Investigating the phase diagram-ionic conductivity isotherm relationship in aqueous solutions of common acids: hydrochloric, nitric, sulfuric and phosphoric acid.

The relationship between phase diagram features around the solid-liquid equilibrium region and ionic conductivity in aqueous solutions is not well understood over the whole concentration range as is the case for acidic aqueous solutions. In this work, we have studied the ionic conductivity (κ) as a function of molar fraction (x) and temperature (T) for four acid/water solutions namely, monoprotic hydrochloric acid (HCl) and nitric acid (HNO3), diprotic sulfuric acid (H2SO4) and triprotic phosphoric acid (H3PO4) along with their binary phase diagrams. The connection between the main features of the phase diagrams and the trends in the ionic conductivity isotherms is established with a new insight on the two pertinent dominant conductivity mechanisms (hopping and vehicular). Ionic conductivity at different temperatures were collected from literature and fitted to reported isothermal (κ vs. x) and iso-compositional (κ vs. T) equations along with a novel semi-empirical equation (κ = f (x, T)) for diprotic and triprotic acids. This equation not only has the best fit for acids with different valency; but also contains four parameters, less than any other similar equation in literature. This work is one of few that advances the understanding of the intricate relationship between structure and ionic transport in various acidic aqueous solutions.

In Situ Electrochemistry of Formate on Cu Thin Films Using ATR-FTIR Spectroscopy and X-ray Photoelectron Spectroscopy.

Formate (HCOO-) is the most dominant intermediate identified during carbon dioxide electrochemical reduction (CO2ER). While previous studies showed that copper (Cu)-based materials that include Cu(0), Cu2O, and CuO are ideal catalysts for CO2ER, challenges to scalability stem from low selectivity and undesirable products in the -1.0-1.0 V range. There are few studies on the binding mechanism of intermediates and products for these systems as well as on changes to surface sites upon applying potential. Here, we use an in situ approach to study the redox surface chemistry of formate on Cu thin films deposited on Si wafers using a VeeMAX III spectroelectrochemical (SEC) cell compatible with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Spectra for surface species were collected in real time as a function of applied potential during cyclic voltammetry (CV) experiments. Results showed the reproducibility of CV curves on freshly prepared Cu/Si wafers with relatively high signal-to-noise ATR-FTIR absorbance features of surface species during these electrochemical experiments. The oxidation reaction of HCOO- to bicarbonate (HCO3-) was observed using ATR-FTIR at a voltage of 0.27 V. Samples were then subjected to reduction in the CV, and the aqueous phase products below the detection limit of the SEC-ATR-FTIR were identified using ion chromatography (IC). We report the formation of glycolate (H3C2O3-) and glyoxylate (HC2O3-) with trace amounts of oxalate (C2O42-), indicating that C-C coupling reactions proceed in these systems. Changes to the oxidation state of surface Cu were measured using X-ray photoelectron spectroscopy, which showed a reduction in Cu(0) and an increase in Cu(OH)2, indicating surface oxidation.

Unraveling the phase diagram-ion transport relationship in aqueous electrolyte solutions and correlating conductivity with concentration and temperature by semi-empirical modeling.

The relationship between structure and ion transport in liquid electrolyte solutions is not well understood over the whole concentration and temperature ranges. In this work, we have studied the ionic conductivity (κ) as a function of molar fraction (x) and Temperature (T) for aqueous solutions of salts with nitrate anion and different cations (proton, lithium, calcium, and ammonium) along with their liquid-solid phase diagrams. The connection between the known features in the phase diagrams and the ionic conductivity isotherms is established with an insight on the conductivity mechanism. Also, known isothermal (κ vs.. x) and iso-compositional (κ vs.. T) equations along with a proposed two variables semi-empirical model (κ = f (x, T)) were fitted to the collected data to validate their accuracy. The role of activation energy and free volume in controlling ionic conductivity is discussed. This work brings us closer to the development of a phenomenological model to describe the structure and transport in liquid electrolyte solutions.

Mechanistic In Situ ATR-FTIR Studies on the Adsorption and Desorption of Major Intermediates in CO2 Electrochemical Reduction on CuO Nanoparticles.

Increasing levels of carbon dioxide (CO2) from human activities is affecting the ecosystem and civilization as we know it. CO2 removal from the atmosphere and emission reduction by heavy industries through carbon capture, utilization, and storage (CCUS) technologies to store or convert CO2 to useful products or fuels is a popular approach to meet net zero targets by 2050. One promising process of CO2 removal and conversion is CO2 electrochemical reduction (CO2ER) using metal and metal oxide catalysts, particularly copper-based materials. However, the current limitations of CO2ER stem from the low product selectivity of copper electrocatalysts due to existing knowledge gaps of the reaction mechanisms using surfaces that normally have native oxide layers. Here, we report systematic control studies of the surface interactions of major intermediates in CO2ER, formate, bicarbonate, and acetate, with CuO nanoparticles in situ and in real time using attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). Spectra were collected as a function of concentration, pH, and time in the dark and the in absence of added electrolytes. Isotopic exchange experiments were also performed to elucidate the type of surface complexes from H/D exchange. Our results show that the organics and bicarbonate form mostly outer-sphere complexes mediated by hydrogen bonding with CuO nanoparticles with Gibbs free energy of adsorption of about -25 kJ mol-1. The desorption kinetics of the surface species indicated relatively fast and slow regions reflective of the heterogeneity of sites that affect the strength of hydrogen bonding. These results suggest that hydrogen bonding, whether intermolecular or with surface sites on CuO nanoparticles, might be playing a more important role in the CO2ER reaction mechanism than previously thought, contributing to the lack of product selectivity.

Engineered interfaces between perovskite La2/3xLi3xTiO3 electrolyte and Li metal for solid-state batteries.

Perovskite La2/3xLi3xTiO3 (LLTO) materials are promising solid-state electrolytes for lithium metal batteries (LMBs) due to their intrinsic fire-resistance, high bulk ionic conductivity, and wide electrochemical window. However, their commercialization is hampered by high interfacial resistance, dendrite formation, and instability against Li metal. To address these challenges, we first prepared highly dense LLTO pellets with enhanced microstructure and high bulk ionic conductivity of 2.1 × 10 - 4 S cm-1 at room temperature. Then, the LLTO pellets were coated with three polymer-based interfacial layers, including pure (polyethylene oxide) (PEO), dry polymer electrolyte of PEO-LITFSI (lithium bis (trifluoromethanesulfonyl) imide) (PL), and gel PEO-LiTFSI-SN (succinonitrile) (PLS). It is found that each layer has impacted the interface differently; the soft PLS gel layer significantly reduced the total resistance of LLTO to a low value of 84.88 Ω cm-2. Interestingly, PLS layer has shown excellent ionic conductivity but performs inferior in symmetric Li cells. On the other hand, the PL layer significantly reduces lithium nucleation overpotential and shows a stable voltage profile after 20 cycles without any sign of Li dendrite formation. This work demonstrates that LLTO electrolytes with denser microstructure could reduce the interfacial resistance and when combined with polymeric interfaces show improved chemical stability against Li metal.

Practical Approach to Enhance Compatibility in Silicon/Graphite Composites to Enable High-Capacity Li-Ion Battery Anodes.

There is an urgent need to improve the energy density of Li-ion batteries to enable mass-market penetration of electric vehicles, grid-scale energy storage, and next-generation consumer electronics. Silicon-graphite composites are currently the most plausible anode material to overcome the capacity limit of graphite or poor cycling performance of silicon. One serious and unrecognized limitation to the use of the composite as an anode is the incompatibility of hydrophobic (natural) graphite with the hydrophilic Si, which adversely affects battery performance. Herein, we report a novel, practical approach to modify the graphite resulting in the formation of a hard carbon coating and graphene sheets that give rise to higher compatibility with Si nanoparticles in the composite. Electrochemical and battery testing of the composite (10 wt % Si) anode shows higher reversible capacity (10% at C/12 and 20% at C/2) than the composite with unmodified graphite reaching ∼600 mAh/g with 95% retention after 100 cycles. The enhanced battery performance is explained by the uniform distribution of Si nanoparticles at the modified graphite surface due to the presence of graphene conductive networks and a thin, oxygen-rich, amorphous carbon layer on the surface of graphite particles, as evidenced by transmission electron microscopy (TEM) images and X-ray photoelectron spectra (XPS). This work provides a new approach to prepare graphite compatible materials that can work with hydrophilic components other than silicon for various applications other than batteries.

Revealing the Role of Poly(vinylidene fluoride) Binder in Si/Graphite Composite Anode for Li-Ion Batteries.

The conventional polyvinylidene fluoride (PVDF) binder works well with the graphite anode, but when combined with silicon in composites to increase the energy density of Li-ion batteries, it results in severe capacity fade. Herein, by using scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses, we reveal that this failure stems from the loss of connectivity between the silicon (or its agglomerates), graphite, and PVDF binder because of the mechanical stresses experienced during battery cycling. More importantly, we reveal for the first time that the PVDF binder undergoes chemical decomposition during the cycling of not only the composite but also the Si-only or even graphite-only electrodes despite the excellent battery performance of the latter. Through X-ray photoemission electron microscopy and X-ray photoelectron spectroscopy techniques, LiF was identified as the predominant decomposition product. We show that the distribution of LiF in the electrodes due to the differences in the interactions between PVDF and either Si or graphite could correlate with the performance of the battery. This study shows that the most suitable binder for the composite electrode is a polymer with a good chemical interaction with both graphite and silicon.

A lipophilic ionic liquid based on formamidinium cations and TFSI: the electric response and the effect of CO2 on the conductivity mechanism.

This work describes the preparation of the new lipophilic ionic liquid tetraoctyl-formamidinium bis(trifluoromethanesulfonyl) imide (TOFATFSI), which is miscible with lower alkanes. In particular, this work focuses on the electric behaviour of TOFATFSI in the particularly challenging highly apolar environment of supercritical CO2. The conductivity and relaxation phenomena are revealed through the analysis of the broadband electric spectra with a particular emphasis on the effect of temperature and CO2 uptake on the IL conductivity. It is found that temperature boosts the conductivity via an increase in the charge carrier mobility. Also, CO2 absorption affects both the conductivity and the permittivity of the material due to the presence of CO2-IL interactions that modulate the nanostructure and the size of the TOFATFSI aggregates, which increases both the mobility and the density of the charge carriers.

Morphology and nanomechanics of conducting plastic crystals.

We present a temperature-controlled scanning probe microscopy characterisation of morphology and mechanical behaviour of conducting N,N'-cyclized pyrazolium salts. These salts are plastic crystals belonging to the pyrazolium trifluoromethanesulfonimide family. Going from a five-membered to a seven-membered ring, and adding a methyl group in alpha to either the five-membered or six-membered rings, allows modulation of the temperature of the phase transitions. Before and after the transitions, the materials' Young moduli, hardnesses and surface roughnesses change. We attribute these macroscopic modifications to specific states: brittle, elastoplastic and viscoplastic, corresponding to variations in the extent of the dislocations present in the crystal lattice planes of the compounds.

The plastic-crystalline phase of succinonitrile as a universal matrix for solid-state ionic conductors.

Solid ionic conductors are actively sought for their potential application in electrochemical devices, particularly lithium batteries. We have found high ionic conductivity for a large variety of salts dissolved in the highly polar medium based on the plastic-crystal form of succinonitrile (N[triple bond]C[bond]CH(2)[bond]CH(2)[bond]C[triple bond]N). Its high diffusivity, plasticity and solvating power allowed the preparation of a large number of materials with high ionic conductivity, reaching values of 3 mS cm(-1) at 25 degrees C (two orders of magnitude above polymers). Their ease of preparation and processing allowed us to study the influence of the solute on ionic conduction within this medium. Comparisons revealed a dichotomy between plastic crystals and conventional polymer media. The usefulness of these new, easily processed electrolytes was asserted in electrochemical tests with lithium intercalation electrodes.