We report the development of in situ (online) EPR and coupled EPR/NMR methods to study redox flow batteries, which are applied here to investigate the redox-active electrolyte, 2,6-dihydroxyanthraquinone (DHAQ). The radical anion, DHAQ 3−• , formed as a reaction intermediate during the reduction of DHAQ 2− , was detected and its concentration quantified during electrochemical cycling. The fraction of the radical anions was found to be concentration-dependent, the fraction decreasing as the total concentration of DHAQ increases, which we interpret in terms of a competing dimer formation mechanism. Coupling the two techniquesEPR and NMRenables the rate constant for the electron transfer between DHAQ 3−• and DHAQ 4− anions to be determined. We quantify the concentration changes of DHAQ during the "high-voltage" hold by NMR spectroscopy and correlate it quantitatively to the capacity fade of the battery. The decomposition products, 2,6-dihydroxyanthrone and 2,6-dihydroxyanthranol, were identified during this hold; they were shown to undergo subsequent irreversible electrochemical oxidation reaction at 0.7 V, so that they no longer participate in the subsequent electrochemistry of the battery when operated in the standard voltage window of the cell. The decomposition reaction rate was found to be concentration-dependent, with a faster rate being observed at higher concentrations. Taking advantage of the inherent flow properties of the system, this work demonstrates the possibility of multi-modal in situ (online) characterizations of redox flow batteries, the characterization techniques being applicable to a range of electrochemical flow systems.
While organic mixed ionic/electronic conductors are widely studied for various applications in bioelectronics, energy generation/storage, and neuromorphic computing, a fundamental understanding of the interactions between the ionic and electronic carriers remains unclear, particularly in the wet state and on electrochemical cycling. Here, we show that operando NMR spectroscopy can selectively probe and quantify ion and water movement during the doping/dedoping of poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) films, the most widely used organic mixed conductor. Na + ions near or within the PEDOT-rich domains experience an anisotropic environment resulting from the underlying partial PSS chain orientation in the polymer films, giving rise to a distinct quadrupolar splitting in the 23 Na NMR spectrum. Operando 23 Na NMR studies reveal a linear correlation between the quadrupolar splitting and the charge stored in the film, which is interpreted in terms of the roles that the Na + ions at the PEDOT/PSS interfaces play in charge balance and electric double layer formation. The observed correlation is quantitatively explained by a competitive binding model, in which holes on the PEDOT backbone are bound to PSS, the hole concentration changes during doping/dedoping inducing variations in the Na + binding percentage at the PEDOT/PSS interfaces. The Na + -to-electron coupling efficiency, measured via 23 Na NMR intensity changes, varies noticeably depending on the cycling history of the film. Operando 1 H NMR spectroscopy confirms that water molecules accompany the ions that are injected into/extracted from the films. These findings shed light on the working principles of organic mixed conductors and demonstrate the utility of operando NMR spectroscopy in revealing structure-property relationships in electroactive polymers.
While organic mixed ionic/electronic conductors are widely studied for various applications in bioelectronics, energy generation/storage, and neuromorphic computing, a fundamental understanding of the interactions between the ionic and electronic carriers remains unclear, particularly in the wet state and on electrochemical cycling. Here, we show that operando NMR spectroscopy can selectively probe and quantify ion and water movement during the doping/dedoping of poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) films, the most widely used organic mixed conductor. Na+ ions near or within the PEDOT-rich domains experience an anisotropic environment resulting from the underlying partial PSS chain orientation in the polymer films, giving rise to a distinct quadrupolar splitting in the 23Na NMR spectrum. Operando 23Na NMR studies reveal a linear correlation between the quadrupolar splitting and the charge stored in the film, which is interpreted in terms of the roles that the Na+ ions at the PEDOT/PSS interfaces play in charge balance and electric double layer formation. The observed correlation is quantitatively explained by a competitive binding model, in which holes on the PEDOT backbone are bound to PSS, the hole concentration changes during doping/dedoping inducing variations in the Na+ binding percentage at the PEDOT/PSS interfaces. The Na+-to-electron coupling efficiency, measured via 23Na NMR intensity changes, varies noticeably depending on the cycling history of the film. Operando 1H NMR spectroscopy confirms that water molecules accompany the ions that are injected into/extracted from the films. These findings shed light on the working principles of organic mixed conductors and demonstrate the utility of operando NMR spectroscopy in revealing structure-property relationships in electroactive polymers.
Supercapacitors are fast-charging energy storage devices of great importance for developing robust and climate-friendly energy infrastructures for the future. Research in this field has seen rapid growth in recent years. Therefore, consistent reporting practices must be implemented to enable reliable comparison of device performance. Although several studies have highlighted the best practices for analysing and reporting data from such energy storage devices, there is yet to be an empirical study investigating whether researchers in the field are correctly implementing these recommendations, and which assesses the variation in reporting between different laboratories. Here, we address this deficit by carrying out the first interlaboratory study of the analysis of supercapacitor electrochemistry data. We find that the use of incorrect formulae and researchers having different interpretations of key terminologies are the primary causes of variability in data reporting. Furthermore, we highlight the more significant variation in reported results for electrochemical profiles showing non-ideal capacitive behaviour. From the insights gained through this study, we make additional recommendations to the community to help ensure consistent reporting of performance metrics moving forward.
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