Understanding the electrochemistry of "water-in-salt" electrolytes: basal plane highly ordered pyrolytic graphite as a model system

Metal-free platforms for molecular thin films as high-performance supercapacitors

Controlling chemical functionalization and achieving stable electrode-molecule interfaces for high-performance electrochemical energy storage applications remain challenging tasks. Herein, we present a simple, controllable, scalable, and versatile electrochemical modification approach of graphite rods (GRs) extracted from low-cost Eveready cells that were covalently modified with anthracene oligomers. The anthracene oligomers with a total layer thickness of ∼24 nm on the GR electrode yield a remarkable specific capacitance of ∼670 F g-1 with good galvanostatic charge-discharge cycling stability (10 000) recorded in 1 M H2SO4 electrolyte. Such a boost in capacitance is attributed mainly to two contributions: (i) an electrical double-layer at the anthracene oligomer/GR/electrolyte interfaces, and (ii) the proton-coupled electron transfer (PCET) reaction, which ensures a substantial faradaic contribution to the total capacitance. Due to the higher conductivity of the anthracene films, it possesses more azo groups (-N[double bond, length as m-dash]N-) during the electrochemical growth of the oligomer films compared to pyrene and naphthalene oligomers, which is key to PCET reactions. AC-based electrical studies unravel the in-depth charge interfacial electrical behavior of anthracene-grafted electrodes. Asymmetrical solid-state supercapacitor devices were made using anthracene-modified biomass-derived porous carbon, which showed improved performance with a specific capacitance of ∼155 F g-1 at 2 A g-1 with an energy density of 5.8 W h kg-1 at a high-power density of 2010 W kg-1 and powered LED lighting for a longer period. The present work provides a promising metal-free approach in developing organic thin-film hybrid capacitors.

Rocking-Chair Aqueous Fluoride-Ion Batteries Enabled by Hydrogen Bonding Competition

Aqueous fluoride ion batteries (FIBs) have garnered attention for their high theoretical energy density, yet they are challenged by sluggish fluorination kinetics, active material dissolution, and electrolyte instability. Here, we present a room temperature rocking-chair aqueous FIBs featuring KOAc-KF binary salt electrolytes, enabling concurrent fluorination and defluorination reactions at both cathode and anode electrodes. Experimental and theoretical results reveal that acetate ions in the electrolyte compete with fluoride ions in hydrogen bonding formation, weakening the excessively strong solvation between H2O and F- ions. This results in the suppression of detrimental HF formation and a reduced desolvation energy of F- ions, enhancing the electrochemical reaction kinetics. The bismuth-based cathode exhibits direct conversion in the optimized electrolyte, effectively suppressing the detrimental disproportionation reactions from Bi2+ intermediates. Additionally, zinc anode undergoes a typical fluorination process, forming solid KZnF3 as the electrode product, minimizing the risks of hydrogen evolution. The proposed aqueous FIBs with the optimized electrolyte demonstrate high discharge capacity, long-term cycling stability and excellent rate capabilities.

4-Fluorobenzyl cyanide, a sterically-hindered solvent expediting interfacial kinetics in lithium-ion batteries

The electrochemical performance of lithium-ion batteries (LIBs) is plagued by sluggish interfacial kinetics. Fortunately, the Li+ solvation structure bridges the bulk electrolyte and interfacial chemistry, providing a pathway for promoting electrochemical kinetics in LIBs. Herein, we improve the interfacial kinetics by tuning the Li+ coordination chemistry based on solvent molecular engineering. Specifically, 4-fluorobenzyl cyanide (FBCN), featuring steric hindrance and a weak Lewis basic center, is designed to construct a bulky coordination structure with Li+, weakening ion-dipole interaction (Li+-solvents) but promoting coulombic attraction (Li+-anions) at a normal Li salt concentration. This sterically-controlled solvation chemistry reduces the interfacial barrier and thus contributes to improved rate performance, as demonstrated practically in LiFePO4//graphite pouch cells. This study provides fresh insights into solvent steric control and coordination chemistry engineering, opening a new avenue for enhancing electrochemical kinetics in LIBs.

Dielectric-free electrowetting on graphene

Electrowetting is a simple way to induce the spreading and retraction of electrolyte droplets. This method is widely used in "device" applications, where a dielectric layer is applied between the electrolyte and the conducting substrate. Recent work, including contributions from our own laboratory, have shown that reversible electrowetting can be achieved directly on conductors. We have shown that graphite surfaces, in particular when combined with highly concentrated electrolyte solutions, show a strong wetting effect. The process is driven by the interactions between the electrolyte ions and the surface, hence models of double-layer capacitance are able to explain changes in the equilibrium contact angles. Herein, we extend the approach to the investigation of electrowetting on graphene samples of varying thickness, prepared by chemical vapor deposition. We show that the use of highly concentrated aqueous electrolytes induces a clear yet subtle electrowetting response due to the adsorption of ions and the suppression of the negative effect introduced by the surface impurities accumulating during the transfer process. The latter have been previously reported to fully hinder electrowetting at lower electrolyte concentrations. An amplified wetting response is recorded in the presence of strongly adsorbed/intercalated anions in both aqueous and non-aqueous electrolytes. The phenomenon is interpreted based on the anion-graphene interactions and their influence on the energetics of the interface. By monitoring the dynamics of wetting, an irreversible behaviour is identified in all cases as a consequence of the irreversibility of anion adsorption and/or intercalation. Finally, the effect of the underlying reactions on the timescales of wetting is also examined.

Phase Behavior and Electrochemical Properties of Highly Asymmetric Redox Coacervates

This work reports the phase behavior and electrochemical properties of liquid coacervates made of ferricyanide and poly(ethylenimine). In contrast to the typical polyanion/polycation pairs used in liquid coacervates, the ferricyanide/poly(ethylenimine) system is highly asymmetric because poly(ethylenimine) has approximately 170 charges per molecule, while ferricyanide has only 3. Two types of phase diagrams were measured and fitted with a theoretical model. In the first type of diagram, the stability of the coacervate was studied in the plane given by the concentration of poly(ethylenimine) versus the concentration of ferricyanide for a fixed concentration of added monovalent salt (NaCl). The second type of diagram involved the plane given by the concentration of poly(ethylenimine) vs the concentration of the added monovalent salt for a fixed poly(ethyleneimine)/ferricyanide ratio. Interestingly, these phase diagrams displayed qualitative similarities to those of symmetric polyanion/polycation systems, suggesting that coacervates formed by a polyelectrolyte and a small multivalent ion can be treated as a specific case of polyelectrolyte coacervate. The characterization of the electrochemical properties of the coacervate revealed that the addition of monovalent salt greatly enhances charge transport, presumably by breaking ion pairs between ferricyanide and poly(ethylenimine). This finding highlights the significant influence of added salt on the transport properties of coacervates. This study provides the first comprehensive characterization of the phase behavior and transport properties of asymmetric coacervates and places these results within the broader context of the better-known symmetric polyelectrolyte coacervates.

Oil palm leaf-derived hierarchical porous carbon for "water-in-salt" based supercapacitors: the effect of anions (Cl- and TFSI-) in superconcentrated conditions

This study investigates the use of a hierarchical porous carbon electrode derived from oil palm leaves in a "water-in-salt" supercapacitor. The impact of anion identity on the electrical performance of the carbon electrode was also explored. The results show that the prepared carbon had a hierarchical porous structure with a high surface area of up to 1840 m2 g-1. When a 20 m LiTFSI electrolyte was used, the carbon electrode had a specific capacitance of 176 F g-1 with a wider potential window of about 2.6 V, whereas the use of a cheaper 20 m LiCl electrolyte showed a higher specific capacitance of 331 F g-1 due to the smaller size of the Cl- anion, which enabled inner capacitance. Therefore, the anion identity has an effect on the electrochemical performance of porous carbon, and this research contributes to the understanding of using "water-in-salt" electrolytes in carbon-based supercapacitors. The study's findings provide insights into developing low-cost, high-performance supercapacitors that can operate in a wider voltage range.

Hydrophobic nanoporous carbon scaffolds reveal the origin of polarity-dependent electrocapillary imbibition

An engineered nanoporous carbon scaffold (NCS) consisting of a 3-D interconnected 85 nm nanopore network was used here as a model material to investigate the nanoscale transport of liquids as a function of the polarity and magnitude of an applied potential ('electro-imbibition'), all in 1 M KCl solution. A camera was used to track both meniscus formation and meniscus jump, front motion dynamics, and droplet expulsion, while also quantifying the electrocapillary imbibition height (H) as a function of the applied potential of the NCS material. Although no imbibition was seen over a wide range of potentials, at positive potentials (+1.2 V vs. the potential of zero charge (pzc)), imbibition was correlated with carbon surface electro-oxidation, as confirmed by both electrochemistry and post-imbibition surface analysis, with gas evolution (O₂, CO₂) seen visually only after imbibition was well underway. At negative potentials, vigorous hydrogen evolution reaction was observed at the NCS/KCl solution interface, well before imbibition began at -0.5 V_pzc, proposed to be nucleated by an electrical double layer charging-driven meniscus jump, followed by processes such as Marangoni flow, adsorption induced deformation, and hydrogen pressure driven flow. This study improves the understanding of electrocapillary imbibition at the nanoscale, being highly relevant in a wide range of multidisciplinary practical applications, including in energy storage and conversion devices, energy-efficient desalination, and electrical-integrated nanofluidics design.

Taming Electrowetting Using Highly Concentrated Aqueous Solutions

Wetting of carbon surfaces is one of the most widespread, yet poorly understood, physical phenomena. Control over wetting properties underpins the operation of aqueous energy-storage devices and carbon-based filtration systems. Electrowetting, the variation in the contact angle with an applied potential, is the most straightforward way of introducing control over wetting. Here, we study electrowetting directly on graphitic surfaces with the use of aqueous electrolytes to show that reversible control of wetting can be achieved and quantitatively understood using models of the interfacial capacitance. We manifest that the use of highly concentrated aqueous electrolytes induces a fully symmetric and reversible wetting behavior without degradation of the substrate within the unprecedented potential window of 2.8 V. We demonstrate where the classical "Young-Lippmann" models apply, and break down, and discuss reasons for the latter, establishing relations among the applied bias, the electrolyte concentration, and the resultant contact angle. The approach is extended to electrowetting at the liquid|liquid interface, where a concentrated aqueous electrolyte drives reversibly the electrowetting response of an insulating organic phase with a significantly decreased potential threshold. In summary, this study highlights the beneficial effect of highly concentrated aqueous electrolytes on the electrowettability of carbon surfaces, being directly related to the performance of carbon-based aqueous energy-storage systems and electronic and microfluidic devices.

Synergistic Inorganic-Organic Dual-Additive Electrolytes Enable Practical High-Voltage Lithium-Ion Batteries

Severe electrolyte decomposition under high voltage can easily lead to degradation of the performance of lithium-ion batteries, which has become a major obstacle to the practical application of high-energy-density batteries. To solve these problems, a dual-functional electrolyte additive comprising inorganic lithium difluorophosphate (LiDFP) and organic 1,3,6-hexanetrinitrile (HTN) was designed and employed to improve the performance of high-voltage Si@C/LiNi_0.5Mn_1.5O₄ full batteries. LiDFP with a lower LUMO energy than the solvent in the electrolyte takes priority in reduction, facilitating the formation of a dense and stable film on the anode, effectively suppressing side reactions of the electrolyte and aiding tolerance to the volume expansion of the Si@C electrode. Additionally, the lower HOMO energy of HTN can improve the oxidation resistance of the electrolyte, with the C≡N functional group of HTN helping to remove the trace water and the byproduct HF from the electrolyte. The Si@C/LiNi_0.5Mn_1.5O₄ full battery with 1 wt % LiDFP and 1 wt % HTN in 1.0 M LiPF₆ traditional electrolyte delivers high capacity retention of 91.57% after 150 cycles at 0.2C, compared to 34.58% capacity retention without any additives. Moreover, the Coulombic efficiency of batteries with electrolyte additives can reach 99.75% on average, compared to their counterparts at ∼96.54%. The synergistic effect of LiDFP and HTN provides a promising strategy for enhancing the performance of high-voltage batteries for practical industrialization.

Auto-oxidation of exfoliated MoS2 in N-methyl-2-pyrrolidone: from 2D nanosheets to 3D nanorods

We have therefore introduced a novel preparation route for MoO3 nanorods from exfoliated 2H-MoS2 via the auto-oxidation of a mixture of N-methyl-2-pyrrolidone and water via the sonication-assisted exfoliation of MoS2.

A Review: Ion Transport of Two-Dimensional Materials in Novel Technologies from Macro to Nanoscopic Perspectives

Ion transport is a significant concept that underlies a variety of technologies including membrane technology, energy storages, optical, chemical, and biological sensors and ion-mobility exploration techniques. These applications are based on the concepts of capacitance and ion transport, so a prior understanding of capacitance and ion transport phenomena is crucial. In this review, the principles of capacitance and ion transport are described from a theoretical and practical point of view. The review covers the concepts of Helmholtz capacitance, diffuse layer capacitance and space charge capacitance, which is also referred to as quantum capacitance in low-dimensional materials. These concepts are attributed to applications in the electrochemical technologies such as energy storage and excitable ion sieving in membranes. This review also focuses on the characteristic role of channel heights (from micrometer to angstrom scales) in ion transport. Ion transport technologies can also be used in newer applications including biological sensors and multifunctional microsupercapacitors. This review improves our understanding of ion transport phenomena and demonstrates various applications that is applicable of the continued development in the technologies described.

Electrochemistry, ion adsorption and dynamics in the double layer: a study of NaCl(aq) on graphite

Graphite and related sp² carbons are ubiquitous electrode materials with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the carbon–electrolyte double layer at technologically relevant concentrations. Here, the (electrified) graphite–NaCl(aq) interface was examined using constant chemical potential molecular dynamics (CμMD) simulations; this approach avoids ion depletion (due to surface adsorption) and maintains a constant concentration, electroneutral bulk solution beyond the surface. Specific Na⁺ adsorption at the graphite basal surface causes charging of the interface in the absence of an applied potential. At moderate bulk concentrations, this leads to accumulation of counter-ions in a diffuse layer to balance the effective surface charge, consistent with established models of the electrical double layer. Beyond ∼0.6 M, however, a combination of over-screening and ion crowding in the double layer results in alternating compact layers of charge density perpendicular to the interface. The transition to this regime is marked by an increasing double layer size and anomalous negative shifts to the potential of zero charge with incremental changes to the bulk concentration. Our observations are supported by changes to the position of the differential capacitance minimum measured by electrochemical impedance spectroscopy, and are explained in terms of the screening behaviour and asymmetric ion adsorption. Furthermore, a striking level of agreement between the differential capacitance from solution evaluated in simulations and measured in experiments allows us to critically assess electrochemical capacitance measurements which have previously been considered to report simply on the density of states of the graphite material at the potential of zero charge. Our work shows that the solution side of the double layer provides the more dominant contribution to the overall measured capacitance. Finally, ion crowding at the highest concentrations (beyond ∼5 M) leads to the formation of liquid-like NaCl clusters confined to highly non-ideal regions of the double layer, where ion diffusion is up to five times slower than in the bulk. The implications of changes to the speciation of ions on reactive events in the double layer are discussed.

CμMD reveals multi-layer electrolyte screening in the double layer beyond 0.6 M, which affects ion activities, speciation and mobility; asymmetric charge screening explains concentration dependent changes to electrochemical properties.

Scalable 18,650 aqueous-based supercapacitors using hydrophobicity concept of anti-corrosion graphite passivation layer

Scalable aqueous-based supercapacitors are ideal as future energy storage technologies due to their great safety, low cost, and environmental friendliness. However, the corrosion of metal current collectors e.g., aluminium (Al) foil in aqueous solutions limits their practical applications. In this work, we demonstrate a low-cost, scalable, and simple method to prepare an anti-corrosion current collector using a concept of hydrophobicity by coating the hydrophobic graphite passivation layer on the Al foil via a roll-to-roll coating technology at the semi-automation scale of production pilot plant of 18,650 cylindrical supercapacitor cells. All qualities of materials, electrodes, and production process are therefore in the quality control as the same level of commercial supercapacitors. In addition, the effects of the graphite coating layer have been fundamentally evaluated. We have found that the graphite-coated layer can improve the interfacial contact without air void space between the activated carbon active material layer and the Al foil current collector. Importantly, it can suppress the corrosion and the formation of resistive oxide film resulting in better rate capability and excellent cycling stability without capacitance loss after long cycling. The scalable supercapacitor prototypes here in this work may pave the way to practical 18,650 supercapacitors for sustainable energy storage systems in the future.

Physicochemical and Electrochemical Properties of Water-in-Salt Electrolytes

Aqueous electrolytes are attractive for applications in electrochemical technologies due to features like being eco-friendly, cost effective, and non-flammable. Very recently, superconcentrated aqueous electrolytes, such as so-called water-in-salt, water-in-bisalt, and hydrate melt, have received a significant attention for electrochemical energy storage due to enhanced stability and much wider electrochemical stability window. This Review focuses on the physicochemical properties of the highly concentrated electrolytes that are derived from several analysis techniques and simulation. A summary of most common features such as ions-water interactions, structure of species present in the electrolyte, conductivity, and viscosity of the electrolytes found in the literature are presented as well. In addition, this Review explains how these characteristics affect the electrochemical behavior of the electrolyte such as double layer structure and electrode/electrolyte interface leading to enhanced electrochemical stability of aqueous electrolytes.

Controlling the flake size of bifunctional 2D WSe2 nanosheets as flexible binders and supercapacitor materials

A new approach using graphene as a conductive binder in electrical supercapacitors has recently been proposed. Graphene shows outstanding properties as a conductive binder, and can be used to replace conductive, additive, and polymer binders. However, graphene follows an EDLC behaviour, which may limit its electrochemical performance. In the process described in this work, we introduced WSe₂ nanoflakes as a new approach to using pseudocapacitive materials as binders. The WSe₂ nanoflakes were produced through liquid phase exfoliation of bulk WSe₂, and the flake size was finely selected using a controlled centrifugation speed. The physical and electrochemical properties of the exfoliated WSe₂ flakes were analysed; it was found that the smallest flakes (an average flake size of 106 nm) showed outstanding electrochemical properties, expanding our understanding of transition metal dichalcogenide (TMD) materials, and we were able to demonstrate the applicability of using WSe₂ as a binder in supercapacitor electrodes. We also successfully replaced conductive additives and polymer binders with WSe₂. The overall performance was improved: capacitance was enhanced by 35%, charge transfer resistance reduced by 73%, and self-discharge potential improved by 9%. This study provides an alternative application of using TMD materials as pseudo capacitive binders, which should lead to the continued development of energy storage technology.