Oxygen-Assisted Supercapacitive Swing Adsorption of Carbon Dioxide.

We report on the supercapacitive swing adsorption (SSA) of carbon dioxide at different voltage windows in the presence of oxygen using activated carbon electrodes, and deliquescent, aqueous electrolytes. The presence of O2 in the CO2/N2 gas mixture results in an up to 11 times higher CO2 adsorption capacity with 3M MgBr2 (at 0.6V) and up to 4-5 times higher adsorption capacity with 3M MgCl2 (at 1V). A tradeoff between high CO2 adsorption capacities and lower coulombic efficiencies was observed at voltages above 0.6V. The energetic and adsorptive performance of the electrodes in the presence of oxygen below 0.5V was similar to the performance with a CO2/N2 mixture without oxygen at 1V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the electrodes demonstrate that the specific capacitance increases while the diffusion resistance decreases in the presence of oxygen. Oxygen concentrations ranging between 5-20% give similar energetic and adsorptive performance. The electrodes exhibit stable performance for up to 100 cycles of operation.

Scaling Supercapacitive Swing Adsorption of CO2 Using Bipolar Electrode Stacks.

Supercapacitive swing adsorption (SSA) modules with bipolar stacks having 2, 4, 8, and 12 electrode pairs made from BPL 4 × 6 activated carbon are constructed and tested for carbon dioxide capture applications. Tests are performed with simulated flue gas (15%CO2 /85%N2) at 2, 4, 8, and 12 V, respectively. Reversible adsorption with sorption capacities (≈58 mmol kg-1) and adsorption rates (≈38 µmol kg-1 s-1) are measured for all stacks. The productivity scales with the number of cells in the module, and increases from 70 to 390 mmol h-1 m-2. The energy efficiency and energy consumption improve with increasing number of bipolar electrodes from 67% to 84%, and 142 to 60 kJ mol-1, respectively. Overall, the results show that SSA modules with bipolar electrodes can be scaled without reducing the adsorptive performance, and with improvement of energetic performance.

High-Voltage Supercapacitive Swing Adsorption of Carbon Dioxide.

Supercapacitive swing adsorption (SSA) with garlic roots-derived activated carbon achieves a record adsorption capacity of 312 mmol kg-1 at a low energy consumption of 72 kJ mol-1 and high mass loadings (>30 mg cm-2 ) at 1.0 V for 85%N2 /15%CO2 mixtures. The activated carbons are inexpensively prepared in a one-step process using potassium carbonate, and air as activators. The adsorption capacity further increases with increasing voltage. At a voltage of 1.4 V, a sorption capacity of 524 mmol kg-1 at an energy consumption of 130 kJ mol-1 can be achieved. The volumetric sorption capacity is also enhanced and reaches values of 85.7 mol m-3 at 1.0 V, and 126 mol m-3 at 1.4 V. Cycle stability for at least 130 h is demonstrated.

High-Pressure Hydrogen Adsorption on a Porous Electron-Rich Covalent Organonitridic Framework.

We report that a porous, electron-rich, covalent, organonitridic framework (PECONF-4) exhibits an unusually high hydrogen uptake at 77 K, relative to its specific surface area. Chahine's rule, a widely cited heuristic for hydrogen adsorption, sets a maximum adsorptive uptake of 1 wt % hydrogen at 77 K per 500 m2 of the adsorbent surface area. High-pressure hydrogen adsorption measurements in a Sieverts apparatus showed that PECONF-4 exceeds Chahine's rule by 50%. The Brunauer-Emmett-Teller (BET) specific surface area of PECONF-4 was measured redundantly with nitrogen, argon, and carbon dioxide and found to be between 569 ± 2 and 676 ± 13 m2 g-1. Furthermore, hydrogen on PECONF-4 has a high heat of adsorption, in excess of 9 kJ mol-1.

Relationships between Electrolyte Concentration and the Supercapacitive Swing Adsorption of CO2.

We quantitatively investigate the influence of the NaCl electrolyte concentration on the adsorptive and energetic characteristics of supercapacitive swing adsorption (SSA) for the separation of CO2 from a simulated flue gas mixture containing 15% CO2 and 85% N2. The investigated concentrations were that of deionized water, 0.010, 0.10, 1.0, 3.0, and 5.0 M NaCl. We find that the energetic metrics strongly improve with the increasing NaCl concentration, whereas the adsorptive metrics improve by a comparatively small degree. The CO2 adsorption capacity increases up to 1.0 M NaCl and then remains constant. The adsorption rate remains near constant for all concentrations, except that it is somewhat smaller for deionized water. The charge efficiency also remains near constant for all experiments with 30 min potentiostatic holding steps but near doubles for pure water when the potential holding step is doubled, because the chemical adsorption equilibrium is reached only after 60 min. The results can be most satisfactorily explained by assuming that both ionic and nonionic adsorption mechanisms contribute to the SSA effect.

Capacitance for Carbon Capture.

Metal recycling: A sustainable, capacitance-assisted carbon capture and sequestration method can turn scrap metal and CO2 into metal carbonates at an attractive energy cost.

Insights into the Hydrothermal Metastability of Stishovite and Coesite.

Hydrothermal experiments aiming at the crystal growth of stishovite near ambient pressure and temperature were performed in conventional autoclave systems using 1 M (molar) NaOH, 0.8 M Na2CO3, and pure water as a mineralizing agent. It was found that the hydrothermal metastability of stishovite and coesite is very different from the thermal metastability in all mineralizing agents and that because of this fact crystals could not be grown. While stishovite and coesite are thermally metastable up to 500 and >1000 °C, respectively, their hydrothermal metastability is below 150 and 200 °C, respectively. The thermally induced conversion of stishovite and coesite leads to amorphous products, whereas the hydrothermally induced conversion leads to crystalline quartz. Both stishovite and coesite are minerals occurring in nature where they can be exposed to hydrothermal conditions. The low hydrothermal stability of these phases may be an important factor to explain the rarity of these minerals in nature.

Design, construction, and testing of a supercapacitive swing adsorption module for CO2 separation.

We present a device which is able to separate gases from a gas stream using supercapacitive energy. When a small bias (1 V) is applied to the electrodes and a CO2/N2 mixture is fed through the device, carbon dioxide is selectively adsorbed to those electrodes, and N2 leaves the device in a purified form. When the module is discharged, the CO2 is quantitatively desorbed, and leaves the device in a concentrated form. A single module is able to concentrate CO2 from 15% in the feed gas to 46% in the effluent gas. The energy invested to charge the electrodes can be largely recovered upon discharge. At a charging current of 1 mA an energy recovery rate of 78% can be reached and the associated total energy consumption of the device is 57 kJ mol-1.

Recovering fine details from under-resolved electron tomography data using higher order total variation ℓ1 regularization.

Over the last decade or so, reconstruction methods using ℓ1 regularization, often categorized as compressed sensing (CS) algorithms, have significantly improved the capabilities of high fidelity imaging in electron tomography. The most popular ℓ1 regularization approach within electron tomography has been total variation (TV) regularization. In addition to reducing unwanted noise, TV regularization encourages a piecewise constant solution with sparse boundary regions. In this paper we propose an alternative ℓ1 regularization approach for electron tomography based on higher order total variation (HOTV). Like TV, the HOTV approach promotes solutions with sparse boundary regions. In smooth regions however, the solution is not limited to piecewise constant behavior. We demonstrate that this allows for more accurate reconstruction of a broader class of images - even those for which TV was designed for - particularly when dealing with pragmatic tomographic sampling patterns and very fine image features. We develop results for an electron tomography data set as well as a phantom example, and we also make comparisons with discrete tomography approaches.

Experimental and theoretical investigation of a mesoporous K(x)WO3 material having superior mechanical strength.

Mesoporous materials with tailored properties hold great promise for energy harvesting and industrial applications. We have synthesized a novel tungsten bronze mesoporous material (K(x)WO3; x ∼ 0.07) having inverse FDU-12 type pore symmetry and a crystalline framework. In situ small angle X-ray scattering (SAXS) measurements of the mesoporous K(0.07)WO3 show persistence of a highly ordered meso-scale pore structure to high pressure conditions (∼18.5 GPa) and a material with remarkable mechanical strength despite having ∼35% porosity. Pressure dependent in situ SAXS measurements reveal a bulk modulus κ = 44 ± 4 GPa for the mesoporous K(x)WO3 which is comparable to the corresponding value for the bulk monoclinic WO3 (γ-WO3). Evidence from middle angle (MAXS) and wide angle X-ray scattering (WAXS), high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy shows that the presence of potassium leads to the formation of a K-bearing orthorhombic tungsten bronze (OTB) phase within a monoclinic WO3 host structure. Our ab initio molecular dynamics calculations show that the formation of the OTB phase provides superior strength to the mesoporous K(0.07)WO3.

Macrocyclic weakly coordinating anions.

Herein, the concept of macrocyclic weakly coordinating anions (M-WCAs) is introduced. Synthetic methodologies are described how to access M-WCAs by thermodynamically controlled self-assembly in high yields, in particular through condensation and alkyne metathesis reactions. The anticipated properties and applications of M-WCAs in solid state and in solution are discussed, specifically for gas storage and separation, homogeneous and heterogeneous catalysis, and as liquid and solid electrolytes.

Size tunable synthesis of solution processable diamond nanocrystals.

Diamond nanocrystals were synthesized catalyst-free from nanoporous carbon at high pressure and high temperature (HPHT). The synthesized nanocrystals have tunable diameters between 50 and 200 nm. The nanocrystals are dispersible in organic solvents such as acetone and are isotropic in nature as seen by dynamic light scattering.

Supercapacitive swing adsorption of carbon dioxide.

An electrical effect, the supercapacitive swing adsorption (SSA) effect is reported, which allows for reversible adsorption and desorption of carbon dioxide by capacitive charge and discharge of electrically conducting porous carbon materials. The SSA effect can be observed when an electrically conducting, nanoporous carbon material is brought into contact with carbon dioxide gas and an aqueous electrolyte. Charging the supercapacitor electrodes initiates the spontaneous organization of electrolyte ions into an electric double layer at the surface of each porous electrode. The presence of this double layer leads to reversible, selective uptake and release of the CO2 as the supercapacitor is charged and discharged.

Synthetic chemistry with periodic mesostructures at high pressure.

Over the last two decades, researchers have studied extensively the synthesis of mesostructured materials, which could be useful for drug delivery, catalytic cracking of petroleum, or reinforced plastics, among other applications. However, until very recently researchers used only temperature as a thermodynamic variable for synthesis, completely neglecting pressure. In this Account, we show how pressure can affect the synthetic chemistry of periodic mesoporous structures with desirable effects. In its simplest application, pressure can crystallize the pore walls of periodic mesoporous silicas, which are difficult to crystallize otherwise. The motivation for the synthesis of periodic mesoporous silica materials (with pore sizes from 2 to 50 nm) 20 years ago was to replace the microporous zeolites (which have pore sizes of <2 nm) in petroleum cracking applications, because the larger pore size of mesoporous materials allows for faster transport of larger molecules. However, these mesoporous materials could not replace zeolite materials because they showed lower hydrothermal stability and lower catalytic activity. This reduced performance has been attributed to the amorphous nature of the mesoporous materials' channel walls. To address this problem, we developed the concept of "nanocasting at high pressure". Through this approach, we produced hitherto-unavailable, periodic mesostructured silicas with crystalline pore walls. In nanocasting, we compress a periodic mesostructured composite (e.g. a periodic mesoporous silica with carbon-filled pores) and subsequently heat it to induce the selective crystallization of one of the two phases. We attain the necessary high pressure for synthesis using piston-cylinder and multianvil apparatuses. Using periodic mesostructured silica/carbon nanocomposites as starting material, we have produced periodic mesoporous coesite and periodic mesoporous quartz. The quartz material is highly stable under harsh hydrothermal conditions (800°C in pure steam), verifying that crystallinity in the channel walls of periodic mesoporous silicas increases their hydrothermal stability. Even without including the carbon phase in the silica pores, we could obtain mesoporous coesite materials. We found similar behavior for periodic mesoporous carbons, which convert into transparent, mesoporous, nanopolycrystalline diamond at high-pressure. We also show that periodic mesoporous materials can serve as precursors for nanocrystals of high-pressure phases. We obtained nearly monodisperse, discrete stishovite nanocrystals from periodic mesoporous silicas and coesite nanocrystals from periodic mesoporous organosilicas. The stishovite nanocrystals disperse in water and form colloidal solutions of individual stishovite nanocrystals. The stishovite nanocrystals could be useful for machining, drilling, and polishing. Overall, the results show that periodic mesoporous materials are suitable starting materials for the synthesis of nanoporous high-pressure phases and nanocrystals of high pressure phases. The substantially enhanced hydrothermal stability seen in periodic mesoporous silicas synthesized at high pressure demonstrates that high pressure can be a useful tool to produce porous materials with improved properties. We expect that synthesis using mesostructures at high pressure can be extended to many other materials beyond silicas and carbons. Presumably, this chemistry can also be extended from mesoporous to microporous and macroporous materials.

Porous covalent electron-rich organonitridic frameworks as highly selective sorbents for methane and carbon dioxide.

Carbon dioxide capture from point sources like coal-fired power plants is considered to be a solution for stabilizing the CO(2) level in the atmosphere to avoid global warming. Methane is an important energy source that is often highly diluted by nitrogen in natural gas. For the separation of CO(2) and CH(4) from N(2) in flue gas and natural gas, respectively, sorbents with high and reversible gas uptake, high gas selectivity, good chemical and thermal stability, and low cost are desired. Here we report the synthesis and CO(2), CH(4), and N(2) adsorption properties of hierarchically porous electron-rich covalent organonitridic frameworks (PECONFs). These were prepared by simple condensation reactions between inexpensive, commercially available nitridic and electron-rich aromatic building units. The PECONF materials exhibit high and reversible CO(2) and CH(4) uptake and exceptional selectivities of these gases over N(2). The materials do not oxidize in air up to temperature of 400 °C.

Synthesis of coesite nanocrystals from ethane bridged periodic mesoporous organosilica at low temperature and extreme pressure.

Coesite nanocrystals have been synthesized from periodic mesoporous organosilica (PMO) with (CH(2))(2) bridges heated at 300 °C for 150 min and 12 GPa. The crystals are not sintered, single crystalline, and have diameters of ca. 100-300 nm. Below 300 °C, an amorphous non-porous organosilica glass was obtained. Heating above 300 °C at 12 GPa results in the rapid crystal growth and micron size coesite crystals were formed.

Catalyst-free synthesis of transparent, mesoporous diamond monoliths from periodic mesoporous carbon CMK-8.

We report on the synthesis of optically transparent, mesoporous, monolithic diamond from periodic mesoporous carbon CMK-8 at a pressure of 21 GPa. The phase transformation is already complete at a mild synthesis temperature of 1,300 degrees C without the need of a catalyst. Surprisingly, the diamond is obtained as a mesoporous material despite the extreme pressure. X-ray diffraction, SEM, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, and Z-contrast experiments suggest that the mesoporous diamond is composed of interconnected diamond nanocrystals having diameters around 5-10 nm. The Brunauer Emmett Teller surface area was determined to be 33 m2 g(-1) according Kr sorption data. The mesostructure is diminished yet still detectable when the diamond is produced from CMK-8 at 1,600 degrees C and 21 GPa. The temperature dependence of the porosity indicates that the mesoporous diamond exists metastable and withstands transformation into a dense form at a significant rate due to its high kinetic inertness at the mild synthesis temperature. The findings point toward ultrahard porous materials with potential as mechanically highly stable membranes.