Metal organic frameworks (MOFs) are extended, nanoporous crystalline compounds consisting of metal ions interconnected by organic ligands. Their synthetic versatility suggest a disruptive class of opto - electronic materials with a high degree of electrical tunability and without the property - degrading disorder of organic conductors. In this project we determined the factors controlling charge and energy transport in MOFs and evaluated their potential for thermoelectric energy conversion. Two strategies for a chieving electronic conductivity in MOFs were explored: 1) using redox active 'guest' molecules introduced into the pores to dope the framework via charge - transfer coupling (Guest@MOF), 2) metal organic graphene analogs (MOGs) with dispersive band structur es arising from strong electronic overlap between the MOG metal ions and its coordinating linker groups. Inkjet deposition methods were developed to facilitate integration of the guest@MOF and MOG materials into practical devices.
Metal organic frameworks (MOFs) have recently attracted great attentions for the thermoelectric (TE) applications, owing to their intrinsic low thermal conductivity, but their TE efficiencies are still low due to the poor electronic transport properties. Here, various synthetic strategies have been designed to optimize the electronic properties of MOFs. Using a series of first principle calculations and band theory, we explore the effect of structural topology and redox matching between the metal and coordinated atoms on the TE transport properties. In conclusion, the presented results provide a fundamental guidance for optimizing electronic charge transport of existing MOFs, and for designing yet to be discovered conductive MOFs for thermoelectric applications.
The Sandia HyMARC team continued its development of new synthetic, modeling, and diagnostic tools that are providing new insights into all major classes of storage materials, ranging from relatively simple systems such as PdHx and MgH2, to exceptionally complex ones, such as the metal borohydrides, as well as materials thought to be very well-understood, such as Ti-doped NaAlH4. This unprecedented suite of capabilities, capable of probing all relevant length scales within storage materials, is already having a significant impact, as they are now being used by both Seedling projects and collaborators at other laboratories within HyMARC. We expect this impact to grow as new Seedling projects begin and through collaborations with other scientists outside HyMARC. In the coming year, Sandia efforts will focus on the highest impact problems, in coordination with the other HyMARC National Laboratory partners, to provide the foundational science necessary to accelerate the discovery of new hydrogen storage materials.
Metal-organic frameworks (MOFs) are highly ordered, functionally tunable supramolecular materials with the potential to improve dye-sensitized solar cells (DSSCs). Several recent reports have indicated that photocurrent can be generated in Grätzel-type DSSC devices when MOFs are used as the sensitizer. However, the specific role(s) of the incorporated MOFs and the potential influence of residual MOF precursor species on device performance are unclear. Herein, we describe the assembly and characterization of a simplified DSSC platform in which isolated MOF crystals are used as the sensitizer in a planar device architecture. We selected a pillared porphyrin framework (PPF) as the MOF sensitizer, taking particular care to avoid contamination from light-absorbing MOF precursors. Photovoltaic and electrochemical characterization under simulated 1-sun and wavelength-selective illumination revealed photocurrent generation that is clearly ascribable to the PPF MOF. Continued refinement of highly versatile MOF structure and chemistry holds promise for dramatic improvements in emerging photovoltaic technologies. (Figure Presented).
Robust time-averaged molecular dynamics has been developed to calculate finiteerature elastic constants of a single crystal. We find that when the averaging time exceeds a certain threshold, the statistical errors in the calculated elastic constants become very small. We applied this method to compare the elastic constants of Pd and PdH0.6 at representative low (10 K) and high (500 K) temperatures. The values predicted for Pd match reasonably well with ultrasonic experimental data at both temperatures. In contrast, the predicted elastic constants for PdH0.6 only match well with ultrasonic data at 10 K; whereas, at 500 K, the predicted values are significantly lower. We hypothesize that at 500 K, the facile hydrogen diffusion in PdH0.6 alters the speed of sound, resulting in significantly reduced values of predicted elastic constants as compared to the ultrasonic experimental data. Literature mechanical testing experiments seem to support this hypothesis.
The well-known synthetic versatility of metal-organic frameworks (MOFs) is rooted in the ability to predict the metal-ion coordination geometry and the vast possibilities to use organic chemistry to modify the linker groups. However, the use of molecules occupying the pores as a component of framework design has been largely ignored. Recent reports show that the presence of these so-called guests can have dramatic effects, even when they are a seemingly innocuous species such as water or polar solvents. We term these guests non-innocent when their presence alters the MOF in such a way as to create a new material with properties different from the MOF without the guests. Advantages of using guest molecules to impart new properties to MOFs include the relative ease of introducing new functionalities, the ability to modify the material properties at will by removing the guest or inserting different ones, and avoidance of the difficulties associated with synthesizing new frameworks, which can be challenging even when the basic topology remains constant. In this article, we describe the Guest@MOF concept and provide examples illustrating its potential as a new MOF design element.
Current methods of detecting material aging rely heavily on accelerated aging studies expensive, bulky, and resource-hungry diagnostics. We are developing compact gas analysis methods based on sensor platforms such as quartz crystal microbalances (QCM), using nanoporous metals and Metal-Organic Frameworks (MOFs), which enhance sensitivity and impart selectivity to analytes. Targeted analytes are O2 and other volatile analytes. In FY16 we installed and tested a new QCM system coupled to a commercial gas mixing system. This instrumentation provides a new multi-use capability that: 1) allows evaluation of detection of novel materials to enable selective detection of volatile species relevant to Enhanced Surveillance; 2) accelerates development of new thin film deposition methods for depositing these materials on sensing devices; and 3) enables in-situ monitoring, with sub-monolayer sensitivity, of the interaction of volatile species with material surfaces subject to aging or corrosion.
As the world transitions from fossil fuels to clean energy sources in the coming decades, many technological challenges will require chemists and material scientists to develop new materials for applications related to energy conversion, storage, and efficiency. Because of their unprecedented adaptability, metal-organic frameworks (MOFs) will factor strongly in this portfolio. By utilizing the broad synthetic toolkit provided by the fields of organic and inorganic chemistry, MOF pores can be customized to suit a particular application. Of particular importance is the ability to tune the strength of the interaction between the MOF pores and guest molecules. By cleverly controlling these MOF-guest interactions, the chemist may impart new function into the Guest@MOF materials otherwise lacking in vacant MOF. Herein, we highlight the concept of the Guest@MOF as it relates to our efforts to develop these materials for energy-related applicatons. Our work in the areas of H2 and noble gas storage, hydrogenolysis of biomass, light-harvesting, and conductive materials will be discussed. Of relevance to light-harvesting applications, we report for the first time a postsynthetic modification strategy for increasing the loading of a light-sensitive electron-donor molecule in the pores of a functionalized MIL-101 structure. Through the demonstrated versatility of these approaches, we show that, by treating guest molecules as integral design elements for new MOF constructs, MOF science can have a significant impact on the advancement of clean energy technologies.