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

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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

Effect of Shuttling Catalyst on the Migration of Hydrogen Adatoms: A Strategy for the Facile Hydrogenation of Graphene

Using density functional theory calculations, we have investigated a strategy for the facile hydrogenation of graphene. We show that the presence of polar hydride molecules, such as H2O, HF, and NH3 (physisorbed molecules that mediate/assist the migration of atomic H adatoms on graphene, named “shuttle gases” in the present work), can provide a favorable catalytic effect (lowering the H migration barrier) and a favorable thermodynamic effect (activating the direct transition to the second-nearest-neighbor site). In comparison with the widely known fact that the migration of chemisorbed H is kinetically unfavorable on graphene, this mechanism for shuttling catalysis provides an easier migration channel. We propose that randomly distributed hydrogen adatoms on graphene can transform into a compactly aggregated hydrogenation domain (similar to graphane, as suggested in the literature) by heat treatment in the presence of a shuttling catalyst

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

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