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Abstract: Cobalt hydroxide and other first-row transition metal hydroxides have gained significant attention as pseudocapacitor materials due to their rapid and reversible redox processes. Their layered structures facilitate interactions between electrolyte anions and cobalt cation sites within the bulk of the material, enabling higher charge density and extending redox activity beyond the particle surface. By controlled precipitation under hydrothermal conditions, the structure and morphology of cobalt hydroxides can be optimized to enhance electrochemical performance. Challenging conventional assumptions, surface area alone is not the primary factor driving increased pseudocapacitive performance. The hexagonal hydrotalcite-like structure, characterized by lower skeletal density and larger basal plane spacing, outperforms the monoclinic cobalt carbonate hydroxide structure, achieving an order of magnitude higher capacitance. In situ X-ray absorption spectroscopy provides critical insights into the pseudocapacitive behavior, revealing enhanced accessibility of Co2+ sites for electrochemical oxidation. While monoclinic cobalt carbonate hydroxide exhibits minimal changes in the Co2+ oxidation state, indicative of surface-limited redox activity, the hydrotalcite-like cobalt hydroxides show substantial shifts in the Co K-edge position, highlighting oxidation of Co2+ sites throughout the bulk.
Keywords: pseudocapacitors, layered-double hydroxides, cobalt hydroxide, redox processes, in situ x-ray absorption spectroscopy
Published in RUNG: 14.03.2025; Views: 621; Downloads: 7
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Abstract: The circular economy (CE) framework is crucial for promoting resource efficiency and minimizing waste, but it cannot achieve its sustainability goals alone. To achieve a truly sustainable future, broader social, environmental and economic aspects need to be taken into account. Rather than separating biological and technological cycles, as the CE principles are depicted in the Ellen MacArthur Foundation’s butterfly diagram (EMF, 2017), we propose an integrated approach that prioritises environmental aspects and includes a conceptual framework, socio-economic changes and implementation processes. It is crucial to re-evaluate the environmental aspects and strengthen their importance within the CE framework. Within this concept we present the case of iron, the most widely used metal, widely available compared to others and has been an essential part of societal development for more than 5,000 years. With its abundance, safety and electrochemical properties, iron is an ideal material for low-carbon energy technologies. We discuss how advanced iron-based technologies have a high potential to be successfully integrated into the CE, we evaluate different electrochemical energy storage systems and present advances in thermochemical Fe-Cl cycles for hydrogen production. An innovative thermal system for hydrogen production based on the thermochemical Fe-Cl cycle was evaluated in a life cycle assessment (LCA) study and shows the importance of choosing sustainable energy sources to minimise environmental impact. Sustainable production methods for iron are also analysed to demonstrate their potential to reduce the carbon footprint of the iron and steel industry. Finally, efforts should focus on minimising environmental impact and optimising resource recovery.
Keywords: circular economy, resource efficiency, sustainability, iron, hydrogen
Published in RUNG: 25.11.2024; Views: 1015; Downloads: 0
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Abstract: This research presents an inventive thermochemical cycle that utilizes a reaction between iron and HCl acid for hydrogen production. The reaction occurs spontaneously at room temperature, yielding hydrogen and a FeCl2 solution as a by-product. Exploring the thermal decomposition of the FeCl2 by-product revealed that, at conditions suitable for utilization of low-temperature industrial waste heat (250 °C), chlorine gas formation can be circumvented. Instead, the resulting by-product is HCl, which is readily soluble in water, facilitating direct reuse in subsequent cycles. The utilization of low-temperature industrial heat not only optimizes resource utilization and reduces operational costs but also aligns with environmentally sustainable production processes. From the kinetic studies the activation energy was calculated to be 45 kJ/mol and kinetics curves were constructed. They showed significant kinetics at room temperature and above but rapid decrease towards lower temperatures. This is important to consider during real-scale technology optimization. The theoretical overall energy efficiency of the cycle, with 100% and 70% heat recuperation, was calculated at 68.8% and 44.8%, respectively. In practical implementation, considering the efficiency of DRI iron reduction technology and free waste heat utilization, the cycle achieved a 41.7% efficiency. Beyond its energy storage capabilities, the Iron-chlorine cycle addresses safety concerns associated with large-scale hydrogen storage, eliminating self-discharge, reducing land usage, and employing cost-effective storage materials. This technology not only facilitates seasonal energy storage but also establishes solid-state energy reserves, making it suitable for balancing grid demands during winter months using excess renewable energy accumulated in the summer.
Keywords: chemical cycles, hydrogen production, thermal decomposition, reaction kinetics, iron, chlorine
Published in RUNG: 12.01.2024; Views: 2998; Downloads: 45
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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: 3436; Downloads: 0  (1 vote)
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Abstract: An electrochemical cycle for the grid energy storage in the redox potential of 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 to maximize the energy efficiency of the electrolysis process. Here we present a study of the influence of electrolysis parameters on the energy efficiency of such electrolysis, performed in an industrial-type electrolyzer. We studied the conductivity of the FeCl2 solution as a function of concentration and temperature and correlated it with the electrolysis energy efficiency. The deviation from the correlation indicated an important contribution from the conductivity of the ion-exchange membrane. Another important studied parameter was the applied current density. We quantitatively showed how 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 L−1 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 was found to be 1.76 Wh g−1.
Keywords: electrolysis, ferrous chloride, iron deposition, energy efficiency
Published in RUNG: 16.02.2022; Views: 3688; Downloads: 81  (1 vote)
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