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Determining the Electrochemical Oxygen Evolution Reaction Kinetics of Fe3S4@Ni3S2 Using Distribution Function of Relaxation Times

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Abstract

We designed a heterostructure of Fe3S4@Ni3S2, as a potent oxygen evolution reaction (OER) electrocatalyst in an alkaline medium. Intriguingly, Fe3S4@Ni3S2 exhibits low onset potential of 290 mV and overpotential of 360 mV at a current density of 10 mA cm−2. We examined the OER kinetics of Fe3S4@Ni3S2 using distribution function of relaxation times (DFRT), which are attained with the help of impedance spectroscopy genetic programming (ISGP). ISGP reveals the occurrences of three events of OER, manifested as peaks in the DFRT, such as active material or pores (P2), charge transfer (P1’), and production rate of intermediates (P1) in case of Fe3S4@Ni3S2 at different faradic overpotentials. The effective resistance of each phenomenon can be easily calculated. It decreases with an increase in conductivity at high overpotentials for all the three, which suggests the high performance of the as-synthesized composite due to faster kinetics. Further, structural investigation of the catalyst employing x-ray photoelectron spectroscopy is elaborated and it is suggested that the catalyst activation takes place by the constant exchange of anions between electrode and electrolyte during electrochemical oxidation.

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Flash‐Sintering Mechanism Studied Through Interrupted Experiments

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Herein, the mass-transfer mechanism of flash sintering during the transient stage is examined using an in-house-made flash and quench (FQ) system. Visual findings of samples during and after FQ experiments and high-resolution electron microscopy are given. Many new observations regarding the flash-sintering nature are presented and discussed. Samples that underwent FQ experiments either show no sign of sintering or local sintering and grain growth due to a hotspot. These findings aid in untying of the two phenomena. Electron microscopy imaging of flash and quenched samples shows atypical microstructures. Such microstructural anomalies include sintering, massive grain growth, and visual findings on the surface. These findings establish flash sintering as a set of phenomena, caused by an abrupt and local increase in temperature (a “flash event”), where only one of which is sintering.

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Electrochemical Activation of Li2MnO3 Electrodes at 0 °C and Its Impact on the Subsequent Performance at Higher Temperatures

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This work continues our systematic study of Li- and Mn- rich cathodes for lithium-ion batteries. We chose Li2MnO3 as a model electrode material with the aim of correlating the improved electrochemical characteristics of these cathodes initially activated at 0 °C with the structural evolution of Li2MnO3, oxygen loss, formation of per-oxo like species (O22−) and the surface chemistry. It was established that performing a few initial charge/discharge (activation) cycles of Li2MnO3 at 0 °C resulted in increased discharge capacity and higher capacity retention, and decreased and substantially stabilized the voltage hysteresis upon subsequent cycling at 30 °C or at 45 °C. In contrast to the activation of Li2MnO3 at these higher temperatures, Li2MnO3 underwent step-by-step activation at 0 °C, providing a stepwise traversing of the voltage plateau at >4.5 V during initial cycling. Importantly, these findings agree well with our previous studies on the activation at 0 °C of 0.35Li2MnO3·0.65Li[Mn0.45Ni0.35Co0.20]O2 materials. The stability of the interface developed at 0 °C can be ascribed to the reduced interactions of the per-oxo-like species formed and the oxygen released from Li2MnO3 with solvents in ethylene carbonate–methyl-ethyl carbonate/LiPF6 solutions. Our TEM studies revealed that typically, upon initial cycling both at 0 °C and 30 °C, Li2MnO3 underwent partial structural layered-to-spinel (Li2Mn2O4) transition.

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Interphases Formation and Analysis at the Lithium–Aluminum–Titanium–Phosphate (LATP) and Lithium–Manganese Oxide Spinel (LMO) Interface during High‐Temperature Bonding

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In this study, fabrication processes of solid electrolyte/cathode interfaces for their use in next‐generation all‐solid‐state lithium‐ion battery (LIB) applications are described. Standard lithium–aluminum–titanium–phosphate (LATP) solid electrolyte and lithium–manganese oxide (LMO) spinel cathode ceramic half cells are assembled using two all‐solid‐state methods: a) co‐sintering the cathode and electrolyte materials via field‐assisted sintering and b) field‐assisted high‐temperature bonding. The morphology and composition of the interfaces are analyzed by scanning electron microscopy (SEM) and energy‐dispersive X‐ray spectroscopy (EDS). This study reveals that the formation of interphases can be significantly decreased by separately performing the densification and joining procedures. Electrochemical impedance spectroscopy (EIS) is applied to understand and determine the effect of the manufactured interfaces on the system conductivity. Based on the results, it is concluded that the high‐temperature bonding technique appears to be a suitable technique for future production of all‐solid‐state LIBs.

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Tri‐Functional Double Perovskite Oxide Catalysts for Fuel Cells and Electrolyzers

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Perovskite oxides are at the forefront of the race to develop catalysts/electrodes for fuel cells and electrolyzers. This work presents trifunctional properties of the double‐perovskite oxide PrBa0.5Sr0.5Co1.5Fe0.5O5+δ and the PrBa0.5Sr0.5Co1.5Fe0.5O5+δ−Ag composite prepared by the glycine nitrate process. The electrocatalytic studies reveal that the Ag‐based composite is an excellent catalyst for both oxygen evolution (OER) and hydrogen evolution reactions (HER) in alkaline solution. The electrochemical impedance spectroscopy analysis through distribution function of relaxation times (DFRT) suggests that the improved activity originates from the suppression of resistance contributed by various relaxation processes. The oxygen reduction reaction (ORR) kinetics in these oxide‐based cathodes has been studied by performing symmetric‐cell measurements at high temperatures using both oxygen‐ion and proton‐conducting cells. DC bias dependence of charge‐transfer processes, oxygen‐surface kinetics, polarization resistances, and activation energies are revealed by DFRT studies. Ag addition in PrBa0.5Sr0.5Co1.5Fe0.5O5+δ leads to enhanced kinetics of OER, HER, and ORR.

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Vibrational and impedance spectroscopic studies of semi-interpenetrating polymer networks solid polymer electrolytes

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Semi-interpenetrating polymer networks (semi-IPNs) with significant ionic conductivity (104 S cm1 at ambient temperature) were studied by vibrational and impedance spectroscopies coupled with advanced analysis procedures. Vibrational spectroscopy recognized the numbers of free ions, ion pairs, ion–polymers and hydrogen bonds within the solid polymer electrolyte matrices (SPE). Electrochemical impedance spectroscopy (EIS) was used to quantify the bulk resistance and bulk relaxation time. The analyses used discrepancy– complexity plots to assess the number of free parameters properly, and EIS was further analyzed using impedance spectroscopy genetic programming (ISGP). Four compositions of PEO-polyurethane/poly(ethylene glycol) dimethyl ether (PEO-PU/PEGDME) were examined with LiClO4 salt. The polymer electrolyte composition of 30/70 PEO-PU/PEGDME resulted in the lowest relaxation times and the highest ionic conductivity. The best salt concentration was observed at an EO/Li ratio of 30 for the PEO-PU/PEGDME:LiClO4 (30/70) semi-IPN matrix. Several lithium salts of different anions were examined at an EO/Li ratio of 10, and the ionic conductivity achieved varied in the order –N(CF3SO2)24 –ClO44 –(CF3SO3)4 –I/I3.

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