Bulk Nafion Research Articles

Structure-Property Relationships for Thin-Ilm Nafion: Model-Predictive Design of Low-Pt PEMFC Catalyst Layers

Proton exchange membrane fuel cells (PEMFCs) are a promising means to decarbonize the global vehicle fleet, but commercialization is limited by costly Pt catalysts, among other challenges. PEMFC designs with low Pt loading typically suffer anomalous performance losses. Previous studies have agreed that poor performance with low Pt-loading is attributed to slow transport through the nano-thin Nafion ionomer found in the catalyst layer (CL). Although it is known that the properties of the Nafion thin films in the CL vary from those previously measured for thicker membranes, research has not yet definitively connected CL polymer properties with PEMFC limitations in low Pt-loaded cells, nor found a means to address the limitations for low cost, higher-performance PEMFCs.This presentation covers parallel efforts to identify and address limiting processes in low-Pt-loaded PEMFC cathode CLs. We begin with an extensive investigation of thin-film Nafion structure-property relationships, as an analog for CL Nafion thin-films. We model nm-scale transport in thin-film Nafion to develop structure-property relationships for thin-film Nafion, based on in situ neutron reflectometry [2], and conductivity measurements [3] on thin film Nafion. These include equilibrium measurements for varying thicknesses and substrates, as well as novel in operando measurements of thin-film Nafion with an H2O flux across the film. Results suggest that conductivity in CL Nafion is a function of not just temperature and relative humidity (as in bulk Nafion membranes), but also film thickness and proximity to the substrate. These insights result in a model that calculates the conductivity as a function of temperature, local water uptake, film thickness, and underlying substrate (Pt or carbon).Simultaneously, we have conducted continuum-scale numerical simulations of PEMFC catalyst layers, to determine the necessary level of detail for predictive yet efficient numerical simulation for device design. Incorporating these structure-property models into a continuum-scale PEMFC cathode half-cell model to locally resolve proton conductivity and O2 diffusion improves model fits to PEMFC data with varying Pt loading [4]. The fits provide new insights into the coupling between kinetic, mass transport, and Ohmic losses in low-Pt-loaded PEMFC cathodes and show that ohmic losses related to proton transport in thin film Nafion play a key role in low-Pt loaded cells. Based on these new fits to low-Pt loaded cells, we present a parametric study of PEMFC CL design parameters with low Pt loading. Incorporating multi-scale modeling of Nafion transport processes enhances the predictive model capabilities and unlocks new insights into CL design. In particular, results suggest that functionally graded catalyst layer designs could significantly improve PEMFC power density with low Pt loading [4]. Weber, A. and Kusoglu, A. Mater. Chem. A. 2014. DOI: 10.1039/c4ta02952fDeCaluwe, S.C. et al. Nano Energy 2018.DOI: 10.1016/j.nanoen.2018.01.008Paul, D.K. et al. Electrochem. Soc. 2014. DOI: 10.1149/2.0571414jesRandall, C.R. and DeCaluwe, S.C., Electrochem. Soc. 2022. DOI: 10.1149/1945-7111/ac8cb4

Molecular Simulation of Nafion Swelling in the Cathode Catalyst Layer of Polymer Electrolyte Fuel Cells

As the world moves towards carbon neutrality, polymer electrolyte fuel cells (PEFCs) have emerged as a vital technology in recent years. The cathode catalyst layer of PEFCs is characterized by its porous structure, which promotes the production of water molecules from oxygen and protons through chemical reactions. The ionomer thin films, predominantly composed of Nafion, are critical in regulating the transport and mobility of molecules and ions in the cathode catalyst layer. Sulfonic groups in the side chains of Nafion enable accumulation of water molecules, causing the ionomer thin films to swell.Previous studies have examined the swelling of Nafion under wet conditions[1][2]. However, precise swelling mechanisms remain unclear. While privious research has analyzed the relationship between relative humidity and the amount of water within bulk Nafion samples[3], the swelling behavior in films with thickness of several nanometers differs significantly, necessitating further investigation[4]. This study therefore investigates the swelling mechanisms and the properties of ionomer thin films used in the cathode catalyst layer of PEFCs. This study aims to establish guidelines for designing the molecular structures of ionomers by elucidating the mechanisms underlying ionomer swelling in the cathode catalyst layer. Grand canonical Monte Carlo (GCMC) simulations and molecular dynamics (MD) simulations were employed to reproduce swelling in Nafion thin films and examine the impact of Nafion's structure on water content and swelling. These molecular simulation techniques enable the elucidation of the influence exerted by polymer structure on water content and swelling at the molecular scale.In this study, we firstly created a simulation model of a Nafion system using LAMMPS[5], the DREIDING Force Field[6], and Mulliken charges[7] in which parameter settings were referred from literature values[8]. The Nafion system contains 10 Nafion molecules with a degree of polymerization of 10, water molecules, and 100 oxonium ions for electrical neutrality. The density of 1.9 g/cm3 achieved during simulations closely agrees with literature values[8].To reproduce Nafion swelling under humid conditions, GCMC simulations and MD simulations were employed. The rationale behind utilizing GCMC simulations lies in the fact that they enable the insertion of water molecules into the Nafion system under constant chemical potential conditions, thereby inducing Nafion swelling. Subsequent to the GCMC simulations, MD simulations were performed under the NPT ensemble, which facilitated the system to reach equilibrium. This alternating cycle of GCMC and MD simulations is repeated to realize the final swelling state. Preliminary results indicate lower water content values than those experimentally measured under similar conditions. Possible causes include inadequate force field parameters that lead to polymer morphology different from the actual Nafion structure.Future work will explore the causes of these discrepancies and examine the effects of Nafion's structure on water content and swelling under various conditions. This will involve testing alternative force field parameters and initial structures to better represent the molecular-level structure of ionomer thin films on carbon support particles in the cathode catalyst. Additionally, we will investigate how different relative humidity conditions, temperatures, and ionomer compositions influence swelling behavior, which could provide insights into optimizing PEFCs performance under various operating environments.[1] G. C. Abuin et al., J. Membr. Sci. 428, 507 (2013).[2] J. Catalano et al., Int. J. Hydrog. Energy 37, 6308 (2012).[3] T. A. Zawadzinski et al., J. Electrochem. Soc. 140, 1981 (1993).[4] M. A. Modestino et al., Macromolecules 46, 867 (2013).[5] A. P. Thompson et al., Comp. Phys. Comm. 271, 10817 (2022).[6] S. L. Mayo et al., J. Phys. Chem. 94, 8897 (1990).[7] S. S. Jang et al., J. Phys. Chem. B 108, 3149 (2004).[8] Y. Kurihara et al., J. Power Sources 414, 263 (2019).Fig. 1 An ionomer model composed of Nafion molecules, oxonium ions, and water molecules. The red and white spheres represent oxygen and hydrogen atoms making up water molecules. The size of the water molecules is emphasized for clarity. Figure 1

Control of self-organization of drop-casted Nafion film for improving proton conduction in a polymer-electrolyte-membrane fuel cell to raise its output power density

A simple drop-cast method to directly deposit Nafion polymer electrolyte membrane (PEM) on nanostructured thin-film catalyst layer composed of stacked Pt nanoparticles prepared by pulsed laser deposition (PLD) was demonstrated. Through optimization of solvent composition and drying temperature of Nafion solution to control self-organization of Nafion, a uniform PEM with better bulk and interface microstructures could be produced, leading to a significant improvement in the output current density of a PEM fuel cell over that using reference commercial PEMs. The formation of facile proton conduction pathways in the bulk Nafion membrane resulted in a 35% reduction in ohmic resistance compared to that with the commercial membrane. Moreover, the infiltration of Nafion in the catalyst layer formed suitable proton transport network to render more catalyst nanoparticles effective and thus lower charge-transfer resistance. With the optimized PLD, drop-cast, and hot-pressing conditions, the current density of PEMFCs using drop-casted PEM reached 1902 mA cm−2 at 0.6 V at 2 atm H2 and O2 pressures with a cathode Pt loading of 100 μg cm−2, corresponding to a power density of 1.14 W cm−2 and a cathode mass-specific power density of 11.4 kW g−1.

(PEFCE&E 21 Best Student Poster - Third Place) Molecular Dynamics Study of Proton Conductivity at an Interface between Nafion and Graphene Sheet

In order to improve the power generation efficiency of polymer electrolyte fuel cells (PEFC), it is necessary to increase the flux of proton transport inside a proton exchange membrane (PEM) under high temperature and low humidity conditions. Nowadays, the Nafion membrane is used as a reference for all proton exchange membranes. This membrane developed by DuPont company demonstrates a high proton conductivity value, chemical stability and prolonged using in fuel cell conditions in comparison with other membranes. However proton transport properties are greatly reduced when the membranes are degraded. Researches have been actively conducted to solve the problem of proton transport reduction in PEM under high temperature and low humidity conditions by two approaches. One of the research focuses is to design alternative materials based on those engineering polymers, for example, polybenzimidazole, poly(ether sulfone), and poly(ether ketone). Another research focus is the introduction of additives. Among the many additives, carbon nanotubes (CNTs) which are next-generation materials with a high aspect ratio and excellent thermal durability and mechanical properties, have been attracting attention. Previous studies have reported that the PEEK (poly ether ether ketone ) / CNT composite membranes show higher durability than Nafion membranes. And the proton transport properties in Nafion / sulfonated CNT composite membranes are improved over the that of Nafion membranes. It is said that the proton transport property is improved by forming a proton path which is made by the influence of the structural characteristics of CNT. However, the proton transport mechanism at the interface between the carbon surface of CNT and the polymer has not been fully understood. The purpose of this study is to clarify the proton transport mechanism in the interface between the carbon surface region and the polymer region and to evaluate the effects of functional groups with carbon surface by using molecular dynamics (MD) simulations. The surface of a CNT was modeled with a graphene sheet (GS). The diameter of the CNTs used in the experiment is about 10-20 nm, which is beyond capability of our MD simulations, and thus the CNT surface was assumed to be a flat surface. The simulation system is shown in Fig. 1. GSs were placed at the top and bottom of the simulation box, and polymer, water molecules, and hydronium ions were placed between them. In order to evaluate the influence of distances between GSs on structural characteristics and transport property, we prepared calculation systems at the wall distances of 4, 6, 8 and 10 nm. We calculated the density distribution in the direction vertical to the GS and the self-diffusion coefficient of the protons at the GS surface region and the center region. Furthermore, a hydrophilic sulfonated GS was prepared by putting sulfo groups to the hydrophobic GS to investigate the effects of the hydrophilic and hydrophobic GS interface on proton transport. The results of the density distribution showed that the surface of pristine GS and sulfonated GS had water clusters, and that proton self-diffusion coefficient of pristine GS and sulfonated GS were larger than that of bulk Nafion. From these facts, it is considered that the diffusion by the grotthuss mechanism increased by improving the connectivity of the proton path on the GS surface and reducing the tortuosity. As a result, the proton transportability is considered to be improved. Figure 1

Electrospun Ion-Conducting Composite Membrane with Buckling-Induced Anisotropic Through-Plane Conductivity.

Fuel cell (FC) is an attractive green alternative for today's fuel combustion systems. In common FCs, a polymer electrolyte membrane selectively conducts protons but blocks the passage of electrons and fuel. Nafion, the current benchmark membrane material, has a superior conductivity owing to unique morphology comprising randomly oriented elongated ionic nanochannels within its Teflon-like matrix. Channel orientation enhances Nafion conductivity, yet there has been no facile method to induce a stable alignment in the desired through-plane (TP) direction. Here, we report an approach based on dual electrospun Nafion-PVDF nanofiber composites that yields a stable TP alignment. It utilizes extreme thinness and strong inherent orientation within electrospun nanofibers, which is readily converted to TP alignment by plunging an electrospun nanofiber mat into a thin slit, resulting in nanofiber buckling and subsequent consolidation. Using TEM and SAXS, we demonstrate a pronounced and sustained TP ion channel orientation in prepared membranes, yielding a highly anisotropic swelling and conductivity exceeding that of bulk Nafion when normalized to Nafion content. The analysis also highlights the importance of PVDF as a stabilizing component, preserving orientation upon annealing, while a similarly prepared pure Nafion membrane loses anisotropy. The approach holds potential to advance the FC technology by overcoming current limitations of ionomeric membranes.

(PEFC&E 2nd Place Best Poster Winner) Molecular Analysis of Cerium Ion Transport Properties in Polymer Electrolyte Membrane

In order to improve the durability of Polymer electrolyte fuel cell (PEFC), to clarify the cause of deterioration and take measures is necessary. Chemical degradation of polymer electrolyte membrane (PEM) is considered as one of the causes of degradation of PEFC. Hydroxyl radical (OH ∙) is generated by decomposition of hydrogen peroxide generated by oxygen reduction reaction (ORR) during PEFC operation. This OH ∙ has high reactivity, and OH ∙ would degrade the polymer membrane. Based on this degradation mechanism, a method of adding a substance that inactivates OH ∙ before reacting with the polymer (radical scavenger) has been put into practical use. One of the most useful radical scavengers is cerium ions. Cerium ions in the PEM have been reported to migrate during operation and the distribution becomes non-uniform, and degradation occurs at points where there are only a few cerium ions in the membrane. Understanding the cerium ion transport mechanism in PEM is important for controlling cerium ion migration. Because investigating the cerium ion transport in operating PEFC is limited in experiments, nanoscale analysis by numerical simulation is useful to understand the properties of cerium ion transport. In this study, we analyze the distribution and transport properties of cerium ions in the membrane by modeling the PEM containing cerium ions using molecular dynamics (MD) simulations. Diffusion due to the concentration gradient, migration due to the ionomer potential gradient, and convective ion transport due to water crossover are considered as the transport factors of the cerium ions in the membrane. In the present study, the effect of water contents and water flux on the convection ion transport is analyzed. Simulation system is composed of polymer chains, cerium ions, and solvent molecules (water molecules and hydronium ions). Nafion® model was used as polymer and the equivalent weight (EW) was set at 1146. The potential based on the DREIDING force field was adopted for the molecular interaction of Nafion®, and particle mesh Ewald method was used to calculate Coulomb force.The cutoff distance of the LJ potential and Coulomb potential was set at r c=12 Å. The positive charge of the hydronium ion and the cerium ion was balanced with the negative charge of the sulfonic acid group in the system. The number of water molecules was determined by the water content λ which represents the number of water molecules per sulfonate group. The water content was set at λ = 4, 8, 12 and 16, which correspond to the water content in the bulk Nafion® membrane at approximately RH = 30%, 70%, 85% and 100%, respectively. Cerium ion concentration is equivalent to 1.2 wt% cerium ion relative to the polymer mass in the membrane, which is the typical concentration for Ce-Nafion composite membranes. Three-dimensional periodic boundary condition was applied to simulation box. The annealing process was applied in order to equilibrate the system. After annealing, the production run was performed for 15 ns using the NVT ensemble. The temperature in the system was 350 K. The time step was set at 1 fs, and the sampling interval was 10,000 steps. The simulation system is shown in Fig. 1. To evaluate the convection ion transport, an external force was applied to water molecules along the uniaxial direction to obtain flux of water. The carbon atom at the base of the side chain in Nafion® was fixed to prevent the flowing of Nafion®. The convection ion transport coefficient, K convection, is defined as the ratio of the mass flux of cerium ions and the mass flux of water molecules. The mass flux can be obtained by multiplying the concentration of X and the mean velocity of X. In addition, structural analysis will be performed to identify the factors that are dominant in cerium ion transportation. Figure 1

Probing interfacial interactions of nafion ionomer: Thermal expansion of nafion thin films on substrates of different hydrophilicity/hydrophobicity

ABSTRACTInterfacial interactions of Nafion ionomer with superhydrophilic (Pt, Au), hydrophilic (SiO2), and hydrophobic (graphene, octyltrichlorosilane [OTS]‐modified SiO2) is investigated, using in situ thermal ellipsometry, by quantification of substrate‐ and thickness‐dependent thermal properties of the ultrathin Nafion films of nominal thickness ranging 25–135 nm. For sub‐50 nm thin Nafion films, the thermal expansion coefficient of films decreased in the order of most hydrophobic to most hydrophilic substrate: OTS > graphene > SiO2 > Au > Pt, implying weaker interpolymer and polymer–substrate interactions for films on hydrophobic substrates. Expansion coefficient of films on SiO2, graphene, and OTS‐modified SiO2 decreased with thickness whereas that of films on Au and Pt substrates increased with thickness. Above ~100 nm of thickness, films on all substrates converged toward a common value representative of bulk Nafion. Thermal transition temperature was found to be higher for films on hydrophilic SiO2 than that for films on hydrophobic graphene and OTS‐modified SiO2 but was not discernible for films on Au and Pt substrates. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 343–352

Proton Conduction and Oxygen Diffusion in Ultra-Thin Nafion Films in PEM Fuel Cell: How Thin?

The success of PEM fuel cells lies in the design of the cathode where C-supported Pt catalyst is covered by a thin layer of ionomer, allowing easy access of oxygen and protons. The high current densities (1-2 A/cm2) are achieved by the optimization of the thickness of the ionomer between two opposing trends: thinner ionomer allows high oxygen through-plane diffusion, while the in-plane conduction of the protons requires reasonable thickness. Anisotropy is likely to develop in such thin films. The purpose of this work was to investigate the effect of a thinner ionomer layer on the diffusion and conductivity and to establish how thin the ionomer layer can be. Oxygen permeability (DH) and proton conductivity in ultra-thin Nafion films (100-1000nm) were respectively characterized on SiO2 substrate via limiting current measurement and electrochemical impedance spectroscopy for a range of temperature and relative humidity conditions. The transport behavior of these films is comparable to the electrode membrane layer of PEM fuel cells. Values of oxygen permeability were shown to increase with decreasing film thickness. The oxygen diffusion through the gas boundary layer becomes significant only at ultra-thin film thickness. The proton conductivity decreases with decreasing thickness and was disproportional to bulk Nafion behavior. Post-test SEM imaging shows buckling of the thin layer on the silicon wafer between the Pt sites, partially explaining the opposing trends in diffusivity and conductivity as film thickness decreases. Anisotropy exists as the thickness becomes comparable to the size of the aqueous regions of the Nafion ionomer. Measurements of thinner layers (<100nm) were experimentally limited, yet the trends indicate that the thinness of the ionomer in the cathode is limited by its ability to allow proton conductance.

Time-resolved total reflection detects mass transfer along thickness direction of bulk Nafion® film

To elucidate a bulk membrane (Nafion®) with respect to channel connectivity or mass transfer (water or ions) through the channels, we propose to utilize time-resolved neutron reflectivity and deuterium exchange method. We performed the experiment on time-of-flight spectrometer iMATERIA at pulsed neutron spallation source J-PARC in Tokai, Japan. To exchange water (D2O or H2O) in the film during RF measurements, we specially designed a sample environment with Si substrates. A bulk film (50 μm thickness) is sandwiched by Si substrate (5 mm thickness) and Al mesh plate. The incident neutron incomes from edge of Si substrate and reflects at a front surface of bulk film. When we exchange water (D2O or H2O), water flows through Al mesh and starts to diffuse in the film. When exchanged water reaches to a front surface of the film, critical angle for total reflection (or critical wave number) changes, according to the chemical composition and therefore the averaged coherent scattering length (bav) at the front surface. Consequently, diffusion coefficient is determined from time-dependence in bav.

Origins of Suppressed Swelling and Increased Mechanical Properties of Ultrathin Nafion

We have previously shown1,2 that there is a suppression in both swelling and water diffusivity when Nafion is confined to thin and ultrathin films. We have also shown that the modulus of ultrathin Nafion films increases upon confinement3,4, as well as upon thermal annealing. Interestingly, we also observe a decrease in swelling and an increase in modulus for ultrathin films aged in high humidity environments. Similar behavior has been shown to occur in bulk Nafion films.5,6 Some researchers suggest that the root cause of such a decrease in swelling and increase in modulus is due to the formation of sulfonic anhydrides, while others suggest that it is due to internal hydrogen bonding of dehydrated sulfonic acids. In this talk, we will present our measurements results on thin and ultrathin Nafion films. We apply polarization-modulation infrared reflection-absorption spectroscopy to study the kinetics of dehydration and formation of sulfonate species in thin Nafion films. We also employ cantilever bending during humidity-induced swelling to probe the mechanical properties of these films. Grazing-incidence small-angle x-ray scattering is used to elucidate changes in morphology upon annealing and/or aging. By combing the results from these techniques, we can paint a more holistic picture of the origins of suppressed swelling and increased mechanical properties in thin and ultrathin Nafion films. S. A. Eastman, S. Kim, K. A. Page, B. W. Rowe, S. Kang, C. L. Soles, and K. G. Yager. Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films, Macromolecules, 45, 7920-7930 (2016).E. M. Davis, C. M. Stafford, and K. A. Page. Elucidating Water Transport Mechanisms in Nafion Thin Films, ACS Macro Letters, 3, 1029-1035 (2014).K. A. Page, J. W. Shin, S. A. Eastman, B. W. Rowe, S. Kim, A. Kusoglu, K. G. Yager, and G. R. Stafford. In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles, ACS Applied Materials and Interfaces, 7, 17874-17883 (2015).K. A. Page, A. Kusoglu, C. M. Stafford, S. Kim, R. J. Kline, and A. Z. Weber. Confinement-Driven Increase in Ionomer Thin-Film Modulus, Nano Letters, 14, 2299-2304 (2014).F. M. Collette, C. Lorentz, G. Gebel, F. Thominette. Hygrothermal aging of Nafion, Journal of Membrane Science, 330, 21-29 (2009).S. Shi, T. J. Dursch, C. Blake, R. Mukundan, R. L. Borup, A. Z. Weber, and A. Kusoglu. Impact of Hygrothermal Aging on Structure/Function Relationship of Perfluorosulfonic-Acid Membranes, Journal of Polymer Science B: Polymer Physics, 54, 570-581 (2016).

Humidity and Temperature Dependences of Oxygen Transport Resistance of Nafion Thin Film on Platinum Electrode

The oxygen transport resistances of Nafion thin films on a Pt electrode and of a bulk Nafion were evaluated to clarify the dominant part of the oxygen transport loss in an ionomer for a cathode catalyst layer of polymer electrolyte fuel cells. To evaluate the oxygen transport resistances of the Nafion thin film, an oxygen flux through the Nafion thin film on the Pt was detected as diffusion limited current density, jd, for an oxygen reduction reaction by using an electrochemical cell equipped with a Pt microelectrode coated with a Nafion film less than 100nm. An interfacial oxygen transport resistance and inner resistivity of the Nafion thin film were separately determined by the relation between jd−1 normalized by oxygen partial pressure and the film thickness. The interfacial resistance is equivalent to 30–70nm of the Nafion film, which are much thicker than a typical ionomer thickness of ∼10nm observed in catalyst layers. From the above results and our recent theoretical results obtained by molecular dynamics techniques, the oxygen permeation fluxes through the ionomers in catalyst layers can be regarded as dominantly controlled by the oxygen permeation at the Pt/ionomer interface.

Proton Conductivity of Graphene-Based Polymer Electrolyte Membrane

A polymer electrolyte membrane fuel cell (PEMFC) is the one of promising alternative energy systems, however, the one of bottlenecks of the PEMFC performance is a poor key transport property, e.g., proton conductivity in catalyst layers. Recently, it has been reported that the addition of a rapidly emerging material, i.e., graphene, to the catalyst layer significantly improves the performance, but the understandings of the role of the graphene on the performance improvements are limited so far. The graphene-based PEM, i.e., Nafion®, is coated on the bulk Nafion® membrane, and the proton conductivity is measured using an impedance spectroscopy with respect to the graphene content from 0 to 3 wt% at the ambient temperature and liquid-water equilibrated state. It shows the proton conductivity enhancement with the increasing graphene content. The proton conductivity is compared using those of the carbon black (CB)- and carbon nanotube (CNT)-based PEM, showing that graphene-based PEM provides greater enhancement. The results suggest that the unique structures of the graphene could improve the proton conducting pathways within PEM. The obtained results provide an insight into an optimal material for improving the proton transport of PEM.

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

Proton Conductivity of Graphene-Based Polymer Electrolyte Membrane

A polymer electrolyte membrane fuel cell (PEMFC) is one of the promising alternative energy systems, but the one of the bottlenecks of the PEMFC performance is poor electrochemical reaction and key transport properties, i.e., proton conductivity in catalyst layers. Recently, it has been reported that an addition of the rapidly emerging material, i.e., graphene, significantly improves the performance1, but the understandings of the role of the graphene on the performance improvements are poor. Here, we examine the role of the graphene in PEM for enhanced performance. The graphene-based, thin PEM, i.e., Nafion®, is coated on the bulk Nafion®membrane, and the proton conductivity of the graphene-based, thin PEM is measured using an impedance spectroscopy with respect to the graphene content from 0 to 3 wt% at the ambient temperature and liquid-water equilibrated state. It shows the proton conductivity improvement with the increasing graphene content. The enhancement is also compared using carbon black, showing that graphene-based PEM provides higher proton conductivity than the carbon-black-coated PEM. The obtained results provides an insight into an optimal material for improving the proton transport of PEM. Experimental Graphene flakes were purchaced from Angstrong materials, were added to Nafion® solution which was purchased from Sigma-Aldrich by weight percentages of 0, 0.1, 0.5, 1, 2, and 3. This solution was then sonicated for 90 minutes in order to ensure desired mixture of graphene with Nafion® solution. The graphene-Nafion® mixture was then sprayed on either side over a dry 2, 7, and 10 mil thick Nafion®membrane which was purchased from Ion Power Inc. was soaked in DI water for one hour to ensure for liquid equilibrium state, and it is placed between the cell. The proton conductivity is measured using an impedance spectroscopy with a frequency range between 10 and 300 kHz with a voltage peak-to-peak variation of 10mV at 0 V. The same wt% of the carbon black is added to the PEM for 2 mil, 7 mil, and 10 mil thick membranes. The real part resistance at the high frequency is used for the proton conductivity, given as σ = 1/ρ where, σ is the proton conductivity, and ρ is the resistivity. Results With the addition of graphene nano-flake particles to the Nafion®, the proton conductivity increases in all the Nafion® thicknesses as shown in Fig 1(a). The proton conductivity increases significantly up to the 2 wt% of graphene, and when it is greater than 2 wt% it increases nearly linearly as it reaches a percolation threshold.2 It is also observed that the proton conductivity improvement of the thicker Nafion® membrane with respect to the graphene content is smaller than those of the thinner membrane. Perhaps, this is caused by the different ratio of the proton conductivity between the bulk membrane and graphene-based Nafion® coating.3Thus, the 2 mil membrane offers more proton conductivity than the 10 mil. The measured proton conductivity of the carbon-black-based Nafion® as a function of the carbon-black content is shown Fig 1(b), comparing with the graphene-based-Nafion® on 7mil thick membrane. The carbon black in Nafion® almost linearly increases the proton conductivity up to 3 wt%, and the degree of the proton conductivity enhancement at the same weight percentage is smaller than that of the graphene. It is also noted that the proton conductivity enhancement by carbon black is much less than the ones of graphene at the low carbon black and graphene content. Perhaps, this is related to the fact that the graphene provides an improved interaction with water compared to the carbon black, which in turn results in the effective proton conductivity enhancement. Also, in the graphene surface, the presence of –O-, -OH, and –COOH functional groups having hydrophilic sites may facilitates the proton transport through hydrogen-bonding networks of the water clusters in the membrane4.

Elucidating Water Transport Mechanisms in Nafion Thin Films.

Ion-exchange membranes are critical components of hydrogen fuel cells, where these ionomers can be confined to nanoscale thicknesses, altering the physical properties of these films from that of bulk membranes. Therefore, it is important to develop methods capable of measuring and elucidating the transport mechanisms under thin film confinement compared to bulk Nafion. In this study, water sorption and diffusion in a Nafion thin film were measured using time-resolved in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Interfacial mass transport limitations were confirmed to be minimal, while restricted water diffusion was observed, where the effective diffusion coefficient of water in the thin Nafion film was many orders of magnitude lower (between 4 and 5 orders of magnitude) than those reported for bulk membranes and was dependent on the initial hydration state of the Nafion. Furthermore, the response of the hydrophobic domains (Teflon backbone) to the swelling of the hydrophilic domains (ionic clusters) was shown to be orders of magnitude slower than that of bulk Nafion.

Multistep Thickening of Nafion Thin Films in Water.

Water sorption kinetics in thin Nafion films prepared on silver and silicon oxide substrates was examined by surface plasmon resonance and neutron reflectivity measurements. It was found that the films thickened in three regimes. The asymptotic swelling ratios in regimes I, II, and III were 1.05, 1.26, and 1.41, respectively. These values were independent of the substrate species and were coincident with the transition points of different hydration states in the bulk Nafion; water binding to sulfonic acid groups, the formation of sphere-like ionic clusters, and bridge formation between clusters. The swelling was much slower in thin films than in the bulk due to the mobility restriction of Nafion near the substrate.

Elucidating the Ionomer-Electrified Metal Interface

The competitive adsorption of Nafion functional groups induce complex potential dependencies (Stark tuning) of vibrational modes of CO adsorbed (CO(ads)) on the Pt of operating fuel cell electrodes. Operando infrared (IR) spectroscopy, polarization modulated IR spectroscopy (PM-IRRAS) of Pt-Nafion interfaces, and attenuated total reflectance IR spectroscopy of bulk Nafion were correlated by density functional theory (DFT) calculated spectra to elucidate Nafion functional group coadsorption responsible for the Stark tuning of CO(ads) on high surface area fuel cell electrodes. The DFT calculations and observed spectra suggest that the side-chain CF3, CF2 groups (i.e., of the backbone and side chain) and the SO3(-) are ordered by the platinum surface. A model of the Nafion-Pt interface with appropriate dihedral and native bond angles, consistent with experimental and calculated spectra, suggest direct adsorption of the CF3 and SO3(-) functional groups on Pt. Such adsorption partially orders the Nafion backbone and/or side-chain CF2 groups relative to the Pt surface. The coadsorption of CF3 is further supported by Mulliken partial charge calculations: The CF3 fluorine atoms have the highest average charge among all types of Nafion fluorine atoms and are second only to the sulfonate oxygen atoms.

Investigation of Transport Properties, Microstructure, and Thermal Behavior of PEFC Catalyst Layers

Several experimental techniques have been used to characterize the properties of ionomer in PFEC catalyst layers (CLs). NMR methods have been applied to determine the water motions at long-range (diffusion) and short-range (relaxation) in CLs. Our preliminary results suggest that the water diffusion in the ionomer phase is affected by the amount of catalyst particles (Pt/C) present in CLs. To further assess the factor(s) that might influence water diffusion in CLs, Transmission Electron Microscopy (TEM) has been conducted to investigate the dispersion of ionomer and catalyst particles in CLs. The composition effect on the CL structure will be discussed. In addition, thermal properties of CLs have been studied by modulated differential scanning calorimetry (MDSC) in an attempt to reveal polymer-Pt/C interactions. When compared with the thermal behavior of bulk Nafion (Nafion-112), additional features are observed in the MDSC 'kinetic' component during heating process. The nature of this feature is currently under investigation.

Super Proton Conductive High-Purity Nafion Nanofibers

In this paper, we report the high proton conductivity of a single high-purity Nafion nanofiber (1.5 S/cm), which is an order of magnitude higher than the bulk Nafion film ( approximately 0.1 S/cm). We also observe a nanosize effect, where proton conductivity increases sharply with decreasing fiber diameter. X-ray scattering provides a rationale for these findings, where an oriented ionic morphology was observed in the nanofiber in contrast to the isotropic morphology in the bulk film. This work also demonstrates the successful fabrication of high-purity Nafion nanofibers ( approximately 99.9 wt %) via electrospinning and higher humidity sensitivity for nanofibers compared to the bulk. These results should have a significant impact on fuel cells and sensors.

Humidity-Dependent Structure of Surface Water on Perfluorosulfonated Ionomer Thin Film Studied by Sum Frequency Generation Spectroscopy

Humidity-dependent water structure on the Nafion thin film surface was determined at room temperature by using sum frequency generation spectroscopy (SFG). When the Nafion thin film was exposed to a low relative humidity (RH) environment, a peak at 3720 cm−1 due to the “dangling OH” or “free OH” of water molecules and a peak centered around 3600 cm−1, which was assigned to be due to water molecules interacting with sulfonate groups of the Nafion surface, were observed. The intensities of these peaks increased as RH was increased. A broad peak centered around 3200−3300 cm−1 due to water adsorbed on hydrophobic, i.e., fluorocarbon, sites of Nafion was observed when the Nafion thin film was exposed to a high RH environment (RH > 60%). The behavior of surface water is compared with that of water in bulk Nafion.