20 research outputs found
Effect of Nanoparticle Surfactants on the Breakup of Free-Falling Water Jets during Continuous Processing of Reconfigurable Structured Liquid Droplets
Structured
liquids, whose 3-D morphology can adapt and respond to external stimuli,
represent a revolutionary materials platform for next-generation energy
technologies, such as batteries, photovoltaics, and thermoelectrics.
Structured liquids can be crafted by the jamming of interfacial assemblies
of nanoparticle (NP) surfactants. Due to the interactions between
functional groups on nanoparticles dispersed in one liquid and polymers
having complementary end-functionality dissolved in a second immiscible
fluid, the anchoring of a well-defined number of polymer chains onto
the NPs leads to the formation of NP surfactants that assemble at
the interface and reduce the interfacial energy. Microfluidic techniques
provide a simple and versatile route to produce one liquid phase in
a second where the shape of the dispersed liquid phase can range from
droplets to tubules depending on the flow conditions and the interfacial
energies. In this study, the effect of NP surfactants on Plateau–Rayleigh
(PR) instabilities of a free-falling jet of an aqueous dispersion
of carboxylic acid functionalized silica NPs into a toluene phase
containing amine-terminated polydimethylsiloxane (PDMS–NH<sub>2</sub>) is investigated. NP surfactants were found to significantly
affect the breakup of laminar liquid jets, resulting in longer jet
breakup lengths and dripping to jetting flow transitions
Minute-MOFs: Ultrafast Synthesis of M<sub>2</sub>(dobpdc) Metal–Organic Frameworks from Divalent Metal Oxide Colloidal Nanocrystals
The material demands for metal–organic
frameworks (MOFs)
for next-generation energy-efficient CO<sub>2</sub> capture technologies
necessitate advances in their expedient and scalable synthesis. Toward
that end, the recently discovered expanded MOF-74, or M<sub>2</sub>(dobpdc), where M = divalent metal cation and dobpdc = 4,4′-dioxido-3,3′-biphenyldicarboxylate,
can now be prepared in minutes via a controlled dissolution–crystallization
route from divalent metal oxides as precursors. We show that the available
surface area of the metal oxide plays a critical role in the precursor
dissolution, which was found to be rate-limiting. Based on this understanding
of the reaction trajectory, we pushed the chemical transformation
to its fringe kinetic limit by configuring the metal oxide precursors
as ligand-free colloidal metal oxide nanocrystals, which allowed MOF
formation in less than 1 min. MOFs prepared by this strategy were
highly crystalline, with BET surface areas on par with conventional
multihour syntheses from metal halide salts. This method was also
applied successfully in the synthesis of M<sub>2</sub>(dobdc) MOFs,
highlighting its generality. Our work challenges the conventional
wisdom that plurality of steps in MOF formation is inherently time-intensive
Redox-Active Supramolecular Polymer Binders for Lithium–Sulfur Batteries That Adapt Their Transport Properties in Operando
Ï€-Stacked
perylene bisimide (PBI) molecules are implemented
here as highly networked, redox-active supramolecular polymer binders
in sulfur cathodes for lightweight and energy-dense Li–S batteries.
We show that the in operando reduction and lithiation of these PBI
binders sustainably reduces Li–S cell impedance relative to
nonredox active conventional polymer binders. This lower impedance
enables high-rate cycling in Li–S cells with excellent durability,
a critical step toward unlocking the full potential of Li–S
batteries for electric vehicles and aviation
Aqueous-Processable Redox-Active Supramolecular Polymer Binders for Advanced Lithium/Sulfur Cells
Lithium/Sulfur (Li/S)
cells are a promising chemistry with potential
to deliver a step-change in energy density compared to state-of-the-art
Li-ion batteries. To minimize the environmental impact of the Li/S
cell manufacturing and to compete with Li-ion cells in both performance
and cost, electrodes cast using an aqueous process are highly desirable.
Here we describe the discovery and application of a lithiated redox-mediating
supramolecular binder based on the well-known n-type semiconductor,
perylene bisimide, that forms high-fidelity sulfur electrodes from
water-processed slurries. A 1.4-fold improvement in sulfur utilization
at 3.0 C and 58% increase in capacity retention after 250 cycles at
1.5 C are reported for the prelithiated, supramolecular binder compared
to control samples. These improvements are attributed to the self-assembly
of lithiated perylene bisimide binders in water to yield nanowire
web morphologies that increase interfacial area between electrode
components and exhibit enhanced electrode-current collector adhesion
Influence of Surface Composition on Electronic Transport through Naked Nanocrystal Networks
Influence of Surface Composition on Electronic Transport
through Naked Nanocrystal Network
Diamine-Appended Mg<sub>2</sub>(dobpdc) Nanorods as Phase-Change Fillers in Mixed-Matrix Membranes for Efficient CO<sub>2</sub>/N<sub>2</sub> Separations
Despite
the availability of chemistries to tailor the pore architectures
of microporous polymer membranes for chemical separations, trade-offs
in permeability and selectivity with functional group manipulations
nevertheless persist, which ultimately places an upper bound on membrane
performance. Here we introduce a new design strategy to uncouple these
attributes of the membrane. Key to our success is the incorporation
of phase-change metal–organic frameworks (MOFs) into the polymer
matrix, which can be used to increase the solubility of a specific
gas in the membrane, and thereby its permeability. We further show
that it is necessary to scale the size of the phase-change MOF to
nanoscopic dimensions, in order to take advantage of this effect in
a gas separation. Our observation of an increase in solubility and
permeability of only one of the gases during steady-state permeability
measurements suggests fast exchange between free and chemisorbed gas
molecules within the MOF pores. While the kinetics of this exchange
in phase-change MOFs are not yet fully understood, their role in enhancing
the efficacy and efficiency of the separation is clearly a compelling
new direction for membrane technology
Polysulfide-Blocking Microporous Polymer Membrane Tailored for Hybrid Li-Sulfur Flow Batteries
Redox
flow batteries (RFBs) present unique opportunities for multi-hour electrochemical
energy storage (EES) at low cost. Too often, the barrier for implementing
them in large-scale EES is the unfettered migration of redox active
species across the membrane, which shortens battery life and reduces
Coulombic efficiency. To advance RFBs for reliable EES, a new paradigm
for controlling membrane transport selectivity is needed. We show
here that size- and ion-selective transport can be achieved using
membranes fabricated from polymers of intrinsic microporosity (PIMs).
As a proof-of-concept demonstration, a first-generation PIM membrane
dramatically reduced polysulfide crossover (and shuttling at the anode)
in lithium–sulfur batteries, even when sulfur cathodes were
prepared as flowable energy-dense fluids. The design of our membrane
platform was informed by molecular dynamics simulations of the solvated
structures of lithium bisÂ(trifluoromethanesulfonyl)Âimide (LiTFSI)
vs lithiated polysulfides (Li<sub>2</sub>S<sub><i>x</i></sub>, where <i>x</i> = 8, 6, and 4) in glyme-based electrolytes
of different oligomer length. These simulations suggested polymer
films with pore dimensions less than 1.2–1.7 nm might incur
the desired ion-selectivity. Indeed, the polysulfide blocking ability
of the PIM-1 membrane (∼0.8 nm pores) was improved 500-fold
over mesoporous Celgard separators (∼17 nm pores). As a result,
significantly improved battery performance was demonstrated, even
in the absence of LiNO<sub>3</sub> anode-protecting additives
Mechanistic Insight into the Formation of Cationic Naked Nanocrystals Generated under Equilibrium Control
Cationic naked nanocrystals (NCs)
are useful building units for
assembling hierarchical mesostructured materials. Until now, their
preparation required strongly electrophilic reagents that irreversibly
sever bonds between native organic ligands and the NC surface. Colloidal
instabilities can occur during ligand stripping if exposed metal cations
desorb from the surface. We hypothesized that cation desorption could
be avoided were we able to stabilize the surface during ligand stripping
via ion pairing. We were successful in this regard by carrying out
ligand stripping under equilibrium control with Lewis acid–base
adducts of BF<sub>3</sub>. To better understand the microscopic processes
involved, we studied the reaction pathway in detail using in situ
NMR experiments and electrospray ionization mass spectrometry. As
predicted, we found that cationic NC surfaces are transiently stabilized
post-stripping by physisorbed anionic species that arise from the
reaction of BF<sub>3</sub> with native ligands. This stabilization
allows polar dispersants to reach the NC surface before cation desorption
can occur. The mechanistic insights gained in this work provide a
much-needed framework for understanding the interplay between NC surface
chemistry and colloidal stability. These insights enabled the preparation
of stable naked NC inks of desorption-susceptible NC compositions
such as PbSe, which were easily assembled into new mesostructured
films and polymer-nanocrystal composites with wide-ranging technological
applications
Materials Genomics Screens for Adaptive Ion Transport Behavior by Redox-Switchable Microporous Polymer Membranes in Lithium–Sulfur Batteries
Selective ion transport across membranes
is critical to the performance
of many electrochemical energy storage devices. While design strategies
enabling ion-selective transport are well-established, enhancements
in membrane selectivity are made at the expense of ionic conductivity.
To design membranes with both high selectivity and high ionic conductivity,
there are cues to follow from biological systems, where regulated
transport of ions across membranes is achieved by transmembrane proteins.
The transport functions of these proteins are sensitive to their environment:
physical or chemical perturbations to that environment are met with
an adaptive response. Here we advance an analogous strategy for achieving
adaptive ion transport in microporous polymer membranes. Along the
polymer backbone are placed redox-active switches that are activated
in situ, at a prescribed electrochemical potential, by the device’s
active materials when they enter the membrane’s pore. This
transformation has little influence on the membrane’s ionic
conductivity; however, the active-material blocking ability of the
membrane is enhanced. We show that when used in lithium–sulfur
batteries, these membranes offer markedly improved capacity, efficiency,
and cycle-life by sequestering polysulfides in the cathode. The origins
and implications of this behavior are explored in detail and point
to new opportunities for responsive membranes in battery technology
development
Assembly of Ligand-Stripped Nanocrystals into Precisely Controlled Mesoporous Architectures
The properties of mesoporous materials hinge on control
of their
composition, pore dimensions, wall thickness, and the size and shape
of the crystallite building units. We create ordered mesoporous materials
in which all of these parameters are independently controlled. Different
sizes (from 4.5 to 8 nm) and shapes (spheres and rods) of ligand-stripped
nanocrystals are assembled using the same structure-directing block
copolymers, which contain a tethering domain designed to adsorb to
their naked surfaces. Material compositions range from metal oxides
(Sn-doped In<sub>2</sub>O<sub>3</sub> or ITO, CeO<sub>2</sub>, TiO<sub>2</sub>) to metal fluorides (Yb,Er-doped NaYF<sub>4</sub>) and metals
(FePt). The incorporation of new types of nanocrystals into mesoporous
architectures can lead to enhanced performance. For example, TiO<sub>2</sub> nanorod-based materials withstand >1000 electrochemical
cycles
without significant degradation