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.     

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.     

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.     

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/${\text{cm}}^2$ demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.     

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

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.     

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。     

Thermal and water transfer in PEMFCs: Investigating the role of the microporous layer

The objective of this work was to investigate experimentally the effects of the microporous layer (MPL) within a PEMFC. The experiments consisted in measuring, at the anode and at the cathode, the average temperature of the electrodes using small platinum wires, heat fluxes using heat flux sensors and water fluxes by means of water balance for two builds of cell; one with porous layers and MPL and another without MPL. Three thermal configurations related to the imposed temperature of the plates were studied. The measurements put forward a new role of the microporous layer on heat transfer. Indeed, the MPL implies an increase of the electrodes temperature by adding a thermal resistance. This higher temperature enables to avoid the saturation at the electrodes and improve the water removal towards the flow field plates. In addition, the effective thermal conductivity of microporous layer, a key parameter for the analysis of heat transfer in the fuel cell, was estimated in situ.     

Effects of cathode gas diffusion layer design on polymer electrolyte membrane fuel cell water management and performance

The effects of a microporous layer (MPL) on performance and water management of polymer electrolyte fuel cells are investigated. The presence of an MPL on the cathode side is found to slightly improve performance, although the voltage gain is less significant than that obtained by wetter reactants. The effect of the MPL on water management depends on the cathode inlet-gas humidity. Differences in water crossover rate are insignificant for wet cathode feed (RH = 75%), while they are significant for dry feed (RH = 25%). A model based on transport resistance of the MPL is proposed to explain the experimental trends observed. Modeling results suggest that the presence of the MPL on the cathode side causes a reduction of the water flux from the cathode catalyst layer to the flow channels, effectively promoting water back diffusion through the membrane. Higher cathode humidity reduces the driving force for water transport from the electrode to the gas channels, also reducing the importance of the water transport resistance due to the presence of the MPL.     

Two-phase transport in the cathode gas diffusion layer of PEM fuel cell with a gradient in porosity

Two-phase transport in the cathode gas diffusion layer (GDL) of a proton exchange membrane fuel cell (PEMFC) is studied with a porosity gradient in the GDL. The porosity gradient is formed by adding micro-porous layers (MPL) with different carbon loadings on the catalyst layer side and on the flow field side. The multiphase mixture model is employed and a direct numerical procedure is used to analyze the profiles of liquid water saturation and oxygen concentration across the GDL as well as the resulting activation and concentration losses. The results show that a gradient in porosity will benefit the removal rate of liquid water and also enhance the transport of oxygen through the cathode GDL. The present study provides a theoretical support for the suggestion that a GDL with porosity gradient will improve the cell performance.     

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.     

Application of water barrier layers in a proton exchange membrane fuel cell for improved water management at low humidity conditions

This study discusses the use of an additional layer in the cathode side of a proton exchange membrane fuel cell (PEMFC) for improved water management at dry conditions. The performance of fuel cells deteriorates significantly when low to no gas humidification is used. This study demonstrates that adding a non-porous material with perforations, such as stainless steel, between the cathode flow field plate and the gas diffusion layer (GDL) improves the water saturation in the cathode GDL and catalyst layer, increases the water content in the anode, and keeps the membrane hydrated. The slight voltage drop in the performance as a result of transport limitations is justifiable since the overall durability of the cell at these extreme conditions is enhanced. The results show that the perforated layer(s) enhances the operational life of the PEMFC under completely dry conditions. These extreme conditions (dry gases without humidification, 90 kPa, 75 °C) were used to accelerate the failure modes in the fuel cells.     

Development of a novel hydrophobic/hydrophilic double micro porous layer for use in a cathode gas diffusion layer in PEMFC

In this study, a gas diffusion layer (GDL) was modified to improve the water management ability of a proton exchange membrane fuel cell (PEMFC). We developed a novel hydrophobic/hydrophilic double micro porous layer (MPL) that was coated on a gas diffusion backing layer (GDBL). The water management properties, vapor and water permeability, of the GDL were measured and the performance of single cells was evaluated under two different humidification conditions, R.H. 100% and 50%. The modified GDL, which contained a hydrophilic MPL in the middle of the GDL and a hydrophobic MPL on the surface, performed better than the conventional GDL, which contained only a single hydrophobic MPL, regardless of humidity, where the performance of the single cell was significantly improved under the low humidification condition. The hydrophilic MPL, which was in the middle of the modified GDL, was shown to act as an internal humidifier due to its water absorption ability as assessed by measuring the vapor and water permeability of this layer.     

From microstructure to the development of water and major reaction sites inside the catalyst layer of the cathode of a proton exchange membrane fuel cell

Microstructures of various sizes and shapes are fabricated on the surface of the catalyst layer (CL) of the cathode of a PEMFC, adjacent to the micro porous layer (MPL). Three major experimental results are: (1) performance is improved by up to 60% and the percentage of the increase is the same as that of the increase in interface area of CL and MPL; (2) the cell suffers no significant performance loss when Pt loading of the cathode is reduced from 1 to 0.25 mg cm⁻² and; (3) transient responses in periodical linear sweep tests show an obvious performance “jump” for all the cathodes with microstructures when approaching steady state, but none for others. Based on observations, a proposal related to the development of water and, consequently, the major reaction sites in the CL is made: there is a general water “surface” inside the CL. Major electrochemical reactions occur above (on the MPL side) of this surface and within a limited height. The surface will “move” from the membrane toward the MPL as more water is produced. The vapor generation rate (current load) relative to the removal rate of the rest of the cell components will determine the steady state position of this water surface.     

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.     

Effects of an MPL on water and thermal management in a PEMFC

In this study, the effects of adding a microporous layer (MPL) as well as the impact of its physical properties on polymer electrolyte fuel cell (PEMFC) performance with serpentine flow channels were investigated. In addition, numerical simulations were performed to reveal the effect of relative humidity and operating temperature. It is indicated that adding an extra between the gas diffusion layer (GDL) and catalyst layer (CL), a discontinuity in the liquid saturation shows up at their interface because of differences in the wetting properties of the layers. In addition, results show that a higher MPL porosity causes the liquid water saturation to decrease and the cell performance is improved. A larger MPL thickness reduces the cell performance. The effects of MPL on temperature distribution and thermal transport of the membrane prove that the MPL in addition to being a water management layer also improves the thermal management of the PEMFC.     

Multi-dimensional liquid water transport in the cathode of a PEM fuel cell with consideration of the micro-porous layer (MPL)

A multi-dimensional two-phase PEM fuel cell model, which is capable of handling the liquid water transport across different porous materials, including the catalyst layer (CL), the micro-porous layer (MPL), and the macro-porous gas diffusion medium (GDM), has been developed and applied in this paper for studying the liquid water transport phenomena with consideration of the MPL. Numerical simulations show that the liquid water saturation would maintain the highest value inside the catalyst layer while it possesses the lowest value inside the MPL, a trend consistent qualitatively with the high-resolution neutron imaging data. The present multi-dimensional model can clearly distinguish the different effects of the current-collecting land and the gas channel on the liquid water transport and distribution inside a PEM fuel cell, a feature lacking in the existing one-dimensional models. Numerical results indicate that the MPL would serve as a barrier for the liquid water transport on the cathode side of a PEM fuel cell.     

Experimental investigation on the performance and durability of hydrogen AEMFC with electrochemical impedance spectroscopy

In situ experimental tests and scanning electron microscope (SEM) observations are done to explore the impact of geometry structure and operating conditions on the performance and durability of alkaline electrolyte membrane fuel cell (AEMFC) accompanied by electrochemical diagnosis. The results show that low anode and cathode polarization resistances can be achieved by selecting a proper ionomer mass fraction (IMF) of the catalyst layer (CL). Increasing the IMF can improve the tolerance to inlet gas humidity. The presence of the anode microporous layer (MPL) greatly improves the cell performance, but cathode MPL plays a reverse effect. After the durability test, the homogeneity of CL is seriously deformed, leading to a great increment of anode and cathode polarization resistances, verified by the electrochemical impedance spectroscopy (EIS) and SEM results. An enhancement of fuel cell durability is achieved by adding nanoscale polytetrafluoroethylene (PTFE) into CL. This work can provide some guidance for the structural and operating parameter optimization of the AEMFC researches in the future.     

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.     

Improving proton exchange membrane fuel cell performance with carbon nanotubes as the material of cathode microporous layer

The cathode microporous layer (MPL) is fabricated by various multiwall carbon nanotubes (CNTs), and its influence on the performance of a proton exchange membrane fuel cell (PEMFC) is evaluated. Three types of CNT with different dimensions are employed in the experiments, and the conventional MPL made by acetylene black (AB) is also considered for the purpose of comparison. The results show that the employment of CNT as MPL composition indeed may improve fuel cell performance significantly in comparison with the case of AB. The type of CNT with the largest tube diameter and straight cylinder in shape exhibits the highest cell performance. The corresponding optimal CNT loading and polytetrafluoroethylene (PTFE) content in the MPL are also evaluated. Results show that the case of cathode MPL composed of 1.5 mg cm^?2 CNT and 20 wt% PTFE exhibits the best performance in all the experimental cases. The present data reveal that the application of CNT for MPL fabrication is beneficial to promote PEMFC performance. Copyright © 2015 John Wiley & Sons, Ltd.