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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Real-time Characterization of the Fine Structure and Dynamics of an Electrical Double Layer at Electrode-electrolyte Interfaces

The chemisorption of species from supporting electrolytes on electrode surfaces is ubiquitous in electrochemical systems and affects the dynamics and mechanism of various electrochemical reactions. The understanding of chemical structure and property of the resulting electrical double layer is vital but limited. In this work, we operando probed the electrochemical interface between a gold electrode surface and a common supporting electrolyte, phosphate buffer, using our newly developed in situ liquid secondary ion mass spectrometry. We surprisingly found that on the positively charged gold electrode surface sodium cations were anchored in the stern layer in a partially dehydrated form by formation of compact ion pairs with the accumulated phosphate anions. The resulting strong and dense adsorption phase was further revealed to retard the electro-oxidation reaction of ascorbate. This finding addressed one major gap in the fundamental science of electrode-electrolyte interfaces that where and how the cations reside in the double layer to impose effects on electrochemical reactions, providing insights into engineering of better electrode-electrolyte interfaces in electrochemical processes.

Zhang, Yanyan↗
DOE OSTI2024

Anionic Effects on Concentrated Aqueous Lithium Ion Dynamics

In this article, the structural dynamics, chemical reactivity, anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO 2 ), and lithium nitrate (LiNO 3 ), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. 2D IR, NMR, and viscosity results show that LiNO 2 exhibits longer correlation times, lower diffusivity, and nearly four times greater viscosity when compared to the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO 2 and LiNO 3 solutions show significantly reduced chemical reactivity with respect to lithium cations coordinating with the methyl thiocyanate when compared to the lithium halide salts.

2D IR↗
DOE OSTI2023

Suppressing the Shuttle Effects with FeCo/SPAN Cathodes and High-Concentration Electrolytes for High-Performance Lithium–Sulfur Batteries

The shuttle effects and the sluggish redox kinetics are two of the main challenges in lithium–sulfur (Li–S) batteries, which significantly reduce the capacity of the batteries and restrict their commercialization. Herein, FeCo/sulfurized polyacrylonitrile (SPAN) is synthesized as a cathode material via the electrospinning technique and a further heat treatment. Synchrotron X-ray absorption spectroscopy confirmed the existence of Fe–S/Fe and Co–S bonds in FeCo/SPAN, which benefit the adsorptive and catalytic activities toward lithium polysulfides (LiPSs). Here, we further investigated the effect of electrolyte concentration in inhibiting the shuttle of LiPSs in Li–S batteries. Small-angle X-ray scattering (SAXS) reveals that more contact ion pairs are formed and fewer free solvent molecules exist with the increase of the electrolyte concentration, which can inhibit the dissolution and shuttle of LiPSs. Finally, the batteries assembled with high-concentration electrolytes (3 M LiTFSI in DOL/DME) exhibit a higher specific capacity retention compared to those assembled with low-concentration electrolytes. This work enriches the route to prepare Li–S batteries with the rational design of cathode materials and electrolytes.

25 ENERGY STORAGE↗
DOE OSTI2021

Active Sites in a Heterogeneous Organometallic Catalyst for the Polymerization of Ethylene

Heterogeneous derivatives of catalysts developed by Ziegler and Natta are important for the industrial production of polyolefin plastics. However, the interaction between precatalysts, alkylaluminum activators, and oxide supports to form catalytically active materials is poorly understood. This is in contrast to homogeneous or well-defined heterogeneous catalysts that 20 contain resolved molecular structures that relate to activity and selectivity in polymerization reactions. This study describes the reactivity of triisobutylaluminum with high surface area aluminum oxide and a zirconocene precatalyst. Triisobutylaluminum reacts with the zirconocene precatalyst to form hydrides and passivates –OH sites on the alumina surface. The combination of passivated alumina and zirconium hydrides formed in this mixture generate ion-pairs that 25 polymerize ethylene.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Crossover from Rouse to Reptation Dynamics in Salt-Free Polyelectrolyte Complex Coacervates

Considerable interest in the dynamics and rheology of polyelectrolyte complex coacervates has been motivated by their industrial application as viscosity modifiers. A central question is the extent to which classical Rouse and reptation models can be applied to systems where electrostatic interactions play a critical role on the thermodynamics. By relying on molecular simulations, we present a direct analysis of the crossover from Rouse to reptation dynamics in salt-free complex coacervates as a function of chain length. This crossover shifts to shorter chain lengths as electrostatic interactions become stronger, which corresponds to the formation of denser coacervates. To distinguish the roles of Coulomb interactions and density, here we compare the dynamics of coacervates to those of neutral, semidilute solutions at the same density. Both systems exhibit a universal dynamical behavior in the connectivity-dominated (subdiffusion and normal diffusion) regimes, but the monomer relaxation time in coacervates is much longer and increases with increasing Bjerrum length. This is similar to the cage effect observed in glass-forming polymers, but the local dynamical slowdown is caused here by strong Coulomb attractions (ion pairing) between oppositely charged monomers. Our findings provide a microscopic framework for the quantitative understanding of coacervate dynamics and rheology.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Conductive Organometallic Polymers from Soluble Superatom Ions

Superatomic crystals comprising ligand-capped, metal chalcogenide clusters and fullerenes are modular materials that exhibit enhanced electronic, magnetic, and thermal conductivity properties. We find that neutral, M 4 S 4 (M = Fe, Co) clusters stabilized with N -heterocyclic carbenes (NHCs) can transfer charge to C 60 fullerene to form binary superatomic crystals. Notably, these compounds are soluble in various organic solvents, allowing their properties to be investigated in solution, unlike traditional fullerene-based superatomic crystals. The ion pairs can be further assembled into organometallic polymers using Janus-bis(NHCs) to cross-link the oxidized M 4 S 4 units. We show that the superatomic polymers are more conductive than both the precursor superatomic crystals and the polymers containing only neutral M 4 S 4 clusters. Similar conductivity values can be obtained when neutral M 4 S 4 –NHC polymers are doped with solutions of C 60 fullerene. These findings demonstrate that next generation superatomic materials can be prepared via the combination of charge transfer and polymerization with appropriate cross-linking agents.

carbon nanomaterials↗
DOE OSTI2021

Formation of Surface Impurities on Lithium–Nickel–Manganese–Cobalt Oxides in the Presence of CO 2 and H 2 O

Surface impurities involving parasitic reactions and gas evolution contribute to the degradation of high Ni content LiNi x Mn y Co z O 2 (NMC) cathode materials. The transient kinetic technique of Temporal Analysis of Products (TAP), density functional theory, and infrared spectroscopy have been used to study the formation of surface impurities on varying nickel content NMC materials (NMC811, NMC622, NMC532, NMC433, NMC111) in the presence of CO 2 and H 2 O. CO 2 reactivity on a clean surface as characterized by CO 2 conversion rate in the TAP reactor follows the order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111. The capacity of CO 2 uptake follows a different order: NMC532 > NMC433 > NMC622 > NMC811 > NMC111. Moisture pretreatment slows down the direct CO 2 adsorption process and creates additional active sites for CO 2 adsorption. Electronic structure calculations predict that the (012) surface is more reactive than the ($10\bar{1}4$) surface for CO 2 and H 2 O adsorption. CO 2 adsorption leading to carbonate formation is exothermic with formation of ion pairs. The average CO 2 binding energies on the different materials follow the CO 2 reactivity order. Water hydroxylates the (012) surface and surface OH groups favor bicarbonate formation. Water creates more active sites for CO 2 adsorption on the ($10\bar{1}4$) surface due to hydrogen bonding. The composition of surface impurities formed in ambient air exposure is dependent on water concentration and the percentage of different crystal planes. Different surface reactivities suggest that battery performance degradation due to surface impurities can be mitigated by precise control of the dominant surfaces in NMC materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Solvent Polarity Independent Symmetry-Breaking Charge Separation in a Slip-Stacked Covalent Terrylene Monoimide Dimer

The design of organic materials capable of efficient photoinduced charge separation in low-polarity environments is a critical challenge for advancing organic photovoltaics. Symmetry-breaking charge separation (SB-CS) offers a promising route to efficient charge separation with minimal energy loss; however, in conventional organic materials this process typically relies on polar solvents to stabilize the charge-separated state. Here, we investigate a slip-stacked terrylene monoimide dimer (TMI 2 ) engineered with intrinsic electronic asymmetry to circumvent this limitation. We demonstrate through a comprehensive suite of ultrafast spectroscopic techniques that TMI 2 undergoes remarkably efficient SB-CS regardless of solvent polarity enabled by the permanent dipole moment of TMI and the accessible intramolecular charge-transfer (ICT) character of its monomer units. Furthermore, analysis of vibronic coherences reveals that the initial state mixing is actively driven by a 193 cm –1 intermolecular mode, while the final SB-CS state is marked by a distinct vibrational fingerprint of the radical ion pair product. Our findings reveal how the intrinsic electronic asymmetry of the TMI monomers that constitute TMI 2 creates an efficient charge separation pathway that precludes the need for external stabilization from a polar solvent. Here, this work establishes a powerful molecular design principle for leveraging intrinsic monomeric asymmetry to achieve efficient charge separation in low-dielectric environments, with significant implications for the development of next-generation organic optoelectronic materials.

Aromatic compounds↗
DOE OSTI2026

Low-Temperature Pyrolysis of Aliphatic Polymers Using a Fluorinated Amorphous Silica–Alumina: Cooperative Reactivity between a Redox-Active Radical and an Aluminum Lewis Site

Fluorinated amorphous silica–alumina (F-ASA) prepared by the thermolysis of Krossing’s Al(OC(CF 3 ) 3 ) 3 (PhF) Lewis superacid supported on silica is a very reactive catalyst that promotes the pyrolysis (cracking) of aliphatic polymer melts to produce low molecular weight hyperbranched oils. Initial spectroscopic studies reported previously (Gao, J.; Perras, F. A.; Conley, M. P. J. Am. Chem. Soc . 2025 , 147, 18145–18154) showed that this material contains a distribution of four-, five-, and six-coordinate aluminum sites and a small amount of Brønsted acid sites, similar to typical amorphous silica–alumina materials that are far less reactive in the pyrolysis of aliphatic polymer melts. The objective of this study was to determine whether other active sites present in F-ASA could facilitate pyrolysis reactions. This study provides evidence for the presence of a redox-active silicon oxycarbide persistent radical ((≡Si) 3 C•) in F-ASA. Mims ENDOR EPR experiments show that (≡Si) 3 C• is located close to aluminum. Contacting F-ASA with thianthrene (Th) results in oxidation to form the [Th •+ ][F–ASA] ion-pair, while reactions with 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) result in H atom transfer to form TEMPO radical and F-ASA-H containing a mildly acidic (≡Si) 3 C–H. Poisoning studies show that both Lewis acidity and (≡Si) 3 C• are required for polymer pyrolysis reactivity. Finally, we propose that F-ASA promotes the formation of alkyl radicals in polymer melts, which are key intermediates in the thermal pyrolysis reactions of aliphatic polymers, involving the cooperative reactivity of both the Lewis acid and (≡Si) 3 C•.

aluminum↗
DOE OSTI2026

Controlled Placement of Trivalent Heteroatoms in MFI Zeolite Frameworks Using Structure-Directing Agents

MFI zeolites contain distinct confining pore environments, either smaller (∼0.55 nm) channels or larger (∼0.70 nm) intersections. The substitution of trivalent heteroatoms generates Brønsted acid sites of varying acid strength, which, together with their location within different pore environments, dictates the stability of transition states via electrostatic (ion-pair) and van der Waals interactions. Here, we report that structure-directing agents (SDAs) that control Al 3+ substitutional patterns during MFI crystallization analogously position other trivalent heteroatoms (Ga 3+ , Fe 3+ , B 3+ ), altering the distribution of acid sites between channels and intersections, as probed by low-temperature (403 K) toluene methylation kinetics. Heteroatom-substituted MFI crystallized with tetrapropylammonium result in low selectivity toward p-xylene (<30%), while those crystallized using (co-)SDAs containing peripheral hydrogen-bonding groups (e.g., ethylenediamine) show high p-xylene selectivity (∼80%). These selectivities are consistent with DFT-calculated Gibbs free energy barriers for intersection-dominant and channel-dominant active site distributions, respectively. Transition states to form each xylene isomer are similar in size and charge; thus, their rate constants decrease with acid strength similarly, causing isomer selectivity to depend strongly on confinement but not on acid strength. These generalizable synthetic strategies enable independently controlling acid site strength and location in MFI zeolites and, in turn, catalytic rates and selectivities.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Dissociation of salts in water under pressure

The investigation of salts in water at extreme conditions is crucial to understanding the properties of aqueous fluids in the Earth. We report first principles (FP) and classical molecular dynamics simulations of NaCl in the dilute limit, at temperatures and pressures relevant to the Earth's upper mantle. Similar to ambient conditions, we observe two metastable states of the salt: the contact (CIP) and the solvent-shared ion-pair (SIP), which are entropically and enthalpically favored, respectively. We find that the free energy barrier between the CIP and SIP minima increases at extreme conditions, and that the stability of the CIP is enhanced in FP simulations, consistent with the decrease of the dielectric constant of water. The minimum free energy path between the CIP and SIP becomes smoother at high pressure, and the relative stability of the two configurations is affected by water self-dissociation, which can only be described properly by FP simulations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells

Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. We find that this synergistically integrated fuel cell reached peak power densities of 1,130 mW cm -2 at 160 °C and 1,740 mW cm -2 at 240 °C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.

25 ENERGY STORAGE↗
DOE OSTI2021

Effects of nanoconfinement and surface charge on iron adsorption on mesoporous silica

Here, we present a combined molecular dynamics (MD) simulation and X-ray absorption fine structure (XAFS) spectroscopic investigation of aqueous iron adsorption on nanoconfined amorphous silica surfaces. The simulation models examine the effects of pore size, pH (surface charge), iron valency, and counter-ion (chloride or hydroxide). The simulation methods were validated by comparing the coordination environment of adsorbed iron with coordination numbers and bond lengths derived from XAFS. In the MD models, nanoconfinement effects on local iron coordination were investigated by comparing results for unconfined silica surfaces and in confined domains within 2 nm, 4 nm, and 8 nm pores. Experimentally, coordination environments of iron adsorbed onto mesoporous silica with 4 nm and 8 nm pores at pH 7.5 were investigated. The effect of pH in the MD models was included by simulating Fe(II) adsorption onto negatively charged SiO 2 surfaces and Fe(III) adsorption on neutral surfaces. The simulation results show that iron adsorption depends significantly on silica surface charge, as expected based on electrostatic interactions. Adsorption on a negatively charged surface is an order of magnitude greater than on the neutral surface, and simulated surface coverages are consistent with experimental results. Pore size effects from the MD simulations were most notable in the adsorption of Fe(II) at deprotonated surface sites (SiO - ), but adsorption trends varied with concentration and aqueous Fe speciation. The coordination environment of adsorbed iron varied significantly with the type of anion. Considerable ion pairing with hydroxide anions led to the formation of oligomeric surface complexes and aqueous species, resulting in larger iron hydroxide clusters at higher surface loadings.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Deconvoluting charge-transfer, mass transfer, and ohmic resistances in phosphonic acid–sulfonic acid ionomer binders used in electrochemical hydrogen pumps

Ion-pair high-temperature polymer electrolyte membranes (HT-PEMs) paired with phosphonic acid ionomer electrode binders have substantially improved the performance of HT-PEM electrochemical hydrogen pumps (EHPs) and fuel cells. Here, blending poly(pentafluorstyrene-co-tetrafluorostyrene phosphonic acid) (PTFSPA) with Nafion™, and using this blend as an electrode binder, improved proton conductivity in the electrode layer resulting in a 2 W cm –2 peak power density of fuel cells at 240 °C (a HT-PEM fuel cell record). However, much is unknown about how phosphonic acid ionomers blended with perfluorosulfonic acid materials affect electrode kinetics and gas transport in porous electrodes. In this work, we studied the proton conductivity, electrode kinetics, and gas transport resistances of 3 types of phosphonic acid ionomers, poly(vinyl phosphonic acid), poly(vinyl benzyl phosphonic acid), and PTFSPA by themselves and when blended with Aquivion® (a perfluorosulfonic acid material).

08 HYDROGEN↗
DOE OSTI2023

The effect of rare earth (RE 3+ ) ionic radii on transparent lanthanide-tellurite glass-ceramics: correlation between ‘hole-formalism’ and crystallization

Rare earth (RE) doped transparent tellurite glass-ceramics (GCs) are widely explored for their application as advanced photonic materials. Specifically, the growth of RE based “anti-glass” crystalline phases in transparent GCs enhances their effective functionality. Enormous studies on the properties of GCs revealed that optimization of glass composition and heat-treatment schedule are the foremost factors that affect the transparency of the GCs. Nevertheless, the direct effect of RE 3+ ions on the crystallization mechanism of glass has hardly been reported. Therefore, a base glass of the composition La 2 O 3 –Gd 2 O 3 –TiO 2 –TeO 2 (LGTT) is doped with RE 3+ ions (Ce 3+ , Pr 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Tb 3+ , and Dy 3+ ) with varied ionic radii. Rietveld refinement from XRD of GCs confirms that (La/Gd) 2 Te 6 O 15 phases are precipitated, where dopants Ce, Pr and Nd occupy La sites, while Sm, Eu, Tb, and Dy occupy Gd sites. The configurational heat-capacity (ΔC p ) from DSC is found to be lower for larger ionic radii REs (Ce, Pr, Nd) than smaller ionic radii REs (Sm, Eu, Tb, Dy) thereby exhibiting higher chemical ordering followed by faster crystal growth rate in the former. However, the transparency retention profile of different RE-doped GCs follows the trend Eu:Tb > Sm:Dy > Nd > Pr > Ce, confirming the ionic radii effect. In addition to the ionic radii effect, in this work, we propose a ‘Hole-Formalism’ concept to explain the observed trend. It is further corroborated with an identical neutron diffraction pattern and ΔC p values for these ion pairs Sm 3+ : Dy 3+ (4f 5 : 4f 9 ) and Eu 3+ : Tb 3+ (4f 6 : 4f 8 ) results in a similar crystallization mechanism, transmission profile and fine-scale microstructures of these RE-based tellurite GCs.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Highly efficient synthesis of zeolite chabazite using cooperative hydration-mismatched inorganic structure-directing agents

Chabazite (CHA type) zeolite is notoriously difficult to synthesize in the absence of organic structure-directing agents owing to long synthesis times and/or impurity formation. The ability to tailor organic-free syntheses of zeolites is additionally challenging due to the lack of molecular level understanding of zeolite nucleation and growth pathways, particularly the role of inorganic cations. In this study, we reveal that zeolite CHA can be synthesized using six different combinations of inorganic cations, including the first reported seed- and organic-free synthesis without the presence of potassium. We show that lithium, when present in small quantities, is an effective accelerant of CHA crystallization; and that ion pairings can markedly reduce synthesis times and temperatures, while expanding the design space of zeolite CHA formation in comparison to conventional methods utilizing potassium as the sole structure-directing agent. Herein, we posit the effects of cation pairings on zeolite CHA crystallization are related to their hydrated ionic radii. We also emphasize the broader implications for considering the solvated structure and cooperative role of inorganic cations in zeolite synthesis within the context of the reported findings for chabazite.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗