We demonstrate here a simple alternative strategy of developing a stable and long-lived aqueous Zn-ion battery. The battery comprises a redox-active anthraquinone-based covalent organic framework (COF) and a graphene oxide composite (COF-GOPH) as the cathode, zinc metal as the anode, and a mixed-ion electrolyte with varying proportions of zinc and lithium ions. This cell configuration contrasts with those of conventional organic batteries with aqueous electrolytes having a single type of cation. Our findings convincingly show that an optimal Li + to Zn 2+ ion ratio is beneficial for Zn 2+ -ion diffusion into the COF. The energy storage mechanism is found to be due to the Zn 2+ -ion intercalation/ deintercalation into the COF with simultaneous reversible redox activity of the framework carbonyl and imine moieties. Additionally, a theoretical analysis of the radial distribution function reveals the preferential insertion of Zn 2+ -ions along with its partial solvation shell into the framework, leading to an optimal coordination of Zn 2+ with oxygen and nitrogen moieties of the COF network. On the other hand, the Li + ions preferentially reside in solution. Irrespective of the electrolyte composition, the composite electrode COF-GOPH performs better than the COF. The best battery performance is obtained with the COF-GOPH in the presence of 0.5 M ZnSO 4 and 0.5 M Li 2 SO 4 electrolyte. The cell shows excellent cyclability and superior capacity with 82% retention even after 500 cycles (from the second cycle onwards). Our studies also reveal a Li + -ion-assisted pseudocapacitance mechanism that is partially responsible for the enhancement in the electrochemical performance in the mixed-ion electrolytes.
We study by computer simulations, and by theory, the coupled rotational and translational dynamics of three important linear diatomic molecules, namely, carbon monoxide (CO), nitric oxide (NO), and cyanide ion (CN−) in water. Translational diffusion of these molecules is found to be strongly coupled to their own rotational dynamics which, in turn, are coupled to similar motions of the surrounding water. In particular, we find that coupled orientational jump motions play an important role in all three cases. While CO and NO show similar features, CN− exhibits certain differences. Our results agree well with the known experimental values of the diffusion coefficient. We examined the validity of hydrodynamic predictions and found them to be inadequate, particularly for rotational diffusion. A mode coupling theory approach is developed and applied to understand the complexity of translation-rotation coupling.
Because of the amphiphilic nature of ethanol in the aqueous solution, ions cause an interesting microheterogeneity where the water molecules and the hydroxy groups of ethanol preferentially solvate the ions, while the ethyl groups tend to occupy the intervening space. Using computer simulations, we study the dynamics of rigid monovalent cations (Li+, Na+, K+, and Cs+) in aqueous ethanol solutions with chloride as the counterion. We vary both the size of the ions and the composition of the mixture to explore size- and composition-dependent ion diffusion. The relative stability of enhanced microheterogeneous configurations makes ion diffusion slower than what would be surmised by using the bulk properties of the mixture, using the Stokes–Einstein relation. We study the structure through partial radial distribution functions and the stability through coordination number fluctuations. The ion diffusion coefficient exhibits sharp re-entrant behavior when plotted against viscosity varied by composition. Our studies reveal multiple anomalous features of ion motion in this mixture. We formulate a mode-coupling theory (MCT) that takes into account the interaction between different dynamical components; MCT can incorporate the effects of heterogeneous dynamics and nonlinearity in composition dependence that arise from the feedback between mutually dependent ion–solvent dynamics.
We study the rotational and translational dynamics of three small important linear molecules, namely, carbon monoxide (CO), nitric oxide (NO), and cyanide ion (CN−) in water–ethanol mixtures, at different compositions. Here, we report a detailed study of the dynamics of these diatomics in water–ethanol binary mixtures for the first time. We find multiple anomalous results, namely, (i) faster rotational motion of CO and NO than CN−, (ii) larger translational diffusion of CO and NO in pure ethanol than in water but the reverse for CN−, (iii) a pronounced anomaly in the composition dependence of translational–rotational dynamics at low ethanol composition, and (iv) a re-entrant type behavior in the viscosity dependence of orientational relaxation. We compare our simulation result-based observations with the existing experimental results wherever available and find that the simulation results are in reasonably good agreement with the experiments. We implement, for the first time, a calculation of the rotational binary friction following the sophisticated scheme of Evans and co-workers. We also calculate the hydrodynamic predictions for the solute molecules. On comparison with the simulation and experimental results, we find that neither the rotational binary friction nor the hydrodynamic expressions (both stick and slip) can reproduce the results. To rectify the situation, we develop a detailed mode-coupling theory and suggest that such an approach if completely implemented can provide a more reliable description than the hydrodynamic approach. Many of our results could be tested in experimental studies because these linear molecules are amenable to spectroscopic studies, such as 2D-IR.
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