Dilute Aqueous Electrolyte Research Articles

Tailoring the Stability and Kinetics of Zn Anodes through Trace Organic Polymer Additives in Dilute Aqueous Electrolyte

Tailoring the Stability and Kinetics of Zn Anodes through Trace Organic Polymer Additives in Dilute Aqueous Electrolyte

Fluorinated interphase enables reversible aqueous zinc battery chemistries.

Metallic zinc is an ideal anode due to its high theoretical capacity (820 mAh g-1), low redox potential (-0.762 V versus the standard hydrogen electrode), high abundance and low toxicity. When used in aqueous electrolyte, it also brings intrinsic safety, but suffers from severe irreversibility. This is best exemplified by low coulombic efficiency, dendrite growth and water consumption. This is thought to be due to severe hydrogen evolution during zinc plating and stripping, hitherto making the in-situ formation of a solid-electrolyte interphase (SEI) impossible. Here, we report an aqueous zinc battery in which a dilute and acidic aqueous electrolyte with an alkylammonium salt additive assists the formation of a robust, Zn2+-conducting and waterproof SEI. The presence of this SEI enables excellent performance: dendrite-free zinc plating/stripping at 99.9% coulombic efficiency in a Ti||Zn asymmetric cell for 1,000 cycles; steady charge-discharge in a Zn||Zn symmetric cell for 6,000 cycles (6,000 h); and high energy densities (136 Wh kg-1 in a Zn||VOPO4 full battery with 88.7% retention for >6,000 cycles, 325 Wh kg-1 in a Zn||O2 full battery for >300 cycles and 218 Wh kg-1 in a Zn||MnO2 full battery with 88.5% retention for 1,000 cycles) using limited zinc. The SEI-forming electrolyte also allows the reversible operation of an anode-free pouch cell of Ti||ZnxVOPO4 at 100% depth of discharge for 100 cycles, thus establishing aqueous zinc batteries as viable cell systems for practical applications.

Unveiling the Formation of Solid Electrolyte Interphase and its Temperature Dependence in "Water-in-Salt" Supercapacitors.

"Water-in-salt" (WIS) electrolytes have emerged as an excellent superconcentrated ionic medium for high-power energy storage systems such as supercapacitors due to their extended working potential compared to the conventional dilute aqueous electrolyte. In this work, we have investigated the performance of WIS supercapacitors using hollow carbon nanoplates as electrodes and compared it to that based on the conventional "salt-in-water" electrolytes. Moreover, the potentiostatic electrochemical impedance spectroscopy has been employed to provide an insightful look into the charge transport properties, which also, for the first time, reveals the formation of a solid-electrolyte interphase (SEI) and their temperature-dependent impedance for charge transfer and adsorption. Furthermore, the effect of temperature on the electrochemical performance of the WIS supercapacitors in the temperature range from 15 to 60 °C has been studied, which presents a gravimetric capacitance of 128 F g-1 and a volumetric capacitance of 197.12 F cm-3 at 55 °C compared to 87.5 F g-1 and 134.75 F cm-3 at 15 °C. The in-depth understanding about the formation of SEI layer and the electrochemical performance at different temperatures for WIS supercapacitors will assist the efforts toward designing better aqueous electrolytes for supercapacitors.

Manipulating Zn anode reactions through salt anion involving hydrogen bonding network in aqueous electrolytes with PEO additive

Abstract The reaction kinetics, reversibility and long-term stability of Zn anodes are crucial for rechargeable aqueous Zn batteries. Polymer electrolyte additives are low cost and effective to modify the deposition behavior of Zn anodes. Here, an interesting phenomenon appears that cycling stability and Coulombic efficiency of Zn anodes are improved by PEO additive in dilute aqueous electrolytes with different Zn salts, while the overpotential and long-term stability of Zn anodes vary significantly with different anions. Systematic electrochemical analysis coupled with NMR and MD simulation was performed to reveal the crucial role of salt anion in tuning the specific solvation structures and hydrogen bonding network of aqueous electrolytes with PEO additive, and interfacial double layer capacitance structures. The reaction kinetics and stability of Zn anodes were, therefore, intensively tuned by salt anions in aqueous electrolytes with PEO additives. The cycle life of Zn anode can be improved by 7–10 times with significantly reduced overpotential in bulky anion such as TFSI- based aqueous electrolyte than compact anion such as SO42- in PEO containing aqueous electrolytes. This work provides useful and low-cost concepts to effectively improve Zn anodes reaction dynamics and cycle life by choosing optimal electrolyte salts and additives.

Downplaying the role of water in the rheological changes of conducting polymers by using water-in-salt electrolytes.

Volumetric changes associated with solvent/electrolyte exchange in electronic conducting polymers (ECPs) play an important role in the mechanical stability of the polymers, as these changes are a critical factor in ECP-based energy storage devices. Thus, the present work explores the hindering of such volumetric deformations for polypyrrole films doped with dodecylbenzenesulphonate (PPy(DBS)) by employing highly concentrated aqueous electrolytes (or water-in-salt electrolytes, WiSEs), and their effects over the corresponding electrochemical capacitor cell energy retention. Electrochemical quartz crystal microbalance with dissipation monitoring measurements for thin PPy(DBS) films in the WiSEs revealed negligible dissipation changes (ΔDn ≈ 0), in contrast with those in dilute aqueous electrolyte (ΔDn ≠ 0), indicating inexpressive structural deformation of PPy(DBS) in the WiSE. This phenomenon is observed for thick freestanding PPy(DBS) films, which presented a maximum bending angle decay from ∼56° (diluted aqueous electrolyte) to 3.5° when working in the WiSE, thus proving the hindering of film bending. The observed trends are reflected in the PPy(DBS) cell energy retention, where the use of a WiSE decreased cell energy fading by 30% after 600 cycles, in comparison with cells based on diluted electrolytes.

Solvation Structure Design for Aqueous Zn Metal Batteries.

Aqueous Zn batteries are promising energy storage devices for large-scale energy-storage due to low cost and high energy density. However, their lifespan is limited by the water decomposition and Zn dendrite growth. Here, we suppress water reduction and Zn dendrite growth in dilute aqueous electrolyte by adding dimethyl sulfoxide (DMSO) into ZnCl2-H2O, in which DMSO replaces the H2O in Zn2+ solvation sheath due to a higher Gutmann donor number (29.8) of DMSO than that (18) of H2O. The preferential solvation of DMSO with Zn2+ and strong H2O-DMSO interaction inhibit the decomposition of solvated H2O. In addition, the decomposition of solvated DMSO forms Zn12(SO4)3Cl3(OH)15·5H2O, ZnSO3, and ZnS enriched-solid electrolyte interphase (SEI) preventing Zn dendrite and further suppressing water decomposition. The ZnCl2-H2O-DMSO electrolyte enables Zn anodes in Zn||Ti half-cell to achieve a high average Coulombic efficiency of 99.5% for 400 cycles (400 h), and the Zn||MnO2 full cell with a low capacity ratio of Zn:MnO2 at 2:1 to deliver a high energy density of 212 Wh/kg (based on both cathode and anode) and maitain 95.3% of the capacity over 500 cycles at 8 C.

The Applications of Water‐in‐Salt Electrolytes in Electrochemical Energy Storage Devices

AbstractWater‐in‐salt electrolytes (WISEs) have attracted widespread attention due to their non‐flammability, environmental friendliness, and wider electrochemical stability window than conventional dilute aqueous electrolytes. When applied in the electrochemical energy storage (EES) devices, WISEs can offer many advantages such as high‐level safety, manufacturing efficiency, as well as, superior electrochemical performances. Therefore, there is an urgent need for a timely and comprehensive summary of WISEs and their EES applications. In this review, the physicochemical and electrochemical properties of the WISEs are first introduced. Then, the research progresses of the WISEs using different metal salts and their analogues are summarized. Next, the current research progresses of WISEs applied in different EES devices (e.g., batteries and supercapacitors) as well as the insights into challenging and future perspectives are systematically discussed.

Cathodic Disintegration as an Easily Scalable Method for the Production of Sn- and Pb-Based Catalysts for CO2 Reduction

CO2 electroreduction to formate powered by renewable energy is an attractive strategy to recycle air-based carbon. At the moment, the electrode materials showing high selectivity for formate at high current density are post transition metals such as In, Sn, Bi, and Pb. Scaling up the CO2 electroreduction technology to industrial size requires, among other things, cheap and clean methods to produce cathode materials in the form of particles to fabricate the square meters of the electrode surface area needed for the industrial electrolyzers. We show here that it is possible to easily produce catalytic powders based on Sn and Pb via a process known as cathodic disintegration, driving the reaction with electric power and avoiding the use of organic solvents, stabilizers, and reducing agents. The catalysts produced with this method are highly selective for the reduction of CO2 to formate and show promise for use in industrial electrolyzers. Moreover, the process of cathodic disintegration is quick and clean, it has a high atom efficiency, it uses dilute aqueous electrolytes as solvents, and it has the possibility to be driven by renewable energy.

Reversible Insertion of Mg‐Cl Superhalides in Graphite as a Cathode for Aqueous Dual‐Ion Batteries

AbstractOxidative anion insertion into graphite in an aqueous environment represents a significant challenge in the construction of aqueous dual‐ion batteries. In dilute aqueous electrolytes, the oxygen evolution reaction (OER) dominates the anodic current before anions can be inserted into the graphite gallery. Herein, we report that the reversible insertion of Mg‐Cl superhalides in graphite delivers a record‐high reversible capacity of 150 mAh g−1 from an aqueous deep eutectic solvent comprising magnesium chloride and choline chloride. The insertion of Mg‐Cl superhalides in graphite does not form staged graphite intercalation compounds; instead, the insertion of Mg‐Cl superhalides makes the graphite partially turbostratic.

Reversible Insertion of Mg-Cl Superhalides in Graphite as a Cathode for Aqueous Dual-Ion Batteries.

Oxidative anion insertion into graphite in an aqueous environment represents a significant challenge in the construction of aqueous dual-ion batteries. In dilute aqueous electrolytes, the oxygen evolution reaction (OER) dominates the anodic current before anions can be inserted into the graphite gallery. Herein, we report that the reversible insertion of Mg-Cl superhalides in graphite delivers a record-high reversible capacity of 150 mAh g-1 from an aqueous deep eutectic solvent comprising magnesium chloride and choline chloride. The insertion of Mg-Cl superhalides in graphite does not form staged graphite intercalation compounds; instead, the insertion of Mg-Cl superhalides makes the graphite partially turbostratic.

Evolution and Reversible Polarity of Multilayering at the Ionic Liquid/Water Interface.

Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from -450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted.

Ruthenium Electrodeposition from Water-in-Salt Electrolytes and the Influence of Tetrabutylammonium

The use of water-in-salt electrolytes is evaluated for the electrodeposition of metallic ruthenium. The mechanisms of proton reduction inhibition by concentrated LiCl and dilute tetrabutylammonium is evaluated. Concentrated LiCl is found to disrupt the hydrogen bonding network within the solution bulk, whereas TBA is found to adsorb onto the electrode surface, blocking proton access. Ruthenium exists as a different complexed species in water-in-salt electrolytes vs dilute aqueous electrolytes, leading to a −300 mV shift in the deposition onset potential. Greater current efficiencies of Ru deposition can be obtained when depositing at proton overpotentials by the use of water-in-salt electrolytes, and TBA can offer further improvements. The grain structure and resistivities of Ru thin films are studied.

Theory of Multicomponent Phenomena in Cation-Exchange Membranes: Part II. Transport Model and Validation

Multicomponent mass-transport in cation-exchange membranes involves the movement of multiple species whose motion is coupled one to another. This phenomenon mediates the performance of numerous electrochemical and water purification technologies. This work presents and validates against experiment a mathematical model for multicomponent mass transport in phase-separated cation-exchange membranes (e.g., perfluorinated sulfonic-acid ionomers). Stefan-Maxwell-Onsager theory describes concentrated-solution transport. Hydrodynamic theory provides constitutive relations for the solute/solvent, solute/membrane, and solvent/membrane friction coefficients. Classical porous-medium theories scale membrane tortuosity. Electrostatic relaxation creates friction between ions. The model uses calculated ion and solvent partitioning between the external solution and the membrane from Part I of this series and incorporates the corresponding ion speciation into the transport coefficients. The proposed transport model compares favorably to properties (e.g., membrane conductivity, transference numbers, electroosmosis, and permeability) measured in dilute and concentrated aqueous binary and ternary electrolytes. The results reveal that the concentration and type of ions in the external solution alter the solvent volume fraction and viscosity in the hydrophilic pathways of the membrane, changing macroscale ionomer conductivity, permeability, and transference numbers. This work provides a physicochemical framework to predict ion-exchange-membrane performance in multicomponent systems exhibiting coupled transport.

Mechanisms for enhanced transition boiling during quenching on a superhydrophilic surface as revealed by in situ characterization on the solid-liquid contact using electrochemical impedance technique

The existence of two “transition” boiling curves has been discovered for over three decades, and has recently been re-examined in the investigations of quenching on superhydrophilic surfaces with enhanced boiling heat transfer, where the stable film boiling regime usually disappears and a significantly extended transitional film boiling sub-regime emerges instead, leading to a two-section shape of the transition boiling curve. It was hypothesized that the critical point at the slope change on the transition boiling curve is related to the transition in intensity/frequency of the solid-liquid contact status. Although it sounds rational, this hypothesis has not yet been proved due to the lack of a quantitative characterization on the solid-liquid contact. In this work, the electrochemical impedance technique was employed to perform in situ measurements on the interfacial solid-liquid contact during quenching on a superhydrophilic surface (over a stainless steel ball) in a very dilute aqueous electrolyte. It was found that the solid-liquid contact proportion in the transitional film boiling sub-regime is around 0.1, which becomes much greater in the transitional nucleate boiling sub-regime. This corresponds to a sharp increase on the contact proportion curve, which was found to occur in accordance with the transition in the boiling curve. The measured proportions thus provide an intuitive evidence to verify, for the first time in the literature, the hypothesized transition of solid-liquid contact mode from “point-contact” to “surface-contact” in the transition boiling regime. This method can be used to give more insights into the mechanisms for enhanced boiling heat transfer on superhydrophilic surfaces with novel micro-/nanostructures.

Stability of aqueous electrolytes based on LiFSI and NaFSI

Through a combination of bulk, interface, and interphase effects, water-in-salt electrolytes, employing a high salt concentration, offer a wider electrochemical stability window than traditional dilute aqueous electrolytes. Here we explore chemical stability, conductivity, viscosity, and electrochemical stability of aqueous solutions of bis(fluorosulfonyl)imide (FSI) salts as electrolytes for lithium-ion and sodium-ion batteries. We demonstrate that the FSI anion is prone to hydrolysis in this environment. However, the kinetics of the reaction strongly depend on salt concentration and nature of the cation. Interestingly, we find that NaFSI solutions are significantly more stable than LiFSI solutions. Highly-concentrated NaFSI solutions also display a wide electrochemical stability window and high conductivity.

The role of electrolyte acid concentration in the electrochemical exfoliation of graphite: Mechanism and synthesis of electrochemical graphene oxide

Electrochemistry has emerged as a major route for graphene and graphene oxide synthesis from graphite. Anodic graphite oxidation is commonly used with dilute mineral acid or aqueous salt electrolytes. In this system, the electrolyte acid concentration appears to be a critical parameter. However, the effect of the acid concentration, particularly at low concentrations, is still not fully understood. To address this issue, we used a packed bed electrochemical reactor to synthesize seven different electrochemical graphite oxide (EGO) products in 2–16 M sulfuric acid. Detailed XRD, XPS, Raman, conductivity and optical microscopy analysis of the products was carried out. We found dilute acid (<10 M) graphite oxides were less crystalline and less oxidized than those produced in stronger acids. The oxygen evolution reaction at the graphite surface appears to affect the structural changes, oxidation mechanism, and electrochemical corrosion of the anode. EGO conductivity is also strongly affected by the electrolyte’s acidity. We show that well oxidized, yet reasonably conductive, single layer graphene oxide can be produced from 7.1 M acid. These results broaden our understanding of graphite electrochemistry and will serve to inform future electrochemical graphene synthesis efforts.

DTAB micelle formation in ionic liquid/water mixtures is determined by ionic liquid cation structure

HypothesisThe high CMCs and low aggregation numbers of ionic micelles in the extreme electrolyte environment of ionic liquids (ILs) seem to be at odds with the effect of dilute aqueous electrolytes, which lower CMCs and promote elongated micelles. We hypothesise that the driving force for micellisation in ILs is determined by their underlying amphiphilic nanostructure, and that this can be controlled by mixing with water. ExperimentsCMCs and micelle sizes of dodecyltrimethylammonium bromide (DTAB) are determined in mixed solvents comprising water and the ionic liquids ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and propylammonium nitrate (PAN) over a wide composition range. Their behaviour is compared with aqueous electrolytes up to their solubility limit. CMCs are determined by a variety of techniques, and their relative strengths critically evaluated. Micelle morphology is determined by small-angle neutron scattering. FindingsIn water-rich mixtures, ILs do behave like simple electrolytes. Counterion binding dominates, both lowering the aqueous CMC and favouring a sphere-rod transition. However, even at modest concentrations, IL cations become incorporated into the micelle, causing the CMC to pass through a minimum, and arresting the sphere-rod transition. The efficiency of the cation depends on its amphiphilicity. As the IL content increases further, its role as a component of the bulk solvent becomes dominant: Only here does IL nanostructure influence micellization, as it increases alkyl chain solubility (EAN, PAN) and hence raises the CMC.

Peering through the Stability Window

In the December issue of Chem, Feng Pan and colleagues perform a detailed study of a super-concentrated aqueous lithium-ion battery electrolyte that more than doubles the intrinsic electrochemical window of water (2.55 V compared to 1.23 V). From molecular dynamics simulations, combined with a suite of extensive characterization methods and electrochemical measurements, the authors correlate the local atomic structure of aqueous LiNO3 electrolyte to its observed enhanced electrochemical stability. They ultimately determine that the increased energy barrier for bulk water desolvation due to super-concentration is primarily responsible.

Separator-Free Gel Electrolytes Based on Water-in-Salt Solutions

Water-in-salt electrolytes (WiSE) are one of a growing family of water-based lithium ion electrolytes that use high concentrations of non-hydrolyzable lithium salts such as LiTFSI. WiSE are non-flammable and have a much wider electrochemical stability window than typical dilute aqueous electrolytes through two anion-based mechanisms: the formation of a passive anode layer through the decomposition of the TFSI- anion and exclusion of water by the anion from the cathode surface. This work details the formation of a polymer gel network containing WiSE as its liquid phase using commercially available polymer and crosslinker components. WiSE gel electrolytes are thought to be a natural evolution of the water-in-salt concept, as a WiSE gel should allow for WiSE-containing batteries to decouple themselves from typical cylindrical and prismatic battery form factors. Cycling performance and mechanical properties of the gel electrolytes will be discussed, along with fundamental investigations into the electrode-electrolyte interface and the minimization of charge-transfer resistance. Additionally, the kinetics of gel reactions in WiSE as a solvent will be discussed.

Experimental Studies of the Capacitive-Potential Relationship for Ionic Liquid – Electrode Interfaces

The electrochemical double layer (EDL) in ionic liquid (IL) solvents plays an important role in myriad energy storage and conversion applications. A clear description of molecular organization within this interfacial region requires a multipronged approach. This presentation will discuss studies of the IL – electrode interface using AC voltammetry, electrochemical impedance spectroscopy (EIS), and vibrational spectroscopy covering slow (re)orientation / movement of ILs within the EDL as it relates to capacitive hysteresis, and make critical comparisons of capacitance vs. potential relationships to theoretical predictions. ILs at an electrified interface have been shown to exhibit capacitive hysteresis depending on the applied potential scan direction (e.g. cathodic or anodic), the extent of which depends on electrode surface material and the measurement technique employed.1-3 Recent work in our group examined several aprotic and protic ILs on metallic, carbon, and semiconducting electrodes demonstrating that capacitive hysteresis is not necessarily a uniform feature of the IL electrified interface. Our studies use large amplitude Fourier transformed alternating current (FT-AC) voltammetry, which demonstrates little (<10 %) hysteresis in the resulting capacitance vs. potential curve respective to the electrochemical potential scan direction. This highlights the benefits of utilizing large amplitude FT-AC voltammetry to probe capacitive currents in ILs. Additional work employs vibrational spectroscopy to study the evolution of the IL EDL under an applied potential. This data provides a vantage point to discuss capacitive hysteresis and how it could relate to molecular (re)orientation in ILs. Our preliminary data show that the IL vibrational profile shows small changes when responding to a potential step of 2 V within the EDL potential window. However, the vibrational profile exhibits noticeable hysteresis in IR peak intensities when undergoing positive- and negative-going applied potential scan directions further demonstrating the slow response times in ILs. It has been suggested that slow (re)orientation of ILs within the EDL could be the mechanism behind the observed hysteresis and that these processes are likely a common feature of the IL – electrode interface.4-5 Depending on the electrochemical technique employed the effective ‘scan rate’ can vary from less than one minute (e.g. FT-AC voltammetry) to several hours (e.g. EIS) when acquiring capacitance data over a range of potentials. Our work using broadband EIS to study the 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate – polycrystalline gold interface demonstrates over a 2 V window that capacitive hysteresis is absent based on several potential scan directions employed compared to Au(111) which exhibits this phenomenon.1 Further this work spanning from 1 MHz to 0.01 Hz suggests the slow process (visible in the complex capacitance plane plot as an arc in the low frequency region) is also absent on the polycrystalline gold electrode compared to the Au(111) single-crystal surface.1 Our work with large amplitude FT-AC voltammetry suggests three possible scenarios to explain contradictions with previous reports: (1) large amplitude FT-AC voltammetry scans the potential window cycle fast enough (~100 mV s-1) relative to slow movement of IL EDL ions to not capture any hysteresis processes, (2) the large perturbation amplitude employed overcomes the energetic barrier for interfacial IL ion movement to exhibit hysteresis processes or (3) capacitive hysteresis and slow (re)orientation are not necessarily common features of the IL – electrode interface. Capacitance curvature in published scientific literature show a wide range of results, even for similar electrochemical systems. Additionally, it has proved challenging to unite theoretical predictions with experimental observations for IL capacitance responses. ILs are concentrated in the sense that they contain no molecular solvent and one viewpoint is they are composed entirely of dissociated ions. However, recent reports challenge this view to suggest they are not well dissociated and behave as dilute electrolytes. Our work examining the 1-butyl-3-methylimidazolum tetrafluoroborate – polycrystalline gold interface corroborates the latter viewpoint. Furthermore, our studies of the IL – semiconductor electrode interface demonstrate a capacitance response analogous to dilute aqueous electrolytes at a semiconductor electrode. Specifically, we report relatively featureless, ‘rising’ capacitance curvature that shows minimal dependence on the nature of the IL (i.e. protic vs. aprotic). Capacitance-potential curvature suggests at the ensemble behavior of the EDL but coupling potential control with spectroscopy can provide a powerful method of probing interfacial chemistry. It is anticipated that this presentation will bring to light some basic questions of IL behavior at an electrified interface.