the flow rate of 1.0–2.0 mL/min, and oxygen at the flow rate of 100–150 mL/min. The DMFC performance increases significantly with an increase in cell temperature.     

Towards operating direct methanol fuel cells with highly concentrated fuel

A significant advantage of direct methanol fuel cells (DMFCs) is the high specific energy of the liquid fuel, making it particularly suitable for portable and mobile applications. Nevertheless, conventional DMFCs have to be operated with excessively diluted methanol solutions to limit methanol crossover and the detrimental consequences. Operation with diluted methanol solutions significantly reduces the specific energy of the power pack and thereby prevents it from competing with advanced batteries. In view of this fact, there exists a need to improve conventional DMFC system designs, including membrane electrode assemblies and the subsystems for supplying/removing reactants/products, so that both the cell performance and the specific energy can be simultaneously maximized. This article provides a comprehensive review of past efforts on the optimization of DMFC systems that operate with concentrated methanol. Based on the discussion of the key issues associated with transport of the reactants/products, the strategies to manage the supply/removal of the reactants/products in DMFC operating with highly concentrated methanol are identified. With these strategies, the possible approaches to achieving the goal of concentrated fuel operation are then proposed. Past efforts in the management of the reactants/products for implementing each of the approaches are also summarized and reviewed.