Carbon nanofiber supported Pt nanoparticles with an accurate size control through copolymer stabilization and chemical reduction for PEM fuel cell application

A one-pot microwave-assisted synthesis method was developed to produce scalable carbon nanofiber (CNF) supported platinum nanoparticle catalysts through an in-situ polymer-based technique. CNF-supported Pt samples were synthesized through electrospinning of poly(acrylonitrile-co-N-vinylpyrrolidone) (P(AN-co-nVP)) copolymer containing PtCl2 salt and consequent microwave reduction within hydrazine hydrate solution and carbonization. The aim of this study was to achieve a precise control on the size and distribution of the Pt nanoparticles by benefiting from a copolymer random templating and rapid microwave reduction. Prior to the application of microwave reduction on nanofibers, the pure effect of various carbonization temperatures (from 600 ̊C up to 1000 ̊C) on growth of Pt particles was studied. The carbonization at 800 ̊C was observed to represent a homogenous particle size distribution and the highest electroactive surface area (ECSA). Two types of samples were synthesized using microwave-assisted reduction – CNT-free and CNT-containing. The microwave irradiation for various time intervals (15s – 120s) was applied on both the CNT-free and the CNT-containing electrospun nanofibers with PtCl2. By selectively changing the process conditions, the minimum average size of 1.751 nm in diameter was obtained in the case of CNT-free samples while 0.862 nm nanoparticles with a narrow size distribution was achieved for the CNT-containing samples for the first time. The mean Pt particle size was increased as a function of microwave irradiation time. The ECSA values obtained for CNT-free samples demonstrated a maximum activity for sample treated for 30 s, despite the smaller Pt particle size in the 15 s-treated sample. This behavior was attributed to the lower amount of accessible Pt particles on the fiber surface. In the case of CNT-containing samples the best catalytic activity (82.55 m2g-1) was observed for 15s microwave reduction, which was hypothesized to be as a result of the significantly higher number of Pt cluster nucleated near the surface of the CNFs, a higher surface area due to the presence of CNTs and a higher electrical conductivity

The alkali-silica reaction damage in concrete at the mesoscale: characterization by X-ray tomography

The alkali-silica reaction (ASR) is the source of one of the most deleterious durability issues of the worldwide concrete infrastructure. It consists of chemical reactions between aggregates and the alkaline pore solution. The ASR products lead to deformations and cracking and, consequently, extensive struc tural damage, causing major safety, economic and environmental issues. Since the first investigations (eight decades ago), some basic ASR (cracking) mechanisms have been discovered. However, a com prehensive understanding has not been achieved yet, due to its complex multiscale nature. One of the major knowledge gaps stems from lack of characterization of the deformations and crack propagation at the mesoscale (few - few ). Results from systematic non-destructive studies have been lacking, although they are necessary to track the ASR cracking. That is because the typical petro graphic characterization techniques rely upon 2D microscopy (electron and optical), which require de structive specimen preparations. X-ray tomography (XT), as a non-destructive technique, has been re cently adopted, although, so far only in limited cases, mainly for simplified model systems and exploited only for qualitative analysis. During this PhD project, embedded within the framework of the Swiss National Science Foundation Sinergia project Nr. 171018 ("Alkali-silica reaction in concrete"), a comprehensive study of ASR crack ing at the mesoscale was carried out using time-lapse XT and various image analysis approaches. The study involved mainly specimens undergoing ASR acceleration in the laboratory. Its key goal was to characterize, for the first time, the spatial-temporal evolution of both ASR products and cracks. To achieve such goal, several challenges in the application of XT needed to be addressed. The result was the development of a new, integrated experimental and image analysis framework. One challenge consisted of enhancing the X-ray attenuation contrast between ASR products and the other material phases of concrete at the mesoscale, the former being normally undistinguishable from the latter in standard X-ray tomograms. Overcoming this challenge allowed analyzing both qualitatively and quan titatively their spatial-temporal distributions. Another challenge dealt with the contrast enhancement between aggregates and the cement paste, typically very low but needed to distinguish between prod ucts/cracks located either inside or outside aggregates. Finally, the qualitative and quantitative analysis of the tomographic time series required the implementation of distinct 3D image registration steps, to enable the time-lapse qualitative comparisons as well as the quantitative analysis. 3D image registration was additionally exploited both for estimating from the time series the global and local ASR-induced deformations and for increasing the robustness of the cracks and products detection (segmentation), compared with what achievable by conventional segmentation approaches. The overall, new XT-based methodology was extensively validated, concerning any eventual spurious effect on the natural ASR (cracking) course. It was possible to show its applicability and it delivered both new information and respective quantitative data about the ASR crack networks, about the extent of ASR products transport along them and the associated cracking itself as well as about the localized deformations, which accompany the cracking. The obtained, 3D datasets about products, cracks and deformations provided a uniquely new integrated knowledge and data base for advancing the basic un derstanding of ASR (cracking) and for supporting the formulation and validation of respective compu tational models at the concrete mesoscale. In addition, the developed XT-based methodology could be immediately exploited by the ASR research community for investigations also at other space-time scales not addressed in this project, e.g., the early stages of products formation and cracking initiation

Study on the internal crack network of the ASR-affected concrete by the tomography-based numerical model

In this paper, we present a novel finite element model to simulate alkali-silica reaction in a realistic concrete meso-structure. Application of the internal ASR loading leads to the evolution of multiple deviated cracks and corresponding macroscopic expansion. A particular crack-extension algorithm and a solution scheme provide numerical stability and allow to model complicated crack patterns while preserving the physics. The predictive validity of the model is demonstrated by matching an analytical solution of a loaded penny-shaped crack. The model is applied to the experimental dataset comprising the time-evolving X-ray tomograms of the ASR-affected concrete. Similar to the tomography data, the model results in an expanded concrete sample with a developed crack network. Two hypotheses on the crack loading and extension mechanisms are tested by comparing the crack statistics. Simulations with varying number of ASR sites and application of the uniaxial loading bring interesting insights. The latter concerns the role of the ASR-sites number, the individual loading amplitude and the difference in crack patterns

Effect of different ions on dissolution rates of silica and feldspars at high pH

The dissolution kinetics of silica-containing minerals in aggregates influences strongly the process of ASR in concrete. In this paper, the effect of different ions on the dissolution rates of SiO2 (amorphous and quartz) and feldspars at high pH values was studied by following the increase of silicon concentrations in dissolution experiments and with a novel approach of measuring the evolution of scratches of polished surfaces. The second method avoided the problem of precipitation in some systems, such as the formation of C-S-H when calcium was present and lithium silicates in the presence of lithium. At high pH values, lithium, calcium and sulfate increased the dissolution rates of silica and feldspars, while iron, magnesium and additional NaCl, KCl or CsCl showed no significant effect. In contrast, aluminium slowed down significantly the dissolution rates of quartz, amorphous silica and Na and K-feldspar at for all temperatures studies: 20, 40 and 60 °C.ISSN:0008-884

A laboratory investigation of cutting damage to the steel-concrete interface

The microstructure of the steel-concrete interface (SCI) in reinforced concrete is closely related to corrosion of reinforcing steel bars. Accordingly, characterization of the SCI is receiving increasing research attention. For microscopical observations of the SCI, a cutting process is needed to create a flat cross-section. Cutting carries the risk of damaging the SCI because of the considerable difference of hardness between concrete and steel. However, studies on characterizing the microstructure of the SCI rarely consider the damage induced by the potentially inappropriate cutting process. This study investigated the damage created by three cutting methods, namely, mechanical sawing, laser cutting, and combined laser-water cutting by the Laser MicroJet technology (LMJ). The SCI of the cut sections was imaged by scanning electron microscopy equipped with a backscattered electron detector. Additionally, the specimens were non-invasively studied by X-ray microtomography before and after cutting, to compare the impact of various cutting techniques on inducing damage to the SCI beneath the cutting surface. The results showed that if a bleed water zone (BWZ) is present, the cutting technique and protocol can significantly influence the morphology of this zone and adjacent regions. This study recommends an optimized mechanical sawing protocol with low feed speed as this led to considerably less SCI damage than laser and LMJ cutting. Moreover, it was found that adjusting the cutting direction can further significantly reduce the damage created during cutting. The least damage was found when the saw blade cut through the steel before cutting the BWZ. The main problem with laser cutting was heat generated even for a relatively low laser power; therefore, a heat-affected zone was created which significantly altered the microstructural features of the SCI not only on the cutting surface but also a certain depth below the surface. In LMJ cutting, this thermal effect was significantly reduced, however, the high-pressure water eroded the porous SCI and caused cracks. These effects can penetrate along the BWZ into the interior material. To complete this study, two applications demonstrate that the optimized mechanical sawing protocol is applicable to concrete specimens with rebars of actual size and corroded rebars.ISSN:0008-884

Rapid microwave-assisted synthesis of platinum nanoparticles immobilized in electrospun carbon nanofibers for electrochemical catalysis

A one-pot microwave-assisted synthesis method was developed to produce carbon nanofiber (CNF) supported platinum (Pt) catalyst nanoparticles as potential electrode materials for polymer electrolyte membrane fuel cells (PEMFC). CNF-supported Pt (Pt/CNF) hybrid structures were prepared through electrospinning of poly(acrylonitrile-co-N-vinylpyrrolidone) (P(AN-co-nVP)) copolymer containing PtCl2 as a precursor and subsequent microwave-assisted chemical reduction in hydrazine hydrate solution and carbonization. First, the effect of carbonization temperatures (600 to 1000 °C) on the growth of Pt nanoparticles and their electrochemical activity was studied. Uniform particle distribution with an average particle size of 4.25 nm and the highest electrochemical surface area (ECSA) (47.4 m2 g–1) were observed for the sample carbonized at 800 °C. Then microwave-assisted chemical reduction of PtCl2 in the nanofibers prior to carbonization was investigated, which resulted in better control of particle size and distribution and higher electrochemical activity. After carbonization at 800 °C of those treated with microwave-assisted chemical reduction with irradiation time of 15 and 30 s, CNF-supported Pt nanoparticles with a particle size of 1.75 and 2.85 nm and catalytic activity of 71.2 m2 g–1 and 74.6 m2 g–1, respectively, were obtained. Finally, the addition of highly conductive SWCNTs into Pt/CNF hybrids was investigated in terms of its effect on Pt particle size and distribution and electrochemical activity of hybrid materials. SWCNT-containing Pt/CNF, obtained by chemical reduction with 15 s microwave irradiation followed by carbonization at 800 °C, had nanoparticles with an average particle size of 0.86 nm and showed high electrochemical active surface area of 90.0 m2 g–1

Alkali-silica reaction products and cracks: X-ray micro-tomography-based analysis of their spatial-temporal evolution at a mesoscale

In this study, we propose a laboratory-scale methodology, based on X-ray micro-tomography and caesium (Cs) as a contrast agent, to advance the understanding of cracking due to alkali-silica reaction (ASR) in concrete. The methodology allows achieving a completely non-destructive and time-lapse characterization of the spatial-temporal patterns of both the cracks and the ASR products. While Cs addition slightly accelerated the ASR kinetics, the crack patterns, with and without Cs addition, were statistically equivalent. Cracks with ASR products appeared first in the aggregates, close to the interface with the cement paste. They propagated afterwards towards the aggregates interior. Some products were then extruded for several mm into air voids and cracks in the cement paste. This process suggests that, in the early stage, the ASR products may be a low-viscosity gel that can flow away from the source aggregate and may settle later elsewhere as a rigid phase, upon calcium uptake.ISSN:0008-884

Use of scratch tracking method to study the dissolution of alpine aggregates subject to alkali silica reaction

Alkali silica reaction (ASR) can significantly affect the service life of concrete. The dissolution of aggregates has a direct impact on gel formation and thus on the macroscopic expansion. The conventional expansion tests and other investigations confirmed ASR reactivity of three aggregates from different locations in Switzerland. The reactive minerals within alpine and composite aggregates were identified using an innovative scratch-tracking method. This method helps to study aggregate dissolution if measuring the amount of released Si is not enough, because: several minerals release Si or Al and/or new phases are probable to form during dissolution experiment. The scratch-tracking method on theses alpine aggregates showed faster dissolution of feldspars and quartz while muscovite was hardly affected. The dissolution of the aggregates in solution confirmed these differences between minerals.ISSN:0958-9465ISSN:1873-393

Quantitative analysis of the evolution of ASR products and crack networks in the context of the concrete mesostructure

Methodological challenges have long been an obstacle for the understanding of damage in concrete due to the alkali-silica reaction (ASR). Time-lapse X-ray tomography could be key to advancements but has not been extensively exploited, mainly due to similar electron densities of the different material phases, which leads to poor contrast in the tomograms. To address this limitation, we propose the implementation of two complementary contrast agents, BaSO4 and CsNO3. After optimizing the contents of both agents synergistically for mortars, we realize for the first time a quantitative characterization of the spatial-temporal distributions of both ASR products and cracks, while differentiating their presence in aggregates and in cement paste. This new methodology allowed showing that similar amounts of ASR products were found inside the aggregates and inside the cement paste. The transport of products from inner aggregate regions into the cement paste along propagating cracks was followed in 4D.ISSN:0008-884

A Universal Approach for Room-Temperature Printing and Coating of 2D Materials

Processing 2D materials into printable or coatable inks for the fabrication of functional devices has proven to be quite difficult. Additives are often used in large concentrations to address the processing challenges, but they drastically degrade the electronic properties of the materials. To remove the additives a high-temperature post-deposition treatment can be used, but this complicates the fabrication process and limits the choice of materials (i.e., no heat-sensitive materials). In this work, by exploiting the unique properties of 2D materials, a universal strategy for the formulation of additive-free inks is developed, in which the roles of the additives are taken over by van der Waals (vdW) interactions. In this new class of inks, which is termed "vdW inks", solvents are dispersed within the interconnected network of 2D materials, minimizing the dispersibility-related limitations on solvent selection. Furthermore, flow behavior of the inks and mechanical properties of the resultant films are mainly controlled by the interflake vdW attractions. The structure of the vdW inks, their rheological properties, and film-formation behavior are discussed in detail. Large-scale production and formulation of the vdW inks for major high-throughput printing and coating methods, as well as their application for room-temperature fabrication of functional films/devices are demonstrated.ISSN:0935-9648ISSN:1521-409