A Nafion-based self-humidifying membrane with ordered dispersed Pt layer

A double-layer Nafion-based membrane consisting of a pure Nafion layer and an ordered dispersed Pt particles layer was investigated. The Pt particles were dispersed under the anode graphite ribs, which provide the sites for the recombination of the permeating _H2${\mathrm{H}}_2$ and _O2${\mathrm{O}}_2$ into water. The electrochemical performances of the ordered Pt particles dispersed membrane in proton exchange membrane fuel cell (PEMFC) were studied and compared with those of the common Pt particles dispersed membrane and the pure Nafion membrane. The results indicate that the ordered Pt dispersed membrane reduces the amount of Pt dosage than the common Pt dispersed membrane and improves the performance of PEMFC operated under dry conditions than the pure Nafion membrane as well.     

Study of the influence of Nafion/C composition on electrochemical performance of PEM single cells with ultra-low platinum load

The electrochemical performance of membrane electrode assemblies (MEAs) with ultra-low platinum load (0.02 mg_Pt cm^?2) and different compositions of Nafion/C in the catalytic layer have been investigated. The electrodes were fabricated depositing the catalytic ink, prepared with commercial catalyst (HiSPEC 2000), onto the gas diffusion layers by wet powder spraying. The MEAs were electrochemically tested using current-voltage curves and electrochemical impedance spectroscopy measurements. The experiments were carried out at 70 °C in H₂/O₂ and H₂/air as reactant gases at 1 and 2 bar pressure and 100% of relative humidity. For all MEAs tested, power density increases when the gasses pressure is increased from 1 to 2 bar. On the other hand, power density also increased when oxygen is used instead of air as oxidant gas in cathode. The lower power density (34 mW cm^?2) and power per Pt loading (0.86 kW g_Pt^?1) corresponds to the MEA prepared without Nafion in anode and cathode catalytic layers working with hydrogen and air at 1 bar pressure as reactants gas. The MEA with 30% wt Nafion/C reached the highest power density (422 mW cm^?2) and power per Pt loading (10.60 kW g_Pt^?1) using hydrogen and oxygen at 2 bar pressure. Finally, electrode surface microstructure and cross sections of MEAs were analyzed by Scanning Electron Microscopy (SEM). Examination of the electrodes, revealed that the most uniform ionomer network surface corresponds to the electrode with 40 wt% Nafion/C, and MEA ionomer-free catalytic layer shows delamination, it leads to low electrochemical performance.     

Effect of loading and distributions of Nafion ionomer in the catalyst layer for PEMFCs

The electrode with various contents of Nafion ionomer for inside and/or on the surface in the catalyst layer, respectively, was designed for proton exchange membrane fuel cell (PEMFC) electrode to investigate the effect of Nafion ionomer distribution in the catalyst layer on cell performance and improve electrode performance. The effect of Nafion ionomer on the electrode of each design was judged by a cyclic voltammetry measurement and the cell performance obtained through a single cell test using H₂/O₂ gases. Electrodes with different ionomer distributions for inside and on the surface in the catalyst layer, respectively, were examined. It is found that the electrode where the Nafion ionomer is impregnated on the surface of catalyst layer shows better cell performance than that where the Nafion ionomer is incorporated in the inside of catalyst layer. The best cell performance among the catalyst layers tested in this study was obtained for the electrode with 0.5 mg cm⁻² of Nafion ionomer inside the catalyst layer and 1.0 mg cm⁻² of Nafion ionomer on the surface of the catalyst layer together.     

Effect and siting of Nafion in a Pt/C proton exchange membrane fuel cell catalyst

This paper explores the effect and siting (location) of Nafion on Pt/C as exists in a PEM fuel cell catalyst layer. The addition of 30 wt% Nafion on Pt/C (Nfn-Pt/C) resulted in a severe loss of BET surface area by filling/blocking the smaller pore structures in the carbon support. Surprisingly, the presence of this much Nafion appeared to have only a minimal effect on the adsorption capability of either hydrogen or CO on Pt. However, the presence of Nafion doubled the amount of time required to purge most of the gas-phase and weakly-adsorbed hydrogen molecules away from the catalyst during hydrogen surface concentration measurements. This strongly chemisorbed surface hydrogen was determined by a H₂/D₂ switch and exchange procedure. Nafion had an even more pronounced effect on the reaction of a larger molecule like cyclopropane. Results from the modeling of cyclopropane hydrogenolysis in an idealized pores suggest that partial blockage of only the pore openings by the Nafion for the meso-macropores is sufficient to induce diffusion limitations on the reaction. The facts suggest that most of the Pt particles are in the meso-macropores of the C support, whereas Nafion is present primarily on the external surface of the C where it blocks significantly the micropores but only partially the meso-macropores.     

A novel proton exchange membrane fuel cell anode for enhancing CO tolerance

A novel proton exchange membrane fuel cell (PEMFC) anode which can facilitate the CO oxidation by air bleeding and reduce the direct combustion of hydrogen with oxygen within the electrode is described. This novel anode consists of placing Pt or Au particles in the diffusion layer which is called Pt- or Au-refined diffusion layer. Thus, the chemical oxidation of CO occurs at Pt or Au particles before it reaches the electrochemical catalyst layer when trace amount of oxygen is injected into the anode. All membrane electrode assemblies (MEAs) composed of Pt- or Au-refined diffusion layer do perform better than the traditionary MEA when 100 ppm CO/H₂ and 2% air are fed and have the performance as excellent as the traditionary MEA with neat hydrogen. Furthermore, CO tolerance of the MEAs composed of Au-refined diffusion layer was also assessed without oxygen injection. When 100 ppm CO/H₂ is fed, MEAs composed of Au-refined diffusion layer have the slightly better performance than traditionary MEA do because Au particles in the diffusion layer have activity in the water gas shift (WGS) reaction at low temperature.     

Accelerated durability tests of catalyst layers with various pore volume for catalyst coated membranes applied in PEM fuel cells

The effect of pore volume on the catalyst layer durability of PEM fuel cell was simulated by soaking the catalyst coated membrane (CCM) into H₂O₂/Fe²⁺ solution. Before this simulation, the CCM with various pore volumes in catalyst layer was fabricated. The structure of catalyst layers was optimized with an increase in pore volume, leading to an improvement of fuel cell performance. However, this treatment causes a negative effect on the lifetime of CCM especially when H₂O₂/Fe²⁺ introduced. As a result, the catalyst layer with high pore volume has a higher detaching rate than that with low pore volume. The detaching of catalyst layers could be attributed to degradation of both the recast Nafion in catalyst layers and the Nafion membrane. The catalyst layer with high pore volume accelerates the recast Nafion degradation. Thus, the durability of membrane electrode assembly should be considered when the catalyst layer is optimized.     

Solving Nafion poisoning of ORR catalysts with an accessible layer: designing a nanostructured core-shell Pt/C catalyst via a one-step self-assembly for PEMFC

A core-shell Pt/C@NCL300 catalyst with an accessible layer was designed to recover lost ORR activity and was constructed via a one-step self-assembly process in this paper. A thin porous layer derived from Nafion was first formed on the surface of Pt/C catalyst to create a shell. This first coating successfully separated the Nafion and Pt particles in the catalysts and reducing the negative impact of Nafion on ORR activity and enhancing the fuel cell performance. The newly fabricated Pt/C@NCL300 catalyst exhibited much higher specific activity than the original Pt/C catalyst in RDE tests under the same conditions and were comparable to the activity of Pt/C electrode without Nafion poisoning. Moreover, the fuel cell with Pt/C@NCL300 catalyst exhibited a higher power density without an obvious increase in proton transport and O₂ transport resistance compared to that of a Pt/C fuel cell with a low Pt loading. This result indicates that coating the Pt/C catalyst with a layer accessible for oxygen and protons is a promising way to effectively promote Pt-based catalysts that work under normal operating conditions.     

Study of Pt electrode dissolution in H2O2-containing H2SO4 solution using an electrochemical quartz crystal microbalance

Pt electrode dissolution has been investigated using an electrochemical quartz crystal microbalance (EQCM) in H₂O₂-containing 0.5 mol dm⁻³ H₂SO₄. The Pt electrode weight-loss of ca. 0.4 μg cm⁻² is observed during nine potential sweeps between 0.01 and 1.36 V vs. RHE. In contrast, the Pt electrode weight-loss is negligible without H₂O₂ (<0.05 μg cm⁻²). To support the EQCM results, the weight-decrease amounts of a Pt disk electrode and amounts of Pt dissolved in the solutions were measured after similar successive potential cycles. As a result, these results agreed well with the EQCM results. Furthermore, the H₂O₂ concentration dependence of the Pt weight-decrease rate was assessed by successive potential steps. These EQCM data indicated that the increase in H₂O₂ accelerates the Pt dissolution. Based on these results, H₂O₂ is known to be a major factor contributing to the Pt dissolution.     

Kinetic effect of the ionomer on the oxygen reduction in carbon-supported Pt electrocatalysts

The mechanism of the oxygen reduction reaction (ORR) on nanoparticulated Pt/C-Nafion electrodes prepared in one step has been studied to simulate the reaction in the cathode of a Polymer Electrolyte Fuel Cell (PEFC). The kinetic parameters have been obtained by hydrodynamic polarization in O₂-saturated 0.01–1.00 M H₂SO₄ and temperatures in the range 25.0–50.0 °C. The ORR current density was maximum and practically independent of the ionomer fraction in the rage 10–55 wt% Nafion. The poorer proton conductivity for lower Nafion fractions and the formation of catalyst areas completely surrounded by Nafion together with adsorption of Pt sites by sulfonate groups for higher Nafion fractions, explain the minor ORR activity in these conditions. The ionomer influence on the O₂ diffusion at high overpotentials for Pt/C-Nafion was negligible when the Nafion content was smaller than 20 wt%. The higher kinetic current density for Pt/C-Nafion (100 mA cm⁻²) with respect to smooth Pt-Nafion (40 mA cm⁻²), together with the smaller activation energy of the former (25 ± 4 kJ mol⁻¹) with respect to the latter (42 ± 5 kJ mol⁻¹) highlighted the better properties attained by the nanosize effect. A remarkable novel result is that the reaction order of H⁺ in HClO₄ is close to unity, whereas in sulfuric acid it is significantly smaller and changes with potential, what has been related to the sulfate adsorption. The anomalous dependence of the charge transfer coefficient with temperature was then explained by the thermal change of the double layer structure and the variation of the coverage of adsorbed species on Pt. The more sensitive effect for Pt/C-Nafion than for smooth Pt-Nafion was ascribed to the stronger interaction between the components when the nanoparticles are involved.     

Pt/CNTs-Nafion reinforced and self-humidifying composite membrane for PEMFC applications

A novel thin three-layer reinforced and self-humidifying composite membrane has been developed for PEMFCs. The membrane has two outer layers of plain Nafion and a middle layer of Pt/carbon nanotubes (Pt/CNTs) dispersed Nafion. The Pt/CNTs present in the membrane provides the sites for the catalytic recombination of H₂ and O₂ permeating through the membrane from the anode and cathode to produce water and improve the mechanical properties of the composite membrane at the same time. The water produced directly humidifies the membrane and allows the operation of PEMFCs with dry reactants. The electrochemical performance and mechanical properties of the composite membranes are compared with those of a commercial Nafion^® membrane. The self-humidifying composite membrane could minimize membrane conductivity loss under dry conditions and improve mechanical strength due to the presence of the Pt/CNTs.     

Optimum compositions of membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell

We investigated the effects of the compositions of catalyst layers and diffusion layers on performances of the membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell. The performances of the MEAs with different thicknesses of Nafion membranes were compared in this work. The optimal compositions in the anode are: 20 wt% Nafion content and 3.6 mg cm⁻² Pt loading in the catalyst layer, and 30 wt% PTFE content and 1 mg cm⁻² carbon black loading in the diffusion layer. In the cathode, MEA with 20 wt% Nafion content in the catalyst layer and 30 wt% PTFE content in the diffusion layer presented the optimal performance. The MEA with Nafion 115 membrane displayed the highest maximum power density of 46 mW cm⁻² among the three MEAs with different Nafion membranes. Copyright © 2009 John Wiley & Sons, Ltd.     

Improved PEM fuel cell performance with hydrophobic catalyst layers

Flooding of catalyst layers is one of the major issues, which effects performance of low temperature proton exchange membrane fuel cells (PEMFC). Rendering catalyst layers hydrophobic one may improve the performance of PEMFC depending on Pt percentage in the catalyst and Polytetrafluoroethylene (PTFE) loading on the electrode. In this study, effect of hydrophobicity in catalyst layers on performance has been investigated by comparing performances of membrane electrode assemblies prepared with 48% Pt/C. Ultrasonic coating technique was used to manufacture highly efficient electrodes. Power density at 0.45 V increased by the addition of PTFE, from 0.95 to 1.01 W/cm² with H₂/O₂ feed; while it slightly increased from 0.52 W/cm² to 0.53 W/cm² with H₂/Air feed. Addition of PTFE to catalyst layers while keeping Pt loading constant, enhanced performance providing improved water management. Kinetic activity increased by decreasing Nafion loading from 0.37 mg/cm² to 0.25 mg/cm² while introducing PTFE (0.12 mg/cm²) to the electrode. Electrochemical impedance spectroscopy (EIS) results proved that charge transfer resistance decreased with hydrophobic catalyst layers for H₂/O₂ feed. This is attributed to enhanced water management due to PTFE presence.     

Control of self-organization of drop-casted Nafion film for improving proton conduction in a polymer-electrolyte-membrane fuel cell to raise its output power density

A simple drop-cast method to directly deposit Nafion polymer electrolyte membrane (PEM) on nanostructured thin-film catalyst layer composed of stacked Pt nanoparticles prepared by pulsed laser deposition (PLD) was demonstrated. Through optimization of solvent composition and drying temperature of Nafion solution to control self-organization of Nafion, a uniform PEM with better bulk and interface microstructures could be produced, leading to a significant improvement in the output current density of a PEM fuel cell over that using reference commercial PEMs. The formation of facile proton conduction pathways in the bulk Nafion membrane resulted in a 35% reduction in ohmic resistance compared to that with the commercial membrane. Moreover, the infiltration of Nafion in the catalyst layer formed suitable proton transport network to render more catalyst nanoparticles effective and thus lower charge-transfer resistance. With the optimized PLD, drop-cast, and hot-pressing conditions, the current density of PEMFCs using drop-casted PEM reached 1902 mA cm⁻² at 0.6 V at 2 atm H₂ and O₂ pressures with a cathode Pt loading of 100 μg cm⁻², corresponding to a power density of 1.14 W cm⁻² and a cathode mass-specific power density of 11.4 kW g⁻¹.     

Direct ESR and spin trapping methods for the detection and identification of radical fragments in Nafion membranes and model compounds exposed to oxygen radicals

Direct electron spin resonance (ESR) and spin trapping methods are used in our laboratory in order to identify radical fragments in fluorinated membranes (Nafion) used in fuel cells and in model compounds when exposed to reactive oxygen species. Oxygen radicals were generated by the Fenton reaction, by the photo-Fenton method, and by UV-irradiation of aqueous H₂O₂ solutions. Direct ESR detection led to the identification of fluorinated radical fragments in UV-irradiated Nafion neutralized by Cu(II), Fe(II), and Fe(III) cations and exposed to H₂O₂. In Nafion exposed to the Fenton reagent in the presence of DMPO (5,5-dimethyl-1-pyrroline) as the spin trap, radical adducts of hydroxyl and carbon-centered radicals (CCRs) were detected. A fluorinated model compound, perfluro-(2-ethoxyethane) sulfonic acid (CF₃CF₂OCF₂CF₂SO₃H, PFEESA), that mimics the side chain of Nafion was chosen for study, with the goal to shed light on the more complicated degradation processes in the polymeric membranes. Analysis of the spin adducts obtained from UV-irradiated PFEESA in the presence of H₂O₂ and MNP (2-methyl-2-nitrosopropane) as the spin trap suggested that the side chain of Nafion is a possible site of attack by oxygen radicals.     

Electrochemical degradation of Nafion ionomer to functionalize carbon support for methanol electro-oxidation

An effective electrochemical route to produce functional groups on carbon surface is demonstrated. Cyclic voltammetric (CV) sweeps are performed in 0.5 M H₂SO₄ electrolyte on electrodes containing carbon cloth, Vulcan XC72R, and Nafion ionomer. With supply of ambient oxygen, the generation of hydroxyl radicals from the oxygen reduction reaction during CV cycles initiates the decomposition of Nafion ionomer that leads to formation of oxygenated functional groups on the carbon surface. Ion chromatography confirms the dissolution of sulfate anions upon CV scans. Raman analysis suggests a minor alteration for the carbon structure. However, X-ray photoelectron spectroscopy indicates a significant increase of oxygenated functional groups in conjunction with notable reduction in the fluorine content. The amount of the oxygenated functional groups is determined by curve fitting of C 1s spectra with known constituents. These functional groups can also be found by immersing the as-prepared electrode in a solution containing concentrated residues from Nafion ionomer decomposition. The functionalized electrode allows a 170% increment of Pt ion adsorption as compared to the reference sample. After electrochemical reductions, the functionalized electrode reveals significant improvements in electrocatalytic abilities for methanol oxidation, which is attributed to the oxygenated functional groups that facilitates the oxidation of CO on Pt.     

Impact of ultra-low Pt loadings on the performance of anode/cathode in a proton-exchange membrane fuel cell

This study focuses on the elaboration of PEMFC electrodes containing ultra-low platinum (Pt) loadings by direct liquid injection metal organic chemical vapor deposition (DLI-MOCVD). DLI-MOCVD offers a large number of advantages for the elaboration of model PEMFC electrodes. First, by using different metal precursors or elaboration temperature, the size of the Pt nanoparticles and thus the intrinsic catalytic activity can easily be tailored in the nanometer range. In this work, Pt nanoparticles (1-5 nm) with remarkable low degree of agglomeration and uniform distribution were deposited onto the microporous side of a commercial gas-diffusion layer (GDL). Second, reduction of the Pt loading is made possible by varying the Pt deposition time and its influence of the cell performance can be extracted without variation of the thickness of the catalytic layer (in previous studies, a decrease of the catalyst utilization was observed when increasing the Pt loading, i.e. the thickness of the catalytic layer (CL)). The electrocatalytic activity of home-made Pt nanoparticles elaborated by DLI-MOCVD was measured in liquid electrolyte or in complete fuel cell operating on H₂/O₂ or H₂/air and compared vs. that of a commercially available electrode containing 500 μg_Pt cm⁻² (Pt_Ref500). At the cathode, the performance of the electrodes containing 104-226 μg of Pt per cm² of electrode compares favorably with that of the Pt_Ref500 in H₂/O₂ conditions. In H₂/air conditions, additional mass-transport losses are detected in the low-current density region but the high effectiveness of our electrodes improves the performance in the high-current density region. At the anode, the Pt loading can be reduced to 35 μg_Pt cm⁻² without any voltage loss in agreement with previous observations.     

Electrochemical preparation of a novel,effective and low cast catalytic surface for hydrogen evolution reaction

The formation of platinum nucleus on the freshly polished aluminum (Al) and anodized aluminum electrodes (Al₂O₃/Al) was studied by cyclic voltammetry. Results showed that the deposition of platinum on freshly polished aluminum from an aqueous 0.5 M phosphate buffer solution containing H₂PtCl₆ takes place rapidly through the electroreduction of dissolved Pt (IV) ions. At shorter deposition times, small particles of platinum crystals were formed on the aluminum and the surface coverage was imperfect. At longer deposition times, the size of the platinum crystals increases while their number decreases due to the coalescence and agglomeration processes. The electrodeposition of Pt on the Al electrode was conveniently carried out over the Al₂O₃/Al electrode. The electrochemical and catalytic activities of the Pt/Al and Pt/Al₂O₃/Al electrodes were studied in 0.1 M H₂SO₄ solution. In cyclic voltammetry, the two pair symmetric peaks appeared in 0.1 M H₂SO₄ solution which was attributed to the formation of strongly (H_s) and weakly bounded hydrogen (H_w). The occurrence of the third anodic hydrogen peak (H_3rd) was revealed at low scan rate and in high concentration of H₂SO₄. At potentials more negative than −0.3 V vs. SCE, the current is mainly due to hydrogen evolution reaction. The influence of the various parameters such as deposition method and amount of platinum, sulfuric acid concentration and medium temperature on the hydrogen evolution reaction is described. Finally the kinetic of the hydrogen evolution reaction is also discussed on the Pt/Al electrode.     

Effect of H2O2 on Pt electrode dissolution in H2SO4 solution based on electrochemical quartz crystal microbalance study

The effect of H₂O₂ on the Pt dissolution in 0.5 mol dm⁻³ H₂SO₄ was investigated using an electrochemical quartz crystal microbalance (EQCM). For the potential cycling at 50 mV s⁻¹, the Pt weight irreversibly decreases in a N₂ atmosphere with H₂O₂, while only a negligible Pt weight-loss is observed in the N₂ and O₂ atmospheres without H₂O₂. The EQCM data measured by the potential step showed that the Pt dissolution in the presence of H₂O₂ depends on the electrode potential and the H₂O₂ concentration. For the stationary electrolysis, the Pt dissolution occurs at 0.61–1.06 and 1.06–1.36 V vs. RHE. It should be noted that the Pt dissolution phenomenon in the presence of H₂O₂ is also affected by the potential scanning time. Based on these results, H₂O₂ is considered not only to contribute to the formation of Pt-oxide causing the cathodic Pt dissolution, but also to participate in the anodic Pt dissolution and the chemical Pt dissolution.     

Hydrogen crossover in high-temperature PEM fuel cells

In this paper, hydrogen crossover was measured in an environment of high-temperature proton exchange membrane (PEM) fuel cells using a steady-state electrochemical method at various temperatures (T) (80–120 °C), backpressures (P) (1.0–3.0 atm), and relative humidities (RH) (25–100%). An H₂ crossover model based on an MEA consisting of five layers – anode gas diffusion layer, anode catalyst layer, proton exchange membrane (Nafion 112 or Nafion 117), cathode catalyst layer, and cathode gas diffusion layer – was constructed to obtain an expression for H₂ permeability coefficients as a function of measured H₂ crossover rates and controlled H₂ partial pressures. The model analysis suggests that the dominant factor in the overall H₂ crossover is the step of H₂ diffusing through the PEM. The H₂ permeability coefficients as a function of T, P, and RH obtained in this study show that the increases in both T and P could increase the H₂ permeability coefficient at any given RH. However, the effect of RH on the permeability coefficient seems to be more complicated. The T effect is much larger than that of P and RH. Through experimental data simulation an equation was obtained to describe the T dependencies of the H₂ permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H₂ crossover through both Nafion 112 and 117 membranes were also evaluated. Both Nafion 112 and Nafion 117 showed similar values of such parameters, suggesting that membrane thickness does not play a significant role in the H₂ crossover mechanism.     

Kinetic study of Nafion degradation by Fenton reaction

To predict the durability of polymer electrolyte membranes in fuel cells, the degradation reactions of Nafion 117 films were studied as oxidation reactions with hydroxyl radicals as oxidation accelerators. The radical species were generated by the Fenton reaction between hydrogen peroxide (H₂O₂) and iron ions (Fe²⁺). The Nafion degradation kinetics were estimated by fluorine ion (F⁻) generation. The H₂O₂ and Nafion degradation reactions fit a pseudo-first-order rate constant. The values of the activation energy and frequency factor are 85 kJ mol⁻¹ and 3.97 × 10⁸ s⁻¹ for H₂O₂ decomposition in the presence of a Nafion film and 97 kJ mol⁻¹ and 9.88 × 10⁸ s⁻¹ for F⁻ generation. The Nafion surface morphology became rough after reaction for 12 h; small cracks, approximately 100 μm in length, were observed at temperatures below 60 °C. These cracks connected to make larger gaps of approximately 1 mm at temperatures above 70 °C. We also found a linear relationship between H₂O₂ consumption and F⁻ generation. The rate constant is temperature dependent and expressed as ln(d[F⁻]/d[decomposed H₂O₂]) = −19.5 × 10³ K⁻¹ + 42.8. F⁻ generated and H₂O₂ consumed along with the Nafion degradation conditions can be predicted using this relation.