Search

High-performance multifunctional materials for water splitting driven by low voltage are crucial for hydrogen evolution reaction (HER), but developing such materials is challenging. Herein, a simple strategy was designed to build a MoS2/Co9S8/MoC@CNT-N (MCM@CNT-N) heterostructure with a large number of interfaces. Regarding the HER, the synthesized MCM@CNT-N heterostructure catalyst showed high efficiency and stable electrocatalytic performance, with a low overpotential of 174.2 mV and a small Tafel slope of 84.7 mV dec−1 at a current density of 10 mA cm−2 in 0.5 M H2SO4. In addition to the function of heterojunctions, the excellent activity is also attributed to the introduction of Co and N atoms and the formation of carbon nanotubes. This work provides a new approach to build efficient and low-cost electrocatalysts for electrochemical reactions.

It still is a challenge to create a superior and easily coupled bifunctional electrocatalyst for water splitting impelled by a low voltage. In this work, the controlled growth of Co2P NAs on the surface of a MXene (Ti3C2Tx)-modified self-supporting electrode is demonstrated as a competent and reliable bifunctional electrocatalyst for efficient water splitting. The heterointerface in Co2P@Ti3C2Tx with an optimized adsorption free energy of H*, H2O, and better conductivity can give enhanced HER (hydrogen evolution reaction) activity, with a low overpotential (42 mV) at 10 mA cm-2. Additionally, the OER (oxygen evolution reaction) activity has also been similarly strengthened by the synergy of Co2P and MXene with an overpotential of 267 mV to arrive at 10 mA cm-2. Furthermore, the excellent bifunctional electrode (Co2P@Ti3C2Tx∥Co2P@Ti3C2Tx) exhibits efficient engineering water-splitting performance (1.46 V@10 mA cm-2) in alkaline solution. This simple design can propose a promising approach to exploit precious-metal-free catalysts for energy conversion.

Preparation of high-activity and earth-abundant bifunctional catalysts for efficient electrochemical water splitting are crucial and challenging. Herein, Co-doped Ni3N nanosheets loaded on nickel foam (Co-Ni3N) were synthesized. The as-prepared Co-Ni3N exhibits excellent catalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline media. Density functional theory (DFT) calculation reveals that Co-Ni3N with redistribution of electrons not only can facilitate the HER kinetics but also can regulate intermediates adsorption energies for OER. Specifically, the Co-Ni3N exhibits high efficiency and stable catalytic activity, with an overpotential of only 30 and 270 mV at a current density of 10 mA cm-2 for the HER and OER in 1 M KOH, respectively. This work provides strong evidence to the merit of Co doping to improve the innate electrochemical performance in bifunctional catalysts, which might have a common impact in many similar metal-metal nitride electrocatalysts.

The effect of yttrium on the reaction of tungsten oxide with CO was studied for the preparing various morphologies and size of tungsten carbide. The reaction rate, phase composition, particles size and morphology, surface area of samples were studied by TG (thermogravimetry), XRD (X-ray diffraction), SEM (scanning electron microscopy) and BET (Brunauer–Emmett–Teller), respectively. It was found that the reaction route was following WO3 → W18O49 → WO2 → WC at lower temperature, while at higher temperature, an intermediate phase W was formed and then carbonized to WC. During the reaction of pure WO3 or mixtures (1 wt.% Y(NO3)3 + WO3) with CO at lower temperature, the final products almost maintained the large irregular block shape. With the increase of temperature, the large WC particles were degraded and the obtained sub-particles became much more dispersed. After the addition of yttrium nitrate, the rod-like sub-particles were transformed into more stable polyhedral-shaped sub-particles with a smaller size. Furthermore, the mechanism of effect of yttrium on prepared tungsten carbides were analyzed in detail.

Tungsten, one of the refractory metals, has lots of excellent properties, making tungsten powders become a choice of raw material for many applications. This work reported a simple method (the salt-assisted hydrogen reduction of WO3 powders) for preparing tungsten powders with desired morphologies and size. By adjusting the amount of additive (Y(NO3)3) and reduction temperature, metallic tungsten particles with different morphologies and size were obtained. It was found that both temperature and additive had apparent effects on the morphology and size of metallic W particles, and the large parallelepiped-shaped powders disappeared and large irregular blocky-shaped powders were degraded under the influence of additive. With the increase of temperature and amount of salt, particles became more dispersed and the size increased at higher temperatures, and the shape of particles was changed from spherical to polyhedral as well. On the contrary, the size of sub-particles became smaller as increasing amount of Y(NO3)3 at lower temperatures. Based on the results, the reaction and refining mechanisms for preparing tungsten were proposed as well.

In this investigation, the Li2CO3-assisted hydrogen reduction method was proposed to produce different morphologies and size of metallic tungsten particles. The effects of temperature and content of Li2CO3 additive were both considered. The experimental results showed that both the temperature and additive had great effects on the morphology and size of produced tungsten particles. As the increases of temperature and fraction of Li2CO3, the morphology of tungsten particles was changed from spherical to polyhedral; while the size of these particles was increased; and the obtained products became more and more dispersed. The addition of Li2CO3 also affected the reduction rate dramatically. It was found that the Li2CO3 additive retarded the initial reduction reaction, but it could obviously improve the later reduction (WO2 to W) at a high temperature, furthermore, the higher fraction of Li2CO3 made this promotion more remarkable. Based on above results, the reduction mechanism was proposed: without additive, the reduction entirely obeys pseudomorphic transformation mechanism at low temperature, and obeys CVT mechanism at high temperature; with additive, the reduction follows a mechanism of mixture of nucleation, pseudomorphic transformation and CVT at low temperature, while at high temperature, the sublimation of WO3, nucleation and CVT mechanisms all play important roles. Keywords: Li2CO3, Nucleation, Reduction, Morphology and size, Tungsten

The effect of DC bias on electrical tree growth and partial discharge (PD) characteristics in epoxy resin (Araldite LY ® 5052/Aradur® HY 5052 supplied by Huntsman) is investigated using three waveforms: AC, AC with positive DC bias, and AC with negative DC bias. Needle-plane samples are used. Tree growth is shown to be accelerated by the combined effect of DC and 50 Hz AC, beyond the AC growth rate. Positive and negative DC biased tests result in 62% and 54% reductions in average time to breakdown, respectively. Different tree structures and stages of development are associated with different partial discharge characteristics, with thick dark tree branches associated with high PD magnitudes, whereas fine tree channel growth is linked with PD magnitudes below 1 pC. AC tests showed five distinct stages of tree growth compared to four stages seen in DC biased tests. In particular, trees growing in the ‘reverse direction’ from the planar to the point electrode, which is observed in the latter stages of AC tests, is not seen in the DC biased tests. It is concluded that for composite voltages, the AC component is the essential driver of tree growth but the DC component can accelerate propagation. AC noise may therefore compromise the reliability of insulation in HVDC networks.

The reduction of metal oxides by hydrogen is a very significant reaction in metallurgical industries. Particularly, hydrogen reduction is one of the effective methods to reduce CO2 emission in iron and steel industry. The mathematical modeling of H2 reduction process would help to define the operating parameters and to get a better control of the reduction process. In this study, a new kinetic model regarding hydrogen reduction of metal oxides controlled by external gas diffusion has been proposed. The model has incorporated numbers of parameters such as temperature, size of the metal oxides, diffusion coefficient constant, hydrogen partial pressure, density of metal oxides and time. Furthermore, the shape of metal oxides (spherical, cylindrical, lamellar particles) was considered as well, and the models for the reduction of metal oxides with different shapes were combined to a formula. Also, the developed model can be applied to describe the reduction kinetics under both isothermal and non-isothermal conditions. Because the model is in the analytic function, therefore, it is convenient for usage and theoretical analysis. Application of the developed model to practical systems validated that its predicted values agreed very well with the experimental data.

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

Space charge accumulation is an important issue for HVDC cable design. The space charge accumulation are determined by the charge transport property which can be monitored by the space charge and conduction current measurement. In this paper, based on PEA system and three-electrode system, the modified combined measurement system was developed to measure the space charge and current in LDPE samples simultaneously. The results show that the current gradually decreased with the accumulation of homocharges; the generation and movement of space charge packet resulted in the current peaks; the reduction of homocharge accumulation and the increase of heterocharge tends to increase the current.

The recycling of high-titanium-bearing blast furnace slag (TiO2 > 23 wt.%) is an urgent problem that has attracted global research attention. To achieve high-efficiency, low-consumption, clean, and high value-added recycling utilization, a new process is proposed herein, which entails reacting a CH4-H2-N2 gas mixture with the titanium-bearing blast furnace slag to initially produce Ti(C, N, O) at a low temperature, whereafter the product is purified and chlorinated. The effects of Fe2O3, urea, and sawdust on the reduction and carbonitriding characteristics are also investigated. The results indicate that the Fe2O3 additive can promote the formation of Ti(C, N, O), while urea and sawdust are instrumental for enhancing gas diffusion in solid powders. The proposed novel recycling process was assessed and it evinced many advantages and great feasibility.

In the present study, a new and simple method to prepare tungsten and tungsten carbides powders was proposed by using WO3 powders as the raw material and CH3OH vapor as the reduction and carbonization agent. Both thermodynamic and experimental studies were carried out, and based on the results, the reaction routes were obtained. It was found that at low and moderate temperatures, the final product of WC was formed via intermediates of W18O49, WO2, partial metallic W and W2C; while at high temperature, WO3 was first reduced to WO2 with the intermediate phase of W18O49, and then reduced to W. The morphology of final products almost retained the same shape as that of raw materials WO3 at low temperature, however, the large particles with blocky shape disappeared and the products were all small W particles with polyhedral shape at high temperature. Furthermore, the reaction mechanisms were proposed: the pseudomorphic transformation mechanism dominated the reaction at low temperature, but at high temperature, the reduction obeyed the chemical vapor transport mechanism, which is beneficial for grain refining of final product.

Tungsten carbides, which can be widely used as cutting and drilling tools, chemical catalysts and aerospace coatings, have attracted widespread attentions. The present work reported a simple method to prepare tungsten carbides by using WO2 as the raw materials and CO as the reduction and carbonization agent. It was found that at lower and moderate temperatures, all the final product was WC, while at higher temperatures, the final products were W2C or W. The reduction and carbonization mechanism was also studied theoretically and experimentally. WO2 was reduced and carburized to WC in one step at lower temperatures; WO2 was first reduced and carburized to W2C and then to WC at moderate temperatures; while at higher temperatures, WO2 was first reduced to metallic W and then carbonized to WC or W2C, but with the temperature increasing, the carbonization became difficult and required much higher CO content.

Molybdenum hemicarbide (Mo2C) can be widely used in the fields of steel, metal ceramics, and catalyst of hydrogen-involved reactions. This work reported a new and simple method for preparing molybdenum hemicarbide by reducing and carbonizing MoO2 with methanol, and both thermodynamic and experimental studies were carried out. Based on the results, the reduction and carbonization routes are obtained: MoO2 was reduced and carburized directly to Mo2C in one step at lower temperatures, and first reduced to metallic Mo and then carburized to Mo2C at higher temperatures. The reduction mechanisms at low and high temperatures were proposed as well: The pseudomorphic transformation mechanism dominated the reaction at low temperatures, but at high temperatures, the reaction obeyed the chemical vapor transport mechanism.

The effect of temperature gradient on space charge behavior is necessary to investigate for HVDC insulation. In this paper, space charge distributions in neat epoxy resin (EP) and EP/SiO 2 nanocomposites (NC) were measured under different DC stresses and temperature gradients. We found that different temperature conditions applied to the electrodes had a great impact on space charge distributions: Only homocharge accumulated near anode at isothermal conditions, on the contrast, at temperature gradient of 60 °C, negative charge injected from cathode (high temperature side) and accumulated in the bulk, heterocharge appeared near anode (low temperature side). Moreover, SiO 2 nano-fillers added to NC could suppress the space charge accumulation significantly, and the pattern of space charge distribution in NC at temperature gradient of 60 °C also shows differently from that of EP. Numerical simulation based on the bipolar charge transport model was employed to study the experimental results. It shows that under temperature gradient, charge extraction plays an important role in heterocharge accumulation near the low temperature side. Moreover, it indicates that unlike the apparently measured conductivity, the charge mobility of NC does not increase rapidly with temperature in the range from 20 °C to 80 °C.

XLPE and its nanocomposites have been subjected to a progressive step voltage until failure occurred, and the results analyzed in terms of an inverse power law ageing expression. The step-stress tests have been conducted at three temperature 20°C,40°C and 60°C, and the exponent n of the inverse power law ageing expression has been estimated for the two materials by a new method, which fully considers the contribution to the failure of the whole sequence of voltage steps, including ageing processes at the lower voltages, the final step and the process of voltage increase. It was found that the value of exponent n increases slightly with increasing temperature for both XLPE and its nano-composites. More importantly the value of n for the nano-composites was found to be substantially lower than that for XLPE at all temperatures. A complementary experiment in which the space charge distribution was measured under the same step-stress protocol as the breakdown tests has also been carried out and the results of both experiments are discussed.

Methane reduction has attracted substantial interest in recent years because an abundant amount of natural gas has been found and methane possesses a strong reduction ability. However, due to the complexity of the reaction, the reductive-kinetics model has not been developed very well. This work reported a new mathematical model for methane reduction. The model is in a form of explicit functions incorporating many parameters to increase its precision. Particularly, it considers the comparison of methane cracking rate and reaction rate. Both the gas diffusion in the product layer and chemical reaction controlled-kinetics formulae were deduced by considering three kinds of shapes (spherical, cylindrical and lamellar) of particles. Also, by employing two parameters (shape coefficient Sc and equivalent diameter d0), the formulae for the same reduction mechanism could be unified to one formula, which was easier to use. The simulation of the model also considers both isothermal and non-isothermal processes of methane reduction. Furthermore, it can describe the reduction of oxides of varied-valence metals. The kinetics of reduction of metal oxides by methane agrees with the results obtained in the practical system.

Titanium oxycarbide can be used as a soluble anode to produce sponge titanium in USTB or MER molten salt electrolysis process. However, the key factor of these two processes is to prepare oxycarbide with appropriate C/O ratio and at a low cost as well. This work reported a simple method of preparing titanium oxycarbide from TiO2 in C-H2 system. Based on the prediction of thermodynamic calculations, titanium oxycarbide was prepared successfully at 1400 °C with the molar ratio of TiO2 to carbon exceeding 1:2, and the ideal electrolytic anode (TiC0.5O0.5) was produced at 1400 °C (with a ratio of 1:3 of TiO2 to carbon). Number of parameters (carbon content, temperature, sample's mass, additive (Fe element), and with ball milling treatment) were considered and discussed in the current study. Furthermore, this method can reduce not only the reaction temperature and time, but also the emission of carbon dioxide.

The effects of charge carrier injection and intrinsic ionic carriers on the conduction process in superclean low-density polyethylene and cross-linked polyethylene (LDPE and XLPE) and the associated development of space charge are investigated. The carrier mobility is measured directly by space charge measurements in a double- and triple-layer structure. It is shown that the difference between superclean LDPE and XLPE in the field-dependence of the current density results from the dominance of ions in XLPE introduced by cross-linking as compared to injected carriers in the case of superclean LDPE.

Partial discharge (PD) characteristics are inherently linked to associated electrical tree growth characteristics. This paper employs both phase-resolved patterns and pulse-sequence analysis (PSA) to study PDs in a short branch-tree grown from a metallic needle tip. It is found that the shape of PSA dV-dV patterns vary with applied voltage; in particular the values of characteristic voltage difference increase with applied voltage. At 10 kV the value of characteristic voltage difference decreases with tree growth, the average number of PDs per cycle increases, the PD magnitudes decrease, and the characteristic wing-like phase-resolved pattern evolves into a turtle-like pattern. This paper proposes a physical model relating PD activity to tree development, considering space charge accumulation inside the tree channel. All key features of experimental results are explained by the model. It is shown that the characteristic voltage differences are determined by the local PD inception and extinction electric fields. These key parameters determining the PD events can be calculated giving a clear physical interpretation of PD measurements, which is invaluable to those using PD as a condition monitoring or asset management tool.

The movement of space charge packets has been reported in polyethylene and epoxy resin under high electric fields by several groups, and has been characterized by the speed of propagation. A number of explanations of the formation and transport of slow space charge packets has been proposed. However, the mechanisms creating fast space charge packets are not clear. Dissado et al have suggested that the formation of fast charge packets is due to discontinuous electromechanical compression and charge injection at the electrode-insulation interface, and the transport of charge packets is related to corresponding relaxation processes. In that model, charges travel in packets because of group polarization. This paper gives an alternative model considering the reduction of charge carrier activation energy due to charge density triggered polymer chain movement and subsequent chain relaxation times. It is shown that the generation and transport of fast charge pulses are readily simulated by a bipolar charge transport model with three additional parameters: reduced activation energy, a charge density threshold, and a polymer chain relaxation time.

Simultaneous imaging of electrical tree growth and partial discharge measurement have allowed 5 stages of tree growth to be identified. Partial discharge (PD) characteristics have been collected from tree growth in needle-plane epoxy resin samples under 15 kV peak AC voltage, and have been shown to behave differently at different stages of tree growth. Initial slow growth is accompanied by large discharges (about 10 pC). Thereafter, growth of fine channels is associated with smaller PDs (about 0.3 pC). It was observed that the tree propagation is further driven by even smaller PDs below the equipment sensitivity. However, when the tree approaches the counter electrode, the PD magnitude increases again and a more substantial tree grows with a low propagation rate. Reverse tree channels then occur from the planar electrode towards the needle, together with higher magnitude PDs characterized by thick and dark branches before breakdown.