Ionomer Distribution Research Articles

Simulation of the catalyst layer in PEMFC based on a novel two-phase lattice model

A lattice model of catalyst layer in proton exchange membrane fuel cells (PEMFCs), consisting of randomly distributed catalyst phase (C phase) and mixed ionomer-pore phase (IP phase), was established by means of Monte Carlo method. Transport and electrochemical reactions in the model catalyst layer were calculated. The newly proposed C-IP model was compared with previously established pore-solid two phase model. The variation of oxygen level and reaction rate along the thickness of catalyst layer with cell current was discussed. The effect of ionomer distribution across catalyst layer was studied by comparing profiles of oxygen level, reaction rate and overpotential, as well as corresponding polarization curves.

Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

This work comprises an extensive coarse-grained molecular dynamics study of self-organization processes that define the mesoscopic structure of catalyst layers used in polymer electrolyte fuel cells. The detailed structural analysis focuses on agglomeration of Pt-decorated primary particles of graphitized carbon black, formation of ionomer domains, emergence of the porous network, and formation of interfaces between the distinct phases. Insights obtained enable us to decisively redraw the existing structural picture of the catalyst layer. As a key result, we found that ionomer forms a thin adhesive film, which partially covers agglomerates of Pt/carbon. Densely arranged charged side chains of ionomer form a highly ordered array on the ionomer film surface. The preferential orientation of these charged side chains depends on the surface wetting properties of the agglomerates. As a major consequence, results on ionomer structure and distribution, presented in this work, seem to invalidate the classical electrolyte-flooded agglomerate model that has been widely applied to catalyst layers in polymer electrolyte fuel cells. Instead, the structural analysis provided defines a need for novel models of proton transport, water distribution, and Pt effectiveness that account for the thin-film morphology of ionomer and the specific arrangement of surface groups.

Improved gas diffusion electrodes for hybrid polymer electrolyte fuel cells

In this study, the performance of the anionic electrodes for hybrid polymer electrolyte fuel cells was improved. The anion exchange membrane (AEM) electrodes were initially characterized as the cathode on a proton exchange membrane (PEM) anode/membrane half-assembly (i.e. hybrid polymer electrolyte fuel cell). The electrode performance was improved by tailoring the ionomer distribution within the electrode structure so as to better balance the electronic, ionic, and reactant transport within the catalyst layer. An ionomer impregnation method was used to achieve a non-uniform ionomer distribution and higher performance. Traditional electrode fabrication methods (i.e. thin-film method) lead to a uniform ionomer distribution. The peak power density at 70 °C for a H 2/O 2 hybrid fuel cell was 44 mW cm −2 using the thin-film electrode, and 120 mW cm −2 using the ionomer impregnated electrode. A hydrophobic additive used in the catalyst layer further improved the electrode performance, giving a peak power density of 315 mW cm −2 for H 2/O 2 at 70 °C. Electrochemical impedance spectroscopy was used as an in situ diagnostic tool to help understand the origin of the electrode improvements. The increase in performance was attributed to improved catalyst utilization due to the creation of facile gas transport domains in the AEM electrode structure. Similarly, the AEM anode prepared by ionomer impregnation with polytetrafluoroethylene resulted in a three-fold increase in the peak power density compared to ones made by the thin-film method, which has no polytetrafluoroethylene.

Complex Capacitance Analysis of Ionic Resistance and Interfacial Capacitance within Cathode Layers of PEMFC and DMFC Electrodes

For PEMFC and DMFC catalyst layers, a complex capacitance analysis of impedance data was developed to evaluate the catalyst/ionomer interfacial capacitance and ionic resistance of ionomer networks, without non-linear data fitting. Assuming no Faradaic reactions, equivalent circuits for the catalyst layers were suggested, and it was confirmed that the plots of the real and imaginary parts as a function of ac frequency are determined by the catalyst/ionomer interfacial capacitances and RC time constants, which are important characteristics for high fuel cell performances. By performing a complex capacitance analysis, the interfacial capacitances and ionic resistances of PEMFC MEAs can be experimentally obtained at various relative humidities, proving that the catalytic activity in fuel cell operation depends on ionic resistances as well as on the catalyst/ionomer interfacial area. In addition, the effects of various MEA preparation methods on the ionomer distributions and DMFC performances were analyzed by a complex capacitance analysis.

Complex Capacitance Analysis of Ionic Resistance and Interfacial Capacitance in PEMFC and DMFC Catalyst Layers

For polymer electrolyte membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) catalyst layers (CLs), a complex capacitance analysis of impedance data was developed to evaluate the catalyst/ionomer interfacial capacitance and ionic resistance of ionomer networks without nonlinear data fitting. First, assuming no faradaic reactions, equivalent circuits for the CLs were suggested, which are similar to electric double-layer capacitor systems with porous carbon electrodes. Then, with the simulated complex capacitances, it was confirmed that the plots of the real and imaginary parts as a function of ac frequency are determined by the catalyst/ionomer interfacial capacitances and the ionic resistances time constants, which are important characteristics for high fuel cell performances. Experimentally, the condition of no faradaic reactions was realized by supplying nitrogen or water to the cathodes instead of air and by fixing the dc potential at 0.4 V during the impedance measurements. By performing a complex capacitance analysis, the interfacial capacitances and ionic resistances of PEMFC membrane electrode assemblies (MEAs) can be obtained at various relative humidities, proving that the catalytic activity in fuel cell operation depends on ionic resistances as well as on the catalyst/ionomer interfacial area. The effects of various MEA preparation methods on the ionomer distributions and DMFC performances were analyzed by a complex capacitance analysis.

PEMFC electrode preparation by electrospray: Optimization of catalyst load and ionomer content

Electrodes for proton exchange membrane fuel cell (PEMFC) have been prepared by means of electrospray deposition, using different ionomer contents and catalyst loads. The electrodes have been mounted in single PEMFC, and tested as cathode. The electroactive platinum area of electrosprayed electrodes has been measured by the hydrogen underpotential deposition method. Polarization curves have been measured to determine the optimal concentration for the platinum catalyst (Pt/C, 20 wt.%) and the ionomer (Nafion ®). Best performance is observed with 15% ionomer content in the electrosprayed cathode due to minimal cell internal resistance. This optimal concentration is lower than that found for electrodes prepared by other standard methods, like airbrushing or impregnation, which is attributed to the improved ionomer distribution within the electrospray deposited catalyst layer. On the other hand, the optimum value for catalyst load is similar to that encountered for electrodes prepared by the other methods, which reflects that electrospray has little or no effect neither on the catalyst nor on the electronic resistance of the catalyst layer.

A Study of the Catalytic Interface for O2 Electroreduction on Pt: The Interaction between Carbon Support Meso/Microstructure and Ionomer (Nafion) Distribution

The catalytic activity of PEM fuel cell electrodes is determined by the complex physicochemical interactions among the components of the electrocatalytic interface: precious metal catalyst, support, and ionomer. In the present study, the effect of the carbon support meso- and microporosity was investigated in relation with both the Nafion content and distribution in the catalyst layer. The ionomer load was between 0.09 and 1.1 mg cm−2 in the catalyst layers prepared either by the Nafion-coated or Nafion-mixed techniques, while the Pt load was kept constant at 0.1 mg cm−2. Three supports were investigated: Vulcan XC-72R, Denka, and graphitized carbon (GC). Employing both the BET (Brunauer−Emmett−Teller) and the BJH (Barrett−Joyner−Halenda) surface measurement techniques, a complete characterization of the support and supported catalyst (Pt/C) pore volume distribution and surface area in the micro- and mesopore size ranges was carried out. It was found that Pt nanoparticles (mean diameter between 4.1−4.9 nm by XRD) reduced the micropore volume of the carbon supports. Therefore, the supports with high BJH mesoporous area (Vulcan XC-72R and GC) yielded Pt/C catalysts with the highest electrochemically active Pt area as well. For the Denka support, characterized by the lowest BJH area, the Nafion-coated procedure gave about 7% larger electrochemically active area compared to the Nafion-mixed method. Regarding the oxygen electroreduction, the effective oxygen permeability in the catalyst layer, the intrinsic kinetic current density, and the area and mass-specific activities at 0.9 VRHE were determined as a function of support type, Nafion incorporation method, and load.

3-d chemical imaging using angle-scan nanotomography in a soft X-ray scanning transmission X-ray microscope

Three-dimensional chemical mapping using angle scan nanotomography in a soft X-ray scanning transmission X-ray microscope (STXM) has been used to investigate the spatial distributions of a low density polyacrylate polyelectrolyte ionomer inside submicron sized polystyrene microspheres. Acquisition of tomograms at multiple photon energies provides true, quantifiable 3-d chemical sensitivity. Both pre-O 1s and C 1s results are shown. The study reveals aspects of the 3-d distribution of the polyelectrolyte that were inferred indirectly or had not been known prior to this study. The potential and challenges for extension of the technique to studies of other polymeric and to biological systems is discussed.

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 H 2/O 2 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.

Low Pt Thin Cathode Layer Catalyst Layer by Reactive Spray Deposition Technology

National Research Council Canada's Institute for Fuel Cell Innovation, NRC-IFCI, has been developing the Reactive Spray Deposition Technology (RSDT) process to optimize composite electrode layer formation and develop novel electrocatalysts and catalyst layers. The RSDT process provides the means necessary to develop the next generation of thin, low platinum or alloy catalyst layers for PEM MEA's. In order to best manage water distribution, mass transport and conductivity, the structure should be a gradient with controlled porosity and controlled distribution of both platinum and ionomer across the catalyst layer. The RSDT process allows good control of the platinum particle size as they are created directly from metal vapors, which prevents agglomeration in the catalyst layer. Additionally, it has the flexibility to build a gradient layer structure across a very thin film catalyst layer (<1 um). In our design, a platinum sub-layer (100 nm) was deposited directly on a Nafion® 117 membrane as a columnar structure. After the sub-layer, the platinum loading was reduced in a co-deposited layer of Pt-Nafion (ionomer)-carbon with the lowest loading closest to the GDL. The combined loading of both layers was <0.05mg/cm2 Pt with this approach. The manufactured catalyst layer has a performance of 0.65V at 1 A/cm2 with 0.05mg/cm2 Pt loading using pure oxygen .

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.

Preparation of Pt supported on mesoporous carbons for the reduction of oxygen in polymer electrolyte membrane fuel cell (PEMFC)

Porous carbon materials (SM-C, HS-C and TM-C) were prepared using commercial colloidal silicas (SM-30, HS-40 and TM-50) and a resorcinol-formaldehyde resin as a removable template and a carbon precursor, respectively. All of the prepared carbons had high surface areas with narrow pore size distributions. In particular, the pore diameter of the carbons could be controlled over a range of mesopore size by the use of an appropriate silica employed as a template. Mesoporous carbon templated using TM-50 had the largest pore size, while that for SM-C, was the smallest. Pt nanoparticles were supported on these mesoporous carbons for use as a catalyst in a polymer electrolyte membrane fuel cell (PEMFC). The crystallite size of the Pt catalyst was found to be closely related to the properties of the corresponding carbon support. A carbon support with a large pore size and a high surface roughness was found to favor the dispersion of Pt crystallite. In a single cell test, the Pt catalysts supported on mesoporous carbons exhibited higher cell performance than that on activated carbon. In particular, the Pt/TM-C catalyst showed the best cell performance among the catalysts tested. In addition to the high surface area of the active metal, the large pore size of the Pt/TM-C appears to have positive effect on the distribution of ionomer, resulting in facile formation of a triple-phase boundary.

Development of a packaging material using antistatic ionomer part 2: charge distributions of potassium ionomer

AbstractGenerally, plastics and plastic films are low in moisture absorption and high in electric insulation. They are inherently easy to be charged with static and can cause a variety of static troubles. We developed a functional packaging material to solve these static problems, by using potassium ionomer. We reported good antistatic performance (e.g. short static decay time, and excellent ash test) of potassium ionomer films in a previous paper. However, a mechanism underlying the antistatic property of potassium ionomer has not yet been fully elucidated. In this study, we measured the space charge distributions of potassium ionomer using the pulsed electro‐acoustic method. As a result of the space charge measurements, we found characteristic charge distribution of potassium ionomer film. On the basis of the existence of this characteristic charge distribution, we speculate that the space electric charge distribution of a potassium ionomer film under a direct current electric field shows apparent electric charge movement. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.

Novel Deposition of Pt∕C Nanocatalysts and Nafion® Solution on Carbon-Based Electrodes via Electrophoretic Process for PEM Fuel Cells

Nanosized platinum particles supported on carbon black carriers (Pt∕C) are popular for use in fabrication of proton exchange membrane fuel cells (PEMFCs). Here, an electrophoretic deposition (EPD) process is proposed to investigate the power performance of Pt∕C nanopowders onto various carbon-based electrodes for the PEMFC applications in a better controlled and cost-effective manner. Novel deposition of Pt∕C nanocatalysts and Nafion® solution via electrophoretic process give rise to higher deposition efficiency and a uniform distribution of catalyst and Nafion ionomer on the electrodes of PEMFCs. Preparation of an EPD suspension with good dispersivity is much desirable for an agreeable overall performance of catalyst coating in terms of types of organic solvents, milling processes, and use of pH adjusting agents and surfactants in the EPD suspension. The EPD suspension was prepared by sonication of mixture of Pt∕C nanopowders, Nafion solution and isopropyl alcohol, the optimal pH value of which was reached by using acetic acid or ammonium hydroxide. The colloidal stability of EPD suspension was achieved at pH ∼10 for an EPD suspension of either Pt∕C catalysts or mixture of Pt∕C catalysts and Nafion ionomer. A nicely distributed deposition of Pt∕C nanocatalysts and Nafion ionomer on both hydrophilic or hydrophobic carbon-based electrodes was successfully obtained by using Pt∕C concentration of 1.0g∕l, electrical field of 300V∕cm, and deposition time of 5min. Microstructural analysis results indicate that Pt∕C nanopowders not only embrace the entire surface of carbon fibers but also infiltrate into the gaps and voids in fiber bundles such that a higher contact area of the same loading of Pt∕C nanocatalysts through the EPD process is thus expected. At present, the EPD process can effectively save more of Pt catalyst loading on electrodes in PEMFC, as compared to conventional methods, such as screen printing, brushing, or spraying through the similar level of power performance for PEMFCs.

Preparation of mesoporous carbon templated by silica particles for use as a catalyst support in polymer electrolyte membrane fuel cells

Mesoporous carbons were prepared using commercial silica particles and a formaldehyde–resorcinol resin as a template and carbon precursor, respectively. By changing the molar ratio of template to carbon precursor, mesoporous carbons with different mesoporosities (MC- X, X represents the molar ratio of template to carbon precursor) were produced. The resulting MCs had a high-surface area and large pore volume. In particular, the highest mesoporosity was observed for MC-3. Pt catalysts-supported on MC- X were prepared using formaldehyde as a reducing agent for use as a cathode catalyst in a polymer electrolyte fuel cell (PEMFC). The size of Pt crystallite was dependent on the properties of corresponding carbon support. As a whole, a carbon support with a high-surface area and high-mesoporosity served the best in terms of a high-dispersion of Pt nanoparticles. In a unit cell test of the PEMFC, a Pt catalyst with a high-mesoporosity and fine dispersion of metal showed an enhanced performance. The findings indicate that the surface area combined with the mesoporosity had a positive influence on the metal dispersion and the distribution of ionomer, leading to the enhanced cell performance.

Fabrication of a thin catalyst layer using organic solvents

The effects of various organic solvents on the performance of polymer electrolyte membrane fuel cell (PEMFC) electrode were investigated. The five catalyst inks were prepared by mixing the 20 wt.% Pt/C, 5 wt.% solubilized Nafion, tetrabutylammonium hydroxide (TBAOH), and different organic solvents such as normal butyl acetate, iso-amyl alcohol, diethyl oxalate, ethylene glycol and ethylene glycol dimethyl ether. Membrane electrode assemblies were prepared by using a transfer method. From the measurements of performances of single cells, the most suitable solvent was selected and the results were discussed in terms of distribution of Nafion ionomer on Pt particles and robustness of the catalyst structure. In addition, reactant transport through electrodes was discussed in relation with decomposition of the solvents.