Dielectric Characterization of H2O and CO2 Uptake by Polyethylenimine Films.

The absorption of CO2 by polyethylenimine polymer (PEI) materials is of great interest in connection with proposed carbon capture technologies, and the successful development of this technology requires testing methods quantifying the amount of CO2, H2O, and reaction byproducts under operating conditions. We anticipate that dielectric measurements have the potential for quantifying both the extent of CO2 and H2O absorption within the PEI matrix material as well as insights into subsequent reaction byproducts that can be expected to occur in the presence of moisture. The complexity of the chemistry involved in this reactive binding process clearly points to the need for the use of additional spectroscopic techniques to better resolve the multiple components involved and to validate the model-dependent findings from the dielectric measurements. Here, we employed noncontact resonant microwave cavity instrumentation operating at 7.435 GHz that allows for the precise determination of the complex dielectric permittivity of CO2 films exposed to atmospheres of controlled relative humidity (RH), and N2:CO2 compositions. We find that the addition of CO2 leads to a considerable increase in dielectric loss of the PEI film relative to loss measured in nitrogen (N2) atmosphere across the same RH range. We attribute this effect to a reaction between CO2 and PEI generating a charged dielectrically active species contributing to the dielectric loss in the presence of moisture. Possible reaction mechanisms accounting for these observations are discussed, including the formation of carbamate-ammonium pairs and ammonium cations stabilized by bicarbonate anions that have sufficient local mobility to be dielectrically active in the investigated microwave frequency range. Understanding of these reaction mechanisms and the development of tools to quantify the amount of reactive byproducts are expected to be critical for the design and optimization of carbon capture materials.

Thickness Dependent CO2 Adsorption of Poly(ethyleneimine) Thin Films for Direct Air Capture.

Mesoporous silica impregnated with polyethyleneimine (PEI) has been shown to be a suitable material for the direct air capture (DAC) of CO2. Factors such as CO2 concentration, temperature, and amine loading impact overall capture capacity and amine efficiency by altering diffusional resistance and reaction kinetics. When studied in the impregnated 3-dimensional sorbent material, internal diffusion impacts the evaluation of the reaction kinetics at the air/amine interface. In this work, we designed a novel tandem quartz crystal microbalance with dissipation (QCM-D) and polarization modulation infrared reflective absorption spectroscopy (PM-IRRAS) instrument. CO2 adsorption kinetics of the PEI-based amine layer in a 2-dimensional geometry were studied at a variety of film thicknesses (10 nm to 100 nm), temperatures (25 °C to 80 °C), and CO2 concentrations (5 % and 0.04 % by mole fraction). Total CO2 capture capacity increased with film thickness but decreased amine efficiency, as additional diffusional resistance for thicker films limits access to available amine sites. The capture capacity of thick films (>50 nm) is shown to be limited by amine availability, while capture of thin films (<50 nm) is limited by CO2 availability. A 50 nm PEI film was shown to be optimal for capture of 0.04 % (400 ppm) CO2. The adsorption profiles for these conditions were fitted to pseudo-first order and Avrami fractional order models. The reaction process switches between a diffusion limited reaction to a kinetic limited reaction at 80 °C when using 5 % CO2 and 55 °C when using 0.04 % CO2. These results offer accurate analysis of adsorption of CO2 at the air/amine interface of PEI films which can be used for the design of future sorbent materials.

Phosphate-functionalized Zirconium Metal-Organic Frameworks for Enhancing Lithium-Sulfur Battery Cycling.

Lithium-sulfur batteries are promising candidates for next-generation energy storage devices due to their outstanding theoretical energy density. However, they suffer from low sulfur utilization and poor cyclability, greatly limiting their practical implementation. Herein, we adopted a phosphate-functionalized zirconium metal-organic framework (Zr-MOF) as a sulfur host. With their porous structure, remarkable electrochemical stability, and synthetic versatility, Zr-MOFs present great potential in preventing soluble polysulfides from leaching. Phosphate groups were introduced to the framework post-synthetically since they have shown a strong affinity towards lithium polysulfides and an ability to facilitate Li ion transport. The successful incorporation of phosphate in MOF-808 was demonstrated by a series of techniques including infrared spectroscopy, solid-state nuclear magnetic resonance spectroscopy, and X-ray pair distribution function analysis. When employed in batteries, phosphate-functionalized Zr-MOF (MOF-808-PO4) exhibits significantly enhanced sulfur utilization and ion diffusion compared to the parent framework, leading to higher capacity and rate capability. The improved capacity retention and inhibited self-discharge rate also demonstrate effective polysulfide encapsulation utilizing MOF-808-PO4. Furthermore, we explored their potential towards high-density batteries by examining the cycling performance at various sulfur loadings. Our approach to correlate structure with function using hybrid inorganic-organic materials offers new chemical design strategies for advancing battery materials.

Chemical Sulfide Tethering Improves Low-Temperature Li-S Battery Cycling.

Demands for energy storage and delivery continue to rise worldwide, making it imperative that reliable performance is achievable in diverse climates. Lithium-sulfur (Li-S) batteries offer a promising alternative to lithium-ion batteries owing to their substantially higher specific capacity and energy density. However, improvements to Li-S systems are still needed in low-temperature environments where polysulfide clustering and solubility limitations prohibit complete charge/discharge cycles. We address these issues by introducing thiophosphate-functionalized metal-organic frameworks (MOFs), capable of tethering polysulfides, into the cathode architecture. Compared to cells with the parent MOFs, cells containing the functionalized MOFs exhibit greater capacity delivery and decreased polarization for a range of temperatures down to -10 °C. We conduct thorough electrochemical analyses to ascertain the origins of performance differences and report an altered Li-S redox mechanism enabled by the thiophosphate moiety. This investigation is the first low-temperature Li-S study using MOF additives and represents a promising direction in enabling energy storage in extreme environments.

Graphene-Metal-Organic Framework Composite Sulfur Electrodes for Li-S Batteries with High Volumetric Capacity.

In an age of rapid acceleration toward next-generation energy storage technologies, lithium-sulfur (Li-S) batteries offer the desirable combination of low weight and high specific energy. Metal-organic frameworks (MOFs) have been recently studied as functionalizable platforms to improve Li-S battery performance. However, many MOF-enabled Li-S technologies are hindered by low capacity retention and poor long-term performance due to low electronic conductivity. In this work, we combine the advantages of a Zr-based MOF-808 loaded with sulfur as the active material with a graphene/ethyl cellulose additive, leading to a high-density nanocomposite electrode requiring minimal carbon. Our electrochemical results indicate that the nanocomposites deliver enhanced specific capacity over conventionally used carbon/binder mixtures, and postsynthetic modification of the MOF with lithium thiophosphate results in further improvement. Furthermore, the dense form factor of the sulfur-loaded MOF-graphene nanocomposite electrodes provides high volumetric capacity compared to other works with significantly more carbon additives. Overall, we have demonstrated a proof-of-concept paradigm where graphene nanosheets facilitate improved charge transport because of enhanced interfacial contact with the active material. This materials engineering approach can likely be extended to other MOF systems, contributing to an emerging class of two-dimensional nanomaterial-enabled Li-S batteries.

Lithium Thiophosphate Functionalized Zirconium MOFs for Li-S Batteries with Enhanced Rate Capabilities.

Zirconium metal-organic frameworks (Zr-MOFs) are renowned for their extraordinary stability and versatile chemical tunability. Several Zr-MOFs demonstrate a tolerance for missing linker defects, which create "open sites" that can be used to bind guest molecules on the node cluster. Herein, we strategically utilize these sites to stabilize reactive lithium thiophosphate (Li3PS4) within the porous framework for targeted application in lithium-sulfur (Li-S) batteries. Successful functionalization of the Zr-MOF with PS43- is confirmed by an array of techniques including NMR, XPS, and Raman spectroscopy, X-ray pair distribution function analysis, and various elemental analyses. During electrochemical cycling, we find that even a low incorporation extent of lithium thiophosphate in Zr-MOFs improves sulfur utilization and polysulfide encapsulation to deliver a sustainably high capacity over prolonged cycling. The functionalized MOF additives also prevent cell damage under abusive cycling conditions and recover high capacities when the cell is returned to lower charge/discharge rates, imperative for future energy storage devices. Our unique approach marries the promising chemical attributes of the purely inorganic Li3PS4 with the stability and high surface area of MOFs, creating a Li-S cathode architecture with a performance beyond the sum of its component parts. More broadly, this novel functionalization strategy opens new avenues for facile syntheses of "designer materials" where chemical components from discrete disciplines can be united and tailored for specific applications.

Lithiated Defect Sites in Zr Metal-Organic Framework for Enhanced Sulfur Utilization in Li-S Batteries.

Lithium sulfur (Li-S) battery technology is one of the most promising candidates for next-generation energy storage devices; however, it is still hindered by limited capacity yield and poor long-term stability. The complexity of these devices has hindered efforts to study electrochemical determinants of battery performance, impeding advancement of the field. Due to the ease of functionalization, metal-organic frameworks (MOFs) are unique platforms to explore such reactions, where integration of defects into the crystalline structure provides a convenient method for introducing synthetic handles. In Zr-based MOFs such as UiO-66, the engineered defect sites contain acidic protons that can be replaced with lithium ions, transforming defected MOFs into a range of materials with tunable lithium content. Our results demonstrate the capability of this facile lithiation procedure to create novel cathode additives and evaluate their influence on Li-S battery performance. By improving ionic conductivity and dispersion of sulfur species, lithiated MOFs enhance both sulfur utilization and capacity retention at a variety of cycling rates compared to the as-synthesized MOFs. Our general synthetic strategy has the potential to be applied to technologies beyond MOFs, including polymeric and inorganic materials. Ultimately, we illustrate that defected MOFs can be used to systematically control lithiation, currently unprecedented in conventional inorganic materials, and provide a window to examine heterogeneous reactions relevant to energy conversion and storage.