Ionomer Membranes Research Articles

Sensing mechanical deformation via ionic polymer metal composites: A primer

Ionic polymer metal composites (IPMCs) were discovered about twenty five years ago by Prof. Oguro and his team in Japan [1], and evidence about metalizing ionomer membranes dates even earlier [2]. Since their discovery, IPMCs have been the object of intensive research across the entire globe [3]-[5]. In its simplest form, an IPMC is constituted by a hydrated ionomeric membrane (that is, an electrically-charged polymer), which is neutralized by mobile counterions in a solution and plated by noble metal electrodes. IPMCs can be used as actuators and sensors, whereby a voltage applied across the electrodes will elicit mechanical deformation, and an imposed mechanical deformation will induce a measurable voltage across the electrodes.

‘Artificial peroxidase’ nanozyme – enzyme based lactate biosensor

In contrast to bienzyme biosensors, we propose the nanozyme-enzyme based ones substituting the enzyme peroxidase with the more active and stable nanoparticles “artificial peroxidase”. The use of catalytically synthesized Prussian Blue based nanozymes simplifies assembling of hydrogen peroxide transducer providing its higher sensitivity. For immobilization of lactate oxidase the composite alkoxysilane – perfluorosulfonated ionomer (PFSI) membranes are proposed achieving the significantly improved operating stability. The resulting nanozyme-enzyme lactate biosensor displays twice higher sensitivity (>0.2 A M−1 cm−2) compared to the Prussian Blue film based one. Nanozymes “artificial peroxidase” are expected to find wide use in elaboration of oxidase based biosensors.

Characterizing Durability of PFSA Membranes of PEM Fuel Cell Based on Numerical Fatigue Crack Propagation

Fatigue crack growth in the Perfluorosulfonic-acid (PFSA) ionomer membranes with mechanical reinforcement and chemical stabilizer represents a key barrier in improving the durability of the polymer electrolyte membrane (PEM) fuel cell. Both ex-situ and in-situ Paris regime fatigue crack behaviors can be evaluated numerically based on the critical plastically dissipated energy (CPDE) ahead of the crack tip. Some fairly good agreement between the finite element (FE) computational crack predictions and the ex-situ experimental crack propagation rates have been found for different polymers in previous work1. In a similar fashion, the fatigue crack growth criteria (e.g., CPDE) of Nafion®-based ionomer membranes can be determined numerically at various environmental conditions (RH and T). The determined fatigue crack criteria, then, will be implemented in a 2D unit fuel cell FE model following the reference work2 with extensions. Thus, in-situ fatigue crack behaviors of the ionomer membranes can be evaluated in response to different operating circumstances. This numerical scheme can be used to evaluate the mechanical integrity of different ionomer membranes in the screening process by the manufacturer/buyer. Moreover, in further implementations, the results from this modeling can be used in exploring the synergy between chemical (e.g., gas-crossovers) and mechanical failures (e.g., fatigue cracks) in PEM fuel cell membranes. REFERENCES G. Ding, A. M. Karlsson, and M. H. Santare, Int. J. Fatigue, 94, 89–96 (2017) http://dx.doi.org/10.1016/j.ijfatigue.2016.09.012.G. Ding, M. H. Santare, A. M. Karlsson, and A. Kusoglu, J. Power Sources, 316, 114–123 (2016) http://dx.doi.org/10.1016/j.jpowsour.2016.03.031.

Cyclic Hygral Swelling and Shrinkage Behavior of Fuel Cell Membranes

Mechanical membrane durability is a key factor for the overall lifetime of PEM fuel cells. It is therefore important to understand the impact of the dynamic membrane hygral swelling and shrinkage characteristics on the in-situ mechanical stresses and the associated mechanical durability. In this work, an in-depth experimental investigation was carried out to determine the cyclic hygral swelling and shrinkage properties of four commonly used types of fuel cell membranes: Nafion® NRE211, two different expanded polytetrafluoroethylene (ePTFE) reinforced perfluorosulfonic acid ionomer membranes, and one hydrocarbon membrane. In addition to the bare membranes, the effect of coating the membranes with catalyst layers on the hygral swelling/shrinkage behavior was also investigated. Tensile specimens were subjected to ex-situ hydration and dehydration cycles using a dynamic mechanical analyzer (TA Instruments Q800 DMA) equipped with an environmental chamber, wherein the dynamic swelling and shrinkage characteristics as well as the residual stresses were measured for each membrane by displacement sensor and load cell, respectively. Hydrocarbon and reinforced PFSA ionomer membranes revealed higher shrinkage than swelling at each RH cycle, resulting in an overall shrinkage. Inside a fuel cell stack where membrane swelling and shrinkage is constrained, the observed shrinkage would result in internal tensile stress in the membrane and adjacent components, and hence reduce the overall membrane mechanical durability. The accumulation of residual stress during successive RH cycles was therefore measured explicitly using a custom-designed constrained shrinkage test and compared to the tensile properties of each individual membrane. For example, the hydrocarbon membranes analyzed in this work showed large residual stresses (severe contraction) during confined shrinkage at 80oC that exceeded their yield stress, and are therefore deemed prone to failure after repeated RH cycles. In addition, the hygral swelling and shrinkage behavior of each membrane was compared to the corresponding catalyst-coated membranes and the reinforcement offered by the catalyst layers was determined. The identified swelling and shrinkage properties, generalized into three different categories of overall cyclic behavior, have vital implications for the development of durable fuel cell membranes and membrane electrode assemblies. In addition to providing good mechanical strength, the membranes are also expected to accommodate or preferably eliminate progressive shrinkage stresses during dynamic fuel cell operation.

Insights into the Evolution of Chemical Degradation in Fuel Cell Membranes Using 4D in Situ Visualization

Ionomer membrane degradation during fuel cell operation is conjointly produced by three different modes, namely, thermal, mechanical, and chemical. Amongst these, chemical degradation is generally considered a major contributor to lifetime limitations.[1] Chemical degradation of the ionomer membrane is caused by the formation of radical species, such as hydroxyl (OH•) and hydroperoxyl (HOO•) groups, which causes polymer chain scission and unzipping leading to membrane thinning and eventual rupture and shorting events. Furthermore, the chemical degradation deteriorates the mechanical properties of the membrane which dramatically accelerates its overall degradation rate. Scanning electron microscopy (SEM) based studies have been conventionally used to conduct two-dimensional characterization of degradation-induced structural features in fuel cell membranes; however, SEM imaging is inherently destructive and thus prohibits any tracking of structural changes over time. Laboratory-based X-ray computed tomography (XCT) is as an alternative imaging technique, which has not only enabled three-dimensional (3D) membrane failure analysis revealing novel insights on membrane failure [2,3], but has also facilitated studies on damage growth characterization, [4] and more recently on evolutionary aspects of mechanical membrane degradation through identical-location imaging. [5] In the present work, an XCT-based 4D in situ imaging method, featuring three dimensions in space and one dimension in degradation time, is applied to investigate the evolution of pure chemical membrane degradation through the use of a custom designed fixture housing a fully operational small-scale fuel cell. [6] Membrane electrode assemblies (MEAs) with three distinct membrane types, namely, i) non-reinforced, ii) mechanically reinforced, and iii) chemically and mechanically mitigated membranes, are separately examined and compared. The MEAs were subjected to open circuit voltage (OCV) hold under high temperature (75°C) and low humidity (30% RH) with 0.3 slpm H2 and 0.5 slpm air supplied to anode and cathode, respectively. Various in situ electrochemical diagnostics were periodically performed to monitor MEA and membrane health. XCT-based 3D datasets were obtained at various stages of degradation to facilitate identical location tracking and analysis of membrane morphology as a function of degradation time and enable the characterization of damage evolution in the three membrane types. Membrane thinning and other damage features within all three membranes are comprehensively examined from various perspectives by studying planar and cross-sectional views extracted from the 3D XCT datasets. Preliminary observations show that electrode-shorting under land regions was the key failure mode for membranes without chemical mitigation, i.e., non-reinforced and mechanically reinforced membranes (Fig. 1). In contrast, the chemically mitigated membrane did not fail up to 850 h of operation, highlighting the scavenging effect against the radicals. No membrane pinhole or crack development was observed in any of the three membranes, which is consistent with previous 3D post mortem results under pure chemical degradation.[3] The present 4D in situ imaging approach further revealed locally amplified membrane thinning preceding the eventual formation of shorts due to absence of membrane material and associated loss of electrode separation. The observed phenomenon further suggests that mechanical stresses, even in the absence of wet/dry cycling, may influence the local shorting events. Overall, the new findings from this work demonstrate the distinct advantage of XCT technology towards improving the fundamental understanding of membrane degradation by capturing critical failure modes and mechanisms at their different developmental stages. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. W. L. Gore & Associates, Inc. is acknowledged for additional support.

Sequential Degradation Mechanism of Stationary PEMFC Under Dehydration Condition

With regard to increasing energy consumption, a polymer electrolyte membrane fuel cell (PEMFC) has received lots of attention due to its pollutant-free fuel and byproduct (water). There are lots of works to improve the performance and stability of PEMFC, however, there remains unsolved problem of long-term durability directly related to cost, which are caused by the corrosive environments inside the membrane-electrode assembly (MEA). In particular, both electrochemical corrosion and chemical deterioration result in a fast degradation of PEMFC, especially under the specific degradation conditions such as fuel starvation, flooding, dehydration, and on-off cycle. Among them, a study on dehydration condition becomes the most important as next-generation fuel cell system demands high temperature (low RH) in stack operation for better tolerance to CO and the simplified water management systems in PEMFC [1]. However, the previously reported degradation mechanisms under dehydration are mostly focused on the membrane decay and the electrochemical analyses were too conventional to elucidate the mechanism. Hence, in the present study we aim at performing sequential deconvolution of the degradation mechanism of PEMFC operating at 300 mA cm-2 under low humidity of RH 30%. During the dehydration degradation process, we conducted electrochemical measurements under both humidity condition: normal condition (RH 100%) and dry condition (RH 30%). In particular, electrochemical measurement under low humidity condition allows us to exclude the influence of the water as a proton transport medium (See Figure 1(a)) and subdivide specific resistance components from the profile of the Nyquist plots by increasing the ionic resistance deliberately in order to focus on only the intrinsic properties of membrane itself as shown in Figure 1(b). As shown in Figure 2(a-b), the current density at 0.7 V of cell voltage under RH 100 % condition decreases from 620.0 to 518.8 cm-2 during initial 300 h of degradation test and additional 300 h of procedure causes additional 17.2% decrement with the increase of charge transfer resistance. Even though these results show the degraded MEA under corrosive condition, these are not enough to identify the successive degradation components. As shown in Figure 2(c), the i-V curves measured under RH 30 % shows the performance also decrease during initial 300 h of degradation process with the increase of ionic resistance from 1.15 to 3.15 ohm cm2. However, after additional 300 h (total 600 h) the abnormal phenomenon showing the temporarily improved performance was confirmed with the decrease of ionic resistance from 3.15 to 1.9 ohm cm2 as shown in Figure 2(d). The unique characteristic could seem to be induced by a shorten proton diffusion path by the membrane thinning [2]. After operating at 300 mA cm-2 for more than 600 h at RH 30%, TEM surface analyses were conducted. As shown in Figure 3(a-e), the Pt particles were grown more largely on cathode than anode, which implies that reverse current decay (cathode carbon corrosion) occurs even in the dehydration degradation. In summary, the degradation mechanism under dehydration can be separately identified in sequential order: (1) ionomer degradation, (2) membrane thinning, and (3) reverse current decay.

(Olin Palladium Award of the Electrochemical Society) Polymer Electrolyte Fuel Cells: Recognition of a Field of Electrochemistry for Technical Contributions Made by Outstanding Technical Teams

The Olin Palladium medal is being awarded at this ECS meeting for projects I have been involved with over the last 35 years, mostly in the field of polymer electrolyte fuel cells (PEFCs). The great advancement of this technology over that period of time, has been the product of outstanding R,D&E work by several teams of scientists and engineers. I believe ECS is recognizing today the impressive advancements in this field of electrochemistry and, particularly, the scientists and engineers who enabled these advancements culminating in reduction to practice of 100 kW level PEFC power sources for electric vehicles (FCEVs). I feel I am receiving this award for the community of key contributors to this field of electrochemistry and I am honored to represent them here. Serving over the years as Technical Team Leader in a series of PEFC R,D&E projects, I learnt to fully recognize the interdisciplinary nature of this technology and the resulting impact on technology development. The foundations for the science and technology of polymer electrolyte fuel cells are provided by a significant number of fields , including: * Catalysis at the solid/electrolyte interface as practiced with nano-dispersed catalyst particles, * ionic and electronic conductivity of mixed conductors, * technology of ionomeric membranes and the science of the water/ionomer system, * physics and chemistry of high surface area carbons, *kinetics of multistep electrochemical processes, * modeling of complex systems involving combined physical & chemical processes, *structural materials and advanced methods of metal forming, *metal corrosion and chemical decay of ionomers and polymers and, * Analysis and design of energy conversion systems. Good control of a wider spectrum of these technical disciplines by members of a fuel cell technical team, carries great value in facilitating communication between technical staff members of different backgrounds and, in intensifying fruitful exchange. A wider “personal horizon” should indeed be assigned special weight in consideration of candidates for staff positions in such teams. My talk in this session will offer a brief survey of PEFC technology highlights over the past 35 years. Starting with the important contributions of the LANL team in the 1980s and 1990s, the development and demonstration of thin film catalyst layers with loading of <0.5mgPtcm-2 had special significance. The catalyst layer has been typically prepared by ultrasound-mixing of an ionomer dispersion and a carbon-supported Pt catalyst , forming a catalyst ink to be applied to the membrane surface. Catalyst layers so prepared indeed enabled achievement of several watts per cm2 using just ~0.5mgPtcm-2. Moving next fast forward to 2015 , a bolder target of ultra-low Pt loading for transport applications (~0.1mgPtcm-2 ) resulted in performance limitations at high current densities, apparently caused by (i) limited rate of oxygen transport through a thin film of ionomer around the catalyst particle and, (ii) catalyst surface deactivation caused by such ionomer coat. A remediation described recently, targets placing the metal catalyst nanoparticle “side-by-side” with a nano-grain or, nano-patch of the ionomer, achieving such nano-separation by using HSACs as catalyst support (GM 2016, Kofu Univ., 2016 and TUM ,2017). With this last development taking place 25 years after the first introduction of low Pt loading technology, some comments will be offered on the dynamics of PEFC science and technology development. Looking for a cost-effective alternative to the PEFC , CellEra (Israel) started in 2010 development of fuel cell technology based on hydroxide ion conducting membranes (HEMFCs) which should allow use of Pt-free electrodes. HEMFCs present some significant technical challenges, including water management and carbonation by CO2 in the air feed stream. Efforts at Cellera (now POCellTech) resulted in HEMFC performance on hydrogen and air (CO2-free) equivalent to that of a PEMFC using similar ultra-low PGM loading of <0.1 mg Pt/cm2. Consequently, advantages of the lower HEMFC stack materials cost can be now realized. A new approach (UDel,2018 ) to the removal of CO2 from the air supply stream by electrochemical pumping, will be described. An activity to be described last , started in 2015 and is devoted to direct ammonia fuel cells (DAFCs) with an alkaline membrane electrolyte , operating around 100°C. A team involving BNL, POCellTech and UDel, demonstrated very recently power density of 600 mW/cm2 generated by such an ammonia/oxygen cell operating near 100°C. Being a carbon-free fuel which is liquefiable at ambient temperature under relatively low pressures, liquid ammonia has been considered recently as a possible medium for storage and distribution of the hydrogen to be made from renewable resources (EPRI Report , 2019).

Durability of Newly Developed Polyphenylene-Based Ionomer Membranes in Polymer Electrolyte Fuel Cells: Accelerated Stress Evaluation

Perfluorosulfonic acid (PFSA) membranes such as Nafion® have been widely used as the electrolyte for polymer electrolyte fuel cells (PEFCs) due to their high proton conductivity and chemical stability. However, their fully fluorinated chemical structure leads to high production cost, low glass transition temperature and high gas permeability, which tend to impede the widespread application of PEFCs. To address these problems, sulfonated hydrocarbon (HC) membranes have been researched over the past decade. In our previous study,1 we investigated a sulfonated benzophenone poly(arylene ether ketone) semiblock copolymer (SPK-bl-1, SPK, Figure 1a)2 and a phenylene poly(arylene ether ketone) semiblock copolymer (SPP-bl-1, SPP, Figure 1b)3 membrane under accelerated open circuit-voltage (OCV) conditions appropriate for FCVs. The results indicate that a simple hydrophilic structure that does not include ketone groups leads to increased chemical stability versus radical attack decomposition.On the other hand, an SPP membrane quickly decomposed when subjected to a Fenton’s reagent test, which may be attributed to the presence of heteroatoms in the hydrophobic structure. Based on these results, we developed a new hydrocarbon membrane that consists solely of phenylene groups, which is designated a sulfonated poly(phenylene) quinquephenylene (SPP-QP, Figure 1c).4 In the present research, we focus on the durability of the SPP-QP membrane, in comparison with previous results for HC membranes (SPP and SPK) under identical accelerated OCV conditions.1 The properties of the membranes are listed in Table 1. We have designated these combinations of MEA components as cell types 1, 2 and 3 cells, as listed in Table 2. The membrane designations, e.g., “SPP-QP type #” (Figure 2), simply refer to the same membrane tested in different cell configurations. During the durability evaluations, the exhaust water from both sides of the cells was collected and then analyzed by ion chromatography (IC). After the durability evaluations, the post-test membrane electrode assemblies (MEAs) were analyzed with nuclear magnetic resonance (NMR) spectroscopy. The cell voltages were continuously measured during the accelerated OCV stress evaluation, and the hydrogen leak current density and the composition of the water that was drained from each cell were measured every 200 h by use of LSV and IC, respectively, as shown in Figures 2a–2c. The cell voltage for the SPP-QP type 1 cell decreased significantly after 700 h. In the case of SPP-QP type 2 and type 3 cells, the cell voltages maintained high values for over 1000 h, as well as those for the SPP cell and SPK cell (Figure 2a). The hydrogen leak current density also behaved nearly the same for each of the cells, except for the SPP-QP type 1 cell, which reached ca. 100 times higher H2 leakage at 800 h than those of the others (Figure 2b). The emitted concentration values of sulfate decreased in the order SPK » SPP > SPP-QP type 1 > SPP-QP type 2 ≈ SPP-QP type 3(Figure 2c). The intrinsic chemical stabilities of the SPP-QP, SPP and SPK membranes were evaluated with Fenton’s reagent. These results indicated that the SPP-QP membranes had much higher intrinsic chemical stability compared to those of SPP and SPK membranes, which was reflected in the results of the accelerated OCV stress evaluation. We also studied approaches to mitigate the mechanical stress by use of GDLs5 and gaskets in order to evaluate the chemical stability of the membranes under accelerated conditions in MEAs.6 Acknowledgement This work was partially supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)” Project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors are grateful to Panasonic Corporation for kindly providing the experimental GDL as soft GDL. Also, the authors are grateful to Honda R&D Co., Ltd. for kindly providing the accelerated OCV stress-evaluation protocol. References1) R. Shimizu, J. Tsuji, N. Sato, J. Takano, S. Itami, M. Kusakabe, K. Miyatake, A. Iiyama, and M. Uchida, J. Power Sources, 367, 63 (2017).2) T. Miyahara, T. Hayano, S. Matsuno, M. Watanabe, and K. Miyatake, ACS applied materials & interfaces, 4, 2881 (2012).3) J. Miyake, T. Mochizuki, and K. Miyatake, ACS Macro Letters, 4, 750 (2015).4) J. Miyake, R. Taki, T. Mochizuki, R. Shimizu, R. Akiyama, M. Uchida, and K. Miyatake, Science Advances, 3, eaao0476 (2017).5) H. Ishikawa, T. Teramoto, Y. Ueyama, Y. Sugawara, Y. Sakiyama, M. Kusakabe, K. Miyatake, and M. Uchida, J. Power Sources, 325, 35 (2016).6) R. Shimizu, K. Otsuji, A. Masuda, N. Sato, M. Kusakabe, A, Iiyama, K. Miyatake, and Makoto Uchida, J. Electrochem. Soc, 166 (7) F3105-F3110 (2019) Figure 1

Investigation of Ce3+ Mobility in NR211 Under RH Gradients

A proton exchange membrane fuel cell (PEMFC) is an attractive candidate for use in transportation and stationary applications as a clean energy source. A key component of a PEMFC is the polymeric ionomer membrane that serves as a proton conductor and prevents reactant gas crossover and electrical shorting. A widely used class of polymeric ionomers are perfluorinated sulfonic acid (PFSA) type ionomers because of their overall superior chemical and mechanical properties. The primary product of an operating fuel cell is water, but there are also small quantities of hydroxyl radical (•OH) and hydrogen peroxide (H2O2) produced from chemical and electrochemical reactions. These species react with non-fluorocarbon atoms or the sulfonic acid moiety to degrade the polymeric ionomer. To mitigate the chemical degradation of the polymeric ionomer, redox active cations such as Ce3+ and Mn2+ can be incorporated in membrane electrode assemblies (MEAs). These cations react at kinetically sufficient rates with •OH to form water. The resulting oxidized cation is subsequently reduced by H2O2 to complete the redox cycle. Within the polymeric ionomer, these cations associate (ion pair) with negatively charged sulfonate (SO3 -) groups. At the same time, cation complexation of the SO3 - groups modulates the local water content (l = nH2O/SO3 -) in the membrane. During fuel cell operation over a period of ten to 100 hours, cations are observed to diffuse on a centimeter length scale. The redistribution of cations can impact the fuel cell performance and mitigation of chemical degradation reactions. The movement of cations within an MEA is dictated by water content (l), water content gradients (Dl), and potential gradients. Therefore, the optimization of fuel cell performance and durability requires understanding how these factors impact cation movement. In this presentation, we will report isotropic diffusion coefficients of Ce3+ and Co2+ within a NR211 membrane over an array of uniform temperature and relative humidity values. The cation concentrations were monitored using FTIR, x-ray fluorescence, and UV-vis spectroscopies. The derived diffusion coefficients from these data show a strong dependence on relative humidity and temperature. These mobility rates are also consistent with redistribution of cations in operating fuel cells. In addition, cation movement under RH gradients will be presented. In this work, a dual-RH chamber apparatus was employed to study cation mobility rates over a range of RH chamber values and temperatures. For example, uniformly Ce3+-doped NR211 membranes clearly exhibit cation migration from the high to the low RH chamber (Figure 1a) and the mobility of cations in the low RH chamber is greatly reduced. Much additional experimentation with Ce3+ patch-doped NR211 strips (Figure 1b) has been conducted to elucidate the influence of water gradients on the rate and extent of cation mobility. Finally, a 1-D in-plane Ce3+ transport model that was developed from these data will be discussed. Figure 1

Chemical and Morphological Origins of Improved Transport in Perfluoro Ionene Chain Extended Ionomers

The performance of proton-conducting ionomer membranes used in electrochemical applications such as fuel cells is complicated by an intricate interplay between chemistry and morphology that is challenging to characterize and control. Here, we report on a class of perfluoro ionene chain extended (PFICE) ionomers that contain either one (PFICE-2) or two (PFICE-3) bis(sulfonyl)imide groups on the side-chain in addition to a terminal sulfonic acid group. PFICE ionomers are promising materials, exhibiting greater water uptake and conductivity over a range of relative humidity values compared to prototypical perfluorinated sulfonic acid (PFSA) ionomers. Advanced in situ synchrotron characterization combined with simulations reveals insights into the connections between molecular structure and morphology that dictate performance. Energy-tunable X-rays with sensitivity to sulfur can decipher the unique bonding environment of different protogenic groups on the polymer side-chain. Guided by simulations, X-ray absorption spectroscopy can be sensitive to hydration level and configuration that dictates proton dissociation. In situ resonant X-ray scattering reveals that PFICE ionomers have a phase-separated morphology with enhanced short-range order that persists in both dry and hydrated state, allowing for improved transport pathways across hydration levels. Furthermore, side-chain chemistry and length can be used as a molecular design parameter to predict phase-separated domain spacing. The enhanced conductivity of PFICE ionomers is attributed to a unique side-chain chemistry and structure promoting hydrogen bonding configurations that facilitate proton dissociation at low water content in combination with a well-ordered phase-separated morphology that forms transport pathways. Overall, these results provide guidelines to design new ionomers with improved transport properties and demonstrate the value of in situ characterization methods such as resonant X-ray scattering and spectroscopy for unraveling the structural features in chemically-heterogeneous materials used in electrochemical systems.

Ionomer Self-Assembly during Drying Process of Alcohol/Water Solutions Using Coarse-Grained Molecular Dynamics Simulations

The cathode catalyst layer (CL) is one of the principal components for proton exchange membrane fuel cells. CL is typically prepared by drying a catalyst ink consisting of Pt-C (platinum on a carbon powder support), ionomer, and solvents (water/alcohol mixtures). The structure and stability of ionomer aggregates in solutions are the key to the physical properties of PEMs, including water distribution, conductivity, and mechanical response of the membrane. Thus, understanding the ionomer aggregation behaviors in solutions is of great importance to gain a deep insight into fundamental morphological characteristics of ionomer membranes. The characterization of the ionomer aggregate structures in solutions has also been extensively explored in detail using experimental measurements. In this study, self-assembly of ionomers in a mixture of 1-propanol (NPA) and water have been investigated using coarse-grained molecular dynamics simulations. The dependence of NPA content on the ionomer structures was studied by systematically changing the NPA content in the system. The self-assembly behavior of ionomers into cylindrical bundle-like aggregates was observed and the aggregate size was found to be dependent on the NPA content in solutions. The effects of drying process on the ionomer aggregations will also be discussed in the presentation using the dimensionless Peclet number, which is used to describe the importance of evaporation to diffusion in a film of initial thickness. Figure 1

Theory of Multi-Ion Transport and Thermodynamic Phenomena in Solvent-Filled Membranes

Solvent-filled membranes, such as Nafion, are an important class of polymer electrolytes because the imbibed solvent imbues the material with high ionic conductivity, while still allowing the membrane to maintain its mechanical stability. Both of these attributes are essential for membrane electrolytes in numerous energy-storage and -conversion devices including redox-flow batteries. The transport phenomena is complicated due to the multiple ions in the electrolytes employed in these applications and the higher concentrations desired for energy density. Furthermore, while there is understanding on the single-ion transport in these materials, there is a need to develop a self-consistent theory for describing the ion transport and ion partitioning and thermodynamics when interacting with the charged polymer membrane. Moreover, there are few quantitative material design guides for polymers that balance the movement of certain species that are desirable, such as current-carrying ions, against transport of contaminants, additives, or redox-active species, which decrease device performance. To address the above gaps and provide material design insights, this work describes the theory of multi-ion transport in polymer-electrolyte membranes. To specify the thermodynamics of the system, we formulate the membrane and electrolyte solution free energies. The model accounts for long-range electrostatic and short-range solvation and physical interactions between ions. Externally validated solution and membrane properties parameterize the model. The model calculations agree with measured water and ion uptake in dilute and concentrated binary and ternary salt electrolytes, with the uptake being a balance between osmotic and elastic pressures that are impacted by ion specificity and membrane pretreatment.1-2 To model the transport phenomena, we use Stefan-Maxwell-Onsager theory. To connect performance to the molecular-level structure of the polymer, hydrodynamic theory provides constitutive relations for the involved ion/solvent and ion/ and solvent/polymer friction coefficients, with classical porous-media theories scaling tortuosity. The model compares favorably to a number of published measured membrane transport properties1-2 (i.e. conductivity, transference numbers, electroosmosis, and water-transport coefficient) in dilute and concentrated binary and ternary electrolytes. Importantly, the model isolates the contribution of each type of molecular-scale to the measured properties. Acknowledgements: This study was funded with support of the Fuel Cell Performance and Durability Consortium (FC-PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02- 05CH11231 and, in part, by the Advanced Research Projects Agency - Energy (ARPA-E), U.S. Department of Energy (DOE) under Award Number DEAR0000149 . References Okada, T.; Satou, H.; Okuno, M.; Yuasa, M., Ion and Water Transport Characteristics of Perfluorosulfonated Ionomer Membranes with H+ and Alkali Metal Cations. J. Phys. Chem. B 2002, 106 (6), 1267-1273.Tang, Z.; Svoboda, R.; Lawton, J. S.; Aaron, D. S.; Papandrew, A. B.; Zawodzinski, T. A., Composition and Conductivity of Membranes Equilibrated with Solutions of Sulfuric Acid and Vanadyl Sulfate. J. Electrochem. Soc. 2013, 160 (9), F1040-F1047.

Contactless actuation of perfluorinated ionomer membranes in salt solution: an experimental investigation

A variety of modeling frameworks have been proposed for ionic polymer metal composites (IPMCs), but the physical underpinnings of their actuation remain elusive. A critical step toward the validation of existing theories and transition to engineering practice entails the design of new experimental paradigms that could support hypothesis-driven research. While several factors exacerbate the complexity of experimenting with IPMCs, the presence of the electrodes plays a major role by hindering the repeatability of the results and bringing a number of difficult-to-measure parameters into the picture. Here, we seek to address these experimental confounds by investigating contactless actuation of perfluorinated ionomer membranes in salt solution. In contrast to IPMCs that bend toward the anode in response to an applied voltage, ionomer membranes display a consistent deflection toward the cathode. Through hypothesis-driven experiments where the membrane width, solution concentration, and voltage applied across the electrodes are systematically varied, we elucidate electrochemistry and mechanics of contactless actuation. The applied voltage and solution concentration have a dominant role on the electrochemistry, while mechanics is mainly affected by the applied voltage and membrane width. Our results depict a complex scenario, which is expected to inform future theoretical inquiries about IPMC actuation.

Ce Cation Migration and Diffusivity in Perfluorosulfonic Acid Fuel Cell Membranes

A hydrogen pump experiment was utilized to simultaneously determine the migration and diffusivity of Ce ions in perfluorosulfonic acid (PFSA) ionomer membranes over a range of temperatures and relative humidities. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). Competing transport phenomena were decoupled by fitting XRF profile data with our previously-developed one-dimensional model, which was updated with improved conductivity and water uptake relations. Measured transport values showed good agreement with the Einstein relation and Okada transport theory, implying that conductivity can be used to estimate migration and diffusivity of cations in other cation/PFSA systems. Results presented here may be used to populate device-level models in order to further understand the effects of cation transport on fuel cell performance and durability to determine the mitigation controls for cation stabilization.

Systematic Characterizations of All-Vanadium Redox Flow Battery Employing Pore-Filled Ion-Exchange Membranes

Recently, the interest in high-performance energy storage system (ESS) is increasing since it is a very essential part of the smart grid for stable and efficient electricity supply. At present, diverse battery technologies such as lithium ion batteries, redox flow batteries (RFBs), and sodium–sulfur batteries are competing with each other to be applied to commercial ESS. Among them, RFBs are considered to be the most promising for a large-scale ESS application. Meanwhile, all-vanadium redox flow batteries (VRBs) utilizing vanadium ions in different oxidation states as the active redox species have been considered as one of the most promising RFBs owing to the controlled crossover rate of the redox species through membranes. Since an ionomer membrane separator is one of the most dominant components determining the charge-discharge characteristics of RFB systems, the development of highly efficient membranes has been considered to be very essential for successful commercialization of VRBs. Among various kinds of membranes, pore-filled ion-exchange membranes (PFIEMs) fabricated by combining a porous substrate and an ionomer are known to have excellent characteristics including high chemical and dimensional stabilities, low manufacturing cost, low mass transfer resistance, etc. In this work, we have developed novel PFIEMs by using a polyethylene (PE)- or a polytetrafluoroethylene (PTFE)-based porous substrate for VRB applications. Especially, we have designed the filling ionomer to possess the high crosslinking degree and ion-exchange capacity, simultaneously. In addition, the prepared PFIEMs and the RFB employing them have been systematically analyzed by various electrochemical methods for evaluating their performances exactly. The detailed results will be presented and discussed at the conference. This work was supported by a grant (No.2017000140002/ RE201702218) from the Environmental Industry Advancement Technology Development Project of Korea Environmental Industry & Technology funded by Korea Ministry of Environment.

Effect of hygral swelling and shrinkage on mechanical durability of fuel cell membranes

Membrane mechanical durability is one of the primary factors limiting the lifetime of ionomer membranes in polymer electrolyte fuel cells. Due to the importance of membrane dimensional stability during relative humidity (RH) cycles on the magnitude of mechanical stress, the present work contributes to an in-depth understanding of the hygral swelling and shrinkage of state-of-the-art membranes and catalyst coated membranes (CCMs). Through conducting ex-situ hydration and dehydration tests, the swelling and shrinkage characteristics for each membrane were determined. An overall, progressive shrinkage in the planar dimension was observed in certain membranes suggesting the creation of residual tensile stress during in-situ operation, hence diminishing the overall membrane mechanical durability. The accumulation of residual stress during successive RH cycles was therefore measured explicitly using a custom-designed constrained shrinkage test and compared to the tensile properties of each individual membrane. Finally, the hygral swelling and shrinkage behavior of each membrane was compared to the corresponding CCM and the reinforcement offered by the catalyst layers was determined. The present findings indicate critical impact of the membrane hygral swelling and shrinkage on the in-situ mechanical stresses and the associated membrane mechanical durability.

Permeability behavior of polyvinyl chloride-ionic liquid ionomer for CO2/CH4 separation

To modification of polyvinylchloride membranes for separation of methane and carbon dioxide gases, 1–vinyl–3-butyilimidazolium halide with three different anions (bromide, [Br]-, thiocyanate, [SCN]-, and tetrafluoroborate, [BF4]-) was grafted onto the polyvinylchloride by free radical graft copolymerization. The prepared membranes were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analyzer (CHN) technique. The permeability of methane and carbon dioxide gases was performed using constant pressure/variable volume method. The effects of variation of anions, feed pressure, and temperature on the membrane performance were investigated. The results indicate that the 1–vinyl–3-butyilimidazolium with [SCN]-anion has better performance to separation of carbon dioxide and methane gases. Also, the enhancement of temperature led to decrease the selectivity of membrane for carbon dioxide/methane gases; whereas, the selectivity of membranes was increased with increasing feed pressure. Also, to confirm the obtained results, the interaction energy between carbon dioxide gas and ionomer membrane with different anions was investigated by density functional theory calculations.

Dendrite Growth Suppression by Zn2+‐Integrated Nafion Ionomer Membranes: Beyond Porous Separators toward Aqueous Zn/V2O5 Batteries with Extended Cycle Life

The dendritic/irregular growth of zinc deposits in the anode surface is often considered as a major intricacy limiting the lifespan of aqueous zinc‐ion batteries. The effect of separators on the evolution of the surface morphology of the anode/cathode is never thoroughly studied. Herein, for the first time, the efficacy of the Zn2+‐integrated Nafion ionomer membrane is demonstrated as a separator to effectively suppress the growth of irregular zinc deposits in the metallic anode of an aqueous Zn/V2O5 battery. The Zn2+‐ions coordinated with the moieties in Nafion result in a high transference number of the Zn2+ cation, all the while facilitating a high ionic conductivity. The Zn2+‐integrated Nafion membrane enables the Zn/V2O5 cell to deliver a high specific capacity of 510 mAh g−1 at a current of 0.25 A g−1, which is close to the theoretical capacity of anhydrous V2O5 (589 mAh g−1). Moreover, the same cell exhibits an excellent cycling stability of 88% retention of the initial capacity even after 1800 charge–discharge cycles, superior to that of the Zn/V2O5 cells comprising conventional porous separators.

Local hydration in ionomer composite membranes determined with confocal Raman microscopy

Water management in electrochemical energy applications like fuel cells has a crucial impact on performance, in particular on the ionic conduction of ionomer membranes. To strengthen the understanding of water management in such devices, we report a novel method for non-destructive measurements of the hydration of composite membranes based on confocal Raman microscopy. Composite membranes were produced by spray-coating of Nafion into a mesh of electrospun poly(vinylidene fluoride-co-hexafluoropropylene)/polyvinylpyrrolidone (PVDF-HFP/PVP) blend nanofibers. Hydration levels of several pure nanofiber meshes and nanofiber/Nafion composites were evaluated by linear least squares fitting of reference Raman spectra to hyperspectral images. We found that spectral contribution of water to nanofiber spectra depends on the PVDF-HFP/PVP ratio and is independent from fiber diameter. Further, we were able to reliably determine nanofiber polymer composition of single fibers based on Raman spectroscopy. Raman imaging of composite membranes was performed at ambient air and fully hydrated conditions to study the local hydration in PVDF-HFP/PVP/Nafion composites as well as in a Nafion XL membrane. 2D through-plane mappings revealed that the nanofiber hydration positively correlated with PVP content. In the Nafion XL membrane, the polytetrafluoroethylene-based reinforcement was verified as a hydrophobic layer sandwiched between Nafion ionomer, which showed a more than 10% reduced hydration compared to the outer Nafion layers. These results motivate the use of confocal Raman microscopy as a novel method to investigate the local water distribution in ionomer composite membranes that are widely used in electrochemical energy conversion.

(Invited) Electrocatalysis of Direct Methanol Fuel Cells with a Special Reference to the Work of Shimshon Gottesfeld

Fuel cells seem firmly advancing towards a broad application in transportation and other areas to take their role of a future clean energy source. It took more than 50 years that to address most of the challenges of fuel cell electrocatalysis in several research cycles. In late sixties electrode kinetics developed diagnostic criteria that allowed in some cases to determine molecular reaction mechanism based on the current – potential relationships. This inspired a considerable fuel cell oriented applied research that culminated in Troisieme Journees Internationales d’Etude des Piles a Combustible, in Bruxelles, 1968. This big meeting did not bring expected advances and the fuel cell research diminished. The interest for direct methanol fuel cell regained momentum slowly through the research directed towards understanding of the role of surface structure and identifying surface intermediates. Advances came from of ex situ structural studies and in particular in situ UV-visible and IR reflectance spectroscopies. In situ FTIR opened new level of details in methanol oxidation. In parallel, the work with well-ordered single crystal surfaces clearly showed inter alia pronounced structural effects in oxidation of methanol on Pt. A broad research effort brought clarifications of reaction mechanism on Pt, poisoning effect of CO intermediate, development of bimetallic electrodes and bifunctional PtRu mechanism. After several reflectance spectroscopy studies Shimshon focused on fuel cell electrocatalysis. Most of the breakthrough in directed basic and applied work on direct methanol fuel cell came from Gottesfeld’s group at Los Alamos. Some of these include: i) Developing new structures for the Pt/C catalyst layer that substantially increase the utilization efficiency of the catalyst having hydrophilicity, thinness, uniformity, and the proper ratio of ionomer and supported catalyst. iii) The catalyst layers are cast from solution as thin films with ionomer itself as a binder and hot pressed directly onto the ionomer membranes, and the hydrophobic gas diffusion backings complete the assembly. iv) Activity of dispersed PtRu catalysts was found to depend on catalyst surface area and a number of metal alloy sites of atomic ratio close to 1:1.v) Patent on achieving CO tolerance using air bleed at the anode inlet was obtained. vi) Modeling studies showed great advantage of a thinner membrane in alleviating resistance problem Recent work brought some breakthroughs very promising for application of DMFC. A consensus on the mechanism of methanol oxidation on a pure Pt electrode includes a parallel pathway mechanism forming a number of reaction intermediates and products (CO2, HCOOH, HCHO); among them adsorbed CO can block Pt sites. Alloying Pt with other oxophilic metals, including Ru, Os, Ir, Rh, and Sn, was broadly studied shown to reduce CO poisoning and increase the catalytic activity. This activity of the Pt alloys can be explained either by the bifunctional model or the electronic effect (also called ligand effect). Most recent breakthrough in this research involve finding that under tensile strain one Pt monolayer (one atom thick layer) placed on Au(111)) exhibits a factor of 7 activity increase and a new reaction mechanism without CO generation, which can eliminate a major drawback of this reaction. Very important is the potentiality of finetuning the catalytic property of Pt monolayer to fabricate electrocatalysts with enhanced activity and ultralow Pt content. This opened up a broad studies of promising core/shell nanoparticle catalysts that will be discussed at the meeting.