Membrane electrode assembly with Pt/SiO2/C anode catalyst for proton exchange membrane fuel cell operation under low humidity conditions

In this work, a novel self-humidifying membrane electrode assembly (MEA) with Pt/SiO₂/C as anode catalyst was developed to improve the performance of proton exchange membrane fuel cell (PEMFC) operating at low humidity conditions. The characteristics of the composite catalysts were investigated by XRD, TEM and water uptake measurement. The optimal performance of the MEA was obtained with the 10 wt.% of silica in the composite catalyst by single cell tests under both high and low humidity conditions. The low humidity performance of the novel self-humidifying MEA was evaluated in a H₂/air PEMFC at ambient pressure under different relative humidity (RH) and cell temperature conditions. The results show that the MEA performance was hardly changed even if the RHs of both the anode and cathode decreased from 100% to 28%. However, the low humidity performance of the MEA was quite susceptible to the cell temperature, which decreased steeply as the cell temperature increased. At a cell temperature of 50 °C, the MEA shows good stability for low humidity operating: the current density remained at 0.65 A cm⁻² at a usual work voltage of 0.6 V without any degradation after 120 h operation under 28% RH for both the anode and cathode.     

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.     

PEM fuel cells operated at 0% relative humidity in the temperature range of 23-120 °C

Operation of a proton exchange membrane (PEM) fuel cell without external humidification (or 0% relative humidity, abbreviated as 0% RH) of the reactant gases is highly desirable, because it can eliminate the gas humidification system and thus decrease the complexity of the PEM fuel cell system and increase the system volume power density (W/l) and weight power density (W/kg). In this investigation, a PEM fuel cell was operated in the temperature range of 23-120 °C, in particular in a high temperature PEM fuel cell operation range of 80-120 °C, with dry reactant gases, and the cell performance was examined according to varying operation parameters. An ac impedance method was used to compare the performance at 0% RH with that at 100% RH; the results suggested that the limited proton transfer process to the Pt catalysts, mainly in the inonomer within the membrane electrode assembly (MEA) could be responsible for the performance drop. It was demonstrated that operating a fuel cell using a commercially available membrane (Nafion^® 112) is feasible under certain conditions without external humidification. However, the cell performance at 0% RH decreased with increasing operation temperature and reactant gas flow rate and decreasing operation pressure.     

Molecular mechanisms involved in creep phenomena of paper

The phenomenon of mechanosorptive creep (i.e., the increasing creep occurring in some hygroscopic materials subjected to moisture cycling) was studied for paper from a molecular point of view. Paper was tested in creep at different loading levels in a constant high humidity of 90% relative humidity (RH) and in a cyclic climate between 30 and 90% RH. Throughout the creep tests, spectra from the mid‐ and near‐IR, as well as dynamic mechanical data, were recorded to determine molecular changes occurring with time. In tensile stress scans the instantaneous, dynamic elastic modulus was found to increase. It is suggested that this increase was due to orientation of the cellulose molecules, which was detected as changes in the mid‐IR spectra at 1160 cm⁻¹ assigned to the C₁ O C₄ stretching. During creep in constant and cyclic humidity, the modulus was found to increase with time, more so for the cyclic humidity. Changes in the mid‐IR spectra at 1184 and 1030 cm⁻¹, which is assigned to CH₂, CH, and C O, may indicate sliding between the cellulose chains. The near‐IR measurements mainly showed differences in the moisture content. In stress scans the moisture content increased with increasing tensile load. In creep at constant 90% RH, the moisture content was also found to increase in a manner similar to the stress scan. In the cyclic humidity with a conditioning time of 70 min at 90% RH the moisture content decreased successively with increasing numbers of cycles. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1590–1595, 2001     

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.     

Nanostructured membrane electrode assemblies from layer‐by‐layer composite/catalyst containing membranes and their fuel cell performances

In this study, two approaches are compared to develop nanostructured membrane electrode assemblies (MEA) using layer‐by‐layer (lbl) technique. The first is based on the direct deposition of polyallylamine hydrochloride (PAH) and sulfonated polyaniline (sPAni) on Nafion support to prepare lbl composite membrane. In the second approach, sPAni is coated on the support in the presence of platinum (Pt) salt, Nafion solution and Vulcan for obtaining catalyst containing membranes (CCMs). SEM and UV–vis analysis show that the multilayers are deposited on both sides of Nafion successfully. Although H₂/O₂ single cell performances of acid doped lbl composite membrane based MEA are found to be at the range of 126 and 160 mW cm^?2 depending on the number of deposited layers, the cell performance of MEA obtained from catalyst containing lbl self‐assembled thin membrane (PAH/sPAni‐H⁺)_10‐Pt is found to be 360 mW cm^?2 with a Pt utilization of 720 mW mgPt^?1. This performance is 82% higher as compared to original Nafion^®117 based MEA (198 mW cm^?2). From the cell performance evaluations for different structured MEAs, it is mainly found out that the use of lbl CCMs instead of composite membranes and fabrication of thinner electrolytes result in a higher H₂/O₂ cell activity due to significant reduction in ohmic resistivity. Also, it is observed that the use of sPAni slightly improves the cell performance due to an increased probability of the triple phase contact and it can lead to superior physicochemical properties such as conductivity and thermal stability. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40314.     

Preparation of a high-performance Pt–Pd/C-electrocatalyst-coated membrane for ORR in PEM fuel cells via a combined process of impregnation and seeding: Effect of electrocatalyst loading on carbon support

The influence of the electrocatalyst loading (2.0–40.7 wt.%) on a carbon support on its suitability as a cathode for proton-exchange membrane (PEM) fuel cells was evaluated at a constant membrane electrocatalyst loading of 0.15 mg/cm². The results clearly demonstrate that different electrocatalyst loadings on a carbon support in the investigated range significantly affect not only the electrocatalyst activity but also the performance in H₂/O₂ fuel cells. Increasing the electrocatalyst loading on a carbon support led to an increase in the particle size of electrocatalyst and the pore size of the membrane electrode assembly (MEA) but a decrease in the particle size distribution and MEA thickness. The maximum oxygen-reduction reaction (ORR) activity and cell performance (745 mA/cm² and 520 mW/cm² at 0.6 V) were obtained at an electrocatalyst loading of 24.1 wt.% on a carbon support.     

A Novel Glass Membrane for Low Temperature H2/O2 Fuel Cell Electrolytes

A novel structure for an H₂/O₂ fuel cell with a proton conducting glass electrolyte and a Pt/C catalyst was developed. The performance of the fuel cell, which was impregnated with a glass electrolyte and a gaseous hydrogen–oxygen feed at low temperature in a humidified atmosphere was significantly improved by introducing membrane electrode assemblies (MEAs) consisting of heteropolyacids (HPAs) (phosphotungstic acid, PWA and phosphomolybdic acid, PMA) doped with a P₂O₅‐SiO₂ glass electrolyte. The HPAs containing porous glass electrolytes show promise for applications in low temperature H₂/O₂ fuel cells. The electrochemical behaviour of these materials was studied by measuring the current–voltage profile from polarisation curves. A maximum power density of ≈ 35 mW cm^–2 was obtained at 30 °C and 30% RH (relative humidity) using a PMA/PWA‐P₂O₅‐SiO₂ glass electrolyte membrane. The impedance measurements displaying the total cell ohmic resistance for 12 h at 0.5 V were evaluated at 30 °C. The resistance value was 3.5 Ω for an operating time of 12 h. This MEA showed the best and the most stable performance for use in an H₂/O₂ fuel cell.     

Electrochemical mass-flow control of hydrogen using a fullerene-based proton conductor

A membrane electrode assembly (MEA) was fabricated using proton conductive hydrogensulfated fullerenol (C₆₀(OSO₃H)_n(OH)_n). Rate-controlled mass flow of hydrogen was performed by applying voltage to both electrodes of the MEA without humidification. The amount of the electrochemically transported hydrogen through the MEA increased as the applied current increased, obeying Faraday's law. Residual gas analysis of the transported hydrogen showed that the transported hydrogen contains trace amounts of water less than 1%.     

Ameliorating effect of silica addition in the anode-catalyst layer of the membrane electrode assemblies for polymer electrolyte fuel cells

Incorporation of silica particles through a sol-gel process into the anode-catalyst layer with a sol-gel modified Nafion-silica composite membrane renders easy retention of back-diffused water from the cathode to anode through the composite membrane electrolyte, increases the catalyst-layer wettability and improves the performance of the Polymer Electrolyte Fuel Cell (PEFC) while operating under relative humidity (RH) values ranging between 18% and 100% with gaseous hydrogen and oxygen reactants at atmospheric pressure. A peak power density of 300 mW cm⁻² is achieved at a load current-density value of 1200 mA cm⁻² for the PEFC employing a sol-gel modified Nafion-silica composite membrane and operating at 18% RH. Under similar operating conditions, the PEFC with a Membrane Electrode Assembly (MEA) comprising Nafion-silica composite membrane with silica in the anode-catalyst layer delivers a peak power density of 375 mW cm⁻². By comparison, the PEFC employing commercial Nafion membrane fails to deliver satisfactory performance at 18% RH due to the limited availability of water at its anode, acerbated electro-osmotic drag of water from anode to cathode and insufficient water back diffusion from cathode to anode causing the MEA to dehydrate.     

Chlorination of Pt–Re/Al2O3 during naphtha reforming

The reforming of a paraffinic naphtha was studied in order to determine the influence of chlorination during the run. Experiments were performed at 505°C, 15 kg cm⁻², WHSV = 4, H₂: HC= 4, and with or without an 8 h initial period of deactivation at 1 kg cm⁻². A commercial Pt–Re/Al₂O₃ (0·3% Pt, 0·3% Re, 0·04% S, 0·15% Cl) catalyst was chlorinated using naphtha feeds with different H₂O/Cl ratios. A model of the chlorination kinetics was developed and represents adequately the experimental results. The acid controlled reactions such as C₂–C₄ and production of C₅ paraffins, disappearance of C₉ paraffins and production of aromatics increase in parallel to the chlorination of the catalyst and the increase is independent of the amount of coke deposited on the catalyst. The sites of chlorine adsorption are different from the sites of coke deposition.     

Process optimization of oxidative coupling of methane for ethylene production using response surface methodology

The statistical design of experiments (DoE) was used in the process study of oxidative coupling of methane (OCM) over Na? W? Mn/SiO₂ catalyst. A set of factors with a certain range was screened using factorial design with respect to three responses: methane conversion, C₂₊ products selectivity and ethylene/ethane ratio. The variances were analyzed and the interaction effects of the process parameters were evaluated. With the understanding of the process, the optimization of the process was further studied using response surface methodology coupled with central composite design (CCD). The optimum conditions were obtained as reaction temperature = 850 °C, gas hourly space velocity = 23 947 cm³ g^?1 h^?1, catalyst pretreatment period = 2 h, dilution ratio = 0.2 and CH₄/O₂ ratio = 7. 40.55% of methane conversion and 79.51% of C₂₊ product selectivity were obtained under these optimum conditions. Experimental runs under optimum conditions were repeated and compared with the simulated values obtained from the model. There was good agreement between the experimental and simulated values. Copyright © 2007 Society of Chemical Industry     

Stability characteristics of Pt1Ni1/C as cathode catalysts in membrane electrode assembly of polymer electrolyte membrane fuel cell

To understand the difference in degradation characteristics between carbon-supported platinum (Pt/C) and platinum–nickel alloy (Pt₁Ni₁/C) cathode catalysts in membrane electrode assemblies (MEAs) of a polymer electrolyte membrane fuel cell (PEMFC), constant current operation of MEA in a single cell was conducted for 1100 h. A significant change in cell potential for the Pt₁Ni₁/C MEA was observed throughout the test. High-resolution transmission electron microscopy showed that sintering and detachment of metal particles in the Pt₁Ni₁/C catalyst occurred more sparingly than in the Pt/C catalyst. Instead, X-ray photoelectron spectroscopy element mapping revealed dissolution of Ni atoms in the Pt₁Ni₁ catalysts even when the Pt₁Ni₁/C catalyst used in the MEA was well synthesized.     

Three-dimensional ordered mesoporous Co–Mn oxide: A highly active catalyst for “storage–oxidation” cycling for the removal of formaldehyde

Three-dimensional (3D) ordered cubic mesoporous Co–Mn oxide (denoted as CoMn-HT) was fabricated using a KIT-6-templating strategy and was tested in a “storage–oxidation” cycling process for the removal of formaldehyde. The formation of a 3D ordered mesoporous structure was confirmed by low-angle XRD, nitrogen adsorption–desorption data, and TEM micrographs. The CoMn-HT catalyst showed promising properties in both the storage and regeneration phases, and remained active in highly humid air (RH = 90%) and at high GHSV (160,000 h^− 1). The excellent catalytic performance of CoMn-HT was associated with its large surface area and 3D ordered mesoporous structure.     

Preparing a catalyst layer in magnetic field to improve the performance of proton exchange membrane fuel cells

Electro catalyst Pt–Co/multi-walled C nanotubes were synthesized by using the modified polyol method with glycol as reducer. The magnetic-field-assisted fabrication of membrane electrode assemblies (MEAs) for proton exchange membrane fuel cells (PEMFCs) was proposed, to orient catalyst layers and increase the efficiency of catalyst utilization. PEMFCs with the magnetic-field-treated MEA (M-MEA) exhibited significant performance improvement over common MEA (C-MEA) without magnetic-field treatment. Under the same operating conditions, the maximum power density of MEA increased from 149.6 to 223.8 mW cm^?2 when C-MEA was replaced by M-MEA. Scanning electron microscope images showed that catalysts exhibited a “cluster-like structure” in M-MEA opposed to a chaotic arrangement in C-MEA. Electrochemical impedance spectroscopy measurements revealed that M-MEA reaction resistance was lower than that of C-MEA. Cyclic voltammetry data showed an increment of almost 29.6 % in electrochemical surface area as a result of the magnetic-field treatment.     

Performance of the Direct Methanol Carbonate Fuel Cell Using Anion Exchange Materials and Non‐Noble Metal Cathode Catalyst

A direct methanol fuel cell using a mixture of O₂ and CO₂ at the cathode was evaluated using anion exchange materials and cathode catalysts of Pt and a non‐Pt catalyst. The MEA based on non‐noble metal catalyst Acta 4020 showed superior performance than Pt/C based MEA in terms of open circuit potential and power density in carbonate environment. The fuel cell performance was improved by applying anion exchange ionomer in the catalyst layer. A maximum power density of 4.5 mW cm^–2 was achieved at 50 °C using 6.0 M methanol and 2.0 M K₂CO₃.     

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.     

Durability of ABPBI‐based MEAs for High Temperature PEMFCs at Different Operating Conditions

High temperature PEMFCs based on phosphoric acid‐doped ABPBI membranes have been prepared and characterised. At 160 °C and ambient pressure fuel cell power densities of 300 mW cm^–2 (with hydrogen and air as reactants) and 180 mW cm^–2 (with simulated diesel reformate/air) have been achieved. The durability of these membrane electrode assemblies (MEAs) in the hydrogen/air mode of operation at different working conditions has been measured electrochemically and has been correlated to the cell resistivity, the phosphoric acid loss rate and the catalyst particle size. Under stationary conditions, a voltage loss of only –25 μV h^–1 at a current density of 200 mA cm^–2 has been deduced from a 1,000 h test. Under dynamic load changes or during start–stop cycling the degradation rate was significantly higher. Leaching of phosphoric acid from the cell was found to be very small and is not the main reason for the performance loss. Instead an important increase in the catalyst particle size was observed to occur during two long‐term experiments. At high gas flows of hydrogen and air ABPBI‐based MEAs can be operated at temperatures below 100 °C for several hours without a significant irreversible loss of cell performance and with only very little acid leaching.     

The effectiveness of platinum/carbon electrocatalysts: Dependence on catalyst layer thickness and Pt alloy catalytic effects

In our previous work, we have proposed a new method to estimate the effective Pt utilization or “effectiveness” (Ef_Pt) using the ratio of the mass activity (MA) for the oxygen reduction reaction in the membrane-electrode assembly (MEA) in the polymer electrolyte fuel cell to that in the channel flow double electrode measurement, MA_max, under similar conditions. In the present research, applying this method, we have focused on elucidating the effect of the thickness of the catalyst layer (CL), the effect of Pt-based alloy catalysts, and effect of the state of dispersion of the Pt/C catalysts in the CL in measurements carried out at 80 °C and various relative humidities (RH), in either O₂ or air. The effect of a thin CL (0.04 mg cm⁻², Pt/C) has improved Ef_Pt by a factor of four, going from 3% to 12%, and the integrated effect of a thin CL and alloying (0.05 mg cm⁻² Pt₃Co) has improved Ef_Pt by a factor of six, going from 3% to 17% for air at 0.85 V, T_cell = 80 °C, and 30% RH. Furthermore, we found that the Ef_Pt values were dependent upon the state of Pt dispersion in the CL. The highest Ef_Pt value obtained thus far for air at 0.85 V, T_cell = 80 °C, and 100% RH was ca. 22%, shown by a low Pt loading CL diluted with added uncatalyzed carbon black (0.04 mg cm⁻², overall average 30 wt%-Pt).     

Esterification of stearic acid with methanol over mesoporous ordered sulfated ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> mixed oxide aerogel catalyst

Aerogel sulfated ZrO₂–SiO₂ mixed oxide solid acid catalyst was prepared by sol–gel method followed by supercritical drying (SCD) in n-propanol solvent, which resulted into higher surface area (170 m²/g), pore volume (0.31 cm³/g) and pore diameter (7.2 nm) having ordered mesoporous structure as well as more number of Brönsted and Lewis acid sites available on larger surface area. The catalyst exhibited 91 % yield of methyl stearate at 60 °C in 7 h, which increased from 71 to 91 % with an increase in the Zr to Si ratio from 1:2 to 2:1 due to increase in acid site concentration. The reaction followed pseudo-first order kinetics under the optimized reaction conditions with a reaction rate of 1.15 mmol h^?1, rate constant of 2.7 × 10^?1 h^?1 and turn over frequency of 9.68 h^?1. The catalyst displayed higher activity (91 %) compared to ion exchange resins (44–68 %), Nafion (58 %), acid clay (61 %) and pure sulfated zirconia (78 %), and was slightly lower as compared to H₂SO₄ (97 %). The study clearly reveals the improved structural, textural and acidic properties of ZrO₂–SiO₂ mixed oxide aerogel prepared via SCD technique.