Modelling Selective CO2 Absorption and Validation via Photosynthetic Bacteria and Chemical Adsorbents for Methane Purification in Anaerobic Fermentation Bioreactors.

This study delves into advanced methane purification techniques within anaerobic fermentation bioreactors, focusing on selective CO2 absorption and comparing photosynthetic bacteria (PNSB) with chemical adsorbents. Our investigation demonstrates that MgO-Mg(OH)2 composites exhibit remarkable CO2 selectivity over CH4, substantiated through rigorous bulk and surface modelling analyses. To address the challenges posed by MgCO3 shell formation on MgO particles, hindering CO2 transport, we advocate for the utilisation of MgO-Mg(OH)2 composites. In on-site experiments, these composites, particularly saturated MgO-Mg(OH)2 solutions (S2), achieved an astonishing 100% CO2 removal rate within a single day while preserving CH4 content. In contrast, solid MgO powder (S3) retained a mere 5% of CH4 over a 10 h period. Although PNSB (S1) exhibited slower CO2 removal, it excelled in nutrient recovery from anaerobic effluent. We introduce a groundbreaking hybrid strategy that leverages S2's swift CO2 removal and S1 PNSB's nutrient recovery capabilities, potentially resulting in a drastic reduction in bioreactor processing time, from 10 days when employing S1 to just 1 day with the use of S2. This represents a remarkable efficiency improvement of 1000%. This pioneering strategy has the potential to revolutionise methane purification, enhancing both efficiency and sustainability. Importantly, it can be seamlessly integrated into existing bioreactors through an additional CO2 capture step, offering a promising solution for advancing biogas production and promoting sustainable waste treatment practices.

Enhancing MgO efficiency in CO2 capture: engineered MgO/Mg(OH)2 composites with Cl-, SO42-, and PO43- additives.

The formation of a MgCO3 shell hampers CO2 capture efficiency in MgO. Our previous studies developed MgO/Mg(OH)2 composites to facilitate CO2 diffusion, improving capture efficiency. However, MgCO3 still formed along the interfaces. To tackle this issue, we engineered the MgO/Mg(OH)2 interfaces by incorporating Cl-, SO42-, and PO43- additives. Novel MgO-H2O-MgX (X = Cl-, SO42-, and PO43-) composites were synthesized to explore the role of additives in preventing MgCO3 formation. MgO-Mg(OH)2-MgCl2 nano-composites displayed enhanced CO2 adsorption and stability. This breakthrough paves the way for effective bio-inspired strategies in overcoming CO2 transport barriers in MgO-based adsorbents.

Layer-dependent stability of 2D mica nanosheets.

We report on the layer-dependent stability of muscovite-type two-dimensional (2D) mica nanosheets (KAl3Si3O10(OH)2). First-principles calculations on mica nanosheets with different layer thicknesses (n = 1, 2, and 3) reveal their layer-dependent stability; odd-numbered 2D mica nanosheets are more stable than even-numbered ones, and the preferable stability of odd-numbered layers originates from electronic effects. A core-shielding model is proposed with a reasonable assumption, successfully proving the instability of the even-numbered mica nanosheets. Raman imaging supports that the population of odd-numbered mica nanosheets is predominant in exfoliated mica products. The alternating charge states with odd/even layers were evidenced by Kelvin probe force microscopy. We also demonstrate a unique photocatalytic degradation, opening new doors for environmental applications of mica nanosheets.

Efficient Synthesis of 2D Mica Nanosheets by Solvothermal and Microwave-Assisted Techniques for CO2 Capture Applications.

Mica, a commonly occurring mineral, has significant potential for various applications due to its unique structure and properties. However, due to its non-Van Der Waals bonded structure, it is difficult to exfoliate mica into ultrathin nanosheets. In this work, we report a rapid solvothermal microwave synthesis of 2D mica with short reaction time and energy conservation. The resulting exfoliated 2D mica nanosheets (eMica nanosheets) were characterized by various techniques, and their ability to capture CO2 was tested by thermogravimetric analysis (TGA). Our results showed an 87% increase in CO2 adsorption capacity with eMica nanosheets compared to conventional mica. Further characterization by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), as well as first-principles calculations, showed that the high specific surface area and deposited K2CO3 layer contribute to the increased CO2 adsorption on the mica nanosheets. These results speak to the potential of high-quality eMica nanosheets and efficient synthesis processes to open new avenues for new physical properties of 2D materials and the development of CO2 capture technologies.

Origin of observed narrow bandgap of mica nanosheets.

Mica nanosheets possess peculiar feature of narrowed bandgap with the decrease of thickness but a conclusive theoretical understanding of the narrowing mechanisms is still under development. In this report, first-principles calculations were carried out to investigate the electronic band structure of mica nanosheets with the deposition of K2CO3. Bulk mica shows an indirect bandgap of 4.90 eV. Mica nanosheets show similar electronic structures to bulk mica with a gradually increased bandgap of 4.44 eV, 4.52 eV and 4.67 eV for 1-layer, 2-layers and 3-layers nanosheets, respectively, which is attributed to the lattice relaxation. K2CO3 is found to have strong affinity towards mica nanosheets. The K2CO3 deposited mica nanosheets showed an increased bandgap with the increase of thickness, consistent with experimental observations. The calculated bandgap of K2CO3 deposited mica for 2-layers and 3-layers nanosheets are 2.60 eV and 2.75 eV, respectively, which are comparable with the corresponding experimental values of 2.5 eV and 3.0 eV. Our theoretical findings support the experimental evidence of surface contamination of mica by K2CO3, and provide new insight into the structure and properties of 2D mica.

Morphology Design and Fabrication of Bio-Inspired Nano-MgO-Mg(OH)2 via Vapor Steaming to Enable Bulk CO2 Diffusion and Capture.

The absorption of CO2 on MgO is being studied in depth in order to enhance carbon engineering. Production of carbonate on MgO surfaces, such as MgCO3, for example, has been shown to hinder further carbon lattice transit and lower CO2 collecting efficiency. To avoid the carbonate blocking effect, we mimic the water harvesting nano-surface systems of desert beetles, which use alternate hydrophobic and hydrophilic surface domains to collect liquid water and convey condensed droplets down to their mouths, respectively. We made CO2-philic MgO and CO2-phobic Mg(OH)2 nanocomposites from electrospun nano-MgO by vapor steaming for 2-20 min at 100 °C. The crystal structure, morphology, and surface properties of the produced samples were instrumentally characterized using XRD, SEM, XPS, BET, and TGA. We observed that (1) fiber morphology shifted from hierarchical particle and sheet-like structures to flower-like structures, and (2) CO2 capture capacity shifted by around 25%. As a result, the carbonate production and breakdown processes may be managed and improved using vapor steaming technology. These findings point to a new CO2 absorption technique and technology that might pave the way for more CO2 capture, mineralization, and fuel synthesis options.

Engineering Electronic Structure and Band Alignment of 2D Mg(OH)2 via Anion Doping for Photocatalytic Applications.

The wide bandgap of 2D Mg(OH)2 inhibits its applications in visible-light photocatalytic applications. Besides, its mismatched band alignment to the redox potential of O2/H2O, brings about low efficacy of water-splitting performance. Therefore, to release the powder of 2D Mg(OH)2 in photocatalytic research, we explore anion doping strategies to engineer its electronic structure. Here, anion doping effects on electronic properties of 2D Mg(OH)2 are investigated by using DFT calculations for seven dopants (F, Cl, S, N, P, SO4, and PO4). We found (1) S, N and P doping remarkably reduces its band gap from 4.82 eV to 3.86 eV, 3.79 eV and 2.69 eV, respectively; (2) the band gap reduction is induced by the electron transfer to the dopant atoms; (3) F, Cl, SO4, and PO4 doping shifts its valence band to be lower than the oxidation potential of O2/H2O to render its band structure appropriate for photocatalytic water splitting. These results suggest that not only electrical conductivity of 2D Mg(OH)2 can be increased but also their band structure be aligned by using the proposed anion doping strategy. These results enable a new photocatalytic materials design approach while offering exciting possibilities in applications of high-current electrolysis, chemical gas sensing, and photocatalysis.

Multi Band Gap Electronic Structure in CH3NH3PbI3.

Organo-lead halide perovskite solar cells represent a revolutionary shift in solar photovoltaics, introducing relatively soft defect containing semiconductors as materials with excellent charge collection for both electrons and holes. Although they are based on the nominally simple cubic perovskite structure, these compounds are in fact very complex. For example, in (CH3NH3)PbI3 the dynamics and ensuing structural fluctuations associated with the (CH3NH3)+ ions and the interplay with the electronic properties are still not fully understood, despite extensive study. Here, using ab-initio calculations, we show that at room and higher temperature, the rotation of CH3NH3 molecules can be viewed as effectively giving local structures that are cubic and tetragonal like from the point of view of the PbI3 framework, though in fact having lower symmetry. Both of these structures are locally polar, with sizable polarization, ~10 µC/cm2 due to the dipoles on the organic. They become energetically degenerate in the volume range, V ~ 250 Å3/f.u-265 Å3/f.u. We also find very significant dependence of the band gap on the local structure. This type of transition is analogous to a transition between two ferroelectric structures, where in-spite of strong electron phonon coupling, there is strong screening of charged defects which can lead to enhanced mobility and charge collection. The results provide insights into the enhanced light absorption near the band edge and good charge collection in this material.

Doping-enhanced lithium diffusion in lithium-ion batteries.

We disclose a distortion-assisted diffusion mechanism in Li3N and Li2.5Co0.5N by first-principles simulations. A B(2g) soft mode at the Γ point is found in α-Li3N, and a more stable α'-Li3N (P3m1) structure, which is 0.71 meV lower in energy, is further derived. The same soft mode is inherited into Li2.5Co0.5N and is enhanced due to Co doping. Consequently, unlike the usual Peierls spin instability along Co-N chains, large lithium-ion displacements on the Li-N plane are induced by a set of soft modes. Such a distortion is expected to offer Li atoms a route to bypass the high diffusion barrier and promote Li-ion conductivity. In addition, we further illustrate abnormal Born effective charges along Co-N chains which result from the competition between the motions of electrons and ion cores. Our results provide future opportunities in both fundamental understanding and structural modifications of Li-ion battery materials.

Proton interaction in phosphate adsorption onto goethite.

The adsorption of phosphate on goethite is generally modeled by assuming a simple ligand exchange reaction with surface hydroxyl groups. This study investigates the binding forms of phosphate on goethite by evaluating the proton interaction and surface charge change during phosphate adsorption. It is found that OH(-) release stoichiometry increases with phosphate coverage, which suggests that different mechanisms predominate at different phosphate loadings. It demonstrates that surface binding changes from monodentate complexation to bidentate complexation with increasing surface phosphate coverage. The net OH(-) release accompanying this transformation is best interpreted with a 2pK(a) multisite model.

Simultaneous adsorption of copper ions and humic acid onto an activated carbon.

In this study, simultaneous adsorption of copper ions and humic acid (HA) from Aldrich onto an activated carbon is investigated. It is found that the HA adsorption in the absence of copper decreases as the pH is increased. It leads to a reduction of 34.7% in the specific surface area of carbon. There exists a critical concentration (CC) of HA for copper adsorption. At HA concentrations < CC, a decrease in copper adsorption is observed; however, the HA improves the adsorption at HA concentrations > CC. An increase in ionic strength can enhance the copper uptake; however, zinc and/or cobalt ions have an insignificant influence on copper adsorption. The adsorption is significantly increased by citric acid, whereas addition of EDTA slightly decreases the uptake. An intraparticle diffusion model is successfully used to describe the copper adsorption kinetics.

Acid/Base-treated activated carbons: characterization of functional groups and metal adsorptive properties.

Surface modification of activated carbons by various physicochemical methods directs an attractive approach for improvement of heavy metal uptake from aqueous solutions. Activated carbons were modified with HCl and HNO3 optionally followed by NaOH. The effects of surface modifications on the properties of the carbons were studied by the specific surface area, carbon pH, and total acidity capacity as well as by scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The modifications bring about substantial variation in the chemical properties whereas the physical properties remain nearly unchanged. NaOH causes an increase in the content of hydroxyl groups, while the HCl treatment results in an increase in the amount of single-bonded oxygen functional groups such as phenols, ethers, and lactones. The HNO3 modification generates a large number of surface functional groups such as carbonyl, carboxyl, and nitrate groups. The HNO3 modification significantly increases the copper adsorption, while the HCl treatment slightly reduces the copper uptake. Most of the copper ions are adsorbed rapidly in the first 2 h; the adsorption equilibrium is established in around 8 h. An intraparticle diffusion model successfully describes the kinetics of copper adsorption onto the carbons.

Constitutive model development and micro-structural topology optimisation for nafion hydrogel membranes with ionic clustering.

The deployment of electroactive ionic polymer hydrogel-metal composites in artificial muscle and BioMEMS applications has recently been intensively investigated. In order to analyse their electromechanical responses to externally applied electrical fields, it is critical to develop a constitutive model linking the macro-mechanical moduli with the micro-mechanical characteristics, and to determine the geometric size and shape of the micro-structural cluster and investigate the effect of cluster morphology on the effective electro-elastic moduli of the polymer hydrogels. As a typical ionic polymer-based hydrogel, the Nafion membrane is studied in this work. Based on the Biot poroelasticity theory, a multi-scale constitutive model which includes both macro and micro characteristics is developed using an asymptotic homogenisation method. The effect of water-volume fraction on the effective elastic moduli of the hydrogel membrane is examined for different equivalent weights. Numerical investigations show that the simulated effective constitutive moduli agree well with experimental data. The presently developed constitutive model is thus validated. In order to determine the micro-structural shape of the polymer skeleton subject to fluid pressure, a representative volume element (RVE) is designed by topology optimisation of the periodic microstructures of the Nafion hydrogels, through the minimisation of the electro-elastic interaction energy between the polymer-based fluorocarbon matrix and the surrounding fluid. This optimal RVE correctly predicts the geometric shapes of the clusters.