Ionomer Adsorption Research Articles

Electrochemical and Ultra-Small Angle X-Ray Scattering Study of Ionomer Adsorption on Catalytic Sites in PEFC Electrodes

The composition of the catalyst ink has a crucial impact on the electrode performance in the polymer electrolyte fuel cells (PEFCs). The presence of the perfluorosulfonic acid (PFSA) ionomer in the membrane electrode assembly (MEA) catalyst layer results in a sluggish oxygen reduction reaction (ORR) kinetics compared with an ionomer-free catalyst layer. Moreover, recently it has been shown that a thin film with high resistance to oxygen transport is formed around the catalyst particles due to its interaction with the ionomer.1 In order to improve the performance of the catalyst layer and eliminate or minimize the loss in both the ionic conductivity and electronic conductivity due to the ionomer-catalyst particles interaction in the next generation fuel cells new encased platinum group metal catalysts have been designed. Nanoporous materials have a wide range of technological applications2-6 and are potential candidates to improve the performance of the PEFC catalysts. Ordered inorganic nanoporous structures with well-defined and uniform pores are used to host the catalyst nanoparticles and block the access of the ionomer aggregates to the catalyst surface. In order to validate this assumption a series of catalyst inks with different ionomer: catalyst ratios (I/C) in isopropyl alcohol aqueous solvent were prepared. The ORR activity of the catalysts in thin film electrodes formed from these inks has been determined. The activity measurements are complemented by ultra-small-angle X-ray scattering (USAXS) studies on the inks and electrodes to determine the effect ionomer on the microstructure of the catalyst aggregates. References C. Cetinbas, R. K. Ahluwalia, N. N. Kariuki, and D.J. Myers, J. Electrochem. Soc., 165(13), F1051 (2018).A. Farghaly, R. K. Khan, and M. M. Collinson, ACS Appl. Mater. Interfaces, 10(25), 21103 (2018).Demir, A. A. Farghaly, M. J. Decuir, M. M. Collinson, and R. B. Gupta, Mater. Chem. Phys., 216, 508 (2018).Nahar, A. A. Farghaly, R. J. A. Esteves, and I. U. Arachchige, Chem. Mater., 29(18),7704 (2017).A. Farghaly and M. M. Collinson, Langmuir, 32(23), 5925 (2016).A. Farghaly, M. Lam, C. J. Freeman, B. Uppalapati, and M. M. Collinson, J. Electrochem. Soc., 163(4) H3083 (2016). Acknowledgements This work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.

Controlling cracking formation in fuel cell catalyst layers

Since cracks in catalyst layers of a fuel cell affect performance and durability, it is important to obtain the factors that control crack formation. Cracking characteristics are likely to differ due to the formation of agglomerates in catalyst inks. Herein, a critical crack thickness (CCT) is examined as a parameter for cracking characteristics by changing the formation of Pt/carbon by the presence or absence of an absorbed ionomer on Pt/carbon particles. The CCT was found to be four times greater for catalyst layers obtained from catalyst inks that are well dispersed by ionomer adsorption than for catalyst layers obtained from catalyst inks with a network of structured agglomerates. This shows that crack behavior can be controlled by ionomer adsorption into the Pt/carbon in catalyst ink. A well-dispersed catalyst ink produce a homogeneous Pt/carbon and ionomer distribution in catalyst layers with high fracture toughness, while catalyst inks with a network of structured agglomerates produce dense aggregates with small primary pores that generate high drying stress, and a self-organizing free ionomer that causes stress concentration in catalyst layers with a high risk of cracking. This research provided useful knowledge for controlling cracks and examining the effects of cracks on performance and durability.

Impact of ionomer adsorption on alkaline hydrogen oxidation activity and fuel cell performance

Summary The adsorption of the ionomer components significantly impacts the activity of hydrogen oxidation reaction (HOR) catalysts under high pH conditions. Two specific adsorptions, i.e., (i) cation-hydroxide-water co-adsorption and (ii) phenyl group adsorption, on the surface of platinum group metal HOR catalysts were identified as the performance-limiting factors for anion-exchange membrane fuel cells (AEMFCs). Here, we review the main characteristics of the surface adsorptions, their impact on the performance of HOR half-cell and AEMFC, and the mitigation strategies. This review emphasizes important aspects in the design of electrocatalysts and ionomers for improved AEMFC performance.

Simple and Reliable Method for Studying the Adsorption Behavior of Aquivion Ionomers on Carbon Black Surfaces.

A better understanding of the interactions of carbon black and perfluorinated sulfonic acid (PFSA) ionomer helps to improve the effectiveness of polymer electrolyte membrane fuel cells. We present a simple and fast method for quantitative PFSA ionomer analysis based on suspension density measurements. After validation of the reliability of our method by thermogravimetric analysis and isothermal titration calorimetry (ITC), we investigate the adsorption equilibrium of short-side-chain PFSA ionomers of different equivalent weights (EW) and polarities on carbon black. The measured adsorption isotherms exhibit a plateau in the ionomer surface concentration for ionomer equilibrium concentrations ≤2 g/L. In this concentration range, the adsorption isotherms are described by the Langmuir model, whereby the surface concentrations in the plateau region are between 0.041 and 0.070 g/g. The plateau value of the ionomer surface concentration increases with EW and therefore with decreasing number of side chains with terminal sulfonic acid group per ionomer molecule, while the amount of adsorbed sulfonic acid groups remains constant for all investigated ionomers, resulting in similar ζ-potentials and sedimentation stability of the suspensions. The free energies of adsorption Δ G calculated from the association constants of the adsorption isotherms agree well with Δ G values obtained by isothermal titration calorimetry (ITC) and thus validate the adsorption isotherm measurement method. From the values of adsorption enthalpy Δ H ((-7.3 ± 0.8) kJ/mol) and entropy Δ S (ca. 100 J/(mol K)), which were extracted from ITC, we conclude that the ionomer adsorption on carbon black is a spontaneous physisorption process.

Characterization of Catalyst Inks By Rheology and Microscopic Particle Properties

Fundamental understanding of the state of catalyst inks and fabrication process is important to fabricate well-established catalyst layers of proton exchange membrane fuel cells (PEMFCs). The catalyst ink is a slurry which contains platinum-supported carbon and ionomer in a solvent. The state of the catalyst ink affects dynamics of materials during the fabrication process of the catalyst layer(1), and then the resultant structure(2). Additionally, the state of the catalyst ink can have time dependence, which means the procedure of particle agglomeration and sedimentation inside the slurry. Therefore, quantitative evaluation of the catalyst ink is required. In this study, a rheological measurement was conducted to characterize catalyst inks used to fabricate PEMFC catalyst layers. The rheological measurement can apply to opaque and high viscosity slurries like catalyst inks. Ionomer dispersion, pseudo-catalyst ink and catalyst ink with various ionomer content were measured to clarify the relationship between the rheological behavior and the types and amount of particles and interparticle interactions in the slurry. The rheological measurement was conducted by using a rheometer with a cone and plate system. Shear rate range was 200 to 3000 /s. The measurement temperature was 25°C. Downward measurement (from upper to lower shear rate) following the upward measurement was conducted. Ionomer dispersion (DE2020, Sigma-Aldrich), carbon black (Ketjenblack, Lion)–ionomer dispersion (pseudo-catalyst ink), and platinum-supported carbon (Pt/C, TEC10E50E, Tanaka Kikinzoku Kogyo)–ionomer dispersion (catalyst ink) were prepared. The volume fraction of carbon to the solvent (45wt.% 1-propanol aqueous solution) was 2% and ionomer to carbon ratio (I/C) was a parameter from 0.3 to 2.0. The viscosities of the slurries as a function of the shear rate are shown in Figure 1. The three types of the slurries showed quite different rheological properties with each other. The ionomer dispersions are Newtonian fluid in all I/C conditions from 0.3 to 2.0 (Fig. 1 (a)). On the other hand, pseudo-catalyst ink (Fig. 1 (b)) and catalyst ink (Fig. 1 (c)) were non-Newtonian fluid when I/C was low. The pseudo-catalyst ink showed a transition from non-Newtonian to Newtonian as I/C increased from 0.3 to 0.5. The ink also showed the transition from Newtonian to non-Newtonian as I/C increased from 1.0 to 2.0. The catalyst ink slightly showed non-Newtonian properties even in I/C 0.5 and 1.0. Additionally, the catalyst ink with I/C 0.3 and 0.5 showed relatively large hysteresis between the upward and downward measurements. The rheological property of the slurries can be the results of the state difference of the particles in the slurries such as the particle agglomeration and/or ionomer adsorption. The measurements of particle size distribution by dynamic light scattering and zeta potential were applied to the slurries (The results are not shown here). The comprehensive discussion to understand the state of the catalyst ink will be presented. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 16K18028. References T. Suzuki, M. Kobayashi, H. Tanaka, M. Hayase and S. Tsushima, ECS Trans., 69, 465 (2015).W. K. Epting, J. Gelb and S. Litster, Adv. Funct. Mater., 22, 555 (2012). Figure 1

Characterization of Catalyst Inks By Rheology and Microscopic Particle Properties

Both rheological measurement and zeta potential measurement of catalyst inks used for catalyst layers of proton exchange membrane fuel cells were conducted and characterized the catalyst inks. The state of the catalyst ink affects dynamics of materials during the fabrication process of the catalyst layers, and then the resultant structure. Therefore, quantitative evaluation of the catalyst ink is required. In this study, ionomer dispersion, pseudo-catalyst ink and catalyst ink with various ionomer content were evaluated to clarify the relationship between the rheological behavior and the types and amount of particles and interparticle interactions in the slurry. Ionomer works as a dispersant like a surfactant. Adding ionomer changed the rheological behavior from non-Newtonian to Newtonian, and hysteresis was suppressed. Ionomer adsorption ratio was affected by the existence of platinum particles on carbon particles. As the result, the rheological behavior of the pseudo-catalyst ink and catalyst ink with the variation of ionomer content was different.

3M Ionomer Adsorption on Polymer Electrolyte Membrane Fuel Cell Electrodes

Previous ionomer adsorption (Figure 1) explorations in our group have shown significantly lower adsorption (lower Keq) of ionomer when platinum (Pt) nanoparticles are present on the carbon support.1 Such differences can suggest a higher affinity of the polymer chain to carbon, which could suggest that acidic groups on the carbon surface might participate in hydrogen bonding on dipole-dipole interactions with the hydrophilic unit in the polymer.1,2 This disagrees with many recent suggestions that the interactions with ionomer are driven by specifc adsorption of sulfonic acid or sulfonate on the Pt. Understanding such interactions can be key to further improve the Pt/Carbon/ionomer electrodes structure and, consequently, proton exchange membrane fuel cell (PEMFC) performance. In this study, the adsorption of ionomer on a series of Pt:C (0:100, 30:70, 50:50 and 70:30) ratios using Vulcan XC-72 Carbon and Ketjenblack (KJB) supports were evaluated to further understand this interaction. The adsorption properties of water-dissolved 3M 825 equivalent-weight (EW) ionomer on both the carbon supports and the series of carbon-supported platinum (Pt/C) catalysts were studied with 400 MHz 19F Nuclear Magnetic Resonance Spectroscopy (NMR) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). A detailed structure characterization of the pristine supports and catalyst materials was achieved through Brunauer-Emmet-Teller (BET) nitrogen adsorption-desorption, transmission electron microscopy (TEM) and X-ray diffraction (XRD). The adsorbed ionomer structure was also explored through NMR and Fourier-Transform Infrared (FTIR). These results will be reported along with context from other studies of ionomer-supported catalyst interactions in the actual catalyst layer. Acknowledgements NMC gratefully acknowledges the support from a Fulbright Fellowship as well as the Office of Naval Research. Research was supported at ORNL by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy. References N. M. Cantillo, L. Li, J. Peng, G. A. Goenaga, and Thomas A. Zawodzinski ECS Trans., 80, 259–268 (2017) http://ecst.ecsdl.org/content/80/8/259.full.pdf.S. Ma, Q. Chen, F. H. Jøgensen, P. C. Stein, and E. M. Skou, Solid State Ionics, 178, 1568–1575 (2007). Figure 1

19F NMR Studies of 3M Ionomer Adsorption on Carbon Supports and Pt/C Catalysts

In this study, the adsorption properties of water-dissolved 3M 825 equivalent-weight (EW) ionomer and carbon (C) supports or carbon-supported platinum (Pt/C) catalysts were studied with 400 MHz 19F Nuclear Magnetic Resonance Spectroscopy (NMR). The supports and catalyst materials (with and without ionomer addition) were also analyzed with Brunauer-Emmet-Teller (BET) nitrogen adsorption-desorption, transmission electron microscopy (TEM) and X-ray diffraction (XRD). 3M ionomer/carbon/Pt interactions were also explored through differential scanning calorimetry (DSC). Using these methods, a series of samples with and without ionomer addition were analyzed to explore the different interactions between components (Pt/Carbon, and Carbon/ionomer). Preliminary DSC results are shown in Figure 1 for the 3M ionomer, ionomer/Pt, ionomer/Vulcan-carbon and the catalyst layer (CL) samples, where the CL is composed of ionomer/Vulcan-carbon/Pt. The 3M ionomer and ionomer/Vulcan-carbon samples, on the other hand, exhibit a weak and broad endothermic peak between 50 and 120°C. This process has been associated with the combination of structural changes in ionic cluster, and water volatilization in PFSA ionomers.1Both experiments with Pt present showed an exothermic feature around 95°C and 135°C for the ionomer/Pt and the CL samples, respectively. The adsorption behavior of 3M ionomer on the carbon and catalyst samples was explored using 19F NMR. Figure 2 shows an NMR spectrum of the 3M PFSA ionomer in water solution after base correction. This figure also includes the chemical shifts and peak assignments. The strongest peak at -76.55 ppm corresponds to the internal reference (trifluoro acetic acid, CF3COOH) in solution. The ratio of the most prominent 3M ionomer peak (-124.13 ppm) associated with the CF2 group in the backbone and the internal reference peak was used for the analytical study, a similar approach to previous studies of Nafion.2–4 Figure 1

19F NMR Studies of 3M Ionomer Adsorption on Carbon Supports and Pt/C Catalysts

In this study, the interactions between 3M 825 equivalent-weight (EW) ionomer and carbon support or carbon-supported platinum (Pt/C) catalyst particles were studied. The support and catalyst materials (with and without ionomer addition) were analyzed with Brunauer-Emmet-Teller (BET) nitrogen adsorption-desorption. 3M ionomer/carbon/Pt interactions were also explored through differential scanning calorimetry (DSC). Progress toward developing methods for studying polymer adsorption onto supports using 19F Nuclear Magnetic Resonance Spectroscopy (NMR) is also reported. Using these methods, a series of ionomer solutions were analyzed to explore the different interactions between components (Pt/Carbon, and Carbon/ionomer).

Characterizing the Pt-Electrolyte Interface in PEM Fuel Cells

Much progress in reducing the platinum loading in proton exchange membrane (PEM) fuel cell cathodes to a target of 0.1 mg-Pt/cm2 has been made. However, large performance losses are observed at high current densities at low cathode platinum loadings. Recent studies have shown that the large losses are caused due to resistances at or near the catalyst-ionomer interface.[1] This resistance can be countered through the use of ionomers designed to interact with Pt in a way that does not limit the oxygen reduction reaction. However, the development of tools to quantify and characterize this resistance is needed to enhance our understanding of this resistance and correlate it with performance loss. In the early 1990’s Feliu et al. developed a method for measuring adsorbed ion charges on Pt single crystals in an aqueous electrolyte.[1] We have validated this CO displacement method on polycrystalline Pt and carbon-supported Pt nanoparticles in an aqueous electrolyte and present here results from implementing this methodology in a fuel cell. Figure 1 illustrates the current transient measured during chronoamperometry as CO gas is introduced to the cathode of a fuel cell. The measured displacement charge at potentials less than 0.2 V represent a displacement of adsorbed protons, corresponding to a positive displacement current. At potentials greater than 0.3 V, a negative current is measured corresponding to the displacement of negatively charged adsorbed species. In the case of a fuel cell cathode, these negative species are the sulfonate group present in the ionomer. The adsorption charge can then be integrated to determine the amount of species adsorbed on the surface at each potential. Using this tool, ionomer adsorption for different ionomer types at different operating conditions can be examined. [1] Kongkanand A, Mathias MF, J. Phys. Chem. Let. 7 (2016) 1127. [2] Feliu JM, Orts JM, Gomez R, Aldaz A, Clavilier J. J. Electroanal. Chem. 372 (1994) 265. Figure 1. Chronoamperometry at different potentials during CO Displacement. Introduction of CO to the cathode occurs at t=120s. Figure 1

(Plenary) Electrochemical Diagnostics and Modeling in Developing the PEMFC Cathode

Reducing the use of precious metal in a proton exchange membrane fuel cell is a key challenge in mass commercialization of fuel cell electric vehicles. However, as Pt loadings are reduced, fuel cell performance losses due to mass transport phenomena become more localized to the Pt and ionomer interface. In this paper, we provide an overview of how we use in-situ electrochemical diagnostics and modeling to understand the performance of low-Pt electrodes, identify their key limiting factors, and guide our catalyst layer development. In particular, diagnostics used to quantify the local-Pt oxygen transport resistance and the ionomer adsorption on the Pt surface will be discussed.

Molecular Dynamics Study of Ionomer Adsorption at a Carbon Surface in Catalyst Ink

Ionomer adsorption at the surface of a graphite sheet in a solvent-saturated environment was examined using molecular dynamics simulations. The amount of ionomer adsorbed on the graphite surface greatly depended on the equivalent weight (EW) of the ionomer and the alcohol content in the solvent. The maximum ionomer coverage was obtained in water-rich solvent for low EW ionomer, while the addition of alcohol to the solvent was necessary to enhance adsorption of high EW ionomer. The ionomer coverage at the surface with ionized functional groups was lower than that of the bare graphite surface, which indicated a significant effect of the electrostatic repulsive interaction between negatively charged functional groups and sulfonic acid groups on the ionomer adsorption. The effect of the alcohol content and EW of the ionomer on ionomer coverage still remained even in the presence of strong electrostatic interactions between sulfonic acid groups and the functionalized carbon surface.

Nano carbon supported platinum catalyst interaction behavior with perfluorosulfonic acid ionomer and their interface structures

The interaction between perfluorosulfonic acid ionomer and supported platinum catalyst is essential. It directly influences platinum accessibility, stability of carbon support and platinum, proton conductivity and electron conductivity in an electrode. In this study, we compare the adsorption behavior of Nafion ionomer on platinized carbon nano fibers (CNFs), carbon nano tubes (CNTs) and amorphous carbon (Vulcan). The interaction is affected by the catalyst surface oxygen groups as well as porosity. Comparisons between the carbon supports and platinized equivalents are carried out. It reveals that the platinization step modifies the surface nature of the carbon supports in terms of specific surface area, crystallinity and especially porosity; therefore, ionomer adsorption over carbon is not always representative for the ionomer adsorption over carbon supported catalyst, though indicative. Moreover, the ionomer may have an adsorption preference to the platinum nano particle rather than to the overall catalyst. This was verified by a close examination on the decomposition temperature of the carbon support and the ionomer. The electrochemical stability of the catalyst ionomer composite electrode suggests that the ionomer adsorption on the overall catalyst is a more stable interface structure for fuel cell application.

Study of Ionomer Adsorption on Carbon Supported Pt Catalyst By the Quartz-Crystal Microbalance (QCM) Method

In polymer electrolyte fuel cells, the three-phase interface in a catalyst layer (CL) is considered to be very important as the electrochemical reaction site. The CL is typically formed using a dispersion of carbon supported platinum catalyst (Pt/C) and ionomer, and the interaction between the ionomer and Pt/C particles probably has an impact on the platinum utilization in the electrode and the dispersion stability. In this study, we focused on the adsorptive property of ionomers onto the Pt/C surface. The amount of adsorbed ionomer on a Pt/C catalyst was measured using the quartz-crystal microbalance (QCM) method. The amount was used as an index of the affinity of the ionomer for the Pt/C catalyst surface. The Pt/C catalyst particles were immobilized on a QCM sensor. The catalyst-immobilized sensor was placed into a solvent, and was left for a while until the QCM oscillation stabilized. An ionomer solution was then added drop-wise into the QCM cell. The change in resonance frequency of the quatz-crystal before and after adding the ionomer dispersion was recorded. The amount of a perfluorosulfonic acid polymer, Flemion® (Asahi Glass), adsorbed on the Pt/C catalyst surface is plotted against the concentration of the ionomer dispersion in Fig.. The amount was calculated based on the change in the resonance frequency of the QCM sensor. It shows the higher the ionomer concentration, the more ionomer was adsorbed. The relationship between the adsorptive property of various ionomers will be discussed in terms of their chemical properties (IEC, chemical structure etc.). The viscoelastic character of the adsorbed ionomer layers will also be described. Figure 1

Adsorption Behavior of Perfluorinated Sulfonic Acid Ionomer on Highly Graphitized Carbon Nanofibers and Their Thermal Stabilities

A systematic adsorption study of perfluorinated sulfonic acid Nafion ionomer on ribbon-type highly graphitized carbon nanofibers (CNFs) was carried out using fluorine-19 nuclear magnetic resonance spectroscopy. On the basis of the values obtained for the equilibrium constant (Keq , derived from Langmuir isotherm), the ionomer has varying affinities for CNFs (Keq between 5 and 22) as compared to Vulcan (Keq = 18), depending on surface treatments. However, the interactions are most likely governed by different adsorption mechanisms depending on hydrophilicity/hydrophobicity of the adsorbent carbon. The ionomer is probably adsorbed via the polar sulfonic group on hydrophilic Vulcan, whereas it is adsorbed primarily via hydrophobic −CF2– backbone on the highly hydrophobic pristine CNFs. Ionomer adsorption behavior is gradually altered from apolar to polar group adsorption for the acid-modified CNFs of decreasing hydrophobicity. This is indicated by the initial decrease and then increase in the value of Keq with the increasing strength of the acid treatment. The corresponding carbon–ionomer composite also showed varying thermal stability depending on Nafion orientation. The specific maximum surface coverage (ΓSmax) of the CNFs is 1 order of magnitude higher than that of Vulcan. The large discrepancy is due to the fact that the ionomers are inaccessible to the internal surface area of Vulcan with high microporosity.

19F NMR studies of Nafion™ ionomer adsorption on PEMFC catalysts and supporting carbons

The adsorption of Nafion™ ionomer on Proton Exchange Membrane Fuel Cell (PEMFC) catalysts and the supporting carbons was studied in aqueous solution by 500 MHz 19Fluorine Nuclear Magnetic Resonance Spectroscopy (NMR). In addition, the materials were characterized with Brunauer-Emmett-Teller (BET) nitrogen adsorption–desorption and X-ray diffraction (XRD). It was observed that Nafion adsorbs strongly on carbon as well as noble metal catalyst: Pt, PtRu. The adsorption was classified into a primary adsorption and a secondary adsorption. At low concentration, the adsorption was found to follow a Langmuir isotherm (primary adsorption), in which important parameters are adsorption equilibrium constant ( K eq) and maximum surface coverage ( Γ max). The K eq turned out to be around ten, and was found to depend on the specific chemical nature of the surface. The Γ max combining with Nafion two-dimensional projection area estimated from a space-filling model implies that only part of the Nafion molecule adsorbs on the surface.