Effects of hot pressing conditions on the performances of MEAs for direct methanol fuel cells

The effects of hot pressing conditions (hot pressing temperature, pressure and time) on the performances of membrane electrode assemblies for direct methanol fuel cells were investigated. The performances of membrane electrode assemblies (MEAs) were characterized by the polarization curves and electrochemical impedance spectra (EIS). The surface morphologies of the electrodes were observed by scanning electron microscopy (SEM). The compression ratios of electrodes were determined by testing the thicknesses of the anodes and the cathodes before and after the hot pressing process. The MEA which was hot pressed at 135 °C under 80 kg cm⁻² for 90 s, showed the highest power density of 46.0 mW cm⁻² at 80 °C and ambient pressure. As the hot pressing temperature, pressure and time increased, the compression ratios of the anodes and cathodes increased, and the activating time required for MEA to reach optimum performance increased, too. The cell resistances of the MEAs hot pressed at higher hot pressing temperature (135 °C) and pressure (120 kg cm⁻²), or for longer time (90 s), decreased because of the good contact between the membrane and electrodes. The MEAs that were hot pressed under higher temperature (135 °C) and higher pressure (120 kg cm⁻²) benefited for long-time cell operating.     

Properties of high-temperature PEFC Celtec-P 1000 MEAs in start/stop operation mode

This paper aims at the comparison of two differently operated Celtec^®-P 1000 MEAs: one MEA is operated in a start/stop cycling mode (12 h operation at 160 °C followed by 12 h shutdown), the other MEA is continuously operated at 160 °C. We demonstrate more than 6000 h total operation time with more than 240 start/stop cycles, while the test is still ongoing. The degradation rate is 0.2 mV cycle⁻¹ or 11 μV h⁻¹ on a time basis. The continuously operated MEA is operated more than 6000 h with a degradation rate of approximately 5 μV h⁻¹. Through separation of the individual cathodic loss terms, a detailed insight into the doubled time-based degradation rate under start/stop cycling conditions could be given. Both Ohmic resistances and oxygen reduction overpotentials are basically identical in MEAs under continuous and start/stop cycling operation. In the start/stop cycled MEA, however, significantly increased cathodic mass transport overpotentials are observed as a result of enhanced corrosion of the cathode catalyst support, which confirms the generally discussed reverse-current mechanism under start/stop cycling conditions. Results from a newly developed MEA demonstrate that this mechanism can successfully be mitigated through improvement of the materials used in the MEA.     

A novel self-humidifying membrane electrode assembly with water transfer region for proton exchange membrane fuel cells

A novel self-humidifying membrane electrode assembly (MEA) with the active electrode region surrounded by a unactive “water transfer region (WTR)” was proposed to achieve effective water management and high performance for proton exchange membrane fuel cells (PEMFCs). By this configuration, excess water in the cathode was transferred to anode through Nafion membrane to humidify hydrogen. Polarization curves and power curves of conventional and the self-humidifying MEAs were compared. The self-humidifying MEA showed power density of 85 mW cm⁻² at 0.5 V, which is two times higher than that of a conventional MEA with cathode open. The effects of anode hydrogen flow rates on the performance of the self-humidifying MEA were investigated and its best performance was obtained at a flow rate of 40 ml min⁻¹. Its performance was the best when the environmental temperature was 40 °C. The performance of the self-humidifying MEA was slightly affected by environmental humidity. The area of WTR was optimized, and feasible area ratio of the self-humidifying MEA was 28%.     

Improvement of the proton exchange membrane fuel cell (PEMFC) performance at low-humidity conditions by adding hygroscopic γ-Al2O3 particles into the catalyst layer

In this study, hygroscopic γ-alumina particles were added into the catalyst layer of membrane electrode assemblies (MEAs) to improve the wettability and performance of PEMFC at low-humidity conditions. Hygroscopic γ-alumina particles with a BET surface area of 442 m² g⁻¹ and an average pore diameter of 9 nm were synthesized by a three-step sol–gel procedure. Uniform Pt/C/γ-alumina catalyst ink was prepared by utilizing an ultrasonic method, and then sprayed on commercial hydrophobic carbon clothes to serve as the catalyst layer. The water contact angles of the catalyst layer with various amounts of γ-alumina additions 0%, 10%, 20% and 40% were measured to be 136°, 109°, 79° and 0°, respectively. Effect of adding γ-alumina particles into the catalyst layer on the single cell performance was investigated under different temperatures of the electrode humidifier. The increased wettability of the cathode catalyst layer with γ-alumina addition reduced the cell performance due to water flooding, which demonstrates the hygroscopic characteristic of γ-alumina particles. On the other hand, when the γ-alumina particles were added into the anode catalyst layer, it was found that the MEA with 10% γ-alumina addition had the highest current density at anode humidifier temperatures ranging from 25 to 55 °C. Nevertheless, the MEA with 40% γ-alumina addition into the anode catalyst layer showed the lowest current density because of the high electrical resistance of the catalyst layer and the water flooding in the anode caused by excess water absorption. The increased wettability of the anode catalyst layer by an appropriate amount of γ-alumina additions also enhances the water adsorption of the anode due to back diffusion.     

Effect of platinum amount in carbon supported platinum catalyst on performance of polymer electrolyte membrane fuel cell

The performance of polymer electrolyte membrane fuel cells fabricated with different catalyst loadings (20, 40 and 60 wt.% on a carbon support) was examined. The membrane electrode assembly (MEA) of the catalyst coated membrane (CCM) type was fabricated without a hot-pressing process using a spray coating method with a Pt loading of 0.2 mg cm⁻². The surface was examined using scanning electron microscopy. The catalysts with different loadings were characterized by X-ray diffraction and cyclic voltammetry. The single cell performance with the fabricated MEAs was evaluated and electrochemical impedance spectroscopy was used to characterize the fuel cell. The best performance of 742 mA cm⁻² at a cell voltage of 0.6 V was obtained using 40 wt.% Pt/C in both the anode and cathode.     

The effect of Nafion content in a graphitized carbon nanofiber-based anode for the direct methanol fuel cell

The performance and stability of a direct methanol fuel cell (DMFC) with membrane electrode assemblies (MEA) using different Nafion^® contents (30, 50 and 70 wt% or MEA30, MEA50 and MEA70, respectively) and graphitized carbon nanofiber (GNF) supported PtRu catalyst at the anode was investigated by a constant current measurement of 9 days (230 h) in a DMFC and characterization with various techniques before and after this measurement. Of the pristine MEAs, MEA50 reached the highest power and current densities. During the 9-day measurement at a constant current, the performance of MEA30 decreased the most (−124 μV h⁻¹), while the MEA50 was almost stable (−11 μV h⁻¹) and performance of MEA70 improved (+115 μV h⁻¹). After the measurement, the MEA50 remained the best MEA in terms of performance. The optimum anode Nafion content for commercial Vulcan carbon black supported PtRu catalysts is between 20 and 40 wt%, so the GNF-supported catalyst requires more Nafion to reach its peak power. This difference is explained by the tubular geometry of the catalyst support, which requires more Nafion to form a penetrating proton conductive network than the spherical Vulcan. Mass transfer limitations are mitigated by the porous 3D structure of the GNF catalyst layer and possible changes in the compact Nafion filled catalyst layers during constant current production.     

Redistribution of phosphoric acid in membrane electrode assemblies for high-temperature polymer electrolyte fuel cells

We demonstrate that the performance of a high-temperature polymer electrolyte fuel cell with a phosphoric acid-based electrolyte is almost independent of the way of introducing the acid into the membrane electrode assembly (MEA). The same power densities were obtained with different MEAs in which the poly(2,5-benzimidazole) membrane was either pre-doped or not and in which either one or two catalyst layers were impregnated with H₃PO₄. Chemical analysis after shut down revealed that in all these MEAs the phosphoric acid distribution between the membrane and the electrodes was nearly the same. An MEA with acid impregnation via the electrodes was started up rapidly from room temperature, delivered a power density of 120 mW cm⁻² at 600 mV (H₂/air, 160 °C, ambient pressure) after only 11 min and was operated for 1000 h (degradation rate: 0.06 mV/h). Based on the analysis of the H₃PO₄ content in the MEA components, reflections on the kinetics of the redistribution of phosphoric acid within the MEA are provided.     

Performance enhancement of membrane electrode assemblies with plasma etched polymer electrolyte membrane in PEM fuel cell

In this work, a surface modified Nafion 212 membrane was fabricated by plasma etching in order to enhance the performance of a membrane electrode assembly (MEA) in a polymer electrolyte membrane fuel cell. Single-cell performance of MEA at 0.7 V was increased by about 19% with membrane that was etched for 10 min compared to that with untreated Nafion 212 membrane. The MEA with membrane etched for 20 min exhibited a current density of 1700 mA cm⁻² at 0.35 V, which was 8% higher than that of MEA with untreated membrane (1580 mA cm⁻²). The performances of MEAs containing etched membranes were affected by complex factors such as the thickness and surface morphology of the membrane related to etching time. The structural changes and electrochemical properties of the MEAs with etched membranes were characterized by field emission scanning electron microscopy, Fourier transform-infrared spectrometry, electrochemical impedance spectroscopy, and cyclic voltammetry.     

Performance of an ultra-low platinum loading membrane electrode assembly prepared by a novel catalyst-sprayed membrane technique

Membrane electrode assemblies (MEAs) with ultra-low platinum loadings are attracting significant attention as one method of reducing the quantity of precious metal in polymer electrolyte membrane fuel cells (PEMFCs) and thereby decreasing their cost, one of the key obstacles to the commercialization of PEMFCs. In the present work, high-performance MEAs with ultra-low platinum loadings are developed using a novel catalyst-sprayed membrane technique. The platinum loadings of the anode and cathode are lowered to 0.04 and 0.12 mg cm⁻², respectively, but still yield a high performance of 0.7 A cm⁻² at 0.7 V. The influence of Nafion content, cell temperature, and back pressures of the reactant gases are investigated. The optimal Nafion content in the catalyst layer is ca. 25 wt.%. This is significantly lower than for low platinum loading MEAs prepared by other methods, indicating ample interfacial contact between the catalyst layer and membrane in our prepared MEAs. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) measurements reveal that our prepared MEA has very thin anode and cathode catalyst layers that come in close contact with the membrane, resulting in a MEA with low resistance and reduced mass transport limitations.     

Optimization of the performance of polymer electrolyte fuel cell membrane-electrode assemblies: Roles of curing parameters on the catalyst and ionomer structures and morphology

In order to understand the origin of performance variations in polymer electrolyte membrane fuel cells (PEMFCs), a series of membrane-electrode assemblies (MEAs) with identical electrode layer compositions were prepared using different electrode curing conditions, their performances were evaluated, and their morphologies determined by scanning electron microscopy (SEM). The polarization curves varied markedly primarily due to differences in morphologies of electrodes, which were dictated by the curing processes. The highest performing MEAs (1.46 W cm⁻² peak power density at 3.2 A cm⁻² and 80 °C) were prepared using a slow curing process at a lower temperature, whereas those MEAs prepared using a faster curing process performed poorly (0.1948 W cm⁻² peak power density at 440 mA cm⁻² and 80 °C). The slowly cured MEAs showed uniform electrode catalyst and ionomer distributions, as revealed in SEM images and elemental maps. The relatively faster cured materials exhibited uneven distribution of ionomer with significant catalyst clustering. Collectively, these results indicate that to achieve optimal performance, factors that affect the dynamics of the curing process, such as rate of solvent evaporation, must be carefully controlled to avoid solvent trapping, minimize catalyst coagulation, and promote even distribution of ionomer.     

Durability improvement at high current density by graphene networks on PEM fuel cell

This study presents a novel structure of catalyst layers of membrane electrode assemblies (MEAs) by adding graphene to platinum on carbon black (Pt/C) to improve the durability at high current density operation (3 A cm⁻²). Graphene displays outstanding low electrical resistance and has the advantage of high electron mobility. It is also used in lithium ion batteries to improve electrical performance such as high rate charge/discharge capability and cycle-life stability. In this study, three MEAs are compared, and graphene is used as an excellent conductive additive in catalyst layers for better electrons transport at high current density operation. The MEA coated Pt/C mixed with 0.1 wt% graphene shows best durability for 0.3 V h⁻¹ which is almost 3.7 times better than that of without graphene additive (1.1 V h⁻¹). The graphene additive effectively extends the durability of the MEA. Furthermore, the MEAs are analyzed by AC impedance. The impedance arc of the MEA coated with Pt/C only is getting worse, but those two coated with graphene show similar and smaller impedance arcs after high current density operation for 80 h.     

Effects of reverse voltage and subzero startup on the membrane electrode assembly of a PEMFC

Effects of reverse voltage and frozen fuel cell startup on the membrane electrode assembly (MEA) were investigated for a proton exchange membrane fuel cell (PEMFC). A single cell was started from a subzero temperature by applying reverse voltage. The voltages applied to the cell were 0.8 and 1.2 V. The fuel cell performance was measured with a polarization curve and by cyclic voltammetry (CV), electrochemical impedance spectra (EIS), linear scan voltammetry (LSV) after each experiment. From the results, it was concluded that the catalyst activity, electrochemical active surface area (ECA) and the membrane were not damaged by the reverse voltage if the voltage was below 0.85 V. In contrast, a reverse voltage improved cell performance slightly. If the reverse voltage was larger than 0.85 V, the cell performance degraded. Another single cell with an active area of 128 cm² was started up at −15 °C by applying reverse voltage. The cell performance and MEA physical characteristic were tested before and after the freeze startup. From the results, the cell performance decayed MEA delamination was observed and the pore size distribution of the MEA changed.     

Study on the membrane electrode assembly fabrication with carbon supported cobalt triethylenetetramine as cathode catalyst for proton exchange membrane fuel cell

An improved fabrication technique for conventional hot-pressed membrane electrode assemblies (MEAs) with carbon supported cobalt triethylenetetramine (CoTETA/C) as the cathode catalyst is investigated. The V-I results of PEM single cell tests show that addition of glycol to the cathode catalyst ink leads to significantly higher electrochemical performance and power density than the single cell prepared by the traditional method. SEM analysis shows that the MEAs prepared by the conventional hot-pressed method have cracks between the cathode catalyst layer and Nafion membrane, and the contact problem between cathode catalyst layer and Nafion membrane is greatly suppressed by addition of glycol to the cathode catalyst ink. Current density-voltage curve and impedance studies illuminate that the MEAs prepared by adding glycol to the cathode catalyst ink have a higher electrochemical surface area, lower cell ohmic resistance, and lower charge transfer resistance. The effects of CoTETA/C loading, Nafion content, and Pt loading are also studied. By optimizing the preparation parameters of the MEA, the as-fabricated cell with a Pt loading of 0.15 mg cm⁻² delivers a maximum power density of 181.1 mW cm⁻², and a power density of 126.2 mW cm⁻² at a voltage of 0.4 V.     

The influence of gas diffusion layer wettability on direct methanol fuel cell performance: A combined local current distribution and high resolution neutron radiography study

The influence of the anode and cathode GDL wettability on the current and media distribution was studied using combined in situ high resolution neutron radiography and locally resolved current distribution measurements. MEAs were prepared by vertically splitting either the anode or cathode carbon cloth into a less hydrophobic part (untreated carbon cloth ‘as received’) and a more hydrophobic part (carbon cloth impregnated by PTFE dispersion). Both parts were placed side by side to obtain a complete electrode and hot-pressed with a Nafion membrane. MEAs with partitioned anode carbon cloth revealed no difference between the untreated and the hydrophobised part of the cell concerning the fluid and current distribution. The power generation of both parts was almost equal and the cell performance was similar to that of an undivided MEA (110 mW cm⁻², 300 mA cm⁻², 70 °C). In contrast, MEAs with partitioned cathode carbon cloth showed a better performance for the hydrophobised part, which contributed to about 60% of the overall power generation. This is explained by facilitated oxygen transport especially in the hydrophobised part of the cathode gas diffusion layer. At an average current density of 300 mA cm⁻², a pronounced flooding of the cathode flow field channels adjacent to the untreated part of GDL led to a further loss of performance in this part of the cell. The low power density of the untreated part caused a significant loss of cell performance, which amounted to less than 40 mW cm⁻² (at 300 mA cm⁻²).     

Highly fluorinated comb-shaped copolymer as proton exchange membranes (PEMs): Fuel cell performance

The fuel cell performance (DMFC and H₂/air) of highly fluorinated comb-shaped copolymer is reported. The initial performance of membrane electrode assemblies (MEAs) fabricated from comb-shaped copolymer containing a side-chain weight fraction of 22% are compared with those derived from Nafion and sulfonated polysulfone (BPSH-35) under DMFC conditions. The low water uptake of comb copolymer enabled an increase in proton exchange site concentrations in the hydrated polymer, which is a desirable membrane property for DMFC application. The comb-shaped copolymer architecture induces phase separated morphology between the hydrophobic fluoroaromatic backbone and the polysulfonic acid side chains. The initial performance of the MEAs using BPSH-35 and Comb 22 copolymer were comparable and higher than that of the Nafion MEA at all methanol concentrations. For example, the power density of the MEA using Comb 22 copolymer at 350 mA cm⁻² and 0.5 M methanol was 145 mW cm⁻², whereas the power densities of MEAs using BPSH-35 were 136 mW cm⁻². The power density of the MEA using Comb 22 copolymer at 350 mA cm⁻² and 2.0 M methanol was 144.5 mW cm⁻², whereas the power densities of MEAs using BPSH-35 were 143 mW cm⁻².     

Performance degradation and microstructure changes in freeze–thaw cycling for PEMFC MEAs with various initial microstructures

When the temperature of a fuel cell vehicle is repeatedly reduced to subzero temperatures, volume changes by water/ice transformations and frost heave mechanism can cause microstructural changes in membrane–electrode assemblies (MEA), and a resultant permanent decrease in the performance of fuel cell stacks. In this study, five MEAs manufactured by different methods, were tested under repeated freeze–thaw (F–T) cycles between −20 °C and 10 °C, and the variations in their electrochemical and microstructural characteristics were analyzed according to the initial microstructures. When the MEAs were prepared by spraying catalyst inks on polymer membranes, no significant microstructural changes were observed. In the case of two supplied MEAs, void formations at the electrolyte/electrode interface or vertical cracks within the catalyst layers were observed after 120 F–T cycles. Void formation seems to be responsible for performance degradation as a result of ohmic loss, but the effect of cracks in the catalyst layers was not confirmed. In 120 F–T cycles, activation overpotentials and concentration overpotentials did not increase significantly for any of the MEAs, even although gradual decreases in the electrochemically active surface area of the platinum catalysts and changes in the porous structure were observed.     

Evaluation of assemblies based on carbon materials modified with dendrimers containing platinum nanoparticles for PEM-fuel cells

Polyamidoamine (PAMAM) dendrimer-encapsulated Pt nanoparticles (G4OHPt) are synthesized by chemical reduction and characterized by transmission electronic microscopy. An H₂–O₂ fuel cell has been constructed with porous carbon electrodes modified with the dendrimer nanocomposites. Electrochemical and physical impregnation methods of electrocatalyst immobilization are compared. The modified surfaces are used as electrodes and gas-diffusion layers in the construction of three different membrane-electrode assemblies (MEAs). The MEAs have been tested in a single polymer-electrolyte membrane-fuel cell at 30 °C and 20 psig. The fuel cell is, then characterized by electrochemical impedance spectroscopy and cyclic voltammetry, and its performance evaluated in terms of polarization curves and power profiles. The highest fuel cell performance is reached in the MEA constructed by physical impregnation method. The results are compared with a 32 cm² prototype cell using commercial electrocatalyst operated at 80 °C, obtaining encouraging results.     

Performance of Pd on activated carbon as hydrogen electrode with respect to hydrogen yield in a single cell proton exchange membrane (PEM) water electrolyser

Palladium (Pd) on activated carbon is used as electrocatalyst coated on Nafion 115 membrane as Hydrogen electrode and RuO₂ is coated on other side of membrane used as oxygen electrode. 5 wt% and 10 wt% Pd on activated carbon is prepared as membrane electrode assembly (MEA) and investigated the performance of the same using inhouse prepared 10 cm² single cell. The performance of the single cell assembly and the hydrogen yield are reported during electrolysis operation at temperatures 27 °C, 45 °C and 65 °C at 0.1, 0.2, 0.3, 0.4, 0.5 A/cm² current densities with respect to voltages.     

Self-humidifying membrane electrode assembly prepared by adding microcrystalline cellulose in anode catalyst layer as preserve moisture

A novel self-humidifying membrane electrode assemblies (MEAs) with the addition of microcrystalline cellulose (MCC) as a hygroscopic agent into anode catalyst layer was prepared to improve the performance of proton exchange membrane fuel cell (PEMFC) under low humidity conditions. The MEAs were characterized by SEM, contact angles and water uptake measurements. The MEAs with addition of MCC exhibit excellent self-humidifying single cell performance, the cell temperature for self-humidification running is up to 60 °C. As an optimized MEA with 4 wt.％ MCC in its anode catalyst layer, its current density at 0.6 V could be up to 760 mA cm⁻² under 20% of relative humidity, and remains at 680 mA cm⁻² after 22 h long time continuous testing, the attenuation of the current density is only 10%. While the current density of the blank MEA without addition of MCC degraded sharply from 300 mA cm⁻² to 110 mA cm⁻², the attenuation of the current density is high up to 70% within 2 h.     

Influence of anisotropic bending stiffness of gas diffusion layers on the degradation behavior of polymer electrolyte membrane fuel cells under freezing conditions

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the degradation behavior of polymer electrolyte membrane fuel cells have been investigated under freezing conditions. We have prepared GDL sheet samples such that the higher stiffness direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I-V performances before and after 1000 temperature cycles between −10 and 1 °C of 90° GDL stack are higher than those of 0° GDL stack, and the voltages of 90° GDL stack are decreased slower than those of 0° GDL stack, indicating a higher durability of 90° GDL stack. Furthermore, the values and increasing rates of high-frequency resistance of 90° GDL stack are lower than those of 0° GDL stack. However, the H₂ and air pressure differences before and after 1000 temperature cycles of 90° GDL stack are very similar to those of 0° GDL stack. The surface of anode catalyst layer (CL) of membrane-electrode assembly (MEA) with catalyst-coated membrane type in 0° GDL stack appears to be more severely damaged than that in 90° GDL stack, especially under the channels, whereas the surfaces of cathode CLs of MEAs in both 0° and 90° GDL stacks are slightly damaged after 1000 temperature cycles.