Influence of ionomer content on the structure and performance of PEFC membrane electrode assemblies

Nafion^® ionomer content of the cathode catalyst-layer of a polymer electrolyte fuel cell (PEFC), made by the “decal” hot pressing method, has been investigated for its effect on performance and structure of the membrane electrode assembly (MEA). Varying Nafion^® content was shown to have an effect on performance within the entire range of polarization curves (i.e. kinetic, ohmic, and mass-transport regions) as well as on the structure. AFM analysis shows the effect of Nafion on the dispersion of carbon aggregates. Further analysis using TEM demonstrates the effect of Nafion on both the dispersion of carbon aggregates and the distribution and thickness of the Nafion ionomer films surrounding the catalyst/carbon aggregates. The MEA structure change correlates well with the MEA performance on both kinetics and mass-transport region. The determining factors on the performance of MEA are the interfacial zone (between the ionomer and catalyst particle), the dispersion of catalyst/carbon aggregates and the distribution/thickness of Nafion films. An optimized Nafion^® content in the range of 27 ± 6 wt.% for the cathode was determined for an E-TEK 20% Pt₃Cr/C catalyst at a loading of 0.20 mg Pt/cm².     

Effect of MEA fabrication techniques on the cell performance of Pt–Pd/C electrocatalyst for oxygen reduction in PEM fuel cell

The effect of three different membrane electrode assembly (MEA) fabrication techniques, catalyst-coated substrate by direct spray (CCS) and catalyst-coated membrane by direct spray (CCM-DS) or decal transfer (CCM-DT), on the performance of oxygen reduction in a proton exchange membrane (PEM) fuel cell was carried out under identical conditions of Pt–Pd/C electrocatalyst loading. The results indicated that the fabrication technique had only a very slight effect on the ohmic resistance of the PEM fuel cell but it significantly affected the charge transfer resistance and open circuit voltage (OCV). The cells prepared by the CCM method, and particularly by decal transfer, exhibited a significantly higher OCV but a lower ohmic and charge transfer resistance compared with the other investigated fabrication techniques. By using cyclic voltammetry with H₂ adsorption, it was found that the electrochemical active area of the electrocatalyst prepared by CCM-DT was higher than those prepared by CCS and CCM-DS by around 1.76- and 1.05-fold, respectively. Under a H₂/O₂ system at 0.6 V, the cells with MEA made by CCM-DT provided the highest cell performance of around 350 mA/cm², significantly greater than those prepared by the CCS and CCM-DS (149 and 42 mA/cm², respectively).     

Degradation of high temperature MEA with PBI-H3PO4 membrane in a life test

The article reports on the results of a 780 h life test of high temperature MEA with PBI-H₃PO₄ membrane. The MEA was loaded by current density 0.2 A cm⁻² for 763 h at 160 °C in hydrogen-air feed. The load was discontinued 14 times during the life test including three complete shut downs. In the course of the life test MEA characteristics were studied by electrochemical methods. Pt particle size growth was evaluated by ex situ measurements of electrochemical hydrogen adsorption/desorption with the cathode catalyst sampled after the life test and with pristine catalyst. Possible changes of electrochemically active surface area (ESA) of carbon support were monitored by electrochemical impedance studies (EIS) performed in the course of the MEA life test. Average Pt particle diameter was found 3.8 and 7.8 nm for pristine catalyst and for catalyst sampled after the life test, respectively. ESA of carbon support remained unchanged, membrane resistance decreased by ∼20%, hydrogen crossover increased by a factor of 14, although remained insignificant. Voltage loss rate in the life test was ∼25 μV h⁻¹. The major cause of the MEA degradation was identified as a loss of Pt ESA by particle size growth.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part II: preparation and characterization of MEAs

Cathode catalyst layers were prepared and characterized as part of membrane electrode assemblies (MEA) and catalyst coated membranes (CCM) on the basis of carbon supported methanol tolerant RuSe _x catalysts. Preparation parameters varied were: catalyst loading (0.5–2 mg RuSe _x cm⁻²), PTFE content (0, 6, 18 wt.%), carbon support (Vulcan XC 72 or BP2000), and fraction of RuSe _x in the carbon supported catalysts (20, 44, 47 wt.%). The MEAs and cathode catalyst layers were electrochemically characterized under Direct Methanol Fuel Cell (DMFC) operating conditions by recording polarization curves, galvanostatic measurements, and impedance spectra. The morphology of the catalyst layers was investigated by means of confocal laser scan microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. MEAs with Ru(44.0 wt.%)Se(2.8 wt.%)/VulcanXC72 cathode catalyst achieved the highest performance of all RuSe _x catalysts investigated, i.e. ∼40 mW cm⁻² at 80 °C under ambient pressure and λ_MeOH = λ_air = 4. This is 40% of the value obtained with commercial platinum cathode catalyst under the same operating conditions. The RuSe _x catalysts investigated are stable over a period of more than 1,000 h. This was confirmed by TEM and XRD measurements, where no increase in mean RuSe _x particle size (∼5 nm) after fuel cell operation was found. Enhancement of specific catalyst activity, mass transport, and active surface offer potential for a further improvement of RuSe _x catalyst layers.     

A 2<Superscript>7-3</Superscript> fractional factorial optimization of polybenzimidazole based membrane electrode assemblies for H<Subscript>2</Subscript>/O<Subscript>2</Subscript> fuel cells

We describe the usefulness of a statistical fractional factorial design to obtain consistent and reproducible behavior of a membrane-electrode-assembly (MEA) based on a phosphoric acid (PA) doped polybenzimidazole (PBI) membrane, which allows a H₂/O₂ fuel cell to operate above 150 °C. Different parameters involved during the MEA fabrication including the catalyst loading, amount of binder, processing conditions like temperature and compaction load and also the amount of carbon in the gas diffusion layers (GDL) have been systematically varied according to a 2^7-3 fractional factorial design and the data thus obtained have been analyzed using Yates’s algorithm. The mean effects estimated in this way suggest the crucial role played by carbon loading in the gas diffusion layer, hot compaction temperature and the binder to catalyst ratio in the catalyst layer for enabling continuous performance. These statistically designed electrodes provide a maximum current density and power density of 1,800 mA cm⁻² and 280 mW cm⁻², respectively, at 160 °C using hydrogen and oxygen under ambient pressure.     

Matrix Material Study for in situ Grown Pt Nanowire Electrocatalyst Layer in Proton Exchange Membrane Fuel Cells (PEMFCs)

The cathode electrocatalyst layers were prepared by in situ growing Pt nanowires (Pt‐NWs) in two kinds of matrixes with various Pt loadings for proton exchange membrane fuel cells (PEMFCs). Commercial carbon powder and 20 wt.% Pt/C electrocatalyst were used as the matrix material for the comparison. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), polarization curves tests, and electrochemical impedance spectroscopy (EIS) were carried out to examine the effects of the matrix materials on the Pt‐NW growing and the electrode performance. The optimum Pt‐NW loadings of 0.30 mg cm⁻² in the carbon matrix (CM) and 0.20 mg cm⁻² for the Pt/C matrix (PM) were obtained. The results indicated that the Pt‐NWs grown in the CM had a better crystalline, longer size length and better catalyst activity than those in the PM. The mechanism of the matrix affection is further discussed in this paper.     

A Study on Process Parameters of Direct Ethanol Fuel Cell

Ethanol is one of the promising future fuels of Direct Alcohol Fuel Cells (DAFC). The electro‐oxidation of ethanol fuel on anode made of carbon‐supported Pt‐Ru electrode catalysts was carried out in a lab scale direct ethanol fuel cell (DEFC). Cathode used was Pt‐black high surface area. The membrane electrode assembly (MEA) was prepared by sandwiching the solid polymer electrolyte membrane, prepared from Nafion^® (SE‐5112, DuPont USA) dispersion, between the anode and cathode. The DEFC was fabricated using the MEA and tested at different catalyst loadings at the electrodes, temperatures and ethanol concentrations. The maximum power density of DEFC for optimized value of ethanol concentration, catalyst loading and temperature were determined. The maximum open circuit voltage (OCV) of 0.815 V, short circuit current density (SCCD) of 27.90 mA/cm² and power density of 10.30 mW/cm² were obtained for anode (Pt‐Ru/C) and cathode (Pt‐black) loading of 1 mg/cm² at a temperature of 90°C anode and 60°C cathode for 2M ethanol.     

Influence of Pyridine‐Polybenzimidazole Film Thickness of Carbon Nanotube Supported Platinum on Fuel Cell Applications

Pyridine‐polybenzimidazole (PyPBI) films of different thickness (∼1.0–2.4 nm) are wrapped on the surfaces of multi‐walled carbon nanotubes (CNTs). To prepare Pt on PyPBI/CNT (Pt‐PyPBI/CNT) catalysts, Pt⁴⁺ ions are immobilized on these PyPBI wrapped CNTs (PyPBI/CNTs) via Lewis acid‐base coordination between Pt⁴⁺ and :N‐ of imidazole groups, followed by reducing Pt⁴⁺ to Pt nanoparticles. The influence of PyPBI film thickness on the Pt particle size, loading and electrochemical surface area, respectively, of Pt‐PyPBI/CNTs is investigated. Fuel cell performances of the PBI/H₃PO₄ based membrane electrode assemblies (MEAs) prepared from these Pt‐PyPBI/CNT catalysts are also evaluated at 160 °C with unhumidified H₂/O₂ gases. Among the catalysts, the Pt‐PyPBI/CNT catalyst with a PyPBI film thickness of ∼1.6 nm (which is around half of the Pt particle size), a Pt loading of ∼44 wt.%, and a Pt particle size of ∼3.3 nm exhibits the best fuel cell performance.     

Gas Diffusion Layers for Proton Exchange Membrane Fuel Cells Using In situ Modified Carbon Papers with Multi‐walled Carbon Nanotubes Nanoforest

Gas diffusion layers (GDLs) in the proton exchange membrane fuel cells (PEMFCs) enable the distribution of reactant gases to the reaction zone in the catalyst layers by controlling the water in the pore channels apart from providing electrical and mechanical support to the membrane electrode assembly (MEA). In the present work, we report the in situ growth of carbon nanotubes nanoforest (CNN) directly onto macro‐porous carbon paper substrates. The surface property as analysed by a Goniometer showed that the CNN/carbon paper surface is highly hydrophobic. CNN/carbon paper was employed as a GDL in an MEA using Nafion‐212 membrane as an electrolyte and evaluated in single cell PEMFCs. While the GDLs prepared by wire‐rod coating process have major performance losses at lower humidities, the in situ CNN/carbon paper, developed in this work, shows very stable performance at all humidity conditions demonstrating a significant improvement for fuel cell performance. The CNN/carbon‐based MEAs showed very stable performance with power density values of ∼1,100 and 550 mW cm^–2, respectively, both using O₂ and air as oxidants at ambient pressure.     

Platinum nanopaticles supported on stacked-cup carbon nanofibers as electrocatalysts for proton exchange membrane fuel cell

Inexpensive stacked-cup carbon nanofibers (SC-CNFs) supported Pt nanoparticles with a loading from 5 to 30 wt.% were prepared through a modified ethylene glycol method. XRD and TEM characterizations show that the average Pt particle sizes increase with increasing metal loading, and they can be controlled <5 nm with a uniform dispersion. A self-developed filtration process was employed to fabricate Pt/SC-CNFs film-based membrane electrode assembly (MEA), and the catalyst transfer efficiency can reach nearly 100% using a super-hydrophobic polycarbonate filter. The thickness of catalyst layer can be accurately controlled through altering Pt loadings of the catalyst and electrode, this is in good agreement with our theoretical calculation. For Pt/SC-CNFs-based-MEAs, Pt cathode loading was found more critical than Pt anode loading on proton exchange membrane fuel cell (PEMFC) performance. The Pt/SC-CNFs-based MEA with an optimized 50 wt.% Nafion content demonstrates higher PEMFC performance than the carbon black-based MEA with an optimized 30 wt.% Nafion content. SC-CNFs possess much larger length-to-diameter ratio than carbon black particles, it makes Pt/SC-CNFs more easily form continuously conductive networks in the Nafion matrix than carbon black. Therefore, the Pt/SC-CNFs-based MEA demonstrates higher Pt utilization than carbon black-based MEA, which implies possible reduction in Pt loading of MEA.     

Enhanced activity and interfacial durability study of ultra low Pt based electrocatalysts prepared by ion beam assisted deposition (IBAD) method

Ultra low loading noble metal (0.04–0.12 mg_Pt/cm²) based electrodes were obtained by direct metallization of non-catalyzed gas diffusion layers via dual ion beam assisted deposition (IBAD) method. Fuel cell performance results reported earlier indicate significant improvements in terms of mass specific power density of 0.297 g_Pt/kW with 250 Å thick IBAD deposit (0.04 mg_Pt/cm² for a total MEA loading of 0.08 mg_Pt/cm²) at 0.65 V in contrast to the state of the art power density of 1.18 g_Pt/kW using 1 mg_Pt(MEA)/cm² at 0.65 V. In this article we report the peroxide radical initiated attack of the membrane electrode assembly utilizing IBAD electrodes in comparison to commercially available E-TEK (now BASF Fuel Cell GmbH) electrodes and find the pathway of membrane degradation as well. A novel segmented fuel cell is used for this purpose to relate membrane degradation to peroxide generation at the electrode/electrolyte interface by means of systematic pre and post analyses of the membrane are presented. Also, we present the results of in situ X-ray absorption spectroscopy (XAS) experiments to elucidate the structure/property relationships of these electrodes that lead to superior performance in terms of gravimetric power density obtained during fuel cell operation.     

Modeling and experimental validation of overpotentials of a direct ethanol fuel cell

Direct ethanol fuel cell (DEFC) is a promising power source for future use in portable electronic equipments. In general, the power density obtained in DEFC is lower than that of direct methanol fuel cell. In the present study, various losses in DEFC are estimated by performing experiments with the prepared membrane electrode (MEA) to obtain current–voltage characteristics and comparing it with the prediction of mathematical model. MEA for the DEFC is prepared using Pt–Ru (40:20 by wt.%)/C as anode catalyst, Pt–black as cathode catalyst with 1 mg/cm² of loadings and cast Nafion^® (SE5112, DuPont) ionomer as proton exchange membrane. The mathematical model for DEFC is developed considering different overpotentials. The activation overpotential term is formulated considering ethanol electrooxidation mechanism proposed in literature and Butler–Volmer equation. The ohmic overpotential is modeled based on proton conductivity of Nafion^® membrane and ohmic losses at the electrodes, current collectors and electrode–current collector interfaces. The concentration overpotential is formulated using Fick's law, modified Butler–Volmer equation and transport process through electrodes and electrocatalyst layers. The experiment data on current–voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is captured by the model.     

The improved methanol tolerance using Pt/C in cathode of direct methanol fuel cell

Membrane electrode assemblies (MEA) were prepared using PtRu black and 60 wt.% carbon-supported platinum (Pt/C) as their anode and cathode catalysts, respectively. The cathode catalyst layers were fabricated using various amounts of Pt (0.5 mg cm⁻², 1.0 mg cm⁻², 2.0 mg cm⁻², and 3.0 mg cm⁻²). To study the effect of carbon support on performance, a MEA in which Pt black was used as the cathode catalyst was fabricated. In addition, the effect of methanol crossover on the Pt/C on the cathode side of a direct methanol fuel cell (DMFC) was investigated. The performance of the single cell that used Pt/C as the cathode catalyst was higher than single cell that used Pt black and this result was pronounced when highly concentrated methanol (above 2.0 M) was used as the fuel.     

Effect of Preparation Conditions of Pt/C Catalysts on Oxygen Electrode Performance in Proton Exchange Membrane Fuel Cells

Supported Pt/C catalyst with 3.2 nm platinum crystallites was prepared by the impregnation—reduction method. The various preparation conditions, such as the reaction temperature, the concentration of precursor H₂PtCl₆ solution and the different reducing agents, and the relationship between the preparation conditions and the catalyst performance were studied. The carbon black support after heat treatment in N₂ showed improved platinum dispersion. The particle size and the degree of dispersion of Pt on the carbon black support were observed by transmission electron microscopy (TEM). The crystal phase composition of Pt in the catalyst was determined by X-ray diffraction (XRD). The surface characteristics of the carbon black support and the Pt/C catalyst were studied by X-ray photoelectron spectroscopy (XPS). The electrochemical characteristics of the Pt/C catalysts were evaluated from current—voltage curves of the membrane electrode assembly (MEA) in a proton exchange membrane fuel cell.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.     

The Influence of Pt Loading,Support and Nafion Content on the Performance of Direct Methanol Fuel Cells: Examined on the Example of the Cathode

Nano‐sized Pt colloids were prepared using the polyol method and supported on Ketjen black EC 600J (KB), Vulcan XC‐72 (VC) and high surface area graphite 300 (HG). The effects of the Nafion ionomer content, and the Pt loading of the cathode catalyst layer as well as the Pt loading on the support on the performance of direct methanol fuel cells (DMFCs), were studied. The membrane electrode assemblies (MEAs) were analysed using current–voltage curves, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and adsorbed CO stripping voltammetry. Optimum Nafion to carbon (N/C) ratios (N/C being defined as the weight ratio of the Nafion ionomer to the carbon) were determined. The optimum N/C ratios were found to depend on the support as follows, 1.4, 0.7 and 0.5 for Pt/KB, Pt/VC and Pt/HG, respectively and to be independent of the Pt/C loading range of 20–80 wt% tested in this work. The highest DMFC performances, as well as the highest electrochemical active surface areas, and improved gas diffusivities, were achieved using these ratios. For the catalysts prepared in this work, the average Pt crystallite size was found to decrease with increasing surface area of the support for a particular Pt loading. MEAs made using KB as support and the optimal N/C ratio of 1.4 showed the best performances, i.e. higher than the VC and HG supports for any N/C ratio. The highest DMFC performance was observed using 60 wt% Pt on KB cathode electrodes of 1 mg Pt cm^–2 loading and an N/C value of 1.4. For all three supports studied, the 60 wt% Pt on carbon loading resulted in the best DMFC performance. This may be linked to the Pt particle size and catalyst preparation method used in this work. In comparison to literature results, high DMFC performances were achieved using relatively ‘low' Pt and Ru loadings. For example, a maximum power density of >100 mW cm^–2 at 60 °C was observed using a 1 mg Pt cm^–2 cathode loading and a 2 mg PtRu cm^–2 anode loading.     

Do not forget the electrochemical characteristics of the membrane electrode assembly when designing a Proton Exchange Membrane Fuel Cell stack

The membrane electrode assembly (MEA) is the key component of a PEMFC stack. Conventional MEAs are composed of catalyzed electrodes loaded with 0.1–0.4 mg_Pt cm⁻² pressed against a Nafion^® membrane, leading to cell performance close to 0.8 W cm⁻² at 0.6 V. Due to their limited stability at high temperatures, the cost of platinum catalysts and that of proton exchange membranes, the recycling problems and material availability, the MEA components do not match the requirements for large scale development of PEMCFs at a low cost, particularly for automotive applications.Novel approaches to medium and high temperature membranes are described in this work, and a composite polybenzimidazole–poly(vinylphosphonic) acid membrane, stable up to 190 °C, led to a power density of 0.5 W cm⁻² at 160 °C under 3 bar abs with hydrogen and air. Concerning the preparation of efficient electrocatalysts supported on a Vulcan XC72 carbon powder, the Bönnemann colloidal method and above all plasma sputtering allowed preparing bimetallic platinum-based electrocatalysts with a low Pt loading. In the case of plasma deposition of Pt nanoclusters, Pt loadings as low as 10 μg cm⁻² were achieved, leading to a very high mass power density of ca. . Finally characterization of the MEA electrical properties by Electrochemical Impedance Spectroscopy (EIS) based on a theoretical model of mass and charge transport inside the active and gas diffusion layers, together with the optimization of the operating parameters (cell temperature, humidity, flow rate and pressure) allowed obtaining electrical performance greater than 1.2 W cm⁻² using an homemade MEA with a rather low Pt loading.     

Enhanced performance and improved interfacial properties of polymer electrolyte membrane fuel cells fabricated using sputter-deposited Pt thin layers

For this study, catalyst layers for polymer electrolyte membrane fuel cells (PEMFC) were prepared by spraying and sputtering to deposit Pt amount of 0.1 and 0.01 mg cm⁻², respectively. These Pt layers were then assembled to fabricate membrane electrode assemblies (MEA) having either single- or double-layered catalysts. The PEM fuel cell with double layers showed a current density of 777 mA cm⁻² at a cell voltage of 0.6 V, which is a higher current density than state-of-the-art fuel cells at 643 mA cm⁻². These results indicate that Pt loading in state-of-the-art PEMFCs could be reduced by approximately 50% with no performance loss by using both spraying and sputtering method in the MEA fabrication process.     

Plasma Sputtering Deposition of PEMFC Porous Carbon Platinum Electrodes

A novel method is proposed to fabricate the active catalytic layers of proton exchange membrane fuel cells (PEMFC). A plasma sputtering technique is used to deposit a porous columnar carbon film (column diameter of 20 nm) followed by the catalyst (platinum) deposition directly on the proton‐conducting membrane. The study of Pt diffusion shows that the optimised catalysed layers correspond to low plasma pressure operation (0.5 Pa) below a platinum loading limit of about 90 μg cm^–2. The initial carbon porosity is then maintained and Pt nanoparticles are present in all parts of the carbon layer. A membrane electrode assembly (MEA) is then achieved by alternate depositions of carbon and platinum onto both sides of the membrane. The results show the importance of the porous carbon structure. A significant increase in the catalyst efficiency is observed compared to a commercial fuel cell when measuring open circuit voltage.