• Energy Research

Abstract Concerns over global climate change have led to strong research emphasis worldwide on reducing the emission of greenhouse gases like CO2. One avenue for carbon emission reduction is using CO2 capture and storage from industrial sources. Having low toxicity and low vapor pressure and being resistant to oxidation, natural amino acids could be a better choice over current carbon capture materials. In this study, we pioneered a unique phase change amino acid salt solvent concept in which amino acid salt solution was turned into a CO2-rich phase and a CO2-lean phase upon simple bubbling with CO2 and most importantly, this solution captured the most CO2 (∼90%) in the CO2-rich phase. Bicarbonate was found to be dominant in the CO2-rich phase, which had a high CO2 loading capacity and good regenerability and cycling properties. Such a phase change amino acid salt solvent may provide unique solutions for industries to reduce CO2 and other harmful emissions.

The ubiquitous and growing global reliance on rare earth elements (REEs) for modern technology and the need for reliable domestic sources underscore the rising trend in REE-related research. Adsorption-based methods for REE recovery from liquid waste sources are well-positioned to compete with those of solvent extraction, both because of their expected lower negative environmental impact and simpler process operations. Functionalized silica represents a rising category of low cost and stable sorbents for heavy metal and REE recovery. These materials have collectively achieved high capacity and/or high selective removal of REEs from ideal solutions and synthetic or real coal wastewater and other leachate sources. These sorbents are competitive with conventional materials, such as ion exchange resins, activated carbon; and novel polymeric materials like ion-imprinted particles and metal organic frameworks (MOFs). This critical review first presents a data mining analysis for rare earth element recovery publications indexed in Web of science, highlighting changes in REE recovery research foci and confirming the sharply growing interest in functionalized silica sorbents. A detailed examination of sorbent formulation and operation strategies to selectively separate heavy (HREE), middle (MREE), and light (LREE) REEs from the aqueous sources is presented. Selectivity values for sorbents were largely calculated from available figure data and gauged the success of the associated strategies, primarily: (1) silane-grafted ligands, (2) impregnated ligands, and (3) bottom-up ligand/silica hybrids. These were often accompanied by successful co-strategies, especially bite angle control, site saturation, and selective REE elution. Recognizing the need to remove competing fouling metals to achieve purified REE "baskets," we highlight techniques for eliminating these species from acid mine drainage (AMD) and suggest a novel adsorption-based process for purified REE extraction that could be adapted to different water systems.

Abstract Solid sorbents can be used to capture CO2 from pre-combustion sources at various temperatures. MgO and CaO are typical medium- and high-temperature CO2 sorbents. However, pure MgO is not active toward CO2. The addition of Na2CO3 increases the operating temperature and significantly increases the reactivity of sorbents to capture CO2. Na2CO3-promoted MgO is a promising medium-temperature CO2 sorbent. In this study, the thermodynamic performance of integrated gasification combined cycle (IGCC) systems with Na2CO3–MgO-based warm gas decarbonation (WGDC) and CaO-based hot gas decarbonation (HGDC) is evaluated and compared with that of an IGCC system with methyldiethanolamine (MDEA)-based cold gas decarbonation (CGDC). Assuming that the average CO2 capture capacities of solid sorbents are one-third of their theoretical maxima, we reveal that the IGCC system undergoes approximately 2.8% and 3.6% improvement on net efficiency when switching from CGDC to WGDC and to HGDC, respectively. The net efficiency of the system is increased by improving the CO2 capture capacity of the sorbent. The IGCC with Na2CO3–MgO experiences more significant increase in efficiency than that with CaO along with the improvement of sorbent average CO2 capture capacity. The efficiency of the IGCC systems reaches the same value when the average CO2 capture capacities of both sorbents are 53% of their theoretical levels. The effects of gas turbine combustor fuel gas inlet temperature on IGCC system performance are analyzed. Results show that the efficiency of the IGCC systems with HGDC and WGDC increases by 0.74% and 0.53% respectively as the fuel gas inlet temperature increases from 250 °C to 650 °C.

Abstract Absorption and desorption of carbon dioxide on Na 2 CO 3 -promoted MgO have been studied at temperatures compatible with warm gas cleanup (300–470 °C) from a pre-combustion syngas. The absorbents are synthesized through the formation and activation of the precipitate resulting from the addition of sodium carbonate to an aqueous solution of magnesium nitrate. The absorbent, which comprises MgO, Na 2 CO 3 and residual NaNO 3 after activation, forms the double salt Na 2 Mg(CO 3 ) 2 on exposure to CO 2 . The thermodynamic properties of the double salt, obtained through computational calculation, predict that the preferred temperature range for absorption of CO 2 with the double salt is significantly higher compared with MgO. Faster CO 2 uptake can be achieved as a result of this higher temperature absorption window. Absorption tests indicate that the double salt absorbent as prepared has a capacity toward CO 2 of 15 wt.% (3.4 mmol CO 2 /g absorbent) and can be easily regenerated through both pressure swing and temperature swing absorption in multiple-cycle tests. Thermodynamic calculations also predict an important effect of CO 2 partial pressure on the absorption capacity in the warm temperature range. The impurity phase, NaNO 3 , is identified as a key component in facilitating CO 2 absorption by these materials. The reason for reported difficulties in reproducing the performance of these materials can be traced to specific details of the synthesis method, which are reviewed in some detail.

Abstract Perovskite iron oxides are promising oxygen carrying materials due to their effectiveness and the low cost of iron. The effect of Ca2+ doping on oxygen ion diffusion in Sr1-xCaxFeO3-δ (x = 0, 0.125, 0.25, 0.375, 0.5) is investigated by combining density functional theory (DFT) calculations and experimental measurements. The oxygen ion diffusion is determined by two key factors of oxygen vacancy formation and migration. The DFT results show that the oxygen vacancy formation energies greatly decrease as Ca2+ content reaches x = 0.125, then gradually decrease with Ca2+ contents up to 0.375, and finally increase as the Ca content reaches x = 0.5. A combination of experimental thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results corroborate this trend for Ca2+ contents between 0 and 0.4. The Fe-O bonding dominates the effect of Ca2+ doping on the oxygen vacancy formation. Shortened Fe-O bonds cause the decrease in the formation energy at lower Ca2+ contents, while the lengthened bonds by FeO6 octahedron distortion cause the increase in the formation energy at higher Ca2+ contents. Kinetically, the oxygen migration barrier is lowered upon Ca2+ doping through the increasing lattice spacing for oxygen diffusion. Therefore, an appropriate Ca2+ doping of x = 0.125–0.375 promotes the oxygen ion diffusion in Sr1-xCaxFeO3-δ. Our findings provide the effective Ca2+ doping value for Sr1-xCaxFeO3-δ and a material design clue for the isovalent A-site doping system of oxygen carriers.

Alkali metal zirconates could be used as solid sorbents for CO2 capture. The structural, electronic, and phonon properties of Na2ZrO3, K2ZrO3, Na2CO3, and K2CO3 are investigated by combining the density functional theory with lattice phonon dynamics. The thermodynamics of CO2 absorption/desorption reactions of these two zirconates are analyzed. The calculated results show that their optimized structures are in a good agreement with experimental measurements. The calculated band gaps are 4.339 eV (indirect), 3.641 eV (direct), 3.935 eV (indirect), and 3.697 eV (direct) for Na2ZrO3, K2ZrO3, Na2CO3, and K2CO3, respectively. The calculated phonon dispersions and phonon density of states for M2ZrO3 and M2CO3 (M = K, Na, Li) revealed that from K to Na to Li, their frequency peaks are shifted to high frequencies due to the molecular weight decreased from K to Li. From the calculated reaction heats and relationships of free energy change versus temperatures and CO2 pressures of the M2ZrO3 (M = K, Na, Li) reacting with CO2, we found that the performance of Na2ZrO3 capturing CO2 is similar to that of Li2ZrO3 and is better than that of K2ZrO3. Therefore, Na2ZrO3 and Li2ZrO3 are good candidates of high temperature CO2 sorbents and could be used for post-combustion CO2 capture technologies.

Due to continuing high demand, depletion of non-renewable resources and increasing concerns about climate change, the use of fossil fuel-derived transportation fuels faces relentless challenges both from a world markets and an environmental perspective. The production of renewable transportation fuel from microalgae continues to attract much attention because of its potential for fast growth rates, high oil content, ability to grow in unconventional scenarios, and inherent carbon neutrality. Moreover, the use of microalgae would minimize “food versus fuel” concerns associated with several biomass strategies, as microalgae do not compete with food crops in the food chain. This paper reviews the progress of recent research on the production of transportation fuels via homogeneous and heterogeneous catalytic conversions of microalgae. This review also describes the development of tools that may allow for a more fundamental understanding of catalyst selection and conversion processes using computational modelling. The catalytic conversion reaction pathways that have been investigated are fully discussed based on both experimental and theoretical approaches. Finally, this work makes several projections for the potential of various thermocatalytic pathways to produce alternative transportation fuels from algae, and identifies key areas where the authors feel that computational modelling should be directed to elucidate key information to optimize the process.

Lithium zirconates have attracted researchers’ interests because they can also be used as solid sorbents for CO2 capture. The structural, electronic, and phonon properties of Li2ZrO3, Li6Zr2O7, and monoclinic phase ZrO2 are investigated by the density-functional theory and phonon dynamics. Their electrochemical properties and their thermodynamics of CO2 absorption/desorption are analyzed. The calculated results show that their optimized structures and calculated bulk moduli as well as cohesive energies are in good agreement with experimental measurements. The calculated band gaps are 3.90 eV (indirect), 3.98 eV (direct), and 3.76 eV (direct) for Li2ZrO3, Li6Zr2O7, and ZrO2, respectively. The calculated Li intercalation voltage and energy densities of Li2ZrO3 are higher than that of Li6Zr2O7, which indicates that as a cathode material Li2ZrO3 is better than Li6Zr2O7. The calculated phonon dispersions and density of states show that there is one soft mode in Li2ZrO3 and two soft modes in Li6Zr2O7. From the calculated thermodynamic properties of these two lithium zirconates reacting with CO2, we found that the performance of Li2ZrO3 as a CO2 sorbent is better than that of Li6Zr2O7. In the first half cycle, sorbents absorbing CO2 to form lithium carbonate, Li6Zr2O7 performs better than Li2ZrO3 because the former releases more heat of reaction and has a lower Gibbs free energy and a higher CO2 capture capacity. However, during the second half cycle, regenerating sorbent from carbonate and zirconia to release CO2, the main product is the thermodynamically favorable Li2ZrO3 rather than forming Li6Zr2O7.

The logic in the design of a halide-mixed APb(I1−xBrx)3 perovskite is quite straightforward: to combine the superior photovoltaic qualities of iodine-based perovskites with the increased stability of bromine-based perovskites. However, even small amounts of Br doped into the iodine-based materials leads to some instability. In the present report, using first-principles computations, we analyzed a wide variety of α-CsPbI2Br and β-CsPbI2Br phases, compared their mixing enthalpies, explored their oxidative properties, and calculated their hole-coupled and hole-free charged Frenkel defect (CFD) formations by considering all possible channels of oxidation. Nanoinclusions of bromine-rich phases in α-CsPbI2Br were shown to destabilize the material by inducing lattice strain, making it more susceptible to oxidation. The uniformly mixed phase of α-CsPbI2Br was shown to be highly susceptible towards a phase transformation into β-CsPbI2Br when halide interstitial or halide vacancy defects were introduced into the lattice. The rotation of PbI4Br2 octahedra in α-CsPbI2Br allows it either to transform into a highly unstable apical β-CsPbI2Br, which may phase-segregate and is susceptible to CFD, or to phase-transform into equatorial β-CsPbI2Br, which is resilient against the deleterious effects of hole oxidation (energies of oxidation >0 eV) and demixing (energy of mixing <0 eV). Thus, the selective preparation of equatorial β-CsPbI2Br offers an opportunity to obtain a mixed perovskite material with enhanced photostability and an intermediate bandgap between its constituent perovskites.