Inverse Layer Dependence of Friction on Chemically Doped MoS_{2}

We present the results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the seemingly universal observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse dependency on the number of layers. Raman spectroscopy measurements revealed signatures of Re intercalation, leading to a decoupling between neighboring MoS2 layers and enhanced electron-phonon interactions, thus resulting in increasing friction with increasing number of layers: a new paradigm in the mechanics of 2D materials.Comment: 15 pages incl. Supplemental Material, 5 figure

A design of experiments (DoE) approach to identify the influencing parameters that determine poly-D,L-lactic acid (PDLLA) electrospun scaffold morphologies

Electrospun fibrous materials have increasing applications in regenerative medicine due to the similarity of fibre constructs to the morphology of certain extracellular matrices. Although experimentally the electrospinning method is relatively simple, at the theoretical level the interactions between process parameters and their influence on the fibre morphology is not yet fully understood. Here, we hypothesised that a design of experiments (DoE) model could determine combinations of process parameters that result in significant effects on poly-D,L-lactic acid (PDLLA) fibre morphology. The process parameters used in this study were applied voltage, needle-to-collector distance, flow rate and polymer concentration. Data obtained for mean fibre diameter, standard deviation of the fibre diameter (stdev, measure of fibre morphology) and presence of 'beading' on the fibres (beads per µm2) were evaluated as a measure of PDLLA fibre morphology. Uniform fibres occurred at standard deviations of ≤ 500 nm, 'beads-on-string' morphologies were apparent between ± 500-1300 nm and large beads were observed at ± 1300-1800 nm respectively. Mean fibre diameter was significantly influenced by the applied voltage and interaction between flow rate and polymer concentration. Fibre morphology was mainly influenced by the polymer concentration, while bead distribution was significantly influenced by the polymer concentration as well as the flow rate. The resultant DoE model regression equations were tested and considered suitable for the prediction of parameters combinations needed for desired PDLLA fibre diameter and additionally provided information regarding the expected fibre morphology

Lithium-ion battery separator membranes based on poly(L-lactic acid) biopolymer

Sustainable materials are increasingly needed in lithium ion batteries in order to reduce their environmental impact and improve their recyclability. This work reports on the production of separators using poly (L-lactic acid) (PLLA) for lithium ion battery applications. PLLA separators were obtained by solvent casting technique, by varying polymer concentration in solution between 8 wt.% and 12 wt.% in order to evaluate their morphology, thermal, electrical and electrochemical properties. It is verified that morphology and porosity can be tuned by varying polymer concentration and that the separators are thermally stable up to 250 ºC. The best ionic conductivity of 1.6 mS/cm was obtained for the PLLA separator prepared from 10 wt.% polymer concentration in solution, due to the synergistic effect of the morphology and electrolyte uptake. For this membrane, a high discharge capacity value of 93 mAh.g-1 was obtained at the rate of 1C. In this work, it is demonstrated that PLLA is a good candidate for the development of separator membranes, in order to produce greener and environmentally friendly batteries in a circular economy context.Work supported by the Portuguese Foundation for Science and Technology (FCT) undes strategic funding UID/FIS/04650/2020 and UID/QUI/0686/2020, project PTDC/FISMAC/28157/2017, and Grants SFRH/BD/140842/2018 (J.C.B.), SFRH/BPD/121526/2016 (D.M.C), CEECIND/00833/2017 (R.G.) and SFRH/BPD/112547/2015 (C.M.C.). Financial support from the Basque Government Industry Department under the ELKARTEK and HAZITEK programs is also acknowledged. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, EGEF and ESF) is gratefully acknowledge

Insights into the Effect of Structural Heterogeneity in Carbonized Electrospun Fibrous Mats for Flow Battery Electrodes by X-Ray Tomography

Electrospun custom made flow battery electrodes are imaged in 3D using X‐ray computed tomography. A variety of computational methods and simulations are applied to the images to determine properties including the porosity, fiber size, and pore size distributions as well as the material permeability and flow distributions. The simulations are performed on materials before and after carbonization to determine the effect it has in the internal microstructure and material properties. It is found that the deposited fiber size is constantly changing throughout the electrospinning process. The results also show that the surfaces of the fibrous material are the most severely altered during carbonization and that the rest of the material remained intact. Pressure driven flow is modeled using the lattice Boltzmann method and excellent agreement with experimental results is found. The simulations coupled with the material analysis also demonstrate the highly heterogeneous nature of the flow. Most of the flow is concentrated to regions with high porosity while regions with low porosity shield other pores and starve them of flow. The importance of imaging these materials in 3D is highlighted throughout

Coaxial Electrospun Cellulose-Core Fluoropolymer-Shell Fibrous Membrane from Recycled Cigarette Filter as Separator for High Performance Lithium-Ion Battery

This paper reports an eco-friendly approach for extracting cellulose acetate (CA) from waste cigarette filter to construct a cellulose-based membrane separator for a high-performance lithium-ion battery. A cellulose/poly(vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofiber membrane was prepared by coaxial electrospinning of a cellulose acetate core and PVDF-HFP shell, then hydrolyzed by LiOH. The cellulose-core/PVD-HFP-shell fibrous membrane shows good tensile strength (34.1 MPa), high porosity (66%), excellent thermal stability (to 200 °C), and super electrolyte compatibility (355% electrolyte uptake). It has a lower interfacial resistance (98.5 Ω) and higher ionic conductivity (6.16 mS cm-1) than those of commercial separators (280.0 Ω and 0.88 mS cm-1). In addition, the rate capability (138 mAh·g-1) and cycling performance (75.4% after 100 cycles) are also superior to those of the commercial separators, demonstrating the cellulose-core fibrous membrane to be a promising separator for a high-power and more secure lithium-ion battery

Wpływ rozpuszczalników na morfologię i wykonanie membran z nanowłókien Nylon 6

Nylon 6 nanofibre membranes were prepared by electrospinning of nylon 6 solutions with various volume ratios of trifluoroethyl alcohol (TFE) and formic acid (FA). The effect of the solvent type on the morphology of nylon 6 nanofibre membranes was investigated. Results showed that all membranes studied showed uniform, defect-free structures with very thin nanofibre diameters. The addition of formic acid led to a significant decrease in average fibre diameters. The average fibre diameters were 660, 186, 87, 62 and 30 nm for nylon 6 nonofibre prepared using the binary solution system and trifluoroethyl alcohol/formic acid (100:0), (75:25), (50:50), (25:75) & (0:100) respectively. In addition, the nylon 6 nanofibre membranes prepared using formic acid showed the highest strength with the highest porosity and the lowest average fibre diameters.Membrany z nanowłókien Nylon 6 przygotowano przez elektroprzędzenie roztworów Nylonu 6 o różnych stosunkach objętości alkoholu trifluoroetylowego (TFE) i kwasu mrówkowego (FA). W pracy zbadano wpływ rodzaju rozpuszczalnika na morfologię membran z nanowłókien Nylon 6. Wyniki wykazały, że wszystkie badane membrany były jednolite, pozbawione wad struktury o bardzo cienkich średnicach nanowłókien. Dodatek kwasu mrówkowego doprowadził do znacznego zmniejszenia średnic włókien. Średnice nanowłókien wyniosły 660, 186, 87, 62 i 30 nm. Nanowłókna przygotowano z zastosowaniem roztworu alkoholu trifluoroetylowego i kwasu mrówkowego: 100:0; 75:25; 50:50; 25:75 i 0:100. Ponadto przygotowane przy użyciu kwasu mrówkowego membrany z nanowłókien wykazały najwyższą wytrzymałość przy największej porowatości i najniższych średnicach włókien

Polymethylmethacrylate/Polyacrylonitrile Membranes via Centrifugal Spinning as Separator in Li-Ion Batteries

Electrospun nanofiber membranes have been extensively studied as separators in Li-ion batteries due to their large porosity, unique pore structure, and high electrolyte uptake. However, the electrospinning process has some serious drawbacks, such as low spinning rate and high production cost. The centrifugal spinning technique can be used as a fast, cost-effective and safe technique to fabricate high-performance fiber-based separators. In this work, polymethylmethacrylate (PMMA)/polyacrylonitrile (PAN) membranes with different blend ratios were produced via centrifugal spinning and characterized by using different electrochemical techniques for use as separators in Li-ion batteries. Compared with commercial microporous polyolefin membrane, centrifugally-spun PMMA/PAN membranes had larger ionic conductivity, higher electrochemical oxidation limit, and lower interfacial resistance with lithium. Centrifugally-spun PMMA/PAN membrane separators were assembled into Li/LiFePO4 cells and these cells delivered high capacities and exhibited good cycling performance at room temperature. In addition, cells using centrifugally-spun PMMA/PAN membrane separators showed superior C-rate performance compared to those using microporous polypropylene (PP) membranes. It is, therefore, demonstrated that centrifugally-spun PMMA/PAN membranes are promising separator candidate for high-performance Li-ion batteries