Crystalline Free-Standing Two-Dimensional Zwitterionic Organic Nanosheets for Efficient Conduction of Lithium Ions

2D organic nanosheets of self-assembled guanidinium derivative for efficient single sodium-ion conduction: rationalizing morphology editing and ion conduction

The resurgence of interest in sodium-ion batteries (SIBs) is largely driven by their natural abundance and favourable cost, apart from their comparable electrochemical performance when compared with lithium-ion batteries (LIBs). The uneven geographic distribution of the raw materials required for LIBs has also contributed to this. The solid-state electrolyte (SSE) is typically one of the vital components for energy storage in SIBs and for achieving high electrochemical performances. SSEs are preferred over liquid electrolytes primarily due to their enhanced safety and stability, apart from the option of achieving higher energy density. A single sodium-ion selective conductor minimises dendrite formation and cell polarisation, among many other benefits over binary ionic conductors in battery operation. Here, we demonstrate the first example of a sulfonated supramolecular organic two-dimensional (2D) nanosheet as a novel class of single sodium-ion conductors prepared from the self-assembly of a functionalised guanidinium ion (AD-1). Solvent-assisted exfoliation of the bulk powder in water yielded nanosheet morphology, whereas nanotube morphology was achieved in isopropanol (IPA). In contrast, self-assembly with systematic water/IPA solvent ratio variations produced marigold, sunflower, and nanorod morphologies. Thermodynamic parameters, crystallinity, elemental composition, and varying natures of hydrogen bonding in five distinct morphologies were determined using microscopic and spectroscopic studies. The single Na+ conducting properties of each morphology are correlated in terms of morphology, crystallinity, and the solvent used to achieve that specific morphology. Importantly, with high crystallinity and directional ion channels, 2D nanosheet morphology exhibits the highest single Na+-ion conductivity of 3.72 × 10-4 S cm-1 with an activation energy of 0.28 eV, showing a moderately high Na+-ion transference number of 0.83 at room temperature without incorporating any additional sodium salts and organic solvents. This report is believed to be the first to show the significance of nanostructure morphologies in achieving high single-Na+-ion transport.

Self-Assembled Conjugated Coordination Polymer Nanorings: Role of Morphology and Redox Sites for the Alkaline Electrocatalytic Oxygen Evolution Reaction

Electrocatalytic water splitting provides a sustainable method for storing intermittent energies, such as solar energy and wind, in the form of hydrogen fuel. However, the oxygen evolution reaction (OER), constituting the other half-cell reaction, is often considered the bottleneck in overall water splitting due to its slow kinetics. Therefore, it is crucial to develop efficient, cost-effective, and robust OER catalysts to enhance the water-splitting process. Transition-metal-based coordination polymers (CPs) serve as promising electrocatalysts due to their diverse chemical architectures paired with redox-active metal centers. Despite their potential, the rational use of CPs has faced obstacles including a lack of insights into their catalytic mechanisms, low conductivity, and morphology issues. Consequently, achieving success in this field requires the rational design of ligands and topological networks with the desired electronic structure. This study delves into the design and synthesis of three novel conjugated coordination polymers (CCPs) by leveraging the full conjugation of terpyridine-attached flexible tetraphenylethylene units as electron-rich linkers with various redox-active metal centers [Co(II), Ni(II), and Zn(II)]. The self-assembly process is tuned for each CCP, resulting in two distinct morphologies: nanosheets and nanorings. The electrocatalytic OER performance efficiency is then correlated with factors such as the nanostructure morphology and redox-active metal centers in alkaline electrolytes. Notably, among the three morphologies studied, nanorings for each CCP exhibit a superior OER activity. Co(II)-integrated CCPs demonstrate a higher activity between the redox-active metal centers. Specifically, the Co(II) nanoring morphology displays exceptional catalytic activity for OER, with a lower overpotential of 347 mV at a current density of 10 mA cm-2 and small Tafel slopes of 115 mV dec-1. The long-term durability is demonstrated for at least 24 h at 1.57 V vs RHE during water splitting. This is presumably the first proof that links the importance of nanostructure morphologies to redox-active metal centers in improving the OER activity, and it may have implications for other transdisciplinary energy-related applications.

Self-assembled cationic organic nanosheets: role of positional isomers in a guanidinium-core for efficient lithium-ion conduction

The growing energy demand with the widespread use of smart portable electronics, as well as an exponential increase in demand for smart batteries for electric vehicles, entails the development of efficient portable batteries with high energy density and safe power storage systems. Li-ion batteries arguably have superior energy density to all other traditional batteries. Developing mechanically robust solid-state electrolytes (SSEs) for lithium-ion conduction for an efficient portable energy storage unit is vital to empower this technology and overcome the safety constraints of liquid electrolytes. Herein, we report the formation of self-assembled organic nanosheets (SONs) utilizing positional isomers of small organic molecules (AM-2 and AM-3) for use as SSEs for lithium-ion conduction. Solvent-assisted exfoliation of the bulk powder yielded SONs having near-atomic thickness (∼4.5 nm) with lateral dimensions in the micrometer range. In contrast, self-assembly in the DMF/water solvent system produced a distinct flower-like morphology. Thermodynamic parameters, crystallinity, elemental composition, and nature of H-bonding for two positional isomers are established through various spectroscopic and microscopic studies. The efficiency of the lithium-ion conducting properties is correlated with factors like nanostructure morphology, ionic scaffold, and locus of the functional group responsible for forming the directional channel through H-bonding in the positional isomer. Amongst the three different morphologies studied, SONs display higher ion conductivity. In between the cationic and zwitterionic forms of the monomer, integration of the cationic scaffold in the SON framework led to higher conductivity. Amongst the two positional isomers, the meta-substituted carboxyl group forms a more rigid directional channel through H-bonding to favor ionic mobility and accounts for the highest ion conductivity of 3.42 × 10^-4 S cm^-1 with a lithium-ion transference number of 0.49 at room temperature. Presumably, this is the first demonstration that signifies the importance of the cationic scaffold, positional isomers, and nanostructure morphologies in improving ionic conductivity. The ion-conducting properties of such SONs having a guanidinium-core may have significance for other interdisciplinary energy-related applications.