Hydrological droughts in the 21st century, hotspots and uncertainties from a global multimodel ensemble experiment

Increasing concentrations of greenhouse gases in the atmosphere are expected to modify the global water cycle with significant consequences for terrestrial hydrology. We assess the impact of climate change on hydrological droughts in a multimodel experiment including seven global impact models (GIMs) driven by biascorrected climate from five global climate models under four representative concentration pathways (RCPs). Drought severity is defined as the fraction of land under drought conditions. Results show a likely increase in the global severity of hydrological drought at the end of the 21st century, with systematically greater increases for RCPs describing stronger radiative forcings. Under RCP8.5, droughts exceeding 40% of analyzed land area are projected by nearly half of the simulations. This increase in drought severity has a strong signal-to-noise ratio at the global scale, and Southern Europe, the Middle East, the Southeast United States, Chile, and South West Australia are identified as possible hotspots for future water security issues. The uncertainty due to GIMs is greater than that from global climate models, particularly if including a GIM that accounts for the dynamic response of plants to CO2 and climate, as this model simulates little or no increase in drought frequency. Our study demonstrates that different representations of terrestrial water-cycle processes in GIMs are responsible for a much larger uncertainty in the response of hydrological drought to climate change than previously thought. When assessing the impact of climate change on hydrology, it is therefore critical to consider a diverse range of GIMs to better capture the uncertainty

2D Covalent Metals: A New Materials Domain of Electrochemical CO₂ Conversion with Broken Scaling Relationship

Toward a sustainable carbon cycle, electrochemical conversion of CO₂ into valuable fuels has drawn much attention. However, sluggish kinetics and a substantial overpotential, originating from the strong correlation between the adsorption energies of intermediates and products, are key obstacles of electrochemical CO₂ conversion. Here we show that 2D covalent metals with a zero band gap can overcome the intrinsic limitation of conventional metals and metal alloys and thereby substantially decrease the overpotential for CO₂ reduction because of their covalent characteristics. From first-principles-based high-throughput screening results on 61 2D covalent metals, we find that the strong correlation between the adsorption energies of COOH and CO can be entirely broken. This leads to the computational design of CO₂-to-CO and CO₂-to-CH₄ conversion catalysts in addition to hydrogen–evolution–reaction catalysts. Toward efficient electrochemical catalysts for CO₂ reduction, this work suggests a new materials domain having two contradictory properties in a single material: covalent nature and electrical conductance

Multiscale Simulation Method for Quantitative Prediction of Surface Wettability at the Atomistic Level

The solid–liquid interface is of great interest because of its highly heterogeneous character and its ubiquity in various applications. The most fundamental physical variable determining the strength of the solid–liquid interface is the solid–liquid interfacial tension, which is usually measured according to the contact angle. However, an accurate experimental measurement and a reliable theoretical prediction of the contact angle remain lacking because of many practical issues. Here, we propose a first-principles-based simulation approach to quantitatively predict the contact angle of an ideally clean surface using our recently developed multiscale simulation method of density functional theory in classical explicit solvents (DFT-CES). Using this approach, we simulate the surface wettability of a graphene and graphite surface, resulting in a reliable contact angle value that is comparable to the experimental data. From our simulation results, we find that the surface wettability is dominantly affected by the strength of the solid–liquid van der Waal’s interaction. However, we further elucidate that there exists a secondary contribution from the change of water–water interaction, which is manifested by the change of liquid structure and dynamics of interfacial water layer. We expect that our proposed method can be used to quantitatively predict and understand the intriguing wetting phenomena at an atomistic level and can eventually be utilized to design a surface with a controlled hydrophobic­(philic)­ity

Achieving Accurate Reduction Potential Predictions for Anthraquinones in Water and Aprotic Solvents: Effects of Inter- and Intramolecular H‑Bonding and Ion Pairing

In this combined computational and experimental study, specific chemical interactions affecting the prediction of one-electron and two-electron reduction potentials for anthraquinone derivatives are investigated. For 19 redox reactions in acidic aqueous solution, where AQ is reduced to hydroanthraquinone, density functional theory (DFT) with the polarizable continuum model (PCM) gives a mean absolute deviation (MAD) of 0.037 V for 16 species. DFT­(PCM), however, highly overestimates three redox couples with a MAD of 0.194 V, which is almost 5 times that of the remaining 16. These three molecules have ether groups positioned for intramolecular hydrogen bonding that are not balanced with the intermolecular H-bonding of the solvent. This imbalanced description is corrected by quantum mechanics/molecular mechanics (QM/MM) simulations, which include explicit water molecules. The best theoretical estimations result in a good correlation with experiments, <i>V</i>(Theory) = 0.903<i>V</i>(Expt) + 0.007 with an <i>R</i>² value of 0.835 and an MAD of 0.033 V. In addition to the aqueous test set, 221 anthraquinone redox couples in aprotic solvent were studied. Five anthraquinone derivatives spanning a range of redox potentials were selected from this library, and their reduction potentials were measured by cyclic voltammetry. DFT­(PCM) calculations predict the first reduction potential with high accuracy giving the linear relation, <i>V</i>(Theory) = 0.960<i>V</i>(Expt) – 0.049 with an <i>R</i>² value of 0.937 and an MAD of 0.051 V. This approach, however, significantly underestimates the second reduction potential, with an MAD of 0.329 V. It is shown herein that treatment of explicit ion-pair interactions between the anthraquinone derivatives and the cation of the supporting electrolyte is required for the accurate prediction of the second reduction potential. After the correction, <i>V</i>(Theory) = 1.045<i>V</i>(Expt) – 0.088 with an <i>R</i>² value 0.910 and an MAD value reduced by more than half to 0.145 V. Finally, molecular design principles are discussed that go beyond simple electron-donating and electron-withdrawing effects to lead to predictable and controllable reduction potentials

Density Functional Physicality in Electronic Coupling Estimation: Benchmarks and Error Analysis

Electronic coupling estimates from constrained density functional theory configuration interaction (CDFT-CI) depend critically on choice of density functional. In this Letter, the orbital multielectron self-interaction error (OMSIE), vertical electron affinity (VEA), and vertical ionization potential (VIP) are shown to be the key indicators inherited from the density functional that determine the accuracy of electronic coupling estimates. An error metric η is derived to connect the three properties, based on the linear proportionality between electronic coupling and overlap integral, and the hypothesis that the slope of this line is a function of VEA/VIP, η = (1/<i>N</i>_testset)­Σ_<i>i</i>^testset|−VE^Ref × OMSIE + ΔVE – ΔVE × OMSIE|_<i>i</i>. Based on η, BH&HLYP and LRC-ωPBEh are suggested as the best functionals for electron and hole transfer, respectively. Error metric η is therefore a useful predictor of errors in CDFT-CI electronic coupling, showing that the physical correctness of the density functional has a direct effect on the accuracy of the electronic coupling

Inner-Sphere Electron-Transfer Single Iodide Mechanism for Dye Regeneration in Dye-Sensitized Solar Cells

During the regeneration of the oxidized dye in dye-sensitized solar cells, the redox couple of I^–/I₃^– reduces the photo-oxidized dye. The simplest mechanism would be a direct charge-transfer mechanism from I^– to D⁺ [D⁺ + I^– → D⁰ + I], called the single iodide process (SIP). However, this is an unfavorable equilibrium because the redox potential of I^•/I^– is 1.224 V vs SHE, which is 0.13 V higher than that of the dye. This led to the postulation of the two iodide process (TIP) [(D⁺···I^–) + I^– → (D···I₂^–) → D⁰ + I₂^–)] for a sufficiently high reducing power, but TIP is not consistent with either the recent experimental data suggesting the first-order kinetics or recent time-resolved spectroscopic measurements. To resolve this conundrum, we used quantum mechanics including Poisson–Boltzmann solvation to examine the electron-transfer process between I^– and D⁺ for the Ru­(dcb)₂NCS₂ or N3 dye. We find that I^– is attracted to the oxidized dye, positioning I^– next to the NCS. At this equilibrium position, the I^– electron is already 40% transferred to the NCS, showing that the redox potential of I^– is well matched with the dye. This matching of the redox potential occurs because I^– is partially desolvated as it positions itself for the inner-sphere electron transfer (ISET). The previous analyses all assumed an outer-sphere electron-transfer process. Thus our ISET-SIP model is consistent with the known redox potentials and with recent experimental reports. With the ISET-SIP mechanism, one can start to consider how to enhance the dye regeneration kinetics by redesigning ligands to maximize the interaction with iodide

Universal Correction of Density Functional Theory to Include London Dispersion (up to Lr, Element 103)

Conventional density functional theory (DFT) fails to describe accurately the London dispersion essential for describing molecular interactions in soft matter (biological systems, polymers, nucleic acids) and molecular crystals. This has led to several methods in which atom-dependent potentials are added into the Kohn–Sham DFT energy. Some of these corrections were fitted to accurate quantum mechanical results, but it will be tedious to determine the appropriate parameters to describe all of the atoms of the periodic table. We propose an alternative approach in which a single parameter in the low-gradient (<i>lg</i>) functional form is combined with the rule-based UFF (universal force-field) nonbond parameters developed for the entire periodic table (up to Lr, <i>Z</i> = 103), named as a DFT-<i>ulg</i> method. We show that DFT-<i>ulg</i> method leads to a very accurate description of the properties for molecular complexes and molecular crystals, providing the means for predicting more accurate weak interactions across the periodic table

Threading Subunits for Polymers to Predict the Equilibrium Ensemble of Solid Polymer Electrolytes

We present a computational method for polymer growth called “threading subunits for polymers (TSP)” that can efficiently sample solid polymer electrolyte structures with extended conformations. The TSP method involves equilibrating subunit (e.g., monomer) conformations that form favorable solvation ion shells, followed by consecutively connecting the subunits and minimizing the structures. The TSP method can sample polymers with good solvent-like conformations and from near-equilibrium structures in which ions are well-dispersed, avoiding unusual ion clustering under ambient conditions. Using the TSP method, the equilibration time can be reduced significantly by effectively sampling the polymer conformations near equilibrium. We anticipate that the TSP method can be applied to simulate various polymer electrolytes

High-Throughput Screening to Investigate the Relationship between the Selectivity and Working Capacity of Porous Materials for Propylene/Propane Adsorptive Separation

An efficient propylene/propane separation is a very critical process for saving the cost of energy in the petrochemical industry. For separation based on the pressure-swing adsorption process, we have screened ∼1 million crystal structures in the Cambridge Structural Database and Inorganic Crystal Structural Database with descriptors such as the surface area of N₂, accessible surface area of propane, and pore-limiting diameter. Next, grand canonical Monte Carlo simulations have been performed to investigate the selectivities and working capacities of propylene/propane under experimental process conditions. Our simulations reveal that the selectivity and the working capacity have a trade-off relationship. To increase the working capacity of propylene, porous materials with high largest cavity diameters (LCDs) and low propylene binding energies (<i>Q</i>_st) should be considered; conversely, for a high selectivity, porous materials with low LCDs and high propylene <i>Q</i>_st should be considered, which leads to a trade-off between the selectivity and the working capacity. In addition, for the design of novel porous materials with a high selectivity, we propose a porous material that includes elements with a high crossover distance in their Lennard-Jones potentials for propylene/propane such as In, Te, Al, and I, along with the low LCD stipulation

CO₂ Hydrate Nucleation Kinetics Enhanced by an Organo-Mineral Complex Formed at the Montmorillonite–Water Interface

In this study, we investigated experimentally and computationally the effect of organo-mineral complexes on the nucleation kinetics of CO₂ hydrate. These complexes formed via adsorption of zwitter-ionic glycine (Gly-zw) onto the surface of sodium montmorillonite (Na-MMT). The electrostatic attraction between the −NH₃⁺ group of Gly-zw, and the negatively charged Na-MMT surface, provides the thermodynamic driving force for the organo-mineral complexation. We suggest that the complexation of Gly-zw on the Na-MMT surface accelerates CO₂ hydrate nucleation kinetics by increasing the mineral–water interfacial area (thus increasing the number of effective hydrate-nucleation sites), and also by suppressing the thermal fluctuation of solvated Na⁺ (a well-known hydrate formation inhibitor) in the vicinity of the mineral surface by coordinating with the −COO^– groups of Gly-zw. We further confirmed that the local density of hydrate-forming molecules (i.e., reactants of CO₂ and water) at the mineral surface (regardless of the presence of Gly-zw) becomes greater than that of bulk phase. This is expected to promote the hydrate nucleation kinetics at the surface. Our study sheds new light on CO₂ hydrate nucleation kinetics in heterogeneous marine environments, and could provide knowledge fundamental to successful CO₂ sequestration under seabed sediments