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Developing highly active and non-precious-metal single-atom catalysts (SAC) for electrochemical water splitting is of great significance to the development of renewable energy technology in future. Herein, 10 first-row transition-metal (TM, from Sc to Zn) single atoms anchored on a graphyne (GY) nanosheet (TM@GY) as electrocatalysts for water splitting are explored through computational screening approach. It is found that the single atoms prefer to tightly bind at the acetylenic-ring center of GY. Meanwhile, these TM@GY catalysts show metallic properties or reduced band gap, favoring electron transfer during the electrochemical processes. Moreover, both Co@GY and Mn@GY catalysts present good catalytic performance for hydrogen evolution reaction (HER) with both the acetylenic C and TM atoms being highly active sites. However, only the Co@GY catalyst shows good oxygen evolution reaction (OER) activity with an overpotential of 0.55 V. Thus, Co@GY could serve as a potential bifunctional electrocatalyst for water splitting. Besides, the d-band center of TM atoms on TM@GY can be turned through controlling TM atoms with different d-electron number and be used to predict the OER performance. This work highlights that GY doped with single non-precious-metals can be considered for designing high-active and low-cost bifunctional electrocatalysts for practical electrochemical reactions. Unlabelled Image • 10 TM@GY are explored as potential catalysts for electrochemical water splitting by the means of computational screening. • The best catalyst for HER is Co@GY with the minimum ∆GH* of 0.04 eV. • The Co@GY catalyst is predicted to be a good bifunctional electrocatalyst for water splitting. • The distinct OER performance can be turned via adjusting TM atoms with different d-electron number. [ABSTRACT FROM AUTHOR]

Graphical abstract An efficient S, N-codoped carbon supported isolated single Mo atoms catalyst is computer-aided designed, and shows excellent HER performance. Highlights • The Mo-ISA/S 6 N1C catalyst was computer-aided designed showing excellent HER performance. • The interactions between H and the Mo atoms can be controlled by the N dopent concentration on carbon support. • The Mo-ISA/N1C catalyst showed the best HER performance with small ΔG H* of 0.04 eV. • The competitive effect of the S-induced tensile strain and charge transfer results in an enhanced HER performance. Abstract The high-efficient electrochemical hydrogen evolution reaction (HER) is a promising pathway to provide clean energy. An electrocatalyst was computer-aided designed with Mo isolated single atoms (Mo-ISA) supported on heteroatom S, N-codoped carbon having excellent HER performance. Here, by performing comprehensive density functional theory (DFT) computations, we investigated the influence of the N dopent concentration and the sites of S atom doping on the HER performance. Our computed results demonstrate that the Mo-ISA/N1C and Mo-ISA/S 6 N1C catalysts possess good stability and high HER reactivity. Moreover, the interactions between hydrogen and the Mo atoms could be controlled by the N dopent concentration on carbon support, and the Mo-ISA/N1C catalyst showed best HER reactivity. In particular, the S atom was introduced into Mo-ISA/N1C at the S 6 site showing an enhanced HER performance due to the competitive effect of the S-induced tensile strain and charge transfer. Our DFT computations open up new opportunities for application of S, N-codoped Mo-based high-efficient catalysts for HER. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The adsorption behaviour and oxidation mechanism of Hg0 on monolayer Ti 2 CO 2 (MXenes) were investigated from First Principles. • Hg0 is chemically adsorbed on the Ov-Ti 2 CO 2 monolayer through atomic orbital hybridization and overlap. • HgO displays high chemical reactivity for its adsorption on monolayer Ti 2 CO 2. • Oxygen vacancies from Ti 2 CO 2 monolayer have promoting effects on Hg0 adsorption and oxidation. Abstract Developing low-cost, high adsorption capacity, and recyclable mercury sorbents is an important step for control of mercury in coal-fired flue gas. However, few sorbents meet all the requirements. Herein, we explore the adsorption behavior and oxidation mechanisms of mercury on 2D Ti 2 CO 2 (MXenes) monolayer and defective Ti 2 CO 2 monolayer with an Ov (Ov-Ti 2 CO 2) through first-principles computations based on density functional theory (DFT). Our calculation results show that Hg0 adsorption on Ti 2 CO 2 and Ov-Ti 2 CO 2 monolayers are mainly physisorption and chemisorption mechanisms, respectively. Moreover, the presence of a single oxygen vacancy on Ti 2 CO 2 monolayer can improve the interaction between the mercury and Ov-Ti 2 CO 2 monolayer, thereby enhancing the adsorption energy of mercury by 66 kJ/mol. Hg0 has a strong interaction with Ti atom on Ov-Ti 2 CO 2 monolayer by the atomic orbital hybridization and overlap. The adsorption of HgO molecule on Ti 2 CO 2 and Ov-Ti 2 CO 2 monolayers are chemisorption processes. Furthermore, electron density difference analysis indicates that the remarkable charge accumulation across the interface is closely related to the strong interaction between HgO molecule and Ti 2 CO 2 or Ov-Ti 2 CO 2 monolayers. The three-step reaction processes (Hg0 → Hg(ads) → HgO(ads) → HgO) are taken place on Ti 2 CO 2 and Ov-Ti 2 CO 2 monolayers, resulting in the formation of gaseous HgO molecule. The energy barrier for the Hg0 oxidation reaction step on Ov-Ti 2 CO 2 monolayer (54.85 kJ/mol) is about half of that on Ti 2 CO 2 monolayer (104.73 kJ/mol). The desorption steps of the resultant HgO molecule from Ti 2 CO 2 and Ov-Ti 2 CO 2 monolayers are the rate-determining steps, which need external energies of about 119.3 and 183.2 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

Electrochemical water splitting to hydrogen is considered as a promising approach for clean H 2 production. However, developing highly active and inexpensive electrocatalysts is an important part of the hydrogen evolution reaction (HER). Herein, we present a multifaceted atom (sp2-and sp-hybridized boron) doping strategy to directly fine-modify the electronic structures of the active site and the HER performance by the density functional theory calculations. It is found that the binding strength between the Co atom and the B doped graphyne nanosheets can be enhanced by doping B atoms. Meanwhile, the Co@B 1 -GY and Co@B 2 -GY catalysts exhibit good thermodynamic stability and high HER catalytic activity. Interestingly, the Co@B 2 -GY catalyst has an ideal HER performance with the ΔG H* value of −0.004 eV. Moreover, the d-band center of the Co atoms is upshifted by the sp2-or sp-hybridized B dopants. The concentrations of the sp-hybridized B atoms have a positive effect on the electrons transformation of the Co atoms. The interaction between the H and Co atoms becomes strong with the increase of the concentrations of the sp-hybridized B atoms and thus the corresponding catalysts show sluggish HER kinetics. This investigation could provide useful guidance for the experimental groups to directly and continuously control the catalytic activity towards HER by precisely doping multifaceted atoms. The sp-hybridized B atoms doped Co@GY (Co@B 2 -GY) catalyst delivers excellent catalytic activity toward HER with the calculated hydrogen adsorption free energy of −0.004 eV. Image 1 • A sp-hybridized B atom doping strategy is performed to the HER electrocatalysis by the DFT calculations. • Doping of sp-hybridized B displays good thermodynamic stability in the HER catalytic process. • Co@B 2 -GY has an ideal HER performance with the ΔG H* value of −0.004 eV. • The d -band center of Co is upshifted by the sp2-or sp-hybridized B dopants. • The interactions between the H and Co atoms are tuned by concentrations of the sp-hybridized B atom. [ABSTRACT FROM AUTHOR]

• FeN 3 -C as an excellent sorbent for Hg0 adsorption is proposed by DFT study. • The incorporated FeN 3 in C endows the carbon with magnetic and metallic properties. • Hg0 can be efficiently adsorbed on FeN 3 -C via the chemisorption mechanism. • The unoccupied Fe d -orbital accepts electron and hybridizes with Hg s -, d -orbitals. • Hg0 adsorption on FeN 3 -C is beneficial at low temperature. The control of mercury (Hg0) is one of the most important environmental problems but a long-standing challenge in environment. Current research efforts for mercury removal mainly focus on the development of economical, effective, and recyclable sorbents, while the carbon-based metal sorbents with magnetism have been rarely studied. Here, we proposed the FeN 3 -embedded carbon (FeN 3 -C) as the efficient sorbent for Hg0 adsorption from first-principles computations. Our calculation results show that the incorporated FeN 3 in carbon endows the carbon with magnetic and metallic properties and that the gas phase Hg0 can be efficiently adsorbed on FeN 3 -C through the chemisorption mechanism with the adsorption energy of −66.27 kJ/mol. Moreover, the unoccupied d -orbital of Fe in FeN 3 -C can hybridize with the Hg s - and d -orbitals and accept electron density from the Hg s - and d -orbitals, which is the "acceptance" process between Hg0 and FeN 3 -C. Additionally, according to the equilibrium constant, Hg0 adsorption on FeN 3 -C is beneficial at low temperature. Our theoretical study not only reveals that the nonprecious FeN 3 -embedded carbon possesses great promise to serve as an efficient sorbent for Hg0 adsorption, but also provides a coordination-engineered strategy for searching more efficient carbon-based metal sorbents. [ABSTRACT FROM AUTHOR]

A general distance synergy of single Ag atoms doped MoS 2 in catalysing HER presents a volcanic trend between the adsorption free energy of hydrogen and the inter-Ag distance, resulting in the optimum HER activity on MoS 2 basal plane. [Display omitted] • A general distance synergy of single Ag atoms doped MoS 2 in catalyzing HER was presented. • A volcanic trend between ΔG H* and the inter-Ag distance was found. • The optimum HER activity with the minimum ΔG H* of 0.03 eV is at the neighboting in-plane S sites. • A proper inter-Ag distance could moderately alter the charge of the S atoms adjacent to the Ag atoms. Doping heteroatoms into the 2D MoS 2 lattice provides an opportunity for regulating the electronic structures of the MoS 2 basal plane and further activating the inert in-plane S atoms for efficient hydrogen evolution reaction (HER). However, the most appropriate type and distribution of the doping atoms in MoS 2 induced the optimum HER performance remains a big challenge. Herein, through the density functional theory calculations, we present a general distance synergy of single Ag atoms doped MoS 2 in catalyzing HER, resulting in the optimum HER activity at the in-plane S sites neighbored to and between the Ag atoms. A general distance synergy between the doped Ag atoms in modifying the HER reactivity of in-plane S atoms adjacent to the Ag atoms is found via the control of the distance between the Ag atoms. This distance synergy shows a volcanic trend between the adsorption free energy of hydrogen (ΔG H*) and the inter-Ag distance, where the Ag d6.38 (the initial distances of 6.38 Å between two Ag atoms doped in the MoS 2 lattice) electrocatalyst with a ΔG H* value of 0.03 eV locates at the top of the volcano. Moreover, the stability and HER catalytic mechanism of the Ag d6.38 electrocatalyst as well as the doping density of Ag atoms in MoS 2 are discussed. This investigation offers useful guidance for the experimental groups to trigger the activity of the MoS 2 basal plane by precisely controlling the distance between the doped heteroatoms. [ABSTRACT FROM AUTHOR]

• A computer-aided design of high-efficient Co@N 1 -GY catalyst for electrochemical water splitting is presented. • The N-induced charge redistribution on the catalysts greatly affects the HER and OER performances. • A feasible strategy for fine-tuning the charge density of the catalysts is provided. Developing inexpensive and high-active electrocatalysts for overall water splitting is important to the development of future clean energy technologies. Many non-noble metal single-atom catalysts (SACs) present unsatisfactory water-splitting catalytic performance because of their intrinsic electronic structures. Here, through the density functional theory calculations, a nitrogen atom doping strategy is applied to directly tune the electronic structure of the SAC and boost their catalytic activities towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). By precisely manipulating N doping modes in graphyne supported single Co atom catalyst (Co@GY), the stability, electronic properties, and HER, as well as OER catalytic activities of these N-doped Co@GY catalysts are investigated. It is found that the N-induced charge redistribution on the surface of the N-doped Co@GY catalysts greatly affects their HER and OER performances. The N-induced charge density not only increases the number of active sites of Co@N 1 -GY and enhances its catalytic activity for HER, but also weakens the chemisorption of oxygenated species and decreases distinctly the over-potential for OER, making Co@N 1 -GY a promising bifunctional electrocatalyst for water splitting. This work offers a feasible strategy for experimental groups to properly tune the electronic structures of catalysts and improve catalytic activity. [ABSTRACT FROM AUTHOR]

• 2D Pc monolayer is a promising support for SACs. • 2D Mo-Pc monolayer can capture N 2 selectively and activate N 2 effectively. • NNR on 2D Mo-Pc monolayer prefers to proceed through distal pathway with biggest energy barrier of 0.70 eV. The current remarkable catalysts for natural and artificial ammonia (NH 3) synthesis are transition-metal (TM) atoms as binding site and catalytic center. Herein several TM atoms were embedded regularly and separately on phthalocyanine monolayer as N 2 capture and reduction catalysts and theirs catalytic activity were investigated in this work by performing density functional theory simulations. The results showed that single molybdenum atom embedded phthalocyanine monolayer exhibited outstanding performances on N 2 capture and reduction to NH 3 at ambient conditions. N 2 reduction reaction prefers to proceed through distal reaction pathway with a rate determining barrier 0.70 V, suggesting its good catalytic performance for N 2 reduction. Our work also demonstrated the Pc monolayers can act as an excellent substrate for the design of single-atom catalysts. Our study provides a novel pathway for achieving conversion of N 2 into NH 3 at ambient conditions. [ABSTRACT FROM AUTHOR]