DMFC performance and methanol cross-over: Experimental analysis and model validation

A combined experimental and modelling approach is proposed to analyze methanol cross-over and its effect on DMFC performance. The experimental analysis is performed in order to allow an accurate investigation of methanol cross-over influence on DMFC performance, hence measurements were characterized in terms of uncertainty and reproducibility. The findings suggest that methanol cross-over is mainly determined by diffusion transport and affects cell performance partly via methanol electro-oxidation at the cathode. The modelling analysis is carried out to further investigate methanol cross-over phenomenon. A simple model evaluates the effectiveness of two proposed interpretations regarding methanol cross-over and its effects. The model is validated using the experimental data gathered. Both the experimental analysis and the proposed and validated model allow a substantial step forward in the understanding of the main phenomena associated with methanol cross-over. The findings confirm the possibility to reduce methanol cross-over by optimizing anode feeding.     

Effect of anode MPL on water and methanol transport in DMFC: Experimental and modeling analyses

The regulation of mass transport through anode diffusion layer is one of the major issue of direct methanol fuel cell. In fact it is critical to maintain an adequate methanol concentration in the anode electrode such that both the rate of methanol crossover and the mass transport loss can be minimized. In the present work the effect of anode micro-porous layer on system operation is investigated both experimentally and theoretically. The developed 2D two-phase isothermal model is validated with respect to three different typologies of measure at the same time, increasing results reliability. Model simulations highlight that anode micro-porous layer can cause an inversion of water diffusion flux through the membrane and enhances methanol gas diffusion mechanism, reducing methanol crossover. Finally the developed model is used as a tool to design an optimized anode diffusion layer.     

Experimental measurements of fuel and water crossover in an active DMFC

This study measured polarization curves as well as the high-frequency resistance of active direct methanol fuel cell (DMFC) operates at around 80 °C with active controls of temperature, methanol concentration, airflow rate, and relative humidity. The relative humidity of the air did not have noticeable impacts on the fuel cell unless the operating temperature was near the evaporation temperature of water (100 °C). The hydrophobic water management layer (WML) between the membrane electrode assembly (MEA) and cathode air channel increases the mass transfer resistance and improves the water retention in MEA. Adding a WML increased the peak power density, decreased the ohmic resistance, and improved the fuel efficiency of the fuel cell, especially when it operated near 100 °C. This study also quantitatively measured methanol and water crossover as well as the fuel efficiency at different operating currents. The fuel efficiency increased significantly with the increase of the current density. Using a hydrophobic fuel management layer (FML) between the anode fuel channel and MEA reduced the fuel and water crossover rates and increased the ohmic resistance due to the decrease of the water content of the Nafion membrane. The FML improved fuel efficiency by reducing the methanol crossover. The combination of the FML and WML enabled the steady operation of DMFC using highly concentrated methanol solutions (up to 75 wt%).     

Application of response surface methodology to optimize and investigate the effects of operating conditions on the performance of DMFC

In this study, the response surface methodology (RSM) has been applied to optimize the operating conditions of direct methanol fuel cell (DMFC). A quadratic model was developed through RSM in terms of related independent variable to describe the current as the response. The input data required in this model has been obtained experimentally. For this purpose, an experimental set up for testing of direct methanol fuel cell has been established to investigate the effects of temperature and flow rate parameters on the cell performance. Two different analyses for operating conditions were performed applying the response surface method to obtain the maximum power. These analyses were based on the unlimited and minimum methanol consumptions. Methanol flow rate, oxygen flow rate, methanol temperature, humidification temperature and cell temperature were the main parameters considered that they were varied between 2 and 50 ml/min, 100-1000 ml/min, 30-70 °C, 30 70 °C and 30-80 °C in the analyses respectively. The maximum current under the unlimited and minimum methanol consumptions was found as 1230 mA and 582 mA based on the contour plots and variance analysis.     

Experimental validation of a methanol crossover model in DMFC applications

A design of experiments (DOEs) coupled with a mathematical model was used to quantify the factors affecting methanol crossover in a direct methanol fuel cell (DMFC). The design of experiments examined the effects of temperature, cathode stoichiometry, anode methanol flow rate, clamping force, anode catalyst loading, cathode catalyst loading (CCL), and membrane thickness as a function of current and it also considered the interaction between any two of these factors. The analysis showed that significant factors affecting methanol crossover were temperature, anode catalyst layer thickness, and methanol concentration. The analysis also showed how these variables influence the total methanol crossover in different ways due to the effects on diffusion of methanol through the membrane, electroosmotic drag, and reaction rate of methanol at the anode and cathode. For example, as expected analysis showed that diffusion was significantly affected by the anode and cathode interfacial concentration, by the thickness of the anode catalyst layer and membrane, and by the diffusion coefficient in the membrane. Less obvious was the decrease in methanol crossover at low cathode flow rates were due to the formation of a methanol film at the membrane/cathode catalyst layer interface. The relative proportions of diffusion and electroosmotic drag in the membrane changed significantly with the cell current of the cell.     

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 algebraic semi-empirical model for evaluating fuel crossover fluxes of a DMFC under various operating conditions

In a fuel cell of low temperature, especially a direct methanol fuel cell (DMFC), fuel crossover phenomenon plays a significant role not only in its performance evaluation and analysis, but also in the optimum control under various operating conditions. A quantitative prediction of the fuel crossover flux thus becomes essential. Generally speaking, the theoretical approaches to the issue will be dramatically complex and less practical. On the other hand, experimental schemes are time-consuming and less capable of further analysis and applications. Consequently, a semi-empirical model that incorporates dominant physical parameters and operating variables is proposed in this paper to adequately evaluate the phenomenon of fuel crossover fluxes. It is stated analytically in the form of an algebraic function, in which the fuel concentration, the current density, and the temperature of the fuel cell are considered. It is therefore more suitable for a variety of in-situ applications. In the proposed model, the methanol concentration gradient in the anode backing layer, the anode catalyst layer, and the membrane are analyzed. The transfer behavior of methanol is modeled on the basis of diffusion and electro-osmosis mechanisms. By means of the proposed model, one can obtain a better prediction and a clearer picture of the effects of operating variables and physical parameters on methanol crossover fluxes.     

A novel multi-porous and hydrophilic anode diffusion layer for DMFC

The effect of hydrophilic treatment within the anode diffusion layer for direct methanol fuel cell (DMFC) has been investigated. By nitrated treatment, the surface structure and wettability of diffusion layer can be tuned. The anode micro-porous surface of carbon paper with hydrophilic adhesive after nitrated treatment presents more multi-hole structures with about 30 μm large pores and about 5 μm small pores, which were significantly larger than commercial carbon cloth and carbon paper without nitrated treatment. FTIR and EDS show that the surface of micro-porous layer has more oxygenic groups and the contact angles test also indicates that it becomes more hydrophilic after nitrated treatment. It is indicated that the anode charge transfer resistance and internal resistance dramatically decrease after nitrated treatment on the EIS test. The performance of assembled cell is also evaluated of which the power density of cell using novel diffusion layer (260 mW/cm²) is significantly higher than cell using commercial diffusion layer. The results indicate that this novel multi-porous and hydrophilic anode diffusion layer is suitable to DMFC.     

A CFD model with semi-empirical electrochemical relationships to study the influence of geometric and operating parameters on DMFC performance

A three-dimensional computational fluid dynamics (CFD) model is developed to investigate the influence of geometric and operating parameters on performance of a direct methanol fuel cell (DMFC). Semi-empirical relationships are introduced to describe the electrochemical behaviors required in the CFD governing equations. Coefficients in these semi-empirical relationships are fitted using experimental data. Two geometric configurations with serpentine channels at the anode and cathode are considered in this work. Temperature, methanol concentration, and methanol flow rate are selected as the operating parameters. Due to the computational effort of CFD, an adaptive metamodeling method is developed to reduce the number of data-fitting iterations for obtaining the coefficients in the semi-empirical relationships. The effectiveness of the method is demonstrated by fitting the model using the experimental data collected from the first geometric configuration of the DMFC and comparing the predicted performance of the second configuration with its experimental performance. A commercial CFD system, Fluent 12.0, was used in this research.     

Polymer blends of SPEEK for DMFC application at intermediate temperatures

Sulfonated poly(ether–ether–ketone) materials (SPEEK) are good proton conductors at high degrees of sulfonation and appropriate for high temperature application due to their glass transition temperatures around 200 °C. Nevertheless, high degrees of sulfonation result in excessive swelling and dissolution of the membranes in hot water, preventing their potential use for direct methanol fuel cells. One possible remedy is their chemical stabilization. For this reason, blends of SPEEK with PVA (polyvinyl alcohol), a hydrophilic polymer, were prepared and tested. Above 25 wt% PVA, the membranes were found to be mechanically stable in boiling water, with acceptable proton conductivities but excessive methanol permeabilities. On the other hand, blends of SPEEK with a hydrophobic polymer, PVB (polyvinyl butyral), resulted in extremely stable membranes in boiling water above a 30 wt% PVB content. Those membranes presented excellent mechanical and methanol barrier properties while proton conductivities were very low. A discussion of possible ways to make optimal use of these materials is presented.     

Experimental investigation on DMFC temporary degradation

Performance degradation is a critical issue of Direct Methanol Fuel Cell technology. The experimental investigations found in the literature show that degradation has both permanent and temporary contributions; the latter can be recovered after operation interruption, but its origins are not fully understood. In a previous work, the authors focused on the anode degradation, where a strong temporary behavior has been identified and discussed. This work aims to further investigate the temporary degradation in the whole fuel cell through performance characterization and appositely developed degradation tests. A 600 h test in cycling operation, with periodic interruptions based on OCV and air-break, highlights a temporary degradation higher than the one attributed to the anode. From additional experimental investigations, platinum oxides formation turns out to be the main cathode temporary degradation mechanism. The reduction of the oxides layer occurs during the air-break periods, due to the low operating cathode potential. Finally, by means of gas chromatographic analyses, anode hydrogen generation is confirmed to occur during the air-break periods in galvanic operation.     

Simultaneous oxygen-reduction and methanol-oxidation reactions at the cathode of a DMFC: A model-based electrochemical impedance spectroscopy study

A model-based electrochemical impedance spectroscopy (EIS) approach that combines an equivalent electrical circuit (EEC) method and a mathematical model derived from the reaction kinetics is proposed to investigate the simultaneous oxygen-reduction reaction (ORR) and methanol-oxidation reaction (MOR) at the cathode of a DMFC. Good agreements between the calculated results and the experimental data validated the proposed method. Detailed kinetic parameters and state variables of the cathode were conveniently extracted and the concerned reaction processes were further analyzed, which demonstrated the comprehensive applicability of this method. The results showed a significant poisoning effect on the ORR by the presence of methanol at the cathode. The results also indicated that whether the methanol permeated from the anode can be completely oxidized by electrochemical reaction at the DMFC cathode depends on the electrode design and operating conditions, even at high potentials.     

Improving the water management and cell performance for the passive vapor-feed DMFC fed with neat methanol

A passive vapor-feed direct methanol fuel cell (DMFC) was experimentally investigated to improve its water management and cell performance when neat methanol was directly used. The effects of different water management approaches, including the addition of a water management layer (WML) and a hydrophobic air filter layer (AFL), and the use of thinner membrane on the cell performance, internal resistance, and fuel efficiency were investigated. The transient discharging behavior and long-term stability of the passive vapor-feed DMFC with the optimized water management were also studied. The results showed that by adding a WML and an AFL, or thinning the membrane thickness, the water management capability can be highly improved, not only enhancing the water recovery from the cathode to the anode, leading to a lower internal resistance and better cell performance, but also curbing the methanol crossover, increasing the fuel efficiency. It is also seen from the long-term constant-voltage test that the discharged current varied with the methanol concentration in the tank and the ambient temperature, while no evident permanent performance degradation was encountered after the 150 h test.     

A comparison of operating strategies to reduce DMFC degradation

Degradation is one of the most critical issues that hinder the commercialization of fuel cells. The experimental investigations of direct methanol fuel cell degradation are complicated by the presence of a temporary contribution that can be recovered adopting suitable operating strategies. In the present work, a methodology to distinguish temporary and permanent degradation is proposed. The capability of different operating strategies to mitigate temporary degradation is evaluated. A strategy, consisting in refresh cycles involving open circuit voltage and cathode air interruption, able to minimize temporary degradation and reduce permanent degradation, is identified and compared with steady‐state operation. Copyright © 2013 John Wiley & Sons, Ltd.     

Single wall nanohorns as electrocatalyst support for vapour phase high temperature DMFC

The use of single wall nanohorns (SWNH) as electrocatalyst support has proved to increase the performance of polymer electrolyte membrane-based fuel cells. In order to investigate in more detail such behavior, the electrochemical characterization of SWNH based electrodes was performed. The use of SWNH in vapour phase high temperature direct methanol fuel cells (HT-DMFC) was also addressed. Cyclic voltammetry experiments have indicated a higher electrochemical activity towards methanol electro-oxidation and a higher tolerance to carbonaceous species accumulation for a SWNH based electrode than for carbon black and commercial corresponding ones. Carbon black electrode presented a better performance than SWNH one for oxygen reduction reaction at low current densities while, at higher overvoltages, SWNH electrode performed better. The exact role of the improved performance of SWNH based electrodes is yet not clear but may be related to a higher water vapour adsorption or electrode morphology. Vapour phase HT-DMFC operation showed the improved performance of the SWNH electrode in agreement with previous works and with the electrochemical characterization performed during this work; despite the higher ohmic resistance observed in comparison with the carbon black based electrode. Moreover, SWNH based electrode showed improved fuel cell stability during longer operation times.     

A novel anode for preventing liquid sealing effect in DMFC

Carbon dioxide removal from the catalyst sites is critical to ensure high power output operation of direct methanol fuel cell (DMFC). In this work, a novel anode which contains water-proof oil, dimethyl silicon oil (DMS) was prepared for preventing liquid sealing effect (LSE) to CO₂. The novel electrode displays outstanding preventing LSE capability than a conventional PtRu/C electrode. The success of the novel anode in preventing LSE is due to DMS, in which the solubility and diffusion coefficient of CO₂ are much higher than that in methanol–water solution, supplying an unoccupied channel for CO₂ transportation.     

Water management of the DMFC passively fed with a high-concentration methanol solution

The methanol barrier layer adopted for high-concentration direct methanol fuel cells (HC-DMFCs) increases water transport resistance, and makes water management in HC-DMFCs more challenging and critical than that in the conventional direct methanol fuel cell (DMFC) without a methanol barrier layer. In the semi-passive HC-DMFC used in this work, oxygen was actively supplied to the cathode side while various concentrated methanol solutions, 4 M, 8 M, 16 M, and neat methanol, were passively supplied from the anode fuel reservoir. The effects of the cathode relative humidity, cathode pressure, and oxygen flow rate on the water crossover coefficient, fuel efficiency, and overall performance of the fuel cell were studied. Results showed that electrolyte membrane resistance, which was determined by its water content, was the predominant factor that determined the performance of a HC-DMFC, especially at a high current density. A negative water crossover coefficient, which indicated that water flowed back from the cathode through the electrolyte membrane to the anode, was measured when the methanol concentration was 8 M or higher. The back flow of water from the cathode is a very important water supply source to hydrate the electrolyte membrane. The water crossover coefficient was decreased by increasing the cathode relative humidity and back pressure. Water flooding at the cathode was not severe in the HC-DMFC, and a low oxygen flow rate was preferred to decrease water loss and yield a better performance. The peak power density generated from the HC-DMFC fed with 16 M methanol solution was 75.9 mW cm⁻² at 70 °C.     

Applications of graphene nano-sheets as anode diffusion layers in passive direct methanol fuel cells (DMFC)

The diffusion layer is an important structure in the membrane electrode assembly (MEA) of direct methanol fuel cells (DMFCs) that provide a support layer for catalysts, electronic channels, and gas–liquid mass transport channels. In this study, three types of carbon-based materials were used to fabricate anode diffusion layers – carbon black Vulcan^® (CBV), M-15 grade graphene nanosheets (GM-15) and C-500 grade graphene nanosheets (GC-500). The microporous layers of cathodes were constructed with CBV. A carbon-based microporous layer with a 2 mg cm^?2 loading was coated onto a PTFE-pretreated carbon cloth, while a Nafion-117 membrane was applied as the electrolyte to the DMFCs. Pt–Ru black and Pt black were used as anode and cathode electrode catalysts, each with loadings of 8 mg cm^?2 and 4 mg cm^?2, respectively. All tests were conducted using MEAs with active areas of 4 cm² and air was supplied to single cells by passive modes. Surface morphology was studied using scanning electron microscopy (SEM), which produced pictures of complex network formations within the structures. CBV consists of nanosized carbon particles, while both GM-15 and GC-500 are made of stacks of graphene sheets with flaky structures that increase catalyst utilization. Performance tests of the DMFCs were conducted using a potentiostat that generated polarization curves. The highest peak power density of 13.7 mW cm^?2 was obtained by the GC-500 anode diffusion layer using 3 M methanol as fuel. The energy efficiency of the passive DMFCs was approximately 10% with a specific energy of approximately 610 Wh kg^?1, which is higher than that of conventional lithium-ion batteries, portraying the bright future of alternative energy sources for use in power applications for portable devices. The high power densities obtained by both graphene-based materials, GM-15 and GC-500, demonstrate that graphene is a material other than state of the art carbon black that has the potential to be used as a DMFC anode support material.     

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.     

Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis

This paper presents the diagnostic results of single polymer electrolyte membrane fuel cell assemblies characterized by polarization curves. Single PEM fuel cell assemblies were investigated through accelerated voltage cycling test at different values of relative humidity. The fuel cells are tested at different humidity level. The cells are discussed in this paper with analysis results at different relative humidity at atmospheric pressure. This represents a nearly fully humidified, a moderately humidified, and a low humidified condition, respectively. This technique is useful for diagnosing the main sources of loss in MEA development work, especially for high temperature/low relative humidity operation where several sources of loss are present simultaneously. All the fuel cells showed better performance in terms of limiting current density value through polarization curves when oxygen was fed to the cathode side of each cell instead of air. The results indicate that the performance of the fuel cell could be depressed significantly by decreasing RH from 100 to 33%. Decrease in RH can result in slower electrode kinetics, including electrode reaction and mass diffusion rates, and higher membrane resistance.