Cell Catalyst Layers Research Articles

Probing Ionomer Swelling and Water Content in Catalyst Layer during Operation Using Small Angle Neutron and X-Ray Scattering

In the fuel cell catalyst layers (CLs), the ionomer plays roles as a binder and a conductor for protons. It is now known that the ionomer not only forms aggregates between the catalyst agglomerates of the porous structure of the CLs, but it also forms a few nanometer thick thin film covering the surface of this catalyst. In this latter form, it modifies the specific activity of the catalyst by altering the structure and behavior of the electrochemical double layer, and it hinders the oxygen transport toward the catalytic sites. In addition, the nanostructure and functional transport properties of O2 and protons of the ionomer depends on its water content which increases with water activity. Especially, the ionomer must be sufficiently hydrated to enable proton transport from the membrane toward the active sites through the CLs and ensure a proper utilization of the electrochemical surface area. However, excess liquid water accumulation in the pores of the CL hinders oxygen diffusion and should be avoided. Therefore, it is necessary to understand the distribution and the quantity of water, being either within the ionomer or in the pores of the catalyst layers, during fuel cell operation to improve their design and constituents. Very recently, and based on our former experience, we pushed forward the in situ and operando Small Angle Scattering techniques to quantify the water content and ionomer swelling in fuel cell CLs during operation. In this work, we report the effect of the conditioning and operating conditions on the ionomer behavior (Figure - Comparison of SANS Spectra obtained in dry state, at 100%RH and during operation at 80%RH, showing the evolution of the ionomer peak related to its swelling). Figure 1

In Situ Measurement of Ionomer Water Content and Liquid Water Saturation in Fuel Cell Catalyst Layers by High-Resolution Small-Angle Neutron Scattering

In fuel cell catalyst layers (CLs), the ionomer plays roles as a binder and proton conductor. The ionomer requires hydration for enabling proton transport; however, the accumulation of excess liquid water in CL pores hinders oxygen diffusion. Therefore, a key to the improvement of CLs is understanding the distribution and the quantity of water in an operando fuel cell. However, there is a lack of information on the water content in conventional Pt-based CLs during operation, especially within the ionomer phase. Here, we present the application of high-resolution operando small-angle neutron scattering (SANS) on custom fuel cells to probe the CL water content. Using this technique, we quantified liquid water saturation and ionomer nanostructure (swelling) in in situ CLs, unique information not accessible via conventional diagnostic tools such as imaging. Even at a low current density (150 mA/cm2), water was observed in the cathode CL as liquid and adsorbed in the ionomer. Improving the understanding of water management in the CL is of high interest in fuel cell community, and our results confirm that operando SANS will be indispensable for the development of catalysts and ionomer-based CLs.

Pore-Scale Modeling of the Fuel Cell Catalyst Layer: The Role of Nafion Thin Films

In this work, a multiphysics pore-scale model of electrochemical reactive transport inside a fuel cell catalyst layer (CL) is presented and is used to elucidate the role of Nafion thin films on the overall performance of the CL. One of the main challenges of modeling transport in CL is the hierarchy of length scales and the multiphysics nature of transport. Consequently, traditional continuum methods, in which volume-averaged quantities are used to encapsulate the microstructural features, cannot give a clear picture of the multiphysics transport processes inside the CL. Alternatively, pore-scale methods, which take into account the true geometry down to the pore level, are suited to capture these processes for such systems. Previously, researchers have mainly employed direct numerical simulation (DNS) techniques for pore-scale modeling of the CL. While these techniques are highly accurate, they are very computationally expensive. This limits the domain size that can be simulated in a reasonable time. In this work, pore network modeling, a computationally cheap technique to simulate pore-scale transport, is used to study transport in the CL. Using this approach, we were able to speed up the simulation runtime up to 20-50x compared to DNS. Consequently, simulation of much larger domains of the CL was made possible, leading to more representative results. Finally, the developed multiphysics pore-scale model was used to study the role of Nafion thin films on the overall performance of the CL. It is notable that the network approach was only applied to the void phase, and the Nafion region was resolved using DNS. Therefore, yet further speed up can potentially be obtained by applying the network approach to the Nafion phase.

Liquid Water Uptake and Contact Angle Measurement of Proton Exchange Membrane Fuel Cell (PEMFC) Electrodes

In this work, capillary rise experiments were performed to assess the wetting properties of free-standing proton exchange membrane fuel cell catalyst layers (CLs) with several ionomer chemistries (equivalent weight -EW- ranging from 540 to 1115). The samples were attached to a micro-balance and then immersed into liquid water to (i) measure the mass gain from the liquid uptake and (ii) estimate the (external) contact angle to water. The results showed that the ionomer EW had an impact on the CL wettability. Specifically, a highly hydrophobic behavior was observed at high ionomer EW (i.e., 1115): 173° contact angle to water. More hydrophilic behavior was observed at low ionomer EW (i.e., 540): 107° contact angle to water. For an intermediate range of ionomer EW (i.e., from 620 to 1100), minimal variation in the CLs wetting properties was observed at an average contact angle of 160°.

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

PEM fuel cells offer the advantages of high power density and energy conversion efficiency, simplicity in design and operation, the added environmental benefits such as zero carbon emissions, and the production of benign by-products such as water when using the H2-O2 fuel cell.1,2 Additionally, reversible fuel cells (flow batteries) offer a viable solution to the highly desired need for economical grid energy storage in order to take full advantage of load leveling.3 As the use of intermittent energy sources such as wind and solar power continue to rise throughout the world, the need for reliable, efficient and economical energy storage solutions will grow.3 Excessive liquid water buildup in the fuel cell catalyst layer (CL) at high current densities can lead to electrode flooding, thus restricting transport of gaseous reactants to the catalyst reaction sites. In order to realize the economic viability of fuel cells, the fuel cell catalyst layer (CL) needs to be redesigned to overcome the negative hydration effects common with PEM fuel cells. Previous studies in our research group addressed the electrode flooding problem by incorporating a hydrophobic phase, such as Teflon, into the cathode CL in order to enable oxygen to be more easily transported to the catalyst reaction site.4 With this approach, fuel cell testing demonstrated improved performance when incorporating Teflon into the CL and showed that optimal amounts of Teflon was highly dependent on the Nafion content.4 The CL structure with Teflon integration is presented in Figure 1A.4 For this research, our hypothesis is based on recent discoveries about the interfacial properties of fluorocarbon-based ionic polymers. Recently, it was discovered that fluorocarbon based polymers exposed to high relative humidity at the air-polymer interface cause the ionic sulfonate groups within Nafion® to rearrange so that the surface is made up of a larger proportion of ionic sulfonate groups.5 Furthermore, when the fluorocarbon-based polymer is exposed to low relative humidity at the air-polymer interface, the ionic sulfonate groups move inward causing the surface to have a higher proportion of hydrophobic Teflon-rich regions.5 We verified that the surface structure of Nafion® and electrospun nanofiber composite membranes depend on the relative humidity (RH) of the gas in contact with the membrane.6,7 By operating an atomic force microscope (AFM) in both contact and tapping modes, we were able to show the dependence of the membrane’s surface by changing the RH, scanning the surface with an AFM and comparing the AFM images.5,7,8 Topography, phase contrast, and conductivity images were captured using the AFM to explore the surface structure of Nafion® and electrospun nanofiber composite membranes.5,7 Based on these observations, it is hypothesized that by heat treatment of the polymer within the fuel cell CL we can engineer the polymer’s surface inside the pores of the CL to be hydrophobic so that oxygen would be able to access the catalyst reaction sites without having to diffuse through a layer of water. The proposed CL structure for this work is shown in Figure 1B.

Finite Thickness Effects on Nafion Properties in the PEMFC Catalyst Layer, Probed By in Situ Neutron Reflectometry

A major technical challenge impeding the optimization of polymer electrolyte membrane fuel cells (PEMFCs) is a lack of understanding regarding Nafion properties inside the fuel cell catalyst layer (CL). Inside the CL, thin Nafion layers (typically on the order of 2 – 80 nm thick) cover solid electrode particles and agglomerates. In this thickness range, dimensional confinement and interfacial effects at solid, liquid, and vapor phases alter the properties of the films. While recent studies have explored water content and transport properties in this thickness range to demonstrate reduced water mobility for films below roughly 60 nm in thickness, the effects of thickness and substrate variation on water uptake and Nafion structure in this range are still debated. Thus far, an unambiguous documentation of Nafion composition and structure for thicknesses typical of the PEMFC CL has not been presented. We present here the use of neutron reflectivity (NR) as an in situ probe of structure and composition for Nafion films with thicknesses of 5 – 200 nm. NR measurements provide simultaneous thickness and composition information for thin-films under controlled conditions, via an SLD depth profile with sub-Ångström resolution for structural elements thicker than roughly 1.5 nm. Neutrons transmit easily through sample chamber walls and have an inherent sensitivity to hydrogen isotopes, with a large SLD variation between H and D isotopes, making NR ideal for in situ measurements of thin-film Nafion hydration. Through the application of multiple NR measurements, elemental conservation, and Bayesian analysis, results presented will document varying water sorption and interfacial structures in thin-film Nafion for varying thickness, substrates, and humidity levels. Results provide an unambiguous confirmation of the reduced water sorption with reducing thickness below 60 nm, a detailed analysis of the sheet-like lamellar phase segregation on SiO2 substrates, as well as an illustration of how substrate modification can affect Nafion properties. Figure 1 shows the variation in water sorption with varying film thickness. As previously observed, for layers that approximate the phase-segregation of bulk Nafion films (“bulk-like”) water uptake decreases steadily for film thicknesses below 60 nm. However, within multi-lamellar polymer structures found at hydrophillic substrate interfaces, the water content reaches a minimum for films roughly 20 nm thick, and increases for films thinner than 15 nm. Furthermore, the average water content within the multi-lamellar structures is slightly higher than in the bulk-like outer layers. Simple numerical simulation results will be presented to discuss the implications of Nafion structural and composition variations in the CL for PEMFC performance. Figure 1

Effect of Water Sorption on the Electronic Conductivity of Porous Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

A method is described for measuring the effective electronic conductivity of porous fuel cell catalyst layers (CLs) as a function of relative humidity (RH). Four formulations of CLs with different carbon black (CB) contents and ionomer equivalent weights (EWs) were tested. The van der Pauw method was used to measure the sheet resistance (RS), which increased with RH for all samples. The increase was attributed to ionomer swelling upon water uptake, which affects the connectivity of CB aggregates. Greater increases in RS were observed for samples with lower EW, which uptake more water on a mass basis per mass ionomer. Transient RS measurements were taken during absorption and desorption, and the resistance kinetics were fit using a double exponential decay model. No hysteresis was observed, and the absorption and desorption kinetics were virtually symmetric. Thickness measurements were attempted at different RHs, but no discernible changes were observed. This finding led to the conclusion that the conducting Pt/C volume fraction does not change with RH, which suggests that effective medium theory models that depend on volume fraction alone cannot explain the reduction in conductivity with RH. The merits of percolation-based models were discussed. Optical micrographs revealed an extensive network of "mud cracks" in some samples. The influence of water sorption on CL conductivity is primarily explained by ionomer swelling, and its effects on the quantity and quality of interaggregate contacts were discussed.

An Improved Agglomerate Model for the PEM Catalyst Layer with Accurate Effective Surface Area Calculation Based on the Sphere-Packing Approach

The complex porous structure of the PEM fuel cell catalyst layer (CL) necessitates the use of multiscale modeling strategies such as the agglomerate approach. In this study a 2D steady-state model for the cathode CL is developed using the spherical agglomerate approach. A new, more accurate, method is introduced to determine the effective agglomerate surface area, which plays a key role in estimating diffusion losses in the CL. Specifically, the reduction in the effective surface area due to overlapping particles is modeled geometrically based on a sphere-packing approach. In addition, the equations for the agglomerate model are reformulated to correctly account for the agglomerate surface area reduction due to overlapping particles. The importance of an accurate geometric model for the effective surface area is demonstrated by investigating the effect of CL composition on performance, and the results show that the new method provides more realistic predictions than the existing approach. Results from the new approach for optimal Pt loading, ionomer loading, and Pt|C ratio show good agreement with experimental results.

Thermo‐Mechanical Response of Fuel Cell Electrodes: Constitutive Model and Application in Studying the Structural Response of Polymer Electrolyte Fuel Cell

AbstractThe characterization of the mechanical properties of fuel cell electrodes through the experimental techniques is a complex task due to the low thickness, constituents' heterogeneous composition, and fragile nature of the film. We present a preliminary investigation on the thermomechanical response of fuel cell catalyst layer (CL) obtained through the numerical experiment. Since the Nafion ionomer is one of the constituents' of the CL, a modified micromechanically motivated viscoplastic model is adopted to characterize the Nafion ionomer in terms of reduced density factor to account for the void content. The catalyst agglomerates are taken as inclusions in the ionomer matrix to form a composite unit which is used to plot the true stress–true strain response. Practicality of this work is tested by implementing the electrode layer as a separate component in the single fuel cell unit cell model. A remarkable difference in the magnitude of stress levels in the membrane is observed under thermal and hydrated conditions with the presence and absence of electrode layer in the simulation domain. The present work will assist in improved understanding of the localized stress distribution in the membrane, which is essential to understand its mechanical endurance.

On the Micro-, Meso-, and Macroporous Structures of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

In this work, N(2) adsorption was employed to investigate the effects of carbon support, platinum, and ionomer loading on the microstructure of polymer electrolyte membrane fuel cell catalyst layers (CLs). Brunauer-Emmett-Teller and t-plot analyses of adsorption isotherms and pore-size distributions were used to study the microstructure of carbon supports, platinum/carbon catalyst powders, and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were chosen for the investigation: Ketjen Black and Vulcan XC-72. CLs with a range of Nafion ionomer loadings were studied in order to evaluate the effect of an ionomer on the CL microstructure. Regions of adsorption were differentiated into micropores associated with the carbon primary particles (<2 nm), mesopores ascribed to the void space inside agglomerates (2-20 nm), and meso- to macroporous space inside aggregates of agglomerates (>50 nm). Ketjen Black was found to possess a significant fraction of micropores, 25% of the total pore volume, in contrast to Vulcan XC-72, for which the corresponding fraction of micropores was 15% of the total pore volume. The microstructure of the carbon support was found to be a significant factor in the formation of the microstructure in the three-component CLs, serving as a rigid porous framework for distribution of platinum and the ionomer. It was found that platinum particle deposition on Ketjen Black occurs in, or at the mouth of, the support's micropores, thus affecting its effective microporosity, whereas platinum deposition on Vulcan XC-72 did not significantly affect the support's microstructure. The codeposition of ionomer in the CL strongly influenced its porosity, covering pores < 20 nm, which are ascribed to the pores within the primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm).