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).
Wood, Brandon C.; Stavila, Vitalie; Poonyayant, Natchapol; Heo, Tae W.; Ray, Keith G.; Klebanoff, Leonard E.; Udovic, Terrence J.; Lee, Jonathan R.I.; Angboonpong, Natee; Pakawatpanurut, Pasit
Internal interfaces in the Li3N/[LiNH2 + 2LiH] solid-state hydrogen storage system alter the hydrogenation and dehydrogenation reaction pathways upon nanosizing, suppressing undesirable intermediate phases to dramatically improve kinetics and reversibility. Finally, the key role of solid interfaces in determining thermodynamics and kinetics suggests a new paradigm for optimizing complex hydrides for solid-state hydrogen storage by engineering internal microstructure.
Mg(BH4)2 is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2 to Mg(BH4)2 complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2 dissociation, likely at Mg-terminated MgB2 surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2 complexes without persistent BxHy intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2 are discussed.
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.
Alkali metal borohydrides can reversibly store hydrogen; however, the materials display poor cyclability, oftentimes linked to the occurrence of stable closo-polyborate intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of metal borohydrides, several alkali metal dodecahydro-closo-dodecaborate salts were isolated in anhydrous form and characterized by diffraction and spectroscopic techniques. Mixtures of Li2B12H12, Na2B12H12, and K2B12H12 with the corresponding alkali metal hydrides were subjected to hydrogenation conditions known to favor partial or full reversibility in metal borohydrides. The stoichiometric mixtures of MH and M2B12H12 salts form the corresponding metal borohydrides MBH4 (M = Li, Na, K) in almost quantitative yield at 100 MPa H2 and 500°C. In addition, stoichiometric mixtures of Li2B12H12 and MgH2 were found to form MgB2 at 500°C and above upon desorption in vacuum. The two destabilization strategies outlined above suggest that metal polyhydro-closo-polyborate species can be converted into the corresponding metal borohydrides or borides, albeit under rather harsh conditions of hydrogen pressure and temperature. (Chemical Equation Presented).
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.
This paper uses molecular dynamics simulations to study surface and interface properties of PdHx that are relevant to hydrogen storage applications. In particular, surface energies, interfacial energies, surface diffusivities, and surface segregations are all determined as a function of temperature and composition. During the course of the calculations, we demonstrated robust molecular dynamics methods that can result in highly converged finite temperature properties. Challenging examples include accurate calculations of hydrogen surface diffusivities that account for all possible atomic jump mechanisms, and constructions of surface segregation composition profiles that have negligible statistical errors. Our robust calculations reveal that the Arrhenius plots of hydrogen surface diffusion is ideally linear at low compositions, and becomes nonlinear at high compositions. The fundamental cause for this behavior has been identified. This nonlinear surface diffusion behavioe is also in good agreement with available experimental data for bulk diffusion. The implication of our calculated properties on hydrogen storage application discussed.
Hydrogen diffusion impacts the performance of solid-state hydrogen storage materials and contributes to the embrittlement of structural materials under hydrogen-containing environments. In atomistic simulations, the diffusion energy barriers are usually calculated using molecular statics simulations where a nudged elastic band method is used to constrain a path connecting the two end points of an atomic jump. This approach requires prior knowledge of the "end points". For alloy and defective systems, the number of possible atomic jumps with respect to local atomic configurations is tremendous. Even when these jumps can be exhaustively studied, it is still unclear how they can be combined to give an overall diffusion behavior seen in experiments. Here we describe the use of molecular dynamics simulations to determine the overall diffusion energy barrier from the Arrhenius equation. This method does not require information about atomic jumps, and it has additional advantages, such as the ability to incorporate finite temperature effects and to determine the pre-exponential factor. As a test case for a generic method, we focus on hydrogen diffusion in bulk aluminum. We find that the challenge of this method is the statistical variation of the results. However, highly converged energy barriers can be achieved by an appropriate set of temperatures, output time intervals (for tracking hydrogen positions), and a long total simulation time. Our results help elucidate the inconsistencies of the experimental diffusion data published in the literature. The robust approach developed here may also open up future molecular dynamics simulations to rapidly study diffusion properties of complex material systems in multidimensional spaces involving composition and defects.
The search for solid electrolytes with sufficiently high ionic conductivities and stabilities is underway to enable the commercial viability of all-solid-state rechargeable batteries. LiCB9H10 and NaCB9H10 compounds exhibit the most impressive superionic conductivities yet among complex-hydride-based materials, including this class of large-polyhedral-anion-based salts. The pseudoaromatic nature of the CB9H10 anions makes them relatively stable like their B12H122-, B10H102-, and CB11H122- cousins, rendering their salts prime candidates for incorporation into next-generation, all-solid-state devices. Preliminary cyclic voltammetry measurements indicate that only cathodic and anodic currents are observed near 0 v corresponding to Li/Na deposition on the Au electrode and Li/Na stripping, respectively, without signifi cant anodic currents, at least ≤ 5 v for both LiCB9H10 (363 K) and NaCB9H10 (303 K). The similar conductivity behaviors with temperature for LiCB9H10 and NaCB9H10 compared to those for LiCB11H12 and NaCB11H12 , and their order-of-magnitude enhancements over disordered NaCB9H10, irrespective of structural symmetries, further reinforces the notion that anion monovalency better facilitates high cation translational mobility in these large- polyhedral-anion-based systems.
We demonstrate that metal-organic frameworks (MOFs) can catalyze hydrogenolysis of aryl ether bonds under mild conditions. Mg-IRMOF-74(I) and Mg-IRMOF-74(II) are stable under reducing conditions and can cleave phenyl ethers containing β-O-4, α-O-4, and 4-O-5 linkages to the corresponding hydrocarbons and phenols. Reaction occurs at 10 bar H2 and 120 °C without added base. DFT-optimized structures and charge transfer analysis suggest that the MOF orients the substrate near Mg2+ ions on the pore walls. Ti and Ni doping further increase conversions to as high as 82% with 96% selectivity for hydrogenolysis versus ring hydrogenation. Repeated cycling induces no loss of activity, making this a promising route for mild aryl-ether bond scission.