Water in the Electrical Double Layer of Ionic Liquids on Graphene.

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILs─with ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectively─on electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene's potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene's quantum capacitance (CQ) to the total interfacial capacitance (Cint) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (CIL ∼ 80 µF cm-2), so that CQ governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (CIL ∼ 6-9 µF cm-2), which governs Cint already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes.

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO2 nanoparticles their phase transformation upon Mg2+ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Nanoheterogeneity of LiTFSI Solutions Transitions Close to a Surface and with Concentration.

Water-in-salt (WIS) electrolytes composed of 21 m LiTFSI have recently emerged as a safe and environmentally friendly alternative to conventional organic electrolytes in Li-ion batteries. Several studies have emphasized the relation between the high conductivity of WIS electrolytes and their nanoscale structure. Combining force measurements with a surface forces apparatus and atomic force microscopy, this study describes the nanoheterogeneity of LiTFSI solutions as a function of concentration and distance from a negatively charged (mica) surface. We report various nanostructures coexisting in the WIS electrolyte, whose size increases with concentration and is influenced by the proximity of the mica surface. Two key concentration thresholds are identified, beyond which a transition of behavior is observed. The careful scrutinization on the concentration-dependent nanostructures lays groundwork for designing novel electrolytes in future energy storage devices.

Effects of Superparamagnetic Iron Nanoparticles on Electrocatalysts for the Reduction of Oxygen.

It is of great research interest to understand the nanostructures contributing to the activity observed in the reduction of oxygen by non-platinum group metal (PGM) electrocatalysts in acidic media. Iron- and nitrogen-containing carbon networks are often the most studied structures, among which single-atom iron-coordinated nitrogen (FeNx) moieties have often been proposed to be the structures leading to the high activity in these non-PGM electrocatalysts. Iron nanoparticles embedded within a carbon support are also formed under certain conditions as a result of the synthetic processes in making non-PGM electrocatalysts. In this study, we present a study to understand the oxygen reduction reaction (ORR) activity of prepared iron- and nitrogen-containing non-PGM electrocatalysts obtained through the pyrolysis of metal-organic framework (MOF) precursors. We studied the structure-property relationship among nanostructures made from the MOF precursor ZIF-8 under different pyrolysis conditions. Density functional theory calculations were used to explain the effect of structural moieties on the ORR activity. Our results suggest that iron-coordinated C-N structures and iron nanoparticles act synergistically to catalyze the ORR.

Oriented LiMn2O4 Particle Fracture from Delithiation-Driven Surface Stress.

The insertion and removal of Li+ ions into Li-ion battery electrodes can lead to severe mechanical fatigue because of the repeated expansion and compression of the host lattice during electrochemical cycling. In particular, the lithium manganese oxide spinel (LiMn2O4, LMO) experiences a significant surface stress contribution to electrode chemomechanics upon delithiation that is asynchronous with the potentials where bulk phase transitions occur. In this work, we probe the stress evolution and resulting mechanical fracture from LMO delithation using an integrated approach consisting of cyclic voltammetry, electron microscopy, and density functional theory (DFT) calculations. High-rate electrochemical cycling is used to exacerbate the mechanical deficiencies of the LMO electrode and demonstrates that mechanical degradation leads to slowing of delithiation and lithiation kinetics. These observations are further supported through the identification of significant fracturing in LMO using scanning electron microscopy. DFT calculations are used to model the mechanical response of LMO surfaces to electrochemical delithiation and suggest that particle fracture is unlikely in the [001] direction because of tensile stresses from delithiation near the (001) surface. Transmission electron microscopy and electron backscatter diffraction of the as-cycled LMO particles further support the computational analyses, indicating that particle fracture instead tends to preferentially occur along the {111} planes. This joint computational and experimental analysis provides molecular-level details of the chemomechanical response of the LMO electrode to electrochemical delithiation and how surface stresses may lead to particle fracture in Li-ion battery electrodes.

Conversion of Co Nanoparticles to CoS in Metal-Organic Framework-Derived Porous Carbon during Cycling Facilitates Na2S Reactivity in a Na-S Battery.

Room-temperature sodium-sulfur batteries have attracted wide interest due to their high energy density and high natural abundance. Polysulfide dissolution and irreversible Na2S conversion are challenges to achieving high battery performance. Herein, we utilize a metal-organic framework-derived Co-containing nitrogen-doped porous carbon (CoNC) as a catalytic sulfur cathode host. A concentrated sodium electrolyte based on sodium bis(fluorosulfonyl)imide, dimethoxyethane, and bis(2,2,2-trifluoroethyl) ether is used to mitigate polysulfide dissolution. We tune the amount of Co present in the CoNC carbon host by acid washing. Significant improvement in reversible sulfur conversion and capacity retention is observed with a higher Co content in CoNC, with 600 mAh g-1 and 77% capacity retention for CoNC and 261 mAh g-1 and 56% capacity retention for acid-washed CoNC at cycle 50 at 80 mAh g-1. Post-mortem X-ray photoelectron spectroscopy, transmission electron microscopy, and selected area electron diffraction suggest that CoS is formed during cycling in place of Co nanoparticles and CoN4 sites. Raman spectroscopy suggests that CoS exhibits a catalytic effect on the oxidation of Na2S. Our findings provide insights into understanding the role Co-based catalysts play in sulfur batteries.

Covalent Ag-C Bonding Contacts from Unprotected Terminal Acetylenes for Molecular Junctions.

Robust molecule-metal linkages are essential for developing high-performance and air-stable devices for molecular and organic electronics. In this work, we report a facile method for forming robust and covalent bonding contacts between unprotected terminal acetylenes and metal (Ag) interfaces. Using this approach, we study the charge transport properties of conjugated oligophenylenes with covalent metal-carbon contacts to silver electrodes formed from unprotected terminal acetylene anchors. We performed single molecule charge transport experiments and molecular simulations on a series of arylacetylenes using gold and silver electrodes. Our results show that molecular junctions on silver electrodes spontaneously form silver-carbynyl carbon (Ag-C) contacts, resulting in a nearly 10-fold increase in conductance compared to the same molecules on gold electrodes. Overall, this work presents a simple, new electrode-anchor pair that reliably forms molecular junctions with stable and robust contacts for molecular electronics.

Preparation of Nonprecious Metal Electrocatalysts for the Reduction of Oxygen Using a Low-Temperature Sacrificial Metal.

Non-platinum group metal (non-PGM) electrocatalysts for the oxygen reduction reaction (ORR) are generally composed of iron, nitrogen, and carbon synthesized through high-temperature pyrolysis. Among the various types of precursors, metal-organic frameworks (MOFs), zeolitic imidazolate framework (ZIF)-8 in particular, have often been used in the synthesis. The pyrolysis of ZIF-8 precursor relies on the use of Zn as a sacrificial metal (SM), and the optimal processing temperatures often exceed 1000 °C to generate active non-PGM catalysts. The high pyrolysis temperature tends to result in heterogeneous active moieties ranging from Fe single atoms to nanoparticles. In this study, we present the synthesis of non-PGM catalysts using Cd as the sacrificial metal instead of Zn. By using Cd, we were able to generate active non-PGM electrocatalysts from the MOF precursors at a low pyrolysis temperature of 750 °C, which helps preserve the single atomic iron active sites.

Origin of Enhanced Cyclability in Covalently Modified LiMn1.5Ni0.5O4 Cathodes.

High-voltage lithium-ion cathode materials exhibit exceptional energy densities; however, rapid capacity fade during cell cycling prohibits their widespread utilization. Surface modification of cathode-active materials by organic self-assembled monolayers (SAMs) has emerged as an approach to improve the longevity of high-voltage electrodes; however, the surface chemistry at the electrode/electrolyte interphase and its dependence on monolayer structure remains unclear. Herein, we investigate the interplay between monolayer structure, electrochemical performance, and surface chemistry of high-voltage LiMn1.5Ni0.5O4 (LMNO) electrodes by the application of silane-based SAMs of variable length and chemical composition. We demonstrate that the application of both hydrophobic and hydrophilic monolayers results in improved galvanostatic capacity retention relative to unmodified LMNO. The extent of this improvement is tied to the structure of the monolayer with fluorinated alkyl-silanes exhibiting the greatest overall capacity retention, above 96% after 100 charge/discharge cycles. Postmortem surface analysis reveals that the presence of the monolayer enhances the deposition of LiF at the electrode surface during cell cycling and that the total surface concentration correlates with the overall improvements in capacity retention. We propose that the enhanced deposition of highly insulating LiF increases the anodic stability of the interphase, contributing to the improved galvanostatic performance of modified electrodes. Moreover, this work demonstrates that the modification of the electrode surface by the selection of an appropriate monolayer is an effective approach to tune the properties and behavior of the electrode/electrolyte interphase formed during battery operation.

Chain length variation to probe the mechanism of accelerator additives in copper electrodeposition.

We evaluate the effect of chain length for a series of alkyl sulfonic acid additives on Cu electrodeposition by using a combination of electrochemical and Raman spectroscopic methods. Rotating disk linear sweep voltammetry revealed the influence of these additives on the bulk concentration of Cu+ and on the exchange current densities of the reduction of Cu2+/Cu+ and Cu+/Cu. We then used in situ shell-isolated, nanoparticle-enhanced Raman spectroscopy to correlate the additives' effects on deposition kinetics with their chemical structures at the electrode surface. The combination of these methods suggests that effective Cu electrodeposition acceleration processes require: (1) direct tethering of mercaptoalkylsulfonate species to the electrode, (2) partial desolvation of Cu2+ by the sulfonate group to minimize its solvent reorganization energy, and (3) stabilization of Cu+ adjacent to the electrode surface by addition of halide. The model provides support for recently proposed theories for the electrodeposition of metals where charge is carried across the electrode interface by the cation, rather than the electron.

Understanding Ca Electrodeposition and Speciation Processes in Nonaqueous Electrolytes for Next-Generation Ca-Ion Batteries.

Electrochemical and analytical techniques were utilized to study Ca electrodeposition in nonaqueous electrolytes. Linear sweep voltammograms obtained at Au and Pt ultramicroelectrodes (UMEs) exhibit an inverse dependence between current density and scan rate, indicative of the presence of a chemical reaction step in a chemical-electrochemical (CE) deposition process. However, the magnitude of change in current density as a function of scan rate is larger at the Au UME than at the Pt UME. COMSOL simulation reveals that the chemical reaction step rate ( kc) obtained at the Pt UME is ∼10 times faster than that at the Au UME. Field desorption ionization mass spectrometry (MS) suggests that dehydrogenation of the borohydride anions by the metal substrate is the chemical reaction step. Pt is more efficient at abstracting hydride from borohydride ions than Au, leading to larger kc. Raman spectroscopy and electrospray ionization MS data show that Ca2+ ions are strongly coordinated with tetrahydrofuran and weakly interacting with BH4- anions. Electron microscopy shows that the surface morphology of Ca electrodeposition is different between Au and Pt, with Au exhibiting a smooth deposit, while a patchier deposit is seen on Pt.

Improving Cell Resistance and Cycle Life with Solvate-Coated Thiophosphate Solid Electrolytes in Lithium Batteries.

Solid electrolytes (SEs) have become a practical option for lithium ion and lithium metal batteries because of their improved safety over commercially available ionic liquids. The most promising of the SEs are the thiophosphates, whose excellent ionic conductivities at room temperature are comparable to those of commercially utilized ionic liquids. Hybrid solid-liquid electrolytes exhibit higher ionic conductivities than their bare SE counterparts because of decreased grain boundary resistance, enhanced interfacial contact with electrodes, and decreased degradation at the interface. In this study, we add lithium bis(trifluoromethane sulfonyl)imide and a highly fluorinated ether solvate electrolyte to the surface of Li7P3S11 (LPS) and Li10GeP2S12 (LGPS) pellets and evaluate their overall cell resistance in Li-Li symmetric cells relative to their bare Li/SE/Li counterparts. Time-resolved electrochemical impedance spectroscopy shows an order of magnitude lower cell resistance for LGPS-solvate than for bare LGPS. In contrast, the LPS-solvate system exhibits a higher cell resistance than bare LPS. Scanning electron microscopy and energy dispersive X-ray spectroscopy show that LGPS allows for the total permeation of the solvate into the bulk SE. Although LPS has smaller grain sizes and higher porosity, it has a higher solubility in 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), which results in an LPS-TTE interlayer on the surface of the pellet, thereby increasing overall cell resistance. Cyclic voltammetry of the bare and hybrid SE cells shows an order of magnitude higher current density for the LGPS-solvate cell over the bare LGPS. Bare LPS shorts after two cycles, whereas the LPS-solvate cell does not short within the timeframe of the experiment (100 cycles). This study suggests that solvates can be used to improve the cell resistance and current density of SEs by altering the grain boundary structures and the interphase between electrode and electrolyte.

Solid-Liquid Lithium Electrolyte Nanocomposites Derived from Porous Molecular Cages.

We demonstrate that solid-liquid nanocomposites derived from porous organic cages are effective lithium ion electrolytes at room temperature. A solid-liquid electrolyte nanocomposite (SLEN) fabricated from a LiTFSI/DME electrolyte system and a porous organic cage exhibits ionic conductivity on the order of 1 × 10-3 S cm-1. With an experimentally measured activation barrier of 0.16 eV, this composite is characterized as a superionic conductor. Furthermore, the SLEN displays excellent oxidative stability up to 4.7 V vs Li/Li+. This simple three-component system enables the rational design of electrolytes from tunable discrete molecular architectures.

Nanoporous Copper-Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO2 to Ethylene and Ethanol.

Electrodeposition of CuAg alloy films from plating baths containing 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields high surface area catalysts for the active and selective electroreduction of CO2 to multicarbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogeneously mixed. The alloy film containing 6% Ag exhibits the best CO2 electroreduction performance, with the Faradaic efficiency for C2H4 and C2H5OH production reaching nearly 60 and 25%, respectively, at a cathode potential of just -0.7 V vs RHE and a total current density of ∼ - 300 mA/cm2. Such high levels of selectivity at high activity and low applied potential are the highest reported to date. In situ Raman and electroanalysis studies suggest the origin of the high selectivity toward C2 products to be a combined effect of the enhanced stabilization of the Cu2O overlayer and the optimal availability of the CO intermediate due to the Ag incorporated in the alloy.

Nonprecious Metal Catalysts for Oxygen Reduction in Heterogeneous Aqueous Systems.

A comprehensive review of recent advances in the field of oxygen reduction electrocatalysis utilizing nonprecious metal (NPM) catalysts is presented. Progress in the synthesis and characterization of pyrolyzed catalysts, based primarily on the transition metals Fe and Co with sources of N and C, is summarized. Several synthetic strategies to improve the catalytic activity for the oxygen reduction reaction (ORR) are highlighted. Recent work to explain the active-site structures and the ORR mechanism on pyrolyzed NPM catalysts is discussed. Additionally, the recent application of Cu-based catalysts for the ORR is reviewed. Suggestions and direction for future research to develop and understand NPM catalysts with enhanced ORR activity are provided.

The Long-Term Stability of KO2 in K-O2 Batteries.

The rechargeable K-O2 battery is recognized as a promising energy storage solution owing to its large energy density, low overpotential, and high coulombic efficiency based on the single-electron redox chemistry of potassium superoxide. However, the reactivity and long-term stability of potassium superoxide remains ambiguous in K-O2 batteries. Parasitic reactions are explored and the use of ion chromatography to quantify trace amounts of side products is demonstrated. Both quantitative titrations and differential electrochemical mass spectrometry confirm the highly reversible single-electron transfer process, with 98 % capacity attributed to the formation and decomposition of KO2 . In contrast to the Na-O2 counterparts, remarkable shelf-life is demonstrated for K-O2 batteries owing to the thermodynamic and kinetic stability of KO2 , which prevents the spontaneous disproportionation to peroxide. This work sheds light on the reversible electrochemical process of K+ +e- +O2 ↔KO2 .

Gold Nanoparticles on Polymer-Wrapped Carbon Nanotubes: An Efficient and Selective Catalyst for the Electroreduction of CO2.

Multiple approaches will be needed to reduce the atmospheric CO2 levels, which have been linked to the undesirable effects of global climate change. The electroreduction of CO2 driven by renewable energy is one approach to reduce CO2 emissions while producing chemical building blocks, but current electrocatalysts exhibit low activity and selectivity. Here, we report the structural and electrochemical characterization of a promising catalyst for the electroreduction of CO2 to CO: Au nanoparticles supported on polymer-wrapped multiwall carbon nanotubes. This catalyst exhibits high selectivity for CO over H2 : 80-92 % CO, as well as high activity: partial current density for CO as high as 160 mA cm-2 . The observed high activity, originating from a high electrochemically active surface area (23 m2 g-1 Au), in combination with the low loading (0.17 mg cm-2 ) of the highly dispersed Au nanoparticles underscores the promise of this catalyst for efficient electroreduction of CO2 .

Effect of the Hydrofluoroether Cosolvent Structure in Acetonitrile-Based Solvate Electrolytes on the Li+ Solvation Structure and Li-S Battery Performance.

We evaluate hydrofluoroether (HFE) cosolvents with varying degrees of fluorination in the acetonitrile-based solvate electrolyte to determine the effect of the HFE structure on the electrochemical performance of the Li-S battery. Solvates or sparingly solvating electrolytes are an interesting electrolyte choice for the Li-S battery due to their low polysulfide solubility. The solvate electrolyte with a stoichiometric ratio of LiTFSI salt in acetonitrile, (MeCN)2-LiTFSI, exhibits limited polysulfide solubility due to the high concentration of LiTFSI. We demonstrate that the addition of highly fluorinated HFEs to the solvate yields better capacity retention compared to that of less fluorinated HFE cosolvents. Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that HFEs exhibiting a higher degree of fluorination coordinate to Li+ at the expense of MeCN coordination, resulting in higher free MeCN content in solution. However, the polysulfide solubility remains low, and no crossover of polysulfides from the S cathode to the Li anode is observed.