Mapping water uptake in organic coatings using AFM-IR

Water uptake kinetics in anti-corrosion organic films with a high resolution microbalance oscillator sensor

Membrane durability is one of the key criteria to be met to successfully commercialize polymer electrolyte fuel cell (PEFC) technology. Commonly used perfluorosulfonic acid (PFSA) ionomer membranes are prone to early degradation under the influence of common degradation stressors encountered in PEFCs. The important functional properties of PFSA membranes such as ionic conductivity and mechanical properties depend on its microstructure due to its unique hydrophobic backbone and side chains with hydrophilic end groups (1). Severe molecular changes to the PFSA membrane structure have been reported due to chemical and mechanical membrane degradation. Such changes in the molecular structure were found to affect thermal stability, water uptake, proton conductivity, and mechanical properties of the membrane; as well as the performance and durability of the fuel cell as previously published by our group (2-5). Although the effects of molecular changes on the degraded membrane characteristics have been explored, the impact on its internal morphology is generally not known. The present objective is therefore to understand the effect of degradation stressors on the morphological changes in order to improve membrane durability. In this work, standard PFSA membranes were subjected to a combined chemical and mechanical accelerated stress test (AST) used for rapid benchmarking of in-situ membrane stability. The chemical phase of the AST generates hydroxyl radicals that attack both the side chain and main chain of the polymer, while the mechanically generated stresses due to humidity cycling accelerate mechanical failures. Beginning-of-life (BOL) and end-of-life (EOL) membrane samples extracted after in-situ degradation were imaged using transmission electron microscopy (TEM). In order to enhance image contrast, the sulfonic acid end group sites in the membrane were selectively exchanged with Pb ions by soaking it in saturated lead acetate solution. The samples embedded in epoxy resin were sliced to thin films (~70-90 nm) using ultra microtome and collected on a Cu grid for imaging. The hydrophilic regions in the Pb exchanged membrane appear as dark spots in the TEM images due to enhanced electron scattering, while the hydrophobic backbone dominated regions appear bright. The BOL membrane micrograph is presented in figure 1(A), exhibiting a regular phase separated morphological layout typical for PFSA ionomer membranes. Since the chemical degradation operates at random locations through out the membrane and commences with ionomer side chain cleavage (2-3), the degraded regions at the nanoscopic level exhibited low concentration of dark spots and the image brightness becomes non-uniform. (figure 1(B)). The lower concentration and absence of dark spots in the degraded region is expected to resemble the absence of ion attractive sulfonic acid groups at that site due to side chain degradation. Also, the dark spots tend to align in a long range order as a result of combined chemical/mechanical degradation. Apart from the severely degraded regions, mildly degraded regions which exhibit BOL morphology were also seen while scanning the membrane at EOL. The images of cracks (not shown here) recorded at EOL reveal that cracks preferentially propagate along the region of lowest concentration of dark spots which represent depletion of SO3 -in the ion-rich hydrophilic domains. Elemental mapping of the degraded zone was carried out by selectively capturing the generated x-rays with characteristic energy due to electron interacting with the sample. The mapping of fluorine in the degraded zone revealed its absence at the most degraded sites (figure 1(C)). This can be correlated to the severe fluoride release from the membrane as a result of combined chemical/mechanical degradation. Overall, the observed effects of combined chemical/mechanical degradation on the membrane morphology contributes to the fundamental understanding of the complex membrane degradation mechanism occurring during PEFC operation. ACKNOWLEDGMENTS Research funding provided by Automotive Partnership Canada (APC), Natural Sciences and Engineering Research Council of Canada (NSERC) and Ballard Power Systems (BPS) is gratefully acknowledged. We also thank BPS for providing material samples and Chan Lim for carrying out AST experiments. REFERENCES [1] Y. Yang, A. Siu, T. J. Peckham, S. Holdcroft, in Advances in Polymer Sciences 215: Fuel cells 1, G. G. Scherer, Editor p. 55 Springer, Berlin Heidelberg 215(2008). [2] C. Lim, L. Ghassemzadeh, F. Van Hove, M. Lauritzen, J. Kolodziej, G.G. Wang, S. Holdcroft, E. Kjeang, J. Power Sources 257102 (2014). [3] L. Ghassemzadeh and S. Holdcroft, J. Am. Chem. Soc. 135 8181 (2013). [4] S. Venkatesan, C. Lim, E. Rogers, S. Holdcroft, and E. Kjeang, Phys. Chem. Chem. Phys. (2015), DOI: 10.1039/C5CP01641J. [5] S. Venkatesan and E. Kjeang, 11th International Fuel Cell Science, Engineering and Technology Conference, ASME, Minneapolis, MN (2013). Figure 1

Experimental study of the quantitative relationship between water uptake and fracture generation in tight reservoirs

Soil water resulting from episodic growing season rainfall evaporates rapidly in semi-arid regions. Plants may mnot benefit from such water additions if near-surface roots are unable to resume water uptake rapidly following periods of soil water deficit. Our objectives were to develop a means of quantifying root uptake responses in the upper soil layer following rewetting after soil water deficit, and to evaluate the existence of genotypic differences among four diverse barley (Hordeum vulgare L.) genotypes in this regard. Plants were grown in replicate soil columns having hydraulically isolated surface and subsoil layers, and instrumented with time-domain reflectometry (TDR) waveguides. The upper 0.05 m soil layer was allowed to dry to —1.8 to —3.0 MPa for 10-14 d, during which time subsoil wetness was maintained at about —0.6 to —0.7 MPa. The time-course of soil water uptake was monitored at 0.5 h intervals following rewetting of the surface layer. Substantial water uptake began 1 d after rewetting following 10 d, and 2-3 d after rewetting following 14 d of water deficit. Rate of water uptake was more rapid in response to a second rewetting 5-7 d later. Consistent genotypic responses in terms of cumulative water uptake on a whole plant and leaf area-specific basis were observed during each trial. These results have application to evaluating drought hardiness and interspecific competitive ability under semi-arid conditions, and to investigations of root physiological and morphological changes that contribute to recovery from water deficit

Organic coating is an efficient method to protect metal surfaces from corrosive atmospheres but is dependent on the presence of a stable interface between the substrate and the polymer coating. However, coatings based on polymers have mostly only limited barrier properties for water and oxygen. Transport of water to the interfacial region is for most coatings inevitable upon prolonged exposure in aqueous environments. Ingress of water molecules can destabilize the substrate / polymer interface and lead to delamination of the polymer coating and initiation of corrosion processes. In this work was spectrochemical imaging of the substrate / polymer interfacial region using a FTIR-microscope with a focal plane array (FPA) detector is performed in order to follow the water up-take at the substrate / polymer interfacial region with micrometer sized lateral resolution. Chemical imaging of the interfacial region between substrates, such as Ge and Ge with thin Zn metallic layers, and organic coatings was performed during exposure to 0.1 M NaCl and humid air (95 %RH). The water uptake and water induced alterations in the substrate / polymer interfacial region on the microscale was followed. Simultaneous electrochemical impedance measurements allowed correlation of electrochemical information from the coated metal with the chemical imaging of the interfacial region.

Wear and Erosion Resistant Ceramic Coatings

Alumina is a relatively low-cost material and easily processable by thermal spraying into wear and corrosion resistant coatings. However, thermally sprayed alumina coatings show inferior corrosion resistance versus chromia coatings, particularly in low and high pH electrolytes. Further, alumina possesses decreased mechanical properties in the as-sprayed state. In the present study, the effect of chromia addition on the properties of the plasma and HVOF sprayed alumina coatings were studied. Pure alumina powder and four different Al2O3-Cr2O3 powders were prepared in two different atmospheres and produced to coatings with APS and HVOF methods. Phase composition of the powders and coatings were studied by X-ray diffraction. Electron microscopy was used for the examination of the microstructure of the powders and coatings. The mechanical properties of the coatings were studied by hardness and abrasion resistance tests. The chemical properties of the coatings were examined by corrosion exposure tests. Results indicate that with chromia addition it is possible to improve the mechanical and chemical properties of the thermally sprayed alumina coatings.

The effect of environmental humidity on radiation-induced degradation of carrageenans

The industrial scale production of high-performance and long term stable lithium ion batteries (LIBs) relies on the usage of precisely tailored materials of high purity. Even small deviations in the material properties or contaminations may lead to detrimental effects regarding the battery’s stability, safety and rate capability. For example, the commonly used conducting salt LiPF6 hydrolyses in the presence of trace amounts of water in the organic solvent, generating HF and further decomposition products [1, 2]. These products can degrade battery components and, consequently, lead to a bad performance of the battery [3]. The components which can contribute to the overall water contamination level inside a LIB are the electrodes, the separator, the housing and the electrolyte itself. Therefore all these components of a LIB have to be dried carefully before assembly. These time and energy intensive processes, contribute to the overall costs of LIBs. In order to ensure a sufficiently low moisture level inside the battery components, while, at the same time, keeping the drying costs low, a detailed understanding of the water sorption and drying behaviour of those battery components is required. While the trace water concentrations in commercially available LIB electrolytes (usually 15 ppm or lower) is commonly precisely monitored, a systematic and comparative study of the hygroscopic and drying behaviour of different electrodes and separators has not been performed yet. This work focusses on the investigation of hygroscopic and drying behaviour of various common anodes, cathodes and separators. Therefore, samples were stored in a defined atmosphere (25 °C, 40 %rh) and their moisture sorption as well as the subsequent drying behaviour (at 120 °C) were studied. The moisture content and drying behaviour of battery components was analysed by a coulometric Karl Fischer titrator. During the sample drying, the evaporated water was transferred into the titrator by a dry argon stream and the water detection was recorded. The results show that the investigated cathodes and anodes differ strongly in their drying behaviour (Fig. 1). Cathodes with LiMn2O4 as active material absorb a high amount of water from the atmosphere and show and a delayed water release while drying. The LiFePO4 cathode also shows strong water uptake, but to a lesser extent than the LiMn2O4 cathode. In contrast, the LiNi1/3Co1/3Mn1/3 cathode shows an amount of water sorption, which is even lower than the water sorption of the commonly used graphite anode. In accordance with the hydrophobic surface behaviour of the polypropylene separator (Celgard PP2500), its water uptake is very low. The glass fibre separator (Whatman GF/C) on the other hand has a large and hydrophilic surface, which results in a very high water content. The water emitted from the samples during the drying process is continuously quantified in the Karl Fischer Titrator. As a consequence time dependent moisture emission rates for the different electrodes and separators are obtained. Fitting and extrapolating the moisture emission rates allows to draw conclusions beyond the runtime of the drying experiments. For example the drying times until reaching threshold levels for the remaining water concentration in the sample can be calculated (Fig. 2). [1] D. Aurbach, a. Zaban, Y. Ein-Eli, I. Weissman, O. Chusid, B. Markovsky, M. Levi, E. Levi, a. Schechter, and E. Granot, J. Power Sources, vol. 68, no. 1, pp. 91–98, Sep. 1997. [2] A. V. Plakhotnyk, L. Ernst, and R. Schmutzler, J. Fluor. Chem., vol. 126, no. 1, pp. 27–31, Jan. 2005. [3] U. Heider, R. Oesten, and M. Jungnitz, J. Power Sources, vol. 81–82, pp. 119–122, Sep. 1999. Figure 1

In association football, players repetitively head the ball for competitive advantages, potentially inducing or exacerbating repetitive acute asymptomatic brain injury pertaining to an elevated risk of neurodegenerative disease development and mortality in later life. During these collisions, the mass of the ball is widely considered an important determinant to collision duress. Many anecdotes cite differences in water uptake characteristics between modern and historical balls, but this has never been scientifically investigated. This study sought to investigate water uptake for a range of leather and synthetic balls. This study exposed seven football types, representative of those used through history, to static whole ball and specimen wettability measurements. All balls demonstrated water uptake. Stitched balls showed more uptake than stitchless balls, and leather balls showed greater uptake than synthetic balls. The magnitude of measured water uptake in leather balls corroborated with historical testing data and showed no evidence of balls doubling in mass, as speculated. All footballs showed similar discharge characteristics. Wettability studies showed greater rate of contact angle changes in leather balls. From differences between stitched and stitchless synthetic balls, two mechanisms of uptake were proposed: absorption (into panel material) and ingress (between panels). This work provides contextual significance to discussions surrounding the historical mass of the ball.

Water-polymer interactions and critical phenomena of swelling in inhomogeneous poly(acrylonitrile-acrylamide-acrylic acid) gels

The commercialization of proton exchange membrane (PEM) fuel cells depends largely upon the development of PEMs whose properties are enhanced over current perfluorinated sulfonic acid PEMs. Understanding how a PEM's molecular weight and morphology affect its relevant performance properties is essential to this effort. Changes in molecular weight were found to have little effect on the phase separated morphologies, water uptake, and proton conductivities of random copolymers. Changes in block length, however, have a pronounced effect on multiblock copolymers, affecting surface and bulk morphologies, water uptake, proton conductivity, and hydrolytic stability, suggesting that multiblock copolymer PEM properties may be optimized by changes in morphology.

A large fraction of secondary aerosol particles are liquid-liquid phase-separated with an organic shell and an inorganic core. This has the potential to regulate the hygroscopicity of such particles, with significant implications for their optical properties, reactivity, and lifetime. However, it is unclear how this phase separation affects the hygroscopic growth of the particles. Here, we showed a large variation in hygroscopic growth (e.g., 1.14-1.32 under a relative humidity (RH) of 90%) of particles from the forest and urban atmosphere, which had different average core-shell ratios. For this reason, a controlled laboratory experiment further quantifies the impact of the organic shell on particle growth with different RH values. Laboratory experiments demonstrated that (NH4)2SO4 particles with thicker secondary organic shells have a lower growth factor at an RH below 94%. Organic shells started to deliquesce first (RH > 50%) and the phase changes of sulfate cores from solid to liquid took place at an RH higher than 80% as deliquescence relative humidity of pure (NH4)2SO4. Our study provides the first direct evidence on an individual particle basis that hygroscopic growth behavior of phase-separated particles is dependent on the thickness of organic shells, highlighting the importance of organic coating in water uptake and possible heterogeneous reactions of the phase-separated particles.

Use of ionic liquids in SECM experiments to distinguish effects of temperature and water in organic coating swelling

Methods for predicting the volume change and swelling-pressure behavior of expansive clays require detailed understanding of coupled interactions between clay microstructure and macrostructure under hydraulic, thermal, and mechanical loads. In this study a suite of water-vapor sorption experiments was conducted using compacted bentonites hydrated in controlled relative humidity (RH) environments maintained under free and constrained volume-change boundary conditions. Emphasis was placed on examining the influences of compaction and predominant exchange cation on the water uptake, volume change, and swelling pressure response. Densely compacted specimens exhibited greater volume changes under free swelling conditions and greater swelling pressures under fully confined conditions. Water uptake, volume change, and swelling pressure were all more significant for Colorado (Ca2+/Mg2+) bentonite than forWyoming (Na+) bentonite. Plastic yielding, evident as a peak in the relationship between swelling pressure and RH, was more evident and occurred at lower RH for the Colorado bentonite. This observation was interpreted to reflect the limited capacity for interlayer swelling in Ca2+/Mg2+ bentonites and corresponding structural collapse induced by the onset of water uptake in larger intra-aggregate and inter-aggregate pores. A semi-quantitative model for the evolution of clay microstructure resulting from interlayer hydration was considered to attribute the experimental observations to differences in the efficiency with which transitions in basal spacing translate to bulk volume changes and swelling pressure. Results provide additional insight and experimental evidence to more effectively model the mechanical behavior of compacted bentonites used as buffer or barrier materials in waste repository applications.