Characterization of direct formic acid fuel cells by Impedance Studies: In comparison of direct methanol fuel cells

We characterized direct liquid fuel cells by electrochemical impedance spectroscopy (EIS) combined with reversible hydrogen electrode (RHE) under fuel cell operating conditions. EIS has been successfully implemented as an in-situ diagnostic tool using an impedance setup with RHE, capable of singling out individual contributions to the overall polarization of fuel cells and separating the anode and cathode contributions. While a direct methanol fuel cell (DMFC) anode was subject to substantial poisoning by reaction intermediates due to better accessibility of methanol to catalyst surface regardless of anode diffusion media, a direct formic acid fuel cell (DFAFC) anode suffered from significant mass transfer limitation depending on the anode diffusion media property and formic acid concentration. The high frequency resistance of a DFAFC cathode increased linearly with an increase of formic acid concentration by membrane dehydration effect. Interestingly, on both the DMFC and DFAFC cathodes, decrease in the mixed charge transfer resistance with an increase of fuel crossover was observed together with a drop in the cathode potential.     

Effect of conditioning method on direct methanol fuel cell performance

We investigated the effect of the conditioning methods on improving the direct methanol fuel cell (DMFC) performance. The DMFC performance after the conditioning was measured using a newly developed single cell having an Ag/Ag₂SO₄ reference electrode, which is not influenced by methanol. As a result, we succeeded in developing an original two-step conditioning method in which the conditioning by fueling H₂ gas is conducted prior to a conventional DMFC conditioning. The anode and cathode characteristics after the two-step conditioning were measured with respect to a reference electrode. Based on the obtained i-E curves, the two-step conditioning is found to improve the methanol oxidation performance at the anode and also suppress the decline of the O₂ reduction performance at the cathode. The high DMFC performance based on the two-step conditioning is well explained by the anode and cathode characteristics.     

An impedance study for the anode micro-porous layer in an operating direct methanol fuel cell

This study presents the benefit to an operating direct methanol fuel cell (DMFC) by coating a micro-porous layer (MPL) on the surface of anode gas diffusion layer (GDL). Taking the membrane electrode assembly (MEA) with and without the anodic MPL structure into account, the performances of the two types of MEA are evaluated by measuring the polarization curves together with the specific power density at a constant current density. Regarding the cell performances, the comparisons between the average power performances of the two different MEAs at low and high current density, various methanol concentrations and air flow rates are carried out by using the electrochemical impedance spectroscopy (EIS) technique. In contrast to conventional half cell EIS measurements, both the anode and cathode impedance spectra are measured in real-time during the discharge regime of the DMFC. As comparing each anode and cathode EIS between the two different MEAs, the influences of the anodic MPL on the anode and cathode reactions are systematically discussed and analyzed. Furthermore, the results are used to infer complete and reasonable interpretations of the combined effects caused by the anodic MPL on the full cell impedance, which correspond with the practical cell performance.     

The effect of anode flow characteristics and temperature on the performance of a direct methanol fuel cell

An experimental direct methanol fuel cell (DMFC), designed and manufactured in-house, was used in this study. The cell is of standard filter-press configuration with parallel rectangular single-pass anode channels. The membrane electrode assembly (MEA), with a suitable Pt–Ru anode electrocatalyst, was purchased from E-TEK Inc. A 1.0 M methanol in water solution was used as the fuel and pure oxygen was used as the oxidant in all experiments. Three graphite anode plates were machined with the same flow channel configuration but each with different depth of channels. The cathode was kept the same for all experiments. Polarisation curves and ac impedance spectra were obtained for varying temperatures and channel depths. To separate the contribution of the oxygen reduction reaction to the overvoltage from the anode and membrane contributions, reference hydrogen electrode (RHE) measurements were taken. By comparing the RHE polarisation with the methanol–oxygen polarisation experiments, it was found that polarisation losses at the oxygen cathode accounted for a 40–50% of the overpotential.The variation in the performance of the cell with flow of methanol/water mix, with temperature and with current density was studied. Polarisation measurements indicate that the medium channel depth flow channels performed better than either the shallow depth or deep depth flow channels indicating that there is a complex relationship between the effect of flow velocity and the influence of the rate of production of product CO₂. AC impedance spectroscopy measurements confirmed the observed polarisation results. This method proved to be able to provide a reliable indication of the performance of the cell even when the cell had not yet achieved steady-state. In the case of the shallow channel depth anode, ac impedance revealed that it required considerably longer to achieve steady-state than the time required for the medium and deep channel depths.     

Operational condition analysis for vapor-fed direct methanol fuel cells

This paper investigates the analysis and design of optimal operational conditions for vapor-fed direct methanol fuel cells (DMFCs). Methanol vapor at a temperature of 35 °C is carried with nitrogen gas together with water vapor at 75 °C. In this experimental condition, stoichiometry of 10 is maintained for each fuel gas. The results show that the optimal operational concentration was 25–30 wt.% under methanol vapor feeding at the anode. The peak power was 14 mW cm² in polarization curves. To analyze major losses, the activation losses of the anode and cathode were measured by an in situ reference electrode and a working electrode. The activation loss of the anode is proportional to the water content and the high methanol concentration caused the activation loss of the cathode to increase due to methanol crossover. In the vapor-fed DMFC, the activation loss of the anode is higher than that of the cathode. Also, depending on the variation of the methanol concentration, the IR loss and Faradaic impedance is measured via impedance analysis. The methanol concentration significantly affects the IR loss and kinetics. Although the IR loss was more than the desired value at the optimal condition (25–30 wt.%), it did not significantly affect the cell’s performance. The cell operated at room temperature and ambient pressure that is a typical operation environment of air-breathing fuel cells.     

Study of pulsed-current loading of direct methanol fuel cells using a new time-domain model based on bi-functional methanol oxidation kinetics

This work describes a non-linear time-domain model of a direct methanol fuel cell (DMFC) and uses that model to show that pulsed-current loading of a direct methanol fuel cell does not improve average efficiency. Unlike previous system level models, the one presented here is capable of predicting the step response of the fuel cell over its entire voltage range. This improved model is based on bi-functional methanol oxidation reaction kinetics and is derived from a lumped, four-step reaction mechanism. In total, six states are incorporated into the model: three states for intermediate surface adsorbates on the anode electrode, two states for the anode and cathode potentials, and one state for the liquid methanol concentration in the anode compartment. Model parameters were identified using experimental data from a real DMFC. The model was applied to study the steady-state and transient performance of a DMFC with the objective to understand the possibility of improving the efficiency of the DMFC by using periodic current pulses to drive adsorbed CO from the anode catalyst. Our results indicate that the pulsed-current method does indeed boost the average potential of the DMFC by 40 mV; but on the other hand, executing that strategy reduces the overall operating efficiency and does not yield any net benefit.     

Effect of design of multilayer electrodes in direct methanol fuel cells (DMFCs)

In a direct methanol fuel cell (DMFC), optimized multilayer electrode design is critical to mitigate methanol crossover and improve cell performance. In this paper, we present a one-dimensional (1-D) two-phase model based on the saturation jump theory in order to explore the methanol and water transport characteristics using various multilayer electrode configurations. To experimentally validate the 1-D model, two different membrane electrode assemblies (MEAs) with and without an anode microporous layer (MPL) are fabricated and tested under various cell current density and methanol feed concentration conditions. Then, 1-D DMFC simulations are performed and the results compared to the experimental data. In general, the numerical predictions are in good agreement with the experimental data; thus, the 1-D DMFC simulations successfully model the effects of the anode MPL that were observed experimentally. In addition to the comparison study, additional numerical simulations are carried out to precisely examine the role of the anode and cathode MPLs and the effect of the hydrophobicity of the anode catalyst layer on the water and liquid saturation distributions inside the DMFCs. This paper demonstrates the quantitative accuracy of the saturation jump model for simulating multilayer DMFC MEAs and also provides greater insight into the operational characteristics of DMFCs incorporating multilayer electrodes.     

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.     

Evaluation of silver-coated stainless steel bipolar plates for fuel cell applications

In this study, computer-aided design and manufacturing (CAD/CAM) technology were applied to develop and produce stainless steel bipolar plates for DMFC (direct methanol fuel cell). Effect of surface modification on the cell performance of DMFC was investigated. Surface modifications of the stainless steel bipolar plates were made by the electroless plating method. A DMFC consisting of silver coated stainless steel as anode and uncoated stainless steel as cathode was assembled and evaluated. The methanol crossover rate (R_c) of the proton exchange membrane (PEM) was decreased by about 52.8%, the efficiency (E_f) of DMFC increased about 7.1% and amounts of methanol electro-oxidation at the cathode side (M_co) were decreased by about 28.6%, as compared to uncoated anode polar plates. These measurements were determined by the transient current and mathematical analysis.     

Numerical analysis of the electrochemical impedance spectra of the cathode of direct methanol fuel cells

A mathematical model is developed to simulate the electrochemical impedance spectra (EIS) of the cathode of a direct methanol fuel cell (DMFC) based on the electrode kinetics and mass transports. Successful simulation of the impedance spectra confirms the usefulness of the model as a diagnostic tool for interpreting the impedance characteristics of the cathode. Numerically, the capacitive semicircle in the impedance pattern is ascribed to the charge transfer process and the inductive semicircle is mainly due to the CO adsorption relaxation. Results show that the impedance pattern is strongly dependent on the electrode potential, which can be used as a criterion for judging the relative effect of the methanol permeation on the cathode. Another capacitive semicircle appears and the charge transfer resistance is changed when the oxygen transport is limited. The effects of the methanol permeation on the impedance pattern are also delineated, indicating that the methanol permeation often leads to larger oxygen transport impedance and the charge transfer resistance of the DMFC cathode depends on the methanol permeation rate.     

Characterization of direct methanol fuel cells by ac impedance spectroscopy

The processes taking place in direct methanol fuel cells (DMFC) are characterized by ac impedance spectroscopy under realistic operating conditions. This method allows the separate examination of anode kinetics, anode mass transport, cathode kinetics, cathode mass transport, and membrane conductivity, making it a valuable diagnostic tool for DMFC development.     

Water management in a passive direct methanol fuel cell

Passive direct methanol fuel cells (DMFCs) are under development for use in portable applications because of their enhanced energy density in comparison with other fuel cell types. The most significant obstacles for DMFC development are methanol and water crossover because methanol diffuses through the membrane generating heat but no power. The presence of a large amount of water floods the cathode and reduces cell performance. The present study was carried out to understand the performance of passive DMFCs, focused on the water crossover through the membrane from the anode to the cathode side. The water crossover behaviour in passive DMFCs was studied analytically with the results of a developed model for passive DMFCs. The model was validated with an in‐house designed passive DMFC. The effect of methanol concentration, membrane thickness, gas diffusion layer material and thickness and catalyst loading on fuel cell performance and water crossover is presented. Water crossover was lowered with reduction on methanol concentration, reduction of membrane thickness and increase on anode diffusion layer thickness and anode and cathode catalyst layer thickness. It was found that these conditions also reduced methanol crossover rate. A membrane electrode assembly was proposed to achieve low methanol and water crossover and high power density, operating at high methanol concentrations. The results presented provide very useful and actual information for future passive DMFC systems using high concentration or pure methanol. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of an active tubular liquid-feed direct methanol fuel cell

A two-dimensional, two-phase, non-isothermal model was developed for an active, tubular, liquid-feed direct methanol fuel cell (DMFC). The liquid-gas, two-phase mass transport in the porous anode and cathode was formulated based on the multi-fluid approach in the porous media. The two-phase mass transport in the anode and cathode channels was modeled using the drift-flux and the homogeneous mist-flow models, respectively. Water and methanol crossovers through the membrane were considered due to the effects of diffusion, electro-osmotic drag, and convection. The model enabled a numerical investigation of the effects of various operating parameters, such as current density, methanol flow rate, and oxygen flow rate, on the mass and heat transport characteristics in the tubular DMFC. It was shown that by choosing a proper tube radius and distance between the adjacent cells, a tubular DMFC stack can achieve a much higher energy density compared to its planar counterpart. The results also showed that a large anode flow rate is needed in order to avoid severe blockage of liquid methanol to the anode electrode due to the gas accumulation in the channel. Besides, lowering the flow rate of either the methanol solution or air can lead to a temperature increase along the flow channel. The methanol and water crossovers are nearly independent of the methanol flow rate and the air flow rate.     

Effect of the cathode gas diffusion layer on the water transport behavior and the performance of passive direct methanol fuel cells operating with neat methanol

The passive operation of a direct methanol fuel cell with neat methanol requires the water that is produced at the cathode to diffuse through the membrane to the anode to compensate the methanol oxidation reaction (MOR). Hence, the anode performance of this type of fuel cell can be limited by the water transport rate from the cathode to the anode. In this work we theoretically show that the water transport from the cathode to the anode depends primarily on the design of the cathode gas diffusion layer (GDL). We investigate experimentally the effects of the design parameters of the cathode GDL, including the PTFE (polytetrafluoroethylene) content in the backing layer (BL), and the carbon loading and the PTFE content in the microporous layer (MPL) on the water transport and the performance of the passive DMFC with the help of a reference electrode. The results indicate that on one hand, these parameters can be adjusted to decrease the water concentration loss of the anode performance, but on the other hand, they can also cause an increase in the oxygen concentration loss of the cathode performance. Hence, an optimal balance in minimizing the both concentration losses is the key to maximize the cell performance.     

Two-step conditioning method for high power-generating direct methanol fuel cells

Power-generation improvement of a direct methanol fuel cell (DMFC) has been investigated through the enhancements of the anode and cathode characteristics using a newly developed “two-step conditioning method”, in which the conditioning is conducted by supplying H₂ gas before the conventional DMFC conditioning. By using the two-step conditioning, the DMFC performances are enhanced. From the polarization curves measured during the DMFC operation using a single cell having an Ag/Ag₂SO₄ reference electrode, the methanol oxidation performance of the anode is proven to be improved with an increase in the methanol concentration. In addition, a decline in the O₂ reduction performance at the cathode due to the methanol crossover is suppressed by our original conditioning method. These results are also supported by the linear sweep voltammetry, and the superior DMFC performances after the two-step conditioning are related to the high speed cleaning of the electrocatalysts. Note that the optimum two-step conditioning leads to 1.4–2.2 times higher maximum power densities than those for the conventional DMFC conditioning even for a shorter conditioning time.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Analysis of the polarization of a direct methanol fuel cell using a pseudo-reversible hydrogen reference electrode

In order to observe the performance of the anode and cathode during actual direct methanol fuel cell (DMFC) operating conditions and to minimize the polarization of the reference electrode, we used a reversible hydrogen reference electrode (RHE) with its instability minimized. For analysis of the I–V polarization curve of each electrode, Tafel plots were used as the diagnostic tool. According to the slopes in the Tafel plot, the I–V polarization curves of each electrode were divided into the several regions. The effects of operating parameters on the performance of each electrode were interpreted in terms of mass transfer and electrode activation. The methanol and oxygen crossover through the membrane significantly affected the performance of the cell.     

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.     

Effect of the cathode open ratios on the water management of a passive vapor-feed direct methanol fuel cell fed with neat methanol

A novel approach has been proposed to improve the water management of a passive direct methanol fuel cell (DMFC) fed with neat methanol without increasing its volume or weight. By adopting perforated covers with different open ratios at the cathode, the water management has been significantly improved in a DMFC fed with neat methanol. An optimized cathode open ratio could ensure both the sufficient supply of oxygen and low water loss. While changing the open ratio of anode vaporizer can adjust the methanol crossover rate in a DMFC. Furthermore, the gas mixing layer, added between the anode vaporizer and the anode current collector to increase the mass transfer resistance, can improve the cell performance, decrease the methanol crossover, and increase the fuel efficiency. For the case of a DMFC fed with neat methanol, an anode vaporizer with the open ratio of 12% and a cathode open ratio of 20% produced the highest peak power density, 22.7 mW cm⁻², and high fuel efficiency, 70.1%, at room temperature of 25 ± 1 °C and ambient humidity of 25-50%.     

Investigations of performance degradation and mitigation strategies in direct methanol fuel cells

This paper addresses a gradual performance loss that is encountered in a direct methanol fuel cell (DMFC) when it is subjected to a continuous operation at constant-load conditions for a period of 600 h. To gain insights into the physico-chemical origins of the degradation process, various analytical techniques are employed and characterization of the membrane electrode assembly (MEA) is carried out before and after the lifetime test. The results reveal that the performance degradation of MEA mainly stems from the degradations at the cathode, and this is further confirmed by individual impedance analyses of cathode and anode as well as by the observation on finding increased quantity of methanol exiting from the cathode with increased operational time. Supplement experiments with the cathodes containing either pre-oxidized Pt catalyst or a fractional amount of Ru catalyst offer new clues to understand the deactivation mechanism. The hydrophobicity losses of gas diffusion layers are prominent in the outlet regions compared to the inlet regions of the DMFC assembly. Further, a couple of restoration techniques are employed to evaluate performance recovery. A periodical on–off switching of applied load and an air-break technique are found to be effective to restore the performance loss that occurs during fuel cell operations.