Initial stage sintering mechanism of NaNbO3 and implications regarding the densification of alkaline niobates

The behavior of submicron- and nano-sized NaNbO₃ powder compacts during conventional sintering was studied using optical dilatometry and microstructure analysis. Microstructure-development trajectories revealed the dominance of grain growth during the initial sintering stage, while densification occurred only during later stages. Surface diffusion with low activation energy in the range of 50–60 kJ/mol was found to be the dominant material-transport mechanism during the initial sintering stage. The early activation of surface diffusion reduced the sintering driving force, decreased the rate of the densifying mechanisms and was thus identified as the main cause for poor densification of NaNbO₃. Same explanation could be valid also for other alkaline niobate based lead-free piezoelectric ceramics. Finally, alternative sintering methods are discussed and the efficiency of the pressure-assisted sintering was demonstrated in successful production of highly-dense fine-grained NaNbO₃ ceramics, with relative density and grain size of 98% and 700 nm, respectively.     

Electrochemical investigations on amorphous Fe-base alloys for alkaline water electrolysis

In this work different amorphous melt-spun Fe-alloys (Fe₈₂B₁₈, Fe₈₀Si₁₀B₁₀, Fe₆₀Co₂₀Si₁₀B₁₀) were investigated as cathode materials for the alkaline electrolysis of water. In particular, the influence of cobalt as well as the metalloids boron and silicon on the activity for the hydrogen evolution reaction (HER) was studied in 1 M KOH at 298 K using cyclic voltammetric, galvanostatic and polarization techniques. The electrocatalytic activity was evaluated in the view of the overpotential. It was found that cyclic voltammetric techniques can be used to activate the melt-spun Fe-alloys strongly. Different cyclic voltammetric activation procedures are discussed and the influence of the sweep rate and the potential window on the HER activity was elucidated. The experimental data indicate that the addition of metalloids and, most importantly, of cobalt improves the HER activity of the materials. Thus, the overpotential can be reduced by 200 mV compared to polycrystalline Ni.     

Rheological performance of pectin-enriched products isolated from red beet (Beta vulgaris L. var. conditiva) through alkaline and enzymatic treatments

The rheological performance of pectin-enriched products extracted from red beet (Beta vulgaris L. var. conditiva) root by-products was evaluated in the present work. They were extracted through an alkaline pre-treatment with or without a subsequent enzymatic (hemicellulase or cellulase) hydrolysis at pH 5.2. Flow assays performed with 2.00% w/v-pectin aqueous systems showed pseudoplastic (flow index, n ≈ 0.4 or 0.8) or Newtonian (n = 1.0) behaviour after fitting of experimental data to Ostwald’s law, also showing poor thickening effect. When Ca²⁺ was added to water with the same pectin concentration, true gels developed as confirmed by the mechanical spectra obtained through dynamic assays. Junction zones of homogalacturonan (HG) side chains mediated by Ca²⁺ were able to build up rigid networks in water.Isolated pectins (2.00% w/v) were also used to constitute milk model systems. Whole and skimmed milk were used at two different concentrations. Milk systems showed more transient and weaker gel networks when compared to Ca²⁺-aqueous systems, and were associated to the formation of a [κ-casein?calcium cross linked low methoxyl pectin] complex dampened by the included milk fat globules. Relaxation spectra of pectin-milk systems were in general extended to large relaxation times (10⁴ s) for all isolated fractions studied, which is typical of structured systems. Since all pectin fractions showed very similar chemical composition and molecular weight (average value and distribution), it was suggested that some differences in the rheological performance of each pectin product came from the different length of arabinans and distribution of rhamnose kinks (RG-I, random coil) as well as from the length of demethylated HG chains (semi-flexible coils).The results of this research show that the pectin-enriched fractions isolated from red beet root wastes are useful as additives in food formulation.     

Polyamide-coated Nafion composite membranes with reduced hydrogen crossover produced via interfacial polymerization

Nafion, a perfluoro-sulfonic acid (PFSA)-based polymer, is a promising material that will help realize the commercialization of proton exchange membrane-based fuel cells (PEMFCs) and proton exchange membrane water electrolyzers (PEMWEs). However, Nafion also exhibits reduced mechanical and dimensional stability and increased hydrogen crossover under cell operating conditions in real operational settings, that is, in a hydrated state or in water at 60–80 °C. These factors may negatively affect cell efficiency and durability and thus must be addressed. To overcome these limitations, polyamide-coated Nafion composite membranes were developed for the first time via interfacial polymerization. 3,5-Diaminobenzoic acid (DABA), which contains carboxyl functional groups, was used as a monomer to add hydrophilicity to the membrane, and the coating layer thickness was controlled by adjusting the DABA content. A nanoscale polyamide (PA) layer was coated on the surface of Nafion-212 to fabricate a membrane, PA-c3-Nafion. PA-c3-Nafion was found to show ion conductivity 13.6% higher than that of a pristine Nafion-212 membrane at 80 °C, while providing improved mechanical performance and dimensional stability. In particular, at 95% RH, the hydrogen permeability of PA-c3-Nafion was 16.4% lower than that of Nafion-212 while, in a fully hydrated state, the hydrogen permeability of PA-c3-Nafion was 21.2% lower than that of Nafion-212. The LSV test results also showed that the degree of hydrogen crossover was significantly lower in PA-c3-Nafion than in Nafion-212.     

Alkali-activated slag concrete: Fresh and hardened behaviour

The behaviour of fresh and hardened alkali-activated slag (AAS) and OPC concretes was compared and the effect of mixing time assessed. OPC and AAS concrete slump and rheological results proved to differ, particularly when the slag was activated with waterglass (WG). The nature of the alkaline activator was the key determinant in AAS concrete rheology. Bingham models afforded a good fit to all the OPC and AAS concretes. In OPC and NaOH-activated AAS concretes, longer mixing had an adverse effect on rheology while improving hardened performance only slightly. In WG-AAS concrete, longer mixing times, improved mechanical properties and also rheological behaviour was enhanced, in which those conditions were required to break down the microstructure. Longer mixing raised thixotropy in OPC and NaOH-activated AAS concretes, but lowered the value of this parameter in waterglass-activated slag concrete.     

Facile synthesis of polyoxometalate-based composite with doped ternary NiCoFe cations as electrocatalyst for oxygen evolution reaction

The construction of efficient and low-cost electrocatalysts for oxygen evolution reactions (OER) to replace precious catalysts is a necessity to achieve economic production of hydrogen. Herein, we report an efficient tri-metallic electrocatalysts for the OER that is prepared by incorporate nickel, cobalt and iron cations on Triton X-100/phosphotungstic acid organic-inorganic composite without utilize any binders or energy consumer procedure. Considering to the synergy effect of simultaneous absorption of NiCoFe cations on composite substrate, the as-made tri-metallic catalyst exhibits excellent OER activity with a small overpotential of 210 and 330 mV at a current density of 10 and 100 mA cm^?2, respectively. Moreover, remarkable trends in electrocatalytic activity of mono-, bi- and tri-metallic electrocatalysts at low (10 mA) and high (100 mA) current density are observed. In addition, this new families of non-precious metal catalyst shows long-term durability in 1 M KOH.     

Nickel-decorated MoS2/MXene nanosheets composites for electrocatalytic oxidation of methanol

Nickel incorporated on MoS₂/MXene composites (NiMoS₂/MXene) via a wet impregnation method is used as an anode electrode material for methanol electro-oxidation. X-ray diffraction, X-ray photoelectron spectra, and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy techniques were used for the confirmation of MoS₂/MXene formation. Textural properties of catalysts were obtained in N₂ adsorption-desorption analysis. Electrochemical measurements in 0.1 M KOH demonstrated the better electrocatalytic activity of NiMoS₂/MXene catalysts. The NiMoS₂/MXene system exhibited enhanced electrocatalytic activity for methanol oxidation due to low onset potential offered by Ni, high tolerance toward CO poisoning by MoS₂, and high conductivity and high mechanical stability of MXene. NiMoS₂-3/MXene catalysts exhibited high current density, electrochemical active surface area, long-term stability, and low Rct value. Based on the electrochemical results, NiMoS₂/MXene catalysts is a highly electroactive anode material. Hence can be utilized in fuel cell applications such as Direct Methanol Fuel Cell (DMFC).     

Microscopic high-speed video observation of oxygen bubble generation behavior and effects of anode electrode shape on OER performance in alkaline water electrolysis

An important difficulty associated with alkaline water electrolysis is the rise in anode overpotential attributable to bubble coverage of the electrode surface. For this study, a system with a high-speed video camera was developed, achieving in-situ observation of bubble generation on an electrode surface, monitoring an area of 1.02 mm² at 6000 frames per second. The relation between polarization curve (current density up to 3.0 A cm^?2) and oxygen bubble generation behavior on nickel electrodes having cylindrical wires and rectangular wires of different sizes (100–300 μm) was clarified. The generated bubbles slide upward, contacting the electrode surface and detaching at the top edge. Observations indicate that small electrodes have short bubble residence time and thin bubble covering layer on the electrode. As a result, the small electrode diameter contributes to smaller overpotential at high current density.     

Nickel hydroxide armour promoted CoP nanowires for alkaline hydrogen evolution at large current density

The development of hydrogen evolution activity (HER) electrocatalyst that can run durably and efficiently under the large current density is of special significance but still challengeable for the massive production of hydrogen. Herein, a CoP/Ni(OH)₂ nanowire catalysts grown on Co foam (CF) with a three-dimensional heterojunction structure has been successfully prepared by electrodepositing nickel hydroxide on the surface of cobalt phosphide. The prepared CoP/Ni(OH)₂–15 min sample reveals a superior HER activity and stability. It merely requires ultralow overpotentials of 108 and 175 mV to 100 and 500 mA cm^?2, respectively. In addition, the long-term stability test shows that the catalyst (CoP/Ni(OH)₂–15 min) can operate stably for at least 70 h at 400 mA cm^?2. Utilizing NiFe-LDH/IF with high OER activity, the NiFe-LDH/IF || CoP/Ni(OH)₂–15 min catalyst system possesses the same outstanding performance for overall water splitting (OWS), which can accomplish ≈ 500 mA cm^?2 at 1.74 V in 1 M KOH electrolyte. Moreover, the NiFe-LDH/IF || CoP/Ni(OH)₂–15 min couple can work for more than 80 h at 500 mA cm^?2, indicating its a great prospect in the area of electrolysis water. Such excellent catalytic performance is mainly attributed to the armor effect of Ni(OH)₂, which can not only promote the rapid decomposition of water molecules, but also prevent the loss of phosphorus and enhance the synergistic effect of CoP and Ni(OH)₂. This work can offer a significant reference for the design with high-performance and durable transition metal phosphide electrocatalysts.     

Efficient MOF- derived V–Ni3S2 nanosheet arrays for electrocatalytic overall water splitting in alkali

The synergistic achievement of low-cost earth-abundant electrocatalysts and high efficiency to meet renewable energy need is highly desirable yet challenging. Here, we developed a simple Ni foam self -templating route for V-doped Ni₃S₂ nanosheet arrays through in situ formation of metal-organic frameworks (MOFs) combined with subsequent conversion. The as-prepared MOF-V-Ni₃S₂/NF catalyst delivers outstanding electrocatalytic performance in the alkaline solution, which requires low overpotentials of 118.1 mV @10 mA cm^?2 and 268 mV @10 mA cm^?2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. The V-doping and MOF-derived 3D hieratical nanostructure play a vital role in the catalytic process, which provides efficient active sites and large surface areas. Furthermore, an alkaline electrolyzer was assembled with two pieces of MOF-V-Ni₃S₂/NF, which achieves efficient water splitting at 1.58 V @10 mA cm^?2. This strategy opens up new channels to synthesize MOF-based bifunctional electrocatalysts toward overall water spitting.