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Redox - cycling – a Tool for Artificial Electrochemical Aging of Solid Oxide Cells
In this work a procedure for accelerated stress tests of Solid Oxide Cell (SOC) by artificial aging of the anode via redox cycling is presented. This approach eliminates the interrelation of different degradation processes and ensures a clear picture of the anode degradation on the total cell performance. The level of oxidation is monitored in situ on bare anode samples by impedance measurements of the Ni network resistance changes during oxidation/reduction cycling which ensures governing of the oxidation level with high reproducibility by selection of appropriate experimental conditions. Once fixed on bare anode samples, the selected redox cycling regime is further applied in full cell configuration. The developed methodology is evaluated by comparative analysis of current-voltage and impedance measurements of artificially aged and calendar aged button cells. The results for 8 redox cycles are comparable to those obtained for more than 600 hours operation in standard conditions.
New Insights on the Nickel State Deposited by Hydrazine Wet-Chemical Synthesis Route in the Ni/BCY15 Proton-Conducting SOFC Anode
Yttrium-doped barium cerate (BCY15) was used as an anode ceramic matrix for synthesis of the Ni-based cermet anode with application in proton-conducting solid oxide fuel cells (pSOFC). The hydrazine wet-chemical synthesis was developed as an alternative low-cost energy-efficient route that promotes ‘in situ’ introduction of metallic Ni particles in the BCY15 matrix. The focus of this study is a detailed comparative characterization of the nickel state in the Ni/BCY15 cermets obtained in two types of medium, aqueous and anhydrous ethylene glycol environment, performed by a combination of XRD, N2 physisorption, SEM, EPR, XPS, and electrochemical impedance spectroscopy. Obtained results on the effect of the working medium show that ethylene glycol ensures active Ni cermet preparation with well-dispersed nanoscale metal Ni particles and provides a strong interaction between hydrazine-originating metallic Ni and cerium from the BCY15 matrix. The metallic Ni phase in the pSOFC anode is more stable during reoxidation compared to the Ni cermet prepared by the commercial mechanical mixing procedure. These factors contribute toward improvement of the anode’s electrochemical performance in pSOFC, enhanced stability, and a lower degradation rate during operation.
ИзтеглиComposites between Perovskite and Layered Co-Based Oxides for Modification of the Thermoelectric Efficiency
The common approach to modify the thermoelectric activity of oxides is based on the concept of selective metal substitution. Herein, we demonstrate an alternative approach based on the formation of multiphase composites, at which the individual components have distinctions in the electric and thermal conductivities. The proof-of-concept includes the formation of multiphase composites between well-defined thermoelectric Co-based oxides: Ni, Fe co-substituted perovskite, LaCo0.8Ni0.1Fe0.1O3 (LCO), and misfit layered Ca3Co4O9. The interfacial chemical and electrical properties of composites are probed with the means of SEM, PEEM/XAS, and XPS tools, as well as the magnetic susceptibility measurements. The thermoelectric power of the multiphase composites is evaluated by the dimensionless figure of merit, ZT, calculated from the independently measured electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (λ). It has been demonstrated that the magnitude’s electric and thermal conductivities depend more significantly on the composite interfaces than the Seebeck coefficient values. As a result, the highest thermoelectric activity is observed at the composite richer on the perovskite (i.e., ZT = 0.34 at 298 K)
One-Step Formation of Reduced Graphene Oxide from Insulating Polymers Induced by Laser Writing Method
Finding a low-cost and effective method at low temperatures for producing reduced graphene oxide (rGO) has been the focus of many efforts in the research community for almost two decades. Overall, rGO is a promising candidate for use in supercapacitors, batteries, biosensors, photovoltaic devices, corrosion inhibitors, and optical devices. Herein, we report the formation of rGO from two electrically insulating polymers, polytetrafluoroethylene (PTFE) and meta-polybenzimidazole fiber (m-PBI), using an excimer pulsed laser annealing (PLA) method. The results from X-ray diffraction, scanning electron microscopy, electron backscattered diffraction, Raman spectroscopy, and Fourier-transform infrared spectroscopy confirm the successful generation of rGO with the formation of a multilayered structure. We investigated the mechanisms for the transformation of PTFE and PBI into rGO. The PTFE transition occurs by both a photochemical mechanism and a photothermal mechanism. The transition of PBI is dominated by a photo-oxidation mechanism and stepwise thermal degradation. After degradation and degassing procedures, both the polymers leave behind free molten carbon with some oxygen and hydrogen content. The free molten carbon undergoes an undercooling process with a regrowth velocity (
Iron oxidation to amplify the Na and Li storage capacities of nano-sized maricite NaFePO4
This study reports an effective approach to improve dramatically the electrochemical performance of nanosized NaFePO4 with a maricite structure, which is commonly considered as electrochemically inactive due to the absence of structural channels for alkaline ion mobility. The approach is based on the complete oxidation under mild conditions (i.e. at low temperatures around 280 °C and traces of oxygen) of the nanosized maricite phase. It is prepared by the phosphate–formate precursor method and is additionally ball-milled with a carbon additive. The oxidation of Fe2+ proceeds at the nanoscale level within the maricite nanoparticles and causes a massive structural transformation of the maricite phase into a monoclinic NASICON phase Na3Fe2(PO4)3 with the preservation of the crystallinity. The oxidized maricite phase exhibits high specific capacities, cycling stability and rate capability when it is used as an electrode in both Na and Li half-cells. The effect of different sodium and lithium electrolytes on the storage performance is investigated as well. It is found that the highest specific capacity (of about 150 mA h g−1) is achieved in Li half-cells using the LiPF6 electrolyte, while in Na half-cells the electrolyte NaFSI/EC:DMC achieves a specific capacity of around 100 mA h g−1. The rate capability is better in Na half-cells than that in Li half-cells. The mechanism of the reversible intercalation/deintercalation of Na+ and Li+ ions is studied by ex situ XRD and TEM analyses. The results show that the maricite is an electrochemically inactive phase, but through manipulation including oxidation or amorphization it becomes an active electrode material.