Electrolyte Fuel Cell Catalyst Layers Research Articles

Probing multiphase reactive transport interactions in the polymer electrolyte fuel cell catalyst layer degradation

The multiphase, multicomponent reactive transport plays a critical role in the degradation of the cathode catalyst layer in Polymer Electrolyte Fuel Cells. This warrants a fundamental understanding of the pore-scale, mechanistic interactions in the catalyst layer. Herein, the interfacial and transport interactions due to the flooding dynamics in the presence of carbon support corrosion, which is a primary degradation mode in the cathode catalyst layer, are delineated based on a comprehensive mesoscale model. The mechanistic interrogation of the degradation-transport interactions reveals the spatio-temporal heterogeneity of the pristine and aged microstructures. An electrode-averaged saturation front of 80 % unveils a critical limit of percolation which results in an onset of limiting current density. Further, it is observed that the reactant severity is amplified at progressively higher stages of corrosion owing to the appearance of dead pores and a thicker ionomer film. We also shed light on the fluid displacement patterns which reveal the existence of a capillary fingering regime. The interplay of the operating current density and carbon corrosion in governing the substrate-ionomer interaction is described through an electrochemical Damköhler number.

Parametric Study of the Influence of Support Type, Presence of Platinum on Support, and Ionomer Content on the Microstructure of Polymer Electrolyte Fuel Cell Catalyst Layers

Ultra-small angle X-ray scattering (USAXS) was employed to investigate the effects of carbon support type, the presence of platinum on carbon, and ionomer loading on the microstructure of polymer electrolyte fuel cell (PEFC) catalyst layers (CLs). Particle size distributions (PSDs), obtained from fitting the measured scattering data were used to interpret the size of carbon aggregates (40–300 nm) and agglomerates (>400 nm) from two-component carbon/ionomer and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were investigated: high surface area carbon (HSC) and Vulcan XC-72. CLs with a range of perfluorosulfonic acid (PFSA) ionomer to carbon (I/C) ratios (0.2–1.2) and also with perfluoroimide acid (PFIA) ionomer were studied to evaluate the effect of ionomer on CL microstructure. The carbon type, the presence of platinum, and ionomer loading were all found to significantly impact carbon agglomeration. The extent of Pt/C agglomeration in the CL was found to increase with increasing ionomer and platinum concentration and to decrease with increasing carbon surface area. Platinum electrochemically-active surface area (ECSA) and local oxygen transport resistance (RnF) were correlated to the CL microstructure to yield relationships affecting electrode performance.

Mechanistic interactions in polymer electrolyte fuel cell catalyst layer degradation

Mechanistic understanding of the coupled performance-durability interactions resulting from the transient landscape of catalyst degradation is elucidated.

High Surface Area Pt/C Electrocatalyst Modification with Ionic Liquids for Improved Ionic Conductivity in Polymer Electrolyte Fuel Cell Catalyst Layer

Oxygen reduction reaction (ORR), specifically in technological case of platinum (Pt) electrocatalyst, is one of the determining factors for commercialization and mass production of affordable fuel cell electric vehicles. ORR takes place at the triple phase boundary of the Pt electrocatalyst, the ionomer (Nafion) or water, and oxygen gas. In order to maximize the electrochemical surface area (ECSA) of the Pt nanoparticles, they are dispersed on high surface area (HSA) carbon supports and a large portion of them are buried inside the micro (< 2nm) and mesopores (< 50nm). Because of the size exclusion, ionomer cannot penetrate pores below 20 nm and water acts as the proton conducting media in these pores1. In addition, the dense polytetrafluoroethylene (PTFE) backbone blocks the oxygen transport. Therefore, the interface design principle in this work for improving catalyst layer ORR performance is to infiltrate micropores with ionic liquids (ILs) to facilitate sufficient proton and oxygen delivery to the Pt active sites. Ionic strength of ILs is within the range of 2.77 – 5.3×103 mol.m- 3 which is up to 7 orders of magnitude higher than that of water1. Higher ionic strength of electrolyte in micropores will ensure that electric double layers are thin and they do not exclude protons from entering the pores as fuel cells typically operate at higher potentials than the potential of zero charge of Pt. We implemented three imidazolium-derived ILs including 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim]+[NTf2]-), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim]+[NTf2]-), and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim]+[NTf2]-) to modify 40 wt.% Pt on HSA Ketjenblack EC-300J catalyst (Pt/C). Our RDE prescreening showed improved proton conduction and accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion making them great candidates for catalyst modification2. After optimization of ink recipe including IL to carbon and water to IPA ratios, the MEAs with 5cm2 active area were fabricated via decal transfer of anode and cathode catalyst layers on Nafion XL membrane. Figure 1a represents the fuel cell polarization curves of the IL-modified catalysts compared to the baseline Pt/C. Pt/C-[C4mim]+[NTf2]- shows the best performance and highest kinetic activity compared to the baseline and other two IL-modified catalyst layers. The open circuit voltages (OCVs) were 0.942, 0.933, 0.948, and 0.929 V for baseline Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-), respectively. Electrochemical impedance spectroscopy (EIS) data was used for measuring cathode catalyst layer ionic conductivity (Figure 1b), as Nyquist spectra represents a 45° line in the high frequency range where real and imaginary parts are equal, followed by a straight 90° line. A physical impedance model obtained from Obermaier et al.3 was fitted to EIS data, specifically at high frequencies to get the catalyst layer’s ionic conductivity. All the modified catalysts showed improved ionic conduction compared to the baseline Pt/C. Conductivity values for catalyst layers of Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-) with approximately 10μm thickness were 1.93, 3.95, 2.34, 2.04 S/m, respectively. Our fuel cell results show that high conductivity of ([C4mim]+[NTf2]-) along with its optimum chemical structure is responsible for improved performance across kinetic and ohmic regions compared to the baseline Pt/C. References Avid, A. & Zenyuk, I. V. “Confinement effects for nano-electrocatalysts for oxygen reduction reaction”. Curr. Opin. Electrochem. 25, 100634 (2021). Avid, A. & Zenyuk, I. V. Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells. ECS Meet. Abstr. MA2020-02, 2155 (2020). Obermaier, M., Bandarenka, A. S. & Lohri-Tymozhynsky, C. A Comprehensive Physical Impedance Model of Polymer Electrolyte Fuel Cell Cathodes in Oxygen-free Atmosphere. Sci. Rep. 8, 4933 (2018). Figure 1

Microstructure Investigation of Polymer Electrolyte Fuel Cell Catalyst Layers Containing Perfluorosulfonated Ionomer.

Perfluorosulfonated ionomers are the most successful ion-exchange membranes at an industrial scale. One recent, cutting-edge application of perfluorosulfonated ionomers is in polymer electrolyte fuel cells (PEFCs). In PEFCs, the ionomers are used as a component of the catalyst layer (CL) in addition to functioning as a proton-exchange membrane. In this study, the microstructures in the CLs of PEFCs were characterized by combined synchrotron X-ray scattering and transmission electron microscopy (TEM) analyses. The CL comprised a catalyst, a support, and an ionomer. Fractal dimensional analysis of the combined ultrasmall- and small-angle X-ray scattering profiles indicated that the carbon-black-supported Pt catalyst (Pt/CB) surface was covered with the ionomer in the CL. Anomalous X-ray scattering revealed that the Pt catalyst nanoparticles on the carbon surfaces were aggregated in the CLs. These findings are consistent with the ionomer/catalyst microstructures and ionomer coverage on the Pt/CB surface obtained from TEM observations.

Interpreting Ionic Conductivity for Polymer Electrolyte Fuel Cell Catalyst Layers with Electrochemical Impedance Spectroscopy and Transmission Line Modeling

A cathode catalyst layer containing optimally distributed ionomer is critical to reduce the platinum loading and increase its utilization in polymer electrolyte fuel cells. Here, electrochemical impedance spectroscopy (EIS) was used to measure effective ionic conductivity of pseudo catalyst layers (PCLs) at a relative humidity (RH) range of 50%–120%. These results are compared to previous work using the hydrogen pump (HP) method. EIS effective ionic conductivity results reported here are higher than those from the HP because in the HP set-up ionic pathways must be effectively connected through the PCL to be counted, whereas in the EIS measurement, ionomer segments that are in contact with the membrane but are not effectively connected all the way through the PCL can be detected. Double layer capacitances and effective ionic conductivities of Pt/C catalyst layers with various supports and ionomer to carbon (I/C) ratios were studied. High surface area carbon support resulted in a lower effective ionic conductivity compared to the graphitized carbon support due to worse ionomer dispersion. Effective ionic conductivities of Pt/C layers were compared to that of PCLs. On average, effective ionic conductivities of Pt/C layers were higher than PCLs because of possible carbon agglomeration within the PCLs.

Interpreting Ionic Conductivity for Polymer Electrolyte Fuel Cell Catalyst Layers with Electrochemical Impedance Spectroscopy and Transmission Line Modeling

An optimal cathode catalyst layer with uniform ionomer distribution is critical to reduce the platinum loading and increase the utilization in polymer electrolyte fuel cells (PEFCs). Having well distributed ionomer with low ionomer to carbon (I/C) ratio is desirable, as it will also minimize local oxygen transport resistance. In this work, we used electrochemical impedance spectroscopy (EIS) method and transmission line model (TLM) to measure and evaluate ionic conductivity of pseudo catalyst layers (PCLs) comprised of Vulcan XC72 carbon black and 3M 825 EW ionomer with I/C ratios of 0.3, 0.6, 1 and 1.4 at a relative humidity (RH) range of 50 to 120 %. These results were compared with our previous hydrogen pump (HP) measurements, where PCL was sandwiched between the two membranes and protons had to transport through the entire PCL to be counted towards current. EIS effective ionic conductivity results reported here are higher than the results of HP because in the HP set-up ionic pathways must percolate all the way through the PCL to be effectively counted, whereas in EIS measurement, ionomer segments that are in contact with the membrane but not percolating all the way through the PCL can be detected. Additionally, three effects were studied: 1) hot press, 2) platinum content and carbon black support, and 3) ionomer content or lack of ionomer. Ionic conductivity increased significantly by hot pressing PCL onto the membrane, however, it was independent with respect to the hot press conditions, such as pressure and temperature. Hot pressing with low I/C ratio had more impact than high I/C ratio because at high I/C ratio even without hot press there is a sufficient amount of contact points between catalyst layer and membrane. At high RHs, catalyst layer containing Pt has higher ionic conductivity compared to PCL because of Pt particles being hydrophilic, which helps to distribute the ionomer more uniformly. To investigate the effect of condensed water, PTFE dispersion was used as a binder for Pt/C layer instead of ionomer, resulting ionic conductivity decreased with increasing applied potential at 100% and 120% RH conditions. At lower RHs, ionic conductivity is independent of the applied potentials.

Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

One of the main challenges of mass deployment of fuel cell electrical cars is the limited power density of the polymer electrolyte fuel cell (PEFC), requiring high loadings of precious Pt metal to drive the oxygen reduction reaction (ORR). Within the catalyst layer of PEFC electrodes, Pt nanoparticles are typically dispersed on porous Ketjenblack carbon due to its high surface area, and the ability of Pt to be deposited into the smaller mesopores of the carbon support; so that they can be protected from ionomer poisoning. Up to 62% of Pt nanoparticles can be buried in the interior of the mesopores1. As the cathode ORR takes place at the triple-phase boundary of the liquid electrolyte, oxygen gas, and solid Pt deposited on the carbon support, a sufficient diffusion of oxygen and protons in the liquid media is vital for the ORR reaction. However, Nafion as the proton conductor cannot penetrate into the micro and mesopores of the support due to the size exclusions; therefore, Pt nanoparticles buried within those pores will not be accessed by the reactants at dry conditions. Studies have shown that electric double layers formed at Pt-water interface can help mediate proton transport increasing proton conductivity by three orders of magnitude. Although, this conductivity is still about two orders of magnitude lower than that of Nafion2 , 3.The principle of electrocatalyst interface design in this work is to impregnate the high surface area (HSA) Ketjenblack carbon pores with a secondary phase, imidazolium-derived ionic liquids (ILs), that can fill the mesopores of the carbon support and provide satisfactory oxygen solubility and proton transfer due to high ΔpKa value and hydrogen-bonded network formation4 , 5. Pt/C-IL catalyst systems are prepared with 1-alkyl-3-metylimidazolium bis(trifluoromethylsulfonyl)imides with short cationic alkyl chain lengths including ([C2mim][NTf2]) and ([C4mim][NTf2]), along with 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim][NTf2]) in order to study the effect of methyl group attachment to the carbon in the 2-position.The effect of the IL molecular structure on the electrochemical behavior of Pt/C catalyst is investigated in half-cell electrochemical set-up, rotating disk electrode (RDE). Figure 1a contains the cyclic voltammogram of pristine and IL-modified Pt/C samples. Pt/C catalyst exhibits electrochemical surface area (ECSA) loss after the IL modification. IL-modified Pt/C samples including Pt/C-[C2mim][NTf2], [C4mim][NTf2], and [C4dmim][NTf2] show ECSA values of 81.29, 78.1 and 78.9 m2 g-1 Pt’ respectively, which are lower than that of the pristine Pt/C. Moreover, IL molecules create a hydrophobic microenvironment that can lead to the repelling of the water molecules from Pt active sites protecting them from being oxidized; therefore, the onset potential of Pt-oxide formation is shifted to higher potentials and the coverage of the non-reactive oxygenated species is decreased, improving the specific activity of the electrocatalysts.Electrochemical impedance spectroscopy (EIS) is also implemented to measure overall effective proton diffusion resistance within the catalyst layers of Pt/C-IL-Nafion and Pt/C-Nafion. The linear portion of the Nyquist plot data at high frequencies shows a noticeably enhanced proton accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion (Figure 1b). References Huang, J., Li, Z. & Zhang, J. Review of characterization and modeling of polymer electrolyte fuel cell catalyst layer: The blessing and curse of ionomer. Front. Energy 11, 334–364 (2017). Zenyuk, I. V & Litster, S. Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells. Electrochim. Acta 146, 194–206 (2014). Thompson, E. L. & Baker, D. Proton Conduction on Ionomer-Free Pt Surfaces. (2011). doi:10.1149/1.3635605 Miran, M. S. et al. Key factor governing the physicochemical properties and extent of proton transfer in protic ionic liquids: ΔpKa or chemical structure? Phys. Chem. Chem. Phys. 21, 418—426 (2018). Moschovi, A. M., Dracopoulos, V. & Nikolakis, V. Inter- and Intramolecular Interactions in Imidazolium Protic Ionic Liquids. J. Phys. Chem. B 118, 8673–8683 (2014). Figure 1

FIB/SEM tomography segmentation by optical flow estimation

Focused ion beam/scanning electron microscopy tomography (FIB/SEM tomography) is the method of choice for the tomographic reconstruction of mesoporous materials systems in various fields such as batteries, fuel cells, filter applications or composite materials. However, due to so called shine-through artifacts in FIB/SEM tomographies of porous materials, their segmentation into pore space and solid material is a nontrivial task. Here, an optical flow-based method that utilizes shine-through artifacts for segmentation is introduced. Subsequently, the performance of the method is discussed by means of tomographic datasets of a polymer electrolyte fuel cell catalyst layer and a lithium ion battery composite electrode. Previous, manual segmentations of these datasets allow the evaluation of the results – for the catalyst layer an accuracy of 86.6% and a precision of 84.0% is reached. In both cases, the optical flow-based approach gives significantly better results than comparable segmentations obtained from gray-value threshold binarization.

Numerical Modeling of Polymer Electrolyte Fuel Cell Catalyst Layer with Different Carbon Supports

Numerical Modeling of Polymer Electrolyte Fuel Cell Catalyst Layer with Different Carbon Supports

Experimental Study on Ionic Conductivity of Carbon Support within Polymer Electrolyte Fuel Cell Catalyst Layers

Conventional polymer electrolyte fuel cells (PEFCs) use carbon supported Platinum (Pt/C) as the catalyst. With decades of efforts, researchers have brought down the loading of Pt below 0.1 mg/cm2 on the anode and 0.4 mg/cm2 on the cathode. However, the high cost of Pt based catalyst still serves as a big barrier for PEFCs’ fully commercialization. Within the catalyst layer, polymer electrolyte ionomer (e.g. Nafion) is usually used as the proton conducting media. During the catalyst layer fabrication process, Pt/C particles are present as aggregates. Due to size exclusion, the polymer electrolyte mainly coats the large mesopores of the Pt/C aggregates, i.e. ionomer covers the secondary pores. In high surface area carbon supports, a significant amount of Pt exists within the smaller mesopores and micropores of the support, which is inaccessible to the ionomer. Although it is known that protons are able to reach Pt within the aggregates when those pores are filled with liquid water, the magnitude and mechanism of proton transport is largely unknown. In past studies, the proton conductivity of continuous Pt surfaces have been studied (1), with hypothesis proposing that proton can transport through Pt surface by surface diffusion or through electric double layer (EDL) at the metal/water interface. It has also been reported that proton conductivity is severely hindered over extended glassy carbon surfaces (2). However, the widely used Pt/C catalyst doesn’t have continuous metal surface, and unlike glassy carbon, the carbon support used in the catalyst layer usually has a mixture of functional groups on the surface (e.g., carboxyl and sulfur species), which will alter the surface ionic conductivity through deprotonation. In this work, we measure the conductivity in ionomer free carbon (Vulcan XC72R) and ionomer free Pt/C (10 wt.% HiSPEC 2000) electrodes to better understand the conduction properties of ionomer-free carbon support surfaces. Our experiments evaluate the effects of both electrode potential and the relative humidity (RH) on the conductivity. Electrochemical impedance spectroscopy (EIS) and transmission line modeling show the proton conductivity of the ionomer-free electrode decreases as electrodes’ potential increase (see Figure 1). The potential dependence of the ionic conductivity is supported by a cyclic voltammetry (CV) analysis. Herein, we present several aspects related with the proton conductivity within the ionomer-free carbon domains of the catalyst layer and their effects on fuel cell performances. Acknowledgement This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. References E. L. Thompson and D. Baker, ECS Trans., 41, 709 (2011).U. Paulus, Z. Veziridis, B. Schnyder, M. Kuhnke, G. Scherer and A. Wokaun, J. Electroanalytical Chemistry, 541, 77 (2003). Figure 1

Analyzing capillary condensation in a polymer electrolyte fuel cell catalyst layer based on a lattice density functional theory

Analyzing capillary condensation in a polymer electrolyte fuel cell catalyst layer based on a lattice density functional theory

Measurement of Gas Transport Properties in PEFC Catalyst Layers Consisted of Different Carbon Materials by Using a Microfluidic Device

Gas transport properties in polymer electrolyte fuel cell catalyst layers consisted of different carbon materials were evaluated by using a microfluidic device. The microfluidic device consists of catalyst coated membrane and Si chip having parallel microchannels. Air and nitrogen were supplied to the two parallel channels in the device, respectively. Gas transport properties were evaluated by measuring outlet partial pressure of oxygen which was diffused through the catalyst layer. Adding multi-walled carbon nanotube in the catalyst layer instead of carbon black resulted in an increase of effective gas diffusivity, while geometrical porosity was decreased.

Review of characterization and modeling of polymer electrolyte fuel cell catalyst layer: The blessing and curse of ionomer

Ionomer impregnation represents a milestone in the evolution of polymer electrolyte fuel cell (PEFC) catalyst layers. Ionomer acts as the binder, facilitates proton transport, and thereby drastically improves catalyst utilization and effectiveness. However, advanced morphological and functional characterizations have revealed that up to 60% of Pt nanoparticles can be trapped in the micropores of carbon support particles. Ionomer clusters and oxygen molecules can hardly enter into micropores, leading to low Pt utilization and effectiveness. Moreover, the ionomer thin-films covering Pt nanoparticles can cause significant mass transport loss especially at high current densities. Ionomer-free ultra-thin catalyst layers (UTCLs) emerge as a promising alternative to reduce Pt loading by improving catalyst utilization and effectiveness, while theoretical issues such as the proton conduction mechanism remain puzzling and practical issues such as the rather narrow operation window remain unsettled. At present, the development of PEFC catalyst layer has come to a crossroads: staying ionomer-impregnated or going ionomer-free. It is always beneficial to look back into the past when coming to a crossroads. This paper addresses the characterization and modeling of both the conventional ionomer-impregnated catalyst layer and the emerging ionomer-free UTCLs, featuring advances in characterizing microscale distributions of Pt particles, ionomer, support particles and unraveling their interactions; advances in fundamental understandings of proton conduction and flooding behaviors in ionomer-free UTCLs; advances in modeling of conventional catalyst layers and especially UTCLs; and discussions on high-impact research topics in characterizing and modeling of catalyst layers.

Tomographic Analysis of Polymer Electrolyte Fuel Cell Catalyst Layers: Methods, Validity and Challenges

Tomographic analysis of catalyst layers is one of the key methods to evaluate morphological properties in polymer electrolyte fuel cell PEFC catalyst layers. The most important applications for tomographic reconstruction are process control during manufacturing and investigations of ageing phenomena. We discuss all methods for tomographic imaging that are applicable for PEFC catalyst layers with an emphasis on focused ion beam / scanning electron microscopy tomography (FIB-SEMt). For now, no single tomographic method suffices to image all morphological properties of interest. Multi-tomographic approaches are therefore necessary. Phase fractions and phase-distributions are the most important properties to calculate within a tomographic reconstruction. We review further parameters that may be extracted from tomographic reconstructions and discuss open issues in first modeling approaches. Finally we discuss the validity of analyses within tomographic representations.

Tomographic Analysis of Polymer Electrolyte Fuel Cell Catalyst Layers: Methods, Validity and Challenges

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.

(Invited) Diagnostics of Microstructure and Properties of Polymer Electrolyte Fuel Cell Catalyst Layer

Rising levels of greenhouse gas emissions, especially carbon dioxide (CO2), are regarded as one of the causes of global warming in recent years. To develop technologies for reducing CO2emission plays an important role for vehicle manufacturers, and Nissan has been developing zero-emission vehicles (ZEVs) such as battery electric vehicles (BEVs) and fuel cell electric vehicles (FCEVs). The biggest issues to penetrate FCEVs into the market are cost reduction of polymer electrolyte fuel cell (PEFC) power system including hydrogen storage subsystem, and development of hydrogen infrastructure. Related to the former issue, one of the most significant challenges is to reduce platinum group metals (PGM) in fuel cell stacks. PGM are used as electrocatalyst and contained in catalyst layers (CLs) of membrane electrode assemblies (MEAs) in the stack. Various kinds of research activities on PEFC electrocatalyst have been reported so far, indicating high oxygen reduction reaction (ORR) activity and durability mainly measured in aqueous electrolyte systems such as a rotating disk electrode method.1-8 On the other hand, electrocatalyst is mixed with electrolyte to form CLs through deposition and solidification process. From industrial point of view, it is needed that performance of the electrocatalyst should be guaranteed as CL in MEA. CL is a multi-scale component, i.e. micro-scale porous electrode consisting of nano-scale materials (Fig. 1), where electrochemical reaction and mass transport occur simultaneously. Micro and nano structure of CL has been revealed gradually by means of characterization techniques remarkably progressed recently, such as electron microscope. 9-10However, it is still under development to characterize and diagnose structure and properties of CLs as overall and average value. To reduce PGM usage, so far Nissan has been doing research and development on CL based on the strategy to understand the correlation between CL/MEA performance and the consisting materials through properties and structure via key mechanism.11-20In this presentation, diagnostics of PEFC CLs related to activity and mass transport are explained, mainly focusing on the unique analysis Nissan has been developing thus far.

Enhancing the quality of the tomography of nanoporous materials for better understanding of polymer electrolyte fuel cell materials

To investigate the nanostructure of polymer electrolyte fuel cell catalyst layers, focused ion beam – scanning electron microscopy (FIB-SEM) tomography is a common technique. However, as FIB-SEM tomography lacks of image contrast between the catalyst layer and its pores, state-of-the-art reconstruction methods by threshold cannot accurately distinguish between these two phases. We show that this inability leads to an underestimation of the porosity by a factor of nearly two, a reconstruction with channel-like artifacts and that these artifacts make it impossible to calculate reliable diffusivities. To overcome this problem, we fill the pores of the catalyst layer with ZnO via atomic layer deposition prior to tomography. By using atomic layer deposition, even smallest pores can be filled with ZnO, which exhibits a good contrast to the catalyst layer in SEM images. As a result, we present the porosity of the catalyst layer (65%) and its three-dimensional representation without typical reconstruction artifacts. Calculated O2 diffusivities (23.05–25.40 × 10−7 m2 s−1) inside the catalyst layer are in good agreement with experimental values from the literature. Furthermore, filling with ZnO permits the identification of large Pt clusters inside the catalyst layer, which were estimated to reduce the catalyst surface area by 9%.

Interfacial Nanostructure of the Polymer Electrolyte Fuel Cell Catalyst Layer Constructed with Different Ionomer Contents

Fibrous carbon materials such as carbon nanotubes (CNTs) and carbon nanofilaments (CNFs) have attracted attention for use in the polymer electrolyte fuel cells (PEFCs). We have applied one type of fibrous carbon materials named Marimo carbon (MC) as the catalyst support. The modified nanocolloidal method was used preparing the Marimo carbon supported Pt catalyst (Pt/MC). And then, ionomer/carbon ratio in the cathode catalyst layer of the PEFC has been investigated for its effect on performance and structure of the membrane electrode assembly (MEA). The morphologies and cell performance of cathode catalyst layer using Marimo carbon changed dramatically with ionomer content. In case of excess ionomer, the air volume between CNFs was plugged up by ionomer, and blocked supply fuel gas diffusion caused decrease cell performance. PEFC cell performance was increase with decreasing ionomer content.

Nano-morphology of a polymer electrolyte fuel cell catalyst layer—imaging, reconstruction and analysis

The oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) of polymer electrolyte fuel cells (PEFC) is one of the major causes of performance loss during operation. In addition, the CCL is the most expensive component due to the use of a Pt catalyst. Apart from the ORR itself, the species transport to and from the reactive sites determines the performance of the PEFC. The effective transport properties of the species in the CCL depend on its nanostructure. Therefore a three-dimensional reconstruction of the CCL is required. A series of two-dimensional images was obtained from focused ion beam — scanning electron microscope (FIB-SEM) imaging and a segmentation method for the two-dimensional images has been developed. The pore size distribution (PSD) was calculated for the three-dimensional geometry. The influence of the alignment and the anisotropic pixel size on the PSD has been investigated. Pores were found in the range between 5 nm and 205 nm. Evaluation of the Knudsen number showed that gas transport in the CCL is governed by the transition flow regime. The liquid water transport can be described within continuum hydrodynamics by including suitable slip flow boundary conditions.