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.     

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.     

A study of water transport as a function of the micro-porous layer arrangement in PEMFCs

Electrochemical losses as a function of the micro-porous layer (MPL) arrangement in Proton Exchange Membrane Fuel Cells (PEMFCs) are investigated by electrochemical impedance spectroscopy (EIS). Net water flux across the polymer membrane in PEMFCs is investigated for various arrangements of the MPL, namely with MPL on the cathode side alone, with MPL on both the cathode and the anode sides and without MPL. EIS and water transport are recorded for various operating conditions, such as the relative humidity of the hydrogen inlet and current density, in a PEMFC fed by fully-saturated air. The cell with an MPL on the cathode side alone has better performance than two other types of cells. Furthermore, the cell with an MPL on only the cathode increases the water flux from cathode to anode as compared to the cells with MPLs on both electrodes and cells without MPL. Oxygen-mass-transport resistances of cells in the presence of an MPL on the cathode are lower than the values for the other two cells, which indicates that the molar concentration of oxygen at the reaction surface of the catalyst layer is higher. This suggests that the MPL forces the liquid water from the cathode side to the anode side and decreases the liquid saturation in GDL at high current densities. Consequently, the MPL helps in maintaining the water content in the polymer membrane and decreases the cathode charge transfer and oxygen-mass transport resistances in PEMFCs, even when the hydrogen inlet has a low relative humidity.     

High performance alkaline-acid direct glycerol fuel cells for portable power supplies via electrode structure design

Alkaline-acid direct glycerol fuel cells (AA-DGFC) were fabricated and primarily proven to be used as portable power generating devices. Pt/C catalyst was used as electrocatalyst for both anode and cathode. The optimal operating condition for cathode was firstly tested. Then the effects of types of backing and microporous layer on the cell performance and stability were investigated to obtain the optimal electrode structure. The cell performance was determined by using both chronoamperometry technique at a constant voltage of 0.4 V, and cell polarization with impedance measurement. The maximum peak power density obtained from the cell was 375 mW cm⁻² and the highest average current density discharged from the cell was 451 mA cm⁻². Non-wetproof carbon cloth is suitable as the backing layer for both the anode and cathode. Although MPL did not directly affect the cell performance, it greatly improved stability of the current discharged during chronoamperometric test. The cathode favors hydrophilic MPL, while hydrophobic MPL was preferred on the anode.     

Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell

Liquid water formation and transport were investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. The level of cathode flow field flooding, under the same operating conditions and cell current, was recognized as a criterion for the water removal capacity of the GDL materials. When compared at the same current density (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that water had been efficiently removed from the catalyst layer.

Visualization of the anode channel was used to investigate the influence of the microporous layer (MPL) on water transport. No liquid water was observed in the anode flow field unless cathode GDLs had an MPL. MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H₂ exit.     

Construction of gradient catalyst layer anode by incorporating covalent organic framework to improve performance of direct methanol fuel cells

Improving cell performance is the most demanded task in direct methanol fuel cell (DMFC) research, although this fuel cell has several intrinsic features like high energy density, moderate operating temperature and environmentally friendly operation. The catalyst layer (CL) is the site of electrochemical reactions directly affecting the cell performance. Accordingly, the structure and preparation of the CL are crucial in optimizing performance. In this study, a novel gradient catalyst layer (G-CL) for the anode of DMFC is developed and better performance is obtained compared with that of single catalyst layer (S-CL) anode. A G-CL anode is composed of an outer CL of covalent organic framework (COF) materials-mixed catalyst near the microporous layer (MPL) and a conventional inner CL near the membrane side. Different loading of Pt catalyst in the two layers. Therefore, in the G-CL structure, there existed a catalyst concentration gradient and porosity gradient. Anode electrodes are characterized morphologically and electrochemically and the performance of individual cells containing such G-CL designs is measured. The results indicate that incorporation of the appropriate amount of COF materials enables the outer CL not only to have a larger electrochemical surface area (ECSA) and expose more catalytic active sites but also holds a strong proton transfer ability to improve the methanol oxidation reaction (MOR) performance. Due to the presence of the inner layer of the CL formed the methanol gradient oxidation process. With the same platinum-ruthenium (Pt–Ru) catalyst loading, the 5 wt.% COF G-CL anode structure exhibited lower methanol crossover and higher power density (nearly 11% increment) compared with that of the S-CL anode with high methanol concentration (8 M) at 60 °C, showing the promising potential in further applications.     

The role of an anode microporous layer in direct ethanol fuel cells at different ethanol concentrations

Ethanol crossover and ethanol electrooxidation kinetic effects on direct ethanol fuel cell (DEFC) performance were determined at different ethanol feed concentrations for cells fabricated with and without an anode microporous layer (MPL). Several characterization techniques were used, including cell performance curves, anode polarization, electrochemical impedance spectroscopy (EIS) and ethanol crossover by the voltammetric method. It was found that the optimum ethanol feed concentration depended on the anode structure design and the cell current density operation. A microporous layer could reduce ethanol crossover but induced high mass transfer resistance, resulting in a slow ethanol electrooxidation reaction rate. However, ethanol crossover was not the dominant factor affecting DEFC performance for the ethanol feed concentration range (0.5–5.0 M) studied. The MEA without an anode MPL exhibited better performance than the one with an MPL for the entire range of ethanol concentration.     

Parameter sensitivity examination for a complete three-dimensional,two-phase,non-isothermal model of polymer electrolyte membrane fuel cell

Parameter sensitivity analysis is carried out for a complete three-dimensional, two-phase, non-isothermal model of polymer electrolyte membrane (PEM) fuel cell with a parallel flow field design. The model couples the two-phase flow of the multi-component reactants and liquid water, species transport, electrochemical reactions, proton and electron transport, and the electro-osmosis transport, back diffusion of water in the membrane, and energy transport. Twenty nine parameters, which are classified into the structural or transport parameters of porous layers (tortuosity, porosity, permeability, proton conductivity, electron conductivity, and thermal conductivity) as well as the electrochemical parameters (anodic and cathodic exchange current densities, anodic and cathodic transfer coefficients for anode and cathode reactions), are used to implement individual parameter investigation. The results show the parameters can be divided in to strongly sensitive, conditional sensitive and weak sensitive parameters according to its effect on the cell polarization curve. The optimization of parameters of cathode gas diffusion layer (GDL) and catalyst layer (CL) is more important to improve cell performance than that of anode GDL and CL because liquid water transport and removal affect significantly membrane hydration and reactant transport. Electrochemical parameters determine the activation potential and the slope of ohmic polarization hence these parameters can be used to fit experimental polarization curve more effectively than the other parameters.     

Understanding the role of cathode structure and property on water management and electrochemical performance of a PEM fuel cell

Water management is vital for the successful development of PEM fuel cells. Water should be carefully balanced within a PEM fuel cell to meet the conflicting requirements of membrane hydration and cathode anti-flooding. In order to understand the key factors that can improve water management and fuel cell performance, the cathodes with different structures and properties are prepared and tested in this study. The experimental results show that even though no micro-porous layer (MPL) is placed between the cathode catalyst layer (CCL) and macro-porous substrate (MaPS), a hydrophobic CCL is effective to prevent cathode flooding and keep membrane hydrated. The impedance study and the analysis of the polarization curves indicate that the optimized hydrophobic micro-porous structure in the MPL or the hydrophobic CCL could be mainly responsible for the improved water management in PEM fuel cells, which functions as a watershed to provide wicking of liquid water to the MaPS and increase the membrane hydration by enhancing the back-diffusion of water from the cathode side to the anode side through the membrane.     

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.     

Investigation of the effect of microporous layers on water management in a proton exchange membrane fuel cell using novel diagnostic methods

Water management remains a significant challenge for the Proton Exchange Membrane Fuel Cell (PEMFC) with respect to performance, lifetime and operational flexibility. In recent years, microporous layers (MPL) have been widely used on the cathode side of the PEMFC in order to improve fuel cell performance and water management capabilities. Many modeling and experimental studies have with limited success attempted to analyze the underlying mechanisms that are responsible for the performance improvement due to the MPL. In this study, porous inserts along with various in-situ experimental techniques are used to investigate the MPLs. It was observed that the anode pressure drop increased when a cathode MPL was present, indicating water cross-over from the cathode towards the anode side. Further testing identified that the MPL improved cell performance due to the reduction of water saturation in the cathode catalyst layer, which resulted in enhanced oxygen diffusion. The influence of the MPL on the anode side was also studied with the aid of porous inserts and other techniques, and it was observed that the anode MPL improves cell voltage stability and reduces water accumulation in the anode catalyst layer. The present investigation provides further important information on the critical role of the MPL in the PEMFC.     

Comparison of the performance and degradation mechanism of PEMFC with Pt/C and Pt black catalyst

Proton exchange membrane fuel cell (PEMFC) has been used in supplying power for Unmanned Underwater Vehicle which operate in a closed environment and dead-ended anode and cathode (DEAC) mode is deemed as an effective way to enhance the fuel utilization rate. Catalyst is an important factor that influences the performance and durability of PEMFC, especially in DEAC mode. In this paper, the degradation characteristics of PEMFCs with Pt black and Pt/C catalyst after 100 h operation have been investigated by electrochemical techniques and morphological characterization methods. It's shown that the degradation of Pt black catalyst layer (CL) was more severe than that of Pt/C CL. The difference of performance degradation is due to the dominant decay mechanism of these two catalysts is different. According to SEM, TEM and XPS results, the decay of Pt black catalyst is mainly caused by Pt agglomeration and oxidation, causing a higher ohmic resistance, higher mass transfer resistance and severer degradation of performance. The degradation of Pt/C catalyst is mainly due to the reduction of electrochemical surface area and carbon corrosion because the larger carbon corrosion makes micropores and the thicker supporting structure, resulting in the performance degradation.     

Effect of PTFE content in microporous layer on water management in PEM fuel cells

The effect of hydrophobic agent (PTFE) concentration in the microporous layer on the PEM fuel cell performance was investigated using mercury porosimetry, water permeation experiment, and electrochemical polarization technique. The mercury porosimetry and water permeation experiments indicated that PTFE increases the resistance of the water flow through the GDL due to a decrease of the MPL porosity and an increase of the volume fraction of hydrophobic pores. When air was used as an oxidant, a maximum fuel cell performance was obtained for a PTFE loading of 20 wt.%. The experimental polarization curves were quantitatively analyzed to determine the polarization resistances resulting from different physical and electrochemical processes in the PEM fuel cell. The polarization analysis indicated that the optimized PTFE content results in an effective water management (i.e., a balancing of water saturations in the catalyst layer and the gas diffusion layer), thereby improving the oxygen diffusion kinetics in the membrane-electrode assembly.     

质子交换膜燃料电池建模及水热传输特性分析

针对质子交换膜燃料电池(PEMFC)水管理开展了研究,建立了一维非等温两相流解析模型,研究了不同电流密度、微孔层接触角和不同加湿方案对电池内部水分布和温度分布的影响,提出了更好的进气加湿方案。结果表明:电流密度增大会导致阳极拖干、阴极水淹加剧,导致电池各部分温度上升。因各层材料亲水性不同,在交界面处能观察到液态水阶跃现象。增大微孔层接触角促进阴极液态水反扩散到阳极,一定程度上缓解阳极变干,但过大的接触角可能导致阴极水淹加剧。通过采取"阳极充分加湿、阴极低加湿"的进气加湿方案可以有效提高电池性能,并且能在一定程度改善电池内部受热,提高电池使用寿命。     

Experimental investigation of the role of a microporous layer on the water transport and performance of a PEM fuel cell

A highly reliable experimental system that consistently closed the overall water balance to within 5% was developed to study the role of a microporous layer (MPL), attached to carbon paper porous transport layer (PTL), on the water transport and performance of a standard 100 cm² active area PEM fuel cell. Various combinations of cells were built and tested with PTLs at the electrodes using either carbon fibre paper with a MPL (SGL 10BB) or carbon fibre paper without a MPL (SGL 10BA). The net water drag coefficient at three current densities (0.3, 0.5 and 0.7 A cm⁻²) for two combinations of anode/cathode relative humidity (60/100% and 100/60%) and stoichiometric ratios of H₂/air (1.4/3 and 1.4/2) was determined from water balance measurements. The addition of a MPL to the carbon fibre paper PTL at the cathode did not cause a statistically significant change to the overall drag coefficient although there was a significant improvement to the fuel cell performance and durability when a MPL was used at the cathode. The presence of a MPL on either electrode or on both electrodes also exhibited similar performance compared to when the MPL was placed at the cathode. These results indicate that the presence of MPL indeed improves the cell performance although it does not affect the net water drag coefficient. The correlation between cell performance and global water transport cannot be ascertained and warrants further experimental investigation.     

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.     

Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.     

Optimal topology and distribution of catalyst in PEMFC

The equations that govern the various transport phenomena occurring in a polymer electrolyte membrane fuel cell (PEMFC) were formulated and implemented in a commercial finite element software, in order to predict the fuel cell current density with respect to the operating conditions. The numerical model showed polarization curves in accordance with literature. The catalyst utilization was then improved by optimizing the platinum distribution (design variable) in the fuel cell, so as to maximize current density (objective function) for a fixed total amount of platinum (constraint). The first analysis showed that, for equal anode and cathode catalyst layer thicknesses, maximal current density was achieved by placing more catalyst in the cathode than in the anode. The second analysis showed that, for equal anode and cathode catalyst layer density, maximal current density was achieved by using a catalyst layer that is thicker on the cathode side than that on the anode side. Finally, a topological optimization of the platinum density within the cathode catalyst layer was performed with a gradient based algorithm, and the results showed that at a high stoichiometric ratio, the best design has most of its platinum placed where the reaction rate is the highest, i.e., close to the membrane layer.     

Enhancement of the cathode/electrolyte interface by a sintering-active barrier layer for solid oxide fuel cells

It's a critical issue for the successful commercialization of solid oxide fuel cells (SOFCs) to achieve long-term operation and thermal cycling stability without significant degradation. Current work reports an almost-dense, sintering-active and Sr-blocking cathode/electrolyte interface, fabricated through a cost-competitive and scalable method of modifying porous Gadolinia doped Ceria (GDC) barrier layer by in-situ grown GDC nanoparticles. Result show that the robust interface enables improved durability and thermal cycling stability. The hydrothermal modified anode supported cell performance only deteriorates by ∼0.5% after 20 times thermal cycles between 200 and 750 °C, which is more prominent enhancement than ∼16% for pristine cell. The modified symmetric cell shows smaller cathode polarization resistance and well-attached cathode/electrolyte interfaces after ∼1200 h of operation, including 4 times thermal cycles. While the pristine cell shows more obvious area specific resistances increases and the peeled-off cathode layer. It is discussed and concluded that the hydrothermal modified sintering-active barrier layer has contributed mainly to the construction of robust cathode/electrolyte interface, yielding the improved performance and prolonged operation.     

Fabrication of a carbon nanofiber sheet as a micro-porous layer for proton exchange membrane fuel cells

A carbon nanofiber sheet (CNFS) has been prepared by electrospinning, stabilisation and subsequent carbonisation processes. Imaging with scanning electron microscope (SEM) indicates that the CNFS is formed by nonwoven nanofibers with diameters between 400 and 700 nm. The CNFS, with its three-dimensional pores, shows excellent electrical conductivity and hydrophobicity. In addition, it is found that the CNFS can be successfully applied as a micro-porous layer (MPL) in the cathode gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC). The GDL with the CNFS as a MPL has higher gas permeability than a conventional GDL. Moreover, the resultant cathode GDL exhibits excellent fuel cell performance with a higher peak power density than that of a cathode GDL fabricated with a conventional MPL under the same test condition.