Analysis of microporous layer characteristics of the anode for high-temperature polymer electrolyte membrane fuel cell

Four commercial gas diffusion layers (GDLs) are applied to high-temperature polymer electrolyte fuel cell anodes to investigate the relationship between microporous layer (MPL) properties and the performance of membrane electrode assembly. Subsequently, the physical properties such as the crack area and porosity of the MPL are analyzed via various methods. It is found that the capillary pressure of phosphoric acid (PA) from several theories is inversely related to the mass transfer capability (MTC) of the MPL. Finally, a new transport property named PA MTC of the MPL is proposed using the PA capillary pressure and MTC in MPL. The results indicate that the voltage at both low and high current densities increases separately as the PA MTC in the MPL adjusts to 0.5 MPa mm^-1. Subsequently, the voltage beyond 0.5 MPa mm^-1 increases in low current density contrary to the voltage at a high current density.     

Ultra-low catalyst loading cathode electrode for anion-exchange membrane fuel cells

A new cathode architecture for anion-exchange membrane fuel cells (AEMFCs) is proposed and fabricated by direct deposition of palladium (Pd) particles onto the surface of the micro-porous layer (MPL) that is interfaced with a backing layer. The MPL is composed of carbon nanotubes while the backing layer is made of a carbon paper. The sputter-deposited electrode with a worm-like shape not only extends the electrochemical active surface area, but also facilitates the oxygen transport. This new cathode, albeit with a Pd loading as low as 0.035 mg cm⁻², enables the peak power density of an AEM direct ethanol fuel cell to be as high as 88 mW cm⁻² (at 60 °C), which is even higher than that using a conventional cathode with a 15-times higher Pd loading. The significance of the present work lies in the fact that the new sputter-deposited electrode is more suitable for fuel-electrolyte-fed fuel cells than the conventional electrode designed for proton-exchange membrane fuel cells (PEMFCs).     

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.     

Effects of a microporous layer on the performance degradation of proton exchange membrane fuel cells through repetitive freezing

The gas diffusion layer (GDL) covered with a microporous layer (MPL) is being widely used in proton exchange membrane fuel cells (PEMFCs). However, the effect of MPL on water transport is not so clear as yet; hence, many studies are still being carried out. In this study, the effect of MPL on the performance degradation of PEMFCs is investigated in repetitive freezing conditions. Two kinds of GDL differentiated by the existence of MPL are used in this experiment. Damage on the catalyst layer due to freezing takes place earlier when GDL with MPL is used. More water in the membrane and catalyst layer captured by MPL causes permanent damage on the catalyst layer faster. More detailed information about the degradation is obtained by electrochemical impedance spectroscopy (EIS). From the point of view that MPL reduces the ohmic resistance, it is effective until 40 freezing cycles, but has no more effect thereafter. On the other hand, from the point of view that MPL enhances mass transport, it delays the increase in the mass transport resistance.     

Ultra thin gas diffusion layer development for PEMFC

This research studies an ultra-thin carbon fiber paper fabrication process for proton exchange membrane fuel cells (PEMFCs). Polyacrylonitrile (PAN) based carbon fibers 6 mm long were dispersed and formed at aerial weights of 15 and 20 g/m² using a slurry molding machine. Polyscrylamide (PAM) and polyvinyl alcohol (PVA) dispersal agent solutions for fiber binding were added to evenly distribute the carbon fibers and increase the paper mechanical strength. The carbon fiber papers were dried after resin impregnation using a convective oven at 120 °C temperature for 10 min. The hot press machine was heated to 160 °C temperature and the workpieces were pressed for 5 min. Graphitization completed the gas diffusion substrate (GDS) process. GDL involves immersing the paper in a 5% polytetrafluoroethylene (PTFE) solution, coating the paper with a micro porous layer (MPL). This study shows the proposed ultra-thin GDL fabrication method is suitable for PEMFC applications and exhibits feasible functionality for fuel cells.     

Cold-start icing characteristics of proton-exchange membrane fuel cells

Understanding the icing characteristics of proton-exchange membrane fuel cells (PEMFCs) is essential for optimizing their cold-start performance. This study examined the effects of start-up temperature, current density, and microporous layer (MPL) hydrophobicity on the cold-start performance and icing characteristics of PEMFCs. Further, the cold-start icing characteristics of PEMFCs were studied by testing the PEMFC output voltage, impedance, and temperature changes at different positions of the cathode gas diffusion layer. Observation of the MPL surface after cold-start failure allowed determination of the distribution of ice formation at the catalytic layer/MPL interface. At fuel cell temperatures below 0 °C, supercooled water in the cell was more likely to undergo concentrated instantaneous freezing at higher temperatures (−4 and −5 °C), whereas the cathode tended to freeze in sequence at lower temperatures (−8 °C). In addition, a more hydrophobic MPL resulted in two successive instantaneous icing phenomena in the fuel cell and improved the cold-start performance.     

Recent progress of the gas diffusion layer in proton exchange membrane fuel cells: Material and structure designs of microporous layer

Proton exchange membrane fuel cells (PEMFCs) have become the most attractive power supply units for stationary and mobile applications. The operation, design characteristics, as well as performance of PEMFCs, are closely related to the multiphase transport of mass, heat, and electricity in the cell, a critical of which is the gas diffusion layer (GDL). It is very important to guarantee the transmission of water and gasses under high current density, and which is the weakness of PEMFCs at present. Microporous layer (MPL) is considered to be the key variable for mass transfer, so varieties of works focus on modification of MPL materials and its structure design. However, there is still a lack of special review to summarize and prospect the progress of MPL in recent years. This review article therefore focuses on the insights and comprehensive understanding of four critical issues of the MPL, the porosity, pore size distribution, wettability, structural design and the durability of MPL. At last, the conclusion and recommendations section summarized the future prospects and recommendations for possible research opportunities.     

Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

In this paper, the effect of porosity-graded micro-porous layer (GMPL) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was studied in detail. The GMPL was prepared by printing micro-porous layers (MPL) with different content of NH₄Cl pore-former and the porosity of the GMPL decreased from the inner layer of the MPLs at the membrane/MPL interface to the outer layer of the MPLs at the gas diffusion electrode/MPL interface. The morphology and porosity of the GMPLs were characterized and the performance of the cell with GMPLs was compared with those having conventional homogeneous MPLs. The result demonstrates that the fuel cells consisting of GMPL have better performance than those consisting of conventional homogeneous MPLs, especially at high current densities. Micro-porous layer with graded porosity is beneficial for the electrode process of fuel cell reaction probably by facilitating the liquid water transportation through large pores and gas diffusion via small pores in the GMPLs.     

Effects of gas diffusion layer (GDL) and micro porous layer (MPL) on cathode performance in microbial fuel cells (MFCs)

This study focused on novel cathode structures to increase power generation and organic substrate removal in microbial fuel cells (MFCs). Three types of cathode structures, including two-layer (gas diffusion layer (GDL) and catalyst layer (CL)), three-layer (GDL, micro porous layer (MPL) and CL), and multi-layer (GDL, CL, carbon based layer (CBL) and hydrophobic layers) structures were examined and compared in single-chamber MFCs (SCMFCs). The results showed that the three-layer (3L) cathode structures had lower water loss than other cathodes and had a high power density (501 mW/m²). The MPL in the 3L cathode structure prevented biofilm penetration into the cathode structure, which facilitated the oxygen reduction reaction (ORR) at the cathode. The SCMFCs with the 3L cathodes had a low ohmic resistance (R_ohmic: 26-34 Ω) and a high cathode open circuit potential (OCP: 191 mV). The organic substrate removal efficiency (71-78%) in the SCMFCs with 3L cathodes was higher than the SCMFCs with two-layer and multi-layer cathodes (49-68%). This study demonstrated that inserting the MPL between CL and GDL substantially enhanced the overall electrical conduction, power generation and organic substrate removal in MFCs by reducing water loss and preventing biofilm infiltration into the cathode structure.     

Efficient electrocatalysts for the oxygen reduction reaction,based on mixed carbonyl-phosphine clusters of iridium

This work reports the synthesis and characterization of two novel electrocatalysts for the oxygen reduction reaction based on iridium clusters containing carbonyl (CO) and phosphine (PPh₃ or PPh(OMe)₂) ligands. The study of the complexes by FTIR, XRD, EDS, XPS, MS, as well as ¹H, ¹³C and ³¹P NMR shows that they preserve the nuclearity of the iridium cluster used as precursor (Ir₄(CO)₁₂), while several of the carbonyl groups were substituted by the corresponding phosphine-type ligands and solvent (o-dichlorobenzene) molecules. The electrochemical characterization by rotating disk electrode measurements in an acid medium (0.5 mol L^?1 H₂SO₄), from which the kinetic parameters could be obtained, indicates that both clusters exhibit an important catalytic activity for the oxygen electroreduction, even in the presence of methanol in concentrations as high as 2.0 mol L^?1. According to these results, the new electrocatalysts are good potential candidates to be evaluated as cathodes in PEMFCs and DMFCs.     

Freezing characteristics of supercooled water in gas diffusion layer of proton exchange membrane fuel cells

The freezing characteristics of supercooled water in a gas diffusion layer (GDL), which are the bases for the cold start-up of proton exchange membrane fuel cells (PEMFCs), were investigated. An experimental apparatus for noncontact temperature measurement and observation systems was developed. GDL and GDL with a microporous layer (MPL) were prepared, and freezing experiments using a water-containing GDL under various cooling rates were performed with variations in polytetrafluoroethylene (PTFE) content and water saturation. Furthermore, based on the experimental results, the freezing initiation probability was theoretically investigated to elucidate the freezing characteristics. Results showed that, with increasing supercooling of water in GDL, the freezing probability of water increased abruptly. The effect of saturation showed a different trend depending on PTFE addition. For the GDL without PTFE, the freezing initiations occurred at approximately 6 °C of supercooling degree, and the probability approached 1.0 at approximately 9.5–11.5 °C, with saturation dependency. In contrast, for both GDL and GDL + MPL containing PTFE, the initiation temperature characteristics were relatively similar, which were approximately 8–12 °C, regardless of the saturation and PTFE content. In these cases, the ice-nucleating activity of water in the GDL was possibly stronger than that in the MPL.     

Proton conduction of novel calcium phosphate nanocomposite membranes for high temperature PEM fuel cells applications

This work describes the synthesis and evaluation of nanocomposite membranes based on calcium phosphate (CP)/ionic liquids (ILs) for high-temperature proton exchange membrane (PEM) fuel cells. Several composite membranes were synthesized by varying the mass ratios of ILs with respect to the CP and all supported on porous polytetrafluoroethylene (PTFE). The membranes exhibit high proton conductivities. Two ionic liquids were investigated in this study, namely, 1-Hexyl-3- methylimidazolium tricyanomethanide, [HMIM][C₄N₃⁻], and 1-Ethyl-3-methylimidazolium methanesulfonate, [EMIM][CH₃O₃S⁻]. At room temperature, the CP/PTFE/[HMIM][C₄N₃⁻] composite membrane possessed a high proton conductivity of 0.1 S cm⁻¹. When processed at 200 °C, and fully anhydrous conditions, the membrane showed a conductivity of 3.14 × 10⁻³ S cm⁻¹. Membranes based on CP/PTFE/[EMIM][CH₃O₃S⁻] on the other hand, had a maximum proton conductivity of 2.06 × 10⁻³ S cm⁻¹ at room temperature. The proton conductivities reported in this work appear promising for the application in high-temperature PEMFCs operated above the boiling point of water.     

Optimal configuration of a two-heat-reservoir heat-engine with heat-leak and finite thermal-capacity

Based on a model of a two-heat-reservoir heat-engine cycle with a finite high-temperature source and bypass heat-leak, in which the maximum work output can be obtained under a given cycle time is determined with the considerations of heat-leak, finite heat-capacity high-temperature source and infinite heat-capacity low-temperature heat-sink with another linear heat-transfer law Q ∝ Δ(T⁻¹). The heat-engine cycles considered are: (1) infinite low-and high-temperature reservoirs without heat-leak; (2) infinite low- and high-temperature reservoirs with heat-leak; (3) finite high-temperature source and infinite low-temperature sink without heat-leak and (4) finite high-temperature source and infinite low-temperature sink with heat-leak. It is assumed that the heat-transfer between the working fluid and the reservoirs obeys another linear heat-transfer law, i.e., the linear phenomenological heat-transfer law, Q ∝ Δ(T⁻¹). It is shown that the existence of heat-leak does not affect the configuration of a cycle with an infinite high-temperature source. The finite heat-capacity of the high-temperature source without heat-leak makes the cycle a generalized Carnot heat-engine cycle. There exists a great difference of the cycle configurations for the finite high-temperature source with heat-leak and the former three cases. Moreover, the relations between the optimal power-output and the efficiency of the former three configurations are derived, and they show that the heat-leak affects the power versus efficiency characteristics of the heat-engine cycles.     

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.     

Optimal operating conditions to maximize the net power of polymer electrolyte membrane fuel cells: The stack-system interface of a commercial 18 kW module

A new, experimental method based on air flow rate rather than current is presented to optimize operating parameters for the stacks and systems of proton exchange membrane fuel cells (PEMFCs) for maximizing their net power. This approach is illustrated for a commercial 18 kW PEMFC module. The impact of contamination pressure drop across the cathode air filter is also investigated on the compressor behavior. It is further shown that a 4V reduction in the compressor voltage reduces its power consumption by 9.1%. Using the 3D graphs of the power-pressure-flow data, it is found that the stack pressure of 180 kPaa is superior to the higher tested pressures as it enhances the net power by 7.0 and 13.7% at different conditions. Application of the present study will lead to the development of PEMFCs with higher power output by optimizing stack pressure, stoichiometry and air flow to properly deliver the system design specifications.     

In-situ sulfonation of targeted silica-filled Nafion for high-temperature PEM fuel cell application

Silica targeted filled in Nafion® membrane is in-situ sulfonated (s-WR-Nafion membrane) to improve the proton conductivity at high-temperature and low relative humidity (RH). The silica introduction as water sorbent in the membrane can both stabilize water balance for polymer electrolyte membrane fuel cells (PEMFCs) and prevent the formation of the water crystalline phase in membrane-electrode assemblies (MEAs) in course of sub-zero temperature operation (so-called cold start) owing to chemical water adsorption. On the other hand, water presence in MEAs provides required operating humidity when temperature increases. Due to the synergistic effect of the targeted filled silica and the efficient –SO₃H based proton conductive channel, high temperature proton conductivity of the Nafion® membrane is therefore significantly improved at low RH. Particularly, proton conductivity of the s-WR-Nafion membrane is raised to 0.07 S/cm at 110 °C and 60% RH. At the same time, mechanical, oxidative and thermal stabilities of the s-WR-Nafion membrane are not decayed due to the integrity of the original structure of Nafion®. As a result, the maximum output power of the s-WR-Nafion membrane-based fuel cell is ca. 65% higher than that of the pristine Nafion® membrane at 110 °C and 20% RH. Above all, the s-WR-Nafion membrane exhibit promising application potential for high-temperature PEMFC.     

A comparative study of Pt/C cathodes in Sn0.9In0.1P2O7 and H3PO4 ionomers for high-temperature proton exchange membrane fuel cells

New Pt/C cathodes with many reaction sites for the oxygen reduction reaction as well as high tolerance to Pt corrosion have been designed for high-temperature proton exchange membrane fuel cells (PEMFCs), wherein a composite mixture of Sn_0.9In_0.1P₂O₇ (SIPO) and sulfonated polystyrene-b-poly(ethylene/butylene)-b-polystyrene (sSEBS) functioned as an ionomer. The microstructure of the Pt-SIPO-sSEBS/C cathode was characterized by homogeneous distribution of the ionomer over the catalyst layer and close contact between the ionomer and the Pt/C powder. As a result, the activation and concentration overpotentials of the Pt-SIPO-sSEBS/C cathode between 100 and 200 °C were lower than those of an H₃PO₄-impregnated Pt/C cathode, which suggests that the present ionomer can avoid poisoning of Pt by phosphate anions and the limitation of gas diffusion through the catalyst layer. Moreover, agglomeration of Pt in the Pt-SIPO-sSEBS/C cathode was not observed during a durability test at 150 °C for 6 days, although it was significant in the Pt-H₃PO₄/C cathode. Therefore, it is concluded that the Pt-SIPO-sSEBS/C electrode is a very promising cathode candidate for high-temperature PEMFCs.     

Anode structure with double-catalyst layers for improving the direct ethanol fuel cell performance

The conventional anode design of direct ethanol fuel cells (DEFCs) usually encounter a problem on the performance stability and ethanol mass transport, i.e., ethanol crossover. Aiming to alleviate these issues, in this study, the anode with different configurations for DEFC was designed and fabricated with different catalyst layer (CL) and microporous layer (MPL) arrangements. The four types of membrane electrode assembly (MEA) is named with MEA-1 (with pretreated carbon paper (PCP) and PtCL), MEA-2 (with PCP, MPL and PtCL), MEA-3 (with PCP, MPL, PtCL and PdCL) and MEA-4 (with PCP, MPL, PtCL, MPL and PdCL). The performance, stability and ethanol crossover of MEAs were tested and measured for continuous long-term operation for 120 h, while the morphological characterization was analyzed. Based on the results, power density for each MEA decreased with time, while ethanol crossover increased gradually. The MEA-3 with additional PdCL shows a highest performance and stability about 20 W/m², and has a lowest ethanol crossover's magnitude. The highest ethanol crossover was obtained using MEA-1 at 3.7 mg/m²·s. Higher ethanol crossover had caused low stability of DEFC performance which result higher irreversible degradation. Moreover, based on characterization, elemental mapping and EDX illustrated phenomena of membrane swelling, delamination of electrode from membrane, and CL loss after stability test for 5 days for all MEAs. The significance of anode structure design was proven in this current study. The anode design of double-layered CL has potential to use at anode structure to reduce ethanol crossover rate, thereby improving DEFC performance and stability.     

Effects of porosity distribution variation on the liquid water flux through gas diffusion layers of PEM fuel cells

Flooding of the membrane electrode assembly (MEA) and dehydrating of the polymer electrolyte membrane have been the key problems to be solved for polymer electrolyte membrane fuel cells (PEMFCs). So far, almost no papers published have focused on studies of the liquid water flux through differently structured gas diffusion layers (GDLs). For gas diffusion layers including structures of uniform porosity, changes in porosity (GDL with microporous layer (MPL)) and gradient change porosity, using a one-dimensional model, the liquid saturation distribution is analyzed based on the assumption of a fixed liquid water flux through the GDL. And then the liquid water flux through the GDL is calculated based on the assumption of a fixed liquid saturation difference between the interfaces of the catalyst layer/GDL and the GDL/gas channel. Our results show that under steady-state conditions, the liquid water flux through the GDL increases as contact angle and porosity increase and as the GDL thickness decreases. When a MPL is placed between the catalyst layer and the GDL, the liquid saturation is redistributed across the MPL and GDL. This improves the liquid water draining performance. The liquid water flux through the GDL increases as the MPL porosity increases and the MPL thickness decreases. When the total thickness of the GDL and MPL is kept constant and when the MPL is thinned to 3 μm, the liquid water flux increases considerably, i.e. flooding of MEA is difficult. A GDL with a gradient of porosity is more favorable for liquid water discharge from catalyst layer into the gas channel; for the GDLs with the same equivalent porosity, the larger the gradient is, the more easily the liquid water is discharged. Of the computed cases, a GDL with a linear porosity 0.4x + 0.4 is the best.     

Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells

A novel membrane electrode assembly (MEA) is described that utilizes a double microporous layer (MPL) structure in the cathode of a passive direct methanol fuel cell (DMFC). The double MPL cathode uses Ketjen Black carbon as an inner-MPL and Vulcan XC-72R carbon as an outer-MPL. Experimental results indicate that this double MPL structure at the cathode provides not only a higher oxygen transfer rate, but enables more effective back diffusion of water; thus, leading to an improved power density and stability of the passive DMFC. The maximum power density of an MEA with a double MPL cathode was observed to be ca. 33.0 mW cm⁻², which is found to be a substantial improvement over that for a passive DMFC with a conventional MEA. A. C. impedance analysis suggests that the increased performance of a DMFC with the double MPL cathode might be attributable to a decreased charge transfer resistance for the cathode oxygen reduction reaction.