Observation on the Ion Association Equilibria in NaNO3 Droplets Using Micro-Raman Spectroscopy

Microwave-modulated graded porous carbon for supercapacitors: Pore size matching and operating voltage expansion

Optimizing the pore structure and its interaction with the electrolytes was vital for enhancing the performance of supercapacitors based on the electrical double layer mechanism. In this study, graded porous carbon material (STP) with outstanding properties was prepared by adjusting the activation temperature and KOH dosage in the microwave pyrolysis process of sargassum thunbergii. The results demonstrated that better electrochemical performance was obtained when 1 M NaNO3 was used as electrolyte and STP-800-3 was employed as electrode material, attributed to its excellent specific surface area (SSA) of 2011.8 m2 g-1, high micropore ratio, and the optimal matching degree between micropore size and electrolyte ion diameter. Moreover, the operating voltage window was expanded to 2.0 V in supercapacitors assembled with 6 M NaNO3 high-concentration electrolyte. Simultaneously, the symmetric supercapacitors exhibited a remarkable specific capacitance of 290.0 F g-1, a high energy density of 39.0 W h kg-1, and outstanding capacity retention at 70.9% after 10,000 charge/discharge cycles based on 6 M NaNO3 electrolyte. Consequently, the results provided valuable technical support and theoretical basis to foster progress of novel and high-performance supercapacitors.

Anion-Induced Interfacial Liquid Layers on LiCoO2 in Salt-in-Water Lithium-Ion Batteries

The incompatibility of lithium intercalation electrodes with water has impeded the development of aqueous Li-ion batteries. The key challenge is protons which are generated by water dissociation and deform the electrode structures through intercalation. Distinct from previous approaches utilizing large amounts of electrolyte salts or artificial solid-protective films, we developed liquid-phase protective layers on LiCoO₂ (LCO) using a moderate concentration of 0.5∼3 mol kg^-1 lithium sulfate. Sulfate ion strengthened the hydrogen-bond network and easily formed ion pairs with Li⁺, showing strong kosmotropic and hard base characteristics. Our quantum mechanics/molecular mechanics (QM/MM) simulations revealed that sulfate ion paired with Li⁺ helped stabilize the LCO surface and reduced the density of free water in the interface region below the point of zero charge (PZC) potential. In addition, in situ electrochemical surface-enhanced infrared absorption spectroscopy (SEIRAS) proved the appearance of inner-sphere sulfate complexes above the PZC potential, serving as the protective layers of LCO. The role of anions in stabilizing LCO was correlated with kosmotropic strength (sulfate > nitrate > perchlorate > bistriflimide (TFSI^-)) and explained better galvanostatic cyclability in LCO cells.

Study on ionic association behavior in sodium nitrate solution

Raman spectroscopy combined with component analysis and molecular dynamics simulation were used to study chemical species and their transformation in aqueous sodium solutions. Study shows that the characteristic vibrational frequency of nitrate ions (ν₁-NO₃^-) blue-shifted from 1043.9 to 1046.9 cm^-1, and the full width at half maximum increased from 6.8 to 10.8 cm^-1 as the concentration increasing. When water/salt molar ratio (WSR) > 30, the relative concentration (RC) of free hydrated ions and solvent shared ion pair accounts for the vast majority, and there is almost no contact ion pair in solution. When WSR less than 30, due to the continuous reduction of the number of water molecules, the hydrated water molecules around the sodium ions and nitrate ions begin to decrease, and solvent shared ion pair or contact ion pair gradually forms. Sodium ions and nitrate ions mainly exist in a monodentate coordination. When WSR > 160, both the relative concentration of contact ion pair and complex structure is close to 0. This work proves that a lower RC of complex structure in solution, a smaller supersaturation of the solution is achieved, meaning aqueous sodium nitrate solution is easier to nucleate crystals.

Hydration and Ion-Pair Formation of NaNO3(aq): A Vibrational Spectroscopic and Density Functional Theory Study

Qualitative and quantitative Raman and infrared measurements on sodium nitrate (NaNO₃) solutions have been carried out over a wide concentration range (5.56 × 10^-6-7.946 mol/L) in water and heavy water. The Raman spectra were measured from 4000 cm^-1 to low wavenumbers at 45 cm^-1. Band fit analysis on the profile of the 1047 cm^-1 band, ν₁(a1') NO3- measured at high resolution at 0.90 cm^-1 produced a small contribution at 1027 cm^-1 of the isotopomer N¹⁶O₂¹⁸O(aq). The effect of solute concentration on the Raman and infrared bands has been systematically recorded. Extrapolation of the experimental data resulted in values for all the nitrate bands of the "free", i.e., fully hydrated NO3-(aq). However, even in dilute solutions, the vibrational symmetry of the hydrated NO3-(aq) is broken and the antisymmetric N-O stretch, which is degenerate for the isolated anion, is split by 56 cm^-1. At concentrations >2.5 mol/L, direct contact between Na⁺ and NO3- was observed and accompanied by large band parameter changes. DFT calculations on NO3-(H₂O)_n (n = 1-3) led to optimized geometries and vibrational frequencies which reproduced the measured ones within an accuracy of 1%. A hydrated gas phase species Na⁺(H₂O)₁₀NO3- was optimized resulting in the geometry and symmetry of the nitrate, which is bound in an antisymmetric bidentate fashion with the nitrate possessing C₁. The ν₁ Na⁺(OH₂) breathing mode in aqueous solution appears at 189 cm^-1, whereas in heavy water, ν₁ Na⁺(OD₂) is shifted to 175.6 cm^-1 due to the isotope effect. DFT calculations on hydrated Na⁺(OH₂)_n gas phase clusters provided realistic Na⁺ hydrate structures with n = 4 and 5, which resembled the measured frequency of ν₁ Na⁺ OH₂ mode quite well. Quantitative Raman analysis employing the symmetric stretching band, ν₁(a1') NO3-, has been carried out down to concentrations as low as 5.56 × 10^-6mol/L. The in-plane deformation mode ν₄(e') in the Raman scattering at higher concentrations has been used as an indicator band for directly coordinated NO3-.

Subsecond measurement on deliquescence kinetics of aerosol particles: Observation of partial dissolution and calculation of dissolution rates

The deliquescence behavior of atmospheric aerosols has significant effects on global climate and atmospheric heterogeneous chemistry but remains largely unclear. The deliquescence kinetics data of micron-sized particles are scarce owing to the difficulty on performing the time-resolved dissolution measurements. In view of this technique bottleneck, an applicable and powerful experimental technique, i. e., vacuum FTIR combining pulsed relative humidity (RH) change technique, is introduced for gaining deliquescence kinetics information of three inorganic salts. For NaCl and (NH₄)₂SO₄ aerosols, a solid-liquid mixing state derived from partial dissolution of NaCl and (NH₄)₂SO₄ crystals is present during deliquescence, and the recrystallization will occur once RH decreases. While for NaNO₃ particles, the recrystallization cannot occur as RH decreases owing to the formed amorphous NaNO₃ solids after dying. The dissolution rates of NaCl, (NH₄)₂SO₄ and NaNO₃ solid particles are calculated, as a first attempt, by the upward pulsed RH mode. The measured rates show a significant dependency on ambient RH with three orders of magnitude. For NaCl particles, the measured J values range from 1.41 × 10^-4 to 7.67 × 10^-1 s^-1 at RH of 73.41-75.15%. The J for (NH₄)₂SO₄ particles is 7.34 × 10^-3 to 2.46 × 10⁰ s^-1 over the RH range of 77.27%-80.13%. The J values for amorphous NaNO₃ solids range from 6.01 × 10^-3 to 2.63 × 10⁰ s^-1 as RH increases from 71.15% to 73.84%. Our results fill in the dataset of atmospheric models describing the kinetics features of deliquescence and provide an insight into dynamic solid-solution transition for PM2.5 particles.

Raman and ab initio analyses of ion pairs in concentrated K[B(OH)4] droplets

In this study, microscopic Raman spectroscopy and Ab initio quantum chemical calculation were used to determine the structural details of ion pairs and their transformation in concentrated K[B(OH)₄] droplets. The Raman experiment shows that the v_sym-B(OH)₄^- undergoes a downward shift with the decrease of WSR. The contact ion pairs (CIPs) change to solvent shared ion pairs when the molar water-to-solute ratio (WSR) is bigger than 6; CIPs are the dominant species when 1.33 < WSR < 6, where K⁺ bonds to [B(OH)₄^-] in bidentate form (CIP-II); the CIPs quickly dehydrate and associate to triple ion pairs (TIPs) when WSR < 5. Raman experiment and ab initio quantum chemical calculation show that TIPs are mainly present in "anionic" type such as {[B(OH)₄^-]K⁺[B(OH)₄^-](H₂O)_n}, where K⁺ bonds to two [B(OH)₄^-] in bidentate or/and tridentate form (TIP-a-II or/and TIP-a-III). When WSR <1.33, most TIPs convert to complex clusters such as chain-like structure. The remaining TIPs associate to six-membered ring structure [B₃O₃(OH)₄^-] and the relative content increases from 0 to 20% when the WSR ranges from 1.33 to 0.55.

Enhancing Double-Beam Laser Tweezers Raman Spectroscopy (LTRS) for the Photochemical Study of Individual Airborne Microdroplets

A new device and methodology for vertically coupling confocal Raman microscopy with optical tweezers for the in situ physico- and photochemical studies of individual microdroplets (Ø ≤ 10 µm) levitated in air is presented. The coupling expands the spectrum of studies performed with individual particles using laser tweezers Raman spectroscopy (LTRS) to photochemical processes and spatially resolved Raman microspectroscopy on airborne aerosols. This is the first study to demonstrate photochemical studies and Raman mapping on optically levitated droplets. By using this configuration, photochemical reactions in aerosols of atmospheric interest can be studied on a laboratory scale under realistic conditions of gas-phase composition and relative humidity. Likewise, the distribution of photoproducts within the drop can also be observed with this setup. The applicability of the coupling system was tested by studying the photochemical behavior of microdroplets (5 µm < Ø < 8 µm) containing an aqueous solution of sodium nitrate levitated in air and exposed to narrowed UV radiation (254 ± 25 nm). Photolysis of the levitated NaNO₃ microdroplets presented photochemical kinetic differences in comparison with larger NaNO₃ droplets (40 µm < Ø < 80 µm), previously photolyzed using acoustic traps, and heterogeneity in the distribution of the photoproducts within the drop.

Decoding Oxyanion Aqueous Solvation Structure: A Potassium Nitrate Example at Saturation

The ability to probe the structure of a salt solution at the atomic scale is fundamentally important for our understanding of many chemical reactions and their mechanisms. The capability of neutron diffraction to "see" hydrogen (or deuterium) and other light isotopes is exceptional for resolving the structural complexity around the dissolved solutes in aqueous electrolytes. We have made measurements using oxygen isotopes on aqueous nitrate to reveal a small hydrogen-bonded water coordination number (3.9 ± 1.2) around a nitrate oxyanion. This is compared to estimates made using the existing method of nitrogen isotope substitution and those of computational simulations (>5-6 water molecules). The low water coordination number, combined with a comparison to classical molecular dynamics simulations, suggests that ion-pair formation is significant. This insight demonstrates the utility of experimental diffraction data for benchmarking atomistic computer simulations, enabling the development of more accurate intermolecular potentials.

Hygroscopicity of Mixed Glycerol/Mg(NO3)2/Water Droplets Affected by the Interaction between Magnesium Ions and Glycerol Molecules

Tropospheric aerosols are usually complex mixtures of inorganic and organic components, which can influence the hygroscopicities of each other. In this research, we applied confocal Raman technology combined with optical microscopy to investigate the relationship between the hygroscopic behavior and the molecular interactions of mixed glycerol/Mg(NO3)2/water droplets. Raman spectra provide detailed structural information about the interactions between glycerol molecules and Mg(2+) ions, as well as information about the interactions between glycerol and NO3(-) ions through electrostatic interaction and hydrogen bonding. The change of the CH2 stretching band of glycerol molecules in mixed droplets suggests that the backbone structures of glycerol mainly transform from αα to γγ in the dehumidifying process, and the additional Mg(2+) ions strongly influence the structure of glycerol molecules. Because the existence of glycerol suppresses the crystallization of Mg(NO3)2·6H2O in the dehumidifying process, Mg(NO3)2 molecules in mixed droplets form an amorphous state rather than forming crystals of Mg(NO3)2·6H2O when the relative humidity is lower than 17.8%. Moreover, in mixed droplets, the molar ratio of NO3(-) to glycerol is higher in the center than in the outer region.

Electroneutrality breakdown and specific ion effects in nanoconfined aqueous electrolytes observed by NMR

Ion distribution in aqueous electrolytes near the interface plays a critical role in electrochemical, biological and colloidal systems, and is expected to be particularly significant inside nanoconfined regions. Electroneutrality of the total charge inside nanoconfined regions is commonly assumed a priori in solving ion distribution of aqueous electrolytes nanoconfined by uncharged hydrophobic surfaces with no direct experimental validation. Here, we use a quantitative nuclear magnetic resonance approach to investigate the properties of aqueous electrolytes nanoconfined in graphitic-like nanoporous carbon. Substantial electroneutrality breakdown in nanoconfined regions and very asymmetric responses of cations and anions to the charging of nanoconfining surfaces are observed. The electroneutrality breakdown is shown to depend strongly on the propensity of anions towards the water-carbon interface and such ion-specific response follows, generally, the anion ranking of the Hofmeister series. The experimental observations are further supported by numerical evaluation using the generalized Poisson-Boltzmann equation.

Temperature-dependent deliquescent and efflorescent properties of methanesulfonate sodium studied by ATR-FTIR spectroscopy

Modeling of aerosols and cloud formation processes in the marine boundary layer (MBL) require extensive data on hygroscopic properties of relevant methanesulfonate particles, which are currently scarce. In this work, methanesulfonate sodium (CH3SO3Na, MSA-Na), the most abundant methanesulfonate salt, was selected, and its deliquescent and efflorescent properties at temperatures relevant to the lower troposphere were studied using an ATR-FTIR flow system. To validate the approach, we investigated hygroscopic properties of NaCl particles, and our measured deliquescent relative humidity (DRH) and efflorescent relative humidity (ERH) of the NaCl particles obtained from the changes in integrated absorbance of water peaks in infrared spectra agreed with literature data well. We then reported DRH and ERH of MSA-Na particles as a function of temperature for the first time using both the changes in integrated absorbance of water peaks and the changes in peak position and shape of CH3SO3(-) symmetric and asymmetric vibrational modes. Our experiments showed that MSA-Na particles present quite different temperature-dependent hygroscopic behaviors from NaCl. Both the DRH and ERH of MSA-Na particles increase with decreasing temperatures. Due to the significant differences in temperature-dependent DRH and ERH, NaCl particles, if processed in MBL by methanesulfonic acid, are expected to deliquesce slightly earlier during a hydration process but effloresce at a much earlier stage during a dehydration process, especially at lower temperatures. This could considerably influence phase, size, and water content of sea salt aerosols and consequently their reactivity, lifetime, and impacts on atmospheric chemistry and climate systems.