Construction of Co1-x S Nanoparticles Embedding in N-Doped Amorphous Carbon@Graphene with Enhanced Li-Ion Storage.

Cobalt sulfide is deemed a promising anode material, owing to its high theoretical capacity (630 mAh g-1 ). Due to its low conductivity, fast energy decay, and the huge volume change during the lithiation process limits its practical application. In this work, a simple and large-scale method are developed to prepare Co1-x S nanoparticles embedding in N-doped carbon/graphene (CSCG). At a current density of 0.2 C, the reversible discharge capacity of CSCG maintains 937 mAh g-1 after 200 cycles. The discharge capacity of CSCG maintains at 596 mAh g-1 after 500 cycles at the high current density of 2.0 C. The excellent performance of CSCG is due to its unique structural features. The addition of rGO buffered volume changes while preventing Co1-x S from crushing/aggregating during the cycle, resulting in multiplier charge-discharge and long cycle life. The N-doped carbon provides a simple and easy way to achieve excellent performance in practical applications. Combined with density functional theory calculation, the presence of Co-vacancies(Co1-x ) increases more active site. Moreover, N-doping carbon is beneficial to the improve adsorption energy. This work presents a simple and effective structural engineering strategy and also provides a new idea to improve the performance of Li-ion batteries.

Theoretical identification of the superior anchoring effect and electrochemical performance of Ti2CS2 by single atom Zn doping for lithium-sulfur batteries.

As one of the promising next-generation energy storage systems, lithium-sulfur (Li-S) batteries have been the subject of much recent attention. However, the polysulfide shuttle effect remains problematic owing to the dissolution of intermediate polysulfide species in the electrolyte and the sluggish reaction dynamics in Li-S batteries. To overcome these issues, this work reports an effective strategy for enhancing the electrochemical performance of Li-S batteries using single atom Zn doping on the S-terminated Ti2C MXenes (Ti2-xZnxCS2). Spin-polarized density functional theory (DFT) calculations were performed to elucidate the interactions of lithium polysulfides (LiPSs) and the Ti2-xZnxCS2 surface in terms of geometric and electronic properties, as well as the delithiation process of Li2S on the Ti2-xZnxCS2 surface. It is found that doping single atom Zn could induce a new Lewis acid-based sites, which could provide proper affinity toward LiPSs. Combined with the metallic character, a low Li diffusion barrier and high catalytic activity for the delithiation process of Li2S, makes Ti2-xZnxCS2 a promising cathode material for Li-S batteries. The results demonstrate the importance of surface chemistry and the electronic structure of MXenes in LiPSs' adsorption and catalysis capability. We believe that our findings provide insights into the recent experimental results and guidance for the preparation and practical application of MXenes in Li-S batteries.

Single Pd atomic catalyst on Mo2CO2 monolayer (MXene): unusual activity for CO oxidation by trimolecular Eley-Rideal mechanism.

The geometric stability, electronic structure and catalytic properties of a single Pd atom deposited on a pristine Mo2CO2 monolayer and a defective Mo2CO2 monolayer with an oxygen vacancy (denoted as Pd/OV-Mo2CO2) are systematically investigated through density functional theory. We find that the oxygen vacancy (OV) can stabilize the single Pd atom and make the Pd/OV-Mo2CO2 system an excellent mono-dispersed atomic catalyst. The Pd dopant serves as an active center which makes the intermediates react productively around it. Three reaction mechanisms are considered for CO oxidation to test the catalytic activity of Pd/OV-Mo2CO2, which exhibits high activity for CO oxidation via a tri-molecular Eley-Rideal (TER) mechanism with a rate-limiting energy barrier of 0.49 eV. The pre-adsorbed CO molecules on the Pd dopant could transfer electrons to the O2-2π* orbitals, which would promote O2 molecule activation and induce O-O bond scission. This work demonstrates that the defective monolayer MXene may serve as a promising sort of support to fabricate single atomic catalysts (SACs) for CO oxidation or other reactions, agreeing well with the experimental reports, which opens up a new avenue for the design and fabrication of SACs of MXene-based materials.

Density functional study on the high catalytic performance of single metal atoms on the NbC(001) surface.

The adsorption and activation of O2 is regarded as the first critical step for the oxygen reduction reaction (ORR), and catalysts with a high performance toward O2 adsorption and activation would provide a theoretical foundation for further investigations. Here, we have studied the adsorption and electronic properties as well as the catalytic activities of group 9-11 single metal atoms deposited on NbC(001), denoted M/NbC(001). According to the location of the d-band centers and the frontier molecular orbital analysis, single metals of Co, Rh, Ir and Ni on NbC(001) exhibit higher activities than other metals (Pd, Pt, Cu, Ag and Au). The quite different catalytic activities of M/NbC(001) may be attributed to the differences in their electro-negativities and work-functions. Meanwhile, the reasonable stabilities of Co, Rh, Ir and Ni on NbC(001) were clarified by investigating the agglomeration resistance and oxidation resistance, and the results indicate that Co and Ni have poor oxidative stability, and Rh and Ir are antioxidants on NbC(001). Further research into the adsorption and activation of O2 confirmed the outstanding properties of Rh/NbC(001) and Ir/NbC(001), which may provide great opportunities to find alternative catalysts.

Repairing single and double atomic vacancies in a C3N monolayer with CO or NO molecules: a first-principles study.

Even the simplest point defect in a two-dimensional (2D) material can have a significant influence on its electronic, magnetic, and chemical properties. Defect repairing in 2D materials has been a focus of concern in recent years. Based on first-principles calculations, the repair of C and N single vacancies with CO or NO molecules in a C3N monolayer has been studied. The repair process consists of two steps, i.e., filling of the vacancy with the first molecule and removal of the extra O atom by a second molecule. Overall, the repair processes of C and N single vacancies by CO or NO molecules are both thermodynamically and kinetically favorable, as evidenced by the significant energy released and the small energy barriers. In addition, the electronic and magnetic properties and the chemical activity of the C3N monolayer before and after the defect repair have been studied systematically. In addition to single vacancies, the repair of double vacancies with CO was also studied; this process is much less kinetically favorable than the case of single vacancies. This study provides useful insight into the effects of simple atomic vacancies on the physical and chemical properties of the C3N 2D semiconductor and also presents a promising strategy for repairing vacancies.

TiC and TiN supported platinum monolayer as high-performance catalysts for CO oxidation: A DFT study.

The reactivity toward CO oxidation of Pt monolayer supported on TiC(001) and TiN(001) is studied by using empirical dispersion-corrected density functional theory calculations. A number of possible reaction pathways for CO oxidation, including the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms, between adsorbed O2 and CO molecules considering the cases that the adsorbed O2 dissociates first or directly reacts with CO. It is found that the dissociation adsorption of O2 molecules as the initial step is more favorable with lower activation barriers compared with the direct reaction mode. Hence the dissociation of adsorbed O2 molecules plays a very key role in the CO oxidation reaction. For both Pt monolayer systems, our analyses also reveal that the reaction is most likely predominant via the initial ER mechanism and the subsequent LH mechanism. Furthermore, by comparing the activation barriers of the rate-limiting steps, CO oxidation on PtML/TiN(001) shows a higher catalytic activity than that on PtML/TiC(001), showing the important role that the support would play in the catalytic reactions. The present results suggest that the TiN supported monolayer Pt catalyst appears to be a good candidate for CO oxidation at low-temperature.

A Simple Wireless Sensor Node System for Electricity Monitoring Applications: Design, Integration, and Testing with Different Piezoelectric Energy Harvesters.

Real time electricity monitoring is critical to enable intelligent and customized energy management for users in residential, educational, and commercial buildings. This paper presents the design, integration, and testing of a simple, self-contained, low-power, non-invasive system at low cost applicable for such purpose. The system is powered by piezoelectric energy harvesters (EHs) based on PZT and includes a microcontroller unit (MCU) and a central hub. Real-time information regarding the electricity consumption is measured and communicated by the system, which ultimately offers a dependable and promising solution as a wireless sensor node. The dynamic power management ensures the system to work with different types of PZT EHs at a wide range of input power. Thus, the system is robust against fluctuation of the current in the electricity grid and requires minimum adjustment if EH unit requires exchange or upgrade. Experimental results demonstrate that this unit is in a position to read and transmit 60 Hz alternating current (AC) sensor signals with a high accuracy no less than 91.4%. The system is able to achieve an operation duty cycle from <1 min up to 18 min when the current in an electric wire varies from 7.6 A to 30 A, depending on the characteristics of different EHs and intensity of current being monitored.

First-principles investigation of H2S adsorption and dissociation on titanium carbide surfaces.

The adsorption and dissociation reactions of H2S on TiC(001) are investigated using first-principles density functional theory calculations. The geometric and electronic structures of the adsorbed S-based species (including H2S, SH and S) on TiC(001) are analyzed in detail. It is found that the H2S is bound weakly, while SH and atomic S are bound strongly on the TiC(001) surface. The transition state calculations show that the formation of SH from H2S (H2S → SH + H) is very easy, while the presence of a co-adsorbed H will inhibit the further dissociation of SH (SH + H → S + H + H). In contrast, the hydrogenation of the adsorbed SH is rather easy (SH + H → H2S). Therefore, the dissociative SH can be removed via the hydrogenation reaction. It is concluded that it is difficult for H2S to dissociate completely to form atomic S and poison the TiC surface. The results will further provide understanding of the mechanism of the sulfur tolerance of the TiC anode of proton exchange membrane fuel cells (PEMFCs).

Mechanisms of direct hydrogen peroxide synthesis on silicon and phosphorus dual-doped graphene: a DFT-D study.

Hydrogen peroxide (H2O2) is an important chemical commodity, with demand growing significantly in chemical synthesis due to its green characteristics. The mechanisms of the direct synthesis of hydrogen peroxide (DSHP) on metal-free silicon and phosphorus dual-doped graphene (Si-P-G) catalyst, based on a dispersion-corrected density functional theory (DFT-D) method, are systematically investigated. The most stable Si-P-G catalyst is presented, with the local region of dopants shown to play an important role in the adsorption and reduction of oxygen. A two-electron pathway is probable for DSHP on Si-P-G according to kinetic and thermodynamic analyses. The hydrogenation of O2 to OOH is the rate-limiting step, with a small barrier energy of 0.66 eV, and the potential energy surface is downhill by Gibbs free energy calculations. All results indicate that Si-P-G is a novel catalyst with high activity and good selectivity for DSHP.

Density functional study on the resistance to sulfur poisoning of Ptx (x = 0, 1, 4 and 8) modified α-Mo2C(0001) surfaces.

The tolerance of sulfur poisoning of α-Mo2C(0001) surfaces with different Pt coverages is investigated combining the density functional theory (DFT) results with thermodynamics data using the ab initio atomistic thermodynamic method. It is found that on Mo2C(0001), Pt clusters tend to form two dimensional planar structures instead of aggregating. The clean Mo2C(0001) surface interacts with sulfides very strongly and is susceptible to sulfur poisoning. With increasing the coverage of Pt on the Mo2C surface, the interaction between sulfur and substrate is weakened. The sulfur tolerance ability increases in the order of Mo2C ≈ Pt1/Mo2C < Pt4/Mo2C < Pt8/Mo2C, where the coverage of Pt on the Mo2C plays a very effective role. The results provide theoretical guidance for designing Mo2C based catalysts with high activity and high sulfur resistance.

Interaction between H2O, N2, CO, NO, NO2 and N2O molecules and a defective WSe2 monolayer.

In this study, the interaction between gas molecules, including H2O, N2, CO, NO, NO2 and N2O, and a WSe2 monolayer containing an Se vacancy (denoted as VSe) has been theoretically studied. Theoretical results show that H2O and N2 molecules are highly prone to be physisorbed on the VSe surface. The presence of the Se vacancy can significantly enhance the sensing ability of the WSe2 monolayer toward H2O and N2 molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the VSe surface with the non-oxygen atom occupying the Se vacancy site. Furthermore, the exposed O atoms of the molecularly chemisorbed CO or NO can react with additional CO or NO molecules, to produce C-doped or N-doped WSe2 monolayers. The calculated energies suggest that the filling of the CO or NO molecule and the removal of the exposed O atom are both energetically and dynamically favorable. Electronic structure calculations show that the WSe2 monolayers are p-doped by the CO and NO molecules, as well as the C and N atoms. However, only the NO molecule and N atom doped WSe2 monolayers exhibit significantly improved electronic structures compared with VSe. The NO2 and N2O molecules will dissociate directly to form an O-doped WSe2 monolayer, for which the defect levels due to the Se vacancy can be completely removed. The calculated energies suggest that although the dissociation processes for NO2 and N2O molecules are highly exothermic, the N2O dissociation may need to operate at an elevated temperature compared with room temperature, due to its large energy barrier of ∼1 eV.

A promising single atom catalyst for CO oxidation: Ag on boron vacancies of h-BN sheets.

Single atom catalysts (SACs) have attracted broad research interest in recent years due to their importance in various fields, such as environmental protection and energy conversion. Here, we discuss the mechanisms of CO oxidation to CO2 over single Ag atoms supported on hexagonal boron-nitride sheets (Ag1/BN) through systematic van der Waals inclusive density functional theory (DFT-D) calculations. The Ag adatom can be anchored onto a boron defect (VB), as suggested by the large energy barrier of 3.12 eV for Ag diffusion away from the VB site. Three possible mechanisms (i.e., Eley-Rideal, Langmuir-Hinshelwood, and termolecular Eley-Rideal) of CO oxidation over Ag1/BN are investigated. Due to "CO-Promoted O2 Activation", the termolecular Eley-Rideal (TER) mechanism is the most relevant one for CO oxidation over Ag1/BN and the rate-limiting reaction barrier is only 0.33 eV. More importantly, the first principles molecular dynamics simulations confirm that CO oxidation via the TER mechanism may easily occur at room temperature. Analyses with the inclusion of temperature and entropy effects further indicate that the CO oxidation via the TER mechanism over Ag1/BN is thermodynamically favorable in a broad range of temperatures.

Effects of a TiC substrate on the catalytic activity of Pt for NO reduction.

Density functional theory calculations are used to elucidate the catalytic properties of a Pt monolayer supported on a TiC(001) substrate (Pt/TiC) toward NO reduction. It is found that the compound system of Pt/TiC has a good stability due to the strong Pt-TiC interaction. The diverse dissociation paths (namely the direct dissociation mechanism and the dimeric mechanism) are investigated. The transition state searching calculations suggest that NO has strong diffusion ability and small activation energy for dissociation on the Pt/TiC. For NO reduction on the Pt/TiC surface, we have found that the direct dissociation mechanisms (NO + N + O → NO2 + N and NO + N + O → N2 + O + O) are easier with a smaller dissociation barrier than those on the Pt(111) surface; and the dimeric process (NO + NO → (NO)2 → N2O + O → N2 + O + O) is considered to be dominant or significant with even a lower energy barrier than that of the direct dissociation. The results show that Pt/TiC can serve as an efficient catalyst for NO reduction.

CO oxidation catalyzed by the single Co atom embedded hexagonal boron nitride nanosheet: a DFT-D study.

A single metal atom stabilized on two dimensional materials (such as graphene and h-BN) exhibits extraordinary activity in the oxidation of CO. The oxidation of CO by molecular O2 on a single cobalt atom embedded in a hexagonal boron nitride monolayer (h-BN) is investigated using first-principles calculations with dispersion-correction. It is found that the single Co atom prefers to reside in a boron vacancy and possesses great stability. There are three mechanisms for CO oxidation: the traditional Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms and the termolecular Eley-Rideal (TER) mechanism proposed recently. Given the relatively small reaction barriers of the rate-limiting steps for the ER, LH and TER mechanisms (0.59, 0.55 and 0.41 eV, respectively), all three mechanisms are able to occur at low temperature. The current study may provide useful clues to develop low cost single atom catalysts.

Density functional study on the mechanism for the highly active palladium monolayer supported on titanium carbide for the oxygen reduction reaction.

The adsorption, diffusion, and dissociation of O2 on the palladium monolayer supported on TiC(001) surface, MLPd/TiC(001), are investigated using ab initio density functional theory calculations. Strong adhesion of palladium monolayer to the TiC(001) support, accompanied by a modification of electronic structure of the supported palladium, is evidenced. Compared with Pt(111) surface, the MLPd/TiC(001) can enhance the adsorption of O2, leading to comparable dissociation barrier and a smaller diffusion barrier of O2. Whilst the adsorption strength of atomic O (the dissociation product of O2) on MLPd/TiC(001) is similar to that on the Pt(111) surface, possessing high mobility, our theoretical results indicate that MLPd/TiC(001) may serve as a good catalyst for the oxygen reduction reaction.

Single Pt atom stabilized on nitrogen doped graphene: CO oxidation readily occurs via the tri-molecular Eley-Rideal mechanism.

Single-atom catalysts, especially with single Pt atoms, have attracted more and more attention due to their high catalytic activity for CO oxidation. The outstanding stability and catalytic activity of a single Pt atom supported on nitrogen doped graphene (Pt/NG) are revealed using first-principles calculations. We find that the stability of a Pt atom on the NG can be promoted by picking an appropriate doping configuration. The exceptionally stable Pt/NG catalyst exhibits excellent catalytic activity for CO oxidation via a new tri-molecular Eley-Rideal mechanism (2CO + O2 → OCO-OCO → 2CO2) with an energy barrier of 0.16 eV for the rate-limiting step of OCO-OCO dissociation, which is more preferable than the other two normal Langmuir-Hinshelwood and Eley-Rideal mechanisms.

Highly efficient Pd-based core-shell nanowire catalysts for O2 dissociation.

The adsorption and dissociation of O2 on the core-shell M@Pd nanowires (M = 3d, 4d, 5d transition metals) are studied using the first-principles density functional method. Suitable core atoms are determined based on the stability of the core-shell NWs and their efficiency for O2 dissociation. With the consideration of the stability and cost, we found that Fe, Co, Ni, Cu, Ru, Ir atoms have lower price than Pd and favor at the core even with O adatom at the surface. The formed M@Pd core-shell nanowires are active for O2 dissociation with activation barriers no larger than 0.25 eV. The results may serve as a guide for the design of efficient Pd-based nanocatalysts for O2 dissociation.

The mechanisms for the high resistance to sulfur poisoning of the Ni/yttria-stabilized zirconia system treated with Sn vapor.

The mechanisms for the resistance to sulfur poisoning at the triple phase boundary (TPB) of the Ni/yttria-stabilized zirconia (YSZ) system treated with Sn vapor are studied using the first-principles method based on density functional theory. Models with Sn dopant or adsorbate are proposed. It is found that the TPB model of the Ni/YSZ system with Sn dopant in Ni can to some extent restrain the diffusion of sulfur from the Ni part to the interface O vacancy by forcing the sulfur atom to diffuse along a longer path, which increases the time for which sulfur remains at the Sn doped Ni surface and allows the O ion to diffuse to the O vacancy at the interface. Once the O ion diffuses to the O vacancy, it forms interface O(2-), which repels the sulfur adsorbate and eliminates the sulfur poisoning. However, as the barriers of sulfur diffusion to the vacancy are still small (0.25 eV or smaller), the Sn dopant in Ni does not efficiently eliminate the sulfur poisoning at the TPB. In contrast, the TPB model of the Ni/YSZ system with an Sn adatom on the Ni can form a physical barrier and prevent effectively sulfur diffusion to the O vacancy at the interface. The diffusion barriers are as large as 1.41 eV, which therefore eliminates the sulfur poisoning at the TPB. The results give a detailed dynamic picture of the mechanism of the high tolerance to sulfur poisoning of the Ni/YSZ anode at the TPB after the pre-exposures to metallic tin vapor.

A density function theory study on the NO reduction on nitrogen doped graphene.

The mechanisms for the catalytic reduction of NO on the metal-free nitrogen doped graphene (NG) support are investigated using the density function theory (DFT) calculations both with and without the van der Waals (vdW) correction. The results indicate that the dimer mechanism is more facile than the direct decomposition mechanism. In the dimer mechanism, a three-step reaction is identified: (i) the coupling of two NO molecules into a (NO)2 dimer, followed by (ii) the dissociation of the (NO)2 dimer into N2O + Oad, then (iii) the O adatom is taken away easily by the subsequent NO. Once the NO2 is desorbed, the remaining N2O can be reduced readily by NO on NG. The reaction processes are also confirmed from the first principles molecular dynamics simulations. The results suggest that the NG is an efficient metal-free catalyst for catalytic reduction of NO.

Importance of oxygen spillover for fuel oxidation on Ni/YSZ anodes in solid oxide fuel cells.

Using first principles simulations and the Monte Carlo method, the optimal structure of the triple-phase boundaries (TPB) of the Ni/Yttria-Stabilized Zirconia (YSZ) anode in solid oxide fuel cells (SOFCs) is determined. Based on the new TPB microstructures we reveal different reaction pathways for H2 and CO oxidation. In contrast to what was believed in previous theoretical studies, we find that the O spillover from YSZ to Ni plays a vital role in electrochemical reactions. The H2 oxidation reaction can proceed very rapidly, by means of both the H and O spillovers, whereas the CO oxidation can only proceed through the O spillover pathway. Further understanding of the roles of defects and dopants allows us to explain puzzling experimental observations and to predict ways to improve the catalytic performance of SOFCs.