Modified Agglomerate Model for Low-Pt Cathode Catalyst Layer in PEMFC

A parametric analysis is presented using a previously validated 1D model for a cathode catalyst layer (CL). The results show that maximum power density at low Pt loading can be maximized with relatively thin CLs (thickness ∼ 2 μm) featuring a high carbon volume fraction (low ionomer-to-carbon weight ratio, I/C) compared to high Pt loading CLs. The shift of the optimal carbon volume fraction (I/C ratio) is caused by the dominant role of the local oxygen transport resistance at low Pt loading, which is lowered by a reduction of the average ionomer film thickness (better ionomer distribution among carbon particles). In contrast, at high Pt loading, higher porosity and pore radius (lower carbon volume fraction) is beneficial due to an increase of bulk effective diffusivity despite thickening of ionomer films. Moreover, the results show that performance at low Pt loading is significantly improved with increasing mass-specific activity. The effect of average saturation and ionomer permeability on performance at low Pt loading is lower compared to dry CL composition and mass-specific activity.

A model for the cathode catalyst layer (CL) is presented, which is validated with previous experimental data in terms of both performance and oxygen transport resistance. The model includes a 1D macroscopic description of proton, electron and oxygen transport across the CL thickness, which is locally coupled to a 1D microscopic model that describes oxygen transport toward Pt sites. Oxygen transport from the channel to the CL and ionic transport across the membrane are incorporated through integral boundary conditions. The model is complemented with data of effective transport and electrochemical properties extracted from multiple experimental works. The results show that the contribution of the thin ionomer film and Pt/ionomer interface increases with the inverse of the roughness factor. Whereas the contribution of the water film and the water/ionomer interface increases with the ratio between the geometric area and the surface area of active ionomer. Moreover, it is found that CLs diluted with bare carbon provide lower performance than non-diluted samples due to their lower electrochemical surface area and larger local oxygen transport resistance. Optimized design of non-diluted samples with a good distribution of the overall oxygen flux among Pt sites is critical to reduce mass transport losses at low Pt loading.

Effect of silicon carbide particle size in the electrolyte matrix on the performance of a phosphoric acid fuel cell

Effects of agglomerate model parameters on transport characterization and performance of PEM fuel cells

Predicting PEMFC performance from a volumetric image of catalyst layer structure using pore network modeling

Self-startup of polymer electrolyte fuel cell (PEFC) without external heating is advantageous for the system. It is generally recognized that the produced ice accumulating in the cathode catalyst layer (CL) reduce the electrochemically active area (ECA) for oxygen reduction reaction (ORR) and the open pore volume for oxygen diffusion. Visualization results obtained from a transparent PEFC shows the importance of the catalyst layer design and gas purge to freeze-starts.1,2 A cyclic voltammetry (CV) technique was developed to investigate the effect of ice formation in the cathode CL on electrochemically active Pt area during and post subzero startup of a PEFC.3Mechanical stress built in the cell during cold start may limit its lifetime. To our knowledge, no work has been done about mechanical stress during cold start in PEFC.A cyclic voltammetry (CV) technique is developed to further investigate the effect of membrane thickness and startup current density on cumulative product water, rate of ice formation in electrode and rate of water absorption by ionomer and membrane in PEFC during cold start. Figure 1a shows typical CVs of the cathode prior to and post cold start. Obviously, the electrochemically active Pt surface area of the cathode CL post cold start is lower than that prior to. It is found that thick membrane offers much more capacity for water to be absorbed by the membrane at −10oC and −20oC. The product water almost does not increase with membrane thickness for starting from −30oC because water absorption by the membrane is negligible. The rate of ice formation in cathode catalyst layer increases with current density but decreases with the startup temperature. The rate of water absorption by the membrane increases with the startup temperature, current density and membrane thickness. A mathematical model is developed to investigate mechanical stress during cold start in PEFC. Ice produced in the catalyst layer forces the catalyst layer to expand. Figure 1b shows the effect of cold-start temperature on stress during cold start. Cold starting from low temperature brings high compress mechanical stress. Cold starting from low temperature also bring high strain in CL, but low strain in the membrane. The membrane is compressed during cold start from -30oC. That is because the amount of water absorbed by membrane is negligible but CL expands a lot. This work demonstrates the effect of thickness of gas diffusion layer (GDL), micro-porous layer (MPL) and CL on the mechanical stress built in the membrane. Membrane constraint is considered in the model.4The simulation result shows that mechanical stressed during purge, cool down and cost start operation do not vary serious due to elastic and porous MPL and GDL. The model is to help elucidate the fundamental physics of ice formation in the PEFC during cold start.Figure 1. (a) Cyclic voltammograms of the cathode prior to and post cold start at startup temperature of −10oC and −30oC. (CCM-25 mm. m Pt=0.4 mg/cm2. Startup conditions: i start=0.05 A/cm2, x H2=14.35, x air=12.06, RHH2=RHair=0%, p H2=p air=1 atm. CV conditions: T CV=T start, n=50 mV/s, F N2=F H2=40 mL/min, RHN2=RHH2=0%.) and (b) Effect of cold-start temperature on stress during cold start (lInitial= 6.0).

In this work, an analytical model of the microporous layer (MPL) and the catalyst layer (CL) interface under compression is developed to investigate the effects of the MPL|CL interfacial morphology on the ohmic and mass transport losses at the MPL|CL interface in a polymer electrolyte fuel cell (PEFC). The model utilizes experimentally measured surface profile data as input. Results indicate that the uncompressed surface morphology of mating materials, the elasticity of PEFC components, and the local compression pressure are the key parameters that influence the characteristics of the MPL and CL contact. The model predicts that a 50% drop in the MPL and CL surface roughness may result in nearly a 40% drop in the MPL|CL contact resistance. The model also shows that the void space along the MPL|CL interface can potentially store a significant amount of liquid water , which could result in performance loss and reduced durability. A 50% drop in the MPL and CL surface roughness is expected to yield nearly a 50% drop in the water storage capacity of the MPL|CL interface. The results of this work provide key insights that will enhance our understanding regarding the complex relation between MPL|CL interfacial structure and cell performance.

A parametric study of cathode catalyst layer structural parameters on the performance of a PEM fuel cell

In polymer electrolyte fuel cells (PEFCs), the cathode catalyst layer (CL) needs a large amount of Pt because of the slow oxygen reduction reaction. Since electron, proton, and oxygen are necessary for the cathode reaction in PEFCs, achieving the optimum structure of cathode CL and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. Some researches reported that when Aquivion is used as the ionomer of the CL, the output voltage increases as compared with the CL using Nafion in the intermediate current density region (1). However, dominant factors improving the cell performance have not been elucidated quantitatively. In this study, the oxygen transport resistance at the Pt surface, which is thought to have a large contribution to the oxygen transport resistance inside the CL (2), was investigated using CLs fabricated with different ionomers. In this study, catalyst coated membranes was fabricated with various cathode structure: 6 types of cathode side CLs using three kinds of Pt-supported carbons (TEC10E30E, TEC10E50E, TEC10E70TPM) having different Pt carrying densities and two types of ionomer, Nafion and Aquivion. All of these CLs were fabricated with I/C = 0.8. First, to distinguish the oxygen transport resistance in the CL from the other resistance such channels, I-V measurement experiments were conducted with different total pressures (0.10MPa, 0.15MPa, 0.20MPa), and the limiting current density method (2) was applied. In the I-V measurement, the flow rate of cathode gas (oxygen and nitrogen mixture) was 2000 sccm with different oxygen concentration (1%, 2%, 3%) and the flow rate of anode hydrogen was 100 sccm with RH 80% (80°C). Next, the oxygen transport resistance at the Pt surface was evaluated. (The details will be described later). In this evaluation, the electrochemical surface area (ECSA [m2/gPt]) in the CL is needed. The ECSA was estimated by CV measurement. In the CV measurement, cathode was filled with N2 gas with RH 100% (80°C). The flow rate of anode hydrogen was 100 sccm with RH100% (80°C). Figure 1 shows I-V characteristics of the CL with Pt loadings of approximately 0.20 mg/cm2. The output current density for the Aquivion based CL is higher than that of the Nafion based CL at a voltages around 0.5 V, whereas the Nafion based CL has superior performance in the operation at 0.1 V where oxygen supply is limited by various transport resistances. In this paper, we focus on the oxygen transport resistance at low voltage, in particular the resistance at the Pt surface, because the Pt surface resistance is considered to be dominant part of oxygen transport resistance under the low Pt loading CL. Figure 2 shows the ECSA estimated from the CV measurement. The values of ECSA are similar with Nafion and Aquivion based CLs. As the Pt carrying density increases, the ECSA decreases. This is because the Pt particles aggregate on the carbon and the surface area becomes smaller. Using the simplified evaluation formula of oxygen transport resistance (2), the total oxygen transport resistance in the CL can be expressed as RCL = RPt + Rdiss + Rpore . The RPt means the resistance at the Pt surface, Rdiss means the resistance of oxygen dissolves into ionomer, and Rpore means the oxygen diffusion resistance in the CL pores. The RPt is formulated as RPt = 1 / (APt × γ × kPt ) (2). Here, APt is the reactive surface area of catalyst Pt per unit area of the CL, γ is ratio of oxygen concentration in the ionomer and CL pore, and kPt [m/s] is the oxygen transfer rate constant at the Pt surface. Figure 3 shows the result evaluating RPt with both ionomer based CLs. The ordinate represents RCL and the abscissa represents 1/APt . Using this result, kPt can be calculated as kPt = 1.5×10 -2 s/m with the Nafion based CL and kPt = 9.1×10 -3 s/m with the Aquivion based CL. As the oxygen transport resistance at the Pt surface is inversely proportional to the kPt , the resistance of Aquivion based CL is estimated to be larger. This indicates a possibility that Aquivion based CL is inferior due to its larger Pt surface resistance. The larger Pt surface resistance may be due to the characteristics of Aquivion: the shorter side chain and the hydrophilic property induce strongly condensed structure of Aquivion at the surface of Pt, and that results in inhibiting the transport of oxygen. Reference (1)C. Park, et al., J. Power Sources, 315,179 (2016). (2)Tabe, et al., ECS Trans., 80(8), 205 (2017). Figure 1

The catalyst layer used in a fuel cell is an archetype of multi-component composite materials used as electrodes in many electrochemical devices. In this talk, we will describe and link together a number of studies of the fuel cell catalyst layer that we have been carried out over the past 10 to 15 years. The first of these was a modeling effort that was initially carried out to describe the geometrically constrained limits on utilization of dispersed catalyst in the layer. To answer our original question, the catalyst components were broken down to constituents (Pt nanoparticles, carbon particles, ionomer) and interactions between components were used to build a hierarchical model of the catalyst layer. The interactions were defined in the context of colloidal interaction theory using descriptions of electronic properties of the constituents derived from experimental measurements of refractive index/dielectric properties across a wide range of frequencies. The model satisfactorily captured some aspects of the structure of the catalyst layer, e.g. the tendency toward segregation of components, and provides a good bound to the utilization of catalysts but failed to provide a quantitatively accurate description of certain transport-related physical properties. In the next phase of the work, the connectivity of ionomer in the catalyst layer was probed using NMR relaxation and diffusion measurements on water in the catalyst layer as a function of water content. Finally, a detailed study of the distribution of components within the catalyst layer, the catalyst layer porosity and the adsorption of ionomer in catalyst layers, all as functions of composition, was undertaken. These three chapters of work correspond to three different probes usng models and experiments of many types to yield a more nuanced picture of the catalyst layer. Future prospects and needs will be presented in the context of still-missing information and where we might look for it. Acknowledgement We gratefully acknowledge the support of this work by the US Army Research Office and the Naval Research Research Office. The significant contributions of Professor J. Adin Mann of Case Western Reserve university and Dr. Karren More of Oak Ridge National Lab are also appreciated.

A collaborative design for the uneven distributions of a flow channel, gas diffusion layer porosity and catalyst layer porosity are newly proposed to improve the utilization ratio of the membrane electrode assembly of the proton exchange membrane fuel cell. The effects of the uneven design of the rib width and of the uneven porosity parameters of the cathode and anode gas diffusion layer and catalyst layer on the fuel cell performance were studied in detail. Numerical simulations were designed and implemented for validation. The results show that the fuel cell performance could be improved through the collaborative design of uneven distributions for different layers. The rib width gradually decreasing and the porosity of the cathode gas diffusion layer and the cathode catalyst layer gradually increasing along the fluid flow direction would contribute to a better design compared to the regular even design. The new uneven design can make the fuel penetrate into the catalyst layer in time to participate in the reaction, improve the utilization rate of the membrane electrode assembly, and greatly improve the performance of the fuel cell.

Proper water management in cathode porous electrode of polymer electrolyte fuel cells (PEFCs) is essential for achieving better performance. Especially at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed in catalyst layer (CL) and gas diffusion layer (GDL). When the open pores in cathode CL and the interface between CL and GDL are filled with liquid water, oxygen cannot be supplied to the reaction sites. This phenomenon, called “water flooding”, is a critical barrier for high efficiency and high power density operation. To alleviate this issue, it is necessary to understand and control liquid water transport inside cathode porous electrode. It is well known that microporous layer (MPL) plays a critical role in suppressing liquid water flooding in PEFCs [1,2]. MPL is a much thinner and finer porous layer, which is typically composed of carbon black (CB) particles and PTFE, and inserted between cathode CL and GDL. Nam et al. [2] experimentally and numerically investigated the vapor condensation and liquid water breakthrough in porous layers, and showed that inserting MPL between CL and GDL reduces the size and saturation level of interfacial water droplets on CL surface. However, there is a risk that MPL addition on cathode side increases oxygen transport resistance through porous layers. In general, MPL is formed by coating the slurry of CB powder and PTFE onto GDL substrate and then heat-treated. Its pore structure is complicatedly affected by various fabrication parameters such as CB loading, PTFE/CB ratio, coating and dry processes. To control interfacial water transport between cathode CL and GDL and minimize water flooding, it is essential to optimize the fabrication parameters of MPL determining its microstructure. In this study, we focused on CB loading as one of the major fabrication parameters of MPL, and analyzed the pore structure of MPLs with different CB loadings using SEM-EDX and mercury porosimetry. Furthermore, the operation test and the optical visualization of liquid droplet behavior in the cathode electrode were conducted using a transparent cell, and the influence of CB loading of MPL on its microstructure, the interfacial water transport and the performance characteristics of PEFC was discussed. In the experiment, the MPLs were coated on the hydrophobized carbon paper (Toray, TGP-H-060) by using the doctor-blade technique. The SEM-EDX observation revealed that the average thickness of two MPLs with 1.0 and 4.0 mg/cm2 CB are 33.7 and 123.6 um, respectively. When the CB loading is increased from 1.0 to 4.0 mg/cm2, the thickness of MPL is enlarged approximately 3.7 times. Subsequently, the operation test of a PEFC was carried out at 70 oC under high-humidity conditions, and the effect of CB loading of MPL on the I-V characteristics was investigated. The MPL addition on the cathode side drastically improves the limiting current density, because it suppresses the formation of liquid water film at the CL/GDL interface and reduces the concentration overpotential. In addition, the cell voltage in the high current density range increases with an increase in CB loading of MPL. Since the thick MPL raises the temperature near the cathode reaction sites, the rate of condensation of product water at the CL decreases in the case of 4.0 mg/cm2CB. Therefore, the thick MPL with high CB loading beneficially reduces water flooding at the cathode CL/MPL interface and improves the cell performance.

A mathematical model for two-phase flow and flooding dynamics in polymer electrolyte fuel cells (PEFCs) has been developed based on recent experimental observations. This three-dimensional PEFC model consists of four submodels to account for two-phase phenomena, including a catalyst coverage model in the catalyst layer, a two-phase transport model in the gas diffusion layer (GDL), a liquid coverage model at the GDL-channel interface, and a two-phase flow model in the gas channel (GC). The multiphase mixture model is employed to describe liquid water transport in the GDL while a mist flow model is used in the gas channel. An interfacial coverage model by liquid water at the GDL/GC interface is developed, for the first time, to account for water droplet emergence on the GDL surface. The inclusion of this interfacial model not only gives the present two-phase model a capability to predict the cathode flooding effect on cell performance, but also ultimately removes the inability of prior two-phase models to correctly capture effects of the gas velocity (or stoichiometry) on cell performance.

Polymer electrolyte fuel cells’ (PEFCs) widespread commercialization is hindered by the devices’ limited durability, in terms caused by the corrosion of the carbon support used in the Pt-based PEFC catalysts. Using unsupported electrocatalysts could mitigate such durability issues, but little is known regarding the manner in which their processing into catalyst layers (CLs) affects pore size distribution (PSD) and PEFC performance. Thus, we have used a computational model to investigate the modes of agglomerate packing in CLs made from unsupported Pt3Ni nanochain ensembles (aerogels) or Pt black, and complemented this analysis with focused ion beam scanning electron microscopy tomography of corresponding real CLs. 3D structures, PSDs and tortuosities were obtained for real and computed CLs and were found to be in good agreement. The Pt black CL mainly exhibits large and straight pores (>100 nm wide), while the Pt3Ni aerogel CL mostly features small and twisted pores (< 100 nm wide) that cause the significantly poorer O2 mass transport (vs. Pt black) observed in PEFC experiments. Moreover, this modeling approach leads to key insights on the working principle of a filler material used for positively shifting the average PSD and improving the PEFC performance of the Pt3Ni CL.

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this application, they must show performance and durability with the requirements for transportation. For transportation applications, the fuel cell will be subjected to frequent power cycling. For example, the DOE/Fuel Cell Tech Team (FCTT) protocol for durability includes load cycling from 0.02 A/cm2 to 1.2 A/cm2 every 0.5 min. The cathode catalyst and catalyst layer have been shown as susceptible to degradation causing loss of performance due to both loss of kinetics for the oxygen reduction reaction and loss of mass transport. Catalyst support-carbon corrosion can result in thinning of the catalyst layer contributing to degradation in performance. To examine the effect of power cycling in situ on carbon corrosion and electrode degradation, we directly measured the catalyst support degradation by measuring CO2 in the cathode outlet by NDIR (Non-Dispersive Infra-Red) while operating a single-cell fuel cell. CO2 present in air was removed by a lime bed prior to introduction to the fuel cell. We operated with a modified DOE/FCTT durability protocol using controlled voltage, and varied the potential limits to explore the effects of the upper potential limit, lower potential limit, the potential step size and time at potential. The upper potential limit was varied from 0.95 to 0.55V; the lower potential limit from 0.40V to 0.80V, with times ranging from 0.5 min to 5 min. The corrosion of three different types of carbon were explored, high surface area (E), vulcan (V), and graphitized (EA). The catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in these steps in potential. A series of carbon corrosion spikes during potential cycling is shown in Figure 1 for E-type carbon, varying the upper potential from 0.95 V to 0.60V while keeping the lower potential constant at 0.40V. Sharp spikes in the carbon corrosion rate are observed during a step increase in cell potential with the magnitude of the spikes decreasing as the high cell potential is reduced from 0.95 V to 0.6 V. The carbon corrosion rate at high cell potential (0.95V) decreases with time at potential, indicating formation of passivating carbon surface oxides. Carbon corrosion was measured during the drive cycle measurements for all three types of carbon, with the relative carbon corrosion rates of E>V>EA. The series of step potential carbon corrosion spikes where the potential was varied for the lower potential from 0.40 V to 0.60V while keeping the upper potential constant at 0.95V shows similar results in terms of the carbon corrosion. The magnitude of the spikes decrease as the lower cell potential is raised. These results indicate that the size of the step in potential has a more significant impact on the carbon corrosion rate than does the absolute value of the potential for normal cathode operating potentials. The peak in CO2 evolution occurs when the cell potential increases from high power operation to low power near open circuit. This correlates with when CO2 evolution is observed during cyclic voltammograms, which occurs during the positive sweep at ~ 0.55 to 0.60 V. The evolution of this CO2 peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface. During long-term operation, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or possibly due to compaction; both effects likely lead to a loss of void volume. This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours. Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion. These results show that carbon corrosion occurs during normal potential operation, and is exacerbated by potential variations during operation. To minimize the carbon corrosion, the size of the steps in potential should be minimized. For a more stable catalyst, another requirement is for the Pt particles to be stabilized on graphitized carbon supports. Acknowledgments Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland Figure 1