Methanol-tolerant cathode electrode structure composed of heterogeneous composites to overcome methanol crossover effects for direct methanol fuel cell

A methanol-tolerant cathode electrode composed of heterogeneous composites was developed to overcome CO poisoning and large O₂ mass transfer overpotential generated by methanol crossover as well as the limitation of a single alloy catalyst with methanol-tolerance in direct methanol fuel cells (DMFCs). Two additives, PtRu black and PTFE particles, were well distributed in the Pt/C matrix of the cathode electrode, and had significant effects upon open circuit voltage (OCV) and performance. A small amount of PtRu black protected the Pt surface during the oxygen reduction reaction (ORR) by decreasing CO poisoning. In addition, hydrophobic PTFE particles reduced the O₂ mass transfer overpotential induced by water and permeated methanol in the cathode. Despite only 0.5 mg cm⁻² of metal catalysts in the cathode, the membrane electrode assembly (MEA) with 3 M methanol showed high performance (0.117 W cm⁻²), which was larger than that of the traditional MEA (0.067 W cm⁻²).     

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.     

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 self-regulated passive fuel-feed system for passive direct methanol fuel cells

Operating a passive direct methanol fuel cell (DMFC) with high methanol concentration is desired because this increases the energy density of the fuel cell system and hence results in a longer runtime. However, the increase in methanol concentration is limited by the adverse effect of methanol crossover in the conventional design. To overcome this problem, we propose a new self-regulated passive fuel-feed system that not only enables the passive DMFC to operate with high-concentration methanol solution without serious methanol crossover, but also allows a self-regulation of the feed rate of methanol solution in response to discharging current. The experimental results showed that with this fuel-feed system, the fuel cell fed with high methanol concentration of 12.0 M yielded the same performance as that of the conventional DMFC running with 4.0 M methanol solution. Moreover, as a result of the increased energy density, the runtime of the cell with this new system was as long as 10.1 h, doubling that of the conventional design (4.4 h) at a given fuel tank volume. It was also demonstrated that this passive fuel-feed system could successfully self-regulate the fuel-feed rate in response to the change in discharging currents.     

Modelling and experimental studies on a direct methanol fuel cell working under low methanol crossover and high methanol concentrations

A number of issues need to be resolved before DMFC can be commercially viable such as the methanol crossover and water crossover which must be minimised in portable DMFCs.     

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.     

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.     

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.     

Performance of air-breathing direct methanol fuel cell with Au-coated aluminum current collectors

Current collectors of the direct methanol fuel cell (DMFC) are of significant importance for portable power sources, and greatly determine the weight energy density and cost of the cell. In this paper, the air-breathing aluminum (Al) current collectors have been developed for powering portable applications. The anode and cathode current collectors with the area of 4.5 cm² were fabricated on the Al substrates utilizing Computer Numerical Control (CNC) technology. To obtain strong anti-corrosion resistance, a 3-μm-Au layer was deposited on the current collectors using chemical plating. Compared with the graphite and stainless steel, the characterization of the Au-coated Al current collector was investigated to exhibit superior characteristics in electric conductivity, weight and electrochemical corrosion resistance. The current collector was applied to a DMFC and the cell performance was experimentally investigated under different operating conditions. The measured maximum power density of the DMFC could reach 19.8 mW cm⁻² at current density of 98 mA cm⁻² with 2 M methanol solutions. The results indicated that the Au-coated Al current collectors presented in this paper might be helpful for the development of portable power sources applied in future commercial applications.     

Development of an air-breathing direct methanol fuel cell with the cathode shutter current collectors

An air-breathing direct methanol fuel cell with a novel cathode shutter current collector is fabricated to develop the power sources for consumer electronic devices. Compared with the conventional circular cathode current collector, the shutter one improves the oxygen consumption and mass transport. The anode and cathode current collectors are made of stainless steel using thermal stamping die process. Moreover, an encapsulation method using the tailor-made clamps is designed to assemble the current collectors and MEA for distributing the stress of the edges and inside uniformly. It is observed that the maximum power density of the air-breathing DMFC operating with 1 M methanol solution achieves 19.7 mW/cm² at room temperature. Based on the individual DMFCs, the air-breathing stack consisting of 36 DMFC units is achieved and applied to power a notebook computer.     

Effects of cathode flow fields on direct methanol fuel cell-simulation study

Flow-field design of direct methanol fuel cell (DMFCs) plays an important role affecting the cell performance. Previous studies suggest that the combination of anode parallel flow field and cathode serpentine flow-field present the best and stable performance. Among these, cathode flow-field holds higher influence than that of anode. However, more detailed experiments needed to be done to find out the reasons. In this study, CFDRC half-cell models are adopted to simulate the flow phenomena within serpentine, parallel and grid flow field. We find that gas is well distributed within serpentine flow field while barren region are observed within parallel flow field. These factors contribute to the cell performance greatly. In addition, the durability test of DMFCs using parallel flow field is improved when the flow rate is increased or the current is uphold at inferior, so the barren region maintained at an acceptable level.     

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.     

Low methanol crossover and high efficiency direct methanol fuel cell: The influence of diffusion layers

This experimental work aims to investigate the possibility to reduce methanol crossover in DMFC modifying diffusion layer characteristics. Improvements in crossover measurement are firstly proposed, permitting to conclude that in the investigated conditions carbon dioxide flow through the membrane can be neglected. The experimental results evidence that introducing appropriate anode and cathode microporous layers determines: a strong reduction in methanol crossover, approximately 45% at low current density; a considerable increment of efficiency; a moderate decrease of power density. The complete experimental analysis demonstrates that methanol transport in both liquid and vapour phases can be controlled modifying properly diffusion layer characteristics in order to increase DMFC efficiency.     

Molecular sieve as an effective barrier for methanol crossover in direct methanol fuel cells

Methanol crossover is still a significant barrier to the commercialization of direct methanol fuel cells with wide-used Nafion® membrane. Herein, molecular sieve is introduced into the design of polymer electrolyte membrane to alleviate methanol crossover. The UZM-9 zeolite with an intermediate window size of 0.42 nm can effectively separate hydrated methanol (ca. 1.10 nm) and hydrated proton (ca. 0.23 nm). The methanol diffusion rate through the membrane is effectively suppressed after modified with UZM-9, which is about four times lower than the origin Nafion® membrane. The resulted peak power density reached 80 mW cm⁻² with 2 mol L⁻¹ methanol solution feed, which is 2.5-fold higher than that of direct methanol fuel cell with commercial Nafion® membrane. These results open a promising route to alleviate methanol crossover in direct methanol fuel cells.     

A theoretical model of the membrane electrode assembly of liquid feed direct methanol fuel cell with consideration of water and methanol crossover

An algebraic model of the membrane electrode assembly of the direct methanol fuel cell is developed, which considers the simultaneous liquid water and methanol crossover effects, and the associated electrochemical reactions. The respective anodic and cathodic polarization curves can be predicted using this model. Methanol concentration profile and flux are correlated explicitly with the operating conditions and water transport rate. The cathode mixed potential effect induced by the methanol crossover is included and the subsequent cell voltage loss is identified. Water crossover is influenced by the capillary pressure equilibrium and hydrophobic property within the cathode gas diffusion layer. The model can be used to evaluate the cell performance at various working parameters such as membrane thickness, methanol feed concentration, and hydrophobicity of the cathode gas diffuser.     

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.     

Application of N-doped carbon nanotube-supported Pt-Ru as electrocatalyst layer in passive direct methanol fuel cell

In this research, nitrogen-doped carbon nanotubes (N-CNT) were prepared through the low-temperature thermal method and used as the support material for the bimetallic catalyst PtRu and Pt nanoparticles. A passive single-cell direct methanol fuel cell (DMFC) was designed and fabricated to investigate and compare the performance of three discrete membrane electrode assemblies (MEA) with carbon black (CB), CNT, and N-CNT as the catalyst support, respectively. Adding N to the structure of CNTs remarkably improves the physical and electrochemical characteristics of the catalyst. More active sites and stronger interaction between support and metal particles lead to the formation of smaller metal clusters and higher surface area as well as superior electrochemical activity. Compared to PtRu/CB and PtRu/CNT, PtRu/N-CNT illustrate 32% and 12% higher surface area, 3 and 1.9 times higher MOR activity, and 62% and 18% higher power output (26.1 mW/cm²), respectively. Moreover, it is revealed that PtRu/N-CNT has long-term stability in the MOR. The research work presented in this paper exhibits the outstanding performance of Pt and PtRu supported on N-CNT in a passive single-cell DMFC.     

An approach for determining the liquid water distribution in a liquid-feed direct methanol fuel cell

In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL.     

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.     

Poly(1-vinylimidazole)/Pd-impregnated Nafion for direct methanol fuel cell applications

Nafion is modified by incorporating poly(1-vinylimidazole)/Pd composites into the ion cluster channels of the membrane. The poly(1-vinylimidazole)/Pd-impregnated (PVI/Pd-impregnated) membranes is characterized by means of X-ray photoelectron spectroscopy (XPS), proton conductivity and methanol permeability measurements and compared with those of the untreated Nafion. The dependence of the membrane proton conductivity and methanol permeability on the poly(1-vinylimidazole) (PVI) and palladium contents in the PVI/Pd-impregnated Nafion is studied. The performance of the cells employing the PVI/Pd-impregnated Nafion is evaluated using a direct methanol fuel cell (DMFC) unit cell. It is found that the best cell performance is obtained when Nafion is impregnated with the PVI/Pd composite solution for 20 h. This result suggests that there exists an optimum content of palladium and PVI in the modified membrane to obtain a high-proton conductivity and low-methanol permeability that result in a high-cell performance.