Fuel Cell Catalyst Layers Research Articles

Physical Modeling of the Proton Density in Nanopores of PEM Fuel Cell Catalyst Layers

In polymer electrolyte fuel cells, a foremost goal is to design catalyst layers with high performance at markedly reduced platinum loading. As a contribution towards this objective, we explore a simplified pore geometry to capture the impact of ionomer structure and metal charging properties on the proton density distribution and conductivity in relevant nanopores. The basic model is a cylindrical tubular pore confined by an ionomer shell and a solid platinum-coated core. The gap region between metal and ionomer is filled with water. We study how the surface charge density at the ionomer and the metal charging relation as well as geometric pore parameters affect the electrochemical performance. The density of charged side chains at the ionomer shell exerts a pronounced impact on the surface charge density at the Pt surface and thereby on the activity of the pore for the oxygen reduction reaction. The key parameter controlling the interplay of surface and bulk charging phenomena is the overlap of the Debye lengths of ionomer and metal surfaces in relation to the width of the gap. It allows distinguishing regions with weak and strong correlation between surface charge densities at ionomer shell and Pt core.

Development of Ordered Mesoporous Carbon Supported Fuel Cell Cathode Catalysts for Improved O2 Transport

Current state of the art PEMFC cathode catalyst layer can be described as a complex, heterogeneous porous system with multiple components across various length scales at which both the electrochemical reaction and mass transport of reactants/products occur. The functional components in the catalyst layer comprises of the catalyst nanoparticles (few nm in diameter) for electrochemical reaction, carbon support (few tens of nm to µm) for electron conduction, ionomer chains (few nm to µm) for proton transport, and pore structure (few nm to few hundred nm) for gas transport. Mass transport of reactants and products are supported by the intrinsic pore structure in the carbon support and the secondary pore structure induced by the ionomer chains in the catalyst ink formulation stage. [1] There is a strong impetus to decrease the catalyst loading to minimize Pt material cost and also, ease the supply-demand scenario. While extensive research into the development of Pt-alloy nanoparticle catalysts has led to improvements in the low current density kinetic region, the current state of the art cathode electrodes suffer from a poor high current density (HCD) performance at low Pt loadings which leads to an increase in stack area and cost. This brings into focus the need to fundamentally understand the functional role and microstructure-property relationships of the various components in the cathode catalyst layer towards mitigating the HCD mass transport losses at low PGM loadings. [2] Recent studies have pointed out to the presence of a local transport resistance in the cathode catalyst layer that affects HCD performance via i) decrease in O2 permeability through the ionomer thin film covering the Pt/C agglomerates, and/or ii) O2 transport within the Pt/C agglomerates. The latter resistance arises due to the necessity of O2to be transported within the tortuous pore structure in the Pt/C agglomerates to reach the active site. [2, 3] Perhaps, the most complicating aspect of the catalyst layer microstructure arises from i) the complex pore structure of the carbon support upon which the Pt-alloy nanoparticles are deposited, and ii) the extensive aggregation of both the porous carbon support & the colloidal ionomer particles. A well optimized cathode catalyst layer could be envisioned as one where the ionomer is distributed around the Pt/C agglomerates with an optimal coverage in such a fashion that there is no direct Pt-ionomer interface formation (to minimize sulfate poisoning), but with Pt nanoparticles present at vantage locations within the agglomerates that are more open, less tortuous and easily accessible by O2. Such a structure would essentially maximize the mass activity in the kinetic regime and minimize the local-O2transport resistance in the mass transport regime. Current state-of-the-art carbon supports for cathode catalyst applications involve moderate to high surface area materials that feature either a disordered-porous (e.g. Ketjen Black) or non-porous (e.g. Vulcan) structure. While the porous carbon supports host the Pt nanoparticles within its micro-meso pores thereby preventing it from coming into direct contact with the sulfate functional groups in the ionomer, the disorder in these pores leads to high tortuous pathways for O2transport that affects the HCD performance. Contrarily, the non-porous carbon supports host Pt nanoparticles on the surface in direct contact with the ionomer leading to a poor low mass activity. In this study, we demonstrate the use of ordered mesoporous carbon (OMC) supports for cathode catalyst applications to mitigate the HCD performance losses. As shown in Figure 1, OMC support features a unique pore structure with the majority of surface area in the mesoporous region along with the presence well-ordered, open, continuous pores for reactant transport. [4] High mass activity is achieved since some or all of the Pt nanoparticles are deposited inside the pores so that there is no direct contact with the ionomer but at the same time the pores have an open, continuous and ordered structure that enables better O2 transport at high current densities. The merits and challenges that arise from the use of OMC supports for cathode catalyst applications will be discussed. References 1) S. Holdcroft, Fuel Cell Catalyst Layers: A Polymer Science Perspective, Chem. Mater. 2014, 26 (1), pp 381-393 2) A. Kongkanand, M. F. Mathias, The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells, J. Phys. Chem. Lett., 2016, 7 (7), pp 1127–1137 3) S. Jomori et al., An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC, J. Electrochem. Soc. 2013, 160 (9), pp F1067-F1073 4) R. Ryoo et al., Ordered Mesoporous Carbon, Adv. Mater. 2001, 13 (9), pp 677-681 Figure 1

A multiscale approach to accelerate pore-scale simulation of porous electrodes

Abstract A new method to accelerate pore-scale simulation of porous electrodes is presented. The method combines the macroscopic approach with pore-scale simulation by decomposing a physical quantity into macroscopic and local variations. The multiscale method is applied to the potential equation in pore-scale simulation of a Proton Exchange Membrane Fuel Cell (PEMFC) catalyst layer, and validated with the conventional approach for pore-scale simulation. Results show that the multiscale scheme substantially reduces the computational cost without sacrificing accuracy.

Measurement of protonic resistance of catalyst layers as a tool for degradation monitoring

The ionic resistance of a PEM fuel cell catalyst layer, as well as the double layer capacity and the high frequency resistance are estimated from the impedance spectra of H2/N2 operated cell, thanks to a volumetric electrode model. The impedance spectra are calculated in the case of a homogeneous distribution of resistivity and capacity, and considering both through plane and in plane heterogeneities.Then, results of the degradation of a MEA subjected to four different accelerated stress tests are presented. They consist of (i) a constant current operation, (ii) 1.2 V potential hold with nitrogen and hydrogen (iii) open circuit voltage (OCV) hold with air and hydrogen, and (iv) a start-up protocol where heterogeneous degradations are observed using a linear segmented fuel cell. This technique can provide useful information about the evolution of catalyst layer microstructure during AST.

423 Study on high efficiency utilization of Pt in PEM fuel cell catalyst layer

423 Study on high efficiency utilization of Pt in PEM fuel cell catalyst layer

Linking Foreign Cationic Contamination of PEM Fuel Cells to the Local Water Distribution

Cationic contamination of polymer electrolyte fuel cells is known to cause performance degradation, particularly due to displacement of protons by foreign cations in the ionomer. Recent findings however, show that mass transport is a critical mechanism in the contamination process. X-ray computed tomography is used to examine the salt precipitation within the cathode diffusion media following in situ cation contamination experiments. Statistical analysis of the tomographic data is performed to examine the salt distribution inside a contaminated dual layer carbon paper substrate (GDL)-MPL. Results confirm the presence of salts on the surface, which is visually observable, but also show subsurface salts that are present up to the GDL-MPL interface. This new finding shows that cationic salts can precipitate in the macroporous carbon paper substrate, and the hydrophobic microporous MPL acts as a barrier to restrict cationic transport from reaching the fuel cell catalyst layer and membrane.

Engineering the Ionic Polymer Phase Surface Properties of a PEM Fuel Cell Catalyst Layer

During high power density operations, the performance of polymer electrolyte membrane fuel cells (PEMFCs) may be limited by high water saturation levels in the cathode catalyst layer due to high wettability of the ionic polymer phase. A new heat-treatment method was used to create and lock-in the surface structure of Nafion 212. Several surface characterization techniques were used to verify the membrane's surface after heat-treatment, including contact angle, atomic force microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. We found that specific heat-treatment conditions led to the formation of either a hydrophobic or hydrophilic surface. The modified membrane's surface remained intact even after the membranes were boiled in water for 1 h. Next, a 4-point conductivity technique was used to verify that the heat-treatment conditions which led to a hydrophobic surface did not negatively impact the membrane's internal conductivity. Finally, this novel heat-treatment method was applied to the cathode catalyst layer of a H2-Air PEMFC to create a hydrophobic polymer-gas interface inside the gas pores of the cathode catalyst layer. Preliminary results showed 33% increase in peak power. The results of this research will guide the design of a new class of PEMFC catalyst layers.

Temperature dependent performance and catalyst layer properties of PtRu supported on modified few-walled carbon nanotubes for the alkaline direct ethanol fuel cell

The performance of PtRu on three differently modified few-walled carbon nanotube (FWCNT) supports for ethanol electro-oxidation is evaluated in alkaline media both with rotating disc electrode (RDE) and direct ethanol fuel cell (DEFC) measurements at various temperatures (0–60°C). FWCNT are modified with oxidative treatment (O-FWCNT), aniline coating (A-FWCNT) and N-doped carbon layer (N-FWCNT). RDE testing shows that A-FWCNT/PtRu outperforms both O-FWCNT/PtRu and N-FWCNT/PtRu especially at high temperatures giving 1.5 times higher current at 60°C. The poisoning resistance of N-FWCNT/PtRu is high over the temperature range, while O-FWCNT/PtRu and A-FWCNT/PtRu become increasingly poisoned with increasing temperature. Alkaline DEFC testing at 30°C and 50°C indicates similar dependence to temperature as in RDE tests. However, only N-FWCNT/PtRu can sustain currents for longer than 20–30h during constant voltage measurement. SEM images of the catalyst layers reveal that both O-FWCNT/PtRu and A-FWCNT/PtRu form a dense structure with little pores for reactant and product transport explaining the quick performance loss, while large pores are formed with N-FWCNT/PtRu facilitating the transport. These results underline that the interactions between the catalyst support and the ionomer in the fuel cell catalyst layer are important in forming a suitable pore structure for efficient mass transfer.

Controlling Nanoparticle Interconnectivity in Thin-Film Platinum Catalyst Layers

The optimization of conventional hydrogen fuel cell catalyst layers suffers from a poor understanding of their composite nanostructure during both initial preparation and its evolution during use. We demonstrate how highly active, ultralow loading platinum (Pt) catalyst layers can be fabricated in a single, solution-processable step using electroless deposition. Growing Pt nanoparticles directly in the surface of a polyelectrolyte Nafion membrane yields a mechanically robust film with tunable optical reflectance and electronic conductivity. Small changes in the polymer hydration and Pt film thickness critically modulate nanoparticle interconnectivity near the percolation threshold. Conductive atomic force microscopy (AFM) and electron microscopy reveal how the film’s dynamic nanoscale morphology allows control over bulk electrochemical and optical properties. Well-defined composition and structure make these layers an experimentally accessible model system for studying thin-film electrocatalyst architectures.

(Invited) Tomographic Analysis of Fuel Cell Catalyst Layers - Methods, Challenges and Validity

The catalyst layer (CLs) are the central part of Polymer Electrolyte Fuel Cells. Consisting of catalyst particles, ionomer, carbon black plus numerous gases and liquid water during operation, the CL contains not less than five different phases with a plethora of mutual physical interactions. Further, all these phases have characteristic length scales in the nanometer range. Due to its nanoscale, multi-phase nature the CL bears complex physical phenomena that are very challenging to describe and understand. The precise morphology of all phases within a CL plays a key role for reactant transport and thus performance: Platinum catalyst particles with poor connection to ion conducting, electron conducting or gas transporting phases cannot contribute to the fuel cell reaction. Also, the precise shape of the pathways of the reaction species determines their effective transport properties. Wetting or non-wetting pore wall areas influence water precipitation and therefore the generation of liquid water networks. Such networks both improve ionic transport and adulterate gas transport. Facing the tremendous influence morphology has on PEMFC performance, cost and durability, tools to image morphology are an obvious necessity. While there is a variety of imaging methods that allow three-dimensional reconstructions, only few are able to resolve the nanostructure of CLs: x-ray tomography (Xt), focused ion beam / scanning electron microscopy tomography (FIB-SEMt) and transmission electron microscopy tomography (TEMt) [1]. Both Xt and FIB-SEMt have been proven to enable differentiating porous and solid phase within PEMFC CLs [2,3]. TEMt allows imaging with resolutions below 1 nm and can be directly used to image Platinum particles [4]. To image all important phases multiple tomographic techniques must be used. In this talk we give an overview on the latest tomographic analysis techniques for PEMFC CL reconstruction with a particular emphasis on FIB-SEMt, multi-scale imaging approaches and validity of the existing reconstruction methods.

A Multi-Scale Modeling Approach to Study and Design PEM Fuel Cells Catalyst Layers with Non-Precious Metals

Building a catalyst layer includes many elements that make understanding and studying it a complex problem. These systems have different elements with multiscale, multiphase and multicomponent issues, as Figure 1 illustrates. Typical PEMFC catalyst layers are composed of three elements: porous carbon to provide electron and gas/liquid transport, ionomer to provide protonic transport and catalyst to facilitate the reduction or oxidation reactions. The replacement of Pt with non-precious-metal-based catalysts (NPMCs) for the ORR is considered to be one key to cost-effective power generation in PEMFCs. Accordingly, developing NPMCs with high performance for ORR (1-4) has been a focal point of research. A major complication in analysis of the NPMC systems under study lies in the unknown nature of the active site. It is difficult to identify structural and electronic parameters based on most characterization techniques. Many questions arise when compared with Pt-based systems such as: Which elements do Pt and NPMC catalyst layers have in common? What makes one metal more suitable than another in the NPMC? What specific catalyst layer composition and structure are optimal? How do we (or can we) correlate energetics, structure and transport issues? Modeling catalyst layers is a complicated task because we need to take into account the phenomena occurring over different time and size scales as represent in Figure 1: (i) heterogeneous electrochemical reactions in the microscale; (ii) proton, electron and gases transport in the mesoscale; and (iii) gases and water transport through membranes and porous media in the macroscale (5-10). The first steps are to identify and classify the main macroscopic parameters that affect the performance by using complex and adequate experimental data sets and to reduce the degrees of freedom for fitting. Once the parameters are identified, we can work at the right scale, with the appropriate physics and computational methodology that the macroscopic results point out in order to complete the picture and resolve the unknowns. The aim of this study is to extend previous results from a 1D model in which we identified the active area and permeability as the most distinguishing elements between Pt-based and NPM-based catalyst layers. On one hand, the active area enters into macroscopic models convolutes with the intrinsic activity of the catalyst site. The latter is related to the specific catalytic properties of the active site; understanding it requires a deep study of the catalyst nature, which is not well known due to the pyrolysis of NPM materials. In this case, a coupled modeling approach using DFT (Density Functional Theory) and MD (Molecular Dynamics) techniques could give us insight in the active site behavior. On the other hand, transport properties and differences can be study by using CFD (Computational Fluid Dynamics) tools to compare fluid transport and local distributions within the catalyst layers with parameters obtained via DFT-MD simulations. This contribution will summarize the previous modeling of transport and describe calculations to probe the final configuration after high temperature treatment of porphyrin and iron porphyrin carbon structures. We aspire to describe interactions of the porphyrin with the carbon substrate during the heating process and give insight into the active site final configuration from a theoretical perspective by combining MD with DFT calculations. Acknowledgement We gratefully acknowledge the support of the NSF EPSCoR program and Colciencias for support of this work.

A Monte Carlo Study on the Effect of Structural and Operating Parameters on the Water Distribution within the Microporous Layer and the Catalyst Layer of PEM Fuel Cells

Depending on the operating conditions, the overall performance and durability of polymer electrolyte membrane fuel cells (PEMFC) can be strongly influenced by liquid water saturation of pores within the gas diffusion layer (GDL). Water agglomerations in the GDL may occur due to material morphology, surface characteristics and local boundary conditions. In the catalyst layer (CL) and microporous layer (MPL) these parameters are considerably different than those of the fibre substrate. The MPL and CL feature by orders of magnitude smaller pores than the substrate and the boundary conditions, e.g. temperature and relative humidity, are different than those in the substrate near the bipolar plate. Thus, different physical behavior within each layer can be observed, e.g. local liquid sorption. Therefore, finding an optimal strategy regarding water management requires knowledge on virtually all the factors involved. To approach this target, we are using and further developing a 3D Monte Carlo (MC) model which simulates water distribution within the GDL, employing the real GDL structure and operating conditions, exploited respectively from tomography and computational fluid dynamics (CFD) studies1. The 3D images are obtained from an assembled fuel cell and therefore, the results include compression and inhomogeneity information of the GDL. Furthermore, recent progress of the model has enabled simulations on the length scales relevant for the MPL and CL in addition to the fibre substrate. GDL substrates usually have an average pore size in the micrometer scale, whereas this value for the MPLs and CLs may go down to the nanometer scale. Therefore, a complete simulation of the water distribution in GDL includes working on at least two scales. Although obtaining accurate results on all three sub-layers is equally important, MPL and CL are the more challenging ones from the point of view of both structure and surface properties acquisition. Furthermore, simulating relevant domains within the material requires expensive computational effort. In this study, we report first results on MC simulations of the water distribution within the MPL and CL pores for different operating conditions. The 3D reconstruction of the material is obtained from the focused ion beam - scanning electron microscope (FIB-SEM) tomography2. By using atomic layer deposition (ALD) of ZnO, a good material contrast between pores and solid particles is achieved and therefore a reliable binarization can be obtained3. The voxel size is set to 45 nm in order to achieve both, an accurate computation with respect to the applied equations and an appropriately sized simulation domain. Our MC model provides the water distribution within the actual MPL and CL structures based on the local boundary conditions. The results show how the water distribution depends on parameters such as contact angle, porosity, local temperature and relative humidity. An example of simulation results is illustrated in the Figure 1.

Indirect and Direct Observation of Ionomer Colloidal Systems with Applications to Fuel-Cell Catalyst Layers

Manufacturing methods for fuel cell catalyst layers are critically important for achieving high material utilization and performance. Typically, electrode systems are cast from an ‘ink’ or ‘colloidal’ state, where the solid material is suspended in a solution containing a solvent and a polymer. For polymer-electrolyte fuel cells, a perfluorinated sulfonic-acid (PFSA) ionomer, typically Nafion, is used in the ink and catalyst layer, where its roles are to act as a binding agent and conduit for proton conduction to the reaction site.1 Recently, it has been suggested that the ionomer film that surrounds the platinum reaction site may contribute to increased resistances that limit performance at low Pt loadings.1,2 Thus, it is of great interest to understand the factors controlling the film formation in catalyst layers to optimize material properties and cell performance. The nature of the dispersion and casting process could have significant influence on the morphology and properties of the dispersion-cast membranes.3-5 To isolate these effects, the morphology of PFSA dispersed in various solvents (glycerol, water/2-propanol, and NMP) was reported previously using small angle neutron scattering (SANS), where catalyst layers from the different solvents exhibit different performance.6 The understanding of local‘ink’ structures as a function of active material (polymer and solid) volume fraction (Φ) and across solvent space and length scales can aid in understanding bulk properties and the impact of ink formulation in cast systems. In this talk, a direct observation of the colloidal suspension of the ionomer in different solvents will be described using confocal laser scanning microscopy (CLSM). CLSM has long been utilized as a tool for characterizing the structure of biphasic colloidal gels. We combine dynamic light scattering, ultra-small angle x-ray scattering, and confocal microscopy to understand PFSA size and structure in dilute (1,3, and 5 wt%) ionomer solutions. The structural formation of nafion membranes are observed in-situ with small angle x-ray scattering. Finally, we contrive a model ‘ink’ composed of attractive and repulsive silica spheres and PFSA suspended in different solvents (water, NMP, and isopropyl-alcohol) and use CLSM to gain a greater understanding about solid|PFSA interactions in model inks. 1. A. Z. Weber and A. Kusoglu, Journal of Materials Chemistry A, 2, 17207 (2014). 2. A. Kongkanand and M. F. Mathias, The Journal of Physical Chemistry Letters, 1127 (2016). 3. T. T. Ngo, T. L. Yu and H.-L. Lin, Journal of Power Sources, 225, 293 (2013). 4. C.-H. Ma, T. L. Yu, H.-L. Lin, Y.-T. Huang, Y.-L. Chen, U. S. Jeng, Y.-H. Lai and Y.-S. Sun, Polymer, 50, 1764 (2009). 5. Y. S. Kim, C. F. Welch, R. P. Hjelm, N. H. Mack, A. Labouriau and E. B. Orler, Macromolecules, 48, 2161 (2015). 6. C. Welch, A. Labouriau, R. Hjelm, B. Orler, C. Johnston and Y. S. Kim, Acs Macro Letters, 1, 1403 (2012).

Advanced Characterization of Electrocatalyst Interfaces

In PEM fuel cells, electrocatalyst interfaces including Pt-C, Pt-ionomer, C-ionomer interfaces, play a critical role for both performance and durability.1-3 Pt-C interface influences the stability and activity of Pt dramatically; electrocatalyst/ionomer interfaces4 in fuel cell catalyst layers determine the electrochemical surface area (ECSA), local transport, and structural stability of catalyst layer. Delineating the structure evolution of the interfaces in response to PEM fuel cell operating environment will provide the science foundation for new electrocatalyst/catalyst layer design therefore improve the performance and durability of PEM fuel cells. We have developed an aberration corrected environmental transmission electron microscopy (ETEM), which allows unprecedented spatial and temporal resolution in the relevant gas environment, such as O2, H2, H2O, CO, CH4, and CO2. With customized intelligent gas delivery system, the gas environment relevant to PEM fuel cell anode and cathode (H2/H2O, O2/H2O) can be simulated and precisely controlled through the residual gas analyzer (RGA). The ETEM opens new avenue to study the dynamic changes of electrocatalyst interfaces. In order to study the surface chemistry of catalysts under different environment, a specially-designed pre-treatment system is integrated into X-ray photoelectron spectroscopy (XPS), which allows us to move catalysts seamlessly between the pre-treatment system and XPS chamber so that surface contamination from external environment will be eliminated. In this talk, we will present our recent progress on identifying both structure and surface chemistry evolution and their correlation in electrocatalysts under PEM fuel cell relevant environment through the combination of ETEM and XPS capabilities. 1. Y. Y. Shao, G. P. Yin and Y. Z. Gao, J. Power Sources, 2007, 171, 558-566. 2. S. Park, Y. Y. Shao, H. Y. Wan, V. V. Viswanathan, S. A. Towne, P. C. Rieke, J. Liu and Y. Wang, J. Phys. Chem. C, 2011, 115, 22633-22639. 3. R. Kou, Y. Y. Shao, D. H. Mei, Z. M. Nie, D. H. Wang, C. M. Wang, V. V. Viswanathan, S. Park, I. A. Aksay, Y. H. Lin, Y. Wang and J. Liu, J. Am. Chem. Soc., 2011, 133, 2541-2547. 4. A. Z. Weber and A. Kusoglu, J. Mater. Chem. A, 2014, 2, 17207-17211.

Performance of Stratified Fuel Cell Catalyst Layer

The development of stratified catalyst layers promises an increase in catalyst layer performance compared to conventional flat catalyst layers.1,2 Irregular catalyst layer thickness and porosity can lead to enhanced gas and water transport in and out of the catalyst layer, respectively. The stratified structure is expected to have the same performance in the kinetic region where the performance is controlled by the overall Pt loading. However at high current densities, the thinner sections of the stratified catalyst layers should allow for better mass transport properties. Electrode fabrication is done in house with a custom designed spray coating procedure and catalyst ink recipe. Various approaches are being explored to achieve the desired electrode structure. One approach is to densify the catalyst layer in localized regions, and is based on Ion Power proprietary manufacturing techniques. This can involve the use of glass epoxy masks during the spray coating process to directly create a patterned electrode on the membrane that has thicker and thinner regions. Figure 1 illustrates the effect of stratification on the fuel cell performance of a MEA. The second approach is to pattern thicker and thinner sections of an electrode using masks. During spray coating, one mask exposes the lands and the other mask exposes the channels. This way the two extreme cases can be investigated and the resulting performance compared. The concept of having more catalyst material in the channels may be beneficial due to rapid reaction times during influx of reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may later become an issue for product water removal and oxygen diffusion. On the other hand, spray coating more of the electrode in the land areas may support better product water removal and oxygen diffusion by leaving a thinner catalyst layer under the channels. The best performance is expected from a combination of ordered thin and thick regions in the catalyst layer. Results from various MEAs tested in a single cell on a fully automated test station to evaluate catalyst performance will be presented. The evaluation of new catalyst ink recipes with reduced ionomer to carbon ratios is included in this study to further increase mass transport by reducing ionomer swelling in the catalyst layer at high relative humidity. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.

A Monte Carlo Study on the Effect of Structural and Operating Parameters on the Water Distribution within the Microporous Layer and the Catalyst Layer of PEM Fuel Cells

Liquid water agglomerations in the catalyst layer and microporouslayer of PEM fuel cells are simulated with a 3D Monte Carlomodel. The simulations are based on real material structures andlocally resolved operating conditions. The 3D material images areobtained from advanced experimental techniques capable of nm-scale imaging. The temperature and the relative humidity profilesare provided by a computational fluid dynamics simulation, basedon a single channel fuel cell model taking into account all relevanttransport and electrochemical processes. The results of this MonteCarlo study confirm the strong dependency of the water saturationin the catalyst layer and microporous layer on the structuralparameters and operating conditions, such as: wettability, pore sizeand local temperature and relative humidity.

The influence of relative humidity on the performance of fuel cell catalyst layers in ethanol sensors

The influence of relative humidity (RH) on the sensing performance of two commercially available fuel cell catalysts were investigated in an ethanol sensor device. Two sets of sensor membrane electrode assemblies (MEAs) were prepared using platinum black (Pt) and 20% platinum supported on carbon (Pt/C). One set employed a Nafion membrane, while the second set employed a porous polyvinyl chloride (PVC) membrane containing sulfuric acid, the latter of which is commonly used in commercial ethanol sensor MEAs. Using our advanced fuel cell sensor testing station, the sensing performance of each set of MEAs was measured and compared with that of a commercial breath alcohol sensor (BrAS) MEAs. It was found that both the Pt and Pt/C outperformed the commercial materials regardless of membrane. The Pt/C-Nafion MEA outperformed the Pt-Nafion MEA both in raw performance at both high humidity and low humidity. The change in sensitivity from 95% RH to 45% RH for the Pt/C-Nafion MEA was also lower compared to the Pt-Nafion MEA. As smaller change in sensitivity would indicate better stability. The porous nature of the Pt/C catalyst reduces the likelihood of flooding at the interface. The Pt/C-PVC MEA outperformed the Pt-PVC MEA in raw performance, but the stability of the Pt-PVC was superior. This was due to the small porosity of the Pt, which reduces water loss in the lower relative humidity environment.

(Industrial Electrochemistry and Electrochemical Engineering Division Student Achievement Award) Engineering the Ionic Polymer Phase Surface Properties of a PEM Fuel Cell Catalyst Layer

During high power density operations, the performance of polymer electrolyte membrane fuel cells (PEMFCs) is limited by high water saturation levels in the cathode catalyst layer due to high wettability of the ionic polymer phase. A new heat treatment was used to create and permanently lock-in the surface structure of a biphasic membrane (Nafion® 212). Several surface characterization techniques were used to verify the engineered membrane’s surface after heat treatment, including contact angle, multi-mode atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). We found that specific heat treatment conditions will lead to the formation of either a hydrophobic or hydrophilic surface layer. The results of this research will guide the design of a new class of PEMFC catalyst layers. Engineered PEMFC catalyst layers are expected to prevent electrode flooding, therefore leading to higher performance and efficiency during high power density operations.

Progress in Soft X-ray Microscopy Characterization of PEM Fuel Cell Catalyst Layers

Progress in Soft X-ray Microscopy Characterization of PEM Fuel Cell Catalyst Layers

Integrated Statistical and Nano-Morphological Study of Effective Catalyst Utilization in Vertically Aligned Carbon Nanotube Catalyst Layers for Advanced Fuel Cell Applications

In this study, a three-dimensional quasi-random nano-structural model based on the Monte Carlo method is proposed to evaluate effective catalyst utilization in fuel cell catalyst layers. Reflecting the intrinsically inhomogeneous nano-morphology of fuel cell catalyst layers, statistical analyses are performed to statistically compare various types of fuel cell electrodes to gain fundamental insight into the effects of morphological nano-structures on the catalyst utilization. For the detailed morphological analysis, a series of multi-component distributions at a 95% confidence level is randomly generated to deduce the statistical variations of the effective transport paths of ternary catalyst components. The statistical nano-morphology model is validated against published experimental data with good agreement. Subsequently, the morphological configuration of the vertically aligned carbon nanotube (VACNT) catalyst layers is simulated to determine the principal nano-design factors for improved catalyst utilization in the same stochastic manner which is used for carbon-supported catalyst layers. In the VACNT catalyst layers, all of the Pt/CNTs are successfully interconnected and therefore, all solid conducting carbon nanotubes can be utilized as electric current paths. Numerical results reveal that despite the relatively poor interconnections of the ion and mass transport paths, the statistical average catalyst utilization of VACNT is significantly improved when compared to conventional catalyst layers. It is also found that the ionic current paths of the VACNT catalyst layers can be considered as a catalyst utilization determining factor and an adequate amount of ionomers are necessary to promote successful ionic conduction. Finally, the average catalyst utilizations for both the regularly patterned in-line and staggered VACNT catalyst layers are compared with results for randomly distributed VACNTs.