Study on Water Splitting of the 214-Type Perovskite Oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga).

We present a study on the electrocatalysis of 214-type perovskite oxides LnSrCoO4 (Ln = La, Pr, Sm, Eu, and Ga) with semiconducting-like behavior synthesized using the sol-gel method. Among these five catalysts, PrSrCoO4 exhibits the optimal electrochemical performance in both the oxygen evolution reaction and the hydrogen evolution reaction, mainly due to its larger electrical conductivity, mass activity, and turnover frequency. Importantly, the weak dependency of LSV curves in a KOH solution with different pH values, revealing the adsorbate evolving mechanism in PrSrCoO4, and the density functional theory (DFT) calculations indicate that PrSrCoO4 has a smaller Gibbs free energy and a higher density of states near the Fermi level, which accelerates the electrochemical water splitting. The mutual substitution of different rare-earth elements will change the unit-cell parameters, regulate the electronic states of catalytic active site Co ions, and further affect their catalytic performance. Furthermore, the magnetic results indicate strong spin-orbit coupling in the electroactive sites of Co ions in SmSrCoO4 and EuSrCoO4, whereas the magnetic moments of Co ions in the other three catalysts mainly arise from the spin itself. Our experimental results expand the electrochemical applications of 214-type perovskite oxides and provide a good platform for a deeper understanding of their catalytic mechanisms.

Regulating Local Atomic Environment around Vacancies for Efficient Hydrogen Evolution.

Defect engineering is essential for the development of efficient electrocatalysts at the atomic level. While most work has focused on various vacancies as effective catalytic modulators, little attention has been paid to the relation between the local atomic environment of vacancies and catalytic activities. To face this challenge, we report a facile synthetic approach to manipulate the local atomic environments of vacancies in MoS2 with tunable Mo-to-S ratios. Our studies indicate that the MoS2 with more Mo terminated vacancies exhibits better hydrogen evolution reaction (HER) performance than MoS2 with S terminated vacancies and defect-free MoS2. The improved performance originates from the adjustable orbital orientation and distribution, which is beneficial for regulating H adsorption and eventually boosting the intrinsic per-site activity. This work uncovers the underlying essence of the local atomic environment of vacancies on catalysis and provides a significant extension of defect engineering for the rational design of transition metal dichalcogenides (TMDs) catalysts and beyond.

Efficient and stable acidic CO2 electrolysis to formic acid by a reservoir structure design.

Electrochemical synthesis of valuable chemicals and feedstocks through carbon dioxide (CO2) reduction in acidic electrolytes can surmount the considerable CO2 loss in alkaline and neutral conditions. However, achieving high productivity, while operating steadily in acidic electrolytes, remains a big challenge owing to the severe competing hydrogen evolution reaction. Here, we show that vertically grown bismuth nanosheets on a gas-diffusion layer can create numerous cavities as electrolyte reservoirs, which confine in situ-generated hydroxide and potassium ions and limit inward proton diffusion, producing locally alkaline environments. Based on this design, we achieve formic acid Faradaic efficiency of 96.3% and partial current density of 471 mA cm-2 at pH 2. When operated in a slim continuous-flow electrolyzer, the system exhibits a full-cell formic acid energy efficiency of 40% and a single pass carbon efficiency of 79% and performs steadily over 50 h. We further demonstrate the production of pure formic acid aqueous solution with a concentration of 4.2 weight %.

Fully Flexible Covalent Organic Frameworks for Fluorescence Sensing 2,4,6-Trinitrophenol and p-Nitrophenol.

Nitrophenols are important nitroaromatic compounds, both important environmental pollutants and dangerous explosives, posing a devastating danger and pollution threat to humans. It is vital to detect efficiently trace nitrophenols in the environment. In this contribution, a series of fully flexible cyclotriphosphazene-based COFs (FFCP COFs: HDADE, HBAPB, and HBPDA), prepared with both a flexible knot and flexible linkers of different lengths, were used for sensing 2,4,6-trinitrophenol (TNP) and p-nitrophenol (p-NP) in real time with excellent sensitivity and selectivity. The quenching constants of HDADE by TNP, HBAPB, and HBPDA by p-NP are 6.29 × 104, 2.17 × 105, and 2.48 × 105 L·mol-1, respectively. The LODs of TNP and p-NP are 1.19 × 10-11, 6.91 × 10-12, and 6.05 × 10-12 mol·L-1. Their sensitivities increase with the linker length, which is better than the corresponding COFs composed of rigid linkers. There is only a photoinduced electron transfer mechanism in the fluorescence quenching of HBPDA by p-NP. Meanwhile, the mechanisms of photoinduced charge transfer and resonance energy transfer exist in the fluorescence quenching of HDADE by TNP and the fluorescence quenching of HBAPB by p-NP.

Cobalt-Free Double Perovskite Oxide as a Promising Cathode for Solid Oxide Fuel Cells.

Double perovskite oxide PrBaFe2O5+δ is a potential cathode material for intermediate-temperature solid oxide fuel cells. To improve its electrochemical performance, the trivalent element Ga is investigated to partially replace Fe, forming PrBaFe2-xGaxO5+δ (PBFGx, x = 0.05, 0.1, and 0.15). The doping effects on physicochemical properties and electrochemical properties are analyzed regarding the phase structures, element valence states, amount of oxygen vacancies, content of oxygen species, oxygen surface exchange coefficients (kchem), electrochemical polarization resistance, and single-cell performance. Specifically, PBFG0.1 exhibits improved kchem, such as a 19% improvement from 4.09 × 10-4 to 4.86 × 10-4 cm s-1 at 750 °C, due to the increased concentration of reactive oxygen species and oxygen vacancies. Consequently, the interfacial polarization resistance is decreased by 28% from 0.057 to 0.041 Ω cm2 at 800 °C. The subreaction steps of the oxygen reduction reaction in the PBFG0.1 cathode are further investigated, which suggests that the oxygen dissociation process is greatly enhanced by doping Ga. Meanwhile, doping Ga increases the peak power density of the anode-supported single cell by 36% from 629 to 856 mW cm-2 at 800 °C. The single cell with the PBFG0.1 cathode also exhibits good stability in 100 h of long-term operation at 750 °C.

Electrochemical performance of CoSe2 with mixed phases decorated with N-doped rGO in potassium-ion batteries.

Potassium-ion batteries (PIBs) have received much attention as next-generation energy storage systems because of their abundance, low cost, and slightly lower standard redox potential than lithium-ion batteries (LIBs). Nevertheless, they still face great challenges in the design of the best electrode materials for applications. Herein, we have successfully synthesized nano-sized CoSe2 encapsulated by N-doped reduced graphene oxide (denoted as CoSe2@N-rGO) by a direct one-step hydrothermal method, including both orthorhombic and cubic CoSe2 phases. The CoSe2@N-rGO anodes exhibit a high reversible capacity of 599.3 mA h g-1 at 0.05 A g-1 in the initial cycle, and in particular, they also exhibit a cycling stability of 421 mA h g-1 after 100 cycles at 0.2 A g-1. Density functional theory (DFT) calculations show that CoSe2 with N-doped carbon can greatly accelerate electron transfer and enhance the rate performance. In addition, the intrinsic causes of the higher electrochemical performance of orthorhombic CoSe2 than that of cubic CoSe2 are also discussed.

Hexagonal perovskite Sr6(Co0.8Fe0.2)5O15 as an efficient electrocatalyst towards the oxygen evolution reaction.

The high overpotential required for the oxygen evolution reaction (OER)-due to the transfer of four protons and four electrons-has greatly hindered the commercial viability of water electrolysis. People have been committed to the development of alternative precious metal-free OER electrocatalysts, especially electrocatalysts for alkaline media. In this study, we report the application of Sr6(Co0.8Fe0.2)5O15 (SCF-H) perovskite oxide with a hexagonal phase structure in the field of OER electrocatalysis. Synthesized by a simple and universal sol-gel method, the SCF-H perovskite oxide shows prominent OER activity with an overpotential of 318 mV at a current density of 10 mA cm-2 and a Tafel slope of only 54 mV dec-1, which is significantly better than the cubic phase structure SrCo0.8Fe0.2O3-δ (SCF-C), benchmark noble-metal oxide RuO2 and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). Compared with cubic SCF-C, the hexagonal SCF-H perovskite oxide has abundant surface oxygen species (O22-/O-), a faster charge transfer rate, and a higher electrochemical surface area. In addition, the DFT calculation results show that the center of the O p-band of SCF-H is closer to the Fermi level than that of SCF-C, which leads to the better OER activity of SCF-H. This work finds that the new hexagonal structure perovskite may become a promising OER electrocatalyst.

Research Progress of FeSe-based Superconductors Containing Ammonia/Organic Molecules Intercalation.

As an important part of Fe-based superconductors, FeSe-based superconductors have become a hot field in condensed matter physics. The exploration and preparation of such superconducting materials form the basis of studying their physical properties. With the help of various alkali/alkaline-earth/rare-earth metals, different kinds of ammonia/organic molecules have been intercalated into the FeSe layer to form a large number of FeSe-based superconductors with diverse structures and different layer spacing. Metal cations can effectively provide carriers to the superconducting FeSe layer, thus significantly increasing the superconducting transition temperature. The orientation of organic molecules often plays an important role in structural modification and can be used to fine-tune superconductivity. This review introduces the crystal structures and superconducting properties of several typical FeSe-based superconductors containing ammonia/organic molecules intercalation discovered in recent years, and the effects of FeSe layer spacing and superconducting transition temperature are briefly summarized.

Stabilizing indium sulfide for CO2 electroreduction to formate at high rate by zinc incorporation.

Recently developed solid-state catalysts can mediate carbon dioxide (CO2) electroreduction to valuable products at high rates and selectivities. However, under commercially relevant current densities of > 200 milliamperes per square centimeter (mA cm-2), catalysts often undergo particle agglomeration, active-phase change, and/or element dissolution, making the long-term operational stability a considerable challenge. Here we report an indium sulfide catalyst that is stabilized by adding zinc in the structure and shows dramatically improved stability. The obtained ZnIn2S4 catalyst can reduce CO2 to formate with 99.3% Faradaic efficiency at 300 mA cm-2 over 60 h of continuous operation without decay. By contrast, similarly synthesized indium sulfide without zinc participation deteriorates quickly under the same conditions. Combining experimental and theoretical studies, we unveil that the introduction of zinc largely enhances the covalency of In-S bonds, which "locks" sulfur-a catalytic site that can activate H2O to react with CO2, yielding HCOO* intermediates-from being dissolved during high-rate electrolysis.

Semiconducting and magnetic properties of FeS-derived compounds (C2H8N2)xFeS and Ax(C2H8N2)yFeS (A = Li, Na).

Several FeS-derived intercalated compounds (C2H8N2)xFeS and Ax(C2H8N2)yFeS (A = Li, Na) were successfully synthesized via a novel ammonothermal method. The powder X-ray diffraction (XRD) measurements reveal that the FeS intercalated samples have the same tetragonal crystal structure as the parent FeS. After intercalation, these three as-synthesized samples do not show superconductor behavior, which is confirmed by the magnetization and the electrical resistivity measurements. (C2H8N2)xFeS exhibits paramagnetic semiconductor behavior, while the newly synthesized Ax(C2H8N2)yFeS (A = Li, Na) shows antiferromagnetic semiconductor behavior. The absence of superconductivity in these FeS-derived compounds should be closely related to the iron vacancies in the FeS layer.

A Self-Doped Oxygen-Free High-Critical-Temperature (High-Tc) Superconductor: SmFFeAs.

A new iron-base superconductor SmFFeAs is synthesized via solid-state metathesis reaction by using SmFCl and LiFeAs as precursors. The compound crystallized in the tetragonal ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters of a = 3.9399(0) Å and c = 8.5034(1) Å. The superconducting diamagnetic transition occurs at 56 K for the parent compound, which confirmed by the resistivity and magnetic susceptibility. The appearance of superconductivity without extrinsic doping could be ascribed to the self-doping owing to the mixed valence of Sm ions. The as-synthesized SmFFeAs serves as a new self-doped parent compound for oxygen-free high-critical-temperature (high-Tc) superconductors.

A facile synthesis of FePS3@C nanocomposites and their enhanced performance in lithium-ion batteries.

FePS3@C nanocomposites are successfully synthesized by a facile two-step route. The as-synthesized samples show excellent thermal and environmental stability. As a novel anode material for lithium batteries, the FePS3@C nanocomposites exhibit a high capacity of 1000 mA h g-1 at a current density of 0.2 A g-1 and excellent reversibility over 100 cycles, indicating their promising applications in lithium storage.

Enhanced electrochemical properties of cellular CoPS@C nanocomposites for HER, OER and Li-ion batteries.

Cellular CoPS@C nanocomposites were successfully synthesized via a facile two-steps route. The performances of the CoPS@C electrode as a non-noble metal electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) show good activity. On the other hand, the electrochemical investigation of CoPS systems for lithium ion batteries (LIBs) is reported for the first time. The CoPS@C nanocomposite as a novel anode can maintain a capacity of about 713 mA h g-1 after 50 cycles at a current density of 0.2 A g-1, indicating its potential applications in lithium storage. Test results also demonstrate that the CoPS@C nanocomposite exhibit more excellent HER, OER and Li storage performances compared to the bulk CoPS sample.

Toward Exploring the Structure of Monolayer to Few-layer TaS2 by Efficient Ultrasound-free Exfoliation.

Tantalum disulfide nanosheets have attracted great interest due to its electronic properties and device applications. Traditional solution-ased ultrasonic process is limited by ultrasound which may cause the disintegration into submicron-sized flake. Here, an efficient multi-step intercalation and ultrasound-free process has been successfully used to exfoliate 1T-TaS2. The obtained TaS2 nanosheets reveal an average thickness of 3 nm and several micrometers in size. The formation of few-layer TaS2 nanosheets as well as monolayer TaS2 sheets is further confirmed by atomic force microscopy images. The few-layer TaS2 nanosheets remain the 1T structure, whereas monolayer TaS2 sheets show lattice distortion and may adopt the 1H-like structure with trigonal prism coordination.

One-step thermolysis synthesis of two-dimensional ultrafine Fe3O4 particles/carbon nanonetworks for high-performance lithium-ion batteries.

To tackle the issue of inferior cycle stability and rate capability for Fe3O4 anode materials in lithium ion batteries, ultrafine Fe3O4 nanocrystals uniformly encapsulated in two-dimensional (2D) carbon nanonetworks have been fabricated through thermolysis of a simple, low-cost iron(iii) acetylacetonate without any extra processes. Moreover, compared to the reported Fe3O4/carbon composites, the particle size of Fe3O4 is controllable and held down to ∼3 nm. Benefitting from the synergistic effects of the excellent electroconductive carbon nanonetworks and uniform distribution of ultrafine Fe3O4 particles, the prepared 2D Fe3O4/carbon nanonetwork anode exhibits high reversible capacity, excellent rate capability and superior cyclability. A high capacity of 1534 mA h g(-1) is achieved at a 1 C rate and is maintained without decay up to 500 cycles (1 C = 1 A g(-1)). Even at the high current density of 5 C and 10 C, the 2D Fe3O4/carbon nanonetworks maintain a reversible capacity of 845 and 647 mA h g(-1) after 500 discharge/charge cycles, respectively. In comparison with other reported Fe3O4-based anodes, the 2D Fe3O4/carbon nanonetwork electrode is one of the most attractive of those in energy storage applications.

One-Step Synthesis of Titanium Oxyhydroxy-Fluoride Rods and Research on the Electrochemical Performance for Lithium-ion Batteries and Sodium-ion Batteries.

Titanium oxyhydroxy-fluoride, TiO0.9(OH)0.9F1.2 · 0.59H2O rods with a hexagonal tungsten bronze (HTB) structure, was synthesized via a facile one-step solvothermal method. The structure, morphology, and component of the products were characterized by X-ray powder diffraction (XRD), thermogravimetry (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), ion chromatograph, energy-dispersive X-ray (EDX) analyses, and so on. Different rod morphologies which ranged from nanoscale to submicron scale were simply obtained by adjusting reaction conditions. With one-dimension channels for Li/Na intercalation/de-intercalation, the electrochemical performance of titanium oxyhydroxy-fluoride for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) was also studied. Electrochemical tests revealed that, for LIBs, titanium oxyhydroxy-fluoride exhibited a stabilized reversible capacity of 200 mAh g(-1) at 25 mA g(-1) up to 120 cycles in the electrode potential range of 3.0-1.2 V and 140 mAh g(-1) at 250 mA g(-1) up to 500 cycles, especially; for SIBs, a high capacity of 100 mAh g(-1) was maintained at 25 mA g(-1) after 115 cycles in the potential range of 2.9-0.5 V.

Facile Synthesis of Mn-Doped ZnO Porous Nanosheets as Anode Materials for Lithium Ion Batteries with a Better Cycle Durability.

Porous Zn1 - x Mn x O (x = 0.1, 0.2, 0.44) nanosheets were prepared by a low-cost, large-scale production and simple approach, and the applications of these nanosheets as an anode material for Li-ion batteries (LIBs) were explored. Electrochemical measurements showed that the porous Zn0.8Mn0.2O nanosheets still delivered a stable reversible capacity of 210 mA h g(-1) at a current rate of 120 mA g(-1) up to 300 cycles. These results suggest that the facile synthetic method of producing porous Zn0.8Mn0.2O nanostructure can realize a better cycle durability with stable reversible capacity.

Synthesis of Mn2O3 nanomaterials with controllable porosity and thickness for enhanced lithium-ion batteries performance.

Mn2O3 has been demonstrated to be a promising electrode material for lithium-ion batteries. Thus, the fabrication of Mn2O3 nanomaterials with high specific capacity and cycling stability is greatly desired. Here we report a simple but effective method to synthesis Mn2O3 nanomaterials from a Mn(OH)2 precursor, which was prepared from manganese acetate in ethylene glycol and water at 180 °C for 12 h. The morphology and sheet thickness of Mn(OH)2 precursor could be tuned by controlling the ethylene glycol/H2O volume ratio, resulting in a further tunable morphology and sheet thickness of the porous Mn2O3 nanomaterials. In the electrochemical tests the prepared Mn2O3 nanomaterials, with the porous architecture and thin thickness exhibited a high and stable reversible capacity, indicating that both small thickness and porous sheets structure are crucial for improving the electrochemical performance of Mn2O3 in terms of specific capacity and stability.

Facile one-pot synthesis of polytypic CuGaS2 nanoplates.

CuGaS2 (CGS) nanoplates were successfully synthesized by one-pot thermolysis of a mixture solution of CuCl, GaCl3, and 1-dodecanethiol in noncoordinating solvent 1-octadecene. Their morphology, crystalline phase, and composition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), respectively. Crystalline structure analysis showed that the as-prepared CGS nanoplates were polytypic, in which the wurtzite phase was interfaced with zincblende domains. The growth process of CGS nanoplates was investigated. It was found that copper sulfide nanoplates were firstly formed and then the as-formed copper sulfide nanoplates gradually transformed to CGS nanoplates with proceeding of the reaction. The optical absorption of the as-synthesized CGS nanoplates was also measured and the direct optical bandgap was determined to be 2.24 eV.