2D transitional-metal nickel compounds monolayer: Highly efficient multifunctional electrocatalysts for the HER,OER and ORR

The atomically dispersed Ni metal on two-dimensional (2D) monolayer species have exhibited impressive catalytic properties towards oxygen evolution and oxygen reduction reactions (OER/ORR). In this work, nickel boride and nickel carbide monolayers, which is inspired by the more different active sites of monolayer surface, such as Ni atom, that efficiently catalyze the reduction of OOH to O₂ to some extent, while B/C atom can work in the reduction of H1 to H₂, are theoretically reported. Remarkably, the density functional theory (DFT) calculations showed that the random combination of Ni atom to two free-metal atoms such as B and C to form catalytically active double sites lead to a remarkable reduction of the first or last hydrogenation free energy barrier step. The resulting Ni₂B₅ and NiC₃ exhibited ultra-low Gibbs free energy (ΔG_H1) of only 0.096 V and 0.018 V for HER and lower onset potentials of only 0.39 V (0.62 V) and 0.60 V (0.31 V) for OER (ORR), respectively, while the NiB₆ exhibited appropriate OER and ORR electrocatalytic activity. The superior bifunctional even multifunctional catalytic performance in the overall water splitting is mainly attributed to both the electron donation from the Ni metal atoms to the key intermediates, which significantly polarizes and weakens the O–H bond, and to the synergistic effect of the B/C atoms that moderates the binding strength of terminal top of H atoms. This work constitutes the DFT study of the water electroreduction processes on diverse nickel compounds monolayer catalytic sites and, consequently, paves the way towards the rational design of highly efficient bifunctional even multifunctional electrocatalysts.     

One-pot hydrothermal synthesis: Enhanced MOR and OER performance using low-cost Mn3O4 electrocatalyst

Mn₃O₄ NPs were synthesized using a simple, rapid, and cost-effective hydrothermal method. High-resolution transmission electron microscopy showed that most of the synthesized NPs were cubic, with some NPs being spherical and hexagonal. The electrochemical results showed that the Mn₃O₄ cubes were exhibited good oxygen evolution activity (OER) and methanol oxidation activity (MOR) and stability in alkaline media, making them a promising and efficient material in energy applications.     

Controllable synthesis of one-dimensional NiS2 nanotube and nanorod arrays on nickel foams for efficient electrocatalytic water splitting

One-dimensional NiS₂ nanotube arrays and nanorod arrays are controllably grown on Ni foam surface. The electrocatalytic test shows that the NiS₂ nanotube arrays require competitive overpotentials of 209 mV for HER and 367 mV for OER (to achieve a current density of 50 mA/cm²), respectively, which are much lower than the NiS₂ nanorod arrays and other NiS₂ nanostructures reported before. Specifically, the NiS₂ nanotube arrays can be employed as an efficient bi-functional catalyst for overall water splitting, with a low cell voltage (1.58 V) to deliver a current density of 10 mA/cm². The outstanding performance can be attributed to the special structural characteristics of nanotubes, which have high specific surface areas along with abundant active sites. The present study not only enriches the morphology of NiS₂ nanostructures for highly efficient electrocatalytic reaction, but also provides an interesting self-assembly path for the synthesis of one-dimensional NiS₂ nanostructures.     

Comparative studies on the electrocatalytic hydrogen evolution property of Cu2SnS3 and Cu4SnS4 ternary alloys prepared by solvothermal method

The development of non-precious metal based electrocatalysts is essential for Hydrogen Evolution Reaction (HER) from the splitting of water. Recently copper tin sulphide (CTS) has gained attention due to its favourable properties in photovoltaic devices and photocatalysis. In this work, CTS has been explored as an electrocatalyst in HER. Cu₂SnS₃ (CTS (A)) and Cu₄SnS₄ (CTS (B)) are prepared by a simple solvothermal method. From the XRD analysis, tetragonal and orthorhombic phase is confirmed for CTS (A) and CTS (B), respectively. Morphology and composition of the prepared samples has been confirmed by SEM and EDAX analysis. Optical study reveals that the band gap energy is around 1.55 and 1.20 eV for CTS (A) and CTS (B), respectively. From photoluminescence studies, CTS (B) has been observed to have a greater recombination of charge carriers than CTS (A). The prepared materials were tested and compared for its performance as electrocatalysts for HER, where the current density of CTS (A) is higher than that of CTS (B). Furthermore, the calculated Tafel slope was 98 mV/dec and 110 mV/dec for CTS (A) and CTS (B), respectively. On the other hand, CTS (A) and CTS (B) have showed good stability even after 500 cycles in acidic medium.     

Self-supported Ni3S2@MoS2 core/shell nanorod arrays via decoration with CoS as a highly active and efficient electrocatalyst for hydrogen evolution and oxygen evolution reactions

A hierarchically porous MoS₂ on Ni₃S₂ nanorod array on Ni foam (MoS₂/Ni₃S₂/NF) was firstly fabricated through a simple microwave-assisted hydrothermal method, and then followed by electrochemical deposition approach in which MoS₂/Ni₃S₂/Ni foam is decorated with CoS (CoSMoS₂/Ni₃S₂/NF). In contrast to conventional hydrothermal approach, microwave irradiation accelerates the synthesis of MoS₂/Ni₃S₂/Ni foam from time of >20 h–2 h. The characterization of CoSMoS₂/Ni₃S₂/NF by scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM) indicate that a whole scale of 1D the Ni₃S₂ nanorods were hierarchically integrated with MoS₂ and CoS nanosheets. The as-synthesized CoSMoS₂/Ni₃S₂/NF hybrid not only endows the ease transport of electrons along Ni₃S₂ nanorods to Ni foam, but also accommodates maximal exposure of active edge sites to the reactants through hierarchically porous CoS doped MoS₂ nanosheets, accomplishing the promoted kinetics and activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). By electrochemical measurements such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscope (EIS), we find that the CoSMoS₂/Ni₃S₂/NF hybrid shows markedly enhanced electrochemical performance for both HER and OER. Specifically, the optimal CoSMoS₂/Ni₃S₂/NF8C possesses the low overpotentials (η₁₀) of 85 and 225 mV at current density (|j|) of 10 mA cm^?2 in 1.0 M KOH and the small 62.3 and 46.1 mV dec^?1 Tafel slope for HER and OER, respectively, outperforming those of most of the current noble metal-free electrocatalysts. These results highlight the fact that CoSMoS₂/Ni₃S₂/NF is a high-performance, noble-metal-free electro-catalyst, and provide a potential avenue toward achieving an enhanced electrocatalytic activity towards both in HER and OER. Yet the duration of the as prepared catalyst in OER still need to be improved.     

Spontaneous synthesis of Ag nanoparticles on ultrathin Co(OH)2 nanosheets@nitrogen-doped carbon nanoflake arrays for high-efficient water oxidation

The slow four-electron transfer of the oxygen evolution reaction (OER) greatly limits the water splitting efficiency, so the development of high-efficiency and stable OER electrocatalysts is of great significance for large-scale alkaline water splitting. Herein, we reported an efficient self-supported OER electrocatalyst of Ag NPs decorated ultrathin Co(OH)₂ nanosheets supported on nitrogen-doped carbon (NC) nanoflake arrays on carbon cloth (Ag/Co(OH)₂@NC/CC). Our designed 3D unique electrode structure with heterointerfaces and hierarchical nanosheets endow Ag/Co(OH)₂@NC/CC with outstanding OER electrocatalytic activity (300 and 350 mV at 50 and 100 mA/cm², 79.8 mV/dec of Tafel slope) and good long-term durability in 1 M KOH. Furthermore, the related results of electrochemical tests show that the improved conductivity, increased electrochemical active sites and enhanced intrinsic active sites explain the superior OER performance. Our study will offer some new ideas in constructing new highly-active materials for various other energy conversion fields.     

Morphological engineering of carbon-based materials: in the quest of efficient catalysts for overall water splitting

This work attempts to optimize the catalytic activity of the carbon-based materials by engineering their morphological structure. Several flake-like quantum dots with different shapes such as triangulene, elliptical, rhomboid, and square, as well as hydrocarbons having sunflower, kekulene, and snow-like structures, are considered and their electrocatalytic activities toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are theoretically evaluated. The activity analysis indicates that the OER overpotentials for the examined carbon materials vary in the range between 0.56 and 1.22 V. Benefiting from the improved electronic properties due to the proper morphology, remarkable catalytic activity was achieved for the snow-like morphology affording overpotentials of 0.56 V for OER and ?0.05 V for HER. In addition to snow-like, other morphologies such as triangulene and square can effectively promote acidic hydrogen evolution via Volmer-Heyrovsky mechanism. On contrary, the high values of free energies for H₂O dissociation step reveal that, under the alkaline condition, the examined carbon materials cannot be considered as efficient HER catalysts.     

Eco-friendly and facile synthesis of novel bayberry-like PtRu alloy as efficient catalysts for ethylene glycol electrooxidation

Direct fuel cells have attracted an increasing notice over last decade, while its large-scale commercial development is also seriously impeded by the lack of cost-efficient electrocatalysts. The shape-controlled syntheses of binary Pt-based nanocrystals bounded with abundant surface active areas and tunable atomic ratio have been of vital importance in the fabrication and modification of outstandingly excellent electrocatalysts. Therefore, embodying the morphology advantages and composition effects are significant for synthesizing the cost-efficient electrocatalysts. In view of this, we herein report our efforts for demonstrating an eco-friendly approach to successfully synthesize a novel type of bayberry-like PtRu nanocatalyst with different compositions. Different from some other reported PtRu binary nanostructures, such as-prepared bayberry-like PtRu nanocrystals with rough surface can meet the requirement of both high mass activity and long-term stability, which shed light for the commercial development of direct fuel cells.     

Facile modified polyol synthesis of FeCo nanoparticles with oxyhydroxide surface layer as efficient oxygen evolution reaction electrocatalysts

The oxygen evolution reaction (OER) at anode requires high overpotential and is still challenging. The metallic core-oxyhydroxide layer structure is an efficient method to lower an overpotential. We synthesized Fe rich FeCo core-Co rich FeCo oxyhydroxide layer with a different particle size of 173 nm, 225 nm, and 387 nm (FeCo 173, 225, 387) through a difference in the reduction rate of Fe/Co precursors using facile modified polyol synthesis. To investigate the effect of conductivity, CoFe₂O₄ nanoparticles of 80–130 nm were synthesized. Among samples, FeCo 173 showed remarkable catalytic performance of 316 mV at a current density of 10 mA/cm² in 0.1 M KOH compared to RuO₂ (408 mV), FeCo 225 (323 mV), FeCo 387 (334 mV), CoFe₂O₄ (382 mV). Moreover, FeCo 173 showed good stability for 60,000 s while RuO₂ showed a gradual increase in overpotential to maintain 10 mA/cm² after 15,000 s in chronopotentiometry. The excellent performance was attributed to Fe-rich metallic core, a small amount of Fe doping into CoOOH, and the synergic effect between the active site of Co rich FeCoOOH and conductive Fe rich metallic core. Following this result, it shows that the use of such FeCo electrodes has advantages in the production of hydrogen via electrochemical water oxidation.     

An extremely facile route to Co2P encased in N,P-codoped carbon layers: Highly efficient bifunctional electrocatalysts for ORR and OER

Exploring low-cost, efficacious and stable bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial yet highly challenging to the wide perspective practicability for rechargeable metal?air batteries together with fuel cells and electrochemical water splitting. Herein, we have successfully prepared a novel nanohybrid consisting of Co₂P nanocores encased in N,P-codoped carbon layers (denoted as Co₂P@NPC) through an extremely facile and effective pyrolysis route utilizing easily available raw materials. The as-prepared Co₂P@NPC exhibits excellent bifunctional electrocatalytic performances for both ORR and OER in alkaline solution. For ORR, Co₂P@NPC shows comparable activity to the commercial 20 wt% Pt/C in terms of the half-wave potential and limited current density under the identical linear sweep voltammetry measurement conditions. Moreover, the ORR durability and methanol tolerance of Co₂P@NPC are much better than those of 20 wt% Pt/C. As regards OER, Co₂P@NPC affords lower activity but better stability than the fresh RuO₂. The high electrocatalytic performances originate from the strong interaction between Co₂P nanocores and N,P-codoped carbon layers, the high BET surface area and the conductive network formed by the crosslinking of carbon coatings.     

Directed growth of nanoarchitectured LiFePO4 electrode by solvothermal synthesis and their cathode properties

We demonstrate a rapid and one-pot solvothermal synthesis of LiFePO₄ hierarchical nanorods and flowers like microstructures in a short reaction time (4-10 min) at temperature as low as 300-400 °C, without any high temperature post-annealing. The ethylene glycol was used as a solvent with hexane and oleic acid as co-solvent and surfactant, respectively. Addition of the co-solvent and/or surfactant played a key role in controlling the morphology and microstructures of the LiFePO₄ nanocrystals. The EG and oleic acid were acted as the size and morphology controlling agents and carbon source when annealed at 600 °C. Sample exhibited about 90% specific capacity at 0.5 °C and showed good cyclic performance. The high-resolution TEM image revealed that these nanorods were self-assembled to form flower like microstructure in presence of oleic acid during the synthesis.     

Electrophoretic behavior of solvothermal synthesized anion replaced Cu2ZnSn(SxSe1-x)4 films for photoelectrochemical water splitting

Quaternary Cu₂ZnSn(S_xSe_1-x)₄ (CZT(S_xSe_1-x)₄) compounds have drawn a great deal of attention for being used in the fabrication of optoelectronic devices such as solar cells, photocatalysts, and photoelectrochemical water splitting. However, one major challenge facing the utilization of this material is to reduce the production cost of synthesis and fabrication of high quality CZT(S_xSe_1-x)₄ films. In the present study, a facile and beneficial solvothermal route has been reported for synthesis of CZT(S_xSe_1-x)₄ compounds. The process of electrophoretic deposition (EPD) of synthesized CZT(S_xSe_1-x)₄ nanoparticles, is systematically compared with each other in order to obtain high quality films with appropriate porosity. The XRD patterns, EDS and Raman spectra confirm the formation of CZT(S_xSe_1-x)₄ phases with no trace of impurities and appreciable crystallinity and also with near stoichiometry composition in all the samples. The obtained particle size for CZTS, CZTSSe and CZTSe samples was in the range of 50–100 nm and also for some agglomerate particles was in the range of 500 nm to 2 μm. Based on the obtained results for thin films prepared using EPD in the present study, the best EPD parameters for each CZTS, CZTSe and CZTSSe samples with 120 V and 5 min as applied voltage and deposition time were reported as the best samples. The obtained photocurrent-potential and current-time curves of CZT(S_xSe_1-x)₄ thin film samples demonstrate that the photocurrents of each CZTS, CZTSe, CZTSSe thin films, are different in the range of ?2.1 to ?6 mA/cm² and also the CZTS and CZTSe samples show a detectable current under the exposure of sunlight that can have an appropriate stability for 3000 s but the CZTSSe sample showed a stable photocurrent just for 2000 s. According to the mentioned results in this study, the CZTS and CZTSe samples can potentially be suitable candidates for further applications.     

Influence of solvent in solvothermal synthesis of Cu3SnS4: Morphology and band gap dependant electrocatalytic hydrogen evolution reaction and photocatalytic dye degradation

Cu₃SnS₄ (CTS) with various morphologies have been synthesized via solvothethermal method. The influence of different solvents such as deionized water (CTS-DI), N,N-dimethylformamide (CTS-DMF) and poly ethylene glycol (CTS-PEG) has been investigated. XRD patterns of all the prepared samples exhibit tetragonal structure with preferential orientation along (1 1 2) direction. It is found that the solvents play a key role to tune the morphology and band gap. The morphology varied as irregular sheet like structure, mesoporous structure and feather like structure for CTS-DI, CTS-DMF and CTS-PEG, respectively. The band gap was found to be 1.33, 1.65 and 1.19 eV for CTS-DI, CTS-DMF and CTS-PEG, respectively. Electrocatalytic hydrogen evolution reaction and photocatalytic dye degradation of MB were tested using the prepared CTS samples. As an electrocatalyst or photocatalyst, it is found that CTS-DMF with mesopours structure shows an enhanced catalytic activity with lower overpotential, a smaller Tafel slope (88 mV/dec), high stability in acidic medium and an enhanced photocatalytic activity. A more realistic mechanism for the photocatalytic activity of CTS-DMF is proposed.     

Investigation of engines radiator heat recovery using different shapes of nanoparticles in H2O/(CH2OH)2 based nanofluids

In this paper, a flat tube of an engine radiator is modeled numerically for improving the cooling process or heat recovery of the engine using nanofluids. Two hydrogen based fluids (water (H₂O) and ethylene glycol or EG ((CH₂OH)₂) and four nanoparticles (CuO, TiO₂, Al₂O₃ and Fe₃O₄) in different shapes (Brick, Cylindrical, Platelet and Spherical) are considered for modeling the nanofluids in four different Reynolds numbers (500, 1000, 1500 and 2000). Hamilton correlation is used to calculate the thermal conductivity of nanofluids in different shapes of nanoparticles. Furthermore, the effect of nanoparticles volume fraction on the Nusselt number for all nanoparticle shapes is discussed in this study. Results show that EG-TiO₂ with platelet shape and larger volume fraction of nanoparticles has the best cooling performance for the engine among other modeled nanofluids.     

Solution processed thin films of non-aggregated TiO2 nanoparticles prepared by mild solvothermal treatment

In this report, non-aggregated anatase TiO₂ nanoparticles were synthesized by mild solvothermal process in 1-butanol. By varying solvothermal reaction temperature and time, TiO₂ particle size was controlled from 5.3 to 9.0 nm, while maintaining pure anatase phase and optical clearance in the concentrated dispersion (5 wt%). Spin coating of TiO₂ dispersions resulted in transparent thin films with thickness controllability, and it was confirmed that the mild solvothermal reaction significantly increased the refractive index of the thin films without post-thermal treatment. In addition to the fabrication of low-temperature processed thin films, the inverse opal TiO₂ films were also fabricated by the colloidal templating method followed by thermal calcination to reveal the improved volume shrinkage of the mesoporous TiO₂ films.     

Electro-catalyst [ZrO2/ZnO/PdO]-NPs green functionalization: Fabrication,characterization and water splitting potential assessment

Present investigation reports the biomimetic synthesis of mixed metallic oxides nanoparticles (NPs) comprising of zirconia, zinc oxide and palladium oxide ([ZrO₂/ZnO/PdO]-NPs) functionalized via nature's bio-factories for the first time. [ZrO₂/ZnO/PdO]-NPs express reduction revealed via Fourier transform infra-red spectroscopy and gas chromatography and mass spectrometry. X-ray diffraction express poly-crystallinity. The fabrication of green NPs is confirmed via energy-dispersive X-ray spectroscopy with irregular morphologies visible by scanning electron micrographs. A direct bandgap of 2.11 eV is obtained for [ZrO₂/ZnO/PdO]-NPs. Promising surficial features are disclosed via X-ray photoelectron spectroscopy. The electro-catalytic potential is studied using linear sweep voltammetry and electrochemical impedance spectroscopy for investigation of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities with low over-potential of 0.37 V and Tafel slope of 74 mV/dec, indicating the suitability for OER. [ZrO₂/ZnO/PdO]-NP electro-catalyst is also tested for HER, demonstrating remarkable results with the overpotential and Tafel slope value of 157 mV and 151 mV/dec.     

Facile synthesis of Ti3+ doped Ag/AgITiO2 nanoparticles with efficient visible-light photocatalytic activity

We successfully synthesized novel Ti³⁺ doped TiO₂ and Ti³⁺ doped Ag/AgITiO₂ nanoparticles with efficient visible-light photocatalytic activity for hydrogen production by facile one-step solvothermal method. The as-prepared Ti³⁺ doped TiO₂ nanoparticles displayed excellent visible-light absorption and visible-light driven hydrogen production activity (115.3 μmol g^?1 h^?1), while the commercial TiO₂ had no visible-light response. Moreover, the as-prepared Ti³⁺ doped Ag/AgITiO₂ nanoparticles in this experiment showed highly enhanced visible-light absorption and efficient visible-light driven activity for hydrogen (571.0 μmol g^?1 h^?1), which was 4.95 times as high as that of the as-prepared TiO₂ nanoparticles. And the surface areas of the as-prepared TiO₂ and Ti³⁺ doped Ag/AgITiO₂ catalysts were up to 138.829 m² g^?1 and 102.988 m² g^?1, much higher than that of the commercial TiO₂ (55.516 m² g^?1). Finally, the visible-light photocatalytic mechanism of the Ti³⁺ doped Ag/AgITiO₂ nanoparticles for hydrogen generation was also proposed in detail.     

Solvothermal synthesis of Li3VO4: Morphology control and electrochemical performance as anode for lithium-ion batteries

In this work, orthorhombic Li₃VO₄ with the controllable morphology has been synthesized by tuning the solvent composition (volume ratios of ethanol to deionized water) in a solvothermal approach. The resulting Li₃VO₄ samples with various morphologies (coral-shaped particle, self-assembled hierarchical microsphere, cube-like particle, sheet-like structure) show then different electrochemical performances when employed as anodes for Li-ion battery applications. The Li₃VO₄ with self-assembled hierarchical microsphere morphology (volume ratio of ethanol to deionized water at 15:15) exhibits the best electrochemical performance. The subsequent carbon coating process on microsphere samples is claimed to significantly improve both the capacities at both low (350–430 mAh g^?1 at 100 mA g^?1) and high current (180–350 mAh g^?1 at 2 A g^?1) conditions, and their excellent cycling stability.     

One-pot solvothermal synthesis of PdCu nanocrystals with enhanced electrocatalytic activity toward glycerol oxidation and hydrogen evolution

The catalytic capability of bimetallic nanocatalysts is closely correlated with their size, shape, and crystal structures. Herein, a facile one-pot solvothermal strategy is designed to fabricate uniform spherical PdCu nanocrystals (NCs) assembled by many smaller grains. Oeylamine (OAm) is acted as the reaction solvent and reductant. KBr and hexadecylpyridinium chloride monohydrate (HDPC) are used as the capping agent and surfactant to avoid the aggregation, respectively. The architectures possess larger electrochemically active surface area (ECSA) of 13.6 m² g^?1 than commercial Pd black (4.4 m² g^?1), showing the improved catalytic ability for glycerol oxidation reaction (GOR) in alkaline electrolyte in contrast with Pd black. Besides, the obtained catalyst exhibits more positive onset potential (?56 mV) and Tafel slope (51 mV decade^?1) toward hydrogen evolution reaction (HER) in acidic media relative to commercial Pd/C, albeit with their performance bellow commercial Pt/C catalyst.     

Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition

Low-temperature (180–240 °C) synthesis of nanocrystalline titanium dioxide (TiO₂) by surfactant-free solvothermal route is investigated. Titanium iso-propoxide is used as the precursor and toluene as the solvent. Different precursors to solvent weight ratios have been used for the synthesis of TiO₂ nanoparticles. For the weight ratios 15/100, 25/100 and 35/100 the X-ray diffractograms show the formation of nanocrystalline TiO₂. The X-ray diffraction and transmission electron microscopy studies shows that the product has anatase crystal structure (for temperatures <200 °C) with average particle size below 15 nm. The films deposited by spray deposition method using these nanoparticles show the crystalline and porous nature of the films. The present method of deposition also avoids the post-treatment (sintering) of the films. The nanoparticles thus prepared and the films can be used for gas sensing and biological applications and also as photo-electrodes for dye-sensitized solar cells.