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.     

Performance characterization of passive direct methanol fuel cells

The performance of a fuel cell is usually characterized by a polarization curve (cell voltage versus current density) under stabilized operating conditions. However, for passive direct methanol fuel cells (DMFC) that have neither fuel pumps nor gas compressors, the voltage at a given current density varies with time because methanol concentration in the fuel reservoir keeps decreasing during the discharging process. The important question brought up by this transient discharging behavior is: under what conditions should the polarization data be collected such that the performance of the passive DMFC can be objectively characterized? In this work, we found that the performance of the passive DMFC became relatively stable as the cell operating temperature rose to a relatively stable value. This finding indicates that the performance of the passive DMFC can be characterized by collecting polarization data at the instance when the cell operating temperature under the open-circuit condition rises to a relatively stable value.     

A strategy of estimating fuel concentration in a direct liquid-feed fuel cell system

Fuel control is one of the most pressing topics to achieve a self-sustainable direct liquid-feed fuel cell system, such as a direct methanol fuel cell (DMFC), and enhance its overall efficiency. In a DMFC system, sensing the methanol concentration generally serves as the basis of the fuel control strategies. This paper proposes a three-dimensional measurement space and constant concentration surfaces (CCS) to develop an algorithm of estimating fuel concentration in a liquid-feed system, which embraces the following merits: (1) it measures only three quantities or indices that correlate with the fuel concentration. The indices can be chosen as current, voltage, temperature, or other quantities that are easily acquired in an operating fuel cell system. The estimation can be accomplished without interrupting the operation of the system, (2) it estimates the fuel concentration in a three-dimensional measurement space; hence it is suitable for situations when one or more operating conditions are varying, (3) it can be performed as a sensor-less approach that requires no additional methanol sensors, thus consuming the less system power, and (4) it is particularly suitable for small and hand-held applications.     

Power management of a direct methanol fuel cell system

The effect of discharge rate of direct methanol fuel cell (DMFC) on fuel efficiency was comparatively investigated using a DMFC single cell and a DMFC system. The results obtained from the single cell were used to model the DMFC system. Several semi-empirical equations were derived that relate discharge current, voltage, power output, energy density and fuel consumption for a nominal 25 W DMFC system. The decrease in fuel efficiency with decreased power output that is observed for the DMFC system is attributable to the increase of methanol crossover that can be observed for an individual cell. A DMFC system can achieve maximum energy density and fuel efficiency at an appropriately high level of power output.     

空气抽吸式甲醇燃料电池传热与传质特性

空气抽吸式直接甲醇燃料电池不仅具有被动式燃料电池的优点,同时又便于将其串联成电堆提高输出电压。建立以阴极为管道抽吸式结构的直接甲醇燃料电池的三维、两相、非等温稳态数值模型,研究了质子交换膜性能、供给甲醇浓度以及电堆规模对电池性能及燃料利用率的影响。对于保温较好的大电堆,采用低甲醇穿透的改性质子交换膜能同时提升燃料利用率和比功率;此类电堆若采用穿透率低的改性膜,则2 mol/L的甲醇浓度就能保证电池在较大的电流密度区间内维持较高的功率与效率。作为影响电池运行温度的重要因素,电堆规模的大小将直接影响质子交换膜种类与甲醇浓度等关键参数的设计与选择。     

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.     

Control of variable power conditions for a membraneless direct methanol fuel cell

The control of a direct methanol fuel cell (DMFC) operating under variable power conditions is important in the development of a commercially applicable device. Fuel cells are conventionally designed for a maximum power output. However variable load cycles can result in fuel cell operation under sub-optimal conditions. In this paper, a simple method of power management using a physical guard is presented. The guard can be used on the anode or cathode electrode, in the membraneless gap or in any combination. This design selectively deactivates specific active regions of the electrode assembly and enables the DMFC to operate at a constant voltage and current density at different absolute power conditions. The guard also serves to control excessive crossover during shutdown and low power operation.     

Lightweight current collector based on printed-circuit-board technology and its structural effects on the passive air-breathing direct methanol fuel cell

To realize lightweight design of the fuel cell system is a critical issue before it is put into practical use. The printed-circuit-board (PCB) technology can be potentially used for production of current collectors or flow distributors. This study develops prototypes of a single passive air-breathing direct methanol fuel cell (DMFC) and also an 8-cell mono-polar DMFC stack based on PCB current collectors. The effects of diverse structural and operational factors on the cell performance are explored. Results show that the methanol concentration of 6 M promotes a higher cell performance with a peak power density of 18.3 mW cm⁻². The combination of current collectors using a relatively higher anode open ratio and inversely a lower cathode open ratio helps enhance the cell performance. Dynamic tests are also conducted to reveal transient behaviors and its dependence on the operating conditions. To validate the real working status of the DMFC stack, it is coupled with an LED lightening system. The performance of this hybrid system is also reported in this study.     

Performance improvement in direct methanol fuel cells using a highly porous corrosion-resisting stainless steel flow field

Power generation with direct methanol fuel cell (DMFC) systems requires only simple equipment, and has the important advantage of using a liquid fuel with higher energy density and easier handling characteristics than hydrogen. However, the power output of DMFC is lower than hydrogen fuel cells. To improve the power output of DMFC it is very important to reduce diffusion polarization at higher current density conditions. This research used a corrosion-resisting type porous stainless steel developed based on the technology for metal–hydride battery electrodes in the separator flow fields for reactants and products in a single cell DMFC and analyzed its influence on performance characteristics.     

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.     

Valveless piezoelectric micropump for fuel delivery in direct methanol fuel cell (DMFC) devices

Fuel cells are being considered as an important technology that can be used for various power applications. For portable electronic devices such as laptops, digital cameras, cell phone, etc., the direct methanol fuel cell (DMFC) is a very promising candidate as a power source. Compared with conventional batteries, DMFC can provide a higher power density with a long-lasting life and recharging which is almost instant. However, many issues related to the design, fabrication and operation of miniaturized DMFC power systems still remain unsolved. Fuel delivery is one of the key issues that will determine the performance of the DMFC. To maintain a desired performance, an efficient fuel delivery system is required to provide an adequate amount of fuel for consumption and remove carbon dioxide generated from fuel cell devices at the same time. In this paper, a novel fuel delivery system combined with a miniaturized DMFC is presented. The core component of this system is a piezoelectric valveless micropump that can convert the reciprocating movement of a diaphragm activated by a piezoelectric actuator into a pumping effect. Nozzle/diffuser elements are used to direct the flow from inlet to outlet. As for DMFC devices, the micropump system needs to meet some specific requirements: low energy consumption but a sufficient fuel flow rate. Based on theoretical analysis, the effect of piezoelectric materials properties, driving voltage, driving frequency, nozzle/diffuser dimension, and other factors on the performance of the whole fuel cell system will be discussed. As a result, a viable design of a micropump system for fuel delivery can be achieved and some simulation results will be presented as well.     

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.     

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.     

Numerical modeling and simulations of active direct methanol fuel cell (DMFC) systems under various ambient temperatures and operating conditions

Direct methanol fuel cells (DMFCs) are potential candidates for portable backup power generation and auxiliary power units owing to their advantageous features, such as ease of fuel storage and delivery. Optimizing each component of a DMFC system is critical to improving the overall system performance and power density. This paper presents an active DMFC system model, in which a one-dimensional DMFC stack model is combined with major system components, including fuel and water tanks, liquid–gas separator, heat exchangers, pumps, and blowers. The model is implemented using a commercial flow-sheet simulator, ASPEN-HYSYS, and then applied to an active DMFC system to analyze the effects of the DMFC operating parameters and heat management. Special emphasis is placed on establishing active control strategies for the DMFC stack temperature, methanol crossover rate, and water recovery by optimizing the system components and operating conditions. Overall, this study helps identify innovative active DMFC system designs and configurations.     

Fuel crossover and internal current in polymer electrolyte membrane fuel cell from water visualization using X-ray radiography

The fuel crossover and internal current in a polymer electrolyte membrane fuel cell undergo a chemical reaction in the cell without power generation. These are the main phenomena for reduced cell voltage at low current density. This fuel crossover also degrades the fuel cell performance, efficiency, and durability. Thus, observation of these phenomena is important for understanding and developing a polymer electrolyte membrane fuel cell. Using X-ray radiography, the water distribution and membrane swelling, which indicate fuel crossover and internal current, in an operating polymer electrolyte membrane fuel cell under open-circuit conditions were examined. The X-ray images effectively demonstrated the transient changes of each phenomenon, which are related to the properties of each component and the operating conditions.     

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.     

An impedance study of an operating direct methanol fuel cell

An electrochemical impedance spectroscopy (EIS) technique was developed to characterize a direct methanol fuel cell (DMFC) under various operating conditions. A silver/silver chloride electrode was used as an external reference electrode to probe the anode and cathode during fuel cell operation and the results were compared to the conventional anode or cathode half-cell performance measurement using a hydrogen electrode as both the counter and reference electrode. The external reference was sensitive to the anode and the cathode as current was passed in a working DMFC. The impedance spectra and DMFC polarization curves were systematically investigated as a function of air and methanol flow rates, methanol concentration, temperature, and current density. Water flooding in the cathode was also examined.     

Exergetic performance coefficient analysis of a simple fuel cell system

In this paper, a fuel cell power generation system fed by hydrogen is analyzed by different performance criteria over the entire range of potential operating conditions. First law efficiency and net power output are considered for conventional energetic indices of performance, and exergy destruction rate is taken into consideration as an exergetic performance criteria. A new exergetic criterion called the exergetic performance coefficient (EPC) is introduced and is applied to the system model based on zero-dimensional approach. The system model consists of the following components: fuel cell stack, afterburner, fuel and air compressors, and heat exchangers. The effects of the operating conditions on the system performance are studied parametrically. The obtained results based on the exergetic performance coefficient criterion are compared with first law efficiency, power output and exergy destruction rate. Results show that design insights of fuel cell systems can be considerably improved when conventional energetic analyses are supplemented with EPC criterion.     

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.     

Effect of operating conditions on water management of membrane electrode assembly for direct methanol fuel cell

介绍了直接甲醇燃料电池(DMFCs)膜电极的水平衡研究对单电池性能和稳定性的影响,研究了电池操作温度,空气流量及电流密度等操作条件对膜电极水平衡的影响.通过调节操作条件改变净水传输系数,进一步表征膜电极水平衡对电池稳定性的影响.结果表明,单电池在60 ℃,阴极常压空气80 mL/min进料,100 mA/cm²条件下工作具有较好的水平衡,最后,测试了单电池在该条件下的稳定性,测试结果表明电池稳定运行200 h后,性能没有明显衰减.