Gennady E. Shter

Photoelectrochemical water splitting in separate oxygen and hydrogen cells

Solar water splitting provides a promising path for sustainable hydrogen production and solar energy storage. One of the greatest challenges towards large-scale utilization of this technology is reducing the hydrogen production cost. The conventional electrolyser architecture, where hydrogen and oxygen are co-produced in the same cell, gives rise to critical challenges in photoelectrochemical water splitting cells that directly convert solar energy and water to hydrogen. Here we overcome these challenges by separating the hydrogen and oxygen cells. The ion exchange in our cells is mediated by auxiliary electrodes, and the cells are connected to each other only by metal wires, enabling centralized hydrogen production. We demonstrate hydrogen generation in separate cells with solar-to-hydrogen conversion efficiency of 7.5%, which can readily surpass 10% using standard commercial components. A basic cost comparison shows that our approach is competitive with conventional photoelectrochemical systems, enabling safe and potentially affordable solar hydrogen production.

Adsorption of sulfur dioxide by active carbon treated by nitric acid: II. Effect of preheating on the adsorption properties

Abstract The adsorption and desorption of sulfur dioxide by active carbon, treated by nitric acid and followed by heating at 200–1000 °C, were studied. The HNO 3 treated carbon has a higher adsorption capacity for sulfur dioxide, than the untreated carbon, in spite of its lesser basicity. This may be the result of interaction of the SO 2 with the oxygenated surface groups of acidic character. Water promotes the adsorption of SO 2 . Heating results in a decrease of the carbon acidity because of decomposition of the surface oxygenated structures. It also leads to an increase of basicity resulting from the formation of basic sites. These changes of the carbon acid-base properties are particularly pronounced for the HNO 3 treated carbon due to higher concentration of the surface oxygenated groups. The increased basicity promotes adsorption of SO 2 and inhibits desorption by an acid-base mechanism different from the mechanism of adsorption on the carbon, which has not been preheated.

Optimal temperatures for catalytic regeneration of activated carbon

Abstract Activated carbons impregnated with a mixture of copper( ii ), iron( iii ) and chromium( iii ) oxide catalysts were prepared in order to facilitate catalytic regeneration of AC saturated with phenols. The temperature ranges of phenol oxidation ( T p ), carbon oxidation ( T c ) and carbon burn-off were determined from DTA/TGA investigations in oxidizing and inert atmospheres. The impregnated catalyst facilitated phenol oxidation but also lowered the T c of the AC. Catalytic regeneration can be conducted between T p (220–240 °C) and T c (320–370 °C). Catalytic regeneration of spent impregnated carbon, at 270 °C in flowing air, restored almost all of its initial adsorption capacity and specific surface area while in the absence of a catalyst the regeneration achieved was only 25–30%. The results obtained from the polythermic investigations (DTA/TGA) correlate well with those from the regeneration experiments and, therefore, may be used for determination of temperature conditions for catalytic regeneration of saturated carbons.

Relationship of solubility parameter (x), powder properties and phase formation in the Nd1+xBa2−xCu3O6.5+x2+δ system

Abstract The relationship between the solubility parameter, x, in the solid solutions Nd 1+x Ba 2−x Cu 3 O 6.5+x 2+δ (Nd123SS) and XRD patterns, powder surface area (SA), particle size, morphology and melting points was investigated. An efficient way to determine the value of x and residual BaCuO2 content during Nd123SS powder synthesis is presented. The method is based on calculation of the orthorhombic splitting (OS) factor from the unit cell parameters obtained from XRD data. The final phase in the Nd123SS system is formed through a diffusion controlled reaction between BaCuO2 and Nd123SS and a kinetic model is developed to describe the formation of the Nd123 (x=0) superconducting powder. Finally, high values of Tc (measured by DC magnetization) were found in the powders. The highest Tc of 98.7 K was measured for the x=0 case. This value is the highest ever reported for the NdBaCuO system. The Tc was found to be insensitive to the value of x, ranging between 98.7 and 94 K for x between 0 and 0.25, respectively. This data is contrast to published results on samples synthesized at higher temperatures, where Tc fell to 40 K at x = 0.25.

Nitrogen-based alternative fuel: an environmentally friendly combustion approach

We report here on a continuous combustion of a low carbon nitrogen-based alternative fuel. The investigated fuel, an aqueous solution of urea and ammonium nitrate, consists of common fertilizer commodities. This nonflammable, nontoxic and nonexplosive fuel underwent combustion at a pressure range of 1 to 25 MPa. The pressure was found to greatly affect pollutant levels in the effluent gas. Molecular nitrogen yield was 99.89% at 25 MPa, and the lowest NOx level was 128 mg MJ−1, below the regulation standard for power generation. The alternative fuel described herein is an excellent gas generator, producing an environmentally friendly working fluid consisting of 73.0% H2O, 21.6% N2, and 5.4% CO2. This fuel has the potential to be a sustainable future renewable energy storage medium as well as an energy carrier.

Experimental investigation of current-limiting device model based on high-Tc superconductors

Abstract The model of an inductive current-limiting device consists of a copper coil and a high-T c superconducting ring which are placed on a ferrite core and are coupled magnetically. The rings were prepared by extrusion of the YBaCuO submicron precursor powder and organic binder, followed by sintering in oxygen atmosphere. The principle of the devices operation is based on a rapid rise of the devices inductance at the transition of the ring from the superconducting to the normal state. The results demonstrate that the device can reduce both the transient and the steady-state fault current significantly. The influence of thermal processes in the ring on the mode of device operation in the circuit is discussed.

Thermal behavior of the phenol–Pd–ACC system

Thermal behavior, morphology features and phenol adsorption as well as Pd catalytic properties in activated carbon cloth (ACC)–Pd–Phenol system and its subsystems were investigated under oxidizing and inert atmospheres. Pd/ACC systems were classified in two types: systems with low Pd loadings (Pd <1.3% wt.) were found to be more active than systems with high Pd loading. Thermal behavior of the systems was described using characteristic temperatures marking the initiation of phenol oxidation (Tp), ACC surface oxidation (Tc) and the carbon gasification (Tg). Analysis of the DTA/TGA profiles was interpreted in terms of desorption, carbonization, oxidative coupling reactions and complete combustion. Data on thermal behavior of systems in argon and air demonstrated a significant catalytic activity of impregnated Pd towards oxidation of ACC surface and its skeleton, and the processes of the phenol/ACC carbonization and oxidation. The results obtained are the basis for development of thermal and catalytic regeneration processes for ACC adsorbent.

The Evolution of Microstructure in Nonhydrolytic Alumina Xerogels

The effect of drying, aging and thermal treatment of alumina xerogels prepared by the nonhydrolytic route was investigated using SAXS, BET and HR-SEM techniques. The microstructure of the fresh xerogels prepared under different procedures varied drastically, ranging from aerogel-like mass fractals to narrow pore size distribution materials. By variation of the drying conditions the N2-BET surface area was varied from an immeasurable low level up to 600 m2/g. The initial microstructure has a significant influence on the xerogel behaviour during the post-drying heating stage. The ability to produce aerogel-like mass fractal materials from the nonhydrolytic systems is discussed. Finally, a brief theoretical treatment of the drying process of mass fractals is presented as well.

Thermal analysis of aqueous urea ammonium nitrate alternative fuel

The thermal decomposition of aqueous ammonium nitrate, aqueous urea and aqueous urea ammonium nitrate was investigated by means of simultaneous TGA/DTA/DTG/MS analysis under ambient pressure, and DSC under applied pressures of 5 and 10 MPa. Aqueous urea ammonium nitrate was previously suggested as a low carbon nitrogen-based alternative fuel. Investigation of the processes which occur in the condensed phase as the temperature increases is crucial in order to understand the combustion mechanism of the suggested alternative fuel. Isomerization of urea into ammonium cyanate as well as urea hydrolysis was inhibited in the presence of ammonium nitrate, hence the fuel is considered to be chemically stable at room temperature. No solid residuals remained above 315 °C. Thermal decomposition of the fuel under ambient pressure was found to involve four principal endothermic stages: (a) water vaporization, (b) urea decomposition along with biuret formation, (c) biuret decomposition, and (d) ammonium nitrate dissociation. The thermal decomposition of the fuel under isobaric conditions of 5 and 10 MPa revealed only an exothermic process with a sharp increase in the heat flow above 300 °C. The present research increases the basic understanding of the suggested nitrogen-based alternative fuel combustion process.

Synthesis of Nd123 superconducting powder via oxalate coprecipitation

Abstract The preparation of pure and fine Nd123 powder was investigated in five ways. Precursor powders were generated by the oxalate coprecipitation. Some of the powders were heated in vacuum (770°C for 20 h at 10–15 Pa), followed by calcination at different temperatures and oxygen partial pressures. The vacuum step was omitted with the remaining powders. Calcination under pure oxygen requires excessively high temperatures ( T >940°C). Under these conditions the powder is badly fused, with grain sizes in the 7–12 μm range. In addition, the orthorhombicity of this powder is low due to the formation of Nd123 solid solution — implying the presence of residual BaCuO 2 . The optimal route to produce the desired powder is in a low partial pressure of 350–1000 ppm oxygen in nitrogen, without a vacuum preheating step. Similar powders can be obtained in 1% O 2 with a vacuum preheating step. At 60 ppm O 2 the reaction between Nd 1+ x Ba 2− x Cu 3 O 6.5+ x /2+δ and BaCuO 2 is incomplete, with about 5% of barium cuprate remaining. Finally, the T c of the powder produced at the optimal conditions was found by DC magnetization to be ∼ 99 K.