Regulating the frontier orbital of iron phthalocyanine with nitrogen doped carbon nanosheets for improving oxygen reduction activity.

Iron phthalocyanine (FePc) has attracted widespread attention for its tunable electronic structure. However, the Fe-N sites suffer from undesirable oxygen reduction activity due to the symmetric geometries. A suitable substrate was thus needed to induce electron redistribution around Fe-N to improve the activity. Herein, ultrathin nitrogen-doped carbon nanosheets (N-CNSs) were prepared by a simple high temperature pyrolysis. Then iron phthalocyanine was loaded on the ultrathin nitrogen-doped carbon nanosheets (FePc@N-CNSs) by a low-cost and simple solution method. This composite catalyst shows an excellent ORR activity with a half potential of 0.88 V, an onset potential of 0.99 V and durability superior to commercial Pt/C. When used as an air cathode catalyst for rechargeable zinc-air batteries, FePc@N-CNS modified batteries outperform Pt/C + RuO2 modified batteries with higher power density and superior constant current charge-discharge cycling stability of 37 hours. The regulated electronic structure of FePc by the N-CNS substrate was revealed further by DFT calculations, which explained the enhanced adsorption of the active center to the intermediates and the increased ORR performance.

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 investigation of synergistically boosting the anchoring and electrochemical performance of lithiophilic/sulfiphilic transition metal carbides for lithium-sulfur batteries.

Lithium-sulfur (Li-S) battery is one of the most promising next-generation energy-storage systems with a high energy density and low cost. However, their commercial applications face several challenges, such as the shuttle effect caused by the soluble lithium polysulfide (LiPSs) intermediates and the sluggish sulfur redox reaction. In this article, we systematically investigated the anchoring and electrochemical performance of a series of transition metal carbides (TMCs: TiC, VC, ZrC, NbC, HfC, TaC) as cathode materials for Li-S batteries by theoretical calculations. The lithiophilic/sulfiphilic non-polar (001) surfaces of TMCs can offer moderate binding strength with LiPS intermediates, ensuring good performance of sulfur immobilization. These TMCs can also facilitate lithium diffusion, indicating the good rate performance of Li-S batteries. We also demonstrated that the studied TMCs can be classified into two classes according to their catalytic activity for Li2S decomposition which originated from their different electronic structural features. Furthermore, TiC, ZrC, and HfC exhibited excellent bifunctional electrochemical activity through reducing the Gibbs free energy for sulfur reduction reactions (SRRs) and lowering the barrier for Li2S decomposition which facilitates accelerating electrode kinetics and elevating utilization of sulfur. Our results offer a systematic approach to designing and screening non-polar materials for high-performance Li-S batteries, based on the rational electronic structure and lattice match strategy.

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.

Urchin-like band-matched Fe2O3@In2S3 hybrid as an efficient photocatalyst for CO2 reduction.

Herein, an urchin-like Fe2O3@In2S3 hybrid composite is designed and synthesized using a facile process. The composite efficiently harvests light in both the ultraviolet and visible regions, and the unique hierarchical structure provides several advantages for photocatalytic applications: (i) a suitable band-matching structure and broadband-light absorbing capacity enable the reduction of CO2 into hydrocarbon, (ii) the extensive network of interfacial contact between nano-sized Fe2O3 and In2S3 significantly increases the separation of charge carriers and enhances the utilization of photogenerated electron-hole pairs, and (iii) an abundance of surface oxygen vacancies provide numerous active sites for CO2 molecule adsorption. The optimized Fe2O3@In2S3 composite generated CO from the photocatalytic reduction of CO2 at a rate of 42.83 µmol·g-1·h-1, and no signs of deactivation were observed during continued testing for 32 h under 300 W Xe lamp irradiation.

Synthesis of CoP@B,N,P co-doped porous carbon by a supramolecular gel self-assembly method for lithium-sulfur battery separator modification.

Lithium-sulfur batteries (LSBs) are regarded as promising next-generation batteries due to their high abundance and high theoretical energy density. However, the commercial application of LSBs is hindered by the shuttle effect of soluble lithium polysulfides (LiPSs). Hence, we synthesised B, N, P co-doped three-dimensional hierarchical porous carbon materials, uniformly dispersed with CoP nanoparticles, and utilized them as the coating material for the PE separator. The catalytic and adsorption capacity of the composite material was significantly enhanced by CoP. Both experimental and theoretical calculations show that the LiPS adsorption capacity of the composite material is significantly enhanced after the introduction of B atoms. As a result, the assembled LSBs with the CoP@BNPC/PE separator show excellent long-term stability (940.8 mA h g-1 after 500 cycles at 1.0 C, and only a 0.026% decay rate per cycle) and superior rate performance (613.6 mA h g-1 at 5.0 C). Our work further proves that a modified separator is an effective strategy to promote the commercialization of LSBs.

Ultrafast Kinetics in a PAN/MgFe2O4 Flexible Free-Standing Anode Induced by Heterojunction and Oxygen Vacancies.

Flexibility and power density are key factors restricting the development of flexible lithium-ion batteries (FLIBs). Interface and defect engineering can modify the intrinsic ion/electron kinetics by regulating the electronic structure. Herein, a polyacrylonitrile/MgFe2O4 (PAN-MFO) electrode with heterojunction and oxygen vacancies was first designed and synthesized as a flexible free-standing anode of FLIBs by electrostatic spinning technology. The PAN carbon nanofiber (PAN-CNF) as the skeleton structure provides fast conductive channels, buffers the volume expansion, and enhances the cycle stability. The heterostructure constructs the internal electric field, facilitates the Li+/charge transfer, intensifies the Li+ adsorption energy, and enhances the interfacial lithium storage. Oxygen vacancies improve the intrinsic conductivity, lower the Li+ diffusion barrier, weaken the Fe-O bonding, and facilitate the conversion reaction. Because of the synergistic effect of the multifunctional structure, the PAN-MFO shows superior cycle and rate performance with ultrafast kinetics. Flexible LiCoO2/PAN-MFO full pouch cells were also assembled that demonstrated a stable cycle performance and power supply in both the plain and bent states.

Fundamental understanding of electrocatalysis over layered double hydroxides from the aspects of crystal and electronic structures.

Layered double hydroxides (LDHs) composed of octahedral ligand units centered with various transition metal atoms display unique electronic structures and thus attract significant attention in the field of electrocatalytic oxygen evolution reactions (OER). Intensive experimental explorations have therefore been carried out to investigate the LDHs synthesis, amorphous control, intrinsic material modifications, interfacing with other phases, strain, etc. There is still the need for a fundamental understanding of the structure-property relations, which could hinder the design of the next generation of the LDHs catalysts. In this review, we firstly provide the crystal structure information accompanied by the corresponding electronic structures. Then, we discuss the conflicts of the active sites on the NiFe LDHs and propose the synergistic cooperation among the ligand units during OER to deliver a different angle for understanding the current structure-property relations beyond the single-site-based catalysis process. In the next section of the OER process, the linear relationship-induced theoretical limit of the overpotential is further discussed based on the fundamental aspects. To break up the linear relations, we have summarized the current strategies for optimizing the OER performance. Lastly, based on the understanding gained above, the perspective of the research challenges and opportunities are proposed.

Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene.

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O2/Ar and 10 vol % H2/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au3+ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au3+ possesses the optimal activity with 1.8 × 10-2 s-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au3+ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au3+ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H2 via constructing frustrated Lewis pairs with O.

Cooperative Catalysis toward Oxygen Reduction Reaction under Dual Coordination Environments on Intrinsic AMnO3 -Type Perovskites via Regulating Stacking Configurations of Coordination Units.

It remains challenging for pure-phase catalysts to achieve high performance during the electrochemical oxygen reduction reaction to overcome the sluggish kinetics without the assistance of extrinsic conditions. Herein, a series of pristine perovskites, i.e., AMnO3 (A = Ca, Sr, and Ba), are proposed with various octahedron stacking configurations to demonstrate the cooperative catalysis over SrMnO3 jointly explored by experiments and first-principles calculations. Comparing with the unitary stacking of coordination units in CaMnO3 or BaMnO3 , the intrinsic SrMnO3 with a mixture of corner-sharing and face-sharing octahedron stacking configurations demonstrates superior activity (Ehalf-wave  = 0.81 V), and charge-discharge stability over 400 h without the voltage gap (≈0.8 V) increasing in zinc-air batteries. The theoretical study reveals that, on the SrMnO3 (110) surface, the active sites switch from coordinatively unsaturated atop Mn (*OO, *OOH) to Mn-Mn bridge (*O, *OH). Therefore, the intrinsic dual coordination environments of Mn-Ocorner and Mn-Oface enable cooperative modulation of the interaction strength of the oxygen intermediates with the surface, inducing the decrease of the *OH desorption energy (rate-limiting step) unrestricted by scaling relationships with the overpotential of ≈0.28 V. This finding provides insights into catalyst design through screening intrinsic structures with multiple coordination unit stacking configurations.

Tuning the ORR activity of Pt-based Ti2CO2 MXenes by varying the atomic cluster size and doping with metals.

The rational design of ideal catalysts for the oxygen reduction reaction (ORR) is of great significance for solving the electrocatalytic potential problems in proton exchange membrane fuel cells (PEMFCs). Ptn (n = 1-4) and Pt3Au alloy subnanoclusters supported on a defective Ti2CO2 monolayer with oxygen vacancies (denoted as v-Ti2CO2) are simulated by using density functional theory to investigate their ORR performance. The geometries, energetics, and electronic properties of the different systems are analyzed. It is found that the supported Pt3Au alloy subnanocluster possesses the best ORR activity. The underlying mechanisms of the improved ORR activity originates from the moderate hybridization between the O 2p and the 5d orbitals of Au and Pt according to the density of states analysis. Our study suggests a facile route for designing low-cost MXene-based electrocatalysts by alloying transition metals with Pt catalysts, which may stimulate realization of suitable alternative catalysts for ORR catalysis.

Tailoring the Electronic Structure of Transition Metals by the V2C MXene Support: Excellent Oxygen Reduction Performance Triggered by Metal-Support Interactions.

The enhancement of oxygen reduction reaction (ORR) activity can significantly boost the performance of fuel cells. MXene-supported transition metals with strong metal-support interactions (SMSI) are an effective strategy to increase the catalytic activity and durability while decreasing the usage of noble metals. Herein, a series of composites of transition-metal atoms (Ni, Pd, Pt, Cu, Ag, and Au) deposited on V2C MXene are designed as potential catalysts for ORR using density functional theory. The calculation results demonstrate that all the transition metals prefer to form a monolayer on V2C (TMML/V2C) with high thermodynamic stability because of SMSI, in which the Pd, Pt, Ag, and Au monolayers exhibit high chemical stability during the ORR process. PtML/V2C exhibits the highest activity toward ORR with the overpotential down to 0.38 V and the largest energy barrier of 0.48 eV. The excellent catalytic performance originates from the modification of the electronic structure by the V2C support because of SMSI. Our studies elucidate the SMSI between transition-metal atoms and V2C MXene from the atomic level and thus rationally design the ORR catalyst at the cathode of fuel cells to enhance the activity while possessing high stability and less Pt usage.

Efficient metal overlayer catalysts on the Nb2C monolayer for CO oxidation from first-principles screening.

Based on the first-principles calculation, the configurations of different metal overlayers on the monolayer Nb2C (MXene) (MML/Nb2C) (M = Rh, Ir, Pd, Pt, Ag, Au) were studied aiming to find a kind of complex system with high CO-tolerance and high CO conversion efficiency. Combined with the stability of the composite systems and their adsorption properties on small gas molecules, AgML/Nb2C was screened out and further tested for CO oxidation reaction. By comparing the energy barriers of different reaction pathways, we concluded that CO oxidation reaction could be carried out on AgML/Nb2C var the LH mechanism with a small energy barrier of 0.35 eV. The rate-determining step was the oxidation of CO by the adsorbed oxygen atom. The AgML/Nb2C showed good activity for CO oxidation, which would provide a theoretical basis for designing the electrode material for the proton exchange membrane fuel cells (PEMFCs).

Surface vacancy on PtTe2 for promoting CO oxidation through efficiently activating O2.

Platinum group metal dichalcogenides (PtTe2) with controllable thickness have been synthesized and confirmed to be promising electric and spintronic materials. Here, using the first-principles calculations, we demonstrate the potential application of PtTe2 as catalyst substrate. Taking CO oxidation as model reaction, the importance of surface vacancy is clarified. It is found that surface vacancy on PtTe2 could improve the stability and catalytic activity of the supported Pt atom. The details of CO oxidation processes indicate that surface vacancy could weaken the adsorption of reactants and speed up the formation and decomposition of OOCO intermediate on Pt catalysts. The underlying mechanisms for the improved activity are unveiled through comprehensively analyzing the charge transfer, density of states, and charge density difference. We hope that the current findings were beneficial for the research and development of efficient catalysts by collocating various single atom/cluster catalysts with different platinum group metal dichalcogenides.

An electronic perturbation in TiC supported platinum monolayer catalyst for enhancing water-gas shift performance: DFT study.

The water-gas shift (WGS) reaction behaviors over the TiC(0 0 1) supported Pt monolayer catalyst (PtML/TiC(0 0 1)) are investigated by using the spin-unrestricted density functional theory calculations. Importantly, we find that the PtML/TiC(0 0 1) system exhibits a much lower density of Pt-5d states nearby the Fermi level compared with that for Pt(1 1 1), and the monolayer Pt atoms undergo an electronic perturbation when in contact with TiC(0 0 1) support that would strongly improve the WGS activity of supported Pt atoms. Our calculations clearly indicate that the dominant reaction path follows a carboxyl mechanism involving a key COOH intermediate, rather than the common redox mechanism. Furthermore, through the detailed comparisons, the results demonstrate that the strong interactions between the monolayer Pt atoms and TiC(0 0 1) support make PtML/TiC(0 0 1) a highly active catalyst for the low-temperature WGS reaction. Following the route presented by Bruix et al (2012 J. Am. Chem. Soc. 134 8968-74), the positive effect derived from strong metal-support interaction in the metal/carbide system is revealed.

Low-temperature preferential oxidation of CO over Ag monolayer decorated Mo2C (MXene) for purifying H2.

Selective removal of CO from reformate streams at low temperature is an important issue to be solved for anode catalysts of proton exchange membrane fuel cells (PEMFCs). Using density functional theory, we demonstrate that the Ag monolayer decorated Mo2C can be used as an effective low cost catalyst with high activity, selectivity and stability for preferential oxidation of CO. Ag monolayer decorated Mo2C can also be used as a filter membrane for separating H2 and CO. Meanwhile, the high selectivity is achieved because H2 hardly takes part in the reaction, and the good activity is obtained since the CO with moderate binding strength could be smoothly removed at various CO concentration. We propose a facile route for removing CO by injecting low O2 supply because it can resist CO poisoning with promoted activity for CO oxidation at higher CO concentration. We anticipate that Ag monolayer decorated Mo2C can be used as a promising H2 purifying pretreater by preferential oxidation of CO connecting to the anode of PEMFCs. We hope the present results could advance the development of catalysts for purifying H2 and inspire more researches on the application of MXene catalysts.

Tunable Electric Properties of Bilayer α-GeTe with Different Interlayer Distances and External Electric Fields.

Based on first-principle calculations, the stability, electronic structure, optical absorption, and modulated electronic properties by different interlayer distances or by external electric fields of bilayer α-GeTe are systemically investigated. Results show that van der Waals (vdW) bilayer α-GeTe has an indirect band structure with the gap value of 0.610 eV, and α-GeTe has attractively efficient light harvesting. Interestingly, along with the decrease of interlayer distances, the band gap of bilayer α-GeTe decreases linearly, due to the enhancement of interlayer vdW interaction. In addition, band gap transition is originated from the electric field-induced near free-electron gas (NFEG) under the application of positive electrical fields. However, when the negative electric fields are applied, there is no NFEG. On account of these characteristics of bilayer α-GeTe, a possible data storage device has been designed. These results indicate that bilayer α-GeTe has a potential to work in new electronic and optoelectronic devices.

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.

Strong metal-support interactions impart activity in the oxygen reduction reaction: Au monolayer on Mo2C (MXene).

The rational design of low-cost, high-efficiency, corrosion-resistant and persistent-activity oxygen reduction reaction (ORR) electrocatalysts is a common goal for the large-scale application of fuel cells. Inspired by the excellent characteristics of MXenes when used as substrate materials and recent experiments of depositing metal nanoparticles on MXenes, we systematically investigated monolayer metal thin films decorated by Mo2C (MXene) (MML/Mo2C, M = Cu, Pd, Pt, Ag and Au) as ORR catalysts using density functional theory. According to the stability and adsorption properties, we speculate that AuML/Mo2C possesses outstanding ORR performance and enhanced durability in comparison with Pt/C catalysts. The ORR on AuML/Mo2C proceeds through a four-electron reduction pathway with comparable or even better activity than Pt(1 0 0), Pt(1 1 1) and commercial Pt/C catalysts both kinetically and thermodynamically. Strong metal-support interactions give rise to larger electronic perturbations in the supported Au monolayer in contact with Mo2C, which strengthen the adsorption of oxygen-containing species and enhance the catalytic activity. Our current results indicate that AuML/Mo2C is a promising ORR catalyst candidate to replace precious Pt/C catalysts due to its good stability, enhanced durability, low cost and high activity. We hope our results will inspire more experimental and theoretical research to further design, explore and apply advanced metal monolayer-supported MXene composites.

Cu3-Cluster-Doped Monolayer Mo2CO2 (MXene) as an Electron Reservoir for Catalyzing a CO Oxidation Reaction.

The catalytic oxidation of CO on Cu3-cluster-decorated pristine and defective Mo2CO2 (MXene) monolayers (Cu3/p-Mo2CO2 and Cu3/d-Mo2CO2) was investigated by first-principles calculations. The stability of the designed catalysts was comprehensively demonstrated via analysis of the energies, geometry distortion, and molecular dynamics simulations at finite temperatures. The difference in the individual adsorption energies, as well as the oxidation and poisoning of Cu3/p(d)-Mo2CO2 under CO and O2 gas atmospheres, was tested to estimate the catalytic ability. We found that Cu3/d-Mo2CO2 might be a superior catalyst with good stability and reactivity for CO oxidation. The active sites of the Cu3 cluster acting as an electron reservoir governed its electron-donating and -accepting ability. Different adsorption configurations of O2 on Cu3/d-Mo2CO2 also gave rise to different reaction activities. The facile rate-limiting energy barrier was attributed to the charge buffer capacity of the Cu3 cluster that mediates the reaction. Our results may provide clues to fabricate MXene-based materials by depositing small clusters on MXenes and exploring the advanced applications of these materials.