Mechanistic Study of Controlled Zinc Electrodeposition Behaviors Facilitated by Nanoscale Electrolyte Additives at the Electrode Interface.

Nanoparticle organic hybrid materials (NOHMs) are liquid-like materials composed of an inorganic core to which a polymeric canopy is ionically tethered. NOHMs have unique properties including negligible vapor pressure, high oxidative thermal stability, and the ability to bind to reactive species of interest due to the tunability of their polymeric canopy. This makes them promising multifunctional materials for a wide range of energy and environmental technologies, including electrolyte additives for electrochemical energy storage (e.g., flow batteries) and the electrochemical conversion of CO2 to chemicals and fuels. Due to their unique transport behaviors in fluid systems, an understanding of the near-electrode surface behavior of NOHMs in electrolyte solutions and their effect on electrochemical reactions is still lacking. In this work, the complexation of zinc (Zn) by NOHMs with an ionically tethered polyetheramine canopy (HPE) (NOHM-I-HPE) was studied using attenuated total reflectance Fourier transform infrared and Carbon-13 nuclear magnetic resonance spectroscopy. Additionally, various electrochemical techniques were employed to discern the role of NOHM-I-HPE during zinc electrodeposition, and the results were compared to those of the electrochemical system containing untethered HPE polymers. Our findings confirmed that NOHM-I-HPE and HPE reversibly complex zinc in the aqueous electrolyte. NOHM-I-HPE and HPE were found to block some of the electrode active sites, reducing the overall current density during electrodeposition, while facilitating the formation of smooth zinc deposits, as revealed by surface imaging and diffraction techniques. Observed variations in the current density responses and the degree of passivation created by the NOHM-I-HPE and HPE adsorbed on the electrode surface revealed that their different packing behaviors at the electrode-electrolyte interface influence the zinc deposition mechanism. The presence of the nanoparticle and ordering offered by the NOHMs as well as the structured conformation of the polymeric canopy allowed the formation of void spaces and free volumes for enhanced transport behaviors. These findings provided insights into how structured electrolyte additives such as NOHMs can allow for advancements in electrolyte design for controlled deposition of metal species from energy-dense electrolytes or for other electrochemical reactions.

Microemulsions as Emerging Electrolytes: The Correlation of Structure to Electrochemical Response.

We describe the structural studies of microemulsions (µEs) prepared from water, toluene, butanol, and polysorbate 20 (PS20) that we recently used as electrolytes. Small-angle neutron scattering was used to monitor the development of the bicontinuous system as a function of the water-to-surfactant mass ratio on a constant oil-to-surfactant dilution line, revealing how the domain size, correlation length, amphiphilicity factor, and bending moduli change with composition. Kratky and Porod analyses are also employed, providing further structural detail of the scattering domains. We demonstrate that controlling the water-to-surfactant ratio with a constant oil-to-surfactant dilution affects the bicontinuous phase, reveals a sizeable compositional region with structural similarities, and provides insight into the correlation of structure to physical properties. Voltammetric results are presented to examine how the evolution of the bicontinuous structure formed in a µE prepared from water, toluene, butanol, and PS20 contributes to the electrochemical response. These findings, therefore, provide essential information that will guide the formulation of µEs as electrolytes for energy storage.

Nanoscale Hybrid Electrolytes with Viscosity Controlled Using Ionic Stimulus for Electrochemical Energy Conversion and Storage.

As renewable energy is rapidly integrated into the grid, the challenge has become storing intermittent renewable electricity. Technologies including flow batteries and CO2 conversion to dense energy carriers are promising storage options for renewable electricity. To achieve this technological advancement, the development of next generation electrolyte materials that can increase the energy density of flow batteries and combine CO2 capture and conversion is desired. Liquid-like nanoparticle organic hybrid materials (NOHMs) composed of an inorganic core with a tethered polymeric canopy (e.g., polyetheramine (HPE)) have a capability to bind chemical species of interest including CO2 and redox-active species. In this study, the unique response of NOHM-I-HPE-based electrolytes to salt addition was investigated, including the effects on solution viscosity and structural configurations of the polymeric canopy, impacting transport behaviors. The addition of 0.1 M NaCl drastically lowered the viscosity of NOHM-based electrolytes by up to 90%, reduced the hydrodynamic diameter of NOHM-I-HPE, and increased its self-diffusion coefficient, while the ionic strength did not alter the behaviors of untethered HPE. This study is the first to fundamentally discern the changes in polymer configurations of NOHMs induced by salt addition and provides a comprehensive understanding of the effect of ionic stimulus on their bulk transport properties and local dynamics. These insights could be ultimately employed to tailor transport properties for a range of electrochemical applications.

Microemulsions: Breakthrough Electrolytes for Redox Flow Batteries.

Aqueous and non-aqueous redox flow batteries (RFBs) have limited energy and current densities, respectively, due to the nature of the electrolytes. New approaches to electrolyte design are needed to improve the performance of RFBs. In this work, we combined a highly conductive aqueous phase and an organic redox-active phase in a microemulsion to formulate a novel RFB electrolyte. As a proof-of-concept, we demonstrate an RFB using this microemulsion electrolyte with maximum current density of 17.5 mA·cm-2 with a 0.19 M posolyte and 0.09 M negolyte at a flow rate of only ∼2.5 ml·min-1, comparable to early vanadium electrolyte RFBs at similar flow rates on a per molar basis. The novel active negolyte component is an inexpensive oil-soluble vitamin (K3). By combining aqueous and organic phases, the solvent potential window and energy density may be increased without sacrificing current density and new redox couples may be accessed. Microemulsion electrolytes show great promise for improved performance and increased energy densities in aqueous RFBs but the path forward is complex. We end with discussion of areas that need work to achieve the potential of these electrolytes.

Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents.

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Dynamics of Glyceline and Interactions of Constituents: A Multitechnique NMR Study.

The dynamics of the organic components of the deep eutectic solvent (DES) glyceline are analyzed using an array of complementary nuclear magnetic resonance (NMR) methods. Fast-field cycling 1H relaxometry, pulsed field gradient diffusion, nuclear overhauser effect spectroscopy (NOESY), 13C NMR relaxation, and pressure-dependent NMR experiments are deployed to sample a range of frequencies and modes of motion of the glycerol and choline components of the DES. Generally, translational and rotational diffusion of glycerol are more rapid than those of choline while short-range rotational motions observed from 13C relaxation indicate slow local motion of glycerol at low choline chloride (ChCl) content. The rates of glycerol and choline local motions become more similar at higher ChCl. This result taken together with pressure-dependent NMR studies show that the addition of ChCl makes it easier to disrupt glycerol packing. Finally, a relatively slow hydroxyl H-exchange process between glycerol and choline protons is deduced from the data. Consistent with this, NOESY results indicate relatively little direct H-bonding between glycerol and choline. These results suggest that the glycerol H-bonding network is disrupted as choline is added, but primarily in regions where there is intimate mixing of the two components. Thus, the local dynamics of most of the glycerol resembles that of pure glycerol until substantial choline chloride is present.

Dynamic Mixing Behaviors of Ionically Tethered Polymer Canopy of Nanoscale Hybrid Materials in Fluids of Varying Physical and Chemical Properties.

An emerging area of sustainable energy and environmental research is focused on the development of novel electrolytes that can increase the solubility of target species and improve subsequent reaction performance. Electrolytes with chemical and structural tunability have allowed for significant advancements in flow batteries and CO2 conversion integrated with CO2 capture. Liquid-like nanoparticle organic hybrid materials (NOHMs) are nanoscale fluids that are composed of inorganic nanocores and an ionically tethered polymeric canopy. NOHMs have been shown to exhibit enhanced conductivity making them promising for electrolyte applications, though they are often challenged by high viscosity in the neat state. In this study, a series of binary mixtures of NOHM-I-HPE with five different secondary fluids, water, chloroform, toluene, acetonitrile, and ethyl acetate, were prepared to reduce the fluid viscosity and investigate the effects of secondary fluid properties (e.g., hydrogen bonding ability, polarity, and molar volume) on their transport behaviors, including viscosity and diffusivity. Our results revealed that the molecular ratio of secondary fluid to the ether groups of Jeffamine M2070 (λSF) was able to describe the effect that secondary fluid has on transport properties. Our findings also suggest that in solution, the Jeffamine M2070 molecules exist in different nanoscale environments, where some are more strongly associated with the nanoparticle surface than others, and the conformation of the polymer canopy was dependent on the secondary fluid. This understanding of the polymer conformation in NOHMs can allow for the better design of an electrolyte capable of capturing and releasing small gaseous or ionic species.

Hierarchical Lignin-Based Carbon Matrix and Carbon Dot Composite Electrodes for High-Performance Supercapacitors.

This work adopts an efficient chemical-wet method to build a three-dimensional (3D) carbon composite as an electrode material for high-performance supercapacitors (SCs). Carbon dots (CDs), prepared by thermal pyrolysis of citric acid and urea under microwaves at 280 °C, are homogeneously coated onto lignin-based activated carbons (ACs), thus forming the 3D composites possessing an interior surface decorated with CD binding sites. Benefiting from the hydrophilicity and ultrafine size of CDs, the affinity of the electrode surface toward aqueous electrolytes is significantly improved with the addition of CDs, leading to the enhanced effective surface area (i.e., abundant electroactive sites) and a decreased ionic diffusion path. The capacitance of the SCs is improved from 125.8 to 301.7 F g-1 with CD addition. The SC with CD addition possesses improved cycle stability with a coulombic efficiency around 100% after 3000 cycles. After cycling, the ion diffusion coefficient of the CD@AC-11 electrode is enhanced by 25.5 times as compared to that of the pristine AC one. This unique and robust carbon framework can be utilized for engineering the desired pore structure and micropore/mesopore fraction within the AC electrodes. This strategy of CD@AC electrodes demonstrates a promising route for using renewable porous carbon materials in advanced energy-storage devices.

Deep Eutectic Solvents: A Review of Fundamentals and Applications.

Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure-property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Electron Transfer in Microemulsion-Based Electrolytes.

The use of flowing electrochemical reactors, for example, in redox flow batteries and in various electrosynthesis processes, is increasing. This technology has the potential to be of central significance in the increased deployment of renewable electricity for carbon-neutral processes. A key element of optimizing efficiency of electrochemical reactors is the combination of high solution conductivity and reagent solubility. Here, we show a substantial rate of charge transfer for an electrochemical reaction occurring in a microemulsion containing electroactive material is loaded inside the nonpolar (toluene) subphase of the microemulsion. The measured rate constant translates to an exchange current density comparable to that in redox flow batteries. The rate could be controlled by the surfactant, which maintains partitioning of reactants and products by forming an interfacial region with ions in the aqueous phase in close proximity. The hypothesized mechanism is evocative of membrane-bound enzymatic reactions. Achieving sufficient rates of electrochemical reaction is the product of an effort designed to establish a reaction condition that meets the requirements of electrochemical reactors using microemulsions to realize a separation of conducting and reactive elements of the solution, opening a door to the broad use of microemulsions to effect controlled electrochemical reactions as steps in more complex processes.

Chemically Cross-Linked Cellulose Nanocrystal Aerogels for Effective Removal of Cation Dye.

In this study, porous aerogels were prepared by directional freeze-drying via cross-linking cellulose nanocrystals (CNCs) with poly(methyl vinyl ether-co-maleic acid) (PMVEMA) and poly(ethylene glycol) (PEG). The thermal properties and physical adsorption performance toward cation methylene blue dye of the obtained CNC aerogels were investigated. The maximum degradation temperature was increased from 324°C of CNCs to 355°C of cross-linked CNC aerogels. The dye adsorption isotherm results showed that the maximum methylene blue adsorption capacity of CNC aerogels was 116.2 mg g-1, according to the Langmuir model, which was mainly due to the electrostatic attractions between negatively charged carboxyl groups or sulfonate groups on the CNC aerogles and cation MB molecules. The reusability test showed that the CNC aerogels contained the same dye adsorption performance in five adsorption/desorption cycles. Overall, this study described an ideal alternative for water purification with high dye adsorption capacity and enhanced physical performance.

Tunable Synthesis of Hollow Metal-Nitrogen-Carbon Capsules for Efficient Oxygen Reduction Catalysis in Proton Exchange Membrane Fuel Cells.

Atomically dispersed metal catalysts anchored on nitrogen-doped (N-doped) carbons demand attention due to their superior catalytic activity relative to that of metal nanoparticle catalysts in energy storage and conversion processes. Herein, we introduce a simple and versatile strategy for the synthesis of hollow N-doped carbon capsules that contain one or more atomically dispersed metals (denoted as H-M-Nx-C and H-Mmix-Nx-C, respectively, where M = Fe, Co, or Ni). This method utilizes the pyrolysis of nanostructured core-shell precursors produced by coating a zeolitic imidazolate framework core with a metal-tannic acid (M-TA) coordination polymer shell (containing up to three different metal cations). Pyrolysis of these core-shell precursors affords hollow N-doped carbon capsules containing monometal sites (e.g., Fe-Nx, CoNx, or Ni-Nx) or multimetal sites (Fe/Co-Nx, Fe/Ni-Nx, Co/Ni-Nx, or Fe/Co/Ni-Nx). This inventory allowed exploration of the relationship between catalyst composition and electrochemical activity for the oxygen reduction reaction (ORR) in acidic solution. H-Fe-Nx-C, H-Co-Nx-C, H-FeCo-Nx-C, H-FeNi-Nx-C, and H-FeCoNi-Nx-C were particularly efficient ORR catalysts in acidic solution. Furthermore, the H-Fe-Nx-C catalyst exhibited outstanding initial performance when applied as a cathode material in a proton exchange membrane fuel cell. The synthetic methodology introduced here thus provides a convenient route for developing next-generation catalysts based on earth-abundant components.

Rechargeable redox flow batteries: flow fields, stacks and design considerations.

Rechargeable redox flow batteries are being developed for medium and large-scale stationary energy storage applications. Flow batteries could play a significant role in maintaining the stability of the electrical grid in conjunction with intermittent renewable energy. However, they are significantly different from conventional batteries in operating principle. Recent contributions on flow batteries have addressed various aspects, including electrolyte, electrode, membrane, cell design, etc. In this review, we focus on the less-discussed practical aspects of devices, such as flow fields, stack and design considerations for developing high performance large-scale flow batteries. Finally, we provide suggestions for further studies on developing advanced flow batteries and large-scale flow battery stacks.

Transport Properties of Perfluorosulfonate Membranes Ion Exchanged with Cations.

In this work, the properties of univalent, that is, Li+, Na+, NH4+, and TEA+ form perfluorosulfonate (PFSA) membranes are studied and compared to the properties of H+ form materials. Properties of these polymer membranes including water uptake, density and conductivity, were investigated for membranes exposed to various water activity levels. The water uptake by the membranes decreased in the order H+ > Li+ > Na+ > NH4+ > TEA+, the same order as the hydration enthalpy (absolute values) of cations. Conductivity values did not strictly follow this order, indicating the importance of different factors besides the hydration level. The partial molar volume of water is derived from the density data as a function of water content for the various membrane forms. This provides further insight into the water, cation, and polymer interactions. Factors that contribute to the conductivity of these membranes include the size of cations, the electrostatic attraction between cations and sulfonate group, and the ion-dipole and hydrogen bonding interactions between cations and water. NH4+ transport is surprisingly high given the low water uptake in NH4+ form membranes. We attribute this to the ability of this ion to develop hydrogen bonded structures that helps to overcome electrostatic interactions with sulfonates. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was used to measure the diffusion coefficient of water in the membranes. FT-IR spectroscopy is employed to probe cation interactions with water and sulfonate sites in the polymer. Overall, the results reflect a competition between the strong electrostatic interaction between cation and sulfonate versus hydration and hydrogen bonding which vary with cation type.

Black Anatase Formation by Annealing of Amorphous Nanoparticles and the Role of the Ti2O3 Shell in Self-Organized Crystallization by Particle Attachment.

We use amorphous titania nanoparticle networks produced by pulsed laser vaporization at room temperature as a model system for understanding the mechanism of formation of black titania. Here, we characterize the transformation of amorphous nanoparticles by annealing in pure Ar at 400 °C, the lowest temperature at which black titania was observed. Atomic resolution electron microscopy methods and electron energy loss spectroscopy show that the onset of crystallization occurs by nucleation of an anatase core that is surrounded by an amorphous Ti2O3 shell. The formation of the metastable anatase core before the thermodynamically stable rutile phase occurs according to the Ostwald phase rule. In the second stage the particle size increases by coalescence of already crystallized particles by a self-organized mechanism of crystallization by particle attachment. We show that the Ti2O3 shell plays a critical role in both black titania transformation and functionality. At 400 °C, Ti2O3 hinders the agglomeration of neighboring particles to maintain a high surface-to-volume ratio that is beneficial for enhanced photocatalytic activity. In agreement with previous results, the thin Ti2O3 surface layer acts as a narrow bandgap semiconductor in concert with surface defects to enhance the photocatalytic activity. Our results demonstrate that crystallization by particle attachment can be a highly effective mechanism for optimizing photocatalytic efficiency by controlling the phase, composition, and particle size distribution in a wide range of self-doped defective TiO2 architectures simply by varying the annealing conditions of amorphous nanoparticles.

Segregated Pt on Pd nanotubes for enhanced oxygen reduction activity in alkaline electrolyte.

Nanoscaled Pt domains were integrated with Pd nanotubes via vapor deposition to yield a highly active electrocatalyst for the oxygen reduction reaction (ORR) in alkaline media. The surface-area-normalized ORR activity of these bi-metallic Pt-on-Pd nanotubes (PtPdNTs) was nearly 6× the corresponding carbon-supported Pt nanoparticle (Pt/C) activity at 0.9 V vs. RHE (1.5 vs. 0.24 mA cmmetal(-2), respectively). Furthermore, the high specific activity of the PtPdNTs was achieved without sacrificing mass-normalized activity, which is more than twice that of Pt/C (0.333 A mgPtPdNT(-1)vs. 0.141 A mgPt/C(-1)) and also greater than that of Pd/C (0.221 A mgPd/C(-1)). We attribute the enhancements in specific and mass activity to modifications of the segregated Pt electronic structure and to nanoscale porosity, respectively.

Vapor synthesis and thermal modification of supportless platinum-ruthenium nanotubes and application as methanol electrooxidation catalysts.

Metallic, mixed-phase, and alloyed bimetallic Pt-Ru nanotubes were synthesized by a novel route based on the sublimation of metal acetylacetonate precursors and their subsequent vapor deposition within anodic alumina templates. Nanotube architectures were tuned by thermal annealing treatments. As-synthesized nanotubes are composed of nanoparticulate, metallic platinum and hydrous ruthenium oxide whose respective thicknesses depend on the sample chemical composition. The Pt-decorated, hydrous Ru oxide nanotubes may be thermally annealed to promote a series of chemical and physical changes to the nanotube structures, including alloy formation, crystallite growth, and morphological evolution. Annealed Pt-Ru alloy nanotubes and their as-synthesized analogs demonstrate relatively high specific activities for the oxidation of methanol. As-synthesized, mixed-phase Pt-Ru nanotubes (0.39 mA/cm(2)) and metallic alloyed Pt64Ru36NTs (0.33 mA/cm(2)) have considerably higher area-normalized activities than PtRu black (0.22 mA/cm(2)) at 0.65 V vs RHE.

Nanostructure enhanced ionic transport in fullerene reinforced solid polymer electrolytes.

Solid polymer electrolytes, such as polyethylene oxide (PEO) based systems, have the potential to replace liquid electrolytes in secondary lithium batteries with flexible, safe, and mechanically robust designs. Previously reported PEO nanocomposite electrolytes routinely use metal oxide nanoparticles that are often 5-10 nm in diameter or larger. The mechanism of those oxide particle-based polymer nanocomposite electrolytes is under debate and the ion transport performance of these systems is still to be improved. Herein we report a 6-fold ion conductivity enhancement in PEO/lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)-based solid electrolytes upon the addition of fullerene derivatives. The observed conductivity improvement correlates with nanometer-scale fullerene crystallite formation, reduced crystallinities of both the (PEO)6:LiTFSI phase and pure PEO, as well as a significantly larger PEO free volume. This improved performance is further interpreted by enhanced decoupling between ion transport and polymer segmental motion, as well as optimized permittivity and conductivity in bulk and grain boundaries. This study suggests that nanoparticle induced morphological changes, in a system with fullerene nanoparticles and no Lewis acidic sites, play critical roles in their ion conductivity enhancement. The marriage of fullerene derivatives and solid polymer electrolytes opens up significant opportunities in designing next-generation solid polymer electrolytes with improved performance.

Ionic conductivity and glass transition of phosphoric acids.

Here we report the low-temperature dielectric and viscoelastic properties of phosphoric acids in the range of H2O:P2O5 1.5-5. Both dielectric and viscosity measurements allow us to determine the glass-transition temperatures of phosphoric acids. The obtained glass-transition temperatures are in good agreement with previous differential scanning calorimetric measurements. Moreover, our analysis reveals moderate decoupling of ionic conductivity from structural relaxation in the vicinity of the glass transition.