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A Rational Design of Isoindigo-Based Conjugated Microporous n-Type Semiconductors for High Electron Mobility and Conductivity
Kayaramkodath Chandran Ranjeesh, Ayman Rezk, Jose Ignacio Martinez, Safa Gaber, Areej Merhi, Tina Skorjanc, Matjaz Finsgar, Gisha Elizabeth Luckachan, Ali Trabolsi, Bilal R. Kaafarani, Ammar Nayfeh, Dinesh Shetty, 2023, original scientific article

Abstract: The development of n-type organic semiconductors has evolved significantly slower in comparison to that of p-type organic semiconductors mainly due to the lack of electron-deficient building blocks with stability and processability. However, to realize a variety of organic optoelectronic devices, high-performance n-type polymer semiconductors are essential. Herein, conjugated microporous polymers (CMPs) comprising isoindigo acceptor units linked to benzene or pyrene donor units (BI and PI) showing n-type semiconducting behavior are reported. In addition, considering the challenges of deposition of a continuous and homogeneous thin film of CMPs for accurate Hall measurements, a plasma-assisted fabrication technique is developed to yield uniform thin films. The fully conjugated 2D networks in PI- and BI-CMP films display high electron mobility of 6.6 and 3.5 cm2 V−1 s−1, respectively. The higher carrier concentration in PI results in high conductivity (5.3 mS cm−1). Both experimental and computational studies are adequately combined to investigate structure–property relations for this intriguing class of materials in the context of organic electronics.
Keywords: conjugated microporous polymers, isoindigo, semiconductors, conductivity, electron mobility
Published in RUNG: 18.08.2023; Views: 172; Downloads: 1
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Efficiency of the grid energy storage technology based on iron-chloride material cycle
Uroš Luin, doctoral dissertation

Abstract: Future high-capacity energy storage technologies are crucial for a highly renewable energy mix, and their mass deployment must rely on cheap and abundant materials, such as iron chloride. The iron chloride electrochemical cycle (ICEC), suitable for long-term grid energy storage using a redox potential change of Fe2+/Fe, involves the electrolysis of a highly concentrated aqueous FeCl2 solution yielding solid iron deposits. For the high overall energy efficiency of the cycle, it is crucial maximizing the energy efficiency of the electrolysis process. The thesis presents a study of the influence of electrolysis parameters on energy efficiency, performed in an industrial-type electrolyzer system. We studied the conductivity of the FeCl2 solution as a function of concentration and temperature and correlated it with the electrolysis energy efficiency as a function of current density. The contribution of the resistance polarization increases with the current density, causing a decrease in overall energy efficiency. The highest energy efficiency of 89 ±3 % was achieved using 2.5 mol dm-3 FeCl2 solution at 70 °C and a current density of 0.1 kA m-2. In terms of the energy input per Fe mass, this means 1.88 Wh g-1. The limiting energy input per mass of the Fe-deposit, calculated by extrapolating experimental results toward Eocell potential, was found to be 1.76 Wh g-1. For optimal long-duration electrolysis efficiency and performance, the optimal catholyte concentration range is 1-2 mol dm-3 FeCl2. We performed in situ X-ray absorption spectroscopy experimental studies to validate theoretical conclusions from literature related to the population and structure of Fe-species in the FeCl2 (aq) solution at different concentrations (1 - 4 mol dm-3) and temperatures (25 - 80 °C). This revealed that at low temperature and low FeCl2 concentration, the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five H2O at a distance of 2.095 (±0.005) Å. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical H2O is substituted by a Cl ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing state-of-the-art conductivity models. An additional steric impediment of the electrolytic cell is caused by the predominant neutral species present in the catholyte solution at high concentration. This correlates with poor electrolysis performance at a very high catholyte concentration (4 mol dm-3 FeCl2), especially at high current densities (> 1 kA m-2). The neutral Fe[Cl2(H2O)4]0 complex negatively affects the anion exchange membrane ion (Cl-) transfer and lowers the concentration of electroactive species (Fe[Cl(H2O)5]+) at the cathode surface. The kinetics of hydrogen evolution from the reaction between Fe powder and HCl acid was studied under the first-order reaction condition. The activation energy was determined to be 55.3 kJ mol-1.
Keywords: ICEC, Power-to-Solid, energy storage, hydrogen, ferrous chloride, electrolysis, Fe deposition, efficiency, XAS, structure and population, ionic species, ion association, conductivity
Published in RUNG: 18.04.2023; Views: 562; Downloads: 20  (1 vote)
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Correlation between FeCl2 electrolyte conductivity and electrolysis efficiency
Luin Uroš, Valant Matjaz, Arčon Iztok, 2022, published scientific conference contribution abstract

Abstract: The electrolysis efficiency is an important aspect of the Power-to-Solid energy storage technology (EST) based on the iron chloride electrochemical cycle [1]. This cycle employs an aqueous FeCl2 catholyte solution for the electro-reduction of iron. The metal iron deposits on the cathode. The energy is stored as a difference in the redox potential of iron species. Hydrogen, as an energy carrier, is released on demand over a fully controlled hydrogen evolution reaction between metallic Fe0 and HCl (aq) [1]. Due to these characteristics, the cycle is suitable for long-term high-capacity and high-power energy storage. In a previous work [2] we revealed that the electrolyte conductivity linearly increases with temperature. Contrary, the correlation between the electrolyte concentration and efficiency is not so straightforward. Unexpectedly small efficiency variations were found between 1 and 2.5 mol dm-3 FeCl2 (aq) followed by an abrupt efficiency drop at higher concentrations. To explain the behavior of the observed trends and elucidate the role of FeCl2 (aq) complex ionic species we performed in situ X-ray absorption studies. We made a dedicated experimental setup, consisting of a tubular oven and PMMA liquid absorption cell, and performed the measurements at the DESY synchrotron P65 beamline. The XAS investigation covered XANES and EXAFS analyses of FeCl2 (aq) at different concentrations (1 - 4 molL-1) and temperatures (25 - 80 °C). We found that at low temperature and low FeCl2 concentration the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å [3]. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the single charged Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well correlates with the existing conductivity models [3]. [1] M. Valant, “Procedure for electric energy storage in solid matter. United States Patent and Trademark Office. Patent No. US20200308715,” Patent No. US20200308715, 2021. [2] U. Luin and M. Valant, “Electrolysis energy efficiency of highly concentrated FeCl2 solutions for power-to-solid energy storage technology,” J. Solid State Electrochem., vol. 26, no. 4, pp. 929–938, Apr. 2022, doi: 10.1007/S10008-022-05132-Y. [3] U. Luin, I. Arčon, and M. Valant, “Structure and Population of Complex Ionic Species in FeCl2 Aqueous Solution by X-ray Absorption Spectroscopy,” Molecules, vol. 27, no. 3, 2022, doi: 10.3390/molecules27030642.
Keywords: Iron chloride electrochemical cycle, Power-to-Solid energy storage, XANES, EXAFS, electrical conductivity, electrolyte complex ionic species structure and population
Published in RUNG: 26.09.2022; Views: 773; Downloads: (1 vote)
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Generalized Theory of Thermal Conductivity for Different Media: Solids to Nanofluids
Swapna Mohanachandran Nair Sindhu, Sankararaman S, 2019, original scientific article

Abstract: The advent of nanotechnology in the 21st century opened a new branch of nanoscience known as nanofluids, finding a wide range of industrial applications especially in heat transfer. Though the theory of thermal conductivity of solids is well established, there is no such conclusive model to explain the thermal conductivity of nanofluids. In the present work we propose a generalized theory for thermal conductivity applicable to materials ranging from heterogeneous solids, porous materials, nanofluids, and ferrofluids. The model could explain the effective thermal conductivity of not only the combination of solids but also solid−fluid mixtures. The proposed theory could successfully link the existing models for porous solid materials and nanofluids as its special cases. The proposed model is verified against experimental data by simulating the theoretical equations
Keywords: thermal conductivity, generalised model, Sankar-Loeb model
Published in RUNG: 05.07.2022; Views: 733; Downloads: 0
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Absolute Porosity Analysis in Carbon Allotropic Nanofluids: A Sankar–Swapna Model Approach
Swapna Mohanachandran Nair Sindhu, SREEJYOTHI S, Sankararaman S, 2020, original scientific article

Abstract: Porous materials have gained significant attention in recent years as a class of material exhibiting interesting chemical and physical properties. The existing methods of porosity analysis have limitations that prevent absolute porosity measurement. Hence, a technique independent of surface physical properties alone can give the absolute porosity of the material. The porosity greatly influences the thermal diffusivity of a material. The manuscript is the first report of employing the Sankar–Swapna model for analyzing the porosity variations in carbon allotropic nanofluids. The model helps not only in getting information about the absolute porosity variations among samples, but also suggests morphological modifications through the thermal diffusivity study using the sensitive single-beam thermal lens technique. The variations in thermal diffusivity and absolute porosity values are also correlated to morphological modifications based on the theoretical model and thereby proposing this as a surrogate method for absolute porosity analysis.
Keywords: absolute porosity, Sankar–Swapna model, thermal diffusivity, thermal lens, thermal conductivity
Published in RUNG: 04.07.2022; Views: 673; Downloads: 0
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Through-plane and in-plane thermal diffusivity determination of graphene nanoplatelets by photothermal beam deflection spectrometry
Humberto Cabrera, Dorota Korte, Hanna Budasheva, Behnaz Abbasgholi N. Asbaghi, Stefano Bellucci, 2021, original scientific article

Abstract: In this work, in-plane and through-plane thermal diffusivities and conductivities of a freestanding sheet of graphene nanoplatelets are determined using photothermal beam deflection spectrometry. Two experimental methods were employed in order to observe the effect of load pressures on the thermal diffusivity and conductivity of the materials. The in-plane thermal diffusivity was determined by the use of a slope method supported by a new theoretical model, whereas the through-plane thermal diffusivity was determined by a frequency scan method in which the obtained data were processed with a specifically developed least-squares data processing algorithm. On the basis of the determined values, the in-plane and through-plane thermal conductivities and their dependences on the values of thermal diffusivity were found. The results show a significant difference in the character of thermal parameter dependence between the two methods. In the case of the in-plane configuration of the experimental setup, the thermal conductivity decreases with the increase in thermal diffusivity, whereas with the through-plane variant, the thermal conductivity increases with an increase in thermal diffusivity for the whole range of the loading pressure used. This behavior is due to the dependence of heat propagation on changes introduced in the graphene nano-platelets structure by compression.
Keywords: graphene nanoplatelets, thermal diffusivity, thermal conductivity, photothermal spectrometry
Published in RUNG: 30.11.2021; Views: 1306; Downloads: 61
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